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JXXX. Interplay of oxygen and hydrogen in catalyst formation for diameter-controlled vertically-aligned carbon nanotube growth.
W. Shi, J. Li, E.S. Polsen, C.R. Oliver, Y. Zhao, E.R. Meshot, H. Fairbrother, A.J. Hart, D.L. Plata.
JXXX. Selective photo-mechanical detachment and retrieval of divided sister cells from enclosed microfluidics for downstream analyses.
Y.C. Chen, H.W. Baac, K.T. Lee, S. Fouladdel, K. Teichert, J.G. Ok, Y.H. Cheng, P. Ingram, A. J. Hart, E. Azizi, L. J. Guo, M. Wicha, E. Yoon.
JXXX. Compression and recovery of carbon nanotube foams described as a phase transition.
X. Liang, J. Shin, D. Magagnosc, Y. Jiang, S. Park, A.J. Hart, K.T. Turner, D.S. Gianola, P. Purohit.
JXXX. Thermophysical phenomena in metal additive manufacturing by selective laser melting: fundamentals, modeling, simulation, and experimentation.
C. Meier, R. Penny, Y. Zou, J.S. Gibbs, A.J. Hart.
JXXX. Rate limits of additive manufacturing by fused filament fabrication and guidelines for high-throughput system design.
J. Go, S. Schiffres, A.G. Stevens, A.J. Hart.
JXXX. Deployable multi-stable structures from twist-coupled Kirigami cells.
N. Nayakanti, S. Tawfick, A.J. Hart.
JXXX. Compression and recovery of carbon nanotube foams described as a phase transition.
X. Liang, J. Shin, D. Magagnosc, Y. Jiang, S. Park, A.J. Hart, K.T. Turner, D.S. Gianola, P. Purohit.
JXXX. Additive manufacturing for engineers and designers: materials, processes, and emerging capabilities.
G. D’Angelo, A.J. Hart, D.B. Pedersen, H.N. Hansen.
Accepted / In Press
JXXX. Liquid imbibition in ceramic-coated carbon nanotube films.
H. Zhao, C. Jacob, H.A. Stone, A.J. Hart. Langmuir
Understanding of the liquid imbibition dynamics in nanoporous materials is important to advances in chemical separations, phase change heat transfer, electrochemical energy storage, and diagnostic assays. We study the liquid imbibition behavior in films of ceramic-coated vertically aligned carbon nanotubes (CNTs). The nanoscale porosity of the films is tuned by conformal ceramic coating via atomic layer deposition (ALD), enabling stable liquid imbibition and precise measurement of the imbibition dynamics without capillary densification of the CNTs. We show that the imbibition rate decreases as the ceramic coating thickness increases, which effectively changes the CNT-CNT spacing and therefore decreases the permeability. We derive a model, based on Darcy’s law, that incorporates an expression for the permeability of nanoscale post arrays, and we show that the model fits the experimental results with high accuracy. The tailorable porosity, along with controllable surface wettability and mechanical stability of coated CNTs, suggest their suitability for application-guided engineering, and for further investigation of imbibition behavior at finer length scales.
JXXX. Additive manufacturing of cellulosic materials with high strength and antimicrobial functionality.
S.W. Pattinson, A.J. Hart. Advanced Materials Technologies.
J122. Ultrathin high-resolution flexographic printing of electronic materials using nanoporous stamps.
S. Kim, H. Sojoudi, H. Zhao, D. Mariappan, G.H. McKinley, K.K. Gleason, A.J. Hart. Science Advances.
S. Kim, H. Sojoudi, H. Zhao, D. Mariappan, G.H. McKinley, K.K. Gleason, A.J. Hart. Ultrathin high-resolution flexographic printing of electronic materials using nanoporous stamps. Science Advances. 2:e1601660. [http://dx.doi.org/10.1126/sciadv.1601660]
Since its invention in ancient times, relief printing, commonly called flexography, has been used to mass-produce
artifacts ranging from decorative graphics to printed media. Now, higher-resolution flexography is essential to
manufacturing low-cost, large-area printed electronics. However, because of contact-mediated liquid instabilities
and spreading, the resolution of flexographic printing using elastomeric stamps is limited to tens of micrometers.
We introduce engineered nanoporous microstructures, comprising polymer-coated aligned carbon nanotubes (CNTs), as a next-generation stamp material. We design and engineer the highly porous microstructures to be wetted by colloidal inks and to transfer a thin layer to a target substrate upon brief contact. We demonstrate printing of diverse micrometer-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO3, and CdSe/ZnS, onto both rigid and compliant substrates. The printed patterns have highly uniform nanoscale thickness (5 to 50 nm) and match the stamp features with high fidelity (edge roughness, ~0.2 mm). We derive conditions for uniform printing based on nanoscale contact mechanics, characterize printed Ag lines and transparent conductors, and achieve continuous printing at a speed of 0.2 m/s. The latter represents a combination of resolution and throughput that far surpasses industrial printing technologies.
J121. Morphology-dependent load transfer governs the strength and failure mechanism of carbon nanotube yarns.
A. Rao, S. Tawfick, M. Bedewy, A.J. Hart. Extreme Mechanics Letters.
A. Rao, S. Tawfick, M. Bedewy, A.J. Hart. Morphology-dependent load transfer governs the strength and failure mechanism of carbon nanotube yarns. Extreme Mechanics Letters. [http://dx.doi.org/10.1016/j.eml.2016.05.003]
The outstanding properties of individual carbon nanotubes (CNTs) have motivated interest in CNT fibers for composite materials, high-strength conductors and multifunctional textiles. However, despite advances in manufacturing, the strength of CNT fiber remains 10–100 fold less than individual CNTs. In light of the complex, multi-scale load transfer in CNT yarns, a hierarchical model taking into consideration the morphology-dependent mechanics is necessary to understand this limitation. We present a coupled analytical and finite element model of three-dimensional morphology and the full tensile behavior of CNT yarns. By incorporating load-induced changes in morphology, simulations of yarns in tension show different load paths such as fracture-type or stick–slip failure depending on the waviness and number density of CNTs. The strength of untwisted pristine CNT yarns is shown to be limited to <10% of the intrinsic CNT strength, even at practical limits to CNT packing density and alignment. Load-induced changes in CNT morphology are verified by tensile testing of CNT yarns along with in-situ X-ray scattering. In addition, the nominal structure of the yarn is shown to strongly influence the improvement in strength achieved by densified and/or cross-linking, and a sublinear relationship between CNT contact enhancement and yarn strength is predicted. Thus, this work provides a means to investigate the complex load transfer mechanisms in CNT assemblies, and to further study the process-structure–property limits of CNT yarns.
J120. Conformal robotic stereolithography.
A.G. Stevens, C.R. Oliver, M. Kirchmeyer, J. Wu, L. Chin, E.S. Polsen, C. Archer, C. Boyle, J. Garber, A.J. Hart. 3D Printing and Additive Manufacturing.
Additive manufacturing by layerwise photopolymerization, commonly called stereolithography (SLA), is attractive due to its high resolution and diversity of materials chemistry. However, traditional SLA methods are restricted to planar substrates and planar layers that are perpendicular to a single-axis build direction. Here, we present a robotic system that is capable of maskless layerwise photopolymerization on curved surfaces, enabling production of large-area conformal patterns and the construction of conformal freeform objects. The system comprises an industrial six-axis robot and a custom-built maskless projector end effector. Use of the system involves creating a mesh representation of the freeform substrate, generation of a triangulated toolpath with curved layers that represents the target object to be printed, precision mounting of the substrate in the robot workspace, and robotic photopatterning of the target object by coordinated motion of the robot and substrate. We demonstrate printing of conformal photopatterns on spheres of various sizes, and construction of miniature three-dimensional objects on spheres without requiring support features. Improvement of the motion accuracy and development of freeform toolpaths would enable construction of polymer objects that surpass the size and support structure constraints imparted by traditional SLA systems.
J119. Real-time imaging of self-organization and mechanical competition in carbon nanotube forest growth.
B. Viswanath, M. Bedewey, E.R. Meshot, S.W. Pattinson, E.S. Polsen, F. Laye, D.N. Zakharov, E. Stach, A.J. Hart. ACS Nano
The properties of carbon nanotube (CNT) materials, and many analogous materials comprising filamentary nanostructures, are governed by the intrinsic filament properties and their hierarchical organization and interconnection. As a result, direct knowledge of the collective dynamics of CNT synthesis and self-organization is essential to engineering improved CNT materials for applications such as membranes and thermal interfaces. Here, we use real time environmental transmission electron microscopy (E-TEM) to observe nucleation and self-organization of CNTs into vertically aligned forests. Upon introduction of the carbon source, we observe a large scatter in the onset of nucleation of individual CNTs and the ensuing growth rates. Experiments performed at different temperatures and catalyst particle densities show the critical role of CNT density on the dynamics of self-organization; low-density CNT nucleation results in the CNTs becoming pinned to the substrate and forming random networks, whereas higher-density CNT nucleation results in self-organization of the CNTs into bundles that are oriented perpendicular to the substrate. We also find that mechanical coupling between growing CNTs alters their growth trajectory and shape, causing significant deformations, buckling, and defects in the CNT walls. Therefore, it appears that CNT-CNT coupling is not-only critical for self-organization, but also directly influences CNT quality and likely the resulting properties of the forest. Our findings show that control of the time-distributed kinetics of CNT nucleation and bundle formation are critical to manufacturing of well-organized CNT assemblies, and that ETEM can be a powerful tool to investigate the mesoscale dynamics of CNT networks.
The handheld micropipette is the most ubiquitous instrument for precision handling of microliter-milliliter liquid volumes, which is an essential capability in biology and chemistry laboratories. The range of one pipette is typically adjustable up to 10-fold its minimum volume, requiring the use and maintenance of multiple pipettes for liquid handling across larger ranges. Here we propose a design for a single handheld pipette adjustable from 0.1 ?l to 1000 ?l (i.e., 10^4-fold) which spans the range of an entire suite of current commercial pipettes. This is accomplished by placing an elastic diaphragm between the existing pipette body and tip, thereby de-amplifying its native volume range while maintaining its simple manual operating procedure. For proof-of-concept, we adapted a commercial pipette (100-1000 ?l nominal range) with a selection of rubber sheets to function as the diaphragms and confirmed the accuracy and precision of drawn volumes are within international ISO-8655 standards across the entire 10^4-fold volume range. The presence of the diaphragms introduces a nonlinear mechanical behavior and a time-dependency due to heat transfer, however, by model and experiment, these are redressed so as to maintain the pipette’s accuracy and precision.
J117. Molecular gastronomy meets 3D printing: layered construction via reverse spherification.
G. D’Angelo, H.N. Hansen, A.J. Hart. 3D Printing and Additive Manufacturing.
G. D’Angelo, H.N. Hansen, A.J. Hart. Molecular gastronomy meets 3D printing: layered construction via reverse spherification. 3D Printing and Additive Manufacturing, 3(3):152-159, 2016.[dx.doi.org/10.1089/3dp.2016.0024]
The potential use of additive manufacturing (AM) techniques for processing of food can span from satisfaction of basic necessities to high-end cuisine and fine dining. The purpose of this study was to explore how AM, specifically extrusion-based layer-wise deposition, can be combined with the reverse spherification technique that is widely used in molecular gastronomy. First, by manual extrusion, we identify suitable recipes and ingredient concentrations to form freestanding features in a liquid bath. Subsequently, a desktop extrusion is adapted for the deposition of a calcium solution into an alginate bath first as a two-dimensional (2D) pathway and then as three-dimensional (3D) geometry by layer-wise deposition. The 2D geometries are measured and compared to a nominal geometry, to elucidate how tool speed and extrusion rate influence form and dimensional accuracy. We demonstrate that motorized extrusion-based AM can be combined with reverse spherification to form stable objects by gelation of fruit-based solutions. In addition, a wider set of manual experiments shows the possibility of combining different flavors and the creation of complex multilayer and multiflavor objects. Additional studies on the deposition precision are required to optimize the process of creating a full 3D geometry. This study shows that 3D printing via reverse spherification can bridge the gap between culinary art and AM technology, and enable new capabilities for creation of dining experiences. This is a step toward the digital design and manufacturing of unique edible objects with complex flavors, textures, and geometries.
J116. Measurement of the dewetting, nucleation, and deactivation kinetics of carbon nanotube population growth by environmental transmission electron microscopy.
M. Bedewy, B. Viswanath, E.R. Meshot, D.N. Zakharov, E.A. Stach, A.J. Hart. Chemistry of Materials.
Understanding the collective growth of vertically aligned carbon nanotube (CNT) populations is key to tailoring their properties for many applications. During the initial stages of CNT growth by chemical vapor deposition (CVD), catalyst nanoparticle formation by thin-film dewetting and the subsequent CNT nucleation processes dictate the CNT diameter distribution, areal density, and alignment. Herein, we use in situ environmental transmission electron microscopy (E-TEM) to observe the catalyst annealing, growth, and deactivation stages for a population of CNTs grown from a model thin film catalyst. Complementary in situ electron diffraction (ED) and TEM imaging show that during the annealing step in hydrogen, reduction of the iron oxide catalyst is concomitant with changes in the thin-film morphology; complete dewetting and the formation of a population of nanoparticles is only achieved upon the introduction of the carbon source (acetylene). The dewetting kinetics, i.e., the appearance of distinct nanoparticles, exhibit a sigmoidal (autocatalytic) curve with 95% of all nanoparticles appearing within 6 seconds. After nanoparticles form, they either nucleate CNTs or remain inactive, with incubation times measured to be as small as 3.5 seconds. TEM imaging also enables observing the build-up of alignment during CNT crowding. In addition, in situ electron energy loss spectroscopy (EELS) shows that the collective rate of carbon accumulation decays exponentially, based on real-time changes in the carbon K-edge near-edge structure. Hence, both the kinetics of catalyst formation and CNT nucleation must be addressed to achieve uniform and high CNT density, and their transient behavior may be a primary cause of the well-known non-uniform density of CNT forests.
J115. Predictive synthesis of freeform carbon nanotube microarchitectures by strain-engineered chemical vapor deposition.
S.J. Park, H. Zhao, S. Kim, M. De Volder, A.J. Hart. Small.
S.J. Park, H. Zhao, S. Kim, M. De Volder, A.J. Hart. Predictive synthesis of freeform carbon nanotube microarchitectures by strain-engineered chemical vapor deposition. Small, 12:4393-4403, 2016. [http://dx.doi.org/10.1002/smll.201601093] [video]
High-throughput fabrication of microstructured surfaces with multi-directional, re-entrant, or otherwise curved features is becoming increasingly important for applications such as phase change heat transfer, adhesive gripping, and control of electromagnetic waves. Toward this goal, curved microstructures of aligned carbon nanotubes (CNTs) can be fabricated by engineered variation of the CNT growth rate within each microstructure, for example by patterning of the CNT growth catalyst partially upon a layer which retards the CNT growth rate. This study develops a fi nite-element simulation framework for predictive synthesis of complex CNT microarchitectures by this strain-engineered growth process. The simulation is informed by parametric measurements of the CNT growth kinetics, and the anisotropic mechanical properties of the CNTs, and predicts the shape of CNT microstructures with impressive fi delity. Moreover, the simulation calculates the internal stress distribution that results from extreme deformation of the CNT structures during growth, and shows that delamination of the interface between the differentially growing segments occurs at a critical shear stress. Guided by these insights, experiments are performed to study the time- and geometry-depended stress development, and it is demonstrated that corrugating the interface between the segments of each microstructure mitigates the interface failure. This study presents a methodology for 3D microstructure design based on “pixels” that prescribe directionality to the resulting microstructure, and show that this framework enables the predictive synthesis of more complex architectures including twisted and truss-like forms.
J114. Highly consistent atmospheric pressure synthesis of carbon nanotube forests by mitigation of moisture transients.
J. Li, M. Bedewy, A.O. White, E.S. Polsen, S. Tawfick, A.J. Hart. Journal of Physical Chemistry C.
J. Li, M. Bedewy, A.O. White, E.S. Polsen, S. Tawfick, A.J. Hart. Highly repeatable atmospheric pressure synthesis of carbon nanotube forests by mitigation of moisture transients. Journal of Physical Chemistry C., 120:11277-11287, 2016. [http://dx.doi.org/10.1021/acs.jpcc.6b02878]
Consistent synthesis of carbon nanotubes (CNTs) using laboratory-scale methods is essential to the development of commercial applications, particularly with respect to the verification of recipes that achieve control of CNT diameter, chirality, alignment, and density. Here, we report that transients in the moisture level and carbon concentration during the chemical vapor deposition (CVD) process for vertically aligned CNT“forests”can contribute significantly to run-to-run variation of height and density. Then,we show that highly consistent CNT forest growth can be achieved by physically decoupling the catalyst annealing and hydrocarbon exposure steps,to allow the gas composition to stabilize between the steps. This decoupling is achieved using a magnetically actuated transfer arm to move the substrate rapidly into and out of the CVD reactor. Compared to a reference process where the sample resides in the furnace throughout the process, the decoupled method gives 21% greater CNT forest height, reduces the run-to-run variance of height by 76%, and results in forests with improved vertical alignment (Herman’s orientation parameter of 0.68 compared to 0.50). Building on this foundation, we study the influence of the moisture level during the CNT growth step and find a 30% improvement in growth rate going from the baseline condition (<15 ppm) to 40 ppm. Interestingly, however, the increased moisture concentration does not improve the catalyst lifetime or the CNT forest density, warranting further study of the role of moisture on CNT nucleation versus growth.
J113. A framework for teaching the fundamentals of additive manufacturing and enabling rapid innovation.
J. Go, A.J. Hart. Additive Manufacturing.
J. Go, A.J. Hart. A framework for teaching the fundamentals of additive manufacturing and enabling rapid innovation. Additive Manufacturing, 10:76-87, 2016.[http://dx.doi.org/10.1016/j.addma.2016.03.001]
The importance of additive manufacturing (AM) to the future of product design and manufacturing systems demands educational programs tailored to embrace its fundamental principles and its innovative potential. Moreover, the breadth and depth of AM spans several traditional disciplines, presenting a challenge to instructors yet giving the opportunity to integrate knowledge via creative and challenging projects. This paper presents our approach to teaching AM at the graduate and advanced undergraduate level, in the form of a 15-week course developed and taught at the Massachusetts Institute of Technology. The lectures begin with in-depth technical analysis of the major AM processes. In lab sessions, students operate and characterize desktop AM machines, and work in teams to design and fabricate a bridge having maximum strength per unit weight while conforming to geometric constraints. To conclude the semester, teams created prototype machines for printing molten glass, soft serve ice cream, biodegradable material, and carbon fiber composites, as well as for large area parallel extrusion and for in situ optical scanning during printing. We conclude that AM education, while arguably rooted in mechanical engineering, is truly multidisciplinary, and that education programs must embrace this context.
J112. High-fidelity replica molding of glassy liquid crystalline polymer microstructures.
H. Zhao*, J.J. Wie*, D. Copic*, C.R. Oliver, A.O. White, S. Kim, A.J. Hart. ACS Applied Materials and Interfaces.
H. Zhao, J.J. Wie, D. Copic, C.R. Oliver, A.O. White, S. Kim, A.J. Hart. High-fidelity replica molding of glassy liquid crystalline polymer microstrucures. ACS Applied Materials and Interfaces, 8(12):8110-8117, 2016. [http://dx.doi.org/10.1021/acsami.6b00785]
Liquid crystalline polymers have recently been engineered to exhibit complex macroscopic shape adaptivity, including optically- and thermally- driven bending, self-sustaining oscillation, torsional motion, and three dimensional folding. Miniaturization of these novel materials is of great interest for both fundamental study of processing conditions and for the development of shape-changing micro-devices. Here, we present a scalable method for high-fidelity replica molding of glassy liquid crystalline polymer networks (LCNs), by vacuum-assisted replica molding, along with magnetic field-induced control of the molecular alignment. We find that an oxygen-free environment is essential to establish high fidelity molding with low surface roughness. Identical arrays of homeotropic and polydomain LCN microstructures are fabricated to assess the influence of molecular alignment on the elastic modulus (E = 1.48 GPa compared to E = 0.54 GPa), and side view imaging is used to quantify the reversible thermal actuation of individual LCN micropillars by high-resolution tracking of edge motion. The methods and results from this study will be synergistic with future advances in liquid crystalline polymer chemistry, and could enable the scalable manufacturing of stimuli-responsive surfaces for applications including microfluidics, tunable optics, and surfaces with switchable wetting and adhesion.
J111. On-demand isolation and manipulation of C. elegans by in vitro maskless photopatterning.
C.R. Oliver, E. Gourgou, D. Bazopoulou, N. Chronis, A.J. Hart. PLoS ONE.
C.R. Oliver, E. Gourgou, D. Bazopoulou, N. Chronis, A.J. Hart. On-demand isolation and manipulation of C. elegans by in vitro maskless photopatterning. PLoS ONE, 11(1), 2016. [http://dx.doi.org/10.1371/journal.pone.0145935]
Caenorhabditis elegans (C. elegans) is a model organism for understanding aging and studying animal behavior. Microfluidic assay techniques have brought widespread advances in C.elegans research; however, traditional microfluidic assays such as those based on soft lithography require time-consuming design and fabrication cycles and offer limited flexibility in changing the geometric environment during experimentation. We present a technique for maskless photopatterning of a biocompatible hydrogel on an NGM (Agar) substrate, enabling dynamic manipulation of the C. elegans culture environment in vitro. Maskless photopatterning is performed using a projector-based microscope system largely built from off-the-shelf components. We demonstrate the capabilities of this technique by building micropillar arrays during C. elegans observation, by fabricating free-floating mechanisms that can be actuated byC. elegans motion, by using freehand drawing to isolate individual C. elegans in real time, and by patterning arrays of mazes for isolation and fitness testing of C. elegans populations. In vitrophotopatterning enables rapid and flexible design of experiment geometry as well as real-time interaction between the researcher and the assay such as by sequential isolation of individual organisms. Future adoption of image analysis and machine learning techniques could be used to acquire large datasets and automatically adapt the assay geometry.
J110. Strain relaxation and resonance of carbon nanotube forests under electrostatic loading.
A. Ya’akobovitz, M. Bedewy, A. Rao, A.J. Hart. Carbon.
A. Ya’akobovitz, M. Bedewy, A. Rao, A.J. Hart. Strain relaxation and resonance of carbon nanotube forests under electrostatic loading. Carbon, 96:250–258, 2016. [http://dx.doi.org/10.1016/j.carbon.2015.09.038]
Electrostatic loading is widely used for sensing and actuation in miniaturized electromechanical systems, yet classical designs involve geometric patterning of solid materials such as silicon and metal films. Conductive nanoporous materials for electrostatics may enable engineering of new functionalities arising from their compliance, internal surface forces, and high surface area. Toward this end, we investigate the response of vertically aligned carbon nanotube (CNT) “forests” to DC and AC electrostatic loads. First, the tensile strain-stress characteristics of patterned CNT forests was determined in a non-contact manner by cyclic DC electrostatic loading, revealing an increase of the effective Young’s modulus with sequential load cycling. Next, we observed resonance can be excited by AC electrostatic loading, and that the resonance frequency increases with sequential sweeps of the AC load frequency. Both the DC and AC measurements indicate that residual stress that arises during CNT growth is relaxed upon electrostatic loading, causing stiffening of the structure. This study shows for the first time that CNT forests can function as bulk electrostatic elements, and their intrinsic low stiffness and quality factor may be suitable for development of wide bandwidth micro-resonators and adsorption-based sensors.
C55. Lego Brick Microfluidics.
C.E. Owens, A.J. Hart. 20th International conference on Miniaturized Systems for Chemistry and Life Sciences (MicroTAS), Dublin, Ireland, October 2016.
C54. Precision locating of additively manufactured parts using embedded kinematic couplings.
R. Penny, A.J. Hart. ASPE Topical Meeting on Dimensional Accuracy and Surface Finish in Additive Manufacturing. June 27-30, 2016. Raleigh, NC.
C53. Plate-to-roll apparatus for continuous relief printing using microstructured nanoporous stamps.
D. Mariappan, S. Kim, A.J. Hart. ASPE Topical Meeting on Precision Mechatronic System Design and Control. April 18-20, 2016. Cambridge, MA.
J109. Art on the nanoscale and beyond.
A.K. Yetisen, A.F. Coskun, G. England, S. Cho, H. Butt, J. Hurwitz, M. Kolle, A. Khademhosseini, A.J. Hart, A. Folch, S.H. Yun. Advanced Materials.
A.K. Yetisen, A.F. Coskun, G. England, S. Cho, H. Butt, J. Hurwitz, M. Kolle, A. Khademhosseini, A.J. Hart, A. Folch, S.H. Yun. Art on the nanoscale and beyond. Advanced Materials, 2015. [http://dx.doi.org/10.1002/adma.201502382]
Methods of forming and patterning materials at the nano- and microscales are finding increased use as a medium of artistic expression, and as a vehicle for communicating scientific advances to a broader audience. While sharing many attributes of other art forms, miniaturized art enables the direct engagement of sensory aspects such as sight and touch for materials and structures that are otherwise invisible to the eye. The historical uses of nano-/microscale materials and imaging techniques in arts and sciences are presented. The motivations to create artwork at small scales are discussed, and representations in scientific literature and exhibitions are explored. Examples are presented using semiconductors, microfluidics, and nanomaterials as the artistic media; these utilized techniques including micromachining, focused ion beam milling, two-photon polymerization, and bottom-up nanostructure growth. Finally, the technological factors that limit the implementation of artwork at miniature scales are identified, and potential future directions are discussed. As research marches toward even smaller length scales, innovative and engaging visualization and artistic endeavors will have growing implications on education, communication, policy making, media activism, and public perception of science and technology.
J108. Anisotropic microwave conductivity dispersion of horizontally aligned multi-walled carbon-nanotube thin film on flexible substrate.
L. Si, H. Wei, L. Min, T. Mingguang, S. Tawfick, A.J. Hart, Z. Qi, X. Hao. IEEE Transactions on Microwave Theory and Techniques.
L. Si, H. Wei, L. Min, T. Mingguang, S. Tawfick, A.J. Hart, Z. Qi, X. Hao. Anisotropic microwave conductivity dispersion of horizontally aligned multi-walled carbon-nanotube thin film on flexible substrate. IEEE Transactions on Microwave Theory and Techniques, 11(63):3588–3594, 2015. [http://dx.doi.org/10.1109/TMTT.2015.2481397]
In this paper, the anisotropic conductivity dispersion of horizontally aligned multi-walled carbon-nanotube (HA-MWCNT) thin films on a kapton substrate are characterized at microwave range. It is well known that a single carbon nanotube (CNT) presents highly 1-D conduction. For a thin film composed of numerous aligned CNTs, the anisotropic conduction may be preserved to some extent, as this is useful for various applications. In this paper, the anisotropic conduction of HA-MWCNT thin films is investigated in a rectangular waveguide measurement setup. The anisotropic surface conductivities of the films along the nanotube axial and radial directions are extracted from the measured S-parameters. The results reveal an anisotropic conductivity ratio of about 2 for the HA-MWCNTs thin film. Furthermore, it is observed that the real part of the extracted conductivity remains constant over the frequency band while the imaginary part increases almost linearly with frequency.
J107. Three-dimensional machining of carbon nanotube forests using water-assisted scanning electron microscope processing.
B. Rajabifar, S. Kim, K. Slinker, G.J. Ehlert, A.J. Hart, and M.R. Maschmann. Applied Physics Letters.
We demonstrate that vertically aligned carbon nanotubes (CNTs) can be precisely machined in a low pressure water vapor ambient using the electron beam of an environmental scanning electron microscope. The electron beam locally damages the irradiated regions of the CNT forest and also dissociates the water vapor molecules into reactive species including hydroxyl radicals. These species then locally oxidize the damaged region of the CNTs. The technique offers material removal capabilities ranging from selected CNTs to hundreds of cubic microns. We study how the material removal rate is influenced by the acceleration voltage, beam current, dwell time, operating pressure, and CNT orientation. Milled cuts with depths between 0–100 microns are generated, corresponding to a material removal rate of up to 20.1um3/min. The technique produces little carbon residue and does not disturb the native morphology of the CNT network. Finally, we demonstrate direct machining of pyramidal surfaces and re-entrant cuts to create freestanding geometries.
J106. Extremely elastic wearable carbon nanotube fiber strain sensor for monitoring of human motion.
S. Ryu, P. Lee, J.B. Chou, R. Xu, R. Zhao, A.J. Hart, and S. Kim. ACS Nano.
S. Ryu, P. Lee, J.B. Chou, R. Xu, R. Zhao, A.J. Hart, and S. Kim. Extremely Elastic Wearable Carbon Nanotube Fiber Strain Sensor for Monitoring of Human Motion. ACS Nano, 9(6):5929–5936, 2015. [http://dx.doi.org/10.1021/acsnano.5b00599]
The increasing demand for wearable electronic devices has made the development of highly elastic strain sensors that can monitor various physical parameters an essential factor for realizing next generation electronics. Here, we report an ultrahigh stretchable and wearable device fabricated from dry-spun carbon nanotube (CNT) fibers. Stretching the highly oriented CNT fibers grown on a flexible substrate (Ecoflex) induces a constant decrease in the conductive pathways and contact areas between nanotubes depending on the stretching distance; this enables CNT fibers to behave as highly sensitive strain sensors. Owing to its unique structure and mechanism, this device can be stretched by over 900% while retaining high sensitivity, responsiveness, and durability. Furthermore, the device with biaxially oriented CNT fiber arrays shows independent cross-sensitivity, which facilitates simultaneous measurement of strains along multiple axes. We demonstrated potential applications of the proposed device, such as strain gauge, single and multiaxial detecting motion sensors. These devices can be incorporated into various motion detecting systems where their applications are limited to their strain.
J105. High speed roll-to-roll manufacturing of graphene using a concentric tube reactor.
E.S. Polsen, D.Q. McNerny, B. Viswanath, S.W. Pattinson, A.J. Hart. Scientific Reports.
E.S. Polsen, D.Q. McNerny, B. Viswanath, S.W. Pattinson, A.J. Hart. High speed roll-to-roll manufacturing of graphene using a concentric tube reactor. Scientific Reports, 5, 10257, 2015. [http://dx.doi.org/10.1038/srep10257] [MIT News] [gizmag]
We present the design of a concentric tube (CT) reactor for roll-to-roll chemical vapor deposition (CVD) on flexible substrates, and its application to continuous production of graphene on copper foil. In the CTCVD reactor, the thin foil substrate is helically wrapped around the inner tube, and translates through the gap between the concentric tubes. We use a bench-scale prototype machine to synthesize graphene on copper substrates at translation speeds varying from 25?mm/min to 500?mm/min, and investigate the influence of process parameters on the uniformity and coverage of graphene on a continuously moving foil. At lower speeds, high-quality monolayer graphene is formed; at higher speeds, rapid nucleation of small graphene domains is observed, yet coalescence is prevented by the limited residence time in the CTCVD system. We show that a smooth isothermal transition between the reducing and carbon-containing atmospheres, enabled by injection of the carbon feedstock via radial holes in the inner tube, is essential to high-quality roll-to-roll graphene CVD. We discuss how the foil quality and microstructure limit the uniformity of graphene over macroscopic dimensions. We conclude by discussing means of scaling and reconfiguring the CTCVD design based on general requirements for 2-D materials manufacturing.
J104. Corrugated paraffin nanocomposite films as large stroke thermal actuators and self-activating thermal interfaces.
D. Copic, A.J. Hart. ACS Applied Materials and Interfaces.
D. Copic, A.J. Hart. Corrugated paraffin nanocomposite films as large stroke thermal actuators and self-activating thermal interfaces. ACS Applied Materials and Interfaces, 7(15):8218–8224, 2015. [http://dx.doi.org/10.1021/acsami.5b01141]
High performance active materials are of rapidly growing interest for applications including soft robotics, microfluidic systems, and morphing composites. In particular, paraffin wax has been used to actuate miniature pumps, solenoid valves, and composite fibers, yet its deployment is typically limited by the need for external volume constraint. We demonstrate that compact, high-performance paraffin actuators can be made by confining paraffin within vertically aligned carbon nanotube (CNT) films. This large-stroke vertical actuation is enabled by strong capillary interaction between paraffin and CNTs and by engineering the CNT morphology by mechanical compression before capillary-driven infiltration of the molten paraffin. The maximum actuation strain of the corrugated CNT-paraffin films (?0.02?0.2) is comparable to natural muscle, yet the maximum stress is limited to ?10 kPa by collapse of the CNT network. We also show how a CNT–paraffin film can serve as a self-activating thermal interface that closes a gap when it is heated. These new CNT–paraffin film actuators could be produced by large-area CNT growth, infiltration, and lamination methods, and are attractive for use in miniature systems due to their self-contained design.
J103. Electrostatic capacitance and Faraday cage behavior of carbon nanotube forests.
A. Ya’akobovitz, M. Bedewy, A.J. Hart. Applied Physics Letters.
A. Ya’akobovitz, M. Bedewy, A.J. Hart. Electrostatic capacitance and Faraday cage behavior of carbon nanotube forests. Applied Physics Letters, 106:053106, 2015. [http://dx.doi.org/10.1063/1.4907609]
Understanding of the electrostatic properties of carbon nanotube (CNT) forests is essential to enable their integration in microelectronic and micromechanical devices. In this study, we sought to understand how the hierarchical geometry and morphology of CNT forests determines their capacitance. First, we find that at small gaps, solid micropillars have greater capacitance, yet at larger gaps the capacitance of the CNT forests is greater. The surface area of the CNT forest accessible to the electrostatic field was extracted by analysis of the measured capacitance, and, by relating the capacitance to the average density of CNTs in the forest, we find that the penetration depth of the electrostatic field is on the order of several microns. Therefore, CNT forests can behave as a miniature Faraday cage. The unique electrostatic properties of CNT forests could therefore enable their use as long-range proximity sensors and as shielding elements for miniature electronic devices.
J102. Mechanism and enhanced yield of carbon nanotube growth on stainless steel by oxygen-induced surface reconstruction.
S. Pattinson, B. Viswanath, D. Zakharov, J. Li, E.A. Stach, A.J. Hart. Chemistry of Materials.
C52. Rate limiting tradeoffs in machine design for extrusion-based additive manufacturing.
J. Go, A.J. Hart. 26th Annual International Solid Freeform Fabrication Symposium.
C51. Dynamic printing of cells and microbeads for custom microfluidic assays.
C.R. Oliver, N. Spielberg, A.J. Hart. 26th Annual International Solid Freeform Fabrication Symposium.
C50. Photopatterning of freeform surfaces using a modular robotic system.
A.G. Stevens, C.R. Oliver, L. Chin, A.J. Hart. 26th Annual International Solid Freeform Fabrication Symposium.
C49. Selective single cell detachment and retrieval for downstream analyses using nanosecond laser pulses in CNT-coated microwell arrays.
Y.C. Chen, H.W. Baac, K.T. Lee, K. Teichert, A.J. Hart, L.J. Guo, E. Yoon. The 19th International Conference on Miniaturized Systems for Chemistry and Life Sciences (MicroTAS 2015).
J101. Enhanced surface capacitance of cylindrical micropillar arrays.
A. Ya’akobovitz, A.J. Hart. Sensors and Actuators.
J100. Scaling the stiffness, strength, and toughness of ceramic-coated nanotube foams into the structural regime.
A. Brieland-Shoultz*, S. Tawfick*, S.J. Park*, M. Bedewy, M.R. Maschmann, J. W. Baur, A.J. Hart. Advanced Functional Materials.
A. Brieland-Shoultz*, S. Tawfick*, S.J. Park*, M. Bedewy, M.R. Maschmann, J. W. Baur, A.J. Hart. Scaling the stiffness, strength, and toughness of ceramic-coated nanotube foams into the structural regime. Advanced Functional Materials, 24(36):5728-5735, 2014. [http://dx.doi.org/10.1002/adfm.201400851]
J99. Strain-engineered manufacturing of freeform carbon nanotube microstructures.
M. De Volder*, S.J. Park*, S. Tawfick, A.J. Hart. Nature Communications.
M. De Volder*, S.J. Park*, S. Tawfick, A.J. Hart. Strain-engineered manufacturing of freeform carbon nanotube microstructures. Nature Communications, 5:4512, 2014. [http://dx.doi.org/10.1038/ncomms5512] [MIT News]
The skins of many plants and animals have intricate microscale surface features that give rise to properties such as directed water repellency and adhesion, camouflage, and resistance to fouling. However, engineered mimicry of these designs has been restrained by the limited capabilities of top–down fabrication processes. Here we demonstrate a new technique for scalable manufacturing of freeform microstructures via strain-engineered growth of aligned carbon nanotubes (CNTs). Offset patterning of the CNT growth catalyst is used to locally modulate the CNT growth rate. This causes the CNTs to collectively bend during growth, with exceptional uniformity over large areas. The final shape of the curved CNT microstructures can be designed via finite element modeling, and compound catalyst shapes produce microstructures with multidirectional curvature and unusual self-organized patterns. Conformal coating of the CNTs enables tuning of the mechanical properties independently from the microstructure geometry, representing a versatile principle for design and manufacturing of complex microstructured surfaces.
J98. Materials, fabrication, and manufacturing of micro/nanostructured surfaces for phase-change heat transfer enhancement.
M. McCarthy, K. Gerasopolous, S.C. Maroo, A.J. Hart. Nanoscale and Microscale Thermophysical Engineering.
M. McCarthy, K. Gerasopolous, S.C. Maroo, A.J. Hart. Materials, fabrication, and manufacturing of micro/nanostructured surfaces for phase-change heat transfer enhancement. Nanoscale and Microscale Thermophysical Engineering, 5:4512, 2014. [http://dx.doi.org/10.1080/15567265.2014.926436]
This paper describes the most prominent materials, fabrication methods, and manufacturing schemes for micro and nano-structured surfaces that can be employed to enhance phase change heat transfer phenomena. The numerous processes include traditional microfabrication techniques such as thin film deposition, lithography, and etching, as well as template-assisted and template-free nano-fabrication techniques. The creation of complex, hierarchical, and heterogeneous surface structures using advanced techniques is also reviewed. Additionally, research needs in the field and future directions necessary to translate these approaches from the laboratory to high performance applications are identified. Particular focus is placed on the extension of these techniques to the design of micro/nano-structures for increased performance, manufacturability, and reliability. The current research needs and goals are detailed, and potential pathways forward are suggested.
J97. Self-ordering of small-diameter metal nanoparticles by dewetting on hexagonal mesh templates.
E.R. Meshot, Z. Zhao, W. Lu, A.J. Hart. Nanoscale.
E.R. Meshot, Z. Zhao, W. Lu, A.J. Hart. Self-ordering of small-diameter metal nanoparticles by dewetting on hexagonal mesh templates. Nanoscale, 6(17):10106-10112, 2014. [http://dx.doi.org/10.1039/C4NR01501K]
Arrays of small-diameter nanoparticles with high spatial order are useful for chemical and biological sensors, data storage, synthesis of nanowires and nanotubes, and many other applications. We show that self-ordered metal nanoparticle arrays can be formed by dewetting of thin films on hexagonal mesh substrates made of anodic aluminum oxide (AAO). Upon heating, the metal (Fe) film dewets onto the interstitial sites (i.e., the node points) between pores on the top surface of the AAO. We investigated the particle morphology and dynamics of dewetting using a combination of atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), and numerical simulations. Templated metal particles are more monodisperse and have higher local order than those formed by the same dewetting process on flat, nonporous alumina. The degree of order depends on the initial film thickness, and for the optimal thickness tested (nominally 2 nm), we achieved uniform coverage and high order of the particles, comparable to that of the AAO template itself. Computational modeling of dewetting on templates with various pore order and size shows that the order of AAO pores is primarily influential in determining particle position and spacing, while the variance in pore size is less impactful. Potential uses of these ordered nanoparticle arrays on porous materials include plasmonic sensors and spatially controlled catalysts.
J96. Direct fabrication of graphene on SiO2 via thin film stress engineering.
D. McNerny, B, Viswanath, D. Copic, F. Laye, C. Prohoda, A. Brieland-Shoultz, E.S. Polsen, N.T. Dee, V.S. Veerasamy, A.J. Hart. Scientific Reports.
D. McNerny, B, Viswanath, D. Copic, F. Laye, C. Prohoda, A. Brieland-Shoultz, E.S. Polsen, N.T. Dee, V.S. Veerasamy, A.J. Hart. Direct fabrication of graphene on SiO2 via thin film stress engineering. Scientific Reports, 4:5049, 2014. [http://dx.doi.org/10.1038/srep05049] [MIT News]
We demonstrate direct production of graphene on SiO2 by CVD growth of graphene at the interface between a Ni film and the SiO2 substrate, followed by dry mechanical delamination of the Ni using adhesive tape. This result is enabled by understanding of the competition between stress evolution and microstructure development upon annealing of the Ni prior to the graphene growth step. When the Ni film remains adherent after graphene growth, the balance between residual stress and adhesion governs the ability to mechanically remove the Ni after the CVD process. In this study the graphene on SiO2 comprises micron-scale domains, ranging from monolayer to multilayer. The graphene has >90% coverage across centimeter-scale dimensions, limited by the size of our CVD chamber. Further engineering of the Ni film microstructure and stress state could enable manufacturing of highly uniform interfacial graphene followed by clean mechanical delamination over practically indefinite dimensions. Moreover, our findings suggest that preferential adhesion can enable production of 2-D materials directly on application-relevant substrates. This is attractive compared to transfer methods, which can cause mechanical damage and leave residues behind.
J95. Synergetic chemical coupling controls the uniformity of carbon nanotube microstructures.
M. Bedewy*, B. Farmer*, A.J. Hart. ACS Nano.
M. Bedewy*, B. Farmer*, A.J. Hart. Synergetic chemical coupling controls the uniformity of carbon nanotube microstructures. ACS Nano, 2014. [http://dx.doi.org/10.1021/nn500698z]
Control of the uniformity of vertically aligned carbon nanotube structures (CNT “forests”), in terms of both geometry and nanoscale morphology (density, diameter, and alignment), is crucial for applications. Many studies report complex and sometimes unexplained spatial variations of the height of macroscopic CNT forests, as well as variations among micropillars grown from lithographically patterned catalyst arrays. We present a model for chemically coupled CNT growth, which describes the origins of synergetic growth effects among CNT micropillars in proximity. Via this model, we propose that growth of CNTs is locally enhanced by active species that are catalytically produced at the substrate-bound nanoparticles. The local concentration of these active species modulates the growth rate of CNTs, in a spatially dependent manner driven by diffusion and local generation/consumption at the catalyst sites. Through experiments and numerical simulations, we study how the uniformity of CNT micropillars can be influenced by their size and spacing within arrays and predict the widely observed abrupt transition between tangled and vertical CNT growth by assigning a threshold concentration of active species. This mathematical framework enables predictive modeling of spatially dependent CNT growth, as well as design of catalyst patterns to achieve engineered uniformity.
J94. Precise control of elastocapillary densification of nanostructures via low pressure condensation.
S. Park, A.J. Schmidt, S. Tawfick, A.J. Hart. Journal of Micromechanics and Microengineering.
S. Park, A.J. Schmidt, S. Tawfick, A.J. Hart. Precise control of elastocapillary densification of nanostructures via low pressure condensation. Journal of Micromechanics and Microengineering, 24(6):065019, 2014. [http://dx.doi.org/10.1088/0960-1317/24/6/065019]
Capillary self-assembly of nanoscale filaments is an emerging means of fabricating complex and hierarchical surface textures. However, via conventional processing methods such as immersion in liquid and atmospheric pressure condensation of liquid onto the substrate, it is challenging to achieve uniform results over large areas and to adapt the process to structures with different dimensions and spacing. Here we study elastocapillary densification of carbon nanotube (CNT) microstructures via controlled low-pressure condensation of liquid and subsequent evaporation, with the structures placed on a temperature controlled substrate. We study the dynamics of liquid infiltration into the microstructures and achieve control over the liquid condensation rate within >1 ?m s?1. We find that the extent of densification depends on the amount of liquid delivered to the substrate as well as the size and spacing of the microstructures. We also show that the low-pressure condensation method can be used to form large arrays of anisotropic CNT microstructures, including thin-walled slanted microwells and tilted microposts.
J93. Simultaneously high stiffness and damping in nanoengineered microtruss composites.
J. Meaud*, T. Sain*, B. Yeom*, S. Park*, A. Brieland-Shoultz, G. Hulbert, N.A. Kotov, A.J. Hart, E.M. Arruda, A.M. Waas. ACS Nano.
J. Meaud*, T. Sain*, B. Yeom*, S. Park*, A. Brieland-Shoultz, G. Hulbert, N.A. Kotov, A.J. Hart, E.M. Arruda, A.M. Waas. Simultaneously high stiffness and damping in nanoengineered microtruss composites. ACS Nano, 8(4):3468-3475, 2014. [http://dx.doi.org/10.1021/nn500284m]
Materials combining high stiffness and mechanical energy dissipation are needed in automotive, aviation, construction, and other technologies where structural elements are exposed to dynamic loads. In this paper we demonstrate that a judicious combination of carbon nanotube engineered trusses held in a dissipative polymer can lead to a composite material that simultaneously exhibits both high stiffness and damping. Indeed, the combination of stiffness and damping that is reported is quite high in any single monolithic material. Carbon nanotube (CNT) microstructures grown in a novel 3D truss topology form the backbone of these nanocomposites. The CNT trusses are coated by ceramics and by a nanostructured polymer film assembled using the layer-by-layer technique. The crevices of the trusses are then filled with soft polyurethane. Each constituent of the composite is accurately modeled, and these models are used to guide the manufacturing process, in particular the choice of the backbone topology and the optimization of the mechanical properties of the constituent materials. The resulting composite exhibits much higher stiffness (80 times) and similar damping (specific damping capacity of 0.8) compared to the polymer. Our work is a step forward in implementing the concept of materials by design across multiple length scales.
J92. Directed growth of primary motor neurons on horizontally aligned carbon nanotube films and striped patterns.
M.J. Roberts, M.K. Leach, M. Bedewy, E.R. Meshot, D. Copic, J.M. Corey, A.J. Hart. Journal of Neural Engineering.
M.J. Roberts, M.K. Leach, M. Bedewy, E.R. Meshot, D. Copic, J.M. Corey, A.J. Hart. Directed growth of primary motor neurons on horizontally aligned carbon nanotube films and striped patterns. Journal of Neural Engineering, 11(3):036013, 2014. [http://dx.doi.org/10.1088/1741-2560/11/3/036013]
Carbon nanotubes (CNTs) are attractive for use in peripheral nerve interfaces because of their unique combination of strength, flexibility, electrical conductivity and nanoscale surface texture. Here we investigated the growth of motor neurons on thin films of horizontally aligned CNTs (HACNTs). Approach. We cultured primary embryonic rat motor neurons on HACNTs and performed statistical analysis of the length and orientation of neurites. We next presented motor neurons with substrates of alternating stripes of HACNTs and SiO2. Main results. The neurons survived on HACNT substrates for up to eight days, which was the full duration of our experiments. Statistical analysis of the length and orientation of neurites indicated that the longest neurites on HACNTs tended to align with the CNT direction, although the average neurite length was similar between HACNTs and glass control substrates. We observed that when motor neurons were presented with alternating stripes of HACNTs and SiO2, the proportion of neurons on HACNTs increases over time, suggesting that neurons selectively migrate toward and adhere to the HACNT surface. Significance. The behavior of motor neurons on CNTs has not been previously investigated, and we show that aligned CNTs could provide a viable interface material to motor neurons. Combined with emerging techniques to build complex hierarchical structures of CNTs, our results suggest that organised CNTs could be incorporated into nerve grafts that use physical and electrical cues to guide regenerating axons.
J91. Extensible-link kinematic model for characterizing and optimizing compliant mechanism motion.
J. Beroz, S. Awtar, A.J. Hart. ASME Journal of Mechanical Design.
J. Beroz, S. Awtar, A.J. Hart. Extensible-link kinematic model for characterizing and optimizing compliant mechanism motion. ASME Journal of Mechanical Design, 136(3):031008, 2014. [http://dx.doi.org/10.1115/1.4026269]
We present an analytical model for characterizing the motion trajectory of an arbitrary planar compliant mechanism. Model development consists of identifying particular material points and their connecting vectorial lengths in a manner that represents the mechanism topology; whereby these lengths may extend over the course of actuation to account for the elastic deformation of the compliant mechanism. The motion trajectory is represented within the model as an analytical function in terms of these vectorial lengths, whereby its Taylor series expansion constitutes a parametric formulation composed of load-independent and load-dependent terms. This adds insight to the process for designing compliant mechanisms for high-accuracy motion applications because: (1) inspection of the load-independent terms enables determination of specific topology modifications that reduce or eliminate certain error components of the motion trajectory; and (2) the load-dependent terms reveal the polynomial orders of principally uncorrectable error components in the trajectory. The error components in the trajectory simply represent the deviation of the actual motion trajectory provided by the compliant mechanism compared to the ideally desired one. A generalized model framework is developed, and its utility demonstrated via the design of a compliant microgripper with straight-line parallel jaw motion. The model enables analytical determination of all geometric modifications for minimizing the error trajectory of the jaw, and prediction of the polynomial order of the uncorrectable trajectory components. The jaw trajectory is then optimized using iterative finite elements simulations until the polynomial order of the uncorrectable trajectory component becomes apparent; this reduces the error in the jaw trajectory by 2 orders of magnitude over the prescribed jaw stroke. This model serves to streamline the design process by identifying the load-dependent sources of trajectory error in a compliant mechanism, and thereby the limits with which this error may be redressed by topology modification.
J90. Rapid anisotropic photoconductive response of ZnO-coated aligned carbon nanotube sheets.
J.G. Ok, J. Lee, H. Baac, S. Tawfick, L.J. Guo, A.J. Hart. ACS Applied Materials and Interfaces.
J.G. Ok, J. Lee, H. Baac, S. Tawfick, L.J. Guo, A.J. Hart. Rapid anisotropic photoconductive response of ZnO-coated aligned carbon nanotube sheets. ACS Applied Materials and Interfaces. 6(2):874-881, 2014. [http://dx.doi.org/10.1021/am404131r]
We investigate the rapid and anisotropic UV-induced photoconductive response of hybrid thin films comprising zinc oxide (ZnO) nanowires (NWs) directly grown on horizontally aligned (HA-) carbon nanotube (CNT) sheets. The films exhibit anisotropic photoconductivity; along the CNTs, conductivity is dominated by the CNTs and the photoconductive gain is lower, whereas perpendicular to the CNTs the photoconductive gain is higher because transport is influenced by ZnO nanoclusters bridging CNT-CNT contacts. Because of the distributed electrical contact provided by the large number of ZnO NWs on top of the HACNT film, this hybrid nanoarchitecture has a significantly greater photocurrent than reported for single ZnO NW-based devices at comparable UV illumination intensity. Moreover, the hybrid architecture where a thin basal film of ZnO ohmically contacts metallic CNTs enables rapid transport of photogenerated electrons from ZnO to CNTs, resulting in sub-second photoresponse upon pulsed illumination. The built-in potential generated across ZnO–CNT heterojunctions competes with the externally applied bias to control the photocurrent amplitude and direction. By tuning the anisotropic conductivity of the CNT network and the morphology of the ZnO or potentially other nanostructured coatings, this material architecture may be engineered in the future to realize high-performance optical and chemical sensors.
J89. Continuum analysis of carbon nanotube array buckling enabled by anisotropic elastic measurements and modeling.
M.R. Maschmann, G.J. Ehlert, S. Tawfick, A.J. Hart, J.W. Baur. Carbon.
M.R. Maschmann, G.J. Ehlert, S. Tawfick, A.J. Hart, J.W. Baur. Continuum analysis of carbon nanotube array buckling enabled by anisotropic elastic measurements and modeling. Carbon, 66:377-386, 2014. [http://dx.doi.org/10.1016/j.carbon.2013.09.013]
For the first time, carbon nanotube (CNT) forests are fully characterized as transversely isotropic continuum material. Each of the five independent elastic constants is experimentally obtained using a combination of nanoindenter-based uniaxial compression and shear testing, in situ SEM compression, and digital image correlation (DIC) of vertically and laterally oriented CNT microstructure columns. Material properties are highly anisotropic, with an axial modulus (165–275 MPa) that is nearly two orders of magnitude greater than the transverse modulus (2.5–2.7 MPa) and the out of plane shear modulus (0.8–1.6 MPa). The Poisson’s ratios along three mutually orthogonal axes, measured directly by simultaneous in situ DIC evaluation of axial and transverse strain, are found to be similarly anisotropic (v12 = 0.35, v23 = 0.1, v21 = 0.005). A Timoshenko beam model is then developed to accurately predict the critical buckling stress of the vertically oriented columns using a subset of these anisotropic properties and considering inelastic column buckling. These results show that the critical bucking stress of CNT microstructures vary predictably with geometry and that continuum models with appropriate material constants may be applied to analyze CNT microstructures and evaluate their stability for many applications.
C48.Fast imaging of carbon nanotube nucleation and growth processes using environmental TEM.
D.N. Zakharov, M. Bedewy, C. Czarnik, A.J. Hart, S. Misawa, E.A. Stach. Microscopy and Microanalysis, Hartford CT, 2014.
C47. Platform for in-vitro photo-patterning of whole animal C. elegans assays and behavior control.
C.R. Oliver, E. Gourgou, D. Bazopoulou, N. Chronis, A.J. Hart. The 18th International Conference on Miniaturized Systems for Chemistry and Life Sciences (MicroTAS 2014).
C46. Design and implementation of a graduate course on additive manufacturing.
J. Go, A.J. Perez, A.J. Hart. 25th Annual International Solid Freeform Fabrication Symposium.
C45. Tunable volume handheld pipette using a pneumatic deamplification mechanism.
J. Beroz, S. Jiang, J. Lewandowski, A.J. Hart. 2014 ASME International Design Engineering Technical Conferences (IDETC).
C44. Self-folding of a laminated paper system.
A. Rao, A.J. Hart. 6th International Meeting on Origami in Science, Mathematics and Education (6OSME).
C43. A modular collapsible folded paper tower.
M.J. Roberts, S. Tawfick, M. Shlian, A.J. Hart. 6th International Meeting on Origami in Science, Mathematics and Education (6OSME).
J88. Robofurnace: A semi-automated laboratory CVD system for high-throughput nanomaterial synthesis and process discovery.
C.R. Oliver, W. Westrick, J. Koehler, A. Brieland-Shoultz, I. Anagnostopoulos Politis, T. Cruz-Gonazalez, A.J. Hart. Review of Scientific Instruments.
C.R. Oliver, W. Westrick, J. Koehler, A. Brieland-Shoultz, I. Anagnostopoulos Politis, T. Cruz-Gonazalez, A.J. Hart. Robofurnace: A semi-automated laboratory CVD system for high-throughput nanomaterial synthesis and process discovery.
Review of Scientific Instruments,84:115105, 2013. [http://dx.doi.org/10.1063/1.4826275] [Video]
Laboratory research and development on new materials, such as nanostructured thin films,often utilizes manual equipment such as tube furnaces due to its relatively low cost and ease of setup. However, these systems can be prone to inconsistent outcomes due to variations in standard operating procedures and limitations in performance such as heating and cooling rates restrict the parameter space that can be explored. Perhaps more importantly, maximization of research throughput and the successful and efficient translation of materials processing knowledge to production-scale systems, relies on the attainment of consistent outcomes. In response to this need, we present a semi-automated lab-scale chemical vapor deposition (CVD)furnace system, called “Robofurnace.” Robofurnace is an automated CVD system built around a standard tube furnace, which automates sample insertion and removal and uses motion of thefurnace to achieve rapid heating and cooling. The system has a 10-sample magazine and motorized transfer arm, which isolates the samples from the lab atmosphere and enables highly repeatable placement of the sample within the tube. The system is designed to enable continuous operation of the CVD reactor, with asynchronous loading/unloading of samples. To demonstrate its performance, Robofurnace is used to develop a rapid CVD recipe for carbon nanotube (CNT) forest growth, achieving a 10-fold improvement in CNT forest mass density compared to a benchmark recipe using a manual tube furnace. In the long run, multiple systemslike Robofurnace may be linked to share data among laboratories by methods such as Twitter. Our hope is Robofurnace and like automation will enable machine learning to optimize and discover relationships in complex material synthesis processes.
J87. Local relative density modulates failure and strength in vertically aligned carbon nanotubes.
S. Pathak, N. Mohan, E. Decolvenaere, A. Needleman, M. Bedewy, A.J. Hart, J.R. Greer. ACS Nano.
S. Pathak, N. Mohan, E. Decolvenaere, A. Needleman, M. Bedewy, A.J. Hart, J.R. Greer. Local relative density modulates failure and strength in vertically aligned carbon nanotubes. ACS Nano, 7(10):8593–8604, 2013. [http://dx.doi.org/10.1021/nn402710j]
Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress–strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material model based on a viscoplastic solid with plastic non-normality and a hardening–softening–hardening plastic flow relation. The hardening slope in compression generated by the model was directly related to the stiffness gradient along the sample height, and hence to the local relative density. These results demonstrate that a microstructural figure-of-merit, the effective relative density, can be used to quantify and predict the mechanical response.
J86. Self-assembly of suspended collagen films and their viability as cell culture substrates.
M.J. Roberts, N. Bhatt, C.M. Voge, E.R. Meshot, J.P. Stegemann, A.J. Hart. Journal of Materials Chemistry B.
M.J. Roberts, N. Bhatt, C.M. Voge, E.R. Meshot, J.P. Stegemann, A.J. Hart. Self-assembly of suspended collagen films and their viability as cell culture substrates. Journal of Materials Chemistry B, 1(37):4711-4718, 2013. [http://dx.doi.org/10.1039/C3TB20800A]
We present the fabrication and mechanical properties of thin collagen networks self-assembled in a suspended configuration over micropost arrays. These collagen “canopies” were formed on arrays of microposts made of PDMS, silicon, and vertically aligned carbon nanotubes (CNT). We reversibly loaded the canopy to an in-plane stress of 32 MPa. We found that human dermal fibroblasts (HDFb) proliferate on the canopy substrates for up to 7 days. This versatile fabrication method for suspended extracellular matrix (ECM) films may enable the development of new assays to probe cell–ECM interactions, along with integration of microelectronic probes.
J85. Enhancing the tensile properties of continuous millimeter-scale carbon nanotube fibers by densification.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. ACS Applied Materials and Interfaces.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Robofurnace: Enhancing the tensile properties of continuous millimeter-scale carbon nanotube fibers by densification. ACS Applied Materials and Interfaces, 5(15):7198–7207, 2013. [http://pubs.acs.org/doi/abs/10.1021/am401524q]
This work presents a study of the tensile mechanical properties of millimeter-long fibers comprising carbon nanotubes (CNTs). These CNT fibers are made of aligned, loosely packed parallel networks of CNTs that are grown in and harvested from CNT forests without drawing or spinning. Unlike typical CNT yarn, the present fibers contain a large fraction of CNTs that span the fibers’ entire gauge length. The fibers are densified after growth and network formation to study how increasing the degree of interaction among CNTs in a network by various methods influences and limits the mechanical behavior of macroscopic CNT materials, particularly for the case in which the continuity of a large fraction of CNTs across the gauge length prevents failure purely by slip. Densification is carried out using various combinations of capillary-driven densification, mechanical pressure, and twisting. All methods of densification increase the fiber density and modify the nanoscale order of the CNTs. The highest strength and stiffness values (1.8 and 88.7 N tex–1, respectively) are observed for capillary-densified fibers, whereas the highest toughness values (94 J g–1) and maximum reversible energy density (1.35 kJ kg–1 or 677 kJ m–3) are observed for fibers densified by mechanical pressure. The results suggest that the path to higher performance CNT materials may lie not only in the use of continuous and long CNTs but also in controlling their density and nanoscale ordering through modification of the as-grown networks, such as by capillary-driven densification.
J84. Decoupled control of carbon nanotube forest diameter and density by continuous-feed convective assembly of catalyst particles.
E.S. Polsen, M. Bedewy, A.J. Hart. Small.
E.S. Polsen, M. Bedewy, A.J. Hart. Decoupled control of carbon nanotube forest diameter and density by continuous-feed convective assembly of catalyst particles. Small, 9: 2564–2575, 2013. [http://dx.doi.org/10.1002/smll.201202878]
The widespread potential application of vertically aligned carbon nanotube (CNT) forests have stimulated recent work on large-area chemical vapor deposition growth methods, but improved control of the catalyst particles is needed to overcome limitations to the monodispersity and packing density of the CNTs. In particular, traditional thin-film deposition methods are not ideal due to their vacuum requirements, and due to limitations in particle uniformity and density imposed by the thin-film dewetting process. Here, a continuous-feed convective self-assembly process for manufacturing uniform mono- and multi-layers of catalyst particles for CNT growth is presented. Particles are deposited from a solution of commercially available iron oxide nanoparticles, by pinning the meniscus between a blade edge and the substrate. The substrate is translated at constant velocity under the blade so the meniscus and contact angle remain fixed as the particles are deposited on the substrate. Based on design of the particle solution and tuning of the assembly parameters, a priori control of CNT diameter and packing density is demonstrated. Quantitative relationships are established between the catalyst size and density, and the CNT morphology and density. The roll-to-roll compatibility of this method, along with initial results achieved on copper foils, suggest promise for scale-up of CNT forest manufacturing at commercially relevant throughput.
J83. Measurement of carbon nanotube microstructure density by optical attenuation and observation of size-dependent density variations.
S.J. Park, A.J. Schmidt, M. Bedewy, A.J. Hart. Physical Chemistry Chemical Physics, 2013.
S.J. Park, A.J. Schmidt, M. Bedewy, A.J. Hart. Measurement of carbon nanotube microstructure density by optical attenuation and observation of size-dependent density variations. Physical Chemistry Chemical Physics 15:11511-11519, 2013. [http://dx.doi.org/10.1039/C3CP51415C]
Engineering the packing density of CNT forest microstructures is vital to applications such as electrical interconnects, micro-contact probes, and thermal interface materials. For CNT forests on centimeter-scale substrates, weight and volume can be used as a measure of bulk density. However, this is not suitable for smaller samples, including individual microstructures, and moreover does not enable mapping of spatial density variations within the forest. We demonstrate that the packing density of CNTs within individual microstructures can be measured by optical attenuation, with spatial resolution equaling the size of the focused spot. For this, a custom optical setup was built to measure the transmission of focused laser beam through CNT microstructures. The transmittance was correlated with the thickness of the CNT microstructures by Beer-Lambert Law to calculate the attenuation coefficient. We reveal that the density of CNT microstructures grown by CVD can depend on their size, and that the overall density of arrays of microstructures is affected significantly by run-to-run process variations. Further, we use the technique to quantify the change in CNT packing density due to capillary densification. This is a useful and accessible metrology technique for CNTs in future microfabrication processes, and will enable direct correlation of density to important properties such as stiffness and electrical conductivity.
J82. Statistical analysis of variation in laboratory growth of carbon nanotube forests and recommendations for improvement of process consistency.
C.R. Oliver*, E.S. Polsen*, E.R. Meshot, S. Tawfick, S.J. Park, M. Bedewy, A.J. Hart. ACS Nano, 2013.
C.R. Oliver, E.S. Polsen, E.R. Meshot, S. Tawfick, S.J. Park, M. Bedewy, A.J. Hart. A statistical analysis of variation in laboratory growth of carbon nanotube forests and recommendations for improvement of process consistency. ACS Nano 7(4):3565–3580, 2013. [http://dx.doi.org/10.1021/nn400507y]
While many promising applications have been demonstrated for vertically aligned carbon nanotube (CNT) forests, lack of consistency in results (e.g., CNT quality, height, and density) continues to hinder knowledge transfer and commercialization. For example, it is well known that CNT growth can be influenced by small concentrations of water vapor, carbon deposits on the reactor wall, and experiment-to-experiment variations in pressure within the reaction chamber. However, even when these parameters are controlled during synthesis, we found that variations in ambient lab conditions can overwhelm attempts to perform parametric optimization studies. We established a standard growth procedure, including the chemical vapor deposition (CVD) recipe, while we varied other variables related to the furnace configuration and experimental procedure. Statistical analysis of 280 samples showed that ambient humidity, barometric pressure, and sample position in the CVD furnace contribute significantly to experiment-to-experiment variation. We investigated how these factors lead to CNT growth variation, and recommend a set of practices to improve process repeatability. Initial results using this approach reduced uncontrolled run-to-run variation in CNT forest height and density by more than 50%.
J81. Laser printing of nanoparticle toner enables digital control of micropatterned carbon nanotube growth.
E.S. Polsen*, A.G. Stevens*, A.J. Hart. ACS Applied Materials and Interfaces, 2013.
E.S. Polsen, A.G. Stevens, A.J. Hart. Laser printing of nanoparticle toner enables digital control of micropatterned carbon nanotube growth. ACS Applied Materials and Interfaces 5(9):3656–3662, 2013. [http://dx.doi.org/10.1021/am400148t]
Commercialization of materials utilizing patterned carbon nanotube (CNT) forests, such as hierarchical composite structures, dry adhesives, and contact probe arrays, will require catalyst patterning techniques that do not rely on cleanroom photolithography. We demonstrate the large scale patterning of CNT growth catalyst via adaptation of a laser-based electrostatic printing process that uses magnetic ink character recognition (MICR) toner. The MICR toner contains iron oxide nanoparticles that serve as the catalyst for CNT growth, which are printed onto a flexible polymer (polyimide), and then transferred to a rigid substrate (silicon or alumina) under heat and mechanical pressure. Then, the substrate is processed for CNT growth under an atmospheric pressure chemical vapor deposition (CVD). This process enables digital control of patterned CNT growth via the laser intensity, which controls the CNT density; and via the grayscale level, which controls the pixelation of the image into arrays of micropillars. Moreover, virtually any pattern can be designed using standard software (e.g. MS Word, AutoCAD, etc.) and printed on demand. Using a standard office printer, we realize isolated CNT microstructures as small as 140 um and isolated catalyst “pixels” as small as 70 um (one grayscale dot), and determine that individual toner microparticles result in features of approximately 5-10 um . We demonstrate that grayscale CNT patterns can function as dry adhesives, and that large-area catalyst patterns can be printed directly onto metal foils or transferred to ceramic plates. Laser printing therefore shows promise to enable high-speed micro patterning of nanoparticle-containing thin films under ambient conditions, possibly for a wide variety of nanostructures via engineering of toners containing nanoparticles of desired composition, size, and shape.
J80. Mechanics of capillary forming of aligned carbon nanotube assemblies.
S. Tawfick, Z. Zhao, M.R. Maschmann, A. Brieland-Shoultz, M. De Volder, J.W. Baur, W. Lu, A.J. Hart. Langmuir, 2013.
S. Tawfick, Z. Zhao, M.R. Maschmann, A. Brieland-Shoultz, M. De Volder, J.W. Baur, W. Lu, A.J. Hart. Mechanics of capillary forming of aligned carbon nanotube assemblies. Langmuir 29 (17): 5190–5198, 2013. [http://dx.doi.org/10.1021/la4002219]
Elastocapillary self-assembly is emerging as a versatile technique to manufacture three-dimensional (3D) microstructures and complex surface textures from arrangements of micro- and nanoscale filaments. Understanding the mechanics of capillary self-assembly is essential to engineering of properties such as shape-directed actuation, anisotropic wetting and adhesion, and mechanical energy transfer and dissipation. We study elastocapillary self-assembly (herein called “capillary forming”) of carbon nanotube (CNT) microstructures, combining in situ optical imaging, micromechanical testing, and finite element modeling. By imaging, we identify sequential stages of liquid infiltration, evaporation, and solid shrinkage; whose kinetics relate to the size and shape of the CNT microstructure. We couple these observations with measurements of the orthotropic elastic moduli of CNT forests to understand how the dynamics of shrinkage of the vapor-liquid interface is coupled to the compression of the forest. We compare the kinetics of shrinkage to the rate of evporation from liquid droplets having the same size and geometry. Moreover, we show that the amount of shrinkage during evaporation is governed by the ability of the CNTs to slip against one another, which can be manipulated by the deposition of thin conformal coatings on the CNTs by Atomic Layer Deposition (ALD). This insight is confirmed by finite element modeling of pairs of CNTs as corrugated beams in contact, and highlights the coupled role of elasticity and friction in shrinkage and stability of nanoporous solids. Overall, this study shows that nanoscale porosity can be tailored via the filament density and adhesion at contact points, which is important to the development of lightweight multifunctional materials.
J79. Mechanical coupling limits the density and quality of self-organized carbon nanotube growth.
M. Bedewy, A.J. Hart. Nanoscale, 2013.
M. Bedewy, A.J. Hart. Mechanical coupling limits the density and quality of self-organized carbon nanotube growth. Nanoscale 5:2928-2937, 2013. [http://dx.doi.org/10.1039/C3NR34067H]
Aligned carbon nanotube (CNT) structures are promising for many applications; however, as-grown CNT “forests” synthesized by chemical vapor deposition (CVD) are typically low-density and mostly comprise tortuous defective CNTs. Here, we present evidence that the density and alignment of self-organized CNT growth is limited by mechanical coupling among CNTs in contact, in combination with their diameter-dependent growth rates. This study is enabled by comprehensive X-ray characterization of the spatially and temporally-varying internal morphology of CNT forests. Based on this data, we model the time evolution and diameter-dependent scaling of the ensuing mechanical forces on catalyst nanoparticles during CNT growth, which arise from the mismatch between the collective lengthening rate of the forest and the diameter-dependent growth rates of individual CNTs. In addition to enabling self-organization of CNTs into forests, time-varying forces between CNTs in contact dictate the hierarchical tortuous morphology of CNT forests, and may be sufficient to influence the structural quality of CNTs. These forces reach a maximum that is coincident with the maximum density observed in our growth process, and are proportional to CNT diameter. Therefore, we propose that improved manufacturing strategies for self-organized CNTs should consider both chemical and mechanical effects. This may be especially necessary to achieve high density CNT forests with low defect density, such as for improved thermal interfaces and high-permeability membranes.
J78. Nanoscale displacement measurement of microdevices via interpolation-based edge tracking of optical images.
A. Ya’akobovitz, D. Copic, J. Beroz, A.J. Hart. Journal of Micromechanics and Microengineering, 2013.
A. Ya’akobovitz, D. Copic, J. Beroz, A.J. Hart. Nanoscale displacement measurement of microdevices via interpolation-based edge tracking of optical images. Journal of Micromechanics and Microengineering 23:045004. [http://dx.doi.org/10.1088/0960-1317/23/4/045004]
We apply an interpolation-based edge-tracking algorithm to measure nanoscale displacements of microdevices, and further enhance its ability to reject noisy signals using averaging of multiple image frames. We present a simulation of a moving edge, and use this simulation to explore the performance of the algorithm as related to the image acquisition and process parameters. Then, we present the application of this technique to two experiments. First, we demonstrate optical measurement of the motion of a compliant mechanism, and smoothly detect lateral step motion change as small as 30.8 nm/s. Second, we measure the anisotropic nanoscale expansion of a liquid crystal network micropillar, which occurs slowly over several minutes. This efficient algorithm can smoothly track motions as small as a few percent of a pixel, which is equivalent to tens of nanometers using images from a video camera attached to a conventional optical microscope. Therefore, this technique can be widely applied to characterization of nanoscale motions using conventional optics, without requiring special features on the device to enable imaging.
J77. Engineering hierarchical nanostructures by elastocapillary self-assembly.
M. De Volder, A.J. Hart. Angewandte Chemie, 2013.
M. De Volder, A.J. Hart. Engineering hierarchical nanostructures by elastocapillary self-assembly. Angewandte Chemie 52:2412–2425, 2013. [http://dx.doi.org/10.1002/anie.201205944]
Surfaces coated with nanoscale filaments such as silicon nanowires and carbon nanotubes are potentially compelling for high-performance battery and capacitor electrodes, photovoltaics, electrical interconnects, substrates for engineered cell growth, dry adhesives, and other smart materials. However, many of these applications require a wet environment or involve wet processing during their synthesis. The capillary forces introduced by these wet environments can lead to undesirable aggregation of nanoscale filaments, but control of capillary forces can enable manipulation of the filaments into discrete aggregates and novel hierarchical structures. Recent studies suggest that the elastocapillary self-assembly of nanofilaments can be a versatile and scalable means to build complex and robust surface architectures. To enable a wider understanding and use of elastocapillary self-assembly as a fabrication technology, we give an overview of the underlying fundamentals and classify typical implementations and surface designs for nanowires, nanotubes, and nanopillars made from a wide variety of materials. Finally, we discuss exemplary applications and future opportunities to realize new engineered surfaces by the elastocapillary self-assembly of nanofilaments.
J76. Carbon nanotubes: present and future commercial applications.
M. De Volder, S. Tawfick, R.H. Baughman, A.J. Hart. Science, 2013.
M. De Volder, S. Tawfick, R.H. Baughman, A.J. Hart. Carbon nanotubes: present and future commercial applications. Science 339:535-589, 2013. [http://dx.doi.org/10.1126/science.1222453]
Worldwide commercial interest in carbon nanotubes (CNTs) is reflected in a production capacity that presently exceeds several thousand tons per year. Currently, bulk CNT powders are incorporated in diverse commercial products ranging from rechargeable batteries, automotive parts, and sporting goods to boat hulls and water filters. Advances in CNT synthesis, purification, and chemical modification are enabling integration of CNTs in thin-film electronics and large-area coatings. Although not yet providing compelling mechanical strength or electrical or thermal conductivities for many applications, CNT yarns and sheets already have promising performance for applications including supercapacitors, actuators, and lightweight electromagnetic shields.
C42. Fluid flow through carbon nanotube forest microchannels.
K.B. Teichert, A.J. Hart. 17th International Conference on Miniaturized Systems for Chemistry and Life Sciences (µTAS 2013).
C41. Fold mechanics of natural and synthetic origami papers.
A. Rao, S. Tawfick, M. Shlian, A.J. Hart. 2013 ASME International Design Engineering Technical Conferences (IDETC).
C40. Extensible-link kinematic model (ELKM) for determining motion characteristics of compliant mechanisms.
J. Beroz, S. Awtar, A.J. Hart. 2013 ASME International Design Engineering Technical Conferences (IDETC).
C39. Deterministic design and manufacturing of carbon nanotube staple yarns.
S. Tawfick, A. Rao, A.J. Hart. 19th International Conference on Composite Materials (ICCM).
C38. High-stroke actuation of aligned CNT-paraffin composite films.
D. Copic, A.J. Hart. 19th International Conference on Composite Materials (ICCM).
C37. Replica molding of liquid crystal polymer microstructures for active surfaces.
D. Copic, A. Ya’akobovitz, A.J. Hart. 19th International Conference on Composite Materials (ICCM).
C36. Fabrication and mechanical properties of carbon nanotube composite microtrusses.
S. Park, A. Brieland-Shoultz, M. Maschmann, S. Tawfick, M. De Volder, J.W. Baur, A.J. Hart. 19th International Conference on Composite Materials (ICCM).
C35. Manufacturing of composite laminates with perforated carbon nanotube forest core.
S. Park, S. Tawfick, A. Brieland-Shoultz, A.J. Hart. 19th International Conference on Composite Materials (ICCM).
C34. Roll-to-roll manufacturing of carbon nanotube forests on metal foils.
E.S. Polsen, D.Q. McNerny, A.J. Hart. 19th International Conference on Composite Materials (ICCM).
C33. Characterization of anisotropic conduction of horizontally aligned carbon nanotube thin films.
S. Li, W. Hua, M. Liang, M. Tuo, S. Tawfick, A.J. Hart, Q. Zhu, H. Xin. 2013 IEEE International Symposium on Antennas and Propagation.
C32. Fabrication and integration of horizontally aligned CNT-C60 films for thin film sensors.
M. De Volder, E.R. Meshot, S. Tawfick, A.J. Hart. 17th International Conference on Solid-State Sensors, Actuators, and Microsystems (Transducers’13).
C31. Direct-write self-assembly of 3D colloidal microstructures.
J.D. Beroz, M. Bedewy, A.J. Hart. Solid-State Sensor, Actuator, and Microsystems Workshop, Hilton Head, SC, 2012
J.D. Beroz, M. Bedewy, A.J. Hart. Direct-write self-assembly of 3D colloidal microstructures. Solid-State Sensor, Actuator, and Microsystems Workshop, Hilton Head, SC, 2012.
We present a direct-write technique for assembly of microscale 3D colloidal crystals on substrates. We use a custom-built high-resolution liquid manipulation system to dispense colloidal suspensions through a capillary tip. Using this system, we establish a liquid bridge between the capillary tip and a temperature-controlled substrate, initiating crystal growth upward from the substrate. We demonstrate construction of cone-shaped and tower structures by controlling the dynamic shape of the liquid meniscus during crystal precipitation. Interplay between lateral capillary forces and granular cohesion governs assembly. Finally, we show that confinement of the meniscus on microfabricated template features enables assembly of discrete article clusters.
J75. Carbon nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy.
H.W. Baac, J.G. Ok, A. Maxwell, K.T. Lee, Y.C. Chen, A.J. Hart, Z. Xu, E. Yoon, L.J. Guo. Scientific Reports, 2012.
H.W. Baac, J.G. Ok, A. Maxwell, K.T. Lee, Y.C. Chen, A.J. Hart, Z. Xu, E. Yoon, L.J. Guo. Carbon nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy. Scientific Reports 2:989, 2012. [http://dx.doi.org/10.1038/srep00989]
We demonstrate a new optical approach to generate high-frequency (>15 MHz) and high-amplitude focused ultrasound, which can be used for non-invasive ultrasound therapy. A nano-composite film of carbon nanotubes (CNTs) and elastomeric polymer is formed on concave lenses, and used as an efficient optoacoustic source due to the high optical absorption of the CNTs and rapid heat transfer to the polymer upon excitation by pulsed laser irradiation. The CNT-coated lenses can generate unprecedented optoacoustic pressures of >50 MPa in peak positive on a tight focal spot of 75 um in lateral and 400 um in axial widths. This pressure amplitude is remarkably high in this frequency regime, producing pronounced shock effects and non-thermal pulsed cavitation at the focal zone. We demonstrate that the optoacoustic lens can be used for micro-scale ultrasonic fragmentation of solid materials and a single-cell surgery in terms of removing the cells from substrates and neighboring cells.
J74. Visualizing strain evolution and coordinated buckling in CNT arrays by in situ digital image correlation.
M.R. Maschmann, G. Ehlert, S.J. Park, D. Mollenhauer, B. Maryuama, A.J. Hart, J.W. Baur. Advanced Functional Materials, 2012.
M.R. Maschmann, G. Ehlert, S.J. Park, D. Mollenhauer, B. Maryuama, A.J. Hart, J.W. Baur. Visualizing strain evolution and coordinated buckling in CNT arrays by in situ digital image correlation. Advanced Functional Materials 22:4686-4695, 2012. [http://dx.doi.org/10.1002/adfm.201200676]
Spatial mapping of strain fields within compressed carbon nanotube (CNT) array columns is achieved using digital image correlation (DIC) analysis of in situ scanning electron microscopy (SEM) image sequences. Full-field displacement and strain maps are generated based upon the motion of the
constituent CNTs, which serve as a traceable high-contrast speckle pattern for DIC analysis. The deformation modes and CNT array buckling characteristics vary systematically as a function of column aspect ratio, including bending, crushing, and bottom-up buckle accumulation behaviors. In spite of disparate appearing deformation modes, strain maps indicate that CNT array buckling consistently initiates at 5% local principal strain for all columns. The ability to quantitatively assess the deformation modes and buckling behavior of CNT arrays at the nanoscale will enable their improved design for highstrain electrical contacts, compliant thermal interfaces, force sensors, energyabsorbing foams, or other applications.
J73. Wide range control of microstructure and mechanical properties of carbon nanotube forests: A comparison between fixed and floating catalyst CVD techniques.
O. Yaglioglu, A. Cao, A.J. Hart, R. Martens, A.H. Slocum. Advanced Functional Materials, 2012.
O. Yaglioglu, A. Cao, A.J. Hart, R. Martens, A.H. Slocum. Wide range control of microstructure and mechanical properties of carbon nanotube forests: A comparison between fixed and floating catalyst CVD techniques. Advanced Functional Materials 22:5028-5037, 2012. [http://dx.doi.org/10.1002/adfm.201200852]
Vertically aligned carbon nanotube (CNT) forests may be used as miniature springs, compliant thermal interfaces, and shock absorbers, and for these and other applications it is vital to understand how to engineer their mechanical properties. Herein is investigated how the diameter and packing density within CNT forests govern their deformation behavior, structural stiffness, and elastic energy absorption properties. The mechanical behavior of low-density CNT forests grown by fixed catalyst CVD methods and high-density CNT forests grown by a floating catalyst CVD method are studied by in situ SEM compression testing and tribometer measurements of force-displacement relationships. Low-density and small-diameter CNT columns (fixed catalyst) exhibit large plastic deformation and can be pre-deformed to act as springs within a specified elastic range, whereas high-density and large-diameter CNT columns (floating catalyst) exhibit significant elastic recovery after deformation. In this work the energy absorption capacity of CNT forests is tuned over three orders of magnitude and it is shown that CNT forest density can be tuned over a range of conventional foam materials, but corresponding stiffness is ?10× higher. It is proposed that the elastic behavior of CNT forests is analogous to open-cell foams and a simple model is presented. It is also shown that this model can be useful as a first-order design tool to establish design guidelines for the mechanical properties of CNT forests and selection of the appropriate synthesis method.
J72. Anisotropic Janus catalysts for spatially controlled chemical reactions.
W. Lv, K. Lee, J. Li, T. Park, A.J. Hart, F. Zhang, J. Lahann. Small, 2012.
W. Lv, K. Lee, J. Li, T. Park, A.J. Hart, F. Zhang, J. Lahann. Anisotropic Janus catalysts for spatially controlled chemical reactions. Small 8(20):3116-3122, 2012. [http://dx.doi.org/10.1002/smll.201200192]
We describe a novel method for manufacturing of multicompartmental (“Janus”) inorganic particles using electrohydrodynamic (EHD) co-jetting of composite polymer-ceramic solutions, followed by thermal calcination at 500 C. A degradable organic polymer serves as a template and ensures homogeneous particle formation by co-jetting. Via this approach, catalytically active additives, such as magnetic nanocrystals can be placed on in one compartment of the Janus catalysts. We demonstrate spatially confined growth of carbon nanotubes on hemispheres of the calcined particles.
J71. Diameter-dependent kinetics of activation and deactivation in carbon nanotube population growth.
M. Bedewy, E.R. Meshot, A.J. Hart. Carbon, 2012.
M. Bedewy, E.R. Meshot, A.J. Hart. Diameter-dependent kinetics of activation and deactivation in carbon nanotube population growth. Carbon 50(14):5106-5116, 2012. [http://dx.doi.org/10.1016/j.carbon.2012.06.051]
We reveal that the collective growth of vertically aligned carbon nanotube (CNT) forests by chemical vapor deposition (CVD) is governed by the size-dependent catalytic behavior of metal nanoparticles, which can be quantitatively related to the activation and deactivation kinetics of subpopulations of CNTs within the forest. We establish this understanding by uniquely combining real-time forest height kinetics with ex situ synchrotron X-ray scattering and mass-attenuation measurements. The growing CNT population is divided into subpopulations, each having a narrow diameter range, enabling the quantification of the diameter-dependent population dynamics. We find that the mass kinetics of different subpopulations are self-similar and are represented by the S-shaped Gompertz model of population growth, which reveals that smaller diameter CNTs activate more slowly but have longer catalytic lifetimes. While competition between growth activation and deactivation kinetics is diameter-dependent, CNTs are held in contact by van der Waals forces, thus preventing relative slip and resulting in a single collective growth rate of the forest. Therefore, we hypothesize that mechanical coupling gives rise to the inherent tortuousity of CNTs within forests and possibly causes structural defects which limit the properties of current CNT forests in comparison to pristine individual CNTs.
J70. Fabrication, densification, and replica molding of 3D carbon nanotube microstructures.
D. Copic, S.J. Park, S. Tawfick, M. De Volder, A.J. Hart. Journal of Visualized Experiments, 2012.
D. Copic, S.J. Park, S. Tawfick, M. De Volder, A.J. Hart. Fabrication, densification, and replica molding of 3D carbon nanotube microstructures. Journal of Visualized Experiments 65:e3980, 2012. [http://dx.doi.org/10.3791/3980]
Our goal is to introduce vertically aligned carbon nanotubes (CNTs), which we refer to as CNT “forests”, as a new microfabrication material. We present details of a suite of related methods recently developed by our group: fabrication of CNT forest microstructures by thermal CVD from lithographically patterned catalyst thin films; self-directed elastocapillary densification of CNT microstructures; and replica molding of polymer microstructures using CNT composite master molds. In particular, our work shows that self-directed capillary densification (“capillary forming”), which is performed by condensation of a solvent onto the substrate with CNT microstructures, significantly increases the packing density of CNTs. This process enables directed transformation of vertical CNT microstructures into straight, inclined, and twisted shapes, which have robust mechanical properties exceeding those of typical microfabrication polymers. This in turn enables formation of nanocomposite CNT master molds by capillary-driven infiltration of polymers. The replica structures exhibit the anisotropic nanoscale texture of the aligned CNTs, and can have walls with sub-micron thickness and aspect ratios exceeding 50:1. Integration of CNT microstructures in fabrication offers further opportunity to exploit the electrical and thermal properties of CNTs, and diverse capabilities for chemical and biochemical functionalization.
J69. High-speed in situ X-ray scattering of carbon nanotube film nucleation and self-organization.
E.R. Meshot, E.A. Verploegen, M. Bedewy, S. Tawfick, A.R. Woll, K.S. Green, M. Hromalik, L.J. Koerner, H.T. Philipp, M.W. Tate, S.M. Gruner, A.J. Hart. ACS Nano, 2012.
E.R. Meshot, E.A. Verploegen, M. Bedewy, S. Tawfick, A.R. Woll, K.S. Green, M. Hromalik, L.J. Koerner, H.T. Philipp, M.W. Tate, S.M. Gruner, A.J. Hart. High-speed in situ X-ray scattering of carbon nanotube film nucleation and self-organization. ACS Nano. 6:5091–5101, 2012. [http://dx.doi.org/10.1021/nn300758f][video1][video2]
The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.
|Dewetting of the catalyst film into nanoparticles||Nucleation and self-organization of the CNT film|
J68. Synthesis of tall carpets of vertically aligned carbon nanotubes via in situ generation of water vapor by preheating of added oxygen.
G.D. Nessim, A. Al-Obeidi, H. Grisaru, E.S. Polsen, C.R. Oliver, T. Zimrin, A.J. Hart, D. Aurbach, C.V. Thompson. Carbon, 2012.
G.D. Nessim, A. Al-Obeidi, H. Grisaru, E.S. Polsen, C.R. Oliver, T. Zimrin, A.J. Hart, D. Aurbach, C.V. Thompson. Synthesis of tall carpets of vertically aligned carbon nanotubes via in situ generation of water vapor by preheating of added oxygen. Carbon 50:4002-4009, 2012. [http://dx.doi.org/10.1016/j.carbon.2012.04.043]
Dense millimeter-tall carpets of vertically aligned carbon nanotubes (VACNTs) were grown using thermal chemical vapor deposition (CVD) from ethylene and hydrogen gases with two or three independently controlled hot zones while introducing controlled ?ows of oxygen. Through preheating, oxygen and hydrogen reacted through a multi-step reaction toform water, enabling the growth of tall CNT carpets. This process showed a large tolerance for variations of O2, H2, and C2H4. The measured water vapor produced was half the theoretical maximum. The residence time strongly affected the decomposition of the gases. The simplicity and robustness of this CVD process provides a simpler alternative to direct addition of water vapor for manufacturing tall carpets of aligned CNTs with a high level of control.
J67. Capillary bending of Janus carbon nanotube micropillars.
S. Tawfick, A.J. Hart, M. De Volder. Nanoscale, 2012.
S. Tawfick, A.J. Hart, M. De Volder. Capillary bending of Janus carbon nanotube micropillars. Nanoscale 4:3852-3856, 2012. [http://dx.doi.org/10.1039/C2NR30802A]
We present a scalable process for the fabrication of slanted carbon nanotube micropillar arrays by inclined metal deposition and capillary self-assembly. Local control of the micropillar angle from vertical to nearly horizontal is achieved, and is explained using a finite element model. These structures may be useful for microscale contacts and anisotropic smart surfaces.
J66. Engineering of micro- and nanostructured surfaces with anisotropic geometries and properties.
S. Tawfick, M. De Volder, D. Copic, S.J. Park, E.S. Polsen, C.R. Oliver, M.J. Roberts, A.J. Hart. Advanced Materials, 2012.
S. Tawfick, M. De Volder, D. Copic, S.J. Park, E.S. Polsen, C.R. Oliver, M.J. Roberts, A.J. Hart. Engineering of micro- and nanostructured surfaces with anisotropic geometries and properties. Advanced Materials. 24:1628–1674, 2012. [http://dx.doi.org/10.1002/adma.201103796]
Widespread approaches to fabricate surfaces with robust micro- and nanostructured topographies have been stimulated by opportunities to enhance interface performance by combining physical and chemical effects. In particular, arrays of asymmetric surface features, such as arrays of grooves, inclined pillars, and helical protrusions, have been shown to impart unique anisotropy in properties including wetting, adhesion, thermal and/or electrical conductivity, optical activity, and capability to direct cell growth. These properties are of wide interest for applications including energy conversion, microelectronics, chemical and biological sensing, and bioengineering. However, fabrication of asymmetric surface features often pushes the limits of traditional etching and deposition techniques, making it challenging to produce the desired surfaces in a scalable and cost-effective manner. We review and classify approaches to fabricate arrays of asymmetric 2D and 3D surface features, in polymers, metals, and ceramics. Analytical and empirical relationships among geometries, materials, and surface properties are discussed, especially in the context of the applications mentioned above. Further, opportunities for new fabrication methods that combine lithography with principles of self-assembly are identi?ed, aiming to establish design principles for fabrication of arbitrary 3D surface textures over large areas.
J65. Photoconductive hybrid films via directional self-assembly of C60 on aligned carbon nanotubes.
E.R. Meshot, K. Patel, S. Tawfick, K.A. Juggernauth, M. Bedewy, E.A. Verploegen, M. De Volder, A.J. Hart. Advanced Functional Materials, 2012.
E.R. Meshot, K. Patel, S. Tawfick, K.A. Juggernauth, M. Bedewy, E.A. Verploegen, M. De Volder, A.J. Hart. Photoconductive hybrid films via directional self-assembly of C60 on aligned carbon nanotubes. Advanced Functional Materials 22(3):577-584, 2012. [http://dx.doi.org/10.1002/adfm.201102393]
Hybrid nanostructured materials can exhibit different properties than their constituent components, and can enable decoupled engineering of energy conversion and transport functions. We demonstrate a novel means of building hybrid assemblies of crystalline C60 and carbon nanotubes (CNTs), wherein aligned carbon nanotube (CNT) films direct the crystallization and orientation of C60 rods from solution. In these hybrid films, the C60 rods are oriented parallel to the direction of the CNTs throughout the thickness of the film. High-resolution imaging shows that the crystals incorporate CNTs during growth, yet grazing-incidence X-ray diffraction (GIXD) shows that the crystal structure of the C60 rods is not perturbed by the CNTs. Growth kinetics of the C60 rods are enhanced 8-fold on CNTs compared to bare Si, emphasizing the importance of the aligned, porous morphology of the CNT films as well as the selective surface interactions between C60 and CNTs. Finally, we show how hybrid C60-CNT films can be integrated electrically and employed as UV detectors having high photoconductive gain, with a responsivity of 10^5 A/W at low biases (± 0.5 V). Our findings that CNTs can induce rapid, directional crystallization of molecules from solution may have broader implications to the science and applications of crystal growth, such as for synthetic polymers, and proteins.
J64. Four degree of freedom liquid dispenser for direct-write capillary self-assembly with sub-nanoliter precision.
J. Beroz, M. Bedewy, M. Reinker, V. Chhajer, S. Awtar, A.J. Hart. Review of Scientific Instruments, 2012.
J. Beroz, M. Bedewy, M. Reinker, V. Chhajer, S. Awtar, A.J. Hart. Four degree of freedom liquid dispenser for direct-write capillary self-assembly with sub-nanoliter precision. Review of Scientific Instruments 83:015104, 2012. [http://dx.doi.org/10.1063/1.3673680][video1(.mpg)][video2(.mpg)]
Capillary forces provide a ubiquitous means of organizing micro- and nanoscale structures on substrates. In order to investigate the mechanism of capillary self-assembly and to fabricate complex ordered structures, precise control of the meniscus shape is needed. We present a precision instrument that enables deposition of liquid droplets spanning from 2 nl to 300 ?l, in concert with mechanical manipulation of the liquid-substrate interface with four degrees of freedom. The substrate has sub-100 nm positioning resolution in three axes of translation, and its temperature is controlled using thermoelectric modules. The capillary tip can rotate about the vertical axis while simultaneously dispensing liquid onto the substrate. Liquid is displaced using a custom bidirectional diaphragm pump, in which an elastic membrane is hydraulically actuated by a stainless steel syringe. The syringe is driven by a piezoelectric actuator, enabling nanoliter volume and rate control. A quantitative model of the liquid dispenser is verified experimentally, and suggests that compressibility in the hydraulic line deamplifies the syringe stroke, enabling sub-nanoliter resolution control of liquid displacement at the capillary tip. We use this system to contact-print water and oil droplets by mechanical manipulation of a liquid bridge between the capillary and the substrate. Finally, we study the effect of droplet volume and substrate temperature on the evaporative self-assembly of monodisperse polymer microspheres from sessile droplets, and demonstrate the formation of 3D chiral assemblies of micro-rods by rotation of the capillary tip during evaporative assembly.
J63. Chemically controlled bending of compositionally anisotropic microcylinders.
S. Saha, D. Copic, S. Bhaskar, N. Clay, A. Donini, A.J. Hart, J. Lahann. Angewandte Chemie, 2012.
S. Saha, D. Copic, S. Bhaskar, N. Clay, A. Donini, A.J. Hart, J. Lahann. Chemically controlled bending of compositionally anisotropic microcylinders. Angewandte Chemie 51(3):660-665, 2012. [http://dx.doi.org/10.1002/anie.201105387]
We report a new type of compositionally anisotropic microcylinders, where defined compartments within the same microcylinder undergo differential expansion due to the site-selective growth of a surface layer. The asymmetric expansion creates surface stresses resulting in significant and controllable bending of the microcylinders, which depends on the particle geometry and the architecture of the surface layers. Using finite element simulations, we verify the observed bending trends and derive a family of performance curves that predict a wide-range tunability of the actuation stroke based on the cylinder geometry and the amount of swelling.
C30. Compliant microgripper with parallel straight-line jaw trajectory.
J.D. Beroz, S. Awtar, M. Bedewy, A.J. Hart. ASPE Annual Meeting, Denver, CO, 2011.
J.D. Beroz, S. Awtar, M. Bedewy, A.J. Hart. Compliant microgripper with parallel straight-line jaw trajectory. ASPE Annual Meeting, Denver, CO, 2011.
We present a compliant gripping mechanism that exhibits a straight-line parallel jaw trajectory. The input (tab) and output (gripper jaw) displacements are proportional, and the proportionality factor can be varied by design. This gripper is a lumped-compliance based flexure mechanism developed from an analogous rigid-link-and-pin-joint mechanism. Transverse errors (?), stemming primarily from linear-elastic structural compliance, are corrected by modifying the mechanism geometry to incorporate kinematic compensation trajectories that redress these errors by superposition. The resulting compliant microgripper features zero kinematic error, and the elastic error to jaw stroke ratio (?/d) is minimized to 2.5×10-5. By comparison, a parallelogram configuration of equivalent beam dimensions has a ratio of 4.35×10-3. Further, a parallelogram gripper would require beam lengths at least one order of magnitude larger than the dimensions of our proposed design to achieve a similar straightness in output path over the same jaw range.
C29. Hygroscopic biomimetic transducers made from CNT-hydrogel composites.
M. De Volder, S. Tawfick, D. Copic, A.J. Hart. 16th International Conference on Solid-State Sensors, Actuators, and Microsystems, Beijing, China, 2011.
M. De Volder, S. Tawfick, D. Copic, A.J. Hart. Hygroscopic biomimetic transducers made from CNT-hydrogel composites.16th International Conference on Solid-State Sensors, Actuators, and Microsystems, Beijing, China, 2011.
Plants as well as other biological organisms achieve directed movements by fibres that constraint and direct the isotropic expansion of a matrix material. In order to mimic these actuators, complex arrangements of rigid fibres must be achieved, which is challenging, especially at small scales. In this paper, a new method to organize carbon nanotubes (CNTs) into complex shapes is employed to create a framework for hydrogel infiltration. These CNT frameworks can be realized as iris, needle and bridge architectures, and after hydrogel infiltration, they show directed actuation in response to water uptake. Finally, we show how the latter can be employed as a novel hygroscopic sensor.
C28. Programmable transformation of vertical carbon nanotubes into 3D microstructures.
M. De Volder, S. Tawfick, S.J. Park, D. Copic, A.J. Hart. 16th International Conference on Solid-State Sensors, Actuators, and Microsystems, Beijing, China, 2011.
M. De Volder, S. Tawfick, S.J. Park, D. Copic, A.J. Hart. Programmable transformation of vertical carbon nanotubes into 3D microstructures.16th International Conference on Solid-State Sensors, Actuators, and Microsystems, Beijing, China, 2011. [http://dx.doi.org/10.1109/TRANSDUCERS.2011.5969791]
Capillary forming of carbon nanotubes (CNTs) enables the fabrication of unique 3D microstructures overlarge areas. In this paper we focus on the simulation as well as on the integration of these structures in MEMS devices. We developed finite element models (FEM) that enables qualitative prediction of shape transformations caused by capillary forming; and show how capillary formed CNT structured can be integrated with conventional lithographic processing for patterning of polymers and metals in concert with CNTs.
J62. Population growth dynamics of carbon nanotubes.
M. Bedewy, E.R. Meshot, M.J. Reinker, A.J. Hart. ACS Nano, 2011.
Understanding the population growth behavior of filamentary nanostructures, such as carbon nanotubes (CNTs), is hampered by the lack of characterization techniques capable of probing statistical variations with high spatial resolution. We present a comprehensive methodology for studying the population growth dynamics of vertically aligned CNT forests, utilizing high-resolution spatial mapping of synchrotron X-ray scattering and attenuation, along with real-time height kinetics. We map the CNT alignment and dimensions within CNT forests, revealing broadening and focusing of size distributions during different stages of the process. Then, we calculate the number density and mass density of the CNT population versus time, which are true measures of the reaction kinetics. We find that the mass-based kinetics of a CNT population is accurately represented by the S-shaped Gompertz model of population growth, although the forest height and CNT length kinetics are essentially linear. Competition between catalyst activation and deactivation govern the rapid initial acceleration and slow decay of the CNT number density. The maximum CNT density (i.e., the overall catalyst activity) is limited by gas-phase reactions and catalyst-surface interactions, which collectively exhibit autocatalytic behavior. Thus, we propose a comprehensive picture of CNT population growth which combines both chemical and mechanical cooperation. Our findings are relevant to both bulk and substrate-based CNT synthesis methods, and provide general insights into the self-assembly and collective growth of filamentary nanostructures.
J61. Hydrogel-driven carbon nanotube microtransducers.
M. De Volder, S. Tawfick, D. Copic, A.J. Hart. Soft Matter, 2011.
We demonstrate the fabrication and integration of active microstructures based on composites of 3D carbon nanotube (CNT) frameworks and hydrogels. The alignment of the CNTs within the microstructures converts the isotropic expansion of the gel into a directed anisotropic motion. Actuation by a moisture-responsive gel is observed by changing the ambient humidity, and is predicted by a finite element model of the composite system. These shape changes are rapid and can be transduced electrically within a microfluidic channel, by measuring the resistance change across a CNT microstructure during expansion of the gel. Our results suggest that combinations of gels with aligned CNTs can be a platform for directing the actuation of gels and measuring their response to stimuli.
J60. Corrugated carbon nanotube microstructures with geometrically tunable compliance.
M. De Volder, S. Tawfick, S.J. Park, A.J. Hart. ACS Nano, 2011.
M. De Volder, S. Tawfick, S.J. Park, A.J. Hart. Corrugated carbon nanotube microstructures with geometrically tunable compliance. ACS Nano 5(9):7310-7317, 2011. [http://dx.doi.org/10.1021/nn202156q]
Deterministic organization of nanostructures into micro-scale geometries is essential for the development of materials with novel mechanical, optical, and surface properties. We demonstrate scalable fabrication of 3D corrugated carbon nanotube (CNT) microstructures, via an iterative sequence of vertically aligned CNT growth and capillary self-assembly. Vertical micro-bellows and tilted micro-cantilevers are created over large areas, and these structures can have thin walls with aspect ratios exceeding 100:1. We demonstrate that corrugated CNT microstructures can be used as out of plane microsprings with compliance determined by the wall thickness and number of folds.
J59. Hierarchical carbon nanowire microarchitectures made by plasma-assisted pyrolysis of photoresist.
M. De Volder, R. Vansweevelt, P. Wagner, D. Reynaerts, C. Van Hoof, A.J. Hart. ACS Nano, 2011.
M. De Volder, R. Vansweevelt, P. Wagner, D. Reynaerts, C. Van Hoof, A.J. Hart. Hierarchical carbon nanowire microarchitectures made by plasma-assisted pyrolysis of photoresist. ACS Nano 5(8):6593-6600, 2011. [http://dx.doi.org/10.1021/nn201976d]
We present a new approach for the fabrication and integration of vertically aligned forests of amorphous carbon nanowires (CNWs), using only standard lithography, oxygen plasma treatment, and thermal processing. The simplicity and scalability of this process, as well as the hierarchical organization of CNWs, provides a potential alternative to the use of carbon nanotubes and graphene for applications in microsystems and high surface area materials. The CNWs are highly branched at the nanoscale, and novel hierarchical microstructures with CNWs connected to a solid amorphous core are made by controlling the plasma treatment time. By multi-layer processing we demonstrate deterministic joining of CNW micropillars into 3D sensing networks. Finally we show that these networks can be chemically functionalized and used for measurement of DNA binding with increased sensitivity.
J58. Structurally programmed capillary folding of vertical carbon nanotube assemblies.
S. Tawfick, M. De Volder, A.J. Hart. Langmuir, 2011.
We demonstrate the fabrication of horizontally aligned carbon nanotube (HA-CNT) networks by spatially programmable folding, which is induced by self-directed liquid infiltration of vertical CNTs. Folding is caused by a capillary buckling instability and is predicted by the elastocapillary buckling height, which scales with the wall thickness as t3/2. The folding direction is controlled by incorporating folding initiators at the ends of the CNT walls, and the initiators cause a tilt during densification which precedes buckling. By patterning these initiators and specifying the wall geometry, we control the dimensions of HA-CNT patches over 2 orders of magnitude and realize multilayered and multidirectional assemblies. Multidirectional HA-CNT patterns are building blocks for custom design of nanotextured surfaces and flexible circuits.
J57. Continuous production of vertically aligned carbon nanotubes on 2D and 3D substrates.
R. Guzman de Villoria, A.J. Hart, B.L. Wardle. ACS Nano, 2011
Vertically aligned carbon nanotubes (VACNTs) have certain advantages over bulk CNT powders and randomly oriented CNT mats for applications in flexible electronic devices, filtration membranes, biosensors and multifunctional aerospace materials. Here, a machine and a process to synthesize VACNTs in a continuous manner are presented showing uniform growth on 2D and 3D substrates, including alumina fibers, silicon wafer pieces, and stainless steel foils. Aligned multiwalled carbon nanotubes (MWNT) are synthesized at substrate feed rates of up to 6.8 cm/min, and the CNTs reach up to 60 ?m in length depending on residence time in the reactor. In addition to the aligned morphology indicative of high yield growth, transmission electron microscopy and Raman spectroscopy reveal that the CNTs are of comparable quality to CNTs grown via a similar batch process. A significant reduction in time, reaction products, gases, and energy is demonstrated relative to batch processing, paving the way for industrial production of VACNTs.
J56. Non-destructive characterization of structural hierarchy within carbon nanotube assemblies.
E.A. Verploegen, A.J. Hart, M. De Volder, S. Tawfick, K.K. Chia, R.E. Cohen. Journal of Applied Physics, 2011.
Understanding and controlling the hierarchical self-assembly of carbon nanotubes (CNTs) is vital for designing materials such as transparent conductors, chemical sensors, high-performance composites, and microelectronic interconnects. In particular, many applications require high-density CNT assemblies that cannot currently be made directly by low-density CNT growth, and therefore require post-processing by methods such as elastocapillary densification. We characterize the hierarchical structure of pristine and densified vertically aligned multi-wall CNT forests, by combining small-angle and ultra-small-angle x-ray scattering (USAXS) techniques. This enables the nondestructive measurement of both the individual CNT diameter and CNT bundle diameter within CNT forests, which are otherwise quantified only by delicate and often destructive microscopy techniques. Our measurements show that multi-wall CNT forests grown by chemical vapor deposition consist of isolated and bundled CNTs, with an average bundle diameter of 16 nm. After capillary densification of the CNT forest, USAXS reveals bundles with a diameter >4 ?m, in addition to the small bundles observed in the as-grown forests. Combining these characterization methods with new CNT processing methods could enable the engineering of macro-scale CNT assemblies that exhibit significantly improved bulk properties.
J55. Fabrication of high-aspect-ratio polymer microstructures and hierarchical textures using carbon nanotube composite master molds.
D. Copic, S.J. Park, S. Tawfick, M. De Volder, A.J. Hart. Lab on a Chip, 2011.
Scalable and cost effective patterning of polymer structures and their surface textures is essential to engineer material properties such as liquid wetting and dry adhesion, and to design artificial biological interfaces. Further, fabrication of high-aspect-ratio microstructures often requires controlled deep-etching methods or high-intensity exposure. We demonstrate that carbon nanotube (CNT) composites can be used as master molds for fabrication of high-aspect-ratio polymer microstructures having anisotropic nanoscale textures. The master molds are made by growth of vertically aligned CNT patterns, capillary densification of the CNTs using organic solvents, and capillary-driven infiltration of the CNT structures with SU-8. The composite master structures are then replicated in SU-8 using standard PDMS transfer molding methods. By this process, we fabricated a library of replicas including vertical micro-pillars, honeycomb lattices with sub-micron wall thickness and aspect ratios exceeding 50:1, and microwells with sloped sidewalls. This process enables batch manufacturing of polymer features that capture complex nanoscale shapes and textures, while requiring only optical lithography and conventional thermal processing.
J54. Fabrication and electrical integration of robust carbon nanotube micropillars by self-directed elastocapillary densification.
M. De Volder, S.J. Park, S. Tawfick, A.J. Hart. Journal of Micromechanics and Microengineering, 2011.
Vertically aligned carbon nanotube (CNT) ‘forest’ microstructures fabricated by chemical vapor deposition (CVD) using patterned catalyst films typically have a low CNT density per unit area. As a result, CNT forests have poor bulk properties and are too fragile for integration with microfabrication processing. We introduce a new self-directed capillary densification method where a liquid is controllably condensed onto and evaporated from the CNT forests. Compared to prior approaches, where the substrate with CNTs is immersed in a liquid, our condensation approach gives significantly more uniform structures and enables precise control of the CNT packing density. We present a set of design rules and parametric studies of CNT micropillar densification by self-directed capillary action, and show that self-directed capillary densification enhances Young’s modulus and electrical conductivity of CNT micropillars by more than three orders of magnitude. Owing to the outstanding properties of CNTs, this scalable process will be useful for the integration of CNTs as a functional material in microfabricated devices for mechanical, electrical, thermal and biomedical applications.
J53. Multidirectional hierarchical nanocomposites made by carbon nanotube growth within layer-by-layer assembled films.
J. Li, S. Srivastava, J.G. Ok, Y. Zhang, M. Bedewy, N.A. Kotov, A.J. Hart. Chemistry of Materials, 2011.
J. Li, S. Srivastava, J.G. Ok, Y. Zhang, M. Bedewy, N.A. Kotov, A.J. Hart. Multidirectional hierarchical nanocomposites made by carbon nanotube growth within layer-by-layer assembled films. Chemistry of Materials 23:1023-1031, 2011. [http://dx.doi.org/10.1021/cm1030443]
We demonstrate fabrication of multidirectional and hierarchical carbon nanotube (CNT) films on diverse substrates, using nanocomposite catalyst films prepared by layer-by-layer (LBL) assembly. CNT density and yield are controlled by the thickness of a montmorillonite clay/poly(dyallyldimethyl ammonium chloride (MTM/PDDA) support film. Using identical methods, few-walled CNTs are grown on flat silicon substrates, carbon fibers, and titanium wire mesh. On flat substrates, unique bilayer CNT forests, reminiscent of microscale “accordions”, form because of diffusion of the Fe catalyst through the support which is then split because of mechanical forces exerted by the growing CNTs. Electrochemical measurements of CNT-coated Ti wires demonstrate an 85-fold enhancement in specific capacitance, and 7.1 F/g for the CNTs alone. This novel approach to substrate engineering for CNT growth can create materials with unique and nonlinear properties by hierarchical ordering of CNTs at multiple length scales, and is scalable to large-area foils and fabrics.
J52. Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature.
G.D. Nessim, M. Seita, D.L. Plata, K.P. O’Brien, A.J. Hart, E.R. Meshot, C.M. Reddy, P.M. Gschwend, C.V. Thompson. Carbon, 2011.
G.D. Nessim, M. Seita, D.L. Plata, K.P. O’Brien, A.J. Hart, E.R. Meshot, C.M. Reddy, P.M. Gschwend, C.V. Thompson. Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature. Carbon, 2011. [http://dx.doi.org/10.1016/j.carbon.2010.10.018]
51. Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation.
H.W. Baac, J.G. Ok, H.J. Park, T. Ling, S.L. Chen, A.J. Hart, L.J. Guo. Applied Physics Letters, 2010.
H.W. Baac, J.G. Ok, H.J. Park, T. Ling, S.L. Chen, A.J. Hart, L.J. Guo. Carbon nanotube composite optoacoustic transmitters for strong and high frequency ultrasound generation. Applied Physics Letters 97:234104, 2010. [http://dx.doi.org/10.1063/1.3522833]
We demonstrate carbon nanotube (CNT) composite-based optoacoustic transmitters that generate strong and high frequency ultrasound. The composite consists of CNTs grown on a substrate, which are embedded in elastomeric polymer used as an acoustic transfer medium. Under pulsed laser excitation, the composite generates very strong optoacoustic pressure: 18 times stronger than a Cr film reference and five times stronger than a gold nanoparticle composite with the same polymer. This enhancement persists over a broadband frequency range of up to 120 MHz and is confirmed by calculation. We suggest the CNT-polymer composites as highly efficient optoacoustic transmitters for high resolution ultrasound imaging.
50. Multiple alkynes react with ethylene to enhance carbon nanotube synthesis, suggesting a polymerization-like formation mechanism.
D.L. Plata*, E.R. Meshot*, C.M. Reddy, A.J. Hart, P.M. Gschwend. ACS Nano, 2010.
D.L. Plata*, E.R. Meshot*, C.M. Reddy, A.J. Hart, P.M. Gschwend. Multiple alkynes react with ethylene to enhance carbon nanotube synthesis, suggesting a polymerization-like formation mechanism. ACS Nano 4(10):7185-7192, 2010. [http://dx.doi.org/10.1021/nn101842g]
Thermal treatments of feedstock gases (e.g., C2H4/H2) used during carbon nanotube (CNT) synthesis result in the formation of a complex mixture of volatile organic compounds and polycyclic aromatic hydrocarbons. Some of these are likely important CNT precursors, while others are superfluous and possibly degrade product quality, form amorphous carbon, and/or contribute to growth termination. To simulate the effect of thermal treatment without this chemical complexity, we delivered trace amounts of individual hydrocarbons, along with ethylene and hydrogen, to a cold-wall atmospheric pressure reactor containing a locally heated metal catalyst (Fe on Al2O3). Using these compound-specific experiments, we demonstrate that many alkynes (e.g., acetylene, methyl acetylene, and vinyl acetylene) accelerate multiwalled CNT formation with this catalyst system. Furthermore, ethylene is required for enhanced CNT growth, suggesting that the alkyne and ethylene may react in concert at the metal catalyst. This presents a distinct CNT formation mechanism where the chemical precursors may be intact during C?C bond formation, such as in polymerization reactions, challenging the widely accepted hypothesis that precursors completely dissociate into C (or C2) units before “precipitating” from the metal. Armed with these mechanistic insights, we were able to form high-purity CNTs rapidly with a 15-fold improvement in yield, a 50% reduction in energetic costs, and order of magnitude reduction in unwanted byproduct formation (e.g., toxic and smog-forming chemicals and greenhouse gases).
49. Bending of nanoscale filament assemblies by elastocapillary densification
Z. Zhao, S. Tawfick, S.J. Park, M. De Volder, A.J. Hart, W. Lu. Physical Review E, 2010.
Z. Zhao, S. Tawfick, S.J. Park, M. De Volder, A.J. Hart, W. Lu. Bending of nanoscale filament assemblies by elastocapillary densification. Physical Review E 82:041605, 2010. [http://dx.doi.org/10.1103/PhysRevE.82.041605]
We report a mechanism by which nanoscale filaments self-assemble into asymmetric aggregates by elastocapillary action. Specifically, capillary rise of liquid into an asymmetric pattern of vertically aligned filaments causes the filaments to deflect laterally during elastocapillary densification. We quantitatively show that the lateral deflection can be controlled precisely by the pattern shape and the coupling strength among the filaments. We exploit this mechanism to fabricate asymmetric micropillars and multidirectional bridges of densely packed carbon nanotubes. Analogous behavior occurs as biological filaments interact with liquids, and these findings enable scalable fabrication of anisotropic filament assemblies for manipulating surface interactions between solids and liquids.
48. Diverse 3D microarchitectures made by capillary forming of carbon nanotubes
M. De Volder*, S. Tawfick* (*equal contribution), S.J. Park, D. Copic, Z. Zhao, W. Lu, A.J. Hart. Advanced Materials, 2010.
M. De Volder*, S. Tawfick* (*equal contribution), S.J. Park, D. Copic, Z. Zhao, W. Lu, A.J. Hart. Diverse 3D microarchitectures made by capillary forming of carbon nanotubes. Advanced Materials 22:4384-4389, 2010. [http://dx.doi.org/10.1002/adma.201001893]
A new technology called capillary forming enables transformation of vertically aligned nanoscale filaments into complex three-dimensional microarchitectures. We demonstrate capillary forming of carbon nanotubes into diverse forms having intricate bends, twists, and multidirectional textures. In addition to their novel geometries, these structures have mechanical stiffness exceeding that of microfabrication polymers, and can be used as masters for replica molding.
47. Electrically addressable hybrid architectures of zinc oxide nanowires grown on aligned carbon nanotubes
J.G. Ok, S. Tawfick, K.A. Juggernauth, K. Sun, Y. Zhang, A.J. Hart. Advanced Functional Materials, 2010
J.G. Ok, S. Tawfick, K.A. Juggernauth, K. Sun, Y. Zhang, A.J. Hart. Electrically addressable hybrid architectures of zinc oxide nanowires grown on aligned carbon nanotubes. Advanced Functional Materials 20:2470-2480, 2010. [http://dx.doi.org/10.1002/adfm.201000249]
The fabrication and characterization of hybrid architectures of ZnO nanowires (ZNWs) grown on organized carbon nanotubes (CNTs), by a two-step chemical vapor deposition (CVD) process involving CNT growth from a hydrocarbon source followed by ZNW growth using a Zn metal source, is reported. The ZNWs grow uniformly and radially from individual CNTs and CNT bundles, and the aligned morphology of the CNTs is not disturbed by the ZNW growth process. The nucleation and growth of ZnO crystals on CNTs are analyzed in relation to the classical vapor–solid mechanism. Importantly, the CNTs make uniform and distributed electrical contact to the ZNWs, with up to a 1000-fold yield advantage over conventional ZNW growth on a flat substrate. Hybrid ZNW/CNT sheets are fabricated by scalable CVD, rolling, and printing methods; and their electrical properties, which are governed by transport through the anisotropic CNT network, are characterized. Functional interaction between the ZNWs and CNTs is demonstrated by photoconductive behavior and photocurrent generation of the hybrid material under UV illumination. There is significant future opportunity to extend these processing methods to fabricate other functional oxides on CNTs, and to build devices that harness the attractive properties of ZNWs and CNTs with high volumetric efficiency over large areas.
46. Characterizing the failure processes that limit the storage of energy in carbon nanotube springs under tension
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Journal of Micromechanics and Microengineering, 2010.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Characterizing the failure processes that limit the storage of energy in carbon nanotube springs under tension. Journal of Micromechanics and Microengineering 20:104012, 2010. [http://dx.doi.org/10.1088/0960-1317/20/10/104012]
We report measurements of the mechanical properties and energy storage capabilities of carbon nanotube (CNT) springs under tensile loading, including correlated measurements of their cyclic loading and electrical resistance behavior. Tests are conducted on fibers of multi-walled CNTs fabricated from 6 mm tall forests. The highest measured strength and stiffness of the fibers are 2 N tex?1 and 70 N tex?1 respectively. The highest recorded energy density is approximately 7 kJ kg?1 or 500 kJ m?3, more than an order of magnitude higher than the gravimetric energy density of steel springs, and half the volumetric energy density of steel springs. The resistance and stress responses of the fibers during loading to failure and cyclic loading demonstrate that disorder at the nanoscale affects the bulk response. CNT springs show limited effects of fatigue under 75 tensile cyclic loading cycles. Improving the structural quality of the CNTs and the organization of the fibers offers potential to significantly increase the energy storage capacity of the springs.
45. Ethanol-promoted high-yield growth of few-walled carbon nanotubes
Y. Zhang, J. Gregoire, R.B. van Dover, A.J. Hart. Journal of Physical Chemistry, 2010.
Y. Zhang, J. Gregoire, R.B. van Dover, A.J. Hart. Ethanol-promoted high-yield growth of few-walled carbon nanotubes. Journal of Physical Chemistry C 114(14):6389-6395, 2010. [http://dx.doi.org/10.1021/jp100358j]
We report the use of a small concentration of ethanol in addition to ethylene as the carbon source for growth of dense vertically aligned “forests” of few-walled carbon nanotubes (CNTs). Through a detailed comparison of CNTs grown with and without ethanol added to the C2H4/H2 feedstock, we quantify several important effects of the ethanol addition. We show that ethanol selectively reduces the number of CNT walls without changing the outer diameter, increases the catalyst lifetime more than 3-fold, and increases the rate of carbon conversion more than 5-fold. Online dewpoint and mass spectrometry measurements of the exhaust stream suggest that ethanol decomposes into active carbon species that enhance growth, and into H2O, which counteracts the accumulation of amorphous carbon and thus prolongs the catalyst lifetime. We performed a systematic study of the effect of the catalyst film thickness, and identify a set of conditions that provides growth of millimeter-tall double-walled CNT forests. Importantly, our study reveals that the chemistry of the CVD atmosphere alone plays a critical role in controlling the structure of CNTs, and that addition of ethanol results in few-walled CNTs over a broad range of growth conditions. These findings are an important step toward the ultimate goal of control of CNT chirality during synthesis as well as toward realization of important large-scale applications of aligned CNT films having high monodispersity and structural quality.
44. Measuring the lengthening kinetics of vertically aligned nanostructures by spatiotemporal correlation of height and orientation
E.R. Meshot*, M. Bedewy* (*equal contribution), K.M. Lyons, A.R. Woll, K.A. Juggernauth, S. Tawfick, A.J. Hart. Nanoscale, 2010.
E.R. Meshot*, M. Bedewy* (*equal contribution), K.M. Lyons, A.R. Woll, K.A. Juggernauth, S. Tawfick, A.J. Hart. Measuring the lengthening kinetics of vertically aligned nanostructures by spatiotemporal correlation of height and orientation. Nanoscale 2:896-900, 2010. [http://dx.doi.org/10.1039/b9nr00343f]
Owing to their inherent tortuosity, the collective height of vertically aligned nanostructures does not equal the average length of the individual constituent nanostructures, and therefore temporal height measurement is not an accurate measure of the genuine growth kinetics. We use high-resolution spatial mapping of alignment by small-angle X-ray scattering (SAXS) to transform real-time measurements of array height to the average length of the nanostructures. Applying this approach to carbon nanotube (CNT) forest growth transforms the kinetics from a sub-linear to a linear relationship with time, highlighting the potential for insights into the limiting growth mechanisms of CNTs and other one-dimensional nanostructures.
43. Nanocomposite microstructures with tunable mechanical and chemical properties
S. Tawfick, X. Deng, A.J. Hart, J. Lahann. Physical Chemistry Chemical Physics, 2010.
S. Tawfick, X. Deng, A.J. Hart, J. Lahann. Nanocomposite microstructures with tunable mechanical and chemical properties. Physical Chemistry Chemical Physics 12:4446-4451, 2010. [http://dx.doi.org/10.1039/c000304m]
We report a two-step chemical vapor deposition (CVD) method for fabrication of hierarchical polymer-coated carbon nanotube (CNT) microstructures having tunable mechanical properties and accessible chemical functionality. Diverse geometries of vertically aligned CNTs were grown from lithographically patterned catalyst films, and the CNT microstructures were chemically functionalized via poly[4-trifluoroacetyl-p-xylylene-co-p-xylylene] made by chemical vapor deposition polymerization. The polymer coating conformally coated the individual CNTs and CNT bundles within the CNT “forest”. The chemical structure of the polymer films was verified by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Simple control of the mechanical properties of the nanocomposite structures can be achieved by adjusting the deposition times during CVD polymerization. Increasing the polymer film thickness from 10 nm to 27 nm resulted in a change of the Young’s modulus from 65 to 80 MPa. These values are substantially higher than the 36 MPa measured for the as-grown CNTs without polymer coating. The effect of the polymer coating in reinforcing the connectivity among CNTs within the structures has been understood using an analytical model. Finally, chemical functionality of the CNT composite structures after CVD polymerization was verified by a 4-fold fluorescence enhancement after binding of a dye to the coated CNT microstructures. This technique can be adapted to a wide variety of reactive coatings and facilitates attachment of chemical groups and functional nanostructures on the surfaces of the CNTs; therefore, this material could serve as a tunable platform for coupling mechanical and chemical responses in materials for environmental and biological sensing.
42. Self-similar organization of arrays of individual carbon nanotubes and carbon nanotube micropillars
M. De Volder, D.O. Vidaud, E.R. Meshot, S. Tawfick, A.J. Hart. Microelectronic Engineering, 2010.
M. De Volder, D.O. Vidaud, E.R. Meshot, S. Tawfick, A.J. Hart. Self-similar organization of arrays of individual carbon nanotubes and carbon nanotube micropillars. Microelectronic Engineering 87(5-8):1233-1238, 2010. [http://dx.doi.org/10.1016/j.mee.2009.11.139]
It is well-known that carbon nanotube (CNT) growth from a dense arrangement of catalyst nanoparticles creates a vertically aligned CNT forest. CNT forests offer attractive anisotropic mechanical, thermal, and electrical properties, and their anisotropic structure is enabled by the self-organization of a large number of CNTs. This process is governed by individual CNT diameter, spacing, and the CNT-to-CNT interaction. However, little information is known about the self-organization of CNTs within a forest. Insight into the self-organization is, however, essential for tailoring the properties of the CNT forests for applications such as electrical interconnects, thermal interfaces, dry adhesives and energy storage. We demonstrate that arrays of CNT micropillars having micron-scale diameters organize in a similar manner as individual CNTs within a forest. For example, as previously demonstrated for individual CNTs within a forest, entanglement of small-diameter CNT micropillars during the initial stage of growth creates a film of entwined pillars. This layer enables coordinated subsequent growth of the pillars in the vertical direction, in a case where isolated pillars would not grow in a self-supporting fashion. Finally, we provide a detailed overview of the self-organization as a function of the diameter, length and spacing of the CNT pillars. This study, which is applicable to many one-dimensional nanostructured films, demonstrates guidelines for tailoring the self-organization which can enable control of the collective mechanical, electrical and interfacial properties of the films.
41. Collective mechanism for the evolution and self-termination of vertically aligned carbon nanotube growth
M. Bedewy, E.R. Meshot, H. Guo, E.A. Verploegen, W. Lu, A.J. Hart. Journal of Physical Chemistry C, 2009.
M. Bedewy, E.R. Meshot, H. Guo, E.A. Verploegen, W. Lu, A.J. Hart. Collective mechanism for the evolution and self-termination of vertically aligned carbon nanotube growth. Journal of Physical Chemistry C 113:20576-20582, 2009. [http://pubs.acs.org/doi/abs/10.1021/jp904152v]
We explain the evolution and termination of vertically aligned carbon nanotube (CNT) “forests” by a collective mechanism, which is verified by temporal measurements of forest mass and height, as well as quantitative spatial mapping of CNT alignment by synchrotron X-ray scattering. We propose that forest growth consists of four stages: (I) self-organization; (II) steady growth with a constant CNT number density; (III) decay with a decreasing number density; and (IV) abrupt self-termination, which is coincident with a loss of alignment at the base of the forest. The abrupt loss of CNT alignment has been observed experimentally in many systems, yet termination of forest growth has previously been explained using models for individual CNTs, which do not consider the evolution of the CNT population. We propose that abrupt termination of CNT forest growth is caused by loss of the self-supporting structure, which is essential for formation of a CNT forest in the first place, and that this event is triggered by accumulating growth termination of individual CNTs. A finite element model accurately predicts the critical CNT number density at which forest growth terminates and demonstrates the essential role of mechanical contact in maintaining growth of self-assembled films of filamentary nanostructures.
40. Early evaluation of potential environmental impacts of carbon nanotube synthesis by chemical vapor deposition
D.L. Plata, A.J. Hart, C.M. Reddy, P.M. Gschwend. Environmental Science and Technology, 2009.
D.L. Plata, A.J. Hart, C.M. Reddy, P.M. Gschwend. Early evaluation of potential environmental impacts of carbon nanotube synthesis by chemical vapor deposition. Environmental Science and Technology 43(21):8367-8373, 2009. [http://pubs.acs.org/doi/abs/10.1021/es901626p]
The carbon nanotube (CNT) industry is expanding rapidly, yet little is known about the potential environmental impacts of CNT manufacture. Here, we evaluate the effluent composition of a representative multiwalled CNT synthesis by catalytic chemical vapor deposition (CVD) in order to provide data needed to design strategies for mitigating any unacceptable emissions. During thermal pretreatment of the reactant gases (ethene and H2), we found over 45 side-products were formed, including methane, volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons (PAHs). This finding suggests several environmental concerns with the existing process, including potential discharges of the potent greenhouse gas, methane (up to 1.7%), and toxic compounds such as benzene and 1,3-butadiene (up to 36000 ppmv). Extrapolating these laboratory-scale data to future industrial CNT production, we estimate that (1) contributions of atmospheric methane will be negligible compared to other existing sources and (2) VOC and PAH emissions may become important on local scales but will be small when compared to national industrial sources. As a first step toward reducing such unwanted emissions, we used continuous in situ measures of CNT length during growth and sought to identify which thermally generated compounds correlated with CNT growth rate. The results suggested that, in future CNT production approaches, key reaction intermediates could be delivered to the catalyst without thermal treatment. This would eliminate the most energetically expensive component of CVD synthesis (heating reactant gases), while reducing the formation of unintended byproducts.
39. Engineering vertically aligned carbon nanotube growth by decoupled thermal treatment of precursor and catalyst
E.R. Meshot, D.L. Plata, S. Tawfick, Y.Y. Zhang, E.A. Verploegen, A.J. Hart. ACS Nano, 2009.
E.R. Meshot, D.L. Plata, S. Tawfick, Y.Y. Zhang, E.A. Verploegen, A.J. Hart. Engineering vertically aligned carbon nanotube growth by decoupled thermal treatment of precursor and catalyst. ACS Nano 3(9):2477-2486, 2009. [http://dx.doi.org/10.1021/nn900446a]
We study synthesis of vertically aligned carbon nanotube (CNT) “forests” by a decoupled method that facilitates control of the mean diameter and structural quality of the CNTs and enables tuning of the kinetics for efficient growth to forest heights of several millimeters. The growth substrate temperature (Ts) primarily determines the CNT diameter, whereas independent and rapid thermal treatment (Tp) of the C2H4/H2 reactant mixture significantly changes the growth rate and terminal forest height but does not change the CNT diameter. Synchrotron X-ray scattering is utilized for precise, nondestructive measurement of CNT diameter in large numbers of samples. CNT structural quality monotonically increases with Ts yet decreases with Tp, and forests grown by this decoupled method have significantly higher quality than those grown using a conventional single-zone tube furnace. Chemical analysis reveals that the thermal treatment generates a broad population of hydrocarbon species, and a nonmonotonic relationship between catalyst lifetime and Tp suggests that certain carbon species either enhance or inhibit CNT growth. However, the forest height kinetics, as measured in real-time during growth, are self-similar, thereby indicating that a common mechanism of growth termination may be present over a wide range of process conditions.
38. Flexible high conductivity carbon nanotube interconnects made by rolling and printing.
S. Tawfick, K.P. O’Brien, A.J. Hart. Small, 2009.
S. Tawfick, K.P. O’Brien, A.J. Hart. Flexible high conductivity carbon nanotube interconnects made by rolling and printing. Small 5(21):2467-2473, 2009. [http://dx.doi.org/10.1002/smll.200900741]
Applications of carbon nanotubes (CNTs) in flexible and complementary metal-oxide-semiconductor (CMOS)-based electronic and energy devices are impeded due to typically low CNT areal densities, growth temperatures that are incompatible with device substrates, and challenges in large-area alignment and interconnection. A scalable method for continuous fabrication and transfer printing of dense horizontally aligned CNT (HA-CNT) ribbon interconnects is presented. The process combines vertically aligned CNT (VA-CNT) growth by thermal chemical vapor deposition, a novel mechanical rolling process to transform the VA-CNTs to HA-CNTs, and adhesion-controlled transfer printing without needing a carrier film. The rolling force determines the HA-CNT packing fraction and the HA-CNTs are processed by conventional lithography. An electrical resistivity of 2?m??·?cm is measured for ribbons having 800-nm thickness, while the resistivity of copper is 100 times lower, a value that exceeds most CNT assemblies made to date, and significant improvements can be made in CNT structural quality. This rolling and printing process could be scaled to full wafer areas and more complex architectures such as continuous CNT sheets and multidirectional patterns could be achieved by straightforward design of the CNT growth process and/or multiple rolling and printing sequences.
37. Nanoscale zirconia as a versatile non-metallic catalyst for graphitization of carbon and growth of single and multi-wall carbon nanotubes
S.A. Steiner, T.F. Baumann, B.C. Bayer, M.A. Worsley, W.J. Moberlychan, E.L. Shaw, A.J. Hart, S. Hofmann, B.L. Wardle. Journal of the American Chemical Society, 2009.
S.A. Steiner, T.F. Baumann, B.C. Bayer, M.A. Worsley, W.J. Moberlychan, E.L. Shaw, A.J. Hart, S. Hofmann, B.L. Wardle. Nanoscale zirconia as a versatile non-metallic catalyst for graphitization of carbon and growth of single- and multi-wall carbon nanotubes. Journal of the American Chemical Society 131:12144-12154, 2009. [http://dx.doi.org/10.1021/ja902913r]
We report that nanoparticulate zirconia (ZrO2) catalyzes both growth of single-wall and multiwall carbon nanotubes (CNTs) by thermal chemical vapor deposition (CVD) and graphitization of solid amorphous carbon. We observe that silica-, silicon nitride-, and alumina-supported zirconia on silicon nucleates single- and multiwall carbon nanotubes upon exposure to hydrocarbons at moderate temperatures (750 °C). High-pressure, time-resolved X-ray photoelectron spectroscopy (XPS) of these substrates during carbon nanotube nucleation and growth shows that the zirconia catalyst neither reduces to a metal nor forms a carbide. Point-localized energy-dispersive X-ray spectroscopy (EDAX) using scanning transmission electron microscopy (STEM) confirms catalyst nanoparticles attached to CNTs are zirconia. We also observe that carbon aerogels prepared through pyrolysis of a Zr(IV)-containing resorcinol?formaldehyde polymer aerogel precursor at 800 °C contain fullerenic cage structures absent in undoped carbon aerogels. Zirconia nanoparticles embedded in these carbon aerogels are further observed to act as nucleation sites for multiwall carbon nanotube growth upon exposure to hydrocarbons at CVD growth temperatures. Our study unambiguously demonstrates that a nonmetallic catalyst can catalyze CNT growth by thermal CVD while remaining in an oxidized state and provides new insight into the interactions between nanoparticulate metal oxides and carbon at elevated temperatures.
36. Low temperature synthesis of vertically aligned carbon nanotubes with electrical contact to metallic substrates enabled by thermal decomposition of the carbon feedstock
G.D. Nessim, M. Seita, K.P. O’Brien, A.J. Hart, R.K. Bonaparte, R.R. Mitchell, C.V. Thompson. Nano Letters, 2009.
G.D. Nessim, M. Seita, K.P. O’Brien, A.J. Hart, R.K. Bonaparte, R.R. Mitchell, C.V. Thompson. Low temperature synthesis of vertically aligned carbon nanotubes with electrical contact to metallic substrates enabled by thermal decomposition of the carbon feedstock. Nano Letters 9(10):3398-3405, 2009. [http://dx.doi.org/10.1021/nl900675d]
Growth of vertically aligned carbon nanotube (CNT) carpets on metallic substrates at low temperatures was achieved by controlled thermal treatment of ethylene and hydrogen at a temperature higher than the substrate temperature. High-resolution transmission electron microscopy showed that nanotubes were crystalline for a preheating temperature of 770 °C and a substrate temperature of 500 °C. Conductive atomic force microscopy measurements indicated electrical contact through the CNT carpet to the metallic substrate with an approximate resistance of 35 k? for multiwall carpets taller than two micrometers. An analysis of the activation energies indicated that thermal decomposition of the hydrocarbon/hydrogen gas mixture was the rate-limiting step for low-temperature chemical vapor deposition growth of CNTs. These results represent a significant advance toward the goal of replacing copper interconnects with nanotubes using CMOS-compatible processes.
35. Diffusional self-organization in exponential layer-by-layer films with micro and nanoscale periodicity
P. Podsiadlo, M. Michel, K. Critchley, S. Srivastava, M. Qin, J.W. Lee, E.A. Verploegen, A.J. Hart, Y. Qi, N.A. Kotov. Angewandte Chemie, 2009.
P. Podsiadlo, M. Michel, K. Critchley, S. Srivastava, M. Qin, J.W. Lee, E.A. Verploegen, A.J. Hart, Y. Qi, N.A. Kotov. Diffusional self-organization in exponential layer-by-layer films with micro and nanoscale periodicity. Angewandte Chemie 48(38):7073–7077, 2009. [http://dx.doi.org/10.1002/anie.200901720]
The layer-by-layer deposition of poly(diallyldimethylammonium chloride) and poly(acrylic acid) incorporating inorganic sheets of sodium montmorillonite clay leads to a stratified structure throughout the film interior, with micrometer-thick polymer complex layers and thin clay strata (see picture; right: cross-section).
34. Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology.
H. Cebici, R. Guzman de Villoria, A.J. Hart, B.L. Wardle. Composites Science and Technology, 2009.
H. Cebici, R. Guzman de Villoria, A.J. Hart, B.L. Wardle. Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology. Composites Science and Technology 69:2649-2656, 2009. [http://dx.doi.org/10.1016/j.compscitech.2009.08.006]
Advanced composites, such as those used in aerospace applications, employ a high volume fraction of aligned stiff fibers embedded in high-performance polymers. Unlike advanced composites, polymer nanocomposites (PNCs) employ low volume fraction filler-like concepts with randomly-oriented and poorly controlled morphologies due to difficult issues such as dispersion and alignment of the nanostructures. Here, novel fabrication techniques yield controlled-morphology aligned carbon nanotube (CNT) composites with measured non-isotropic properties and trends consistent with standard composites theories. Modulus and electrical conductivity are maximal along the CNT axis, and are the highest reported in the literature due to the continuous aligned-CNTs and use of an unmodified aerospace-grade structural epoxy. Rule-of-mixtures predictions are brought into agreement with the measured moduli when CNT waviness is incorporated. Waviness yields a large (10×) reduction in modulus, and therefore control of CNT collimation is seen as the primary limiting factor in CNT reinforcement of composites for stiffness. Anisotropic electron transport (conductivity and current-carrying capacity) follows expected trends, with enhanced conductivity and Joule heating observed at high current densities.
33. High-yield growth of vertically aligned carbon nanotubes on a continuously moving substrate
R. Guzman de Villoria, S.L. Figueredo, A.J. Hart, S.A. Steiner, A.H. Slocum, B.L. Wardle. Nanotechnology, 2009.
R. Guzman de Villoria, S.L. Figueredo, A.J. Hart, S.A. Steiner, A.H. Slocum, B.L. Wardle. High-yield growth of vertically aligned carbon nanotubes on a continuously moving substrate. Nanotechnology 20:405611, 2009. [http://dx.doi.org/10.1088/0957-4484/20/40/405611]
Abstract: Vertically aligned carbon nanotube (CNT) arrays are grown on a moving substrate, demonstrating continuous growth of nanoscale materials with long-range order. A cold-wall chamber with an oscillating moving platform is used to locally heat a silicon growth substrate coated with an Fe/Al2O3 catalyst film for CNT growth via chemical vapor deposition. The reactant gases are introduced over the substrate through a directed nozzle to attain high-yield CNT growth. Aligned multi-wall carbon nanotube arrays (or ‘forests’) with heights of 1 mm are achieved at substrate speeds up to 2.4 mm s?1. Arrays grown on moving substrates at different velocities are studied in order to identify potential physical limitations of repeatable and fast growth on a continuous basis. No significant differences are noted between static and moving growth as characterized by scanning electron microscopy and Raman spectroscopy, although overall growth height is marginally reduced at the highest substrate velocity. CNT arrays produced on moving substrates are also found to be comparable to those produced through well-characterized batch processes consistent with a base-growth mechanism. Growth parameters required for the moving furnace are found to differ only slightly from those used in a comparable batch process; thermal uniformity appears to be the critical parameter for achieving large-area uniform array growth. If the continuous-growth technology is combined with a reaction zone isolation scheme common in other types of processing (e.g., in the manufacture of carbon fibers), large-scale dense and aligned CNT arrays may be efficiently grown and harvested for numerous applications including providing interlayers for advanced composite reinforcement and improved electrical and thermal transport.
32. Storing elastic energy in carbon nanotubes
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Journal of Micromechanics and Microengineering, 2009.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Storing elastic energy in carbon nanotubes. Journal of Micromechanics and Microengineering 19:094015, 2009. [http://dx.doi.org/10.1088/0960-1317/19/9/094015]
Abstract: The potential performance of carbon nanotubes (CNTs) as springs for elastic energy storage is evaluated. Models are used to determine an upper bound on the energy density that can be stored in defect-free individual CNTs and in assemblies of such CNTs. The models reveal that optimal energy density may be achieved in small-diameter single-walled CNTs loaded in tension, with a maximum theoretical energy density for CNT groupings of 7.8 × 106 kJ m?3. Millimeter-scale CNT springs are constructed using 3 mm tall forests of multi-walled CNTs as the starting material, and tensile tests are performed to measure the springs’ stiffness, strength and elastic properties. The measured strain energy density of these continuous CNT fibers is comparable to the energy density of steel springs.
31. Automated spin-assisted layer-by-layer assembly of nanocomposites.
S. Vozar, Y.C. Poh, T. Serbowicz, M. Bachner, P. Podsiadlo, M. Qin, E. Verploegen, N.A. Kotov, A.J. Hart. Review of Scientific Instruments, 2009.
S. Vozar, Y.C. Poh, T. Serbowicz, M. Bachner, P. Podsiadlo, M. Qin, E. Verploegen, N.A. Kotov, A.J. Hart. Automated spin-assisted layer-by-layer assembly of nanocomposites. Review of Scientific Instruments 80:023903, 2009. [http://dx.doi.org/10.1063/1.3078009]
Abstract: We present the design and verification of a desktop system for the automated production of nanostructured thin films via spin-assisted layer-by-layer (spin-LBL) assembly. The utility of this system is demonstrated by fabricating polyvinyl alcohol/clay nanocomposites. Ellipsometry measurements demonstrate that the automated spin-LBL method creates composites with bilayer thickness and growth rate comparable to traditional dip-LBL; however, the cycle time of the spin-LBL method is an order of magnitude faster. Small angle X-ray scattering analysis shows that the clay platelets in spin-LBL nanocomposites are more highly aligned than in dip-LBL composites. This method can significantly increase the throughput of laboratory-scale LBL discovery and processing, can enable testing of functional properties of LBL nanocomposites over wafer-scale areas, and can be scaled to larger substrates for commercial production.
30. High-yield growth of aligned carbon nanotubes on ceramic fibers for multifunctional enhancement of structural composites.
N. Yamamoto, A.J. Hart, E.J. García, S.S. Wicks, H.M. Duong, A.H. Slocum, B.L. Wardle. Carbon, 2009.
N. Yamamoto, A.J. Hart, E.J. García, S.S. Wicks, H.M. Duong, A.H. Slocum, B.L. Wardle. High-yield growth of aligned carbon nanotubes on ceramic fibers for multifunctional enhancement of structural composites. Carbon 47(3):551-560, 2009. [http://dx.doi.org/10.1016/j.carbon.2008.10.030]
Abstract: We present an in-depth study of CNT growth on commercially-available woven alumina fibers, and achieve uniform growth of dense aligned CNTs on commercially-available cloths up to 5 × 10 cm in area. By systematically varying the catalyst concentration, catalyst pre-treatment time, and sample position within the tube furnace, we isolate key factors governing CNT morphology on fiber surfaces and classify these morphologies as related to the processing conditions. Synthesis employs a low-cost salt-based catalyst solution and atmospheric pressure thermal CVD, which are highly attractive approaches for commercial-scale processing. The catalyst solution concentration determines the uniformity and density of catalyst on the fibers, H2 exposure mediates formation of catalyst clusters, and thermal decomposition of the reactant mixture activates the catalyst particles to achieve uniform aligned growth. Under conditions for aligned CNT growth, uniform radially-aligned coatings are achieved with shorter CNT length, and these split into “mohawks” as the CNT length increases. Radially-aligned growth for 5 min adds a typical CNT mass fraction of 3.8% to the initial sample mass, and a uniform morphology exists throughout the weave. Composites prepared by standard layup techniques using these CNT “fuzzy” alumina fibers are attractive as integral armor layers having enhanced ballistic and impact performance, and serve as a model system for later implementation of this technology using carbon fibers.
29. Exposure to nanoscale particles and fibers during fabrication and machining of hybrid CNT advanced composites.
D. Bello, B.L. Wardle, K. Ahn, N. Yamamoto, E.J. García, A.J. Hart, M.J. Ellenbecker. Journal of Nanoparticle Research, 2009.
D. Bello, B.L. Wardle, K. Ahn, N. Yamamoto, E.J. García, A.J. Hart, M.J. Ellenbecker. Exposure to nanoscale particles and fibers during fabrication and machining of hybrid CNT advanced composites. Journal of Nanoparticle Research 11(1):231-249, 2009. [http://dx.doi.org/10.1007/s11051-008-9499-4]
Abstract: This study investigated airborne exposures to nanoscale particles and fibers generated during dry and wet abrasive machining of two three-phase advanced composite systems containing carbon nanotubes (CNTs), micron-diameter continuous fibers (carbon or alumina), and thermoset polymer matrices. Exposures were evaluated with a suite of complementary instruments, including real-time particle number concentration and size distribution (0.005–20 ?m), electron microscopy, and integrated sampling for fibers and respirable particulate at the source and breathing zone of the operator. Wet cutting, the usual procedure for such composites, did not produce exposures significantly different than background whereas dry cutting, without any emissions controls, provided a worst-case exposure and this article focuses here. Overall particle release levels, peaks in the size distribution of the particles, and surface area of released particles (including size distribution) were not significantly different for composites with and without CNTs. The majority of released particle surface area originated from the respirable (1–10 ?m) fraction, whereas the nano fraction contributed ~10% of the surface area. CNTs, either individual or in bundles, were not observed in extensive electron microscopy of collected samples. The mean number concentration of peaks for dry cutting was composite dependent and varied over an order of magnitude with highest values for thicker laminates at the source being >1 × 106 particles cm?3. Concentration of respirable fibers for dry cutting at the source ranged from 2 to 4 fibers cm?3 depending on the composite type. Further investigation is required and underway to determine the effects of various exposure determinants, such as specimen and tool geometry, on particle release and effectiveness of controls.
28. High-conductivity polymer nanocomposites obtained by tailoring the characteristics of carbon nanotube fillers.
N. Grossiord, J. Loos, L.C. van Laake, M. Maugey, C. Zakri, C.E. Koning, A.J. Hart. Advanced Functional Materials, 2008.
N. Grossiord, J. Loos, L.C. van Laake, M. Maugey, C. Zakri, C.E. Koning, A.J. Hart. High-conductivity polymer nanocomposites obtained by tailoring the characteristics of carbon nanotube fillers. Advanced Functional Materials 18(20):3226-3234, 2008.
27. Tuning of vertically aligned carbon nanotube diameter and areal density through catalyst pre-treatment.
G.D. Nessim, A.J. Hart, J.S. Kim, D. Acquaviva, J. Oh, C.D. Morgan, M. Seita, J.S. Leib, C.V. Thompson. Nano Letters, 2008.
G.D. Nessim, A.J. Hart, J.S. Kim, D. Acquaviva, J. Oh, C.D. Morgan, M. Seita, J.S. Leib, C.V. Thompson. Tuning of vertically aligned carbon nanotube diameter and areal density through catalyst pre-treatment. Nano Letters 8(11):3587-3593, 2008.
26. Fabrication and characterization of ultra-high volume fraction aligned carbon nanotube polymer composites.
B.L. Wardle, D.S. Saito, E.J. García, A.J. Hart, R.G. de Villoria, E.A. Verploegen. Advanced Materials, 2008.
B.L. Wardle, D.S. Saito, E.J. García, A.J. Hart, R.G. de Villoria, E.A. Verploegen. Fabrication and characterization of ultra-high volume fraction aligned carbon nanotube polymer composites. Advanced Materials 20(14):2710-2714, 2008.
25. Exponential growth of LBL films with incorporated inorganic sheets.
P. Podsiadlo, M. Michel, J. Lee, E. Verploegen, N.W.S. Kam, J. Lee, Y. Qi, A.J. Hart, P.T. Hammond, N.A. Kotov., Nano Letters, 2008.
P. Podsiadlo, M. Michel, J. Lee, E. Verploegen, N.W.S. Kam, J. Lee, Y. Qi,, A.J. Hart, P.T. Hammond, N.A. Kotov. Exponential growth of LBL films with incorporated inorganic sheets, Nano Letters 8(6):1762-1770, 2008.
24. Joining prepreg composite interfaces with aligned carbon nanotubes.
E.J. García, B.L. Wardle, A.J. Hart. Composites Part A, 2008.
E.J. García, B.L. Wardle, A.J. Hart. Joining prepreg composite interfaces with aligned carbon nanotubes. Composites Part A 39(6):1065-1070, 2008.
23. Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown in situ.
E.J. García, B.L. Wardle, A.J. Hart, N. Yamamoto. Composites Science and Technology, 2008.
E.J. García, B.L. Wardle, A.J. Hart, N. Yamamoto. Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown in situ. Composites Science and Technology 68:2034-2041, 2008.
22. Long carbon nanotubes grown on the surface of fibers for hybrid composites.
E.J. García, A.J. Hart, B.L. Wardle. AIAA Journal, 2008.
E.J. García, A.J. Hart, B.L. Wardle. Long carbon nanotubes grown on the surface of fibers for hybrid composites. AIAA Journal 46(6):1405-12, 2008.
21. Particle exposure levels during growth and subsequent handling of vertically-aligned carbon nanotube films.
D. Bello, A.J. Hart, K. Ahn, M. Hallock, N. Yamamoto, E.J. García, B.L. Wardle, M.J. Ellenbecker. Carbon, 2008.
D. Bello, A.J. Hart, K. Ahn, M. Hallock, N. Yamamoto, E.J. García, B.L. Wardle, M.J. Ellenbecker. Particle exposure levels during growth and subsequent handling of vertically-aligned carbon nanotube films. Carbon 46:974-981, 2008.
20. Abrupt self-termination of vertically-aligned carbon nanotube growth.
E.R. Meshot, A.J. Hart. Applied Physics Letters, 2008.
E.R. Meshot, A.J. Hart. Abrupt self-termination of vertically-aligned carbon nanotube growth. Applied Physics Letters 92:113107, 2008.
19. Conductive carbon nanotube composite microprobes.
O. Yaglioglu, R. Martens, A.J. Hart, A.H. Slocum. Advanced Materials, 2008.
O. Yaglioglu, R. Martens, A.J. Hart, A.H. Slocum. Conductive carbon nanotube composite microprobes. Advanced Materials 20:357-362, 2008.
18. 2D and 3D growth of carbon nanotubes on substrates, from nanometre to millimetre scales.
A.J. Hart, H.K. Taylor, A.H. Slocum. International J. Nanomanufacturing, 2007.
A.J. Hart, H.K. Taylor, A.H. Slocum. 2D and 3D growth of carbon nanotubes on substrates, from nanometre to millimetre scales. International J. Nanomanufacturing 1(6):701-709, 2007.
17. Characterizing the morphologies of mechanically-manipulated multi-wall carbon nanotube films by small-angle X-ray scattering.
B.N. Wang, R.D. Bennett, E. Verploegen, A.J. Hart, R.E. Cohen. J. Physical Chemistry C, 2007.
B.N. Wang, R.D. Bennett, E. Verploegen, A.J. Hart, R.E. Cohen. Characterizing the morphologies of mechanically-manipulated multi-wall carbon nanotube films by small-angle X-ray scattering. J. Physical Chemistry C 111(48):17933-17940, 2007.
16.Suspended heated silicon platform for rapid thermal control of surface reactions with application to carbon nanotube synthesis.
L.C. van Laake, A.J. Hart, A.H. Slocum., Review of Scientific Instruments, 2007.
L.C. van Laake, A.J. Hart, A.H. Slocum. Suspended heated silicon platform for rapid thermal control of surface reactions with application to carbon nanotube synthesis, Review of Scientific Instruments 78:083901, 2007.
15. Fabrication and nanocompression testing of aligned CNT/polymer nanocomposites.
E.J. García, A.J. Hart, B.L. Wardle, A.H. Slocum. Advanced Materials, 2007.
E.J. García, A.J. Hart, B.L. Wardle, A.H. Slocum. Fabrication and nanocompression testing of aligned CNT/polymer nanocomposites. Advanced Materials 19:2151-2156, 2007.
14. Desktop growth of carbon nanotube monliths with in situ optical imaging.
A.J. Hart, L.C. van Laake, A.H. Slocum. Small, 2007.
A.J. Hart, L.C. van Laake, A.H. Slocum. Desktop growth of carbon nanotube monoliths with in situ optical imaging. Small 3(5):772-777, 2007.
13. Quantitative characterization of the morphology of multi-wall carbon nanotube films by small-angle X-ray scattering.
B.N. Wang, R.D. Bennett, E. Verploegen, A.J. Hart, R.E. Cohen. J. Physical Chemistry C, 2007.
B.N. Wang, R.D. Bennett, E. Verploegen, A.J. Hart, R.E. Cohen. Quantitative characterization of the morphology of multi-wall carbon nanotube films by small-angle X-ray scattering. J. Physical Chemistry C 111(16):5859-5865, 2007.
12. Fabrication of composite microstructures by capillarity-driven wetting of aligned carbon nanotube polymers
E.J. García, A.J. Hart, B.L. Wardle, A.H. Slocum. Nanotechnology, 2007.
E.J. García, A.J. Hart, B.L. Wardle, A.H. Slocum. Fabrication of composite microstructures by capillarity-driven wetting of aligned carbon nanotubes with polymers. Nanotechnology 18(16):165602, 2007.
2006 and prior
11. Method of characterizing electrical contact properties of carbon nanotube coated surfaces
O. Yaglioglu, A.J. Hart, R. Martens, A.H. Slocum. Review of Scientific Instruments, 2006.
O. Yaglioglu, A.J. Hart, R. Martens, A.H. Slocum. Method of characterizing electrical contact properties of carbon nanotube coated surfaces. Review of Scientific Instruments 77:095105, 2006.
10. Creating patterned carbon nanotube catalysts through the microcontact printing of block copolymer micellar thin films.
R.D. Bennett, A.J. Hart, A.C. Miller, P.T. Hammond, D.J. Irvine, R.E. Cohen. Langmuir, 2006.
R.D. Bennett, A.J. Hart, A.C. Miller, P.T. Hammond, D.J. Irvine, R.E. Cohen. Creating patterned carbon nanotube catalysts through the microcontact printing of block copolymer micellar thin films. Langmuir 22:8273-8276, 2006.
9. Controlling the morphology of carbon nanotube films by varying the areal density of catalyst nanoparticles using block copolymer micellar thin films
R.D. Bennett, A.J. Hart, R.E. Cohen. Advanced Materials, 2006.
R.D. Bennett, A.J. Hart, R.E. Cohen. Controlling the morphology of carbon nanotube films by varying the areal density of catalyst nanoparticles using block copolymer micellar thin films. Advanced Materials 18:2274-2279, 2006.
8. Force output, control of film structure, and micro-scale shape replication by carbon nanotube growth under mechanical pressure
A.J. Hart, A.H. Slocum. Nano Letters, 2006.
A.J. Hart, A.H. Slocum. Force output, control of film structure, and micro-scale shape replication by carbon nanotube growth under mechanical pressure. Nano Letters 6:1254-1260, 2006.
7. Rapid growth and flow-mediated nucleation of millimeter-scale aligned carbon nanotube structures from a thin-film catalyst (with cover feature).
A.J. Hart, A.H. Slocum. J. Physical Chemistry B, 2006.
A.J. Hart, A.H. Slocum. Rapid growth and flow-mediated nucleation of millimeter-scale aligned carbon nanotube structures from a thin-film catalyst (with cover feature). J. Physical Chemistry B 110(16):8250-8257, 2006.
6. Uniform and selective CVD growth of carbon nanotubes and naofibers on arbitrarily microstructured silicon surfaces
A.J. Hart, B.O. Boskovic, A.T.H. Chuang, V.B. Golovko, J. Robertson, B.F.G. Johnson, A.H. Slocum. Nanotechnology, 2006.
A.J. Hart, B.O. Boskovic, A.T.H. Chuang, V.B. Golovko, J. Robertson, B.F.G. Johnson, A.H. Slocum. Uniform and selective CVD growth of carbon nanotubes and nanofibers on arbitrarily microstructured silicon surfaces. Nanotechnology 17:1397-1403, 2006.
5. Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation.
A.J. Hart, A.H. Slocum, L. Royer. Carbon, 2006.
A.J. Hart, A.H. Slocum, L. Royer. Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation. Carbon, 44(2):348-359, 2006.
4. Experimental determination of kinematic coupling repeatability in industrial and laboratory conditions.
P.J. Willoughby, A.J. Hart, A.H. Slocum. SME J. Manufacturing Systems, 2005.
P.J. Willoughby, A.J. Hart, A.H. Slocum. Experimental determination of kinematic coupling repeatability in industrial and laboratory conditions. SME J. Manufacturing Systems 24:108-121, 2005.
3. Segmented and shielded structures for reduction of thermal expansion-induced tilt errors.
A.J. Hart, A.H. Slocum, J. Sutin. J. Int’l. Soc. Precision Engineering and Nanotechnology, 2004.
A.J. Hart, A.H. Slocum, J. Sutin. Segmented and shielded structures for reduction of thermal expansion-induced tilt errors. J. Int’l. Soc. Precision Engineering and Nanotechnology, 28:443-458, 2004.
2. Kinematic coupling interchangeability.
A.J. Hart, A.H. Slocum, P.J. Willoughby. J. Int’l.Soc. Precision Engineering and Nanotechnology, 2004.
A.J. Hart, A.H. Slocum, P.J. Willoughby. Kinematic coupling interchangeability. J. Int’l.Soc. Precision Engineering and Nanotechnology, 28:1-15, 2004.
1. Linear motion carriage with aerostatic bearings preloaded by inclined iron core linear electric motor.
A.H. Slocum, M. Basaran, R. Cortesi, A.J. Hart. J. Int’l. Soc. Precision Engineering and Nanotechnology, 2003.
Papers in Conference Proceedings (2010 and prior)
27. Tuning the tensile properties of carbon nanotube springs by densification.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. 10th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Leuven, Belgium, 2010.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Tuning the tensile properties of carbon nanotube springs by densification.10th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Leuven, Belgium, 2010.
26. A temperature sensor from a self-assembled carbon nanotube bridge.
M. De Volder, S. Tawfick, D. Reynaerts, C. Van Hoof, A.J. Hart. IEEE Sensors Conference, Waikoloa, HI, 2010.
M. De Volder, S. Tawfick, D. Reynaerts, C. Van Hoof, A.J. Hart. A temperature sensor from a self-assembled carbon nanotube bridge.IEEE Sensors 2010 Conference, Waikoloa, HI, 2010. [http://dx.doi.org/10.1109/ICSENS.2010.5690868]
Recent studies show that carbon nanotubes (CNTs) can be used as temperature sensors, and offer great opportunities towards extreme miniaturization, high sensitivity, low power consumption, and rapid response. Previous CNT based temperature sensors are fabricated by either dielectrophoresis or piece-wise alignment of read-out electronics around randomly dispersed CNTs. We introduce a new deterministic and parallel microsensor fabrication method based on the self-assembly of CNTs into three-dimensional microbridges. We fabricated prototype microbridge sensors on patterned electrodes, and found their sensitivity to be better than -0.1 %/K at temperatures between 300K and 420K. This performance is comparable to previously published CNT based temperature sensors. Importantly, however, our research shows how unique sensor architectures can be made by self-assembly, which can be achieved using batch processing rather than piecewise assembly.
25. Collective mechanisms limiting the indefinite growth of carbon nanotube assemblies.
M. Bedewy, E.R. Meshot, E.S. Polsen, S. Tawfick, A.J. Hart. SAMPE Fall Technical Conference, Salt Lake City, UT, 2010.
M. Bedewy, E.R. Meshot, E.S. Polsen, S. Tawfick, A.J. Hart. Collective mechanisms limiting the indefinite growth of carbon nanotube assemblies.SAMPE Fall Technical Conference, Salt Lake City, UT, 2010.
While carbon nanotubes (CNTs) have been produced industrially in ton-scale quantities for nearly two decades, scalable manufacturing processes that precisely control the structure, length, and alignment of CNTs are needed to realize the exceptional properties of CNTs at larger scales. Specifically, vertically aligned CNT forests are a model system for further understanding what limits the growth of indefinitely long CNTs, and are building blocks for novel microstructures and multifunctional thin films. This paper will present our current understanding of the limiting mechanisms of CNT forest growth by chemical vapor deposition (CVD). Interactions among a population of CNTs govern their collective growth behavior, and in combination with the performance of individual catalyst particles, prevent indefinite CNT growth using known methods. To understand these effects, we examine the dynamics of catalyst formation, CNT forest self-organization, steady growth, and termination using a holistic approach, combining in situ and ex situ X-ray scattering with spatiotemporal mapping of CNT forest mass, height, and density. While indefinite CNT growth may remain a dream, the present findings have enabled scalable production of CNT forests and horizontally-aligned thin films via continuous CVD, rolling, and printing schemes. These technologies have been realized in our laboratory using prototype bench-scale machines and show promise for cost-effective manufacturing of large-area, organized CNT films.
24. Hybrid architectures of metal oxide nanostructures grown on aligned carbon nanotubes.
J.G. Ok, K.A. Juggernauth, K. Sun, A.A. McLane, Y. Zhang, S. Tawfick, A.J. Hart. Annual meeting of the Microbeam Analysis Society (Microscopy & Microanalysis), Portland, OR, 2010.
22. Continuous manufacturing of aligned carbon nanotube film.
E.S. Polsen, S. Tawfick, E.R. Meshot, A.J. Hart. 38th North American Manufacturing Research Conference (NAMRC38), Kingston, Ontario, 2010.
We present a continuous manufacturing process for low-cost and scalable production of vertically aligned carbon nanotube (CNT) forests, along with continuous rolling of the CNTs to create horizontally aligned (HA) CNT ribbons and sheets. These methods are capable of producing uniform films over waferscale dimensions and beyond, and the CNTs can be patterned by lithography and plasma etching to achieve feature dimensions down to the micron scale. In this paper, we describe the design, manufacturing and related process physics of (1) a chemical vapor deposition (CVD) chamber apparatus for continuous CNT forest growth on a re-circulating catalyst-coated substrate; and (2) a mechanical rolling machine for CNT densification and transfer printing. Together, these innovations enable integration of CNT high quality and ordered CNTs growth with electronics and composites manufacturing on arbitrary substrates, including flexible plastics, fiber laminates, and metal foils.
21. Characterizing the failure processes that limit the storage of energy in carbon nanotube springs under tension.
F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Washington, DC, 2009.
We report correlated measurements of cyclic loading behavior with electrical resistance of energy-storing carbon nanotube (CNT) springs under tensile loading. Tests are conducted on fibers of MWCNTs fabricated from 6 mm tall forests. The highest measured strength and stiffness of the fibers are 0.5 N/tex and 24 N/tex. The resistance and stress responses of the fibers during loading to failure and cyclic loading demonstrate that disorder at the nanoscale affects the bulk response. CNT springs perform well under cyclic tensile loading and show limited effects of fatigue.
20. Fabrication of robust carbon nanotube microstructures by elastocapillary densification.
M. De Volder, D. Vidaud, S.J. Park, S. Tawfick, A.J. Hart. 35th International Conference on Micro & Nano Engineering, Ghent, Belgium, 2009.
19. Controlled growth orientation of carbon nanotube pillars by catalyst patterning in microtrenches.
M. De Volder, S. Tawfick, A.J. Hart. 15th International Conference on Solid-State Sensors, Actuators, and Microsystems, Denver, CO, 2009.
We present a novel method for controlling the growth orientation of individual carbon nanotube (CNT) microstructures on a silicon wafer substrate. Our method controls the CNT forest orientation by patterning the catalyst layer used in the CNTs growth on slanted KOH edges. The overlap of catalyst area on the horizontal bottom and sloped sidewall surfaces of the KOH-etched substrate enables precise variation of the growth direction. These inclined structures can profit from the outstanding mechanical, electrical, thermal, and optical properties of CNTs and can therefore improve the performance of several MEMS devices. Inclined CNT microstructures could for instance be used as cantilever springs in probe card arrays, as tips in dip-pen lithography, and as sensing element in advanced transducers.