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. [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.

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. [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.

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%.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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]

Watch the VIDEO! Read the accompanying protocol.

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.

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

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 flows 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.

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.

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 identified, aiming to establish design principles for fabrication of arbitrary 3D surface textures over large areas.

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.

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 μl, 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.

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.

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 (δ), 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 (δ/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.

M. De Volder, S. Tawfick, D. Copic, A.J. Hart. Hygroscopic biomimetic transducers made from CNT-hydrogel composites. 16^th International Conference on Solid-State Sensors, Actuators, and Microsystems, Beijing, China, 2011. 
[http://dx.doi.org/10.1109/TRANSDUCERS.2011.5969612]

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.

M. De Volder, S. Tawfick, S.J. Park, D. Copic, A.J. Hart. Programmable transformation of vertical carbon nanotubes into 3D microstructures. 16^th 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.

M. Bedewy, E.R. Meshot, M.J. Reinker, A.J. Hart. Population growth dynamics of carbon nanotubes. ACS Nano 5(11):8974-8989, 2011. [http://dx.doi.org/10.1021/nn203144f]

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.

M. De Volder, S. Tawfick, D. Copic, A.J. Hart. Hydrogel-driven carbon nanotube microtransducers. Soft Matter 7:9844-9847, 2011. [http://dx.doi.org/10.1039/C1SM06215H]

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.

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.

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.

S. Tawfick, M. De Volder, A.J. Hart. Structurally programmed capillary folding of vertical carbon nanotube assemblies. Langmuir 27(10):6389-6394, 2011. [http://dx.doi.org/10.1021/la200635g]

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 t^3/2. 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.

R. Guzman de Villoria, A.J. Hart, B.L. Wardle. Continuous production of vertically aligned carbon nanotubes on 2D and 3D substrates. ACS Nano 5(6):4850-4857, 2011 [http://dx.doi.org/10.1021/nn2008645]

E.A. Verploegen, A.J. Hart, M. De Volder, S. Tawfick, K.K. Chia, R.E. Cohen. Non-destructive characterization of structural hierarchy within carbon nanotube assemblies. Journal of Applied Physics 109:094316, 2011. [http://dx.doi.org/10.1063/1.3584759]

 

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 μm, 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.

D. Copic, S.J. Park, S. Tawfick, M. De Volder, A.J. Hart. Fabrication of high-aspect-ratio polymer microstructures and hierarchical textures using carbon nanotube composite master molds. Lab on a Chip 11(10):1831-1837, 2011. [http://dx.doi.org/10.1039/c0lc00724b]

 

 

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.

M. De Volder, S.J. Park, S. Tawfick, A.J. Hart. Fabrication and electrical integration of robust carbon nanotube micropillars by self-directed elastocapillary densification. Journal of Micromechanics and Microengineering 21:045033, 2011. [http://dx.doi.org/10.1088/0960-1317/21/4/045033]

 

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.

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.

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. [http://dx.doi.org/10.1016/j.carbon.2010.10.018]

Despite significant progress in carbon nanotube (CNT) synthesis by thermal chemical vapor deposition (CVD), the factors determining the structure of the resulting carbon filaments and other graphitic nanocarbons are not well understood. Here, we demonstrate that gas chemistry influences the crystal structure of carbon filaments grown at low temperatures (500°C). Using thermal CVD, we decoupled the thermal treatment of the gaseous precursors (C₂H₄/H₂/Ar) and the substrate-supported catalyst. Varying the preheating temperature of the feedstock gas, we observed a striking transition between amorphous carbon nanofibers (CNFs) and crystalline CNTs. These results were confirmed using both a hot-wall CVD system and a cold-wall CVD reactor. Analysis of exhaust gases (by ex situ gas chromatography) showed increasing concentrations of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) that correlated with the structural transition (characterized using high resolution transmission electron microscopy). This suggests that the crystallinity of carbon filaments may be controlled by the presence of specific gas phase precursor molecules (e.g., VOCs and PAHs). Thus, direct delivery of these molecules in the CVD process may enable selective CNF or CNT formation at low substrate temperatures. The inherent scalability of this approach could impact many promising applications, especially in the electronics industry.

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.

D.L. Plata*, E.R. Meshot* (*equal contribution), C.M. Reddy, A.J. Hart, P.M. Gschwend. ACS Nano 4(10):7185-7192, 2010. [http://dx.doi.org/10.1021/nn101842g]

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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).

 

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (T_s) primarily determines the CNT diameter, whereas independent and rapid thermal treatment (T_p) 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 T_s yet decreases with T_p, 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 T_p 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.

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 mΩ · 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.

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.

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 kΩ 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.

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).

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.

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.

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.

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.

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.

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 μm), 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 μm) 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.

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.

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.

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.

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.

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.

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.

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.

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.

E.R. Meshot, A.J. Hart. Abrupt self-termination of vertically-aligned carbon nanotube growth. Applied Physics Letters 92:113107, 2008.

O. Yaglioglu, R. Martens, A.J. Hart, A.H. Slocum. Conductive carbon nanotube composite microprobes. Advanced Materials 20:357-362, 2008.

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.

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.

L.C. van Laake, A.J. Hart, A.H. Slocum. A suspended heated silicon platform for rapid thermal control of surface reactions with application to carbon nanotube synthesis, Review of Scientific Instruments 78:083901, 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.

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.

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.

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.

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.

R.D. Bennett, A.J. Hart, A.C. Miller, P.T. Hammond, D.J. Irvine, R.E. Cohen. Patterning of block copolymer micellar thin films using microcontact printing and applications in carbon nanotube synthesis. Langmuir 22:8273-8276, 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.

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.

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.

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.

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.

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.

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.

A.J. Hart, A.H. Slocum, P.J. Willoughby. Kinematic coupling interchangeability. J. Int’l.Soc. Precision Engineering and Nanotechnology, 28:1-15, 2004.

A.H. Slocum, M. Basaran, R. Cortesi, A.J. Hart. Linear motion carriage with aerostatic bearings preloaded by inclined iron core linear electric motor. J. Int’l. Soc. Precision Engineering and Nanotechnology 27:382-394, 2003.

F.A. Hill, T.F. Havel, A.J. Hart, C. Livermore. Tuning the tensile properties of carbon nanotube springs by densification. 10^th International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Leuven, Belgium, 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.

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.

J.G. Ok, K.A. Juggernauth, K. Sun, A.A. McLane, Y. Zhang, S. Tawfick, A.J. Hart. Hybrid architectures of metal oxide nanostructures grown on aligned carbon nanotubes. Annual meeting of the Microbeam Analysis Society (Microscopy & Microanalysis), Portland, OR, 2010. [http://dx.doi.org/10.1017/S1431927610063014]

K.A. Juggernauth, K. Sun, A.J. Hart, S. Daly. Hybrid structures of NiTi and carbon nanotubes. Annual meeting of the Microbeam Analysis Society (Microscopy & Microanalysis), Portland, OR, 2010. [http://dx.doi.org/10.1017/S143192761006246X]

E.S. Polsen, S. Tawfick, E.R. Meshot, A.J. Hart. Continuous manufacturing of aligned carbon nanotube film. 38^th 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.

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. International Workshop on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS), Washington, DC, 2009. [http://dx.doi.org/10.1088/0960-1317/20/10/104012]

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.

M. De Volder, D. Vidaud, S.J. Park, S. Tawfick, A.J. Hart. Fabrication of robust carbon nanotube microstructures by elastocapillary densification. 35^th International Conference on Micro & Nano Engineering, Ghent, Belgium, 2009.

M. De Volder, S. Tawfick, A.J. Hart. Controlled growth orientation of carbon nanotube pillars by catalyst patterning in microtrenches. 15^th International Conference on Solid-State Sensors, Actuators, and Microsystems, Denver, CO, 2009. [http://dx.doi.org/10.1109/SENSOR.2009.5285636]

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.

“Principles and methods of integration of carbon nanotubes in miniaturized systems” A.J. Hart, S.J. Park, M. De Volder, S. Tawfick, E.R. Meshot. Chapter in MEMS and Nanotechnologies: From Science to Electronic Systems. CRC Press, to be published in 2012.

“Electronic, optical, and optoelectronic applications of carbon nanotubes” A.J. Hart, A. Rinzler, and J. Kong. Chapter 6 in WTEC/NSF Report on Carbon Nanotube Manufacturing and Applications, 2007, http://www.wtec.org/cnm.