Ink-jet printing of carbon nanotube thin film transistors

Ink-jet printing is an important process for placing active electronics on plastic substrates. We demonstrate ink-jet printing as a viable method for large area fabrication of carbon nanotube (CNT) thin film transistors (TFTs). We investigate different routes for producing stable CNT solutions ("inks"). These consist of dispersion methods for CNT debundling and the use of different solvents, such as N -methyl-2-pyrrolidone. The resulting printable inks are dispensed by ink-jet onto electrode bearing silicon substrates. The source to drain electrode gap is bridged by percolating networks of CNTs. Despite the presence of metallic CNTs, our devices exhibit field effect behavior, with effective mobility of ∼0.07 cm2 /V s and ON/OFF current ratio of up to 100. This result demonstrates the feasibility of ink-jet printing of nanostructured materials for TFT manufacture. © 2007 American Institute of Physics.

The effect of carbon nanotube orientation on erosive wear resistance of CNT-epoxy based composites

Aligned carbon nanotube (CNT) polymer composites are envisioned as the next-generation composite materials for a wide range of applications. In this work, we investigate the erosive wear behavior of epoxy matrix composites reinforced with both randomly dispersed and aligned carbon nanotube (CNT) arrays. The aligned CNT composites are prepared in two different configurations, where the sidewalls and ends of nanotubes are exposed to the composite surface. Results have shown that the composite with vertically aligned CNT-arrays exhibits superior erosive wear resistance compared to any of the other types of composites, and the erosion rate reaches a similar performance level to that of carbon steel at 20° impingement angle. The erosive wear mechanism of this type of composite, at various impingement angles, is studied by Scanning Electron Microscopy (SEM). We report that the erosive wear performance shows strong dependence on the alignment geometries of CNTs within the epoxy matrix under identical nanotube loading fractions. Correlations between the eroded surface roughness and the erosion rates of the CNT composites are studied by surface profilometry. This work demonstrates methods to fabricate CNT based polymer composites with high loading fractions of the filler, alignment control of nanotubes and optimized erosive wear properties. © 2014 Elsevier Ltd. All rights reserved.

Carbon nanotube Schottky diode and directionally dependent field-effect transistor using asymmetrical contacts

We demonstrate the fabrication and operation of a carbon nanotube (CNT) based Schottky diode by using a Pd contact (high-work-function metal) and an Al contact (low-work-function metal) at the two ends of a single-wall CNT. We show that it is possible to tune the rectification current-voltage (I-V) characteristics of the CNT through the use of a back gate. In contrast to standard back gate field-effect transistors (FET) using same-metal source drain contacts, the asymmetrically contacted CNT operates as a directionally dependent CNT FET when gated. While measuring at source-drain reverse bias, the device displays semiconducting characteristics whereas at forward bias, the device is nonsemiconducting. © 2005 American Institute of Physics.

Efficient carbon nanotube field emitter using electrospun carbon nanofibers as a flexible electrode

Thermal-stable, conductive, and flexible carbon fabric (CF), which is composed of thin carbon fibers prepared by electrospinning, was used for the substrate of carbon nanotube (CNT) field emitter arrays. The field emitter arrays were prepared by chemical vapor deposition (CVD). The current density-electric field characteristics revealed that the CNT field emitter arrays on CF produced a higher current density at a lower turn-on voltage compared to ones on a Si substrate. This emitter integrated with a gate electrode based on hierarchy-structured carbon materials, CNTs on CF, can be used for light sources, displays, and other electronic devices. © 2009 Materials Research Society.

Carbon nanotube array: a new MIP platform.

Here we demonstrate that a free-standing carbon nanotube (CNT) array can be used as a large surface area and high porosity 3D platform for molecular imprinted polymer (MIP), especially for surface imprinting. The thickness of polymer grafted around each CNT can be fine-tuned to imprint different sizes of target molecules, and yet it can be thin enough to expose every imprint site to the target molecules in solution without sacrificing the capacity of binding sites. The performance of this new CNT-MIP architecture was first assessed with a caffeine-imprinted polypyrrole (PPy) coating on two types of CNT arrays: sparse and dense CNTs. Real-time pulsed amperometric detection was used to study the rebinding of the caffeine molecules onto these CNT-MIPPy sensors. The dense CNT-MIPPy sensor presented the highest sensitivity, about 15 times better when compared to the conventional thin film, whereas an improvement of 3.6 times was recorded on the sparse CNT. However, due to the small tube-to-tube spacing in the dense CNT array, electrode fouling was observed during the detection of concentrated caffeine in phosphate buffer solution. A new I-V characterization method using pulsed amperometry was introduced to investigate the electrical characterization of these new devices. The resistance value derived from the I-V plot provides insight into the electrical conductivity of the CNT transducer and also the effective surface area for caffeine imprinting.

Fabrication of high-Q film bulk acoustic resonator (FBAR) filters withcarbon nanotube (CNT) electrodes

FBAR devices with carbon nanotube (CNT) electrodes have been developed withthe aim of taking advantage of the low density and high acoustic impedance ofthe CNTs compared to other known materials. The influence of the CNTs on thefrequency response of the FBAR devices was studied by comparing two identicalsets of devices, one set comprised FBARs fabricated with chromium/gold bilayerelectrodes, and the second set comprised FBARs fabricated with CNT electrodes.It was found that the CNTs had a significant effect on attenuating travellingwaves at the surface of the FBARs membranes due to their high elastic stiffness.Finite element analysis of the devices fabricated was carried out using COMSOLMultiphysics, and the numerical results confirmed the experimental resultsobtained. © 2010 IEEE.

ZnO-Based FBAR resonators with carbon nanotube electrodes

Film bulk acoustic resonator (FBAR) devices with carbon nanotube (CNT) electrodes directly grown on a ZnO film by thermal chemical vapor deposition have been fabricated. CNT electrodes possess a very low density and high acoustic impedance, which reduces the intrinsic mass loading effect resulting from the electrodes' weight and better confines the longitudinal acoustic standing waves inside the resonator, in turn providing a resonator with a higher quality factor. The influence of the CNTs on the frequency response of the FBAR devices was studied by comparing two identical sets of devices; one set comprised FBARs fabricated with chromium/ gold bilayer electrodes, and the second set comprised FBARs fabricated with CNT electrodes. It was found that the CNTs had a significant effect on attenuating traveling waves at the surface of the FBARs' membranes because of their high elastic stiffness. Three-dimensional finite element analysis of the devices fabricated was carried out, and the numerical simulations were consistent with the experimental results obtained. © 2011 IEEE.

Design of carbon nanotube fiber microelectrode for glucose biosensing

BACKGROUND: Carbon nanotube (CNT) fiber directly spun from an aerogel has a unique, well-aligned nanostructure (nano-pore and nano-brush), and thus provides high electro-catalytic activity and strong interaction with glucose oxidase enzyme. It shows great potential as a microelectrode for electrochemical biosensors. RESULTS: Cyclic voltammogram results indicate that post-synthesis treatments have great influence on the electrocatalytic activity of CNT fibers. Raman spectroscopy and electrical conductivity tests suggest that fibers annealed at 250 °C remove most of the impurities without damaging the graphite-like structure. This leads to a nano-porous morphology on the surface and the highest conductivity value (1.1 × 10 5 S m -1). Two CNT fiber microelectrode designs were applied to enhance their electron transfer behaviour, and it was found that a design using a 30 nm gold coating is able to linearly cover human physiological glucose level between 2 and 30 mmol L -1. The design also leads to a low detection limit of 25 μmol L -1. CONCLUSIONS: The high performance of CNT fibers not only offers exceptional mechanical and electrical properties, but also provides a large surface area and electron transfer pathway. They consequently make excellent bioactive microelectrodes for glucose biosensing, especially for potential use in implantable devices. © 2011 Society of Chemical Industry.

Device and circuit-level performance of carbon nanotube field-effect transistor with benchmarking against a nano-MOSFET

The performance of a semiconducting carbon nanotube (CNT) is assessed and tabulated for parameters against those of a metal-oxide-semiconductor field-effect transistor (MOSFET). Both CNT and MOSFET models considered agree well with the trends in the available experimental data. The results obtained show that nanotubes can significantly reduce the drain-induced barrier lowering effect and subthreshold swing in silicon channel replacement while sustaining smaller channel area at higher current density. Performance metrics of both devices such as current drive strength, current on-off ratio (Ion/Ioff), energy-delay product, and power-delay product for logic gates, namely NAND and NOR, are presented. Design rules used for carbon nanotube field-effect transistors (CNTFETs) are compatible with the 45-nm MOSFET technology. The parasitics associated with interconnects are also incorporated in the model. Interconnects can affect the propagation delay in a CNTFET. Smaller length interconnects result in higher cutoff frequency. © 2012 Tan et al.

Mechanics of capillary forming of aligned carbon nanotube assemblies.

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 dynamic 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.

Hydrogel-driven carbon nanotube microtransducers

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. © 2011 The Royal Society of Chemistry.

Corrugated carbon nanotube microstructures with geometrically tunable compliance

Deterministic organization of nanostructures into microscale 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 microbellows and tilted microcantilevers are created over large areas, and these structures can have thin walls with aspect ratios exceeding 100:1. We show these structures can be used as out-of-plane microsprings with compliance determined by the wall thickness and number of folds. © 2011 American Chemical Society.

Fabrication of high-aspect-ratio polymer microstructures and hierarchical textures using carbon nanotube composite master molds

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. © 2011 The Royal Society of Chemistry.

Fabrication and electrical integration of robust carbon nanotube micropillars by self-directed elastocapillary densification

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. © 2011 IOP Publishing Ltd.