Lakshman Randeniya

Composite yarns of multiwalled carbon nanotubes with metallic electrical conductivity.

Unique macrostructures known as spun carbon-nanotube fibers (CNT yarns) can be manufactured from vertically aligned forests of multiwalled carbon nanotubes (MWCNTs). These yarns behave as semiconductors with room-temperature conductivities of about 5 x 10(2) S cm(-1). Their potential use as, for example, microelectrodes in medical implants, wires in microelectronics, or lightweight conductors in the aviation industry has hitherto been hampered by their insufficient electrical conductivity. In this Full Paper, the synthesis of metal-CNT composite yarns, which combine the unique properties of CNT yarns and nanocrystalline metals to obtain a new class of materials with enhanced electrical conductivity, is presented. The synthesis is achieved using a new technique, self-fuelled electrodeposition (SFED), which combines a metal reducing agent and an external circuit for transfer of electrons to the CNT surface, where the deposition of metal nanoparticles takes place. In particular, the Cu-CNT and Au-CNT composite yarns prepared by this method have metal-like electrical conductivities (2-3 x 10(5) S cm(-1)) and are mechanically robust against stringent tape tests. However, the tensile strengths of the composite yarns are 30-50% smaller than that of the unmodified CNT yarn. The SFED technique described here can also be used as a convenient means for the deposition of metal nanoparticles on solid electrode supports, such as conducting glass or carbon black, for catalytic applications.

Incorporation of Si and SiOx into diamond-like carbon films: Impact on surface properties and osteoblast adhesion

The interaction of human osteoblast cells with diamond-like carbon films incorporating silicon and silicon oxide (SiO(x), 1 < or = x < or = 1.5) and synthesized using the direct-current plasma-activated chemical vapour deposition method was investigated. Cell culture studies were performed for films with Si contents ranging from approximately 4 at.% to 15 at.%. Substantial differences between Si-incorporated and SiO(x)-incorporated films were found for the bonding environments of Si atoms and the hybridization of underlying carbon structures. However, osteoblast-attachment studies did not show statistically significant trends in properties of cell growth (count, area and morphology) that can be attributed either to the Si content of the films or to the chemical structure of the films. The surface energy decreased by 40% as the Si content of the SiO(x) incorporated DLC films increased to 13 at.%. The cell adhesion properties however did not change in response to lowering of the surface energy. The incorporation of both Si and SiO(x) leads to a beneficial reduction in the residual stress of the films. The average roughness of the films increases and the hardness decreases when Si and SiO(x) are added to DLC films. The impact of these changes for load-bearing biomedical applications can be determined only by carefully controlled experiments using anatomic simulators.

Harnessing the influence of reactive edges and defects of graphene substrates for achieving complete cycle of room-temperature molecular sensing.

Molecular doping and detection are at the forefront of graphene research, a topic of great interest in physical and materials science. Molecules adsorb strongly on graphene, leading to a change in electrical conductivity at room temperature. However, a common impediment for practical applications reported by all studies to date is the excessively slow rate of desorption of important reactive gases such as ammonia and nitrogen dioxide. Annealing at high temperatures, or exposure to strong ultraviolet light under vacuum, is employed to facilitate desorption of these gases. In this article, the molecules adsorbed on graphene nanoflakes and on chemically derived graphene-nanomesh flakes are displaced rapidly at room temperature in air by the use of gaseous polar molecules such as water and ethanol. The mechanism for desorption is proposed to arise from the electrostatic forces exerted by the polar molecules, which decouples the overlap between substrate defect states, molecule states, and graphene states near the Fermi level. Using chemiresistors prepared from water-based dispersions of single-layer graphene on mesoporous alumina membranes, the study further shows that the edges of the graphene flakes (showing p-type responses to NO₂ and NH₃) and the edges of graphene nanomesh structures (showing n-type responses to NO₂ and NH₃) have enhanced sensitivity. The measured responses towards gases are comparable to or better than those which have been obtained using devices that are more sophisticated. The higher sensitivity and rapid regeneration of the sensor at room temperature provides a clear advancement towards practical molecule detection using graphene-based materials.

SWCNT Networks on Nanoporous Silica Catalyst Support: Morphological and Connectivity Control for Nanoelectronic, Gas-Sensing, and Biosensing Devices

Effective control of morphology and electrical connectivity of networks of single-walled carbon nanotubes (SWCNTs) by using rough, nanoporous silica supports of Fe catalyst nanoparticles in catalytic chemical vapor deposition is demonstrated experimentally. The very high quality of the nanotubes is evidenced by the G-to-D Raman peak ratios (>50) within the range of the highest known ratios. Transitions from separated nanotubes on smooth SiO(2) surface to densely interconnected networks on the nanoporous SiO(2) are accompanied by an almost two-order of magnitude increase of the nanotube density. These transitions herald the hardly detectable onset of the nanoscale connectivity and are confirmed by the microanalysis and electrical measurements. The achieved effective nanotube interconnection leads to the dramatic, almost three-orders of magnitude decrease of the SWCNT network resistivity compared to networks of similar density produced by wet chemistry-based assembly of preformed nanotubes. The growth model, supported by multiscale, multiphase modeling of SWCNT nucleation reveals multiple constructive roles of the porous catalyst support in facilitating the catalyst saturation and SWCNT nucleation, consistent with the observed higher density of longer nanotubes. The associated mechanisms are related to the unique surface conditions (roughness, wettability, and reduced catalyst coalescence) on the porous SiO(2) and the increased carbon supply through the supporting porous structure. This approach is promising for the direct integration of SWCNT networks into Si-based nanodevice platforms and multiple applications ranging from nanoelectronics and energy conversion to bio- and environmental sensing.

Manufacturing polymer/carbon nanotube composite using a novel direct process

A direct process for manufacturing polymer carbon nanotube (CNT)-based composite yarns is reported. The new approach is based on a modified dry spinning method of CNT yarn and gives a high alignment of the CNT bundle structure in yarns. The aligned CNT structure was combined with a polymer resin and, after being stressed through the spinning process, the resin was cured and polymerized, with the CNT structure acting as reinforcement in the composite. Thus the present method obviates the need for special and complex treatments to align and disperse CNTs in a polymer matrix. The new process allows us to produce a polymer/CNT composite with properties that may satisfy various engineering specifications. The structure of the yarn was investigated using scanning electron microscopy coupled with a focused-ion-beam system. The tensile behavior was characterized using a dynamic mechanical analyzer. Fourier transform infrared spectrometry was also used to chemically analyze the presence of polymer on the composites. The process allows development of polymer/CNT-based composites with different mechanical properties suitable for a range of applications by using various resins.

Thin film composites of nanocrystalline ZrO2 and diamond-like carbon: Synthesis, structural properties and bone cell proliferation

We report on the synthesis of thin composites of diamond-like carbon (DLC) and nanocrystalline ZrO(2) deposited using pulsed direct current plasma-enhanced chemical vapor deposition at low temperatures (<120 degrees C). Films containing up to 21at.% Zr were prepared (hydrogen was not included in the calculation) and their structural and surface properties were determined using a number of spectroscopic methods and contact angle measurements. Bone cell adhesion to the films was studied using a 3 day cell culture with osteoblasts. These nanocomposites (DLC-ZrO(2)) consist of tetragonal ZrO(2) nanocrystals with an average size of 2-5 nm embedded in an amorphous matrix consisting predominantly of DLC. The surface water contact angle of the films increased from approximately 60 degrees to 80 degrees as the Zr content increased from 0 to 21at.%. The cell culture study revealed that although the cell counts were not significantly different, the morphology of the osteoblasts growing on the DLC-ZrO(2) nanocomposites was markedly different from that of cells growing on DLC alone. Cells growing on the DLC-ZrO(2) surfaces were less spread out and had a smaller cell area in comparison with those growing on DLC surfaces. In some areas on the DLC-ZrO(2) surfaces, large numbers of cells appeared to coalesce. It is postulated that the difference in cell morphology between osteoblasts on DLC-ZrO(2) surfaces and DLC surfaces is related to the presence of very small tetragonal nanocrystals of ZrO(2) in the composite film.

Plasma-produced phase-pure cuprous oxide nanowires for methane gas sensing

Phase-selective synthesis of copper oxide nanowires is warranted by several applications, yet it remains challenging because of the narrow windows of the suitable temperature and precursor gas composition in thermal processes. Here, we report on the room-temperature synthesis of small-diameter, large-area, uniform, and phase-pure Cu 2O nanowires by exposing copper films to a custom-designed low-pressure, thermally non-equilibrium, high-density (typically, the electron number density is in the range of 1011–1013 cm−3) inductively coupled plasmas. The mechanism of the plasma-enabled phase selectivity is proposed. The gas sensors based on the synthesized Cu 2O nanowires feature fast response and recovery for the low-temperature (∼140 °C) detection of methane gas in comparison with polycrystalline Cu 2O thin film-based gas sensors. Specifically, at a methane concentration of 4%, the response and the recovery times of the Cu 2O nanowire-based gas sensors are 125 and 147 s, respectively. The Cu 2O nanowire-based gas sensors have a potential for applications in the environmental monitoring, chemical industry, mining industry, and several other emerging areas.

Spinning CNT based composite yarns using a dry spinning process

A process for manufacturing polymer carbon nanotube (CNT) based composite yarns is reported. The aligned CNT structure was combined with a polymer resin and, after being stressed through the spinning process, the resin was cured and polymerized, with the CNT structure acting as reinforcement in the composite yarn. The present method obviates the need of special and complex treatments to align and disperse CNTs in a polymer matrix. The process allows development of polymer/CNT based composite yarns with different mechanical properties suitable for a range of applications by using various resins.

Alloy Hybrid Carbon Nanotube Yarn for Multifunctionality

Abstract This chapter discusses fabrication of carbon nanotube yarn and coating of metals and nonmetals onto the yarn to increase electrical conductivity. An increase in weight due to the metal particle coating is the trade-off for achieving increased electrical conductivity. Electrical conductors made from the hybrid alloy yarn may have multiple applications including in electrical conductors and transmission lines. Carbon nanotube yarns also show potential as microelectrodes and robust and flexible chemiresistors for molecular detection. The decoration of metal nanoclusters on the yarn provides high sensitivity and selectivity to gases that are present in industrial and environmental emissions. Overall, coating yarn with particles produces a multifunctional material that opens up new design possibilities for applications.

Investigation of Biomimetic Apatite Growth on DLC-ZrO2 Thin Films Prepared by MOCVD

Thin films of diamond-like carbon (DLC) containing zirconium dioxide (DLC-ZrO2) have been deposited onto conducting (100) silicon wafer substrates using a pulsed direct-current metal–organic plasma activated chemical vapour deposition (MOPACVD) technique. DLC-ZrO2 thin films were immersed in simulated body fluid (SBF) at 37° C. The formation of apatite as a function of time was determined using a number of characterization techniques including XPS, FTIR, XRD, SEM and EDX. The XPS results confirmed the presence of calcium and phosphorus on the DLC-ZrO2 film surfaces after immersion in SBF. FTIR and XRD results confirmed that biomimetic apatite was formed on DLC-ZrO2 as an amorphous film. Ball-like particles composed of Ca and P were observed on the film surface. EDX results also confirmed that Ca and P were deposited on the film surface.