We have created a thermionic cathode structure that consists of a thin tungsten ribbon; carbon nanotubes (CNTs) on the ribbon surface; and a thin layer of low work function barium strontium oxide coating on the CNTs. This oxide coated CNT cathode was designed to combine the benefits from the high field enhancement factor from CNTs and the low work function from the emissive oxide coating. The field emission and thermionic emission properties of the cathode have been characterized. A field enhancement factor of 266 and a work function of 1.9 eV were obtained. At 1221 K, a thermionic emission current density of 1.22A/cm2 in an electric field of 1.1 V/μm was obtained, which is four orders of magnitude greater than the emission current density from the uncoated CNT cathode at the same temperature. The high emission current density at such a modest temperature is among the best ever reported for an oxide cathode.

Nucleation and growth processes of carbon nanotubes (CNTs) in iron catalyzed chemical vapor deposition (CVD) have been observed by means of in-situ environmental transmission electron microscopy. Our atomic scale observations demonstrate that solid state iron carbide (Fe3C) nanoparticles act as catalyst for the CVD growth of CNTs. Iron carbide nanoparticles are structurally fluctuated in CVD condition. Growth of CNTs can be simply explained by bulk diffusion of carbon atoms since nanoparticles are carbide.

By applying optical near-field effects in a laser-assisted chemical vapor deposition (LCVD) process, self-aligned growth of ultra-short single-walled carbon nanotubes (SWNTs) was realized in a well controlled manner at a relatively low substrate temperature due to the nanoscale heating enhancement induced by the optical near-field effects. Bridge structures containing single suspending SWNT channels were successfully fabricated. Ultra-sharp tip-shaped metallic electrodes were used as optical antennas in localizing and enhancing the optical fields. Numerical simulations using High Frequency Structure Simulator (HFSS) reveal significant enhancement of electrical fields at the metallic electrode tips under laser irradiation, which induces localized heating at the tips. Numerical simulations were carried out to optimize SWNT growth conditions, such as electrode tip sharpness and film thickness, for maximal enhancement of electrical near field and localized heating.

As carbon nanotubes (CNTs) inevitably contain defects, an understanding of their effect on the electrochemical behavior is crucial. We consider, through Cyclic Voltammetry and Raman Spectroscopy, the influence of both intrinsic and extrinsically introduced defects. Bamboo and hollow multi-walled carbon nanotube morphologies provided examples of the former while the controlled addition of Argon and Hydrogen ions was used for studying extrinsic defects. We show that the electrocatalytic response of the hollow type CNTs can be tailored significantly, while bamboo type CNTs have innately high reactive site densities and are less amenable to modification. Argon irradiation also differs greatly from that of Hydrogen irradiation. CNT irradiation with Argon appears to positively charge CNTs, while Hydrogen irradiation neutralizes defects further allowing for the tuning of CNT defect density. The work has implications in the design of nanotube and nanowire based chemical sensors.

Glancing angle deposition (GLAD) is a physical vapor deposition technique that depends on a highly oblique flux angle to create porous, large surface area thin films via self-shadowing. Control of the deposition parameters may provide a means to tune film porosity for zinc oxide sensors and photovoltaic devices. However, the self-shadowing mechanism requires a collimated particle flux, and therefore GLAD performs best under high vacuum. Creating structured films with sputtered GLAD is difficult since the high chamber pressure (>1 mTorr) necessary to maintain the sputter plasma reduces the mean-free-path of flux particles to less than 100 mm. By using an aperture to reduce the angle subtended by the target from the perspective of the substrate, maintaining an argon plasma pressure of 1.4 mTorr, and reducing the throw distance to less than 50 mm we were able to produce structured, polycrystalline, zinc thin films via GLAD. At oblique flux angles, highly porous films consisting of randomly oriented nanorods are grown. The nanorods have diameters between 10-100 nanometers with lengths up to several micrometers. Annealing at temperatures up to 250 °C in air produces polycrystalline zinc oxide with minimal changes to the film structure. We present details of the thin film fabrication process for the convoluted nanorod film morphology. We report characterization results for films produced at several deposition angles before and after annealing using scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM).

In this paper we report about the synthesis of single wall carbon nanohorns and highly convoluted graphite sheets by AC powered arc discharge carried out between pure graphite electrodes. The arc is ignited in air and the arched electrodes are surrounded by a cylindrical collector which collects the synthesized material and contributes to control the synthesis environment. With the purpose of studying the effect of the process variables, in this work we have explored the effect of the powering frequency on the structure of the synthesized material and on the yield of the process. Preliminary experimental results on tests carried out at constant voltage, show that the process yield is strongly influenced by the powering frequency and that higher yields are obtained at low frequency. The structure of the resulting soot has been characterized by transmission electron microscopy. Two kinds of microstructures are found by TEM observation constituted by highly convoluted graphene sheets, having locally the nanohorn morphology, and better organized nano-balls where also graphite nano-sheets can be locally found. The relative abundance of the two kinds of particles appears to depend on the powering frequency with a larger amount of the latter observed in samples synthesized at high frequency.

We have observed gate-controlled N-shaped negative differential resistance (NDR) and photoconductivity enhancement in carbon nanotube (CNT) based addressable intra-connects. The intra-connects – bridges spanning across planar electrodes – were measured at room temperature. Individual single-walled CNT (SWCNT) channels were grown using chemical vapor deposition (CVD) precisely between very sharp metal tips on the pre-fabricated electrodes without post-processing. The electrodes were made of cobalt. Methane and H2 gas mixture were introduced into quartz tube for an hour at 900 C, with flow rate of 1900 sccm for methane and 20 sccm for H2. We have investigated two different cases: in one case, the source-drain current-voltage, Ids-Vds, characteristics were linear. The other case exhibited nonlinear Ids-Vds characteristics. Raman scattering of the intra-connects indicated that each were made of SWCNT with radial breathing mode (RBM) at 191.9 cm-1 and 176.2 cm-1, respectively. Current-voltage Ids-Vds characteristics were measured for various Vgs from -10 V to +10 V. Negative differential resistance (NDR) was found in the Ids-Vgs curves for gate bias in the region of -3>Vgs>-6 V. The NDR peak was shifted to the negative side as the source-drain voltage was increased from Vds=0 to 0.75 V. Otherwise, the intra-connects exhibited characteristics of an ordinary p-type channel. The experiments were repeated under white light illumination. The light increases the carrier density in the channel but not in the metal electrodes and allowed us to study the effective doping of the channel without affecting the work function of the SWCNT/metal contact. The overall channel conductance increased under the light irradiation. Under illumination, the devices became more stable, as well. In summary, we have investigated contact properties between a SWCNT intra-connect and metal electrodes in a well controlled layout settings.