Alireza Nojeh

Electric-field-directed growth of carbon nanotubes in two dimensions

The interplay between mechanical and electronic properties in carbon nanotubes leads to interesting characteristics in devices such as a nanotube cross structure. The fabrication of nanotube devices has often been based on random growth or deposition of nanotubes and subsequently searching for those in desired locations with proper orientations. Obviously we want to be able to make such devices controllably. We present data on extending the technique of one-dimensional alignment of nanotubes using electric field to two dimensions in order to make more complicated structures such as nanotube crosses. It appears that nanotubes assemble in the regions of the most intense electric field. Also, they tend to follow the local field lines, even in nonuniform fields. Furthermore, they tend to grow away from negative toward positive polarities.

Nonlocal Continuum Modeling and Molecular Dynamics Simulation of Torsional Vibration of Carbon Nanotubes

This paper investigates the size effects in the dynamic torsional response of single-walled carbon nanotubes (SWCNTs) by developing a modified nonlocal continuum shell model. The purpose is to facilitate the design of devices based on CNT torsion by providing a simple, accurate, and efficient continuum model that can predict the frequency of torsional vibrations and the propagation speed of torsional waves. To this end, dispersion relations of torsional waves are obtained from the proposed nonlocal model and compared to classical models. It is seen that the classical and nonlocal models predict nondispersive and dispersive behavior, respectively. Molecular dynamics simulations of torsional vibrations of (6, 6) and (10, 10) SWCNTs are also performed, the results of which are compared with the classical and nonlocal models and used to extract consistent values of the nonlocal elasticity constant. The superiority and accuracy of the nonlocal elasticity model in predicting the size-dependent dynamic torsional response of SWCNTs are demonstrated.

Transforming carbon nanotube forest from darkest absorber to reflective mirror

Carbon nanotube (CNT) forests are known to be among the darkest materials on earth. They can absorb the entire visible range of electromagnetic wave more efficiently than any other known black material. We have attempted controlled mechanical processing of the CNTs and, surprisingly, observed mirror-like reflection from the processed area with 10%–15% reflectivity, a level higher than typical reflectivity of pure forests by over two orders of magnitude, for a wide range of the spectrum (570–1100 nm). Patterning of micro mirrors in the forest is demonstrated to show its potential application for producing monolithically integrated reflector-absorber arrays in the material.

Solar electron source and thermionic solar cell

Common solar technologies are either photovoltaic/thermophotovoltaic, or use indirect methods of electricity generation such as boiling water for a steam turbine. Thermionic energy conversion based on the emission of electrons from a hot cathode into vacuum and their collection by an anode is also a promising route. However, thermionic solar conversion is extremely challenging as the sunlight intensity is too low for heating a conventional cathode to thermionic emission temperatures in a practical manner. Therefore, compared to other technologies, little has been done in this area, and the devices have been mainly limited to large experimental apparatus investigated for space power applications. Based on a recently observed “Heat Trap” effect in carbon nanotube arrays, allowing their efficient heating with low-power light, we report the first compact thermionic solar cell. Even using a simple off-the-shelf focusing lens, the device delivered over 1 V across a load. The device also shows intrinsic storage ...

Field-emission-assisted approach to dry micro-electro-discharge machining of carbon-nanotube forests

This work investigates dry micro-electro-discharge machining (μEDM) of vertically aligned carbon nanotube (CNT) forests that are used as cathodes in the process, as opposed to conventional μEDM where the material to be machined forms the anode, toward achieving higher precision in the patterned microstructures. The new configuration with the reversed polarity is observed to generate higher discharge currents in the process, presumably due to effective field-emission from CNTs. This effect allows the process to be performed at very low discharge energies, approximately 80× smaller than in the conventional normal-polarity case, with the machining voltage and tolerance down to 10 V and 2.5 μm, respectively, enabling high-precision high-aspect-ratio micropatterning in the forests. The new approach is also demonstrated to make the process faster, cleaner, and more stable than conventional processing. Spectroscopic analyses of the forests processed by reverse μEDM show no evidence of significant crystalline det...

ELECTRON EMISSION FROM CARBON NANOTUBES

Carbon nanotubes, nanometer-diameter tubes made of carbon atoms, have garnered significant attention from researchers over the past 15 years due to their outstanding properties such as excellent electronic transport characteristics and mechanical strength. Because of their ability to carry extremely high current densities, their high aspect ratio, and small tip radius that enhances an external electric field greatly, one particularly promising area of interest has been the usage of nanotubes as electron sources. In this article we will try to provide an overview of the subject from various experimental and theoretical angles.

Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen

The working principle of dry micro-electro-discharge machining of vertically aligned carbon-nanotube forests is investigated by evaluating the effect of oxygen on the process. The machining experiments with controlled oxygen/nitrogen ratios indicate a correlation between the peak current of discharge pulses and the oxygen concentration, suggesting not only a vital role for oxygen in the process, but also a removal mechanism fundamentally different from that in typical electro-discharge machining based on direct melting and evaporation of the sample material. The highest surface quality and uniformity in the machined forest microstructures as well as smooth machining without short circuiting are achieved at an approximate oxygen concentration of 20% under the discharge condition of 30 V and 10 pF, revealing that air is an optimal medium for the removal process. Elemental and molecular analyses show no evidence of significant crystalline deterioration or contamination in the nanotubes processed with the tec...

Secondary electron yield of multiwalled carbon nanotubes

Secondary electron yield from individual multiwalled carbon nanotubes is investigated for a wide range of primary beam energies (0.5–15 keV). By using a simple experimental procedure under an optical microscope, we make suspended nanotubes, which are free from interaction with the substrate during electron yield measurements. It is found that the secondary electron yield from isolated suspended nanotubes is less than unity and decreases as a function of primary electron energy.

Performance evaluation of wireless networks on chip architectures

The performance benefits of conventional Network-on-Chip (NoC) architectures are limited by the high latency and energy dissipation in long distance multihop communication between embedded cores. To alleviate these problems, wireless on-chip networks are envisioned. Using miniaturized on-chip antennas as an enabling technology, wireless NoCs (WiNoCs) can be designed. In this paper we elaborate on the design methodology and technology requirements for a WiNoC and evaluate its performance. It is demonstrated that a WiNoC outperforms its wireline counterpart in terms of network throughput and latency, and that energy dissipation improves by an order of magnitude.

Scanning electron microscopy of field-emitting individual single-walled carbon nanotubes

Carbon nanotubes are promising electron emitters because of their sharp geometries that lead to significant external field enhancement, as well as their mechanical strength. However, distinguishing the emission due to an individual single-walled carbon nanotube (SWCNT) from that due to surrounding structures is a challenge. Here, we demonstrate how a scanning electron microscope (SEM) can be used to view the emission from individual SWCNTs by applying an external field close to the onset of field-emission and then scanning the tube with the electron beam of the SEM. The stimulated emission is revealed in the SEM image as localized bright spots.