Simulations of infrared and optical rectification by geometrically asymmetric metal-vacuum-metal junctions for applications in energy conversion devices

We use a transfer-matrix methodology to simulate the rectification of infrared and optical radiation by geometrically asymmetric metal-vacuum-metal junctions in which one of the metals is flat while the other is extended by a tip. We determine in particular the power this junction could provide to an external load and the efficiency with which the energy of incident radiations is converted. We consider first situations in which the external radiation is monochromatic, with typical frequencies in the infrared and optical domains. We then consider situations in which the external radiation consists of a full range of frequencies, with amplitudes that are representative of a focused beam of solar radiation. We investigate in particular how the efficiency of the rectification is affected by the aspect ratio of the tip, the work function of the metallic elements and the occurrence of polarization resonances. Our results demonstrate that the rectification of infrared and optical radiation is possible using devices of the type considered in this work. They finally provide a quantitative analysis of the efficiency of this rectification.

Single-Walled Carbon Nanotubes of Controlled Diameter and Bundle Size and Their Field Emission Properties

Field emission studies were conducted on as-produced CoMoCAT single-walled carbon nanotube/silica composites with controlled nanotube diameter and bundle size. It has been observed that the as-produced nanotube material does not need to be separated from the high-surface area catalyst to be an effective electron emitter. By adjusting the catalytic synthesis conditions, single-walled carbon nanotubes (SWNT) of different diameters and bundle sizes were synthesized. A detailed characterization involving Raman spectroscopy, optical absorption (vis-NIR), SEM, and TEM was conducted to identify the nanotube species present in the different samples. The synthesis reaction temperature was found to affect the nanotube diameter and bundle size in opposite ways; that is, as the synthesis temperature increased the nanotube average diameter became larger, but the bundle size became smaller. A gradual and consistent reduction in the emission onset field was observed as the synthesis temperature increased. It is suggested that the bundle size, more than the nanotube diameter or chirality, determines the field emission characteristics of these composites. This is a clear demonstration that field emission characteristics of SWNT can be controlled by the nanotube synthesis conditions.