B. L. Weiss

Systematic study of microwave absorption, heating, and microstructure evolution of porous copper powder metal compacts

We present a systematic study of the absorption, heating behavior, and microstructure evolution of porous copper powder metal compacts subjected to 2.45 GHz microwave radiation and explain our observations using known physical mechanisms. Using a single-mode microwave system, we place the compacts in pure electric (E) or magnetic (H) fields and compare the heating trends. We also investigate the effect of particle size on the same. The observed trends and the differences between E- and H-field heating are reflected in the dramatic changes in the conductivity, permittivity, and permeability of the samples. These property changes are effected by the microstructure evolution during heating in the two types of fields. We also find that the observed dependence of the initial microwave heating on particle size is suggestive of single-particle behavior.

Nanoscale Devices for Rectification of High Frequency Radiation from the Infrared through the Visible: A New Approach

We present a new and viable method for optical rectification. This approach has been demonstrated both theoretically and experimentally and is the basis fot the development of devices to rectify radiation through the visible. This technique for rectification is based not on conventional material or temperature asymmetry as used in MIM (metal/insulator/metal) or Schottky diodes, but on a purely sharp geometric property of the antenna. This sharp “tip” or edge with a collector anode constitutes a tunnel junction. In these devices the rectenna (consisting of the antenna and the tunnel junction) acts as the absorber of the incident radiation and the rectifier. Using current nanofabrication techniques and the selective atomic layer deposition (ALD) process, junctions of 1 nm can be fabricated, which allow for rectification of frequencies up to the blue portion of the spectrum. To assess the viability of our approach, we review the development of nanoantenna structures and tunnel junctions capable of operating in the visible region. In addition, we review the detailed process of rectification and present methodologies for analysis of diode data. Finally, we present operational designs for an optical rectenna and its fabrication and discuss outstanding problems and future work.

Electron emission from disordered tetrahedral carbon

Electron field-emission tests have been performed on films grown by a modified microwave plasma assisted chemical vapor deposition diamond process. This modification includes the addition of N2 and O2 during the growth stage. Characterization of these films shows the presence of a disordered tetrahedral carbon structure. Raman spectroscopy indicates a disturbance in the cubic symmetry of the lattice and x-ray diffraction indicates a disordered tetrahedral structure. Field-emission testing indicate that current densities of 0.5 mA/cm2 can be obtained for applied fields of 5–8 V/μm. The results are explained in terms of a change in the band structure and the formation of electronic states in the band gap.

A new model for the replacement process in electron emission at high fields and temperatures

Abstract A new model is presented to resolve the Nottingham-Fleming and Henderson controversy concerning the average energy of the replacement electrons in field emission. In addition to thermal excitation, we introduce the tunnelling-state contribution as a mechanism to vacate levels available for replacement electrons. It is found that the condition for a steady-state current is not satisfied without the tunnelling-state contribution. The present result of the net energy exchange Δϵ per electron obtained as a function of both temperature and field shows much improved agreement with experimental data. The inversion temperature T i as a function of field is now in good quantitative agreement with existing experimental data. Contrary to the assertion of Nottingham [Phys. Rev. 59 (1941) 907] that the replacement energy is equal to the chemical potential of the emitter, our results favor the argument of Fleming and Henderson [Phys. Rev. 58 (1940) 887] that the replacement process takes place in the available energy states below the Fermi energy in the emitter. Non-equilibrium effects in the emitter due to fields and temperature gradients evaluated within the relaxation time approximation are noticeable only for large fields.

New analysis of electron energy exchange and cooling in semiconductors

Energy exchange Δe is investigated in field emission from semiconductors. For the first time, a formal theory is developed for the replacement process of the injected charge carriers. It leads to analytic expressions for Δe, which exhibit the dependence on field, temperature, and doping concentration in a parametric form. The analytic and numeric results reveal the important feature that Δe is positive for all temperatures. This implies that field emission from semiconductors always produces cooling of an emitter. When Joule heating is included, there is still a net cooling for a wide range of emitted current densities.

Theoretical analysis of triple junction field emission for a type of cold cathode

Triple junction of metal-dielectric-vacuum is theoretically analysed. Electric field enhancement is observed in the vicinity of a metal-dielectric contact and considered to be due to the presence of dielectric. The dielectric enhancement in the problem gives rise to a new concept that dielectric can enhance the electric field, which is opposite to the usual view that dielectric reduces the electric field. This new type of enhancement is attributed to the polarization of dielectric. It is also found that the metal-dielectric-vacuum path is more favorable than the metal-vacuum path for field electrons. The present results suggest that the triple junction can be a new type of field emission source

Fabrication of thin-film cold cathodes by a modified chemical vapor deposition diamond process

Thin-film cold cathodes have been grown on molybdenum by a modified microwave assisted plasma chemical vapor deposition diamond process. Electron field-emission tests have been performed on the devices. The modification from the chemical vapor deposition diamond process includes the addition of N2 and O2 into the plasma during the growth stage. Characterization of these films indicates a disordered tetrahedral carbon structure. Raman spectroscopy shows a disturbance in the cubic symmetry of the lattice and x-ray diffraction indicates a disordered tetrahedral structure. Electron emission testing indicate low turn-on voltages. Current densities from 1 to 8 mA/cm2 can be obtained for applied fields of 5–8 V/μm. The results are explained in terms of a change in the electronic band structure and the formation of states in the band gap.

Field emission cooling of thermoelectric semiconductor PbTe

We report a theoretical analysis of the cooling effect due to field emission from n-type PbTe, a typical thermoelectric material. We show that, by calculating the average energies of field and replacement electrons, the energy exchange in field emission from n-type PbTe always produces cooling. The resultant heat loss leads to cooling of up to a few tenths of an electron volt per field emitted electron. For n-type PbTe, the Nottingham energy exchange in field emission can be comparable to or greater than the Peltier coefficient.

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.

Classical and quantum responsivities of geometrically asymmetric metal-vacuum-metal junctions used for the rectification of infrared and optical radiations

The authors study the rectification properties of geometrically asymmetric metal-vacuum-metal junctions in which a combination of static and oscillating biases is established between a cathode that is extended by a hemispherical protrusion and a flat anode. The static current-voltage characteristics of this device are established using a transfer-matrix methodology. The rectification properties of the device are, however, analyzed in the framework of a classical model that is based on the Taylor-expansion of static current-voltage data. This enables the impedance and the classical responsivity of the device to be established. The authors then investigate how the impedance and the classical responsivity of this junction are affected by the work function of the materials, the gap spacing between the cathode and the anode, and the aspect ratio of the protrusion. They also consider the efficiency with which the energy of incident radiations can be converted using this device. The authors finally compare the responsivity obtained using this classical approach with the quantum responsivity one can define from the currents actually achieved in an oscillating barrier. This work provides additional insight for the development of a device that could be used for the energy conversion of infrared and optical radiations.The authors study the rectification properties of geometrically asymmetric metal-vacuum-metal junctions in which a combination of static and oscillating biases is established between a cathode that is extended by a hemispherical protrusion and a flat anode. The static current-voltage characteristics of this device are established using a transfer-matrix methodology. The rectification properties of the device are, however, analyzed in the framework of a classical model that is based on the Taylor-expansion of static current-voltage data. This enables the impedance and the classical responsivity of the device to be established. The authors then investigate how the impedance and the classical responsivity of this junction are affected by the work function of the materials, the gap spacing between the cathode and the anode, and the aspect ratio of the protrusion. They also consider the efficiency with which the energy of incident radiations can be converted using this device. The authors finally compare the r...