The Effects of Electronic and Photonic Coupling on the Performance of a Photothermionic-Photovoltaic Hybrid Solar Device

This work presents a detailed analysis of the photothermionic-photovoltaic hybrid solar device. The electrons in this hybrid device gain energy from both the solar photons and thermophotons generated within the device, and hence the device has the potential to offer a voltage boost compared to individual photothermionic or photovoltaic devices. We show that the gap size between the photothermionic emitter and the photovoltaic collector crucially affects the device performance due to the strong dependence of the electronic and photonic coupling strengths on this gap size. We also investigate how the current matching constraint between the thermionic and photovoltaic stages can affect the hybrid solar device performance by studying different hybrid device configurations. Moreover, the hybrid devices are compared with the single photothermionic solar device with a metallic collector. Interestingly, we observe that the addition of a photovoltaic stage meant to enable the hybrid device to capture the entire terrestrial solar spectrum does not necessarily lead to higher overall conversion efficiency.Comment: 38 Pages, 11 Figure

Micro glow plasma for localized nanostructural modification of carbon nanotube forest

This paper reports the localized selective treatment of vertically aligned carbon nanotubes, or CNT forests, for radial size modification of the nanotubes through a micro-scale glow plasma established on the material. An atmospheric-pressure DC glow plasma is shown to be stably sustained on the surface of the CNT forest in argon using micromachined tungsten electrodes with diameters down to 100 lm. Experiments reveal thinning or thickening of the nanotubes under the micro glow depending on the process conditions including discharge current and process time. These thinning and thickening effects in the treated nanotubes are measured to be up to �30% and �300% in their diameter, respectively, under the tested conditions. The elemental and Raman analyses suggest that the treated region of the CNT forest is pure carbon and maintains a degree of crystallinity. The local plasma treatment process investigated may allow modification of material characteristics in different domains for targeted regions or patterns, potentially aiding custom design of micro-electro- mechanical systems and other emerging devices enabled by the CNT forest

Tuning the thermal conductivity of silicon nanowires by surface passivation

Using large scale molecular dynamics simulations, we study the thermal conductivity of bare and surface passivated silicon nanowires (SiNWs). For the smaller cross-sectional widths $w$, SiNWs become unstable because of the surface amorphousization and even evaporation of a certain fraction of Si atoms when $w \leq 2$ nm. Our results suggest that the surface (in--)stability is related to a large excess energy $\Delta$ of the surface Si atoms with respect to the bulk Si. This is because the surface Si atoms being less coordinated and having dangling bonds. As a first step of our study, we propose a practically relevant method that uses $\Delta$ as a guiding tool to passivate these dangling bonds and thus stabilizes SiNWs. The surface stabilization is achieved by passivation of Si atoms by hydrogen or oxygen. These passivated SiNWs are then used for the calculation of the thermal conductivity coefficient $\kappa$. While the expected trend of $\kappa \propto w$ is observed for all SiNWs, surface passivation provides an added flexibility of tuning $\kappa$ with the surface coverage concentration $c$ of passivated atoms. Analyzing the phonon band structures via spectral energy density, we discuss separate contributions from the surface and the core to $\kappa$. The effect of passivation on SiNW stiffness is also discussed

Thermionic Energy Conversion in the Twenty-First Century: Advances and Opportunities for Space and Terrestrial Applications

Thermionic energy conversion (TEC) is the direct conversion of heat into electricity by the mechanism of thermionic emission, the spontaneous ejection of hot electrons from a surface. Although the physical mechanism has been known for over a century, it has yet to be consistently realized in a manner practical for large-scale deployment. This perspective article provides an assessment of the potential of TEC systems for space and terrestrial applications in the twenty-first century, overviewing recent advances in the field and identifying key research challenges. Recent developments as well as persisting research needs in materials, device design, fundamental understanding, and testing and validation are discussed

Optical anisotropy in micromechanically rolled carbon nanotube forest

The bulk appearance of arrays of vertically aligned carbon nanotubes (VACNT arrays or CNT forests) is dark as they absorb most of the incident light. In this paper, two postprocessing techniques have been described where the CNT forest can be patterned by selective bending of the tips of the nanotubes using a rigid cylindrical tool. A tungsten tool was used to bend the vertical structure of CNTs with predefined parameters in two different ways as stated above: bending using the bottom surface of the tool (micromechanical bending (M2B)) and rolling using the side of the tool (micromechanical rolling (M2R)). The processed zone was investigated using a Field Emission Scanning Electron Microscope (FESEM) and optical setup to reveal the surface morphology and optical characteristics of the patterned CNTs on the substrate. Interestingly, the polarized optical reflection from the micromechanical rolled (M2R) sample was found to be significantly influenced by the rotation of the sample. It was observed that, if the polarization of the light is parallel to the alignment of the CNTs, the reflectance is at least 2 x higher than for the perpendicular direction. Furthermore, the reflectance varied almost linearly with good repeatability (~10%) as the processed CNT forest sample was rotated from 0° to 90°