Enhanced oxygen reduction and evolution reaction with hematite nanoparticle decorated carbon nano... more Enhanced oxygen reduction and evolution reaction with hematite nanoparticle decorated carbon nanotube array cathode for nonaqueous Li–O2batteries.
Renewable Energy and the Environment Optics and Photonics Congress, 2012
ABSTRACT Optimizing the antireflection coatings of porous silicon (PS ARC) is found to be sensiti... more ABSTRACT Optimizing the antireflection coatings of porous silicon (PS ARC) is found to be sensitive to their thicknesses. The optimal reflectance for a 400-nm-thin, PS ARC is calculated to be as low as 1.55%.
Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer, 2009
Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy co... more Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional thermoelectric figure of merit, ZT = (S^2 σT)/k where S, σ, k are the Seebeck coefficient, electrical conductivity and thermal conductivity respectively. These physical properties are interdependent, and hence making ZT of a material high is very challenging work. However, when the size of nanostructure is comparable to the wavelength and mean free path of energy carriers, it is feasible to avoid such interdependence to enhance ZT energy conversion. [1–3]
ABSTRACT The relation between filling ratio and length of Si nano wires is investigated via chara... more ABSTRACT The relation between filling ratio and length of Si nano wires is investigated via characterizing the optical and electrical performances of solar cells. To enhance photovoltaic performances, we suggest the optimal parameters in nano wired solar cell.
The optical properties of silicon nanowires (Si NWs) coated with metallic nanoparticles (NPs) wer... more The optical properties of silicon nanowires (Si NWs) coated with metallic nanoparticles (NPs) were theoretically investigated. A three dimensional (3D) model of a solar cell has been proposed, in which the Si NWs were geometrically modeled to be periodically aligned nano-pillars in a square lattice. Hemispherical metallic NPs were coated onto a sidewall of Si NWs. We numerically studied the
We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhanc... more We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhancement and optical loss in periodically aligned, silicon nanowire (SiNW) arrays integrated with a silver back reflector (Ag BR). Optimizing the embedded depth of the wire bottoms into a silver reflector achieved a highly efficient SiNW solar cell. Compared to the SiNW solar cell employing a flat back reflector, the embedded depth of ~20 nm resulted in the relative increase of ~5% in ultimate solar cell efficiency.
The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been cons... more The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been considered ideal for the efficient harnessing solar energy. Our hybrid approach uses full spectrum solar energy via lossless coupling between PV and TE devices while collecting waste energy from thermalization and transmission losses from PV devices. Achieving lossless coupling makes the power output from the hybrid device equal to the sum of the maximum power outputs produced separately from individual PV and TE devices. TE devices need to have low internal resistances enough to convey photo-generated currents without sacrificing the PV fill factor. Concomitantly, a large number of p-n legs are preferred to drive a high Seebeck voltage in TE. Our simple method of attaching a TE device to a PV device has greatly improved the conversion efficiency and power output of the PV device (~30% at a 15°C temperature gradient across a TE device).
Cost-efficient fabrication of UV/Vis nanowire (NW) photodiodes enabled by ZnO/Si radial heterojun... more Cost-efficient fabrication of UV/Vis nanowire (NW) photodiodes enabled by ZnO/Si radial heterojunction ... Fig. 2 Schematic illustration of the fabrication procedure for preparing the novel NW photodiode. ... the NW photodiodes show the rapid response with respect to incident light. ...
A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the ref... more A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc's model. The optical bandgap of the SiNW arrays (112 nm in average diameter) was measured to be ~1.19 eV, which is larger than the ~1.08 eV bandgap of bulk Si. Further decreasing the wire diameter to 68 nm caused an increase of the bandgap to ~1.24 eV, which is closer to the optimal bandgap (~1.40 eV) required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays.
Silicon nanowires (NWs) and microwires (MWs) are cost-effectively integrated on a 4-inch wafer us... more Silicon nanowires (NWs) and microwires (MWs) are cost-effectively integrated on a 4-inch wafer using metal-assisted electroless etching for solar cell applications. MWs are periodically positioned using low-level optical patterning in between a dense array of NWs. A spin-on-doping technique is found to be effective for the formation of heavily doped, thin n-type shells of MWs in which the radial doping profile is easily delineated by low voltage scanning electron microscopy. Controlled tapering of the NWs results in additional optical enhancement via optimization of the tradeoff between increased light trapping (by a graded-refractive-index) and increased reflectance (by decreasing areal density of NWs). Compared to single NW (or MW) arrayed cells, the co-integrated solar cells demonstrate improved photovoltaic characteristics, i.e. a short circuit current of 20.59 mA cm(-2) and a cell conversion efficiency of ∼ 7.19% at AM 1.5G illumination.
Nanowires have been taken much attention as a nanoscale building block, which can perform the exc... more Nanowires have been taken much attention as a nanoscale building block, which can perform the excellent mechanical function as an electromechanical device. Here, we have performed atomic force microscope (AFM)-based nanoindentation experiments of silicon nanowires in order to investigate the mechanical properties of silicon nanowires. It is shown that stiffness of nanowires is well described by Hertz theory and that elastic modulus of silicon nanowires with various diameters from ~100 to ~600 nm is close to that of bulk silicon. This implies that the elastic modulus of silicon nanowires is independent of their diameters if the diameter is larger than 100 nm. This supports that finite size effect (due to surface effect) does not play a role on elastic behavior of silicon nanowires with diameter of >100 nm.
A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering ... more A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering method was used in the initial growth of AZO films and a rapid annealing process was performed on the as-deposited AZO film under N2 atmosphere for 3min. An as-deposited AZO film had an optical transmittance of 84.78% at 550nm and a resistivity of 7.8×10−3 Ω cm.
... 100-nm-thick Ti layers were thermally evaporated onto the wafers, and the backside temperatur... more ... 100-nm-thick Ti layers were thermally evaporated onto the wafers, and the backside temperature of the wafers was observed to increase (≥200 °C) by the bombardment of hot Ti atoms during the evaporation so that a Si-containing Ti layer was formed by thermal diffusion of Si ...
Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowire... more Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.
Single-crystal, Al-catalyzed silicon nanowires were grown under atmospheric pressure using the di... more Single-crystal, Al-catalyzed silicon nanowires were grown under atmospheric pressure using the dimpled feature of the Al metal that remained after removal of an anodic aluminum oxide (AAO) template directly formed on a Si substrate. Upon annealing in a hydrogen-rich atmosphere, the dimpled morphology of Al was transformed into a smooth, rounded shape in which Si nanodots were periodically embedded due to Si migration from the substrate. The positions of the nanodots were exactly the same as the positions of sawtooth features on the dimpled surface. Although Al-catalyzed silicon nanowires have been known to grow only under vacuum due to the tendency of Al to oxidize, these silicon nanodots, surrounded by residual Al, showed excellent resistance to oxidation under atmospheric pressure. These nanodots were also capable of acting as catalysts for the growth of nanowires, and played a role in determining the diameter of the nanowires. A thinner residual Al layer made it easier to form Si...
Enhanced oxygen reduction and evolution reaction with hematite nanoparticle decorated carbon nano... more Enhanced oxygen reduction and evolution reaction with hematite nanoparticle decorated carbon nanotube array cathode for nonaqueous Li–O2batteries.
Renewable Energy and the Environment Optics and Photonics Congress, 2012
ABSTRACT Optimizing the antireflection coatings of porous silicon (PS ARC) is found to be sensiti... more ABSTRACT Optimizing the antireflection coatings of porous silicon (PS ARC) is found to be sensitive to their thicknesses. The optimal reflectance for a 400-nm-thin, PS ARC is calculated to be as low as 1.55%.
Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer, 2009
Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy co... more Thermal properties of one dimensional nanostructures are of interest for thermoelectric energy conversion. Thermoelectric efficiency is related to non dimensional thermoelectric figure of merit, ZT = (S^2 σT)/k where S, σ, k are the Seebeck coefficient, electrical conductivity and thermal conductivity respectively. These physical properties are interdependent, and hence making ZT of a material high is very challenging work. However, when the size of nanostructure is comparable to the wavelength and mean free path of energy carriers, it is feasible to avoid such interdependence to enhance ZT energy conversion. [1–3]
ABSTRACT The relation between filling ratio and length of Si nano wires is investigated via chara... more ABSTRACT The relation between filling ratio and length of Si nano wires is investigated via characterizing the optical and electrical performances of solar cells. To enhance photovoltaic performances, we suggest the optimal parameters in nano wired solar cell.
The optical properties of silicon nanowires (Si NWs) coated with metallic nanoparticles (NPs) wer... more The optical properties of silicon nanowires (Si NWs) coated with metallic nanoparticles (NPs) were theoretically investigated. A three dimensional (3D) model of a solar cell has been proposed, in which the Si NWs were geometrically modeled to be periodically aligned nano-pillars in a square lattice. Hemispherical metallic NPs were coated onto a sidewall of Si NWs. We numerically studied the
We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhanc... more We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhancement and optical loss in periodically aligned, silicon nanowire (SiNW) arrays integrated with a silver back reflector (Ag BR). Optimizing the embedded depth of the wire bottoms into a silver reflector achieved a highly efficient SiNW solar cell. Compared to the SiNW solar cell employing a flat back reflector, the embedded depth of ~20 nm resulted in the relative increase of ~5% in ultimate solar cell efficiency.
The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been cons... more The optimal hybridization of photovoltaic (PV) and thermoelectric (TE) devices has long been considered ideal for the efficient harnessing solar energy. Our hybrid approach uses full spectrum solar energy via lossless coupling between PV and TE devices while collecting waste energy from thermalization and transmission losses from PV devices. Achieving lossless coupling makes the power output from the hybrid device equal to the sum of the maximum power outputs produced separately from individual PV and TE devices. TE devices need to have low internal resistances enough to convey photo-generated currents without sacrificing the PV fill factor. Concomitantly, a large number of p-n legs are preferred to drive a high Seebeck voltage in TE. Our simple method of attaching a TE device to a PV device has greatly improved the conversion efficiency and power output of the PV device (~30% at a 15°C temperature gradient across a TE device).
Cost-efficient fabrication of UV/Vis nanowire (NW) photodiodes enabled by ZnO/Si radial heterojun... more Cost-efficient fabrication of UV/Vis nanowire (NW) photodiodes enabled by ZnO/Si radial heterojunction ... Fig. 2 Schematic illustration of the fabrication procedure for preparing the novel NW photodiode. ... the NW photodiodes show the rapid response with respect to incident light. ...
A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the ref... more A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc's model. The optical bandgap of the SiNW arrays (112 nm in average diameter) was measured to be ~1.19 eV, which is larger than the ~1.08 eV bandgap of bulk Si. Further decreasing the wire diameter to 68 nm caused an increase of the bandgap to ~1.24 eV, which is closer to the optimal bandgap (~1.40 eV) required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays.
Silicon nanowires (NWs) and microwires (MWs) are cost-effectively integrated on a 4-inch wafer us... more Silicon nanowires (NWs) and microwires (MWs) are cost-effectively integrated on a 4-inch wafer using metal-assisted electroless etching for solar cell applications. MWs are periodically positioned using low-level optical patterning in between a dense array of NWs. A spin-on-doping technique is found to be effective for the formation of heavily doped, thin n-type shells of MWs in which the radial doping profile is easily delineated by low voltage scanning electron microscopy. Controlled tapering of the NWs results in additional optical enhancement via optimization of the tradeoff between increased light trapping (by a graded-refractive-index) and increased reflectance (by decreasing areal density of NWs). Compared to single NW (or MW) arrayed cells, the co-integrated solar cells demonstrate improved photovoltaic characteristics, i.e. a short circuit current of 20.59 mA cm(-2) and a cell conversion efficiency of ∼ 7.19% at AM 1.5G illumination.
Nanowires have been taken much attention as a nanoscale building block, which can perform the exc... more Nanowires have been taken much attention as a nanoscale building block, which can perform the excellent mechanical function as an electromechanical device. Here, we have performed atomic force microscope (AFM)-based nanoindentation experiments of silicon nanowires in order to investigate the mechanical properties of silicon nanowires. It is shown that stiffness of nanowires is well described by Hertz theory and that elastic modulus of silicon nanowires with various diameters from ~100 to ~600 nm is close to that of bulk silicon. This implies that the elastic modulus of silicon nanowires is independent of their diameters if the diameter is larger than 100 nm. This supports that finite size effect (due to surface effect) does not play a role on elastic behavior of silicon nanowires with diameter of >100 nm.
A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering ... more A low-resistive Al-doped ZnO (AZO) film was achieved by rapid thermal annealing. A co-sputtering method was used in the initial growth of AZO films and a rapid annealing process was performed on the as-deposited AZO film under N2 atmosphere for 3min. An as-deposited AZO film had an optical transmittance of 84.78% at 550nm and a resistivity of 7.8×10−3 Ω cm.
... 100-nm-thick Ti layers were thermally evaporated onto the wafers, and the backside temperatur... more ... 100-nm-thick Ti layers were thermally evaporated onto the wafers, and the backside temperature of the wafers was observed to increase (≥200 °C) by the bombardment of hot Ti atoms during the evaporation so that a Si-containing Ti layer was formed by thermal diffusion of Si ...
Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowire... more Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.
Single-crystal, Al-catalyzed silicon nanowires were grown under atmospheric pressure using the di... more Single-crystal, Al-catalyzed silicon nanowires were grown under atmospheric pressure using the dimpled feature of the Al metal that remained after removal of an anodic aluminum oxide (AAO) template directly formed on a Si substrate. Upon annealing in a hydrogen-rich atmosphere, the dimpled morphology of Al was transformed into a smooth, rounded shape in which Si nanodots were periodically embedded due to Si migration from the substrate. The positions of the nanodots were exactly the same as the positions of sawtooth features on the dimpled surface. Although Al-catalyzed silicon nanowires have been known to grow only under vacuum due to the tendency of Al to oxidize, these silicon nanodots, surrounded by residual Al, showed excellent resistance to oxidation under atmospheric pressure. These nanodots were also capable of acting as catalysts for the growth of nanowires, and played a role in determining the diameter of the nanowires. A thinner residual Al layer made it easier to form Si...
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