Diameter Of Nanowires Research Articles

Formation of CuOx Nanowires by Anodizing in Sodium Bicarbonate Solution

Mechanism of copper nanostructuring by oxides and hydroxide formation during anodizing is not fully understood. At the same time, the search for novel copper anodizing regimes and electrolytes is ongoing due to multiple potential applications. In this work copper anodizing in two electrode setups, in stirred 0.01 M solution of NaHCO3 at 20 °C and at voltages ranging from 5 to 40 V was explored. The morphology and composition of prepared materials were studied using FE-SEM imaging and XRD measurements. Anodizing at potentials in a range of 15–30 V led to formation of nanowires composed of crystalline Cu2O, CuO, Cu(OH)2 and malachite Cu2CO₃(OH)2. The latter was formed due to anion incorporation from the electrolyte. The diameter of nanowires strongly depended on the applied voltage, and was 35 ± 6 nm for samples prepared at 15 V, and 45 ± 9 nm for 30 V. At higher applied voltages oscillations of current density were observed, suggesting partial delamination of the formed oxidized layer, with subsequent self-healing.

(Invited) Metal Assisted Chemical Etching of Silicon Nanowires: Quantum Confinement for Photon Emission and Phonon Transport

Recent advancements in the growth and formation of semiconducting nanostructures, particularly group 14 metalloid semiconductors such as silicon and germanium have shown how quantum effects can be engineered and controlled (1,2) to modify thermal and optical properties (3). In particular, the nanostructuring of silicon materials mediates phonon scattering and confinement, including reduction of thermal conductivity (4,5) and Si-based nanowire heterostructures have been employed as solar cell materials and nanoelectronic power sources. Crystal sizes below the Bohr radius also results in light emission, and effective bandgap modification can influence absorbed thermopower in nanoscale silicon, and for photovoltaics. Reducing size to the confinement regime can also perturbate phonon transport, and this has recently been show to occur in nanocrystallites on rough surfaces of silicon nanowires.We report intense red luminescence from mesoporous n+-Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching (6-9), and also demonstrate that nanoscale confinement is a cause for limited photon or heat transport in silicon nanowires formed by MACE. n+-Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure (10). p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surface-bound nanocrystals. Both mesoporous n+-Si NWs and rough, solid p-Si NWs exhibit red luminescence at ~1.7 and ~1.8 eV, respectively. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. When quantum confinement effects are introduced in crystalline materials such as the nanocrystalline mesoporous NWs, the electron and phonon transport can be significantly altered in tandem with quantum confinement that causes intense red light emission. The data confirm that phonon confinement and scattering for silicon nanowires is due to surface-bound and internal nanostructure, rather than simply a NW diameter reduction in NW materials. References (1) S. Armatas and M. G. Kanatzidis, Science, 313, 817 (2006).(2) Kiraly, S. Yang and T. J. Huang, Nanotechnology, 24, 245704 (2013)(3) T. Canham, Appl. Phys. Lett., 57, 1046 (1990).(4) I. Hochbaum, R. Chen, R. D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar and P. Yang, Nature, 451, 163 (2008).(5) I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W. A. Goddard III and J. R. Heath, Nature, 451, 168 (2008).(6) O'Dwyer, W. McSweeney and G. Collins, ECS J. Solid State Sci. Technol., 5, R3059 (2016).(7) McSweeney, H. Geaney and C. O'Dwyer, Nano Res., 8, 1395 (2015).(8) McSweeney, C. Glynn, H. Geaney, G. Collins, J. D. Holmes and C. O'Dwyer, Semicon. Sci. Technol., 31, 014003 (2015).(9) G. Chadwick, V. Mogili, C. O’Dwyer, J. Moore, J. Fletcher, F. Laffir, G. Armstrong and D. A. Tanner, RSC Adv., 3, 19393 (2013).(10) C. Glynn, K.-M. Jones, V. Mogili, W. McSweeney, and C. O'Dwyer, ECS J. Solid State Sci. Technol. 6, N3029 (2017).

Impact of Electrical Current on Single GaAs Nanowire Structure

The impact of electrical current on the structure of single free‐standing Be‐doped GaAs nanowires grown on a Si 111 substrate is investigated. Single nanowires have been structurally analyzed by X‐ray nanodiffraction using synchrotron radiation before and after the application of an electrical current. The conductivity measurements on single nanowires in their as‐grown geometry have been realized via W‐probes installed inside a dual‐beam focused ion beam/scanning electron microscopy chamber. Comparing reciprocal space maps of the 111 Bragg reflection, extracted perpendicular to the nanowire growth axis before and after the conductivity measurement, the structural impact of the electrical current is evidenced, including deformation of the hexagonal nanowire cross section, tilting, and bending with respect to the substrate normal. For electrical current densities below 30 A mm−2, the induced changes in the reciprocal space maps are negligible. However, for a current density of 347 A mm−2, the diffraction pattern is completely distorted. The mean cross section of the illuminated nanowire volume is reconstructed from the reciprocal space maps before and after the application of electrical current. Interestingly, the elongation of two pairs of opposing side facets accompanied by shrinkage of the third pair of facets is found. The variations in the nanowire diameter, as well as their tilt and bending, are confirmed by scanning electron microscopy. To explain these findings, material melting due to Joule heating during voltage/current application accompanied by anisotropic deformations induced by the W‐probe is suggested.

Study of Microwave-Induced Ag Nanowire Welding for Soft Electrode Conductivity Enhancement

Silver nanowire (AgNW)-coated thin films are widely proposed for soft electronics application due to their good conductivity, transparency and flexibility. Here, we studied the microwave welding of AgNW-based soft electrodes for conductivity enhancement. The thermal effect of the microwave to AgNWs was analyzed by dispersing the nanowires in a nonpolar solution, the temperature of which was found to be proportional with the nanowire diameters. AgNWs were then coated on a thin film and welded under microwave heating, which achieved a film conductivity enhancement of as much as 79%. A microwave overheating of AgNWs, however, fused and broke the nanowires, which increased the film resistance significantly. A soft electrode was finally demonstrated using the microwave-welded AgNW thin film, and a 1.13 µA/mM sensitivity was obtained for glucose sensing. Above all, we analyzed the microwave thermal effect on AgNWs to provide a guidance to control the nanowire welding effect, which can be used for film conductivity enhancement and applied for soft and bio-compatible electrodes.

Surfactant assisted electrochemical growth of ultra-thin CuSCN nanowires for inverted perovskite solar cell applications

In this report, a simple surfactant-assisted electrodeposition method is developed to obtain ultra-thin CuSCN nanowires with controlled thickness and diameter. The morphology and thickness of the CuSCN nanowires are controlled by changing the electrochemical potential and deposition time. The phase pure β-CuSCN is confirmed from XRD and Raman analysis. The packing fraction and diameter of the CuSCN nanowires are confirmed using FESEM micrographs. The formation of tetragonal shaped ultra-thin CuSCN nanowire is observed at 3 min deposition time for all the applied potentials. The chemical composition and oxidation state of the grown CuSCN nanowires are confirmed from XPS spectra. The maximum short circuit current of 21.09 mA/cm2 and power conversion efficiency of 16.99% is achieved for the thickness of 186 nm and packing fraction of 3067 nanowires/μm2. The low hysteresis and high charge transfer nature of the CuSCN nanowire boost the device efficiency more than 60% comparing with CuSCN nanowire grown at short deposition time and low applied potential. The large-area (0.75 cm2) device showed excellent stability up to 1400 h with CuSCN nanowires and carbon back contact. It is observed that the packing fraction and thickness of the CuSCN nanowire significantly influences the photovoltaic performance of the device. The present investigation provides a pathway for the application of CuSCN nanowire and carbon back contacts for efficient large-area solar cell fabrication.

Emergence and tunability of transmission gap in the strongly disordered regime of a dielectric random scattering medium.

Light transmission characteristics in a strongly disordered medium of dielectric scatterers, having dimensionalities similar to those of self-organized GaN nanowires, is analyzed employing finite difference time domain analysis technique. While photonic bandgap like transmission gaps have already been reported for several quasi-crystalline and weakly disordered media, the results of this work show that in spite of the lack of any form of quasi-crystallinity, distinct transmission gaps can be attained in a strongly disordered medium of dielectric scatterers. In fact, similar to the case of a two-dimensional photonic crystal, transmission gap of a uniform random medium of GaN nanowires can be tuned from ultra-violet to visible regime of the spectrum by varying diameter and fill-factor of the nanowires. Comparison of transmission characteristics of periodic, weakly disordered, correlated strongly disordered and uniform strongly disordered arrays having nanowires of identical diameters and fill factors suggest that in spite of the dominance of multiple scattering process, the underlying Mie and Bragg processes contribute to the emergence and tunability of transmission gaps in a strongly disordered medium. Without any loss of generality, the findings of this work offer significant design latitude for controlling transmission properties in the strong disorder regime, thereby offering the prospect of designing disorder based novel photonic and optoelectronic devices and systems.

Decoupling electron and phonon transport in single-nanowire hybrid materials for high-performance thermoelectrics.

Organic-inorganic hybrids have recently emerged as a class of high-performing thermoelectric materials that are lightweight and mechanically flexible. However, the fundamental electrical and thermal transport in these materials has remained elusive due to the heterogeneity of bulk, polycrystalline, thin films reported thus far. Here, we systematically investigate a model hybrid comprising a single core/shell nanowire of Te-PEDOT:PSS. We show that as the nanowire diameter is reduced, the electrical conductivity increases and the thermal conductivity decreases, while the Seebeck coefficient remains nearly constant-this collectively results in a figure of merit, ZT, of 0.54 at 400 K. The origin of the decoupling of charge and heat transport lies in the fact that electrical transport occurs through the organic shell, while thermal transport is driven by the inorganic core. This study establishes design principles for high-performing thermoelectrics that leverage the unique interactions occurring at the interfaces of hybrid nanowires.

Effects of Electrolyte Additives and Nanowire Diameter on the Electrochemical Performance of Lithium‐Ion Battery Anodes based on Interconnected Nickel–Tin Nanowire Networks

Tin‐based nanowire electrodes present desirable properties as lithium‐ion battery anodes, because they undergo volume changes without pulverization. However, they suffer from limited mass loading and propensity for surface parasitic reactions. Herein, the electrochemical performances of nickel–tin 3D‐interconnected nanowire network (NiSn 3DNWN) electrodes are evaluated with the nanowire diameters of 40, 105, and 230 nm, respectively, that attain mass loadings of up to 3 mg cm−2. To mitigate the surface parasitic reactions, the effects of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) additives are investigated as a function of the nanowire diameter and additive concentration. The results show that the FEC and VC of all compositions improve the capacity retentions and coulombic efficiencies (CEs) of the NiSn 3DNWN electrodes. In 10 vol% FEC, the electrodes demonstrate a similar capacity of ≈550 mAh g−1, but the capacity retentions after 100 cycles are 73.68%, 53.79%, and 51.70% for the 40, 105, and 230 nm NiSn 3DNWN, respectively. However, the 105 nm‐diameter nanowire electrode has the highest average CE of 96.55%. Electrochemical impedance spectroscopy and post‐cycling investigations reveal that the FEC has the most profound effect on the interfacial resistances, which is reflected in the rate performances of the tested electrodes.

Magnetic reversal modes in cylindrical nanostructures: from disks to wires

Cylindrical magnetic nanowires are key elements of fast-recording and high-density 3D-storage devices. The accurate tuning of the magnetization processes at the nanoscale is crucial for the development of future nano-devices. Here, we analyzed the magnetization of Ni nanostructures with 15–100 nm in diameter and 12–230 nm in length and compared our results with experimental data for periodic arrays. Our modelling led to a phase diagram of the reversal modes where the presence of a critical diameter (d ≈ 30 nm) triggered the type of domain wall (DW) formed (transverse or vortex); while a critical length (L ≈ 100 nm) determined the number of DWs nucleated. Moreover, vortex-DWs originated from 3D skyrmion tubes, reported as one of the best configurations for storage devices. By increasing the diameter and aspect-ratio of nanowires with L > 100 nm, three reversal modes were observed: simultaneous propagation of two vortex-DWs; propagation of one vortex-DW; or spiral rotation of both DWs through “corkscrew” mechanism. Only for very low aspect-ratios (nanodisks), no skyrmion tubes were observed and reversal occurred by spiral rotation of one vortex-DW. The broad range of nanostructures studied allowed the creation of a complete phase diagram, highly important for future choice of nanoscaled dimensions in the development of novel nano-devices.

Electrical Characteristics of Vertical GaN Nanowire Vacuum Field Emitter Devices

The electrical characteristics of simulated vertical gallium nitride (GaN) nanowire (NW) vacuum field emission diodes (VFEDs) and GaN NW vacuum field emission transistors (VFETs) with flat- and sharp-tip GaN NW field emitters (FEs) are presented by investigating their dependence on geometry and material parameters. For a flat-tip GaN NW VFED, the turn-on voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> ) was decreased by decreasing the GaN NW diameter ( D), the anode to emitter distance (d), the work function of the anode metal (φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> ), or by increasing the GaN NW height ( h). Variations in doping concentration (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ) of the GaN NW do not impact V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> . The sharp-tip GaN NW VFEDs exhibited decreased V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> as the degree of sharpness ( d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> ) was increased from 0 to 0.5 μm. The saturation emitter current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) was only affected by variations in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> , and was increased for increased N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> . The transfer (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> ) characteristics of sharp-tip VFETs exhibited reduced threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ), increased transconductance (G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tm, max</sub> ), and ON-/ OFF-current ratio, compared with the flat-tip ones. The output (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> ) characteristics of both types of devices showed nonlinear behavior at the low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> regime.

Ultralow Threshold, Single-Mode InGaAs/GaAs Multiquantum Disk Nanowire Lasers.

We present single-mode nanowire (NW) lasers with an ultralow threshold in the near-infrared spectral range. To ensure the single-mode operation, the NW diameter and length are reduced specifically to minimize the longitudinal and transverse modes of the NW cavity. Increased optical losses and reduced gain volume by the dimension reduction are compensated by an excellent NW morphology and InGaAs/GaAs multiquantum disks. At 5 K, a threshold low as 1.6 μJ/cm2 per pulse is achieved with a resulting quality factor exceeding 6400. By further passivating the NW with an AlGaAs shell to suppress surface nonradiative recombination, single-mode lasing operation is obtained with a threshold of only 48 μJ/cm2 per pulse at room temperature with a high characteristic temperature of 223 K and power output of ∼0.9 μW. These single-mode, ultralow threshold, high power output NW lasers are promising for the development of near-infrared nanoscale coherent light sources for integrated photonic circuits, sensing, and spectroscopy.

Fabrication and Characterization of CuO nanowires on V-shaped Microgroove surfaces

This study developed a facile and scalable thermal oxidation method to prepare CuO nanowires on the electro-discharge machining (EDM) processed V-shaped microgrooves. The formation feasibility, surface morphology, and wetting properties of nanowires on V-shaped microgrooves were explored by the variation of thermal oxidation temperatures from 300 to 600 °C, and oxidation times from 2 h to 8 h. Nanowires were found to successfully synthesized on the V-shaped microgrooves surfaces when annealing temperature was 400 °C or higher. The microvoids or microcavities on the EDM processed V-shaped microgrooves surfaces contributed to the stress grain boundary (GB) diffusion of copper atoms, and facilitated the growth of nanowires. The diameter of nanowires increased monotonically when the annealing temperatures increased, whereas it almost kept unchanged with increasing annealing times. The length of nanowires was influenced positively by both annealing temperature and annealing time. All the nanowire samples showed hydrophobic properties of water.

Vertical Sandwich GAA FETs With Self-Aligned High-k Metal Gate Made by Quasi Atomic Layer Etching Process

We presented and demonstrated a new type of vertical nanowire (NW) and nanosheet (NS) field-effect transistors (FETs), named vertical sandwich gate-all-around FETs or VSAFETs, which were formed with the process compatible in the main stream industry. The VSAFETs with self-aligned high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${k}$ </tex-math></inline-formula> metal gates (HKMGs) were fabricated with epitaxy of Si/SiGe/Si sandwich structure, an isotropic quasi-atomic-layer-etch (qALE) process, and gate replacement process. The gate-length of VSAFETs is mainly determined by the thickness of SiGe film grown by epitaxy. The NW diameter and NS thickness could be obtained by the isotropic qALE method, which can be used to the Si-selective etching of SiGe. As a result, p-type NS and NW VSAFETs with good device characteristics were fabricated. Device performance and the influence of silicide, Ge fraction, Si cap, and high thermal process were investigated; threshold voltage tuning and reliability were also discussed.

Structural manipulation of silver nanowire transparent conductive films for optoelectrical property optimization in different application fields

Silver nanowire films as a kind of bright flexible transparent conductive material are expected to have the sheet resistance as low as possible, the critically high optical transmittance and the wide range haze for different applications. Herein, we fabricated a series of silver nanowire transparent conductive films with different structural parameters and sheet resistances. By analyzing the relationship between the structural parameters and optical properties, it was found that the long and thick nanowires led to high transmittance and haze, while long and thin nanowires resulted in high transmittance and low haze. Experimental and theoretical results give instructions of designing silver nanowire films from structural manipulation for various applications in need of high haze, low haze with high transmittance or low haze with high conductivity. Through modification of the diameter, length and density of the silver nanowires, the transparent conductive films with sheet resistance below 75 ohm/square and transmittance above 80% were achieved, while haze at the wavelength of 550 nm could be controlled between 1% and 25%. This work provides insight for practical application of silver nanowires in optoelectronic devices.

Improving the yield of GaAs nanowires on silicon by Ga pre-deposition

GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor−liquid−solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume.

Influence of Colloidal Au on the Growth of ZnO Nanostructures.

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles.

Self-catalyst growth and characterization of wurtzite GaAs/InAs core/shell nanowires

Research on nanowire (NW) growth of III-V semiconductors including GaAs and InAs demonstrated the ability to grow in both wurtzite (WZ) and zincblende (ZB) polymorphs. However, the control of crystal phase in self-catalyzed NW growth is still a remaining challenge. In this study we report a controlled growth of GaAs/InAs core/shell nanowires in WZ phase using self-catalyzed molecular beam epitaxy (MBE). The GaAs NWs having pure WZ crystal were achieved and attributed to the effect of small wetting angle, which is realized by supplying high V/III ratio. Furthermore, the WZ formation is shown to be uninfluenced by NW diameter. For the wetting angle larger than 90°, mixed phase starts to be observed. The InAs shell is planarly grown on six m-plane facets of the GaAs core NWs leading to the same strain states along a and c axes of growth plane, in which tensile and compressive strains are observed for GaAs core and InAs shell, respectively. High-resolution transmission electron microscopy (HR-TEM) reveals a few misfit dislocations (~0.03 nm−1) at GaAs/InAs interface indicating insignificant strain relief via creating misfit dislocation. Electrical characterization of the hetero-wires shows that the InAs shell exhibits n-type conduction. A room-temperature sheet carrier concentration at zero gate-bias of 2.5 × 1012 cm−2 and the corresponding carrier mobility of ~ 900 cm2 V−1 s−1 are demonstrated.

Simulation of optical absorption in conical nanowires.

The optical absorptance from arrays of GaAs nanowires (NWs) was examined by the finite element method. Absorptance in cylindrical NWs, frustum nanocones (with base wider than the top) and inverted frustum nanocones (with top wider than the base) was compared. The introduction of higher order HE1n modes, the red-shift of the HE1n modes along the NW length due to NW tapering, and the red-shift of the modes due to increase of the overall NW diameter all contribute to a broadening of the absorption spectrum in conical NWs as compared to NWs with a constant diameter. The optical reflectance versus NW top diameter shows a minimum due to a balance between reflectance from the top of the NWs and reflectance from the substrate between NWs. The optimum geometry for photovoltaic energy conversion was determined from the total photocurrent. An optimum photocurrent of 26.5 mAcm-2 was obtained, corresponding to a conical NW morphology with base diameter of 200 nm, top diameter of 110 nm, and length of 2000 nm. An optimized inverse tapered conical morphology gave comparable performance.

Controlling plasmon propagation and enhancement via reducing agent in wet chemistry synthesized silver nanowires.

Silver nanowires with varying diameters and submillimeter lengths were obtained by changing a reducing agent used during hydrothermal synthesis. The control over the nanowire diameter turns out to play a critical role in determining their plasmonic properties, including fluorescence enhancement and surface plasmon polariton propagation. Advanced fluorescence imaging of hybrid nanostructures assembled of silver nanowires and photoactive proteins indicates longer propagation lengths for nanowires featuring larger diameters. At the same time, with increasing diameter of the nanowires, we measure a substantial reduction of fluorescence enhancement. The results point at possible ways to control the influence of plasmon excitations in silver nanowires by tuning their morphology.

Design and Modeling of High-Efficiency GaAs -Nanowire Metal-Oxide-Semiconductor Solar Cells beyond the Shockley-Queisser Limit: An NEGF Approach

The present work proposes a $\mathrm{Ga}\mathrm{As}$-nanowire-based vertical metal-oxide-semiconductor (MOS) solar cell of quantum scale to achieve very high efficiency beyond the Shockley-Queisser (SQ) limit. Photogeneration and carrier transport in such devices are analytically modeled by adopting nonequilibrium Green's function formalism based on second quantization field operators for the incident photons and generated photocarriers. The study suggests that the utilization of photogenerated light and heavy holes to harvest solar energy is capable of providing significantly higher power conversion efficiency above the SQ limit. Such superior efficiency is achieved due to the resonance of incident photon modes with the energy gap between three-dimensional-quantized electron states and two-dimensional-quantized hole subbands. The power conversion efficiency, along with other relevant solar-cell-performance parameters, such as open-circuit voltage, short-circuit current, fill factor, external quantum efficiency, and responsivity, is observed to depend significantly on the nanowire diameter and top-oxide thickness, which, in turn, controls the quantization effect in such MOS devices. The results show that the power conversion efficiency of 50% and above can be achieved in the present tosylate-modified poly(3,4-ethylenedioxythiophene) (PEDOT-Tos)/${\mathrm{Si}\mathrm{O}}_{2}/\mathrm{Ga}\mathrm{As}$-nanowire MOS solar cell for a combination of nanowire diameter and oxide thickness in the range of 18--14 nm and 4--2 nm, respectively. Thus, the proposed device scheme offers an alternative design route for next-generation solar cells with superior efficiency by engineering the quantization effect.