Design of Contact Electrodes for Semiconductor Nanowire Solar Energy Harvesting Devices

Transparent, low-resistive contacts are critical for efficient solar energy harvesting devices. It is important to reconsider the material choices and electrode design as devices move from 2D films to 1D nanostructures. In this paper, we study the effectiveness of indium tin oxide (ITO) and metals, such as Ag and Cu, as contacts in 2D and 1D systems. Although ITO has been studied extensively and developed into an effective transparent contact for 2D devices, our results show that effectiveness does not translate to 1D systems. Particularly with consideration of resistance requirement, nanowires with metal shells as contacts enable better absorption within the semiconductor as compared to ITO. Furthermore, there is a strong dependence of contact performance on the semiconductor band gap and diameter of nanowires. We found that metal contacts outperform ITO for nanowire devices, regardless of the sheet resistance constraint, in the regime of diameters less than 100 nm and band-gaps greater than 1 eV. These metal shells optimized for best absorption are significantly thinner than ITO, which enables for the design of devices with high nanowire number density and consequently higher device efficiencies

Plasmonic Core–Multishell Nanowire Phosphors for Light-Emitting Diodes

White LEDs have replaced traditional incandescent lamps in many places. They are extensively studied to increase the efficiency to even greater values. In this work, we proposed and studied theoretically core–shell metal–semiconductor nanowires as phosphor components in white LEDs. Due to the coupling of the optical density of states to the surface plasmon resonance in the metal, the emission in the core–shell nanowires studied demonstrated a 5-fold enhancement for red phosphors and a 120-fold enhancement for green phosphors, compared to the bare semiconductor nanowires. Due to the coupling of the plasmon resonance oscillations at the metal surface with the electric fields of the incident light, the studied core–shell nanowires also show an absorbance of 0.6–0.9 for blue light compared to the absorbance of 0.2–0.4 observed in the core–shell quantum dots. We have predicted that the external quantum efficiency (EQE) can be enhanced by almost 11 times for red phosphors, by 36 times for yellow phosphors, and as high as 4 orders of magnitude for the green phosphors relative to the bare semiconductor nanowires, when carefully choosing the semiconductor and metal materials and dimensions. Finally, we have also explored the concept of core–shell–shell nanowires and have shown that these nanowires improve values of the EQE values by as much as 60% relative to the core–shell nanowires for red phosphors and 3 times for yellow phosphors, due to the addition of another enhanced electric field from the semiconductor core to the Purcell factor

Aluminum Plasmonics for Enhanced Visible Light Absorption and High Efficiency Water Splitting in Core–Multishell Nanowire Photoelectrodes with Ultrathin Hematite Shells

The poor internal quantum efficiency (IQE) arising from high recombination and insufficient absorption is one of the critical challenges toward achieving high efficiency water splitting in hematite (α-Fe₂O₃) photoelectrodes. By combining the nanowire (NW) geometry with the localized surface plasmon resonance (LSPR) in semiconductor–metal–metal oxide core–multishell (CMS) NWs, we theoretically demonstrate an effective route to strongly improve absorption within ultrathin (sub-50 nm) hematite layers. We show that Si–Al–Fe₂O₃ CMS NWs exhibit photocurrent densities comparable to Si–Ag–Fe₂O₃ CMS and outperform Fe₂O₃, Si–Fe₂O₃ CS and Si–Au–Fe₂O₃ CMS NWs. Specifically; Si–Al–Fe₂O₃ CMS NWs reach photocurrent densities of ∼11.81 mA/cm² within a 40 nm thick hematite shell which corresponding to a solar to hydrogen (STH) efficiency of 14.5%. This corresponds to about 93% of the theoretical maximum for bulk hematite. Therefore, we establish Al as an excellent alternative plasmonic material compared to precious metals in CMS structures. Further, the absorbed photon flux is close to the NW surface in the CMS NWs, which ensures the charges generated can reach the reaction site with minimal recombining. Although the NW geometry is anisotropic, the CMS NWs exhibit polarization independent absorption over a large range of incidence angles. Finally, we show that Si–Al–Fe₂O₃ CMS NWs demonstrate photocurrent densities greater than ∼8.2 mA/cm² (STH efficiency of 10%) for incidence angles as large as 45°. These theoretical results strongly establish the effectiveness of the Al-based CMS NWs for achieving scalable and cost-effective photoelectrodes with improved IQE, enabling a novel route toward high efficiency water splitting

Fabrication of Sub-25 nm Diameter GaSb Nanopillar Arrays by Nanoscale Self-Mask Effect

GaSb individual nanowires and nanowire arrays are considered as intriguing candidates for electronic and photonic applications. In this paper, we report a new mask-free method to fabricate large area GaSb nanopillar arrays through reactive ion etching of GaSb substrates facilitated by O₂ plasma. We have shown that nanoscale oxide self-masks could form thereby facilitating the formation of GaSb nanopillars. We have achieved GaSb nanowires with diameters less than 25 nm and an aspect ratio of 24. Additionally, GaSb nanopillar arrays with desired heights, diameters, and density can be obtained by choosing the plasma chemistry and/or controlling etching parameters, such as bias power and pressure. The nanopillar arrays prepared also exhibit tunable broadband antireflection properties