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/cm(2) 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/cm(2) (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.

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.

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.

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Semiconductor nanowires (NWs) are sub-wavelength structures which exhibit strong optical (Mie) resonances in the visible range. In addition to such optical resonances, the localized surface plasmon resonances (LSPRs) in metal-semiconductor core-shell (CS) and core-multishell (CMS) NWs can be tailored to achieve novel negative-index metamaterials (NIM), extreme absorbers, invisibility cloaks and sensors. Particularly, in this review, we focus on our recent theoretical studies which highlight the versatility of CS and CMS NWs for: 1) the design of negative-index metamaterials in the visible range and 2) plasmonic light harvesting in ultrathin photocatalyst layers for water splitting. Utilizing the LSPR in the metal layer and the magnetic dipole (Mie) resonance in the semiconductor shell under transverse electric (TE) polarization, semiconductor-metal-semiconductor CMS NWs can be designed to exhibit spectrally overlapping electric and magnetic resonances in the visible range. NWs exhibiting such double resonances can be considered as meta-atoms and arrayed to form polarization dependent, low-loss NIM. Alternatively, by tuning the LSPR in the TE polarization and the optical resonance in the transverse magnetic (TM) polarization of metal-photocatalyst CS and semiconductor-metal-photocatalyst CMS NWs, the absorption within ultrathin (sub-50 nm) photocatalyst layers can be substantially enhanced. Notably, aluminum and copper based NWs provide absorption enhancement remarkably close to silver and gold based NWs, respectively. Further, such absorption is polarization independent and remains high over a large range of incidence angles and permittivity of the medium. Therefore, due to the tunability of their optical properties, CS and CMS NWs are expected to be vital components for the design of nanophotonic devices.