Anodic aluminum oxide (AAO) templates are emerging as a platform for simple, costeffective, high-throughput top-down nanofabrication of regular arrays of nanostructures. Thus far, however, AAO pattern transfer has largely been restricted to smooth and chemically inert surfaces, mostly Silicon substrates. Here, we present a more generalizable strategy for preparing free-standing through-hole ultrathin alumina membranes (UTAMs) and transferring them to both smooth and rough substrates, thereby enabling the fabrication of centimeter-scale arrays of nanostructures with sub-100 nm feature diameters on almost arbitrary substrates. To validate the utility of our procedures, we transferred UTAMs to surfaces relevant for photocatalytic applications and prepared plasmonic photocathodes consisting of dense arrays of size-controlled sub-100 nm Au and Ni nanodots on top of chemically non-inert NiO x thin films. To demonstrate the functionality of the fabricated structures, we used a plasmonic photocathode consisting of an array of sub-50 nm Au nanodots on NiO x /Al substrates to drive direct, plasmon-enhanced photoelectrocatalysis and found excellent device performance. We also successfully decorated very rough fluorine-doped tin oxide substrates with an array of high-density sub-100 nm nanodots. Our results extend the opportunities for AAO masks to serve as generic templates for novel applications that were previously prohibited by lack of methods to transfer to the required substrate.

Graphene is a two-dimensional material with intriguing electrical and optical properties for infrared photonic devices. However, single layer graphene (SLG) suffers from a very low optical absorption of ∼1–2% depending on the substrate, which significantly limits its efficiency as photonic devices. In this Letter, we address this challenge by coating SLG with self-assembled, pseudoperiodic ultrahigh refractive index (n = 8–9 at λ = 1600–5000 nm) semimetal Sn nanostructures for highly effective, broad-band infrared photon management in SLG, offering a new approach for light trapping beyond plasmonics and high refractive index dielectric photonics. The infrared absorption in SLG on fused quartz (SiO2) is greatly increased from <1.5% to >15% in a very broad spectral range of λ = 900–2000 nm due to the near-field electromagnetic interactions between the ultrahigh refractive index Sn nanostructures and SLG, a significant advantage over relatively narrow-band plasmonic resonances for photon management in SLG. The optical absorption enhancement in SLG has also been confirmed by field-enhanced Raman peaks from SLG and supported by higher photoconductivities both at an infrared wavelength of λ = 1550 nm and at a visible wavelength of λ = 650 nm. This work also opens the door to the investigations of ultrahigh refractive index semimetal nanostructures for nanoscale photon management.

In recent years, Ge and Ge 1−x Sn x materials and devices have achieved rapid progress in integrated photonics. However, conventional heteroepitaxy of active photonic devices compromises the area on Si for CMOS electronics, limiting the scale of integration. Furthermore, it is not possible to grow GeSn epitaxially on amorphous and/or flexible substrates toward 3D photonic integration in mid-infrared (MIR) regime. Here, we present low-temperature crystallization of direct bandgap, high crystallinity Ge 1−x Sn x (0.08 < x < 0.26) on amorphous dielectrics insulators (GeSnOI) toward 3D and flexible MIR integrated photonics. Utilizing eutectically-enhanced crystallization (EEC), an extraordinarily large average grain size of ∼100 µm has been achieved in blanket GeSn films crystallized on SiO 2 layers, flexible glass, and polyimide substrates alike. Furthermore, using Sn nanodot enhanced composition enhancement (NICE), we have achieved an average Sn composition as high as 26 at.% to further extend the optical response of GeSn toward λ = 3-5 µm. The achieved Sn composition of 8-26 at.% far exceeds that of the equilibrium solubility limit of <1 at.%, even though the crystallization temperature of 350-450 • C far exceeds the typical epitaxial growth temperature of GeSn. This result indicates that crystallization from amorphous GeSn (a-GeSn) may offer better metastability compared to direct epitaxial growth of GeSn. Attesting to the high crystallinity, a peak optical gain of 2,900 cm −1 with a lifetime approaching 0.1 ns is achieved at λ = 2,200-2,350 nm at 300 K. The gain lifetime is on the same order as epitaxial GeSn, and it is >100x longer than the direct gap transition in Ge, confirming the indirect-to-direct band gap transition in GeSn at ∼9 at. Sn composition. Moreover, a prototype p-GeSn/n-Si photodiode from a-GeSn crystallization achieves 100 mA/W responsivity at λ = 2,050 nm and T = 300 K, approaching the level of some commercial PbS detectors. The device also demonstrates photovoltaic behavior and a low dark current density of 1 mA/cm 2 at −1 V reverse bias, comparable to epitaxial Ge/Si photodiodes. These results indicate that crystallization of GeSnOI offers a promising solution for active devices toward 3D MIR photonic integration and/or MIR photonics on flexible substrates.

We present a systematic approach for the design and optimization of nanoparticle-pigmented solar selective absorbers for operation at 750 °C. Using the scattering and absorption cross-sections calculated by Lorenz-Mie scattering theory as input, we employ a four-flux radiative transfer method to investigate the solar selectivity mechanism and optimize the optical-to-thermal conversion efficiency (ηtherm) as a function of the metallic nanoparticle material, the nanoparticle diameter, the volume fraction, and the coating thickness. Among the nanoparticle material candidates in this study, C54-TiSi2 is the best option with an optimized ηtherm = 87.0% for a solar concentration ratio of C = 100 and ηtherm = 94.4% for C = 1000 at 750 °C. NiSi is also a promising candidate comparable to TiSi2 in thermal efficiency. Experimentally, an un-optimized 200 nm-diameter TiSi2 nanoparticle-silicone solar selective coating has already achieved ηtherm = 89.8% for C = 1000 at 750 °C. This performance is consistent with the theoretical model and close to the thermal efficiency of the commercial Pyromark 2500 coatings (90.1%). We also demonstrate that Ni/NiSi core-shell structures embedded in the SiO1.5 matrix is thermally stable at 750 °C for 1000 h in air. These results indicate that silicide cermet coatings are promising to achieve high optical performance and high temperature thermal stability simultaneously.