Highly Reflective Periodic Nanostructure Based on Thermal Evaporated Tungsten Oxide and Calcium Fluoride for Advanced Photonic Applications

Abstract

In this paper, we report the fabrication and characterization of high-quality distributed Bragg reflectors (DBRs) deposited by low-energetic thermal evaporation. This technique allows deposition of high-quality thin films with an accurate control of thickness at the nanoscale. We investigated, for the first time, the use of tungsten oxide (WO3) and calcium fluoride (CaF2) as high (2.15) and low (1.4) refractive index materials, respectively, for DBR fabrication. They consist of nine pairs of WO3/CaF2 layers, with a central wavelength λc tuned at 640 nm and a maximum reflectance of 99.6%. A passive microcavity consisting of a λ/2 thick spacer of CaF2 sandwiched between 8.5 pairs of WO3/CaF2 has also been fabricated. We investigated the optical behavior of this microresonator after post-deposition thermal treatment at different temperatures, showing a blue shift of the resonant mode and an increase in its intensity. Indeed, heat treatment induced a compaction of the DBR multilayer, leading to more defined and clearer interfaces, as demonstrated by scanning electron microscopy cross section measurements. This behavior results in an improvement of the microcavity quality factor (up to 400) as the temperature increases, which is currently the highest value obtained for microcavities obtained by thermal evaporation processes, so far. These results represent a further step forward toward the development of high-reflectivity mirrors and high-quality microresonators fabricated by the low-energy deposition technique. Such a structure could find extensive application in nano-photonics, optoelectronics, and sensors, even based on soft organic materials.

Engineering Hexagonal/Monoclinic WO3 Phase Junctions for Improved Electrochemical Hydrogen Evolution Reaction

“…4,16,17 These negative aspects can be strongly reduced by an effective strategy using nanostructures, with high surface-to-volume ratio and low resistance to further improve the electrochemical properties, of WO 3 as cathodes for HER. WO 3 nanostructures can be effectively synthesized by hydrothermal, 18 sputtering, 19 thermal evaporation, 20 sol−gel, 21 and electrodeposition 22 methods. Nevertheless, in its pristine form, nanostructured WO 3 does not give excellent HER performances, as the atomic hydrogen adsorption free energy (ΔG H ) on the W site is high, leading to a poor HER activity, as explained by Sabatier's principle, for which ΔG H close to zero gives better catalytic performances.…”

“…Indeed, WO 3 is an n- type semiconductor with abundant reserves, high electrochemical stability in acidic environments, tunable band gap, and good proton conduction; unfortunately, as a semiconductor, it has poor electron transport ability and few active sites hindering high HER performances. ,, These negative aspects can be strongly reduced by an effective strategy using nanostructures, with high surface-to-volume ratio and low resistance to further improve the electrochemical properties, of WO 3 as cathodes for HER. WO 3 nanostructures can be effectively synthesized by hydrothermal, sputtering, thermal evaporation, sol–gel, and electrodeposition methods. Nevertheless, in its pristine form, nanostructured WO 3 does not give excellent HER performances, as the atomic hydrogen adsorption free energy (Δ G H ) on the W site is high, leading to a poor HER activity, as explained by Sabatier’s principle, for which Δ G H close to zero gives better catalytic performances. ,, Many efforts have been made with the aim of modulating the WO 3 electronic structure, such as decoration with Pt clusters, , the embedding of W-based compounds on conductive supports such as reduced graphene oxide (RGO) and carbon nanotubes (CNTs), and the realization of heterostructures by coupling WO 3 with other transition-metal oxides. − Although these solutions are effective for enhancing the HER activity, their exploitation on a large scale is limited by the complex synthesis and assembly processes.…”

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

“…The precursors are separately heated in effusion cells until sublimation, after which the gaseous elements condense and react on the deposition substrate. MBE has been used to grow high-quality epitaxial single crystal dielectrics such as CaF 2 − and SrTiO 3 , that can form quasi vdW interfaces with 2D TMDs and serve as back gate dielectrics. While high-quality films can be produced, epitaxial growth typically require specially prepared substrates, and its direct, pinhole-free growth on 2D materials remains challenging.…”

A Transparent Nano-Polycrystalline ZnWO4 Thin-Film Scintillator for High-Resolution X-ray Imaging

“…In contrast, ZnWO 4 has lower emission intensity than recently reported inorganic scintillating and perovskite materials. , The perovskite scintillators are severely restricted due to their vulnerability to moisture and heat. , Furthermore, because ZnWO 4 has a much lower melting point than other inorganic scintillating materials, it can be efficiently used in the thermal evaporation method. , Additionally, ZnWO 4 has a high density (ρ = 7.87 g/cm 3 ), short decay time, high stability, and low cost. ,− Since ZnWO 4 has a high X-ray absorption capability, , it can be used in thin-film scintillators. There is a growing body of research on fabricating transparent ceramic thin films using thermal evaporation with sintering. − The thermal evaporation method is an efficient approach for conveniently depositing thin and uniform films on complex structures. − The main purpose of this study was to demonstrate that nano-polycrystalline ZnWO 4 thin-film scintillators can be utilized in high-resolution X-ray imaging. In addition, the proposed method can be used to fabricate thin-film scintillators with various shapes, thereby demonstrating its potential for use in different high-resolution X-ray imaging applications.…”