Optical Properties of Zinc Sulfide Nanopowders and ZnS/Ag 2 S Heteronanostructures
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Optical Properties of Zinc Sulfide Nanopowders and ZnS/Ag2S Heteronanostructures S. I. Sadovnikova, * and I. D. Popova a Institute
of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620990 Russia *e-mail: [email protected] Received May 12, 2020; revised June 3, 2020; accepted June 9, 2020
Abstract—Nanopowders of zinc sulfide (ZnS) and ZnS/Ag2S heteronanostructures were synthesized by chemical deposition from aqueous solutions. A change in the ratio between the concentrations of reagents allowed us to obtain ZnS nanopowders with a particle mean size from 2 to 10 nm. The Ag2S nanoparticle size in the resulting heteronanostructures was 9–30 nm and the ZnS surface layer thickness was equal to 4–5 nm. The diffuse reflectance spectra for nanostructured ZnS and ZnS/Ag2S heteronanostructures were measured. Based on the analysis of the obtained spectra, the bandgap energy Eg in the studied sulfide nanostructures was estimated. Upon a decrease in the nanoparticle size from 10 to 2 nm, the Eg value in ZnS nanopowders increased in a range of 3.17–3.36 eV. An increase in the content of Ag2S in the ZnS/Ag2S heteronanostructures led to a decrease in the bandgap energy and a decrease in the luminescence intensity. Keywords: zinc sulfide, silver sulfide, heteronanostructure, optical absorption, bandgap DOI: 10.1134/S1063783420110268
INTRODUCTION Zinc (ZnS) and silver (Ag2S) sulfides are widely used semiconductor compounds [1–9]. They are applied in IR sensors and detectors, lasers, luminophores, solar cells, light-emitting diodes, photochemical cells, catalysts, resistance switchers, and nonvolatile memory storage devices [10–17]. The bandgap energy Eg of cubic ( F 43m space group) α-ZnS having a sphalerite structure (B3 type) is 3.50–3.76 eV [9]. The exciton diameter in bulk zinc sulfide is 4.8–5.2 nm [9]. A low-temperature semiconductor phase of silver sulfide exists below 450 K and has a monoclinic (P21/c space group) acanthitetype structure (α-Ag2S). The bandgap energy of acanthite-structure silver sulfide α-Ag2S is 0.9–1.1 eV and the exciton diameter is 2.8 nm [5–8]. An increase in the bandgap energy of nanostructured sulfide and heteronanostructures is observed upon a decrease in the nanoparticle size and in the design of sulfide composites of two different metals [1, 2, 9]. A decrease in the nanoparticle size to the value less than that of exciton allows one to control the bandgap energy. The exciton diameter for ZnS and Ag2S is small (~5.0 and ~2.8 nm, respectively); therefore, it is difficult to obtain stable nanoparticles with such size. The preparation of semiconductor nanocomposites consisting of zinc and silver sulfide nanoparticles is a more simple method. Ideally, the bandgap energy of heteronanostructures based on
ZnS and Ag2S can vary from 1.0 to 4.0 eV, which corresponds to a change in the position of optical absorption edge from IR to near-UV range. The development of such heteronanostructures will allow one to extend the spectral sensitivity of sulfide semicond
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