Indium selenide monolayer: a two-dimensional material with strong second harmonic generation

Abstract

Because of their unique physical properties, two-dimensional (2D) materials have attracted extensive interest as potential candidates for next generation electrical and optoelectronic applications; in particular, large optical second harmonic generation (SHG) has been recently discovered in a number of 2D materials, which allows them to have valuable applications in electro-optic modulators and switches, frequency conversion, and so forth. Here, we performed a detailed first-principles study on the geometries, electronic structures and SHG properties of a recently prepared In₂Se₃ monolayer. Our results reveal that In₂Se₃ with a single-layered structure may be a material with the strongest SHG response among IIIA–VIA semiconductors reported to date, and the maximum magnitude of the static SHG coefficient is predicted to be 208.8 pm V⁻¹. The extraordinary SHG is strongly correlated with a special arrangement of two kinds of In–Se polyhedra, namely, [InSe₆]⁹⁻ octahedra and [InSe₄]⁵⁻ tetrahedra, on the opposite sides of the In₂Se₃ monolayer. Further investigations on other In–Se compounds with different structures and stoichiometries indicate that besides the configuration of the In–Se building block, the SHG response can be significantly influenced by the interlayer interactions, and the magnitude of the SHG coefficient of the In₂Se₃ monolayer decreases gradually as the interlayer distance decreases until it approaches that of In₂Se₃ bulk in the R3m phase. Our present findings provide a deep understanding of the SHG properties in IIIA–VIA 2D semiconductors and are also helpful for designing new 2D materials with tunable SHG susceptibility.