Thickness-dependent mechanical, electronic, and optical properties of λ-Cu2Se and ζ-Cu2Se: First-principles calculations

Abstract

The physical properties of two-dimensional materials are usually influenced by the number of layers. Recently, experimentally fabricated semiconducting Cu2Se monolayers have emerged as intriguing materials with outstanding photoelectric characteristics. In this work, the thickness-dependent mechanical, electronic, and optical properties of λ-Cu2Se and ζ-Cu2Se are systematically investigated based on first-principles calculations. The Young's moduli of both λ-Cu2Se and ζ-Cu2Se increase gradually as the number of layers increases, while Poisson's ratios barely change. Both λ-Cu2Se and ζ-Cu2Se maintain semiconducting properties which band gaps decrease with increasing thickness. Various bandgaps (0.38∼1.33 eV) and CBM/VBM energy levels (-4.70∼-3.91/-5.26∼-5.08 eV) are very promising for designing Cu2Se-based electronic and optoelectronic devices. Meanwhile, the absorption coefficients increase gradually for both λ-Cu2Se and ζ-Cu2Se from ML to bulk, with the highest values in bulk. Both λ-Cu2Se and ζ-Cu2Se have sizable optical absorptions within the visible and ultra-violet ranges. Our finds can provide references for their further device applications.