Highly Anisotropic Mechanical and Optical Properties of 2D Layered As₂S₃ Membranes

Abstract

Two-dimensional (2D) materials with strong in-plane anisotropy are of interest for enabling orientation-dependent, frequency-tunable, optomechanical devices. However, black phosphorus (bP), the 2D material with the largest anisotropy to date, is unstable as it degrades in air. In this work we show that As₂S₃ is an interesting alternative, with a similar anisotropy to bP, while at the same time having a much higher chemical stability. We probe the mechanical and optical anisotropy in As₂S₃ by three distinct angular-resolved experimental methods: Raman spectroscopy, atomic force microscopy (AFM), and resonance frequency analysis. Using a dedicated angle-resolved AFM force-deflection method, an in-plane anisotropy factor of [Formula: see text] is found in the Young's modulus of As₂S₃ with <i>E</i>_{<i>a</i>-axis} = 79.1 ± 10.1 GPa and <i>E</i>_{<i>c</i>-axis} = 47.2 ± 7.9 GPa. The high mechanical anisotropy is also shown to cause up to 65% difference in the resonance frequency, depending on crystal orientation and aspect ratio of membranes.