Thickness-Dependent Optical Properties and In-Plane Anisotropic Raman Response of the 2D β-In₂S₃

Recent years have witnessed major advances in development of massive nonlayer structured ultrathin materials, providing great enrichment to the 2D nanomaterial family. The intriguing physical and chemical properties brought by nonlayered nanomaterials have attracted tremendous research interest. In this work, a systematica study of the optical properties of 2D nonlayered β-diindium trisulfide (β-In₂S₃) is reported. The thickness-dependent photoluminescence (PL), Raman spectra, and absorption property are measured for ultrathin β-In₂S₃ flakes and found distinguished from its bulk counterpart. These peculiar features originate from the superficial indium oxide in ultrathin β-In₂S₃, as revealed by low-temperature PL and X-ray photoelectron spectroscopy measurement. The Raman vibration modes are identified by Raman spectra measurements combined with calculations using density functional theory. Importantly, the tetragonal β-In₂S₃ flakes exhibit strong in-plane anisotropic Raman response under the angle-resolved polarized Raman spectroscopy measurements. The results in this paper provide an in-depth understanding of the emerging 2D nonlayered material In₂S₃ and pave a fundamental step for its potential applications in future electronics and optoelectronics.