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

Abstract

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.

Memtransistors Based on Non-Layered In2S3 Two-Dimensional Thin Films With Optical-Modulated Multilevel Resistance States and Gate-Tunable Artificial Synaptic Plasticity

“…The binding energies for In3d5/2 and In3d3/2 are 443.2 and 450.7 eV with a ΔE of 7.5 eV, while those of S2p3/2 and S2p1/2 are 160.5 and 161.7 eV with a ΔE of 1.2 eV, which agrees well with results from the previously reported β-In2S3 [27], [28]. The above characterizations reveal the pure tetragonal phase of β-In2S3 and agree well with the results of 2D β-In2S3 flakes synthesized by similar PVD approach in our previous study [24].…”

“…For 2D In2S3 nanosheet, the surface is abundant with dangling bonds that mostly formed by vacancies. These vacancies in β-In2S3 include tetrahedrally coordinated S vacancies, octahedrally coordinated In vacancies, In interstitial on the position of a tetrahedral structural vacancy, and In vacancies in both octahedral and tetrahedral positions, which have been theoretically predicted and experimentally investigated in previous studies [24], [34], [35]. The results of gate-tunable properties of the memtransistors (presented and discussed later) suggest the migration of positive charge defect on the surface of In2S3 2DTFs.…”

“…Optical characterization might be an effective approach to distinguish 2D material from its bulk counterpart. However, experimental results of our previous work [24] and an earlier paper [23] have both demonstrated that Raman peak position of β-In2S3 does not shift as its thickness reduces from bulk to few-layer. Also, there are no experimental evidences so far that the bandgap energy of β-In2S3 is thickness-dependent.…”

“…Previous studies of β-In2S3 single crystal have reported a bandgap about 1.94 eV and a number of vacancy-related energy states near the band edges [24,34], resulting in ultrasensitive optical response to light from visible and near-infrared [23,25]. In this regard, we conducted a test of resistance states in response to the illumination of laser light in 405, 532, 635 and 808 nm.…”

“…β-In2S3 is an emerging 2D nonlayered material with medium bandgap and unique defective structure [23]. Our previous studies have revealed the unique thicknessdependent optical properties of 2D β-In2S3 and the ultrasensitive broad-range optical response of In2S3 based van der Waals heterostructure [24], [25]. In this letter, a highquality β-In2S3 analogue of van der Waals thin films was grown through physical vapor deposition (PVD) method.…”