Thickness-dependent Raman active modes of SnS thin films

Itsuki Yonemori,Katsunori Wakabayashi,Kosuke Nagashio,Sudipta Dutta

doi:10.1063/5.0062857

Abstract

Tin sulfide (SnS) thin films have been reported to show strong layer number dependence on their ferroelectricity and Raman spectra. Identifying the number of layers and stacking structures is crucial for optoelectronic device fabrication. Here, we theoretically study the electronic and phononic properties of SnS thin films using first-principles calculations. We identify the characteristic Raman active phonon modes and their dependence on the number of layers and stacking sequences. The clear separation between surface modes and bulk modes is clarified for SnS thin films. In addition, we have clarified the relation between stacking structures and Raman active modes for bilayer SnS. Our results will serve the experimental characterization of such thin monochalcogenide systems through Raman spectra and will expedite their device fabrication.

Highlights

Ferroelectric materials are the fundamental building blocks for the application of nonvolatile memory, sensor, and nonlinear optoelectronic devices.1–19 In order to create transparent and flexible devices, it is necessary to search materials that can retain their physical properties even when made into thin films.20–24 Recently, materials with ferroelectricity in the out-of-plane direction (MoTe2,1,25 WTe2,2,25 and CuInP2S63,11,12,14) and the in-plane/outof-plane direction (α − In2Se34–9,13,26 and SnTe10) have been studied
Higashitarumizu et al reported that SnS can maintain the ferroelectricity in the in-plane direction even with a few thin layers due to the asymmetry in the crystal structure, unlike three-dimensional ferroelectric materials
We perform first-principles calculations based on density functional theory as implemented in the Vienna Abinitio Simulation Package (VASP)41 to perform structural optimization and to investigate the electronic states and phonon modes

Summary

INTRODUCTION

Ferroelectric materials are the fundamental building blocks for the application of nonvolatile memory, sensor, and nonlinear optoelectronic devices. In order to create transparent and flexible devices, it is necessary to search materials that can retain their physical properties even when made into thin films. Recently, materials with ferroelectricity in the out-of-plane direction (MoTe2,1,25 WTe2,2,25 and CuInP2S63,11,12,14) and the in-plane/outof-plane direction (α − In2Se34–9,13,26 and SnTe10) have been studied. In order to create transparent and flexible devices, it is necessary to search materials that can retain their physical properties even when made into thin films.. Higashitarumizu et al reported that SnS can maintain the ferroelectricity in the in-plane direction even with a few thin layers due to the asymmetry in the crystal structure, unlike three-dimensional ferroelectric materials.. For transparent and flexible electronic device fabrication, precise control over the number of layers is necessary. From an experimental point of view, Raman spectroscopy is a useful tool to analyze the number of layers and stacking geometry by precise detection of phonon vibration modes of samples. We report on the electronic structures and phonon vibration modes of bulk and single- and multi-layered structures of SnS as obtained from the first-principles calculations. The numerical data of phonon frequency for bulk and few-layer SnS are given

METHOD

BULK AND MONOLAYER STRUCTURES

FEW-LAYER STRUCTURE

BILAYER STRUCTURE

SUMMARY