# SnSe1‐xSx Alloys: Anisotropic Van der Waals Semiconductors with Tunable Bandgaps

## TL;DR

This paper explores SnSe1-xSx alloys, showing how their properties can be tuned for use in various electronic applications.

## Contribution

The study demonstrates the synthesis and tunable properties of SnSe1-xSx alloys using a mixed powder precursor.

## Key findings

- SnSe1-xSx alloys show full miscibility and tunable lattice constants and bandgaps.
- Anisotropic vibrational modes and electronic structure are confirmed through polarized Raman spectroscopy.
- The alloys maintain non-degenerate direct valleys for valleytronics applications.

## Abstract

Alloying is one of the main tools of bandgap engineering, allowing the tuning of crystal structure, lattice parameters, and electronic structure of 3D and 2D/layered semiconductors. Among the latter, it can play a key role in tailoring the properties of tin monochalcogenides, a class of van der Waals semiconductors of interest for optoelectronics, thermoelectrics, ferroelectrics, and valleytronics. Here, the study investigates the synthesis and properties of large flakes of the anion substitution alloys SnSe1‐xSx. Alloy flakes across a wide range of compositions are obtained systematically by repeated growth from the same mixed (SnS, SnSe) powder precursor. Combined experiment and theory show full miscibility for all compositions, along with tunable lattice constants, bandgaps, and vibrational modes. Atomic resolution imaging demonstrates the accumulation of S and Se in alternating layers in the SnSe1‐xSx unit cell, attributed to growth kinetics. Polarized Raman spectroscopy confirms anisotropic vibrational modes; the calculated and measured band structure shows systematic changes in the band edge energies and anisotropic electronic structure due to the anisotropic in‐plane lattice of the monochalcogenides. Cathodoluminescence, finally, indicates that a unique configuration of two non‐degenerate, direct valleys along orthogonal k‐space directions persists all the way from SnS to SnSe, making SnSe1‐xSx alloys interesting for valleytronics.

Consecutive growths from a mixed (SnS‐SnSe) powder precursor produced series of samples of large, single‐crystalline SnSe1‐xSx alloy flakes with systematically varying composition. Advanced experiments on such samples, combined with theoretical calculations, enabled the comprehensive investigation of the crystal structure, vibrational modes, band structure, and optoelectronic properties of anisotropic SnSe1‐xSx alloys, showing promising properties for optoelectronics, ferroelectrics, thermoelectrics, and valleytronics.