Engineering two-dimensional materials from single-layer NbS$_2$

TL;DR

This study demonstrates the synthesis and characterization of two novel two-dimensional NbS$_2$-derived materials, Nb$_{5/3}$S$_3$-2D and Nb$_2$S$_3$-2D, from a single-layer NbS$_2$ using different pathways, revealing unique structures influenced by surface effects.

Contribution

It introduces new methods to create phase-pure 2D NbS$_2$-derived materials and provides detailed structural insights through combined experimental and theoretical analysis.

Findings

Successful synthesis of Nb$_{5/3}$S$_3$-2D and Nb$_2$S$_3$-2D from single-layer NbS$_2$

Distinct stacking sequences and structures compared to bulk materials

Surface and interface effects significantly influence 2D structure formation

Abstract

Starting from a single layer of NbS$_2$ grown on graphene by molecular beam epitaxy, the single unit cell thick 2D materials Nb$_{5/3}$S$_3$-2D and Nb$_2$S$_3$-2D are created using two different pathways. Either annealing under sulfur-deficient conditions at progressively higher temperatures or deposition of increasing amounts of Nb at elevated temperature result in phase-pure Nb$_{5/3}$S$_3$-2D followed by Nb$_2$S$_3$-2D. The materials are characterized by scanning tunneling microscopy, scanning tunneling spectroscopy and X-ray photoemission spectroscopy. The experimental assessment combined with systematic density functional theory calculations reveals their structure. The 2D materials are covalently bound without any van der Waals gap. Their stacking sequence and structure are at variance with expectations based on corresponding bulk materials highlighting the importance of surface and interface effects in structure formation.