Topological defects and their induced metallicity in monolayer semiconducting γ-phase group IV monochalcogenides

Abstract

γ-Phase group IV monochalcogenides (γ-MX), predicted to be stable with semiconducting characteristics, have been synthesized by chemical vapor deposition but showing metallicity. The ubiquitous topological defects introduced during the growth process could bring about key influences on the electronic behaviors, but their structures and properties remain unexplored. Taking monolayer γ-GeSe as an example, first-principles calculations were performed to investigate the structural, thermodynamic, and electronic properties of dislocation cores (DCs) and grain boundaries (GBs). Various derivative DCs emerge depending on different arrangements of atoms. The calculated low-energy DCs are then used to determine preferential structures of GBs versus tilt angle, with special attention paid to the whole family of 60° twin GBs, showing distinct hexagons or Ge—Ge bonds. Furthermore, electronic structures are calculated for thermodynamically favored 21.8° with closely packed dislocations and 60° twin GBs. Most of them show a strong resonance between the bulk and dislocation states, rendering the systems metallic, while some display semiconducting behaviors with reduced band gap. These electronic properties are universal for other γ-MXs. The simulated scanning tunneling microscopy images show characteristic fingerprints to help identify their existence in practice. Our results show that topological defects in γ-MX with versatile properties should be carefully engineered for their potential applications.