Point Defects and Grain Boundaries in Rotationally Commensurate MoS₂ on Epitaxial Graphene

Abstract

With reduced degrees of freedom, structural defects are expected to play a greater role in two-dimensional materials in comparison to their bulk counterparts. In particular, mechanical strength, electronic properties, and chemical reactivity are strongly affected by crystal imperfections in the atomically thin limit. Here, ultra-high vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS 2 grown on epitaxial graphene (EG) at the atomic scale. Five types of point defects are observed with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect density is observed to be lower than MoS 2 grown on other substrates, and is likely attributed to the van der Waals epitaxy of MoS 2 on EG.Grain boundaries (GBs) with 30° and 60° tilt angles resulting from the rotational commensurability of MoS 2 on EG are more easily resolved by STM than atomic force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap reduction to ~0.8 eV and ~0.5 eV is observed with STS for 30° and 60° GBs, respectively. In addition, atomic resolution STM images of these GBs are found to agree well with proposed structure models. This work offers quantitative insight into the structure and properties of common defects in MoS 2 , and suggests pathways for tailoring the performance of MoS 2 /graphene heterostructures via defect engineering. 2 TOC Figure

Resonant and bound states of charged defects in two-dimensional semiconductors

“…[18] They are among the most abundant point defects in our samples but their density varies considerably between different CVD preparations. Similar STM contrast has been observed in CVD-grown MoS 2 [19], MOCVD-and MBE-grown WSe 2 [20,21], MBE-grown MoSe 2 [21] and natural bulk MoS 2 (0001) [22]. Some of these reports suggest that the observed defect might be charged [18,21,22] but their chemical origin remains unclear.…”

The Effect of Twin Grain Boundary Tuned by Temperature on the Electrical Transport Properties of Monolayer MoS2

“…The observed behavior is common in highly-disordered systems and a signature of hopping transport via localized states [29][30][31]. The VRH transport is found to be consistent with the larger electrical conductivity of GB, due to localized mid-gap states that arises from twin GB can increase the hopping bond and conductivity [15,16,32,33]. The conductivity versus the inverse of temperature (…”

Large Spin-Orbit Splitting of Deep In-Gap Defect States of Engineered Sulfur Vacancies in Monolayer WS2

“…Yet, the defect assignment is not straightforward because their STM contrast is dominated by their electronic states, and tip-dependent contrast inversion makes it difficult to assign lattice sites. Both of these complications have lead to recent contradictory defect identification in TMDs by STM [7,18,21].…”