skip to main content


Title: Formation of metal vacancy arrays in coalesced WS2 monolayer films
Defects have a profound impact on the electronic and physical properties of crystals. For two-dimensional (2D) materials, many intrinsic point defects have been reported, but much remains to be understood about their origin. Using scanning transmission electron microscopy imaging, this study discovers various linear arrays of W-vacancy defects that are explained in the context of the crystal growth of coalesced, monolayer WS2. Atomistic-scale simulations show that vacancy arrays can result from steric hindrance of bulky gas-phase precursors at narrowly separated growth edges, and that increasing the edge separation leads to various intact and defective growth modes, which are driven by competition between the catalytic effects of the sapphire substrate and neighboring growth edge. Therefore, we hypothesize that the arrays result from combined growth modes, which directly result from film coalescence. The connections drawn here will guide future synthetic and processing strategies to harness the engineering potential of defects in 2D monolayers.  more » « less
Award ID(s):
1654107
NSF-PAR ID:
10231401
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
2D materials
Volume:
8
Issue:
1
ISSN:
2053-1583
Page Range / eLocation ID:
011003
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. null (Ed.)
    Atomistic simulation techniques have become an indispensable tool to acquire a fundamental understanding of growth and structural characteristics of two-dimensional (2D) materials of interest, thereby accelerating experimental research in the same field. A new ReaxFF reactive force field presented here is the first comprehensive empirical potential that is explicitly designed to capture the most prominent features of 2D WSe2 solid-phase chemistry, such as defect formation as a function of local geometry and chalcogen chemical potential, vacancy migration and phase transition, thus enabling cost-effective and reliable characterization of 2D WSe2 at large length scales and time scales much longer than what is accessible by first-principles theory. This potential, validated using extensive first-principles energetics data on both periodic and nonperiodic systems and experimental measurements, can accurately describe the mechanochemical coupling between monolayer deformations and vacancy energetics, providing valuable atomistic insights into the morphological evolution of a monolayer in different environments in terms of loading conditions and various concentrations and distributions of defects. Since understanding how growth is affected by the local chemical environment is vital to fabricating efficient and functional atomically thin 2D WSe2, the new ReaxFF description enables investigations of edge-controlled growth of single crystals of 2D WSe2 using reactive environments closely matching experimental conditions at a low computational cost. 
    more » « less
  2. Topological crystalline insulators (TCIs) are new materials with metallic surface states protected by crystal symmetry. The properties of molecular beam epitaxy grown SnTe TCI on SrTiO3(001) are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, low‐energy and reflection high‐energy electron diffraction, X‐ray diffraction, Auger electron spectroscopy, and density functional theory. Initially, SnTe (111) and (001) surfaces are observed; however, the (001) surface dominates with increasing film thickness. The films grow island‐by‐island with the [011] direction of SnTe (001) islands rotated up to 7.5° from SrTiO3[010]. Microscopy reveals that this growth mechanism induces defects on different length scales and dimensions that affect the electronic properties, including point defects (0D); step edges (1D); grain boundaries between islands rotated up to several degrees; edge‐dislocation arrays (2D out‐of‐plane) that serve as periodic nucleation sites for pit growth (2D in‐plane); and screw dislocations (3D). These features cause variations in the surface electronic structure that appear in STM images as standing wave patterns and a nonuniform background superimposed on atomic features. The results indicate that both the growth process and the scanning probe tip can be used to induce symmetry breaking defects that may disrupt the topological states in a controlled way.

     
    more » « less
  3. Graphene with in-plane nanoholes, named holey graphene, shows great potential in electrochemical applications due to its fast mass transport and improved electrochemical activity. Scalable nanomanufacturing of holey graphene is generally based on chemical etching using hydrogen peroxide to form through-the-thickness nanoholes on the basal plane of graphene. In this study, we probe into the fundamental mechanisms of nanohole formation under peroxide etching via an integrated experimental and computational effort. The research results show that the growth of nanoholes during the etching of graphene oxide is achieved by a three-stage reduction–oxidation–reduction procedure. First, it is demonstrated that vacancy defects are formed via a partial reduction-based pretreatment. Second, hydrogen peroxide reacts preferentially with the edge-sites of defect areas on graphene oxide sheets, leading to the formation of various oxygen-containing functional groups. Third, the carbon atoms around the defects are removed along with the neighboring carbon atoms via reduction. By advancing the understanding of process mechanisms, we further demonstrate an improved nanomanufacturing strategy, in which graphene oxide with a high density of defects is introduced for peroxide etching, leading to enhanced nanohole formation. 
    more » « less
  4. Abstract

    The bulk-boundary correspondence, which links a bulk topological property of a material to the existence of robust boundary states, is a hallmark of topological insulators. However, in crystalline topological materials the presence of boundary states in the insulating gap is not always necessary since they can be hidden in the bulk energy bands, obscured by boundary artifacts of non-topological origin, or, in the case of higher-order topology, they can be gapped altogether. Recently, exotic defects of translation symmetry called partial dislocations have been proposed to trap gapless topological modes in some materials. Here we present experimental observations of partial-dislocation-induced topological modes in 2D and 3D insulators. We particularly focus on multipole higher-order topological insulators built from circuit-based resonator arrays, since crucially they are not sensitive to full dislocation defects, and they have a sublattice structure allowing for stacking faults and partial dislocations.

     
    more » « less
  5. null (Ed.)
    Abstract Topological insulator lasers (TILs) are a recently introduced family of lasing arrays in which phase locking is achieved through synthetic gauge fields. These single frequency light source arrays operate in the spatially extended edge modes of topologically non-trivial optical lattices. Because of the inherent robustness of topological modes against perturbations and defects, such topological insulator lasers tend to demonstrate higher slope efficiencies as compared to their topologically trivial counterparts. So far, magnetic and non-magnetic optically pumped topological laser arrays as well as electrically pumped TILs that are operating at cryogenic temperatures have been demonstrated. Here we present the first room temperature and electrically pumped topological insulator laser. This laser array, using a structure that mimics the quantum spin Hall effect for photons, generates light at telecom wavelengths and exhibits single frequency emission. Our work is expected to lead to further developments in laser science and technology, while opening up new possibilities in topological photonics. 
    more » « less