Scalable faceted voids with luminescent enhanced edges in WS2 monolayers

Disentangling doping and strain effects at defects of grown MoS2 monolayers with nano-optical spectroscopy

The role of defects in two-dimensional semiconductors and how they affect the intrinsic properties of these materials have been a widely researched topic over the past few decades. Optical characterization techniques such as photoluminescence and Raman spectroscopies are important tools to probe the physical properties of semiconductors and the impact of defects. However, confocal optical techniques present a spatial resolution limitation lying in a μm-scale, which can be overcome by the use of near-field optical measurements. Here, we use tip-enhanced photoluminescence and Raman spectroscopies to unveil the nanoscale optical properties of grown MoS2 monolayers, revealing that the impact of doping and strain can be disentangled by the combination of both techniques. A noticeable enhancement of the exciton peak intensity corresponding to trion emission quenching is observed at narrow regions down to a width of 47 nm at grain boundaries related to doping effects. Besides, localized strain fields inside the sample lead to non-uniformities in the intensity and energy position of photoluminescence peaks. Finally, two distinct MoS2 samples present different nano-optical responses at their edges associated with opposite strains. The edge of the first sample shows a photoluminescence intensity enhancement and energy blueshift corresponding to a frequency blueshift for E2g and 2LA Raman modes. In contrast, the other sample displays a photoluminescence energy redshift and frequency red shifts for E2g and 2LA Raman modes at their edges. Our work highlights the potential of combining tip-enhanced photoluminescence and Raman spectroscopies to probe localized strain fields and doping effects related to defects in two-dimensional materials.

Interplay between Thermal Stress and Interface Binding on Fracture of WS2 Monolayer with Triangular Voids

The defect engineering of two-dimensional (2D) materials has become a pivotal strategy for tuning the electrical and optical properties of the material. However, the reliable application of these atomically thin materials in practical devices require careful control of structural defects to avoid premature failure. Herein, a systematic investigation is presented to delineate the complex interactions among structural defects, the role of thermal mismatch between WS₂ monolayer and different substrates, and their consequent effect on the fracture behavior of the monolayer. Detailed microscopic and Raman/PL spectroscopic observations enabled a direct correlation between thermal mismatch stress and crack patterns originating from the corner of faceted voids in the WS₂ monolayer. Aberration-corrected STEM-HAADF imaging reveals the tensile strain localization around the faceted void corners. Density functional theory (DFT) simulations on interfacial interaction between the substrate (Silicon and sapphire -Al₂O₃) and monolayer WS₂ revealed a binding energy between WS₂ and Si substrate is 20 times higher than that with a sapphire substrate. This increased interfacial interaction in WS₂ and substrate-aided thermal mismatch stress arising due to difference in thermal expansion coefficient to a maximum extent leading to fracture in monolayer WS₂. Finite element simulations revealed the stress distribution near the void in the WS₂ monolayer, where the maximum stress was concentrated at the void tip.

Elucidation of Novel Potassium-Mediated Oxidation and Etching of Two-Dimensional Transition Metal Dichalcogenides

Preparation of edge-rich two-dimensional (2D) transition metal dichalocogenides (TMDs) has been actively investigated with the aim to improve their electrical and catalytic properties. Here, we elucidate the role of potassium ions in oxidation of TMDs and suggest a consequent novel anisotropic etching mechanism driven by self-running oxide droplets. We discover that potassium-mediated oxidation of MoS₂ leads to the formation of K-intercalated hexagonal-phase molybdenum oxides (h-K_xMoO₃), whereas orthorhombic-phase oxides are formed in the absence of potassium ions. Metastable h-K_xMoO₃ appears to have decomposed into oxide droplets at higher temperature. Self-running of the oxide droplets leads to layer-by-layer anisotropic etching of MoS₂ along the armchair direction. The motion of the droplets appears to be triggered by the surface energy instability between the oxide droplets and the underlying MoS₂ layer. This study opens new possibilities to design and manufacture novel edge-rich 2D TMDs that do not follow the equilibrium Wulff shape by modulating their oxidation with the assistance of alkali metals and also offers fundamental insights into the interactions between nanodroplets and 2D materials toward edge engineering.

The deviations of evaporation modes in two different morphologies of 2D WS2 film

Exploring the inverse process of materials growth, evaporation of atoms from the material, is a crucial method to investigate the physical properties of two dimensional (2D) nanomaterials. Here, the evaporation modes of two different morphologies of 2D WS2 film, stacked film and normal film, were investigated by thermal annealing. It is found that the atomic evaporation rate increases and the crystallinity deteriorates when annealing temperature rises. During the evaporation process, atom evaporation firstly starts from the boundaries and defects. The evaporation rate is proportional to the free energy of S and W atoms, and inversely proportional to the local S-W atomic concentration. There is a striking difference in the evaporation modes between stacked film and normal film; layer-by-layer peeling off the surface only appears in normal film. These results imply that the interlayer coupling strength of stacked film is greater than that of the normal film with uniform thickness.

In situ high temperature atomic level dynamics of large inversion domain formations in monolayer MoS2

Here we study the high-temperature formation and dynamics of large inversion domains (IDs) that form in monolayer MoS2 using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) with an in situ heating stage. We use temperatures above 700 °C to thermally activate rapid S vacancy migration and this leads to a formation mechanism of IDs that differs from the one at room temperature, where S vacancy migration is limited. We show that at high temperatures the formation of IDs occurs from intersected networks of long S vacancy line defects, whose strain fields are non-orthogonal and trigger large scale atomic reconstructions. Once formed, the IDs are influenced by the dynamic behaviour of nearby line defects and voids. With Mo and S atoms undergoing movement, the two types of ID grain boundaries can shift to allow further expansion of the ID area along the adjoining line defects. We reveal that IDs serve as metastable configurations between line defect rearrangements and eventual void formation under electron beam irradiation during heating. The formation of voids near to the IDs causes them to revert back to pristine lattice, which has the effect of restricting the ID domain size to a certain range (e.g. 3-5 nm in our observation) instead of continuously enlarging. This study provides insights into how the MoS2 IDs form and evolve at high temperature and can benefit the tailoring of electronic properties of two dimensional materials by structural manipulation.