Defect Density Quantification in Monolayer MoS2 Using Scanning Helium Microscopy (SHeM)

Sulphur vacancy defects mediate a wide range of optoelectronic properties in MoS2, with precise control of defect density allowing for tuneable optoelectronic devices. However, accurate measurement of defect density in monolayer and few-layer samples poses a challenge due to their small scattering cross-sections to photon or electron probes.Conventional lab-based techniques such as Raman and photoluminescence can infer approximate defect density in micro-scale samples via optoelectronic properties, but they require calibration using stoichiometric beam-line XPS. We introduce an ultra-low energy (~64 meV), non-damaging, lab-based technique to quantify the surface defect density in micron-scale monolayer MoS2. Here we show that a recently developed technique, scanning helium microscopy (SHeM), can be used to directly measure vacancy-type defect density in 2D materials by performing atom diffraction from a microscopic spot. SHeM uses a neutral, inert, and thermal energy probe of helium-4 atoms to measure ordered and disordered atom-surface scattering allowing the level of surface order to be inferred. The presented method enables rapid, non-damaging, and material-agnostic lab-based quantification of defect density in 2D materials, a crucial step towards the wider adoption of 2D semiconductors in devices.