Characterising Atomic-Scale Surface Disorder on 2D Materials Using Neutral Atoms

TL;DR

This paper demonstrates that scanning helium microscopy (SHeM) can detect atomic-scale surface disorder caused by trace contamination on 2D MoS2, revealing the fragility of surface order even under ultra-high vacuum conditions.

Contribution

It introduces SHeM as a non-destructive, ultra-sensitive technique for characterising surface cleanness of 2D materials at atomic scale, capable of detecting minute contamination effects.

Findings

Minute contamination causes atomic-scale disorder on MoS2 surfaces.

Flat regions are more susceptible to contamination than edges.

SHeM effectively tracks surface order decay over time.

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2, have the potential to be widely used in electronic devices and sensors due to their high carrier mobility and tunable band structure. In 2D TMD devices, surface and interface cleanness can critically impact the performance and reproducibility. Even sample surfaces prepared under ultra-high vacuum (UHV) can be contaminated, causing disorder. On such samples, trace levels of submonolayer contamination remain largely overlooked, and conventional surface characterisation techniques have limited capability in detecting such adsorbates. Here, we apply scanning helium microscopy (SHeM), a non-destructive and ultra-sensitive technique, to investigate the surface cleanness of 2D MoS2. Our measurements reveal that even minute amounts of adventitious carbon induce atomic-scale disorder across MoS2 surfaces, leading to the disappearance of helium diffraction. By tracking helium reflectivity over time, we quantify the decay of surface order across different microscopic regions and find that flat areas are more susceptible to contamination than regions near edges. These findings highlight the fragility of surface order in 2D materials, even under UHV, and establish SHeM as a powerful tool for non-damaging microscopic 2D material cleanness characterisation. The approach offers a new route to wafer-scale characterisation of 2D material quality.