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

TL;DR

This work addresses the challenge of detecting and quantifying submonolayer surface contaminants on 2D TMDs, particularly MoS$_2$, which can degrade electronic properties even under ultra-high vacuum. It introduces scanning helium microscopy (SHeM) as a non-invasive wafer-scale probe of atomic-scale surface order, using a $64\,\mathrm{meV}$ helium beam to observe Bragg diffraction and diffuse scattering, complemented by temperature-programmed helium reflectivity to study desorption kinetics with an activation energy of about $1\,\mathrm{eV}$. The study shows that adventitious carbon induces atomic-scale disorder on MoS$_2$, erasing diffraction; cleaning at modest temperatures ($80$–$200\,\circ\mathrm{C}$) restores order, while recontamination occurs under UHV with region-dependent rates, faster on more crystalline (flat) regions. Overall, SHeM provides a powerful, non-destructive, micron-scale method for real-time cleanness characterization of 2D materials, enabling improved reproducibility for wafer-scale device fabrication and potentially extending to other lightly bound surface contaminants.$\lbrace$ $\mathcal{O}(1)$–$\mathcal{O}(10)$ sentences overall$\rbrace$

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.

Figures (6)

• Figure 1: Schematics of helium atom diffraction from crystalline surfaces under different levels of contamination. (a) For an ordered crystalline surface, such as the surface of an MoS2 crystal, helium atoms scatter undergo Bragg scattering (ordered scattering). (b) In the presence of adsorbates, the atoms are scattered randomly, reducing the intensity of ordered scattering and increasing the intensity of disordered diffuse scattering. (c) A full coverage of disordered adsorbates results in completely diffuse scattering.
• Figure 2: SHeM measurements of MoS2 under different levels of contamination. (a-d) SHeM image of the sample as exfoliated (a), after heating to $200\degreeCelsius$ for an hour (b), after cooling back down to room temperature and exposure to air for $\sim30mins$ (c), and after heating again to $200\degreeCelsius$ for an hour (d). The scale bar is $500µm$. (e) HAMD measurements from spot D on the clean sample surface at $200\degreeCelsius$ and contaminated surface at room temperature. (a) exhibits only diffuse contrast, with slowly varying intensity, (b) exhibits diffraction contrast, with many bright spots and rapidly changing intensity even for relatively flat regions, (c) exhibits mostly diffuse contrast with some regions of diffraction evident, (d) exhibits the same contrast as (b).
• Figure 3: SHeM measurements of monolayer MoS2. (a-b) A monolayer of MoS2-on-hBN as prepared in air (a), and after heating in vacuum (b). The monolayer was identified by optical microscopy and is highlighted as region E, the remaining bright region in (b) is the hBN buffer. The scale bar is $10µm$. (c) Diffraction scans from the monolayer MoS2 at room temperature and $240\degreeCelsius$. The same change in contrast is seen as with the bulk sample in figure \ref{['fig:clean_and_recontaminate']}, with the distinction between the MoS2 crystal and the substrate barely visible prior to cleaning, but with the crystal exhibiting high reflectivity after cleaning. All measurements of monolayer samples were taken with the Cambridge A-SHeM barr_design_2014vonJeinsen2023.
• Figure 4: Spatial resolution of the contamination process on MoS2. Two regions of the MoS2 crystal from figure \ref{['fig:clean_and_recontaminate']} are shown, one corresponding to a generally flat region (left, panels (a) to (f)) and the other a delaminated region (right, panels (g) to (l)). After cleaning the sample surface at a high temperature, (a) & (g), diffractive contrast was observed. The sample was cooled to room temperature while remaining under vacuum and the specular signal monitored for 5 different points on each sample (f) & (l). At intervals, micrographs of both regions were acquired demonstrating the changing contrast, (b)-(e) & (h)-(k). Through both the specular intensity and contrast in micrographs it was observed that surface order remained for longer on the 'delaminated' regions than the 'flat' regions, and that complete recontamination was observed for all regions despite the sample remaining under UHV conditions. Scale bar, $100µm$. As with the results in figures \ref{['fig:clean_and_recontaminate']} and \ref{['fig:monolayer_mos2']}, we also performed helium atom diffraction measurements to confirm our interpretation of the observed contrast, these are given in the supplementary information.
• Figure 5: Disappearance of contamination on MoS2 by thermal heating. (a) Temperature programmed helium reflectivity measurements of the MoS2 crystal, with background subtracted. Helium reflectivity starts to increase at $\sim80\degreeCelsius$ due to the desorption of adventitious carbon, reaches a maximum increase rate at $\sim120\degreeCelsius$ and rolls off at $\sim200\degreeCelsius$. (b)-(d) Helium micrographs of the bottom right corner of the MoS2 sample in figure \ref{['fig:clean_and_recontaminate']} at room temperature, $80\degreeCelsius$ and $200\degreeCelsius$, imaged separately to the data in (a). We already observe diffraction contrast across the sample at $80\degreeCelsius$, with no significant change when the temperature is increased to $200\degreeCelsius$. The blue spot is the same as spot D in figure \ref{['fig:clean_and_recontaminate']}, where the helium reflectivity is measured. Scale bar, $300µm$.