[Paper Review] Helium atom micro-diffraction as a characterisation tool for 2D materials

Nick von Jeinsen, Aleksandar Radić+8 more|arXiv (Cornell University)|Sep 30, 2024

Hydrogen Storage and MaterialsMaterials Science3 citations

TL;DR

This paper demonstrates helium atom micro-diffraction (SHeM) as a non-invasive, surface-sensitive technique for characterizing 2D materials with sub-micron spatial resolution. It enables quantitative measurement of monolayer-MoS₂ substrate interactions, thermal expansion coefficients, electron-phonon coupling, and vacancy-type defect densities without sample damage or preparation, leveraging neutral helium's low energy and chemically inert nature to probe only the outermost atomic layers.

ABSTRACT

We present helium atom micro-diffraction as an ideal technique for characterization of 2D materials due to its ultimate surface sensitivity combined with sub-micron spatial resolution. Thermal energy neutral helium scatters from the valence electron density, 2-3A above the ionic cores of a surface, making the technique ideal for studying 2D materials, where other approaches can struggle due to small interaction cross-sections with few-layer samples. Sub-micron spatial resolution is key development in neutral atom scattering to allow measurements from device-scale samples. We present measurements of monolayer-substrate interactions, thermal expansion coefficients, the electron-phonon coupling constant and vacancy-type defect density on monolayer-MoS2. We also discuss extensions to the presented methods which can be immediately implemented on existing instruments to perform spatial mapping of these material properties.

Motivation & Objective

• To address the challenge of characterizing 2D materials with conventional non-contact techniques that struggle due to weak interaction cross-sections.
• To demonstrate the suitability of helium atom micro-diffraction for probing the outermost atomic layers of 2D materials with minimal perturbation.
• To enable quantitative measurement of key material properties such as defect density, substrate interactions, and thermal expansion in monolayer MoS₂.
• To extend the technique’s utility for device-scale samples by achieving sub-micron spatial resolution.
• To validate the method’s capability for measuring surface cleanliness and contamination without altering the sample.

Proposed method

• The study employs a Scanning Helium Microscope (SHeM) with a thermal energy beam of neutral ⁴He atoms at ~64 meV, yielding a de Broglie wavelength of 0.06 nm.
• The technique exploits the scattering of low-energy helium atoms from the valence electron density ~2–3 Å above ionic cores, ensuring surface-only sensitivity.
• Real-space imaging and diffraction measurements are performed at a single reciprocal-space (ΔK) value per rastered position, enabling simultaneous topographic and diffractive contrast.
• Spatial resolution of ~300 nm is achieved, with in-plane angular resolution of 7.9°, improved by a factor of 2 over prior SHeM implementations.
• Monolayer MoS₂ samples were mechanically exfoliated and transferred onto hBN/SiO₂ substrates, with defect densities tuned via controlled thermal annealing in Ar/H₂.
• Defect density was calibrated using stoichiometric XPS, and results were correlated with helium diffraction patterns and real-space imaging.

Figure 1 : Reflection mode optical (top-left) and real-space SHeM (bottom) images of bulk MoS 2 , hBN/SiO 2 and monolayer MoS 2 /hBN/SiO 2 (Shaded intersection). A 2D micro diffraction measurement (top-right) was taken to identify diffraction conditions for the real space imaging. An obvious change

Experimental results

Research questions

• RQ1 Can helium atom micro-diffraction provide quantitative, non-invasive characterization of monolayer MoS₂'s structural and thermal properties at the surface?
• RQ2 To what extent does the substrate (SiO₂ vs. hBN) influence the structural integrity and electronic properties of monolayer MoS₂?
• RQ3 Can helium atom micro-diffraction accurately quantify vacancy-type defect densities in 2D materials without sample preparation or damage?
• RQ4 How does the technique’s sub-micron spatial resolution enable mapping of material properties across device-scale samples?
• RQ5 Can helium atom micro-diffraction detect surface contamination and surface cleanliness with high sensitivity and without altering the sample?

Key findings

• Helium atom micro-diffraction successfully measured the thermal expansion coefficient of monolayer MoS₂, revealing its dependence on substrate interactions.
• The electron-phonon coupling constant in monolayer MoS₂ was quantified using temperature-dependent helium diffraction, showing measurable changes with lattice dynamics.
• Vacancy-type defect density in monolayer MoS₂ was measured with a range of ~0.1 to 1.8 × 10¹⁴ cm⁻², correlating with annealing parameters and validated by XPS.
• The use of hBN as a buffer layer preserved the structural integrity of monolayer MoS₂, while direct SiO₂ contact induced significant disorder, as confirmed by diffraction and real-space imaging.
• Surface contamination was detected with high sensitivity via real-space contrast and diffraction pattern changes, with no sample modification or cleaning required.
• The technique demonstrated spatial resolution of ~300 nm, enabling sub-micron scale mapping of structural and defect properties across device-relevant samples.

Figure 2 : Real-space images and diffraction scans of monolayer-MoS 2 /hBN/SiO 2 (red), few-layer hBN/SiO 2 (black) and SiO 2 (blue) before and after heating (left to right) to $240\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for 8 hours to remove physisorbed species. For monolayer-MoS 2 and hBN we see o