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Scanning helium microscopy (SHeM) is an emerging technique that uses a beam of neutral atoms to image and analyse surfaces. The low energies ($\sim$64 meV) and completely non-destructive nature of the probe particles provide exceptional sensitivity for studying delicate samples and thin devices, including 2D materials. To date, around five such instruments have been constructed and are described in the literature. All represent the first attempts at SHeM construction in different laboratories, and use a single detection device. Here, we describe our second generation microscope, which is the first to offer multi-detector capabilities. The new instrument builds on recent research into SHeM optimisation and incorporates many improved design features over our previous instrument. We present measurements that highlight some of the unique capabilities the instrument provides, including 3D surface profiling, alternative imaging modes, and simultaneous acquisition of images from a mixed species beam.

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., Comment: Draft version, 11 pages, 6 figures, 2 tables

2D materials continue to be pivotal in the advancement of modern devices. Their optoelectronic, mechanical and thermal properties can be finely modulated using a variety of methods, including strain, passivation, doping and defect density. Vacancy-type defect density, as found in the prototypical MoS2, in 2D materials is inherently difficult to measure due to their thickness. Here we show that helium atom micro-diffraction using a 5 {\mu}m beam size can be used to measure defect density in ~15x20 {\mu}m monolayer MoS2 quickly and easily at low cost compared to standard methods. We find that diffracted helium intensity is inversely proportional to defect density and that the method can be used as a standalone measure of vacancy-type defect density in monolayer MoS2, with the ability to spatially map defect density. We compare our results to photoluminescence (PL) spectroscopy and stoichiometric beam-line XPS. Furthermore, we provide computational and theoretical evidence that the method is agnostic to sample chemistry and can therefore immediately be applied to the measurement of vacancy-type defect density on the surface of any crystalline material. Our results demonstrate helium atom micro-diffraction as a rapid, low cost and lab-based alternative to often prohibitively expensive and time-consuming beam-line techniques such as XPS, improving accessibility to 2D materials science and device fabrication.

Free-jet atomic, cluster and molecular sources are typically used to produce beams of low-energy, neutral particles and find application in a wide array of technologies, from neutral atom microscopes to instruments for surface processing. However, understanding and maximising the intensity derived from such sources has proven challenging, partly because of gas interference effects attenuating the beam intensity. Here, we describe a semi-analytic method to estimate the resulting attenuation in low-pressure free-jet sources. We explicitly calculate two contributions to the interference, that arise from the interaction of the beam with (i) background gas and (ii) atoms backscattered from the skimmer. We demonstrate that the design of the skimmer has an insignificant impact for room temperature beams. Finally, we develop a number of design recommendations for the skimmer and vacuum chamber that optimise beam properties for spectroscopy and microscopy when using cryogenic-temperature atom or molecular beams., Comment: 18 pages, 9 figures main text, 7 figures appendices. Submitted to Physical Review A

We explore the ultrahigh-vacuum (UHV) compatibility of Formlabs `Clear Resin' via vat photopolymerization (VPP). We report on a method for using VPP additive manufacturing, specifically Formlabs' widely available stereolithographic (SLA) printing using their `Clear Resin' material, to rapidly and cheaply prototype components for use in high-vacuum (HV) environments. We present pump down curves and residual gas analysis to demonstrate the primary vacuum contaminant from freshly printed SLA plastics is water with no evidence of polymers outgassing from the material and thus the vacuum performance can be controlled with simple treatments which do not involve surface sealing. An unbaked vacuum system containing SLA printed components achieved 1.9e-8 mbar base pressure whilst retaining structural integrity and manufacturing accuracy. Outgassing rates in the HV test chamber and preliminary results in a UHV chamber indicate that our method can be extended to achieve ultrahigh-vacuum compatibility. We further report on the effect of atmospheric exposure to components and present evidence to suggest that water re-ad/absorption occurs exclusively on the surface, by showing that the bulk mass changes of the material is irreversible on the timescale investigated (< 2 weeks)., Comment: 17 pages, 7 figures, 1 table. Poster presentation at EVC-17/ECOSS-37 (2024). Accepted at Vacuum on 5th Nov. 2024

Scanning Helium Microscopy (SHeM) offers a combination of spatial and angular resolution via a pinhole-collimated beam of thermal energy, neutral helium-4 atoms for non-destructive imaging. This thesis introduces a novel 3D imaging mode, "heliometric stereo", enabling true-to-size 3D surface reconstruction using an adapted photometric stereo algorithm. Stereolithography (SLA) 3D printed plastics are explored for SHeM pinhole plates due to limitations in traditional machining. FormLabs "Clear Resin" via SLA printing proves ideal for rapid prototyping of vacuum components, with a developed baking protocol ensuring vacuum compatibility. The study indicates re-wetting of such plastics is a surface process over weeks. Developing 3D image reconstruction for both single and multi-detector setups required a real-space point tracking method. The point tracking method facilitates facet angle measurement in various materials, including technological and biological crystals. It has since become integral to SHeM imaging protocols for sample manipulator debugging. The thesis also details a multi-detector SHeM instrument, referred to as B-SHeM. While primarily designed to perform heliometric stereo reconstructions, the instrument also enables the range of novel SHeM experiments such as mixed-species beams to investigate inelastic scattering. The heliometric stereo methods implemented in the work have motivated the development of a GPU accelerated version of the in-house Monte-Carlo based ray tracing framework, which is the de-facto standard for SHeM image analysis. GPU parallelisation was explored as a method for decreasing simulation time and enabling previously inaccessible simulations involving complex scattering distributions and high resolution, realistic sample geometries. Preliminary testing on an analogous problem yielded a potential performance increase of up to 380 times., Comment: Submitted for the degree of Master of Philosophy (MPhil) in Physics at the University of Cambridge in 2021. 110 pages, 75 figures

Three-dimensional mapping of surface structures is important in a wide range of biological, technological, healthcare and research applications, including taxonomy, microfluidics and fabrication. Neutral helium atom beams have been established as a sensitive probe of topography and have already enabled structural information to be obtained from delicate samples, where conventional probes would cause damage. Here, we demonstrate empirical reconstruction of a complete surface profile using measurements from a scanning helium microscope (SHeM), using the heliometric stereo method and a single detector instrument geometry. Results for the surface profile of tetrahedral aluminum potassium sulphate crystals demonstrate the areas of surfaces and facet orientations can be recovered to within 5% of the expected values., Comment: 7 pages (including bibliography), 5 figures (including Appendix). Published at Applied Physics Letters on 14th May 2024 (doi.org/10.1063/5.0206374)