arXiv subjects

Sam Lambrick

Publications and source records attributed to Sam Lambrick.

5 recordsLinked to original sources

arXiv2025

Defect-modified acoustic phonons in a single layer of MoS2

The thermal, mechanical, and electronic performance of atomically thin semiconductors is governed by their low-energy phonons, yet the impact of atomic-scale disorder on these modes remains poorly understood. Here, we report the first measurement of acoustic phonon dispersions in a quasi-freestanding monolayer semiconductor (MoS2), using helium-3 spin-echo spectroscopy. We identify a defect-driven regime change at a critical wavevector, $q_c$, marking the breakdown of continuum elastic behavior. At this length scale, the flexural mode transitions from continuum bending to defect-pinned standing waves, while the hybridized Rayleigh wave becomes vibrationally disordered in its dispersion and linewidth. We observe multiple defect-induced Van Hove singularities deep within the Brillouin zone and strongly suppressed acoustic group velocities, providing direct experimental evidence that four-phonon processes drive thermal transport in mono- and few-layer MoS2. These results offer a microscopic explanation for the anomalously low thermal conductivity widely observed in transition-metal dichalcogenides and demonstrate how atomic-scale disorder dictates energy flow in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2025

Heliometric stereo: a new frontier in surface profilometry

Accurate and reliable measurements of three-dimensional surface structures are important for a broad range of technological and research applications, including materials science, nanotechnology, and biomedical research. Scanning helium microscopy (SHeM) uses low-energy (64 meV) neutral helium atoms as the imaging probe particles, providing a highly sensitive and delicate approach to measuring surface topography. To date, topographic SHeM measurements have been largely qualitative, but with the advent of the heliometric stereo method - a technique that combines multiple images to create a 3D representation of a surface - quantitative maps of surface topography may now be acquired with SHeM. Here, we present and discuss two different implementations of heliometric stereo on two separate instruments, a single detector SHeM and a multiple-detector SHeM. Both implementations show good accuracy (5% and 10% respectively) for recovering the shape of a surface. Additionally, we discuss where heliometric stereo is most applicable, identify contrast features that can limit its accuracy, and discuss how to mitigate these limitations with careful design and sample choices that be readily implemented on current instruments.

physics.ins-det↗

arXiv2024

Helium atom micro-diffraction as a characterisation tool for 2D materials

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.

physics.app-ph↗

arXiv2024

Measuring vacancy-type defect density in monolayer semiconductors

Two-dimensional (2D) materials have attracted wide-spread interest due to their unique and tunable properties. Their optoelectronic, mechanical, and thermal properties are greatly influenced by crystal defects, which are, in turn, used to control these properties. However, experimental quantification of the density of defects, whether deliberately introduced or inherent, is very difficult in these atomically thin materials. Here we show that helium atom micro-diffraction can be used to measure the defect density in 15x20um monolayer MoS2, a prototypical 2D semiconductor, quickly and easily compared to standard methods. We present a simple analytic model, the lattice gas equation, that fully captures the relationship between atomic Bragg diffraction intensity and defect density. The model, combined with ab initio scattering calculations, shows that our technique can immediately be applied to a wide range of 2D materials, independent of sample chemistry or structure. Additionally, wafer-scale characterization is immediately possible.

physics.app-ph↗

arXiv2024

On the application of components manufactured with stereolithographic 3D printing in high vacuum systems

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).

physics.app-ph↗