@article{Ruckhofer2022,

title = {Evolution of ordered nanoporous phases during h-BN growth: controlling the route from gas-phase precursor to 2D material by in situ monitoring},

author = { Adrian Ruckhofer, Marco Sacchi, Anthony Payne, Andrew P. Jardine, Wolfgang E. Ernst, Nadav Avidor and Anton Tamtögl},

url = {

DOI

    https://doi.org/10.1039/D2NH00353H },

doi = {10.1039/D2NH00353H},

year  = {2022},

date = {2022-09-21},

journal = {NanoScience Horizons},

abstract = {Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) can be grown by chemical vapour deposition (CVD). However, the high temperatures and fast timescales at which the conversion from a gas-phase precursor to the 2D material appears, make it extremely challenging to simultaneously follow the atomic arrangements. We utilise helium atom scattering to discover and control the growth of novel 2D h-BN nanoporous phases during the CVD process. We find that prior to the formation of h-BN from the gas-phase precursor, a metastable (3 × 3) structure is formed, and that excess deposition on the resulting 2D h-BN leads to the emergence of a (3 × 4) structure. We illustrate that these nanoporous structures are produced by partial dehydrogenation and polymerisation of the borazine precursor upon adsorption. These steps are largely unexplored during the synthesis of 2D materials and we unveil the rich phases during CVD growth. Our results provide significant foundations for 2D materials engineering in CVD, by adjusting or carefully controlling the growth conditions and thus exploiting these intermediate structures for the synthesis of covalent self-assembled 2D networks.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{nokey,

title = {Observation of diffuse scattering in scanning helium microscopy},

author = {S. M. Lambrick, M. Bergin, D. J. Ward, M. Barr, A. Fahy, T. Myles, A. Radić, P. C. Dastoor, J. Ellis and A. P. Jardine},

url = {https://doi.org/10.1039/D2CP01951E},

doi = {10.1039/D2CP01951E},

year  = {2022},

date = {2022-09-20},

journal = {Phys. Chem. Chem. Phys},

volume = {24},

pages = {26539-26546},

abstract = {In understanding the nature of contrast in the emerging field of neutral helium microscopy, it is important to identify if there is an atom–surface scattering distribution that can be expected to apply broadly across a range of sample surfaces. Here we present results acquired in a scanning helium microscope (SHeM) under typical operating conditions, from a range of surfaces in their native state, i.e. without any specialist sample preparation. We observe diffuse scattering, with an approximately cosine distribution centred about the surface normal. The ‘cosine-like’ distribution is markedly different from those distributions observed from the well-prepared, atomically pristine, surfaces typically studied in helium atom scattering experiments. Knowledge of the typical scattering distribution in SHeM experiments provides a starting basis for interpretation of topographic contrast in images, as well as a reference against which more exotic contrast mechanisms can be compared.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Allison2022,

title = {Perturbation theory of scattering for grazing-incidence fast-atom diffraction},

author = { William Allison and Salvador Miret-Artés and Eli Pollak},

url = {https://doi.org/10.1039/D2CP01013E },

year  = {2022},

date = {2022-06-17},

journal = {Phys. Chem. Chem. Phys.},

publisher = {The Royal Society of Chemistry},

abstract = {Recent grazing-incidence, fast atom diffraction (GIFAD) experiments have highlighted the well known observation that the distance between classical rainbow angles depends on the incident energy. The GIFAD experiments imply an incident vertical scattering angle, facilitating an analytic analysis using classical perturbation theory, which leads to the conclusion that the so called “dynamic corrugation” amplitude, as defined by Bocan et al., Phys. Rev. Lett., 2020 125, 096101 is, within first-order perturbation theory, proportional to the tangent of the rainbow angle. Therefore it provides no further information about the interaction than is gleaned from the rainbow angle and its energy dependence. Perhaps more importantly, the resulting analytic theory reveals how the energy dependence of rainbow angles may be inverted into information on the force field governing the interaction of the incident projectile with the surface.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{complexelements,

title = {Complex optical elements for scanning helium microscopy through 3D printing},

author = {M Bergin and T A Myles and A Radic and C J Hatchwell and S M Lambrick and D J Ward and S D Eder and A Fahy and M Barr and P C Dastoor},

url = {https://doi.org/10.1088/1361-6463/ac3a3e

https://doi.org/10.5281/zenodo.5496454},

doi = {10.1088/1361-6463/ac3a3e},

year  = {2022},

date = {2022-03-03},

journal = {Journal of Physics D: Applied Physics},

volume = {55},

number = {9},

pages = {095305},

abstract = {Developing the next generation of scanning helium microscopes requires the fabrication of optical elements with complex internal geometries. We show that resin stereolithography (SLA) 3D printing produces low-cost components with the requisite convoluted structures whilst achieving the required vacuum properties, even without in situ baking. As a case study, a redesigned pinhole plate optical element of an existing scanning helium microscope was fabricated using SLA 3D printing. In comparison to the original machined component, the new optical element minimised the key sources of background signal, in particular multiple scattering and the secondary effusive beam.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Avidor2021,

title = {Probing surface motion above ambient temperature with helium spin-echo spectroscopy},

author = {Nadav Avidor},

url = {https://www.nature.com/articles/s42254-021-00387-2},

doi = {10.1038/s42254-021-00387-2},

year  = {2021},

date = {2021-10-12},

journal = {Nature Reviews Physics},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tamtögl2021,

title = {Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene},

author = {Anton Tamtögl and Emanuel Bahn and Marco Sacchi and Jianding Zhu and David J. Ward and Andrew P. Jardine and Stephen J. Jenkins and Peter Fouquet and John Ellis and William Allison},

url = {https://www.nature.com/articles/s41467-021-23226-5},

doi = {10.1038/s41467-021-23226-5},

year  = {2021},

date = {2021-05-25},

journal = {Nature Communications},

volume = {12},

pages = {3120},

abstract = {The interfacial behaviour of water remains a central question to fields as diverse as protein folding, friction and ice formation. While the properties of water at interfaces differ from those in the bulk, major gaps in our knowledge limit our understanding at the molecular level. Information concerning the microscopic motion of water comes mostly from computation and, on an atomic scale, is largely unexplored by experiment. Here, we provide a detailed insight into the behaviour of water monomers on a graphene surface. The motion displays remarkably strong signatures of cooperative behaviour due to repulsive forces between the monomers, enhancing the monomer lifetime ( ≈ 3 s at 125 K) in a free-gas phase that precedes the nucleation of ice islands and, in turn, provides the opportunity for our experiments to be performed. Our results give a molecular perspective on a kinetic barrier to ice nucleation, providing routes to understand and control the processes involved in

ice formation.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lambrick2021,

title = {True-to-size surface mapping with neutral helium atoms},

author = {Sam M. Lambrick and Adrià Salvador Palau and Poul Erik Hansen and Gianangelo Bracco and John Ellis and Andrew P. Jardine and Bodil Holst},

url = {https://journals.aps.org/pra/abstract/10.1103/PhysRevA.103.053315},

doi = {10.1103/PhysRevA.103.053315},

year  = {2021},

date = {2021-05-19},

journal = {Physical Review A},

volume = {103},

pages = {053315},

abstract = {Three-dimensional mapping of microscopic surface structures is important in many applications of technology and research, including areas as diverse as microfluidics, MEMS, and geoscience. However, on the nanoscale, using established techniques for such imaging can be extremely challenging. Scanning helium microscopy is a technique that uses neutral helium atoms as a probe, enabling completely nondestructive imaging. The technique is broadly applicable and ideal for many otherwise difficult-to-image materials, such as insulators, ultrathin nanocoatings, and biological samples. Here we present a method for implementation and operation of a stereo helium microscope, by applying the photometric stereo method of surface reconstruction to helium microscopy. Four detectors around the sample are typically required, but we show how sample rotation can be used to perform stereo reconstruction with a single-detector instrument, or to improve the quality of the reconstructed surface by increasing the number of independent measurements. We examine the quality of the reconstructed surface and

show that for low aspect ratio good absolute height is recovered. For features with height/width ∼1 the shape of the surface is still recovered well (8% error) despite multiple scattering and masking of the helium beam by surface topography. Therefore, it is possible to perform accurate reconstruction of the shape of nanoscale structures with a height to width ratio of at least unity.

},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Kelsall2021,

title = {Ultrafast Diffusion at the Onset of Growth: O/Ru(0001)},

author = {Jack Kelsall and Peter S. M. Townsend and John Ellis and Andrew P. Jardine and Nadav Avidor},

url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.155901},

doi = {10.1103/PhysRevLett.126.155901},

year  = {2021},

date = {2021-04-12},

journal = {Physical Review Letter},

volume = {126},

pages = {155901},

abstract = {Nanoscopic clustering in a 2D disordered phase is observed for oxygen on Ru(0001) at low coverages

and high temperatures. We study the coexistence of quasistatic clusters (with a characteristic length of

∼9 Å) and highly mobile atomic oxygen which diffuses between the energy-inequivalent, threefold hollow

sites of the substrate. We determine a surprisingly low activation energy for diffusion of 385 +/- 20 meV.

The minimum of the O − O interadsorbate potential appears to be at lower separations than previously

reported.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{holst2021material,

title = {Material Properties Particularly Suited to be Measured with Helium Scattering: Selected Examples from 2D Materials, van der Waals Heterostructures, Glassy Materials, Catalytic Substrates, Topological Insulators and Superconducting Radio Frequency Materials},

author = {Bodil Holst and Gil Alexandrowicz and Nadav Avidor and Giorgio Benedek and Gianangelo Bracco and Wolfgang E Ernst and Daniel {Far'ias} and Andrew Peter Jardine and Kim Lefmann and Joseph R Manson and others},

doi = {https://doi.org/10.1039/D0CP05833E},

year  = {2021},

date = {2021-01-01},

journal = {Physical Chemistry Chemical Physics},

publisher = {Royal Society of Chemistry},

abstract = {Helium Atom Scattering (HAS) and Helium Spin-Echo scattering (HeSE), together helium scattering, are well established, but non-commercial surface science techniques. They are characterised by the beam inertness and very low beam energy (<0.1 eV) which allows essentially all materials and adsorbates, including fragile and/or insulating materials and light adsorbates such as hydrogen to be investigated on the atomic scale. At present there only exist an estimated less than 15 helium and helium spin-echo scattering instruments in total, spread across the world. This means that up till now the techniques have not been readily available for a broad scientific community. Efforts are ongoing to change this by establishing a central helium scattering facility, possibly in connection with a neutron or synchrotron facility. In this context it is important to clarify what information can be obtained from helium scattering that cannot be obtained with other surface science techniques. Here we present a non-exclusive overview of a range of material properties particularly suited to be measured with helium scattering: (i) high precision, direct measurements of bending rigidity and substrate coupling strength of a range of 2D materials and van der Waals heterostructures as a function of temperature, (ii) direct measurements of the electron–phonon coupling constant λ exclusively in the low energy range (<0.1 eV, tuneable) for 2D materials and van der Waals heterostructures (iii) direct measurements of the surface boson peak in glassy materials, (iv) aspects of polymer chain surface dynamics under nano-confinement (v) certain aspects of nanoscale surface topography, (vi) central properties of surface dynamics and surface diffusion of adsorbates (HeSE) and (vii) two specific science case examples – topological insulators and superconducting radio frequency materials, illustrating how combined HAS and HeSE are necessary to understand the properties of quantum materials. The paper finishes with (viii) examples of molecular surface scattering experiments and other atom surface scattering experiments which can be performed using HAS and HeSE instruments.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{raghavan2021alkali,

title = {Alkali metal adsorption on metal surfaces: new insights from new tools},

author = {Arjun Raghavan and Louie Slocombe and Alexander Spreinat and David J Ward and William Allison and John Ellis and Andrew P Jardine and Marco Sacchi and Nadav Avidor},

doi = {https://doi.org/10.1039/D0CP05365A},

year  = {2021},

date = {2021-01-01},

journal = {Physical Chemistry Chemical Physics},

publisher = {Royal Society of Chemistry},

abstract = {The adsorption of sodium on Ru(0001) is studied using 3He spin-echo spectroscopy (HeSE), molecular dynamics simulations (MD) and density functional theory (DFT). In the multi-layer regime, an analysis of helium reflectivity, gives an electron–phonon coupling constant of λ = 0.64 ± 0.06. At sub-monolayer coverage, DFT calculations show that the preferred adsorption site changes from hollow site to top site as the supercell increases and the effective coverage, θ, is reduced from 0.25 to 0.0625 adsorbates per substrate atom. Energy barriers and adsorption geometries taken from DFT are used in molecular dynamics calculations to generate simulated data sets for comparison with measurements. We introduce a new Bayesian method of analysis that compares measurement and model directly, without assuming analytic lineshapes. The value of adsorbate–substrate energy exchange rate (friction) in the MD simulation is the sole variable parameter. Experimental data at a coverage θ = 0.028 compares well with the low-coverage DFT result, giving an effective activation barrier Eeff = 46 ± 4 meV with a friction γ = 0.3 ps−1. Better fits to the data can be achieved by including additional variable parameters, but in all cases, the mechanism of diffusion is predominantly on a Bravais lattice, suggesting a single adsorption site in the unit cell, despite the close packed geometry.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{ward2021inter,

title = {Inter-adsorbate forces and coherent scattering in helium spin-echo experiments},

author = {David J Ward and Arjun Raghavan and Anton Tamtögl and Andrew P Jardine and Emanuel Bahn and John Ellis and Salvador Miret-Art{`e}s and William Allison},

doi = {https://doi.org/10.1039/D0CP04539J},

year  = {2021},

date = {2021-01-01},

journal = {Physical Chemistry Chemical Physics},

publisher = {Royal Society of Chemistry},

abstract = {



In studies of dynamical systems, helium atoms scatter coherently from an ensemble of adsorbates as they diffuse on the surface. The results give information on the co-operative behaviour of interacting adsorbates and thus include the effects of both adsorbate–substrate and adsorbate–adsorbate interactions. Here, we discuss a method to disentangle the effects of interactions between adsorbates from those with the substrate. The result gives an approximation to observations that would be obtained if the scattering was incoherent. Information from the experiment can therefore be used to distinguish more clearly between long-range inter-adsorbate forces and the short range effects arising from the local lattice potential and associated thermal excitations. The method is discussed in the context of a system with strong inter-adsorbate interactions, sodium atoms diffusing on a copper (111) surface.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{tamtogl2020nanoscopic,

title = {Nanoscopic diffusion of water on a topological insulator},

author = {Anton Tamtögl and Marco Sacchi and Nadav Avidor and Irene Calvo-Almazán and Peter SM Townsend and Martin Bremholm and Philip Hofmann and John Ellis and William Allison},

doi = {https://doi.org/10.1038/s41467-019-14064-7},

year  = {2020},

date = {2020-01-01},

journal = {Nature communications},

volume = {11},

number = {1},

pages = {1--9},

publisher = {Nature Publishing Group},

abstract = {The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of inter-molecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi2Te3. Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi2Te3. Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{lambrick2020multiple,

title = {Multiple scattering in scanning helium microscopy},

author = {SM Lambrick and L Vozdeck{`y} and Matthew Bergin and John E Halpin and Donald A MacLaren and Paul C Dastoor and Stefan A Przyborski and AP Jardine and DJ Ward},

doi = {https://doi.org/10.1063/1.5143950},

year  = {2020},

date = {2020-01-01},

journal = {Applied Physics Letters},

volume = {116},

number = {6},

pages = {061601},

publisher = {AIP Publishing LLC},

abstract = {Using atom beams to image the surface of samples in real space is an emerging technique that delivers unique contrast from delicate samples. Here, we explore the contrast that arises from multiple scattering of helium atoms, a specific process that plays an important role in forming topographic contrast in scanning helium microscopy (SHeM) images. A test sample consisting of a series of trenches of varying depths was prepared by ion beam milling. SHeM images of shallow trenches (depth/width < 1) exhibited the established contrast associated with masking of the illuminating atom beam. The size of the masks was used to estimate the trench depths and showed good agreement with the known values. In contrast, deep trenches (depth/width > 1) exhibited an enhanced intensity. The scattered helium signal was modeled analytically and simulated numerically using Monte Carlo ray tracing. Both approaches gave excellent agreement with the experimental data and confirmed that the enhancement was due to localization of scattered helium atoms due to multiple scattering. The results were used to interpret SHeM images of a bio-technologically relevant sample with a deep porous structure, highlighting the relevance of multiple scattering in SHeM image interpretation.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{bergin2020observation,

title = {Observation of diffraction contrast in scanning helium microscopy},

author = {M Bergin and SM Lambrick and H Sleath and DJ Ward and John Ellis and AP Jardine},

doi = {https://doi.org/10.1038/s41598-020-58704-1},

year  = {2020},

date = {2020-01-01},

journal = {Scientific reports},

volume = {10},

number = {1},

pages = {1--8},

publisher = {Nature Publishing Group},

abstract = {Scanning helium microscopy is an emerging form of microscopy using thermal energy neutral helium atoms as the probe particle. The very low energy combined with lack of charge gives the technique great potential for studying delicate systems, and the possibility of several new forms of contrast. To date, neutral helium images have been dominated by topographic contrast, relating to the height and angle of the surface. Here we present data showing contrast resulting from specular reflection and diffraction of helium atoms from an atomic lattice of lithium fluoride. The signature for diffraction is evident by varying the scattering angle and observing sharp features in the scattered distribution. The data indicates the viability of the approach for imaging with diffraction contrast and suggests application to a wide variety of other locally crystalline materials.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}