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The pace of modern technological development - from microelectronics to catalysts, to coatings - is driven by
fundamental breakthroughs in fabricating novel nano-structured materials. Such materials often promise unique or
enhanced physical characteristics, such as strong magnetisation or unusual reactivity. These characteristics will
derive from, but can also be compromised by, subtle, atomic-scale structural variations. In each case,
nano-resolved characterization is essential and full understanding of a material demands multiple state-of-the-art
experimental probes. The probes used in my research include beams of electrons, atoms, ions, and photons, as
well as physical devices such as the tip of an atomic force microscope. Each probe has its advantages and its
limitations and the best studies of nanostructured material combine the results from several probes in order to develop
a complete description.
A selection of my recent projects is given below. The uppermost projects derive from my time in Glasgow, whilst
some of those below are based on collaboration with colleagues in Cambridge, where more information can be found.
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The objective of this ambitious project was the design and construction of a helium-atom based
microscope - i.e. a microscope that uses a focused beam of helium atoms (rather than photons or
electrons) to image a sample. The principal motivation is the unique combination of de-Broglie
wavelength (<1Å) and momentum (~1Å-1) provided by a thermal energy He beam, lending
sensitivity to both atomic-scale structure and dynamics. In addition, typical beam energies of ~20meV
eliminate probe-induced damage, giving a truly non-invasive probe. Unlike neutrons, a He probe is
exclusively surface sensitive and scatters from the outermost electronic contours of a sample. As a
consequence, the existing reciprocal-space technique of helium atom scattering (HAS) is already used
routinely as a non-perturbing probe of surface nucleation, growth, alloying, adsorption and diffusion
processes. It is of particular advantage in high-precision structural
and dynamical studies of surfaces and in observation of delicate or organic systems, insulators, or
systems with surface hydrogen. Conventional HAS, however, lacks spatial resolution, restricting its
application to homogeneous surface systems.
The key technological advance that enables microscopy with helium atoms was the development of
an atom-optical element that can focus and manipulate the atomic beam. As ground-state helium has
neither spin nor charge, electromagnetic focusing is not possible and instead we developed an
ultra-smooth atom-optical mirror (illustrated to the right). The mirror surface is fabricated from a
chemically-prepared, ultra-thin silicon single crystal that is then carefully bent into a pseudo-
parabolic shape. Once focused, a helium beam can be scanned across
a surface in a similar manner to the operation of a scanning electron microscope - an imaging mode that
we term "Scanning Helium Microscopy" (SHeM). The technique extends the capabilities of HAS to
heterogeneous or amorphous "real-world" systems and creates a host of new experimental
opportunities.
This project is run within the Cambridge Surface Physics group, where a
more detailed explanation can be found. It is supported both by the EPSRC and also the European Commission in a multi-national project
entitled Imaging with Neutral Atoms (INA).
Publications: 1-3,5,6,10,17-19,21,29.
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