Structural transitions in the 309-atom magic number Lennard-Jones cluster

The thermal behaviour of the 309-atom Lennard-Jones cluster, whose structure is a complete Mackay icosahedron, has been studied by parallel tempering Monte Carlo simulations. Surprisingly for a magic number cluster, the heat capacity shows a very pronounced peak before melting, which is attributed to several coincident structural transformation processes. The main transformation is somewhat akin to surface roughening, and involves a cooperative condensation of vacancies and adatoms that leads to the formation of pits and islands one or two layers thick on the Mackay icosahedron. The second transition in order of importance involves a whole scale transformation of the cluster structure, and leads to a diverse set of twinned structures that are assemblies of face-centred-cubic tetrahedra with 6 atoms along their edges, i.e., one atom more than the edges of the 20 tetrahedra that make up the 309-atom Mackay icosahedron. A surface reconstruction of the icosahedron from a Mackay to an anti-Mackay overlayer is also observed, but with a lower probability.Comment: 7 pages, 4 figure

Effect of substrate surface topography on forensic development of latent fingerprints with iron oxide powder suspension

This is a pre-print version of the article. The official published version can be accessed from the link below - Copyright @ 2010 Wiley-BlackwellLatent fingerprint deposition and effectiveness of detection are strongly affected by the surface on which prints are deposited. Material properties, surface roughness, morphology, chemistry and hydrophobicity can affect the usefulness or efficacy of forensic print development techniques. Established protocols outline appropriate techniques and sequences of processes for broad categories of operational surfaces. This study uses atomic force microscopy and scanning electron microscopy to investigate a series of surfaces classified as smooth, non-porous plastic. Latent prints developed with iron oxide powder suspension are analysed on a range of scales from macro to nano to help elucidate the interaction mechanisms between the latent fingerprint, development agent and underlying surface. Differences between surfaces have a strong effect, even within this single category. We show that both average roughness and topographical feature shape, characterised by skew, kurtosis and lay, are important factors to consider for the processing of latent fingerprints. Copyright (C) 2010 John Wiley & Sons, Ltd.This work is part-funded by the UK Home Office project 7088762

Quantum spill out in few-nanometer metal gaps: Effect on gap plasmons and reflectance from ultrasharp groove arrays

Plasmons in ultranarrow metal gaps are highly sensitive to the electron density profile at the metal surfaces. Using a fully quantum mechanical approach, we study the effects of electron spill-out on gap plasmons and reflectance from ultrasharp metal grooves. We demonstrate that the mode index of ultranarrow gap plasmons converges to the bulk refractive index in the limit of vanishing gap and, thereby, rectify the unphysical divergence found in classical models. Surprisingly, spill-out also significantly increases the plasmonic absorption for few-nanometer gaps and lowers the reflectance from arrays of ultrasharp metal grooves. These findings are explained in terms of enhanced gap plasmon absorption taking place inside the gap 1-2 {\AA} from the walls and delocalization near the groove bottom. Reflectance calculations taking spill-out into account are shown to be in much better agreement with measurements compared with classical models

Plasma-enhanced atomic layer deposition of nanostructured gold near room temperature

A plasma-enhanced atomic layer deposition (PE-ALD) process to deposit metallic gold is reported, using the previously reported Me3Au(PMe3) precursor with H-2 plasma as the reactant. The process has a deposition window from 50 to 120 degrees C with a growth rate of 0.030 +/- 0.002 nm per cycle on gold seed layers, and it shows saturating behavior for both the precursor and reactant exposure. X-ray photoelectron spectroscopy measurements show that the gold films deposited at 120 degrees C are of higher purity than the previously reported ones (<1 at. % carbon and oxygen impurities and <0.1 at. % phosphorous). A low resistivity value was obtained (5.9 +/- 0.3 mu Omega/cm), and X-ray diffraction measurements confirm that films deposited at 50 and 120 degrees C are polycrystalline. The process forms gold nanoparticles on oxide surfaces, which coalesce into wormlike nanostructures during deposition. Nanostructures grown at 120 degrees C are evaluated as substrates for free-space surface-enhanced Raman spectroscopy (SERS) and exhibit an excellent enhancement factor that is without optimization, only one order of magnitude weaker than state-of-the-art gold nanodome substrates. The reported gold PE-ALD process therefore offers a deposition method to create SERS substrates that are template-free and does not require lithography. Using this process, it is possible to deposit nanostructured gold layers at low temperatures on complex three-dimensional (3D) substrates, opening up opportunities for the application of gold ALD in flexible electronics, heterogeneous catalysis, or the preparation of 3D SERS substrates