Structural and chemical evolution of Au-silica core-shell nanoparticles during 20 keV helium ion irradiation: a comparison between experiment and simulation.

Au-silica core-shell nanoparticles have been irradiated with 20 keV He+ ions up to a maximum fluence of 4.7 × 1017 ions/cm2. The nanoscale structural and crystallographic evolution induced by He+ ion irradiation was followed at various stages using Transmission Electron Microscopy (TEM). During irradiation satellite Au clusters are formed around the main Au core, which remained crystalline even after the maximum He+ ion fluence. The spherical silica shell deformed into a hemisphere due to He+ ion irradiation. Three dimensional Monte-Carlo simulations, based on the binary collision approximation, have been performed on stacked infinite layers and an individual particle. The stacked layers results show that the He+ beam interacts with most of the nanoparticle and Au migrates in the direction of beam incidence agreeing with experimental findings. The individual particle results match the experiment in terms of the volume which is sputtered away however additional mechanisms, not included in the simulations, are present in the experiment during the satellite formation and silica shell deformation. These results show the ability for 20 keV He+ ions to be used for the modification of nanostructures. Furthermore, these results contribute to a quantitative understanding of the dynamic evolution of materials observed using microscopy techniques based on He+ ions.

Prevalence of active Charcot disease in the East Midlands of England.

AIMS: To undertake a prospective point prevalence study of the prevalence of active Charcot neuro-inflammatory osteoarthropathy (Charcot disease) in a circumscribed part of England and to audit the time elapsing between disease onset and first diagnosis. METHODS: The prevalence of active Charcot disease of the foot during a single month was assessed by specialist foot care teams at seven secondary care services in the East Midlands region of England. RESULTS: A total of 90 cases were identified, representing 4.3 per 10 000 of the 205 033 total diabetes population of the region. The time elapsed from first presentation to any healthcare professional until diagnosis was also assessed. While the diagnosis was suspected or confirmed in one-third of patients within 2 weeks, it was not made for 2 months or more in 23 patients (24%). CONCLUSIONS: Non-specialist professionals should have greater awareness of the existence of this uncommon complication of diabetes in the hope that earlier diagnosis will lead to lesser degrees of deformity.

Normal modes and mode transformation of pure electron vortex beams.

Electron vortex beams constitute the first class of matter vortex beams which are currently routinely produced in the laboratory. Here, we briefly review the progress of this nascent field and put forward a natural quantum basis set which we show is suitable for the description of electron vortex beams. The normal modes are truncated Bessel beams (TBBs) defined in the aperture plane or the Fourier transform of the transverse structure of the TBBs (FT-TBBs) in the focal plane of a lens with the said aperture. As these modes are eigenfunctions of the axial orbital angular momentum operator, they can provide a complete description of the two-dimensional transverse distribution of the wave function of any electron vortex beam in such a system, in analogy with the prominent role Laguerre-Gaussian (LG) beams played in the description of optical vortex beams. The characteristics of the normal modes of TBBs and FT-TBBs are described, including the quantized orbital angular momentum (in terms of the winding number l) and the radial index p>0. We present the experimental realization of such beams using computer-generated holograms. The mode analysis can be carried out using astigmatic transformation optics, demonstrating close analogy with the astigmatic mode transformation between LG and Hermite-Gaussian beams.This article is part of the themed issue 'Optical orbital angular momentum'.