Nanospectroscopy of single silicon nanowire surface using energy filtered X-ray photoElectron emission microscopy

Bailly, Aude; Barrett, Nick; Renault, Olivier; Zagonel, Luiz Fernando; Gentile, Pascal; Pauc, Nicolas; Baron, Thierry; Dhalluin, Florian; Cezar, Julio Criginski; Brookes, Nicholas France

Engineering of nanowires (NWs) is of prime importance in order to tailor their properties for nanotechnology applications. This implies to get a better insight into the properties of a single NW and therefore requires specific experimental techniques. Here we illustrate the capabilities of X-ray PhotoElectron Emission Microscopy (XPEEM) to undertake surface studies of isolated single NW. The instrument used is the NanoESCA featuring a fully electrostatic PEEM column together with an aberration-corrected energy filter (double hemispherical energy analyser) [1]. It is installed at the Nanocharacterisation Centre of the CEA Leti-Minatec and is periodically moved to the European Synchrotron Radiation Facility (ESRF) to benefit from the high brightness and energy tuning of ID08, a soft X-ray beamline. The combination of energy filtering and synchrotron radiation as excitation source gives the possibility of acquiring core level images of a given element with both a high lateral and energy resolutions thus giving information on the element spatial distribution and chemical state. Furthermore images obtained from low energy secondary electrons at the photoemission threshold may reveal subtle differences in local work function at the surface.
150 nm-diameter silicon NWs were grown using the vapour-liquid-solid process from gold/silicon nanoparticles. After growth the silicon NWs were dispersed on gold patterns for the XPEEM study. The Si2p, Au4f and O1s core levels were imaged in order to spatially map the element distribution. The NWs sidewalls turn out to be oxidised and also partially covered by gold. This last point has been evidenced by localized spectra at photoemission threshold taken along the NWs and revealing two distinct photoemission edges: one characteristic of Si NWs and the other one close to that of the gold substrate. This spectromicroscopic investigation tends to support a recent study showing that gold, which is actually part of the catalyst used to promote the growth of the NWs, can diffuse along the NW sidewalls limiting its length and altering its shape and surface properties [2].
[1] M. Escher et al., J. Phys.: Condens. Matter 17 (2005) S1329.
[2] J.B. Hannon et al., Nature 440 (2006) 69.

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