The Scanning Tunnelling Microscope is an established surface science tool, combining unprecedented resolution with real space mapping. One of its major drawbacks, however, is that it gives no chemical information, but Photon Emission is able to probe the inelastic channel, which for metals contain invaluable chemical information. It has already been shown that flat metal surfaces produce differing emission spectra. Real surfaces may not be flat, so it is important to known how curvature changes the yield of photons, and the emission spectrum. For this reason, a two sphere model was developed. The mechanism for Photon Emission, that of excitation of localised surface plasmons followed by either radiation or dielectric loss, was split into separate problems so that the dependence of the overall photon emission on materials and curvature is clear. Experiments were performed on small silver particles, and for what was later believed to be a silver tip it was found that the Photon Emission was approximately proportional to both the tin and particle radius. Light emission was observed from clusters containing as few as 30 atoms, and the first chemically specific photon maps were presented, which distinguished silver particles from carbide deposits. Emission was also seen on the Si(111)-7x7 surface. The new theory compared well with spectra from flat surfaces, and when extended to two spheres it also showed that the Photon Emission was approximately proportional to both the tip and particle radius. It was predicted that the onset of emission would occur at a lower tip bias for gold particles than for silver particles, and this was confirmed experimentally. Metal-specific spectra will be produced if the plasmon modes do not move in energy when curvature varies, staying close to the travelling surface plasmon energy. This should occur for small radius tips and particles, and the movement of modes should be reduced for tungsten tips.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598626 |
| Date | January 1997 |
| Creators | Downes, A. R. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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