Light emission from gold and silver thin films in a scanning tunneling microscope: role of contamination and interpretation of grain structure in photon maps

Scanning tunnelling microscope (STM) tip-induced light emission from Au and Ag has been studied. Thin film samples similar to100nm thick were prepared by thermal evaporation at 0.5nm/s onto a room-temperature glass substrate to produce grains of 20-50nm in lateral dimension at the surface. Light emission from the samples in the STM was quasi-simultaneously recorded with the topography, at 1.8V tip bias and 3-40nA current, alternating pixel by pixel at the same bias. Typically, a surface scan range of 150 nm x 150 nm was surveyed. Au, W and PtIr tips were used. Light emission from An and Ag metal surfaces in ambient atmosphere is found to be reduced, often totally inhibited, by surface adsorbed contamination. Reference to a result from Smolyaninov explains how a workfunction decrease and corresponding gap increase as little as 0.1 nm arising from contamination can reduce light output by 25% while a 1 nm gap increase will essentially prevent observable emission. Even with a freshly prepared clean sample the tip of the STM picks up and sheds contamination. This is seen in on- and off-switching of light emission while topographic imaging suffers down- and up-steps respectively of 2-5nm height but otherwise survives. Contamination problems and tip instability mean that operation in ambient though less reliable is in some ways more informative than that in vacuum. The light emission pattern, when observed in ambient or in 10(-8) Torr vacuum, shows no meaningful correlation with local (sub-granular) surface topography but there is a high correlation of grain boundaries between the topographic images and photon maps. A software program has been written to detect grain boundaries in each and correlate them. To explain the observed constancy of light emission from individual grains it is suggested that the tip induced plasmon modes of the tip-grain system are determined importantly by grain geometry, particularly thickness. Thus, interference of contributions from the top and bottom faces of the grains dictates the intensity of light emitted from an individual grain and by implication the grains are sufficiently isolated from each other to behave individually. A corollary is that coupling differences and also variability of grain lateral dimensions may fine tune the resonance and account for the observed variety in output intensity. Dark grains may be a special case where destructive interference is substantially complete. (C) 2004 Elsevier B.V. All rights reserved.