Surface Science 274 (1992) 199-204 North-Holland i~:ii~:~.:::~:: ................................... :::,:::~:~i~ii~ i surface science ~i~:::':" ................ '..,::~:-::.-~i~ ~.::..'.~:~:~,~::.:::.~:::~:.:~::: ::::;:;:;:~,~: :~:.~::.:~:~:~:~ ~::~:~'~i :!:: ~..,. :.,.:, ~ ~:~:.,... ~!~,..:.~.., ...... Photon emission from nano-granular gold excited by electron tunneling N. Venkateswaran, K. Sattler, J. Xhie and M. Ge Department of Physics and Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA Received 3 December 1991; accepted for publication 26 February 1992 Using a scanning tunneling microscope, photon emission induced by electron tunneling into a granular gold surface was investigated. The samples consisted of aggregated small particles ~ 25 nm in diameter. We measured the intensity of light emission as a function of the applied bias voltage and observed resonance peaks at 1.9 and 2.7 V. The maxima lie at energies corresponding to the local and the extended plasmons of the granular gold surface. 1. Introduction Light emission from metal-insulator-metal (MIM) junctions has been studied extensively [1- 3]. The physics of electron-to-photon conversion is modelled as follows: (a) tunneling electrons excite surface plasmons (SP's), (b) the SP's scat- ter into radiative states via surface roughness, and (c) the radiative relaxation of the SP's gener- ates photons of the appropriate energy. The question regarding elastic versus inelastic tunneling in MIM junctions is still under debate [2]. During the inelastic process the electrons lose energy in the tunneling barrier and thus produce SP's in the metal electrode. In elastic tunneling [3] the electrons penetrate into the metal elec- trode as hot electrons where they relax and excite SP's. Both cases show the plasmon line at the same energy, but the efficiency for electron-to- photon conversion is lower for the inelastic case. In microscopic systems, the dimensionality or size can modulate the tunneling process and the subsequent conversion to photons. For example, for aggregated small particles, size effects occur which influence the emission intensity falling off with decreasing particle size. This makes light emission experiments on nano-grain materials difficult, in particular when an instrument like the scanning tunneling microscope (STM) is used where the tunneling current is restricted to rela- tively small values of ~ nA as compared to ~ mA in MIM junctions. The STM allows tunneling into selected sur- face features providing photon emission charac- teristics, in addition to the surface morphology. In MIM junctions, the surface exhibits roughness on many length scales, often leading to irrepro- ducibilities on otherwise identical samples. The correlation between surface features and emitted light, provided by the STM, is therefore an obvi- ous advantage. Also, in MIM junctions, the re- quirement of an oxide barrier restricts the choice of the substrate. On the other hand, in STM junctions the substrate can be chosen to allow the formation of nano-granular metal particles with narrow size distributions. We report experiments on such samples which we produced by the depo- sition of gold vapor on mica. An MIM junction consists of an insulator sandwiched between two flat metal electrodes. In an STM junction the insulator is replaced by a vacuum or air gap, and a sharp metallic tip is substituted for one of the metal electrodes. Un- like MIM junctions, with the STM the collective 0039-6028/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved