Surface Science 438 (1999) 297–304 www.elsevier.nl/locate/susc STM light emission from Ag /Si(111) Y. Suzuki, H. Minoda, N. Yamamoto * Physics Department, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan Abstract A light detection system combined with UHV scanning tunneling microscopy (STM) was constructed using an ellipsoidal mirror for light collection. The system was applied to the light emission from a flat (111) surface of a silver film with a tungsten tip in the STM. Improvement in the stability and quality of the emission spectra was achieved by cooling the sample to 80 K and decreasing the tunneling current from 500 to 50 nA. We also succeeded in observing a photon map of the silver surface at 80 K. The observed spectra taken under various sample bias voltages were compared with those calculated using the theory. The peaks in the spectra could be assigned as the multipole modes of the local plasmon formed between the tip and the surface. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Light emission; Scanning tunneling microscopy; Silicon; Silver 1. Introduction current of 300 nA. Two peaks appeared in the emission spectra at 1.9 and 2.4 eV, and their position does not depend on the tip bias. Those STM is a useful tool for studying physical peaks were interpreted to be due to localized properties of surfaces on an atomic scale because plasmon modes excited in the interface region of its high spatial resolution. Light emission from between the tip and the sample surface. A theoret- STM was first reported by Coombs et al. [1], who ical approach was made by Johansson and Monreal observed enhanced light emissions from an inter- [3] for the light emission from STM. In the calcula- face region between a tip and a metal surface. This tion by their theory, the peak at around 2.4 eV enables us to study optical properties of surfaces appears for a tip radius of 30 nm. Berndt et al. [4] in high spatial resolution, in addition to informa- observed emission spectra from a silver film with tion obtained from STM such as surface topo- a tungsten tip and a silver-covered tungsten tip, graphy and local electronic states. which gave different spectra; a single broad peak In the light emission studies, a silver film has appeared with a tungsten tip, and two sharp peaks frequently been used, because of its high emission appear in the longer wavelength region with a efficiency. Gimzewski et al. [2] have observed the silver-covered tip. This indicates that the emission dependence of the emission spectrum on the tip spectrum is significantly affected by the tip material. bias voltage using a polycrystalline silver film. The dependence of the emission spectra on the bias They used an iridium tip with a constant tunneling voltage was also observed in the W tip–Ag sample system by Berndt et al. [5]. However, the appear- ance of the peaks seems not to be systematic for * Corresponding author. the variation of the bias voltage, and the properties E-mail address: nyamamot@surface.phys.titech.ac.jp (N. Yamamoto) of the emission peaks are not yet fully understood. 0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0039-6028(99)00594-4