Spatial mapping of surface plasmons in nanoscale Ag islands on graphite using Scanning Probe Energy Loss Spectroscopy

A scanning STM tip operated at high voltage can be used to obtain localized spectroscopic information about surfaces via energy loss measurements [1]. In this technique, known as Scanning Probe Energy Loss Spectroscopy (SPELS), the STM tip is used as a localized source of field-emitted electrons, which, upon backscattering from a surface, are analyzed by an energy-dispersive detector to obtain localized energy loss spectra. Characteristic surface excitations such as plasmons and excitons (as well as secondary electrons) can be probed with a spatial resolution below 50 nm and an energy resolution approaching 0.3 eV [2]. We report the development of a new generation SPELS instrument utilizing a 400-Channel detector, allowing sufficiently fast sampling of the energy loss spectra to allow us to obtain 2D spatially-resolved maps of energy loss features in a reasonable timeframe. We demonstrate the new instrument by mapping plasmons in (thermally evaporated) Ag nano-islands on the surface of graphite and illustrate the various mechanisms give rise to the contrast obtained in the energy-resolved maps. [1] A. Pulisciano, S.J. Park and R. E. Palmer, Appl. Phys. Lett. 93, 213109 (2008). [2] F. Festy and R. E. Palmer, Appl. Phys. Lett. 85, 5034 (2004).