Surface electric field‐induced Raman spectroscopy from adsorbate and substrate modes

Abstract

AbstractRaman spectroscopy due to electric fields at surfaces, which manifests itself as surface‐enhanced Raman scattering (SERS) from adsorbate modes, and as electric field‐induced Raman scattering (EFIRS) from substrate modes, is discussed. In SERS experiments with benzene adsorbed on small Ni/SiO2 and Pt/SiO2 clusters, using exciting radiation of different wavelengths, we observed not only very strong peaks of the normally Raman active vibrational modes but also normally infrared‐active and some inactive modes. Although the frequencies of the modes are only slightly shifted with respect to liquid benzene, the relative intensites of the Raman lines are considerably changed and in addition they are dependent on the excitation wavelength. The observation of metal‐carbon vibrations indicates a strong interaction between the adsorbate and the metal atoms. Therefore, in addition to the electric field, a metal‐molecule electronic charge transfer may be responsible for the enhancement mechanism.EFIRS is based on the symmetry‐forbidden scattering from the longitudinal optical (LO) substrate phonon. It allows the detection of the static electric field in the surface region of semiconductor substrates, which originates from electronic band bending. By EFIRS the band bending can be monitored at surfaces and interfaces for coverages from submonolayer up to more than 100 monolayers, depending on the overlayer material. Here, the Sb/GaAs(110) interface is treated as an example. Considerable band bending dynamics is observed for overlayer thicknesses up to 100 monolayers. Simultaneous analysis of the overlayer structure by Raman scattering from the Sb vibration modes allows the interpretation of the band bending dynamics.