STM studies on dye molecules embedded in ordered liquid crystal structures and an approach for laser-assisted scanning tunneling microscopy

Abstract

Scanning tunneling microscopy, although capable of yielding very high resolution on periodic structures, very often provides only moderate resolution on singular features. Our work aims at the use of laser light to improve the identification of individual molecules. We report on scanning tunneling microscopy measurements performed on dye molecules dissolved in a liquid crystal and adsorbed onto highly oriented pyrolytic graphite. Either localized perturbations of the liquid crystal structure with the size of single molecules or more or less extended ordered domains of well resolved dye molecules were reproducibly imaged for several dyes. To study light-induced resonant effects the influence of non-resonant absorption leading to thermal expansion of tip and sample has to be suppressed. Therefore, an electro- optical system was realized using an ArPLU- and a dye laser of different wavelengths power-modulated with a relative phase shift of 180 degree(s). Preliminary results obtained with this setup are presented documenting the efficiency of the compensation.