Scanning tunneling microscopy and barrier-height observation of stearic acid thin films prepared by hot-wall technique

Stearic acid thin films prepared by the hot-wall technique have been observed and barrier heights (φ) have been measured using scanning tunneling microscopy (STM), which is operated in air. Three kinds of substrates are used to prepare stearic acid films: highly oriented pyrolytic graphite (HOPG), gold thin film and indium-tin-oxide (ITO) film. X-ray diffraction analysis reveals that molecules in films are oriented normal to the substrate. The thickness of films we used is about 30 nm. Using STM and φ images of films, we confirmed that the film surface morphology varies with the kind of substrate. Islands of stearic acid are sparsely formed on the HOPG and gold thin film substrates; however, in the case of the ITO substrate, small islands (about 3 nm) nearly cover the entire substrate surface. Under certain circumstances, it is more effective to observe the stearic acid morphology in terms of values of φ with the STM images in the constant-current mode.