STM Light Emission Spectroscopy of Self-Assembled Monolayer of Alkanethiol on Au Film

Abstract

In this paper, scanning tunneling microscope (STM) light emission (STM-LE) from alkanethiol self-assembled monolayer (SAM)-covered Au film has been probed in the Kretschmann configuration. The films were deposited on the smooth plane of a hemispherical glass prism. STM-LEs from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were investigated. Our experimental results showed that the tip-side emission was scarcely found, and the prism-side emissions were successfully detected due to the enhancement of Kretschmann configuration. It was also found from the experimental study that the peak intensity of STM-LE spectra become smaller, accompanying the redshift of the peak position with the rise in thickness of the alkanethiol SAM film. The main focus of this paper is to explore the behavior of electron tunneling into the SAM-covered Au film. In order to explain these phenomena, we have designed different models regarding the tip–sample gap structure. Among these models, the antenna factor successfully explains the cutoff energy shift of STM-LE spectra. According to this model, the cutoff energy shows redshift when the electronic transition occurs at the Au–S interface layer and the amount of the shift depends on the strength of the transitions.