Capacitance Behavior of Alkanethiol Self-Assembled Monolayer Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have studied the capacitance behavior of alkanethiol self-assembled monolayer (SAM)-covered Au film by scanning tunneling microscope light emission (STM-LE) spectroscopy in the Kretschmann geometry. Although, the STM-LE from tip-sample gap into the vacuum (tip-side emission) is fundamentally weak and very difficult to detect, we have succeeded in detecting the STM-LE radiated into the prism (prism-side emission) by virtue of the enhancement of prism-coupled geometry. Our experimental results shows that the cutoff energy of STM-LE spectra have been redshifted with increase in thickness (chain length) of the SAM film. In order to explain the cutoff energy shift, we have designed a two-layer tunnel junction model by considering the capacitance response of the SAM film depending on the molecular chain length. It has been found from the model analysis, that the capacitance of the SAM changes with changing of the molecular thickness. Hence, it is concluded that the shift of the cutoff energy has originated from the change of the capacitance of the SAM film depending on the molecular chain length.