Alternating current scanning tunneling spectroscopy of self-assembled monolayers on gold

We demonstrate the spectroscopic capabilities of an ac scanning tunneling microscope. This ac scanning tunneling microscope has a bias voltage modulation frequency tunable over the range dc–20 GHz and is thus capable of recording images and local spectra of insulating as well as conducting substrates. In this article we detail the spectral sensitivity and the tip-sample separation dependence of the spectra recorded by simultaneously tuning the modulation and detection frequencies. Spectra of an evaporated gold film and of self-assembled monolayers of CH3(CH2)15SH and of CH3O2C(CH2)15SH on gold are presented.