Prism-coupled scanning tunneling microscope light emission spectroscopy of Au film covered with self-assembled alkanethiol monolayer

We have investigated scanning tunneling microscope (STM) light emission from bare and alkanethiol self-assembled monolayer (SAM)-covered Au films in the Kretschmann geometry. The films were deposited on the flat bottom of a hemispherical prism, and the STM light emissions from the tip-sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. The prism-side emission was much stronger than the tip-side emission for the bare Au film. Theoretical analysis revealed that this enhancement of emission intensity is caused by the fact that surface plasmon polaritons (SPPs) localized at the Au surface become radiative on the prism side. This geometry was applied to the investigations of STM light emission from the Au film covered with an alkanethiol SAM. The prism-side emission was successfully detected by virtue of the enhancement of STM light emission.