Fourier transform infrared scanning tunneling microscopy: Measuring vibrational modes at the nanoscale

Scanning probe microscopy has enabled unprecedented surface imaging capability down to atomic resolution. Recent advances in photon technologies has led to successful integration of optical spectroscopy with various scanning probe microscopy. In this work, we use the scanning tunneling microscope tip as a nano antenna and a multiplexed signal from a bench-top Fourier-transform interferometer to discover vibrational modes at the nanoscale. By back illuminating our gold on sapphire samples, we are able to excite molecular self-assembled monolayers into vibrationally excited states evanescently, thus creating a perturbation in the conductance of the STM junction. We use cyanide on gold as a proof-of-concept system because of its distinct vibrational spectra, its stability under ambient conditions, and our control of the molecules interactions. In previous work, we have demonstrated direct control of the gold cyanide molecular packing structures – cubic or hexagonal close packed – and can contribute a vibrational mode from Raman and infrared spectroscopy to each phase. This custom-built instrument performs operates at ambient temperature and pressure, enabling us to image chemical structure with single-molecule resolution on a variety of systems.