Spatially Resolved Surface Vibrational Spectroscopies

Herein, we discuss the STM-IETS (scanning tunneling microscopy-inelastic electron tunneling spectroscopy) technique in the following order. After briefly mentioning conventional IETS (inelastic electron tunneling spectroscopy), STM-IETS experimental results are introduced focusing on similarities and differences to conventional IETS. The working principle behind STM-IETS for the detection of vibrational modes is considered by reviewing recent progress in STM-IETS theoretical calculations. In addition, experimental setups to improve the quality of the IET (inelastic electron tunneling) spectrum, including low-temperature measurements, electronics (especially the use of the lock-in amplifier), and IET signal mapping are reviewed. With this information, we discuss in detail STM-IETS measurements performed on an alkanethiol self-assembled monolayer (SAM) formed on an Au(111) surface. This molecule is often employed as a standard sample for the examination of IETS observations with atom-scale electrodes, which are used in single-molecule electronics investigations. STM-IETS reveals not only C − H stretching mode, which often appears as a prominent feature in IET spectra, but also other vibrational features in the so-called fingerprint region including vibrational modes which are beneficial for distinguishing functional groups. A comparison with recent calculations shows excellent agreement. In addition, partial deuteration of the molecule can provide more information about the site of the molecule where the excitation of the vibrational mode occurs. A selection rule or propensity for IETS detection is then discussed on the basis of a combination of these experimental investigations and theoretical simulations.