A straightforward optical alignment protocol for STM-based single molecule spectroscopy

Abstract

A light-accessed scanning tunneling microscope (STM) is a powerful spectroscopic tool that enables chemical analysis at the single molecular level, but it requires highly precise optical alignments to pinpoint the nano-scale tunneling gap, leaving experimental challenges. Here we present straightforward procedures to align the optical setup for STM-luminescence and STM-based tip-enhanced Raman spectroscopy (TERS) performed with a reflection geometry in an ultrahigh vacuum chamber. Observing real-space images of the metal tip apex through a spectrograph set to the zeroth-order diffraction enables “ in situ” optimization of the detection path and introduction of the excitation light of TERS to the nanogap. The best spatial overlap with the nanogap can be achieved by monitoring plasmon-enhanced, low-frequency inelastic scattering of the metal. This protocol allows us to overcome such difficulties in STM-based spectroscopy and facilitates physicochemical study of single adsorbates on nontransparent substrates.