A UHV-LT-STM System for Optical Experiments

The combination of optical techniques and scanning tunneling microscopy (STM) provides insight into a diverse set of physical processes including surface chemistry, surface-photon interactions, and spin scattering in semiconductors. We present a novel cryogenic temperature, ultrahigh vacuum STM which incorporates a maneuverable, high numeric aperture lens, with sub-5 micron spot size, in proximity to the tunnel junction. Modifications to our microscope have been made to improve upon the 12.5 K base temperature and 10 pm tip stability. Our initial efforts are focused on studies of photo-chemical reactions and chemical identification by tip-enhanced Raman spectroscopy (TERS). UHV deposition techniques have been developed for a variety of molecules well suited for TERS studies including azulene, azobenzene, methylene blue, and C60. Electrochemically and chemically-etched Ag and Au tips are optimized for field enhancement with characterization by scanning electron microscopy and collection of the plasmon emission from the tip. Raman spectra have been collected from molecule-coated tips in vacuum to test the optical setup for TERS. http://www.physics.ohio-state.edu/$\sim $jgupta