The general theme of this project is to monitor the emergence of bulk properties from the atomic scale. We are using our scanning tunneling microscopes (STMs) to study the properties of metal nanoclusters whose size can be varied from a few atoms to thousands of atoms. Quantum confinement of electrons in the cluster leads to a discrete spectrum of states with a level spacing that depends on the size. By isolating the nanocluster from the metal substrate by using an ultrathin insulating layer (e.g., Cu₂N islands), we can measure this spectrum with tunneling spectroscopy. The optical plasmon response of metal nanoclusters is also subject to quantum confinement effects. We can monitor the plasmon energy using STM-induced luminescence techniques.

Inelastic light scattering (the Raman effect) gives spectroscopic information about molecular vibrational modes and magnetic excitations in a wide variety of systems. One limitation of the technique is that only a small fraction of incident photons lose energy during the scattering process; Raman signals are thus very small. Huge enhancements in Raman scattering have been seen from molecules adsorbed on rough metal surfaces. Such enhancements have enabled Raman spectroscopy at the single molecule level. This effect arises from the locally enhanced electric field in the vicinity of sharp metal protuberances. Comparable enhancement may be expected at the apex of a sharp metal tip. Our goal is to realize the tip-enhanced Raman effect in a high resolution, low temperature scanning tunneling microscope. Such a combination offers a way to image vibrational excitations with sub-molecular spatial resolution.

Spin polarized STM

Spin-polarized carriers are excited in certain semiconductors upon illumination with circularly polarized light. Conventional far-field optical methods measure dynamical properties of an ensemble of spins whose mutual coherence can be limited by spin-scattering with magnetic impurities or other carriers. An ability to spatially resolve spin polarization would help to better understand such processes by focusing on spin-scattering at the single-impurity level. Using ferromagnetic or antiferromagnetic metal tips, the tunnel current in an STM can be spin-polarized. We envision illuminating a semiconductor surface with light, and imaging the resultant spin polarization with the STM tip in the vicinity of single impurities or magnetic adsorbates. 

STM study of Transport through molecule

Electron transport through organic molecules have been studied via various techniques. Since STM has advantages in terms of atom/molecule manipulation and I/V measurement in atomic precision, it can allow us to study forming and characterizing contact between single molecules and metal atoms.  By using ultrathin insulating film (Cu₂N) on metal substrate, we can tune interaction between single molecules and metal substrate (the insulating layer suppress local density of state of metal substrate). These advantages open opportunity to study electronic structure of single molecules, charge transfer of molecule due to chemical bonding with metal atoms, and electron/spin transport through single molecules in atomic level.