Single Molecule Imaging and Spectroscopy
[Work supported by the National Science Foundation and by Research Corporation]

Experiments at the level of individual molecules have recently attracted much interest across traditional disciplinary boundaries. The expectation stands that a profound understanding of molecular-scale processes and unique engineered structures may be obtained if standard ensemble measurements and 'top-down' fabrication schemes are abandoned.
Few specific tools have so far been developed for experiments in this exciting and rapidly expanding field. Techniques based on scanning probes promise to become key tools for measurements on single molecules adsorbed on solid substrates. The capability of scanning tunneling microscopy (STM), for instance, to locally inject carriers into single molecules or even individual molecular bonds opens up a wide range of possibilities for atomic resolution imaging, spectroscopy and manipulation. Of prime importance among them is the imaging of molecular structures with chemical specificity, i.e., with the capability of identifying intramolecular units joined by a single bond.
Figure 1: (Left) Home-built, compact STM head for vacuum operation. (Right) Our new low-temperature UHV STM: (1) LHe cooled cryoshield; (2) Access door; (3) STM scan head; (4) Cu sample stage; (5) Spring suspension; (6) Precooling clamp; (7) SS capillary cable guides.

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Figure 2: What makes a good STM tip?? (Left) Conventional metal STM tip; (Right) Cleaved semiconductor tip, and STM image on Si(111) 7x7 obtained with an InAs probe.
We are developing novel techniques, based on STM, by which such chemical specificity may be achieved. One of them, energy-filtered STM, uses inelastic scattering of tunneling electrons injected into single molecular bonds as a fingerprint for identifying that bond. We are also developing novel scanning probe instrumentation that allows us to test our proposed STM imaging and spectroscopy schemes under extreme conditions. Highlights are a compact UHV-STM head for operation in a 4He bath cryostat, and a larger version of a low-T, UHV STM that will be attached to an existing UHV sample preparation system (in development).