Seminar on
Modern Optics and Spectroscopy


Martin Gruebele, University of Illinois Urbana-Champaign

"STM-assisted single molecule absorption spectroscopy"

April 22, 2008

12:00 noon - 1:00 p.m. Grier Room 34-401


Abstract:

Scanning tunneling microscopy provides exquisite spatial resolution, but only moderate energy resolution via current-voltage derivative curves.  Absorption spectroscopy can resolve individual molecular energy levels, but usually provides no spatial information.  We combined the two techniques by using an STM tip to detect the electron density change when a molecular absorption line is resonantly excited by a laser.  There are plenty of technical difficulties, beginning with laser heating effects, and I show how they can be overcome by combining laser frequency modulation, rear illumination, tip field enhancement, and lock-in detection of the STM current signal.  The results is a technique that simultaneously returns topographic images and absorption spectra with sub-molecular resolution.  The ability to tell where a macromolecule actually absorbs can be usefully applied, for example to detecting defects in the molecule invisible in ordinary STM scans.  Future applications to energy transfer from one quantum dot to another will also be discussed, along with ultrathin gold surfaces needed to satisfy the requirements of both STM and absorption spectroscopy.

TUESDAYS, 12:00-1:00, GRIER ROOM (34-401)
Refreshments served following the seminar

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Co-sponsored by the George R. Harrison Spectroscopy Laboratory,
the Department of Electrical Engineering and Computer Science and
the School of Science, Massachusetts Institute of Technology.