We are developing scanning probe microscopies to study molecular films such as may be used in molecular electronic devices. In particular we are interested in charge transport in the films under the influence of light. We are studying the dynamics of suface plasmons on metal and semiconductor surfaces and their interactions with microstructures (including optical microcavities). Our interest is in both fundamental effects (such as microcavity QED modification of surface plasmon lifetimes) and in the advancement of the technology of integrated optics.
While an STM is capable of selecting a single atom or molecule into which to inject current, positional stability is usually inadequate for true spectroscopic measurements either for electron current or for light. The laboratory is developing techniques with which to maintain position above a point of interest by recognizing features in the surrounding topography. Evanescent-wave coupling is used to illuminate the junction region with light from various types of laser sources. Emitted light is detected with photon-counting techniques.
Another major tool in Gregory's laboratory is the crossed-fiber tunnel junction, in which electronic tunneling can occur between optically transparent gold films deposited on the surface of tapered optical fibers. At the same time, photons are supplied to the junction via evanescent coupling from the fiber through the gold film. Atomic or molecular films are deposited on to the gold and these films become the junction barrier material when the fibers are brought close together. The barrier material also provides mechanical stability to the junction, in contrast to the STM situation. However, the microscopic character of the crossed-fiber junction ensures that single molecules are selected and can be studied.
At sufficiently high bias voltages, a tunnel current can excite optical levels within molecules. The radiative decay of these levels can, at least in principle, be detected as fluorescence. Interestingly (and also problematically) the behavior of junctions under biases of several volts can be rather complex, so this in itself is also being studied. In the lower-bias regime, we are studying surface-enhanced Raman scattering from vibrational excitations of the molecules created by the electron tunnel current.
It is hoped that these kinds of studies will give information which is of relevance to the use of molecular films in sensors and eventually in "molecular electronics."