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45421-AC5
Energy Transfer Studied by Atomic Resolution Absorption Spectroscopy
The final goal of this project is to do single molecule absorption spectroscopy detected by a scanning tunneling microscope, in order to look at energy transfer among pairs of single molecules. For the first annual period, considerable progress has been made in several of the directions needed.
We have been able to image individual carbon nanotubes and CdSe quantum dots under laser illumination, and we have recorded absorption spectra of carbon nanotubes on passivated Si (100) surfaces. A paper featured on the March cover of the Journal of Physical Chemistry has reported the latter results.
We have successfully used both amplitude and frequency modulation of the laser beam to suppress signals from surface heating. In addition, rear-illumination through a prismatic wedge reduces tip heating, whereas the sharp tip enhances the evanescent wave at the location of the molecule, increasing the absorption signal all the way to saturation, with laser powers of only a few mW. To allow complete wavelength scans of single molecule spectra, we have put into operation a tunable OPO system, which allows scans between 1100-1270 nm at powers above 50 mW. We characterized a series of nanotubes of varying length and diameter, and find that we can now assign nanotube chirality parameters based on an ordinary STM scan and the absorption spectrum down to a choice of 1-2 chiralities. To get to the level of unique assignments, we will need to take full wavelength scans, which will become possible with the OPO system.
We recently observed position dependent absorption within a single carbon nanotube, a sign of an electronic structure transition such as from metallic to semiconducting behavior. Thus we can pinpoint the source of absorption with submolecular precision, at the “functional group” level. This was reported in a brief report in Materials Today. Although we have not quite achieved atomic resolution with our single molecule absorption technique, we are now at the 5 Å level, a factor of 20 better than the next-best single molecule optical techniques such as near-field illumination.
We have perfected the stamping methods that are necessary for putting on the surface sufficient numbers of isolated (as opposed to clumped) molecules, so that pairs of molecules can be investigated with reasonable probability. For CdSe clusters, it turns out that coating a Si wafer with a toluene solution of clusters, then evaporating, and stamping onto the Si substrate works best. For nanotubes, we find that a teflon or fiberglass applicator produces good results.
Finally, we have studied thin metal surfaces deposited onto atomically flat sapphire, both with a Nb/Cr adhesion layer, and by electron beam as well as sputtering deposition. We find that sputtering/beam deposition produce opposite trends in surface roughness vs. film thickness, with a cross-over at a film thickness of about 10 nm. For the purposes of single molecule absorption spectroscopy on surfaces, either sputtered 15 nm films or ebeam films of 6 nm have the necessary conductivity and smoothness properties for our experiments.
