45421-AC5
Energy Transfer Studied by Atomic Resolution Absorption Spectroscopy
The final goal of this project is to do single molecule absorption spectroscopy at room temperature, and to look at energy transfer among single molecules. We detect the absorption by looking for electron density changes in the excited molecule with a scanning tunneling microscope. During the final annual period, we collected continuous absorption spectra vs. wavelength, and we completed characterization of the ultrathin metal film surfaces needed for the energy transfer project. These surfaces are simultaneously transparent, conductive, and flat to less 1 Å rms over regions of 10s of nanometers. Three publications resulted from ACS-PRF funding of this project and acknowledge the fund, and a fourth and final one is in preparation. We have been able to image individual carbon nanotubes and CdSe quantum dots under laser illumination, and we have now recorded absorption spectra of carbon nanotubes on passivated Si (100) surfaces. A paper featured on the March 2007 cover of the Journal of Physical Chemistry reported our measurement of absorption images on carbon nanotubes. We have also imaged defects in carbon nanotubes by single molecule absorption spectroscopy, and initial results are published in Materials Today. Our work creating flat, transparent conducting substrates for the single molecule absorption experiments has just been submitted to the Journal of Physical Chemistry. We have successfully used both amplitude and frequency modulation of the laser beam to suppress signals from surface heating that interfere with the absorption signal. 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 now use a diode laser system scanned between 1200-1260 nm at powers above 10 mW. We combine this with a reference laser at 1300 nm, modulating between the two to subtract background signals from the resonant absorption signal. We characterized a series of nanotubes of varying length and diameter. We have now achieved full wavelength scans of the single molecule absorption spectrum (to be submitted for publication). We have perfected several stamping and aerosol deposition methods that are necessary for putting on the surface sufficient numbers of isolated (as opposed to clumped). Our newest development is to use an aerosol spray producing ca. 5 micrometer droplets containing a few quantum dots or carbon nanotubes to repeatedly spray the surface. This works well for the metal surfaces that can be handled out of vacuum. For the passivated silicon surfaces, we still use in-vacuum stamping methods (fiberglass, teflon or silicon stamp) described in earlier reports. Finally, we have completed our study of thin metal surfaces deposited onto atomically flat sapphire, with and without a Nb adhesion layer, and by electron beam as well as sputtering deposition. We developed a Monte Carlo lattice model that quantitatively describes the deposition process, and shows how minimal roughness of the layer may be obtained. It is possible to create layers 10 nm thick (>60% transparency) with < 0.6 Å rms roughness, and good conductivity for STM probing. We are currently completing the work by looking at co-deposited nanotubes/quantum dots of different sizes, exciting only one resonantly with the laser, while scanning the other one. Erin Carmichael has completed her PhD with ACS-PRF funding, while graduate student Greg Scott now has taken over the project.
