Single-Molecule Spectroscopy, Polymer-Based Nanoscale Photonics, Energy and Charge Transfer in
Nanoscale Architectures, and Chiroptical Properties of Single Molecules 

Our research focuses on the physical properties and processes of single molecules and single nanoscale architectures.  One important research area involves studying hybrid quantum dot/polymer nanostructures for use in optoelectronic devices. Critical issues associated with effective integration of semiconductor nanocrystals (quantum dots) into sensors, displays, and related structures are the lack of solution-phase processibility without aggregation, fluorescence intermittency (blinking), and effective charge separation.  The solution to these obstacles involves the direct connection of electrically conducting polymer ligands to the quantum dot surface. Single-molecule spectroscopy and scanning-probe microscopies are utilized in our group to characterize the physical and optical properties of these new hybrid materials.  We showed (JACS, 128, 3506, 2006) that oligo-phenylene vinylene ligands directly connected to CdSe quantum dots exhibit enhanced energy transfer and reduced blinking.  More recently (JPCB, 110, 14167, 2006), we showed that the degree of blinking supression is directly connected to the number of conjugated organic ligands on the surface of the quantum dot.

Another interesting question involves three-dimensional confinement effects in polymeric systems with specific emphasis on materials that can be used in a nanoscale optoelectronic context. Semiconducting polymers, commonly used in macroscopic organic light-emitting diodes (OLEDs), appear as an obvious material for molecular scale optoelectronics, however difficulties in controlling chain organization at the single-molecule level result in many undesirable photophysical properties (such as broad emission spectra and poor photochemical stability) for these kinds of applications. The figure at right shows a high-resolution fluorescence image from single molecules of a common semiconducting polymer (MEH-PPV) isolated from microdroplets of ultradilute solution. The highly uniform ‘donut’-like spatial intensity patterns characteristic of a dipolar emissive source with the “antenna” orientation perpendicular to the substrate. In these samples, the transition moment orientation of every molecule is nearly identical (to within a few percent). In addition to the novel orientation, the photophysical properties of these oriented species are profoundly altered and are comparable –if not superior in many respects to inorganic quantum dots. Shown in the figure at right is a comparison of the bulk spectra (gray) and the room-temperature emission spectrum from a z-oriented single MEH-PPV molecule.

Other important research interests include chiroptical properties of single molecules, properties of conjugated organic molecules such as polyfluorenes and polyvinylenes, and energy transfer in light harvesting dendrimer complexes.