My research all involves the technique of microphotoluminescence. My primary effort is in studying fluorescence from single semiconductor nanocrystals, with an additional small-scale project studying novel organometallic luminophores.
The physics of single molecules and single quantum objects, such as semiconductor quantum dots, enjoys widespread current interest. In CdSe nanocrystals (NCs) a few nanometers in diameter, the exciton confinement energy is greater than the electron-hole Coulomb interaction energy, so they offer an excellent system in which to study the effects of quantum confinement.[1,2] These NCs produce very bright, stable fluorescence with high quantum efficiency, so fluorescence from individual NCs has been widely studied. Applications from biological fluorescence markers [3] to single-photon sources [4] are being actively pursued.
One intriguing property of single semiconductor NCs is that they exhibit intermittent fluorescence — under steady illumination, individual NCs switch between a dark (or "off") state, in which nonradiative recombination of excitons occurs much more rapidly than radiative recombination, so no light is emitted, and a bright ("on”) state, in which the reverse is true. These on or off states last for milliseconds to many minutes at a time. [1,2] A power-law distribution of off-time durations and a truncated power-law distribution of on-times has been repeatedly observed. Although this blinking behavior is not fully understood, the dark state is thought to occur when the electron escapes from the NC core and gets trapped either at the surface or in the environment, leaving the core charged; the bright state is recovered when the electron tunnels back to the core.
I am studying fluorescence intermittency from two angles:
(In collaboration with Prof. Marija Drndic's group at the University of Pennsylvania)
My coworkers and I showed that rod-shaped semiconductor nanocrystals, or nanorods (NRs), also blink, with very similar statistics as NQDs. NRs show truncated power law on-time statistics with a truncation time that increases with decreasing aspect ratio. [5] Reprint
Recently we used correlated transmission electron microscopy and fluorescence to show that small clusters with 2-3 NRs have indistinguishable blinking statistics from single NRs, while clusters with 5 - 15 NRs have increased truncation times and altered on- and off-time exponents.[6]
In collaboration with Matthew Pelton and Xiaohua Wu of Argonne National Laboratory's Center for Nanoscale Materials, I am undertaking measurements of nanorod blinking on sub-microsecond timescales, to look for the transition between two different time regimes predicted by the diffusion-based theories of R. A. Marcus and coworkers.[2]
My students and I are measuring the blinking statistics of different sizes of spherical core/shell nanocrystals (NCs) using several different excitation energies, corresponding to 532 nm, 488 nm, and 405 nm laser light. This work is nearly complete and a preprint will be posted when ready.
With Marija Drndic's group, we have also measured the intensity dependence of blinking from both NRs and NCs. The NR measurements appear in the first paper on NR blinking [5]; we are in the process of writing up the NC measurements. A preprint will be posted when ready.