Date: Tuesday, November 12, 2013
Time: 11:00 am
Speaker: Gordana Dukovic, University of Colorado, Boulder
Title: Photophysics and Photochemistry of Nanoscale Semiconductors
Location: 67-3111 Chemla room
Colloidal semiconductor nanocrystals are remarkably versatile materials that exhibit a high degree of tunability in electronic structure, optical spectra, and surface properties. They are generally strong light absorbers and have potential applications in solar energy harvesting. My research group is focused on the relationships between photophysics and photochemistry of nanoscale semiconductors with a particular emphasis on light-driven reactions involved in solar water splitting. This seminar will focus on our work in two areas: solar photochemistry of II-VI nanocrystals and development of nanoscale oxynitride semiconductors.
We have assembled hybrid nanostructures that couple light harvesting II-VI nanocrystals with redox catalysts that perform proton reduction and water oxidation. The functionality of such hybrid structures depends strongly on the charge transfer dynamics between the photoexcited nanocrystal and the catalyst. We utilize ultrafast transient absorption spectroscopy to examine the dynamics of nanocrystal → catalyst charge transfer and the competitiveness of this process with the energy-wasting charge recombination. I will discuss our recent findings identifying kinetic bottlenecks for photochemistry and describe our ongoing efforts to design nanocrystal structure and surface chemistry to enhance the kinetic competitiveness of the photochemical pathways.
The second half of the seminar will focus on our efforts to synthesize and characterize zinc-gallium oxynitride ((Ga1-xZnx)(N1-xOx)) nanocrystals with potential applications in solar water splitting. (Ga1-xZnx)(N1-xOx) is a solid solution of ZnO and GaN. Remarkably, although both constituent semiconductors absorb light in the ultraviolet, the mixture can absorb visible light with a band gap that varies with composition (value of x). The origin of this effect is not currently understood. We have developed a synthesis of oxynitride nanocrystals with a wide range of compositions (0.2<x<0.99) using nanocrystals with simpler composition as precursors. I will discuss our current efforts to understand the relationship between composition, elemental distribution, and electronic structure in these materials.