[MOS] November 25, 2014, Alexander Weber-Bargioni, Lawrence Berkeley National Laboratory

[Zina M Queen]

Seminar on
Modern Optics and Spectroscopy
Mapping the propagation of excitons through organic and inorganic light harvesting nano composites using nano optics

Alexander Weber-Bargioni,
Lawrence Berkeley National Laboratory

Tuesday, November 25, 2014
12:00 – 1:00 p.m.
Controlling individual excitons and their deliberate movement through a material will provide the access to a new parameter space for the development of next generation light harvesting materials. E.g. with such control the captured energy in form of an excitons could be transported to predetermined sites in the material where the energy can be efficiently harvested. Nano materials have in principle the potential to realize this vision since the material property determining electronic structure can be tuned via geometry, material composition, interfaces and environment. However, the lack of spatial resolution has so far prevented the insight needed to develop engineering rules for the nm world to control the transport of optically excited electronic states at their native length scale.  To study the local exciton transport we use optical antennae to locally excite our sample optically and map spatially independent the energy flow by detecting either the local photo luminescence or the local photo current. We use this approach to study exciton transport through three model systems: Inorganic nano wires, 2-D assemblies of inorganic nano crystals, and through organic PV materials. First I will present our results on exciton transport and local exciton properties in InP nanowire system and demonstrate that the local exciton recombination velocity is mediated by charge puddles on nanowire surfaces. CdSe Quantum Dot assemblies are another system of great interest to study exciton transport since they are an excellent absorber material system for light harvesting purposes. We determined exciton transport length through well ordered 2-D films of CdSe Nano Crystals of 80 nm and 120 nm for the 1-D case, mediated by Foerster Resonance Energy Transfer (FRET). To develop a better understanding of FRET between quantum dots (which is still not really understood) we used a graphene Field Effect Transistor to study FRET between individual quantum dots and graphene. In this device we can systematically tune with high precision the distance between graphene and quantum dot and the electronic structure of the exciton adsorber (graphene), while building the currently smallest light switch in the world.  Exciton diffusion is also a key hurdle for the systematic development of Organic Photo Voltaic. We used our techniques to directly measure the exciton diffusion length in polymer (P3HT) and small molecule (rubrene) organic photo voltaic materials and show a crystallinity dependent exciton diffusion length that correlates to the OPV dedvice power conversion efficiency. Furthermore we have evidence that local electric field gradient can modify the exciton diffusion length in organic semiconductors, where the exciton binding energy is large (1 eV) and the transport is mainly mediated by tunneling processes.
Grier Room, MIT Bldg 34-401
Refreshments served after the lecture

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