Alexander Weber-Bargioni, Molecular Foundry LBNL
Event Date and Time: 
Tue, 2016-05-17 14:30 - 15:30
Local Contact: 
Sarah Burke
Intended Audience: 

Electronic and Optoelectronic processes in matter are governed by the electronic structure and are well understood for most bulk material systems. Even individual nano building blocks made from these materials are reasonably well characterized. However, the effect of defects and specifically hierarchically ordered defects in quantum confined material systems on the electronic properties or the effect of order in heterogeneous nano building block assemblies on the resulting optoelectronic properties are largely uncharted territory. Herein lies a path to control electronic and optoelectronic processes at their native length scale, enabling the improvement of current material functionality or the discovery of entirely new material properties.

The first part of my talk will explore point defects and linear defects in two dimensional transition metal dichalcogenides (TMDs). My main motivation for exploring 2-D materials is ability to directly visualize via STM the electron wave function and how they can be tuned via defects on length scales comparable to screening length scale in these materials. We identify individual Se vacancies – on both, the SPM facing and the substrate facing surface. Both result in particular electronic wave function related to states located at the atomic defect. These defect states form atomically sharp type 1 heterojunctions with the surrounding pristine MoSe2, and form an excellent test bed to study catalytic activity with atomic precision and explains single photon emission for these defects. We also identified hierarchically ordered defects in from of Mirror Twin Boundaries (MTB) in MoSe2, which form truly 1-D metal channels embedded in the surrounding semiconductor. At low temperatures these 1-D metallic states open a band gap at the Fermi level of 100meV. The new band gap frontier states exhibit a spatial modulation along the channels with a periodicity of three times the lattice constant.  Density Functional Theory calculation confirm that the observed charge modulation is a result of the formation of a charge density wave. By charging up the charge density wave we observe the creation of solitons – a self-reinforcing wave - and are able to measure its’ energetic dispersion.

In the second part of my talk I will focus on the deliberate transport of excitons through 2-D nano building block assemblies. As shown in nature’s photosystem II, energy in form of an exciton is transported to predetermined sites with almost 100% efficiency where the energy can be efficiently harvested. Our goal is to map exciton transport through artificial nano building block assemblies and eventually control the transport. 

Using nano optics, modified confocal microscopy and scanning probe microscopy we study exciton transport through two systems: 2-D assemblies of CdSe quantum dot assemblies, 2-D assemblies of lead halide perovskite QD assemblies.

CdSe Quantum Dot assemblies 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.

Lead halide perovskite are a fascinating new material class for light harvesting and lead halide quantum dots exhibit an enormous, size determined, tunable band edge with PL quantum efficiency of up to 100% and a large absorption cross section. FRET mediated exciton transport reached up to a record 300 nm.

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