Date: Tuesday, October 7, 2014
Time: 11:00 am
Speaker: Naomi S. Ginsberg, UC Berkeley
Title: Navigating Space-Time to Correlate Local Structure to Function in Molecular Materials
Location: 67-3111 Chemla Room
Part and parcel with the promise of synthetic and mechanical flexibility and cost-effective and energy-efficient solution processing, the materials used in organic electronics have complex, heterogeneous physical structure. With the heterogeneity in physical structure comes heterogeneity in the electronic structure, whose weakest local attributes often determine or limit device performance. A cross-cutting theme in my lab is to correlate the nanoscale physical properties in organic semiconducting thin films to their local optical properties in order to inform and improve the non-equilibrium processes used to deposit the materials to make effective devices.
I will first describe how we have measured ultrafast exciton dynamics in small-molecule polycrystalline thin films used to make organic transistors by using polarized transient absorption microscopy. In so doing we infer the nanoscale structure of a complex domain interface that explains current limitations to charge carrier mobility. In the second part of the seminar, I will describe how we use both sub-diffraction optical far-field and near-field microscopies to uncover heterogeneity in the optical properties of more disordered conjugated polymer films used in photovoltaics. Our far-field approach will enable the mapping of exciton migration with nanometer and picosecond precision in order to correlate it to the underlying physical structure of the materials.
Our near-field approach represents an unusual development in nano-imaging, as it leverages the focus and rapid scanning of a keV electron beam and the non-invasiveness and spectral selectivity of optical fields. I will show how we image nanoscale features in luminescent polymer blends through electon-beam-induced (cathodoluminescence) activation of an oxide thin film scientillator that locally excites the sample through resonant energy transfer. I will also describe studies in which we use enhancement of the scintillator cathodoluminescence as a contrast mechanism to image the fields induced around aluminum nanostructures. Recent technical advances will soon permit imaging of dynamics in liquids, with a focus on the motions and interactions of biomolecules at the nanoscale.