Tracking long-range energy flow on its native nanometer and picosecond scales

October 30, 2017

For Immediate Release
Berkeley, CA

Nanoscale excitation spot graphic, Ginsberg Research.
Above graphic: A nanoscale excitation spot is prepared in an organic semiconductor using two ultrafast laser pulses. Photoexcitations that migrate beyond the boundary of this original volume are quenched with another pulse. Those remaining fluoresce to indicate how far the distribution expands as a function of time. Image by Naomi Ginsberg and Connor Bischak.

In a new study, appearing in the November 2017 issue of Nature Materials, a research team led by UC Berkeley Associate Professor of Chemistry and Physics, Naomi S. Ginsberg, has announced the development and implementation of the most direct method to-date to track the nanoscale process of energy flow that punctuates the initial picoseconds after light absorption in some natural and artificial light harvesting systems. The research results are also available online at the Nature Materials website.

Until now, measuring excitation migration has been challenging because of the mismatch between nanoscale migration lengths and the optical diffraction limit (in hundreds of nanometers). Instead of using conventional bulk methods, the research team optically defined sub-diffraction regions of study by transforming a challenging super-resolution fluorescence microscopy into a time-resolved ultrafast approach.

Exciton migration is an important process in many opto-electronic systems, especially in natural and artificial light harvesting, where the transfer of optical excitations to reaction centers or p-n junctions is a critical step in energy conversion. Representative measures of exciton migration in organic semiconductors have been challenging because of the mismatch between diffusion lengths (5-20 nm) and the optical diffraction limit. In a novel approach, the research team has successfully resolved exciton transport on its characteristic nm and ps scales in electronically-coupled materials.

By coupling these measurements with spectral information and simulation, it is now possible to analyze the corresponding nature of migration with a spatially and energetically disordered environment to best describe not only how nanoscale energy landscapes affect exciton migration but also what model best describes the material's spatio-energetic heterogeneities on relevant scales. Ginsberg and co-authors Sam Penwell, Lucas Ginsberg, and Rodrigo Noriega found that exciton migration trajectories are appropriately modeled diffusively in the organic semiconductor they studied because the variation in an exciton's energy at any given site is comparable to the variation in mean site energy among the sites that together compose the material.

According to Dr. Ginsberg, "Our approach will enable previously unattainable correlation of local material structure to photoexcitation migration character. This is applicable to both photovoltaic, and display-destined organic semiconductors and also to the quintessential excitation migration in photosynthesis."

The research was carried out at the Materials Sciences Division of the Lawrence Berkeley National Laboratory and was supported in part by grants from the David and Lucile Packard Fellowship for Science and Engineering, The Dow Chemical Company, STROBE NSF Science and Technology Center, US Department of Energy, the Philomathia foundation, the Alfred P Sloan Foundation and the Camille and Henry Dreyfus Teacher-Scholar Program.

For further information regarding this research, contact Dr. Naomi Ginsberg at