A Polaron Paradigm for Perovskite Nanocrystal Stokes Shifts

With the support of the Macromolecular, Supramolecular and Nanochemistry Program and the Chemical Structure and Dynamics Program in the Division of Chemistry, Professor Masaru Kuno of the University of Notre Dame will investigate the origin of ubiquitous energy differences between the peak absorption and emission energies of recently discovered perovskite nanocrystals. These nanocrystals have applications, ranging from next-generation lighting to solar energy conversion and harvesting. However, unanswered questions exist regarding the identity of their absorbing and emitting states. As part of this effort, Professor Kuno and collaborators will work with local K-12 teachers to develop hands on kits that demonstrate to the principles of light emitting diodes as well as how semiconductor nanocrystals are currently being used to make commercial quantum dot televisions. These kits will be part of an outreach effort to student from underrepresented groups in science from the South Bend area. Absorption and emission energy differences are prevalent in semiconductor nanocrystals. Conventional wisdom suggests that they arise from the existence of size-dependent, band edge exciton fine structure. However, predicted bright/dark exciton fine structure splittings are much too small to account for experimentally-observed, 20-100 meV Stokes shifts. Professor Kuno and coworkers have recently put forward a model that attributes the origin of perovskite absorption/emission Stokes shifts to a polaron emitting state. This model rationalizes perovskite nanocrystal Stokes shifts in terms of size-dependent, polaron binding energies and simultaneously sheds new light on their band edge absorbing and emitting states. This fundamental information has potential long term implications for the successful commercialization of these materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.