Call for applications for a position at ND

The call for applications for the following positions at Notre Dame is open:

• Origin of near-universal, absorption/emission Stokes shifts in perovskite nanocrystals

For applications please contact the supervisors.

Origin of near-universal, absorption/emission Stokes shifts in perovskite nanocrystals

Background and motivation

The proposed work investigates the origin of near-universal, absorption/emission Stokes shifts in perovskite (e.g., CsPbBr3) nanocrystals (NCs). Observed shifts are energy differences between absorbing and emitting states, which indicate that they differ. Moreover, because no (absorbing) transition is seen where emission occurs, the emitting state is “dark” in absorption. Clues to the origin of the Stokes shift and identity of the emitting state emerge from observed size- and composition-dependencies. Stokes shifts are furthermore intrinsic to the NCs as exhaustive measurements exclude extrinsic origins such as residual size distributions and/or solvatochromatic effects, involving slow degrees of freedom. Despite significant interest in applying perovskite NCs to light emitting diodes, single photon sources for quantum information science, and solar energy conversion, very little is known about the origin of their Stokes shifts, let alone the identity of the emitting state. This is to be contrasted to more conventional NCs such as CdSe where band edge exciton fine structure quantitatively accounts for both global and resonant Stokes shifts, with emission emerging from a dark exciton. Although similar perovskite NC fine structure might account for their shifts, both theory and experiment predict bright/dark fine structure splittings easily an order of magnitude too small to account for experiment. No explicit rationalizations of perovskite NC Stokes shifts therefore exist. This leaves absent a fully self-consistent picture of their electronic structure. The proposed effort addresses an open question in the field by developing a self-consistent explanation for ubiquitous perovskite NC Stokes shifts, their observed size- and temperature-dependencies, and, ultimately, the identity of the emitting state.

This effort seeks to reveal the identity of perovskite NC emitting states to self-consistently rationalize observed size (l)-, temperature (T)-, and composition-dependent Stokes shifts. Motivated by recent results, it is hypothesized that perovskite NC emitting states are, in fact, polarons, which result from the lattice accommodation of photogenerated charges. Polaron binding energies and lifetimes, in turn, are the origin of observed l-, T-, and composition-dependent absorption/emission Stokes shifts and excited state lifetimes. This represents a significant departure from more conventional descriptions of NC band edge states, which exclusively involve exciton fine structure and dark exciton emitting states. The proposed work is part of a joint experimental and theoretical study, involving detailed ensemble/single NC absorption, emission, and Stokes shift measurements, accompanied by theoretical work to more fully establish a recently developed polaron model.

Opportunities

• The student will work mainly at the University of Notre Dame (ND), USA.
• Modelling and experimental research within the international collaboration between ND and Università Cattolica (UCSC), with one year spent at UCSC.
• Opportunity to obtain the Ph.D. title from ND and from UCSC.

Supervisors

Prof. Masaru Kuno, Univ. Notre Dame, US, mkuno@nd.edu
Prof. Luca Gavioli, UCSC, Italy, luca.gavioli@unicatt.it

How to apply

For applications please contact the supervisors.