Excitons in Perovskite Nanostructures

The bright emission observed in cesium lead halide perovskite nanocrystals (NCs) has recently been explained in terms of a bright exciton ground state, a claim that would make these materials the first known examples in which the exciton ground state is not an optically forbidden dark exciton. This unprecedented claim has been the subject of intense experimental investigation that has so far failed to detect the dark ground-state exciton in CsPbBr₃ NCs. I will discuss the exciton fine structure created by the crystal feld and the short-range and long-range electron−hole exchange interaction and will show that the only Rashba terms provide an explanation for the observed bright exciton level order in CsPbBr₃ NCs. The size dependence of the exciton fine structure calculated for perovskite NCs shows that the bright−dark level inversion caused by the Rashba effect is suppressed by the enhanced electron−hole exchange interaction in small NCs. [1]<br/>Another interesting phenomenon connected with presence of the Rashba spin-orbit terms is creation of helical exciton states in orthorhombic perovskites, which are split from each other. The splitting can be described as a Zeeman effect in an effective magnetic field, whose direction and magnitude depend on the exciton momentum. The selective excitation of these states by helical light gives rise to CD. Using experimentally determined material parameters, we calculate significant circular dichroism of order 30% in orthorhombic perovskites under off-normal, top illumination. These calculations suggest the effect is observable and CD can be measured in non-chiral perovskite nanostructures such as layered-2D perovskites or nanoplatelets. [2]<br/>Large spin-orbit Rashba terms in the conduction and valence bands leads to the exciton with complex dispersion – Rashba exciton. The Rashba terms in case when they have different signs flip the order of the bright and dark excitons at zero exciton momentum. They also affect the exciton dispersion at momentum not equal to zero and creates the exciton dispersion minima at momentum not equal to zero. [3] However, the exciton dispersion minima occur at nonzero momentum.<br/>Finally, I will discuss the thickness-dependent fine structure of excitons in perovskite nanoplatelets. Our theoretical model introduces the effect of the strong special confinement on the band-edge Bloch functions. The predicted fine structure is very different from that observed in 3D nanocrystals, and is in good agreement with experimental observation. [4]<br/><br/>[1] M. A. Becker, <i>et al.</i> “Bright triplet excitons in caesium lead halide perovskites,” <i>Nature</i>, <b>553</b>, 189-193 (2018)<br/>[2] P. C. Sercel, Z. V. Vardeny, Al. L. Efros, “Circular dichroism in non-chiral metal halide perovskites” Nanoscale 2020, DOI/10.1039/D0NR05232A<br/>[3] M. W. Swift, J. L. Lyons, Al. L. Efros, P. C. Sercel “Rashba exciton in a 2D perovskite quantum dot” Nanoscale 2021, doi.org/10.1039/D1NR04884H<br/>[4] M. Gramlich, M. W. Swift, C. Lampe, J. L. Lyons, M. Döblinger, Al. L. Efros, P. C. Sercel, and A. S. Urban, “Dark and Bright Excitons in Halide Perovskite Nanoplatelets” Advanced Science 2021, to be published.