Prof. Sasha Efros: "Semiconductor Nanocrystals: from discovery to modern development"

The Nobel Prize in Chemistry 2023 was awarded to Moungi G. Bawendi, Louis E. Brus and Ale-ksey I Ekimov "for the discovery and synthesis of quantum dots”. Nanocrystals (NCs) quantum dots are the most heavily studied of the nanoscale semiconductors. The size dependence of NC optical properties was discovered independently more than 30 years ago in two different materials: in semiconductor-doped glasses by Ekimov et al (1981), and in aqueous solutions by Brus et al (1983). Theoretical description of these optical properties was reported by Efros et al.( 1982). I will briefly discuss the history of this dis-covery, the main obstacles in the development of this field, and the critical breakthroughs along the way [1].
Today, semiconductor NCs have become much more than objects of scientific curiosity. The demonstration of tunable, room-temperature lasing using NC quantum dot solids, the development of NC-based light-emitting diodes and photovoltaic cells, quantum dots TV produced by Samsung, and the first commercial products in the area of NC bio-labeling are just a few illustrations of the broad techno-logical potential of these materials.
Recently new class of CPbX3 (X=Cl, Br. I) NCs was discovered which unusual optical proper-ties can be connected with a ground bright exciton state [2]. We calculate the lowest quantum confined levels of electrons and holes and the spectra of the allowed optical transitions. The symmetry of the ground exciton state has been analyzed, and the radiative decay time has been calculated. The results of our theoretical calculations have explained the 200 ps radiative decay time and polarization properties measured in experiments on single CsPb(BrCl2) quantum dots.
Finally, I will discuss effects of discontinuity of dielectric constants at interfaces and surfaces which is a common feature of nanostructures and semiconductor heterostructures. Near such interfaces, a charged particle experiences a singular self-interaction potential, which can be interpreted as interaction with fictitious mirror charges. The singularity of this interaction at the interface presents an obstruction to a perturbative approach. We develop a non-perturbative theory that provides a self-consistent description of carrier propagation across an interface with dielectric discontinuity. The approach is based on cur-rent-density conservation at the interface and formulated in terms of general boundary conditions (GBC) for the wave function, characterized by a single phenomenological parameter W.[3]. Using these results, we describe the photo effect at the semiconductor/vacuum interface and the energy spectrum of quantum wells (QWs) at the interface with the vacuum or a high-k dielectric. For a surface of liquid helium, we estimate the parameter W, and match the resulting electron spectrum with the existing experimental data and theoretical analysis.

References
[1] Al. L. Efros and L. E. Brus, Nanocrystal quantum dots: from discovery to modern development. ACS Nano 15, 6192–6210 (2021).
[2] M. A. Becker, et al., Bright triplet excitons in cesium lead halide perovskites. Nature, 553, 189 (2018).
[3] . M. Beltukov, A. V. Rodina, A. Alekseev and Al. L. Efros, Appl. Phys. Rev. 12, 041415 (2025)