Excitonic landscape in layered semiconductors
Dr. Paulina Plochocka
Wroclaw University
Laboratoire National des Champs Magnétiques Intenses Toulouse, CNRS
The optical properties of low-dimensional semiconductor nanostructures are often governed by excitons – quasi-particles formed by a photo-generated electron and hole bound together by Coulomb attraction. Excitonic effects are particularly pronounced in two-dimensional (2D) van der Waals semiconductors, where simultaneous quantum and dielectric confinement enhance the binding energy of the electron-hole pair to values as large as several hundred meV. This strong excitonic binding provides an unprecedented platform for studying exciton quasiparticles, which exhibit different charges, spins, or spatial configurations.
Here I will explore the excitonic landscape in 2D semiconductors and van der Waals heterostructures. One prominent example is the 2D Ruddlesden–Popper metal-halide perovskites (2DP), where the soft, polar, and low-symmetry lattice creates a unique environment for electron-hole interactions, offering a fascinating playground for studying exciton physics. Some aspects of this system, however, remain challenging to fully understand. I will highlight the controversy surrounding the unexpectedly high light emission efficiency of this material and show that it can be explained by the interplay between phonons and the exciton fine structure.
I will also demonstrate that the van der Waals nature of 2DP allows for easy integration with other 2D materials, particularly transition metal dichalcogenides. The nontrivial band alignment in such stacks facilitates charge transfer, leading to the formation of interlayer excitons. Strong excitonic effects in both materials enable energy and spin transfer between the layers, which can be controlled by aligning the excitonic states appropriately.
Finally, I will discuss the excitonic properties in homo-bilayer transition metal dichalcogenides. These homo-bilayers can host dipolar interlayer excitons with static dipole moments, which can be easily tuned by an external electric field. Additionally, I will present how the interaction between two dipolar excitons with opposite dipole moments can lead to the formation of a new type of interlayer exciton, namely a quadrupolar exciton, whose energy shifts quadratically in an electric field, in contrast to dipolar interlayer states. By combining our experimental results with microscopic many-body theory, I will show how the electric field can effectively tune the spatial characteristics of the excitonic species, switching between intra-, inter-, and hybrid exciton states.
Hosted by Prof. Janko