Seminarium Fizyki Ciała Stałego

The talk will focus on the electronic properties of the excitons in new emerging materials as atomically thin transition metals dichalcogenides and solid-state perovskite.Recently, the stacking of atomic monolayers of TMDs has emerged as an effective way to engineer their properties. In principle, the staggered band alignment of such heterostructures should result in the formation of inter-layer excitons with long lifetimes and robust valley polarization. Since single layer TMDs suffer from very short exciton lifetimes and rapid valley depolarization, TMDs heterostructures can circumvent these drawbacks, paving the way for implementation of valleytronic and spintronic concepts. In this talk I will discuss the optical properties of excitons in MoS₂/MoSe₂ van der Waals heterostructure. First, I will demonstrate a long lived inter-layer exciton emission. Under circularly polarized excitation, the inter-layer exciton emission is intriguingly counter polarized; the emitted light has the opposite helicity compared to the excitation. This surprising effect could be partially explained by the formation of the Moiré excitons in van der Waals heterostructures. To support this idea I will demonstrate splitting of the intralayer exciton and trion in a monolayer MoSe₂ assembled in a heterostructure with MoS₂ and encapsulated in hBN. Such a splitting, observed for the first time, is a direct consequence of the Moiré pattern formed between MoSe₂ and MoS₂. Secondly, I will demonstrate the results of the magneto-photoluminescence spectroscopy of interlayer excitons, which exhibits a non-trivial dependence of the valley polarization as a function of the magnetic field. The measured trends can be accounted for by considering that the valley polarization of energetic levels split by the valley Zeeman effect stems from the interplay between exchange interaction and phonon mediated intervalley scatteringIn the second part of the talk I will demonstrate that the absorption measurements performed in very high magnetic fields up to 150T gives a direct access to basic electronic properties such as exciton binding energy and effective mass of the carriers for Organic-Inorganic or fully inorganic Tri-halide Perovskites. I will show that for all the family of these materials, the binding energy of the exciton is smaller than or comparable with the thermal energy at 300K, explaining the excellent performance of the devices. Finally, I will focus on the exciton fine structure in single crystal MAPbBr₃.