ABSTRACT:
Atomically thin layers of semiconducting transition metal dichalcogenides (S-TMDs) MX2, where M = Mo, W, Re, Hf, etc. and X = S, Se, or Te, represent a new class of materials, which are of vivid interest, primary in the area of semiconductor physics and nanoscience, as well as optoelectronic applications. The studies of thin films of S-TMDs are greatly inspired by and profit from the research and developments focused on graphene-based systems.
In my talk, I will give a concise overview of excitonic complexes apparent in S-TMDs. Particularly, the differences between βbrightβ and βdarkishβ MLs of Mo- and W-based S-TMDs will be discussed in terms of optical transitions. I will demonstrate that, in S-TMD MLs the s-type Rydberg series of excitonic states follows a simple energy ladder: πππ =βπ
π¦β/(π+πΏ)2. The effective Rydberg energy in the formula, π
π¦β is very close to the Rydberg energy scaled by the dielectric constant of the medium surrounding the ML and by the reduced effective electron-hole mass, whereas the ML polarizability is accounted for only by πΏ.
The second part of my talk will be devoted to the study of emission lines apparent in the low-temperature photoluminescence (PL) spectra of n-doped WS2 ML embedded in hexagonal BN (hBN) layers. Results of measurements in external magnetic fields and first-principles calculations will be reviewed. It will be demonstrated that apart from the neutral exciton line, all observed emission lines are related to the negatively charged excitons. Consequently, emissions due to both the bright (singlet and triplet) and dark (spin- and momentum-forbidden) negative trions as well as the phonon replicas of the latter optically-inactive complexes are identified. The semidark trions and negative biexcitons are also distinguished. On the basis of their experimentally extracted and theoretically calculated g-factors, three distinct families of emissions are identified due to exciton complexes in WS2: bright, intravalley-, and intervalley-dark. The g-factors of the spin-split subbands in both the conduction and valence bands are also determined.
The last part of my presentation will be dedicated to the analysis of the excitonic ladder in both the monolayer (ML) and the bilayer (BL) of MoSe2 encapsulated in hBN. I will demonstrate that the measured experimentally ladder of excitonic s states in the ML can be reproduced using the kΒ·p approach with the Rytova-Keldysh potential that describes the electron-hole interaction. The analogous calculations for the BL case require taking into account the out-of-plane dielectric response of the MoSe2 BL, which is neglected for the ML.
BIOGRAPHY:
Dr. Maciej Molas is an Associate Professor at the Faculty of Physics, University of Warsaw. His research interests are devoted to properties of two-dimensional layered materials. His current projects are “Excitonic complexes in two-dimensional van del Waals structures” and “Magnetic phenomena in flatlands: Investigations of atomically thin magnetic layers.
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