Optical properties of transition metal dichalcogenide bilayers

Li, Zhijie (2023): Optical properties of transition metal dichalcogenide bilayers. Dissertation, LMU München: Fakultät für Physik

Abstract

Two-dimensional transition metal dichalcogenide (TMD) semiconductors exhibit unique optoelectronic properties. Rational assembly of individual monolayers into homobilayers or heterostructures provides versatile means for realizations of a wide range of novel quantum materials with tunable optical and transport phenomena based on unique spin-valley degrees of freedom and strong electron correlations. Monolayer crystals synthesized by chemical vapor deposition (CVD) serve as elementary building blocks of layered van der Waals systems with distinct properties determined by layer number, composition and orientation. In the first part of this work, bilayers of WSe2 were obtained with CVD synthesis in two contrasting crystal structures and studied with cryogenic optical spectroscopy. Control of CVD growth parameters resulted in high-quality large-area homobilayer stacks with contamination-free interfaces and strictly parallel and antiparallel alignment. Using complementary optical spectroscopy techniques at room and cryogenic temperatures, and employing theoretical calculations, we identified distinct signatures of momentum-direct excitons in optical absorption and momentum-indirect excitons in the photoluminescence of WSe2 homobilayers with parallel and antiparallel crystallographic orientation. The study not only relates optical properties of homobilayers to crystal symmetry and stacking, it also highlights the role of crystal structure for the formation of hybrid interlayer exciton states. In the second part, we report studies of MoSe2-WSe2 heterostructures built as stacks of separately grown monolayers. For such nearly-commensurate heterostacks, theory predicts atomic reconstruction of the rigid moiré lattice into periodic domains of nanoscale patterns. In finite-size samples of heterostacks, however, we found effects of lattice reconstruction on mesoscopic length scales, with direct consequences for local optical properties dictated by excitons. Using extensive optical spectroscopy studies, correlative secondary electron imaging of reconstructed domains, and theoretical modeling, we identified the coexistence of nanoscale quantum arrays, quantum wires, and extended domains of only one atomic registry in the same sample as the main source of the diverse spectral features reported for excitons in MoSe2-WSe2 heterostacks. This notion of mesoscopic reconstruction provides a unifying perspective on exciton phenomena in heterobilayers with and without moiré effects. Finally, the last part of the work presents the results on MoSe2-WSe2 heterostacks obtained as vertically stacked triangular crystals from CVD-synthesis. With combined studies by cryogenic optical spectroscopy and high-resolution transmission electron microscopy, we identified extended moiré-free domains of energetically favored Hhh and RMh registries in H- and R-type stacks, in addition to grain boundaries of alternative registries and moiré-type cores. Remarkably, optical spectroscopy suggests that the RMh registry hosts the majority of exciton population with in-plane dipolar emission, as opposed to the predominance of RXh exciton features with out-of-plane luminescence in exfoliation-stacked MoSe2-WSe2 heterostructures. Overall, the work provides a unifying perspective on diverse signatures of excitons in semiconductor homobilayers and heterobilayers in parallel and antiparallel alignment. The results contribute to improved understanding of exciton phenomena that dictate the optical properties of layered semiconductor van der Waals systems.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: Two dimensional transition metal dichalcogenide, chemical vapor deposition, cryogenic-temperature spectroscopy, degree of polarization, g-factors
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 24. April 2023
• 1. Bericht­erstatter:in: Högele, Alexander