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Theoretical and experimental study of optical properties of 2D material heterostructures

Subject Area Theoretical Condensed Matter Physics
Experimental Condensed Matter Physics
Term from 2018 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 403180436
 
Final Report Year 2024

Final Report Abstract

Two-dimensional (2D) materials research opened up a vast realm of experimental and theoretical investigations and discoveries, among that a plethora of heterostructuring and fewlayer-system related physics. The established concept of 2D excitons has drawn a lot of researchers’ attention. 2D species confined to their monolayer host crystal exhibit a different screening situation with respect to their configuration in bulk crystals of that type. Thus, 2D excitons in many of these so-called van-der-Waals materials feature relatively high binding energies, rendering them attractive for utilization in room-temperature applications. Foremost, the material class of transition-metal dichalcogenides (TMDCs) was studied worldwide, and their application scenarios ranging from photovoltaics to quantum photonics were addressed early on. Much focus lied on the monolayer configuration. But later on, the possibility to create and examine a variety of bilayer and multilayer configurations enhanced the field of heterostructure (HS) exciton studies. With monolayer, bilayer, fewlayer and heterosystem excitons at one’s disposal, key optical studies were also conducted in this project. Some of these 2D-materials excitons exist with different dipolar orientation and spatial distributions, even across interfaces in some HSs. In stacked van-der-Waals materials, these excitons often appear with interesting properties, to some extent governed by moiré potential landscapes. The project’s research was pursued in order to establish a better understanding of excitonic properties in homemade TMDC samples as well as supplied ones. Among them were mono- and bilayer CVD grown systems and high-quality hexagonal-boron-nitride (hBN) encapsulated stacks. For our tungsten-selenidebased testbed structures, incorporating for instance suitable other 2D materials, such as molybdenum- or tungsten-based ones, we discovered with the help of monolayer and bilayer systems, how the radiation patterns of distinct monolayer exciton species can differ from each other, but also from bilayer excitons, such as those attributed to moiré states and interlayer species formed across the interface. Theoretical works examined the more volatile telluride based monolayers, which on the experimental side degraded on relatively fast timescales but are attractive for type-I or type-II heterojunction-based optoelectronic devices. Over the course of years, we learned how to improve the TMDC features through ultrathin hBN encapsulation, and additionally how to cap these ultrathin sheets under a glass-like coating, such as by nanometers thin alumina, for conservation and protection purposes. Optical experiments such as Raman characterization, angle-resolved or polarization-resolved photoluminescence and time-resolved examinations set the stage for more complex experiments, which have been successfully pursued continuously with worldwide partners, among others within the frame of the DFG-SPP2244 priority program.

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