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Description
Atomically thin transition metal dichalcogenides (TMDs) are new and exciting materials which share many interesting features with graphene, yet since they belong to semiconductors family they are much better suited for optoelectronic applications such as light emitters and photodetectors. Moreover, due to peculiarities of their structure, the polarization of light absorbed (or emitted) by those materials is related to the motion of electrons inside them in a very specific way. This unique property opens a new field of optoelectronics called “valleytronics” in analogy to spintronics. In spintronics the storage and manipulation of information is based on electron''s spin, while TMDs offer the additional possibility to encode information in an electron''s valley. The aim of this project is to combine state-of-the-art experimental methods to perform a comprehensive study of valley relaxation in a variety of novel TMD systems. On one hand, as a main tool to measure the valley relaxation the researchers will use the valley noise spectroscopy – a novel technique recently pioneered by the Principal Investigator. On the other hand, the focus will be put on TMD structures prepared with unique methods, including Molecular Beam Epitaxy (MBE). The use of MBE technique to fabricate atomically thin TMD layers of high quality was recently pioneered by researchers from the Host Institution (the University of Warsaw), which is currently the only place in the world where such capability is available. Combination of the experience of the Principal Investigator related to unique measurement techniques, together with novel fabrication methods developed at Host Institution, is anticipated to give unprecedented insight into the valley relaxation mechanisms in monolayer TMD structures. This, in turn, will advance the quest for robust and efficient valleytronic memories and information processing devices.
Summary of project results
The project was devoted to atomically thin transition metal dichalcogenides (TMDs) and other socalled van der Waals compounds. Those are new and exciting materials that have graphene-like crystalline structure, however - since they belong to the semiconductors family - they are much better suited for optoelectronic applications such as light emitters and photodetectors. Additionally, many of them combine interesting optical characteristics with unusual magnetic properties rendering them a promising platform for future thin-film electronics and an interesting playground for material science physicists. Studies performed within the project have provided valuable information on the optical and magnetic properties of several 2D semiconductor systems. Many previous results of optical experiments obtained with TMD monolayers have been successfully described in the literature with relatively simple, so-called single-particle model of optical excitations. However, we have shown that under careful examination it turns out that more complex many-body interactions in
monolayer TMDs play a crucial role and are indispensable in the proper understanding of several fundamental properties of that material system. For example, under specific conditions, an optical excitation (the transfer of the energy from photon to electron in the material)
involves not one electron - which is the most common situation in semiconductors - but several of them which significantly changes the optical features of the material. Those many-body interactions can be also used in a practical manner, as they allow tuning of the magnetic properties of monolayer TMDs by relatively simple electrostatic tuning of the charge density. In the case of other 2D materials, in particular in the case of magnetic systems, our results contribute to a better understanding of the non-trivial behavior of layered metamagnets and they open the path toward optical studies of their complex phase diagrams. They also provide practical means for studies of built-in strain in magnetic 2D structures. All these properties are important not only from the scientific point of view but are also crucial for the practical design of future information processing and storage devices based on those materials.
The realization of the project has resulted in several important deliverables, among which are:
- establishing advanced experimental setup for optical studies of spin and valley properties of novel 2D semiconductor systems;
- establishing the fabrication capability at the Host Institution for complex heterostructures containing 2D semiconductor monolayers;
- demonstration of the many-body nature and intervalley correlations of specific quasiparticle states in monolayer Transition Metal Dichalcogenides (TMDs);
- demonstration of enhanced electron magnetic susceptibility due to manybody interactions in monolayer TMDs;
- observation of complex spin dynamics and phase transitions in layered metamagnets such as CrPS4 or NiPS3 via studies of optical polarization properties at various temperatures and magnetic fields;
- application of Optically Detected Magnetic Resonance (ODMR) for studies of built-in strain in magnetic 2D semiconductor structures;
- preparation of three bachelor theses related to novel 2D materials (two finished and one being prepared) and one master thesis (in progress);
- establishing new collaborations between the Host Institution and external parties
Altogether, the results of the project constitute a substantial step forward in studies of novel 2D materials and provide valuable knowledge for other researchers interested in those systems. From a long-term perspective, the findings of the project are expected to constitute a valuable input in the search for the best material systems for electronics, optoelectronics, and valleytronics based on 2D materials. If successful, such a search can result in a significant acceleration of modern electronics development, which in turn can result in improved electronics user experience, cheaper and better devices, and improved availability of modern technology.