Advanced two-dimensional transition metal chalcogenides

• Applicant: Dr. Thais Chagas Peixoto Silva, Ph.D.
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 530222608

Monolayers of transition metal chalcogenides (TMCs) are an important class of two-dimensional (2D) materials that are promising candidates for devices. These materials are formed by a transition metal and a chalcogen and their properties depend on their composition and atomic coordination. Molecular beam epitaxy (MBE) on single-crystalline metal surfaces is an established method for the preparation of 2D-TMCs in high quality. The focus of this project is Ta-S compounds, as these two elements can give rise to a variety of phases. The starting point of the project is to gain a deeper understanding of the atomic process involved in the synthesis of TMCs. This will be applied to grow high-quality, phase-pure, extended, well-oriented, and defect-free 2D materials. This opens the way to investigate recently discovered but still underexplored phases and on top of that phase engineer the established phases, converting them into new materials with new functionalities. In our preliminary exploration of the rich phase diagram of Ta-S, we observed a Kagome lattice, an important model system in quantum physics as they are the most frustrated 2D magnetic lattice. The typical Kagome bands, consisting of a Dirac cone and a flat band, can lead to the emergence of exotic many-body states. We will fully determine its structure and stoichiometry here, followed by an investigation of the electronic properties focusing on the predicted bands and half-metallic edge state. This proposal also aims to develop a new synthesis method that will allow the preparation of Janus membranes as TaSSe with high quality: We will use tantalum monosulfide as a seed, which subsequently will be post-selenized, adding the required Se layer from below. This synthesis will allow an investigation of the predicted piezoelectric and superconducting properties of TaSSe. Our knowledge of nucleation and growth processes obtained for tantalum sulfide compounds will be generalized to other TMCs on closed-packed metal surfaces. Particularly, we will investigate molybdenum sulfide compounds at their nucleation step as our previous work indicates similarities in the initial stages of growth. Once similar grow routes were determined, the exploration of the preparation parameters will be carried out to obtain various phases of Mo-S, similar to the ones observed for Ta-S compounds. The proposed methodology to achieve these goals is MBE under highly controlled conditions to synthesize these materials since it is compatible with surface science techniques, enabling a complete characterization. We will employ scanning tunneling microscopy (STM) and spectroscopy (STS) as the main tools of characterization as they allow the distinction of different phases within the same sample and the study of how their structural and electronic properties vary under specific conditions. Complementary surface science techniques will also be used to gain further insight into the physical properties of these materials.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Professor Dr. Andres Santander-Syro
• Co-Investigators: Professor Dr. Thomas Werner Michely; Professor Dr. Michael Rohlfing