Advanced two-dimensional transition metal chalcogenides – extension proposal

• Applicant: 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. With the goal of gaining a deeper understanding of the atomic processes involved in the synthesis of TMCs, the subsequent growth steps beyond nucleation were investigated. The smallest island entities were identified, and the transition point at which embedded nucleation seeds are lifted from the surface was determined. A systematic analysis of the growth onditions was conducted, providing insight into edge termination and island orientation. In the exploration of the rich phase diagram of Ta-S, the growth of the kagome lattice, an important model system in quantum physics as they are the most frustrated 2D magnetic lattice, was optimized. Density functional theory (DFT) calculations and our experimental observations suggest that this material is Ta2S3, a predicted Chern insulator. The development of a new synthesis method for high-quality SeTaS Janus membranes is also proposed. Tantalum mono-sulfide will be used as a seed, which will subsequently be post-selenized by adding the required Se layer from below. Through this synthesis, the predicted piezoelectric and superconducting properties of SeTaS will be investigated. Previously unexplored Ta-Se phases were revealed through these experiments, opening opportunities for the investigation of these novel phases. Our knowledge of nucleation and growth processes obtained for tantalum sulfide compounds will be generalized to other TMCs on closed-packed metal surfaces. Particularly, molybdenum sulfide compounds will be investigated at their nucleation step, as similarities in their growth dynamics have been indicated by previous work. In exploring the sample’s preparation parameters lateral-heterostructures were also observed, with the observation of phase boundaries and mirror-twin boundaries, which will be also further explored in this research proposal. 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: Brazil, France, Saudi Arabia
• Cooperation Partners: Professor Mario Mazzoni; Professor Dr. Andres Santander-Syro; Professor Dr. Udo Schwingenschlögl
• Co-Investigators: Professor Dr. Thomas Werner Michely; Professor Dr. Michael Rohlfing