Graduation Year
2024
Document Type
Dissertation
Degree
Ph.D.
Degree Name
Doctor of Philosophy (Ph.D.)
Degree Granting Department
Physics
Major Professor
Matthias Batzill, Ph.D.
Committee Member
Sarath Witanachchi, Ph.D.
Committee Member
Inna Ponomareva, Ph.D.
Committee Member
Stephen E. Saddow, Ph.D.
Keywords
TMDs, 2D materials, STM, XPS, MBE
Abstract
The interest in studying 2D materials has increased over time due to their unique physical and chemical properties, which have the potential to lead to numerous applications in technology. For instance, their small size makes them useful for technology that requires greater efficiency, faster processing speeds, and less physical space. Transition metal dichalcogenides (TMDs) are a type of 2D material that is composed of one transition metal layer sandwiched between two chalcogen layers. The study of TMDs is of significant importance due to their layer-dependent properties and the fact that different phases exhibit distinct properties, including electronic properties. However, it is notable that the majority of TMDs lack intrinsic magnetic properties. If these TMDs demonstrate magnetic properties in the 2D region, this would be of considerable benefit for several applications, including spintronic and magnetic data storage. As a result, researchers have recently commenced investigations into the potential for inducing different properties in TMDs. One of the principal methods for inducing distinctive properties in TMDs is through doping with other transition metals. There are two primary approaches for exposing TMDs to transition metals: co-deposition or post-growth. In the co-deposition method, TMD is deposited simultaneously with the transition metal. In contrast, in the post-growth method, TMD is initially deposited, followed by the deposition of the transition metal.
In this study, TMDs are modified by exposing them to metal vapor in both ways, co-deposition and post-growth. The systems are studied: WSe2 co-deposited with V, post-growth Cr on VSe2, post-growth Mn on VSe2, and post-growth Mn on NbSe2. Then it was observed how these additional TM atoms affected the initial structure of the TMDs. The experimental modifications are explained by using density functional theory (DFT) calculations. The combination of experimental results with the DFT calculations provides a more comprehensive understanding of the reasons behind the observed structural preferences. The WSe2 co-deposited with the V system forms mirror twin boundaries (MTBs) due to the susceptibility of V-doped WSe2 to the incorporation of additional V-atoms at interstitial sites. In the remaining three systems, transition metal (TM) atoms are inserted into the van der Waals (vdW) gap of TMD layers, forming covalent bonds with the Se atoms. This resulted in the opening of a new pathway for the design of new pseudo-2D nanomaterials. Post-growth Cr or Mn on VSe2 bilayers exhibit different superstructures related to the concentrations of inserted TM atoms, as these TM atoms are arranged periodically inside the vdW gap. Nevertheless, superstructures are not observed in Mn-inserted NbSe₂ systems, as Mn atoms are inserted randomly and not arranged in a periodic manner.
It is of interest to determine the impact of these modifications on the intrinsic properties of host TMDs, with a particular focus on magnetic properties. For low V concentrations in WSe2, a diluted ferromagnetic order is observed, and MTBs do not appear to affect the magnetic properties. These MTBs in the V: WSe2 system result in the pinning of the Fermi level, thereby preventing V-induced electronic doping. In the Cr/Mn: VSe2 structure, the charge states of the intercalated Cr and Mn atoms are +3 and +2, respectively. These atoms are octahedrally coordinated, which places them in a high-spin state. For dilute Cr atoms, experimental and computed magnetic moments are high. When the concentration of Cr is increased, the average magnetic moment decreases, suggesting the antiferromagnetic order between the Cr ions. In the Mn: NbSe2 structure, the charge state of the intercalated Mn atom is 2+, which is a high spin state.
Scholar Commons Citation
Pathirage, Vimukthi Viraj Perera, "Modification of 2D Chalcogenides by Metal Vapor Reaction" (2024). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/10662
