Peter A. Dowben
Date of this Version
Evans, P. E. Symmetry and Interface Considerations for Interactions on MoS2. Ph.D. Dissertation, University of Nebraska-Lincoln, Lincoln, NE, 2020.
The critical role of symmetry, in adsorbate-MoS2 interactions, has been demonstrated through a variety of electronic structure, topology, and catalytic studies of MoS2 and MoS2 composites.A combination of density functional theory and experiment exhibiting diiodobenzene isomer dependent adsorption rates highlight frontier orbital symmetry as key to adsorption on MoS2. It is clear that the geometry and symmetry of MoS2 influences the creation and stability of surface defects, that in turn affect catalytic activity and a myriad of other applications. We have shown that surface reactions such the methanol to methoxy reaction can create defects on MoS2. From experiment, it is clear moving forward, if there is to be a better understanding of reactions and catalysis, theory must include further considerations of frontier orbital symmetry and input from band structure. Extensive electronic band structure studies have shown bulk MoS2 differs from the monolayer material, as observed with other transition metal dichalcogenides (TMDs). Further differences in the electronic behavior of monolayer and bulk MoS2 are illustrated by variations in behavior that occurs with adsorbed metals om MoS2. Experiments show metal to bulk TMD interactions exhibit electronic behavior fitting the expected conventional models while monolayer to metal interactions show a distinctly opposite trend. Undoubtedly, more complete interaction theory on MoS2 and other TMDs will require a better detailing of the difference in band structure between monolayer and bulk. Added to the differences in electronic structure, between monolayer and bulk, bulk MoS2 has the volume reservoir that permits defect removal by annealing, while defects will persist for monolayer MoS2. Monolayer MoS2 and other monolayer material are generally accompanied by an underlying substrate. The influence of these underlying substrates on the monolayer systems must now be taken into account along with band bending and interface charge transfer effects. More complete theory which includes underlying mechanisms of frontier orbital symmetry in addition to monolayer and bulk electronic properties is necessary for accurately predicting the catalytic behavior and application of TMDs.
Advisor: Peter A. Dowben