Abstract by: Talat S. Rahman

Unraveling and Manipulating Functionalities of 2D Materials: Old Stuff with New Promises

About eight years back, in the wake of graphene, single-layer molybdenum disulfide (MoS₂) emerged as a promising materials for next generation applications (optoelectronic and catalysis), because of its low-dimensionality, intrinsic direct band-gap which lies in the visible, and presence of quasiparticle excitations such as excitons, biexcitons, and trions with binding energies uncharacteristically high for semiconductors. MoS₂ is also a leading hydro-desulphurization catalyst, for reasons that remain elusive. Efforts are underway to further tune these physical and chemical properties through defects, dopants, support, and stacking (homo- and hetero-structures), not just for MoS₂ but also for a range of 2D materials, including other transition metal dichalcogenides (TMDs). In this talk, I will present some results which provide a framework for manipulating the functionality of these interesting van der Waals materials. With regard to optical properties, I will present results of our analysis of the excitation spectrum and the ultrafast charge dynamics in both single- and bi-layer TMDs obtained through the application of combined time-dependent density functional theory and many-body theory. In particular, I will show how reduced electron screening in these systems leads to the large binding energy of excitons (hundreds of meVs), trions (tens of meVs) and biexcitons (tens of meVs), in rather good agreement with available experimental data. I will also show that ultrafast (10-100 fs) transfer processes are possible in these materials as a result of strongly-delocalized hole orbitals. With emphasis on the chemical properties of defect-laden single layer MoS₂ and hexagonal boron nitride (h-BN), I will examine modulations of the local atomic environment under which these inert materials could serve as a catalyst for several technologically important reactions. Lastly, I would draw attention to defect-laden h-BN which has been proposed as a single photon emitter.

* Work supported through grants from DOE and NSF