Large scale production and light induced dynamics of two dimensional semiconductors

Seminar | December 13 | 10-11 a.m. | 775 Tan Hall

 Fang Liu Ph.D.

 College of Chemistry

Two dimensional (2D) semiconductors and their artificial structures hold great promises for electronic, optoelectronic, and quantum devices. We developed a facile method to disassemble van der Waals (vdW) single crystals layer-by-layer into monolayers with near-unity yield, high quality, and macroscopic dimensions limited only by bulk crystal sizes. It enables us to exfoliate a range of vdW crystals and assemble the monolayers into heterostructures of atomically thin pn junctions, as well as artificial lattices with dramatic enhancement in nonlinear optical responses. This approach takes us one step closer to mass production and commercialization of 2D materials. Using the macroscopic 2D crystals, we quantified several key dynamics in the 2D semiconductors and their heterojunctions. Due to poorly screened Coulomb potential in 2D geometry, the quasiparticle bandgap (Eg) is reduced significantly in the presence of carriers, an effect named band renormalization. Using time and angle resolved photoemission spectroscopy (TR-ARPES), we quantified bandgap renormalization directly in macroscopic single crystal MoS2 monolayer and WS2/MoS2 heterobilayer on dielectric substrate. We have also determined momentum-resolved intervalley conduction band (CB) electron transfer in the WS2/MoS2 heterobilayer, revealing fast scattering dynamics across multiple CB valleys on fs time scale, assisted by fast phonon scattering. The interlayer charge transfer is accompanied by momentum specific band renormalization. These findings suggest the presence of both direct and indirect interlayer excitons and reveal constraints on achieving long-lived spin-valley polarization, a key aspect in spintronics for future information processing.