Optical properties of atomically thin semiconductors layers and heterostructures

TYPESpecial Seminar - Solid State Institute, Technion
Speaker:Prof. Tony F. Heinz
Affiliation:Departments of Applied Physics and Photon Science, Stanford University, U.S.A.
Location:Solid State Auditorium(Entrance)
Remark:Host: Professor Efrat Lifshitz
Abstract:In this paper we discuss recent advances in our understanding of the optical properties of monolayers of the transition metal dichalcogenide (TMDC) materials, including MoS2, MoSe2, MoTe2, WS2, WSe2. These materials share several unusual  characteristics,  including  a  transition  from  an  indirect-gap  material  in  the  bulk  to  a  direct-gap,  emissive material at monolayer thickness. They also exhibit selectivity to excitation of the degenerate K or K' valley under circularly polarized radiation. In our discussion we will highlight progress in understanding two types of interactions in these materials: the manybody interactions between charge carriers in one layer and interactions between carriers that arise when two monolayer sheets of materials stacked on one another to form a bilayer. The  many-body  electronic  interactions  in  monolayer  TMDC  crystals  play  a  central  role  in  defining  their  optical properties. Here we will stress recent spectroscopic studies in which we have identified the progression of excited exciton states in precise absorption measurements. This study directly reveals exciton binding energies of several hundred meV. A strongly non-hydrogenic disposition of levels is also observed. Another  unusual  type  of interaction associated  with these  materials  concerns the  electronic  states  and  transitions expected in stacks of TMDC monolayers. We will present results of studies of the optical response of stacks composed of two monolayers the same material (but with an adjustable twist angle) and bilayers of two different crystals.