George Kioseoglou

Department of Materials Science and Technology, Un. of Crete

Two-dimensional materials beyond graphene: optical and electronic properties

Abstract

 

Two-dimensional materials beyond graphene: optical and electronic properties
 
George Kioseoglou
Department of Materials Science and Technology
           
 
Transition-metal dichalcogenides, MX2 (M=Mo, W and X=S, Se) have emerged as a new class of two dimensional atomic crystals with remarkable physical properties. Unlike graphene, layered MX2 have a sizeable bandgap. More interestingly, when thinned down to a monolayer, they transform from indirect to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular polarization, doping dependent charged excitons and strong photocurrent response.
            To study the optical and electronic properties of these 2D materials, we use a number of spectroscopic techniques such as micro-PL, Raman, differential reflectivity and absorption. For all of the MX2 materials, excitonic emission is observed in the visible regime with the main emission occurring at the A-exciton. Additionally, identification of the B-exciton yields spin-orbit energies ranging from 160 meV to 400 meV. We also use energy and helicity dependent optical pumping to analyze the coupling of the valley and spin indices to the depolarization of emitted light. The circular polarization of the photoluminescence for the Mo-X2 family is very high for photo-excitation near the bandgap, and has a power-law decrease as the photo-excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model of depolarization that explains the wide variation in values for the optical polarization reported in the literature. The same energy-dependent optical pumping study for the W-X2 family, on the other hand, reveals a quite different behavior that is still under investigation.


Date: 26/3/2015
Time:12:00 (coffee & cookies will be served at 11:45)
Place:FORTH Seminar Room 1