Séminaire de groupe
|Optical properties of MoS2. Excitons beyond the bandgap|
|Physics and Materials Science Research Unit|
|jeudi 05 février 2015 , 10h25|
|Salle de séminaire du groupe de Physique Statistique|
Monolayer MoS2 is a prominent candidate to be a semiconductor alternative to graphene in the realm of ultra-thin materials. It is characterized by an optical bandgap of 1.8 eV, which makes it suitable for use in nano-scale transistors. Besides the bandgap, we find many interesting characteristics on the electronic properties of monolayer MoS2 . The sensitivity of the band structure to external strain paves the way to tune not only the bandgap value, but also its direct/indirect character. Moreover the particular topology of the electronic structure of monolayer MoS2 has also non-trivial consequences in the optical properties. Due to spin-orbit interaction, the optical spectra is characterized by two excitonic peaks at the absorption threshold. In addition, we find a sharp peak in the continuum. This exciton has been detected by means of second-harmonic generation spectroscopy. Its origin can be a Van Hove singularity in the density of states, localized between the G and K points of the Brillouin zone [2, 3]. In recent photo-current experiments, this exciton has exhibited a much higher energy of dissociation than the excitons at lower energy . Another important feature of monolayer MoS2 is the capability of generating light with defined circular polarization. This makes MoS2 single-layers suitable for the study of “valleytronics”, which consists in the generation of carrier populations with a well-defined angular momentum . We present the state-of-the art calculations concerning the electronic and optical properties of monolayer MoS2, in comparison with few-layers and bulk MoS2. We discuss which approaches are suitable for the theoretical study of the band structure and the exciton states, as for instance, (non) self-consistency in the GW method. Moreover, the influence of the electron-phonon interaction in the electronic structure of MoS2 layers will be presented. We also introduce the recent advances in the influence of substrate in the optical response.  A. Molina-SÃ¡nchez, D. Sangalli, K. Hummer, A. Marini, and L. Wirtz, “Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2”, Phys. Rev. B 88, 045412 (2013).  A. Molina-SÃ¡nchez, K. Hummer, and L. Wirtz, “Theory of the vibrational and electronic properties of layered MoS2: From single-layer to bulk”, to appear in Surf. Sci. Reports (2014).  L. M. Malard, T. V. Alencar, A. P. Barboza, K. F. Mak, A. M. de Paula, “Observation of intense second harmonic generation from MoS2 atomic crystals”, Phys. Rev. B, 87, 201401 (2013).  A. R. Klots et. al., “Probing excitonic states in ultraclean suspended two-dimensional semiconductors by photocurrent spectroscopy”, Scientific Reports 4, 6608 (2015).  D. Xiao, et al., “Coupled Spin and Valley Physics in Monolayers of MoS2, and other group-VI dichalcogenides”, Phys. Rev. Lett. 108, 196802 (2012).