Groupe de Physique Statistique

Equipe 106, Institut Jean Lamour

                     
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Séminaire de groupe

Optical properties of MoS2. Excitons beyond the bandgap
Alejandro Molina-Sánchez
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 [1]. 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 [4]. 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 [5]. 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. [1] A. Molina-Sánchez, 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). [2] A. Molina-Sánchez, 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). [3] 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). [4] A. R. Klots et. al., “Probing excitonic states in ultraclean suspended two-dimensional semiconductors by photocurrent spectroscopy”, Scientific Reports 4, 6608 (2015). [5] D. Xiao, et al., “Coupled Spin and Valley Physics in Monolayers of MoS2, and other group-VI dichalcogenides”, Phys. Rev. Lett. 108, 196802 (2012).



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