CeNS Colloquium

Dr. Bernhard Urbaszek
Laboratory of Physics and Chemistry of Nano-Objects (INSA-CNRS-UPS), Toulouse

In strong analogy to graphene, the physical properties of transition metal dichalcogenides (TMDCs) change drastically when thinning the bulk material down to one monolayer (ML). The closely related ML materials MoS₂, MoSe₂, WSe₂ and WS₂ have a direct optical bandgap in the visible region around 1.8 eV and show strong optical absorption (>10% / ML). TMDC MLs are atomically flat, 2D materials for electronic devices such as transistors, non-linear optics (frequency doubling) and optoelectronics (photodetectors, LEDs and solar cells). The properties of different ML materials can be combined in heterostructure devices. During this seminar a general introduction to TMDC ML properties will be given [1].

Current micro-and nano-electronics is based on the manipulation ofthe electron charge and spin. ML TMDCs provide unique and convenientaccess to controlling in addition the electron valley degree of freedom in k-space in the emerging field of 'valleytronics'. The valley index (K+ or K-) of an electron in ML TMDC is stabilized by the large spin-orbit splitting and is expected to be robust. We show how the electron valley index can be initialized and read out optically in time and polarization resolved photoluminescence measurements [2]. The polarization properties of bilayers will be discussed for comparison [3].

When electrons and holes are simultaneously present in a ML, they will form excitons as the Coulomb interaction is enhanced by the strong quantum confinement, the large effective masses and the reduced dielectric screening in these ideal 2D systems. We determine large exciton binding energies of typically 0.5eV i.e. the energy of the lowest lying optical transition (optical bandgap) is about 0.5eV below the electronic bandgap. We will demonstrate a variation over several orders of magnitude of the non-linear and linear optical response of ML WSe₂. This is achieved by tuning the optical excitation ON and OFF resonance with respect to the ground (1s) and excited (2s, 2p, ...) exciton states. We show that at these particular energies, identified in 1 and 2-photon excitation spectroscopy, the light-matter interaction is strongly enhanced [4].

[1] http://en.wikipedia.org/wiki/Transition_metal_dichalcogenide_monolayers
[2] PRL 112, 047401 (2014) & PRB 90, 075413 (2014)
[3] G. Wang et al http://arxiv.org/abs/1409.8553
[4] G. Wang et al http://arxiv.org/abs/1404.0056