Abstract:

The spectacular progress in controlling the electronic properties of graphene has triggered research in alternative atomically thin two-dimensional crystals. Monolayers (ML) of transition-metal dichalcogenides such as MoS2 have emerged as very promising nanostructures for optical and electronic applications for mainly two reasons. First, the indirect bulk semiconductor MoS2 becomes direct when thinned to 1ML, resulting in efficient optical absorption and emission. Second, inversion symmetry breaking (usually absent in graphene) together with the large spin-orbit interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (¦Ò+ or ¦Ò?) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two nonequivalent k valleys (K+ or K?, respectively). In this talk we will review our recent results based on cw and time-resolved photoluminescence experiments performed on MoS2 and WSe2 MLs. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90% polarized emission at 4 K is observed with ~40% polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T [1,2]. We have performed the first time resolved photoluminescence polarization measurements in MoS2 [2] and WSe2 [3] MLs providing vital information on the electron and exciton valley dynamics [4]. Moreover the combination of non-linear and linear optical spectroscopy techniques allows us to uncover the excited exciton spectrum which does not follow the usual Rydberg series. An exciton binding energy as large as 600 meV has been measured for WSe2 ML [5]. We will also present micro-Raman and photoluminescence results measured at 300 K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate [6].
[1] Sallen et al, Phys. Rev. B 86, 081301(R) (2012)
[2] Lagarde et al, Phys. Rev. Lett. 112 47401 (2014)
[3] Wang et al, ArXiv 1402.6009 (2014)
[4] Glazov et al, ArXiv 1403.0108 (2014)
[5] Wang et al, ArXiv?1404, 0056 (2014)
[6] Wang et al, Phys. Rev. B 88, 121301(R) (2013)
 
 
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