Optical and high field magneto optical studies on atomically thin sheets of WS2, WSe2, MoSe2 and MoTe2: A new family of semiconductors

Layered transition metal dichalcogenides of the form MX2 (M = W, Mo; X = S, Se, Te) have attracted considerable interest recently as a new family of semiconductors. Their band structure undergoes a transition from indirect to direct band gap as their thickness is reduced from bulk to atomically thin monolayers (MLs). This makes them ideal for studying many physical phenomena in two dimensions, such as realization of tightly bound quasiparticles eg. excitons and trions. Also, different K and K' valleys in the band structure can be addressed using circularly polarized light excitation making them interesting in the emerging area of spin-valley based electronics, namely valleytronics.

In this talk, I will present the optical and magneto-optical studies which we have performed on layered WS2, WSe2, MoSe2 and MoTe2.[1-4] We used photoluminescence, photoluminescence excitation, reflectivity and transmission measurements in micro-measurement mode to investigate these materials as a function of layer thickness, under temperatures ranging from 4 K to 300 K, and under magnetic fields up to 30 tesla. Apart from the typical semiconductor behavior such as narrowing of exciton band gap and increase of lineshape broadening with increasing temperatures that these materials demonstrate, many new interesting effects are observed. A few noteworthy effects are an interesting behavior of exciton binding energy and band gap with layer thickness[1,2,4] a possible observation of Fano-resonance in ML MoSe2[4] and lifting of valley degeneracy with magnetic field. A presence of quantum dot like narrow line emitting centers have been observed at the corners of WSe2 flakes which demonstrate the effect of photon antibunching in the emission lines, the unambiguous attribute of single photon emitters.[3]

References:
[1] A. Arora et al., Nanoscale 7, 10421 (2015) [studies on WSe2]
[2] I. Lezama, A. Arora et al., Nano Lett. 15, 2336 (2015) [studies on MoTe2]
[3] M. Koperski, A. Arora et al., Nature Nanotech. 10, 503, (2015) [single photon emitters in   
     WSe2]
[4] A. Arora et al., Nanoscale (accepted, 2015), preprint at http://arxiv.org/abs/1509.06439 
     [studies on MoSe2]