Physics

Dr. Ashish Arora
University of Münster, Germany

Abstract:

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 [1-7]. 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 [2,3,5], evidence of dark excitons in monolayer WSe2 [2], a possible observation of Fano-resonance in ML MoSe2 [3], and a lifting of valley degeneracy with magnetic field [1,4,6]. I will also discuss the discovery of a new magneto-optical effect depicting the control of valley coherence using magnetic field [4]. The first evidence of the presence of single-photon emitters at the corners of WSe2 flakes, which demonstrate the effect of photon antibunching in the emission lines will also be presented [6].

References:
[1] A. Arora et al., Nano Letters 16, 3624 (2016) [magneto-optics of monolayer MoTe2]
[2] A. Arora et al., Nanoscale 7, 10421 (2015) [optics of monolayer WSe2]
[3] A. Arora et al., Nanoscale 7, 20769 (2015) [optics of monolayer MoSe2]
[4] R. Schmidt,* A. Arora* et al., Phys. Rev. Lett 117, 077402 (2016) [magneto-optics of monolayer WS2, *Equal contribution]
[5] I. Lezama, A. Arora et al., Nano Lett. 15, 2336 (2015) [optics of MoTe2]
[6] M. Koperski, A. Arora et al., Nat. Nanotech. 10, 503, (2015) [single photon emitters in WSe2]
[7] G. Plechinger, A. Arora et al., Nat. Commun. 7, 12715 (2016) [optics of monolayer WS2]