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CAMP Seminar: Raman inquiry into transition metal dichalcogenide semiconductors and semimetals

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Jun Yan, University of Massachusetts, Amherst
06 September 2016 from 3:30 PM to 4:30 PM
339 Davey Lab
Contact Name
Kin Fai Mak
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Molybdenum and tungsten based transition metal dichalcogenides (TMDCs) with polymorphs in the hexagonal (H) and distorted octahedral (T’) phases are a class of layered materials with fascinating electronic properties.  H-TMDCs host massive Dirac fermions and have become a material system of choice for realizing valleytronics; bulk and monolayer T’-TMDCs are predicted to be Weyl semimetals and topological insulators respectively.  In this seminar I will discuss our recent studies using Raman spectroscopy as a powerful non-destructive tool to probe the vibrational and symmetry properties of H- and T’- TMDCs.  Using helicity-resolved measurements, we show that in H-(Mo,W)(S,Se)2 atomic layers, due to conservation of pseudo-angular momentum,  the in-plane zone-center optical phonons switch the circular polarization of incident photons completely 1; in drastic contrast to valley-polarized luminescence that maintains the circular polarization of incident photons. In H-MoTe2, we found that the anti-Stokes Raman can be made stronger than the Stokes peak, providing an interesting mechanism for laser cooling of atomically thin TMDC semiconductors 2. In T’-MoTe2, we found the complex-looking phonon structures can be classified into zigzag modes and mirror modes respectively. This is confirmed by our measurements in bulk and monolayer samples. We further discovered that the low energy shear modes in bulk T’-MoTe2 provide a sensitive tool for probing the breaking of inversion symmetry in the crystal, which is key to the existence of Weyl fermions in non-magnetic systems 3.  


1.           Chen, S.-Y., Zheng, C., Fuhrer, M. S. & Yan, J. Helicity resolved Raman scattering of MoS2, MoSe2, WS2 and WSe2 atomic layers. Nano Lett. 15, 2526–2532 (2015).

2.           Goldstein, T. et al. Raman scattering and anomalous Stokes–anti-Stokes ratio in MoTe2 atomic layers. Sci. Rep. 6, 28024 (2016).

3.           Chen, S.-Y., Goldstein, T., Venkataraman, D., Ramasubramaniam, A. & Yan, J. Activation of new Raman modes by inversion symmetry breaking in type II Weyl semimetal candidate T’-MoTe2. Nano Lett. DOI: 10.1021/acs.nanolett.6b02666 (2016).