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J Phys Condens Matter. 2019 Aug 30. doi: 10.1088/1361-648X/ab3fec. [Epub ahead of print]

Thermal Expansion Coefficient and Phonon Dynamics in Coexisting Allotropes of Monolayer WS2 Probed by Raman Scattering.

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IIT Mandi, Mandi, 175001, INDIA.
Physics, Indian Institute of Technology Mandi, School of basic sciences, Mandi, Himachal Pradesh, 175001, INDIA.
School of Engineering, IIT Mandi, Mandi, Himachal Pradesh, INDIA.
School of Engineering , Indian Institute of Technology Mandi, Mandi, 175001, INDIA.
School of Basic Sciences , IIT Mandi, Mandi, Himachal Pradesh, INDIA.


We report a comprehensive temperature dependent Raman measurements on three different phases of monolayer WS2 from 4K to 330K in a wide spectral range. Our studies reveal the anomalous nature of the first as well as the higher order combination modes reflected in the disappearance of the few modes and anomalous temperature evaluation of the phonon self-energy parameters attributed to the detuning of resonance condition and development of strain due to thermal expansion mismatch with the underlying substrate. Our detailed temperature dependence studies also decipher the ambiguity about assignment of the two modes in literature near ~ 297 cm-1 and 325 cm-1. Mode near 297 cm-1 is assigned as first order Raman mode, which is forbidden in the backscattering geometry and 325 cm-1 is assigned to the combination of and mode. We also estimated thermal expansion coefficient by systematically disentangling the substrate effect in the temperature range of 4K to 330K and probed its temperature dependence in 1H, 1T and 1T' phases.


2D materials; Raman scattering; Thermal expansion


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