I have done in detail Raman studies for charge transfer between few layer MoS2 and donor and acceptor aromatic molcules (like TCNE, MV etc.).In general FWHM increase with increasing conc. of electron donor and acceptor molecules.But In case of few layer MoS2 especially with donors FWHM decreaing with increasing conc. of Molecules.I would like to know why it is happening? in case of graphene (FWHM increases with increasing conc.), it is well reported, but in case few layer MoS2 experimentally not reported about FWHM.
I appreciate, if anyone share views or articles in which i can get above information.
Thanks a lot.
Your observation (narrowing of the Raman spectral lines) is due to a fluctuating environment and the phenomenon is thus referred to as motional narrowing. Motional narrowing is often discussed in the context of the NMR spectroscopy, however the underlying mechanism equally applies to Raman spectroscopy.
The theory of motional (and exchange) narrowing of spectral lines is due to Anderson:
[1] PW Anderson, J. Phys. Soc. Jpn 9, 316 (1954).
See also:
[2] CP Slichter, Principles of Magnetic Resonance, 3rd edition (Springer, Berlin, 1990).
In the context of Raman spectroscopy, Smith et al. explicitly refer to the phenomenon of motional narrowing:
[3] JD Smith, RJ Saykally, and PL Geissler, J. Am. Chem. Soc. 129, 13847 (2007).
See in particular Ref. [12] (Eaves et al.) herein.
As you know, MoS2 has two main peaks of A1g and E2g. I guess the decreased FWHM with increase electron concentration is for E2g than A1g. The reason that the increased electron density in few layer MoS2 induces the reduction of van der Waals interaction between layers. Therefore, the resulting the reduced screening effect of out-of-plane phonon strength make the FWHM of E2g mode short with blue shift because in-plane mode do not have obstacle for its own mode.
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