Resonance Raman scattering has been widely used to study electronic band structures and to investigate the nature of electron-phonon interactions in semiconductors [1]. A key issue in those studies is the role played by intermediate exciton states, which are usually explored by varying the incident photon energy across the interband transition energy [2]. Some studies use an alternative approach in which the exciton is tuned to resonate with the laser at a fixed energy, either by applying high hydrostatic pressure at a constant temperature or by varying the temperature at ambient pressure. Below we present a study of both temperature and pressure dependence [3], which greatly differ in their effect on the shift of excitonic-transition energies and broadening parameters. Those are analyzed under a single framework and complemented by a calculated Stokes and anti-Stokes Raman probability profiles of the resonant A 1g phonon.

The studied system is a crystalline 2H-MoS2, a transition metal dichalcogenide layer-type indirect semiconductor which exhibits direct transition around 2eV, characterized by an A-B Excitonic pair. Recently, the thickness dependence of the optical and electronic properties of this material has been widely explored [4]. Particularly interesting is a cross-over to a direct gap material which has been found in the limit of one monolayer. Our study clearly points to the additional insight which may be gained by a pressure and temperature -dependent resonant Raman study at the one monolayer level.

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[1] M. Cardona, in Light Scattering in Solids II, edited by M. Cardona and G. Güntherodt, Berlin: Springer- Verlag, 1983, p. 19.;P. Yu and M. Cardona Fundamentals of Semiconductors , Berlin: Springer-Verlag, 1999, p. 393.
[2] T. Sekine, T. Nakashizu, M. Izumi, K. Toyoda, K. Uchinokura, E. Matsuura, J. Phys. Soc. Jpn. 49 Suppl. A, 551 (1980).
[3] T. Livneh and E. Sterer, Phys. Rev. B 81 195209 (2010).
[4] K.F. Mak, C. Lee, J. Hone, J. Shan and T.F. Heintz Phys. Rev Let. 105 136805 (2010) and references therein.