Phonon dispersion and optical properties of layered materials: influence of the environment

Since graphene and related hexagonal monolayers (BN, MoS2) are atomically thin 2D materials, their properties can be strongly influenced by the environment. In particular, the substrate on which they are deposited can modify the phonon dispersion and the optical properties.
Besides the direct "mechanical" interaction via chemisorption or physisorption, the dielectric (or metallic) screening by the substrate plays an important role. It can reduce the Kohn-Anomaly at the high-symmetry point K of the highest optical phonon branch of graphene by partially screening the electron-phonon coupling. We give several examples how this effect can be observed in Raman and HREEL spectra: graphene on boron nitride [1], graphene on Ir(111)
[2], graphene on Ni(111) [3].
Furthermore, we present ab-initio calculations of the electronic band-structure and optical absorption spectra of mono and multi-layers of molybdenum disulfide: we discuss the role of spin-orbit coupling and inter-layer coupling on the splitting of the valence band maximum. The absorption spectra display prominent excitonic effects [4]. The excitonic binding energy strongly depends on the number of layers and the dielectric environment. [1] F. Forster, A. Molina-Sánchez, S. Engels, A. Epping, K. Watanabe, T. Taniguchi, L. Wirtz, and
C. Stampfer, Phys. Rev. B 88, 085419 (2013).
[2] M. Endlich, A. Molina-Sánchez, L. Wirtz, and J. Krüger, Phys. Rev. B 88, 205403 (2013).
[3] A. Allard and L. Wirtz, Nano Letters 10, 4335 (2010).
[4] A. Molina-Sánchez, D. Sangalli, K. Hummer, A. Marini, and L. Wirtz, Phys. Rev. B 88, 045412
(2013).