Title: Theory of optical and vibrational properties of 2D materials: influence of the environment

　　Speaker: Prof. Ludger Wirtz (Physics and Materials Science Research Unit, University of Luxembourg)

　　Time: Oct. 23,2014, 10:30AM

Venue:No. 101 Meeting Room, IOS, CAS 

　　Abstract: 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, graphene on Ir(111), graphene on Ni(111).

　　Furthermore, we present calculations of the optical and vibrational properties of mono-layer, few- layer, and bulk MoS2. Some surprises occur: the phonon dispersion displays an anomalous Davydov splitting which we analyze in terms of the short and medium range interatomic force constants for S-S and S-Mo atom pairs in neighboring layers. The optical absorption spectra are calculated using the methods of many-body perturbation theory (GW and Bethe-Salpeter Equation). The spectra display a rich structure of excitonic peaks in the band-gap and in the continuum of interband transitions.

　　Biography：Ludger Wirtz studied physics at Bonn University, Germany, and at the University of Tennessee in Knoxville, USA. He received the PhD from the Vienna University of Technology, Austria, in 2001. After a postdoctoral stay in the group of Prof. A. Rubio at the University of the Basque Country, San Sebastian, Spain, he joined the Institute of Electronics, Microelectronics, and Nanotechnology (IEMN) in Lille, France, as a permanent researcher of the CNRS in 2004. In February 2012 he was appointed professor of theoretical solid-state physics at the University of Luxembourg. His research interests include semiclassical methods in mesoscopic physics, the theory of ion-surface interaction, and the theoretical/computational spectroscopy of nanostructured materials (2D materials, nanotubes, nanocrystals).