Changes induced by doping on the Raman spectrum of monolayer and bilayer graphene
Résumé
In this communication, we report data recorded by in situ Raman experiments on single-layer (SLG) and bilayer (BLG) graphene during exposure to rubidium vapor. By this way, we have been able to follow continuously the changes of the G and 2D bands features over a broad doping range (up to about 1014 electrons/cm2). From the comparison with theoretical predictions, we state that the evolution of the G-mode in SLG is understood as resulting of the competition between adiabatic and non-adiabatic effects in presence of a substrate pinning effect which inhibits the charge-induced lattice expansion of graphene layer [1]. For BLG, the added electrons are shown to be first confined in the top layer before a more symmetric charge repartition at high level doping. The evolution of the 2D band of BLG suggests an unexpected increase of the phonon dispersion upon doping [1]. On the other hand, the ultimate surface exposure provided by graphene monolayer makes it the ideal sensor platform but also exposes its intrinsic properties to any environmental perturbations. In this work, we demonstrate that the charge carrier density of graphene exfoliated on a SiO2/Si substrate can be finely and reversibly tuned between electron and hole doping with visible photons. This photo-induced doping happens under moderate laser power conditions but is significantly affected by the substrate cleaning method. In particular, it requires hydrophilic substrates and vanishes for suspended graphene. These findings also suggest that Raman spectroscopy is not always as non-invasive as generally assumed [2]. [1] R. Parret et al., ACS Nano 7, 165 (2013) [2] A. Tiberj et al., Scientific Reports 3:2355 (2013)