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 hole and electron 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 suggest that optically gated graphene devices operating with a sub-second time scale can be envisioned and that Raman spectroscopy is not always as non-invasive as generally assumed.

Abstract In this communication, we will illustrate how Raman spectroscopy can be used to study the doping of graphene. We will first report data recorded by in situ Raman experiments on single-layer (SLG) 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). Previous theoretical predictions have shown that the evolution of the G-mode in SLG results from the competition between adiabatic and non-adiabatic effects. We emphasize that a possible substrate pinning effect, which inhibits the charge-induced lattice expansion of graphene layer, can strongly influence the G band position [1]. In the second part, we will show 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]. References [1] R. Parret, M. Paillet, J.-R. Huntzinger, D. Nakabayashi, T. Michel, A. Tiberj, J.-L. Sauvajol, A.-A. Zahab, ACS Nano, 7 (2013) 165. [2] A. Tiberj, M. Rubio-Roy, M. Paillet, J.-R. Huntzinger, P. Landois, M. Mikolasek, S. Contreras, J.-L. S!auvajol, E. Dujardin, A.-A. Zahab, Scientific Reports, 3 (2013) 2355.

The ultimate surface exposure provided by graphene monolayer makes it the ideal sensor platform but also exposes its intrinsic properties to any nvironmental perturbations. We show in this work that structural and electronic characterization of graphene in air by Raman spectroscopy is significantly affected by the substrate surface cleaning method and moderate laser power conditions. In particular, 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. The amplitude of this photo-induced doping is found to require hydrophilic substrates and to vanish in suspended graphene. These findings suggest that optically gated graphene devices operating with a sub-second time scale can be envisioned but also that Raman spectroscopy might not be as non-invasive as generally assumed.

The optical properties of single-wall carbon nanotubes are very promising for developing novel opto-electronic components and sensors with applications in many fields. Despite numerous studies performed using photoluminescence or Raman and Rayleigh scattering, knowledge of their optical response is still partial. Here we determine using spatial modulation spectroscopy, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of individual semiconducting single-wall carbon nanotubes. These quantitative measurements permit determination of the oscillator strength of the different excitonic resonances and their dependencies on the excitonic transition and type of semiconducting nanotube. A non-resonant background is also identified and its cross-section comparable to the ideal graphene optical absorbance. Furthermore, investigation of the same single-wall nanotube either free standing or lying on a substrate shows large broadening of the excitonic resonances with increase of oscillator strength, as well as stark weakening of polarization-dependent antenna effects, due to nanotube-substrate interaction.

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)

Double-wall carbon nanotubes (DWNTs) appear as prototypal nanomaterial for studying coupling interactions [1-4] and screening effects at the nanoscale [5]. In this context, we prepared suspensions of individualized DWNTs in sodium deoxycholate (DOC) from pristine DWNTs (p-DWNTs) and covalently functionalized DWNTs (f-DWNTs). F-DWNTs are prepared by the diazonium route which has been shown to selectively functionalize outer-walls [6, 7]. The samples were studied by UV-Vis-NIR absorption, Raman spectroscopy, and photoluminescence (PL). The optical absorption of DWNTs suspensions in surfactant have been measured up to 2500 nm in order to confirm that the DWNTs are individualized and the covalent functionalization of outer-walls. Raman spectra were measured with several excitation wavelengths between 532 nm and 800 nm. According to [8, 9], we identified the (n,m) of small diameter inner-walls by using radial-breathing-like modes (RBLM) frequencies. Compared with the RBM of individualized HiPCO single-wall nanotubes (SWNTs) samples prepared using the same protocol, we observed a slight upshift of RBLM of inner-walls [1-3]. We measured PL maps with excitation wavelengths in the range 700-950 nm and compared them with results from the literature [11, 12] as well as data obtained on HiPCO SWNTs suspension. The results obtained suggest that inner-walls are luminescent. In order to confirm this conclusion, preliminary results of combined PL/Raman experiments performed at the individual DWNT level will be presented.

The ultimate surface exposure provided by graphene monolayer makes it the ideal sensor platform but also exposes its intrinsic properties to any environmental perturbations. We show in this work that structural and electronic characterization of graphene in air by Raman spectroscopy is significantly affected by the substrate surface cleaning method and moderate laser power conditions. In particular, 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. The amplitude of this photo-induced doping is found to require hydrophilic substrates and to vanish in suspended graphene. These findings suggest that optically gated graphene devices operating with a sub-second time scale can be envisioned but also that Raman spectroscopy might not be as non-invasive as generally assumed.