Ahmed Azmi Zahab

Direct measurement of the absolute absorption spectrum of individual semiconducting single-wall carbon nanotubes.

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.

Reversible optical doping of graphene

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.

In situ Raman probing of graphene over a broad doping range upon rubidium vapor exposure.

We report in situ Raman scattering experiments on single-layer graphene (SLG) and Bernal bilayer graphene (BLG) during exposure to rubidium vapor. The G- and 2D-band evolutions with doping time are presented and analyzed. On SLG, the extended doping range scanned (up to about 10(14) electrons/cm(2)) allows the observation of three regimes in the evolution of the G-band frequency: a continuous upshift followed by a plateau and a downshift. Overall the measured evolution is interpreted as the signature of the competition between dynamic and adiabatic effects upon n-doping. Comparison of the obtained results with theoretical predictions indicates however that a substrate pinning effect occurs and inhibits charge-induced lattice expansion of SLG. At low doping, a direct link between electrostatic gating and Rb doping results is presented. For BLG, the added electrons are shown to be first confined in the top layer, but the system evolves with time toward a more symmetric repartition of the added electrons in both layers. The results obtained on BLG also confirm that the slope of the phonon dispersion close to the K point tends to be slightly reduced at low doping but suggest the occurrence of an unexpected increase of the phonon dispersion slope at higher electron concentration.

About the indexing of the structure of single-walled carbon nanotubes from resonant Raman scattering

We review the information that we have obtained from combined Raman spectroscopy and electron diffraction experiments on individual free-standing single-walled carbon nanotubes (SWNT). From these data, we define Raman criteria allowing the indexing of SWNTs from their Raman features only and we show the efficiency of this approach in assigning the structure of individual carbon nanotubes. These results on index-identified individual SWNTs are compared with theoretical predictions.