LETTERS Reversible Oxidation Effect in Raman Scattering from Metallic Single-Wall Carbon Nanotubes

Raman scattering from individual single-wall carbon nanotube (SWNT) bundles was measured using confocal optical microscopy with 632 nm laser excitation. The Raman scattering from metallic SWNTs was found to depend sensitively on adsorbed oxidizing molecules. Tangential mode Raman lines of HNO 3 -treated SWNTs exhibited different line shapes from those of H 2 SO 4 /H 2 O 2 -treated SWNTs. However, the line shapes became identical after high power laser irradiation, which heats the bundle to ∼750 K based on measurement of the breathing mode Stokes to anti-Stokes Raman intensity ratio. Thermal annealing of both samples at 873 K caused the same change in Raman spectra as with laser irradiation, which is attributed to degassing of doping adsorbates. A mechanism of charge transfer between carbon nanotubes and adsorbate molecules is proposed to explain the large change in Raman scattering from metallic SWNTs upon chemical doping and degassing. Raman scattering from degassed SWNT bundles was found to show a linear dependence on excitation intensity, suggesting that Raman cross sections do not change with temperature in the range 300-750 K

Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers

The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G-, D-, and 2D-phonon modes. The G-mode phonons exhibit reduced energies (1580 cm-1) and increased widths (14 cm-1) compared to the response of graphene monolayers supported on the SiO2 covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2x10^11 cm-2 for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show little local disorder, based on the very weak Raman D-mode response. The two-phonon 2D mode of the free-standing graphene monolayers is downshifted in frequency compared to that of the supported region of the samples and exhibits a narrowed, positively skewed line shape

Excitons and high-order optical transitions in individual carbon nanotubes

We examine the excitonic nature of high-lying optical transitions in single-walled carbon nanotubes by means of Rayleigh scattering spectroscopy. A careful analysis of the principal transitions of individual semiconducting and metallic nanotubes reveals that in both cases the lineshape is consistent with an excitonic model, but not one of free-carriers. For semiconducting species, side-bands are observed at ~200 meV above the third and fourth optical transitions. These features are ascribed to exciton-phonon bound states. Such side-bands are not apparent for metallic nanotubes,as expected from the reduced strength of excitonic interactions in these systems

Graphene Oxidation: Thickness Dependent Etching and Strong Chemical Doping

Patterned graphene shows substantial potential for applications in future molecular-scale integrated electronics. Environmental effects are a critical issue in a single layer material where every atom is on the surface. Especially intriguing is the variety of rich chemical interactions shown by molecular oxygen with aromatic molecules. We find that O2 etching kinetics vary strongly with the number of graphene layers in the sample. Three-layer-thick samples show etching similar to bulk natural graphite. Single-layer graphene reacts faster and shows random etch pits in contrast to natural graphite where nucleation occurs at point defects. In addition, basal plane oxygen species strongly hole dope graphene, with a Fermi level shift of ~0.5 eV. These oxygen species partially desorb in an Ar gas flow, or under irradiation by far UV light, and readsorb again in an O2 atmosphere at room temperature. This strongly doped graphene is very different than graphene oxide made by mineral acid attack.Comment: 15 pages, 5 figure

Imaging Stacking Order in Few-Layer Graphene

Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the electronic properties of FLG. We demonstrate an accurate and efficient method to characterize stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetra-layer graphene. We find that 15% of exfoliated graphene tri- and tetra-layers is comprised of micron-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. These domains are stable and remain unchanged for temperatures exceeding $800^{\circ}$C.Comment: submitted to Nano Letters; supplementary information about infrared spectroscopy of ABA and ABC graphene trilayers are included