Evolution of G-band modes of single metallic carbon nanotubes with the Fermi level shift is examined by simultaneous Raman and electron transport studies. Narrow Lorentzian line shape and upshifted frequencies are observed near the van Hove singularities. However, all G modes soften and broaden at the band crossing point. The concurrent appearance of an asymmetric Fano line shape at this point indicates that phonon-continuum coupling is intrinsic to single metallic tubes. The apparent Lorentzian line shapes of as-synthesized metallic tubes are induced by O2 adsorption causing the Fermi level shift.

Raman spectra of electrostatically gated single-layer graphene are measured from room temperature to 560 K to sort out doping and thermally induced effects. Repeated heating cycles under Ar led to convergent first-order temperature coefficients of the G-band (χ(G) = -0.03 cm(-1)/K) and the 2D-band (χ(2D) = -0.05 cm(-1)/K) frequencies, which are independent of doping level as long as the Fermi level does not shift with temperature. While the intrinsic behavior may be different (e.g., χ(G) ∼ -0.02 cm(-1)/K near room temperature), these values appear more appropriate in describing responses of most graphene samples on SiO(2) substrates. The more negative χ(G) value than theoretical expectations may be explained by interactions with the substrate reducing the lattice thermal expansion contribution to the temperature dependence of G-band frequency. Enhanced interactions with the substrate may also be responsible for zero-charge, room-temperature G-band line width increase and 2D-band frequency downshift.

Diversity in the Raman G-band phonon modes within individual metallic carbon nanotubes of the same chirality is examined. Comparisons between Raman spectra of as-synthesized nanotubes with those obtained under electrochemical gate potential are made. We show that most of the distribution in line width and peak position of the G-band modes within a single chirality type can be explained by variations in where the Fermi level lies with respect to the band crossing point (i.e., where the nanotube is at zero charge). Varying degree of charge transfer from adsorbed O 2 is likely to be the main source of ∼2 eV or larger range of Fermi level positions. On average, the Fermi level of individual metallic nanotubes lies on the order of 1 eV below the band crossing point. Both charge transfer and physical disorder are evident upon O 2 adsorption. Implications of these findings on electron-phonon coupling and charge transfer processes are discussed.

This article reports on the first attempt of a systematic study on the synthesis of carbon dots (C-dots) for the potential applications in labeling and detection of molybdenum ion (Mo ). Carbon dots (C-dots) were synthesized directly via a simple hydrothermal method using lemon juices as carbon precursor with different temperatures to control the luminescence of C-dots. The obtained C-dots had strong green light emission and the ability to use its luminescence properties as probes for Mo detection application, which is based on Mo induced luminescence quenching of C-dots. This analysis system exhibits strong sensitivity and good selectivity for Mo ion, and a detection limit as low as 20 ppm is achieved. These results suggest that the present C-dots have potential application in optoelectronic, labeling and luminescent probing of Mo ions.

We have examined how electrical characteristics and charging dependent Raman G-band phonon softening in individual metallic carbon nanotubes are influenced by covalent defects. In addition to decreasing electrical conductance with increasing on/off current ratio eventually leading to semiconducting behavior, adding covalent defects reduces the degree of softening and broadening of longitudinal optical (LO) phonon mode of the G-band near the charge neutrality point where the bands cross. On the other hand, the transverse optical (TO) mode softening is enhanced by defects. Implications on the interpretation of Raman G-band phonon softening and on utilizing Raman spectroscopy to examine covalent functionalization are discussed.