Martin Hulman

Raman spectroscopy of small-diameter nanotubes

Results based on Raman measurements of small-diameter nanotubes (NTs) are presented and discussed in this paper. The NTs with diameters from 1 nm down to 0.4 nm were produced either as the inner tubes in the double-wall carbon NTs (DWCNTs) or as tubes embedded in the channels of the zeolite crystals. While analysing the Raman spectra attention was paid to the radial breathing mode (RBM), the D line and the G band. For both NT systems the RBM frequency was found to follow the same functional diameter dependence as the tubes with larger diameters. However, in contrast to the latter, the diameters of the thin tubes obtained from density functional theory calculations must be taken into account to explain satisfactorily the observed line positions. The resonance behaviour of the RBM intensities was recorded for the tubes in zeolites. It allows us to ascribe a position of the RBM to a particular NT. This result also demonstrates the breakdown of a simple tight-binding approach to the electronic structure but agrees with predictions from ab initio calculations. The D line of the outer tubes in DWCNTs is dispersive, similar to the single-wall carbon NTs. However, the rate of dispersion is reduced for the inner tubes in DWCNTs. This is attributed to the fact that the inner and outer tubes are probed with the same laser excitation. The linear shift due to the increasing laser energy is compensated by the negative shift due to the NT diameter. The latter is smaller for the inner NTs which leads to a stronger compensation of their dispersive behaviour. This effect is even stronger for the NTs in zeolites. In the extreme case, the strong Raman lines are not dispersive at all. This unexpected behaviour was explained by the detailed ab initio calculation of the phonon structure. The G bands of the inner semiconducting tubes were observed as new features in the Raman spectra of DWCNTs. On the other hand, no lines of metallic inner tubes were found. G bands of semiconducting as well as metallic NTs were detected for the zeolite samples. In either case, Raman lines due to the recently proposed Peierls-like mechanism for the thin metallic tubes were not indentified. This mechanism must therefore cause a significant reduction in Raman intensity.

Raman spectroscopy of fullerenes and fullerene-nanotube composites

The discovery of fullerenes in 1985 opened a completely new field of materials research. Together with the single–wall carbon nanotubes (SWCNTs) discovered later, these curved carbon networks are a playground for pure as well as applied science. We present a review of Raman spectroscopy of fullerenes, SWCNTs and composite materials. Beginning with pristine C60, we discuss intercalated C60 compounds and polymerized C60, as well as higher and endohedral fullerenes. Concerning SWCNTs, we show how the diameter distribution can be obtained from the Raman spectra and how doping modifies the spectra. Finally, the Raman response of C60 encapsulated into SWCNTs (C60 peapods) is discussed.

Raman spectroscopy of single-wall carbon nanotubes and graphite irradiated by γ rays

Graphite and single-wall carbon nanotubes irradiated by γ rays of energy of 1.3 MeV were investigated by Raman spectroscopy. Irradiation generates defects in the lattice as confirmed by the increase of the intensity of the defect-induced D line in both materials. On the other hand, the intensity of the radial breathing mode of nanotubes is lowered. The intensity of the G line does not change for graphite but increases for carbon nanotubes. For the latter, this behavior cannot be explained by the defect-mediated double-resonance mechanism. Softening of the q=0 selection rule is suggested as a way to explain the results.

Diatomic metal encapsulates in fullerene cages: A Raman and infrared analysis of C84 and Sc2@C84 with D2d symmetry

Raman scattering and infrared absorption of the C84 and Sc2@C84 isomers 23:D2d were studied at room temperature and 95 K. The results are compared to the response of pristine and doped C60. According to the lower symmetry and the higher number of atoms C84 exhibits much more vibrational modes than C60, in particular at wave numbers above 500 cm−1. For lower energies the vibrational structure of C84 resembles a downshifted and split C60 spectrum. After the encapsulation of two scandium atoms the overall vibrational structure and the number of C84 modes was preserved as a result of the similar geometric structure. From the very good correlation of the C84 and Sc2@C84 cage modes metal to fullerene charge transfer induced shifts could be analyzed. The lines were found less shifted compared to the C60 modes in exohedral doped A6C60 (A=K,Rb,Cs). Increased line widths of low energy cage modes were attributed to an additional intramolecular relaxation channel related to the dynamics of the encapsulated scandium i...

Light-induced instability of PbO-filled single-wall carbon nanotubes

We investigated single-wall carbon nanotubes filled with lead oxide, PbO, by transmission electron microscopy and Raman spectroscopy. It is concluded that PbO crystallizes in the orthorombic phase forming nanowires inside the nanotubes. The positions of the PbO Raman lines are downshifted as compared to the bulk material as a result of the reduced dimensionality. As a consequence of the filling, nanotubes become sensitive to the laser irradiation. At higher laser power densities, they oxidize and the free PbO nanowires are left in the sample.

Reduced graphite oxide in supercapacitor electrodes.

The current energy needs have put the focus on highly efficient energy storage systems such as supercapacitors. At present, much attention focuses on graphene-like materials as promising supercapacitor electrodes. Here we show that reduced graphite oxide offers a very interesting potential. Materials obtained by oxidation of natural graphite and subsequent sonication and reduction by hydrazine achieve specific capacitances as high as 170 F/g in H2SO4 and 84F/g in (C2H5)4NBF4/acetonitrile. Although the particle size of the raw graphite has no significant effect on the physico-chemical characteristics of the reduced materials, that exfoliated from smaller particles (<75 μm) result more advantageous for the release of the stored electrical energy. This effect is particularly evident in the aqueous electrolyte. Graphene-like materials may suffer from a drop in their specific surface area upon fabrication of electrodes with features of the existing commercial devices. This should be taken into account for a reliable interpretation of their performance in supercapacitors.

Effects of Charge Impurities and Laser Energy on Raman Spectra of Graphene

The position and width of the Raman G-line was analyzed for unintentionally doped single-layered graphene samples. Results indicate a significant heating of the monolayer by the laser beam. Moreover, a weak additional component was resolved in the G-band. The position of the line is independent of the level of doping of the sample. We conclude that this new component is due to the phonons coupled to the intraband electronic transitions.

Resonance Raman investigation of single wall carbon nanotubes

Resonance Raman scattering for the radial breathing mode of single wall carbon nanotubes was investigated experimentally and theoretically. 14 different Raman lines were identified as a fine structure of the mode response. The strongest lines were assigned to achiral and chiral tubes. The metallic tubes were suggested to give the strongest contribution.

Raman spectroscopy of template grown single wall carbon nanotubes in zeolite crystals

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy and ab initio density functional calculations. In the experiment up to 19 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. Even though the observed Raman spectra were very rich on lines only two types of nanotubes with different chiralities, (5,0) and (4,2), were found to be responsible for the observed response. The frequencies of the radial breathing modes were reliably assigned. Even though the (5,0) is metallic, the A1g mode does not couple to the electronic continuum and the Peierls-type mechanism does not shift the mode toward lower frequencies. A strong response was also observed for frequencies around 1250 cm−1. The positions of two peaks assigned to the (5,0) do not depend on the laser energy whereas only one peak was observed for the (4,2) nanotube. Its frequency shifts wi...

Superposition of quantum and classical rotational motions in Sc2C2@C84 fullerite

The superposition of the quantum rotational motion (tunneling) of the encapsulated Sc(2)C(2) complex with the classical rotational motion of the surrounding C(84) molecule in a powder crystal of Sc(2)C(2)@C(84) fullerite is investigated by theory. Since the quantum rotor is dragged along by the C(84) molecule, any detection method which couples to the quantum rotor (in casu the C(2) bond of the Sc(2)C(2) complex) also probes the thermally excited classical motion (uniaxial rotational diffusion and stochastic meroaxial jumps) of the surrounding fullerene. The dynamic rotation-rotation response functions in frequency space are obtained as convolutions of quantum and classical dynamic correlation functions. The corresponding Raman scattering laws are derived, and the overall shape of the spectra and the width of the resonance lines are studied as functions of temperature. The results of the theory are confronted with experimental low-frequency Raman spectra on powder crystals of Sc(2)C(2)@C(84) [M. Krause et al., Phys. Rev. Lett. 93, 137403 (2004)]. The agreement of theory with experiment is very satisfactory in a broad temperature range.