C. Fantini

Double resonance Raman spectroscopy of single-wall carbon nanotubes

A review of double resonance Raman spectroscopy is presented. Non-zone centre phonon modes in solids can be observed in the double resonance Raman spectra, in which weak Raman signals appear in a wide frequency region and their combination or overtone modes can be assigned. By changing the excitation laser energy, we can derive the phonon dispersion relations of a single nanotube.

Exciton antennas and concentrators from core-shell and corrugated carbon nanotube filaments of homogeneous composition.

There has been renewed interest in solar concentrators and optical antennas for improvements in photovoltaic energy harvesting and new optoelectronic devices. In this work, we dielectrophoretically assemble single-walled carbon nanotubes (SWNTs) of homogeneous composition into aligned filaments that can exchange excitation energy, concentrating it to the centre of core-shell structures with radial gradients in the optical bandgap. We find an unusually sharp, reversible decay in photoemission that occurs as such filaments are cycled from ambient temperature to only 357 K, attributed to the strongly temperature-dependent second-order Auger process. Core-shell structures consisting of annular shells of mostly (6,5) SWNTs (E(g)=1.21 eV) and cores with bandgaps smaller than those of the shell (E(g)=1.17 eV (7,5)-0.98 eV (8,7)) demonstrate the concentration concept: broadband absorption in the ultraviolet-near-infrared wavelength regime provides quasi-singular photoemission at the (8,7) SWNTs. This approach demonstrates the potential of specifically designed collections of nanotubes to manipulate and concentrate excitons in unique ways.

Atypical Exciton–Phonon Interactions in WS2 and WSe2 Monolayers Revealed by Resonance Raman Spectroscopy

Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS2 and WSe2 using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS2 monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related XA and XB excitons), whereas Raman bands of WSe2 are not enhanced at XA/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS2 and WSe2 exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between XC and XA excitons with phonons explains the different Raman responses of WS2 and WSe2, and the absence of Raman enhancement for the WSe2 modes at XA/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).

Intervalley scattering by acoustic phonons in two-dimensional MoS2 revealed by double-resonance Raman spectroscopy

Double-resonance Raman scattering is a sensitive probe to study the electron-phonon scattering pathways in crystals. For semiconducting two-dimensional transition-metal dichalcogenides, the double-resonance Raman process involves different valleys and phonons in the Brillouin zone, and it has not yet been fully understood. Here we present a multiple energy excitation Raman study in conjunction with density functional theory calculations that unveil the double-resonance Raman scattering process in monolayer and bulk MoS2. Results show that the frequency of some Raman features shifts when changing the excitation energy, and first-principle simulations confirm that such bands arise from distinct acoustic phonons, connecting different valley states. The double-resonance Raman process is affected by the indirect-to-direct bandgap transition, and a comparison of results in monolayer and bulk allows the assignment of each Raman feature near the M or K points of the Brillouin zone. Our work highlights the underlying physics of intervalley scattering of electrons by acoustic phonons, which is essential for valley depolarization in MoS2.

Carbon nanotube population analysis from Raman and photoluminescence intensities

In the absence of standard single-wall carbon nanotube samples with a well-known (n,m) population, we provide both a photoluminescence excitation (PLE) and resonance Raman scattering (RRS) analysis that together can be used to check the calculations for PLE and RRS intensities for carbon nanotubes. We compare our results with available models and show that they describe well the chirality dependence of the intensity ratio, confirming the differences between type 1 and type 2 semiconducting tubes [(2n+m)mod3]=1and2, respectively, and the existence of a node in the radial breathing mode intensity for type 2 carbon nanotubes with chiral angles between 20° and 25°.

Raman scattering study of RETiTaO6 dielectric ceramics

In this work we have investigated microwave-dielectric ceramics RETiTaO6 (RE=Al, Y, In, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er or Yb) by Raman scattering measurements at room temperature. The observed Raman-active phonons were analysed in conformity with two orthorhombic structures proposed for these ceramics, namely aeschynite (for lanthanide ions with atomic number between 58 and 66) or euxenite (for Y, Ho, Er and Yb). The results indicate that the phonon spectra remained relatively unchanged with respect to the RE ion substitution within each structure. For the three remaining materials with RE=La, Al and In, which are known to present multiphases, the Raman phonon spectra showed significant differences in comparison with those of ceramics with aeschynite and euxenite structure. # 2003 Elsevier Ltd. All rights reserved.

The influence of oxygen-containing functional groups on the dispersion of single-walled carbon nanotubes in amide solvents

Surface composition plays an important role in carbon nanotube dispersibility in different environments. Indeed, it determines the choice of dispersion medium. In this paper the effect of oxidation on the dispersion of HiPCO single-walled carbon nanotubes (SWNTs) in N-methyl-pyrrolidinone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-dodecyl-pyrrolidinone (N12P) and cyclohexyl-pyrrolidinone (CHP) was systematically studied. During the oxidation process, similar amounts of carboxylic acid and phenolic groups were introduced to mostly already existing defects. For each solvent the dispersion limits and the absorption coefficients were estimated by optical absorption analysis over a range of SWNT concentrations. The presence of acid oxygenated groups increased SWNT dispersibility in NMP, DMF and DMA, but decreased in N12P and CHP. The absorption coefficients, however, decreased for all solvents after oxidation, reflecting the weakening of the effective transition dipole of the π-π transition with even limited extension functionalization and solvent interaction. The analysis of the results in terms of Hansen and Flory-Huggins solubility parameters evidenced the influence of dipolar interactions and hydrogen bonding on the dispersibility of oxidized SWNTs.

The effect of environment on the radial breathing mode of supergrowth single wall carbon nanotubes

It has been shown that “supergrowth” single wall carbon nanotubes (SWNTs) exhibit a radial breathing mode frequency ωRBM dependence on tube diameter dt given by ωRBM=227/dt. This result gave rise to two distinct scenarios for SWNTs: one for the supergrowth radial breathing mode and another for all the other samples reported in the literature. Here we show that, by dispersing the supergrowth SWNTs in surfactant or bringing them into interacting bundles, it is possible to merge these two scenarios, where now the supergrowth SWNT properties are similar to all SWNT properties reported so far in the literature.

Raman and infrared study of hydroxyl sites in natural uvite, fluor-uvite, magnesio-foitite, dravite and elbaite tourmalines

We present the Raman and infrared spectra of different tourmaline species in the spectral range associated with the hydroxyl stretching modes, investigated through polarized Raman spectroscopy. Different lineshapes are observed for the OH spectra in uvite, fluor-uvite, magnesio-foitite, dravite and elbaite samples, and can be related to the coordination of OH in the two different structural V[O(3)]- and W[O(1)]-occupied sites. Local arrangements around the two different OH sites were assigned, and different ion substitutions for these five tourmaline species were identified. Our work with polarized Raman spectroscopy revealed that OH-stretching modes are described by totally symmetric, irreducible representations.

Influence of thermal treatment on the Raman, infrared and TL responses of natural topaz

Abstract Analyses of thermoluminescence (TL) and thermally stimulated exoelectron emission have proved that the natural Brazilian topaz is a promising material for dosimetry of ionizing radiation. Topaz is an aluminum fluorosilicate with a general composition of Al 2 (SiO 4 )(F,OH) 2 whose main defect is the presence of OH − groups substituting for the F − ions. Analyses of topaz samples from Santo Antonio do Jacinto, MG, Brazil, showed that their TL responses are strongly affected by the temperature of pre-irradiation thermal treatment. This behavior was correlated to the changes observed in the infrared and Raman spectra and a model could be proposed for the charge trapping and recombination process in colourless topaz.