We calculate the double resonant (DR) Raman spectrum of graphene, and determine the lines associated to both phonon-defect processes, and two-phonons ones. Phonon and electronic dispersions reproduce calculations based on density functional theory corrected with GW. Electron-light, -phonon, and -defect scattering matrix elements and the electronic linewidth are explicitly calculated. Defect-induced processes are simulated by considering different kind of idealized defects. For an excitation energy of $\epsilon_L=2.4$ eV, the agreement with measurements is very good and calculations reproduce: the relative intensities among phonon-defect or among two-phonon lines; the measured small widths of the D, $D'$, 2D and $2D'$ lines; the line shapes; the presence of small intensity lines in the 1800, 2000 cm$^{-1}$ range. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. According to the present findings, the intensity ratio between the $2D'$ and 2D lines can be used to determine experimentally the electronic linewidth. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could possibly be used to identify the kind of defects present in actual samples. Charged impurities outside the graphene plane provide an almost undetectable contribution to the Raman signal.

/pdf/theory-of-double-resonant-raman-spectra-in-graphene-55hadiikn1.pdf

Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands

Graphene-based materials (GBMs), with graphene, their most known member, at the head, constitute a large family of materials which has aroused the interest of scientists working in different research fields such as chemistry, physics, or materials science, to mention a few, arguably as no other material before. In this review, we offer a general overview on the most relevant synthetic approaches for the covalent and non-covalent functionalization and characterization of GBMs. Moreover, some representative examples of the incorporation into GBMs of electroactive units such as porphyrins, phthalocyanines, or ferrocene, among others, affording electron donor–acceptor (D–A) hybrids are presented. For the latter systems, the photophysical characterization of their ground- and excited-state features has also been included, paying particular attention to elucidate the fundamental dynamics of the energy transfer and charge separation processes of these hybrids. For some of the presented architectures, their application in solar energy conversion schemes and energy production has been also discussed.

/pdf/chemical-functionalization-and-characterization-of-graphene-24q864fkcw.pdf

Chemical functionalization and characterization of graphene-based materials

Graphene/elastomer nanocomposites

sp2 hybridized carbons constitute a broad class of solid phases composed primarily of elemental carbon and can be either synthetic or naturally occurring Some examples are graphite, chars, soot, graphene, carbon nanotubes, pyrolytic carbon, and diamond-like carbon They vary from highly ordered to completely disordered solids and detailed knowledge of their internal structure and composition is of utmost importance for the scientific and engineering communities working with these materials Multiwavelength Raman spectroscopy has proven to be a very powerful and non-destructive tool for the characterization of carbons containing both aromatic domains and defects and has been widely used since the 1980s Depending on the material studied, some specific spectroscopic parameters (eg, band position, full width at half maximum, relative intensity ratio between two bands) are used to characterize defects This paper is addressed first to (but not limited to) the newcomer in the field, who needs to be guided due to the vast literature on the subject, in order to understand the physics at play when dealing with Raman spectroscopy of graphene-based solids We also give historical aspects on the development of the Raman spectroscopy technique and on its application to sp2 hybridized carbons, which are generally not presented in the literature We review the way Raman spectroscopy is used for sp2 based carbon samples containing defects As graphene is the building block for all these materials, we try to bridge these two worlds by also reviewing the use of Raman spectroscopy in the characterization of graphene and nanographenes (eg, nanotubes, nanoribbons, nanocones, bombarded graphene) Counterintuitively, because of the Dirac cones in the electronic structure of graphene, Raman spectra are driven by electronic properties: Phonons and electrons being coupled by the double resonance mechanism This justifies the use of multiwavelength Raman spectroscopy to better characterize these materials We conclude with the possible influence of both phonon confinement and curvature of aromatic planes on the shape of Raman spectra, and discuss samples to be studied in the future with some complementary technique (eg, high resolution transmission electron microscopy) in order to disentangle the influence of structure and defects

https://hal.archives-ouvertes.fr/hal-01596681/document

A Guide to and Review of the Use of Multiwavelength Raman Spectroscopy for Characterizing Defective Aromatic Carbon Solids: from Graphene to Amorphous Carbons

Improving electrochemical activity of graphene is crucial for its various applications, which requires delicate control over its geometric and electronic structures. We demonstrate that precise control of the density of vacancy defects, introduced by Ar+ irradiation, can improve and finely tune the heterogeneous electron transfer (HET) rate of graphene. For reliable comparisons, we made patterns with different defect densities on a same single layer graphene sheet, which allows us to correlate defect density (via Raman spectroscopy) with HET rate (via scanning electrochemical microscopy) of graphene quantitatively, under exactly the same experimental conditions. By balancing the defect induced increase of density of states (DOS) and decrease of conductivity, the optimal HET rate is attained at a moderate defect density, which is in a critical state; that is, the whole graphene sheet becomes electronically activated and, meanwhile, maintains structural integrity. The improved electrochemical activity can b...

Quantitative correlation between defect density and heterogeneous electron transfer rate of single layer graphene

difference to the term CS,x ,r epresenting the Raman cross section ofI(x)/I(G) associated with the distortion of the crystal lattice after defect introduction per unit of damaged area, where x = D or D � . We observed that CS,D = 0

/pdf/raman-study-on-defective-graphene-effect-of-the-excitation-1fotxr9e4q.pdf

Rebecca Davey

Papers

Raman study on defective graphene: Effect of the excitation energy, type, and amount of defects