We present an analysis of deep-UV Raman measurements of graphite, graphene and carbon nanotubes. For excitation energies above the strong optical absorption peak at the M point in the Brillouin zone (≈ 4.7 eV), we partially suppress double-resonant scattering processes and observe the two-phonon density of states of carbon nanomaterials. The measured peaks are assigned to contributions from LO, TO, and LA phonon branches, supported by calculations of the phonon dispersion. Moreover, we gain access to the infrared-active E1u mode in graphite. By lowering the excitation energy and thus allowing double-resonant scattering processes, we demonstrate the rise of the 2D mode in graphite with ultra-short phonon wave vectors.PACS numbers: 63.22. Rc, 61.46.Fg, 78.67.Ch Graphite, graphene and carbon nantoubes (CNT) have experienced increasing interest in fundamental research in the last decade. In this context, Raman spectroscopy has been established as a powerful experimental technique, since it provides access to both the electronic and vibrational properties of carbon materials 1,2 . Due to its high sensitivity, it is possible to probe properties like the crystallographic orientation of graphene 3,4 , the number of graphene layers 5 , doping 6 and strain 3,7,8 , as well as the diameter and chiral indices (n, m) of CNT 9 .In general, the Raman spectra of graphene, graphite or CNT in the two-phonon region are dominated by doubleresonant (DR) Raman modes [10][11][12] . Especially in singlelayer graphene, the prominent 2D mode outperforms the intensity of the first-order G mode by a factor of up to five.13 However, these strong DR Raman modes are very unique only for graphitic materials. In common semiconductors, away from optical resonances, the secondorder spectrum typically shows the two-phonon density of states 14 . A possible route to observe the two-phonon density of states also in graphene, graphite, and CNT is the suppression of DR Raman modes. Here, Raman spectroscopy with photon energies in the ultraviolet (UV) spectral range seems most promising, as the strong optical absorption around the M point in the Brillouin zone is then suppressed. Thus, all so-called 'inner' DR scattering processes are selectively inhibited.In this work, we investigate the Raman process in graphene, graphite, and CNT under UV excitation. For excitation energies well above the M -point transition energy of approximately 4.7 eV, we can selectively suppress the dominant Raman processes that are commonly identified with 'inner' DR Raman scattering. In these cases the two-phonon density of states (pDOS) is observed. Therefore, we gain access to phonon frequencies at high-symmetry points in the Brillouin zone, in particular the infrared-active E 1u mode in graphite, which are otherwise not accessible in either first-order or DR Raman scattering in the visible optical range. By lowering the excitation energy towards the M -point transition energy, we can initiate inner double-resonance processes and therefore demonstrate the onset of the 2D m...
The reaction of negatively charged SWCNTs with diazonium salts was analyzed in a combined experimental and computational DFT study.
We present measurements of the D Raman mode in graphene and carbon nanotubes at different laser excitation energies. The Raman mode around 1050 -1150 cm −1 originates from a doubleresonant scattering process of longitudinal acoustic (LA) phonons with defects. We investigate its dependence on laser excitation energy, on the number of graphene layers and on the carbon nanotube diameter. We assign this Raman mode to so-called 'inner' processes with resonant phonons mainly from the Γ−K high-symmetry direction. The asymmetry of the D mode is explained by additional contributions from phonons next to the Γ − K line. Our results demonstrate the importance of inner contributions in the double-resonance scattering process and add a fast method to investigate acoustic phonons in graphene and carbon nanotubes by optical spectroscopy.
We analyzed the vibrational and electronic properties of diamondoid oligomers via resonance Raman spectroscopy. The compounds consist of lower diamondoids such as adamantane or diamantane that are interconnected with double bonds. Therefore, all oligomers have ethylene-like centers strongly influencing the character of the optical transitions. The double bond localizes the HOMO (highest occupied moluecular orbital) in between the diamondoids accompanied by a significant decrease of optical transition energies. Comparing Raman spectra of the compounds to pristine diamondoids, we find several characteristic modes originating from the ethylene moieties. Supported by DFT (density functional theory) computations, we attribute these modes to highly localized vibrations that can partially be derived from the vibrational modes of parent ethylene. We further observe two new Raman modes in the compounds: a dimer breathing mode and a rotational mode of the entire ethylene moieties.
The electronic properties of sp/sp diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.5 eV-1 eV. The π → π transition is responsible for large absorption in both gas and crystalline phases. It further causes a large increase in the Raman intensity of the C=C stretch vibration when excited resonantly. By resonance Raman spectroscopy we were able to determine the C=C bond length of the trishomocubane dimer to exhibit 1.33 Å in the ground and 1.41 Å in the excited state.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.