Some optical properties of graphite from IR to millimetric wavelengths (original) (raw)
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A polycrystalline graphite model for the 2175 Å interstellar extinction band
Monthly Notices of the Royal Astronomical Society, 2009
A random, hydrogen-free, assembly of microscopic sp 2 carbon chips, forming a macroscopically homogeneous and isotropic solid, is proposed as a model carrier for the ultraviolet (UV) interstellar extinction band. The validity of this model is based on the calculation of the Bruggeman average dielectric function of a mixture of the known parallel and perpendicular dielectric functions of graphite. The π absorption feature of Rayleigh-sized spheres of this mixture falls near 4.6 μm −1 (2175 Å), but its width is 1.5 μm −1 , somewhat larger than the astronomically observed average, 1 μm −1. This is confirmed by measurements of the reflectance of an industrial material, polycrystalline graphite. A better fit to the interstellar feature position and width is obtained with a hypothetical material, having the same dielectric functions as natural graphite, except for less extended wings of the π resonance. Physically, this could result from changes in the electronic band structure due to previous thermal histories. In this model, the Frölich feature central wavelength depends only on the π resonance frequency, while its width depends only on the damping constant of the same resonance. This explains the range of observed feature widths at constant feature wavelength.
Band structure and optical properties of graphite
Solid State Communications, 1996
We report the first ever calculation of the frequency dependent anisotropic dielectric function of graphite. We have used the LMTO-ASA method including the combined correction terms. On comparison with the experimental data, we find good agreement as far as the anisotropy is concerned. The individual Ed and e&w) are also in reasonable agreement with the data.
Ab initio study of the optical absorption and wave-vector-dependent dielectric response of graphite
Physical Review B, 2004
We performed ab initio calculations of the optical absorption spectrum and the wave-vector-dependent dielectric and energy-loss functions of graphite in the framework of the random-phase approximation. In the absorption spectrum, the most prominent peaks were analyzed in terms of interband transitions from specific regions of the Brillouin zone. The inclusion of the crystal local-field effects (LFE) in the response had an important influence on the absorption spectrum for light polarization parallel to the c axis. The calculated electron energy-loss spectra, even without LFE, were in very good agreement with existing momentumdependent energy-loss experiments concerning the peak positions of the two valence-electron plasmons. Important aspects of the line shape and anisotropy of the energy-loss function at large momentum transfer q were also well described: the splitting of the total ͑ + ͒ plasmon and the appearance of peaks originating from interband transitions. Finally, the role of the interlayer interaction was studied, in particular with regard to its effect on the absorption spectrum for light polarization parallel to c, and to the position of the higher-frequency + plasmon. PACS number s): 81.05.Uw, 71.45.Ϫd, 78.67.Ϫn, 79.20.Uv FIG. 1. (a) Graphite; ABA stacking sequence of graphene sheets. (b) Two-dimensional section of the Brillouin zone at the zone center.
On the stellar sources of presolar graphite
2006
Primitive meteorites contain graphite spherules whose anomalous isotopic compositions indicate a stellar origin. The isolation of presolar graphite grains is difficult and they have been less well studied than presolar silicon carbide grains. While previous isotopic measurements have been made on graphite from the Murchison meteorite and mostly on low-density grains, a new separation of graphite from the Orgueil meteorite and the application of a new ion microprobe, the NanoSIMS, have provided a wealth of C, N, O, Al, and Si isotopic data on grains with a range of densities. These data confirm that low-density grains come from supernovae. These grains are characterized by 15 N, 18 O, 28 Si and 49 Ti excesses, high inferred 26 Al/ 27 Al ratios, as well as evidence for the initial presence of the short-lived radionuclides 22 Na, 41 Ca, and 44 Ti. All these signatures point to an origin in Type II supernovae. The Ne-E(L) component, almost pure 22 Ne, is characteristic of presolar graphite and led to its discovery. In SN grains it is due to the * Speaker
Optical Properties of Graphene from the THz to the Visible Spectral Region
2012
Recent terahertz technology achievements, such as THz time-domain spectroscopy and high power THz generation, have opened the door to many exciting applications in biology science, communication and sensing technology, etc [22]. Graphene is considered a promising candidate for THz detection and generation devices for many reasons. For example, the frequency of graphene plasmon and the bandgap of graphene nanoribbon are in the range of terahertz [23-24]. People have already proposed many exciting applications utilizing THz properties of graphene [4, 25-26]. In this chapter, I investigate the basic optical properties of graphene in the THz range.
Applied Physics A, 2017
Absorption coefficient of graphite bulk pressed from 1-5 micrometers size crystalline grains was measured in UV-Vis-NIR range with three different methods: i) determination of pulsed laser ablation rate as the function of laser fluence for different wavelengths ˙(248nm, 337 nm, 532 nm and 1064 nm, respectively); ii) production of aerosol particles by UV laser ablation of the bulk graphite in inert atmosphere and determination of the mass-specific absorption coefficient with a four-wavelength (266 nm, 355 nm, 532 nm and 1064 nm, respectively) photoacoustic spectrometer and iii) spectroscopic ellipsometry in 250-1000 nm range. Taking into account the wide range of the absorption coefficients available in for the different carbon structures, an overall relatively good agreement was observed for the three methods. The ellipsometric results fit well with the ablation rate measurement, the data obtained with photoacoustic method are also similar in the UV and NIR region, however the values were somewhat higher in visible and near-UV range. Taking into account the limitations of the methods, they can be promising candidates for the determination of absorption coefficient when the samples are strongly scattering and there is no possibility to perform transitivity measurements.
2015
In this research we investigate how the conductivity of graphene changes in response to mid-infrared photoexcitation. Our p-type sample was formed through chemical vapor deposition. Pump/probe methodology produced the time-resolved Terahertz transmission, from which the photoconductivity was calculated. We probed the sample with energies above and below the Fermi energy, which was determined by Fourier transform infrared spectroscopy. Our results support a model in which heating of the electron gas, leading to high carrier scatt ering rates, is responsible for a decrease in conductivity. We observe this negative photoconductivity at all pump energies, allowing us to rule out the possibility of population inversion to explain the results.
1982
The carbon E-edge measurement by high-resolution electron-energy-loss spectroscopy is reported and compared with secondary-electron-emission spectra. A very good correlation has been found between the two techniques, which gives a complementary view of the final density of states. The comparison of the results allows identification of the onset of the extended x-ray absorption fine-structure regime. From the fine structure above the E edge we obtain radial distribution functions which are sensitive to the momentum-transfer direction. This is explained
2001
Certain dust particles in space are expected to appear as clusters of individual grains. The morphology of these clusters could be fractal or compact. In this paper we study the light scattering by compact and fractal polycrystalline graphite clusters consisting of touching identical spheres. We compare three general methods for computing the extinction of the clusters in the wavelength range 0.1 - 100 micron, namely, a rigorous solution (Gerardy & Ausloos 1982) and two different discrete-dipole approximation methods -- MarCODES (Markel 1998) and DDSCAT (Draine & Flatau 1994). We consider clusters of N = 4, 7, 8, 27,32, 49, 108 and 343 particles of radii either 10 nm or 50 nm, arranged in three different geometries: open fractal (dimension D = 1.77), simple cubic and face-centred cubic. The rigorous solution shows that the extinction of the fractal clusters, with N 5 micron, the rigorous solution indicates that the extinction from fractal and compact clusters are of the same order o...
Graphite, Tubular PAHs, and the Diffuse Interstellar Bands
The Astrophysical Journal, 2006
Planar polycyclic aromatic hydrocarbons (PAHs) are candidates for the diffuse interstellar band (DIB) carriers. Nanometer-size Fe metallic particles, expected to be present during astrophysical graphite growth, are catalytic for formation of a related species-large tubular PAH molecules. We propose that tubular PAH molecules are a component of the interstellar medium. Electronic structure calculations, on a specific family of tubular PAH molecules derived from elongated C 60 , reveal intense electronic transitions in the visible and near-IR, which vary systematically with length. We analyze these molecules as DIB carriers within the known constraints.