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Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride ; Wirtz, Ludger ; et al in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(8), 081413 The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with ... [more ▼] The anisotropy of the valence energy-loss function of hexagonal boron nitride (hBN) is shown to be largely enhanced by the highly inhomogeneous character of the excitonic states. The energy loss with momentum transfer parallel to the BN layers is dominated by strongly bound, quasi-two-dimensional excitons. In contrast, excitations with momentum transfer perpendicular to the layers are influenced by weakly bound three-dimensional excitons. This striking phenomenon is revealed by a combined study using high-precision nonresonant inelastic x-ray scattering measurements supported by ab initio calculations. The results are relevant in general to layered insulating systems. [less ▲] Detailed reference viewed: 128 (7 UL)Coupling of excitons and defect states in boron-nitride nanostructures ; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2011), 83(14), 144115 The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN ... [more ▼] The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the "free" exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra. [less ▲] Detailed reference viewed: 135 (4 UL)Phonon surface mapping of graphite: Disentangling quasi-degenerate phonon dispersions ; ; et al in Physical Review B (2009), 80(8), The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct ... [more ▼] The two-dimensional mapping of the phonon dispersions around the K point of graphite by inelastic x-ray scattering is provided. The present work resolves the longstanding issue related to the correct assignment of transverse and longitudinal phonon branches at K. We observe an almost degeneracy of the three TO-, LA-, and LO-derived phonon branches and a strong phonon trigonal warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which exhibits a trigonal warping effect opposite to that of the electronic band structure. We determined the electron-phonon coupling constant to be 166 (eV/A degrees)(2) in excellent agreement to GW calculations. These results are fundamental for understanding angle-resolved photoemission, double-resonance Raman and transport measurements of graphene-based systems. [less ▲] Detailed reference viewed: 95 (1 UL)Electron-electron correlation in graphite: A combined angle-resolved photoemission and first-principles study ; ; et al in Physical Review Letters (2008), 100(3), 037601 The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The ... [more ▼] The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory underestimates the slope of the bands and the trigonal warping effect. Including electron-electron correlation on the level of the GW approximation, however, yields remarkable improvement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semimetallic graphite and points toward a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials. [less ▲] Detailed reference viewed: 102 (1 UL)Tight-binding description of the quasiparticle dispersion of graphite and few-layer graphene ; ; Wirtz, Ludger et al in Physical Review B (2008), 78(20), A universal set of third-nearest-neighbor tight-binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of N stacked sp(2) graphene layers (N=1...infinity) with AB ... [more ▼] A universal set of third-nearest-neighbor tight-binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of N stacked sp(2) graphene layers (N=1...infinity) with AB stacking sequence. The present TB parameters are fit to ab initio calculations on the GW level and are universal, allowing to describe the whole pi "experimental" band structure with one set of parameters. This is important for describing both low-energy electronic transport and high-energy optical properties of graphene layers. The QP bands are strongly renormalized by electron-electron interactions, which results in a 20% increase in the nearest-neighbor in-plane and out-of-plane TB parameters when compared to band structure from density-functional theory. With the new set of TB parameters we determine the Fermi surface and evaluate exciton energies, charge carrier plasmon frequencies, and the conductivities which are relevant for recent angle-resolved photoemission, optical, electron energy loss, and transport measurements. A comparision of these quantitities to experiments yields an excellent agreement. Furthermore we discuss the transition from few-layer graphene to graphite and a semimetal to metal transition in a TB framework. [less ▲] Detailed reference viewed: 118 (0 UL)Low energy quasiparticle dispersion of graphite by angle-resolved photoemission spectroscopy ; ; et al in PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS (2007), 244(11), 4129-4133 The low energy electron dispersion in graphite is measured by angle-resolved photoemission spectroscopy. The measured photoemission intensity maxima are compared to a tight-binding calculation of the ... [more ▼] The low energy electron dispersion in graphite is measured by angle-resolved photoemission spectroscopy. The measured photoemission intensity maxima are compared to a tight-binding calculation of the electronic band structure. We observe a strong trigonal warping of the equi-energy contour which is well reproduced by the calculations. Furthermore we clearly show that the concept of Dirac Fermions breaks down for AB stacked graphite. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. [less ▲] Detailed reference viewed: 98 (1 UL)Vibrational properties of hexagonal boron nitride: Inelastic X-ray scattering and ab initio calculations ; ; et al in Physical Review Letters (2007), 98(9), 095503 The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations ... [more ▼] The phonon dispersion relations of bulk hexagonal boron nitride have been determined from inelastic x-ray scattering measurements and analyzed by ab initio calculations. Experimental data and calculations show an outstanding agreement and reconcile the controversies raised by recent experimental data obtained by electron-energy loss spectroscopy and second-order Raman scattering. [less ▲] Detailed reference viewed: 123 (2 UL)Excitons in boron nitride nanotubes: Dimensionality effects Wirtz, Ludger ; ; in Physical Review Letters (2006), 96(12), 126104 We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and ... [more ▼] We show that the optical absorption spectra of boron nitride (BN) nanotubes are dominated by strongly bound excitons. Our first-principles calculations indicate that the binding energy for the first and dominant excitonic peak depends sensitively on the dimensionality of the system, varying from 0.7 eV in bulk hexagonal BN via 2.1 eV in the single sheet of BN to more than 3 eV in the hypothetical (2,2) tube. The strongly localized nature of this exciton dictates the fast convergence of its binding energy with increasing tube diameter towards the sheet value. The absolute position of the first excitonic peak is almost independent of the tube radius and system dimensionality. This provides an explanation for the observed "optical gap" constancy for different tubes and bulk hexagonal BN. [less ▲] Detailed reference viewed: 114 (1 UL)Raman spectroscopy of single-wall boron nitride nanotubes ; ; et al in Nano Letters (2006), 6(8), 1812-1816 Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant ... [more ▼] Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A(1) tangential mode at 1370 cm(-1). This is 5 cm(-1) higher than the E-2g mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction. [less ▲] Detailed reference viewed: 112 (2 UL)Raman spectra of BN nanotubes: Ab initio and bond-polarizability model calculations Wirtz, Ludger ; ; et al in Physical Review. B, Condensed Matter and Materials Physics (2005), 71(24), We present ab initio calculations of the nonresonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are ... [more ▼] We present ab initio calculations of the nonresonant Raman spectra of zigzag and armchair BN nanotubes. In comparison, we implement a generalized bond-polarizability model where the parameters are extracted from first-principles calculations of the polarizability tensor of a BN sheet. For light polarization along the tube axis, the agreement between model and ab initio spectra is almost perfect. For perpendicular polarization, depolarization effects have to be included in the model in order to reproduce the ab initio Raman intensities. [less ▲] Detailed reference viewed: 110 (0 UL)Optical absorption of hexagonal boron nitride and BN nanotubes Wirtz, Ludger ; ; in AIP Conference Proceedings (2005), 786 present calculations for the optical absorption spectra of hexagonal boron nitride (hBN) and BN nanotubes, using many-body perturbation theory. Solution of the Bethe-Salpeter equation for hBN leads to ... [more ▼] present calculations for the optical absorption spectra of hexagonal boron nitride (hBN) and BN nanotubes, using many-body perturbation theory. Solution of the Bethe-Salpeter equation for hBN leads to optical absorption and loss spectra where the positions and shapes of the peaks are strongly dominated by excitonic effects. The binding energy of the first exciton is about 0.71 eV. Comparison of the calculations with recently measured optical absorption and EELS demonstrates that DFT underestimates the "true" band gap of BN by 2.25 eV. This band gap difference can be partially (hot not completely) reproduced by a calculation of the quasi-particle band-structure on the level of the GW-approximation. We show, how the lower dimensionality of BN nanotubes leads to a much stronger excitonic binding energy and at the same time to a larger quasi-particle gap. This leaves the position of the first absorption peak almost unchanged. However, the difference in the series of excitonic peaks allows the spectroscopic distinction between BN nanotubes and bulk BN. [less ▲] Detailed reference viewed: 130 (2 UL)Formation and electronic properties of BC3 single-wall nanotubes upon boron substitution of carbon nanotubes ; ; et al in Physical Review B (2004), 69(24), We report a detailed experimental and theoretical study on the electronic and optical properties of highly boron-substituted (up to 15 at.%) single-wall carbon nanotubes. Core-level electron energy-loss ... [more ▼] We report a detailed experimental and theoretical study on the electronic and optical properties of highly boron-substituted (up to 15 at.%) single-wall carbon nanotubes. Core-level electron energy-loss spectroscopy reveals that the boron incorporates into the lattice structure of the tubes, transferring similar to1/2 hole per boron atom into the carbon derived unoccupied density of states. The charge transfer and the calculated Fermi-energy shift in the doped nanotubes evidence that a simple rigid-band model can be ruled out and that additional effects such as charge localization and doping induced band-structure changes play an important role at this high doping levels. In optical absorption a new peak appears at 0.4 eV which is independent of the doping level. Compared to the results from a series of ab initio calculations our results support the selective doping of semiconducting nanotubes and the formation of BC3 nanotubes instead of a homogeneous random boron substitution. [less ▲] Detailed reference viewed: 44 (0 UL)The phonon dispersion of graphite revisited Wirtz, Ludger ; in Solid State Communications (2004), 131(3-4), 141-152 We review calculations and measurements of the phonon dispersion relation of graphite. First-principles calculations using density-functional theory are generally in good agreement with the experimental ... [more ▼] We review calculations and measurements of the phonon dispersion relation of graphite. First-principles calculations using density-functional theory are generally in good agreement with the experimental data since the long-range character of the dynamical matrix is properly taken into account. Calculations with a plane-wave basis demonstrate that for the in-plane optical modes, the generalized-gradient approximation (GGA) yields frequencies lower by 2% than the local-density approximation (LDA) and is thus in better agreement with experiment. The long-range character of the dynamical matrix limits the validity of force-constant approaches that take only interaction with few neighboring atoms into account. However, by fitting the force-constants to the ab initio dispersion relation, we show that the popular 4th-nearest-neighbor force-constant approach yields an excellent fit for the low frequency modes and a moderately good fit (with a maximum deviation of 6%) for the high-frequency modes. If, in addition, the non-diagonal force-constant for the second-nearest neighbor interaction is taken into account, all the qualitative features of the high-frequency dispersion can be reproduced and the maximum deviation reduces to 4%. We present the new parameters as a reliable basis for empirical model calculations of phonons in graphitic nanostructures, in particular carbon nanotubes. (C) 2004 Elsevier Ltd. All rights reserved. [less ▲] Detailed reference viewed: 84 (0 UL)Phonon and plasmon excitation in inelastic electron tunneling spectroscopy of graphite ; ; et al in Physical Review B (2004), 69(12), The inelastic electron tunneling spectrum (IETS) of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy (STS) at 6 K. The observed spectral features are in very good ... [more ▼] The inelastic electron tunneling spectrum (IETS) of highly oriented pyrolitic graphite has been measured with scanning tunneling spectroscopy (STS) at 6 K. The observed spectral features are in very good agreement with the vibrational density of states of graphite calculated from first principles. We discuss the enhancement of certain phonon modes by phonon-assisted tunneling in STS based on the restrictions imposed by the electronic structure of graphite. We also demonstrate the local excitation of surface plasmons in IETS, which are detected at an energy of 40 meV. [less ▲] Detailed reference viewed: 122 (1 UL)Optical absorption and electron energy loss spectra of carbon and boron nitride nanotubes: a first-principles approach ; Wirtz, Ludger ; et al in Applied Physics A : Materials Science & Processing (2004), 78(8), 1157-1167 We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density ... [more ▼] We present results for the optical absorption spectra of small-diameter single-walled carbon and boron nitride nanotubes obtained by ab initio calculations in the framework of time-dependent density-functional theory. We compare the results with those obtained for the corresponding layered structures, i.e. the graphene and hexagonal boron nitride sheets. In particular, we focus on the role of depolarization effects, anisotropies, and interactions in the excited states. We show that the random phase approximation reproduces well the main features of the spectra when crystal local field effects are correctly included, and discuss to what extent the calculations can be further simplified by extrapolating results obtained for the layered systems to results expected for the tubes. The present results are relevant for the interpretation of data obtained by recent experimental tools for nanotube characterization, such as optical and fluorescence spectroscopies, as well as polarized resonant Raman scattering spectroscopy. We also address electron energy loss spectra in the small-q momentum-transfer limit. In this case, the interlayer and intertube interactions play an enhanced role with respect to optical spectroscopy. [less ▲] Detailed reference viewed: 88 (0 UL)Raman spectroscopy of single-wall BN nanotubes ; Wirtz, Ludger ; et al in AIP Conference Proceedings (2003), (685), 384-388 We present results on the vibrational properties of BN-SWNTs together with a study of the synthesis material by transmission electron microscopy. Phonon modes have been investigated by Raman spectroscopy ... [more ▼] We present results on the vibrational properties of BN-SWNTs together with a study of the synthesis material by transmission electron microscopy. Phonon modes have been investigated by Raman spectroscopy with laser excitation wavelengths in the range from 363.8 to 676.44 nm. The assignment of the modes is guided by ab-initio calculations. [less ▲] Detailed reference viewed: 92 (0 UL)Vibrational properties of boron nitride nanotubes: Effects of finite length and bundling Wirtz, Ludger ; in IEEE Transactions on Nanotechnology (2003), 2 We present ab initio calculations of phonons in single-wall. boron nitride nanotubes. Raman and infrared active modes of isolated and infinitely long tubes are evaluated according to the non-symmorphic ... [more ▼] We present ab initio calculations of phonons in single-wall. boron nitride nanotubes. Raman and infrared active modes of isolated and infinitely long tubes are evaluated according to the non-symmorphic rod groups of BN nanotubes. For tubes of finite length, the selection rules are less restrictive and give rise to additional modes which may be observed in Raman and IR spectroscopy with an intensity depending on the tube length. Bundling of tubes is shown to have little effect on the phonon frequencies. However, arranging tubes in a large periodic array (larger than the wave-length of incoming light) gives rise to a strong frequency shift (LO-TO splitting) of some modes due to the establishing of a macroscopic electric field. Modes of A(1) symmetry experience a shift for laser light along the tube axis and E-1 modes are split for light incidence in the perpendicular direction. [less ▲] Detailed reference viewed: 99 (0 UL)Optical absorption in small BN and C nanotubes Wirtz, Ludger ; ; et al in AIP Conference Proceedings (2003), 685 We present a theoretical study of the optical absorption spectrum of small boron-nitride and carbon nanotubes using time-dependent density-functional theory and the random phase approximation. Both for C ... [more ▼] We present a theoretical study of the optical absorption spectrum of small boron-nitride and carbon nanotubes using time-dependent density-functional theory and the random phase approximation. Both for C and BN tubes, the absorption of light polarized perpendicular to the tube-axis is strongly suppressed due to local field effects. Since BN-tubes are wide band-gap insulators, they only absorb in the ultra-violet energy regime, independently of chirality and diameter. In comparison with the spectra of the single C and BN-sheets, the tubes display additional fine-structure which stems from the (quasi-) one-dimensionality of the tubes and sensitively depends on the chirality and tube diameter. This fine structure can provide additional information for the assignment of tube indices in high resolution optical absorption spectroscopy. [less ▲] Detailed reference viewed: 87 (0 UL)Ab initio calculations of the lattice dynamics of boron nitride nanotubes Wirtz, Ludger ; ; et al in Physical Review B (2003), 68(4), We present an extensive first-principles study of the phonons in boron nitride nanotubes using density functional perturbation theory in the local density approximation. Based on the nonsymmorphic rod ... [more ▼] We present an extensive first-principles study of the phonons in boron nitride nanotubes using density functional perturbation theory in the local density approximation. Based on the nonsymmorphic rod-group symmetry of the tubes, the Raman- and infrared-active modes at the Gamma point of the one-dimensional Brillouin zone are evaluated. For zigzag and chiral nanotubes, the set of infrared-active modes is a subset of the Raman- active modes. In particular, the radial breathing mode is not only Raman but also infrared active. However, for armchair tubes, the sets of infrared-and Raman- active modes are disjoint. This may serve to spectroscopically distinguish between macroscopic samples of zigzag-chiral and armchair nanotubes. We present the frequencies of the active modes of zigzag, chiral, and armchair tubes as a function of the tube diameter and compare the results with the frequencies obtained by the zone-folding method, i.e., the rolling of a single hexagonal BN sheet into a tube. Except for the high-frequency tangential modes, the zone-folding results are in very good agreement with the ab initio calculations. The radial breathing mode frequency can be derived by folding a sheet of finite width. Finally, we show that the effects of bundling on the phonon frequencies are small. This demonstrates that the obtained results for isolated BN tubes may serve as a basis for an accurate assignment of phonon modes in spectroscopic measurements. [less ▲] Detailed reference viewed: 112 (1 UL)Band structure of boron doped carbon nanotubes Wirtz, Ludger ; in AIP Conference Proceedings (2003), 685 We present ab initio and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a ... [more ▼] We present ab initio and self-consistent tight-binding calculations on the band structure of single wall semiconducting carbon nanotubes with high degrees (up to 25 %) of boron substitution. Besides a lowering of the Fermi energy into the valence band, a regular, periodic distribution of the p-dopants leads to the formation of a dispersive "acceptor"-like band in the band gap of the undoped tube. This comes from the superposition of acceptor levels at the boron atoms with the delocalized carbon pi-orbitals. Irregular (random) boron-doping leads to a high concentration of hybrids of acceptor and unoccupied carbon states above the Fermi edge. [less ▲] Detailed reference viewed: 101 (0 UL) |
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