Anisotropic excitonic effects in the energy loss function of hexagonal boron nitride

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 ▲]

Electron-electron correlation in graphite: A combined angle-resolved photoemission and first-principles study

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 ▲]

Low energy quasiparticle dispersion of graphite by angle-resolved photoemission spectroscopy

in Physica Status Solidi B. Basic Research (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 ▲]

Raman spectroscopy of single-wall boron nitride nanotubes

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 ▲]

Optical absorption of hexagonal boron nitride and BN nanotubes

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 ▲]

The phonon dispersion of graphite revisited

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 ▲]

Formation and electronic properties of BC3 single-wall nanotubes upon boron substitution of carbon nanotubes

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 ▲]

Band structure of boron doped carbon nanotubes

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 ▲]

Ab initio calculations of the lattice dynamics of boron nitride nanotubes

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 ▲]

Optical absorption in small BN and C nanotubes

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 ▲]