Showing papers by "Francesco Mauri published in 2008"

Structure, stability, edge states, and aromaticity of graphene ribbons.

Abstract: We determine the stability, the geometry, the electronic, and magnetic structure of hydrogen-terminated graphene-nanoribbon edges as a function of the hydrogen content of the environment by means of density functional theory. Antiferromagnetic zigzag ribbons are stable only at extremely low ultravacuum pressures. Under more standard conditions, the most stable structures are the mono- and dihydrogenated armchair edges and a zigzag edge reconstruction with one di- and two monohydrogenated sites. At high hydrogen concentration "bulk" graphene is not stable and spontaneously breaks to form ribbons, in analogy to the spontaneous breaking of graphene into small-width nanoribbons observed experimentally in solution. The stability and the existence of exotic edge electronic states and/or magnetism is rationalized in terms of simple concepts from organic chemistry (Clar's rule).

Ab Initio Study of the Hydroxylated Surface of Amorphous Silica: A Representative Model

Abstract: A new complete, representative model for the hydroxylated surface of amorphous silica is presented and characterized by means of periodic DFT calculations. This model accounts for the experimentally encountered ring size distribution, Si-O-Si and O-Si-O angles, silanols density, and distribution (isolated, associated, geminals). Properties such as NMR shifts, dehydrogenation energies, OH vibrational frequencies, and the interaction with water are investigated. The results are compared with former experimental and theoretical results. This new representative model for this complex surface would probably help the investigation of its reactivity toward amino acids or other organic molecules, opening new perspectives in the understanding of the chemistry of amorphous materials.

Impact of the electron-electron correlation on phonon dispersion: Failure of LDA and GGA DFT functionals in graphene and graphite

Abstract: We compute the electron-phonon coupling (EPC) of selected phonon modes in graphene and graphite using various ab initio methods. The inclusion of nonlocal exchange-correlation effects within the $GW$ approach strongly renormalizes the square EPC of the ${\text{A}}_{1}^{\ensuremath{'}}$ $\mathbf{K}$ mode by almost 80% with respect to density-functional theory in the LDA and GGA approximations. Within GW, the phonon slope of the ${\text{A}}_{1}^{\ensuremath{'}}$ $\mathbf{K}$ mode is almost two times larger than in GGA and LDA, in agreement with phonon dispersions from inelastic x-ray scattering and Raman spectroscopy. The hybrid B3LYP functional overestimates the EPC at $\mathbf{K}$ by about 30%. Within the Hartree-Fock approximation, the graphene structure displays an instability under a distortion following the ${\text{A}}_{1}^{\ensuremath{'}}$ phonon at $\mathbf{K}$.

Boroxol Rings in Liquid and Vitreous B2O3 from First Principles.

Abstract: We investigate the structural and vibrational properties of glassy ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$ using first-principles molecular dynamics simulations. In particular, we determine the boroxol rings fraction $f$ for which there is still no consensus in the literature. Two numerical models containing either a low or a high level of boroxol rings are tested against a gamut of experimental probes (static structure factor, Raman, $^{11}\mathrm{B}$ and $^{17}\mathrm{O}$ NMR data). We show that only the boroxol-rich model ($f=75%$) can reproduce the full set of observables. Total-energy calculations show that at the glass density, boroxol-rich structures are favored by about $6\text{ }\text{ }\mathrm{kcal}/(\mathrm{mol}\text{ }\text{ }\mathrm{\text{boroxol}})$. Finally, the liquid state is explored in the 2000--4000 K range and a reduction of $f$ to 10%--20% is obtained.

High-Tc superconductivity in superhard diamondlike BC5.

Abstract: Using density functional theory calculations we show that the recently synthesized superhard diamondlike ${\mathrm{BC}}_{5}$ is superconducting with a critical temperature of the same order as that of ${\mathrm{MgB}}_{2}$. The average electron-phonon coupling is $\ensuremath{\lambda}=0.89$, the phonon-frequency logarithmic average is $⟨\ensuremath{\omega}{⟩}_{\mathrm{log} }=67.4\text{ }\text{ }\mathrm{meV}$, and the isotope coefficients are $\ensuremath{\alpha}(\mathrm{C})=0.3$ and $\ensuremath{\alpha}(\mathrm{B})=0.2$. In ${\mathrm{BC}}_{5}$, superconductivity is mostly sustained by concerted vibrations of the B atom and its C neighbors.

Giant nonadiabatic effects in layer metals: raman spectra of intercalated graphite explained.

Abstract: The occurrence of nonadiabatic effects in the vibrational properties of metals has been predicted since the 1960s, but hardly confirmed experimentally. We report the first fully ab initio calculations of nonadiabatic frequencies of a number of conventional (hcp Ti and Mg) and layered metals (MgB2, CaC6, and other intercalated graphites). Nonadiabatic effects can be spectacularly large (up to 30% of the phonon frequencies) in both cases, but they can only be experimentally observed in the Raman spectra of layered compounds. In layered metals nonadiabatic effects are crucial to explaining the observed Raman shifts and linewidths. Moreover, we show that those quantities can be used to extract the electron momentum-relaxation time.

Theoretical infrared absorption coefficient of OH groups in minerals

Abstract: The integrated molar absorption coefficient of isolated and localized OH groups in selected minerals is theoretically investigated within the density functional theory framework. The overall decrease in absorption coefficient of stretching modes observed with increasing frequency is consistent with the experimental observations. It is related to a decrease in the magnitude of the hydrogen Born effective charge tensor projected along the OH bond as a function of increasing H-bonding. The scatter of theoretical data shows that the use of a general calibration of infrared absorbances in minerals cannot lead to accurate water contents. In contrast, the combination of theoretical modeling and experimental measurements should improve the determination of the hydrogen distribution among structurally distinct OH defects in nominally anhydrous minerals.

1H, 13C, and 15N solid-state NMR studies of imidazole- and morpholine-based model compounds possessing halogen and hydrogen bonding capabilities

Abstract: The halogen and hydrogen bonding interactions present in solid 1-(2,3,3-triiodoallyl)imidazole (1), morpholinium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)imidazole·morpholinium iodide (3), morpholine (4), imidazole (5), and 1-(3-iodopropargyl)imidazole (6) have been investigated by solid-state 1H, 13C, and 15N NMR spectroscopies. Comparison of the 15N CP MAS NMR spectrum of 3 with that of 2 indicates that protonated morpholine is present in solid 3, but this conclusion must be taken with caution as GIPAW calculations predict a 15N chemical shift for morpholine similar to that of the morpholinium cation. Conclusive evidence for the presence of a morpholinium cation in crystalline 3 was obtained by recording the static 15N NMR spectrum of this host−guest complex and comparing the morpholinium/morpholine part of the spectrum with the static spectra of 3 and 4 as obtained from ab initio calculations of NMR parameters based on the X-ray structures of these compounds. Concerning the imidazolyl group,...

17O solid-state NMR and first-principles calculations of sodium trimetaphosphate (Na3P3O9), tripolyphosphate (Na5P3O10), and pyrophosphate (Na4P2O7).

Abstract: The assignment of high-field (18.8 T) (17)O MAS and 3QMAS spectra has been completed by use of first-principles calculations for three crystalline sodium phosphates, Na 3P 3O 9, Na 5P 3O 10, and Na 4P 2O 7. In Na 3P 3O 9, the calculated parameters, quadrupolar constant ( C Q), quadrupolar asymmetry (eta Q), and the isotropic chemical shift (delta cs) correspond to those deduced experimentally, and the calculation is mandatory to achieve a complete assignment. For the sodium tripolyphosphate Na 5P 3O 10, the situation is more complex because of the free rotation of the end-chain phosphate groups. The assignment obtained with ab initio calculations can however be confirmed by the (17)O{ (31)P} MAS-J-HMQC spectrum. Na 4P 2O 7 (17)O MAS and 3QMAS spectra show a complex pattern in agreement with the computed NMR parameters, which indicate that all of the oxygens exhibit very similar values. These results are related to structural data to better understand the influence of the oxygen environment on the NMR parameters. The findings are used to interpret those results observed on a binary sodium phosphate glass.

Maximum T c at the verge of a simultaneous order-disorder and lattice-softening transition in superconducting CaC 6

Abstract: In order to account for the large drop of superconducting critical temperature, ${T}_{c}$, and for the dramatic increase in residual resistivity, ${\ensuremath{\varrho}}_{0}$, previously reported in ${\text{CaC}}_{6}$ at ${P}_{\text{cr}}\ensuremath{\approx}9\text{ }\text{GPa}$, we studied the room-temperature crystal structure of bulk ${\text{CaC}}_{6}$ samples as a function of pressure up to 13 GPa by means of synchrotron x-ray diffraction in diamond anvil cells. At ${P}_{\text{cr}}$, we found no change of the trigonal $R\overline{3}m$ space group symmetry, but a large increase in isothermal compressibility, $\ensuremath{\kappa}$, from $\ensuremath{-}0.0082\text{ }{\text{GPa}}^{\ensuremath{-}1}$ to $\ensuremath{-}0.0215\text{ }{\text{GPa}}^{\ensuremath{-}1}$, accompanied by a large broadening of Bragg peaks. With both effects being reversible, it follows that superconductivity in ${\text{CaC}}_{6}$ is maximized at the verge of a peculiar order-disorder phase transition concomitant to a large lattice softening. Space group symmetry considerations supported by ab initio calculations of the relaxed structure within the density functional theory lead us to conclude that the disordered phase is presumably characterized by a random off-centering of the Ca atoms in the $ab$ plane with respect to the C honeycomb layers.

Ga self‐interstitials in GaN investigated by ab‐initio calculations of the electronic g ‐tensor

Abstract: In GaN, an exceptionally important role is played by Ga-interstitials being mobile at room temperature. Despite rather large formation energy, they have been observed in irradiated wurtzite-GaN: at least two similar, but clearly distict ODEPR-signals L5 and L6/L6* have been identified (via exceptionally large, nearly isotropic hyperfine splittings of about 4 GHz) as interstitial in two diffent lattice configurations. However, judging from experimental data and total energy calculations alone, the exact microscopic configuration remains unclear. In this theoretical work, the situation is elucidated by ab-initio calculating the complete set of EPR parameters, hyperfine splittings as well as g -tensors, for the stable structural configurations of the Ga-interstitial using a gauge-including projector augmented plane wave (GI-PAW) approach. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The Edges of Hydrogen Terminated Graphene Ribbons Energetics, Bandstructure and Magnetization

Abstract: In this thesis we have examined the edges of hydrogen-terminated single-sheet graphene ribbons by means of ab initio density functional theory calculations. Edges in different crystal symmetry directions, with different planar reconstructions and different edge-hydrogen densities were considered. The studies concentrated on the analysis of the formation energy, the bandstructure and the magnetism of the ribbons. Following our primary goal, the identification of the energetically most favorable edge configuration under different chemical conditions, the formation energy was translated to a broad sector in thermodynamical phase space via the chemical potential of molecular hydrogen. These considerations reveal that at room temperature and under ambient hydrogen pressure the monohydrogenated armchair ribbon represents the most stable configuration. In the area of negative formation energy where spontaneous breaking is possible, the dihydrogenated armchair ribbon becomes most favorable. None of the armchair ribbons analyzed in this study showed magnetism whereas zigzag ribbons can be magnetic or not depending on the edge configuration. The region where magnetism occurs, however could be reduced to low pressure, high temperature conditions. The zero-point formation energies calculated here confirm our expectations based on the analysis of the Clar structures of the ribbons.

Pressure-induced order-disorder phase transition in superconducting CaC6

Abstract: By means of synchrotron X-ray diffraction, we studied the effect of high pressure, P, up to 13 GPa on the room temperature crystal structure of superconducting CaC6 In this P range, no change of the pristine space group symmetry, R ¯, is found However, at 9 GPa, ie close to the critical value at which a large Tc reduction was reported recently, we observed a compressibility jump concomitant to a large broadening of Bragg peaks The reversibility of both effects upon depressurization and symmetry arguments give evidence of an order-disorder phase transition of second order, presumably associated with the Ca sublattice, which provides a full account for the above Tc reduction

Thermal transport in isotopically disordered carbon nanotubes

Abstract: We present a study of the phononic thermal conductivity of isotopically disordered carbon nanotubes. In particular, the behavior of the thermal conductivity as a function of the system length is investigated, using Green's function techniques to compute the transmission across the system. The method is implemented using linear scaling algorithms, which allows us to reach systems of lengths up to $L=2.5\mu$m (with up to 400,000 atoms). As for 1D systems, it is observed that the conductivity diverges with the system size $L$. We also observe a dramatic decrease of the thermal conductance for systems of experimental sizes (roughly 80% at room temperature for $L = 2.5$ $\mu$m), when a large fraction of isotopic disorder is introduced. The results obtained with Green's function techniques are compared to results obtained with a Boltzmann description of thermal transport. There is a good agreement between both approaches for systems of experimental sizes, even in presence of Anderson localization. This is particularly interesting since the computation of the transmission using Boltzmann's equation is much less computationally expensive, so that larger systems may be studied with this method.