Ab initiostudy of the processes of nitrogen functionalisation in graphene.

Nitrogen functionalisation of graphene is studied with the help ofab initioelectronic structure methods. Both static formation energies and energy barriers obtained from nudged elastic band calculations are considered. If carbon defects are present in the graphene structure, low energy barriers on the order of 0.5 eV were obtained to incorporate nitrogen atoms inside the sheet. For defect-free graphene, much larger barriers in the range of 3.70-4.38 eV were found, suggesting an external energy source is required to complete this type of incorporation.

Colossal C130 Fullertubes: Soluble [5,5] C130-D5h(1) Pristine Molecules with 70 Nanotube Carbons and Two 30-Atom Hemifullerene End-caps.

We report the seminal experimental isolation and DFT characterization of pristine [5,5] C130-D5h(1) fullertubes. This achievement represents the largest soluble carbon molecule obtained in its pristine form. The [5,5] C130 species is the highest aspect ratio fullertube purified to date and now surpasses the recent gigantic [5,5] C120-D5d(1). In contrast to C90, C100, and C120 fullertubes, the longer C130-D5h has more nanotubular carbons (70) than end-cap fullerenyl atoms (60). Starting from 39,393 possible C130 isolated pentagon rule (IPR) structures and after analyzing polarizability, retention time, and UV-vis spectra, these three layers of data remarkably predict a single candidate isomer and fullertube, [5,5] C130-D5h(1). This structural assignment is augmented by atomic resolution STEM data showing distinctive and tubular "pill-like" structures with diameters and aspect ratios consistent with [5,5] C130-D5h(1) fullertubes. The high selectivity of the aminopropanol reaction with spheroidal fullerenes permits facile separation and removal of fullertubes from soot extracts. Experimental analyses (HPLC retention time, UV-vis, and STEM) were synergistically used (with polarizability and DFT property calculations) to down select and confirm the C130 fullertube structure. Achieving the isolation of a new [5,5] C130-D5h fullertube opens the door to application development and fundamental studies of electron confinement, fluorescence, and metallic character for a fullertube series of molecules with systematic tubular elongation. This [5,5] fullertube family also invites comparative studies with single-walled carbon nanotubes (SWCNTs), nanohorns (SWCNHs), and fullerenes.

Visible Out-of-plane Polarized Luminescence and Electronic Resonance in Black Phosphorus.

Black phosphorus (BP) is unique among layered materials because of its homonuclear lattice and strong structural anisotropy. While recent investigations on few-layer BP have extensively explored the in-plane (a, c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE), and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. An unexpected b-polarized luminescence emission is detected in the visible, far above the fundamental gap. PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman with the enhancement of the Ag phonon modes scattering efficiency. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

ABINIT: Overview and focus on selected capabilities.

abinit is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which abinit relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The abinit DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library libpaw. abinit has strong links with many other software projects that are briefly mentioned.

Momentum-Resolved Dielectric Response of Free-Standing Mono-, Bi-, and Trilayer Black Phosphorus.

Black phosphorus (BP), a 2D semiconducting material of interest in electronics and photonics, exhibits physical properties characterized by strong anisotropy and band gap energy that scales with reducing layer number. However, the investigation of its intrinsic properties is challenging because thin-layer BP is photo-oxidized under ambient conditions and the energy of its electronic states shifts in different dielectric environments. We prepared free-standing samples of few-layer BP under glovebox conditions and probed the dielectric response in a vacuum using scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS). Thresholds of the excitation energy are measured at 1.9, 1.4, and 1.1 eV for the mono-, bi-, and trilayer BP, respectively, and these values are used to estimate the corresponding optical band gaps. A comparison of our results with electronic structure calculations indicates that the variation of the quasi-particle gap is larger than that of the exciton binding energy. The dispersion of the plasmons versus momentum for one- to three-layer BP and bulk BP highlights a deviation from parabolic to linear dispersion and strong anisotropic fingerprints.

Second-Order Raman Scattering in Exfoliated Black Phosphorus.

Second-order Raman scattering has been extensively studied in carbon-based nanomaterials, for example, nanotube and graphene, because it activates normally forbidden Raman modes that are sensitive to crystal disorder, such as defects, dopants, strain, and so forth. The sp2-hybridized carbon systems are, however, the exception among nanomaterials, where first-order Raman processes usually dominate. Here we report the identification of four second-order Raman modes, named D1, D1', D2 and D2', in exfoliated black phosphorus (P(black)), an elemental direct-gap semiconductor exhibiting strong mechanical and electronic anisotropies. Located in close proximity to the Ag1 and Ag2 modes, these new modes dominate at an excitation wavelength of 633 nm. Their evolutions as a function of sample thickness, excitation wavelength, and defect density indicate that they are defect-activated and involve high-momentum phonons in a doubly resonant Raman process. Ab initio simulations of a monolayer reveal that the D' and D modes occur through intravalley scatterings with split contributions in the armchair and zigzag directions, respectively. The high sensitivity of these D modes to disorder helps explaining several discrepancies found in the literature.

Graft-induced midgap states in functionalized carbon nanotubes.

Covalent addition of functional groups onto carbon nanotubes is known to generate lattice point defects that disrupt the electronic wave function, resulting namely in a reduction of their optical response and electrical conductance. Here, conductance measurements combined with numerical simulations are used to unambiguously identify the presence of graft-induced midgap states in the electronic structure of covalently functionalized semiconducting carbon nanotubes. The main experimental evidence is an increase of the conductance in the OFF-state after covalent addition of 4-bromophenyl grafts on many single- and double-walled individual nanotubes, the effect of which is fully suppressed after thermodesorption of the adducts. The graft-induced current leakage is thermally activated and can reach several orders of magnitude above its highly insulating pristine-state level. Ab initio simulations of various configurations of functionalized nanotubes corroborate the presence of these midgap states and show their localization around the addends. Moreover, the electronic density of these localized states exhibits an extended hydrogenoid profile along the nanotube axis, providing access for long-range coupling between the grafts. We argue that covalent nanotube chemistry is a powerful tool to prepare and control midgap electronic states on nanotubes for enabling further studies of the intriguing properties of interacting 1D localized states.

Direct observation of ultrafast long-range charge separation at polymer-fullerene heterojunctions.

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Bromophenyl functionalization of carbon nanotubes: an ab initio study.

We study the thermodynamics of bromophenyl functionalization of carbon nanotubes with respect to diameter and metallic/insulating character using density-functional theory (DFT). On the one hand, we show that the functionalization of metallic nanotubes is thermodynamically favoured over that of semiconducting ones, in agreement with what binding energy calculations previously suggested. On the other hand, we show that the activation energy for the grafting of a bromophenyl molecule onto a semiconducting zigzag nanotube ranges from 0.72 to 0.75 eV without any clear diameter dependence within numerical accuracy. This implies that this functionalization is not selective with respect to diameter at room temperature, which explains the contradictory experimental selectivities reported in the literature. This contrasts with what is suggested by the clear diameter dependence of the binding energy of a single bromophenyl molecule, which ranges from 1.52 eV for an (8, 0) zigzag nanotube to 0.83 eV for a (20, 0) zigzag nanotube. Also, attaching a single bromophenyl to a nanotube creates states in the gap close to the functionalization site. It therefore becomes energetically favourable for a second bromophenyl to attach close to the first one on semiconducting nanotubes. The para configuration is found to be favoured for resulting bromophenyl pairs and their binding energy is found to decrease with increasing diameter, ranging from 4.35 eV for a (7, 0) nanotube to 2.26 eV for a (29, 0) nanotube. An analytic form for this radius dependence is derived using a tight binding Hamiltonian and first order perturbation theory. The 1/R(2) dependence obtained (where R is the nanotube radius) is verified by our DFT results within numerical accuracy. Finally, bromophenyl pairs are shown to be favoured by only 50 meV with respect to separate moieties on (9, 0) metallic nanotubes, which suggests that pair formation is not significantly favoured on some metallic nanotubes. This result explains the observation of stable isolated moieties at room temperature in nanotube samples containing random nanotube chiralities.

Large electronic bandwidth in solution-processable pyrene crystals: the role of close-packed crystal structure.

We examine the interdependence of structural and electronic properties of two substituted pyrene crystals by means of combined spectroscopic probes and density-functional theory calculations. Substituted pyrenes are useful model systems to unravel the interplay of crystal structure and electronic properties in organic semiconductors. To study the effect of steric encumbrance on the crystalline arrangement of two 1,3,6,8-tetraalkynylpyrene derivatives, one features linear n-hexyl side groups while the other contains branched trimethylsilyl groups. Both derivatives form triclinic crystal structures when grown from solution, but the electronic dispersion behavior is significantly different due to differences in π-π overlap along the π-stacking axis. Both systems display dispersion of around 0.45 eV in the valence band, suggesting a high intrinsic hole mobility. However, the direction of the dispersion is different: it is primarily along the π-stacking axis in the trimethylsilyl-substituted derivative, but less aligned with this crystal axis in the hexyl-substituted molecule. This is a direct consequence of the differences in co-facial π electron overlap revealed by the crystallographic studies. We find that photophysical defects, ascribed to excimer-like states, point to the importance of localized trap states.

First principles elaboration of low band gap ladder-type polymers.

Ladder-type polymers, obtained by small modifications of the atomic structure of ladder-type polythiophene, are studied using density-functional theory calculations. Within the local-density and GW approximations, it is found that upon a simple substitution of the sulfur atoms by nitrogen and boron atoms, the band structure of the resulting polymer exhibits band overlap between the occupied and the unoccupied states. However, the three-parameter Becke hybrid functional predicts these polymers to be small band gap semiconductors. Finally, results of time-dependent density-functional theory are reported on increasing length oligomers, indicating that the polymers would have very low excitation energies.

Fullerene in a metal-organic matrix: design of the electronic structure.

We present a theoretical study of a new hybrid compound, where the C60 molecules are encapsulated in a recently discovered metal-organic framework (MOF). Being placed in a rigid skeleton, the fullerene molecules form a cubic crystal, while the intermolecular distance of the fullerenes is tuned by the choice of appropriate organic linkers of the MOF structure. The resulting C60 crystal shows a density of conduction states considerably higher than any of the fullerene crystals considered so far, which is a key factor influencing the transition temperature of the superconducting state. This constitutes a new approach of tuning the density of states of a fullerene crystal.

Theory of tunnel ionization in complex systems.

A quasianalytical theory of tunnel ionization is developed that is applicable to general complex systems, such as large molecules. Our analysis reveals strong deviations from conventional tunnel ionization theories, dependent upon the system's geometry, angular momentum, and polarizability. A comparison of our theory with recent C(60) ionization experiments yields reasonable agreement.

A first principles calculations and experimental study of the ground- and excited-state properties of ladder oligo(p-aniline)s.

The molecular structure of three ladder oligo(p-aniline)s, 5,11-diethyl-6,12-dimethylindolo[3,2-b]carbazole (DIMER 2P), 14-ethyl-5,8-dihydro-diindolo[3,2-b:2',3'-h]carbazole (TRIMER 2P), and 5,8,14-triethyl-diindolo[3,2-b:2',3'-h]carbazole (TRIMER 3P) were investigated by first principles calculations at the Hartree-Fock (HF6-31G*) and density functional theory (DFTB3LYP6-31G*) levels. It is found that the agreement between theoretical and x-ray geometrical parameters is good and rather similar for both theoretical methods. The nature and the energy of the first two singlet-singlet electronic transitions have been obtained by Zerner intermediate neglect of differential overlap/spectroscopy semiempirical calculations performed on the HF6-31G* and DFTB3LYP6-31G* optimized geometries, as well as time-dependent density functional theory (TDDFT) calculations performed on the DFTB3LYP6-31G* optimized structures. For all the compounds and for all the theoretical approaches, it is observed that the S(1)<--S(0) electronic transition (pipi*) is weakly allowed and polarized along the short axis (y) of the molecule. On the other hand, the S(2)<--S(0) electronic transition of each oligomer possesses a much larger oscillator strength and is polarized along the long (x) molecular axis. It is found that TDDFT calculations provide the best overall agreement between the energies and the corresponding optical transitions obtained from the absorption bands (0-0 peaks) measured in dichloromethane as well as providing a good evaluation of the bathochromic shifts caused by the increase in the conjugation length or by the presence of extra alkyl chains on the nitrogen atoms in TRIMER 3P compared to TRIMER 2P.

First-principles study of the rotational transitions of H2 physisorbed over benzene.

In the ongoing search for promising compounds for hydrogen storage, novel porous metal-organic frameworks (MOFs) have been discovered recently [M. Eddadoudi, J. Kim, N. L. Rosi, D. Vodak, J. Wachter, M. O'Keeffe, and O. M. Yaghi, Science 295, 469 (2002); N. L. Rosi, J. Eckert, M. Eddadoudi, D. Vodak, J. Kim, M. O'Keeffe, and O. M. Yaghi, Science 300, 1127 (2003)]. Binding sites in these MOFs were deduced from inelastic neutron scattering (INS) spectroscopy of the rotational transitions of the adsorbed molecular hydrogen. In light of this discovery, it is important to have a fundamental understanding of hydrogen adsorption at different sites in this class of MOF materials. As a first step, here we study the case of H(2) adsorbed on benzene as a model of the organic linkers in the microporous crystal. We access the density functional theory results by comparing with correlated ab initio methods, e.g., second-order Møller-Plesset and coupled cluster with noniterative triple excitations. Different approximations for the exchange-correlation potentials were accessed for a set of relevant properties (binding energy, energetically favored configuration, and distance between the adsorbents and adsorbates). In particular, theoretical rotational spectra of the adsorbed H(2) were obtained that could be compared to the experimental INS spectra.

A six-year experience with the omnicarbon valve in North American patients.

AIM: We studied the results of an all-carbon monoleaflet valve prosthesis (the Omnicarbon) in a North American population. METHODS: Patients were recalled to our valve clinic for complete evaluation, including echocardiography, laboratory tests, and physician examination. This experience includes 108 Omnicarbon valve implants. We report the results of single aortic (AVR) or mitral (MVR) valve replacement. RESULTS: Patients' ages ranged from 40 to 83 (mean: 63 +/- 9 years), and most were male (60%, 59/98). Preoperatively, 71% were NYHA Classes III/IV, while most patients are now Classes I/II (86%). AVR predominated (63%, 62/98), and many patients (44%, 43/98) underwent cardiac procedures either previously or concomitant with valve replacement. Hospital mortality was 6.1% (6/98). Predicted hospital mortality using the Parsonnet additive risk model averaged 12.1%. Currently, four patients cannot be located (96% accountability). Overall, hematology indicated low hemolysis-lactate dehydrogenase: 691 +/- 184 IU/L (112%+/- 30% upper normal), reticulocytes: 1.8%+/- 0.7%, and red blood cells (10(6)/mm(3)): 4.41 +/- 0.50 (males)/4.16 +/- 0.50 (females). International normalized ratio averaged 2.67 +/- 0.72. Doppler echocardiography values were acceptable and comparable to other mechanical valves. Five-year survival (hospital death included) is 86%+/- 4%. At 5 years, freedom from any thromboembolic event is 99%+/- 1%, and freedom from bleeding is 97%+/- 2%. Endocarditis and nonstructural dysfuction also occurred at low rates (99%+/- 1% freedom at five years), and no structural failure or hemolytic anemia was observed during the 343 patient-years (mean: 3.7 +/- 1.6 years). CONCLUSIONS: Good hematology and hemodynamics, along with remarkably low complication rates, demonstrate that the Omnicarbon valve meets contemporary performance expectations.