Excited states properties of organic molecules: from density functional theory to the GW and Bethe-Salpeter Green's function formalisms.

Many-body Green's function perturbation theories, such as the GW and Bethe-Salpeter formalisms, are starting to be routinely applied to study charged and neutral electronic excitations in molecular organic systems relevant to applications in photovoltaics, photochemistry or biology. In parallel, density functional theory and its time-dependent extensions significantly progressed along the line of range-separated hybrid functionals within the generalized Kohn-Sham formalism designed to provide correct excitation energies. We give an overview and compare these approaches with examples drawn from the study of gas phase organic systems such as fullerenes, porphyrins, bacteriochlorophylls or nucleobases molecules. The perspectives and challenges that many-body perturbation theory is facing, such as the role of self-consistency, the calculation of forces and potential energy surfaces in the excited states, or the development of embedding techniques specific to the GW and Bethe-Salpeter equation formalisms, are outlined.

Many-body Green's function GW and Bethe-Salpeter study of the optical excitations in a paradigmatic model dipeptide.

We study within the many-body Green's function GW and Bethe-Salpeter formalisms the excitation energies of a paradigmatic model dipeptide, focusing on the four lowest-lying local and charge-transfer excitations. Our GW calculations are performed at the self-consistent level, updating first the quasiparticle energies, and further the single-particle wavefunctions within the static Coulomb-hole plus screened-exchange approximation to the GW self-energy operator. Important level crossings, as compared to the starting Kohn-Sham LDA spectrum, are identified. Our final Bethe-Salpeter singlet excitation energies are found to agree, within 0.07 eV, with CASPT2 reference data, except for one charge-transfer state where the discrepancy can be as large as 0.5 eV. Our results agree best with LC-BLYP and CAM-B3LYP calculations with enhanced long-range exchange, with a 0.1 eV mean absolute error. This has been achieved employing a parameter-free formalism applicable to metallic or insulating extended or finite systems.

Short-range to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: a Bethe-Salpeter study.

We study, using the Bethe-Salpeter formalism, the singlet excitation energies of the zincbacteriochlorin-bacteriochlorin dyad, a paradigmatic photosynthetic complex. In great contrast with standard time-dependent density functional theory calculations with (semi)local kernels, charge transfer excitations are correctly located above the intramolecular Q-band transitions found to be in excellent agreement with experiment. Further, the asymptotic Coulomb behavior towards the true quasiparticle gap for charge transfer excitations at long distance is correctly reproduced, showing that the present scheme allows us to study with the same accuracy intramolecular and charge transfer excitations at various spatial ranges and screening environments without any adjustable parameter.

Molecular fingerprints in the electronic properties of crystalline organic semiconductors: from experiment to theory.

By comparing photoemission spectroscopy with a nonperturbative dynamical mean field theory extension to many-body ab initio calculations, we show in the prominent case of pentacene crystals that an excellent agreement with experiment for the bandwidth, dispersion, and lifetime of the hole carrier bands can be achieved in organic semiconductors, provided that one properly accounts for the coupling to molecular vibrational modes and the presence of disorder. Our findings rationalize the growing experimental evidence that even the best band structure theories based on a many-body treatment of electronic interactions cannot reproduce the experimental photoemission data in this important class of materials.

Chemically induced mobility gaps in graphene nanoribbons: a route for upscaling device performances.

We report a first-principles based study of mesoscopic quantum transport in chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence of quasi-bound states related to boron impurities results in mobility gaps as large as 1 eV, driven by strong electron-hole asymmetrical backscattering phenomena. This phenomenon opens new ways to overcome current limitations of graphene-based devices through the fabrication of chemically doped graphene nanoribbons with sizes within the reach of conventional lithography.

Anomalous doping effects on charge transport in graphene nanoribbons.

We present first-principles calculations of quantum transport in chemically doped graphene nanoribbons with a width of up to 4 nm. The presence of boron and nitrogen impurities is shown to yield resonant backscattering, whose features are strongly dependent on the symmetry and the width of the ribbon, as well as the position of the dopants. Full suppression of backscattering is obtained on the pi-pi* plateau when the impurity preserves the mirror symmetry of armchair ribbons. Further, an unusual acceptor-donor transition is observed in zigzag ribbons. These unconventional doping effects could be used to design novel types of switching devices.

Effect of the chemical functionalization on charge transport in carbon nanotubes at the mesoscopic scale.

We present first-principles calculations of quantum transport in chemically functionalized metallic carbon nanotubes with lengths reaching the micrometer scale and random distributions of functional groups. Two typical cases are investigated, namely, a sp2-type bonding between carbene groups (CH2) and the nanotube sidewalls and a sp3-type bonding of nanotubes with paired phenyl groups. For similar molecular coverage density, charge transport is found to range from a quasi-ballistic-like to a strongly diffusive regime, with corresponding mean free paths changing by orders of magnitude depending on the nature of the chemical bonding.

Charge transport in chemically doped 2D graphene.

We report on a numerical study of electronic transport in chemically doped 2D graphene materials. By using ab initio calculations, a self-consistent scattering potential is derived for boron and nitrogen substitutions, and a fully quantum-mechanical Kubo-Greenwood approach is used to evaluate the resulting charge mobilities and conductivities of systems with impurity concentration ranging within [0.5, 4.0]%. Even for a doping concentration as large as 4.0%, the conduction is marginally affected by quantum interference effects, preserving therefore remarkable transport properties, even down to the zero temperature limit. As a result of the chemical doping, electron-hole mobilities and conductivities are shown to become asymmetric with respect to the Dirac point.

Thermal stability of graphene and nanotube covalent functionalization.

We study kinetic factors governing the diffusion and desorption of covalently grafted phenyl and dichlorocarbene radicals on graphene and carbon nanotubes. Our ab initio calculations of reaction rates show that isolated phenyls can easily desorb and diffuse at room temperature. On the contrary, paired phenyls are expected to remain grafted to the surface up to a few hundred degrees Celsius. In the case of dichlorocarbene, no clustering is observed; at room temperature, the isolated radicals remain covalently attached to small-diameter nanotubes but desorb easily from graphene. Our results on the thermal behavior of side moieties on graphitic surfaces could be used to optimize the tradeoff between reactivity and conductance of nanotubes in the process of covalent functionalization.

Preserved conductance in covalently functionalized silicon nanowires.

We study by means of ab initio simulations the Landauer conductance of covalently functionalized silicon nanowires. We show that in the case of alkyl side chains, the most common linkers, silicon nanowires remain quasiballistic over a large energy range. More reactive side molecules, such as alkenyl or phenyl conjugated radicals, amino and alkoxide groups, are less favorable as they induce resonant backscattering in the valence bands mainly. Such results provide strong support for the use of selectively functionalized nanowires in (opto)electronic devices and molecular sensors.

Conductance, surface traps, and passivation in doped silicon nanowires.

We perform ab initio calculations within the Landauer formalism to study the influence of doping on the conductance of surface-passivated silicon nanowires. It is shown that impurities located in the core of the wire induce a strong resonant backscattering at the impurity bound state energies. Surface dangling bond defects have hardly any direct effect on conductance, but they strongly trap both p- and n-type impurities, as evidenced in the case of H-passivated wires and Si/SiO2 interfaces. Upon surface trapping, impurities become transparent to transport, as they are electrically inactive and do not induce any resonant backscattering.

Superconductivity in doped cubic silicon.

Although the local resistivity of semiconducting silicon in its standard crystalline form can be changed by many orders of magnitude by doping with elements, superconductivity has so far never been achieved. Hybrid devices combining silicon's semiconducting properties and superconductivity have therefore remained largely underdeveloped. Here we report that superconductivity can be induced when boron is locally introduced into silicon at concentrations above its equilibrium solubility. For sufficiently high boron doping (typically 100 p.p.m.) silicon becomes metallic. We find that at a higher boron concentration of several per cent, achieved by gas immersion laser doping, silicon becomes superconducting. Electrical resistivity and magnetic susceptibility measurements show that boron-doped silicon (Si:B) made in this way is a superconductor below a transition temperature T(c) approximately 0.35 K, with a critical field of about 0.4 T. Ab initio calculations, corroborated by Raman measurements, strongly suggest that doping is substitutional. The calculated electron-phonon coupling strength is found to be consistent with a conventional phonon-mediated coupling mechanism. Our findings will facilitate the fabrication of new silicon-based superconducting nanostructures and mesoscopic devices with high-quality interfaces.

Surface segregation and backscattering in doped silicon nanowires.

By means of ab initio simulations, we investigate the structural, electronic, and transport properties of boron and phosphorus doped silicon nanowires. We find that impurities always segregate at the surface of unpassivated wires, reducing dramatically the conductance of the surface states. Upon passivation, we show that for wires as large as a few nanometers in diameter, a large proportion of dopants will be trapped and electrically neutralized at surface dangling bond defects, significantly reducing the density of carriers. Important differences between p- and n-type doping are observed. Our results rationalize several experimental observations.

Room temperature peierls distortion in small diameter nanotubes.

By means of ab initio simulations, we investigate the phonon band structure and electron-phonon coupling in small 4-A diameter nanotubes. We show that both the C(5,0) and C(3,3) tubes undergo above room temperature a Peierls transition mediated by an acoustical long wavelength and an optical q=2k(F) phonon, respectively. In the armchair geometry, we verify that the electron-phonon coupling parameter lambda originates mainly from phonons at q=2k(F) and is strongly enhanced when the diameter decreases. These results question the origin of superconductivity in small diameter nanotubes.

Role of the dopant in the superconductivity of diamond.

We present an ab initio study of the recently discovered superconductivity of boron doped diamond within the framework of a phonon-mediated pairing mechanism. The role of the dopant, in substitutional position, is unconventional in that half of the coupling parameter lambda originates in strongly localized defect-related vibrational modes, yielding a very peaked Eliashberg alpha2F(omega) function. The electron-phonon coupling potential is found to be extremely large, and T(C) is limited by the low value of the density of states at the Fermi level. The effect of boron isotope substitution is explored.

Exceptional ideal strength of carbon clathrates.

We study by means of ab initio calculations the ideal tensile and shear strengths of the C-46 clathrate phase. While its bulk modulus and elastic constants are smaller than in diamond, its strength is found to be in all directions larger than the critical stresses associated with the diamond [111] planes of easy slip. This can be related to the frustration by the clathrate cage structure of the diamond to graphite instability under nonhydrostatic stress conditions [corrected] The criteria for designing strong materials are discussed.

Superconductivity in doped sp3 semiconductors: the case of the clathrates.

We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba(8)@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I(8)@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp(3) silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C60.

Tailoring band gap and hardness by intercalation: an ab initio Study of I(8)@Si-46 and related doped clathrates.

We present an ab initio study of the structural and electronic properties of the recently synthesized I(8)@Si-46 clathrate which is shown to be a degenerate p-type doped system. The intercalation significantly opens the band gap to a 1.75 eV value within the density functional theory. We study further the intercalation by other neighboring elements. A quasiparticle study reveals that such systems can display a band gap in the "green-light" energy range. Finally, we show that the bulk modulus can be increased to values equivalent to the one of the diamond phase.

Quasiparticle effects on tunneling currents: a study of C2H4 adsorbed on the Si(001)- (2 x 1) surface.

We present a first-principles calculation of the quasiparticle electronic structure of ethylene adsorbed on the dimer reconstructed Si(001)-(2x1) surface. Within the GW approximation, the self-energy corrections for the adsorbate states are found to be about 1.5 eV larger than those for the states derived from bulk silicon. The calculated quasiparticle band structure is in excellent agreement with photoemission spectra. Finally, the effects of the quasiparticle corrections on the scanning tunneling microscope images of the adsorbed molecules are shown to be important as the lowering of the C2H4 energy levels within GW strongly reduces their tunneling probability.

Structure and energy of the 90 degrees partial dislocation in diamond: a combined Ab initio and elasticity theory analysis.

The core structure and stability of the 90 degrees partial dislocation in diamond is studied within isotropic elasticity theory and ab initio total energy calculations. The double-period reconstruction is found to be more stable than the single-period reconstruction for a broad range of stress states. The analysis of the ab initio results shows further that elasticity theory is valid for dislocation spacings as small as 10-20 A, thus allowing ab initio calculations to provide reliable parameters for continuum theory analysis.