Papers by Davide Ceresoli
Physical Review B, 2016
We investigated a suspended bilayer graphene where the bottom (top) layer is doped by boron (nitr... more We investigated a suspended bilayer graphene where the bottom (top) layer is doped by boron (nitrogen) substitutional atoms by using Density Functional Theory (DFT) calculations. We found that at high dopant concentration (one B-N pair every 32 C atoms) the electronic structure of the bilayer does not depend on the B-N distance but on the relative occupation of the bilayer graphene sub-lattices by B and N. We found that a large built in electric field is established between layers, giving rise to an energy gap. We further investigated the transport properties and found that intralayer electron current is weakly influenced by the presence of these dopants while the inter-layer current is significantly enhanced for biases allowing the energy alignment of N and B states. This effect leads to current rectification in asymmetric junctions.
NaCl (and other alkali halide) crystal surfaces have the peculiar property of repelling their own... more NaCl (and other alkali halide) crystal surfaces have the peculiar property of repelling their own melt. As a result they let themselves be wetted only partially by their own liquid at the melting point TM. We recently investigated the physical reasons for this unusual behavior. We found them through theory and molecular dynamics simulation to stem from the conspiracy of three factors. First, the solid NaCl(100) surface is exceptionally anharmonic,but also exceptionally stable. It can in fact survive even well above the melting point, for unlike most other surfaces it does not spontaneously melt. Second, the solid-liquid interface is very costly, due to a 27% density difference between solid and liquid. Third, the surface tension of liquid NaCl is relatively high. This last feature is due to an unexpected entropy deficit, that can in turn be traced to incipient molecular charge order in the outermost regions of the molten salt surface[1,2].[1] T. Zykova-Timan, D. Ceresoli, U. Tartaglino, E. Tosatti, Phys. Rev. Lett. 94, 176105 (2005) [2] T. Zykova-Timan, D. Ceresoli, U. Tartaglino, E. Tosatti, J. Chem. Phys. 123, 164701 (2005)
Aps Meeting Abstracts, Mar 1, 2009
Conventional density-functional approaches often fail in offering an accurate description of the ... more Conventional density-functional approaches often fail in offering an accurate description of the spin-resolved energetics in transition metal complexes, due to spurious self-interaction errors (SIE). Previous studies have shown that a self-consistent DFT + U approach [1] can accurately correct SIE in TM complexes, providing excellent agreement with high-level quantum chemistry calculations. In this work we report a systematic evaluation of DFT + U in a series of small manganese oxide clusters (MnOx, x=1-4) and their anions, focusing on structural, electronic and magnetic properties. It is found that DFT + U succeeds in systems where the valence electrons are essentially localized on the TM ion, but progressively worsens when the coordination number increases and more covalency comes into play between the TM center and neighboring atoms, pointing to the importance of choosing the appropriate correlated manifold when Hubbard corrections are added. [1] H. J. Kulik, M. Cococcioni, D. A. Scherlis, and N. Marzari, Phys. Rev. Lett., 2006, 97, 103001 .
Aps Meeting Abstracts, Mar 1, 2010
We present first principles, density functional theory calculations of the EPR spectrum of mononu... more We present first principles, density functional theory calculations of the EPR spectrum of mononuclear and binuclear organic transition metal complexes, that constitute building block of more complicated catalysts for the water splitting reaction. We apply here the modern theory of orbital magnetization and we obtain the EPR g-tensor by computing the derivative of the orbital magnetization with respect to the electronic spin flip. This method allowed us to incorporate self-interaction corrections in the Hamiltonian, at the level of DFT+U. We found that the DFT+U method improves the agreement with respect to experiment, of the EPR g-tensor and hyperfine couplings parameters for high spin complexes. We also discuss the success and failures of DFT+U as an energy functional, and the importance of benchmarking improved- or post-DFT methods (such as hybrid functionals) against the EPR spectra of transition metals complexes.
Physical Review B Condensed Matter and Materials Physics, Apr 1, 2007
We describe the effective Lorentz forces acting on the ions of a generic insulating system in a m... more We describe the effective Lorentz forces acting on the ions of a generic insulating system in a magnetic field, in the context of Born-Oppenheimer ab initio molecular dynamics. The force on each ion includes an important contribution of electronic origin, which depends explicitly on the velocity of all other ions. It is formulated in terms of a Berry curvature, in a form directly suitable for future first principles classical dynamics simulations based, e.g., on density functional methods. As a preliminary analytical demonstration we present the dynamics of an H2 molecule in a field of intermediate strength, approximately describing the electrons through Slater’s variational wave function.
The high temperature surface properties of alkali halide crystals are very unusual. Through molec... more The high temperature surface properties of alkali halide crystals are very unusual. Through molecular dynamics simulations based on Tosi-Fumi potentials, we predict that crystalline NaCl(100) should remain stable without any precursor signals of melting up to and even above the bulk melting point $T_m$. In a metastable state, it should even be possible to overheat NaCl (100) by at least 50 K. The reasons leading to this lack of surface self-wetting are investigated. We will briefly discuss the results of calculations of the solid-vapor and liquid-vapor interface free energies, showing that the former is unusually low and the latter unusually high, and explaining why. Due to that the mutual interaction among solid-liquid and liquid-vapor interfaces, otherwise unknown, must be strongly attractive at short distance, leading to the collapse of any liquid film attempting to nucleate at the solid surface. This scenario naturally explains the large incomplete wetting angle of a drop of melt on NaCl(100).
Angewandte Chemie, 2016
Three NIR-emitting neutral Ir(III) complexes [Ir(iqbt)2 (dpm)] (1), [Ir(iqbt)2 (tta)] (2), and [I... more Three NIR-emitting neutral Ir(III) complexes [Ir(iqbt)2 (dpm)] (1), [Ir(iqbt)2 (tta)] (2), and [Ir(iqbt)2 (dtdk)] (3) based on the 1-(benzo[b]thiophen-2-yl)-isoquinolinate (iqtb) were synthesized and characterized (dpm=2,2,6,6-tetramethyl-3,5-heptanedionate; tta=2-thienoyltrifluoroacetonate; dtdk=1,3-di(thiophen-2-yl)propane-1,3-dionate). The compounds emit between λ=680 and 850 nm with high luminescence quantum yields (up to 16 %). By combining electrochemistry, photophysical measurements, and computational modelling, the relationship between the structure, energy levels, and properties were investigated. NIR-emitting, solution-processed phosphorescent organic light-emitting devices (PHOLEDs) were fabricated using the complexes. The devices show remarkable external quantum efficiencies (above 3 % with 1) with negligible efficiency roll-off values, exceeding the highest reported values for solution-processible NIR emitters.
We compute the orbital magnetization in real materials by evaluating a recently discovered formul... more We compute the orbital magnetization in real materials by evaluating a recently discovered formula for periodic systems, within density functional theory. We obtain improved values of the orbital magnetization in the ferromagnetic metals Fe, Co, and Ni, by taking into account the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the EPR $g$-tensor in molecules and solids. The present approach reproduces the $g$-tensor obtained by linear response (LR), when the spin-orbit can be treated as a perturbation. However, it can also be applied to radicals and defects with an orbital-degenerate ground-state or containing heavy atoms, that can not be properly described by LR.
J Phys Chem B, 2001
ABSTRACT Dehydroxylation and silanization processes on the silica surface are studied by ab initi... more ABSTRACT Dehydroxylation and silanization processes on the silica surface are studied by ab initio molecular dynamics. The (100) and (111) surfaces of β-cristobalite are used as two possible models of the hydroxylated amorphous surface. The activation energy and latent heat for the dehydroxylation reactions of the (100) surface computed by constrained ab initio molecular dynamics are in reasonable agreement with experimental data on the amorphous surface. Adhesion reactions of silanes are simulated aiming at elucidating the binding mechanism of organosilanes used for instance as silica−polymer coupling agents. The simulation have provided insights on the occurrence of multiple silica−silane bonds and on the role of hydrolization of silane by physisorbed water in the adhesion on the wet surface.
Physical Review B Condensed Matter and Materials Physics, 2010
Within density-functional theory we compute the orbital magnetization for periodic systems evalua... more Within density-functional theory we compute the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula. For the ferromagnetic metals Fe, Co, and Ni we explicitly calculate the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the electron paramagnetic resonance g tensor in paramagnetic systems. Here the method can also be applied in cases where linear-response theory fails, e.g., radicals and defects with an orbital-degenerate ground state or those containing heavy atoms.
Aps Meeting Abstracts, 2006
Emission of neutral cations/anions from solid akali halides can in principle be provoked by donat... more Emission of neutral cations/anions from solid akali halides can in principle be provoked by donating/subtracting electrons to the surface of alkali halide crystals, but generally constitutes a very endo-energetic process. The amount of energy required is expected to decrease for atoms located in less favorable positions, such as at surface steps and kinks, where the local coordination is smaller. The most favorable case of all should be represented by corner atoms of alkali halides cubes, where ionic binding is the weakest. We carried out first principles density functional calculations and simulations of magic-size neutral and charged NaCl nanocubes, to ascertain the stability and extractibility of neutralized corner ions. We find that detachment of neutral Na atoms upon donation of an excess electron is possible for a very modest energetic cost of the order of 0.6 eV, in line with early experimental observations. The atomic and electronic structure of the resulting Na^+ vacancy is analyzed in detail.
Physical Review Letters, 2000
We present extensive modeling of the amorphous silica surface, aimed at connecting its structural... more We present extensive modeling of the amorphous silica surface, aimed at connecting its structural and chemical features. beta-cristobalite surfaces are initially studied to model the hydroxylated surfaces. A model reconstruction of the (111) surface is used to define a path leading to the formation of two-membered silicon rings upon dehydroxylation. Subsequently, a realistic model of the amorphous dehydroxylated (dry) surface is produced, by full ab initio annealing of an initial model generated by classical simulation. The presence of surface two-membered silicon rings emerges naturally. A calculation of IR activity yields an associated peak doublet in agreement with experimental data.
We present the results of muon-spin relaxation measurements of spin excitations in the lowdimensi... more We present the results of muon-spin relaxation measurements of spin excitations in the lowdimensional quantum Heisenberg antiferromagnet Cu(pyz)(NO3)2. Using density functional theory we propose the muon sites and assess the degree of perturbation the muon probe causes on the system. Our measurements of the dynamics of the system show that in the temperature range TN < T < J (between the ordering temperature TN and the exchange energy scale J) the transport of spin excitations detected by the muon is diffusive over much of the range of applied fields. We also identify a possible cross-over in the probe response to the fluctuation spectrum at higher applied fields, to a regime where the muon detects transport with a ballistic character. This behavior is contrasted with that found for T > J and that in the related two-dimensional system Cu(pyz)2(ClO4)2.
Surface Science, 2006
Alkali halide (100) surfaces are anomalously poorly wetted by their own melt at the triple point.... more Alkali halide (100) surfaces are anomalously poorly wetted by their own melt at the triple point. We carried out simulations for NaCl(100) within a simple (BMHFT) model potential. Calculations of the solid-vapor, solid-liquid and liquid-vapor free energies showed that solid NaCl(100) is a nonmelting surface, and that the incomplete wetting can be traced to the conspiracy of three factors: surface anharmonicities stabilizing the solid surface; a large density jump causing bad liquid-solid adhesion; incipient NaCl molecular correlations destabilizing the liquid surface, reducing in particular its entropy much below that of solid NaCl(100). Presently, we are making use of the nonmelting properties of this surface to conduct case study simulations of hard tips sliding on a hot stable crystal surface. Preliminary results reveal novel phenomena whose applicability is likely of greater generality.
The Journal of Chemical Physics, 2014
By partitioning the electron density into subsystem contributions, the Frozen Density Embedding (... more By partitioning the electron density into subsystem contributions, the Frozen Density Embedding (FDE) formulation of subsystem DFT has recently emerged as a powerful tool for reducing the computational scaling of Kohn-Sham DFT. To date, however, FDE has been employed to molecular systems only. Periodic systems, such as metals, semiconductors, and other crystalline solids have been outside the applicability of FDE, mostly because of the lack of a periodic FDE implementation. To fill this gap, in this work we aim at extening FDE to treat subsystems of molecular and periodic character. This goal is achieved by a dual approach. On one side, the development of a theoretical framework for periodic subsystem DFT. On the other, the realization of the method into a parallel computer code. We find that periodic FDE is capable of reproducing total electron densities and (to a lesser extent) also interaction energies of molecular systems weakly interacting with metallic surfaces. In the pilot calculations considered, we find that FDE fails in those cases where there is appreciable density overlap between the subsystems. Conversely, we find FDE to be in semiquantitative agreement (but still within chemical accuracy) with Kohn-Sham DFT when the inter-subsystem density overlap is low. We also conclude that to make FDE a suitable method for describing molecular adsorption at surfaces, kinetic energy density functionals that go beyond the GGA level must be employed.
Surface Science, 2006
We performed first principles density functional calculations and simulations of magic-size neutr... more We performed first principles density functional calculations and simulations of magic-size neutral NaCl nanocubes, and computed the the extraction of a Na neutral corner atom after donating an electron. The atomic structure of the resulting Na corner vacancy is presented.
Journal of Physical Chemistry B, 2001
... Moreover, the two main faces of β-cristobalite can sustain the two types of silanol groups id... more ... Moreover, the two main faces of β-cristobalite can sustain the two types of silanol groups identified experimentally on the amorphous silica surface, namely the single silanols (a single hydroxyl attached to a surface Si) typical of the (111) surface and the “geminal” silanols (two ...
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Papers by Davide Ceresoli