Macrocyclic dyes such as phthalocyanine and porphyrin molecules are modeled on (1010) wurzite surfaces using the DFT and molecular dynamics approaches. It is found that the single dye anchored on the wurtzite surface stabilizes in an inclined geometry with its core facing the surface at a tilting angle of ca 60∘. The tilting of the dye relative to the crystal surface has a dual effect on the charge transfer from a chromophore to the semiconductor. Increasing the tilting angle leads to a stronger coupling between the lowest level of the semiconductor conduction band and dye’s LUMO, thus raising the tunneling probability of the electron injection. By contrast, the electrostatic interaction between units upon the tilting of macrocycles results in a lowering of the molecule LUMO level with respect to the conduction band minimum of the wurzite crystal, which may hinder the electron transfer. The type of a linker and peripheral substituents significantly affect the mutual conformation of the moieties, and their proper choice can facilitate the photoinduced charge transfer reactions.

A phthalocyanine molecule adsorbed on the (101[combining macron]0) surface of wurtzite CdSe is theoretically modeled by the DFT method. We have found that a linker does not affect substantially the redox properties of phthalocyanine, while saturation of the macrocycle with peripheral substituent groups causes a downward shift in the energy position of its frontier orbitals that can hinder electron injection to the CdSe surface. Tilting of the phthalocyanine molecule relative to the surface also leads to the lowering of its molecular electronic levels relative to the bands of CdSe. At a tilting angle of 30°, the LUMO level of the dye appears to be lower than the conduction band minimum of cadmium selenide, which makes the electron transfer to its hybridized surface unfavorable. By contrast, the HOMO level of the phenylbutyric acid linker provides a suitable intermediate channel for the hole transfer from the valence band of CdSe to the phthalocyanine that points to the possible acceptor behavior of the phthalocyanine molecule in its hybrids with CdSe nanostructures.

We observe using ab initio methods that localized surface plasmon resonances in icosahedral silver nanoparticles enter the asymptotic region already between diameters of 1 and 2 nm, converging close to the classical quasistatic limit around 3.4 eV. We base the observation on time-dependent density-functional theory simulations of the icosahedral silver clusters Ag-55 (1.06 nm), Ag-147 (1.60 nm), Ag-309 (2.14 nm), and Ag-561 (2.68 nm). The simulation method combines the adiabatic GLLB-SC exchange-correlation functional with real time propagation in an atomic orbital basis set using the projector-augmented wave method. The method has been implemented for the electron structure code GPAW within the scope of this work. We obtain good agreement with experimental data and modeled results, including photoemission and plasmon resonance. Moreover, we can extrapolate the ab initio results to the classical quasistatically modeled icosahedral clusters.

Adsorption of benzene on oxygen rich and reduced SnO2 surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals approach. Rather than preferential adsorption sites, it is found that the adsorption potential energy surface is flat at both surfaces. The bridging oxygen atoms on the stoichiometric surface induce both covalent and ionic bonding leading to weak chemisorptions, whereas bonding on the reduce surface is closer to physisorption. Deformation of the benzene adsorbate due to adsorption is negligible and only small opposite charge transfer is found explaining the differences between the two surfaces.

Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability and systematic convergence properties. However, as a unique feature GPAW also facilitates a localized atomic-orbital basis set in addition to the grid. The efficient atomic basis set is complementary to the more accurate grid, and the possibility to seamlessly switch between the two representations provides great flexibility. While DFT allows one to study ground state properties, time-dependent density-functional theory (TDDFT) provides access to the excited states. We have implemented the two common formulations of TDDFT, namely the linear-response and the time propagation schemes. Electron transport calculations under finite-bias conditions can be performed with GPAW using non-equilibrium Green functions and the localized basis set. In addition to the basic features of the real-space PAW method, we also describe the implementation of selected exchange-correlation functionals, parallelization schemes, ΔSCF-method, x-ray absorption spectra, and maximally localized Wannier orbitals.

We model a Kohn-Sham potential with a discontinuity at integer particle numbers derived from the GLLB approximation of Gritsenko et al. We evaluate the Kohn-Sham gap and the discontinuity to obtain the quasiparticle gap. This allows us to compare the Kohn-Sham gaps to those obtained by accurate many-body perturbation theory based optimized potential methods. In addition, the resulting quasiparticle band gap is compared to experimental gaps. In the GLLB model potential, the exchange-correlation hole is modeled using a GGA energy density and the response of the hole to density variations is evaluated by using the common-denominator approximation and homogeneous electron gas based assumptions. In our modification, we have chosen the PBEsol potential as the GGA to model the exchange hole, and add a consistent correlation potential. The method is implemented in the GPAW code, which allows efficient parallelization to study large systems. A fair agreement for Kohn-Sham and the quasiparticle band gaps with semiconductors and other band gap materials is obtained with a potential which is as fast as GGA to calculate.

Siloxane is a favorable candidate as an anchor group that can be used to bind organic molecules to SnO2 surfaces, with a wide range of practical applications. Therefore, adsorption geometries and energies of siloxane coupler on the SnO2 (110) surface have been investigated in this study using quantum-chemical periodic density functional theory (DFT) calculations. We present a comparative study of different siloxane adsorption arrangements on pristine and fluorine doped SnO2 surface. According to the calculations, the surface doping with fluorine leads to stabilization of the siloxane network at the stannic oxide surface. The trend is analyzed in terms of additional charge provided by F impurities to the chemisorbed oxygen atoms thus increasing the ionicity of their bonding. Implications of the current findings for the design of organic-metal oxide interface with better thermo-stability and improved electronic properties are discussed.

Bismuth adsorbate-stabilized (2x1) and (2x4) reconstructions of the GaAs(100) surfaces have been studied by first-principles calculations, valence-band and core-level photoelectron spectroscopies, and scanning tunneling microscopy. It is demonstrated that large Bi atom size leads to the formation of the pseudogap at the Fermi energy and to the lower energy of an adsorbate-derived surface band, which contributes to the stabilization of the exceptional Bi/GaAs(100)(2x1) reconstruction. It is proposed that the Bi/GaAs(100)(2x4) reconstructions include asymmetric mixed Bi-As dimers, in addition to the Bi-Bi dimers. Based on the calculations, we solve the atomic origins of the surface core-level shifts (SCLSs) of the Bi 5d photoemission spectra from the Bi/GaAs(100)(2x4) surfaces. This allows for resolving the puzzle related to the identification of two SCLS components often found in the measurements of the Bi 5d and Sb 4d core-level emissions of the Bi/III-V and Sb/III-V(100)(2x4) surfaces. Finally, the reason for the absence of the common (2x4)-beta 2 structure and additional support for the stability of the (2x1) structure on the Bi/III-V(100) surfaces are discussed in terms of Bi atom size and subsurface stress.

Adsorption of three small hydrocarbons molecules (acetylene, ethylene and ethane) on two SnO2(110) surfaces is studied by employing density functional theory calculations, slab model and linear combination of atomic orbitals (LCAO) approach. Acetylene and ethylene adsorb preferentially on top of a 5-fold tin atom, while ethane aligns itself with the rows of bridging oxygens and settles between two surface tin atoms. Adsorption energies and changes occurring in the structural characteristics and electronic structure of both the hydrocarbons and SnO2 surface indicate chemisorption of unsaturated hydrocarbons and physisorption of ethane on reduced surfaces. In all cases of chemisorption there are two contributing mechanisms: charge transfer creating ionic bonding (down-shift of levels) and covalent bonding (broadening of levels).

We have studied nitrogen interstitial defects in GaAs with first-principles calculations. On the basis of calculated formation energies we have determined the most common nitrogen defects and the transition levels for various charge states. The lowest energy interstitial-type defects are found to be N-N and N-As split interstitials for most of the experimentally relevant conditions. We have also compared two different methods of obtaining the potential correction needed in an accurate calculation of the formation energies and transition levels.

First-principles phase diagrams of bismuth-stabilized GaAs- and InP(100) surfaces demonstrate for the first time the presence of anomalous (2x1) reconstructions, which disobey the common electron counting principle. Combining these theoretical results with our scanning-tunneling-microscopy and photoemission measurements, we identify novel (2x1) surface structures, which are composed of symmetric Bi-Bi and asymmetric mixed Bi-As and Bi-P dimers, and find that they are stabilized by stress relief and pseudogap formation.

The GaAs/GaAsN interface band offset is calculated from first principles. The electrostatic potential at the core regions of the atoms is used to estimate the interface potential and align the band structures obtained from respective bulk calculations. First, it is shown that the present method performs well on the well-known conventional/conventional AlAs/GaAs (001) superlattice system. Then the method is applied to a more challenging nonconventional/conventional GaAsN/GaAs (001) system, and consequently type I band lineup and valence-band offset of about 35 meV is obtained for a nitrogen concentration of about 3%, in agreement with the recent experiments. We also investigate the effect of strain on the band lineup. For the GaAsN layer longitudinally strained to the GaAs lattice constant, the type II lineup with a nearly vanishing band offset is found, suggesting that the anisotropic strain along the interface is the principal cause for the often observed type I lineup.

First-principles calculations show that measured surface core-level shifts (SCLSs) of the GaAs(100)(2x4) surfaces can be described within the initial state effects. The calculated As 3d and Ga 3d SCLSs for the beta 2 and alpha 2 reconstructions of the GaAs(100)(2x4) surfaces are in reasonable agreement with recent measurements. In particular, the results confirm that both the lower and the higher binding energy SCLSs, relative to the bulk emission in the As 3d photoelectron spectra, are intrinsic properties of the GaAs(100)(2x4) surfaces. The most positive and most negative As shifts are attributed to the third layer As atoms, which differs from the previous intuitive suggestions. In general, calculations show that significant SCLSs arise from deep layers, and that there are more than two SCLSs. Our previously measured As 3d spectra are fitted afresh using the calculated SCLSs. The intensity ratios of the SCLSs, obtained from the fits, show that as the heating temperature of the GaAs(100)(2x4) surface is increased gradually, the area of the alpha 2 reconstruction increases on the surface, but the beta 2 phase remains within the whole temperature range, in agreement with previous experimental findings. Our results show that the combination of the experimental and theoretical results is a prerequisite for the accurate analysis of the SCLSs of the complex reconstructed surfaces.

The path-integral Monte Carlo approach is used to study the coupled quantum dynamics of the electron and nuclei in hydrogen molecule ion. The coupling effects are demonstrated by comparing differences in adiabatic Born-Oppenheimer and nonadiabatic simulations, and inspecting projections of the full three-body dynamics onto the adiabatic Born-Oppenheimer approximation. Coupling of the electron and nuclear quantum dynamics is clearly seen. The nuclear pair correlation function is found to broaden by 0.040a(0), and the average bond length is larger by 0.056a(0). Also, a nonadiabatic correction to the binding energy is found. The electronic distribution is affected less than the nuclear one upon inclusion of nonadiabatic effects.

The path-integral Monte Carlo simulation method is used to examine one and two electrons in a system of two coupled disc-like quantum dots (QD) in a zero magnetic field. With this approach we are able to evaluate the one-electron distributions and two-electron correlation functions, and finite temperature effects on both. Increase of temperature broadens the distributions as expected, the effect being smaller for correlated electrons than for single ones. The simulated one- and two-particle distributions of a single and two coupled quantum dots are also compared to those from other theoretical methods. For the one-particle distributions we find a good agreement with those from the DFT approach. The effect of the third dimension or the thickness of the almost two-dimensional disc-like QDs is small for the one-particle distributions, but it is clearly seen in the electron-electron correlation or the two-particle distribution function at low temperatures. The mutual Coulomb energy of the two electrons is found to be temperature-independent, and also, independent of the correlation effects on the dynamics. Computational capacity is found to become the limiting factor in simulations with increasing accuracy or increasing number of particles, and in case of fermions in particular. This and other aspects of PIMC and its capability for this type of calculations are also discussed.

We have studied the structural and electronic properties of As-rich GaAs1−x Nx and N-rich GaN1−yAsy alloys in a large composition range using first-principles methods. We have systematically investigated the effect of the impurity atom configuration near both GaAs and GaN sides of the concentration range on the total energies, lattice constants and bandgaps. The N (As) atoms, replacing substitutionally As (N) atoms in GaAs (GaN), cause the surrounding Ga atoms to relax inwards (outwards), making the Ga–N (Ga– As) bond length about 15% shorter (longer) than the corresponding Ga–As (Ga–N) bond length in GaAs (GaN). The total energies of the relaxed alloy supercells and the bandgaps experience large fluctuations within different configurations and these fluctuations grow stronger if the impurity concentration is increased. Substituting As atoms with N in GaAs induces modifications near the conduction band minimum, while substituting N atoms with As in GaN modifies the states near the valence band maximum. Both lead to bandgap reduction, which is at first rapid but later slows down. The relative size of the fluctuations is much larger in the case of GaAs1−x Nx alloys. We have also looked into the question of which substitutional site (Ga or N) As occupies in GaN. We find that under Ga-rich conditions arsenic prefers the substitutional N site over the Ga site within a large range of Fermi level values.

We have studied the structural and electronic properties of As-rich GaAs1-xNx and N-rich GaN1-yAsy alloys in a large composition range using first-principles methods. We have systematically investigated the effect of the impurity atom configuration near both GaAs and GaN sides of the concentration range on the total energies, lattice constants and bandgaps. The N ( As) atoms, replacing substitutionally As (N) atoms in GaAs (GaN), cause the surrounding Ga atoms to relax inwards (outwards), making the Ga-N (Ga-As) bond length about 15% shorter (longer) than the corresponding Ga-As (Ga-N) bond length in GaAs (GaN). The total energies of the relaxed alloy supercells and the bandgaps experience large fluctuations within different configurations and these fluctuations grow stronger if the impurity concentration is increased. Substituting As atoms with N in GaAs induces modifications near the conduction band minimum, while substituting N atoms with As in GaN modifies the states near the valence band maximum. Both lead to bandgap reduction, which is at first rapid but later slows down. The relative size of the fluctuations is much larger in the case of GaAs1-xNx alloys. We have also looked into the question of which substitutional site (Ga or N) As occupies in GaN. We find that under Ga-rich conditions arsenic prefers the substitutional N site over the Ga site within a large range of Fermi level values.

A characteristic feature of the state‐of‐the‐art of real‐space methods in electronic structure calculations is the diversity of the techniques used in the discretization of the relevant partial differential equations. In this context, the main approaches include finite‐difference methods, various types of finite‐elements and wavelets. This paper reports on the results of several code development projects that approach problems related to the electronic structure using these three different discretization methods. We review the ideas behind these methods, give examples of their applications, and discuss their similarities and differences. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface properties of solids and the interactions between molecules and solid surfaces are important for many technical applications. They also involve a range of physical and chemical phenomena of fundamental scientific interest. The importance of oxygen chemistry at SnO2 surfaces follows from the fact that SnO2 is used as an active material in gas sensor applications. The operation principle of these sensors is usually based on measurable conductance response of the material, which is understood in terms of reactions of gas molecules with different oxygen species adsorbed onto the surface. The role of the lattice oxygen, but in particular, the bridging oxygen atoms on SnO2 surfaces, is also active. Detailed understanding of the reaction mechanisms of various oxygen species at SnO2 surfaces is important, as it offers a way to improve the sensitivity and selectivity of the sensors.Oxygen adsorption-desorption kinetics at the SnO2 surface is studied experimentally using O-2-temperature-programmed desorption (TPD) method together with conductance measurements in the case of SnO2 powder and polycrystalline thick films made from the powder. In addition, CO-TPD is studied and the transient behaviour of various oxygen species is considered. Molecular beam epitaxy (MBE) was also used to fabricate polycrystalline and monocrystalline thin films with the SnO2(101) face on single crystal sapphire substrate. Simultaneous surface potential and conductance measurements during heating and cooling in different ambient atmospheres were used to characterize the monocrystalline SnO2(101) surface after various surface treatments. (c) 2005 Elsevier B.V. All rights reserved.

We have carried out electronic structure calculations on N interstitials in GaAs alloys using the first-principles plane-wave pseudopotential (PWPP) and projector augmented-wave (PAW) electronic structure methods in the framework of the density functional theory (DFT). Both the ultrasoft pseudopotential (USPP) method in connection with the generalized gradient approximation (GGA) and the PAW method in connection with the local density approximation (LDA) have been employed. Effects of the single nitrogen atom and nitrogen dimer related interstitial defects on the atomic and electronic structures of GaAs have been studied. Total energies, electronic band structures, and local densities of states have been evaluated. In general, energies of the defects with the NN dimer at the center of the Ga or As tetrahedron are more than 2 eV lower per nitrogen atom than those with a single N impurity at the same sites. We have also found that there are metastable defect candidates with a single N impurity in the middle of a particular edge of the Ga or As tetrahedra. Considering the modifications of the atomic structure of GaAs, our calculations show that the relaxations of the nearest neighbor atoms around the NN dimer at the center of the Ga or As tetrahedron, are essentially smaller than those around the single N impurity at the center of an edge of the Ga tetrahedron. Finally, these defect states induce drastic modifications into the electronic structure of GaAs. Interestingly, the NN dimer related defects cause noticeable changes only to the conduction bands near the conduction band edge, while the single N impurity related defects mainly modify the valence band edge and also induce localized and delocalized states into the band gap. Notably, the NN dimer and N impurity related defects lead to redshift and blueshift behavior of the band gap, respectively. The blueshift behavior is tentatively supported by the recent photoluminescence experiments.