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We study the spectral properties of two kinds of derivatives of the carbon fullerene C60, small fullerenes and Si-heterofullerenes, by ab initio calculations. The principal method of study is the time-dependent density-functional theory in its full time-propagation form. C20, C28, C32, C36, and C50, the most stable small fullerenes in the range of C20–C50, are found to have characteristic features in their optical absorption spectra, originating from the geometry of the molecules in question. The comparison of measured and calculated absorption spectra is found to be a useful tool in differentiating between different, almost isoenergetic ground state structure candidates of small fullerenes. Substitutionally doped fullerenes are of interest due to their enhanced chemical reactivity. It is suggested that the doping degree can be obtained by studying the absorption spectra. For example, it is observed that the spectra gradually change when doping C60 up to C48Si12 so that absorption in the visible and near infrared regions increases. [ABSTRACT FROM AUTHOR]

We model tunneling currents through step barrier structures representative of ferroelectric tunnel junctions wherein one of the electrodes contributes to the barrier potential profile or an extra layer is grown between the ferroelectric barrier and one of the electrodes. We study current density–voltage (J–V) and tunneling electroresistance (TER) curves using the Tsu-Esaki formula with numerically calculated transmission. This method is computationally robust, and the same results cannot be obtained with the standard methods usually applied for interpreting experimental I–V curves. Our results predict that the effect of resonant tunneling produces asymmetry in the J–V curves and negative differential resistance characteristics. We show that the asymmetry of the J–V curves can be tuned by adjusting the barrier heights and widths and that changing the extra barrier width affects the asymmetry the most. The barrier widths and the main barrier height affect the magnitude of the tunneling current the most. The change in the tilt of the main barrier is suggested to produce a significant TER of the order of 102. Our numerical method provides a systematic way to study trends in tunneling currents through step barrier structures across a wide range of barrier parameters and bias voltages, in contrast to the necessity of employing several different approximations when using analytical formulae. Therefore, our method provides the means for interpreting existing and future experiments and can be used as a tool for designing new devices with desired functionalities. [ABSTRACT FROM AUTHOR]

Tunnel junctions are interesting for both studying fundamental physical phenomena and providing new technological applications. Modeling of the tunneling current is important for understanding the tunneling processes and interpreting experimental data. In this work, the tunneling current is modeled using the Tsu-Esaki formulation with numerically calculated transmission. The feasibility of analytical formulae used for fitting experimental results is studied by comparing them with this model. The Tsu-Esaki method with numerically calculated transmission provides the possibility to calculate tunneling currents and fit experimental I-V curves for wide bias voltage and barrier width ranges as opposed to the more restricted analytical formulae. I-V curve features typical of tilted barrier structures are further analyzed to provide insight into the question, which of the phenomena can be explained with this simple barrier model. In particular, a small change in the effective barrier width is suggested as a possible explanation for experimental I-V curve features previously interpreted by a change in the tilt and height of the barrier. [ABSTRACT FROM AUTHOR]

We present an all-electron method for time-dependent density functional theory which employs hierarchical nonuniform finite-element bases and the time-propagation approach. The method is capable of treating linear and nonlinear response of valence and core electrons to an external field. We also introduce (i) a preconditioner for the propagation equation, (ii) a stable way to implement absorbing boundary conditions, and (iii) a new kind of absorbing boundary condition inspired by perfectly matched layers. [ABSTRACT FROM AUTHOR]

We have calculated the absorption characteristics of different hybrid systems consisting of Ag, Ag2, or Ag3 atomic clusters and poly(methacrylic acid) using the time-dependent density-functional theory. The polymer is found to have an extensive structural-dependency on the spectral patterns of the hybrid systems relative to the bare clusters. The absorption spectrum can be “tuned” to the visible range for hybrid systems with an odd number of electrons per silver cluster, whereas for hybrid systems comprising an even number of electrons per silver cluster, the leading absorption edge can be shifted up to ∼4.5 eV. The results give theoretical support to the experimental observations on the absorption in the visible range in metal cluster-polymer hybrid structures. [ABSTRACT FROM AUTHOR]

We present for static density functional theory and time-dependent density functional theory calculations an all-electron method which employs high-order hierarchical finite-element bases. Our mesh generation scheme, in which structured atomic meshes are merged to an unstructured molecular mesh, allows a highly nonuniform discretization of the space. Thus it is possible to represent the core and valence states using the same discretization scheme, i.e., no pseudopotentials or similar treatments are required. The nonuniform discretization also allows the use of large simulation cells, and therefore avoids any boundary effects. [ABSTRACT FROM AUTHOR]

Optical absorption spectra have been calculated for a series of boron nitride fullerenelike cage structures BnNn of sizes n=12–36. The method used is a real-time, real-space implementation of the time-dependent density-functional theory, involving the full time propagation of the time-dependent Kohn-Sham equations. The spectra are found to be a possible tool for distinguishing between different boron nitride fullerene species and isomers. The trends and differences in the spectra are found to be related to the general geometry of the molecules. Comparison between local-density and generalized-gradient approximations for electron exchange-correlation functionals shows that both of them produce essentially the same spectral characteristics. [ABSTRACT FROM AUTHOR]

Quantum aspects, such as electron tunneling between closely separated metallic nanoparticles, are crucial for understanding the plasmonic response of nanoscale systems. We explore quantum effects on the response of the conductively coupled metallic nanoparticle dimer. This is realized by stretching a nanorod, which leads to the formation of a narrowing atomic contact between the two nanorod ends. Based on first-principles time-dependent density-functional-theory calculations, we find a discontinuous evolution of the plasmonic response as the nanorod is stretched. This is especially pronounced for the intensity of the main charge-transfer plasmon mode. We show the correlation between the observed discontinuities and the discrete nature of the conduction channels supported by the formed atomic-sized junction. [ABSTRACT FROM AUTHOR]

We use Ehrenfest dynamics and time-dependent density functional theory to calculate electronic stopping power Se of energetic ions in graphitic targets from first principles. By treating core electrons as valence electrons within the projected augmented wave framework, we demonstrate that this approach provides an accurate description of Se for a wide range of ions and ion energies, even when not only valence, but also core electron excitations are essential. Our impact-parameter-dependent approach capable of describing the stopping of both low- and high-energy ions is a significant step forward in Se calculations, as it makes it possible to monitor projectile charge state during impacts, estimate contributions of core and valence electron excitations to Se, and it gives a quantitative description of electronic stopping in the cross-over region for bulk solids and nanostructures from first principles. [ABSTRACT FROM AUTHOR]

The role of self-interstitials in the response of layered carbon materials such as graphite, bilayer graphene and multiwalled carbon nanotubes to irradiation has long remained a puzzle. Using density-functional-theory methods with an exchange and correlation functional which takes into account the interlayer van der Waals interaction in these systems without any material-specific empirical parameters, we study the energetics and migration of single- and di-interstitials in graphite and bilayer graphene. We show that two classes of interstitials, "bound" and "free," can coexist. The latter are mobile at room and lower temperatures, which explains the experimental data and reconciles them with the results of atomistic simulations. Our results shed light on the behavior of graphite and carbon nanotubes under irradiation and have implications for irradiation-mediated processing of bilayer graphene. [ABSTRACT FROM AUTHOR]

We present an approach for generating local numerical basis sets of improving accuracy for first-principles nanoplasmonics simulations within time-dependent density functional theory. The method is demonstrated for copper, silver, and gold nanoparticles that are of experimental interest but computationally demanding due to the semi-core d-electrons that affect their plasmonic response. The basis sets are constructed by augmenting numerical atomic orbital basis sets by truncated Gaussian-type orbitals generated by the completeness-optimization scheme, which is applied to the photoabsorption spectra of homoatomic metal atom dimers. We obtain basis sets of improving accuracy up to the complete basis set limit and demonstrate that the performance of the basis sets transfers to simulations of larger nanoparticles and nanoalloys as well as to calculations with various exchange-correlation functionals. This work promotes the use of the local basis set approach of controllable accuracy in first-principles nanoplasmonics simulations and beyond. [ABSTRACT FROM AUTHOR]

We derive equations for nonadiabatic Ehrenfest molecular dynamics within the projector augmented-wave (PAW) formalism. The discretization of the electrons is time-dependent as the augmentation functions depend on the positions of the nuclei. We describe the implementation of the Ehrenfest molecular dynamics equations within the real-space PAW method. We demonstrate the applicability of our method by studying the vibration of NaCl, the torsional rotation of H2C=NH2+ in both the adiabatic and the nonadiabatic regimes, and the hydrogen bombardment of C40H16. [ABSTRACT FROM AUTHOR]

The intrinsic n-type doping limits of tin oxide (SnO2) and indium oxide (In2O3) are predicted on the basis of formation energies calculated by the density-functional theory using the hybrid-functional methodology. The results show that SnO2 allows for a higher n-type doping level than In2O3. While n-type doping is intrinsically limited by compensating acceptor defects in In2O3, the experimentally measured lower conductivities in SnO2-related materials are not a result of intrinsic limits. Our results suggest that by using appropriate dopants in SnO2 higher conductivities similar to In2O3 should be attainable. [ABSTRACT FROM AUTHOR]

We present the implementation of the time-dependent density-functional theory both in linear-response and in time-propagation formalisms using the projector augmented-wave method in real-space grids. The two technically very different methods are compared in the linear-response regime where we found perfect agreement in the calculated photoabsorption spectra. We discuss the strengths and weaknesses of the two methods as well as their convergence properties. We demonstrate different applications of the methods by calculating excitation energies and excited state Born–Oppenheimer potential surfaces for a set of atoms and molecules with the linear-response method and by calculating nonlinear emission spectra using the time-propagation method. [ABSTRACT FROM AUTHOR]

We suggested recently [V. Virkkala et al., Phys. Rev. B 88, 035204 (2013)] that the band-gap narrowing in dilute GaAs1-xNx, alloys can be explained to result from the broadening of the localized N states due to the N-N interaction along the zigzag chains in the (110) directions. In that study our tight-binding modeling based on first-principles density-functional calculations took into account the random distribution of N atoms in a natural way. In this work we extend our modeling to GaAs1-xBix alloys. Our results indicate that Bi states mix with host material states. However, the states near the valence-band edge agglomerate along the zigzag chains originating from individual Bi atoms. This leads to Bi-Bi interactions in a random alloy broadening these states in energy and causing the band-gap narrowing. [ABSTRACT FROM AUTHOR]

The origin of the band gap bowing in dilute nitrogen doped gallium based III-V semiconductors is largely debated. In this paper we show the dilute GaAs1-xNx and GaP1-xNx as representative examples that the nitrogen-induced states close to the conduction band minimum propagate along the zigzag chains on the {110} planes. Thereby states originating from different N atoms interact with each other resulting in broadening of the nitrogen-induced states which narrows the band gap. Our modeling based on ab initio theoretical calculations explains the experimentally observed N concentration dependent band gap narrowing both qualitatively and quantitatively. [ABSTRACT FROM AUTHOR]

We have recently shown that by using a scaling approach for randomly distributed topological defects in graphene, reliable estimates for transmission properties of macroscopic samples can be calculated based even on single-defect calculations [A. Uppstu et al, Phys. Rev. B 85, 041401 (2012)]. We now extend this approach of energy-dependent scattering cross sections to the case of adsorbates on graphene by studying hydrogen and carbon adatoms as well as epoxide and hydroxy 1 groups. We show that a qualitative understanding of resonant scattering can be gained through density functional theory results for a single-defect system, providing a transmission "fingerprint" characterizing each adsorbate type. This information can be used to reliably predict the elastic mean free path for moderate defect densities directly using ab initio methods. We present tight-binding parameters for carbon and epoxide adsorbates, obtained to match the density-functional theory based scattering cross sections. [ABSTRACT FROM AUTHOR]

Band structures of GaAs1-xNx and GaSb1-xNxalloys are studied in the framework of the density functional theory within the hybrid functional scheme (HSE06). We find that the scheme gives a clear improvement over the traditional (semi)local functionals in describing, in a qualitative agreement with experiments, the bowing of electron energy band gap in GaAs1-xNxalloys. In the case of GaSb1-xNx alloys, the hybrid functional used makes the study of band structures possible ab initio without any empirical parameter fitting. We explain the trends in the band gap reductions in the two materials that result mainly from the positions of the nitrogen-induced stales with respect to the bottoms of the bulk conduction bands. [ABSTRACT FROM AUTHOR]

We show that transport in low-dimensional carbon structures with finite concentrations of scatterers can be modeled by utilizing scaling theory and effective cross sections. Our results are based on large-scale numerical simulations of carbon nanotubes and graphene nanoribbons, using a tight-binding model with parameters obtained from first-principles electronic structure calculations. As shown by a comprehensive statistical analysis, the scattering cross sections can be used to estimate the conductance of a quasi-one-dimensional system both in the Ohmic and localized regimes. They can be computed with good accuracy from the transmission functions of single defects, greatly reducing the computational cost and paving the way toward using first-principles methods to evaluate the conductance of mesoscopic systems, consisting of millions of atoms. [ABSTRACT FROM AUTHOR]

Ab initio methods are used to study the spin-resolved transport properties of graphene nanoribbons (GNRs) that have both chemical and structural edge disorder. Oxygen edge adsorbates on ideal and protruded ribbons are chosen as representative examples, with the protrusions forming the smallest possible structural disorder consistent with the edge geometry. The impact of the oxygen adsorbate dominates the transport properties of armchair nanoribbons. For zigzag nanoribbons, the transmission properties are markedly affected by the protrusion alone, leading to spin-polarized transport and a smaller perturbation from the oxygen adsorbate. Armchair nanoribbons also exhibit, as a function of their width and the threefold family structure, a repeating pattern related to the existence of the spin polarization and to the variation in the width of the band gap. [ABSTRACT FROM AUTHOR]

We consider a system consisting of two macroscopic superconductors coupled to each other through a weak link that incorporates a non-superconducting two-dimensional quantum dot. We find that Coulomb interactions between mid-gap states and electrons in the quantum dot give rise to a new phase-dependent contribution to the total energy of the system, and hence to the supercurrent. Since the number of electrons confined in the quantum dot depends on the gate voltage Vg, even the supercurrent can be controlled with Vg. [Copyright &y& Elsevier]

Finite MoS2 nanoparticles are known to support metallic edge states that are responsible for their catalytic activity. In this work we employ time-dependent density-functional theory (TDDFT) to study the influence of such edge states on the optical properties of triangular MoS2 monolayer flakes. We find that the edge states support collective plasmon-like excitations that couple strongly to the optical field leading to pronounced absorption peaks below the onset of interband transitions on the basal plane. Additionally, structural relaxation of the flakes can significantly distort the edge states. Thus, we observe that while an evenly-spaced edge configuration supports one-dimensional (1D) plasmon modes similar to those of an ideal 1D electron gas, the relaxed structures show mixed plasmon and single-electron excitations in the low-energy response. Our findings illustrate the sensitivity of the optical response of MoS2 nanostructures to the details of the edge configuration. [ABSTRACT FROM AUTHOR]