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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 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]

Successful application of two-dimensional transition metal dichalcogenides in optoelectronic, catalytic, or sensing devices heavily relies on the materials' quality, that is, the thickness uniformity, presence of grain boundaries, and the types and concentrations of point defects. Raman spectroscopy is a powerful and nondestructive tool to probe these factors but the interpretation of the spectra, especially the separation of different contributions, is not straightforward. Comparison to simulated spectra is beneficial, but for defective systems first-principles simulations are often computationally too expensive due to the large sizes of the systems involved. Here, we present a combined first-principles and empirical potential method for simulating Raman spectra of defective materials and apply it to monolayer MoS2 with random distributions of Mo and S vacancies. We study to what extent the types of vacancies can be distinguished and provide insight into the origin of different evolutions of Raman spectra upon increasing defect concentration. We apply to our simulated spectra the phonon confinement model used in previous experiments to assess defect concentrations, and show that the simplest form of the model is insufficient to fully capture peak shapes, but a good match is obtained when the type of phonon confinement and the full phonon dispersion relation are accounted for. [ABSTRACT FROM AUTHOR]

Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cells require appropriate depth and lateral distributions of alkali metal dopants and gallium to attain world record photovoltaic energy conversion. The two requirements are interdependent because sodium is known to hamper In/Ga interdiffusion in polycrystalline films. However, such a fact is challenged by recent findings where sodium appears to enhance In/Ga interdiffusion in monocrystalline films. This contribution reviews closely the two cases to the benefits of grain boundary engineering in CIGS. A computational model reveals why Na induces In accumulation at CIGS grain boundaries, confining Ga to grain interiors. The positive technological implications for wider gap chalcopyrites are stressed. [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]

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]

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 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]

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 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]

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]

We present first principle calculations on formation and binding energies for Cu and Zn as solute atoms forming small clusters up to nine atoms in Al-Cu and Al-Zn alloys. We employ a density-functional approach implemented using projector-augmented waves and plane wave expansions. We find that some structures, in which Cu atoms are closely packed on {100}-planes, turn out to be extraordinary stable. We compare the results with existing numerical or experimental data when possible. We find that Cu atoms precipitating in the form of two-dimensional platelets on {100}-planes in the fcc aluminum are more stable than three-dimensional structures consisting of the same number of Cu-atoms. The preference turns out to be opposite for Zn in Al. Both observations are in agreement with experimental observations. [ABSTRACT FROM AUTHOR]

Point defects and complexes may affect significantly physical, optical, and electrical properties of semiconductors. The Cu(In,Ga)Se2 alloy is an absorber material for low-cost thin-film solar cells. Several recently published computational investigations show contradicting results for important point defects such as copper antisite substituting indium (CuIn), indium vacancy (VIn), and complexes of point defects in CuInSe2. In the present work effects of the most important computational parameters are studied especially on the formation energies of point defects. Moreover, related to defect identification by the help of their calculated properties possible explanations are discussed for the three acceptors, occuring in photoluminescence measurements of Curich samples. Finally, new insight into comparison between theoretical and experimental results is presented in the case of varying chemical potentials and of formation of secondary phases. [ABSTRACT FROM AUTHOR]