Electronic structure and thermal behavior of a magicNa59+cluster

We have made theoretical investigations of the atomic geometry, electronic structure, and lattice dynamics of the $\mathrm{C}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surface. The atomic geometry and electronic structure have been calculated by using the local density approximation of the density functional theory and ab initio pseudopotentials. Using our calculated atomic and electronic structures, surface lattice dynamics was studied by employing two different approaches: a linear response approach based on density functional perturbation theory, and an adiabatic bond-charge model. The phonon dispersion curves obtained from the two models are compared against each other in detail. In general, results from both models are in agreement with each other, with small differences due mainly to differences in the conceptual natures of these models. Our results from both methods are in good agreement with recent electron energy loss spectroscopy measurements. We have also provided a comparison of a few characteristic phonon modes on this surface with those on the $\mathrm{Si}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ and $\mathrm{Ge}(001)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces.

A first-principles study of the effects of temperature on the atomic and electronic structures of Mg-montmorillonite with H2O molecule adsorption

The design of two-dimensional superatomic materials, which form their atomic structures through covalently bonded clusters with variable chemical compositions, will enable the development of new materials with promised electronic properties that are beneficial for modern nanoelectronics. This paper presents ab initio calculations of the atomic and electronic structures of both bulk and 2D Re6Se8Cl2. The calculations were carried out using density functional theory, incorporating noncollinear spin density and the pseudopotential method. The results include data on the atomic structure, band gap value, formation energy of the Re6Se8Cl2 2D layer, and the redistribution of atomic charges within the structures. The differences in effective masses for electrons and holes in the two-dimensional and bulk Re6Se8Cl2 materials are demonstrated, along with an explanation of how these differences impact their transport properties. The findings are expected to be of great significance for the design, synthesis, and implementation of new two-dimensional superatomic materials with controlled properties in modern nanoelectronics.

The atomic and electronic structures of pristine, Mn- and Nb-doped grain boundaries in SrTiO3 are investigated by atomistic simulations and cluster calculations. The atomic structures of (310) symmetric tilt grain boundaries in SrTiO3 are determined by atomistic simulation using empirical potentials. The defect energies of Mn(Nb)-doped models are calculated and discussed in relation to the concentration profiles of Mn(Nb) in SrTiO3 grain boundaries. The local electronic structures near Mn(Nb)-doped grain boundaries in SrTiO3 are determined using embedded cluster calculations based on the density functional theory. The charge density of each system is calculated to elucidate the electronic structure of the grain boundary. The calculation results agree well with previous experimental observations of the atomic structures and grain boundary charges near the Mn(Nb)-doped grain boundary in SrTiO3.

We present a systematic theoretical study of several III-V semiconductor (110) surfaces based on accurate, self-consistent total-energy and force calculations, using density-functional theory and ab initio pseudopotentials. We study GaP, InP, GaAs, and InAs and analyze the theoretical trends for the equilibrium atomic structures, photoelectric thresholds, and surface band structures. The influence of the basis-set completeness on these results is examined. The thoeretical results are compared with experimental low-energy electron-diffraction analyses and photoemission and inverse-photoemission data.

The atomic structure of a bulk metallic glass resolved by scanning tunneling microscopy and ab-initio molecular dynamics simulation

We report results obtained by a systematic study of Sb adsorption on the relaxed GaAs(110) surface, using density-functional theory within the local-density approximation (LDA) and norm-conserving, fully separable, ab-initio pseudopotentials. The GaAs(110) surface is simulated by a slab geometry wherein the atomic structure of the Sb atoms at the preferred adsorption positions and the uppermost substrate layer is optimized by minimizing the total energy, in contrast to previously reported theoretical approaches obtaining the surface bandstructure for given geometrical equilibrium structures. Sb coverages of Q=1/2 and Θ=1 are considered. We give a detailed analysis of the total-energy surface of the Sb/GaAs(110) system and identify stable and metastable adsorption sites. The resulting equilibrium geometries are discussed: We interpret these results in terms of the Sb-Sb interaction within the chains parallel to the [1¯11] direction and of possible structural instabilities in such chains. The atomic positions are compared with results of LEED analysis, stating an overall agreement except the buckling of the chain atoms. The resulting electronic properties (surface bandstructure, photothreshold, Schottky barrier) are discussed within the context of experimental data available from STM, photoemission spectroscopy, etc.

A systematic theoretical study of clean and sodium covered GaAs(110) surfaces based on accurate, self-consistent total-energy and force calculations using the density-functional theory and ab-initio pseudopotentials is presented. The atomic and electronic structures of the clean and adsorbed surfaces are examined with plane wave basis sets. The adsorbed semiconductor surface is studied for different coverages. The atomic positions of the adsorbate and of the top three substrate layers are fully relaxed and detailed predictions of the surface geometries are given. During the formation of the metal-semiconductor contacts two different types of electronic states appear in the fundamental band gap; adatom related states and metal induced surface states. For the alkali adatoms an efficient and highly anisotropic diffusion parallel to the atomic chains of the GaAs(110) surface is found.

With the increasing cognition of the importance of organic molecules, they are widely applied in printing, biological and pharmacological fields, because of their special capabilities of harvesting solar light, scavenging free radicals, and chelating metal ions. During the past decades, the unique photoelectronic and photochemical properties of organic molecules, such as phthalocyanine, cyanidin, and their relevant derivatives, attract tremendous attention, because they provide an excellent opportunity to solve the worldwide energy crisis by converting directly the solar light to electricity. The surface morphology and electronic interaction of these organic molecules with other molecules, surfaces or interfaces play a critical role in determining the performance of the electronic and optical devices based on organic molecules. In this thesis, we focus on the investigation of several selected organic molecules, and their interaction with molecules, inorganic semiconductors, and organic semi-conductors, by using first-principles electronic structure calculations based on density functional theory, and time-dependent density functional theory. Particular attention is paid to the atomic structure, electronic and optical properties of organic molecules and the corresponding interfaces. The focus of this thesis is on the following aspects of the organic molecules: (i) The complexation mechanism of flavonoids with metal ions. The most likely chelation site for Fe is the 3-hydroxyl-4-carbonyl group, followed by 4-carbonyl-5-hydroxyl group and the 3'-4' hydroxyl (if present) of quercetin. A complex of two quercetin molecules with a single Fe ion is energetically more stable, however, six orbitals of Fe in the three quercetin complex are saturated by three perpendicular molecules to form and octahedral configurations. Furthermore, the optical absorbance spectra serves as signatures to identify various complexes. (ii) The electronic coupling between a dye molecule (Cyanidin) and a TiO2 nanowire. Upon molecular adsorption on TiO2 [010]-wire, cyanidin will be deprotonated into the quinonoidal form. This results in its highest occupied molecular orbitals being located in the middle of TiO2 bandgap and its lowest occupied molecular orbitals being close to the TiO2 conduction band minimum, in turn enhancing visible light absorption. Moreover, the excited electrons are injected into TiO2 conduction band within a time scale of 50 fs with negligible electron-hole recombination. (iii) The atomic and electronic structure of copper (fluoro-)phthalocyanine and graphene. When adsorbed on graphene, F16CuPc molecules prefer to form a close-packed hexagonal lattice with two-ordered alternating α and β stripes, whereas CuPc would like to form a square lattice. In addition, phthalocyanine adsorption modifies the electronic structure of graphene introducing intensity smoothing at 2-3 eV below and a small peak at ∼0.4 eV above the Dirac point in the density of states. And finally, (iv) the electronic interaction between CuPc and fullerene. For CuPc/C60 molecular complex, CuPc prefers to lie flat on the C60 surface rather than taking the standing-up molecular orientation. The favorable adsorption site for CuPc is the bridge site of C60 with one N-Cu-N bond of CuPc being parallel to a C-C bond of C60. Based on the analysis of the molecular complex, we predict that CuPc/C60(001) thin film heterojunction adopting the lying-down molecular orientation should have a higher efficiency of charge transfer in comparison with the relevant CuPc/C60(111) heterojunction with the standing-up arrangement.

Atomic and electronic structures of an Ag-containing 4A zeolite

Using empirical atomistic simulations and density functional theory (DFT), we examine the atomic and electronic structure of pure- and tin-doped indium oxide in various degrees of amorphisation. Atomic structures ranging from maximally amorphous (within fixed periodic boundary conditions) to fully crystalline are prepared using liquid-quench molecular dynamics simulations in which the cooling/quench rate is the governing parameter. The final structures are reoptimized using DFT and the electronic structure (band gaps and carrier effective masses) are compared to the crystalline material. We find that the conduction bands of ${\text{In}}_{2}{\text{O}}_{3}$ are quite resilient in several aspects to changes in the atomic structure. This suggests that local coordination geometries around indium and oxygen are less critical to transparent conductivity than previously thought.

First-principles study of W, N, and O adsorption on TiB2(0001) surface with disordered vacancies

Introduction Li2S-P2S5 glass has been attracted attention as the solid electrolyte for lithium ion battery because of its high ionic conductivity and wide electrochemical potential window. It is known that the ratio of Li2S and P2S5 affects Li-ion conductivity, and suggested that the atomic structure is related to Li ion conductivity. Atomic structures of Li2S-P2S5 glasses calculated by Reverse Monte Carlo (RMC) modeling previously reported.1,2RMC calculation results in structures satisfying the requirement for experimental data in totality, however, its local structure might have unreasonable configuration. In this study, we calculated reasonable Li3PS4, Li7P3S11 and Li4P2S7glass structure using combination of RMC calculations and density functional theory (DFT) calculations. Method Cubic cell whose lattice constant is about 22.5 Å and number of atoms is 567 were used for the initial structure of Li7P3S11 glass structure. Density is equal to experimental result. Li atoms, PS4 and P2S7molecules were randomly allocated in the cell. At first, RMC calculation was performed for the initial random structure. RMC calculation was carried out for the two structure-factor data S(Q) of Li7P3S11 glass using RMC++ code. S(Q) data were obtained from neutron diffraction and X-ray diffraction. Secondly, DFT calculation was performed for RMC structure using VASP code. In DFT part, internal atomic positions optimized until residual forces become less than 0.02 eV/Å. These RMC and DFT calculations were repeated until the difference of atomic structures between RMC and DFT become to be less than 0.1 Å. For comparison, smaller cells, 10 types of 325-atom cells and 100 types of 105-atom cells were calculated in the same way. Results and Discussion The structures calculated by RMC had good agreement with S(Q) in any cases as shown in Fig. 1, however, the densities of states are obviously different. While the last RMC structure had about 1.5 eV band gap (shown in blue line in Fig. 2), the 1st RMC structure was metallic (shown in red line in Fig. 2). The largest difference between 1st and last RMC structure is configuration of Li atoms. Figure 3 shows the pair distribution function of Li and Li. The last RMC g(r) is flat around 1, which means that Li atoms exist homogeneously in the cell. In contrast, the 1st RMC g(r) has a peak at 2.5 Å which is equal to the constraint distance between Li atoms setup condition in RMC. This suggests that Li configuration does not affect to the S(Q) in this system even if neutron diffraction is considered. Figure 4 shows the relative energies of Li7P3S11 based on Li2S and P2S5 energies against band gaps. The energies of glass structures are higher than crystal and lower than the mixture of Li2S and P2S5. Most energy fell within 0.03 eV/atom and band gaps fell within 0.6 eV. In summary, the combination of RMC and DFT calculation results proper atomic structures satisfying experimental data within satisfactory accuracy. Acknowledgment This work was supported by the RISING project of the New Energy and Industrial Technology Development Organization (NEDO).

Sum Frequency Generation (SFG) spectra of nanocrystalline porous silicon (por-Si) exposed to different chemical treatments are studied. We report the first SFG studies of por-Si in direct contact with a liquid. SFG is excited by a regeneratively amplified Ti:sapphire system (787 nm, 120 fs, 1 kHz). The sum frequency is generated by combining this light with infrared that is generated with an optical parametric amplifier (OPA) that delivers 100-200 &#956;J pulses at 1370-1770 nm. Por-Si is made from a 10-20 &#937; cm <i>p</i>-type Si(001) wafer. Comparisons are made to planar Si(001) as well as GaAs(001). First principle electronic structure theory based on density functional theory (DFT) is used to study the adsorption and optical response functions from the system of ethanol molecule adsorbed on Si(001) and Si(111) surfaces. Equilibrium atomic geometries are obtained through molecular dynamics and total energy minimization methods. Electron energy structure and optical properties are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted differential optical absorption spectra for chemisorbed Si(001) and Si(111) surfaces are analyzed in comparison with SFG data measured on differently treated porous silicon. Substantial modifications of the surface atomic and electron energy structures of silicon surfaces due to chemisorption are shown to provide the dominant contributions to the SFG response.

The replacement of Pt group metal (PGM) electrocatalysts with earth-abundant PGM-free materials in polymer electrolyte fuel cell (PEFC) cathodes faces several technological hurdles, including the need for improved long-term durability. In PGM systems, catalyst durability is achieved by maintaining Pt catalyst surface area that is in electrical contact with the electrode (electrochemical surface area, ESA).1,2 This ESA can be probed directly by electrochemical methods. Loss of electrical contact (via carbon corrosion or Pt particle detachment) as well as Pt surface area loss (via Pt dissolution into ionomer, particle agglomeration, surface blocking, or Ostwald ripening) lead to a loss in oxygen reduction reaction (ORR) activity in aged PGM cathodes. State-of-the-art PGM-free catalysts are highly heterogeneous systems due to pyrolysis processing and contain a great diversity of atom-scale structures. As such, there is still much debate regarding the atomic scale structure of the ORR active site. It has been proposed that a number of sites in the materials are ORR active, with varying degrees of activity and durability, further complicating study. Unlike their PGM counterparts, PGM-free materials do not have an easily electrochemically probed property such as ESA that can be determined and correlated to measured activity loss. These complexities necessitate different approaches to identifying and predicting durability properties and degradation mechanisms in PGM-free systems. One proposed approach for understanding PGM-free catalyst durability3 is combining quantum chemical modeling with a TEM beam-damage model4 to identify how local atomic structure affects the kinetics of atom removal. Calculation of the knock-on displacement threshold energy (KODTE) could serve as a durability descriptor for a given atomic structure, similar to how thermodynamic limiting potential serves as a computationally-accessible activity descriptor of ORR active site structures. This descriptor would go beyond relative thermodynamic stability of active site structures and actively probe the kinetics of bond breaking in the system. As part of the DOE Electrocatalysis Consortium (ElectroCat), an automated workflow for calculating the KODTE for a given atomic scale structure using ab initio molecular dynamics has been developed and applied to a variety of possible PGM-free ORR active site structures as well as C-host materials. These methods have identified that N are most susceptible to removal and that edge-hosted structures are more susceptible than bulk-hosted structures. Utilized methodology, as well as identified trends from these calculations and comparisons to available experiments will be discussed.