Local Electron Density Research Articles

Peculiarities of Electronic Structure of Silicon–on–Insulator Structures and Their Interaction with Synchrotron Radiation

Structures with strained and unstrained silicon layers were studied by ultrasoft X − ray emission spectroscopy and X − ray absorption near edge structure spectroscopy with the use of synchrotron radiation techniques the SOI (silicon − on − insulator). Analysis of X − ray data has shown a noticeable transformation of the electron energy spectrum and local partial density of states distribution in valence and conduc- tion bands in the strained silicon layer of the SOI structure. USXES Si L 2,3 spectra analysis revealed a decrease of the distance between the L ′ 2v и L 1v points in the valence band of the strained silicon layer as well as a shift of the first two maxima of the XANES first derivation spectra to the higher energies with respect to conduction band bottom E c . At the same time the X − ray standing waves of synchrotron radiation ( λ ~ 12 − 20 nm) are formed in the silicon − on − insulator structure with and without strains of the silicon layer. Moreover the synchrotron radiation grazing angle θ changing by 2 ° leads to a change of the electromagnetic field phase to the opposite.

Perspective: Treating electron over-delocalization with the DFT+U method.

Many people in the materials science and solid-state community are familiar with the acronym "DFT+U." For those less familiar, this technique uses ideas from model Hamiltonians that permit the description of both metals and insulators to address problems of electron over-delocalization in practical implementations of density functional theory (DFT). Exchange-correlation functionals in DFT are often described as belonging to a hierarchical "Jacob's ladder" of increasing accuracy in moving from local to non-local descriptions of exchange and correlation. DFT+U is not on this "ladder" but rather acts as an "elevator" because it systematically tunes relative energetics, typically on a localized subshell (e.g., d or f electrons), regardless of the underlying functional employed. However, this tuning is based on a metric of the local electron density of the subshells being addressed, thus necessitating physical or chemical or intuition about the system of interest. I will provide a brief overview of the history of how DFT+U came to be starting from the origin of the Hubbard and Anderson model Hamiltonians. This history lesson is necessary because it permits us to make the connections between the "Hubbard U" and fundamental outstanding challenges in electronic structure theory, and it helps to explain why this method is so widely applied to transition-metal oxides and organometallic complexes alike.

PROPERTIES OF POLYCYCLIC AROMATIC HYDROCARBONS IN THE NORTHWEST PHOTON DOMINATED REGION OF NGC 7023. III. QUANTIFYING THE TRADITIONAL PROXY FOR PAH CHARGE AND ASSESSING ITS ROLE

Polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer/IRS spectral map of the northwest photon dominated region (PDR) in NGC 7023 is analyzed. Here, results from fitting the 5.2–14.5 μm spectrum at each pixel using exclusively PAH spectra from the NASA Ames PAH IR Spectroscopic Database (www.astrochem.org/pahdb/) and observed PAH band strength ratios, determined after isolating the PAH bands, are combined. This enables the first quantitative and spectrally consistent calibration of PAH charge proxies. Calibration is straightforward because the 6.2/11.2 μm PAH band strength ratio varies linearly with the ionized fraction (PAH ionization parameter) as determined from the intrinsic properties of the individual PAHs comprising the database. This, in turn, can be related to the local radiation field, electron density, and temperature. From these relations diagnostic templates are developed to deduce the PAH ionization fraction and astronomical environment in other objects. The commonly used 7.7/11.2 μm PAH band strength ratio fails as a charge proxy over a significant fraction of the nebula. The 11.2/12.7 μm PAH band strength ratio, commonly used as a PAH erosion indicator, is revealed to be a better tracer for PAH charge across NGC 7023. Attempting to calibrate the 12.7/11.2 μm PAH band strength ratio against the PAH hydrogen adjacency ratio (duo+trio)/solo is, unexpectedly, anti-correlated. This work both validates and extends the results from Paper I and Paper II.

Toward the Prediction of Water Exchange Rates in Magnetic Resonance Imaging Contrast Agents: A Density Functional Theory Study.

We present a theoretical investigation of Gd-Owater bonds in different complexes relevant as contrast agents in magnetic resonance imaging (MRI). The analysis of the Ln-Owater distances, electron density (ρBCP), and electron localization function (ELF) at the bond critical points of [Ln(DOTA)(H2O)](-) and [Ln(DTPA-BMA)(H2O)] indicates that the strength of the Ln-Owater bonds follows the order DTPA-BMA > DOTA (M isomer) > DOTA (m isomer). The ELF values decrease along the 4f period as the Ln-Owater bonds get shorter, in line with the labile capping bond phenomenon. Extension of these calculations to other Gd(3+) complexes allowed us to correlate the experimentally observed water exchange rates and the calculated ρBCP and ELF values. The water exchange reaction becomes faster as the Gd-Owater bonds are weakened, which is reflected in longer bond distances and lower values of ρBCP and ELF. DKH2 calculations show that the two coordinated water molecules may also have significantly different (17)O hyperfine coupling constants (HFCCs).

A high-performance white-light-emitting-diodes based on nano-single crystal divanadates quantum dots.

We report a high-performance phosphors-free white light-emitting-diodes (w-LEDs) using Ba2V2O7 or Sr2V2O7 quantum dots that directly heteroepitaxially grown on common quartz substrates by polymer assisted deposition (PAD). The quantum efficiency of quantum dots is as high as 95%. More importantly, electronic local functions, band structure and partial density of states have been firstly calculated to study the luminescent and heteroepitaxial growth mechanisms by the Ab-initio Simulation. Additionally, the glaring white light excited at a wavelength of 325 nm was experimentally observed, which unambiguously demonstrated that such quantum dots can be efficient w-LEDs for solid state lighting.

Theoretical investigations of sp–sp2 hybridized capped graphyne nanotubes

The aim of this paper is to demonstrate a new material of a capped graphyne nanotube with sp–sp2 hybridization. In order to prove the possible existence of this structure, frequency, binding energy and kinetic stability calculations are carried out by means of density functional theory (DFT) calculation and molecular dynamics (MD) confirmation. For the understanding of the geometric structure and electronic properties, C–C bond lengths of this structure are analyzed. The energy gap between the highest occupied molecular orbital (HOMO) state and the lowest unoccupied molecular orbital (LUMO) state is calculated, and electrostatic potential (ESP) derived charges and local electron density distributions of the capped graphyne nanotube are explored. In addition, the changing trend of energy gap with the increasing electric field is used to describe the field emission properties of the structure. Finally, we calculate work functions and ionization potentials under the different electric fields to estimate potential field emission properties directly.

The use of multi-element aerosol particles for determining temporal variations in temperature and electron density in laser-induced plasmas in support of quantitative laser-induced breakdown spectroscopy

Quantitative laser-induced breakdown spectroscopy (LIBS) analysis operates on the assumption that the sample is completely dissociated and diffused within the highly energetic plasma on time-scales of analyte analysis, resulting in analyte emission ideally at the bulk plasma temperature and a signal that is linear with analyte mass concentration. However, recent studies focusing on aerosol analysis have found the heat and mass diffusion rates within laser-induced plasmas to be finite, resulting in particle-rich, locally perturbed areas within the hot bulk plasma. The goal of this study is to observe any related plasma differences, by calculating the bulk and local (i.e. analyte rich regions) plasma temperatures and electron density, to better understand the time frame of equilibrium between the local and bulk plasma properties. This study also seeks to determine whether the presence of large quantities of a matrix element can significantly alter the local plasma conditions, thereby generating matrix effects. We report the temporal profiles of particle-derived species, adding additional insight into the effect of local perturbation of plasma properties, with the conclusion that significant plasma residence (tens of microseconds) is necessary to minimize such effects.

Mechanism of CO preferential oxidation catalyzed by Cu n Pt (n = 3–12): a DFT study

The CO preferential oxidation reaction (PROX) is particularly well suited for hydrogen purification for proton exchange membrane fuel cell applications. In this work, the mechanism of CO-PROX catalyzed by Cu n Pt (n = 3–12) clusters has been studied by density functional theory calculations. The calculated results indicate that the most favored adsorption site of H2 for all clusters is on the Pt sites, and O2 prefers to bind on Cu sites and CO bind on Pt sites. The lowest energy barrier for hydrogen dissociation is 0.02 eV. Smaller H–Pt bond length of Cu n PtH2 corresponds to larger H–H bond length. CO-PROX occurs via the main intermediates of COOH and OH. Cu6Pt is proposed as the most effective catalyst for CO-PROX. To understand the high catalytic activity of Cu n Pt clusters, the nature of the interaction between adsorbate and substrate is also analyzed by detailed electronic local density of states. These findings enrich applications of Cu-based materials to the field of high-activity catalysts.

Simulation study of one-dimensional self-organized pattern in an atmospheric-pressure dielectric barrier discharge

A two-dimensional fluid model is developed to simulate the one-dimensional self-organized patterns in an atmospheric-pressure dielectric barrier discharge (DBD) driven by sinusoidal voltage in argon. Under certain conditions, by changing applied voltage amplitude, the transversely uniform discharge can evolve into the patterned discharge and the varied self-organized patterned discharges with different numbers and arrangements of discharge channels can be observed. Similar to the uniform atmospheric-pressure DBD, the patterned discharge mode is found to undergo a transition from Townsend regime, sub-glow regime to glow regime with increasing applied voltage amplitude. In the different regimes, charged particles and electric field display different dynamical behaviors. If the voltage amplitude is increased over a certain value, the discharge enters an asymmetric patterned discharge mode, and then transforms into the spatially chaotic state with out-of-order discharge channels. The reason for forming the one-dimensional self-organized pattern is mainly due to the so-called activation-inhibition effect resulting from the local high electron density region appearing in discharge space. Electrode arrangement is the reason that induces local high electron density.

Universal statistical approach to radiative and collisional processes with multielectron ions in plasmas

The universal statistical approach for calculation of radiative and collisional processes with multielectron ions in plasmas is developed. It is based on the atomic structure representation similar to that used in a condensed medium. The distribution of local atomic electron density determines the set of elementary excitations with classical plasma frequency. The statistical method is tested by the calculations of the total electron impact single ionization cross-sections, ionization rates and radiative losses of various ions. In the coronal limit the radiative losses of heavy plasma impurities with any type of multielectron ions are determined by the excitation of collective atomic oscillations due to collisions with plasma electrons. It is shown that for low plasma densities the tungsten ions total radiative loss scatter within universal statistical approach does not exceed similar results of current complex numerical codes in the wide range of plasma temperatures. The general expression for the radiative losses in the case of the intermediate state between limiting cases of coronal and Boltzmann population distributions is derived as well. The total electron impact ionization cross-sections and ionization rates for ions of various charge stages for a wide range of elements from Ar to U are compared with experimental and conventional complex code data showing satisfactory agreement. As the universal statistical method operates in terms of collective excitations, it implicitly includes direct and indirect ionization processes.

Radiative transition, local field enhancement and energy transfer microcosmic mechanism of tellurite glasses containing Er3+, Yb3+ ions and Ag nanoparticles

Er3+-doped, Er3+/Yb3+ co-doped tellurite glass with and without Ag NPs were synthesized by melt-quenching method. The high resolution transmission electron microscopy (HR-TEM) and selected area electron diffractions (SAED) manifest growth of Ag NPs. The UV–vis–NIR absorption spectroscopy and fluorescence spectroscopy were measured. The optical band gap and multiphonon relaxation rate constants were calculated. The electronic band structure and local density of state (DOS) of Ag NPs are calculated. The fluorescence emission and enhancement mechanism including localized surface plasmon resonance (LSPR) and energy transfer (ET) microcosmic mechanism were discussed. The electric field distributions of Ag NPs are emulated by FDTD solutions software. Local field enhancement (LFE) induced by LSPR and lightning rod effect was found to be responsible for the fluorescence enhancement while energy transfer from Ag NPs to rare-earth was considered ignorable in the samples without photoluminescent emission.

Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces.

Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green's functions for finite geometries and electron plane wave expansion for periodic systems. The "particle in a box" picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

Oxidation state and interfacial effects on oxygen vacancies in tantalum pentoxide

First-principles density-functional theory calculations are used to study the atomistic structure, structural energetics, and electron density near the O monovacancy (VOn; n = 0,1+,2+) in both bulk, amorphous tantalum pentoxide (a-Ta2O5), and also at vacuum and metallic Ta interfaces. We calculate multivariate vacancy formation energies to evaluate stability as a function of oxidation state, distance from interface plane, and Fermi energy. VOn of all oxidation states preferentially segregates at both Ta and vacuum interfaces, where the metallic interface exhibits global formation energy minima. In a-Ta2O5, VO0 is characterized by structural contraction and electron density localization, while VO2+ promotes structural expansion and is depleted of electron density. In contrast, interfacial VO0 and VO2+ show nearly indistinguishable ionic and electronic signatures indicative of a reduced VO center. Interfacial VO2+ extracts electron density from metallic Ta, indicating that VO2+ is spontaneously reduced at the expense of the metal. This oxidation/reduction behavior suggests careful selection and processing of both oxide layer and metal electrodes for engineering memristor device operation.

Production rate enhancement of size-tunable silicon nanoparticles by temporally shaping femtosecond laser pulses in ethanol.

This paper proposes an efficient approach for production-rate enhancement and size reduction of silicon nanoparticles produced by femtosecond (fs) double-pulse ablation of silicon in ethanol. Compared with a single pulse, the production rate is ~2.6 times higher and the mean size of the NPs is reduced by ~1/5 with a delay of 2 ps. The abnormal enhancement in the production rate is obtained at pulse delays Δt > 200 fs. The production-rate enhancement is mainly attributed to high photon absorption efficiency. It is caused by an increase in localized transient electron density, which results from the first sub-pulse ionization of ethanol molecules before the second sub-pulse arrives. The phase-change mechanism at a critical point might reduce nanoparticle size.

Inclusion of effects of self-consistency of the electron density within the LDA + U + SO method implemented in the temperature Green’s function formalism in the basis of the Wannier functions

The LDA + U + SO method (local electron density approximation + Coulomb correlations taken into account in the static mean field limit + spin-orbit interaction) has been formulated and implemented in computer codes in the temperature Green’s function formalism in the basis of the Wannier functions. A formula for the approximate inclusion of the effect of self-consistency of the electron density on the parameters of the Hamiltonian has been proposed. It has been shown that the results obtained for NiO, GdNi2, Pu, and US by this method are in good agreement with the results obtained by methods with the complete self-consistency of the electron density.

Introducing PROFESS 3.0: An advanced program for orbital-free density functional theory molecular dynamics simulations

Orbital-free density functional theory (OFDFT) is a linear-scaling first-principles quantum mechanics method used to calculate the ground-state energy of a given system. Here we present a new version of PRinceton Orbital-Free Electronic Structure Software (PROFESS) with new features. First, PROFESS 3.0 provides a set of new kinetic energy density functionals (KEDFs) which are designed to model semiconductors or transition metals. Specifically, PROFESS 3.0 includes the Huang–Carter (HC) KEDF [1], a density decomposition method with fixed localized electronic density [2], the Wang–Govind–Carter (WGC) decomposition KEDF [3], and the Enhanced von Weizsacker (EvW)-WGC KEDF [4]. Other major new functions are included, such as molecular dynamics with different statistical mechanical ensembles and spin-polarized density optimizers.

On the mechanism of the preferential oxidation of carbon monoxide over Cu n Pd (n = 3–12) catalysts

Preferential oxidation of CO (CO-PROX) is an important practical process for the purification of H2 for use in proton exchange membrane fuel cells. In order to clarify the mechanism of CO-PROX, the reaction promoted by CunPd (n = 3–12) clusters has been studied by density functional theory calculations. The CO-PROX reaction via carboxylic and hydroxyl intermediates is found to be the most likely mechanism. The theoretical results show that CO oxidation is promoted by the attack of surface OH groups, which is different from the usual oxidation of CO with O2. The Cu6Pd cluster is predicted to have a lower activation barrier compared with other CunPd clusters and is therefore proposed as the most effective nanocatalyst. To gain insights into the high catalytic activity of the CunPd nanoparticles, the nature of the interaction between adsorbate and substrate has been analyzed by the detailed electronic local density of states. The results should be helpful in developing efficient catalysts for the CO-PROX reaction.

The molecular structure of methylfluoroisocyanato silane: a combined microwave spectral and theoretical study.

The structure of methylfluoroisocyanato silane (Me-SiHF-NCO) has been deduced by a combination of microwave (MW) spectra including data from (12,13)C, (14,15)N, and (28,29,30)Si isotopomers, and ab initio calculations. The rotational constants (RC) for the most abundant isotopes are A = 6301.415(45), B = 1535.078(39), and C = 1310.485(39) MHz. The symmetric quartic centrifugal distortion constants have been identified, using the I(r) representation for C1 symmetry, which includes the 3-fold rotor. The spectra of the isotopomer combinations gave a partial substitution structure where the C2Si3, Si3N4, and N4C9 bond lengths are 1.8427(70), 1.7086(77), and 1.2120(90)Å; although the C2Si3N4 angle is close to tetrahedral (109.71° (52)), the Si3N4C9 angle is wide (157.69° (18)). The rotational constants are only consistent with a trans-orientation for each of the dihedral angles (HC2Si3N4, C2Si3N4C9, and Si3N4C9O10). The structural analysis was completed by calculations of the equilibrium structure, using MP3 in conjunction with an aug-cc-pVTZ basis set (434 Cartesian basis functions). This gave A = 6240.324, B = 1518.489, and C = 1297.819 MHz. The equilibrium structure bond lengths for C2Si3, Si3N4, and N4C9 were 1.8485, 1.7147, and 1.1947 Å, with the C2Si3N4 and Si3N4C9 angles 109.55 and 156.67°, respectively. Although the SiNC polynomial bending surface is complex, the data points can be fit to the simple form V(x) = 50.36(91)x(4) - 7.53(44)x(5), with comparatively little loss of accuracy. The A-rotational constant is strongly influenced by the Si3N4C9 angle, and smaller bases lead to this angle being nearly linear. The theoretical results suggest a very heavily polarized molecule, which is supported by the positions of the local electron density minima within the bonds and electron density calculations.

Fast Shape-Based Global and Local Electron Density Map Search

The Electron Microscopy DataBank (EMDB) is growing rapidly, accumulating biological structural data obtained cryo-electron microscopy (cryo-EM). Cryo-EM is an emerging technique for determining large biomolecular complexes and subcellular structures. Together with the Protein Data Bank (PDB), EMDB is becoming a fundamental resource of the tertiary structures of biological macromolecules. To take full advantage of this indispensable resource, the ability to search the database by structural similarity is essential. However, unlike high-resolution structures stored in PDB, methods for comparing low-resolution EM density maps are not well established. Here, we developed a novel computational method for efficiently searching EM maps. The method uses a compact fingerprint representation of EM maps based on the 3D Zernike descriptor, which is a mathematical series expansion for representing isosurface shape of EM maps. The method was implemented in a web server, named EM-SURFER (http://kiharalab.org/em-surfer/), which allows users to search against the entire EMDB with over 2400 entries in a few seconds. By combing with map segmentation, the method can also identify corresponding local regions in EM maps. Examples of search results from different types of query structures are discussed. The unique capability of EM-SURFER to detect 3D shape similarity of low-resolution EM maps should prove invaluable in structural biology.

Scanning tunneling microscopy and spectroscopy at very low temperatures

During past decades, an increasing number of laboratories is using cryogenic scanning tunneling microscopy and spectroscopy (STM/S) to probe different kinds of electronic systems. Measurements in a dilution refrigerator are particularly useful to study superconductors, because temperatures of order of 100 mK are well below most critical temperatures and effectively reduce thermally excited quasiparticles. The local electronic density of states is then obtained at atomic level with a resolution in energy of some tens of μeV. Visualizing spatial variations of the local density of states allows characterizing vortex cores and the vortex lattice. Vortex core electronic features provide the anisotropy of the superconducting properties, and help understanding the influence of competing orders such as charge density waves. Here we will review results in dichalcogenide superconductors, in the magnetic borocarbide TmNi2B2C and in thin films, discussing in some detail a few relevant aspects of thermal depinning and melting in thin films.