The electronic structure of silver clusters

In this paper, the electronic structures of Ag(n) (n = 3-12) are studied by using Density Functional Theory with the Local Spin Density Approximation. The investigation shows that 4d electrons are very localized, while 5s electrons is very extensive in the cluster bonding state. 5s electrons are mainly responsible for the bonding, while 4d electrons are less. 5s electrons are primarily responsible for the properties of silver clusters, and 4d electrons have less but important influence on the properties due to an obvious hybridization between 4d and 5s. The d character in cluster properties becomes more and more obvious with the increase of cluster size.     

Electronic structure studies of nanoferrite Cu(x)Co(1-x)Fe2O4 by X-ray absorption spectroscopy

Pure and mixed cobalt copper ferrites are of great interest due to their widespread application in electronics and medicine. We report on the electronic structure of a nanoferrite Cu(x)Co(1-x)Fe2O4 (0.0 < or = x < or = 1.0) system studied by X-ray absorption spectroscopy. These magnetic nanoferrites (average crystallite size approximately 31-43 nm) were synthesized by an auto combustion method and are characterized by high resolution X-ray diffraction and near edge X-ray absorption fine structure measurements at the O K and Co, Cu, and Fe L-edges. The O K-edge spectra suggest that there is a strong hybridization between O 2p and 3d electrons of Co, Cu and Fe cations and Fe L3,2-edge spectra indicate that Fe ions coexist in mixed valence states (Fe3+ and Fe2+) at tetrahedral and octahedral sites of the spinel structure. Copper and cobalt ions are distributed in the divalent state in octahedral sites of the spinel structure. The origin of high saturation magnetization and coercivity in cobalt-copper ferrites are explained in light of these results.     

Room-temperature ferromagnetism in Cu doped GaN nanowires

We report magnetism in Cu doped single-crystalline GaN nanowires. The typical diameter and the length of the Ga1-xCuxN nanowires (x = 0.01, 0.024) are 10-100 nm and tens of micrometers, respectively. The saturation magnetic moments are measured to be higher than 0.86 microB/Cu at 300 K, and the Curie temperatures are far above room temperature. Anomalous X-ray scattering and X-ray diffraction measurement make it clear that Cu atoms substitute the Ga sites, and they largely take part in the wurtzite network of host GaN. X-ray absorption and X-ray magnetic circular dichroism spectra at Cu L(2,3) edges show that doped Cu has local magnetic moment and the electronic configuration of it is mainly 3d9 but mixed with a small portion of trivalent component. It seems that the ionocovalent bonding nature of Cu 3d orbital with surrounding semiconductor medium makes Cu atom a mixed electron configuration and local magnetic moments. These outcomes suggest that the Ga1-xCuxN system is a room-temperature ferromagnetic semiconductor.     

Excited-state absorption and anisotropy properties of two-photon absorbing fluorene derivatives

The electronic structure of fluorene derivatives N-(7-benzothiazol-2-yl-9,9-bis-decyl-9H-fluoren-2-yl)-acetamide (1); 9,9-didecyl-2,7-bis-(N,N-benzothiazoyl)fluorene (2); 4,4'-{[9,9-bis(ethyl)-9H-fluorene-2,7-diyl]di-2,1-ethenediyl}bis(N,N-diphenyl)benzeneamine (3); and 4,4',4"{[9,9-bis(ethyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}tris(N,N-diphenyl)benzeneamine (4) were investigated by a steady-state spectral technique, quantum-chemical calculations, and a picosecond pump-probe method. These derivatives are of interest for their relatively high two-photon absorption. The steady-state excitation anisotropy spectra reveal the nature of the ground-state absorption bands. Semiempirical quantum-chemical calculations of the fluorene derivatives (AM1, ZINDO/S) show good agreement with experimental data. The spectral positions and alignment of various electronic transitions of derivatives 1-4 were estimated from their excited-state absorption and anisotropy spectra.     

Quantum chemical treatment of nivalenol and its tautomers

Nivalenol, a highly poisonous mycotoxin, and its possible tautomers have been considered theoretically by RHF/6-31G/d,p) and B3LYP/6-31G(d,p) calculations together with a semi-empirical PM3 method. The calculations revealed that some of the tautomers are more stable and exothermic than nivalenol. The calculated IR spectra as well as some geometrical and physicochemical properties of the structures considered have been presented.     

An analytical model for calculating microdosimetric distributions from heavy ions in nanometer site targets

The analytical model of Xapsos used for calculating microdosimetric spectra is based on the observation that straggling of energy loss can be approximated by a log-normal distribution of energy deposition. The model was applied to calculate microdosimetric spectra in spherical targets of nanometer dimensions from heavy ions at energies between 0.3 and 500 MeV amu(-1). We recalculated the originally assumed 1/E(2) initial delta electrons spectrum by applying the Continuous Slowing Down Approximation for secondary electrons. We also modified the energy deposition from electrons of energy below 100 keV, taking into account the effective path length of the scattered electrons. Results of our model calculations agree favourably with results of Monte Carlo track structure simulations using MOCA-14 for light ions (Z = 1-8) of energy ranging from E = 0.3 to 10.0 MeV amu(-1) as well as with results of Nikjoo for a wall-less proportional counter (Z = 18).     

Ab initio study of electronic,magnetic and optical properties of CuWO4 tungstate

Ab initio calculations based on the density-functional theory have been employed to study the electronic and magnetic properties of copper tungstate CuWO₄, as well as its optical characteristics in the ultraviolet region, up to 40 eV. The electronic structure around the band gap is dominated by the O p- and the Cu d-states and it is quite different from the recent spin-restricted calculations of the same compound. The most stable antiferromagnetic state and the values of magnetic moments at Cu sites and O(3) oxygen atomic sites (closest to Cu²⁺ ions in zigzag antiferromagnetic chains) are in agreement with experiments. The gap is found to be indirect with the acceptable value only after use of the LDA + U rotationally invariant self-consistent full potential linearized augmented plane wave (FP LAPW) approach. The optical spectra are analyzed, compared, and interpreted in terms of calculated band structures. It is shown that absorption process involves significant energy flow from the O ions to the Cu ions.     

Study of Band Structure Properties of Pnictide LaO1−xF x FeAs (x = 0, 0.2) Superconducting Compound

This paper reports on the band structure properties and changes in band structure of fluorine-doped LaO_1?x F _x FeAs (x = 0, 0.2) compound, measured using X-ray photoemission spectroscopy (XPS). The band structure of the superconducting compound is compared with nonsuperconducting parent compound LaOFeAs. With fluorine doping, a shift of the shallow core level is observed in XPS spectra, which may be a response of the band structure due to fluorine doping in the system. The balance of the chemical potential shift with the screening effect of conduction electrons near the Fe and As ions is discussed using nearly unchanged Fe 2p and As 3d core-level spectra. The La 3d core-level spectra shift towards the high energy, ～0.36 eV, may be due to the chemical potential shift caused by fluorine doping. In our valence band spectra, a small peak at around 0.2 eV is observed, which disappeared with the fluorine doping in the system, indicating a change of Fe 3d state from low spin to high spin states and also confirming the nature of Fe 3d electrons as itinerant, which is responsible for superconductivity in these compounds.     

Structural determination of individual chemical species in a mixed system by iterative transformation factor analysis-based X-ray absorption spectroscopy combined with UV-visible absorption and quantum chemical calculation

A multitechnique approach using extended X-ray absorption fine structure (EXAFS) spectroscopy based on iterative transformation factor analysis (ITFA), UV-visible absorption spectroscopy, and density functional theory (DFT) calculations has been performed in order to investigate the speciation of uranium(VI) nitrate species in acetonitrile and to identify the complex structure of individual species in the system. UV-visible spectral titration suggests that there are four different species in the system, that is, pure solvated species, mono-, di-, and trinitrate species. The pure EXAFS spectra of these individual species are extracted by ITFA from the measured spectral mixtures on the basis of the speciation distribution profile calculated from the UV-visible data. Data analysis of the extracted EXAFS spectra, with the help of DFT calculations, reveals the most probable complex structures of the individual species. The pure solvated species corresponds to a uranyl hydrate complex with an equatorial coordination number (CNeq) of 5, [UO2(H2O)5]2+. Nitrate ions tend to coordinate to the uranyl(VI) ion in a bidentate fashion rather than a unidentate one in acetonitrile for all the nitrate species. The mononitrate species forms the complex of [UO2(H2O)3NO3]+ with a CNeq value of 5, while the di- and trinitrate species have a CNeq value of 6, corresponding to [UO2(H2O)2(NO3)2]0 (D2h) and [UO2(NO3)3]- (D3h), respectively.     

Direct Observation and Interpretation of Carrier Dynamics of Molecular Magnetic Superconductors

The temperature-dependent time-resolved reflectance spectra of the organic superconductors (OSs) κ- and λ-(BETS)₂MCl₄ (M=Fe, Ga) and κ-(ET)₂Cu[N(CN)₂]Br are reported. The π electrons of the organic molecules (BETS or ET) are responsible for the electrical conduction in these salts, while only the Fe-containing salts have localized spins on the d electrons in the Fe atoms (S=5/2) with π–d interaction. Analysis of the spectra provided the relaxation time τ of the carriers, as well as information concerning the number of carriers at the Fermi energy N(E _F). The carrier dynamics were classified in terms of high-T _C and low-T _C OSs. In these salts, the results indicate that (1) the electron-phonon interaction is important in the occurrence and stability of the superconducting phase, and (2) electron correlation can be related to the enhancement of T _C.     

Spin‐Polarized Semiconductors: Tuning the Electronic Structure of Graphene by Introducing a Regular Pattern of sp3 Carbons on the Graphene Plane

First‐principles calculations (generalized gradient approximation, density functional therory (DFT) with dispersion corrections, and DFT plus local atomic potential) are carried out on the stability and electronic structures of superlattice configurations of nitrophenyl diazonium functionalized graphene with different coverage. In the calculations, the stabilities of these structures are strengthened significantly since van der Waals interactions between nitrophenyl groups are taken into account. Furthermore, spin‐polarized and wider‐bandgap electronic structures are obtained when the nitrophenyl groups break the sublattice symmetry of the graphene. The unpaired quasi‐localized p electrons are responsible for this itinerant magnetism. The results provide a novel approach to tune graphene's electronic structures as well as to form ferromagnetic semiconductive graphene.     

Magnetic excitations of rare Earth atoms and clusters on metallic surfaces

Magnetic anisotropy and magnetization dynamics of rare earth Gd atoms and dimers on Pt(111) and Cu(111) were investigated with inelastic tunneling spectroscopy. The spin excitation spectra reveal that giant magnetic anisotropies and lifetimes of the excited states of Gd are nearly independent of the supporting surfaces and the cluster size. In combination with theoretical calculations, we argue that the observed features are caused by strongly localized character of 4f electrons in Gd atoms and clusters.     

四面体掺杂尖晶石结构Co1-xRexCr2O4(Re=Li、Na、K、Rb)的电子结构和光学性质

采用基于密度泛函理论(DFT)的平面波超软赝势方法, 计算了CoCr₂O₄及Li、Na、K和Rb四面体掺杂CoCr₂O₄的基态结构、电子结构和光学性质。计算结果表明: 一价离子四面体掺杂都导致晶格有微小的畸变, 使体系的稳定性降低, Rb掺杂的体系最稳定; 电子态密度的计算结果表明: 掺杂体系的导带主要有Co-3d和Cr-3d轨道电子构成, 掺杂离子改变了CoCr₂O₄导带的电子结构, 主要引起了导带Co-3d态密度峰的下移, 随着掺杂浓度的增大, 费米能级进入价带更深; 光学性质计算表明: 掺杂体系的吸收光谱发生红移, 并在低能区有很强的吸收, 表明掺杂能极大地提高CoCr₂O₄对可见光的吸收和光催化效率。     

First-principles study of small oxidized silver clusters

The structure and stability of clusters Ag(n)O+(m), Ag(n)O(m), and Ag(n)O(m)- (n = 1-6, m = 1-2) are studied by using the first-principles method. Calculated results show that the properties studied are strongly dependent on the size and charge state of the clusters, some of which show the odd-even alteration. Generally, the oxidation drives the transition of silver clusters from two-dimensional (2D) to three-dimensional (3D) structures. It is found that the structure with dissociated O2 is more stable if the silver cluster can provide oxygen with enough electrons.     

Assembling nanowires from Mo-S clusters and effects of iodine doping on electronic structure

Using ab initio calculations, we find high stability of octahedral Mo6S8 clusters, which can further be condensed to form Mo3nS3n+2 (n, an integer) nanowires. These linear structures are energetically more favorable compared with other closed-packed polyhedral isomers of Mo-S clusters. The octahedral units in nanowires are stabilized by strong Mo-Mo interactions and p-d hybridization between Mo 4d and S 2p orbitals. There is a free electron-like band that crosses the Fermi energy in infinite nanowires and leads to their metallic character. Iodine doping acts as electron donor and can be used to tailor the electronic conductivity. For Mo12S8I4 nanowires, both electrons and holes are found to contribute to conduction. These nanowires are energetically more favorable than the experimentally obtained Mo12S6I12 nanowires.     

A 2-level anisotropic electronic system in superfluid 4He

No Heading Theory for treating a two-level electronic system in superfluid helium is described. Numerical density functional theory calculations in 3-D are then used to calculate electronic absorption spectra for both spherical and angularly anisotropic chromophores. The results can be used in rationalizing the experimentally observed absorption spectra in small helium droplets. It is shown that anisotropic with the liquid does not lead to considerable changes in the linear absorption spectrum when compared to a purely spherical case.PACS numbers: 33.20 Kf, 33.70 Jg     

Oscillation of absorption bands of Zn(1-x)Mn(x)S clusters: an experimental and theoretical study

Electroporation of synthetic vesicles is utilized for the preparation of molecular size uncapped Zn(1-x)Mn(x)S clusters. The absence of caps permits (i) continued growth of the Zn(1-x)Mn(x)S clusters formed, (ii) the assessment of their true absorption spectra unaltered by stabilizing ligands, and (iii) the previously inaccessible live observation of the growth of the clusters in the molecular size regime. Upon cluster growth, the UV spectra exhibit novel, time-dependent, oscillation of red and blue shifts of the characteristic absorption band. The structure and electronic properties of Zn(N-1)MnS(N) clusters with N = 1-9 are calculated using the first-principles DMol(3) package. On the basis of similarities between the oscillating trend of the experimentally observed absorption spectra and that of the calculated highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of Zn(N-1)MnS(N) clusters with N = 1-9, the wavelengths of the sequential spectral peaks can be assigned to Zn(2)MnS(3), Zn(3)MnS(4), Zn(4)MnS(5), Zn(6)MnS(7), and Zn(8)MnS(9), respectively. Our results demonstrate that both the cluster size and the composition can be used to tune the optical properties.     

X-ray absorption fine structure combined with X-ray fluorescence spectrometry. Improvement of spectral resolution at the absorption edges of 9-29 keV

X-ray absorption near-edge structure (XANES) suffers from core-hole lifetime broadening at a higher energy absorption edge, such as Sn K (29 keV, Gamma(K) = 8.49 eV). To overcome this problem, emitted Sn Kalpha1 fluorescence from sample was counted using high-energy-resolution fluorescence spectrometer in the XANES measurements. Experimental energy resolution (5.0 eV) was consistent with theoretical values based on the Rowland configuration of the spectrometer. The absorption edge became steeper compared to conventional spectra. The white-line peak due to Sn(II) species became remarkably sharper and more intense in the Sn Kalpha1-detecting Sn K-edge XANES for Pt-Sn/SiO2. To support the semiclassical theory of resonant Raman scattering for the explanation of observed elimination of lifetime width, more resolved XANES data at Cu K, Pb L3, and Sn K in this work were convoluted (filtered) with a Lorentzian of each core-hole lifetime width. The processed data resembled generally well corresponding XANES spectrum measured in transmission mode. The verification based on ab initio XANES calculations was also performed.     

Angular dependence of the coherent peak position in the polarization bremsstrahlung spectrum of relativistic electrons in polycrystalline targets

The spectra of the polarization bremsstrahlung (PB) in the X-ray range induced by 7-MeV electrons in polycrystalline Al, Cu, and Ni polycrystalline films have been measured and the angular dependences of the PB characteristics have been studied. The experimental data agree well with the results of theoretical calculations and show good prospects for the development of a new method of diagnostics of polycrystalline materials based on the measurement of characteristics of the coherent PB component.