Cu-N共掺杂ZnO体系的稳定性,电子结构和磁性的第一性原理研究

采用第一性原理框架下的全势缀加平面波方法研究了Cu-N共掺杂的ZnO体系的电子结构和磁学性能.基于总能的计算表明,Cu-N共掺杂体系在基态下具有稳定的铁磁性,这个铁磁性的起源可以利用双交换理论进行解释.同时,电子结构的计算表明,掺杂体系在基态下呈现的是半金属特性.在Zn1-xCuxO1-yNy (x=0.0625, y=0.0625)中体系的总的磁矩为2 μB.这些磁矩主要来源于Cu-3d电子和O-2p电子以及N-2p电子的相互作用.     

Investigation of the density of localized states in a-Si using the field effect technique

The electronic properties of amorphous solids are largely determined by the distribution of localized states $\text{N}$ (?) in the mobility gap. In this paper, the field effect technique is applied to the experimental study of $\text{N}$ (?) in specimens of a-Si prepared by the glow discharge method and by vacuum evaporation. The experimental approach and the analysis of the results are discussed in some detail. $\text{N}$ (?) curves, extending over an energy range of up to 0.5 eV have been obtained for a series of glow discharge specimens, deposited at substrate temperatures between 310 and 570 K. The results show structure in the gap states, a well-defined minimum almost in the centre of the mobility gap and a rapid rise in $\text{N}$ (?), 0.18 eV below ?_c, which is identified with the onset of band tail states. The field effect data confirm that the predominant conduction mechanism at room temperature changes from hole hopping to transport in extended electron states, as the Fermi level is moved through the minimum in $\text{N}$ (?) The effects of annealing on the state distribution have been investigated, showing that $\text{N}$ (?_f) can be reduced by one or two orders of magnitude. The nature of the gap states is discussed and the divacancy is suggested as a basic model for the electronic states involved.     

First-principles study on structural and electronic properties of Prussian blue cathode material for sodium-ion battery

Abstract

Prussian blue, Fe[Fe(CN)₆] currently attracts a huge attention as a promising material in the application of large-scale energy storage because of its cost-effective and environmental friendly material. Besides, open framework structure and stability are the main causes for PB performance. In this study, effects of sodium cation insertion/deinsertion toward the structural and electronic properties were analyzed. The use of Hubbard U method successfully delivered a good approximation on electronic properties of the transition-metal ions in PB. The calculated band gap of 1.84?eV was in good agreement with theoretical and experimental results. Upon the insertion of the sodium cation, volume of the cathode material expanded between 2% and 4% showing that this cathode material has good cycle retention. From partial density of states, Fe 3d dominated the conduction and valence band. Furthermore, the redox reaction mechanism of Prussian blue can also be depicted. The voltage obtained from energy calculation for the first and second insertion of cation was 3.32 and 3.42?V, respectively.     

Resonance Raman Scattering of Br2 Doped Single-Walled Carbon Nanotube Bundles

Abstract

We have measured Raman spectra of bromine doped single-walled carbon nanotubes (SWNTs) using various laser lines to clarify the electronic states of the doped SWNT. In the case of evacuated sample after full doping, two breathing mode peaks were observed simultaneously by visible laser excitations. We assigned the higher frequency peak to the doped SWNT bundles, and the other peak to the undoped portions in the sample. Intensity ratio between them decreased with decreasing excitation energy, and in the infrared region, the breathing mode band of the doped bundle was not observed. These results can be explained by a simple rigid band model.     

Magnetic Properties Near the Ferromagnetic-Nonferromagnetic Phase Boundary in Potassium Clusters Incorporated into Zeolite LTA

Abstract

Magnetic and optical properties are investigated for K clusters incorporated into zeolite LTA at loading densities of K atoms, n, between 1.0 and 3.2 per cluster. No magnetic ordering is observed at n < 2. The spontaneous magnetization due to ferromagnetism is observed suddenly at n > 2. It is confirmed that the 1p-like quantum electronic state of K cluster plays an essential role in the ferromagnetic phase.     

Electronic Structure of Some Possible Organic Ferromagnets

A semiempirical method has been used to study the electronic structures of some π-bonded purely organic materials that are either known or proposed to have multiplet ground states. These materials are the precursors of organic ferromagnets. Narrow energy bands formed from the radical π-orbitals of these materials are necessary for the π-electrons to be unpaired. Favorable topological arrangements of atoms in these materials are needed to enable the spins of these electrons to align parallel, and hence yield macroscopic ferromagnetic ordering. Following ideas based on these studies, a new model for a possible organic ferromagnet is proposed and studied.     

Evidence of hopping of charge carriers in the clustered state of manganites

The electrical resistivity and magnetization measurements as a function of both temperature and magnetic field provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered in the La_0.55Y_0.15Ca_0.3MnO₃ compound. The intermediated-T regime, called clustered state, involves the presence of both ferromagnetic and paramagnetic phases and is shown to be closely related to the colossal magnetoresistivity effect. An extra hopping conducting process is proposed to exist in this clustered state. We use both the Abeles’ model and microstructural parameters to estimate the hopping activation energy between ferromagnetic clusters. The size (density) temperature dependence and the shape of the ferromagnetic metallic clusters are revealed to play a major role in the clustered state of this manganite.     

Structural,electrochemical and optical properties of 4′-n-alkyl-4-cyanobiphenyl (nCB) – A computational approach

The structures of nCB (n = 6 & 7 where n is the number of carbon atoms in the alkyl chain) have been optimized using the Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6–31G+(d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The photo sensitivity of liquid crystalline alkyl cyanobiphenyl has been presented on ultraviolet (UV) absorption based approach through Density functional theory (DFT) calculations. The structural and electrochemical properties such as HOMO (H), LUMO (L), and energy gap (Eg = E_L – E_H) have been investigated. A comparison of dimers during the different modes of interactions suggests an absorption maxima at longer wavelength for 7CB, indicating the high photo sensitivity. Further, the 6CB dimers exhibit a lower band gap; hence its conductivity is high in comparison with the 7CB dimers.     

Raman study of lattice dynamics in quasicrystals

We present the first Raman investigation of icosahedral quasicrystals. Broad structured bands in the energy range up to ~500 cm^− 1 have been observed in a series of AlCuFe, AlPdMn and AlPdRe systems. Original information on the vibrational density of states g(ω) was obtained for AlPdRe; for AlCuFe and AlPdMn estimated g(ω) shows a good agreement with the previous neutron results, but demonstrates finer structure. Strong increase in the parameter of electron-vibrational coupling for the low-energy vibrations and its correlation with changes in electronic conductivity has been observed in the series from AlCuFe to AlPdRe. This suggests the increase of the degree of localization for these vibrational excitations and involved electronic states.     

New Transport Mechanisms for Conducting Polymers: The Role of Disorder

Abstract

A unified view of the metallic state in polyaniline and heavily-doped polacetylene is presented. We first consider a single randomly-protonated strand of the emeraldine form of polyaniline. We show explicitly that the disorder inherent in this system is described by the random-dimer model of Dunlap, Wu and Phillips¹. The random dimer model is simply a tight-binding model for a binary alloy in which pairs of lattice sites are assigned one of two values at random. The random dimer model is shown to possess a narrow band of conducting states that can ultimately lead to dramatic increases in the conductivity if the Fermi level is appropriately tuned. It is demonstrated explicitly that the location in the energy band of the conducting states of the random-dimer model for polyaniline coincides with recent calculations of the location of the Fermi level in the protonated form of the polymer^2,3. We argue then that the random-dimer model is capable of explaining the insulator-metal transition in polyaniline. In the context of polyacetylene, we propose a ‘dirty metal’ picture for the metallic state which is consistent with both the closing of the band gap and the existence of a band of extended states. Our model is based on the observation that a random distribution of solitons closes the band gap at ～5-6% doping and is formally equivalent to the random dimer model. The location of the extended states in the disordered soliton lattice is computed. It is shown that because these states lie in the vicinity of the band edge, they are capable of explaining the sudden onset of the experimentally-observed Pauli susceptibility and the subsequent depinning of the solitons in the metallic state. We close by noting that because the random dimer model applies to any lattice in which the defects are extended and possess a plane of symmetry, it quite generally describes any polymeric system in which the stable defects are solitonic or bi-polaronic in nature     

Distribution of electronic states in amorphous Cd-As thin films on the basis of optical measurements

Transmission and fundamental reflectivity studies, completed on amorphous Cd-As thin films, allowed us to obtain parameters describing the fundamental absorption edge, i.e. the optical pseudogap E_G^opt, Urbach energy E_U and exponential edge parameter E_T. All these data, together with the results of earlier transport measurements, have been utilized in developing simple models of electronic structure (distribution of electronic states) for amorphous Cd-As thin films of various compositions.     

Quantum chemical investigation on complexation of palladium with iminopyridyl ligands: Structural,thermochemical, and electronic aspects

The main purpose of this research is to investigate computationally the structural, thermochemical and electronic properties in complexation process of dichloride {N-[(5-methylthiophen-2-yl)methylidene]?2-(pyridine-2-yl)ethanamine-κ²N,N′}palladium(II) complex. In the first step, we have concentrated on comparative survey of ability of density functional theory (DFT) and also semi-empirical approaches to reproduce the crystal structure of palladium(II) complex. Comparison of our calculated structural parameters of aforementioned complex with the available crystallographical data reveals that both functionals (B3LYP and M06) can well-reproduce x-ray structure of the complex with a near accuracy while PM6-D2 semi-empirical calculated values are not in a reliable agreement with the crystallographical data.

In the next step, we have shown the thermodynamical superiority in using THF as a polar solventin complexation reaction via polarized continuum model (PCM) computations which is in confirmation with experimental observations. Additionally, the bond orders of some selected key bonds in C₁₃H₁₄N₂Sligand andPdCl₂(C₁₃H₁₄N₂S) complex have been evaluated comparatively to analyze the electronic behavior of coordination.

Finally, we focused on topological analysis of electron density function via quantum theory of atoms in molecules (QTAIM) approach to explore the strength and nature of metal-ligand interactions on bond and ring critical points (BCPs).Strictly speaking, QTAIM calculations have been performed to determine the electronic density, its Laplacian and other electronic energy density indicators on some key BCPs to interpret the electronic features of complexation.     

Coherent transport in high performance double-spacer magnetic junctions

Spin-resonant tunneling mediated by quantized interlayer states has been studied using the tight-binding model for a composite magnetic junction with a thin non-magnetic metallic spacer, coherently inserted between the tunnel barrier and one of the ferromagnetic electrodes (F/I/N/F). An explicit analytical formula for the quantum transmission coefficient is derived and used for calculation of tunnel magnetoresistance (TMR) ratio in the Landauer formalism. The existence of strong magnetoresistance peaks is shown either at varying the on-site atomic energy and the width (number of atomic planes) of the N-spacer. The strongest TMR enhancement in the shallow band regime suggests a search for new spacer materials to optimize the device performance.     

Localized Excitations in Competing Bond-Order-Wave,Charge-Density-Wave and Spin-Density Wave Systems

Abstract

The localized excitations in the competing bond-order-wave(BOW), charge-density-wave(CDW), and spin-density-wave(SDW) systems are investigated within the Bogoliubov-de Gennes formalism using an extended Peierls-Hubbard model. An extensive study of localized excitations over a wide range of the on-site e-ph coupling λ₂ and the Hubbard interaction U leads to the following observations: (a) As λ₂ increases at fixed U, the number of bound states inside the gap changes from two to four for the STE case and from two to three for the polaron case. (b) A non-monotonic dependence of the lattice relaxation energy on λ₂ is predicted within the lattice relaxation approach developed by two of us earlier, and is attributed to a crossover from the weak-coupling to strong-coupling behavior showing up as the emergence of new bound states inside the gap. Moreover, the non-radiative transition rate of STE is also calculated and is used to tentatively interpret the very short life-time of STE in PtCl complexes.     

Interpretation of Anomalous Absorption Spectra. A Theoretical Study of the Geometric,Electronic and Optical Properties of Poly[3-(4-Octylphenyl)-Thiophene]

Abstract

The geometric, electronic, and optical properties of poly-[3-(4-octylphenyl)-thiophene] are studied. The effect of the side groups as well as doping is analyzed. Band structures, oscillator strengths and optical absorption spectra are calculated. Absorption spectra reported for the doped polymer show up to four peaks below the band-gap energy.     

Excitation Photon Energy Dependent Relaxation Processes of the Photoexcited States in Wide Gap MX-Chain Complexes

Abstract

Photo-induced absorption (PA) spectra of [Pt (en)₂] [Pt (en)₂X₂] (SO₂)₂. 6H₂O (X=Cl and Br) have been measured. Large excitation photon energy dependent changes were found in the features of the PA spectra. The excitation photon energy dependences are considered to be due to the difference between the relaxation processes of the charge transfer excitons and those of the higher energy excited states. Several new PA bands have been found.     

Application of High-Sensitivity UV photoemission Spectroscopy to Examine the Electronic Structure of Thermophilic Rhodopsin

Abstract

The electronic structures of materials such as HOMO and LUMO are essential information to understand their functions. Photoemission spectroscopy, which is the most standard technique to examine the electronic structures of various materials, has been not well applied to biomolecules mostly due to sample charging and poor sensitivity. Very recently, we have developed high-sensitivity photoemission spectroscopy (HS-PES) using low photon energy. In this study, HS-PES has been successfully applied to a thermophilic rhodopsin (TR) film. The high sensitivity of our technique enabled to observe the HOMO region of retinal part in TR without any problem due to sample charging. This technique is expected to explore the electronic structures of various proteins.     

Magnetic properties and sensitized visible and NIR luminescence of DyIII and EuIII coordination polymers by energy transfer antenna ligands

Two coordination polymers, {[Ln(2-stp)(4,4′-bipy)(H₂O)].(H₂O)}, [Ln = Dy (1) and Eu (2), 2-stp = 2-sulfoterephthalate and 4,4′-bipy = 4,4′-bipyridine] have been characterized by solid state UV-vis, FTIR spectra, X-ray single crystal diffraction and solid state photoluminescence. Variable-temperature magnetic susceptibility and isothermal magnetization as function of external magnetic field is also studied for both complexes. After ligand-mediated excitation, both complexes show the characteristic visible and NIR luminescence of the corresponding Ln^III ions (Ln = Dy, Eu) which is due to efficient energy transfer from the ligands to the central Ln^III ions via an antenna effect. The indirect energy transfer in both complexes has been investigated and discussed in detail.     

Paramagnons in the PrNi system with an induced magnetic moment

The recent studies of the PrNi system have shown that, at a temperature slightly higher than the ferromagnetic ordering temperature (T _C ～ 21 K), softening of some part of magnetic excitations occurs near the Γ point of the Brillouin zone. The magnetic modes observed have been qualitatively described by a model taking into account the crystal field and the exchang interaction within the mean-field random-phase approximation. To refine the model parameters and obtain a complete set of excitation modes, inelastic neutron scattering measurements on PrNi single crystals and polycrystals have been performed at T = 23 K. An acoustic branch related to the level with high excitation energy has been observed for the first time. The character of its dispersion suggests that the low-energy acoustic mode is responsible for the magnetic phase transition.     

The temperature dependence f photoconductivity in a-Si

The photoconductivity and its dependence on light intensity have been investigated in a-Si as a function of temperature between 100 and 500 K. In most experiments a photon energy of 2 eV was used. Specimens were deposited on to a substrate held at a temperature between 300 and 600 K by the r.f. decomposition of silane. Graphs of the photocurrent versus 1/T show a photoconductive maximum and the general features of the curves are similar to those found for the chalcogenide glasses. The main emphasis of the paper lies in the interpretation of the results in the light of the information on transport properties and the density of state distribution obtained from drift mobility and field effect experiments. It is shown that recombination takes place predominantly between two groups of localized states, which have been identified in the previous work. The initial state at ?_A is situated about 0.18 eV below ge_C in the electron tail states, the final state lies in a density of state maximum, 0.4 eV above ?_V. Above about 250 K, the photocurrent is carried by electrons in extended states, but below this temperature transport is by phonon assisted hopping through states near ?_A. A recombination process involving two states of a structural defect centre is discussed on the basis of the results and appears to be a feasible interpretation.