X-ray and electron spectroscopy investigation of the core-shell nanowires of ZnO:Mn

ZnO/ZnO:Mn core-shell nanowires were studied by means of X-ray absorption spectroscopy of the Mn K- and L_2,3-edges and electron energy loss spectroscopy of the O K-edge. The combination of conventional X-ray and nanofocused electron spectroscopies together with advanced theoretical analysis turned out to be fruitful for the clear identification of the Mn phase in the volume of the core-shell structures. Theoretical simulations of spectra, performed using the full-potential linear augmented plane wave approach, confirm that the shell of the nanowires, grown by the pulsed laser deposition method, is a real dilute magnetic semiconductor with Mn²⁺ atoms at the Zn sites, while the core is pure ZnO.     

Recent developments in high energy and spatial resolution analysis of polymers by XPS

Historically, obtaining useful XPS spectra from polymers has been perceived to be difficult. Problems have included low signal intensities from monochromatic sources, poor spatial resolution, and difficulties with charge compensation. Recent improvements in XPS instrumentation now make it possible to routinely analyze samples which previously would have been considered challenging or impossible. This is particularly true for multi-component polymer samples, which can be difficult to characterize using only large-area spectroscopy. The status of current XPS instrumentation is overviewed, with a variety of examples from polymer analysis. Examples include the use of imaging XPS in the analysis of PVC/PMMA blends, quantification using the valence band region, chemical derivatization of PVMK using hydrazine and trifluorophenylmethyl hydrazine, and high spatial resolution analysis of fiber mats. The importance of high spatial resolution analyses in the evaluation of heterogeneous samples is discussed.     

石墨烯能带中的重叠矩阵效应:Tight-Binding方法在模拟中的研究

采用紧束缚方法计算了石墨烯的价带(π)和导带(π*),考虑了非正交基矢下重叠矩阵效应,重叠积分参量s越小,导带越靠近费米面,而价带越远离费米面.在重叠积分参量s≤0.1时,基本保持了原子在实际空间中重叠所引起的能带的改变,太大(s=0.4)则会导致物理上失效.计算了石墨烯的能态密度,在费米面ε=0处(对应Dirac点)的能态密度为零,并且在Dirac点附近呈线性变化.     

Surface structure of sphalerite studied by medium energy ion scattering and XPS

The reactivity of high-Fe containing sphalerite (Zn_1−xFe_xS), the major source of Zn, is of great interest for industrial applications. Since the initial reactivity depends on the physical and chemical properties of the surface, it is important to understand the structure of cleaved and fractured surfaces. Zn_1−xFe_xS zincblende (1 1 0) oriented samples cleaved in air and in vacuum were studied with medium energy ion scattering (MEIS) in order to study surface relaxation and reconstruction associated with the possible formation of S dimers. The experimental results are presented together with ion scattering Monte Carlo simulations that have been performed using the different models of the surface structure. The MEIS blocking patterns are different for the air- and vacuum-cleaved specimens. Models for the air-cleaved samples found S atoms in the first layer that are relaxed outwards by 0.08 Å and Zn(Fe) atoms relaxed inwards by 0.51 Å, with some lateral translation of both species. Results for the vacuum-cleaved sample indicate S atoms have been displaced laterally by 0.5 Å at the surface. X-ray photoelectron spectroscopic (XPS) measurements provide evidence for a high binding energy species indicative of S-S bonds in the near-surface region that are consistent with the ion scattering structural data for both cleaving protocols.     

YNi4Cu: XPS measurements and electronic structure calculation

The valence band and the core levels of the YNi₄Cu compound are studied using the X-ray photoemission spectroscopy. The valence band is compared with the theoretical calculation by the spin-polarized Tight Binding Linear Muffin Tin Orbital method. The dominance of the Ni 3d and Cu 3d states down to 5 eV below the Fermi level is found. The modification of the bands’ widths and positions can be well explained by the d–d repulsion model. The Ni 2p and Y 3d peaks resemble the results for pure metals.     

A study of the ultrafast optical response of magnetized GaAs/AlGaAs quantum wire structures

We have studied analytically the ultrafast optical response of GaAs/AlGaAs quantum wire subjected to a moderately strong transverse magnetic field. The energy dispersion relations have been numerically calculated and show a significant deviation from parabolic behaviour as the magnetic field is increased. The effective semiconductor Bloch equation technique is used to calculate the induced polarization and differential transmission spectra in the quantum wire. The calculated induced polarization is used to study the optical coherent transient phenomenon of optical nutation. The analysis demonstrates that the magnetic field effectively alters the optical response of the semiconductor quantum wire nanostructures. It is observed that the nutating signal frequency enhances with an increasing magnetic field. The results are useful to explain magnetic field effects on the transient optical properties of semiconductor nanostructures.     

Comparison of the background corrected valence band XPS spectra of Fe and Co aluminides and silicides with their electronic structures

The background corrected valence band XPS spectra and the electronic structures of FeAl, FeSi, CoAl and CoSi were studied. Clean surfaces of the polycrystalline samples were obtained by in situ fracturing of the samples in an XPS spectrometer. The energy loss parts of the Fe 2p, Co 2p and valence band spectra were removed by the deconvolution method using Al 2s or Si 2s spectra as response functions. CoAl exhibited a satellite peak in the Co 2p region, but the other compounds had no clear satellite peaks in the Co 2p and Fe 2p regions. The experimentally background corrected valence band spectra were compared with the calculated spectra using the first-principle band calculation. There were large discrepancies between the spectra above the binding energy of 5 eV. These indicated that the experimental spectra could not be explained by the electronic structures of the ground states alone.     

Electronic structure of CeF3 and TbF3 by valence-band XPS and theory

Electronic structures of the rare earth trifluorides CeF₃ () and TbF₃ (Pnma) were examined by high-resolution valence-band X-ray photoelectron spectroscopy (VB-XPS) and all-electron periodic-crystal DFT theory including the spin-polarization (SP) combined with spin-orbit (SO) coupling using a second-variational treatment. Calculations using the Perdew-Burke-Ernzerhof (PBE) functional and the LDA+U method were carried out and compared. The results show that a complete analysis does require a full DFT-SP-SO treatment to obtain a quantitative account for the observed VB-XPS spectra, with an additional insight of the theory with regard to the nature of the topmost orbitals, and the bonding-antibonding character of orbitals within the VB and sub-VB levels. The band structure at the bottom of the conduction band (BCB) shows a strong dispersion in TbF₃ but not in CeF₃, predicting photoconductivity in TbF₃.     

Ab-initio investigation of Ni(Fe)/ZrO2(0 0 1) and Ni-Fe/ZrO2(0 0 1) interfaces

The atomic and electronic structures of Me/ZrO₂(0 0 1) interfaces, where Me is Ni, Fe or a Ni-Fe alloy, are investigated by the plane wave pseudopotential method within density-functional theory. The work of separation of metal films from oxide substrate for the O- and Zr-terminated Me/ZrO₂(0 0 1) interfaces is calculated. High adhesion at both Me/(ZrO₂)_O and Me/(ZrO₂)_Zr interfaces is found. The effect of oxygen vacancies on the adhesion at the metal-ceramic interfaces is also investigated. It is shown that Ni(Fe)-O interaction at the O-terminated interface weakens in the presence of interfacial oxygen vacancies. At interfaces with Ni-Fe alloys the adhesion depends strongly on the composition of the interfacial layers and their magnetic properties.     

Superconducting interaction charge in thallium-based high-TC cuprates: Roles of cation oxidation state and electronegativity

Superconductivity in the Tl-based cuprates encompasses a notably broad range of measured optimal transition temperatures T_C0, ranging from lowest in the charge-depleted Tl-1201 compounds (Tl_1–x(Ba/Sr)_1+yLa_1–yCuO_5–δ), such as Tl_0.7LaSrCuO₅ (37 K) and TlBa_1.2La_0.8CuO₅ (45.4 K), to highest in the Tl-1223 compound TlBa₂Ca₂Cu₃O_9±δ (133.5 K). Seven Tl-based cuprates are considered and compared using the model of superconductive pairing via electronic interactions between two physically separated charge reservoirs, where T_C0∝(ση/A)^1/2ζ⁻¹ is determined by the superconducting interaction charge fraction σ, the number η of CuO₂ layers, and the basal-plane area A, each per formula unit, and the transverse distance ζ between interacting layers. Herein it is demonstrated that σ follows from the elemental electronegativity and the oxidation state of Tl, and other structurally analogous cations. The comparatively lower elemental electronegativity of Tl, in conjunction with its oxidation state, explains the higher σ and T_C0 values in the Tl-based compounds relative to their Bi-based cuprate homologs. A derivation of σ is introduced for the optimal Tl₂Ba₂Ca_η–1Cu_ηO_2η+4 (for η=1, 2, 3) compounds, which exhibit a Tl oxidation state at or near +3, obtaining the fundamental value σ₀=0.228 previously established for YBa₂Cu₃O_6.92. Also reported is the marked enhancement in σ associated with Tl⁺¹ and analogous inner-layer cations relative to higher-valence cations. For a model proposition of σ=σ₀, the fractional Tl⁺¹ content of the mixed-valence compound, TlBa₂Ca₂Cu₃O_9±δ, is predicted to be 1/3 at optimization, in agreement with existing data. Charge depletion is illustrated for the two Tl-1201 compounds, where σ<σ₀ values are determined according to substitution of Ba⁺² or Sr⁺² by La⁺³, and/or Tl depletion. Additionally, statistical analysis of calculated and experimental transition temperatures of 48 optimal superconductors shows an absence of bias in determining σ, A, and ζ.     

Electronic and optical properties of CuGaS2: First-principles calculations

Electronic structure and optical properties of CuGaS₂ are calculated using the full potential linearized augmented plane wave plus local orbitals method. The calculated equilibrium lattice is in reasonable agreement with the experimental data. The electronic structures indicate that CuGaS₂ is a semiconductor with a direct bandgap of 0.81802 eV. Furthermore, other experiments and theory also show that this material has a direct bandgap. It is noted that there is quite strong hybridization between Ga 3d and S 3s orbitals, which belongs to the (GaS₂)⁻. The complex dielectric functions are calculated, which are in good agreement with the available experimental results.     

Electronic structure and charge distribution in : The ab initio approach

The full-potential linearized augmented plane wave (FP-LAPW) method was used for spin-polarized electronic structure calculations of stoichiometric superconductor. The generalized gradient approximation plus Hubbard correction (GGA + U) was employed for electrons. The reference calculations of were carried out in the spin-polarized mode to enable direct comparison. The determined total and atomic-projected densities of states (DOS), as well as charge distribution and on-site magnetic moments were analyzed and compared in detail. It was shown that in Dy123 system DOS in the close vicinity of the Fermi level is higher than in Y123. Also more efficient transfer of charge from CuO₂ planes was noticed in Dy123. The strongly localized magnetic spin moment of Dy is responsible for relatively weak magnetic interactions in the system, mostly in the CuO₂ planes. The obtained computational results are in agreement with the reported good superconducting properties of DyBa₂Cu₃O₇ compound.     

First-principle investigation on multiferroicity and interfacial coupling of nonstoichiometric tetragonal La2/3Sr1/3MnO3/BiCoO3 interface

The energetically stable tetragonal nonstoichiometric La_2/3Sr_1/3MnO₃/BiCoO₃ (LSMO/BCO) interface, whose chemical formula is (BiO)₃(CoO₂)₄/(LaO)₃(SrO)₁(MnO₂)₃, was investigated by using first-principle calculations. A magnetoelectric coupling effect, i.e., an electric control of the magnetism, was approved by a maximum variation of 17.9% in magnetic moments tuned by the electric polarization reversal. The interfacial coupling was controlled by the polar shifts of the interfacial Co and/or O atoms. A huge variation in spin-polarization from 6.3% to 72.7% was achieved upon switching the electric polarization. These findings are useful for magnetoelectric coupled spintronic device applications.     

Pressure induced structural phase transition and electronic properties of actinide monophospides: Ab-initio calculations

We have investigated the structural and electronic properties of monophospides of thorium, uranium and neptunium. The total energy as a function of volume is obtained by means of the self-consistent tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). From the present study with the help of total energy calculations it is found that ThP, UP and NpP are stable in NaCl-type structure at ambient pressure. The structural stability of ThP, UP and NpP changes under the application of pressure. We predict a structural phase transition from NaCl-type (B₁-phase) structure to CsCl-type (B₂-phase) structure for these phospides in the pressure range of 37.0-24.0 GPa (ThP-NpP). We also calculate lattice parameter (a₀), bulk modulus (B₀), band structure and density of states. From energy band diagram it is observed that ThP, UP and NpP exhibit metallic behavior. The calculated equilibrium lattice parameters and bulk modulus are in good agreement with experimental and theoretical work.     

Boron carbon nanotube superlattices: Geometry, electronic structure and quantum conductance

The stable boron carbon nanotube superlattices (BCNTSLs) that are constructed by periodically connecting carbon nanotube (CNT) and boron nanotube (BNT) with different lengths and diameters are predicted by employing the density functional first-principles calculations. The geometrical and electronic structures as well as quantum conductance of BCNTSLs are studied. It is found that the superlattices can be metallic or semiconducting depending on tube diameters and the ratio of BNT to CNT segments in a periodic unit. The confined states in the superlattice are observed. The present study could offer a useful way for designing some functional nanodevices.     

Anisotropic charge transport properties of chrysene derivatives as organic semiconductor: A computational study

We used density functional theory to calculate the angular resolution anisotropic charge mobility of the substituted chrysene molecules, viz, 4,10‐diphenoxychrysene (DPC), 4,10‐bis(phenylsulfanyl)chrysene (BPSC), and ethyl 8,9,12‐trimethoxychrysene‐6‐carboxylate (ETCC). The highest occupied molecular orbital–lowest unoccupied molecular orbital gap for DPC, BPSC, and ETCC was calculated to be 3.92, 3.83, and 3.81 eV, respectively, which inferred the compounds to be wide‐band‐gap semiconductors indicating that the compounds should have high stability in atmospheric conditions. The fact is also supported by electronic band‐structure calculation. In addition, higher electron affinity of studied compounds as compared with the bare chrysene molecule imparts enhancement of n‐type character in the compounds. The maximum hole ( ) and electron mobilities ( ) for DPC compound were found to be 0.739 cm²V^?1s^?1 and 0.319 cm²V^?1s^?1, respectively, at Φ = 0°. On the other hand, in the case of BPSC crystal, comparatively larger anisotropic electron mobility (0.709 cm²V^?1s^?1 at Φ = 0° and Φ = 179.90°) than the hole mobility (0.208 cm²V^?1s^?1 at Φ = 127.19° and Φ = 307.10°) was noted. Similarly, in ETCC, the parallel dimers were found to contribute maximum and of 0.052 and 0.102 cm²V^?1s^?1, respectively, at Φ = 0°. The substitution of ‐SPh in BPSC and ‐OCH₃ and ‐CO₂CH₂CH₃ in ETCC have relatively more impact on band reduction than ‐OPh in DPC, thus facilitating electron transport in BPSC and ETCC.     

Reaction of acetonitrile with the silicon(0 0 1) surface: A combined XPS and FTIR study

The adsorption of acetonitrile on the Si(0 0 1) surface has been investigated using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). XPS and FTIR spectra indicate that adsorbed acetonitrile forms two correlated binding configurations, a CN species with a strong FTIR absorption at 1540 cm⁻¹ and a CCN (ketenimine) species that has a very strong FTIR absorption at 1952 cm⁻¹. The CCN FTIR peak at 1952 cm⁻¹ shows a striking polarization dependence, with the infrared transition dipole almost entirely in the plane of the sample and parallel to the SiSi dimer axis. Our data suggests that the primary CCN structure results from cleavage of two C-H bonds, forming a structure in which the N and terminal C atom are both linked to the surface. Temperature-dependent experiments help to elucidate the complicated reaction mechanism for acetonitrile adsorbing onto the Si(0 0 1) surface. Dosing at higher temperature increases the amount of CCN relative to CN species while heating leads to direct transformation of the CN to the CCN species. Our results indicate that previous studies, which considered only products formed by cleavage of a single C-H bond, have misidentified the primary ketenimine product. A reinterpretation of the earlier results, combined with data presented here, sheds new light onto the products and mechanism of interaction of acetonitrile with Si(0 0 1).     

Sputter deposition of indium tin oxide onto zinc pthalocyanine: Chemical and electronic properties of the interface studied by photoelectron spectroscopy

The interface chemistry and the energy band alignment at the interface formed during sputter deposition of transparent conducting indium tin oxide (ITO) onto the organic semiconductor zinc phtalocyanine (ZnPc), which is important for inverted, transparent, and stacked organic light emitting diodes, is studied by in situ photoelectron spectroscopy (XPS and UPS). ITO was sputtered at room temperature and a low power density with a face to face arrangement of the target and substrate. With these deposition conditions, no chemical reaction and a low barrier height for charge injection at this interface are observed. The barrier height is comparable to those observed for the reverse deposition sequence, which also confirms the absence of sputter damage.     

太阳能材料CuInS2的晶体结构、电子和光学特性

用全电势线性缀加平面波法加局域轨道方法调查了黄铜矿半导体CuInS2的结构、电子和光学特性。我们计算的带隙0.17 eV是直接的,其它实验和理论也表明这种材料有一个直接带隙。在 In 4d和S 3p轨道之间有相当强的杂化,构成了(InS2)4-阴离子。我们计算的反射率光谱,介电函数的实部和虚部,消光系数和折射率和实验结果取得了很好的一致。