Valence band photoemission study of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te

The electronic structures of the copper chalcogenide compounds, Cu₂S, Cu₂Se and Cu₂Te have been investigated by taking photoemission data with synchrotron photon sources. The band calculations are done using the full-potential linear-muffin-tin-orbital method. Since the crystal structures are not clarified well, several simplified structure models are used. The calculated densities of states are compared with the observed spectra. The analysis shows that a sharp peak at −3.5 eV is due to the Cu 3d states, and that the tails at the high and low energy sides of the Cu 3d peak are due to the chalcogen p states.     

Electronic band structure of pseudo-binary AlB2-like hexagonal silicides SrNixSi2−x as novel low-TC superconductors

The very recently discovered pseudo-binary hexagonal silicide SrNi_xSi_2−x, which exhibits low-T_C superconductivity, was examined theoretically to understand the effect of the unusual doping type (partial replacement of Si by Ni) on the electronic band structure of this material. Besides, the possible factors of the stabilization of the hexagonal AlB₂-type structure of SrSi₂ upon substitution of Ni for Si, and the solubility limit of Ni in SrNi_xSi_2−x are discussed in terms of competing Si–Si, Si–Ni, and Ni–Ni bonds.     

Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectron spectroscopy

The spinel CoFe₂O₄ has been synthesized by combustion reaction technique. X-ray photoelectron spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the powder and bulk sample is most typically represented by the formula (Co_0.4Fe_0.6)[Co_0.6Fe_1.4]O₄, where cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The results demonstrate that most of the iron ions are trivalent, but some Fe²⁺ may be present in the powder sample. The Co 2p_3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40% and 15%, attributes to Co²⁺ in octahedral sites, tetrahedral sites and Co³⁺ in octahedral sites. The O 1s spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.     

Effects of Cu2−xS phase removal on surface potential of Cu2ZnSnS4 thin-films grown by electroplating

Cu₂ZnSnS₄ (CZTS) has an optical band gap of 1.4–1.5 eV, which is similar to that of Cu(In,Ga)Se₂ (CIGS), and a high absorption coefficient (>10⁴ cm⁻¹) in the visible light region. In previous reports, CIGS thin-film solar cells have been shown to improve the performance of the device since the secondary phase is removed by Potassium cyanide (KCN) etching treatment. Therefore, in this study we applied a KCN etching treatment on CZTS and measured the effects. We confirmed the removal of Cu_2−xS via Kelvin probe force microscopy (KPFM) and Raman scattering spectroscopy. The effects of the experiment indicate that we can define with precision the location of the secondary phases, and therefore the control of the secondary phases will be easier and more efficient. Such capabilities could improve the solar cell performance of CZTS thin-films.     

Photoluminescence and X-ray diffraction studies on Cu2O

Cuprous oxide (Cu₂O) crystals were grown by the two-step crystallization method in air atmosphere conditions from polycrystalline thin copper foils. The method comprises two stages; in the first one the copper plates are oxidized at 1020 °C by some hours in line with its initial thickness. In the second stage, the growth of large crystalline areas is promoted by annealing the Cu₂O samples at 1100 °C for long periods. Raman scattering an X-ray measurements demonstrates the existence of the single-phase Cu₂O. The effects on the crystalline structure and photoluminescence (PL) response were studied as a function of the conditions used in the second stage of the synthesis method. PL spectra were taken from 10 to 180 K to define the main radiative recombination paths. Besides the near band excitonic transitions, two strong emission bands at 720 and 920 nm associated with relaxed excitons at oxygen and copper vacancies were detected. Both excitonic-vacancy bond transitions presented similar intensities that are related to the growth method. X-ray and Raman scattering measurements help to assess the samples crystalline quality.     

Electronic and pressure dependent vibrational properties of silicide Sr(Ni0.5Si0.5)2

We report a detailed ab initio study of the structural, electronic, and volume dependent elastic and lattice dynamical properties of Sr(Ni_0.5Si_0.5)₂. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The phonon dispersion curves along the high-symmetry directions and phonon frequencies with their Grüneisen parameters at the Brillouin zone center are computed by using density functional perturbation theory while the elastic constants are calculated in metric-tensor formulation. The band structure, and partial densities of states and Fermi surface topology are also discussed.     

Electronic and optical properties of kesterite Cu2ZnSnS4 under in-plane biaxial strains: First-principles calculations

The electronic structures and optical properties of Cu₂ZnSnS₄ (CZTS) under in-plane biaxial strain were systematically investigated using first-principles calculations based on generalized gradient approximation and hybrid functional method, respectively. It is found that the fundamental bandgap at the Γ point decreases linearly with increasing tensile biaxial strain perpendicular to c-axis. However, a bandgap maximum occurs as the compressive biaxial strain is 1.5%. Further increase of compressive strain decreases the bandgap. In addition, the optical properties of CZTS under biaxial strain are also calculated, and the variation trend of optical bandgap with biaxial strain is consistent with the fundamental bandgap.     

Electronic structure and Fermi surface of new K intercalated iron selenide superconductor KxFe2Se2

Using the ab initio FLAPW-GGA method we examine the electronic band structure, densities of states, and the Fermi surface topology for a very recently synthesized ThCr₂Si₂-type potassium intercalated iron selenide superconductor K_xFe₂Se₂. We found that the electronic state of the stoichiometric KFe₂Se₂ is far from that of the isostructural iron pnictide superconductors. Thus the main factor responsible for experimentally observed superconductivity for this material is the deficiency of potassium, i.e. the hole doping effect. On the other hand, based on the results obtained, we conclude that the tuning of the electronic system of the new K_xFe₂Se₂ superconductor in the presence of K vacancies is achieved by joint effect owing to structural relaxations and hole doping, where the structural factor is responsible for the modification of the band topology, whereas the doping level determines their filling.     

A theoretical study of the oxygen K-edge near-edge X-ray absorption ne structure of N2O/Ir(110)

A multiple-scattering cluster method is employed to calculate the oxygen K-edge near-edge X-ray absorption fine structure of N₂O/Ir(110) and its monolayer. Two peaks and one weak resonance appear in both cases. The selfconsistent field DV-X calculations of the peaks and resonance show that the physical origin of the pre-edge peak x is different from those of the main peak 1 and the other weak resonance σ₁. This setup is intrinsic to the N₂O monolayer, owing to the interaction between the neighbouring molecular chains in the monolayer and partly to the adsorbed atomic oxygen, according to both the theoretical and experimental data.     

Investigations on co-evaporated Cu2SnSe3 and Cu2SnSe3-ZnSe thin films

Cu₂SnSe₃ is an important precursor material for the growth of Cu₂ZnSnSe₄, an emerging solar cell absorber layer via solid state reaction of Cu₂SnSe₃ and ZnSe. In this study, we have grown Cu₂SnSe₃ (CTSe) and Cu₂SnSe₃-ZnSe (20%) films onto soda-lime glass substrates held at 573 K by co-evaporation technique. The effect of annealing of these films at 723 K for an hour in selenium atmosphere is also investigated. XRD studies of as-deposited Cu₂SnSe₃ and Cu₂SnSe₃-ZnSe films indicated SnSe as secondary phase which disappeared on annealing. The direct optical band gap of annealed Cu₂SnSe₃ and Cu₂SnSe₃-ZnSe films were found to be 0.90 eV and 0.94 eV respectively. Raman spectroscopy studies were used to understand the effect of ZnSe on the properties of Cu₂SnSe₃.     

(Fe1-xNix)2P电子结构与磁学特性的X射线近边吸收谱研究

利用X射线近边吸收谱对Fe₂P,Ni₂P及其掺杂物(Fe_1-xNi_x)₂P(x=01,025,05)中Fe,Ni,P的K边进行了研究.结合多重散射理论近边计算,讨论了金属原子不同位置格点3f,3g对近边谱特征的贡献,得出当Ni原子取代Fe原子时将优先占据Fe(3f)格点位置;根据第一性原理对能态的计算发现,不考虑磁性时不同格点P的pDOS未占据态电子结构与P-K近边吸收谱实验相符合;与考虑铁磁性Fe₂P 的DOS相比较后结果显示Fe₂P的磁性主要来源于Fe(3g)格点,铁磁性Ni₂P计算的Ni不同格点原子磁矩均接近于0,与它一般显顺磁性结论相一致. 关键词： X射线近边吸收谱 电子结构 多重散射理论 态密度     

“In situ” XPS study of band structures at Cu2O/TiO2 heterojunctions interface

In an attempt to investigate influence of the defects on electronic structure of Cu₂O/TiO₂ heterojunctions, thin Cu₂O layers were successively deposited on TiO₂ that has different levels of defect concentrations, and the resultant band bending and offset characteristics were studied by in situ X-ray photoelectron spectroscopy (XPS). The TiO₂ substrates with defects were prepared by Ar⁺ sputtering, followed by annealing at different temperatures in oxygen atmosphere. Presence of the defects in TiO₂ surface dramatically influences on the band bending and band offset at the interface: more defects are on TiO₂ surface, less band bending are at the interface, inducing smaller conduction band offsets. On the reduced TiO₂ surface, Cu₂O was disproportionately decomposed to form CuO and Cu.     

X-ray photoelectron spectroscopy analysis for undegraded and degraded Gd2O2S:Tb phosphor thin films

This paper presents the X-ray Photoelectron Spectroscopy (XPS) analysis for the undegraded and degraded Gd₂O₂S:Tb³⁺ thin film phosphor. The thin films were grown with the pulsed laser deposition (PLD) technique. XPS measurements were done on Gd₂O₂S:Tb³⁺ phosphor thin films before and after electron degradation. The XPS technique has proven the presence of Gd₂O₃ on the degraded and undegraded thin film spots. The presence of the SO₂ bonding was also detected after degradation. This clearly indicates that surface reactions did occur during prolonged electron bombardment in an oxygen atmosphere.     

Theoretical determination of the K2 electronic structure

A theoretical study of the electronic structure of K₂⁺ is presented. Potential energy curves for the ground and various electronic excited states have been computed in the framework of a model potential method over a wide range of internuclear distances. Spectroscopic constants for the lowest short-range bound states have been determined and they are compared with available experimental and theoretical values. Long-range structures (wells and humps) have been also predicted, some of which being described from a long-range model.     

Doctor-bladed Cu2ZnSnS4 light absorption layer for low-cost solar cell application

The doctor-blade method is investigated for the preparation of Cu₂ZnSnS₄ films for low-cost solar cell application. Cu₂ZnSnS₄ precursor powder, the main raw material for the doctor-blade paste, is synthesized by a simple ball-milling process. The doctor-bladed Cu₂ZnSnS₄ films are annealed in N₂ ambient under various conditions and characterized by X-ray diffraction, ultraviolent/vis spectrophotometry, scanning electron microscopy, and current-voltage (J-V) meansurement. Our experimental results indicate that (i) the X-ray diffraction peaks of the Cu₂ZnSnS₄ precursor powder each show a red shift of about 0.4°; (ii) the high-temperature annealing process can effectively improve the crystallinity of the doctor-bladed Cu₂ZnSnS₄, whereas an overlong annealing introduces defects; (iii) the band gap value of the doctor-bladed Cu₂ZnSnS₄ is around 1.41 eV; (iv) the short-circuit current density, the open-circuit voltage, the fill factor, and the efficiency of the best Cu₂ZnSnS₄ solar cell obtained with the superstrate structure of fluorine-doped tin oxide glass/TiO₂/In₂S₃/Cu₂ZnSnS₄/Mo are 7.82 mA/cm², 240 mV, 0.29, and 0.55%, respectively.     

X-ray spectroscopic study of the electronic structure of visible-light responsive N-, C- and S-doped TiO2

The electronic origins of the visible-light response of N-, C- and S-doped TiO₂ have been studied using X-ray absorption, X-ray emission, and X-ray photoelectron spectroscopies. New electronic states are observed in the bulk band gap, above the valence band edge of pure TiO₂, which can be directly related to the visible-light absorption of the N-, C- and S-doped TiO₂ materials.     

Electronic structure of non-centrosymmetric AgCd2GaS4 and AgCd2GaSe4 single crystals

X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion irradiated (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals grown, respectively, by the Bridgman method and the method of direct crystallization have been measured in the present work. The X-ray photoelectron spectroscopy (XPS) results reveal high chemical stability of (0 0 1) surfaces of AgCd₂GaS₄ and AgCd₂GaSe₄ single crystals. Electronic structure of AgCd₂GaS₄ has been calculated employing the full potential linearized augmented plane wave method. For the AgCd₂GaS₄ compound, the X-ray emission bands representing the energy distribution of the valence Ag d-, Cd d-, Ga p- and S p-like states were recorded and compared on a common energy scale with the XPS valence-band spectrum. The theoretical and experimental data regarding the occupation of the valence band of AgCd₂GaS₄ were found to be in excellent agreement to each other. Second harmonic generation (SHG) efficiency of AgCd₂GaS₄ by using the 320 ns CO laser at 5.5 μm has been recorded within the temperature range 80–300 K. Substantial increase of the photoinduced SHG which in turn is substantially dependent on the temperature has been detected for the AgCd₂GaS₄ compound.     

Elastic, electronic, and optical properties of hypothetical SnNNi3 and CuNNi3 in comparison with superconducting ZnNNi3

The elastic, electronic, and optical properties of MNNi₃ (M=Zn, Sn, and Cu) have been calculated using the plane-wave ultrasoft pseudopotential technique, which is based on the first-principle density functional theory (DFT) with generalized gradient approximation (GGA). The optimized lattice parameters, independent elastic constants (C₁₁, C₁₂, and C₄₄), bulk modulus B, compressibility K, shear modulus G, and Poisson's ratio υ, as well as the band structures, total and atom projected densities of states and finally the optical properties of MNNi₃ have been evaluated and discussed. The electronic band structures of the two hypothetical compounds show metallic behavior just like the superconducting ZnNNi₃. Using band structures, the origin of features that appear in different optical properties of all the three compounds has been discussed. The large reflectivity of the predicted compounds in the low energy region might be useful in good candidate materials for coating to avoid solar heating.     

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 band structure calculations of the low-temperature phases of Ag2Se, Ag2Te and Ag3AuSe2

Ab initio band structure calculations were performed for the low-temperature modifications of the silver chalcogenides β-Ag₂Se, β-Ag₂Te and the ternary compound β-Ag₃AuSe₂ by the local spherical wave (LSW) method. Coordinates of the atoms of β-Ag₂Se and β-Ag₃AuSe₂ were obtained from refinements using X-ray powder data. The structures are characterized by three, four and five coordinations of silver by the chalcogen, a linear coordination of gold by Se, and by metal-metal distances only slightly larger than in the metals. The band structure calculations show that β-Ag₃AuSe₂ is a semiconductor, while β-Ag₂Se and β-Ag₂Te are semimetals with an overlap of about 0.1-0.2 eV. The Ag 4d and Au 5d states are strongly hybridized with the chalcogen p states all over the valence bands. β-Ag₂Se and β-Ag₂Te have a very low DOS in the energy range from about −0.1 to +0.5 eV. The calculated effective mass β-Ag₂Se is about 0.1-0.3 m_e for electrons and 0.75 m_e for holes, respectively.