Fine structures in the X-ray photoemission spectra of MnO,FeO, CoO,and NiO single crystals

The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced.     

Electroreflectance observation of localized and itinerant electron states in NiO

We have observed electroreflectance spectra in NiO at 6 eV which we believe represent transitions from the oxygen 2_p derived valence band to the nickel 4s derived conduction band. We also observe the spectrum seen earlier by McNatt near 4 eV but interpret it differently in terms of transitions from the localized 3d⁸ state to the 4s band. These interpretations are consistent with the recent model of Adler and Feinleib.     

Low temperature μSR studies in NiO and CoO

Recent measurements on NiO and CoO show a complex temperature dependence. In this work, the local μ⁺ fields are examined over an extended temperature range. Only one precession frequency of 61.3 MHz was observed in the μ⁺ spin precession in zero field in NiO, similar to the case in MnO. The signal broadens at 200 K and is difficult to observe above 250 K. In contrast, in CoO, at least three lines are observed: two sharp lines at 54.2 MHz, and at 78 MHz, with an additional small peak at 162 MHz. Above 40 K, the 54.2 MHz signal (CoO) vanishes, but the 78 MHz signal survives to 110 K. However, at 270–280 K, a new signal at 13 MHz is also observed. Dipole field calculations of these 3d‐oxides (MnO, NiO and CoO) for various lattice sites (symmetric sites and O‐bonded positions) are examined and compared with the experimental results, with considerations toward the dynamics of the muons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Small-polaron versus band conduction in some transition-metal oxides

In this paper an attempt is made to establish the nature of free charge carriers and of charge carriers bound to centres in p-type NiO, CoO and MnO and in n-type MnO and α-Fe₂O₃.

For free charge carriers, d.c. conductivity, Seebeck coefficient and Hall effect are considered. Effects arising from inhomogeneous conduction and impurity conduction are discussed. Impurity conduction appears to have a strong influence on transport properties in the case of α-Fe₂O₃, less so in NiO, whereas no influence of this effect has been found in CoO and MnO. It is shown that NiO and CoO do not exhibit the features characteristic of small-polaron conductors but rather can be consistently conceived of as large-polaron band semiconductors. It is suggested that magnetic resistance due to exchange coupling between charge-carrier spin and cation spins plays an important role. The anomalous behaviour of the Hall effect in NiO and α-Fe₂O₃ is extensively discussed. In contradistinction to NiO, CoO and n-type MnO, free charge carriers in p-type MnO seem to have small-polaron character.

For charge carriers bound to centres, dielectric loss, high-frequency conduction and optical absorption are considered. The dielectric loss data relate to Li or Na centres in NiO, CoO and MnO and to Ti, Zr, Sn, Ta, Nb and presumably oxygen vacancy centres in α-Fe₂O₃. It is concluded from the dependence of dielectric loss on frequency and temperature that bound charge carriers are small polarons. It is shown for the cases of NiO and α-Fe₂O₃ that apart from small-polaron effects, disorder due to locally varying electric fields determines the nature of dielectric loss. The small-polaron character of bound charge carriers in NiO is corroborated by the behaviour of high-frequency conduction and optical absorption due to centres and also by the magnitude of impurity conduction.     

Cation diffusion,semiconductivity and nonstoichiometry in (Co,Ni)O crystals

The diffusivities of ⁶⁰Co and ⁵⁷Ni, and the electrical semiconductivity in (Co,Ni)O crystals have been determined as a function of temperature and composition in an air atmosphere. Both ⁶⁰Co and ⁵⁷Ni diffuse by a vacancy mechanism. Their diffusivities increase exponentially with an increase cation fraction of cobalt in the mixed oxides at a constant temperature. Using Manning's model of diffusion in a random alloy, the relative extent of off-stoichiometry and, therefore, the concentration of cation vacancies relative to NiO as a function of c in (Co_cNi_1?c)O were evaluated from the diffusion data. The change in enthalpy of formation of a cation vacancy as a function of the cation composition in mixed oxides has also been deduced from the experimental results. From a comparison of the cation composition dependency characteristic of the electrical semiconductivity with that of the cation vacancy cocentration, it was deduced that the apparent number of effective negative charge on a cation vacancy varies almost linearly from two in NiO to almost unity in CoO. Interdiffusion experiments show that a net flow of cations in one direction in a CoO?NiO diffusion couple will produce a gradient in the oxygen activity that can alter the diffusion behavior of the sample.     

Spectroscopic ellipsometry study of above-band gap optical constants of layered structured TlGaSe2, TlGaS2 and TlInS2 single crystals

Spectroscopic ellipsometry measurements on TlGaSe₂, TlGaS₂ and TlInS₂ layered crystals were carried out on the layer-plane (0 0 1) surfaces, which are perpendicular to the optic axis c^?, in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the pseudodielectric function as well as pseudorefractive index and pseudoextinction coefficient were found as a result of analysis of ellipsometric data. The structures of critical points in the above-band gap energy range have been characterized from the second derivative spectra of the pseudodielectric function. The analysis revealed four, five and three interband transition structures with critical point energies 2.75, 3.13, 3.72 and 4.45 eV (TlGaSe₂), 3.03, 3.24, 3.53, 4.20 and 4.83 eV (TlGaS₂), and 3.50, 3.85 and 4.50 eV (TlInS₂). For TlGaSe₂ crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure.     

Electroreflectance and reflectance study of 2H-MoSe2

We report the first electrolyte electroreflectance (EER) study of 2HMoSe₂ in the energy range 0.7 to 6 eV. The reflectivity (R) was also measured in the range of 0.7 to 9.5 eV and the real and imaginary parts of the dielectric constant were determined using the Kramers-Kronig relations. All the measurements were done at room temperature. Single crystals of 2H-MoSe₂ were grown using the direct combination of the elements in a sealed silica tube combined with chemical vapor transport using Br as the transport agent. The EER spectrum exhibits sharp structure in the vicinity of the excitonic transitions A, A′, B and B′ as well as higher lying interband transitions; the transition energies are determined with better accuracy than has been possible to date. A comparison of the R and EER spectra helped identify the various features in the EER spectrum.     

Charge transfer and dd excitations in transition metal oxides

Electronic excitations in transition-metal oxides MnO, FeO, CoO and NiO are investigated by inelastic X-ray scattering and optical reflectivity measurements. The dielectric functions are derived from the experimental data as a function of the momentum transfer, q. Based on the derived q-dependent dielectric functions, two types of the charge transfer excitations, i.e., dipolar and non-dipolar charge transfer, are clearly identified. We show that the Mott gaps around 5 eV are defined by the former whereas the latter occurs at higher energies of 8–12 eV. Based on a molecular orbital analysis, we associate the dipolar and the non-dipolar excitations with non-local charge transfer and conventional charge transfer, respectively. These types of excitations are shown to be common for the 3d metal monoxides. On the other hand, the dd excitations observed in NiO and CoO at energy <4 eV do not appear in FeO and MnO. The reasons are addressed in this report.     

Electronic structure and optical spectra of oxyanion crystals

Band structure of lithium, sodium, and potassium nitrites, nitrates, chlorates, perchlorates, sulfites, and sulfates together with the state density and the imaginary part of the complex dielectric permittivity is calculated by the DFT-LDA method with the Perdew and Zunger (PZ) correlation potential using the software package CRYSTAL06 on the uniform grid comprising 512 points of the Brillouin zone in the approximation of the direct interband transitions and constant matrix element. Good agreement of experimental and theoretical optical spectra is established. It is demonstrated that the main specific features of the optical spectra at energies of 5–12 eV are determined by the complex anion structure and depend only slightly on the cation structure. The cation excitation energies are 25‒28 eV for sodium salts and 15‒18 eV for potassium ones.     

Piezo optic properties of nickel and platinum

The piezoreflectance spectra of nickel and platinum were measured over the range 1.25–5.2 eV with the direction of incident light polarised parallel and perpendicular to the strain axis. Strain amplitudes of 0.12 × 10^?3 and 0.6 × 10^?4 respectively were employed at a frequency of 68kHz. The results showed enhanced structure in the differential absorption spectra of nickel at 1.75, 3.75 and 5 eV. Weaker structure was also found in the range 2.0–2.5 eV. From the results it is possible to discount the assignment of the higher energy structures as due to symmetry point transitions at the X and L points. A summary of possible assignments for the observed structures is given.The results for platinum show an essentially featureless spectrum in the visible range from which it may be concluded that the earlier prediction of interband transitions in this metal in the visible regions are incorrect.     

Infrared probe of the electronic structure and charge dynamics of Na0.7CoO2

We present measurements of the optical spectra on Na(0.7)CoO(2) single crystals. The optical conductivity shows two broad interband transition peaks at 1.6 eV and 3.1 eV, and a weak midinfrared peak at 0.4 eV. The intraband response of conducting carriers is different from that of a simple Drude metal. A peak at low but finite frequency is observed, which shifts to higher frequencies with increasing temperature, even though the dc resistivity is metallic. The origin of the interband transitions and the low-frequency charge dynamics have been discussed and compared with other experiments.     

Preparation of NiO and CoO nanoparticles using M2+-oleate (M = Ni,Co) as precursor

The preparation of NiO and CoO nanoparticles was reported. The dot-like NiO and flower-like CoO nanoparticles were obtained using M²⁺-oleate (M = Ni, Co) as precursor via thermal decomposition method. Transmission electron microscopic (TEM) images monitored the growth of NiO and CoO nanoparticles. When the reaction complex including M²⁺-oleate (M = Ni, Co) precursor, oleic acid and 1-octadecene was heated to the refluxing temperature (320 °C), the formed NiO and CoO nanoparticles were needle-like and very small, indicating low growth speed. However, when the reaction complex was kept refluxing for 30 min, dot-like NiO and flower-like CoO nanoparticles were observed, suggesting the accelerated growth at this refluxing stage. The difference of the morphology of the resultant NiO and CoO nanoparticles resulted from the difference of their growth mode. Selected-area electron diffraction (SAED) patterns showed the face-centered cubic structures of NiO and CoO nanoparticles. The magnetic property of the nanoparticles was studied using vibrating sample magnetometer (VSM).     

Pressure dependent optical absorption edge shift and compressibility of CdCr2Se4 single crystals

The pressure shift of the optical absorption edge (dE_g/dp = (1.1 ± 0.1) × 10^?6 eV bar^?1) and the compressibility (κ = (1.3 ± 0.) × 10^?6 bar^?1) of single crystalline CdCr₂Se₄ have been measured at ambient temperature. These data suggest an interpretation of the fundamental absorption in terms of either p → p interband or p → localized d charge transfer transitions, but exclude excitations involving s-band states.     

Effects of alloying on the band structure of gold: Piezoreflectance measurements on some AuCo and AuV alloys

Piezoreflectance measurements have been made on a series of gold cobalt and gold vanadium alloys with impurity concentrations of up to 4% using a strain amplitude of 4 × 10^?4 at a frequency of 68 kHz. The spectra show that the main interband transitions occur at 2.4, 3.5 and 4.5 eV. The deformation potentials with impurity concentration have been found for these transitions. An additional interband transition was found to occur at an energy of 1.8eV and this was enhanced significantly with impurity concentration. This may be due to the impurity causing a broadening of the d bands and hence a smearing of the interband threshold or alternatively it may be caused by a d band to Fermi level transition along the Δ direction close to the X point.     

Electron-hole interaction of the d core levels in III–V semiconductors

Recently Aspnes and Olson reported the first electroreflectance spectra obtained on GaP in the vacuum ultraviolet. They identified the structures above 20 eV as originating from critical points occuring between the flat Ga 3d corelevels and the sp³ conduction bands. We show that photoemission (E.D.C.) and optical spectroscopy (absorption, electroreflectance) give different results. The one-electron calculation fails to explain the experimental data. Electron-hole interaction and relaxation effects have to be included.     

Ellipsometry study of interband transitions in TlGaS2xSe2(1−x) mixed crystals (0≤x≤1)

In this paper, the spectroscopic ellipsometry measurements on TlGaS_2xSe_2(1?x) mixed crystals (0≤x≤1) were carried out on the layer-plane (001) surfaces with light polarization E⊥c^? in the 1.2–6.2 eV spectral range at room temperature. The real and imaginary parts of the dielectric function, refractive index and extinction coefficient were calculated from ellipsometric data using the ambient-substrate optical model. The critical point energies in the above-band gap energy range have been obtained from the second derivative spectra of the dielectric function. Particularly for TlGaSe₂ crystals, the determined critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure. The effect of the isomorphic anion substitution (sulfur for selenium) on critical point energies in TlGaS_2xSe_2(1?x) mixed crystals was established.     

Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.     

Luminescence characteristics of the Pr<Superscript>3+</Superscript> ion in SrB<Subscript>4</Subscript>O<Subscript>7</Subscript> and SrB<Subscript>6</Subscript>O<Subscript>10</Subscript>

Spectral and kinetic characteristics of the luminescence and luminescence excitation spectra of polycrystalline SrB₄O₇:Pr (1%) and SrB₆O₁₀:Pr (1%) samples are studied at 150–170 K. The samples show an intense luminescence band in the vicinity of 405 nm (¹ S ₀→¹ I ₆ transitions of Pr³⁺) and shorter wavelength bands also assigned to transitions from the ¹ S ₀ level. The main luminescence decay constant is ～2×10^?7 s. The excitation spectra of the ¹ S ₀ luminescence in these crystals are significantly different. The SrB₄O₇:Pr crystal shows three well-resolved bands at 6.14, 6.55, and 6.91 eV in the region of the 4f ²→4f ¹5d transitions and a complex structure in the region of interband transitions (7.1–20 eV), whereas the SrB₆O₁₀:Pr crystal shows a weakly structured band at 6.31 eV and no excitation in the region of the interband transitions. The physical mechanisms that may be responsible for the observed features of the spectra are discussed.     

Luminescence of LaBr3: Ce,Hf crystals under photon excitation in the ultraviolet,vacuum ultraviolet,and X-ray ranges

This study has been carried out using synchrotron radiation, time-resolved luminescence ultraviolet and vacuum ultraviolet spectroscopy, optical absorption spectroscopy, and thermal activation spectroscopy. It has been found that, in scintillation spectrometric crystals LaBr₃: Ce,Hf characterized by a low hygroscopicity, along with Ce³⁺ centers in regular lattice sites, there are Ce³⁺ centers located in the vicinity of the defects of the crystal structure. It has also been found that the studied crystals exhibit photoluminescence (PL) of new point defects responsible for a broad band at wavelengths of 500–600 nm in the PL spectra. The minimum energy of interband transitions in LaBr₃ is estimated as E _g ～ 6.2 eV. The effect of multiplication of electronic excitations has been observed in the range of PL excitation energies higher than 13 eV (more than 2E _g ). Thermal activation studies have revealed channels of electronic excitation energy transfer to Ce³⁺ impurity centers.     

Electronic transitions in α, θ and γ polymorphs of ultraporous monolithic alumina

Spectroscopy of α, θ, and γ phases of high‐purity ultraporous alumina has been studied at cryogenic temperatures of 7 K in the near‐IR–VUV range of spectra with synchrotron radiation excitation. The UV photoluminescence (PL) spectra are dominated by optical transitions of self‐trapped excitons, while the PL excitation spectra are assigned to free excitons and interband transitions. The analysis of PL excitation spectra indicates a tendency to fundamental bandgap narrowing in order of 9.36 eV (α) to 7.60 eV (θ) and 6.85 eV (γ). Structural defects related to oxygen vacancies are responsible for the visible F⁺/F transitions decrease in order γ > θ > α. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)