X-ray Photoelectron Spectroscopy Core-level Spectra Research Articles

Single crystal growth and electronic structure of TlPbI3

High-quality inclusion-free TlPbI3 single crystals have been grown using Bridgman–Stockbarger method. The electronic structure of TlPbI3 is studied by using the possibilities of X-ray photoelectron spectroscopy (XPS). For the TlPbI3 crystal, XPS core-level and valence-band spectra for both pristine and Ar+ ion-bombarded surfaces are recorded. The present XPS data indicate that the TlPbI3 single crystal surface is somewhat sensitive with respect to Ar+ ion-bombardment. In particular, the XPS measurements reveal that thallium and lead atoms are in the formal valence +1 and +2, respectively, on the pristine TlPbI3 single crystal surface. Further, the 3.0 keV Ar+ ion-bombardment of the surface induces partial transformation of lead ions to lower valence state, namely Pb0; however, no partial loss of iodine atoms belonging to TlI8 polyhedra occurs due to the Ar+ ion-bombardment of the TlPbI3 surface because after such a treatment thallium remains exclusively in the formal valence +1. The present XPS results indicate that low hygroscopicity is characteristic of the TlPbI3 single crystal surface. Photoinduced birefringence profiles in TlPbI3 are explored.

Growth, structure and optical properties of Tl4HgBr6 single crystals

Tl4HgBr6 single crystals were grown using solution-fusion method. The crystal structure of the ternary bromide was refined. Tl4HgBr6 crystallizes in the non-centrosymmetric space group P4nc with the lattice parameters a=8.9539(8)Å and c=8.7884(8)Å and it is isostructural to the Tl4HgI6 compound. The non-centrosymmetric structure of the Tl4HgBr6 compound was also confirmed by the existence of a modest second harmonic generation effect (0.4–0.5pm/V) and by the value of piezoelectric coefficient (0.9pm/V). The electronic structure of Tl4HgBr6 was explored using X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). In particular, for the Tl4HgBr6 crystal, we have measured XPS core-level and valence-band spectra for both pristine and Ar+ ion-bombarded surfaces. The XPS data reveal low hygroscopicity of Tl4HgBr6, the property that is very important when handling this material in optoelectronic devices working at ambient conditions. The present XPS data indicate that the Tl4HgBr6 single crystal surface is rather sensitive with respect to Ar+ ion-bombardment: such a treatment reduces significantly mercury content in the topmost surface layers. Comparison on a common energy scale of the XPS valence-band spectrum of Tl4HgBr6 and the XE Br Kβ2 band, representing peculiarities of the energy distribution of the Br 4p states, reveals that the main contribution of the valence Br p states occurs in the upper portion of the valence band, with also their significant contributions in other valence band regions. The measurements of spectral distribution of the absorption coefficient indicate that the Tl4HgBr6 compound is a semiconductor with the bandgap energy value of 2.43eV at 300K, and the bandgap energy increases up to 2.48eV when temperature decreases to 100K.

Combustion synthesis of Mg–Er ferrite nanoparticles: Cation distribution and structural, optical, and magnetic properties

Rare earth ion (Er3+)-doped magnesium ferrite nanoparticles of basic composition MgFe2−xErxO4 (x=0, 0.02, 0.04, and 0.06) were synthesized for the first time by a combustion method with use of glycine as fuel. The synthesized nanoparticles were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-diffuse reflectance spectroscopy, and vibrating sample magnetometer analysis in order to study the structural, compositional, morphological, and magnetic changes with addition of dopant. The X-ray diffraction pattern revealed a single phase with cubic spinel structure, and from the Scherrer formula and the Williamson–Hall formula, the average grain sizes ranged from 35 to 56nm and from 31 to 54nm, respectively. The lattice parameter (a) increases with the increase of the Er3+ concentration x in the lattice. The cation distributions among the tetrahedral (A) and octahedral (B) sites of spinel-type Er-doped magnesium ferrites were also investigated. The Fourier transform infrared spectroscopy spectra of synthesized samples illustrate that the higher-frequency bands lying in the range from 550 to 620cm−1 and the lower-frequency bands lying in the range from 410 to 450cm−1 are associated with the asymmetric stretching modes of the AB2O4 type of spinel transition metal oxides. Information about the chemical elements and oxidation states of the samples was obtained from high-resolution core-level X-ray photoelectron spectroscopy spectra of Mg 1s, Fe 2p, Er 4d, and O 1s. Further information about the morphology of the nanoparticles was obtained by scanning electron microscopy. From UV-diffuse reflectance spectroscopy studies, the optical band gaps were found to range from 1.81 to 1.96eV. The magnetic hysteresis curves clearly indicate the soft ferromagnetic nature of the samples. Various magnetic properties such as saturation magnetization, coercivity, and remanent magnetization obtained from M–H loops were observed to increase with Er3+ substitution.

Modification of band offsets of InGaZnO4/Si heterojunction through nitrogenation treatment

The effect of nitrogen on the band offset of sputtering-derived InZnGaO4 (IGZO)/Si heterostructures has been systematically investigated by x-ray photoelectron spectroscopy (XPS) measurements. Elemental analysis indicates that nitrogen has been successfully incorporated into the IGZO film. By using In 3d5/2, In 3d3/2 and Ga 3d core level (CL) XPS spectra as references, values of valence band offsets (ΔEv) of have been determined to be 2.56 ± 0.02 and 2.44 ± 0.02 eV for IGZO/Si and IGZO:N/Si heterojunctions, respectively. Using the experimental band gap of 3.59 and 3.50 eV of the IGZO/Si and IGZO:N/Si, the calculated values of conduction band offset (ΔEc) is 0.09 ± 0.01 and 0.06 ± 0.01 eV, respectively. The results indicate that nitrogen incorporation leads to the reduction in band gap and ΔEv and the slight effect on the ΔEc has also been detected.

Electronic structure of the heavy fermion superconductor Ce2PdIn8: Experiment and calculations

The electronic structure of a heavy-fermion superconductor Ce2PdIn8 was investigated by means of X-ray photoelectron spectroscopy (XPS) and ab initio density functional band structure calculations. The Ce 3d core-level XPS spectra point to stable trivalent configuration of Ce atoms that is also reproduced in the band structure calculations within the generalized gradient approximation GGA+U approach. Analysis of the 3d9f2 weight in the 3d XPS spectra within the Gunnarsson-Schönhammer model suggests that the onsite hybridization energy between Ce 4f and the conduction band states, Δfs, is ∼120 meV, which is about 30 meV larger than Δfs in isostructural Ce2TIn8 compounds with T = Co, Rh, and Ir. Taking into account a Coulomb repulsion U on both the Ce 4f and Pd 4d states in electronic band structure calculations, a satisfactory agreement was found between the calculated density of states (DOS) and the measured valence band XPS spectra.

Atomic-layer deposited thulium oxide as a passivation layer on germanium

A comprehensive study of atomic-layer deposited thulium oxide (Tm2O3) on germanium has been conducted using x-ray photoelectron spectroscopy (XPS), vacuum ultra-violet variable angle spectroscopic ellipsometry, high-resolution transmission electron microscopy (HRTEM), and electron energy-loss spectroscopy. The valence band offset is found to be 3.05 ± 0.2 eV for Tm2O3/p-Ge from the Tm 4d centroid and Ge 3p3/2 charge-corrected XPS core-level spectra taken at different sputtering times of a single bulk thulium oxide sample. A negligible downward band bending of ∼0.12 eV is observed during progressive differential charging of Tm 4d peaks. The optical band gap is estimated from the absorption edge and found to be 5.77 eV with an apparent Urbach tail signifying band gap tailing at ∼5.3 eV. The latter has been correlated to HRTEM and electron diffraction results corroborating the polycrystalline nature of the Tm2O3 films. The Tm2O3/Ge interface is found to be rather atomically abrupt with sub-nanometer thickness. In addition, the band line-up of reference GeO2/n-Ge stacks obtained by thermal oxidation has been discussed and derived. The observed low reactivity of thulium oxide on germanium as well as the high effective barriers for holes (∼3 eV) and electrons (∼2 eV) identify Tm2O3 as a strong contender for interfacial layer engineering in future generations of scaled high-κ gate stacks on Ge.

Electronic structure of Cu2CdGeSe4 single crystal as determined from X-ray spectroscopy data

We report on studies of the electronic structure of Cu2CdGeSe4 single crystal by using X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). In particular, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+ ion-irradiated surfaces of the Cu2CdGeSe4 single crystal have been recorded. Our XPS results indicate that Cu2CdGeSe4 is rather rigid with respect to Ar+ ion-irradiation: such a treatment does not induce any significant changes of values of the binding energies of core-level electrons as well as of the shapes of the XPS core-level and valence-band spectra of the Cu2CdGeSe4 surface. For the Cu2CdGeSe4 single crystal, the XES bands representing the energy distribution of mainly the valence Cu d, Cd d, Ge p and Se p states have been measured and compared on a common energy scale with its XPS valence-band spectrum. Such a comparison allows for concluding that the Cu 3d and Cd 4d states contribute mainly at the top and at the bottom of the valence band, respectively, while the Ge 4p and Se 4p states in the central and upper portions of the band. These experimental findings are found to be in agreement with literature data of band-structure calculations for this compound.

The influence of oxygen adsorption on the NEXAFS and core-level XPS spectra of the C60 derivative PCBM.

Fullerenes have been a main focus of scientific research since their discovery due to the interesting possible applications in various fields like organic photovoltaics (OPVs). In particular, the derivative [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) is currently one of the most popular choices due to its higher solubility in organic solvents compared to unsubstituted C60. One of the central issues in the field of OPVs is device stability, since modules undergo deterioration (losses in efficiency, open circuit voltage, and short circuit current) during operation. In the case of fullerenes, several possibilities have been proposed, including dimerization, oxidation, and impurity related deterioration. We have studied by means of density functional theory the possibility of oxygen adsorption on the C60 molecular moiety of PCBM. The aim is to provide guidelines for near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) measurements which can probe the presence of atomic or molecular oxygen on the fullerene cage. By analysing several configurations of PCBM with one or more adsorbed oxygen atoms, we show that a joint core level XPS and O1s NEXAFS investigation could be effectively used not only to confirm oxygen adsorption but also to pinpoint the bonding configuration and the nature of the adsorbate.

Optimisation of tribological performance of sputtering Zr/ZrC coatings

An intelligent approach, which uses adaptive network based fuzzy inference systems (ANFIS) based on experimental designs, is used to characterise the tribological behaviour of undoped and Zr doped diamond-like carbon (DLC) films that are deposited using magnetron sputtering. An orthogonal array experiment is used and the effect of the deposition parameters on the films is determined. The films are analysed using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Friction and wear tests are performed, using a pin-on-disk tribometer. This study identifies a group of highly developed hillock-like textures and lower wear volume loss is evident in the undoped and Zr doped films. The C1s core level XPS spectra show that the undoped and Zr doped films formed have a relative content of sp3 and sp2 hybrids. It is found that a value that is close to the estimated value, 0·5±0·05, of sp3/sp2 ratio results in better tribological properties in the undoped and Zr doped DLC films. These predicted values and the experimental results, for which an ANFIS predicts the tribological behaviour of the DLC films, are similar. The experimental results demonstrate that the tribological properties of DLC multilayer films are accurately predicted by an ANFIS. The results obtained for the ANFIS model are also compared to those for an ANN model and a Fuzzy system and it is shown that an ANFIS gives more reliable modelling of these sputtering processes and is more accurate and flexible than ANN and Fuzzy system models, which verifies the reliability and feasibility of this approach.

Surface state modulation through wet chemical treatment as a route to controlling the electrical properties of ZnO nanowire arrays investigated with XPS

ZnO is a wide bandgap semiconductor that has many potential applications including solar cell electrodes, transparent thin film transistors and gas/biological sensors. Since the surfaces of ZnO materials have no amorphous or oxidised layers, they are very environmentally sensitive, making control of their semiconductor properties challenging. In particular, the electronic properties of ZnO nanostructures are dominated by surface effects while surface conduction layers have been observed in thin films and bulk crystals. Therefore, the ability to use the ZnO materials in a controlled way depends on the development of simple techniques to modulate their surface electronic properties. Here, we use monochromatic x-ray photoelectron spectroscopy (XPS) to investigate the use of different wet chemical treatments (EtOH, H2O2) to control the electronic properties of ZnO nanowires by modulating the surface depletion region. The valence band and core level XPS spectra are used to explore the relationship between the surface chemistry of the nanowires and the surface band bending.

Electronic structure of β-RbNd(MoO4)2 by XPS and XES

β-RbNd(MoO4)2 microplates have been prepared by the multistage solid state synthesis method. The phase composition and micromorphology of the final product have been evaluated by XRD and SEM methods. The electronic structure of β-RbNd(MoO4)2 molybdate has been studied employing the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). For the molybdate, the XPS core-level and valence-band spectra, as well as XES bands representing energy distribution of the Mo 4d- and O 2p-like states, have been measured. It has been established that the O 2p-like states contribute mainly to the upper portion of the valence band with also significant contributions throughout the whole valence-band region. The Mo 4d-like states contribute mainly to a lower valence band portion.

Electronic structure of cage-like compound UB12 – Theory and XPS experiment

The electronic structure of cubic UB12 has been investigated by X-ray photoelectron spectroscopy (XPS) and first principles calculations employing the fully relativistic full-potential local-orbital (FPLO-9) method. The calculated densities of states (DOS) revealed unique fairly sharp five-peaks of the U 5f states with the lowest energy peak crossing the Fermi level. The calculated and experimental valence band XPS spectra are in very good accord, indicating a substantial delocalization of the U 5f electrons. On the other hand, our experimental U 4f core-level XPS spectrum exhibits both the 1-eV and 7-eV satellites, pointing to some localization effects of the U 5f electrons, which may exhibit a mixed-valence behavior. Our Fermi surfaces of UB12 calculated for ambient and such high pressure as 55GPa show only negligible differences, which can explain the earlier reported stability of the cubic crystal structure of the studied system under high pressure.

Structural investigation on GexSb10Se90−x glasses using x-ray photoelectron spectra

The structure of GexSb10Se90−x glasses (x = 7.5, 10, 15, 20, 25, 27.5, 30, and 32.5 at. %) has been investigated by x-ray photoelectron spectroscopy (XPS). Different structural units have been extracted and characterized by decomposing XPS core level spectra, the evolution of the relative concentration of each structural unit indicates that, the relative contributions of Se-trimers and Se-Se-Ge(Sb) structure decrease with increasing Ge content until they become zero at chemically stoichiometric glasses of Ge25Sb10Se65, and then the homopolar bonds like Ge-Ge and Sb-Sb begin to appear in the spectra. Increase of homopolar bonds will extend band-tails into the gap and narrow the optical band gap. Thus, the glass with a stoichiometric composition generally has fewer defective bonds and larger optical bandgap.

Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (Ts), from 25 to 600 °C. The effect of Ts on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ∼ 531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing Ts. The SE analyses indicate that the index of refraction (n) of Ga2O3 films increases with increasing Ts due to improved structural quality and packing density of the films. The n(λ) of all the Ga2O3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (∼200 Ω-cm) for amorphous Ga2O3 films grown at Ts = RT-300 °C and decreased to ∼1 Ω-cm for nanocrystalline Ga2O3 films grown at Ts ≥ 500–600 °C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga2O3 films is derived.

Thickness effect on the optical properties of Bi/As2S3 bilayer thin films

The optical properties of bilayer thin films of Bi/As2S3 with various thicknesses which were prepared from Bi and As2S3 by thermal evaporation technique under high vacuum were characterized by Fourier Transformed Infrared Spectroscopy, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The prepared bilayer films of 905nm, 910nm and 915nm thickness have As2S3 as bottom layer (900nm) and Bi as top layer (5,10,15nm). X-ray diffraction studies confirm the amorphous nature of the prepared films. The optical properties show a strong dependence on the film thickness. It was found that the optical band gap increases with film thickness. The obtained lower values of Urbach energy indicate that as thickness increases, more ordered films can be produced. The reduction in disorder in bonding network is amply supported by the way of increase in band gap, increase in Tauc parameter (B1/2) and reduction in Urbach energy from the analysis of transmittance spectra. The change in XPS core level spectra and Raman spectra also shows the changes due to thickness.

The effect of Se doping on spectroscopic and electrical properties of GeTe

Selenium doped thin films of GeTe alloys were investigated for their structural modifications by X-ray Diffraction, Fourier Transform Infrared Spectroscopy, X-ray photoelectron Spectroscopy (XPS) and Raman Spectroscopy. The band gap increase from 0.69 to 1.10eV with increasing Se addition signifies the possibility of band gap tuning in the material. Disorder decreases, band widens and conductivity saturates about 0.20at.% of Se addition. Structural changes are explained by the bond theory of solids. The as-deposited films are amorphous and 0.50at.% Se alloy forms a homogeneous amorphous phase with a mixture of GeSe and TeSe bonds. The XPS core level spectra and Raman spectra investigation clearly indicate the formation of GeSe, GeTe2 and TeSe bonds with Se addition. Crystallization temperature is found to be increasing with Se and the 0.10at.% Se alloy is found to have a higher resistance contrast compared to other Se concentration alloys. Up to 0.10at.% of Se addition can enhance GeTe phase change memory properties.

Characterization of Fluorinated Multi-Walled and Single-Walled Carbon Nanotubes Using High Resolution XPS and EDX

Pristine and fluorinated multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) were characterized using energy dispersive X-ray spectra (EDX) and X-ray photoelectron spectroscopy (XPS), respectively. The fluorine percentages of fluorinated multi-walled carbon nanotubes (F-MWCNTs) and fluorinated single-walled carbon nanotubes (F-SWCNTs) were 10.42% and 9.67% respectively by EDX. The absorption and de-absorption of fluorine properties were studied using high resolution C 1s and F 1s core level XPS and valence band spectra. The fluorine can be completely dissociated from F-MWCNTs, but partially dissociated from SWCNTs. There was 5.79% fluorine in atomic percent remaining associated with the F-SWCNTs when annealing the nanotubes to 500 °C measured by EDX. The results of F 1s core level XPS indicated that the binding energy of fluorine associated on SWCNTs was shifted from 687.0 eV to 688.3 eV after annealing the nanotubes to 500 °C. The results of valence band spectra showed that the binding energy of F 2p and F 2s shifted from 7.5 eV and 31.0 eV to 8.8 eV and 32.5 eV respectively in SWCNTS. However, the two peaks disappeared in annealed MWCNTs.

Crystal structure and bonding characteristics of In-doped β-Zn4Sb3

The effects of indium impurity on the crystal structure and bonding characteristics of In-doped β-Zn4Sb3 were investigated by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD Rietveld refinement indicates that the indium impurity preferentially substitutes one of Sb atoms in Sb–Sb dimer at the 12c Sb(2) site and simultaneously leads to the increase of Zn occupancy. The observations of binding energy shift and a new valence state in Sb 3d core-level XPS spectra can be attributed to the charge transfer from In and Zn to Sb. As a result, more electropositive Zn atoms are needed to maintain the charge balance. The reduction of the lattice thermal conductivity is ascribed to the formation of the asymmetric Sb–In bond, resulting in much low lattice thermal conductivity of 0.49W−1K−1 of Zn4Sb2.96In0.04.

Antibacterial properties and corrosion resistance of AISI 420 stainless steels implanted by silver and copper ions

Silver or copper ions are often chosen as antibacterial agents. But a few reports are concerned with these two antibacterial agents for preparation of antibacterial stainless steel (SS). The antibacterial properties and corrosion resistance of AISI 420 stainless steel implanted by silver and copper ions were investigated. Due to the cooperative antibacterial effect of silver and copper ions, the Ag/Cu implanted SS showed excellent antibacterial activities against both Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) at a total implantation dose of 2×1017 ions/cm2. Electrochemical polarization curves revealed that the corrosion resistance of Ag/Cu implanted SS was slightly enhanced as compared with that of un-implanted SS. The implanted layer was characterized by X-ray photoelectron spectroscopy (XPS). Core level XPS spectra indicate that the implanted silver and copper ions exist in metallic state in the implanted layer.

Effectiveness of a hot-filament chemical vapor deposition method for preparation of a boron-doped superconducting diamond film with higher superconducting transition temperature

We have investigated the effectiveness of a hot-filament chemical vapor deposition (HFCVD) method for preparation of a boron-doped superconducting diamond film with higher superconducting transition temperature (Tc). A boron-doped superconducting diamond film has been fabricated on a diamond (111) substrate using the HFCVD method, and studied by means of scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and resistivity. The film consists of grains with an average size of 200mm. Analyses of valence band and boron 1s core-level XPS spectra indicate the formation of boron-doped diamond film on the substrate. From the resistivity measurements, the film is found to be a superconductor with onset Tc of 7.1K. Carrier concentration determined by Hall conductivity measurements is 1.1×1021cm−3. The value of Tc is higher compared with that in boron-doped superconducting diamond films prepared by a commonly used microwave plasma-assisted chemical vapor deposition (MPCVD) method, at the same carrier concentration [A. Kawano et al., Phys. Rev. B 82 (2010) 085318]. The result of higher Tc in the film by the HFCVD method is consistent with the previous one [Wang et al., Diamond Relat. Mater. 15 (2006) 659], suggesting that the HFCVD method is effective for preparation of boron-doped superconducting diamond films showing higher Tc.