Core Level X-ray Photoelectron Spectra Research Articles

An interpretation for the components of 2p 3/2 core level x-ray photoelectron spectra of the cations in some inverse spinel oxides

In an effort to reconcile the various interpretations for the cation components of the 2p 3/2 observed in x-ray photoelectron spectroscopy (XPS) of several spinel oxide materials, the XPS spectra of both spinel alloy nanoparticles and crystalline thin films are compared. We observed that different components of the 2p 3/2 core level XPS spectra, of these inverse spinel thin films, are distinctly surface and bulk weighted, indicating surface-to-bulk core level shifts in the binding energies. Surface-to-bulk core level shifts in binding energies of Ni and Fe 2p 3/2 core levels of NiFe2O4 thin film are observed in angle-resolved XPS. The ratio between surface-weighted components and bulk-weighted components of the Ni and Fe core levels shows appreciable dependency on photoemission angle, with respect to surface normal. XPS showed that the ferrite nanoparticles Ni x Co1−x Fe2O4 (x = 0.2, 0.5, 0.8, 1) resemble the surface of the NiFe2O4 thin film. Surface-to-bulk core level shifts are also observed in CoFe2O4 and NiCo2O4 thin films but not as significantly as in NiFe2O4 thin film. Estimates of surface stoichiometry of some spinel oxide nanoparticles and thin films suggested that the apportionment between cationic species present could be farther from expectations for thin films as compared to what is seen with nanoparticles.

Core and Shell Contributions to the Phonon Spectra of CdTe/CdS Quantum Dots

The parameters of the shell and interface in semiconductor core/shell nanocrystals (NCs) are determinant for their optical properties and charge transfer but are challenging to be studied. Raman spectroscopy was shown earlier to be a suitable informative probe of the core/shell structure. Here, we report the results of a spectroscopic study of CdTe NCs synthesized by a facile route in water, using thioglycolic acid (TGA) as a stabilizer. Both core-level X-ray photoelectron (XPS) and vibrational (Raman and infrared) spectra show that using thiol during the synthesis results in the formation of a CdS shell around the CdTe core NCs. Even though the spectral positions of the optical absorption and photoluminescence bands of such NCs are determined by the CdTe core, the far-infrared absorption and resonant Raman scattering spectra are dominated by the vibrations related with the shell. The physical mechanism of the observed effect is discussed and opposed to the results reported before for thiol-free CdTe Ns as well as CdSe/CdS and CdSe/ZnS core/shell NC systems, where the core phonons were clearly detected under similar experimental conditions.

Impact of pre-annealing time on the growth and properties of Ag2ZnSnSe4 thin films

In this study, we demonstrate the pre-annealing treatment of stacked precursors at 250 °C for different durations (5, 10, 15, 20, 30, and 60 min), subsequently selenized at 400 °C for 1 min and correlate its effects on structural, morphological, and optoelectronic properties of Ag2ZnSnSe4 films (AZTSe) with a detailed characterization. After investigating pre-annealing treatment duration, the grown AZTSe films show various binary/ternary phases and appreciable transformation in surface morphology. However, stacked layers pre-annealed for 20 min and selenized at 400 °C for 1 min show a single-phase kesterite AZTSe with two main Raman peaks at 181 and 198 cm−1 with compact and larger grains (~0.53 μm). The bond length and bond angles for single-phase AZTSe films are evaluated using Rietveld refinement of X-ray diffraction data. The core-level X-ray photoelectron spectra confirm the oxidation formula as Ag2+1Zn+2Sn+4Se4−2 and uniform distribution of elements is established by secondary ion mass spectrometer measurements, for precursor films pre-annealed at 250 °C for 20 min and selenized at 400 °C for 1 min. They possess n-type conductivity with a maximum mobility of 79.4 cm2(Vs)−1 and a direct band gap of 1.37 eV.

Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole-phonon interaction and minority carrier diffusion.

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole-phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers (Vk center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3d5/2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron-hole recombination in KI hole traps).

Experimental determination of electron attenuation lengths in complex materials by means of epitaxial film growth: Advantages and challenges

Accurate electron attenuation lengths are of critical importance in using electron spectroscopic methods to quantitatively characterize complex materials. Here, the authors show that analysis of core-level and valence-band x-ray photoelectron spectra excited with monochromatic AlKα x-rays from the substrate and measured as a function of film thickness can be used to determine electron attenuation lengths in epitaxial SrTiO3 films on Ge(001). Closely lattice-matched epitaxial heterojunctions are ideal systems for determining attenuation lengths provided the films grow in a layer-by-layer fashion, leading to atomically flat surfaces, and the buried interfaces are atomically abrupt. In principle, either the rate of attenuation of substrate peak intensities or the rate of increase of film peak intensities can be used for this purpose. However, the authors find that structural nonuniformities in the films reduce the accuracy of electron attenuation lengths determined from photoelectrons that originate within the films. A more reliable source of information is found in photoelectrons from the substrate which traverse the film. By using the energy dependence of calculated electron attenuation lengths from the NIST database in combination with Ge 3d core and Ge-derived valence-band intensities, the authors determine electron attenuation length as a function of kinetic energy for SrTiO3.

Band bending at the ZnO(0001)-Zn surface produced by electropositive, electronegative and atmospheric adsorbates

We have analysed the surface band bending (SBB) at the clean ZnO(0001)-Zn surface and after its modification by electropositive, electronegative and atmospheric adsorbates. The evolution of the band bending with increasing adsorption was monitored by measuring core-level and valence-band X-ray photoelectron spectra. We found that the SBB produced by the adsorption of electropositive elements was always downwards, reaching in all cases the limit imposed by the crossing of the conduction-band minimum with the Fermi level. The adsorption of electronegative elements produced always upward band bending, but in this case a pinning of the SBB was observed that kept the valence-band maximum far from the Fermi level. A self-consistent equation relating the adsorbate level occupation with the band bending is shown to qualitatively reproduce the observed SBB pinning effect. In contrary to the commonly assumed upwards SBB for the ZnO/air interface, a downward SBB was observed after exposing the clean surface to atmospheric air. A downward SBB was also observed when the surface was bombarded with 1 keV Ar+ in vacuum.

X-ray induced vanadium L3M23M45 auger electron spectra as tool to trace vanadium oxidation state: Study of surface stoichiometry recovery of V2O3 film

Ratio of two peaks in X-ray induced vanadium L3M23M45 Auger electron spectra has been used to monitor oxidation state of V2O3 thin film surface followed by vacuum annealing. The V2O3 film was grown with sol-gel method on the Al2O3 (0001) substrate. The O 1s, V 2p core-level X-ray photoelectron spectra and V L3M23M45 Auger electron spectra were measured from the film as function of annealing temperature. Brief exposure of the film to the air turned surface vanadium to 4+ oxidation state. Annealing in UHV at 455 K completely converted vanadium to 3+. The binding energy (BE) difference between O 1s and V 2p3/2 core-level changed from 14.0 eV to 14.9 eV as annealing temperature increased from room temperature to 455 K. Intensity ratio of the two peaks in V L3M23M45 Auger electron spectra, peak originated from O 2p region to that from V 3d region, changes from 0.91 to 0.63. In combination with measurement from VO2 film, relation between intensity ratio of V L3M23M45 doublet and the oxidation state of vanadium was established by comparison with BE difference between O 1s and V 2p3/2 core-level. V L3M23M45 Auger electron spectra can be useful tool for scanning Auger microscope (SAM) to get high spatial resolution vanadium ion distribution. A few points to consider for application toward SAM mapping have been discussed.

X-Ray Photoelectron Spectroscopy Study of Surface-Modified Silicate Glass

Using characteristics of valence electron and core level X-ray photoelectron spectra at binding energies in the range from 0 to ~1000 eV, we have determined the quantitative elemental composition of the surface of silicate glass samples containing Si, Al, Na, K, Ca, Mg, Fe, S, O, and C, whose surface was modified in different ways. We have identified the oxidation states of the ions in the glasses, which, as expected, correspond to Si4+, Al3+, Na+, K+, Ca2+, and Mg2+. It has been shown that chemical modification of the glass surface produces the largest changes for potassium and sodium: their concentration considerably decreases in comparison with that in the starting glass.

The effects of core-level broadening in determining band alignment at the epitaxial SrTiO3(001)/p-Ge(001) heterojunction

Chemical effects at the surface and interface can broaden core-level spectra in X-ray photoemission for thin-film heterojunctions, as can electronic charge redistributions. We explore these effects and their influence on the measurement of valence and conduction band offsets at the epitaxial SrTiO3(001)/p-Ge(001) heterojunction. We observe a clear broadening in Ge 3d and Sr 3d core-level X-ray photoelectron spectra relative to those of clean, bulk Ge(001), and homoepitaxial SrTiO3(001), respectively. Angle-resolved measurements indicate that this broadening is driven primarily by chemical shifts associated with surface hydroxylation, with built-in potentials playing only a minor role. The impact of these two interpretations on the valence band offset is significant on the scale of transport energetics, amounting to a difference of 0.2 eV.

Single Silver Adatoms on Nanostructured Manganese Oxide Surfaces: Boosting Oxygen Activation for Benzene Abatement.

The involvement of a great amount of active oxygen species is a crucial requirement for catalytic oxidation of benzene, because complete mineralization of one benzene molecule needs 15 oxygen atoms. Here, we disperse single silver adatoms on nanostructured hollandite manganese oxide (HMO) surfaces by using a thermal diffusion method. The single-atom silver catalyst (Ag1/HMO) shows high catalytic activity in benzene oxidation, and 100% conversion is achieved at 220 °C at a high space velocity of 23 000 h-1. The Mars-van Krevelen mechanism is valid in our case as the reaction orders for both benzene and O2 approach one, according to reaction kinetics data. Data from H2 temperature-programmed reduction and O core-level X-ray photoelectron spectra (XPS) reveal that Ag1/HMO possesses a great amount of active surface lattice oxygen available for benzene oxidation. Valence-band XPS and density functional theoretical calculations demonstrate that the single Ag adatoms have the upshifted 4d orbitals, thus facilitating the activation of gaseous oxygen. Therefore, the excellent activation abilities of Ag1/HMO toward both surface lattice oxygen and gaseous oxygen account for its high catalytic activity in benzene oxidation. This work may assist with the rational design of efficient metal-oxide catalysts for the abatement of volatile organic compounds such as benzene.

Electronic structure of single-crystal solid solutions Pb1-xBaxTiO3 (0 ≤ x ≤ 1) from X-ray photoelectron spectroscopy and real-space multiple electron scattering calculations

Valence band and core level X-ray photoelectron spectra (XPS) are measured in single-crystal solid solutions Pb1-xBaxTiO3 (x = 0, 0.021, 0.146, 0.24, 0.541, 0.784, 0.906, 1). Valence band densities of states (DOS) and theoretical valence band XPS are calculated for Pb1-xBaxTiO3 (x = 0, 0.12, 0.25, 0.5, 0.875, 1) via the method of multiple electron scattering in real space. Experimental and theoretical valence band XPS are in good agreement. The increase of ionicity of the TiO bonds in Pb1-xBaxTiO3 upon the growth of relative Ba content x is predicted theoretically and shown experimentally using the energy position of the charge transfer satellites in the Ti2p XPS. The increase of Ti2p-spectrum line widths against relative Ba content x is observed.

Laser induced Bi diffusion in As40S60 thin films and the optical properties change probed by FTIR and XPS

Amorphous chalcogenide semiconducting materials are playing a pivotal role in modern technology. Such type of materials are very sensitive to electromagnetic radiations which is useful for infrared optics. In the present report, Bi doped in As40S60 thin films (As40S60, Bi06As40S54) of 800 nm thickness were prepared by thermal evaporation method. The Bi06As40S54 thin film is subjected to laser irradiation for photo induced study. The X-ray diffraction study reveals no structural change due to laser irradiation. The optical parameters are affected by both Bi addition and laser irradiation which brings a change in the transmitivity and absorption coefficient. The indirect optical band gap is found to be increased by 0.08 eV with laser irradiation with the decrease in disorderness. The Tauc parameter and Urbach energy which measures the degree of disorderness changes with Bi doping and irradiation. The refractive index is modified by the illumination process which is useful for optical applications. The optical property change is well supported by the X-ray photoelectron core level spectra.

Electronic and magnetic structure of epitaxialFe3O4(001)/NiOheterostructures grown on MgO(001) and Nb-dopedSrTiO3(001)

We study the underlying chemical, electronic and magnetic properties of a number of magnetite based thin films. The main focus is placed onto NiO/Fe$_3$O$_4$(001) bilayers grown on MgO(001) and Nb-SrTiO$_3$(001) substrates. We compare the results with those obtained on pure Fe$_3$O$_4$(001) thin films. It is found that the magnetite layers are oxidized and Fe$^{3+}$ dominates at the surfaces due to maghemite ($\gamma$-Fe$_2$O$_3$) formation, which decreases with increasing magnetite layer thickness. From a layer thickness of around 20 nm on the cationic distribution is close to that of stoichiometric Fe$_3$O$_4$. At the interface between NiO and Fe$_3$O$_4$ we find the Ni to be in a divalent valence state, with unambiguous spectral features in the Ni 2p core level x-ray photoelectron spectra typical for NiO. The formation of a significant NiFe$_2$O$_4$ interlayer can be excluded by means of XMCD. Magneto optical Kerr effect measurements reveal significant higher coercive fields compared to magnetite thin films grown on MgO(001), and a 45$^{\circ}$ rotated magnetic easy axis. We discuss the spin magnetic moments of the magnetite layers and find that the moment increases with increasing thin film thickness. At low thickness the NiO/Fe$_3$O$_4$ films grown on Nb-SrTiO$_3$ exhibits a significantly decreased spin magnetic moments. A thickness of 20 nm or above leads to spin magnetic moments close to that of bulk magnetite.

Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes

The origin, the nature, and the electronic structure of surface defects causing surface states on metal oxides and their role in solar water splitting have been under scrutiny for several decades. In the present study, the surface of hematite films is treated with an oxygen plasma and then subject to a detailed investigation with electroanalytical methods and element orbital specific X-ray spectroscopy. We observe a systemic variation of photoelectrochemical properties with oxygen treatment time. Fe 2p and O 1s core level X-ray photoelectron spectra and resonant valence band photoemission at the Fe 3p edge reveal the filling of prevalent oxygen vacancies with concomitant oxidation of Fe2+ to Fe3+ upon the oxygen treatment. The dc bias dependent impedance spectra confirm how a prevalent capacitive surface state, which evolves parallel with the photocurrent onset potential, becomes diminished upon oxygen treatment. Surface states of iron induce higher reactivity toward water oxidation than oxygen surface st...

Atomistic aspects of carrier concentration variation in post-annealed indium tin oxide films

Post-annealing environment-dependent optical and electrical properties of indium tin oxide films grown on glass were examined. X-ray diffraction measurements revealed that all of the films exhibited poly-crystallinity after annealing at 400 °C for 10 min O2, in-air and N2. The optical property measurements yielded >80% transmittances for all the films except for the as-grown and O2-annealed films, even though there were no significant optical band-gap energy differences. In the Hall measurements, all of the films exhibited n-type characteristics. However, the film annealed under the N2 environment showed the best electrical properties (highest carrier concentration and conductivity). The physical origin of electrical property variations due to annealing environment differences was explained by examining the core-level x-ray photoelectron spectra.

Defect-induced optical and electrical property modification in amorphous InGaZnO4 films

We report the physical characteristics of InGaZnO4 films deposited at various temperatures. The films were deposited on Al2O3(0001) substrates using pulsed laser deposition technique. Based on X-ray diffraction and field emission scanning electron microscopy measurements, the crystal structure changed from amorphous to polycrystalline as deposition temperature increased to 550°C. Furthermore, UV–vis measurements revealed a decrease in tail state i.e. improvement of local ordering, resulting in an increase of optical band-gap energy as deposition temperature increased. The core-level X-ray photoelectron spectra also showed an increase (decrease) in metal-oxide (oxygen deficiency) bond as the deposition temperature increased. The carrier concentration, Hall mobility and conductivity variation with deposition temperature are related to the competition between oxygen deficiency and grain boundary formation.

Enhancing mechanism of visible-light absorption for photovoltaic response and photoluminescence of Zn2SnO4 with high solubility of Bi3+

The enhanced visible-light absorption for photovoltaic response and room temperature photoluminescence of Bi3+-doped Zn2SnO4 nanoparticles are studied. The valence states of Bi3+ and Zn2+/Sn4+ are detected by their core level X-ray photoelectron spectra. BiZn+ and BiSn− from substitutions of Bi3+ for Zn2+/Sn4+ interattract and form noncompensated n–p codopant band to improve visible light absorption and facilitate electron transition for photovoltaic response and photoluminescence. Abundant surface states and weak built-in electric field near surface improve together the separation of photoinduced charges of n-type nanoparticle semiconductors. The investigation of high dopant solubility to enhance visible light absorption could be applied to other wide band gap ternary oxides.

Tl1−xIn1−xSnxSe2 (x = 0, 0.1, 0.2, 0.25) single-crystalline alloys as promising non-linear optical materials

Novel materials for the infrared two-photon absorption—Tl1−xIn1−xSnxSe2 single crystals (x = 0, 0.1, 0.2, 0.25) were grown. Their optoelectronic properties including two-photon absorption at wavelength equal to 9.4 μm at different temperatures were studied. From the spectral data it was established that band gap energy increases with increasing SnSe2 content in the solid solutions reaching its maximum for the largest content (x = 0.25). The infrared two photon absorption has achieved its maximal value at x = 0.1. The temperature dependence of the optical band gap at various compositions in the range of 77–300 К is practically linear. The band gap value decreases with the increase of T causing the spectral shift of the absorption edge to the low-energy region. Additionally X-ray photoelectron core-level spectra for pristine and Ar+-ion irradiated surfaces of Tl1−xIn1−xSnxSe2 single crystals have been measured.

Iron Resonant Photoemission Spectroscopy on Anodized Hematite Points to Electron Hole Doping during Anodization

Anodization of α-Fe(2)O(3) (hematite) electrodes in alkaline electrolyte under constant potential conditions the electrode surface in a way that an additional current wave occurs in the cyclic voltammogram. The energy position of this current wave is closely below the potential of the anodization treatment. Continued cycling or exchanging of the electrolyte causes depletion of this new feature. The O 1s and Fe 2p core-level X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra of such conditioned hematite exhibit a chemical shift towards higher binding energies, in line with the general perception that anodization generates oxide species with dielectric properties. The valence band XPS and particularly the iron resonant valence band photoemission spectra, however, are shifted towards the opposite direction, that is, towards the Fermi energy, suggesting that hole doping on hematite has taken place during anodization. Quantitative analysis of the Fe 2p resonant valence band photoemission spectra shows that the spectra obtained at the Fe 2p absorption threshold are shifted by virtually the same energy as the anodization potential towards the Fermi energy. The tentative interpretation of this observation is that anodization forms a surface film on the hematite that is specific to the anodization potential.

Absence of ferromagnetism in Mn-doped tetragonal zirconia

In a recent letter, it has been predicted within first principle studies that Mn-doped ZrO2 compounds could be good candidates for spintronics application because expected to exhibit ferromagnetism far beyond room temperature. Our purpose is to address this issue experimentally for Mn-doped tetragonal zirconia. We have prepared polycrystalline samples of Y0.15(Zr0.85−yMny)O2 (y = 0, 0.05, 0.10, 0.15, 0.20) by using standard solid state method at equilibrium. The obtained samples were carefully characterized by using x-ray diffraction, scanning electron microscopy, elemental color mapping, x-ray photoemission spectroscopy, and magnetization measurements. From the detailed structural analyses, we have observed that the 5% Mn doped compound crystallized into two symmetries (dominating tetragonal and monoclinic), whereas higher Mn doped compounds are found to be in the tetragonal symmetry only. The spectral splitting of the Mn 3s core-level x-ray photoelectron spectra confirms that Mn ions are in the Mn3+ oxidation state and indicate a local magnetic moment of about 4.5 μB/Mn. Magnetic measurements showed that compounds up to 10% of Mn doping are paramagnetic with antiferromagnetic interactions. However, higher Mn doped compound exhibits local ferrimagnetic ordering. Thus, no ferromagnetism has been observed for all Mn-doped tetragonal ZrO2 samples.