Core-level Photoelectron Spectra Research Articles

Thermal and photochemical degradation of CsGeI3 and CsGeBr3: XPS and optical studies

The toxicity of complex lead halides significantly limits the commercialization and practical application of perovskite solar cells. In recent days, lead-free complex lead halides with a perovskite structure have become increasingly popular, among which germanium systems are the least studied. Here we present for the first time a systematic study of the thermal and photochemical stability of perovskite thin films of CsGeI3 and CsGeBr3. The measurements of optical absorbance and X-ray photoelectron spectra (XPS) of core levels and valence bands of Ge-based halide perovskites after light soaking and heat stress were thoroughly performed. It has been found the effect of thermal and photochemical degradation of Ge perovskites which is accompanied by reduction of I:Ge and Br:Ge ratios and appearance of tetravalent metal species fixed in XPS Ge 2p and Ge 3d spectra. The XPS O 1s measurements of irradiated and annealed Ge-based perovskites and their comparison with spectra of GeO2 have shown that formation of Ge4+-species is not due to metal oxidation but owing to defect-mediated hole-doping. While the Ge2+→Ge4+ process lowers the thermal and photochemical stability of Ge-based perovskites and limits their use in photovoltaics, the temperature induced hole doping is favorable for thermoelectric applications.

Liquid-jet photoemission spectroscopy as a structural tool: site-specific acid-base chemistry of vitamin C.

Liquid-jet photoemission spectroscopy (LJ-PES) directly probes the electronic structure of solutes and solvents. It also emerges as a novel tool to explore chemical structure in aqueous solutions, yet the scope of the approach has to be examined. Here, we present a pH-dependent liquid-jet photoelectron spectroscopic investigation of ascorbic acid (vitamin C). We combine core-level photoelectron spectroscopy and ab initio calculations, allowing us to site-specifically explore the acid-base chemistry of the biomolecule. For the first time, we demonstrate the capability of the method to simultaneously assign two deprotonation sites within the molecule. We show that a large change in chemical shift appears even for atoms distant several bonds from the chemically modified group. Furthermore, we present a highly efficient and accurate computational protocol based on a single structure using the maximum-overlap method for modeling core-level photoelectron spectra in aqueous environments. This work poses a broader question: to what extent can LJ-PES complement established structural techniques such as nuclear magnetic resonance? Answering this question is highly relevant in view of the large number of incorrect molecular structures published.

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.

Properties and modification of native oxides of InP(100)

Properties of oxidized InP surfaces, which are known to cause less electrical and optical losses than other III–V oxides, are relevant to develop the passivation of current and future applications of III–V crystals (e.g. lasers, detectors). We report that a proper low-temperature (LT) (<400 °C) gas exposure with NH3 or O2 enables beneficial modifications of InP native oxides, of which formation is difficult to avoid in devices. Effects of the gas exposure depend on the doping of InP. NH3 exposure without a plasma source at 250 °C or lower temperature increases photoluminescence (PL) intensity of native-oxide covered n-InP crystals, which interestingly provide a stronger PL signal than n-InP with the HCl-cleaned surface. In contrast, O2 exposure around 300 °C increases PL signal of native-oxide covered p-type InP. Core-level photoelectron spectra reveal that N atoms are incorporated into the native oxide system of InPO4/InP during LT NH3 exposures. Scanning tunneling microscopy shows a band bending and a tendency to crystallization at native-oxide covered InP surfaces. Photoelectron spectra, which are analyzed with recent calculations, show larger variation in the bonding environment for the host In atoms and for incorporated N atoms, as compared to the P bonding sites in the InPO4 native oxide.

Inverse photoemission of the light actinide metals and dioxides

Inverse photoemission spectroscopy (IPES) operating in the isochomat mode (≈10 eV) has been used to probe the unoccupied states in the actinide dioxides of thorium, uranium and plutonium, and the actinide metals of thorium, uranium, α-plutonium and a plutonium-gallium alloy, δ-Pu(Ga). The IPE spectra of the dioxides show a signal predominantly occurring from the 5f unoccupied states and where possible have been curve fit to obtain the 5f5/2 – 5f7/2 spin orbit coupling and intensity values. Combined with the HeI Ultraviolet Photoelectron Spectroscopy of the actinide dioxides the band gap values have been determined. The IPE spectra of the metals show a weak signal on a large background contribution with little difference between the α- and δ-Pu(Ga) phases. The IPE spectra of the metals have been used to aid interpretation of the possible screening mechanisms in the 4f core level photoelectron spectra. Photoelectric work functions for these materials are also reported. UK Ministry of Defence © Crown Owned Copyright 2021/AWE

Determining valence band offsets in heterojunctions using a single core-level x-ray photoelectron spectrum

The behavior and functionality of semiconductor heterojunctions depend critically on the alignments of the valence and conduction bands at the various interfaces. Traditionally, band alignment has been measured by x-ray photoelectron spectroscopy using pairs of distinct core levels, one from each side of the interface, to track the valence band maxima in the two materials. Here, we demonstrate that band alignment across an interface can also be determined using a single core-level photoelectron spectrum for an element that is common to both materials. The energy splitting between the photoemission features originating in the two materials is shown to be dominated by the difference in electrostatic potential across the interface, thereby leading to a reliable determination of the band alignment.

Core and Valence Level Photoelectron Spectroscopy of Nanosolvated KCl.

The solvation of alkali and halide ions in the aqueous environment has been a subject of intense experimental and theoretical research with multidisciplinary interests; yet, a comprehensive molecular-level understanding has still not been obtained. In recent years, electron spectroscopy has been increasingly applied to study the electronic and structural properties of aqueous ions with implications, especially in atmospheric chemistry. In this work, we report core and valence level (Cl 2p, Cl 3p, and K 3p) photoelectron spectra of the common alkali halide, KCl, doped in gas-phase water clusters in the size range of a few hundred water molecules. The results indicate that the electronic structure of these nanosolutions shows a distinct character from that observed at the liquid–vapor interface in liquid microjets and ambient pressure setups. Insights are provided into the unique solvation properties of ions in a nanoaqueous environment, emerging properties of bulk electrolyte solutions with growing cluster size, and sensitivity of the electronic structure to varying solvation configurations.

Surface Chemistry of Gold Nanoparticles Produced by Laser Ablation in Pure and Saline Water.

Pulsed laser ablation in liquid (PLAL) is a powerful method for producing nanoparticle colloids with a long-term stability despite the absence of stabilizing organic agents. The colloid stability involves different reactivities and chemical equilibria with complex ionic-specific effects at the nanoparticle/solvent interface which must be strongly influenced by their chemical composition. In this work, the surface composition of PLAL-produced gold nanoparticles in alkaline and saline (NaBr) water is investigated by X-ray photoelectron spectroscopy on free-flying nanoparticles, exempt from any substrate or radiation damage artifact. The Au 4f photoelectron spectra with a depth profiling investigation are used to evaluate the degree of nanoparticle surface oxidation. In alkaline water, the results preclude any surface oxidation contrary to the case of nanoparticles produced in NaBr solution. In addition, the analysis of Br 3d core-level photoelectron spectra agrees with a clear signature of Br on the nanoparticle surface, which is confirmed by a specific valence band feature. This experimental study is supported by DFT calculations, evaluating the energy balance of halide adsorption on different configurations of gold surfaces including oxidation or adsorbed salts.

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.

Experimental and Theoretical Photoemission Study of Indole and Its Derivatives in the Gas Phase

The valence and core-level photoelectron spectra of gaseous indole, 2,3-dihydro-7-azaindole, and 3-formylindole have been investigated using VUV and soft X-ray radiation supported by both an ab initio electron propagator and density functional theory calculations. Three methods were used to calculate the outer valence band photoemission spectra: outer valence Green function, partial third order, and renormalized partial third order. While all gave an acceptable description of the valence spectra, the last method yielded very accurate agreement, especially for indole and 3-formylindole. The carbon, nitrogen, and oxygen 1s core-level spectra of these heterocycles were measured and assigned. The double ionization appearance potential for indole has been determined to be 21.8 ± 0.2 eV by C 1s and N 1s Auger photoelectron spectroscopy. Theoretical analysis identifies the doubly ionized states as a band consisting of two overlapping singlet states and one triplet state with dominant configurations corresponding to holes in the two uppermost molecular orbitals. One of the singlet states and the triplet state can be described as consisting largely of a single configuration, but other doubly ionized states are heavily mixed by configuration interactions. This work provides full assignment of the relative binding energies of the core level features and an analysis of the electronic structure of substituted indoles in comparison with the parent indole.

Adsorption and reaction layers when turning AISI 304 using various cooling strategies

Abstract This paper deals with the surface chemistry of face-turned workpieces of AISI 304 using various cooling conditions. The workpieces were machined by means of LN2 cooling, CO2 snow cooling, conventional emulsion, sub-zero MWF as well as dry machining. With regard to the machining process, the tool temperature, the process forces, and the surface topography are investigated. The surface chemistry is studied using X-ray photoelectron spectroscopy (XPS). These investigations focus firstly on a detection of the elements occurring on the surface, their concentration and the dominant binding state of the main components. Especially the oxidation state of the surface due to the thermal load in the tool-work interface during machining is analyzed on the basis of O 1s and Fe 2p3 core level photoelectron spectra. Secondly, sputter depth profiling using XPS is performed in order to qualitatively analyze the surface depth distribution of the detected elements. Adsorption and reaction layers can occur during turning of AISI 304 up to a depth of approximately 7 nm depending on the cooling conditions used. The use of emulsion, CO2 snow and dry machining results in a slightly higher carbon concentration. Regarding the oxidation in the reaction layer it can be concluded, that the higher the temperatures in the tool at distance of 1 mm to the contact zone during machining, the more oxidation occurs in these layers.

Probing energy landscapes in multilayer heterostructures: Challenges and opportunities

Quantitative characterization of energy landscapes at buried interfaces is essential for assessing their functionality. This perspective highlights recent developments in reconstructing internal potential profiles based on hard x-ray photoemission experiments that have proved to be a rich source of information. We show that band-edge profiles can be reconstructed from core-level photoelectron spectra by performing a comprehensive search for the best-fit set of associated layer-resolved spectra. The use of hard x-rays allows heterostructures to be probed over length scales comparable to relevant electronic screening lengths. Significantly, this information takes our understanding of such systems to a new level that is not currently achievable using any other experimental technique.

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.

Electronic Structure of Lanthanide-Doped Bismuth Vanadates: A Systematic Study by X-ray Photoelectron and Optical Spectroscopies

Monoclinic BiVO 4 has emerged in recent years as one of the most promising materials for photocatalytic evolution of oxygen under solar irradiation. However, it is in itself unable to phototcatalyze reduction of water to hydrogen due to the placement of the conduction band edge below the potential required for H 2 O/H 2 reduction. As a consequence, BiVO 4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO 4 powders have been shown to be able to both reduce and to oxidize water under solar irradiation, but to date there has been no comprehensive study of the electronic properties of lanthanide-doped bismuth vanadates aimed at establishing the systematic trends in the electronic structure in traversing the lanthanide series. Here, the accessible family of lanthanide-doped BiVO 4 quaternary oxides of stoichiometry Bi 0.5 Ln 0.5 VO 4 (Ln = La to Lu, excluding Pm) has been studied by X-ray powder diffraction, X-ray photoemission spectroscopy, and diffuse reflectance optical spectroscopy. The compounds all adopt the tetragonal zircon structure, and lattice parameters decrease monotonically in traversing the lanthanide series. At the same time, there is an increased peak broadening in the diffraction patterns as the mismatch in ionic radius between Bi 3+ and the Ln 3+ ions increases across the series. Valence band X-ray photoemission spectra show that the final state 4f n-1 structure associated with ionization of lanthanide 4f n states is superimposed on the valence band structure of BiVO 4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO 4 , 4f-related states appear above the top of the main valence band of BiVO 4 and account for the small bandgap in the Ce compound. In all cases, the 4f structure is characteristic of the lanthanide element in the Ln(III) oxidation state. Vanadium 2p and lanthanide 3d or 4d core level photoelectron spectra of those compounds where the lanthanide may in principle adopt a higher (Ln = Ce, Pr, Tb) or lower (Ln = Eu, Yb) oxidation state further confirm the prevalence of the Ln(III) valence state throughout. The visible region optical properties of all samples were studied by diffuse reflectance spectroscopy, with a particular focus on the optical bandgap and the details of transitions associated with localized 4f states. Taken together, the results demonstrate the remarkable tunability of optical and electronic properties for these quaternary materials.

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.

Valence and Core-Level X-ray Photoelectron Spectroscopy of a Liquid Ammonia Microjet.

Photoelectron spectroscopy of microjets expanded into vacuum allows access to orbital energies for solute or solvent molecules in the liquid phase. Microjets of water, acetonitrile and alcohols have previously been studied; however, it has been unclear whether jets of low temperature molecular solvents could be realized. Here we demonstrate a stable 20 μm jet of liquid ammonia (−60 °C) in a vacuum, which we use to record both valence and core-level band photoelectron spectra using soft X-ray synchrotron radiation. Significant shifts from isolated ammonia in the gas-phase are observed, as is the liquid-phase photoelectron angular anisotropy. Comparisons with spectra of ammonia in clusters and the solid phase, as well as spectra for water in various phases potentially reveal how hydrogen bonding is reflected in the condensed phase electronic structure.

Charge Ordering in Ladder Vanadate NaV2O5 Evidenced by Core-level Photoemission

Core-level X-ray photoelectron spectrum of the ladder vanadate NaV2O5 is calculated with an extended Hubbard model, adopting a numerically exact diagonalization method based on the Lanczos algorith...

Revisiting the origin of satellites in core-level photoemission of transparent conducting oxides: The case of n -doped SnO2

The longstanding problem of interpretation of satellite structures in core level photoemission spectra of metallic systems with a low density of conduction electrons is addressed using the specific example of Sb-doped SnO$_2$. Comparison of {\it ab initio} many-body calculations with experimental hard X-ray photoemission spectra of the Sn 4$d$ states shows that strong satellites are produced by coupling of the Sn core hole to the plasma oscillations of the free electrons introduced by doping. Within the same theoretical framework, spectral changes of the valence band spectra are also related to dynamical screening effects. These results demonstrate that, for the interpretation of electron correlation features in the core level photoelectron spectra of such narrow-band materials, going beyond the homogeneous electron gas electron-plasmon coupling model is essential.