Core-level X-ray Photoelectron Spectroscopy Research Articles

Graphitic carbon nitride sheets sandwiched metal oxides: A novel platform for S-Scheme heterojunction generation for efficient photodegradation of volatile organics

This study reports the preparation of new kind of S-Scheme heterojunction (SSHJ) photocatalyst by distributing two semiconducting metal oxides (titanium dioxide and ZnO) onto the surface of graphitic carbon nitride (G-CN) two dimensional (2D) sheets. The composite photocatalysts, designated as T/Z@ GCN SSHJ, were prepared with different mass ratios of T,Z and G-CN.X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy were used to characterize the microstructure, morphology, chemical composition, electronic states, and optical properties of the T/Z@ GCN SSHJ composite. TEM image of the typical T/Z@ GCN SSHJ composite shows the presence of distribution of particles with sizes in the range 40–80 nm on the lamellar surface of G-CN inferring the formation of a heterostructure Z(0 D) /G-CN(2D)/T(0D) SSHJ composite. The comparison of the core level XPS spectrum of Ti 2p, Zn 2p, C1s, O1s and N1s of T/Z@ GCNSSHJ composite with the binary composites (T+G-CN and Z+G-CN) and pristine components clearly suggests that that Z, T and G-CN are kept together through van der Walls interactions. The T/Z@ GCN SSHJ composite photocatalyst with T: Z: G-CN mass ratio of 1.0: 0.80: 0.12 exhibited enhanced photocatalytic degradation (%) over binary and pristine single components for all the tested volatile organic compounds (VOCs), namely, acetaldehyde (ACD), ethyl acetate (EA), toluene (T) and benzaldehyde (BA). On comparing the % removal of VOCs after 20 minute of light irradiation, BA showed the highest removal % (94.3 %) and the others take the following order: ACD (70.9 %) >EA (56.45 %)> TL (40.12 %). The study explains the enhanced performance of T/Z@ G-CN HJ composite towards VOC photodegradation in terms of S-Scheme based charge transfer and internal electric field driven efficient charge separation. The charge transfer process of T/Z@ GCN HJ composite photocatalysts is discussed through S-Scheme HJ formation and supported with electronic states and band energy levels of the constituent components derived through the results of XPS and DRS analysis. This study gives insight into the design for regulating the hetero-interfaces for enhancing the photocatalysis and expected to further understand into charge transfer mechanism at multi-junction-based photocatalysts.

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.

Growth optimization, optical, and dielectric properties of heteroepitaxially grown ultrawide-bandgap ZnGa2O4 (111) thin film

Ultrawide bandgap ZnGa2O4 (ZGO) thin films were grown on sapphire (0001) substrates at various growth temperatures with a perspective to investigate the electrical and optical characteristics required for high-power electronic applications. Due to the variation in the vapor pressure of Zn and Ga, severe loss of Zn was observed during pulsed laser deposition, which was solved by using a zinc-rich Zn0.98Ga0.02O target. A pure phase single-crystalline ZGO thin film was obtained at a deposition temperature of 750 °C and an oxygen pressure of 1 × 10−2 Torr. The out-of-plane epitaxial relationship between the sapphire and ZGO thin film was obtained from φ-scan. The x-ray rocking curve of the ZGO thin film grown at 750 °C exhibits a full width at half maximum of ∼0.098°, which indicates a good crystalline phase and quality of the thin film. Core-level x-ray photoelectron spectroscopy of ZGO grown at 750 °C indicated that Zn and Ga were in the 2+ and 3+ oxidation states, respectively, and the atomic ratio of Zn/Ga was estimated to be ∼0.48 from the fitted values of Zn-2p3/2 and Ga-2p3/2. The high-resolution transmission electron microscopy images revealed a sharp interface with the thickness of the ZGO film of ∼265 nm, and the signature of minor secondary phases was observed. The bandgap of the ZGO film at different growth temperatures was calculated from the ultraviolet-diffuse reflectance spectroscopy spectra, and its value was obtained to be ∼5.08 eV for the 750 °C grown sample. The refractive index (n) and the extinction coefficient (k) were determined to be ∼1.94 and 0.023 from the ellipsometric data, respectively, and the real dielectric function (ɛr) was estimated to be ∼6.8 at energy 5 eV. The ultrawide bandgap and dielectric function of ZGO recommend its possible potential applications in deep-ultraviolet optoelectronic devices and high-power electronics.

Investigation of the surface band structure and the evolution of defects in β-(AlxGa1−x)2O3

This study examines the electronic and luminescent properties of β-(AlxGa1−x)2O3 (0 ≤ x ≤ 0.42) thin films grown on (0001) sapphire using laser-MBE, with a focus on the evolution of defect energy levels and their impact on surface Fermi level pinning and luminescence. X-ray photoelectron spectroscopy (XPS) and cathodoluminescence (CL) have been employed to analyze surface band bending and defect evolution as a function of aluminum content. The results have revealed a pinned Fermi level at 3.6 eV above the valence band maximum despite the increase in the bandgap. The consequent upward band bending has been confirmed by a peak shift in the core level XPS. The defects that lead to the Fermi level pinning effect are attributed to E2*, which is related to a Ga vacancy or Ga vacancy-O vacancy complex. In addition, CL spectroscopy and depth-resolved CL have demonstrated consistent blue and ultraviolet emissions across the Al content range and a similar suppression of electron concentration on blue and ultraviolet emissions in β-(AlxGa1−x)2O3 and β-Ga2O3. Based on the observed evolution of defects with Al content, the blue band emission is attributed to electron transition in the donor–accepter pair.

Electronic structure of CeCu4In from band structure calculations and X-ray photoelectron spectroscopy

We present a combined new experimental and theoretical study of the electronic structure for the heavy fermion CeCu4In based on X-ray photoelectron spectroscopy (XPS) data and ab-initio band structure calculations. The compound crystallizes in the orthorhombic CeCu4.38In1.62 type of structure (space group Pnnm). Below the Fermi energy the total density of states contains mainly 3d–states of Cu atoms that hybridized with the Ce 4f electronic states. The Ce core-level XPS spectra point to a stable trivalent configuration of Ce atoms in CeCu4In, consistently with the magnetic susceptibility data. For more detailed information about electronic states the fully relativistic band structure was calculated within the density functional theory (DFT) for the first time. Based on these calculations we present calculated photoemission spectra, which very well reproduce the experimental ones. The Fermi level is located at a deep decrease in the density of electronic states and reaches a value of 3.69 states/(eV f.u.). This value is much smaller than the one obtained experimentally, which indicates the importance of many-body effects, which are not properly taken into account in ab-initio calculations. The valence band is formed mainly by Cu(3d) electrons with a small contribution of Ce(4f) electrons in the immediate vicinity of the Fermi level.

Impact of Synthesis Parameters upon the Electronic Structure in PVD-Deposited CdxZn1-xO Composite Thin Films: An XPS-XANES Investigation.

The impact of different synthesis parameters, such as thickness, postsynthesis annealing temperature, and oxygen gas flow rate, upon the electronic structure is discussed in detail in the present experimental investigation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy techniques are used to evaluate the surface electronic properties along with the presence and stability of the CdO2 surface oxide in CdxZn1-xO (x = 0.4) composite thin films. The thin films were synthesized with varying thicknesses using a Cd0.4Zn0.6O (CZO) ceramic and Cd0.4Zn0.6 (CZ) metallic targets and oxygen gas flow rates during magnetron sputtering. The Zn L3,2 edge and O K edge XANES spectra are affected by the oxygen gas flow rate. For the zero rate, an increase in intensity is observed in the Zn L3,2 edge, and notable changes occur in the overall spectral features of the O K edge. In the films synthesized in the presence of oxygen, highly probable O 2p → antibonding Zn 3d electronic transitions decrease the probability of the Zn 2p1/2 → antibonding Zn 3d electronic transition by filling the vacant antibonding Zn 3d states, leading to the reduction in overall intensity in the Zn L3,2 edge. Scanning electron microscopy reveals grain growth with increasing annealing temperature. The annealing induces orbital hybridization, generating new electronic states with higher transition probabilities and intensity enhancement in both Zn L3,2 and O K edges. The presence of the CdO2 surface phase is confirmed by analyzing the Cd 3d5/2 and O 1s XPS core levels. The CdO2 surface phase is observed in the films synthesized using the CZO target for all thicknesses, while the CZ target is only observed for higher thicknesses. Further postsynthesis annealing treatment results in the disappearance of the CdO2 phase. The CdO2 surface phase can be controlled by varying the film thickness and postsynthesis annealing temperature.

Surface Structure to Tailor the Electrochemical Behavior of Mixed-Valence Copper Sulfides during Water Electrolysis.

The semiconducting behavior of mixed-valence copper sulfides arises from the pronounced covalency of Cu-S bonds and the exchange coupling between CuI and CuII centers. Although electrocatalytic study with digenite Cu9S5 and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured Cu9S5 and CuS are prepared and used as electrode materials to study their electrochemistry. Powder X-ray diffraction (PXRD) and microscopic studies have found the exposed surface of Cu9S5 to be d(0015) and d(002) for CuS. Tetrahedral (Td) CuII, distorted octahedral (Oh) CuII, and trigonal planar (Tp) CuI sites form the d(0015) surface of Cu9S5, while the (002) surface of CuS consists of only Td CuII. The distribution of CuI and CuII sites in the lattice, predicted by PXRD, can further be validated through core-level Cu 2p X-ray photoelectron spectroscopy (XPS). The difference in the electrochemical response of Cu9S5 and CuS arises predominantly from the different copper sites present in the exposed surfaces and their redox states. In situ Raman spectra recorded during cyclic voltammetric study indicates that Cu9S5 is more electrochemically labile compared to CuS and transforms rapidly to CuO/Cu2O. Contact-angle and BET analyses imply that a high-surface-energy and macroporous Cu9S5 surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of Cu9S5 and CuS to CuO/Cu2O/Cu(OH)2 electroactive species. The performance of the in situ formed copper-oxides towards electrocatalytic water-splitting is superior compared to the pristine copper sulfides. In this study, the redox chemistry of the Cu9S5/CuS has been correlated to the atomic arrangements and coordination geometry of the surface exposed sites. The structure-activity correlation provides in-depth knowledge of how to interpret the electrochemistry of metal sulfides and their in situ potential-driven surface/bulk transformation pathway to evolve the active phase.

Photoemission study of plutonium oxycarbide

Surface films of plutonium oxycarbide have shown oxidation retardation properties. The plutonium oxycarbide film analyzed in this study has a stoichiometry of PuC0.5O0.3 and is homogenous within the depth probed by x-ray photoelectron spectroscopy. The electronic structure of this plutonium oxycarbide surface film has been investigated using core level x-ray photoelectron spectroscopy and valence band ultraviolet photoelectron spectroscopy. The conduction band has been probed with inverse photoemission spectroscopy. The core level Pu 4f7/2 spectrum consists of two components occurring from the poor and well-screened final states. The valence band spectra display peaks attributable to occupied ligand states and both localized and itinerant Pu 5f electron behavior.

Band alignment and interfacial charge transfer in sol-gel derived anatase/rutile heterophase TiO2: explaining the synergistic photocatalytic activity.

In this study, we have synthesized nanostructured titanium dioxide (TiO2) photocatalysts under different configurations, viz., anatase, rutile and anatase/rutile heterophase (Eg = 3.14-2.96 eV) through a sol-gel route. The photocatalytic performance of the heterophase samples was better than that of their phase-pure counterparts. Photocatalytic performance was maximum for sample T12H (50% rutile) with a rate constant, k = (7.38 ± 1.48) × 10-2 min-1. This is ca. 47% greater than that estimated via an extrapolation method using the values for phase pure samples indicating synergistic effects. High-resolution transmission electron microscope (HRTEM) analysis and the phase-dependent broadening of core-level X-ray photoelectron spectroscopy (XPS) analysis indicate the formation of well-defined anatase/rutile interfaces. Interfacial charge transfer is the critical factor for this synergistic effect. To analyze the charge transfer pathways, the possible band alignment scheme was analyzed through the precise determination of band alignments from XPS, employing the Ti 2p3/2 core level as a reference. We found a staggered rutile band alignment with the valence band edge of rutile lying above that of anatase, indicating the transfer of photogenerated free electrons from the conduction band of rutile to that of anatase and hole transfer from the valence band of anatase to rutile. These inferences are validated through Mott-Schottky analysis as well.

Electronic structure and chemical states of green synthesized silica nanoparticles from biomasses

Silica nanoparticles (SiO2 NPs) were synthesized from the sugarcane bagasse (SB), pinewood stem (PS), walnut shell (WS), and rice husk (RH) using ambient burning, hydro-ball-milling, filtration, and post-annealing at 650 °C. The broadened XRD peak pattern (2θ = 22°) indicates the amorphous nature of all four samples, further confirmed by the circles of the SAED pattern. Microscopic images (snap-shots, SEM, TEM) revealed the agglomeration of SiO2 NPs with an average grain size of 40–80 nm. All samples' bandgap (Eg) was found in the 5.69–5.78 eV range determined from the Tauc's plot. The symmetric stretching of O–Si–O at 440 cm−1, Si–O at 880 cm−1, and asymmetric stretching of Si–O–Si peaks at 1062 cm−1 indicate the formation of polymorphic SiO2 NPs. Electronic and chemical states of SiO2 NPs were investigated by X-ray photoelectron spectroscopy (XPS), and the chemical bonding of SiOx and Si–O–Si occurred at 99.5 eV and 528.6 eV, respectively. Silicon (Si) concentration was observed to be ∼14.35 %, ∼18.88 %, ∼21.25 %, and ∼9.5 %, estimated from core-level XPS analysis. The pinewood stem extracted SiO2 NPs were observed to have a regular spherical shape with a size of 40 nm. The different atomic concentration of Si in all samples establishes their utilization in medical and industrial applications.

Oxygen vacancies mediated changes in ferromagnetic, optical, Photoluminescent and electronic structure properties of Ho-doped CeO2 nanoparticles

The undoped CeO2 and Ce1-xHoxO2 (x = 0.03, 0.05 and 0.07) nanoparticles prepared using microwave assisted co-precipitation route are investigated using X-Ray Diffraction (XRD), UV–Vis–NIR, Raman, photoluminescence, X-ray Photoelectron Spectroscopy (XPS) and SQUID-VSM magnetometer. The XRD validates the inclusion of Ho3+ at the Ce3+/Ce4+ site while maintaining the crystal structure with development of defects and oxygen vacancies with expansion of the CeO2 nanolattice. The composition of the elements with desired stoichiometry is confirmed by Energy Dispersive X-ray spectra analysis. The absorption spectroscopy analysis reports the Urbach energy along with refractive index increases with Ho3+ doping and the optical band gap is decreased indicating red shift. All the nanoparticles are subjected to PL investigations to ascertain the oxygen vacancy and defect structure of nanoparticles. Raman active modes at ∼500 - 630 cm−1 verifies the existence of oxygen vacancies (Vo)., which are seen to rise with concentration of Ho-doping in the CeO2 lattice. When Ce4+ transforms into Ce3+, oxygen vacancies are generated in the lattice to maintain the charge neutrality, as can be seen by the Ce-3d, O-1s, and Ho-4d core level XPS spectra. To demonstrate that Ho3+ doping favours ferromagnetic ordering in the CeO2 lattice, the magnetic properties using the Bound Magnetic Polaron model and F centre exchange mechanism are discussed. The development of various complexes that involve host and doped cations are mediated by Vo suggests the existence of F0, F+, and F2+ centres. The weak ferromagnetic behaviour in Ho3+-doped CeO2 nanoparticles reinforce the relationship between Vo to explain the observed magnetic properties of nanoparticles.

Antisites disorder mediated magnetization relaxation and polydispersity in La2NiMnO6 crystallites

Antisite disorders play a critical role in determining the magnetic ground state of the La2NiMnO6 double perovskite compounds that show multifunctional behaviours and spin-glass phases at lower temperatures. The spin-glass (SG) state and magnetization relaxation in these compounds have been studied extensively; however, the polydispersity of the SG state remains elusive. Employing ac-magnetic susceptibility χ(ω,T) and temperature-dependent electron spin resonance (ESR) spectroscopy, herein, we probe the polydispersity of the SG phase in La2NiMnO6 crystallites. We employed X-ray photoelectron spectroscopy (XPS) to -investigate the crystallite size-dependent variable fractions of Mn3+/Mn4+ (Ni2+/Ni3+) cations and their influence on completing exchange interactions. Mn-2p core-level XPS spectra suggest a dominant Mn4+ + Ni2+→Mn3+ + Ni3+ charge transfer peak that appeared as a satellite peak at lower binding energy than the Mn core-level. The observed differences in saturation magnetization (MS) as a function of crystallite size indicate the variation in the degree of antisites disorder. Competing magnetic interactions driven by mixed valence and disorder facilitate a spin-glass (SG) phase at lower temperatures. Utilizing ESR measurement across paramagnetic → competing ferro/antiferromagnetic phase, a modified g-factor that ranges from 2.050-2.037 is observed in the paramagnetic region. The line width of the ESR signals is found to be increased across the transition suggesting spin-freezing characteristics. Cole-Cole plots obtained from χ(ω,T) data indicate collective spin relaxation dynamics in the proximity of the freezing temperature. Our findings suggest the importance of the size effect and its relation with the degree of antisites disorders in these crystallites. These experimental results enable the La2NiMnO6 compound to find its possible applications in new areas of material research.

Unraveling the atomic and electronic structure of nanocrystals on superconducting Nb(110): Impact of the oxygen monolayer

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Electronic and thermal properties of Ag-doped single crystal zinc oxide via laser-induced technique

The doping of ZnO has attracted lots of attention because it is an important way to tune the properties of ZnO. Post-doping after growth is one of the efficient strategies. Here, we report a unique approach to successfully dope the single crystalline ZnO with Ag by the laser-induced method, which can effectively further post-treat grown samples. Magnetron sputtering was used to coat the Ag film with a thickness of about 50 nm on the single crystalline ZnO. Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was chosen to irradiate the Ag-capped ZnO samples, followed by annealing at 700 °C for two hours to form ZnO:Ag. The three-dimensional (3D) information of the elemental distribution of Ag in ZnO was obtained through time-of-flight secondary ion mass spectrometry (TOF-SIMS). TOF-SIMS and core-level x-ray photoelectron spectroscopy (XPS) demonstrated that the Ag impurities could be effectively doped into single crystalline ZnO samples as deep as several hundred nanometers. Obvious broadening of core level XPS profiles of Ag from the surface to depths of hundred nms was observed, indicating the variance of chemical state changes in laser-induced Ag-doped ZnO. Interesting features of electronic mixing states were detected in the valence band XPS of ZnO:Ag, suggesting the strong coupling or interaction of Ag and ZnO in the sample rather than their simple mixture. The Ag-doped ZnO also showed a narrower bandgap and a decrease in thermal diffusion coefficient compared to the pure ZnO, which would be beneficial to thermoelectric performance.

Bonding of activated ethylene–propylene rubber to surface–modified stainless steel

Film formation of self synthesized Polymer EPM–g–VTMDS (ethylene–propylene rubber, EPM, grafted with vinyltetramethyldisiloxane, VTMDS) was studied regarding bonding to adhesion promoter vinyltrimethoxysilane (VTMS) on oxidized 18/10 chromium/nickel–steel (V2A) stainless steel surfaces. Polymer films of different mixed solutions including commercial siloxane and silicone, dimethyl, vinyl group terminated crosslinker (HANSA SFA 42100, CAS# 68083-19-2, 0.35 mmol Vinyl/g) and platinum, 1,3-diethenyl-1,1,3,3-tetramethyldisiloxane complex Karstedt's catalyst (ALPA–KAT 1, CAS# 68478-92-2) were spin coated on V2A stainless steel surfaces with adsorbed VTMS thin layers in order to analyze film formation of EPM–g–VTMDS at early stages. Surface topography and chemical bonding of the high performance polymers on different oxidized V2A surfaces were investigated with X–ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM) and surface enhanced Raman spectroscopy (SERS). AFM and SEM as well as XPS results indicated that the formation of the polymer film proceeds via growth of polymer islands. Chemical signatures of the essential polymer contributions, linker and polymer backbones, could be identified using XPS core level peak shape analysis and also SERS. The appearance of signals which are related to Si–O–Si can be seen as a clear indication of lateral crosslinking and silica network formation in the films on the V2A surface.

Boron doping of diamond-like carbon films by pulsed laser deposition technique for photovoltaic application

Diamond-like carbon (DLC) films were deposited by pulsed laser deposition technique using a glassy carbon disc as carbon source material. Boron doping in DLC films was carried out by ablating boron disc sequentially during the synthesis of DLC films. Field emission scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and Raman measurements were utilized to characterize the films. Hot-probe study indicated the films to be p-type. Core level XPS spectra of B1s appeared at ∼ 189.5 eV and indicated that boron atoms are in form of B-C network connect. Raman studies indicated a dilatation of host lattice with the incorporation of boron in DLC matrix favouring graphitization. Band gap of the thin films decreased from ∼2.0 eV (pure DLC) to ∼1.76 eV with increased incorporation of boron. A photovoltaic cell structure (glass/indium tin oxide)/B:DLC/ZnO/Ag) conceived here, indicated an open circuit voltage ∼110 mV, short circuit current density ∼7.5 mA/cm2 with a fill factor ∼0.3, and a low conversion efficiency (η) ∼ 0.23%.

Lattice and electronic structure of ScN observed by angle-resolved photoemission spectroscopy measurements

Scandium nitride (ScN) has recently attracted much attention for its potential applications in thermoelectric energy conversion, as a semiconductor in epitaxial metal/semiconductor superlattices, as a substrate for GaN growth, and alloying it with AlN for 5G technology. This study was undertaken to better understand its stoichiometry and electronic structure. ScN (100) single crystals 2 mm thick were grown on a single crystal tungsten (100) substrate by a physical vapor transport method over a temperature range of 1900–2000 °C and a pressure of 20 Torr. The core level spectra of Sc 2p3/2,1/2 and N 1s were obtained by x-ray photoelectron spectroscopy (XPS). The XPS core levels were shifted by 1.1 eV toward higher values as the [Sc]:[N] ratio varied from 1.4 at 1900 °C to ∼1.0 at 2000 °C due to the higher binding energies in stoichiometric ScN. Angle-resolved photoemission spectroscopy measurements confirmed that ScN has an indirect bandgap of ∼1.2 eV.

Valence State Tuning of Gold Nanoparticles in the Dewetting Process: An X-ray Photoelectron Spectroscopy Study.

Gold nanoparticles (AuNPs) are commonly synthesized using the citrate reduction method, reducing Au3+ into Au1+ ions and facilitating the disproportionation of aurous species to Au atoms (Au0). This method results on citrate-capped AuNPs with valence single states Au0. Here, we report a methodology that allows obtaining AuNPs by the dewetting process with three different valence states (Au3+, Au1+, and Au0), which can be fine-tuned with ion bombardment. The chemical surface changes and binding state of the NPs were investigated using core-level X-ray photoelectron spectroscopy (XPS). This is achieved by recording high-resolution Au 4f XPS spectra as a function of ion dose exposure. The results obtained show a time-dependent tuning effect on the Au valence states using low-energy 200 V acceleration voltage Ar+ ion bombardment, and the valence state conversion kinetics involves the reduction from Au3+ and Au1+ to Au0. Proper control of the reduction in the valence states is critical in surface engineering for controlling catalytic reactions.

Properties and device performance of BN thin films grown on GaN by pulsed laser deposition

Wide and ultrawide-bandgap semiconductors lie at the heart of next-generation high-power, high-frequency electronics. Here, we report the growth of ultrawide-bandgap boron nitride (BN) thin films on wide-bandgap gallium nitride (GaN) by pulsed laser deposition. Comprehensive spectroscopic (core level and valence band x-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman) and microscopic (atomic force microscopy and scanning transmission electron microscopy) characterizations confirm the growth of BN thin films on GaN. Optically, we observed that the BN/GaN heterostructure is second-harmonic generation active. Moreover, we fabricated the BN/GaN heterostructure-based Schottky diode that demonstrates rectifying characteristics, lower turn-on voltage, and an improved breakdown capability (∼234 V) as compared to GaN (∼168 V), owing to the higher breakdown electrical field of BN. Our approach is an early step toward bridging the gap between wide and ultrawide-bandgap materials for potential optoelectronics as well as next-generation high-power electronics.

Electronic structure of YbFe4Al8 antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe4Al8 using X-ray photoelectron spectroscopy (XPS) and first-principles calculations. The XPS valence band of YbFe4Al8 consists of two contributions coming from divalent (Yb2+) and trivalent (Yb3+) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4d XPS spectra. We study several collinear antiferromagnetic models of YbFe4Al8 from the first-principles and for comparison we also consider LuFe4Al8 with a fully filled 4f shell. We predict that only Fe sublattices of YbFe4Al8 carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mulliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4f orbitals on Yb valence and Fe magnetic moments.