3d Core Level Research Articles

Studies of Ultrafast Molecular Dynamics by Femtosecond Extreme Ultraviolet Absorption Spectroscopy

Transient absorption spectroscopy is one of the most widely used experimental techniques for time-resolved studies of ultrafast phenomena. Recent advances in the generation of short-wavelength pulses of ultrashort duration have made transient absorption probing of atomic core level transitions possible. In this Highlight Review, we introduce our recent work on using extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the ultrafast molecular dynamics induced by intense laser fields. While investigations of intense laser-molecule interaction have primarily focused on elucidating electron dynamics, it is important to understand the vibrational dynamics that are triggered by intense laser fields because of their coupling to electron dynamics. Probing the iodine 4d core level, our XUV transient absorption studies of iodoalkanes irradiated by intense laser fields elucidate ultrafast dissociative ionization dynamics and vibrational wave packet dynamics. Time-dependent XUV spectral shifts and global analysis of time-resolved XUV transient absorption spectra can be used to reconstruct C–I dissociation dynamics and identify multiple dissociation channels. In addition, XUV transient absorption spectroscopy also unravels both small- and large-amplitude coherent vibrational motion, whose origin can be identified by analyzing the phase of the oscillatory XUV transition energies. Our studies show that XUV transient absorption spectroscopy is a powerful technique for the study of ultrafast molecular dynamics.

Tuning of ferromagnetic behavior of GaN films by N ion implantation: An experimental and first principle-based study

Magnetic properties of N-ion implanted GaN films (150 nm) have been reported. It is found that GaN films grown by the MOCVD technique show strong room temperature ferromagnetic behavior, which can be tuned by implanting N-ions at different fluences (1 × 1015 to 5 × 1016 ions-cm−2). Presence of implanted N at interstitial sites of the GaN host matrix is indicated from the strain observed in GaN by analysis of XRD data. PL spectra show presence of different types of defects in the as deposited film and engineering of defects after N-ion implantation. XPS spectra of Ga 3d-core level and valence band reveal the bonding of implanted N with the host Ga and/or N. The origin of ferromagnetic behavior is ascribed to unpaired electrons created at N sites due to Ga vacancies. First principle-based calculations also confirm ferromagnetism due to Ga vacancies and the reduction of magnetic behavior in Ga deficient GaN with N-ion implantation at interstitial site. The systematic reduction in the saturation magnetic moment value after N-ion implantation is explained on the basis of pairing of the unpaired electrons due to the bond formation of interstitial N with Ga and N present in the host matrix.

A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

Simultaneous Observation of Carrier-Specific Redistribution and Coherent Lattice Dynamics in 2H-MoTe2 with Femtosecond Core-Level Spectroscopy.

We employ few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to reveal simultaneously the intra- and interband carrier relaxation and the light-induced structural dynamics in nanoscale thin films of layered 2H-MoTe2 semiconductor. By interrogating the valence electronic structure via localized Te 4d (39-46 eV) and Mo 4p (35-38 eV) core levels, the relaxation of the photoexcited hole distribution is directly observed in real time. We obtain hole thermalization and cooling times of 15 ± 5 fs and 380 ± 90 fs, respectively, and an electron-hole recombination time of 1.5 ± 0.1 ps. Furthermore, excitations of coherent out-of-plane A1g (5.1 THz) and in-plane E1g (3.7 THz) lattice vibrations are visualized through oscillations in the XUV absorption spectra. By comparison to Bethe-Salpeter equation simulations, the spectral changes are mapped to real-space excited-state displacements of the lattice along the dominant A1g coordinate. By directly and simultaneously probing the excited carrier distribution dynamics and accompanying femtosecond lattice displacement in 2H-MoTe2 within a single experiment, our work provides a benchmark for understanding the interplay between electronic and structural dynamics in photoexcited nanomaterials.

Development of UHV pulsed laser deposition set-up for in-situ photoelectron spectroscopic study at ARPES beamline, Indus-1 synchrotron radiation source, India

We have developed an Ultra-High Vacuum (UHV) compatible Pulsed Laser Deposition (PLD) set-up at Angle Resolved Photo Electron Spectroscopy (ARPES) beamline (BL-3) at Indus-1 synchrotron radiation source, Raja Ramanna Centre for Advanced Technology, Indore, India. The set-up is successfully tested and integrated at beamline for in-situ photoelectron spectroscopic (PES) studies of thin films. In present PLD set-up, quick in-situ transfer of deposited thin films to analysis chamber enables the surface sensitive PES measurements on atomically cleaned surfaces without the requirement of any sputtering. This set-up has the capability of thin film depositions using six number of one-inch targets with precise control of various deposition parameters at a base pressure of ∼1x10−9 mbar. To explore the capability of the system, a set of thin films of HfO2, 5 % Ga2O3 doped ZnO (GZO) and GZO/HfO2 are deposited and the band offset of GZO/HfO2 heterostructure is investigated by in-situ PES measurements using synchrotron radiation. The film surface is found to be contamination free, which demonstrates the capability of the set-up for in-situ PES studies. The Hf 4f and Zn 3d core level peaks along with valence band maxima positions are used to determine the valence band offset. The valence band offset is found to be 0.31 ± 0.08 eV, which is the first result reported on this heterostructure system.

Swift heavy ion irradiation-induced modifications in the electrical and surface properties of β-Ga2O3

The electrical device characteristics of Ni/β-Ga2O3 vertical Schottky barrier diodes (SBDs) were measured in situ during the irradiation of 120 MeV Ag7+ swift heavy ions (SHIs). These devices exhibit SHI irradiation-induced degradation with 120 MeV Ag7+ ions in the ion fluence ranges of 1 × 1010 to 1 × 1012 ions/cm2. The height of the Schottky barrier is found to decrease from 1.11 to 0.93 eV, and the ideality factor increases from 1.16 to 2.06. These changes indicate the degradation of the device with SHI irradiation. A significant four orders increase is observed in the leakage current density from 4.04 × 10−8 to 1.98 × 10−4 A/cm2 at −1 V, and the series resistance also increases from 3.38 × 103 to 1.15 × 104 Ω. X-ray photoelectron spectroscopy measurements show that the Ga ions are present in divalent and trivalent states with the spectral features having the binding energies centered at 20.2 eV and 19.9 eV (Ga 3d core-levels) before and after ion irradiation. The O 2s peak shifts to 23.7 eV, and there is an increase in intensity and peak broadening due to the change in the trivalent to divalent state of Ga due to the irradiation. The O(I) peak appears at 530.7 eV in the pristine sample with the Ga–O bonding with the Ga3+ state in pure Ga2O3. Moreover, there is a significant change in the intensity and the peak width of O(II) centered at 533.0 eV after ion irradiation at the fluence of 1 × 1012 ions/cm2. This indicates that there is an increase in the surface adsorbed/lattice oxygen, resulting in GaO.

A facile synthesis and characterization of surfactant (CTAB/TSC/TX-100) assisted Ir-Sn bimetallic nanoparticles

The present work is mainly focused on the synthesis and characterization of surfactant-assisted Ir-Sn BMNP (bimetallic nanoparticles) using modified polyol method. CTAB (Cetyltrimethylammonium bromide), TSC (Trisodium citrate), and TX-100 (Triton X-100) as cationic, anionic, and nonionic surfactant respectively are used to study their effect on the particle size, morphology, and their stability. The synthesized particles are characterized by XRD, FE-SEM, EDAX, elemental mapping, TEM, HR-TEM, XPS and FT-IR, TGA, DTG, and DTA techniques. UV-Vis spectroscopy is used to monitor the synthesis of nanoparticles. The XRD patterns of all samples confirm that Ir-Sn BMNP are amorphous in nature. The influence of surfactant on the morphology and particle size of Ir-Sn BMNP samples was examined by the FE-SEM technique. The FE-SEM figures demonstrate that the particles of bimetal are in nano size, highly dispersed, and pseudo spherical in shape with smooth surfaces. The approximate crystallite (particle/grain) size of CTAB, TSC, and TX-100 assisted samples are ~1.39 nm (39 nm), ~2.2 nm (59 nm), and ~5.79 nm (89 nm) respectively, which are estimated from TEM and FE-SEM images. The presence and distribution of Ir and Sn elements in the BMNP is determined through EDAX and elemental mapping respectively. These techniques show that CTAB assisted Ir-Sn BMNP sample is in alloy form but TSC and TX-100 samples are in core-shell form, where the Ir core is surrounded by Sn shell. The XPS confirms the metallic state (Ir0), and the dual valence state of Sn (Sn2+ and Sn4+) is in the Sn 3d core level.

Electronic structure studies on single crystalline Nd2PdSi3, an exotic Nd-based intermetallic: evidence for Nd 4f hybridization

In the series R2PdSi3, Nd2PdSi3 is an anomalous compound in the sense that it exhibits ferromagnetic order unlike other members in this family. The magnetic ordering temperature is also unusually high compared to the expected value for a Nd-based system, assuming 4f localization. Here, we have studied the electronic structure of single crystalline Nd2PdSi3 employing high resolution photoemission spectroscopy and ab initio band structure calculations. Theoretical results obtained for the effective on-site Coulomb energy of 6 eV corroborate well with the experimental valence band spectra. While there is significant Pd 4d–Nd 4f hybridization, the states near the Fermi level are found to be dominated by hybridized Nd 4f–Si 3p states, which is possibly responsible for the ferromagnetism in Nd compound. Nd 3d core level spectrum exhibits multiple features manifesting strong final state effects due to electron correlation, charge transfer and collective excitations. These results serve as one of the rare demonstrations of hybridization of Nd 4f states with the conduction electrons possibly responsible for the exoticity of this compound.

Molybdenum diselenide thin films grown by atomic layer deposition: An XPS analysis

Molybdenum diselenide (MoSe2) thin films were deposited on annealed titanium foils by atomic layer deposition using suitable precursors. In this paper, a detailed x-ray photoelectron spectroscopy analysis of the MoSe2 film is presented. Survey spectra, Mo 3d, Se 3d, Mo 3p, Se LMM, Se 3p, C 1s, and Se 4s core level along with the valence band spectra were measured. Quantitative analysis indicates a surface composition of MoSe1.8, suggesting a deficiency of selenium on the surface.

XPS of the surface chemical environment of CsMAFAPbBrI trication-mixed halide perovskite film

Perovskite-based materials have been considered as the most promising materials in several solar-driven processes, especially photovoltaics. Some features such as high sunlight harvesting and improved carrier transport have been highlighted to have an impact on the efficiency of the above topics, but limited studies have pointed out their surface composition, which mainly influence the above abilities. As a starting point to recognize the surface environment and chemical states of these materials, x-ray photoelectron spectroscopy measurements were performed. Trication-mixed halide perovskite films based on Cs0.08MA0.18FA0.78PbBr0.42I2.58 were prepared by a one step process and deposited on an indium tin oxide substrate by spin-coating. Survey spectra, Pb 4f, I 3d, Br 3d, Cs 3d, C 1s, N 1s, Pb 4d, and Pb 5d core levels, and valence band spectra were measured for the semiconductor sample. Results exhibit symmetrical peaks, indicating that a homogenous solid solution was achieved. Main chemical states of this kind of perovskites such as Cs-Br and Pb-I species are also shown. We deduced that the later signals are associated with the [PbI6]4− octahedra existing in the perovskite lattice, while the former one is related to Cs+ bounded to [PbBr6]4− units on the perovskite surface. Interestingly, the NR4+ signal was also identified, associated with the presence of methylammonium and formamidinium cations.

Surface Composition of a Highly Active Pt3Y Alloy Catalyst for Application in Low Temperature Fuel Cells

AbstractCurrently, platinum is the most widely used catalyst for low temperature proton exchange membrane fuel cells (PEMFC). However, the kinetics at the cathode are slow, and the price of platinum is high. To improve oxygen reduction reaction (ORR) kinetics at the cathode, platinum can be alloyed with rare earth elements, such as yttrium. We report that Pt3Y has the potential to be over 2 times more active for the ORR compared with Pt inside a real fuel cell. We present detailed photoemission analysis into the nature of the sputtered catalyst surface, using synchrotron radiation photoelectron spectroscopy (SRPES) to examine if surface adsorbates or impurities are present and can be removed. Pretreatment removes most of the yttrium oxide in the surface leaving behind a Pt overlayer which is only a few monolayers thick. Evidence of a substochiometric oxide peak in the Y 3d core level is presented, this oxide extends into the surface even after Ar+ sputter cleaning in‐situ. This information will aid the development of new highly active nanocatalysts for employment in real fuel cell electrodes.

Cadmium selenide by XPS

Cadmium selenide was analyzed using x-ray photoelectron spectroscopy. The specimen is a powder purchased from Chemsavers. The sample was fixed to a stainless-steel sample holder with copper 3MTM double-sided adhesive tape. Survey spectra, Cd 3d, Se 3d, O 1s, Cd 3p, Cd MNN, Se LMM, Se 3p, C 1s, and Cd 4d core level along with the valence band spectra were recorded. Results point out a stoichiometry of 0.95 on surface.

On the Real Nature of Rh Single‐Atom Catalysts Dispersed on the ZrO2 Surface

AbstractThe nature of a Rh single‐atom catalyst (SAC) stabilized on the surface of tetragonal zirconia, t‐ZrO2, is investigated here by performing extensive DFT calculations on various possible structural models and comparing the resulting spectral properties with existing data from the literature. The models considered include a Rh atom adsorbed on the clean surface, (Rh)ads, Rh atoms stabilized by the reaction with surface OH groups to form direct Rh−O bonds, (RhO)ads, and (RhO2)ads, the interaction of a Rh atom with an OH group, (RhOH)ads, and a Rh atom replacing Zr in the lattice, (Rh)subZr. Also a t‐ZrO2 supported Rh6 cluster has been considered for comparison with Rh single atoms. Surface heterogeneity has been taken into account by computing the various Rh species at terraces and at step sites of the zirconia surface. Based on the calculated adsorption energies, Rh 3d core level binding energies, and stretching frequencies of adsorbed CO and comparison with the experimental data we conclude that the potential candidates for Rh SAC on t‐ZrO2 are (RhO)ads and (RhOH)ads species.

Ultrafast dissociative ionization and large-amplitude vibrational wave packet dynamics of strong-field-ionized di-iodomethane.

We employ few-cycle pulses to strong-field-ionize di-iodomethane (CH2I2) and femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to investigate the subsequent ultrafast dissociative ionization and vibrational wave packet dynamics. Probing in the spectral region of the I 4d core-level transitions, the time-resolved XUV differential absorption spectra reveal the population of several electronic states of CH2I2 + by strong-field ionization. Global analysis reveals three distinct time scales for the observed dynamics: 20 ± 2 fs, 49 ± 6 fs, and 157 ± 9 fs, ascribed to relaxation of the CH2I2 + parent ion from the Franck-Condon region, dissociation of high-lying excited states of CH2I2 + to I+ (3P2), CH2I, and I2 + (2Π3/2,g), and dissociation of CH2I2 + to I (2P3/2) and CH2I+, respectively. Oscillatory features in the time-resolved XUV differential absorption spectra point to the generation of vibrational wave packets in both the residual CH2I2 and the CH2I2 + parent ion. Analysis of the oscillation frequencies and phases reveals, in the case of neutral CH2I2, C-I symmetric stretching induced by bond softening and I-C-I bending driven by a combination of bond softening and R-selective depletion. In the case of CH2I2 +, both the fundamental and first overtone frequencies of the I-C-I bending mode are observed, indicating large-amplitude I-C-I bending motion, in good agreement with results obtained from ab initio simulations of the XUV transition energy along the I-C-I bend coordinate. These results show that femtosecond XUV absorption spectroscopy is well-suited for studying ultrafast photodissociation and vibrational wave packet dynamics.

TiZrN thin films under CO2 and thermal treatment characterized by x-ray photoelectron spectroscopy

TiZrN thin films were prepared by DC magnetron sputtering on silicon (111) substrates. Samples were subjected to different temperature conditions (200, 400, and 600 °C) under a flow of carbon dioxide to emulate a corrosive atmosphere. Each treatment was performed in situ. X-ray photoelectron spectroscopy was used to examine the surface chemical changes on TiZrN thin films before and after the treatments. Survey spectra and C 1s, O 1s, N 1s, Ti 2p, and Zr 3d core level spectra were measured for each sample. Results show remarkable differences in all spectra when the sample was heated over 400 °C. At these temperatures, it is evident that the coating undergoes the greatest chemical change since metals cease to be nitrides and oxidize.

Electronic structure of CaMn1-xNbxO3 (x=0.02, 0.04, 0.06 and 0.08) perovskites. Self-organization of terminal layers

Electronic structure of niobium doped CaMn1-xNbxO3 (x = 0.02, 0.04, 0.06 and 0.08) powder and fresh chip manganites was studied by x-ray absorption near edge spectroscopy (XANES) at Mn K-edge, and by x-ray photoelectron spectroscopy (XPS) of Mn 3s -, Mn 2p -, O 1s -, Ca 2p- and Nb 3d-core levels. These XPS spectra for all the studied perovskites are very similar. The O 1s and Ca 2p spectra for powders differ from those for fresh chips because of the quick oxidation and carbonization of powder surface. The Mn 3s, Mn 2p and Nb 3d spectra for the powders and fresh chips are almost the same. According to Mn 3s spectra the formal valence of surface manganese ions is 3.82 for all studied oxides indicating the presence of oxygen vacancies in the samples. Niobium is in pentavalent state as evidenced by XPS measurements, but the negative chemical shift of the Nb 3d5/2 core level with respect to Nb2O5 oxide is observed. High similarity of the XPS spectra of the samples with different Nb doping is probably a result of the self-organization of terminal layers.

Precise determination of surface band bending in Ga-polar n-GaN films by angular dependent X-Ray photoemission spectroscopy

We present a systematic study of surface band bending in Ga-polar n-GaN with different Si doping concentrations by angular dependent X-ray photoelectron spectroscopy (ADXPS). The binding energies of Ga 3d and N 1 s core levels in n-GaN films increase with increasing the emission angle, i. e., the probing depth, suggesting an upward surface band bending. By fitting the Ga 3d core level spectra at different emission angles and considering the integrated effect of electrostatic potential, the core level energy at the topmost surface layer is well corrected, therefore, the surface band bending is precisely evaluated. For moderately doped GaN, the electrostatic potential can be reflected by the simply linear potential approximation. However, for highly doped GaN samples, in which the photoelectron depth is comparable to the width of the space charge region, quadratic depletion approximation was used for the electrostatic potential to better understand the surface band bending effect. Our work improves the knowledge of surface band bending determination by ADXPS and also paves the way for studying the band bending effect in the interface of GaN based heterostructures.

MICRO-EMULSION METHOD FOR THE SYNTHESIS AND CHARACTERIZATION OF La, Mn-DOPED Ba3Co2 Fe24O41 Z-TYPE HEXA-NANOFERRITES

In this report, micro-emulsion method was used to obtain La, Mn-doped Ba3Co2 [Formula: see text][Formula: see text] single-phase Z-type hexa-nanoferrites by maintaining annealing temperature at 1200∘C. X-ray diffraction (XRD) analysis shows lattice parameters, crystallite size, and volume are significantly enhanced by introducing La and Mn ions instead of Ba and Fe, respectively. Moreover, La and Mn ion doping improved the preferred orientation of atoms through the basal plane of Z-type hexaferrite. Field emission scanning electron microscopy (FESEM) study reveals that the surface morphology of La- and Mn-doped Ba3Co2 [Formula: see text][Formula: see text], Z-type ferrites show ultrafine and small particle size. The analysis of X-Ray absorption near edge spectroscopy (XANES) suggests that transition in [Formula: see text] has occurred from 3d core level to empty electronic state above the Fermi level. Furthermore, it is observed that in the Z-type ferrites Co exist as divalent ion and Fe as trivalent ion by analyzing XANES. Vibrating samples magnetometer (VSM) indicated that the samples are ferromagnetic as they have narrow hysteresis loops. The decrease in coercivity is due to paramagnetic dopant element Manganese (Mn).

Point Defect-Mediated Interface Formation and Appearance of a Cooper Minimum for AlOx Atomic-Layer-Deposited Films on CH3NH3PbI3

We report on the interaction of CH3NH3PbI3 substrates with AlOx films prepared by atomic layer deposition at room temperature. We use synchrotron radiation-based photoemission spectroscopy and study the Pb 4f, I 3d, and Al 2p core levels as well as the corresponding valence band states (Pb 5d and O 2p). A Cooper minimum is observed for both the Pb 5d and O 2p states, and it indicates that the bonding at the interface must be covalent. We focus on the electronic properties of the substrate and its interface and identify a delicate charge balance between polaronic and excitonic states in MAPI and AlOx. The perovskite vacancy sites, identified by changes in the core-level intensities, mediate the charge balance and covalent interaction.

Combined effects of oxygen pressures and RF powers on the electrical characteristics of ITO-based multilayer transparent electrodes

The opto-electrical properties of plasma-treated ITO films were investigated as functions of oxygen pressures (3.33 and 26.66 Pa) and radio-frequency (RF) plasma powers (50 and 150 W). The plasma-treated samples showed higher transmittance than the untreated sample. After the low-oxygen treatment, the carrier concentration increased with increasing plasma RF power and the figure of merit (FOM) also increased from 1.17 × 10−3 to 1.65 × 10−3 Ω−1. This was not the case for the high-oxygen treatment. The ITO surfaces became smoother with increasing RF power. It was shown that unlike the high-oxygen treated samples, the In 3d and Sn 3d core levels for the low-oxygen-treated ITO films were shifted to higher energies compared to that of the untreated sample. ITO films of ITO/Ag/ITO multilayers (IAI) were plasma-treated at different conditions. Their transmittance maxima were in the range of 91.58–92.32% at 492–501 nm. The bottom-ITO-treated sample gave 23.1% higher mobility and 19% lower sheet resistance than the untreated IAI. The wetting characteristics of deionized (DI) water on the ITO films showed that the surface energy of ITO films increased from 34.22 to 71.14 mJ/m2 when the oxygen pressures increased from 0 to 26.66 Pa.