Effect of temperature on polaronic transport in CeO2 thin-film.

The outstanding catalytic property of cerium oxide (CeO2) strongly depends on the polaron formation due to the oxygen vacancy (V̈O) defect and Ce4+ to Ce3+ transformation. Temperature plays an important role in the case of polaron generation in CeO2 and highly influences its electrical transport properties. Therefore, a much needed attention is required for detailed understanding of the effect of temperature on polaron formation and oxygen vacancy migration to get an idea about the improvement in the redox property of ceria. In this work, we have probed the generation of polarons in CeO2 thin-film deposited on a silicon (Si) substrate using the resonance photoemission spectroscopy (RPES) study. The RPES data show an increase in polaron density at the substrate-film interface of the thermally annealed film, indicating the formation of an interfacial Ce2O3 layer, which is, indeed, a phase change from the cubic to hexagonal structure. This leads to a modified electronic band structure, which has an impact on the capacitance-voltage (C-V) characteristics. This result nicely correlates the microscopic property of polarons and the macroscopic transport property of ceria.

Assessing the prospect of XAFS experiments of metalloproteins under in vivo conditions at Indus-2 synchrotron facility, India.

The feasibility of X-ray absorption fine-structure (XAFS) experiments of ultra-dilute metalloproteins under in vivo conditions (T = 300 K, pH = 7) at the BL-9 bending-magnet beamline (Indus-2) is reported, using as an example analogous synthetic Zn (0.1 mM) M1dr solution. The (Zn K-edge) XAFS of M1dr solution was measured with a four-element silicon drift detector. The first-shell fit was tested and found to be robust against statistical noise, generating reliable nearest-neighbor bond results. The results are found to be invariant between physiological and non-physiological conditions, which confirms the robust coordination chemistry of Zn with important biological implications. The scope of improving spectral quality for accommodation of higher-shell analysis is addressed.

Role of C and B4C barrier layers in controlling diffusion propagation across the interface of Cr/Sc multilayers.

The optical performance of low-bilayer-thickness metallic multilayers (ML) can be improved significantly by limiting the intermixing of consecutive layers at the interfaces. Barrier layers are supposed to exhibit a decisive role in controlling diffusion across the interfaces. The element-specific grazing incidence extended X-ray absorption fine structure technique using synchrotron radiation has been used in conjunction with grazing incidence X-ray reflectivity and diffuse X-ray scattering measurements to study the impact of the two most common barrier layers, viz., C and B4C, at the interfaces of Cr/Sc MLs. The diffusion propagation is reduced by both the barrier layers; however, it is found that the improvement is more significant with the B4C barrier layer. It is seen that C forms an intermixed layer with Sc and leads to carbide formation at the interface, which then acts as shielding and prevents further interdiffusion, while B4C hardly penetrates into Sc and stops the overlap between Sc and Cr directly by wetting the corresponding interface. Thus, the above measurements reveal crucial and precise information regarding the elemental diffusion kinetics at the interfaces of Cr/Sc MLs in a non-destructive way, which is very important for technological applications of these MLs as X-ray optical devices.

Comprehensive study on the origin of orthorhombic phase stabilization in Gd-doped HfO2 and DFT calculations.

In recent times, ultra-thin films of hafnium oxide (HfO2) have shown ferroelectricity (FE) attributed to the orthorhombic (o) phase of HfO2 with space group Pca21. This polar o-phase could be stabilized in the doped thin film of the oxide. In the present work, both polar and non-polar o-phases of HfO2 could be stabilized in Gd-doped bulk polycrystalline HfO2. Rietveld analysis of XRD data shows that the relative population of o-phases in the presence of the monoclinic (m) phase of HfO2 increases with increasing Gd-content. The local environment around the host atom has been investigated by time differential perturbed angular correlation (TDPAC) spectroscopy, synchrotron based X-ray near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) measurements showed a reduction in grain size with increasing Gd-dopant indicating a solute drag effect. It could be established that the segregation of the Gd-dopant in the grain boundary is a thermodynamically favorable process and the solute drag effect plays an important role in nucleation of the o-phase in bulk HfO2. Stabilization of Gd in both Pbca and Pca21 phases of HfO2 was supported by defect formation energy calculations using density functional theory (DFT). The present study has important implications in future applications of HfO2 in ferroelectric devices and in understanding the role of dopants in stabilizing the o-phase of HfO2 in the bulk.

Modulation of intrinsic defects in vertically grown ZnO nanorods by ion implantation.

Intrinsic defects created by chemically inert gas (Xe) ion implantation in vertically grown ZnO nanorods are studied by optical and X-ray absorption spectroscopy (XAS). The surface defects produced due to dynamic sputtering by ion beams control the fraction of O and Zn with ion fluence, which helps in tuning the optoelectronic properties. The forbidden Raman modes related to Zn interstitials and oxygen vacancies are observed because of the weak Fröhlich interaction, which arises due to disruption of the long-range lattice order. The evolution of the lattice disorder is identified by O K-edge and Zn K-edge scans of XAS. The hybridization strength between the O 2p and Zn 4p states increases with ion fluence and modulates the impact of intrinsic defects. The ion irradiation induced defects also construct intermediate defects bands which reduce the optical bandgap. Density functional theory (DFT) calculations are used to correlate the experimentally observed trend of bandgap narrowing with the origin of electronic states related to Zn interstitial and O vacancy defects within the forbidden energy gap in ZnO. Our finding can be beneficial to achieve enhanced conductivity in ZnO by accurately varying the intrinsic defects through ion irradiation, which may work as a tuning knob to control the optoelectronic properties of the system.

Phase evolution in thermally annealed Ni/Bi multilayers studied by X-ray absorption spectroscopy.

The thin films of Ni and Bi are known to form NiBi3 and NiBi compounds spontaneously at the interface, which become superconducting below 4.2 K and show ferromagnetism either intrinsically or due to Ni impurities. Formation of NiBi3 and NiBi is a slow diffusion reaction, which means the local environment around Ni and Bi atoms may vary with time and temperature. In this report, we assess the feasibility of using X-ray Absorption Spectroscopy (XAS) as a tool to track the changes in local bonding environment in NiBi3 and NiBi. Thermal annealing at temperatures up to 500 °C was used to induce changes in the local environment in NiBi3 system. Consequent decomposition of NiBi3 into NiO and Bi has been tracked through changes in structural and magnetization behavior, which matched well with the findings of XAS. In addition, the magnetic hysteresis measurements indicated that NiO should be the dominant phase when NiBi3 is annealed at 500 °C. This was corroborated from XAS and was found to be >90%. The shift in K-edge of Ni in annealed samples was attributed to increasing charge state on Ni atom, which was ascertained by Bader charge analysis using Density Functional Theory (DFT). This study correlating macroscopic properties of NiBi3 with local bonding environment of the system indicates that XAS can be a very reliable tool for studying dynamics of diffusion in the NiBi3 system.

Effect of germanium auto-diffusion on the bond lengths of Ga and P atoms in GaP/Ge(111) investigated by using X-ray absorption spectroscopy.

The germanium auto-diffusion effects on the inter-atomic distance between the nearest neighbors of the Ga atom in GaP epilayers are investigated using high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy. The GaP layers grown on Ge (111) are structurally coherent and relaxed but they show the presence of residual strain which is attributed to the auto-diffusion of Ge from the results of secondary ion mass spectrometry and electrochemical capacitance voltage measurements. Subsequently, the inter-atomic distances between the nearest neighbors of Ga atom in GaP are determined from X-ray absorption fine-structure spectra performed at the Ga K-edge. The estimated local bond lengths of Ga with its first and second nearest neighbors show asymmetric variation for the in-plane and out-of-plane direction of GaP/Ge(111). The magnitude and direction of in-plane and out-of-plane microscopic residual strain present in the GaP/Ge are calculated from the difference in bond lengths which explains the presence of macroscopic residual tensile strain estimated from HRXRD. Modified nearest neighbor configurations of Ga in the auto-diffused GaP epilayer are proposed for new possibilities within the GaP/Ge hetero-structure, such as the conversion from indirect to direct band structures and engineering the tensile strain quantum dot structures on (111) surfaces.

Insight into the charging-discharging of magnetite electrodes: in situ XAS and DFT study.

The structural changes of Fe3O4 nanoparticle electrodes in Li ion batteries during charging-discharging cycles have been investigated using in situ X-ray absorption spectroscopy (XAS). Chemometric methods viz., Principal Component Analysis (PCA) and Multivariate Curve Resolution-Alternate Least Square (MCR-ALS) have been used for analysis of the in situ XANES data during the charge-discharge cycle, which help to identify the various species formed during the lithiation-delithiation of Fe3O4. The concentration variation of the different species has also been determined and the detailed intercalation-conversion mechanism of the Fe3O4 electrodes during the first discharge has been established. Subsequently, the first charge and second discharge cycles were also studied to apprehend the difference in redox reaction between the first discharge and subsequent cycles. The above studies clearly identify the four species involved in the whole intercalation-conversion process of Fe3O4 electrode of a Li ion battery and also indicate the irreversibility of the conversion reaction in subsequent cycles which may be one of the reasons for capacity fading of these electrodes. The above results have also been corroborated with density functional theory (DFT)based ab inito calculations.

Probing the solute-drag effect and its role in stabilizing the orthorhombic phase in bulk La-doped HfO2 by X-ray and gamma ray spectroscopy.

The recent observation of ferroelectricity in ultra thin films of hafnium oxide (HfO2) has been attributed to the orthorhombic (o) phase of HfO2 with space group Pca21. Although this oxide is polymorphic in nature, this polar o-phase is known to be stabilized in the doped thin film oxide. The objective of the present experiment is to stabilize the o-phases in La doped bulk polycrystalline HfO2 and investigate their evolution with the doping concentration through Time Differential Perturbed Angular Correlation (TDPAC), X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements. The present work reports the presence of both the polar Pca21 phase and the antipolar Pbca phase at different La-concentrations. Two o-phases of HfO2 with space groups Pca21 and Pbca, difficult to distinguish by other complimentary methods, could be unambiguously identified by utilizing the atomic scale sensitivity of the electric field gradient (EFG) embedded in TDPAC spectroscopy. The determination of the oxidation state and the local environment of La-atoms by XANES and EXAFS measurements illuminates the microscopic role of the dopant in stabilizing the o-phase. The "solute drag model" proposes a critical crystallite size for the nucleation of the o-phase in bulk HfO2 and explains the role of the La-dopant in stabilizing the o-phase. Thus the present study shows the possibility of stabilizing the polar o-phase and hence attaining ferroelectricity in bulk HfO2 to augment the scope of future application for this ferroelectric device.

First high-pressure XAFS results at the bending-magnet-based energy-dispersive XAFS beamline BL-8 at the Indus-2 synchrotron facility.

The static focusing optics of the existing energy-dispersive XAFS beamline BL-8 have been advantageously exploited to initiate diamond anvil cell based high-pressure XANES experiments at the Indus-2 synchrotron facility, India. In the framework of the limited photon statistics with the 2.5 GeV bending-magnet source, limited focusing optics and 4 mm-thick diamond windows of the sample cell, a (non-trivial) beamline alignment method for maximizing photon statistics at the sample position has been designed. Key strategies include the selection of a high X-ray energy edge, the truncation of the smallest achievable focal spot size to target size with a slit and optimization of the horizontal slit position for transmission of the desired energy band. A motor-scanning program for precise sample centering has been developed. These details are presented with rationalization for every step. With these strategies, Nb K-edge XANES spectra for Nb2O5 under high pressure (0-16.9 GPa) have been generated, reproducing the reported spectra for Nb2O5 under ambient conditions and high pressure. These first HPXANES results are reported in this paper. The scope of extending good data quality to the EXAFS range in the future is addressed. This work should inspire and guide future high-pressure XAFS experiments with comparable infrastructure.

Operando X-ray absorption spectroscopy study of the Fischer-Tropsch reaction with a Co catalyst.

This article describes the setting up of a facility on the energy-scanning EXAFS beamline (BL-09) at RRCAT, Indore, India, for operando studies of structure-activity correlation during a catalytic reaction. The setup was tested by operando X-ray absorption spectroscopy (XAS) studies performed on a Co-based catalyst during the Fischer-Tropsch reaction to obtain information regarding structural changes in the catalyst during the reaction. Simultaneous gas chromatography (GC) measurements during the reaction facilitate monitoring of the product gases, which in turn gives information regarding the activity of the catalyst. The combination of XAS and GC techniques was used to correlate the structural changes with the activity of the catalyst at different reaction temperatures. The oxide catalyst was reduced to the metallic phase by heating at 400°C for 5 h under H2 at ambient pressure and subsequently the catalytic reaction was studied at four different temperatures of 240, 260, 280 and 320°C. The catalyst was studied for 10 h at 320°C and an attempt has been made to understand the process of its deactivation from the XANES and EXAFS results.

First results from the XMCD facility at the Energy-Dispersive EXAFS beamline of the Indus-2 synchrotron source.

Setting up of the X-ray Magnetic Circular Dichroism (XMCD) measurement facility with hard X-rays at the Energy-Dispersive EXAFS beamline (BL-08) at the Indus-2 synchrotron source is reported. This includes the design and development of a water-cooled electromagnet having a highest magnetic field of 2 T in a good field volume of 125 mm3 and having a 10 mm hole throughout for passage of the synchrotron beam. This also includes the development of an (X-Z-θ) motion stage for the heavy electromagnet for aligning its axis and the beam hole along the synchrotron beam direction. Along with the above developments, also reported is the first XMCD signal measured on a thick Gd film in the above set-up which shows good agreement with the reported results. This is the first facility to carry out XMCD measurement with hard X-rays in India.

Structural studies of spray pyrolysis synthesized oxygen deficient anatase TiO2 thin films by using X-ray absorption spectroscopy.

The present work focuses on synthesis and X-ray absorption studies of single phase oxygen deficient anatase TiO2 thin films. These films are prepared in a two-step method viz. the synthesis of near stoichiometric anatase TiO2 films using an open atmospheric spray pyrolysis method followed by vacuum annealing at their corresponding synthesis temperatures (Ts = 450 °C, 500 °C) for different time durations (t = 2, 4, 6, 8 hours). XRD and Raman studies of these films ascertained the formation and retention of the anatase phase post annealing, indicating that there was no phase change due to prolonged annealing. Extended X-ray absorption spectra (EXAFS) and X-ray absorption near edge spectra (XANES) revealed the presence of an oxygen vacancy and its effect on the local co-ordination. The co-ordination number obtained from EXAFS analysis revealed that the number density of the oxygen vacancy is higher in the case of thin films synthesized at 450 °C than in the case of the films synthesized at 500 °C. As the oxygen vacancy leads to changes in local co-ordination, which in turn have a profound effect on the pre-edge features of the X-ray absorption spectra (XAS), theoretically simulated XAS spectra of pure anatase TiO2 and oxygen deficient anatase TiO2 were generated using FEFF and were found to match with the experimentally observed spectra. In addition, the ambiguities in whether a change in the metal-oxygen bond length has any effect on the pre-edge features or not were delineated in the present study by comparing the pre-edge peak positions of the oxygen deficient TiO2 films. Our results matched with those of some of the researchers who have studied the rutile phase TiO2, wherein it was concluded that in the case of the titanium-oxygen system, the mean Ti-O bond length does have an effect on the pre-edge peak position. It was observed that as the Ti-O mean bond length increases, the pre-edge peak positions shift towards lower energy, which is in concurrence with the literature available for other Ti-O systems with similar geometry. The second pre-edge peak intensity, which is a measure of disorder, is higher for TiO2450 °C-2 h and TiO2500 °C-2 h thin films. This is attributed to the annealing effect, which suggests that TiO6 octahedrons are the most disordered for thin films annealed for 2 h and become more ordered upon annealing for longer times.

Oxygen vacancy mediated cubic phase stabilization at room temperature in pure nano-crystalline zirconia films: a combined experimental and first-principles based investigation.

We report here the stabilization of the cubic phase under ambient conditions in the thin films of zirconia synthesized by electron beam evaporation. The cubic phase stabilization was achieved without the use of chemical stabilizers and/or concurrent ion beam bombardment. Films of two different thicknesses (660 nm and 140 nm) were deposited. While the 660 nm as-deposited films were in the cubic phase, as indicated by X-ray diffraction and Raman spectroscopy, the 140 nm as-deposited films were amorphous and the transformation to the cubic phase was obtained after thermal annealing. Extended X-ray absorption fine structure measurements revealed the existence of oxygen vacancies in the local structure surrounding zirconium for all films. However, the amount of these oxygen vacancies was found to be significantly higher for the amorphous films as compared to that for the films in the cubic phase (660 nm as-deposited and 140 nm annealed films). The stabilization of the cubic phase is attributed to the breaking of the oxygen-zirconium bonds due to the presence of the oxygen vacancies, which results in the suppression of the soft X2- mode of vibration of the oxygen sub-lattice. Our first-principles modeling under the framework of density functional theory shows that the cubic structure with oxygen vacancies is indeed more stable under ambient conditions than its pristine (without vacancies) counterpart due to breaking of the oxygen bonds. The requirement of a critical amount of these vacancies for cubic phase stabilization is discussed.

Doping effect on the local structure of metamagnetic Co doped Ni/NiO:GO core-shell nanoparticles using X-ray absorption spectroscopy and the pair distribution function.

Core-shell nanoparticles of Co (0%, 3%, and 5%) doped Ni/NiO and incorporated (5 mg) graphene oxide (GO) sheets were synthesized by a sol-gel auto-combustion method. X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and pair distribution function measurements were employed for the determination of the local structure and structural properties of the samples. Due to the effect of Co doping the bond lengths of all coordinate atoms were varied. The microstructural features in the core-shell structured particles were evaluated with high-resolution transmission electron microscopy (HRTEM). Magnetic properties of the samples revealed that both the interface of Ni/Co and NiO crystal lattices and the weight fraction of Ni have significant impact on their magnetic properties at 5 K to 300 K. Experimental results show that Co doping and GO incorporation into Ni/NiO suppress the antiferromagnetic charge ordering and lead to a spin-flop metamagnetic behavior at 5 K to 300 K temperatures. Above 5 K, the step-like transitions transform into broad ones. This step-like feature is correlated with the collapse of the balance between the magnetic energy and elastic energy at the core-shell interface. It is confirmed from M-T measurements that the blocking temperature of Ni/NiO was reduced with Co content.

Local crystal structure in the vicinity of Cr in doped AlN thin films studied by X-ray absorption spectroscopy.

This article reports the detailed X-ray absorption spectroscopy (XAS) study of Al1-xCrxN (x = 4, 6, 11%) thin films synthesized by the reactive magnetron co-sputtering technique. All these films were crystallized with a hexagonal wurtzite structure with preferential orientation along the a-axis without the formation of any secondary phases. Surface chemical analysis to evaluate the Cr concentration was carried out using X-ray photoelectron spectroscopy. The study confirmed the presence of AlN and Cr in bonding with N. The local crystal structure around the Cr dopant in the as-synthesized and annealed thin films has been analyzed by both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. From XAS, it was found that Cr replaced the Al atom in the AlN lattice and led to a localized CrN species with distorted tetrahedral AlN in the absence of Cr clustering. The bond lengths of (Cr-N)ax, (Cr-N)bs and Cr-Al, extracted from the EXAFS fitting, were found to decrease with the Cr concentration for both the as-synthesized and annealed thin films due to the enhancement of p-d hybridization between the dopant and the host atoms. However, in the annealed 11% Cr film, the bond lengths are larger than the other and tend to match the Cr-N geometry in CrN.

Unraveling doping induced anatase-rutile phase transition in TiO2 using electron, X-ray and gamma-ray as spectroscopic probes.

The present work reports the microscopic details of anatase (A) to rutile (R) phase transformation in a Mn-doped TiO2 system. Titanium dioxide (TiO2) powder was synthesized at three different dopant percentages, namely 1, 5, and 10 atom% of Mn, by a coprecipitation technique. Time differential perturbed angular correlation (TDPAC) spectroscopy was used to identify the formation of the rutile-like phase (R*) during the phase-transition process and revealed interface nucleation to be promoted by the Mn dopant. Electron paramagnetic resonance (EPR) spectroscopy, synchrotron-based X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) studies showed that Mn exhibited a mixed valence states of 2+ and 4+ at different stages of the annealing process. The rutile onset temperature gradually decreased with the increase in the Mn content. The present report proposes the mechanism for the phase transformation and details the effect of Mn on the A to R phase-transformation process. This can assist in gaining a fundamental understanding of the A to R phase-transformation process and the role of the dopant in stabilizing one phase over the other.

Insight into growth of Au-Pt bimetallic nanoparticles: an in situ XAS study.

Au-Pt bimetallic nanoparticles have been synthesized through a one-pot synthesis route from their respective chloride precursors using block copolymer as a stabilizer. Growth of the nanoparticles has been studied by simultaneous in situ measurement of X-ray absorption spectroscopy (XAS) and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at Indus-2 SRS at RRCAT, Indore, India. In situ XAS spectra, comprising both X-ray near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) parts, have been measured simultaneously at the Au and Pt L3-edges. While the XANES spectra of the precursors provide real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed in the intermediate stages of growth. This insight into the formation process throws light on how the difference in the reduction potential of the two precursors could be used to obtain the core-shell-type configuration of a bimetallic alloy in a one-pot synthesis method. The core-shell-type structure of the nanoparticles has also been confirmed by ex situ energy-dispersive spectroscopy line-scan and X-ray photoelectron spectroscopy measurements with in situ ion etching on fully formed nanoparticles.

Effect of oxygen partial pressure in deposition ambient on the properties of RF magnetron sputter deposited Gd2O3 thin films.

Gadolinium oxide is an excellent optical material that offers high transmission in a wide wavelength range of 200-1600 nm and exhibits a high bulk refractive index of ∼1.80 at 550 nm. In the present study, a set of Gd2O3 thin films has been deposited on fused silica substrates by RF sputtering of a Gd2O3 target under various O2 to Ar flow ratios. The samples have been characterized by grazing incidence x-ray diffraction (GIXRD) to study the long range structural behavior, by GIXR to study density and surface roughness of the films, by atomic force microscopy measurements to study morphological properties, by Rutherford backscattering measurements for compositional studies, and by transmission spectrophotometry and spectroscopic ellipsometry techniques to study their optical properties. It has been observed that the films deposited with 10% oxygen partial pressure have low density, high surface roughness, and high void content, which results in a low value of refractive index of this film, and film quality improves as oxygen partial pressure is further increased. Extended x-ray absorption fine structure measurement with synchrotron radiation has also been employed to extract local structural information around Gd sites, which has in turn been used to explain some of the observed macroscopic properties of the films.

Growth of block copolymer stabilized metal nanoparticles probed simultaneously by in situ XAS and UV-Vis spectroscopy.

The growth of Au and Pt nanoparticles from their respective chloride precursors using block copolymer-based reducers has been studied by simultaneous in situ measurement of XAS and UV-Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at INDUS-2 SRS at RRCAT, Indore, India. While the XANES spectra of the precursor give real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed at the intermediate stages of growth. The growth kinetics of both types of nanoparticles are found to be almost similar and are found to follow three stages, though the first stage of nucleation takes place earlier in the case of Au than in the case of Pt nanoparticles due to the difference in the reduction potential of the respective precursors. The first two stages of the growth of Au and Pt nanoparticles as obtained by in situ XAS measurements could be corroborated by simultaneous in situ measurement of UV-Vis spectroscopy also.