CeO2 Thin Films Research Articles

Enhancement of room temperature ferromagnetic behavior of rf sputtered Ni-CeO2 thin films

Ni-doped CeO2 thin films were prepared under Ar+ atmosphere on glass substrates using rf magnetron sputtering. To assess the properties of the prepared thin films, the influence of various amounts of Ni dopant on structural, morphological, optical, vibrational, compositional and magnetic properties of the CeO2 films were studied by using X-Ray diffraction (XRD), atomic force microscope (AFM), photoluminescence (PL), micro-Raman, X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM). XRD patterns for all the samples revealed the expected CeO2 cubic fluorite-type structure and Ni ions were uniformly distributed in the samples. AFM images of the prepared samples indicate high dense, columnar structure with uniform distribution of CeO2. Room-temperature photoluminescence (PL) and micro-Raman spectroscopic studies revealed an increase of oxygen vacancies with higher concentration of Ni in CeO2. XPS results confirm the presence of Ni2p, O1s and Ce and depict that cerium is present as both Ce4+ and Ce3+ oxidation states in Ce1−xNixO2 (x=15%) thin film. Field dependent magnetization measurements revealed a paramagnetic behavior for pure CeO2, while a ferromagnetic behavior appeared when Ni is doped in CeO2 films. Doping dependent magnetization measurements suggest that the observed ferromagnetism is due to the presence of metallic Ni clusters with nanometric size and broad size distribution.

Understanding the photoelectrochemical properties of nanostructured CeO2/Cu2O heterojunction photoanode for efficient photoelectrochemical water splitting

Nanostructured thin films of CeO2 sensitized by overlayering thin layers of Cu2O with varying thickness have been studied for the first time as photoelectrode in photoelectrochemical (PEC) water splitting. Effective mass calculations for electrons and holes in bulk CeO2 and Cu2O using first principles based on Density Functional Theory (DFT) have also been attempted to explain the enhanced charge separation at CeO2/Cu2O heterojunction interface. All samples were characterized by X-ray diffractometer (XRD), Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM) and UV–Visible spectrophotometer. The photoelectrochemical activity of the samples was investigated in a three electrode quartz cell system and maximum photocurrent density of 2.89 mA cm−2 at 0.7 V/SCE was obtained for the CeO2/Cu2O heterojunction with overall thickness of 397 nm. Improved conductivity and better separation of the photogenerated charge carriers at the CeO2/Cu2O heterojunction as compared to individual components may be responsible for the higher photocurrent density. The possible mechanism for the enhanced photocurrent density has been explained using heterojunction model based on density functional theory calculations.

Electrical Properties of Al2O3 Incorporated CeO2 Thin Films Deposited by RF Magnetron Sputtering

The Al2O3 incorporated CeO2 thin films (Al2O3 molar fraction of 0.0 to 0.6) were prepared by the RF magnetron sputtering with the combinatorial mask system using the two targets of Al2O3 and CeO2 in Ar + 10% O2. The deposited films were annealed in N2 and O2 at 500°C for 30 minutes. The minimum leakage current density of 1.6 x 10-8 A/cm2 measured at the electric field of 1MV/cm was obtained for the N2 annealed sample with the Al2O3 incorporation of 0.2, which value was significantly lower than that of the film without the Al2O3 incorporation by two orders of magnitude. The variation of the flat band voltage with the incorporated Al2O3 molar fraction suggested the generation of the fixed positive charge, which was probably arisen from the lower Ce oxide in the film and/or the lower oxide of Si diffused from the substrate into the film.

High plasticity reversible resistive switching in heteroepitaxial metal/CeO2−x/Nb:SrTiO3/Ti/Pt structures

We report on the characterization of resistive switching devices based on epitaxial CeO2 thin films as a functional material. CeO2 epitaxial thin films were grown by the pulsed laser deposition technique on conductive substrates. Platinum and titanium nitride top electrodes (TE) were successively deposited. Very good performances, in terms of resistivity switching and multilevel operation capability, were obtained using the Pt TE. The dependence of the low resistance and high resistance state on the TE material and on the CeO2 film thickness were explained. The electrical characteristics of these heterostructures make them promising as synapse for neuromorphic computation, but suggest also their use with multi-valued digital systems or multibit memory cells.

Enhanced Visible Light Adsorption of Heavily Nitrogen Doped CeO2 Thin Film via Ion Beam Assisted Deposition

CeO2 is a promising material for the utilization of solar light in photocatalytic reactions. However, the major obstacle for these studies is the lack of reliable methods to incorporate the desired elements such as nitrogen into the crystal lattice of CeO2. In the present study, a nitrogen-doped CeO2 thin film was synthesized by IBAD technique. With this technique, the nitrogen was heavily and uniformly doped into the CeO2 thin films. XPS analysis clearly demonstrates the greatest N concentration of 25 mol% can be doped in CeO2 thin films which is much higher than that obtained via traditional methods. The high resolution N 1s spectrum shows that nitrogen dopants are uniformly doped into CeO2 lattice by substituting O. The XRD results indicate ion bombardment on the growing film surface does not alter the crystal structure of the film by itself. Instead, the heavy nitrogen doping induces smaller grain size of CeO2. The SEM images show that with the increase of N doping, the surface become smoother with smaller particle size. The heavily nitrogen doping also induces a red shift of the visible light absorbance from 380 nm to 450 nm.

Interaction of Zr with oxidized and partially reduced ceria thin films

The growth and electronic properties of Zr on the ceria thin films were studied by X-ray photoelectron spectroscopy, low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and work function measurements. Metallic zirconium was vapor-deposited on the well-ordered fully oxidized CeO2(111) and partially reduced CeO2-x(111) (0<x<0.5) thin films, which were epitaxially grown on a Ru(0001) substrate, under ultrahigh vacuum (UHV) conditions. The results show that the deposition of Zr on both ceria surfaces leads to electron transfer from Zr to ceria, accompanied by partial reduction of Ce from Ce4+ to Ce3+ states and oxidation of metallic Zr to Zr4+. Moreover, with increasing the Zr coverage, the reduction degree of ceria films increases and eventually only Ce3+ is observed at a high coverage of Zr. The STM results suggest that Zr grows two-dimensionally (2D) on the CeO2(111) thin film at low coverages due to the strong interaction between Zr and CeO2(111).

On the number, binding energies, and mutual intensities of Ce3d peaks in the XPS analysis of cerium oxide systems: A response to Murugan et al., Superlatt. Microstruct. 85 (2015) 321

I discuss on an XPS analysis of CeO2 thin films offered by Murugan et al. in a recent article, Superlatt. Microstruct. 85 (2015) 321. I argue that the authors’ interpretation is not an accurate picture of the chemical composition of their material, especially not the mapping of the Ce(IV)-Ce(III) redox couple, because such interpretation largely overlooks the quantum mechanics principles and electron correlation phenomena that govern the binding energies and mutual intensities of photoemission peaks in the Ce3d spectrum. I propose that the spectral feature that the authors observe at ∼522 eV on the binding energy scale is not the O1s component of lattice cerium oxide, as the authors infer, but the AlKα3,4-related satellite of the AlKα1,2 – excited O1s peak, which they observe at ∼532.5 eV.

Ultrafast pump-probe spectroscopy studies of CeO2 thin film deposited on Ni-W substrate by RF magnetron sputtering

This study presents the first investigation of rapid dynamical processes that occur in pure CeO2 thin film, using ultra fast pump-probe spectroscopy at room temperature. For this purpose we have used a single (200) oriented CeO2 film deposited on biaxially textured Ni-W substrate by RF magnetron sputtering technique. The ultrafast transient spectra show initial sharp rise transition followed by an exponential photon decay. This rise time is about 10 ps irrespective of the probe wavelengths range 500–800 nm. The initial decay constant (τ) at 500 nm probe wavelength is found to be 171 ps, while at 800 nm probe wavelength it is 107.5 ps. The ultrafast absorption spectra show two absorption peaks at 745 and 800 nm, and are attributed to the electronic transitions from 2F7/2–2F5/2 and 1S0–1F3 respectively. The relatively high intensity absorption peak at 745 nm indicates dominant f–f electronic transition. Further, the absorption peak at 745 nm splits into two distinct peaks with respect to delay time, and is attributed to the charge transfer in between Ce4+ and Ce3+ ions. These results indicate that CeO2 itself is a potential candidate and can be used for optical applications.

Defect induced magnetic transition in Co doped CeO2 sputtered thin films

Cobalt-doped cerium dioxide thin films exhibit room temperature ferromagnetism due to high oxygen mobility in doped CeO2 lattice. CeO2 is an excellent doping matrix as there is a possibility of it losing oxygen while retaining its structure. This leads to increased oxygen mobility within the fluorite CeO2 lattice, leading to formation of Ce3+ and Ce4+ species. Magnetic ceria materials are important in several applications from magnetic data storage devices to magnetically recoverable catalysts. In this paper, the room temperature ferromagnetism of rf sputtered Co doped CeO2 thin films is reported whereas undoped CeO2 thin films exhibit paramagnetic behavior. The ferromagnetic properties of the Co doped films were explained based on oxygen vacancies created by Co ions in Ce sites. This is further supported by X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman. Change in surface morphology due to Co doping of the samples were analyzed using atomic force microscopy (AFM).

Effect of iron doping on the structural and optical properties of CeO2 films

Undoped and Fe-doped CeO2 thin films were fabricated by spin coating on F-doped tin oxide glass substrates followed by annealing at 500 °C for 15 h. The concentration of the dopant was varied from 2 to 10 % iron by weight (metal basic). Glancing angle X-ray diffraction and laser Raman microspectroscopy indicated that the films consisted only of CeO2 without any impurity phases of FexOy. The thickness of the films was determined by dual-beam focused ion beam milling to be ~150 nm for undoped film, while Fe-doped CeO2 films have thickness of ~200 nm for all dopant samples. The transmission spectra from UV–visible spectrophotometry showed a redshift of the absorption edge of the doped films, and the optical indirect band gap of the films decreased from 3.48 to 3.20 eV with increasing dopant concentration. Furthermore, the results have been proposed in the diagram of electron–hole trapping.

Analysis of the grain boundary conductivity of singly and doubly doped CeO2 thin films at elevated temperature

Acceptor-doped cerium oxides are a promising solid electrolyte material for solid oxide fuel cells (SOFCs) due to their exceptionally high oxygen ion conductivity. Typical solid electrolytes are polycrystalline with a number of grain boundaries, and it is common for grain boundaries to block the motion of mobile oxygen ions as they migrate from one grain to the next. Accordingly, it is important precisely to measure the grain boundary conductivities of doped cerium oxides, particularly at the temperature of relevance (>500 °C). Here, we undertake a quantitative analysis of the transport properties of singly and doubly doped cerium oxides. So that we may precisely extract the grain boundary contribution out of the total conductivity at an elevated temperature (>500 °C), both epitaxial and polycrystalline thin films of the oxides are grown via pulsed layer deposition (PLD) on two different insulating, single-crystal substrates: Al2O3(0001) and SiO2(001), and these are then characterized by a range of analysis tools, in this case TEM, SEM, XRD, ICP-MS and AFM. These samples with controlled microstructures, in combination with in-plane ionic conductivity measurements by impedance spectroscopy as a function of the thickness, temperature and pO2, enable us precisely to measure the grain boundary conductivity of the doped ceria at temperatures ranging from 620 to 700 °C. The impact of the number and type of dopants on the transport properties are also explored.

Surface potentials of (111), (110) and (100) oriented CeO2−x thin films

Differently oriented CeO2 thin films were prepared by radio frequency magnetron sputter deposition from a nominally undoped CeO2 target. (111), (110) and (100) oriented films were achieved by deposition onto Al2O3(0001)/Pt(111), MgO(110)/Pt(110) and SrTiO3:Nb(100) substrates, respectively. Epitaxial growth is verified using X-ray diffraction analysis. The films were analyzed by in situ photoelectron spectroscopy to determine the ionization potential, work function, Fermi level position and Ce3+ concentration at the surface in dependence of crystal orientation, deposition conditions and post-deposition treatment in reducing and oxidizing atmosphere. We observed a very high variation of the work function and ionization potential of more than 2eV for all surface orientations, while the Fermi level varies by only 0.3eV within the energy gap. The work function generally decreases with increasing Ce3+ surface concentration but comparatively high Ce3+ concentrations remain even after strongly oxidizing treatments. This is related to the presence of subsurface oxygen vacancies.

Low-Temperature Dissociation of CO2 on a Ni/CeO2(111)/Ru(0001) Model Catalyst

The adsorption of CO2 on CeO2-x(111) and Ni/CeO2-x(111)/Ru(0001) surfaces has been studied with reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). On the maximal-oxidized CeO2(111) surface physisorbed linear CO2 and a CO2– species are identified at 97 K. The reduced CeO2-x(111) surface exhibits higher reactivity toward adsorbed CO2, which leads to higher coverages of CO2– and promotes CO2 dissociating into CO and an active oxygen species at higher temperature, reoxidizing the reduced CeO2-x(111) films. Deposition of Ni on the maximal-oxidized CeO2 thin films leads to slight reduction of ceria films. Adsorption of CO2 on Ni/CeO2-x(111) films causes dissociation at 97 K and leads to Ni-CO adsorbates plus partial oxidation of Ni nanoparticles. This process is inhibited when Ni nanoparticles on CeO2 are fully oxidized. In contrast to the results reported for CO2 adsorption on Ni single-crystals, where the dissociation temperature was found to be higher than 240 K, ...

Structure and stability of Gd-doped CeO2 thin films on yttria-stabilized zirconia

25nm thick Gd-doped ceria thin films were grown on yttria-stabilized zirconia (YSZ) substrates with (110) and (111) orientation by pulsed laser deposition to study both their crystalline structure and interfacial stability. The films were characterized by high-energy grazing incidence x-ray diffraction, x-ray reflectivity and x-ray photoelectron spectroscopy before and after annealing to 1400K under ultra-high vacuum (UHV) conditions. The films were found to be epitaxial to the YSZ substrates, exhibiting good crystalline quality without defects like twinning, and low surface roughness. Upon reduction due to the annealing in ultrahigh vacuum (UHV), both samples showed an increase in lattice parameter while maintaining their original crystalline quality. The x-ray reflectivity measurements gave evidence for interdiffusion after annealing by the presence of an additional interfacial layer with reduced electron density. X-ray photoelectron spectroscopy revealed an increase in the concentration of Ce3+ and also yttrium at the surface upon annealing, indicating a slight reduction of the surface as well as diffusion of yttrium to the surface.

Effect of La0.6Sr0.4Co0.2Fe0.8O3-δ microstructure on oxygen surface exchange kinetics

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is a mixed ionic-electronic conducting perovskite (ABO3) material which has attracted significant research interest in recent years as a cathode for solid oxide fuel cell (SOFC) applications at intermediate temperatures (500°C–750°C). Fundamental understanding of the oxygen surface exchange kinetics and diffusion in LSCF cathodes is necessary to optimize the cell performance at the required operating temperatures. In this study, we controlled the surface microstructure and crystalline orientation of LSCF thin films on (100) Gd2O3-doped CeO2 thin films (GDC) prepared by pulsed laser deposition on yttria-stabilized zirconia (YSZ) single crystal substrates. Tailoring of the LSCF microstructure was achieved by modifying the underlying GDC thin film surface morphology and microstructure. To clarify the effect of the LSCF microstructure on its oxygen surface exchange kinetics, we employed the 18O/16O oxygen isotope exchange technique in conjunction with secondary ion mass spectroscopy (SIMS) depth profile analysis. Comparison of the qualitative features of the measured concentration of the 18O profile and evaluation of the oxygen surface exchange coefficient k* were performed and the results correlated to the LSCF microstructure.

Enhanced superconducting properties in MOD-YBCO thick films with CeO2 interlayer

In YBa2Cu3O7-x (YBCO) film there exists thickness effect: the critical current density of YBCO film drops precipitously as the coating thickness increases, especially in the case that the thickness of YBCO film exceeds 1 m. In this paper, we introduce very thin layers of CeO2 into YBCO layers and successfully fabricate the structure of YBCO/YBCO/CeO2/YBCO superconducting thick film. Firstly, YBCO films with two layers are fabricated on a LaAlO3 substrate by a multiple coatings process using a trifluoroacetate metal organic deposition method. Secondly, CeO2 thin films are deposited on YBCO films by RF-sputtering. Finally, we prepare the third YBCO film on CeO2interlayer. No cracks are observed in scanning electron microscopy images of these films; further, the majority of the grains in the films are well-textured and c-axis oriented. The full-width-half-maximum of the out-of-plane texture is measured to be 1.395 for the multilayer YBCO film at a thickness of 2 m Using this multilayer technology, we achieve Jc values of up to 1.36 MA/cm2 (77 K, self-field) in films as thick as 2 m, for an extrapolated critical current of 272 A/cm. We attribute the enhanced performance of the thick YBCO film to the CeO2 interlayer which playsan important role in transmission texture and stress relaxation.

Stabilization of perpendicular magnetic anisotropy in CeO2 films deposited on Co/Pt multilayers

We report the stabilization of perpendicular magnetization, and anisotropy, in transparent CeO2 thin films integrated onto Co/Pt multilayered films.

Microstructural, optical and gas sensing characterization of laser ablated nanostructured ceria thin films

Ceria thin films are deposited on quartz substrates at various substrate temperatures (923–1073 K) with optimized deposition parameters by using pulsed laser deposition technique. Prepared thin films are characterized by X-ray diffraction (XRD), atomic force microscopy, Raman spectroscopy and transmission UV–Vis spectroscopy to study the effect of substrate temperature. The XRD studies reveal the polycrystalline nature of CeO2 thin films. The preferred orientation is observed along (111) plane. The atomic force microscopy studies show the formation of uniform and dense nanocrystallites with smooth surface morphology. The RMS roughness of the thin film is increased with the increase of substrate temperature. The formation of CeO2 with cubic structure is confirmed with Raman peak appeared at 463 cm−1 due to the F2g active mode. The optical transmission studies reveal that the optical band gap decrease with increasing the substrate temperature. Acetone gas sensing characterization has been carried out at elevated temperatures.

Single Oriented CeO2 Buffer Layer Deposition On Biaxially Textured Ni-W Substrate By RF Magnetron Sputtering

Considerable attention has been gained on the deposition of CeO2 thin films with (200) single orientation as hetero-epitaxial buffer layer on (200) oriented biaxially textured flexible Ni substrates, in the fabrication of superconductor and semiconducting epitaxial thin films for device applications. In this work we have deposited (200) oriented CeO2 thin films on biaxially textured Ni-W substrate in a single-step process by RF magnetron sputtering, using CeO2 target. X-ray diffraction analysis shows that for the CeO2 thin film deposited at RF sputtering power below 200 W and for the substrate temperature of 700 o C, the film assumes single (200) orientation. For the substrate temperature below 700 o C and RF sputtering powers above 200 W the film shows polycrystalline nature with (111) and (200) orientations. The Raman spectrum of single oriented (200) CeO2 thin film shows only one sharp peak at about 464 cm -1 corresponds to the presence of F2g mode of CeO2. The ellipsometry studies reveal the value refractive index and optical band gap of single oriented film as 2.52 and 3.41 eV, respectively. Copyright © 2015 VBRI Press.

Room temperature ferromagnetism of Ni doped cerium oxide single crystalline thin Films deposited by using rf magnetron sputtering

CeO2 and Ni–CeO2 thin films with cubic fluorite structure were prepared by rf magnetron sputtering. Crystallinity and optical properties of the prepared thin films were studied using X-ray diffraction, UV–vis and Photo Luminescence analysis. Vibrational and magnetic properties of the films were analysed through Raman and VSM analyses. The results from XRD and Raman spectroscopy indicate that doped Ni species were dispersed on the ceria surface. Room temperature magnetic measurements reveal paramagnetic nature of pure CeO2 thin films and ferromagnetic nature with reasonable saturation magnetisation (4.03×10−5emu/cm3) of Ni doped CeO2 thin films. It is believed that the ferromagnetic contributions exhibited in the M–H loops originate from the absorptive oxygen on the surface rather than the oxygen vacancies in the lattice.