Doped CeO2 Films Research Articles

Synthesis and characterization of Nd-doped CeO2 thin films grown by spray pyrolysis method: Structural, optical and electrochemical properties

In the present research, we successfully synthesized thin films of Cerium Oxide (CeO2) and Neodymium (Nd) doped CeO2 on both glass and ITO substrates using the chemical spray pyrolysis method. Their structural, morphological, optical and electrochemical properties were experimentally studied. X-ray diffraction analyses showed that all the layers exhibited the cubic fluorite phase characteristic of CeO2. Raman spectroscopy reveals a downward F2g band shift in all Nd3+-doped CeO2 films due to oxygen vacancies. The asymmetry and broadening of the Raman modes arise from the injection of Nd3+ ions in CeO2 thin films, which leads to variations in the presence of Ce3+ and oxygen vacancies. Additionally, the morphology of the films changes with different levels of Nd-doping. The UV–Vis-NIR spectra obtained in the wavelength scale between 300 nm and 1800 nm showed a noticeable reduction in the band gap from 3.46 eV to 3.30 eV. The cyclic voltammetry (CV) results indicate that the addition of Nd improves the ion storage capacities (ISC) of the cerium oxide thin film. Even after the intercalation/deintercalation of lithium ions, the doped cerium oxide thin films remain fully transparent. These significant findings broaden the potential applications of this oxide, particularly for electrochromic devices.

Luminescence from metal-oxide-semiconductor devices with Eu3+-doped CeO2 films on silicon: From broadband to monochromatic emission

We report on the transformation from broadband to monochromatic orange-red emission for the electroluminescence (EL) from the metal-oxide-semiconductor (MOS) devices with europium (Eu3+)-doped CeO2 films annealed at various temperatures as the light-emitting layers. Under the bias voltages exceeding 8 V, the 800 °C-annealed MOS device exhibits dominant Eu3+-related emission at 590 nm with a full width at half maximum of only ∼2 nm. In contrast, the 400 °C-annealed MOS device is activated into broadband emissions associated with the defects in the CeO2 host. Also, the two distinct EL behaviors can co-exist by annealing the CeO2:Eu3+ film at 600 °C. It has been demonstrated that the defect- and Eu3+-dependent EL are related to the Poole-Frenkel (P–F) and trap-assisted tunneling (TAT) electrical conduction mechanisms, respectively. The MOS device with a tunable EL is significant due to its potential applications, e.g., optoelectronic integration.

(Digital Presentation) Characterization of Anisotropic Strain in Anelastic Material By Raman Spectroscopy

Raman spectroscopy has recently been applied for non-destructive characterization of strain in crystalline thin films. It makes use of the numerical value of the mode Grüneisen parameter γ , which equals (minus) the ratio of the relative change in the frequency of a Raman-active vibrational mode and the strain-induced relative change in the unit cell volume. In the presence of in-plane, compressive biaxial strain, aliovalent doped CeO2-films exhibit values of γ which are ~30% smaller than the literature values for the bulk ceramics under isostatic stress. This discrepancy is due in large part to the negative contribution from the anelastic (time-dependent) mechanical properties of oxygen-deficient ceria, thereby raising questions concerning the relationship between Raman mode frequency and anelastic strain. Here we propose a way to "separate" anelastic and elastic contributions to the Grüneisen parameter.500-650nm thick films , x=0.1-0.2, were deposited by RF magnetron sputtering on (100) p-Si wafers (1Ω-cm, tSi=280mm). The films (S-films) were in the fluorite phase with pronounced (111) texture and exhibited biaxial , compressive strain. The in- and out-of-plane lattice parameters (aS || , aS ⊥ ) of the S-films were determined by XRD at different sample tilt angles. The in-plane stress (σbi) in the S-films was determined by profilometry from the change in Si-wafer curvature following film deposition. From σbi and the film biaxial modulus, the elastic component of the in-plane compressive strain (ue ) was calculated (~ -0.2%).Strain-free (relaxed) R-films were obtained by partial substrate removal, forming 2 mm diameter self-supported membranes. Pronounced membrane buckling indicated that residual strain was negligible (aR || ≈ aR ^ ). From the increase in area upon substrate removal, measured with optical profilometry, the total S-film in-plane strain (ut ) was calculated from aS ||/aR-1 ≈ -0.5%. The anelastic strain component may be calculated as ua= ut-ue . The small, anisotropic strain-induced blue shift of the frequency of the F2g -phonon mode for S-films suggests that the F2g mode responds primarily to the elastic component of the strain.As a control, 430nm thick films of mechanically elastic Y2O3 were deposited by sputtering on a Si wafer covered with a 1.5 µm thick PECVD deposited SiO2 layer. This layer supported the Y2O3-films upon membrane formation, but prevented complete strain relaxation. The Y2O3-films were in the double fluorite phase with strong (222) texture and, as with the S-films, were under in-plane compressive strain. The Y2O3 S-film lattice parameters were determined by acquiring XRD-patterns at different sample tilt angles, while membrane in- and out-of-plane lattice parameters were determined with XRD in the transmission and reflection modes, respectively. The values of γ calculated from the Raman frequency of the [Fg +Ag ] peak position were close to the literature values for bulk yttria under isostatic stress.This work should serve as a sample protocol for strain characterization of doped ceria thin films. It supports the concept that the discrepancy between the behavior of Raman modes of isostatically and anisotropically strained doped ceria is related to anelasticity. Our results shed light upon strain characterization by Raman spectroscopy and may lay the foundation for future studies to elucidate selective sensitivity to various strain components.

Characterization of Gd doped CeO2 thin films grown by ultrasonic spray pyrolysis

Abstract Gd concentration effect on the optical, structural, surface and electrical properties of Gd doped CeO2 thin films are coated on glass substrates by spray pyrolysis technique with precursors containing cerium nitrate hexahydrate [Ce(NO3)3·6H2O] and gadalinium nitrate hexahydrate [Gd(NO3)3·6H2O] as a source material. The structural, morphologicaly and elemental characterizations of the films have been carried out using x-ray diffraction, field emission scanning electron microscopy and energy dispersive x-ray analysis, respectively. An optical energy band gap is calculated in the range 3.3–3.8 eV from UV–Vis transmittance spectra. Surface morphologies and roughness values of the films have been investigated by atomic force microscopy. Thicknesses have been determined using spectroscopic ellipsometry technique and Cauchy–Urbach model. A four-probe setup has been used to determine the electrical resistivity values. All samples showed x-ray diffraction peaks corresponding to a cubic fluorite structure. The results indicate that the deposited films have a preferential growth in the (0 0 2) direction. The observed transmittance in the visible region has been situated between 60% and 70%. Photoluminescence spectra showed an ultraviolet emission band around 365–400 nm and the visible emission peaks between 469 and 590 nm. Raman active peaks for undoped and Gd doped CeO2 films were determined at ~465 cm−1. Finally, it has been concluded that Gd doped CeO2 films have value to work on and may be promising materials for optoelectronic applications.

Formation of multicomponent alloy particles in doped ceria under I2+ ion irradiation and thermal annealing

Multicomponent alloys resembling the ε-phase particles in nuclear fuel have the potential to immobilize 99Tc in a stable waste form. Formation of such multicomponent alloys under extreme conditions is studied in a non-activated model system: CeO2 doped with 2 wt.% Mo, 1.5 wt.% Ru, 0.75 wt.% Pd, 0.5 wt.% Re and 0.25 wt.% Rh. The doped CeO2 films were irradiated with I2+ ions (610°C, 1016 and 5×1016 I2+/cm2). For selected samples post-irradiation heat treatment was conducted (900°C, 1100°C). Analytical transmission electron microscopy revealed the formation of 5-20 nm precipitates containing Pd, Mo, and Re sometimes associated with cavities. Iodine was found to segregate to grain boundaries and cracks. After heat treatment at 1100°C, the CeO2 matrix had recrystallized. Precipitates containing Pd, Mo, Re, and Ru were observed near the interface with the polycrystalline yttria-stabilized zirconia substrate. It follows that Pd is the most mobile element, followed by Mo and Re. While irradiation promotes precipitation, high-temperature effects are more significant.

Catalytic Performance of Ag2O and Ag Doped CeO2 Prepared by Atomic Layer Deposition for Diesel Soot Oxidation

The catalytic behaviour of Ag2O and Ag doped CeO2 thin films, deposited by atomic layer deposition (ALD), was investigated for diesel soot oxidation. The silver oxide was deposited from pulses of the organometallic precursor (hfac)Ag(PMe3) and ozone at 200 °C with growth rate of 0.28 Å/cycle. Thickness, crystallinity, elemental composition, and morphology of the Ag2O and Ag doped CeO2 films deposited on Si (100) were characterized by ellipsometry, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM), respectively. The catalytic effect on diesel soot combustion of pure Ag2O, CeO2, and Ag doped CeO2 films grown on stainless steel foil supports was measured with oxidation tests. Nominally CeO2:Ag 10:1 doped CeO2 films were most effective and oxidized 100% of soot at 390 °C, while the Ag2O films were 100% effective at 410 °C. The doped films also showed much higher stability; their performance remained stable after five tests with only a 10% initial reduction in efficiency whereas the performance of the Ag2O films reduced by 50% after the first test. It was concluded that the presence of Ag+ sites on the catalyst is responsible for the high soot oxidation activity.

The Interplay of Strain and Defect Association on the Conductivity Rare Earth Substituted CeO2

In solid state oxygen ion conductors such as CeO2 and ZrO2, aliovalent cations are used to introduce oxygen vacancies which allow ionic conduction, but which also limit the maximum achievable conductivity due to defect-defect interactions. Materials are optimised based on the size and concentration of the substituted cations, in order to minimise defect association and maximise ionic transport. To push the performance of such materials beyond their compositional limit, significant attention has turned to the application of mechanical strain. Computational studies have shown that the energy barrier for migration in CeO2 and ZrO2 can be manipulated using lattice strain. A number of studies are in broad agreement, showing that significant enhancements in the conductivity under tensile strain can be achieved.[1],[2] However, experimentally, the results on strained thin films and multilayers have been both controversial and inconstant, with vastly different findings in nominally similar systems.[3] Typically strained layers due to interfacial mismatch are grown and the thickness varied to control the volume fraction of the strained material. However this leads to difficulties controlling the density of grain boundaries, and isolating changes in the conductivity in the layers.[4] These complications mean that comparisons between experimental and computational work are challenging. Furthermore, much of the work so far on the effects of strain on the transport properties have focused on the so-called ‘optimised’ ionic conductors, such as commercial electrolytes Y-stabilised ZrO2 or Gd- or Sm-doped CeO2. Very little attention has been payed to the ‘non-optimised’ electrolytes where the defect-defect interactions are much more significant. However, as the defect association limits the conductivity for these materials a much larger enhancement in the transport properties (or a less significant reduction) due to strain may be possible. We have fabricated both ‘optimised’ and ‘non-optimised’ doped-CeO2 films by pulsed laser deposition on MgO substrates. The dopants used were La, Gd, and Yb, which represent an ionic radii mismatch with the host Ce lattice of 19.6%, 8.6%, and 1.5% respectively. Epitaxy was maintained using a double BaZrO3 and SrTiO3 buffer layer resulting in films free from grain boundaries. The thickness of the films was kept constant and the strain varied by thermal annealing (from 600°C to 1000°C) to relax the intrinsic strain occurring during growth. Crucially, this means that the strain in these materials can be tailored and the conductivity measured without complications due to grain boundary effects or changes in the volume fraction of the interface and surface. The out-of-plane and in-plane strain in the films was characterised in detail using X-ray diffraction and Raman spectroscopy, and the microstructure evaluated by high-resolution transmission electron microscopy. Impedance spectroscopy measurements showed that the conductivity of the relaxed films was in excellent agreement with bulk rare earth substituted CeO2. With increasing in-plane compressive strain, the conductivity of the films was reduced and the activation energy increased. The observed change in the activation energy with strain is in excellent agreement with computational work, and consistent with other experimental findings. In addition we show that the effect is more pronounced for Yb-substituted CeO2 than La- or Gd-substituted CeO2, suggesting that larger changes in the conductivity with strain can be obtained with ‘non-optimised’ dopant cations. [1] A. Kushima and B. Yildiz, “Oxygen ion diffusivity in strained yttria stabilized zirconia: where is the fastest strain?,” J. Mater. Chem., vol. 20, no. 23, p. 4809, 2010. [2] R. A. De Souza, A. Ramadan, and S. Hörner, “Modifying the barriers for oxygen-vacancy migration in fluorite-structured CeO2 electrolytes through strain: a computer simulation study,” Energy Environ. Sci., vol. 5, no. 1, p. 5445, 2012. [3] K. Wen, W. Lv, and W. He, “Interfacial lattice-strain effects on improving the overall performance of micro-solid oxide fuel cells,” J. Mater. Chem. A, vol. 3, no. 40, pp. 20031–20050, 2015. [4] G. F. Harrington, A. Cavallaro, D. W. McComb, S. J. Skinner, and J. A. Kilner, “The effects of lattice strain, dislocations, and microstructure on the transport properties of YSZ films,” Phys. Chem. Chem. Phys., vol. 19, pp. 14319–14336, 2017.

Defect assisted room temperature ferromagnetism on rf sputtered Mn doped CeO2 thin films

Pure and manganese doped cerium oxide (CeO2 & Mn-CeO2) thin films were deposited on glass substrates by employing rf magnetron sputtering having CeO2 and Mn-CeO2 targets. Deposited films were characterized by using X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), atomic force microscope (AFM) and vibrating sample magnetometer (VSM). XRD patterns reveal the single crystalline nature of films with oriented growth in (220) plane and peak shift to the lower angles for the Mn doped CeO2 films. Mn doped cerium oxide exhibits strong luminescence at ~490–520nm, possibly due to defects like oxygen vacancy created by Mn doping. Oxygen vacancy is responsible for ferromagnetic property of Mn doped CeO2 thin films whereas pure CeO2 thin films exhibit paramagnetic behavior. The enhanced magnetic properties are attributed to formation of that complex between the Mn ion and an oxygen vacancy which is confirmed by XPS analyses.

Near-infrared emissions from Yb3+-doped CeO2 and Ce2Si2O7 films based on silicon substrates subjected to thermal treatment

Photoluminescence properties of Yb3+-doped CeO2 films annealed in different atmospheres were investigated. CeO2:Yb3+ films were deposited by electron-beam evaporation technique. Near-infrared emission around 970 nm was observed after annealing the films both in air and in Ar–H2 atmosphere, which is attributed to the Yb3+:2F5/2 → 2F7/2 transition. Optimization of the Yb3+ concentration for the 970 nm luminescence yield was also investigated. Characterized by different methods, Ce2Si2O7 was formed in the films annealed in reducing atmosphere, which was expected to be more applicable for the silicon-based optoelectronic applications.

Near-infrared luminescence of Tm3+-doped CeO2 films based on silicon substrates

CeO2/Tm2O3 multilayer films were deposited on silicon substrates by electron-beam evaporation. Tm3+ ions were doped in CeO2 after the films were annealed in oxygen atmosphere at 1000 °C. The doping concentration of Tm3+ varies in the range of 0.1–3 mol%. A series of near-infrared emission peaks were observed under the excitation of 330 nm, which correspond to 1G4–3H5, 3H4–3H6, 1G4–3H4, 3H5–3H6, 3F2–3H5, 3H4–3F4, 1G4–3F3,2 and 3F4–3H6 transitions of Tm3+, respectively. The dominant transition of 3H4–3H6 near 805 nm was within optical transmission window. The luminescence properties and the crystal structure of CeO2:Tm3+ films were investigated by excitation and emission spectroscopy and X-ray diffraction. Meanwhile, the substitution process of Ce4+ by Tm3+ was illustrated, and lattice expansion of the matrix CeO2 gave rise to the increase in FWHM of CeO2 diffraction peaks. In addition, the effect of Tm3+ concentration on photoluminescence was also studied, and the optimal concentration of Tm3+ was 0.5 mol%.

Structural characteristics, luminescent properties and energy transfer processes of CeO2:Er3+ thin films

CeO2:Er3+ films were deposited by electron-beam evaporation technique. Photoluminescence properties of Er3+ doped CeO2 films annealed in air atmosphere were investigated. Visible and near-infrared emission peaks were assigned to the transitions from 2H11/2, 4S3/2, 4F9/2, 4I11/2 and 4I13/2 levels to 4I15/2 level, respectively. The structure and the luminescent properties of CeO2:Er3+ films were characterized by X-ray diffraction, photoluminescence and photoluminescence excitation spectra. In addition, luminescence intensities of Er3+ and the intensity ratio of red/green emission peaks were also investigated. They were strong affected by the doping concentration of Er3+. With the good luminescent properties, CeO2:Er3+ films should be promising candidates for multifunctional applications.

Photocatalytic degradation of methyl orange by CeO2 and Fe-doped CeO2 films under visible light irradiation.

Undoped CeO2 and 0.50–5.00 mol% Fe-doped CeO2 nanoparticles were prepared by a homogeneous precipitation combined with homogeneous/impreganation method, and applied as photocatalyst films prepared by a doctor blade technique. The superior photocatalytic performances of the Fe-doped CeO2 films, compared with undoped CeO2 films, was ascribed mainly to a decrease in band gap energy and an increase in specific surface area of the material. The presence of Fe3+ as found from XPS analysis, may act as electron acceptor and/or hole donor, facilitating longer lived charge carrier separation in Fe-doped CeO2 films as confirmed by photoluminescence spectroscopy. The 1.50 mol% Fe-doped CeO2 film was found to be the optimal iron doping concentration for MO degradation in this study.

Structural, morphological and optical properties of spray deposited Mn-doped CeO2 thin films

Cerium oxide and manganese (Mn) doped cerium oxide thin films on glass substrates were prepared by home built spray pyrolysis system. The effect of Mn doping on the structural, morphological and optical properties of CeO2 films were studied. It was found that both the undoped and doped CeO2 films were polycrystalline in nature but the preferential orientation and grain size changed upon doping. Atomic force micrograph showed a complete changeover of surface morphology from spherical to flake upon doping. A water contact angle result displayed the hydrophobic nature of the doped CeO2 film. Optical properties indicated an increase in band-gap and a decrease in transmittance upon doping owing to Moss–Burstein effect and inverse Moss–Burstein effects. Other optical properties such as refractive index, extinction coefficient and dielectric constant as a function of doping were analysed and reported.

The influence of Er3+ doping on the structural and optical properties of CeO2 thin films grown by PED

Erbium doped CeO2 thin films were deposited on both Corning glass substrates and indium doped tin oxide (ITO) coated glass substrates by pulsed e-beam deposition (PED) method at room temperature. Structural features of Er doped CeO2 thin films were studied with X-ray diffraction (XRD) and micro-Raman spectra. The XRD patterns of all films showed polycrystalline nature and cubic crystalline structure. Raman active peaks for both undoped CeO2 and Er doped CeO2 films were determined at ∼465cm−1. The Raman shift observed in this study can also be assigned to Raman active modes of CeO2 that are shifted from the original position due to different doping concentration. The optical properties of CeO2 films and Er doped CeO2 films, which were determined from transmittance and reflectance measurements at room temperature, were very similar in character. The refractive indices and extinction coefficients, which were calculated from 3.5 to 1.25eV (300–1000nm), were between 1.5–3 and 0.05–0.2, respectively. The optical band gaps were deduced from the absorption coefficient according to solid band theory. The electrochromic measurements revealed that 2% Er doped CeO2 films grown on ITO+WO3 substrates had highest charge density compared to the other samples. Long-time cyclic voltammetry (CV) and chronoamperometry (CA) measurements were carried out to investigate the stability of this film.

Growth and characterization of doped CeO2 buffers on Ni-W substrates for coated conductors using metal organic deposition method

CeO2 and Ce0.8M0.2O2−d films (M = Mn, Y, Gd, Sm, Nd and La) with (00l) preferred orientation have been prepared on biaxially textured Ni-W substrates by metal organic decomposition (MOD) method. The factors influencing the formation of cracks on the surface of these CeO2 and doped CeO2 films on Ni-W substrates were explored by X-ray diffraction (XRD), scanning electron microscopy (SEM) analysis, atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The results indicate that many factors, such as the change of the ionic radii of doping cations, the transformation of crystal structure and the formation of oxygen vacancies in lattices at high annealing temperature, may be related to the formation of cracks on the surface of these films. However, the crack formation shows no dependence on the crystal lattice mismatch degree of the films with Ni-W substrates. Moreover, the suppression of surface cracks is related to the change of intrinsic elasticity of CeO2 film with doping of cations with a larger radius. SEM and AFM investigations of Ce0.8M0.2O2−d (M = Y, Gd, Sm, Nd and La) films reveal the dense, smooth and crack-free microstructure, and their lattice parameters match well with that of YBCO, illuminating that they are potentially suitable to be as buffer layer, especially as cap layer in multi-layer architecture of buffer layer for coated conductors.

Magnetic and Magneto-Optical Properties of Sputtered Co-CeO<sub>2</sub> Thin Films on Al<sub>2</sub>O<sub>3</sub> (0001) Substrates with (100) Orientation<sup> </sup>

Diluted magnetically doped CeO2 films is an attractive dilute magnetic oxide which would facilitate the practical realization of spintronic devices and may also be used to explore novel magneto-optical applications. In this experiments, 3 at% cobalt-doped CeO2 films with the stoichiometry of Ce0.97Co0.03O2-δ (CCO) were deposited by magnetron sputtering methods on Al2O3 (0001) substrates. The structural, magnetic, and magneto-optical properties were investigated. The results indicate that CCO films with CeO2 (100) orientation can readily be obtained via magnetron sputtering on Al2O3 (0001) substrates. Films are ferromagnetic at room temperature, which is anisotropic with an out-of-plane magnetization easy axis. Magneto-optical measurements exhibit a giant Faraday rotation of about 4800 deg/cm at 650 nm wavelength in out-of-plane direction. The excellent room-temperature ferromagnetism and the giant Faraday rotation in CCO films show highly potential applications in novel magneto-optical devices as well as in spintronics.

Ferromagnetism in sputtered Co doped CeO2 thin film on Al2O3 (0001) substrate

Diluted Co doped CeO2 films with the stoichiometry of Ce0.97Co0.03O2-d have been prepared via long range magnetron sputtering under varied O2 flux. Films were thoroughly characterized using X-ray diffraction (XRD), atomic force microscopy (AFM), optical transmission spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and magnetic measurements. The results indicate all film are ferromagnetism (FM) at room temperature, of which the saturated magnetization moments (MS) are changed according to the O2 pressure adopted in deposition. This phenomenon is well consistent with the oxygen vacancy (VO, or so called F-center) mediated FM mechanism which was recently proposed to interpret FM found in insulating diluted magnetic oxides (DMOs) system. For further exploration of the oxygen dependent of the MS, oxygen non-stoichiometry was characterized by XPS. It shows that film deposited at lower O2 pressure contains less oxygen weight or more oxygen vacancies, lead to an enhanced MS and vice versa.

Microstructural Correlation with Electrical Properties for Y2O3 Doped CeO2 Thin Films

ABSTRACTHigh quality textured 0.58% Y2O3 doped CeO2 films with (001), (111)/(001) and (110) were prepared using an e-beam deposition technique on substrates of (001) LaAlO3, r-cut sapphire, and fused silica, respectively. The composition and stoichiometry of the films were verified by Rutherford backscattering spectroscopy analysis. Both x-ray diffraction and transmission electron microscopy analyses gave consistent microstructural information. Complex impedance measurements have been performed to study the electrical properties of these films as a function of temperature. The conductivities of the films were dominated by grain boundaries of high conductivities as compared to that of the bulk ceramic of the same dopant concentration. The activation energies for the film conductivities were only slightly higher than that for the bulk lattice conductivities, but much lower than that for the bulk grain boundary conductivity. These results have been discussed in terms of the differences of the grain size and grain boundary microstructures between the films and the bulk ceramics.