Cerium Oxide Thin Films Research Articles

Structural and morphological properties of CeO2 films deposited by radio frequency magnetron sputtering for back-end-of-line integration

Cerium oxides have potential applications ranging from low-temperature gas sensing to photodetection. A back-end-of-line integration of the material into complementary metal-oxide-semiconductor device fabrication processes has many advantages but places limits on material deposition, most notably the thermal budget for deposition and annealing. Here, we investigate thin cerium oxide films deposited by radio frequency (RF) magnetron sputtering at substrate temperatures of 300°C and RF magnetron powers between 30 W and 70 W without any post-deposition annealing steps. Our investigation of the structural and morphological properties reveals a columnar texture of the thin films, and we find that the material is composed predominantly of CeO2 (111), with a large degree of crystallinity. We discuss implications for resistive gas sensing applications.

The influence of pH electrolyte on the electrochemical deposition and properties of cerium oxide thin films

Thin cerium oxide layers cathodically deposited on zinc were studied. These electrochemically deposited coatings provide effective corrosion protection and require less deposition time compared to chemical conversion coatings. The kinetics of their formation depend on various factors, such as temperature, applied current density, electrolyte composition, pH, and agitation. In this study, the effect of bath solution pH was investigated, with coatings produced at room temperature from aqueous cerium nitrate solutions. Chronopotentiometry, linear sweep voltammetry, and electrochemical impedance spectroscopy (EIS) were used to monitor the deposition process, while scanning electron microscopy and energy-dispersive spectroscopy were performed to characterize the deposits. Corrosion resistance was evaluated through potentiodynamic tests in a 3.5% NaCl solution. The results indicate that the electrolyte pH significantly influences the cerium electrodeposition process. Based on the findings, the cerium oxide coating obtained from a pH 4 solution exhibited lower corrosion current density, higher polarization resistance, and superior anti-corrosion performance.

Influence of Post-Annealing Treatment on Some Physical Properties of Cerium Oxide Thin Films Prepared by the Sol–Gel Method

In this study, thin films of Cerium Oxide CeO2 were fabricated using the sol–gel technique and deposited onto a glass substrate. The annealing process was carried out at various temperatures ranging from 200 to 600 °C to investigate the structural, morphological, and optical properties of the films and their interrelations. X-ray diffraction (XRD) patterns revealed the crystalline nature of the prepared films, with film quality exhibiting enhancement with increasing annealing temperature. The average crystallite size, dislocation density, microstrain, and lattice constant were determined from XRD patterns. Higher annealing temperatures were found to increase the crystallite size values from 4.71 to 15.33 nm and decrease the dislocation density and microstrain of the unit cell. Scanning electron microscope (SEM) images illustrated the uniformity of the films, presenting a spheroid shape. Optical properties such as transmittance, absorbance, reflectance, the direct band gap, extinction coefficients, the refractive index, and optical conductivity were assessed using optical measurements. The direct optical band gap of the CeO2 film was observed to decrease from 3.99 to 3.75 eV with increasing film thickness. Using the Wemple and DiDomenico (WDD) single-oscillator model, dispersion energy parameters were calculated based on the refractive index. The nonlinear optical properties of the CeO2 thin films were evaluated using these dispersion energy parameters. The improvement of optical parameters holds significance in standardizing CeO2 thin films for various optoelectronic applications.

Low temperature hydrogen sensor with high sensitivity based on CeOx thin film

In this work, a 500 nm-thick cerium oxide thin film was prepared by electron beam evaporation. It was found that the deposition of 7 nm thick Pd catalyst was required for obtaining a sensor response to hydrogen. The Pd/CeOx sensing structure has a high response of 5000 towards 25 ppm H2 at a working temperature of 200 °C and exhibits a sensor response of 1.3 at temperatures near ambient. Furthermore, the sensing structure exhibited excellent response/recovery kinetics. The results confirm that the CeOx-based materials are a promising material for the fabrication of room-temperature hydrogen sensors.

Metal-organic decomposed lanthanum cerium oxide thin films: A study of chemical and surface properties

Lanthanum cerium oxide thin films have been deposited on n-type (100) silicon wafers using the spin coating technique. The post-deposition annealing was performed over spin-coated films. The compositional studies were conducted by performing Fourier transform infrared spectroscopy, Energy dispersive X-ray spectroscopy, and X-ray diffraction. Ellipsometry was used to determine the thickness and atomic force microscopy was performed to understand the surface morphology of the deposited thin films. The effect of rapid thermal annealing at different temperatures over the structural and chemical properties of the LaCeO2 thin films was observed. The FTIR spectra show the formation of silicates at the interface of the oxide and semiconductor, while XRD results show that the films become crystalline with increased annealing temperature. The refractive index was observed to be reduced with an increase in the annealing temperature and a minor reduction in the physical thickness as per the sample standard deviation of 0.2981[Formula: see text]nm.

Synthesis and Characterization of Cerium Oxide Thin Films Fabricated via the Facile Doctor Blade Method

In this study, we present an in-depth investigation of cerium oxide (CeO2) thin films synthesized using the doctor blade approach, with polyethylene glycol employed as a binder. A comprehensive characterization employing X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV-visible spectroscopy, and atomic force microscopy (AFM) has been conducted to elucidate the structural, chemical, and morphological attributes of the fabricated films. The XRD analysis reveals distinctive wide diffraction peaks indicative of a face-centered cubic CeO2 crystalline structure existing in a singular phase. The morphological analysis using AFM delineates a mean square roughness of 34.54 nm, providing valuable insights into the surface topography of the CeO2 thin films. Additionally, the direct correlation between the material’s band gap, determined as 1.92 eV through UV-visible spectroscopy, and its nanostructural features is established using spectroscopic ellipsometry in conjunction with AFM studies. This approach offers a unique perspective on the optical characteristics of CeO2 films, enhancing our understanding of their nanostructures and facilitating the optimization of their performance for energy applications. Furthermore, the synergistic utilization of scanning electron microscopy (SEM) and spectroscopic ellipsometry contributes to a comprehensive understanding of the growth modes and surface characteristics of the thin films. The integration of these advanced techniques not only refines the fabrication process but also provides crucial insights into the intricate interplay between morphology and optical properties, crucial for optimizing thin films for various applications.

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.

A novel room-temperature nitrogen dioxide gas sensor based on silver-doped cerium oxide thin film

A novel room-temperature nitrogen dioxide gas sensor based on silver-doped cerium oxide thin film

Pulsed laser crystallization of sol-gel derived cerium oxide thin films

AbstractCerium oxide sol-gel-derived thin films were crystallized by pulsed excimer laser annealing (ELA). X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman Spectroscopy (RS), and Fourier transform infrared (FTIR) were employed to analyze the effect of laser fluence and number of pulses on the structural and morphological properties of the irradiated films. XRD and RS results, supported by thermal simulations, confirmed crystallization into cubic cerium oxide starting from 40 mJcm−2. SEM micrographs showed that an increased number of pulses induced the formation of porous and complex nanostructured surfaces, different from the morphology obtained on thermally annealed films. Finally, water contact angle measurements revealed that the films showed a characteristic hydrophobic “petal effect”. Graphical Abstract

Hydrogen diffusion in cerium oxide thin films fabricated by pulsed laser deposition

Cerium oxide (CeO2) is well known to be reducible by hydrogen (H2), yet the diffusion and solution properties of hydrogen in ceria at elevated temperatures have remained challenging to evaluate. We therefore fabricated nanometer-thin (∼100 nm) cerium oxide films on Si(111) substrates by pulsed laser deposition (PLD) and quantitatively investigated the H depth distributions therein by means of resonant 1H(15N,αγ)12C nuclear reaction analysis (NRA) before and after annealing in H2 gas at 773–973 K. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) reveal that the as-deposited films exhibit single phase CeO2 structure and partially reduced stoichiometry (CeO1.69). H2 annealing does not largely change the H content of the as-deposited films; in all conditions several atomic percent of hydroxyl (OH) are found to exist in a thin (∼4 nm) surface layer, whereas stably bound hydrogen in the bulk of the films is almost uniformly distributed and of much smaller quantity (∼0.2 at.%) than the oxygen vacancy concentration in the partially reduced ceria. Its low concentration and high thermal stability identify this bulk H species as likely being strongly bound to defects in the polycrystalline films rather than as a hydride species that interacts weakly with O-vacancies. The H diffusion coefficient and activation energy in the ceria films are determined as > 10−18 m2 s−1 at 773–973 K and <1.69 eV, respectively. The observed diffusion activation energy is somewhat larger than theoretical predictions for thermal diffusion of H in stoichiometric bulk CeO2, suggesting that defects and oxygen vacancies in the PLD-fabricated ceria thin films possibly have an impact on the H mobility.

Oxidation and Reduction of Polycrystalline Cerium Oxide Thin Films in Hydrogen.

This study investigates the oxidation state of ceria thin films' surface and subsurface under 100 mTorr hydrogen using ambient pressure X-ray photoelectron spectroscopy. We examine the influence of the initial oxidation state and sample temperature (25-450 °C) on the interaction with hydrogen. Our findings reveal that the oxidation state during hydrogen interaction involves a complex interplay between oxidizing hydride formation, reducing thermal reduction, and reducing formation of hydroxyls followed by water desorption. In all studied conditions, the subsurface exhibits a higher degree of oxidation compared to the surface, with a more subtle difference for the reduced sample. The reduced samples are significantly hydroxylated and covered with molecular water at 25 °C. We also investigate the impact of water vapor impurities in hydrogen. We find that although 1 × 10-6 Torr water vapor oxidizes ceria, it is probably not the primary driver behind the oxidation of reduced ceria in the presence of hydrogen.

Tunability of bandgap from UV to visible region by transition metal ion implantation in CeO2 and their electronic structures

The present report examines the tunability of optical band gap by 150 keV Fe ion implantation in cerium oxide (CeO2) thin films and their electronic structures. X-ray diffraction (XRD) confirms the stable FCC structure and these films lose their crystalline nature with increasing ion fluences. Atomic force microscopy (AFM) shows the morphology changes. Raman measurement exhibits the presence of defect states. UV–vis spectroscopy reveals that the optical band gap reduces from 3.14 eV to 2.70 eV. These results are well correlated with the electronic structure studies from the x-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS). The XPS indicates the existence of Ce3+ due to the replacement of Fe3+ which reduces the Ce4+ to Ce3+ ions. XAS at the Ce-M4,5 edge reveals the change in Ce4+→Ce3+ with Fe ion fluence and the O-K edge spectra show that the vacancies are introduced after Fe ion implantation. The edge shifting of the O-K edge confirmed the presence of Ce4+-VO-Ce3+ and Ce3+-VO-Fe3+ networks in Fe-implanted thin films. This reversible ability enables the CeO2-based nanomaterial for energy and environmental-related applications.

Evaluation of polycrystalline cerium oxide electrodes for electrochemiluminescent detection of sarcosine

Prostate cancer (PCa) is widely spread in male population, especially over 65 years. Currently used medical methods of PCa diagnosis often lead to false-positive results thus new non-invasive methods for PCa detection, such as urine tests for cancer metabolites, are actively studied. Herein, nanostructured polycrystalline cerium oxide thin films (CeO2/GC) prepared by magnetron sputtering on a glassy carbon substrate are tested for electrochemiluminescent (ECL) detection of sarcosine exploiting the oxidative-reduction mechanism using Ru(bpy)32+ as luminophore. Non-functionalized CeO2/GC electrodes revealed a higher ECL signal stability compared to bare glassy carbon electrodes. Moreover, CeO2/GC electrodes were successfully applied for rapid and sensitive detection of different sarcosine concentrations ranging from 50 to 5000 µM. These results open new possibilities for developing sensing platforms for sarcosine detection based on the CeO2/GC working electrode via surface modification and functionalization, aiming to further investigate and improve their sensitivity and selectivity.

Metal-organic decomposed cerium oxide thin film in mixed ambient at different temperatures for MOS capacitor

Post-deposition annealing of CeO2 thin films in nitrogen/oxygen/nitrogen ambient from 400 until 1000 °C successfully transformed the films from oxygen poor to oxygen-rich state, in which an excess of oxygen was detected at higher temperatures as the oxygen interstitials. The adsorbed nitrogen attaching to oxygen vacancies in the CeO2 thin films would inhibit further oxidation of the films, and thus controlling the film thickness in addition to expanding the lattice. The increase of temperature has enhanced adsorption and diffusion of nitrogen and oxygen, decreasing the attachment of nitrogen to the oxygen vacancies. The dissociated nitrogen would reside as dangling bonds that caused a difficulty in controlling the oxidation and hence interfacial layer growth was expected. An exaggerated film growth dominated by interfacial layer happened at the highest temperature, surpassing the CeO2 layer thickness. The lowest interface trap density was attained but a degradation in the current-voltage characteristic happened due to the presence of excess oxygen and nitrogen as the scattering center for the incoming electrons during forward bias operation. A better performance was achieved by the film annealed at 800 °C via the demonstration of the lowest leakage current level, surpassing other films. Detailed explanation was discussed in this work.

Cerium oxide thin films: synthesis, characterization, photocatalytic activity and influence on microbial growth

The resistance of surfaces to biofouling remains a significant advantage for optical devices working in natural conditions, increasing their lifetime and reducing maintenance costs. This paper reports on the functionalities of transparent CeO2 thin films with thicknesses between 25 and 600 nm deposited by reactive magnetron sputtering on the glass substrate. The CeO2 photocatalytic performance exhibited an efficiency of 30% on imidacloprid degradation under 6 h of UV radiation and increased linearly with the irradiation time, suggesting a complete degradation within 48 h. The films did not alter the growth rate of the green algae Chlorella vulgaris after 72 h short-term exposure. The tested CeO2 films proved to efficiently inhibit with high efficiency the Staphylococcus aureus biofilms and planktonic growth (reducing the counts of bacterial cells by 2 to 8 logs), demonstrating the promising potential of these materials for obtaining antimicrobial and antibiofilm surfaces, with broad applications for the biomedical, ecological and industrial fields.

Tailoring of defects dependent magnetic properties of swift heavy ion irradiated CeO2 for spintronics application

The present investigation reports the swift heavy ion induced effects on cerium oxide (CeO2) thin films. These thin films were deposited on Si (111) substrates by the electron-beam evaporation method and irradiated by a 100 MeV O7+ ion beam with different ion fluences. X-ray diffraction analysis of these films confirms the stable fluorite phase of CeO2 even after the higher fluence of irradiations. Raman measurement also supports the presence of the F2g phase of CeO2 and the presence of defect states. The Gaussian deconvolution of photoluminescence (PL) spectra reveals various defect-associated peaks. The broad peaks in the PL spectra are associated with oxygen vacancies and are red-shifted (494–520 nm) with ion fluences. The surface morphological images show the modification in the surface roughness with ion irradiation and the re-growth of smaller circular-formed nanoparticles on the surface is observed at the fluence of 5 × 1011 ions/cm2. Magnetic measurements show an enhancement in magnetic ordering with ion irradiation. All the samples demonstrate room temperature ferromagnetism with magnetic saturation (Ms) up to 14.57 emu/cm3. The saturation magnetization in irradiated thin films is directly correlated to the area under the peak of defect-associated PL emission. The mechanism such as the oxygen vacancy-based F-center exchange model is considered to understand the enhancement of ferromagnetism in ion irradiated CeO2 thin films. Some popular theoretical models are also employed to determine various magnetic parameters.

Samarium doped cerium oxide thin films deposited by pulsed laser deposition

Samarium doped ceria thin films are an important material for the electrochemical devices. Fluorite structured Samarium Doped Cerium Oxide (SDC) thin films were deposited on Si (100) at 500 °C and 600 °C substrate temperatures by PLD (Pulsed Laser Deposition) from Sm0,24 Ce0,13 O0.63 target. X-ray Diffraction (XRD) proved the existence of SDC crystalline compound in both samples, deposited at 500 and 600 °C but not in the target. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) investigations revealed a high quality of the films, with lower roughness, of under 5 nm in the case of the sample deposited at 600 °C. The beginning of sintering process was evidenced by SEM in the case of samples deposited at 600 °C. X-ray Photoelectron Spectroscopy (XPS) indicated the increasing proportion of Ce3+ for PLD films comparatively to the target, in both cases, of substrate temperatures of 500 and of 600 °C, higher in the first case. Spectroscopic Ellipsometry (SE) investigation permitted the calculation of a refractive index under 2.2 in the visible (Vis)-near infrared (near-IR) domain for both samples. Nanoindentation tribometric measurements concluded that both elasticity modulus and hardness decreases with substrate temperature increasing. The obtained films are candidates for potential uses as electrolyte material for a new generation of electrochemical devices.

Synergistic interactions between water and the metal/oxide interface in CO oxidation on Pt/CeO2 model catalysts

The significance of catalytic active interfacial sites in metal supported on metal oxide systems has been well established in prototypical CO oxidation reactions. Here, we investigate the role of water molecules in the CO oxidation reaction using Pt nanoparticles (Pt NPs) supported by a two-dimensional (2D) cerium oxide thin-film. The 2D model CeO2 thin films were fabricated using the e-beam evaporation technique and a monolayer of Pt nanoparticles deposited on CeO2 thin films by the Langmuir-Blodgett (LB) method. The well-defined reaction conditions, such as humid and dry conditions, were achieved using an ultra-high vacuum (UHV) batch chamber. Interestingly, we found a strong metal-support interaction (SMSI) in the Pt/CeO2 catalyst under dry reaction conditions, which was increased further by the presence of water molecules. However, water had a detrimental effect on a Pt thin-film model catalyst under humid reaction conditions. By altering the partial pressures of water molecules and CO, the reaction mechanism was established under humid conditions. The current results demonstrate that hydroxyl intermediates from water molecules provide an alternative pathway to the Pt/CeO2 catalyst, resulting in increased overall turnover rates. These findings shed light on the synergistic interplay of water and metal-oxide interfaces.

Ammonia sensing characteristics of a cerium oxide thin film coated with platinum nanoparticles

This paper reports on ammonia (NH3) sensing characteristics of a new semiconducting metal oxide (SMO) sensor. employing a cerium oxide (CeO2) thin film and platinum (Pt) nanoparticles (NPs) as a sensing layer. The thickness of CeO2 thin film is 30 nm and the particle size of Pt NPs is about 4–6 nm. Based on the enhanced surface area/volume (SA/V) ratio and catalytic ability of Pt metal, the sensing behaviors are efficiently improved owing to the smaller Pt NPs. A high sensing response of 5.81 under 1000 ppm NH3/air gas and a low detectable content of 1 ppm NH3/air are acquired at 275 ℃. In addition, a relatively fast sensing action with the response (recovery) times of 15 s (85 s) and 6 s (25 s) are observed upon exposure to 1000 and 1 ppm NH3/air gas, respectively, at 300 ℃. The Langmuir isotherm is used to study the related surface coverage under introduced ammonia gas. The proposed Pt NP/CeO2 sensor shows prominent stability for long-term operation without intentional passivation treatment. The proposed sensor device’s benefits include a simple device structure, relatively easy manufacturing, a wide sensing range of ammonia gas content (1–1000 ppm NH3/air), and lower cost.

In situ NAP-XPS study of CO2 and H2O adsorption on cerium oxide thin films

Insights of CO2 and H2O activation on cerium oxide based catalysts are of great interest to many important catalytic processes, which are widely studied in model systems under ultrahigh vacuum conditions. Here, in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was used to study CO2 and H2O adsorption on fully oxidized CeO2 and partially reduced CeO2−x surfaces. It is found that CO2 only weakly interacts with the stoichiometric CeO2 but strongly adsorbs on partially reduced CeO2−x surfaces with surface carbonate formation. H2O dissociatively adsorbs reversely on the CeO2 surface with H2O exposure pressure above 1 × 10−3 mbar, while H2O strongly dissociates on the partially reduced CeO2−x surfaces. The surface dissociated OH and H species are stable on the CeO2−x surface up to 600 K. Both CO2 and H2O adsorption cause clear reoxidation of partially reduced CeO2−x. This study provides new insights into CO2 and H2O interaction with cerium oxide surfaces at near ambient pressure conditions.