Eu Thin Films Research Articles

Growth of ultimate high-density scintillating films for high-resolution X-ray imaging at synchrotrons

Scintillators are X-ray to visible light converters applied in X-ray imaging detectors used at synchrotrons. Drastically improved performances of the 4th generation synchrotron sources enable us to perform X-ray imaging experiments at higher energies. For high spatial resolution X-ray imaging (micrometer to submicrometer), thin scintillating films are required. Consequently, especially for high X-ray energies (30keV to 100keV), the detection efficiency is limited due to the low X-ray absorption efficiency of the thin films. We have used Liquid Phase Epitaxy (LPE) to grow thin films of one of the ultimate high-density materials, Lu2Hf2O7:Eu3+, which demonstrate scintillating properties and thus combine high spatial resolution and maximized absorption efficiency. X-ray imaging experiments demonstrate that the Modulation Transfer Function (MTF) reaches 10% at 900 lp/mm, and radiographs visually confirm the promising imaging properties. We present structural, luminescent, and scintillation characterization of Lu2Hf2O7:Eu thin films grown on ZrO2:Y substrates, showing that the films crystallize in the disordered fluorite structure and the europium ions incorporate into the structure and enhance the luminescence intensity. This contribution is a first step toward developing promising ultra-dense, hafnate-based scintillating screens for high spatial resolution X-ray imaging.

Dispersion properties of (La0.06Ga0.94)2O3:Eu thin films

The dispersion of the refractive index of (La0.06Ga0.94)2O3:Eu thin films obtained by radio-frequency ion-plasma sputtering has been studied. It was found that the films have a polycrystalline structure corresponding to the monoclinic structure of β- Ga2O3. It is shown that the freshly deposited films are characterised by an abnormal dispersion, and after annealing in argon, a normal dispersion of the refractive index is observed. It was found that at normal dispersion, the spectral dependence of the refractive index in the visible region of the spectrum is mainly determined by electronic transitions from the 2p-state oxygen band, which form the upper filled level of the valence band to the bottom of the conduction band formed by hybridised 2p-states of oxygen and 4s-states of gallium. Two single-oscillator approximation models were analysed and compared for the films annealed in argon, and the approximation parameters, dispersion energy, degree of chemical bond ionicity, coordination number, and static refractive index were determined.

Effect of annealing temperature on Eu2+ and Eu3+ ratios in AlN:Eu thin films

Eu-doped aluminum nitride (AlN:Eu) thin films were fabricated and characterized in a way of white light generation. AlN exhibits blue luminescence originating from defect levels. Furthermore, Eu2+ demonstrates green luminescence, while Eu3+ emits red light. Consequently, white luminescence is anticipated in Eu-doped AIN thin films. The concept is to control the ratio of Eu2+/Eu3+ in the thin film to vary the emission colors. We aimed to manipulate the Eu valence ratio in AIN:Eu thin films by changing the annealing temperature and examining the resultant luminescence characteristics. Our findings revealed an increase in defect-related luminescence in AIN samples annealed at higher temperatures, along with a rise in the Eu2+ fraction. We explore these outcomes in relation to the structural models surrounding Al and Eu ions, as elaborated in the main text.

Temperature dependence of luminescence characteristics from Eu doped Ga2O3 thin films excited by synchrotron radiation source

Ga2O3 thin film with Eu doping was prepared on p-Si substrate by pulsed laser deposition to investigate the temperature dependence of photoluminescence from Eu3+ and host. The obtained Ga2O3:Eu thin film has a polycrystalline monoclinic structure and smooth surface. The film exhibits multiple sharp emissions originating from Eu3+ dopants, as well as broad defect-related emissions in the UV-green region from Ga2O3 host. With increasing the temperature from 21 K to RT, the red emission from Eu3+ keeps unchanged in the wavelength, and remains ∼55% PL intensity. Meanwhile, Ga2O3 defect-related UV and blue emissions experience a strong thermal quenching and a distinct red shift following the Varshni equation and Bose–Einstein expression. These experimental data will provide reliable guide for fabricating efficient luminescent devices based on rare-Earth doped Ga2O3.

Revealing the Anisotropy in Protonation-Induced Electronic Phase Transitions of Rare-Earth Nickelates within a Marine Environment

Although the discovery of the electrochemical protonation-induced electronic phase transition of rare-earth nickelates (ReNiO3) enables potential application in sensing the ocean electric field that simulates the working principle of the ampullary organ of marine animals, whether such a functionality is anisotropic is previously overlooked. Herein, we demonstrate the anisotropy in the protonation-induced electronic phase transition in ReNiO3 (Re = Sm, Nd, and Eu) thin films as electrochemically triggered in an ocean environment. A larger elevation in the material resistivity triggered by an electric field within an ocean environment is observed for ReNiO3/LaAlO3(110), compared to ReNiO3/LaAlO3(001) and ReNiO3/LaAlO3(111). This is attributed to the orientation-related in-plane oxygen atomic density that results in more effective in-plane proton diffusion along the adjacent oxygen position, as further confirmed by the electrochemical cyclic voltammetry characterization. In addition, the larger activation energy associated to the anisotropic in-plane electronic structures of ReNiO3/LaAlO3(110) is also expected to promote the formation of electron-localized orbital configurations upon hydrogenation. As demonstrated, anisotropy sheds light on another possibility that can be further introduced to regulate the protonation-induced electronic phase transition properties of ReNiO3 for its potential applications such as ocean electric field sensing or biosensing.

UV light blocking and conversion by porous europium-doped titanium dioxide (TiO2-Eu) thin films for potential protection of photovoltaic devices

In recent years, transparent thin films capable of screening and converting ultraviolet (UV) photons into visible spectrum gained significant interest in the protection of photovoltaic devices. We investigated the optical properties and UV screening capability of europium-doped titanium oxide (TiO2−Eu) thin films deposited by the spin-coating method from a solution precursor for the first time in this study. We showed that TiO2-Eu thin films demonstrate europium concentration-dependent optical properties, and the quantum yield of the optimized sample was found to be ∼10.2%. Transmittance and photoluminescence measurements suggested that TiO2−Eu thin film can effectively block UV photons (∼30.5% at 320 nm) at glass substrate and convert them to the red emission thanks to 5D0 →7Fj (j = 0, 1, 2, 3, and 4) Eu (III) electronic transitions. Photodegradation experiments with methylene blue dye revealed that TiO2−Eu thin films offer better UV protection compared to uncoated samples. We strongly believe that porous TiO2−Eu thin films can be effectively utilized as a UV blocking and light conversion coating.

Magneto-optical property and magnetic anisotropy of (100) oriented R0.5Bi2.5Fe5O12 (R = Eu, Sm, and Pr) thin films prepared by metal–organic decomposition

Bi-substituted rare-earth iron garnets, R3−xBixFe5O12 (Bi:RIG), where R represents one of the rare-earth elements, exhibit the excellent magneto-optical (MO) properties that increase with Bi content x. In addition, magnetic properties of Bi:RIGs, such as the magnetization, the magnetic anisotropy, and the magnetostriction, could be controlled by choosing rare-earth elements. In this paper, we report on R0.5Bi2.5Fe5O12 (Bi2.5:RIG, R = Pr, Sm, and Eu) thin films on Gd3Ga5O12 (GGG) (100) single crystal substrates prepared by the metal–organic decomposition method. XRD analysis reveals that Bi2.5:RIG thin films are grown along the same orientation with GGG substrates, and their lattice constants are dependent on the ionic radii of the rare-earth ions. MO measurements show that Faraday spectra of the Bi2.5:RIG thin films have a typical spectral structure observed for Bi:RIGs. The magnetic anisotropy constants, the uniaxial magnetic anisotropy energy Ku, and the magnetocrystalline anisotropy energy K1 of Bi2.5:RIG (R = Y, Pr, Nd, Sm, and Eu) thin films are investigated by using the ferromagnetic resonance measurement.

Local strain-dependent Zeeman splitting in GaN:Eu

The electronic spins of rare-earth materials are attractive candidates for spin qubits and quantum memories. To access individual spins, tuning of the g-factor is desirable. Here, we report on local strain-dependent g-factors of the 5D0–7F2 transitions of Eu3+ centers in GaN:Eu thin films. We have found a clear correlation between the effective g-factor and the emission energy shift induced by the local strain. The combination of micro-photoluminescence and scanning electron microscope/electron backscattering diffraction measurements has revealed that the compressive strain of 0.2%–0.4%, relative to a surrounding reference point, induces an energy shift of about 3 meV. The strain decreases the g-factor of the emission at 1.991 eV from 2.5 to 1.5, while the strain increases the g-factor of the emission at 1.994 eV from 1.1 to 1.7. The result suggests that the g-factor can be tuned by the local strain. On the basis of the strain-induced energy shift and the g-factor, we have identified the optical sites. The 5D0–7F2 transitions observed in this study consist of three optical sites with C3v symmetry and one site with C1h symmetry.

ANALYSIS OF SPECTRAL AND KINETIC CHARACTERISTICS OF CATHODOLUMINESCENCE OF Y2O3:Eu THIN FILMS AS A RED COMPONENT OF FULL-COLOR HIGH-RESOLUTION FLUORESCENT DISPLAYS

The spectra and kinetics of the rising and decay of cathodoluminescence (CL) of thin Y 2 O 3 :Eu films obtained by the method of RF-magnetron sputtering are studied. Based on the shape of the CL spectra at different energies and excitation current densities, the possibility of the formation of irregular solutions of yttrium and europium oxide and the features of the structure of surface and bulk layers are shown. The dependence of the CL intensity on the energy of the exciting electrons and the current density of the electron irradiation has been investigated. The CL decay time constant for the 612 nm luminescence is determined, the value of which is in the range (1.8 – 4.1) ms. It is shown that this quantity is a complex function of the type of film deposition atmosphere, the concentration of the activator, and the duration of the exciting pulses. The features of CL rising are investigated and it is proposed to analyze the structural perfection of Y 2 O 3 :Eu thin films based on the delay in CL rising. Key words : yttrium oxide, cathodoluminescence, thin films.

Structural, surface and luminescent properties of SrF2:Eu annealed thin films

The effect of calcined temperature on the structural, surface and luminescent properties of a SrF2:Eu thin film were investigated for possible application as a downshifting layer for solar cell application. The film was deposited on sodium lime glass by a spin coating method and obtained after heat-treatment under Ar/H2 atmosphere. The thin film's structure as well as the morphology were investigated with X-ray diffraction and scanning electron microscopy, respectively. The elemental composition of the film was obtained with X-ray photoelectron spectroscopy (XPS). The XPS depth profile showed that oxygen and silicon segregated into the 600 °C calcined film and formed an intermediate layer. The XPS results for the SrF2 thin films confirmed that the elements of thin film existed in their stable form on the surface. The electric dipole allowed transitions of Eu2+ ions dominated the film's photoluminescent (PL) spectra at low calcination temperatures. The 5D0-7F0 PL transition of Eu3+ was, however, observed at elevated temperatures. The results suggested that Eu might be a good sensitizer for other lanthanide ions, such as Tb3+ and Er3+, at low temperature treatments.

Effect of pulsed laser annealing on optical and structural properties of ZnO:Eu thin film

ZnO:Eu thin film fabricated by pulsed laser deposition was treated by pulsed UV laser. The effect of laser fluence from 70 to 125 mJ cm−2 on film properties was investigated. The results showed that the surface morphology was clearly modified and the film treated at laser fluence of 70 mJ cm−2 was more densified compared to the other annealed films. Laser treatment led to a reduction of carbon content and to a stoichiometric ZnO:Eu composition on the surface. Refractive index significantly decreased and extinction coefficient was blue shifted for higher laser fluences. Strong characteristic Eu3+ transitions were obtained using excitation at 266 nm. The highest photoluminescence intensity was observed for the annealed film at a laser fluence of 70 mJ cm−2. The influence of ambient composition (N2 and O2) and temperature (room temperature and 150 °C) to photoluminescence response was examined for optical gas sensor application. The laser-untreated film showed higher sensitivity to the presence of oxygen at both temperatures compared to the treated film. The stability of photoluminescence intensity at elevated temperature was improved for laser-treated areas.

Red Y2O3:Eu-Based Electroluminescent Device Prepared by Atomic Layer Deposition for Transparent Display Applications.

Y2O3:Eu is a promising red-emitting phosphor owing to its high luminance efficiency, chemical stability, and non-toxicity. Although Y2O3:Eu thin films can be prepared by various deposition methods, most of them require high processing temperatures in order to obtain a crystalline structure. In this work, we report on the fabrication of red Y2O3:Eu thin film phosphors and multilayer structure Y2O3:Eu-based electroluminescent devices by atomic layer deposition at 300 °C. The structural and optical properties of the phosphor films were investigated using X-ray diffraction and photoluminescence measurements, respectively, whereas the performance of the fabricated device was evaluated using electroluminescence measurements. X-ray diffraction measurements show a polycrystalline structure of the films whereas photoluminescence shows emission above 570 nm. Red electroluminescent devices with a luminance up to 40 cd/m2 at a driving frequency of 1 kHz and an efficiency of 0.28 Lm/W were achieved.

Structure and Photoluminescence of Optically Transparent Y2O3:Eu Thin Films Prepared on Sapphire Substrates by RF Magnetron Sputtering

Structure and Photoluminescence of Optically Transparent Y₂O₃:Eu Thin Films Prepared on Sapphire Substrates by RF Magnetron Sputtering

Concentration quenching of photoluminescence in optically transparent Lu2O3:Eu thin films deposited to be used as scintillators

Photoluminescent thin films of europium doped lutetium oxide (Lu2O3:Eu) -an established material for x-ray scintillators- have been grown on glass substrates employing radio-frequency magnetron sputtering deposition technique. Optically transparent, crystalline films were obtained by only controlling the deposition process parameters. Neither any pre-treatment on glass substrate nor any post-deposition treatment on the films was applied. To our knowledge, depositing transparent but crystalline films of Lu2O3:Eu on glass substrates in such a direct way is an achievement that is reported for the first time in the literature. Using separate sputtering targets for the matrix constituent (Lu2O3) and the dopant enabled us to control the doping level of europium atoms in the films. By varying the magnetron power applied to the Eu2O3 target, we produced a series of Lu2O3:Eu thin films with different Eu concentrations. Significant quenching of luminescence by concentration was observed in the films. Detailed chemical and microstructural analyses revealed that europium atoms segregated from the matrix at high concentrations, an important contributor to concentration quenching. Some remarks have been given on finding the europium concentration that will give best scintillator performance.

Control of Eu Oxidation State in Y2O3-xSx:Eu Thin-Film Phosphors Prepared by Atomic Layer Deposition: A Structural and Photoluminescence Study.

Structural and photoluminescence studies were carried out on Eu-doped Y2O3−xSx thin films grown by atomic layer deposition at 300 °C. (CH3Cp)3Y, H2O, and H2S were used as yttrium, oxygen, and sulfur precursors, respectively, while Eu(thd)3 was used as the europium precursor. The Eu oxidation state was controlled during the growth process by following the Eu(thd)3 pulse with either a H2S or O3 pulse. The Eu(thd)3/O3 pulse sequence led to photoluminescence emission above 550 nm, whereas the Eu(thd)3/H2S pulse sequence resulted in emission below 500 nm.

Synthesis and Structure of Y2O3:Eu Thin Films

Synthesis and Structure of Y2O3:Eu Thin Films

Features of Deposition and Formation of the Structure of Y2O3:Eu Thin Films Obtained by Radio-Frequency Sputtering

Features of Deposition and Formation of the Structure of Y2O3:Eu Thin Films Obtained by Radio-Frequency Sputtering

Influence of Annealing Temperature and Time on the Photoluminescence and the Structure of Y2O3:Eu Thin Films

Y2O3:Eu thin films were deposited on p-type (100)-oriented silicon wafers by using the radiofrequency (RF) magnetron sputter deposition technique and then post-annealed in an air ambient at various temperatures and holding times. X-ray diffraction data showed that the as-deposited films had a cubic Y2O3 structure textured with the (222) plane. The as-deposited films were found to be under internal compressive stress, and the annealing treatment led to relaxation of the stress in the films. Photoluminescence (PL) was observed from the films annealed at temperatures ≥ 400°C, and the strongest peak occurred at 613 nm in the red range. The PL intensity increased as the annealing temperature was increased, resulting from the improved crystalline quality of the Y2O3:Eu films annealed at higher temperatures. The PL intensity reached a maximum at 1200°C and then decreased due to the formation of the Y2SiO5 phase. The Y2O3:Eu films were also annealed at 1200°C for different periods of time from 1 to 10 h. The PL intensity of the 1200°C-annealed films was decreased for longer holding time resulting from the formation of the Y2SiO5 phase.

Morphological, structural and room temperature optical properties of ZnO:Eu layers deposited by RF-Sputtering

The structural and optical properties of ZnO:Eu thin films deposited on silicon and glass substrates by the magnetron sputtering method have been studied by scanning electron microscopy, energy dispersive X-ray analysis, Auger electron spectroscopy, Raman spectroscopy and photoluminescence measurements. Intense red emission of Eu3+ dopant in ZnO films is issued by the band-to-band excitation and energy transfer from the host ZnO to europium ions.

Optical and structural properties of ZnO:Eu thin films grown by pulsed laser deposition

Eu doped ZnO thin films were deposited by pulsed laser deposition on fused silica substrate at room temperature and 300 °C in an oxygen ambient of pressure of 10 Pa. The films exhibited a wurtzite structure. The average transmittance exceeded 80% in the visible region confirming a good optical quality of the films. The characteristic emission of both Eu2+ and Eu3+ was observed when indirectly excited. The increase of substrate temperature to 300 °C led to higher crystallites size, lower efficiency of energy transfer from ZnO to Eu ions and lower resistivity of the film. XPS revealed the ratio of Eu divalent and trivalent states as 35%:65% and 27%:73% for sample deposited at room temperature and 300 °C, respectively. AFM showed higher RMS for the sample prepared at 300 °C.