Eu-doped ZnO Research Articles

Effect of surfactants on morphology, structure and photoluminescence properties of Eu-doped ZnO microsphere

The influence of polyvinylpyrrolidone (PVP) and monoethanolamine (MEA) as surfactants on photoluminescence (PL) of Eu-doped ZnO (ZnO:Eu) has been studied. X-ray diffraction (XRD) results show that there are more Eu3+ ions to replace Zn2+ ions into ZnO lattice for ZnO:Eu in the presence of surfactant MEA. From the spectral analysis for ZnO:Eu with different surfactants by Gaussian deconvolution, it can be seen that the use of MEA as a surfactant affects the energy transfer from ZnO host to Eu3+ ions and leads to three new emission peaks, including UV emission (389 nm), violet emission (411 nm) and green emission (506 nm).

Photocatalytic Degradation of Eriochrome Black-T Azo Dye Using Eu-Doped ZnO Prepared by Supercritical Antisolvent Precipitation Route: A Preliminary Investigation

Photocatalytic Degradation of Eriochrome Black-T Azo Dye Using Eu-Doped ZnO Prepared by Supercritical Antisolvent Precipitation Route: A Preliminary Investigation

Annealing effect on the structural and optical behavior of ZnO:Eu3+ thin film grown using RF magnetron sputtering technique and application to dye sensitized solar cells

Eu-doped ZnO (ZnO:Eu3+) thin films deposited by RF magnetron sputtering have been investigated to establish the effect of annealing on the red photoluminescence. PL spectra analysis reveal a correlation between the characteristics of the red photoluminescence and the annealing temperature, suggesting efficient energy transfer from the ZnO host to the Eu3+ ions as enhanced by the intrinsic defects levels. Five peaks corresponding to 5D0–7FJ transitions were observed and attributed to Eu3+ occupancy in the lattice sites of ZnO thin films. As a proof of concept a dye sensitized solar cell with ZnO:Eu3+ thin films of high optical transparency was fabricated and tested yielding a PCE of 1.33% compared to 1.19% obtained from dye sensitized solar cells (DSSC) with pristine ZnO without Eu produced indicating 11.1% efficiency enhancement which could be attributed to spectral conversion by the ZnO:Eu3+.

Pd-Functionalized ZnO:Eu Columnar Films for Room-Temperature Hydrogen Gas Sensing: A Combined Experimental and Computational Approach.

Reducing the operating temperature to room temperature is a serious obstacle on long-life sensitivity with long-term stability performances of gas sensors based on semiconducting oxides, and this should be overcome by new nanotechnological approaches. In this work, we report the structural, morphological, chemical, optical, and gas detection characteristics of Eu-doped ZnO (ZnO:Eu) columnar films as a function of Eu content. The scanning electron microscopy (SEM) investigations showed that columnar films, grown via synthesis from a chemical solutions (SCS) approach, are composed of densely packed columnar type grains. The sample sets with contents of ∼0.05, 0.1, 0.15, and 0.2 at% Eu in ZnO:Eu columnar films were studied. Surface functionalization was achieved using PdCl2 aqueous solution with additional thermal annealing in air at 650 °C. The temperature-dependent gas-detection characteristics of Pd-functionalized ZnO:Eu columnar films were measured in detail, showing a good selectivity toward H2 gas at operating OPT temperatures of 200-300 °C among several test gases and volatile organic compound vapors, such as methane, ammonia, acetone, ethanol, n-butanol, and 2-propanol. At an operating temperature OPT of 250 °C, a high gas response Igas/Iair of ∼115 for 100 ppm H2 was obtained. Experimental results indicate that Eu doping with an optimal content of about 0.05-0.1 at% along with Pd functionalization of ZnO columns leads to a reduction of the operating temperature of the H2 gas sensor. DFT-based computations provide mechanistic insights into the gas-sensing mechanism by investigating interactions between the Pd-functionalized ZnO:Eu surface and H2 gas molecules supporting the experimentally observed results. The proposed columnar materials and gas sensor structures would provide a special advantage in the fields of fundamental research, applied physics studies, and ecological and industrial applications.

Characterization and photocatalytic activity of Eu:ZnO & Au/Eu:ZnO nanoparticles prepared by laser ablation in water

Herein we report on the photocatalytic activity of surfactant-free ZnO, Eu:ZnO, Au/ZnO, and Au/Eu:ZnO nanoparticles prepared by laser ablation of undoped and Eu-doped ZnO sintered targets and gold metal in distilled water. The hexagonal crystal structure of the ablated ZnO NPs was confirmed by the XRD analysis. The TEM images showed the formation of aggregated NPs with a nearly round shape and size in the range of 8–60 nm. The optical absorbance spectra indicated a slight widening of the bandgap energy in the Eu-doped ZnO NPs. The photoluminescence spectra of all samples displayed a sharp UV exciton emission of ZnO. The ablation yield was obviously enhanced in Eu-doped ZnO target compared to the undoped one, as revealed by the XRD, optical absorbance and PL spectra. The photocatalytic activity of the ablated NPs was evaluated by the decolorization of the R6G organic dye pollutant, under UV irradiation. The AU/Eu:ZnO NPs showed the highest photocatalytic efficiency of about 93% within 45 min. These results point out that laser ablation of Eu-doped sintered target and gold metal in distilled water can be an effective method to prepare ultra-pure Au/Eu:ZnO NPs with improved photocatalytic efficiency.

Efficient luminomagnetic and conductive Eu and Dy doped ZnO phosphors for multifunctional devices

In this study, europium (Eu) and dysprosium-(Dy) doped phosphors, i.e., ZnO:Eu, ZnO:Dy, and Eu:Dy co-doped zinc oxide (ZnO:Eu, Dy) semiconductor phosphors, with optimized concentrations, were synthesized using the conventional solid-state reaction method in an oxygen environment. Detailed structural, morphological, and compositional investigations were conducted using X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray measurements, respectively. Photoluminescence (PL) emission spectra were recorded under different excitation wavelengths to assess the optical characteristics. Electrical analysis was conducted using an inductance (L), capacitance (C), and resistance (R) (LCR) meter and a Keithley source meter, and magnetic analysis was performed using a vibrating sample magnetometer (VSM). XRD confirmed the presence of Eu and Dy ions because the diffraction peaks matched perfectly with those for Eu2O3 and Dy2O3, respectively. The PL results indicated the broad emissions from the samples in the visible region (400–700 nm) under different ultraviolet excitation wavelengths. The VSM results demonstrated that both the Dy-doped and Eu:Dy co-doped ZnO samples exhibited paramagnetic behavior at room temperature, but not the undoped and Eu-doped ZnO samples. The ZnO:Dy sample had superior parameters in terms of the electrical measurements such as the AC/DC conductivity, capacitance, current density, and dielectric constant compared with the ZnO:Eu and ZnO:Eu, Dy samples. These detailed investigations of the optical, magnetic, and electrical properties demonstrate that ZnO:Dy and ZnO:Eu, Dy may be promising materials for use in optical devices.

Effects of post-annealing on photoluminescence of Eu-doped ZnO microsphere for single-component white-light materials

The influence of post-annealing on optical properties of Eu-doped ZnO (ZnO:Eu3+) microsphere synthesized by hydrothermal method has been studied. The intrinsic deep defect emission of ZnO and the transition emission of both 5D0→7F1 and 5D0→7F2 related to Eu3+ have been observed under excitation at 325 nm. The photoluminescence (PL) spectra reveal that ZnO annealed in N2 is more favorable for Eu3+ being doped into ZnO, which is helpful to greatly enhance the red light emission of ZnO:Eu3+. Alkaline conditions are more conducive to the formation of intrinsic deep defects in ZnO lattice, but they are not so conducive to the doping of Eu3+ ions into ZnO lattice. From Gaussian deconvolution analysis of PL spectrum for ZnO:Eu3+, it can be seen that intrinsic defects of ZnO in N2 atmosphere may efficiently assist energy transfer from ZnO host to Eu3+ ions to enhance Eu3+ intrinsic red light emission. There are blue light (446 nm), green light (525 nm) and stronger red light (618 nm) which are simultaneously obtained. This work could pave a way for high performance of white light emission application.

Photoluminescence and Structural Characteristics of Eu-doped ZnO–Li3NbO4

Photoluminescence and Structural Characteristics of Eu-doped ZnO–Li3NbO4

Room-temperature ferromagnetism and hydrogen shallow donors in rare-earth Eu-doped ZnO nanorods

Prominent room-temperature ferromagnetism in rare-earth Eu-doped ZnO nanorods has been investigated by means of magnetization and electron paramagnetic resonance (EPR) measurements. The magnetism of the unhydrogenated Eu-doped ZnO nanorods is explained by a spin-1/2 species, attributed to the Zn vacancies. In the hydrogenated Eu-doped ZnO nanorods, a spin-1 species, attributed to the Zn vacancies coupled to the hydrogen shallow donors, would give rise to a slightly enhanced magnetization. The hydrogen shallow donors, identified by the electron paramagnetic resonance (EPR) measurements, can be taken to be responsible for the changes in the spin states and magnetization of the Eu-doped ZnO nanorods.

Photoluminescence and photocatalytic properties of europium doped ZnO nanoparticles

The present study describes the effect of europium doping in enhancing the optical properties of ZnO nanoparticles. Eu doped ZnO (EZ) nanoparticles were prepared by the soft-solution method with varying the dopant concentration. The structure and phase analysis have been carried out by x-ray diffraction (XRD) measurement which shows the size of the particles lies in 30–60 nm range. Improved crystallinity in ZnO is also observed along with lower angle shift of XRD peaks and increase in lattice parameters like unit cell volume which may be explained as the substitutional effect of Eu at Zn sites. Microstructure of the sample has been studied by transmission electron microscopy (TEM) and the images show the average size of EZ nanoparticles is in the range of 30–50 nm. Both XRD and TEM studies give the evidence of Eu incorporation in ZnO lattice. The EZ nanoparticles exhibit intense red emission upon excitation of 400 nm light which are characteristic 4f-intra-shell (5D0-7F2) transitions of Eu3+ other than the defect emissions of host ZnO. The dopant ions are located in ZnO by substituting Zn2+ at a low-symmetry site (C3v) and also near the surface in some distorted lattice sites. The photocatalytic activity has been evaluated by degradation of hazardous methyl orange (MO) dye with exposure to UV light. Among the tested samples, the sample with optimal doping concentration shows notable photocatalytic activity and achieved 90% degradation of MO dye within 180 min of UV irradiation. This photocatalytic activity of EZ has been found to be better as compared to that of ZnO in identical experimental conditions.

Defects-induced nonlinear saturable absorption mechanism in europium-doped ZnO nanoparticles synthesized by facile hydrothermal method

In the present work, the effect of europium (Eu) dopant on the third-order nonlinear optical (NLO) responses of ZnO nanoparticles (NPs) is investigated. The NPs are synthesized by facile hydrothermal technique using saponin as a capping agent. The samples are characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Energy-dispersive spectroscopy (EDS), UV–visible spectroscopy and Room-temperature photoluminescence spectroscopy (RTPL). The synthesized NPs exhibit wurtzite crystal structure with good crystallinity. Williamson–Hall analysis of XRD pattern revealed the presence of dopant induced compressive strain in the crystal lattice of Eu doped ZnO NPs. The capping of saponin on the surface of NPs is confirmed by FTIR study. A significant decrease in the optical band gap of Eu-doped ZnO NPs attributed to the presence of extended defect states below the conduction band. An enhancement in the blue luminescence is observed for the highest doping level (5 at. wt% of Eu), and this is attributed to defects-related emission in the NPs. The third-order NLO properties of the NPs are explored by a single beam Z-scan technique by employing continuous-wave laser of wavelength 532 nm. The open aperture Z-scan curves showed a remarkable switching from reverse saturable absorption to saturable absorption in 5 at. wt% Eu-doped ZnO NPs is caused due to the ground-state bleaching effect. Further, the closed aperture Z-scan curves exhibit the positive nonlinear refractive index in the 5 at. wt% Eu-doped ZnO NPs attributed to the thermo-optical effects.

Using Different Ions in the Hydrothermal Method to Enhance the Photoluminescence Properties of Synthesized ZnO-Based Nanowires

ZnO films with a thickness of ~200 nm were deposited on SiO2/Si substrates as the seed layer. Then Zn(NO3)2-6H2O and C6H12N4 containing different concentrations of Eu(NO3)2-6H2O or In(NO3)2-6H2O were used as precursors, and a hydrothermal process was used to synthesize pure ZnO as well as Eu-doped and In-doped ZnO nanowires at different synthesis temperatures. X-ray diffraction (XRD) was used to analyze the crystallization properties of the pure ZnO and the Eu-doped and In-doped ZnO nanowires, and field emission scanning electronic microscopy (FESEM) was used to analyze their surface morphologies. The important novelty in our approach is that the ZnO-based nanowires with different concentrations of Eu3+ and In3+ ions could be easily synthesized using a hydrothermal process. In addition, the effect of different concentrations of Eu3+ and In3+ ions on the physical and optical properties of ZnO-based nanowires was well investigated. FESEM observations found that the undoped ZnO nanowires could be grown at 100 °C. The third novelty is that we could synthesize the Eu-doped and In-doped ZnO nanowires at temperatures lower than 100 °C. The temperatures required to grow the Eu-doped and In-doped ZnO nanowires decreased with increasing concentrations of Eu3+ and In3+ ions. XRD patterns showed that with the addition of Eu3+ (In3+), the diffraction intensity of the (002) peak slightly increased with the concentration of Eu3+ (In3+) ions and reached a maximum at 3 (0.4) at%. We show that the concentrations of Eu3+ and In3+ ions have considerable effects on the synthesis temperatures and photoluminescence properties of Eu3+-doped and In3+-doped ZnO nanowires.

The enhancement of humidity sensing performance based on Eu-doped ZnO

In this work, a high performance impedance-type humidity sensor based on Europium-doped ZnO with abundant surface oxygen vacancy defects was synthesized by sol-gel method. Response of the Eu-doped ZnO with different molar ratio were investigated by exposing them to humidity environments in wide range of 11–95% RH at room temperature. The Eu-doped ZnO (2 mol%) exhibits a three orders impedance change, along with short response/recovery time (5 s/19 s), low hysteresis and best linearity. Complex impedance spectra indicates that dopant Eu can enhance humidity sensing performance of ZnO, which is resulted from the introduction of Eu3+ ions into ZnO structure to produce more defects of surface oxygen vacancy and more active sites on the surface of ZnO. The results show that this is a feasible method to achieve high humidity sensing performance by Eu doped ZnO, which make it a promising candidate for humidity sensing materials and broaden the use of ZnO materials.

Correspondence between host crystal conditions and emission spectrum shape of Eu3+ ions doped in ZnO and ZnGa2O4 films

We investigated the photoluminescence of Eu-doped ZnO and ZnGaxO1 + 1.5x (x = 1.8–2.8) films upon bandgap excitation to identify the occupation sites of Eu3+ ions through varying the host crystal material, substrate material, and reactant gas. Eu3+ emissions from ZnGaxO1 + 1.5x:Eu films deposited on Si and SiO2 substrates had similar spectral shapes, but their intensities were different, correlated with different defect emission levels. When depositions were carried out in H2O gas, the Eu3+ emissions from the c-axis oriented ZnO:Eu films consisted of a very sharp main peak (P1) at 612 nm and a sub-peak (P2) at 619 nm for 5D0 → 7F2 transition, whereas those from randomly oriented polycrystalline ZnGaxO1 + 1.5x:Eu films were broader and could be divided into four components (P1, P2, P3, and P4), of which the wavelengths of P1 and P2 matched those of the ZnO:Eu films. The availability of both Ga3+ and Zn2+ sites in the ZnGa2O4 lattice expands the variance of chemically distinct sites that can be occupied with Eu3+ compared with only divalent Zn2+ sites in the ZnO crystal. Films deposited with O2 exhibited lower emission intensities with a narrower spectrum width than those deposited with H2O. This behavior was correlated with the X-ray diffraction peak width of ZnGa2O4(311). High-quality crystal lattices formed with O2 allowed only a few sites to accommodate Eu3+ ions. In contrast, OH− and H+ derived from H2O will modify crystallinity to generate various emission-active sites, resulting in intense and broad emissions. Possible sites for the four components are discussed.

Tuning red emission and photocatalytic properties of highly active ZnO nanosheets by Eu addition

Eu-doped ZnO nanosheets were synthesized via a facile and surfactant-free chemical solution deposition method. The detailed characterizations revealed that the as-synthesized samples were well-crystalline which assembled by nanosheets with 25 nm thickness. The Eu ions were successfully doped into the ZnO nanosheets, and the red emissions of Eu3+ ions attributed to the energy of 5D0→7F0, 5D0→7F1 and 5D0→7F2 4f–4f intrashell transitions were observed in PL spectra. The appropriate Eu doping would increase the separation efficiency of electron-hole pairs and promote the formation of the superoxide ions during the photolysis process, which eventually improved the photodegradation of Rhodamine B under UV light irradiation. And the photocatalytic degradation of RhB was also analyzed by a pseudo first order rate. A possible explanation for the modification of bandgap was given based on the modified effect of Eu3+ ions on the electronic structure of ZnO nanosheets, which mainly due to two mechanisms of Burstein-Moss effect and electron-ion/electron scattering.

Heat-treatment controlled structural and optical properties of sol-gel fabricated Eu:ZnO thin films

We report on the fabrication of sol-gel derived Eu-doped ZnO films and the effects of heat-treatment on their structural and optical properties. X-ray diffraction data analysis confirms the decrease in strain in the thin films with increasing heat-treatment temperatures, and the compressive stress of the samples is found to become tensile for heat-treatments between 650 °C and 800 °C. X-ray-absorption fine-structure spectra performed at the Eu L3-edge confirm the absence of clustering of Eu-ions. The photoluminescence (PL) spectra of undoped and doped films consist of near-band edge emissions along with the defect state emissions of ZnO. Intense red emission (612 nm) is observed under UV excitation attributed to the D05→F27 energy level transition of Eu3+ ions in the doped films. The elemental composition and the existence of Eu2+ as well as Eu3+ has been discussed with the help of X-ray photoelectron spectroscopy. ZnO → Eu3+ energy transfer has also been confirmed with the help of PL excitation spectra in these transparent ZnO thin films for applications in integrated optic devices operating in the UV–visible region.

Investigated electronic structure and magnetic ordering of rare earth impurities (Eu, Gd) in ZnO

First-principles calculations of the electronic structure of substitutional rare earth (RE) impurity (Eu and Gd) in wurtzite ZnO have been performed using density functional theory within a Hubbard potential correction to the RE 4f states. For Eu-doped ZnO, the magnetic coupling between Eu ions in the nearest neighbor sites is ferromagnetic (FM). The room temperature (RT) ferromagnetism (FM) can be enhanced by an appropriate hole doping into the sample. The ZnO:Gd is found to favor the antiferromagnetic (AFM) phase. The FM can be achieved by high electron doping. The native defects effect (V[Formula: see text], V[Formula: see text]) on the FM is also studied. The oxygen vacancies seem to play an important role in the generation of the FM in both ZnO:Eu and ZnO:Gd, which is in good agreement with recent experimental results.

The Effect of Synthesis Pressure on Properties of Eu-Doped ZnO Nanopowders Prepared by Microwave Hydrothermal Method

The Effect of Synthesis Pressure on Properties of Eu-Doped ZnO Nanopowders Prepared by Microwave Hydrothermal Method J. Rosowska, J. Kaszewskia,∗, B. Witkowski, Ł. Wachnicki and M. Godlewski Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL–02668 Warsaw, Poland Dept. of Mathematics and Natural Sciences College of Science, Cardinal S. Wyszynski University, Dewajtis 5, 01-815 Warsaw, Poland

Switching photoluminescence channels between dopant Eu2+ and Eu3+ ions in ZnO thin films by varying the post-annealing conditions

An Eu-doped ZnO (ZnO:Eu) is an extensively investigated optical material. While most studies reported photoluminescence (PL) from Eu3+ ions resulting from the electronic transitions between 4f levels (5D0→7FJ), the evidence of emissions from Eu2+ ions remains limited. Here, we report that a violet emission in the wavelength region between 450 and 500 nm emerged from the sputter-deposited ZnO:Eu films that were post-annealed at 900 °C in an O2 ambient. This emission peak was away from the band edge and the defect green emissions of ZnO, and it was identified as being from Eu2+ ions. Simultaneous emergence of the red emission bands at wavelengths longer than 600 nm indicated that Zn vacancies (VZn) were created during a high-temperature annealing and that producing VZn promoted substitution at Zn2+ sites with Eu2+ ions. In contrast, when annealing was done in a vacuum, the defect emissions were attenuated and the PL spectra showed only band-edge emissions. Here, it can be interpreted that this reduced state, where some oxygen atoms have been removed from the ZnO lattice, has a short lifetime of excitons. In contrast, loading hydrogen atoms into the a ZnO lattice by annealing in an H2 ambient at 350 °C generated a weak Eu3+ emission at 612 nm along with an orange emission band ranging from 550 to 650 nm, which was from the OH termination at the surfaces and interfaces of the ZnO crystals. In this system, a codoping of H+ with Eu3+ ions assists the substitution at Zn2+ sites through a charge compensation. The present results, thus, demonstrate that the emergence of Eu2+ and Eu3+ emissions can be simply controlled by selecting the annealing conditions.

Eu-doped ZnO nanoparticles for dielectric, ferroelectric and piezoelectric applications

Europium doped ZnO nanorods were successfully synthesized using low cost wet-chemical precipitation method. The crystalline phases and structural analysis were investigated by powder X-ray diffraction (XRD) study. XRD analysis confirmed the formation of wurtzite hexagonal crystal system for both pure and Europium doped ZnO. The crystallite size, lattice strain, stress and energy density were evaluated by line broadening analysis methods such as Scherrer and Williamson-Hall (W-H) methods. Transmission electron microscopy (TEM) confirmed that pure and Europium doped ZnO nanoparticles were grown in the shape of rods. The average diameter of pure ZnO nanorods was found to be ∼85.79 nm. The average diameter (∼78.92 nm) of the Eu-doped ZnO nanorods measured using TEM analysis was found to be in co-relation with W-H methods. Detailed analyses of frequency and temperature dependence of dielectric constant, dielectric loss and AC conductivity of pure and Europium doped ZnO were performed. Improved ferroelectric to paraelectric phase transition temperature at Tc = 230 °C was observed for Eu-ZnO sample. In ferroelectric measurement of Europium doped ZnO, remnant polarization and coercive field were found to be 0.11 μC/cm2 and 5.81 kV/cm, respectively. In addition, butterfly loop and effective piezoelectric coefficient (d33) versus applied voltage curve were traced for Europium doped ZnO nanorods. The mean value of d33 was evaluated to be 43.38 pm/V. I-V studies were carried out to measure the effect of Europium doping on DC conductivity of ZnO.