Photoluminescence characteristics of ZnO doped with Eu powders

In this work, we have investigated the photoluminescence spectra of europium-doped zinc oxide crystallites prepared by a vibrating milled solid-state reaction method. X-ray diffraction, scanning electron microscopy, luminescence spectra and time-resolved spectra analysis were used to characterize the synthetic ZnO:Eu³⁺ powders. XRD results of the powders showed a typical wurtzite hexagonal crystal structure. A second phase occurred at 5 mol% Eu₂O₃-doped ZnO. The ⁵D₀-⁷F₁ (590 nm) and ⁵D₀-⁷F₂ (609 nm) emission characteristics of Eu³⁺ appeared after quenching with more than 1.5 mol% Eu₂O₃ doping. The Commission Internationale d’Eclairage (CIE) chromaticity coordinates of a ZnO:Eu³⁺ host excited at λ_ex=467 nm revealed a red-shift phenomenon with increase in Eu³⁺ ion doping. The lifetime of the Eu³⁺ ion decreased as the doping concentration was increased from 1.5 to 10 mol%, and the time-resolved ⁵D₀→⁷F₂ transition presents a single-exponential decay behavior.     

Micro-structural investigations and paramagnetic susceptibilities of zinc oxide, europium oxide and their nanocomposite

Nanoparticles of zinc oxide (ZnO), europium oxide (Eu₂O₃) and their nanocomposite system {(ZnO)_0.55(Eu₂O₃)_0.45} have been prepared by pyrophoric reaction and chemical co-precipitation methods. The precursor materials used for the synthesis were Zn(NO₃)₂·6H₂O and bulk Eu₂O₃. For nanocrystallization, the as-prepared samples were annealed at 500 and 600 °C for 6 h. The X-ray diffractograms (XRD) confirmed the formation of desired phases of the nanoparticles of ZnO, Eu₂O₃ and nanocomposite of {(ZnO)_0.55 (Eu₂O₃)_0.45}. Particle sizes of all the samples have been estimated from the width of the XRD peaks using the Debye-Scherrer equation. Particle sizes, crystallographic phases, etc. extracted from the high resolution transmission electron microscopy of a few selected samples are in agreement with those obtained from the XRD. Field emission scanning electron microscopy showed that ZnO nanoparticles are more-or-less spherical in shape. Average magnetic susceptibilities of all the annealed samples measured in the temperature range of 300-14 K indicate that all the samples including the zinc oxide, which is normally diamagnetic in the bulk state, are paramagnetic and the data are tried to analyze by the Curie-Weiss law. Photo-luminescence data recorded at room temperature of all the samples indicate that the optical property of the ZnO nanoparticles are not affected by Eu₂O₃ nanoparticles in the nanocomposite system though its bulk magnetization is substantially enhanced by incorporating the Eu₂O₃ nanoparticles.     

Probing of surface Eu ions present in ZnO:Eu nanoparticles by covering ZnO:Eu core with Y2O3 shell: Luminescence study

Eu³⁺ doped ZnO nanoparticles are known to have significance extent of surface Eu³⁺ ions due to a large difference in ionic radii. Effect of such Eu³⁺ ions on the luminescence properties of ZnO:Eu nanoparticles has been understood from the luminescence studies of ZnO:Eu nanoparticles covered with Y₂O₃ shell. Based on the asymmetric ratio of luminescence and extent of energy transfer, it is established that when ZnO:Eu nanoparticles are covered with Y₂O₃ shell, a part of Eu³⁺ ions present with ZnO:Eu core migrate to Y₂O₃ shell and occupy Y³⁺ lattice positions.     

Red luminescence of Eu3+ doped ZnO nanoparticles fabricated by laser ablation in aqueous solution

Fabrication of Eu³⁺-doped ZnO nanoparticles by laser ablation in liquid medium is reported. Sintered disks made of mixed powders of ZnO and Eu₂O₃ are used for targets, and surfactant of sodium dodecyl sulfate or LiOH is included in solution. Round-shaped nanoparticles with the diameter of 5??30?nm are synthesized. When the ZnO host is photoexcited, broad green photoluminescence (PL) of oxygen vacancies in the ZnO host as well as red PL of Eu³⁺ is observed at room temperature. The red PL peak of Eu³⁺ included in the ZnO host lattice is different from that of the source material of Eu₂O₃. Energy transfer from the ZnO host to Eu³⁺ is demonstrated in site-selectively excited PL spectra and its excitation spectra. This result shows that the liquid-phase laser ablation is useful for doping active centers into nanoparticles.     

Microstructural and optical properties of europium-doped zinc oxide nanowires

Single-crystal Eu³⁺-doped wurtzite ZnO micro- and nanowires were synthesized by chemical vapor deposition. The nanostructures grew via a self-catalytic mechanism on the walls of an alumina boat. The structure and properties of the doped ZnO were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, scanning and transmission electron microscopy, and photoluminescence (PL) methods. A 10-min synthesis yielded vertically grown nanowires of 50–400 nm in diameter and several micrometers long. The nanowires grew along the ±[0001] direction. The Eu³⁺ concentration in the nanowires was 0.8 at.%. The crystal structure and microstructure of were compared for Eu³⁺-doped and undoped ZnO. PL spectra showed a red shift in emission for Eu³⁺-doped (2.02 eV) compared to undoped ZnO nanowires (2.37 eV) due to Eu³⁺ intraionic transitions. Diffuse reflectance spectra revealed widening of the optical bandgap by 0.12 eV for Eu³⁺-doped compared to undoped ZnO to yield a value of 3.31 eV. Fourier-transform infrared spectra confirmed the presence of europium in the ZnO nanowires.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

PL and EL characterizations of ZnO:Eu, Li films derived by sol-gel process

ZnO:Eu³⁺, Li⁺ films prepared by the dip-coating method were characterized by photoluminescence (PL) and electroluminescence (EL). When the ZnO:Eu³⁺, Li⁺ films were excited using UV light with energy corresponding to the band-to-band excitation of the host matrix, the PL spectra showed emissions from both ZnO and Eu³⁺ ions, while their EL spectra showed emissions only from Eu³⁺ ions, and no emission from ZnO could be detected. It is found that the EL emission intensity B is dependent on the applied voltage, B=B_o exp(−bV^−1/2). With increasing frequency, the EL intensity dramatically increases at lower frequencies (<1000 Hz), and then increases gradually at higher frequencies (>1000 Hz).     

Ultra-small sized Y2O3:Eu3+ nanocrystals: One-step polyoxometalate-assisted synthesis and their photoluminescence properties

A novel hydrothermal approach for the preparation of europium(III)-doped yttrium oxide (Y₂O₃:Eu³⁺) nanocrystals was reported. The as-synthesized Y₂O₃:Eu³⁺ nanocrystals with diameter of about 5 nm are highly uniform and dispersed in water. The Y₂O₃:Eu³⁺ nanocrystals were characterized by high-resolution transmission electron microscopy and fluorescence spectroscopy. Due to their well dispersity in water, low toxicity, and good photoluminescence, the Y₂O₃:Eu³⁺ nanocrystals can potentially be used in high-definition displays and fluorescence probe in bioimaging.     

Photoluminescence and phosphorescence properties of MAl2O4:Eu, Dy (M=Ca, Ba, Sr) phosphors prepared at an initiating combustion temperature of 500 °C

Eu²⁺ and Dy³⁺ co-doped calcium aluminate, barium aluminate and strontium aluminate phosphors were synthesized at an initiating combustion temperature of 500 °C using urea as an organic fuel. The crystallinity of the phosphors was investigated by using X-ray diffraction (XRD) and the morphology was determined by a scanning electron microscope (SEM). The low temperature monoclinic structure for both CaAl₂O₄ and SrAl₂O₄ and the hexagonal structure of BaAl₂O₄ were observed. The effect of the host materials on the photoluminescence (PL) and phosphorescence properties were investigated by using a He-Cd Laser and a Cary Eclipse fluorescence spectrophotometer, respectively. The broad band emission spectra observed at 449 nm for CaAl₂O₄:Eu²⁺, Dy³⁺, 450 nm (with a shoulder-peak at 500 nm) for BaAl₂O₄:Eu²⁺, Dy³⁺ and 528 nm for SrAl₂O₄:Eu²⁺, Dy³⁺ are attributed to the 4f⁶5d¹ to 4f⁷ transition in the Eu²⁺ ion in the different hosts.     

Rapid synthesis and luminescence of the Eu, Er codoped ZnO quantum-dot chain via chemical precipitation method

ZnO quantum-dot chains codoped with Eu³⁺ and Er³⁺ were synthesized by the chemical precipitation method and the codoping effects on the structures, morphologies and optical properties of the powders were briefly investigated. The X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) results indicated the Eu³⁺ and Er³⁺ were incorporated into the crystal lattice of ZnO host. Transmission electron microscope (TEM) measurements showed the sizes of the ZnO quantum dots decreased with the increase of Eu³⁺ and Er³⁺ doping concentration, and the quantum-dot chains were formed by codoping with Eu³⁺ and Er³⁺. The green emissions in the photoluminescence spectra were attributed to 4f-4f of Er³⁺ inner shell ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, and the characteristic red emissions of Eu³⁺ ions were attributed to the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions, respectively. Moreover, the red emission of the Eu³⁺ ions gradually decreased with the Er³⁺ ions doping concentration increased, which may be due to the different energy storage centers in the powders.     

Optical and structural properties of Eu-diffused and doped ZnO nanowires

Single crystal ZnO nanowires diffused with europium (Eu) from a solid source at 900 °C for 1 h or doped with Eu during growth have been characterized. The ZnO nanowires were grown by chemical vapor deposition on Si substrates employing Au as a catalyst. The diameter of the resulting nanowires was 200 nm with a length of 1 μm. Photoluminescence spectra excited by a He–Cd laser at room temperature showed the green luminescence at 515 nm in Eu-diffused nanowires. A small red shift of near-band-edge emission of ZnO nanowires was observed in the diffused wires, but sharp emission from Eu³ ions was not present. Transmission electron microscopy shows crystalline Eu₂O₃ formation on the diffused nanowire surface, which forms a coaxial heterostructure system. When Eu was incorporated during the nanowire growth, the sharp ⁵D_O–⁷F₂ transition of the Eu³⁺ ion at around 615 nm was observed.     

Effect of Eu contents on the structure and properties of BaLa2ZnO5:Eu phosphors

A series of Eu³⁺ doped BaLa₂ZnO₅ was prepared by a Pechini-type sol-gel method. The effect of Eu³⁺ concentration on the structure and the luminescent properties of the samples BaLa_2−xEu_xZnO₅ were investigated by powder X-ray diffraction (XRD) and photoluminescence (PL) spectra. The sample adopts the tetragonal or orthorhombic structure as the Eu³⁺ concentration reaches below 1.2 or over 1.6, respectively. The sample is a mixture of tetragonal and orthorhombic phases as the contents of dopants lie between 1.2 and 1.6. The phosphor offers the strongest red light as content of Eu³⁺ is 0.4. In addition, the crystallization processes of the complex precursors were examined by simultaneous thermogravimetry and differential thermal analysis (TG-DTA), and also the optimal heating temperature was investigated.     

Preparation and characterization of nanosized (Y,Bi)VO4:Eu and Y(V,P)O4:Eu red phosphors

Nanosized luminescent (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ were synthesized at low temperatures either by a coprecipitation method or by a hydrothermal method from aqueous solutions. The effect of Bi³⁺ ion or P⁵⁺ ion content in the lattice, annealing temperature effects on the crystal structure and the particle size, and the luminescence property of (Y,Bi)VO₄:Eu³⁺ and Y(V,P)O₄:Eu³⁺ nanoparticles were examined with a field-enhanced scanning electron microscopy, XRD, and a spectrofluorometer. The pristine YVO₄:Eu³⁺, (Y,Bi)VO₄:Eu³⁺, or Y(V,P)O₄:Eu³⁺ nanoparticles are 35-50 nm in size. The luminescence spectrum of the Eu³⁺ ion was used to probe its position in the crystal lattice. The dopant ions enter the same lattice sites in the nanocrystalline as in the corresponding bulk material, resulting similar spectral features between them. Photoluminescence intensity is weak for the pristine nanoparticles. Annealing the nanoparticles at temperatures up to 1000 °C results in the increased luminescence intensity (>80% of micrometer-sized phosphors) with the minimal particle growth and the improved particle crystallinity.     

Electrical and optical spectroscopy on ZnO:Co thin films

Magnetic oxide semiconductors, for example the highly transparent and intrinsically n-type conducting zinc oxide doped with the 3d transition metal Co (ZnO:Co), are promising for the emerging field of spintronics [1]. We investigated n-conducting ZnO:Co thin films with a Co content of nominal 0.02, 0.20, or 2.00 at. %. The substitution of Co cations in the tetrahedral sites of wurtzite ZnO with Zn was confirmed at low temperature by the 1.877 eV photoluminescence between crystal field split d-levels of Co²⁺ (d⁷) ions. Based on theoretical studies, it is predicted that the formation of electron levels with zinc interstitials (I_Zn) or hole levels with zinc vacancies (V_Zn) is necessary to induce ferromagnetism, whereas the formation of electron levels with oxygen vacancies (V_O) is detrimental for ferromagnetism in ZnO:Co [2]. Cobalt generates a hole level in ZnO [3]. We investigated the generation of electron levels in n-conducting ZnO:Co in dependence on the Co content by means of deep level transient spectroscopy (DLTS). However, because of the ambiguous categorization of deep defects in n-conducting ZnO (V_O, I_Zn), an optimization of defect-related ferromagnetism in ZnO:Co is not possible at the moment. PACS 78.30.Fs; 91.60.Ed; 91.60.Mk     

Synthesis, characterization, and luminescent properties of Lu2O3:Eu phosphors

Eu-doped lutetia (Lu₂O₃:Eu) nano-phosphors were synthesized by the sol-gel combustion process from a mixed aqueous solution of europium and lutetium nitrates, using organic glycine as the fuel. Powder X-ray diffraction shows that cubic Lu₂O₃:Eu crystallites are directly obtained by the sol-gel combustion process without further calcination. Electron microscopy reveals that the as-prepared phosphors are agglomerated and have a fluffy, fine, and porous morphology, consisting of primary particle size of 8-10 nm. The excitation spectrum is characterized by three dominant bands centered at 395, 466, and 534 nm, respectively. Both the photoluminescent and radioluminescent spectra are very similar and exhibit intense emission peaks centered at 612 nm due to ⁵D₀→⁷F₂ transition of Eu³⁺ ions. The energy transfer from Lu₂O₃ host to Eu³⁺ activator is more efficient in the case of calcined phosphors than for the as-prepared phosphors due to their improved lattice perfection.     

Effects of x and R3+ on the luminescent properties of Eu3+ in nanocrystalline YVxP1-xO4:Eu3+ and RVO4:Eu3+ thin-film phosphors

Using inorganic oxides and salts instead of alkoxides as the main starting materials, we prepared nanocrystalline YV_xP_1-xO₄:Eu³⁺ and RVO₄:Eu³⁺ (0x1; R=Y,La,Gd) thin-film phosphors by the Pechini sol–gel dip-coating process. The resulting films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and photoluminescence excitation and emission spectra as well as luminescence decay. The results of XRD showed that a solid solution formed in the YV_xP_1-xO₄:Eu³⁺ film series from x=0 to x=1 with zircon structure. The same structure also held for the GdVO₄:Eu³⁺ film, but the LaVO₄:Eu³⁺ film crystallized with a different structure, monazite. AFM and SEM studies revealed that the phosphor films consisted of spherical particles ranging from 90 to 400 nm depending on the film compositions. With the increase of x values in YV_xP_1-xO₄:Eu³⁺ films, the integrated emission intensity and the red (⁵D₀–⁷F₂)-to-orange (⁵D₀–⁷F₁) intensity ratio of Eu³⁺ increase due to the increased energy-transfer probability from VO₄^3- to Eu³⁺ and the increased polarizability of the surrounding oxygen ions, respectively. The x values also have an influence on the decay behavior of Eu³⁺. The YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films showed very similar luminescence properties due to their same crystal structures. However, the LaVO₄:Eu³⁺ film exhibited a much different emission property from those of the YVO₄:Eu³⁺ and GdVO₄:Eu³⁺ films due to the structural effects. PACS 73.63.Bd; 78.55.Hx; 78.66.Nk; 81.15.Lm; 81.20.Fw     

Preparation, characterization and luminescence properties of porous Si3N4 ceramics with Eu2O3 as sintering additive

Porous Si₃N₄ ceramics with photoluminescence properties were prepared by pressureless sintering using α-Si₃N₄ powder as raw material and Eu₂O₃ as sintering additive. Chemical composition, phase formation, microstructure and photoluminescence properties of porous Si₃N₄ ceramics were studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence measurements (PL/PLE). The results show that single Eu₂O₃ additive promotes α→β transformation but not significant densification. A broad band emission center at 570 nm assigned to Eu²⁺ is observed, Eu³⁺ in Eu₂O₃ is (partially) converted to Eu²⁺ by reaction with Si₃N₄, which results in a lower β aspect ratio and β-content compared to the other Ln (Ln=lanthanide) oxide additives.     

Spectral and surface investigations of Ca2V2O7:Eu3+ nanophosphors prepared by citrate-gel combustion method: a potential red-emitting phosphor for near-UV light-emitting diodes

In the present work, red-emitting Ca₂V₂O₇:xEu³⁺ (x = 0.5–6.0 mol%) nanophosphors, in the form of powders, were synthesized by the citrate-gel combustion method using metal nitrates as precursors and citric acid as fuel. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy, photoluminescence (PL) and cathodoluminescence (CL) spectroscopy were used to study the structure, morphology and spectral properties of the samples. The chemical compositions and electronic states of the powders were analyzed with X-ray photoelectron spectroscopy. The average crystallite sizes estimated using the XRD data were found to be in the range of 30–45 nm, and were cross verified by TEM. The lattice parameters determined by the POWD program were approximated as a = 7.242 Å, b = 6.674 Å, c = 6.932 Å and V = 291.24 Å³, respectively. Under UV (395 nm) (PL) and electron (CL) excitation, the nanophosphors show characteristic emission from the Eu³⁺ ion (⁵D₀ → ⁷F_j, j = 1–5) with the main peaks at 612 and 616 nm. The maximum emission intensity was recorded from the sample with an Eu³⁺ concentration of 4 mol% and a critical energy distance of 19.084 Å between the donor and the acceptor. Above this concentration, there was a reduction in the intensity due to dipole–dipole induced concentration quenching effects. The potential applications of this phosphor as a high color-purity phosphor in light-emitting diodes are evaluated.     

Preparation, characterization and room temperature photoluminescence properties of a zinc oxide nanosheet network grown on glass substrate by a facile chemical route

An ultralong and ultrathin zinc oxide nanosheet network grown on glass substrate is prepared using an organic CTAB (cetyltrimethylammonium bromide, CH₃(CH₂)₁₅N⁺(CH₃)₃Br⁻) and the simple chemical materials (Zn(AC)₂·2H₂O and NaOH) by hydrothermal method. The morphology and microstructure of ZnO nanosheet network have been characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–vis spectroscopy. The results revealed that the product grown on glass substrate was the ultralong and ultrathin zinc oxide nanosheet network and the crystalline hexagonal wurtzite ZnO crystal structure. The zinc oxide nanosheet network exhibits room temperature photoluminescence (RTPL) characteristics and three blue emissions located at 452, 459, and 469 nm, and a green emission located at about 494 nm were observed.