Visible cathodoluminescence of Er ions in β-Ga(2)O(3) nanowires and microwires

Erbium doped β-Ga(2)O(3) nanowires and microwires have been obtained by a vapour-solid process from an initial mixture of Ga(2)O(3) and Er(2)O(3) powders. X-ray diffraction (XRD) analysis reveals the presence of erbium gallium garnet as well as β-Ga(2)O(3) phases in the microwires. Scanning electron microscopy (SEM) images show that the larger microwires have a nearly rectangular cross-section. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analysis show good crystal quality of the β-Ga(2)O(3) nanowires. The nanostructures have been studied by means of the cathodoluminescence technique in the scanning electron microscope. Er intraionic blue, green and red emission lines are observed in luminescence spectra even at room temperature, which confirms the optical activity of the rare earth ions in?the grown structures. Mapping of the main 555?nm emission intensity shows a non-homogeneous distribution of Er ions in the microstructures.     

Study of the relationship between crystal structure and luminescence in rare-earth-implanted Ga2O3 nanowires during annealing treatments

A systematical analysis of the correlation between the crystalline quality and the luminescence of rare-earth-implanted β-Ga₂O₃ nanostructures with potential applications in visible and ultraviolet photonics is presented. Europium ions led to red emission while gadolinium ions are efficient ultraviolet emitters. Different degrees of lattice recoveries of the nanostructures have been achieved after implantation by rapid thermal annealing treatments carried out at different temperatures. The recovery process has been analyzed by transmission electron microscopy (TEM), high-resolution TEM, and Raman techniques. High-fluence implantation with either of the two rare earth ions induces partial amorphization of the structures. Partial recrystallization of the nanostructures above 500 °C is revealed by Raman analysis. Nearly complete recovery of the crystal structure is obtained in the annealing temperature range 900–1100 °C, coincident with the expected value for bulk Ga₂O₃. Cathodoluminescence and photoluminescence allowed comparison of the Eu³⁺ and Gd³⁺ intraionic luminescence lines after annealing at different temperatures and their correlation with the crystallinity. It has been found that the width of the Eu³⁺ luminescence lines clearly correlates with the width of the Raman peaks, both decreasing with annealing temperature, which shows the possibility of using the luminescence of this rare earth as a probe for lattice disorder. On the other hand, our results suggest that Gd³⁺ lines are much less sensitive to disorder.     

Rare earth doped zinc oxide nanowires

Zinc oxide (ZnO) nanowires were grown via thermal transport and subsequently doped with different concentrations of Tm, Yb, and Eu using ion implantation and post annealing. High ion fluences lead to morphology changes due to sputtering; however, freestanding nanowires become less damaged compared to those attached to substrates. No other phases like rare earth (RE) oxides were detected, no amorphization occurs in any sample, and homogeneous doping with the desired concentrations was achieved. Photoluminescence measurements demonstrate the optical activation of trivalent RE-elements and the emission of the characteristic intra-4f-luminescence of the respective RE atoms, which could be assigned according to the Dieke-diagram. An increasing RE concentration results into decreasing luminescence intensity caused by energy transfer mechanisms to non-radiative remaining implantation defect sites. Furthermore, low thermal quenching was observed due to the considerable wide band gap of ZnO.     

Preparation and luminescence of nano-sized In(2)O(3) and rare-earth co-doped SiO(2) thin films

The sol-gel method was used to prepare SiO(2) thin films co-doped with In(2)O(3) nano-particles and Eu(3+). The formation of nano-sized In(2)O(3) particles after annealing at 900?°C was confirmed by the x-ray diffraction technique. A novel phase transition from a hexagonal rhombic centered to a body centered cubic structure of In(2)O(3) nano-particles was observed at around 1100?°C. It is found that the particle size and the particle density of In(2)O(3) can be tuned by changing the annealing temperature and the indium doping concentration, respectively. The characteristic emission bands from Eu(3+) ions can be observed at room temperature and the luminescence intensity is increased 20 times by introducing In(2)O(3) nano-particles into Eu(3+)-doped silica films. The integrated luminescence intensity was gradually enhanced by increasing the In(3+) concentration, suggesting effective energy transfer from nano-sized In(2)O(3) to Eu(3+) ions.     

Eu(3+) doped gadolinium oxysulfide (Gd(2)O(2)S) nanostructures-synthesis and optical and electronic properties

One-dimensional Eu(3+) doped gadolinium oxysulfide (Gd(2)O(2)S:Eu(3+)) nanotubes/nanorods have been synthesized via precursors of Gd(OH)(3) nanostructures using a hydrothermal technique. The blue-shifts in the optical spectra for the Gd(2)O(2)S:Eu(3+) system corresponding to the fundamental absorption and Eu(3+)-X(2-) ligand (X = O/S) charge transfer bands (CTBs) are significant (～0.22-0.36?eV) with respect to the bulk counterpart. The nanotubes are good candidates for investigating the size-induced electrical and optical properties of functional oxysulfides. In order to identify the origin and nature of the electronic transitions observed in the visible region, optical and photo-induced impedance measurements have been extended to the nanotubes in this report.     

Study on luminescent properties of Eu3+ doped Gd2WO6, Gd2W2O9 and Gd2(WO4)3 nanophosphors prepared by co-precipitation

Eu3+ doped Gd2WO6, Gd2W2O9 and Gd2(WO4)3 nanophosphors with different concentrations have been prepared by co-precipitation. XRD (X-ray diffraction) and SEM (scanning electron microscopy) were used to investigate the structure and morphology. The emission spectra, excitation spectra and fluorescence decay curves were measured, and partial J-O parameters and quantum efficiencies of Eu3+ 5D0 energy level were calculated. Furthermore, concentration quenching curves of Eu3+ in different hosts were drawn. The photoluminescent properties of Eu3+ doped Gd2WO6, Gd2W2O9 and Gd2(WO4)3 nanophosphors have been studied. The results indicate that Eu3+ 5D0-7F2 red luminescence can be effectively excited by 395 nm and 465 nm in Gd2WO6 and Gd2W2O9 hosts, similar to the familiar Gd2(WO4)3:Eu. Especially Gd2W2O9:Eu has strong red emission and high quenching concentration, so it has potential applications for trichromatic white LED as red fluorescent materials.     

Study on energy transfer and energy migration of Ca2Gd8(SiO4)6O2:Dy3+ phosphor films

Being a kind of rare-earth-metal silicate with oxidapatite structure, Ca2R8(SiO4)6O2 (R = Y, Gd, La) is a promising material doped with rare earth, and widely used as phosphors. In this thesis, Ca2Gd8(SiO4)6O2:Dy3+ films were prepared by the sol-gel method. X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectra, and lifetimes were used to characterize the resulting films. AFM study indicated that the phosphor films consisted of 120 nm homogeneous particles. By combining the model of Burshtein for donor-donor migration and the V-F-B model for donor-acceptor energy transfer, the experimental luminescence decay curve of 6P(J) state of Gd3+ was re-simulated. It is found that concentration quenching of Gd3+ can be due to the result of the joint action of donor-donor (Gd3+-Gd3+) energy migration and donor-acceptor (Gd3+-Dy3+) energy transfer.     

Luminescence study on Eu(3+) doped Y(2)O(3) nanoparticles: particle size, concentration and core-shell formation effects

Nanoparticles of Eu(3+) doped Y(2)O(3) (core) and Eu(3+) doped Y(2)O(3) covered with Y(2)O(3) shell (core-shell) are prepared by urea hydrolysis for 3?h in ethylene glycol medium at a relatively low temperature of 140?°C, followed by heating at 500 and 900?°C. Particle sizes determined from x-ray diffraction and transmission electron microscopic studies are 11 and 18?nm for 500 and 900?°C heated samples respectively. Based on the luminescence studies of 500 and 900?°C heated samples, it is confirmed that there is no particle size effect on the peak positions of Eu(3+) emission, and optimum luminescence intensity is observed from the nanoparticles with a Eu(3+) concentration of 4-5?at.%. A luminescence study establishes that the Eu(3+) environment in amorphous Y (OH)(3) is different from that in crystalline Y(2)O(3). For a fixed concentration of Eu(3+) doping, there is a reduction in Eu(3+) emission intensity for core-shell nanoparticles compared to that of core nanoparticles, and this has been attributed to the concentration dilution effect. Energy transfer from the host to Eu(3+) increases with increase of crystallinity.     

Enhanced luminescence from encapsulated silicon nanocrystals in SiO2 with rapid thermal anneal

It is well known that Si ion implantation into SiO₂ and subsequent high-temperature anneals induce the formation of embedded luminescent Si nanocrystals. In this work, the potentialities of rapid thermal annealing to enhance the photoluminescence intensity have been investigated. Ion implantation was used to synthesize specimens of SiO₂ containing excess Si with different concentrations. Si precipitation to form nanocrystals in implanted samples takes place with a conventional furnace anneal. The photoluminescence intensity and the peak energy of emission from Si nanocrystals depend on implanted ion dose. Moreover, the luminescence intensity is strongly enhanced with a rapid thermal annealing prior to a conventional furnace anneal. The luminescence intensity, however, decreases when rapid thermal annealing follows conventional furnace annealing. It is found that the order of heat treatment is an important factor in intensities of the luminescence and that the luminescence peak energy is found to be dependent, but only by a small factor, on the thermal history of specimens. Enhancement is found to be typical for low dose samples. Furthermore, the observation that the prolonged anneal induces saturation and a blue shift of the luminescence strongly indicates the emission is not simply due to electron-hole recombination inside the Si nanocrystals. Based on our experimental results, we discuss the mechanism for the enhancement of the photoluminescence, together with the mechanism of photoemission.     

Enhanced emissions of Eu(3+) by energy transfer from ZnO quantum dots embedded in SiO(2) glass

SiO(2):Eu(3+) based bulk composites containing ZnO quantum dots were synthesized by an in situ sol-gel process. The quantum dots homogeneously distributed among the SiO(2) glass matrix exhibited a broad ultraviolet emission band centered at 385?nm. The ZnO ultraviolet luminescence intensity decreased monotonically with increasing Eu(3+) doping concentration, while the Eu(3+) visible emission was intensified significantly by the precipitation of ZnO quantum dots, ascribed to the energy transfer from ZnO to Eu(3+). The Eu(3+) luminescence at 612?nm for the sample with 20?mol% ZnO was about ten times stronger than that for the sample without ZnO. The influence of ZnO or Eu(3+) concentration on the energy transfer process is discussed.     

Red light from ZrO2:Eu nanostructures

Zirconia nanocrystals doped with europium ions were developed envisaging optical applications. The nanostructures were produced using zirconyl nitrate (ZrO(NO₃)₂·H₂O) and europium nitrate (Eu(NO₃)₃·5H₂O) as cation precursors, and urea (C₂H₅NO₂) as the fuel, by the combustion synthesis process. The lanthanide-doped nanostructures were characterized by X-ray diffraction, transmission electron microscopy, Raman spectroscopy and photoluminescence. X-ray diffraction revealed the presence of tetragonal and monoclinic crystalline ZrO₂ phases. The latter was found to be a minority phase as identified by Raman and corroborated by the observed europium luminescence when compared to the intraionic emission in crystalline tetragonal fibres grown by the laser floating zone technique. Bright red europium luminescence is observed at room temperature when the combustion synthesized zirconia powders are excited with ultraviolet radiation. The spectroscopic properties of the europium ions in the powders are ascertained by comparing combined excitation–emission measurements with those from crystalline fibres.     

Electronegativity and point defect formation in the ion implanted SiO2 layers

The metal-oxide-silicon (MOS) diode structure containing ion implanted electropositive (M⁺) and electronegative (M⁻) ions is one of the most promising candidates for a new type of high-efficiency electroluminescence (EL) devices which can be integrated with standard silicon CMOS technology. The implantation process creates defects in the SiO₂ layer. After implantation, an annealing process leads to the diffusion of implanted elements and broadening of the SiO₂/Si interface. The influence of different implanted ions (Gd, F, K) was investigated by EL measurements and correlated to different defects in the oxide layer. Implanted electronegative ions (such as F) lead to defects comprising O₂ molecules and peroxy radicals (POR). On the other hand, electropositive ions (Gd and K) increase the number of oxygen vacancy defects.     

Effect of Oxide Assisted Metal Nanoparticles on Microstructure and Morphology of Gallium oxide Nanowires

Several researches have been reported about the characteristic of β-Ga2O3 nanowires which was synthesized on nickel oxide particle. But indeed, recent researches about synthesis of β-Ga2O3 nanowires on oxide-assisted transition metal are limited to nickel or cobalt oxide catalyst. In this work, Gallium oxide (β-Ga2O3 ) nanowires were synthesized by a simple thermal evaporation method from gallium powder in the range of 700 - 1000℃ using the iron, nickel, copper, cobalt and zinc oxide as a catalyst, respectively. The β-Ga2O3 nanowires with single crystalline without defects were successfully synthesized at the reaction temperature of 850, 900 and 950℃ in all the catalysts. But optimum experimental condition in synthesis of nanowires varied with the kind of catalyst. As increasing synthesis temperature,the morphology of gallium oxide nanowires changed from nanowires to nanorods, and its diameter increased. From these results, we could be proposed that the growth mechanism of β-Ga2O3 nanowires was changed with synthesis temperature of nanowires. Microstructure and morphology of Synthesized nanowire was characterized by HR-TEM, FE-SEM, EDX and XRD.     

掺Eu的γ-La2S3的低温合成机理及其发光特性研究

采用CS2硫化法制备γ-La2S3时,通过掺入Eu离子,使γ-La2S3的生成温度降低500℃以上,在700℃就能得到稳定的γ-La2S3。掺入的Eu离子,充当γ-La2S3成核的晶种,从而降低γ-La2S3的生成温度。一部分Eu2+通过不等价取代La3+,同时在晶格中形成S空位以保持电中性;另一部分Eu2+则填入γ-La2S3的S4四面体空洞,从而稳定了γ-La2S3的晶体结构,使其能够在1100℃下仍能保持稳定。在硫化反应过程中,Eu的掺杂使相变过程发生变化,La2S3不再经过β→γ相变,而直接由介稳相LaS2转变为γ-La2S3。光谱分析表明,在244nm紫外光激发下,掺Eu的γ-La2S3在280、390以及570nm处有3个发射带,且390nm发射峰非常强,可能源于缺陷发光,而Eu2+的发光很弱或不发光。     

Investigation of the emission mechanism in milled SrAl₂O₄:Eu, Dy using optical and synchrotron X-ray spectroscopy

There currently exists much debate as to the active state related to the "long afterglow" effect in europium doped oxide materials. Redox couples that consist of Eu(+/2+) and Eu(2+/3+) are discussed, but no common answer is currently accepted. Here, we present a comparison of the optical properties of a commercially available SrAl(2)O(4):Eu, Dy phosphor, as a function of nanoparticle size reduction via dry mechanical milling. X-ray and optical spectroscopic data indicate a significant decrease in phosphorescence efficiency and an increase in laser stimulated emission efficiency as near surface Eu(2+) ions are oxidized to Eu(3+) as a consequence of increased exposure during the milling process. These results show evidence only for Eu(2+/3+) oxidation states, suggesting the mechanism related to long afterglow effect does not arise from Eu(+) species. We also suggest that size reduction, as a rule, cannot be universally applied to improve optical properties of nanostructures.     

Structural properties of fluorozirconate-based glass ceramics doped with multivalent europium

The structure/property relationships of fluorochlorozirconate glass ceramics as a function of divalent and trivalent europium (Eu) co-doping and thermal processing have been investigated; the influence of doping ratio on the formation of barium chloride (BaCl(2)) nanocrystals therein was elucidated. X-ray absorption near-edge structure spectroscopy shows that the post-thermal annealing changes the Eu valence of the as-poured glass slightly, but during the melting process Eu(3+) is more strongly reduced to Eu(2+), in particular, when doped as a chloride instead of fluoride compound. The Eu(2+)-to-Eu(3+) doping ratio also plays a significant role in chemical equilibrium in the melt. X-ray diffraction measurements indicate that a higher Eu(2+) fraction leads to a BaCl(2) phase transition from hexagonal to orthorhombic structure at a lower temperature.     

Characterization of implantation induced defects in Si-implanted SiO2 film

Si-implanted thermal SiO2 layers and their annealing behaviour were investigated. In the results of variable-energy positron annihilation spectroscopy, the defects caused by ion implantation are manifested as a particularly low S parameter in the Si ion implantation region of the SiO2 layer. Compared with Fourier transform infrared measurements, it suggests that the decrease of the line shape S parameter after implantation is related to the compaction of implanted layers induced by the breaking of the SiO2 network structure. The presence of blue band emission (430-470 nm) in the implanted SiO2 layer is associated with neutral oxygen vacancy. An increase of the S parameter in the implanted layers is observed after annealing at different temperatures, but it is impossible completely recover the pre-implantation condition after a thermal treatment.     

Effect of structure, particle size and relative concentration of Eu(3+) and Tb(3+) ions on the luminescence properties of Eu(3+) co-doped Y(2)O(3):Tb nanoparticles

Eu(3+) co-doped Y(2)O(3):Tb nanoparticles were prepared by the combustion method and characterized for their structural and luminescence properties as a function of annealing temperatures and relative concentration of Eu(3+) and Tb(3+) ions. For Y(2)O(3):Eu,Tb nanoparticles annealed at 600 and 1200?°C, variation in the relative intensity of excitation transitions between the (7)F(6) ground state and low spin and high spin 4f(7)5d(1) excited states of Tb(3+) is explained due to the combined effect of distortion around Y(3+)/Tb(3+) in YO(6)/TbO(6) polyhedra and the size of the nanoparticles. Increase in relative intensity of the 285?nm peak (spin-allowed transition denoted as peak B) with respect to the 310?nm peak (spin-forbidden transition denoted as peak A) with decrease of Tb(3+) concentration in the Y(2)O(3):Eu,Tb nanoparticles heated at 1200?°C is explained based on two competing effects, namely energy transfer from Tb(3+) to Eu(3+) ions and quenching among the Tb(3+) ions. Back energy transfer from Tb(3+) to Eu(3+) in these nanoparticles is found to be very poor.     

Luminescent properties of rare earth ions in one-dimensional oxide nanowires

Rare-earth doped one-dimensional oxide nanowires including LaPO4, La2O3, and Gd2O3 were synthesized by the hydrothermal method. Their luminescent properties including local environments, electronic transitions, energy transfer, and frequency up-conversion luminescence processes were systematically studied. In LaPO4:Eu and La2O3:Eu nanowires, different symmetry sites of Eu3+ ions were identified, which obviously differed from those of the corresponding micrometer-sized particles. This was attributed to crystal field degeneration in the fringe along the length axis. In LaPO4:Eu nanowires, the electronic transition rate of 5D1-sigmaJ7FJ increased approximately 2 times over that of the zero-dimensional nanoparticles and micrometer-sized particles, which was related to the variation of dipole field induced by shape anisotropy. Considering the nonradiative relaxations, meanwhile, the luminescent quantum efficiency for 5D1-sigmaJ7FJ transitions of Eu3+ in nanowires increased 100% over that in nanoparticles and 20% over that in micrometer particles. In Gd2O3:Eu3+, LaPO4:Ce3+, and LaPO4:Tb3+ nanowires and micrometer-sized particles, the electronic transition rate of rare earths had only a little variation. In LaPO4:Ce3+/Tb3+ nanowires, the energy transfer rate of Ce3+--> Tb3+ decreased 3 times compared to that in micrometer rods. Despite this, the brightness for the 5D4-7F5 green emissions of Tb3+ increased several times due to decreased energy transfer from the excited states higher than 5D4 to some defect levels. In Gd2O3:Er3+/Yb3+ nanocrystals, as the shape varied from nanopapers to nanowires, the relative intensity of up-conversion luminescence of 2H(11/2)/4S(3/2)-4I(15/2) and 4F(9/2)-4I(15/2) to the infrared down-conversion luminescence of 4I(13/2)-4I(15/2) increased remarkably, indicating efficient up-conversion luminescence. Our present results indicate that rare-earth-doped oxide nanowires is a type of new and efficient phosphors.     

Recent progress on the spectroscopy of rare earth ions in core-shells, nanowires, nanotubes, and other novel nanostructures

Research and development of nanoscale luminescent and laser materials are part of the rapidly advancing nanoscience and nanotechnology. Because of unique spectroscopic properties and luminescent dynamics of f-electron states, doping luminescent rare earth ions into nano-hosts has been demonstrated as an optimistic approach to developing highly efficient and stable nanophosphors for various applications. In this article, we review the most recent progress in spectroscopic measurements of rare earth ion-activated low-dimensional nanostructures including nanolayers, core-shells, nanowires, nanotubes, and nanodisks. Among a large volume of work reported in the literature on many members of the rare earth series including Ce3+, Pr3+, Nd3+, Eu3+, and Er3+, we focus on recent findings in the spectroscopic and luminescence properties of Eu3+ doped nanolayers, core-shells, and nanotubes, because Eu3+ ions have been extensively studied and widely used as an ideal probe for fundamental understanding of nano-phenomena. Specifically, the dependence of the optical properties of rare earth ions on nanostructures is discussed in detail.