Visible luminescence of Al2O3 nanoparticles embedded in silica glass host matrix

This paper deals with the sol-gel elaboration and defects photoluminescence (PL) examination of Al₂O₃ nanocrystallites (size ∼30 nm) confined in glass based on silica aerogel. Aluminium oxide aerogels were synthesized using esterification reaction for hydrolysis of the precursor and supercritical conditions of ethyl alcohol for drying. The obtained nanopowder was incorporated in SiO₂ host matrix. After heating under natural atmosphere at 1150 °C for 2 h, the composite Al₂O₃/SiO₂ (AS) exhibited a strong PL bands at 400-600 and 700-900 nm in 78-300 K temperature range. PL excitation (PLE) measurements show different origins of the emission. It was suggested that OH-related radiative centres and non-bridging oxygen hole centres (NBOHCs) were responsible for the bands at 400-600 and 700-900 nm, respectively.     

Spherical Zn2SiO4:Eu@SiO2 phosphor particles in core-shell structure: Synthesis and characterization

Spherical SiO₂ particles have been coated with Zn₂SiO₄:Eu³⁺ phosphor layers by a Pechini sol-gel process. The microstructure and luminescent properties of the obtained Zn₂SiO₄:Eu³⁺@SiO₂ particles were well characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), photoluminescence (PL) spectra, and lifetime. The results demonstrate that the Zn₂SiO₄:Eu³⁺@SiO₂ particles, which have regular and uniform spherical morphology, emitted an intensive red light emission at 613 nm under excitation at 395 nm. Besides, the effects of the Eu³⁺ concentration, annealing temperature and charge compensators of Li⁺ ions on the PL emission intensities were investigated in detail.     

High luminance of new green emitting phosphor, Mg2SnO4:Mn

Mg₂SnO₄, which has an inverse spinel structure, was adopted as the host material of a new green emitting phosphor. Luminescence properties of the manganese-doped magnesium tin oxide prepared by the solid state reaction were investigated under vacuum ultraviolet (VUV) ray and low-voltage electron excitation. The Mg₂SnO₄:Mn phosphor exhibited green luminescence with the emission spectrum centered at 500 nm due to spin flip transition of the d-orbital electron associated with the Mn²⁺ ion. Optimum Mn concentration of Mg₂SnO₄:Mn under VUV excitation with 147 nm wavelength and electron beam excitation with 800 V excitation voltage are 0.25 and 0.6 mol%, respectively. The emission intensities of Mg₂SnO₄:Mn phosphors under the two excitation sources are higher than those of Zn₂SiO₄:Mn and ZnGa₂O₄:Mn phosphors. At 0.25 mol% of Mn concentration, on the other hand, the decay time is shorter than 10 ms.     

Preparation and characterization of Zn2SiO4: Mn phosphors with hydrothermal methods

Mn²⁺-doped Zn₂SiO₄ phosphors had been prepared by hydrothermal method in stainless-steel autoclaves. Effects of synthesized methods, reaction temperature, ambience of heat treatment on the structure and the luminescence properties of this silicate were studied with X-ray diffraction apparatus (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) and fluorescence spectrum. Results show that Zn₂SiO₄ nanocrystalline can be obtained by hydrothermal method at relatively low temperatures. The absorption pattern shows an absorption edge at about 380 nm originated from ZnO crystals and two absorption bands at about 215 and 260 nm. Mn²⁺-doped Zn₂SiO₄ has a luminescence band with the wavelength at about 522 nm under 255 nm excitation, and the luminescent intensity increases after being heat treated.     

Enhanced photoluminescence of ZnO-SiO2 nanocomposite particles and the analyses of structure and composition

Stable blue-green photoluminescent ZnO-SiO₂ nanocomposite particles exhibiting quantum efficiency as high as 34.8% under excitation at 360 nm were prepared using a spray-drying process from a feed solution that contained both luminescent ZnO nanoparticles synthesized by a sol-gel method and commercially-available SiO₂ nanoparticles. The effects of silica nanoparticle size and SiO₂-to-ZnO concentration ratio on the PL properties of the composite particles were investigated. The internal structure and chemical composition were investigated in detail using elemental mapping, which revealed that ZnO nanoparticles were well-dispersed within silica nanoparticle matrix. At a LiOH concentration of 0.23 M, the predicted ZnO crystallite diameter before and after spray drying was approximately constant at 3.3 and 3.6 nm, respectively. This result indicates that ZnO particle growth was inhibited and therefore the PL property of ZnO nanoparticles was stably preserved in the composite.     

Sol–gel synthesis and luminescent properties of SiO2/Zn2SiO4 and SiO2/Zn2SiO4:V composite materials

Undoped and vanadium-doped Zn₂SiO₄ particles embedded in silica host matrix were prepared by a simple solid-phase reaction after the incorporation of ZnO and ZnO:V nanoparticles, respectively, in silica monolith using the sol–gel method with supercritical drying of ethyl alcohol in two steps. After supercritical drying and annealing in the temperature range between 1423 and 1473 K in an air atmosphere, the photoluminescence (PL) measurements show a band centered at about 760 nm in the case of non-doped Zn₂SiO₄ which is attributed to energy transfer from Zn₂SiO₄ particles to NBOHs interface defects. In the case of vanadium doped Zn₂SiO₄, the PL reveals a band centered at about 540 nm attributed to the vanadium in the interfaces between Zn₂SiO₄ particles and SiO₂ host matrix. Photoluminescence excitation (PLE) measurements show different origins of the emission bands. The PLE band (～240–350 nm) may be understood as an energy transfer process from O^2? to V⁵⁺ which occurs intrinsically in the vanadyl group.     

Magnetic properties of the novel nanocomposite (Zn0.15Ni0.85Fe2O4)0.15/(SiO2)0.85 at room temperature

The value of the effective magnetic anisotropy constant of the ferrimagnetic nanoparticles Zn_0.15Ni_0.85Fe₂O₄ embedded in a SiO₂ silica matrix, determined through ferromagnetic resonance (FMR), is much higher than the magnetocrystalline anisotropy constant. The higher value of the anisotropy constant is due to the existence of surface anisotropy. However, even if the magnetic anisotropy is high, the ferrimagnetic nanoparticles with a 15% concentration, which are isolated in a SiO₂ matrix, display a superparamagnetic (SPM) behavior at room temperature and at a frequency of the magnetization field equal to 50 Hz. The FMR spectrum of the novel nanocomposite (Zn_0.15Ni_0.85Fe₂O₄)_0.15/(SiO₂)_0.85, recorded at room temperature and a frequency of 9.060 GHz, is observed at a resonance field (B_0r) of 0.2285 T, which is substantially lower than the field corresponding to free electron resonance (ESR) (0.3236 T). Apart from the line corresponding to the resonance of the nanoparticle system, the spectrum also contains an additional weaker line, identified for a resonance field of ∼0.12 T, which is appreciably lower than B_0r. This line was attributed to magnetic ions complex that is in a disordered structure in the layer that has an average thickness of 1.4 nm, this layer being situated on the surface of the Zn_0.15Ni_0.85Fe₂O₄ nanoparticles that have a mean magnetic diameter of 8.9 nm.     

A new self-activated yellow-emitting phosphor Zn2V2O7 for white LED

A new self-activated yellow-emitting Zn₂V₂O₇ phosphor was synthesized by high temperature solid-state reaction. X-ray powder diffraction (XRD) analysis confirmed the sample with monoclinic formation of Zn₂V₂O₇. The excitation and emission spectra indicated the phosphor can be efficiently excited by near ultraviolet (NUV) light in 220–400 nm range and exhibit a bright broad yellow emission with the highest emission intensity at 531 nm. The broad emission band from 400 to 650 nm can be attributed to the charge transfer transition in the VO₄ tetrahedra, which suggests that the phosphor is a promising yellow phosphor applied for white light-emitting diodes (WLED).     

Synthesis and characterization of Mn-doped Zn2SiO4/SiO2 phosphor particles in core-shell structure

Manganese-doped zinc silicate (Zn₂SiO₄:Mn) is a kind of phosphor material that has a photo-luminescent (PL) and cathode-luminescent (CL) properties with intensive green light emission at 520 nm. The particles consisting of SiO₂@Zn₂SiO₄:Mn (SiO₂ core-Zn₂SiO₄:Mn shell) were synthesized via colloidal process and forced precipitation. After drying, the Zn/Mn precipitates were coated on the surface of SiO₂ particles. The Zn/Mn precipitates reacted with SiO₂ and transformed to Zn₂SiO₄:Mn by suitable calcination. The microstructure, crystalline phase, and luminescent characteristics of the products were studied. Besides, a CL device consisting of the core-shell powder was characterized.     

Luminescence and energy transfer processes in Lu2SiO5:Ce scintillator

The energy transfer processes in Lu₂SiO₅:Ce³⁺ luminescence was investigated through the temperature dependent luminescence under excitation with VUV-UV. Ce1 center emission peaking at 393 and 422 nm and Ce2 center emission peaking at 462 nm were observed. Ce2 center emission is enhanced with the temperature, which can be explained by the rate of energy transfer from Ce1 center increases when the temperature rises. The Ce1 emission shows the thermal quenching effect under the direct excitation of Ce³⁺ at 262 nm. However, under the interband excitation of 183 nm, the Ce1 center emission exhibits undulating temperature dependence. This is because the emission is governed by thermal quenching and possible thermal enhancement of the transport of free carriers with the rising temperature.     

Preparation and characterizations of SiO2-coated nanoparticles of Mn0.4Zn0.6Fe2O4

Core/shell nanoparticles consisting of a magnetic core of zinc-substituted manganese ferrite (Mn_0.4Zn_0.6Fe₂O₄) and a shell of silica (SiO₂) are prepared by a sol-gel method using tetraethyl orthosilicate (TEOS) as a precursor material for silica and salts of iron, manganese and zinc as the precursor of the ferrite. Three weight percentages of the shell materials of SiO₂ are used to prepare the coated nanoparticles. The X-ray diffractograms (XRD) of the coated and uncoated magnetic nanoparticles confirmed that the magnetic nanoparticles are in their mixed spinel phase in an amorphous matrix of silica. Particles sizes of the samples annealed at different temperatures are estimated from the width of the (3 1 1) line of the XRD pattern using the Debye-Sherrer equation. The information regarding the crystallographic structure together with the particles sizes extracted from the high-resolution transmission electron microscopy (HRTEM) of a few selected samples are in agreement with those obtained from the XRD. HRTEM observations revealed that particles are coated with silica. The calculated thickness is in agreement with that obtained from the HRTEM pictures. Hysteresis loops observed in the temperature range 300 down to 5 K and Mössbauer spectra at room temperature indicate superparamagnetic relaxation of the nanoparticles.     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Structural and magnetic properties of NiCuZn ferrite/SiO2 nanocomposites

Ni_0.53Cu_0.12Zn_0.35Fe₂O₄/SiO₂ nanocomposites with different weight percentages of NiCuZn ferrite dispersed in silica matrix were prepared by microwave-hydrothermal method using tetraethylorthosilicate as a precursor of silica, and metal nitrates as precursors of NiCuZn ferrite. The structure and morphology of the composites were studied using X-ray diffraction and scanning electron microscopy. The structural changes in these samples were characterized using Fourier Transform Infrared Spectrometer in the range of 400-1500 cm⁻¹. The bands in the range of 580-880 cm⁻¹ show a slight increase in intensity, which could be ascribed to the enhanced interactions between the NiCuZnFe₂O₄ clusters and silica matrix. The effects of silica content and sintering temperature on the magnetic properties of Ni_0.53Cu_0.12Zn_0.35Fe₂O₄/SiO₂ nanocomposites have been studied using electron spin resonance and vibrating sample magnetometer.     

White light-emitting Mg0.1Sr1.9SiO4:Eu phosphors

Divalent europium-activated strontium orthosilicate Sr₂SiO₄:Eu²⁺ and Mg_0.1Sr_1.9SiO₄:Eu²⁺ phosphors were synthesized through the solid-state reaction technique. Their luminescent properties under ultraviolet excitation were investigated. The X-ray diffraction (XRD) results show that these phosphors are of α′-Sr₂SiO₄ phase with a trace of β-Sr₂SiO₄. Doping of Eu²⁺ ion into the crystal lattice results in the lattice constant being expended, while Mg²⁺ makes the lattice constant shrinking. A solid solution with the same crystal structure is formed when Eu²⁺ or Mg²⁺ substitutes part of Sr²⁺ ions and occupies the same lattice sites. The Sr₂SiO₄:Eu²⁺ phosphors show two emission spectra peaked at 535 and 473 nm originated from the 5d-4f transition of Eu²⁺ ion doped in two different Sr²⁺ sites in the host lattice. By substitution of 0.1 mol of Sr²⁺ with Mg²⁺, these two emission bands are tuned to be in the blue and yellow region (459 and 564 nm for Mg_0.1Sr_1.88SiO₄:Eu_0.02), respectively. The tuning effect is discussed. With a combination of the blue and yellow emission bands the phosphors show white color, indicating that these phosphors may become promising phosphor candidates for white light-emitting diodes (LEDs).     

Luminescence of Ce and Pr doped Sr2Mg(BO3)2 under VUV-UV and X-ray excitation

This report presents the luminescence properties of Ce³⁺ and Pr³⁺ activated Sr₂Mg(BO₃)₂ under VUV-UV and X-ray excitation. The five excitation bands of crystal field split 5d states are observed at about 46 729, 44 643, 41 667, 38 314 and 29 762 cm⁻¹ (i.e. 214, 224, 240, 261 and 336 nm) for Ce³⁺ in the host lattice. The doublet Ce³⁺ 5d→4f emission bands were found at about 25 840 and 24 096 cm⁻¹ (387 and 415 nm). The influence of doping concentration and temperature on the emission characteristics and the decay time of Ce³⁺ in Sr₂Mg(BO₃)₂ were investigated. For Pr³⁺ doped samples, the lowest 5d excitation band was observed at about 42017 cm⁻¹ (238 nm), a dominant band at around 35714 cm⁻¹ (280 nm) and two shoulder bands were seen in the emission spectra. The excitation and emission spectra of Ce³⁺ and Pr³⁺ were compared and discussed. The X-ray excited luminescence studies show that the light yields are ∼3200±230 and ∼1400±100 photons/MeV of absorbed X-ray energy for the samples Sr_1.86Ce_0.07Na_0.07Mg(BO₃)₂ and Sr_1.82Pr_0.09Na_0.09Mg(BO₃)₂ at RT, respectively.     

Laser-excited spectra of Lu2SiO5:Ce scintillator

The emission spectra of Lu₂SiO₅:Ce single crystal under the excitation of 266 nm laser were investigated. The emission spectra of LSO single crystal show no temperature quenching from 20 to 300 K, under the excitation of 266 nm laser with 2 mJ pulse energy. With rising temperature, the Ce1 emission is slightly decreased, while the Ce2 emission is slightly increased. These results show the emissions of Ce1 and Ce2 is not only dependent on the concentration ratio but also influenced by the possible energy transfer processes, including Ce1 to Ce2, intrinsic STHs to Ce2 and the phonon-assisted transfer processes. The spectral thermal broadening and the spectral overlap become evident at high temperature, leading to the enhancement of energy transfer. When the excitation power lowers, the ratio of Ce1 and Ce2 emission increases, and is close to the Xe lamp ultraviolet (UV) excitation, suggesting that the energy transfer from Ce1 center to Ce2 center may be also dependent on the excitation power.     

Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce

Radioluminescence and thermally stimulated luminescence measurements on Lu₂O₃, Lu₂SiO₅ (LSO) and Lu₂SiO₅:Ce³⁺ (LSO:Ce) reveal the presence of intrinsic ultraviolet luminescence bands. Characteristic emission with maximum at 256 nm occurs in each specimen and is attributed to radiative recombination of self-trapped excitons. Thermal quenching of this band obeys the Mott-Seitz relation yielding quenching energies 24, 38 and 13 meV for Lu₂O₃, LSO and LSO:Ce, respectively. A second intrinsic band appears at 315 nm in LSO and LSO:Ce, and at 368 nm in Lu₂O₃. Quenching curves for these bands show an initial increase in peak intensity followed by a decrease. Similarity in spectral peak position and quenching behavior indicate that this band has a common origin in each of the samples and is attributed to radiative recombination of self-trapped holes, in agreement with previous work on similar specimens. Comparison of glow curves and emission spectra show that the lowest temperature glow peaks in each specimen are associated with thermal decay of self-trapped excitons and self-trapped holes. Interplay between the intrinsic defects and extrinsic Ce³⁺ emission in LSO:Ce is strongly indicated.     

Fabrication and characterization of Au/SiO2 nanocomposite films

Au/SiO₂ nanocomposite films were prepared by radio frequency sputtering technique and annealing. The above nanocomposite films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). The surface of the nanocomposite films was uniform with the particle diameter of 100-300 nm. The size of Au crystallites increased on increasing annealing time. The luminescent behavior of the nanocomposite films was characterized by photoluminescence (PL) with different excitation wavelengths. Two emission peaks at around 525 nm and 560 nm were observed with the excitation wavelength at 325 nm. An intensive emission peak at around 325 nm was observed with the excitation wavelength at 250 nm, which is related to the defective structure of the amorphous SiO₂ layer because of oxygen deficiency, and could be applied to many fields, such as ultraviolet laser and ultraviolet detector.     

Controlled synthesis and relationship between luminescent properties and shape/crystal structure of Zn2SiO4:MN phosphor

Mn-doped Zn₂SiO₄ phosphors with different morphology and crystal structure, which show different luminescence and photoluminescence intensity, were synthesized via a low-temperature hydrothermal route without further calcining treatment. As-synthesized zinc silicate nanostructures show green or yellow luminescence depending on their different crystal structure obtained under different preparation conditions. The yellow peak occurring at 575 nm comes from the β-phase zinc silicate, while the green peak centering at 525 nm results from the usual α-phase zinc silicate. From photoluminescence spectra, it is found that Zn₂SiO₄ nanorods have higher photoluminescence intensity than Zn₂SiO₄ nanoparticles. It can be ascribed to reduced surface-damaged region and high crystallinity of nanorods.     

Emission color variation of M2SiO4:Eu (M=Ba, Sr, Ca) phosphors for light-emitting diode

The luminescent properties of alkaline earth orthosilicates M₂SiO₄ (M=Ba, Sr, Ca) doped with Eu²⁺ ions are investigated. Two emission bands are assigned to the f-d transitions of Eu²⁺ ions doped into two different cation sites in host lattices confirmed by electron paramagnetic resonance signal. Two emission bands show the different emission color variation with substituting M²⁺ cations with smaller cations. This behavior is discussed in terms of two competing factors of the crystal field strength and covalence. Also the decay times are in order of 600-1000 ns. These phosphors with maximum excitation of around 370 nm can be applied as a color-tunable phosphor for light-emitting diode based on ultraviolet chip/phosphor technology.