Rare-earth doped sol-gel derived oxyfluoride glass-ceramics: Structural and optical characterization

Glass-ceramics containing RE³⁺-doped BaF₂ nanocrystals (RE = Eu, Sm, Dy, Ho and Pr) with the size below 10 nm size have been made by using the controlled crystallization at higher temperatures of the RE³⁺-doped SiO₂-BaF₂ xerogels. Photoluminescence measurements have indicated the incorporation of the RE³⁺ ions in both silica network and in the BaF₂ nanocrystals. Thermoluminescence measurements have shown a peak whose position depends on the nature of RE³⁺-dopant as it follows: 140 °C (for Ho³⁺, Dy³⁺), 340 °C (for Sm³⁺) and 370 °C (for Eu³⁺); in Eu³⁺-doped SiO₂ glass the TL peak is shifted to 383 °C. The peaks in glass-ceramics were assigned to the recombination of the electrons thermal released from the RE³⁺-electron traps located in both glass-matrix and BaF₂ nanocrystals. Within the series the trivalent lanthanide ions act as increasingly deeper electron trapping centres.     

Luminescence properties of Sr5(PO4)3Cl:Ni nanocrystals

Strontium chloroapatite (Sr₅(PO₄)₃Cl) nanocrystals doped with Ni²⁺ ions were synthesized by a precipitation method. The obtained nanocrystals appeared to be rod-like with diameters of  50 nm and lengths of 150–350 nm by transmission electron microscopy (TEM). The photoluminescence (PL) properties of Ni²⁺ ions in strontium chloroapatite were studied by photoluminescence spectroscopy.     

Structure and luminescence properties of Eu/Tb codoped oxyfluoride glass ceramics containing Sr2GdF7 nanocrystals

Eu/Tb codoped transparent oxyfluoride borosilicate glass ceramics containing Sr₂GdF₇ nanocrystals were fabricated under a reductive atmosphere and the conversion of Eu³⁺ ions to Eu²⁺ ions was observed. The Sr₂GdF₇ nanocrystals with an average size of 32 nm were homogeneously precipitated in the oxyfluoride borosilicate glass matrix, which could be evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) spectroscopy. The enhancement of photoluminescence emission intensity, reduction of the relative emission intensities between ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁, and long fluorescence lifetimes of Eu²⁺, Eu³⁺, and Tb³⁺ ions revealed that more rare earth ions were partitioned into the low phonon energy environment Sr₂GdF₇ nanocrystals. Under ultraviolet excitation, pure and bright white light emission was obtained in the oxyfluoride borosilicate glass ceramic, which may be a potential blue, green and red-emitting phosphor for white LEDs.     

Eu ions and CaF2-containing transparent glass-ceramics

Transparent glass-ceramics have been prepared by heat-treating 45SiO₂-20Al₂O₃-10CaO-25CaF₂ glasses doped with Eu²⁺ ions (in mol%). The precipitated crystalline phase in the glass-ceramics was CaF₂. TEM observation showed the precipitated crystalline phase had a size of 11-18 nm and dispersed in the amorphous phase without clustering. Fluorescence measurements showed that Eu²⁺ ions entered into the CaF₂ crystalline phase and gave a much stronger emission in the glass-ceramics than in the corresponding glass.     

Luminescence concentration quenching and site-occupancy of Eu2+ ions in Na2Ca2Si3O9 phosphors derived from 45S5 glass-ceramics

The phosphors of Na₂Ca_2-2xEu_2xSi₃O₉ (x = 0, 0.03, 0.05, 0.07, 0.09) were first synthesized by sol-gel method. The crystal phase formations of the phosphors were detected by X-ray powder diffraction (XRD) measurements and the structure refinement. The photoluminescence spectra, the concentration quenching, the luminescence decay curves and the luminescence color chromaticity were measured, respectively. The excitation spectra indicate that the phosphors can be effectively excited by near UV-LED chips. Two kinds of Eu²⁺ sites centered at 545 nm and 505 nm were discussed by analyzing the spectra, concentration-dependent luminescence intensity and lifetimes. This is a potential tool for monitoring the bioactivity of 45S5 glass-ceramics in situ.     

Electrodeposition and characterization of CaF2 and rare earth doped CaF2 films

Calcium fluoride (CaF₂) and the rare earth doped CaF₂ films have been electrodeposited on indium tin oxide (ITO) electrode through electrochemical generation of acid in aqueous solutions near room temperature. For CaF₂ film electrodeposition, the local pH at ITO surface is lowered by the electrochemical oxidation of ascorbate anion, and then the Ca-ethylene diamine tetraacetic acid disodium complexes which are close to the ITO electrode are decomposed to release free Ca²⁺ to react with F⁻to form CaF₂ deposit onto ITO. In the same way, RE-doped CaF₂ films have been electrodeposited onto ITO electrode. The morphology of films studied by scanning electron microscopy revealed that they are agglomerated and dense. The films showed a little [111] preferred orientation by X-ray diffraction. It was also proved that doped RE ions were distributed homogeneously in the film by energy dispersive spectroscopy mapping. The optical properties of the electrodeposited CaF₂, CaF₂:Eu and CaF₂:Tb films were studied by photoluminescence, and all films exhibit intense emission peaks.     

Photoluminescence characteristics of Ti2S3 nanoparticles embedded in sol–gel derived silica xerogel

Ti₂S₃ nanocrystallites embedded in sol–gel derived silica xerogel have been prepared. Their photoluminescence (PL) characteristics have been evaluated and compared with those of pure silica xerogel. UV–vis absorption spectra, transmission electron micrograph, excitation spectra and PL spectra of the doped and undoped samples have all been investigated. Two emission peaks have been observed from the doped samples, one at 440 nm (λ_ex=380 nm) while the other at 600 nm (λ_ex=550 nm). The latter has been assigned to the Ti₂S₃ nanocrystallites in the silica xerogel. Therefore, a novel luminescence property can be observed by introducing the semiconductor nanoparticles into the silica xerogel.     

Transparent glass ceramic containing Er:CaF2 nano-crystals prepared by sol-gel method

Er³⁺ doped SiO₂-CaF₂ transparent glass ceramic was prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared spectra (IR). After heat-treatment at 900 °C, the Si-OH bonds and other organic groups were basically eliminated. The CaF₂ crystallites in the sample heat-treated at 900 °C are 10-20 nm in size, distributed homogeneously among the amorphous silica matrix. The efficient upconversion emission for Er^3+.4F_9/2 → ⁴I_15/2 transition was recorded under 980 nm excitation, which could be ascribed to the incorporation of Er³⁺ ions into the CaF₂ nano-crystals with low phonon energy.     

Synthesis and structural characterization of Tm:Lu2O3 nanocrystals. An approach towards new laser ceramics

Nanocrystals of Tm³⁺ Thulium doped cubic sesquioxides, Tm:Lu₂O₃, with a maximum size around 30 nm have been synthesized by a modified Pechini sol-gel method. The calcination temperature for the synthesis is 1073 K. Electron microscopy was used to analyze the presence of aggregates, and the type of boundary between the nanocrystals. The linear coefficient of thermal expansion for these nanocrystals has been determined to be around 7.5 × 10⁻⁶ K⁻¹. The growth of the nanocrystals has been studied in terms of temperature and time. Nanocrystals start to grow at temperatures around 1267 K. Finally, the grain growth activation energy of this material has been evaluated to be 76 kJ/mol, indicating a diffusion growth mechanism. Linear thermal expansion of prepared nanocrystals is ≈7.5 × 10⁻⁶ K⁻¹.     

Preparation and characterization of sol-gel derived ZnTiO3 nanocrystals

Nanometer scale cubic ZnTiO₃ has been synthesized by sol-gel method at a lower temperature (600 °C). X-ray diffraction (XRD) analysis shows that the average crystalline size of the sample is ∼8-10 nm. The cubic to hexagonal phase transition of ZnTiO₃ is clearly observed from the XRD patterns of the sample. The thermal behavior of the sample was characterized by the thermogravimetric/differenthermal analysis (TG/DTA), XRD patterns, and the infrared spectra (IR). Transmission electron microscope (TEM) observations of the sample reveal a high microstructural uniformity.     

Formation of size-controlled silicon nanocrystals in plasma enhanced chemical vapor deposition grown SiOxNy/SiO2 superlattices

Size controlled silicon nanocrystals (SiNC) in silicon oxynitride matrix were prepared using plasma enhanced chemical vapor deposition. The as-deposited superlattices (SLs) and the corresponding bulk films were treated by thermal annealing. Hydrogen effusion was performed during the heating up by choosing a sufficiently low heating ramp. The phase separation of the layers into SiNCs and surrounding oxynitride matrix was studied at temperatures of up to 1150 °C. The influence of the annealing temperature on SiO_xN_y/SiO₂ - SLs with varying SiO_xN_y layer thickness was investigated by several analytical techniques including variable angle spectroscopic ellipsometry, photoluminescence (PL) spectroscopy, x-ray photoelectron spectroscopy, Fourier transform infrared spectrometry (FTIR) and transmission electron microscopy (TEM). Before annealing FTIR investigations show in addition to the expected Si-O bonds also the formation of nitrogen and hydrogen related bonds. The shift of the Si-O-Si stretching vibration to higher wave numbers after annealing indicates phase separation. The disappearance of the hydrogen related bonds indicates the hydrogen effusion. The PL signal is rising significantly with increasing annealing temperature and the PL peak position is strongly related to the thickness of the SiO_xN_y sublayers due to quantum confinement effects. TEM investigations confirm the size-controlled growth of SiNCs within the oxynitride matrix. The role of incorporated nitrogen and hydrogen is discussed.     

Preparation and photoluminescence properties of Eu-doped Ga2O3 nanorods

GaOOH:Eu³⁺ nanorods with different aspect ratios were prepared by hydrothermal method at 140 °C. - and β-Ga₂O₃:Eu³⁺ were converted from as-prepared GaOOH:Eu³⁺ particles by calcination at 500 and 850 °C, respectively. The products were characterized with X-ray diffraction (XRD), transmission electron microscope (TEM) and photoluminescence (PL). Results show that solution pH values play a key role in the formation of the GaOOH:Eu³⁺ powders with different morphologies and - and β-Ga₂:Eu³⁺ inherit the morphology of GaOOH:Eu³⁺ exactly. The photoluminescence characteristics of β-Ga₂O₃:Eu³⁺ were also investigated. Experimental results reveal that the color purity of β-Ga₂O₃:Eu³⁺ nanorods with high aspect ratio is enhanced in comparison with β-Ga₂O₃:Eu³⁺ nanorods with low aspect ratio.     

Formation of the distributed NiSiGe nanocrystals nonvolatile memory formed by rapidly annealing in N2 and O2 ambient

In this work, electrical characteristics of the Ge-incorporated Nickel silicide (NiSiGe) nanocrystals memory device formed by the rapidly thermal annealing in N₂ and O₂ ambient have been studied. The trapping layer was deposited by co-sputtering the NiSi₂ and Ge, simultaneously. Transmission electron microscope results indicate that the NiSiGe nanocrystals were formed obviously in both the samples. The memory devices show obvious charge-storage ability under capacitance-voltage measurement. However, it is found that the NiSiGe nanocrystals device formed by annealing in N₂ ambient has smaller memory window and better retention characteristics than in O₂ ambient. Then, related material analyses were used to confirm that the oxidized Ge elements affect the charge-storage sites and the electrical performance of the NCs memory.     

Red emission in Pr doped CaBi4Ti4O15 ferroelectric ceramics

In this paper, Pr doped CaBi₄Ti₄O₁₅ ceramics were prepared by a traditional solid state method. Crystal structure and morphologies of the ceramics were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The photoluminescence properties of the samples were investigated by a spectrofluorometer. Three excitation bands are located at wide range of wavelength, which are 300-430 nm, 440-510 nm and 550-570 nm respectively. Upon the excitation of 494 nm light, the samples shows an emission peak centered at 614 nm, corresponding to ¹D₂ → ³H₄ transition. A 614 nm red emission excited under the wave with long wavelength of Pr doped CaBi₄Ti₄O₁₅ makes it useful in the white LEDs. In addition, it is an intrinsic ferroelectric and piezoelectric material; the enhanced ferroelectric properties were obtained by Pr doping. As a multifunctional materials, Pr doped CaBi₄Ti₄O₁₅ may be useful in white LEDs, sensor, and optical-electro integration.     

Controlled synthesis and possible formation mechanism of leaf-shaped SnS2 nanocrystals

A sort of novel leaf-shaped SnS₂ nanocrystals have been synthesized by controlled reaction of CH₃CH₂OCS₂K with SnCl₄·5H₂O in aqueous solution in the absence of surfactant. The leaf-shaped nanocrystals consist of very thin sheets with several microns in length and 100–300 nm in width. The structure, composition, and morphology of the as-prepared leaf-shaped SnS₂ nanocrystals were characterized by XRD, Raman spectra, XPS, and FE-SEM, respectively. Elemental analysis, FT-IR spectra, and TEM were further used to study the formation mechanism of the leaf-shaped SnS₂ nanocrystals. A possible mechanism of producing lamellae – breaking into nanosheets – rolling and assembling was suggested for the leaf-shaped SnS₂ nanocrystals formation based on experimental results.     

Preparation of SiO2/TiO2 composite fibers by sol-gel reaction and electrospinning

Submicron scale composite fibers of SiO₂/TiO₂ with various compositions have been prepared by electrospinning a sol-gel precursor of tetraethyl orthosilicate(TEOS) and titanium(IV) isopropoxide(TiP), followed by calcination. Any gelator or binder has not been used in this direct preparation process for composite fibers, and the maximum amount of titania for suitable fiber formation was about 50 mol%. The sintered composite fibers were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) spectroscopy. Our results show that the surface morphology and crystallization behavior of electrospun fibers are largely influenced by the calcination temperature and the content of TiO₂. XRD results also reveal that the anatase phase in composite fibers can be preserved even after high temperature processing at lower content (x = 0.1, 0.2) of titania.     

Facile synthesis of rice-like anatase TiO2 nanocrystals

Rice-like anatase TiO₂ nanocrystals have been synthesized by hydrothermal treating Ti(OH)₃ precursor in deionized water, which is easy to form in-situ a stable TiO₂ aqueous dispersion and no post-synthetic purification process is required. The influences of synthetic parameters, such as reaction time and pH value, on the morphologies of the resulting nanocrystals have been investigated. It is found that the crystallinity and sizes of TiO₂ nanocrystals increase with the reaction time increased. The pH value is important for controlling the sizes and shapes of TiO₂ nanocrystals. As pH value increases from 2 to 11, spherical, rice-like, and rod-like TiO₂ nanocrystals are obtained, respectively. The formation mechanism of rice-like anatase TiO₂ nanocrystals is supposed to be a hydrothermal crystallization and Ostwald ripening process.     

Preparation of KMgF3 and Eu-doped KMgF3 nanocrystals in water-in-oil microemulsions

In this study, KMgF₃:Eu²⁺ luminescent nanocrystals (NCs) were prepared in water/cetyltrimethylammonium bromide (CTAB)/2-octanol microemulsions. The KMgF₃:Eu²⁺ NCs were characterized by transmission electron microscopy (TEM), X-ray diffractometer (XRD), fluorescence spectrum, infrared spectroscopy (IR) and elementary analysis. The results showed that the size of the KMgF₃:Eu²⁺ NCs was hardly affected by water content and surfactant (CTAB) concentration. The emission spectrum showed that the position of the 362 nm peak is due to the K⁺ sites substituted Eu²⁺. Two emission peaks located at 589 and 612 nm can be attributed to Eu³⁺, which exist at two different types of Eu³⁺ centers: one is Eu³⁺ at a K⁺ site, the other is clustering of Eu³⁺ ions in the interstices of KMgF₃ host lattice.     

Structural investigations of sol–gel derived silicate gels using Eu ion-probe luminescence

Undoped and Eu³⁺-doped CaF₂–SiO₂ gels were prepared by the sol–gel method and their optical properties have been studied. The UV–VIS–NIR absorption and photoluminescence spectra have shown the bands typical for the Eu³⁺ ions transitions. When the Eu-doped gel is annealed at temperatures up to 800 °C (i.e. above the CaF₂ crystallisation peak at 460 °C) the photoluminescence spectra intensity increase, the 590 nm (⁵D₀→⁷F₁) and 620 nm (⁵D₀→⁷F₂) luminescence bands become comparable and a structuring of the 620 nm band is observed. The phonon sidebands peaks associated with the ⁵F₀→⁷D₂ transition of the Eu³⁺ ion were observed at around 1000 and 620 cm⁻¹ and have been assigned to the Si–O and Ca–O bonds, respectively. A phonon sideband signal in the range of 300–400 cm⁻¹ was attributed to Ca–F bonds in the precipitated CaF₂ phase. From the optical absorption, photoluminescence and phonon sidebands spectra we have concluded that in the gels annealed at 800 °C, the Eu³⁺ ions are incorporated into the silica network and in the precipitated CaF₂ phase.     

Raman study of stress effect on Ge nanocrystals embedded in Al2O3

Ge nanocrystals (NCs) embedded in Al₂O₃ were grown by RF-sputtering. X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy techniques were used to characterize the stresses on the NCs. While small NCs (< 10 nm) have been observed to be spherical and fully relaxed in the matrix, the larger ones (> 17 nm) demonstrated a compressive stress effect. This could be linked to the crystal structure of the adjacent Al₂O₃ matrix.