Fabrication and luminescence behavior of phosphate glass ceramics co-doped with Er3+ and Yb3+

Transparent phosphate glass ceramics co-doped with Er³⁺ and Yb³⁺ in the system P₂O₅Li₂OCaF₂TiO₂ were successfully synthesized by melt-quenching and subsequent heating. Formation of the nanocrystals was confirmed by X-ray powder diffraction. Judd–Ofelt analyses of Er³⁺ ions in the precursor glasses and glass ceramics were performed to evaluate the intensity parameters Ω_2,4,6. Under 975 nm excitation, intense upconversion (UC) and infrared emission (1545 nm) were observed in the glass ceramics by efficient energy transfer from Yb³⁺ to Er³⁺. The luminescence processes were explained and the emission cross section was calculated by Fuchtbauer–Ladenburg (F–L) formula. The results confirm the potential applications of Er³⁺/Yb³⁺ co-doped glass ceramics as laser and fiber amplifier media.     

Spectroscopic properties of Co2+: ZnAl2O4 nanocrystals in sol–gel derived glass–ceramics

Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl₂O₄) nanocrystals doped with tetrahedrally coordinated Co²⁺ ions were obtained by the sol–gel method for the first time. The gels of composition SiO₂–Al₂O₃–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl₂O₄ nanocrystals were precipitated. Co²⁺ ions were located in tetrahedral sites in ZnAl₂O₄ nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl₂O₄ crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co²⁺ doped glass–ceramic and Co²⁺ doped bulk crystal was analysed. The crystal field parameter D_q of 423 cm⁻¹ and the Racah parameters B of 773 cm⁻¹ and C of 3478.5 cm⁻¹ were calculated for tetrahedral Co²⁺ ions.     

Quantum dots of Cd0.5Mn0.5Te semimagnetic semiconductor formed by the cold isostatic pressure method

Cd_0.5Mn_0.5Te is a semimagnetic semiconductor, which crystallizes in the zinc-blende structure (ZB) and exhibits a magnetic spin glass like transition at 21 K. Under pressure it shows a first-order phase transition around 2.6 GPa to the NaCl like structure. In this work, the pressure cycled method using a Paris–Edinburgh cell up to 8 GPa has been applied to Cd_0.5Mn_0.5Te samples in order to obtain recovered nanocrystals. The nanoparticles have been characterized by EDX and electron microscopy. The X-ray and electron diffraction results confirmed the existence of nanocrystals in the ZB phase with an average size of 7 nm. Magnetization measurements made in the range of 2–300 K at low field show that the temperature of the magnetic transition decreases when the crystallites’ size is reduced.     

Size dependence of ground-state energy of the excitons in spherical Y2O3

The exciton energies of rare earth oxides (Ln₂O₃) have rarely been calculated by the theory. Experimentally, the blue-shift of exciton energy in nanocrystals deviates from the traditional size confinement effect. Herein, the dependence of the ground-state energy of an exciton in Y₂O₃ spheres on particle radius was calculated by using a variational method. In the model, an exciton confined in a sphere surrounded by a dielectric continuum shell was considered. The ground-state energy of exciton comprises kinetic energy, coulomb energy, polarization energy and exciton–phonon interaction energy. The kinetic and coulomb energy were considered by the effective mass and the dielectric continuum and the exciton–phonon interaction energy was given by the intermediate coupling method. The numerical results demonstrate that the present model is roughly consistent with the experimental results. The confinement effect of the kinetic energy is dominant of the blue-shift of the exciton energy in the region of R < 5 nm, while confinement effect of the coulomb energy is dominant of the blue-shift of the exciton energy in the region of R > 5 nm. The polarization energy contributes largely to the exciton energy as the particle size is smaller than ~ 10 nm, while the exciton–phonon interaction energy takes only a little contribution in all the range.     

Optical characteristics of Tb-doped InBO3 nanocrystals

InBO₃ nanocrystals doped with Tb³⁺ ions are prepared via the sol–gel method. The structure, morphology, and optical properties of the nanocrystals are characterized by X-ray diffraction, high-resolution transmission electron microscopy, field-emission scanning electron microscopy, and photoluminescence analysis. The results show that a hexagonal InBO₃ phase forms at above 650 °C. A second phase of In₂O₃ begins to appear with Tb doping of over 3 mol%. The ⁵D₄→⁷F₅ (553 nm) transitions of Tb³⁺ ions in the InBO₃ host are observed at 2 mol%. The decay time of Tb-doped InBO₃ nanocrystals is about 2.1 ms. For Tb-doped InBO₃ nanocrystals excited at 237 nm and 553 nm wavelengths, the 2 mol% doping level yields the highest saturation of green emission. The emission shifts from green to yellow when the doping concentration is increased from 1 to6 mol%, due to the ⁵D₄→⁷F₅ transition.     

Preparation of functionalized and gold-coated cobalt nanocrystals for biomedical applications

We present the synthesis, structural and magnetic characterization of cobalt-based nanocrystals with controlled size (5–25 nm; ±1 nm) and tailored morphologies (spheres, discs with specific aspect ratio of 5×20 nm and core-shell structures). The reproducible synthesis route for Au-coated, high moment, cobalt nanocrystals presented here opens up possibilities for a number of biomedical applications and surface functionalities.     

Preparation and properties of transparent SnO–P2O5 glasses

Colorless, transparent SnO–P₂O₅ (SP) glasses with 60–70 mol% SnO compositions were prepared by melting at 880–1000 °C in Ar atmosphere using commercial SnO and P₂O₅ powders as raw materials and vitreous carbon crucibles. SP glasses are characterized by glass transition temperature, onset of crystallization, thermal expansion coefficient and weight loss after immersion test. The viscosity of 67SnO–33P₂O₅ glass was measured by a penetration method in the range of 10^7.9–10^10.5 Pa s at 267–290 °C. The results of optical properties show that the transparent SP glasses have high refractive indexes over 1.75 and high transmission over 80% in the visible and IR region of 380 and 2700 nm.     

Desvitrification on an oxyfluoride glass doped with Tm3+ and Yb3+ ions under Ar laser irradiation

Desvitrification in a Tm³⁺ and Yb³⁺ codoped oxyfluoride glass has been obtained by exciting with a continuous Argon laser radiation increasing the average laser power from 144 to 2900 mW. Excitation spectra inside a locally damaged zone in a 1 mol% Tm³⁺ and 2.5 mol% Yb³⁺ codoped glass have been measured under excitation in the wavelength range 750–830 nm detecting the ²F_5/2 (Yb³⁺) level. This curve is the result of the contribution of two different kinds of centers, the fluoride nanocrystals and the glassy phase of the glass ceramic sample created due to the irradiation. The weight of the contributions of each of the centers depends on the excitation wavelength, and from the analysis of the decay of the luminescence it can be concluded that approximately 80% of the Tm³⁺ ions are located in the nanocrystals and therefore less than 20% in the glassy phase.     

The broadband NIR emission properties of Bi doped La2O3–Al2O3–SiO2 glass

Bi₂O₃ doped 65SiO₂–20Al₂O₃–15La₂O₃ (in mole%) glasses were prepared by the traditional melting–quenching method. The spectroscopic properties and mechanism of NIR broadband emission in these glasses were investigated in this work. Three excitation wavelengths of 500, 700 and 800 nm were used to test emission spectra. The emission band under 500 nm excitation can be regarded as combination of emission bands under 700 and 800 nm excitation. 2.0 mole% is found to be the optimal Bi₂O₃ doping level in this glass. Under 500 nm excitation its emission peak, FWHM and lifetime of emission band are 1160 nm, 300 nm and 569 μs, respectively. The longest fluorescent lifetime reaches 620 μs under 700 nm excitation. The valence state of Bi in these glasses is suggested to be lower than +3 by X-ray photoelectron spectroscopy. With the help of energy matching, we infer that both Bi⁰ and Bi⁺ centers are responsible for the NIR fluorescence of Bi₂O₃ doped 65SiO₂–20Al₂O₃–15La₂O₃ glass.     

Upconversion luminescence dynamics of Er-doped fluoride crystals for optical converters

Erbium (Er)-doped fluoride crystals (YLF, BYF, CaF₂, etc.) are well-known as active media for solid-state lasers emitting in IR and VIS spectral domains, and as materials for efficient near-IR to VIS upconversion. In this paper, we report on the study of conversion of IR light from an ～1.5 μm spectral region to an ～1 μm spectral domain in low-phonon RE-doped fluoride crystals CaF₂ (RE=Er³⁺ Yb³⁺). Energy transfer processes taking place at selective pulsed and CW laser excitation are investigated experimentally. It is shown that in the CaF₂:RE crystals efficient conversion of IR radiation from the ～1.5 μm region to the ～1 μm region occurs, and these crystals are perspective for using in spectral converters for enhancing solar cell efficiency.     

Broadband 2.84 μm luminescence properties and Judd–Ofelt analysis in Dy3+ doped ZrF4–BaF2–LaF3–AlF3–YF3 glass

2.84 μm luminescence with a bandwidth of 213 nm is obtained in Dy³⁺ doped (ZrF₄–BaF₂–LaF₃–AlF₃–YF₃) ZBLAY glass. Three intensity parameters and radiative properties have been determined from the absorption spectrum based on the Judd–Ofelt theory. The 2.84 μm emission characteristics and energy transfer mechanism upon excitation of a conventional 808 nm laser diode are investigated. The prepared Dy³⁺ doped ZBLAY glass possessing high predicted spontaneous transition probability (45.92 s^?1) along with large calculated emission cross section (1.17×10^?20 cm²) has potential applications in 2.8 μm laser.     

High lithium ion conducting glass-ceramics in the system Li2S–P2S5

Highly ion-conductive Li₂S–P₂S₅ glass-ceramic electrolytes were prepared by controlling the compositions and heat treatment temperatures of the glasses. The 70Li₂S·30P₂S₅ (mol%) glass-ceramic heated at 360 °C showed the highest conductivity of 3.2 × 10^− 3 S cm^− 1 at room temperature and the lowest activation energy of 12 kJ mol^− 1 for conduction in the binary system Li₂S–P₂S₅. The outstanding property was attributed to both the precipitation of the new crystal as a metastable phase and the increase in crystallinity of the phase. With increasing heat treatment temperatures, the metastable phase changed into thermodynamically stable phases such as the Li₄P₂S₆ crystal by heat treatment up to 550 °C, resulting in low conductivities of the glass-ceramics. It was, thus, found that the formation of superionic metastable phases by heating the Li₂S–P₂S₅ glasses is responsible for the marked enhancement on the conducting properties of the glass-ceramics.     

Ionic conductivities of Nasicon-type glass-ceramic superionic conductors in the system Na2O–Y2O3–XO2–SiO2 (X = Ti,Ge, Te)

Glass-ceramics of the titanium-, germanium- or tellurium-containing Na₅RSi₄O₁₂-type (R = rare earth; Y) Na⁺-superionic conductors (N5YXS) were prepared by crystallization of glasses with the composition Na_3+3xY_1 ? xX_ySi_3 ? yO₉ (X = Ti; NYTiS, Ge; NYGeS, X = Te; NYTeS), and the effects of X elements on the separation of the phase and the microstructural effects on the conduction properties of glass-ceramics were discussed. The combination of x and y was most varied in N5YGeS and more limited in the order of N5YTeS > N5YTiS. Their conductivities and activation energies are of the order of 10^? 2 S/cm at 300 °C and of 15 to 24 kJ/mol, respectively. The conductivity of the glass-ceramic N5YXS decreases giving the order N5YGeS > N5YTeS > N5YTiS. It is considered that this order corresponds to the N5 single phase region. Large enhancement of electrical conductivity was observed in the glass-ceramics as the grain growth was promoted with increase of heating temperature and heating time for crystallization.     

Luminescence behavior of Tb ions in transparent glass and glass-ceramics containing CaF2 nanocrystals

Transparent glass-ceramics have been fabricated by heat treatment with 1.0 mol% Tb₂O₃-doped 45SiO₂-20Al₂O₃-10CaO-25CaF₂ aluminosilicate glass. The precipitated crystalline phase in the glass-ceramics is CaF₂ nanocrystals with a size of 15-27 nm and dispersed in the amorphous phase. Both photo- and radioluminescence measurements show that the increase of Tb³⁺ 545 nm line intensity by a factor of 4 and 3.5 is obtained in glass-ceramics with respect to the as-made glass, respectively.     

Self-limited growth of the CaF nanowire on the Si(5 5 12)-2 × 1 template

The atomic structure and interfacial bonding of the ordered-and-isolated CaF nanowires on Si(5 5 12)-2 × 1 have been disclosed by scanning tunneling microscopy and synchrotron photoemission spectroscopy. Initially, CaF molecules dissociated from thermally deposited CaF₂ molecules are adsorbed preferentially on the chain structures of Si(5 5 12)-2 × 1 held at 500 °C. With increasing CaF₂ deposition amount, one-dimensional (1D) CaF nanowires composed of (113) and (111) facets are formed. The line density of these CaF nanowires increases as a function of deposition amount. Finally, at a submonolayer coverage, the surface is saturated with these 1D nanowires except for the (225) subunit, while the original period of Si(5 5 12)-2 × 1, 5.35 nm, is preserved. It has been deduced by the present studies that, owing to these preferential adsorption of CaF and facet-dependent growth of a CaF layer within a unit periodic length of Si(5 5 12)-2 × 1, such a self-limited growth of the CaF nanowire with a high aspect ratio becomes possible.     

Type B epitaxy of Ge on CaF2(111) surface

It was known experimentally that type B orientation, which is rotated 180° about the [111] axis, dominated the heteroepitaxial growth of Ge(111) on a CaF₂(111) substrate at an elevated temperature. We performed first principles calculations using density functional theory to determine the energetics of the Ge(111)/CaF₂(111) interface and found that the type B orientation of the Ge film is most likely a result of a direct bonding between Ge atoms and Ca²⁺ at the CaF₂ surface with the top F^? layer depleted. Our theoretical prediction is supported by our X-ray diffraction experiments on {111} < 121> biaxially textured Ge/CaF₂ samples.     

High lithium ion conductivity glass-ceramics in Li2O–Al2O3–TiO2–P2O5 from nanoscaled glassy powders by mechanical milling

Mechanical milling (MM) has been used to prepare the nanosized Li_1.4Al_0.4Ti_1.6(PO₄)₃ (denoted LATP) glassy powders, which was converted into glass-ceramics through thermal treating at 700–1000 °C. The XRD, TEM, FESEM and ac impedance techniques were used to characterize the products. The results showed that completely amorphous products were prepared by MM for 40 h, and single-phase LiTi₂(PO₄)₃-type structured glass-ceramics were obtained by further heat treatment. The lithium ion conductivity of the glass-ceramics increased with the growth of the crystalline phase and decrease of the grain size. The highest bulk conductivity (σ_b) of 1.09 × 10^− 3 S cm^− 1 with an energy of activation as low as 0.28 eV was obtained at room temperature for the specimen treated at 900 °C for 6 h. The high conductivity, easy fabrication and low cost make the LATP glass-ceramics promising to be used as inorganic solid electrolyte for all-solid-state Li-ion rechargeable batteries.     

Characterization of luminescent chromophore obtained from silica spheres

Blue light emitting chromophores have been separated from silica spheres by soaking them into acetone for 120 days. The luminescent chromophores were not obtained from other solvents, including ether, methanol, ethanol, 2-propanol, chloroform and tetrahydrofuran. According to the Fourier transform infrared spectrum, the luminescent material is composed of C–OH, –CH₂, –CH₃, C=O, and Si–O–Si. UV–visible absorption peak of the chromophore is at 5.17 eV (240 nm). Field emission scanning electron microscope images show small cracks on the surface of aged spheres. The luminescence peak was at 2.81 eV (441 nm) for excitation energy between 3.88 and 3.35 eV and slightly shifted toward lower energy for excitation energy lower than 3.35 eV. The deconvoluted luminescent spectrum shows two emission bands at 3.08 and 2.74 eV, which are well-matched the oxygen deficient center model. Compared to the absorption peak (5.17 eV) and the emission peak (2.81 eV), large Stokes shift (2.36 eV) is observed.     

Entropy-driven adsorption of carbon nanotubes on (0 0 1) and (1 1 1) surfaces of CeO2 islands grown on sapphire substrate

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO₂ surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO₂ islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO₂ surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO₂ and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO₂ islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO₂ surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.     

Er–Pr doped tellurite glass nanocomposites for white light emitting diodes

In this paper, optical glass nanocomposites (nanoparticles sizes up to 100 nm) with composition TeO₂–WO₃–PbO–xEr₂O₃–yPr₆O₁₁ (x = 0.30 mol%, y = 0.70 mol%) embedded into polymer matrices was reported. The two types of polymers chosen for present study were: photopolymer oligoetheracryalte (OEA) and polymethylmethacrylate (PMMA), respectively. The incorporation of the titled nanoparticles into the polymer matrices is analyzed optically. The fluorescence spectra of the nanocomposites were compared with the fluorescence spectra of bulk glasses. Based on the comparison of Er^3 + and Pr^3 + ions' energy level schemes, possible energy transfer processes were identified. The prepared glasses are promising candidates for the white light emitting diodes applications.