Nanocrystalline Phosphors Research Articles

Spectral and surface investigations of Mn2+ doped SrZnO2 nanocrystalline phosphors

A yellow light-emitting SrZnO2:Mn2+ nanophosphor was synthesized through a combustion process using metal nitrates as precursors and urea as fuel. The nanocrystals have sizes ranging from 25 to 30 nm. X-ray diffraction, transmission electron microscopy, photoluminescence (PL) and cathodoluminescence spectra were utilized to characterize the present samples. Crystallographically the occupancy of cations in SrZnO2 has been explained. Under the UV and the electron-beam excitation, the SrZnO2:Mn2+ nanoparticle shows a yellow emission due to the characteristic transition of the Mn2+ (4T1 → 6A1). To study oxygen-related defects and the surface chemical composition, the X-ray photoelectron spectroscopy technique was used. Surface chemical study suggests that oxygen vacancies may be generated by surface shrinkage due to the size reduction. For post-combustion annealing at 900 °C, the PL intensity of the nanopowders was increased due to better crystallinity and improved activity of the recombination centers.

Synthesis and luminescent properties of novel red-emitting CaGd4O7: Eu3+ nanocrystalline phosphors

We reported the novel red-emitting phosphors of trivalent europium (Eu3+) ions activated CaGd4O7 (CG) prepared by a sol–gel method. After the samples were annealed at 1500°C, the X-ray diffraction patterns confirmed their monoclinic structure and the transmission electron microscopy image showed the nanocrystallinity of the CG: Eu3+ phosphor. The photoluminescence excitation spectra exhibited the efficient energy transfer from the Gd3+ to Eu3+ by overlapping the f–f transition of Gd3+ excitation band with charge transfer band of Eu3+ ions. The photoluminescence (PL) spectra showed the emission band in the entire visible region at lower Eu3+ concentration and the red emission at higher Eu3+ concentration. All the decay curves were well fitted by single exponential functions. The temperature dependent luminescence measurements were performed for identifying the thermal stability of CG: Eu3+ phosphors at elevated temperatures. The cathodoluminescent (CL) spectra also presented a similar behavior to that observed in PL spectra. Form the PL, CL, and decay analysis the optimum concentration of Eu3+ ions in the CG host lattice was found to be 5mol%. The CIE chromaticity coordinates varied from yellow to red in visible spectral region by increasing the Eu3+ concentration.

Photoluminescence properties of gadolinium oxide nanophosphor

In this paper, we report low temperature solution combustion synthesis and characterization of rare earth oxide (Gd2O3) nanocrystalline phosphor. During the synthesis, glycine was used as a fuel. The final product was examined by well characterized techniques such as powder X-ray diffraction pattern (PXRD), UV-Visible absorption spectroscopy, photoluminescence (PL) measurements etc. The average crystallite size of the Gd2O3 nanophosphor was estimated using Scherrer's formula and found to be 36 nm. The UV-Visible absorption peaks observed at ~243 and ~305 nm. Further the band gap of 900 °C for 3 h calcined Gd2O3 nanophosphor were estimated by Wood and Tauc relation and it was found to be ~5.4 eV. A broad PL emission band at ~380 nm was observed at 230 nm excitation. According to Wang's proposal, this emission band may be attributed to recombination of a delocalized electron close to the conduction band with a single charged state of surface oxygen vacancy.

Controlled synthesis of Eu3+-doped La2O2S nanophosphors by refluxing method

A novel refluxing method is presented to synthesise Eu3+-doped La2O2S nanocrystalline phosphors by employing lanthanum formate, europium formate and sulphur powder as reactants and triethylene tetramine with n-dodecanethiol as solvents. The sizes and morphologies of La2O2S : Eu3+ phosphors could be controlled from nanoflakes (10 ± 2 nm in thickness), micrometre vesicles (1 µm in diameter), to spherical nanocrystals (average 100 nm in diameter) by simply adjusting the solvent ratios of n-dodecanethiol to triethylene tetramine. The photoluminescence excitation and emission spectra were used to characterise the as-synthesised nanocrystalline phosphors. The results show that Eu3+ occupies C3V site in Eu3+-doped La2O2S nanocrystals, which is the same as in bulky La2O2S : Eu3+ crystals. The fluorescent decay curves for 5D0 7F2 transition (624 nm) of Eu3+-doped La2O2S spherical nanocrystals show a long decay time (1.2 ms) and a short decay time (0.4 ms), which embodies the fluorescent decay feature of nanosized phosphors. The as-synthesised La2O2S : Eu3+ nanocrystals display strong red emission at 624 nm, suggesting potential applications in the fields of light displays, light emitting diodes and devices.

Citrate sol-gel combustion preparation and photoluminescence properties of YAG:Ce phosphors

Yellow-emitting YAG:Ce3+ nanocrystalline phosphors were prepared by citrate sol-gel combustion method using citric acid as the fuel and chelating agent. The influence of mole ratio of citric acid to metallic ions (MRCM), pH value of the solution, calcination temperature and Ce-doped concentration on the structures and properties of as-prepared powders were investigated in detail. Higher crystallinity and better luminescence performance powders were obtained at MRCM=2, pH=3 and the calcination temperature of 1200 °C. The phosphors exhibited the characteristic broadband visible luminescence of YAG:Ce. The optimum concentration of Ce3+ was 1.0 mol.%, and the concentration quenching was derived from the reciprocity between electric dipole and electric quadrupole (d-q). Especially, the pH value of the solution was a key factor to obtain a stable sol-gel system and then obtain pure and homogeneous rare earth ions doped YAG phosphors at a lower temperature. The Y3Al5O12:Ce0.03 phosphor with optimized synthesis-condition and composition had a similar luminescence intensity with the commercial phosphor YAG:Ce.

Molten salt synthesis and luminescent properties of YVO4:Eu nanocrystalline phosphors

YVO4:Eu nanocrystalline phosphors were successfully prepared at 400 °C in equal moles of NaNO3 and KNO3 molten salts. NaOH concentration and annealing temperature played important roles in phase purity and crystallinity of the nanocrystallines, and the optimum NaOH concentration and annealing temperature were 6:40 and 400 °C, respectively. The nanocrystallines were well crystallized with a cubic morphology in an average grain size of 18 nm. Upon excitation of the vanadate groups at 314 nm, YVO4:Eu nanocrystallines exhibited the characteristic emission of Eu3+, which indicated that there was an energy transfer from vanadate groups to Eu3+. Moreover, the influence of superficial effect, especially the dangling bonds on the structure and luminescent properties of the nanocrystallines was discussed in detail.

Nanocrystalline CaYAlO4:Tb3+/Eu3+ as promising phosphors for full-color field emission displays

Eu(3+) and/or Tb(3+)-doped CaYAlO(4) phosphor samples were synthesized by Pechini-type sol-gel method. X-Ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), photoluminescence (PL) and cathodoluminescence (CL) spectra were used to characterize the samples. For CaYAlO(4):Tb(3+), it is shown that the Tb(3+)-doping concentration has a significant effect on the (5)D(3)/(5)D(4) emission intensity of Tb(3+), which is attributed to the cross relaxation from (5)D(3) to (5)D(4). Under the 4f(8)→ 4f(7)5d excitation of Tb(3+) or low-voltage electron beams excitation, the CaYAlO(4):Tb(3+) phosphors show tunable luminescence from blue to cyan, and then to green with the change of Tb(3+)-doping concentration. The CaYAlO(4):Eu(3+) samples exhibit a reddish-orange emission of Eu(3+) corresponding to (5)D(0,1)→(7)F(0,1,2,3) transitions. Furthermore, a white emission can be realized in the single phase CaYAlO(4) host by reasonably adjusting the doping concentrations of Tb(3+) and Eu(3+) under low-voltage electron beams excitation. Compared with the commercial blue (Y(2)SiO(5):Ce(3+)) and green (ZnO:Zn) phosphors, CaYAlO(4):0.1%Tb(3+) and CaYAlO(4):5%Tb(3+) phosphors have higher CL intensity and stability under continuous electron bombardment. Due to the excellent CL properties and good CIE chromaticity coordinates, the as-prepared Tb(3+)/Eu(3+)-doped CaYAlO(4) nanocrystalline phosphors have potential application in FEDs devices.

Ion beam induced amorphization and bond breaking in Zn2SiO4:Eu3+ nanocrystalline phosphor

This paper reports on the ionoluminescence (IL) of Zn2SiO4:Eu3+ nanophosphors bombarded with 100MeV Si7+ ions with fluences in the range (3.91–21.48)×1012ionscm−2. The prominent IL emission peaks recorded at 580, 590, 612, 650 and 705nm are attributed to the luminescence centers activated by Eu3+ ions. It is observed that IL intensity decreases and saturates with increase of Si7+ ion fluence. Fourier transform infrared (FT-IR) studies confirm surface/bulk amorphization for a fluence of (3.91–21.48)×1013ionscm−2. These results show degradation of SiO (2ν3) bonds present on the surface of the sample and/or due to lattice disorder produced by dense electronic excitation under heavy ion irradiation. These results are discussed in detail.

Sol–gel synthesis, structural and optical characteristics of Sr1−x Zn2Si2yO7+δ: xEu2+ as a potential nanocrystalline phosphor for near-ultraviolet white light-emitting diodes

In this research, a new blue-emitting phosphor Eu2+-doped SrZn2Si2O7 was developed for white light-emitting diodes via the sol–gel process. Thermogravimetric-differential thermal analysis, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), and photoluminescence (PL) spectra were used to characterize the resulting phosphors. The obtained phosphor is efficiently excited in the wavelength range of 340–400 nm which matches to a near-UV-emitting InGaN chip and emits strong band blue light peaking at 481 nm because of 4f65d1(2D) → 4f7(8S7/2) transition of Eu2+ ions. The effects of the activator concentration and excess Si (y > 1) on the luminescence properties were evaluated. It was found that, when the Eu2+ content (x) and the Si concentration (2y) were 0.04 and 2.4, respectively, the optimum phosphor can be achieved. Also, the mechanism of concentration quenching was determined to be the dipole–dipole interaction using Dexter’s theory. Finally, the mean crystallite size of the products was estimated to be approximately 30 nm using Scherrer’s equation, which was confirmed by the TEM observations.

Thermoluminescence studies of solution combustion synthesized Y 2O 3:Nd 3+ nanophosphor

Thermoluminescence (TL) of neodymium doped yttrium oxide (Y 2O 3:Nd 3+) nanocrystalline phosphors, synthesized by solution combustion route, using disodium ethylene diamine tetra acetic acid (EDTA-Na 2) as fuel, was studied at low temperature (<350 °C). Powder X-ray diffraction (PXRD) pattern of Y 2O 3:Nd 3+ revealed the cubic crystalline phase and the average crystallites sizes were found to be in the range of 18–24 nm. The morphology of the samples was studied by scanning electron microscopy (SEM) and was foamy and fluffy in nature. Fourier transformed infrared spectroscopy (FTIR) revealed prominent absorption with peaks at 3400, 1435, 875 and 565 cm −1. Optical absorption studies showed that the energy gap of the synthesized sample was found to be 5 eV. Thermoluminescence of γ-irradiated Y 2O 3:Nd 3+ showed two well resolved TL glows with peaks at 587 and 628 K and they were analyzed by glow curve shape method and the activation energies were found to be 1.83 eV and 2.2 eV respectively.

Upconversion Based Tunable White-Light Generation in Ln:Y2O3 Nanocrystalline Phosphor (Ln = Tm/Er/Yb)

We report the generation of efficient white light based on upconversion (UC) in Tm(3+)/Er(3+)/Yb(3+):Y(2)O(3) nanocrystalline phosphor synthesized by simple and cost effective solution combustion technique on 976nm laser excitation. The calculated color coordinates (using 1931 CIE standard) for samples annealed at different temperatures vary from (0.16, 0.30) to (0.32, 0.33) with dopant concentration, annealing temperature and the pump power; thus providing a wide color tunability including the white one. White emission is observed even at a very low laser power (60mW). The maximum upconversion efficiency obtained for white emission is 2.79% with the color coordinates (0.30, 0.32) at laser power of 420mW which is quite close to the standard white color coordinates.

Optical temperature sensing behavior of enhanced green upconversion emissions from Er–Mo:Yb 2Ti 2O 7 nanophosphor

The Er–Mo:Yb 2Ti 2O 7 nanocrystalline phosphor has been prepared by sol–gel method and used as an optical thermometry. By Mo codoping, the green upconversion (UC) emission intensity increased about 250 times than that of Er:Yb 2Ti 2O 7 under a 976 nm laser diode excitation. It indicates that such green enhancement arises from the high excited state energy transfer (HESET) with the | 2F 7/2, 3T 2> state of Yb 3+–MoO 4 2− dimer to the 4F 7/2 level of Er 3+. The fluorescence intensity ratio (FIR) of the two green UC emissions bands was studied as a function of temperature in a range of 290–610 K, and the maximum sensitivity and the temperature resolution were approximately 0.0074 K −1 and 0.1 K, respectively. It suggests that the Er–Mo:Yb 2Ti 2O 7 nanophosphor with a higher green UC emissions efficiency is a promising prototype for applications in optical temperature sensing.

Interaction of biological and nonbiological surfaces. New opportunities for research and technologies using synchrotron radiation

The applicability of synchrotron radiation to implementation of medical ideas associated with introduction of nonbiological objects into a human body—implants, drug nanocapsules, and X-ray therapeutic means (metal nanoparticles and nanocrystalline phosphors and scintillators)—is considered. Synchrotron radiation presents new possibilities of analyzing the surface with a resolution comparable to the sizes of biological nanostructures involved in interactions such as chemical and biochemical modification of implant surfaces in vitro; activation of implant surface integration with biological tissue in a tissue culture (in vitro) and in vivo; biochemical modification of therapeutic nanoparticle surface in vitro; immunocamouflage by host proteins; attachment of “molecular targets”, i.e., antibodies, to target tissue to provide targeted delivery vehicles; and local activation of X-ray therapeutic drugs and drug nanocapsules in biological tissues. A functional block diagram of a medical technological station is given.

Luminescence enhancement of Y2O3:Eu3+ and Y2SiO5:Ce3+,Tb3+ core particles with SiO2 shells

This paper reports on the luminescence and microstructural features of oxide nano-crystalline (Y2O3:Eu3+) and submicron-sized (Y2SiO5:Ce3+,Tb3+) phosphor cores, produced by two different synthesis techniques, and subsequently coated by an inert shell of SiO2 using a sol–gel process. The shells mitigate the detrimental effect of the phosphor particle surfaces on the photoluminescence emission properties, thereby increasing luminous output by 20–90%, depending on the core composition and shell thickness. For Y2O3:Eu3+, uniformly shaped, narrow particle size distribution core/shell particles were successfully fabricated. The photoluminescence emission intensity of core nanoparticles increased with increasing Eu3+ activator concentration and the luminescence emission intensity of the core/shell particles was 20–50% higher than that of the core particles alone. For Y2SiO5:Ce3+,Tb3+, the core/shell particles showed enhancement of the luminescence emission intensity of 35–90% that of the core particles, depending on the SiO2 shell thickness.

Size dependent luminescence of nanocrystalline Y2O3:Eu and connection to temperature stimulus

In the emission spectra of nanocrystalline Y2O3:Eu with decreasing the particle size, the 5D0–7F0 transition of Eu3+ shifts towards blue. This size effect is explained by lattice expansion of the nanocrystallines, more precisely, by larger Eu–O distance. Meanwhile, increasing temperature also expands the lattice constant and shifts the emission peak to blue. Then nephelauxetic effect is employed to organize the size effect and temperature dependence. It is also introduced to indicate the change of Eu3+ coordination number in the nanosized phosphor. Based on the experimental results, possible applications of temperature sensing and stress mapping with Eu3+ doped nanocrystalline phosphors are proposed.

Luminescence characterization and electron beam induced chemical changes on the surface of ZnAl2O4:Mn nanocrystalline phosphor

Luminescence characteristics and surface chemical changes of nanocrystalline Mn2+ doped ZnAl2O4 powder phosphors are presented. Stable green cathodoluminescence (CL) or photoluminescence (PL) with a maximum at ∼512nm was observed when the powders were irradiated with a beam of high energy electrons or a monochromatic xenon lamp at room temperature. This green emission can be attributed to the 4T1→6A1 transitions of the Mn2+ ion. Deconvoluted CL spectra resulted in two additional emission peaks at 539 and 573nm that may be attributed to vibronic sideband and Mn4+ emission, respectively. The luminescence decay of the Mn2+ 512nm emission under 457nm excitation is single exponential with a lifetime of 5.20±0.11ms. Chemical changes on the surface of the ZnAl2O4:Mn2+ phosphor during prolonged electron beam exposure were monitored using Auger electron spectroscopy. The X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the possible compounds formed on the surface as a result of the prolonged electron beam exposure. The XPS data suggest that the thermodynamically stable Al2O3 layer was formed on the surface and is possibly contributing to the CL stability of ZnAl2O4:Mn phosphor.

Sol–gel synthesis, characterization and luminescence properties of SrMgAl2SiO7:Eu2+ as a novel nanocrystalline phosphor

In this research, a new SrMgAl(2) SiO(7):Eu(2+) phosphor was synthesized via the sol-gel method. The phase-forming processes were studied by thermogravimetric-differential thermal analysis and X-ray diffraction technique. Scanning electron microscopy showed that there is uniform morphology and microstructure owing to the sol-gel route. Spectrophotometry and colorimetry analyses illustrated that, under short ultraviolet excitation, the main emission peak occurred at 421 nm and also a relatively pure blue color was observed that was ascribed to the 4f(6) 5d(1) ((2) D) → 4f(7) ((8) S(7/2)) transition of Eu(2+) with color coordination of x = 0.187, y = 0.077. Finally, it was found that the color and phase purity of the synthesized powder increased as calcinations time increased.

Nanocrystalline Phosphors for Lighting and Detection Applications

We report the development of new nanoparticle phosphors and quantum dot structures designed for applications to enhance the color rendering and efficiency of high brightness white LEDs, as well as for bio-sensing applications. The intrinsic problem of self-absorption, high toxicity, and high sensitivity to thermal quenching of conventional quantum dot systems has prevented their adoption to LED devices. Doped Cd-free quantum dots may circumvent these issues due to their distinct Stokes shift and improved stability at high temperature. We report on the modification of Mn-doped ZnSe/ZnS core-shell quantum dots for application to the (blue diode + yellow emitter) white LED system. Band gap tuning for 460 nm excitation, inorganic shell growth and in-situ monitoring for enhanced efficiency, and analysis of thermal stability will are reported.

Magnetic-field-induced optical bistability in multifunctional Gd_2O_3:Er^3+∕Yb^3+ upconversion nanophosphor

The effect of an external magnetic field (0-1 T) on the upconversion emission (lambda(exc)=976 nm) of Gd(2)O(3):Er(3+)Yb(3+) nanocrystalline phosphor has been studied. Optical bistability (hysteresis behavior in the intensity of the optical emission) for different transitions of the Er(3+) ion has been observed for a complete cycle of the magnetic field between 0 and 1 T. The phosphor shows paramagnetic behavior, consistent with the presence of Gd(3+) ions, at room temperature. Interaction between induced magnetization in the Gd(2)O(3) host and the intrinsic magnetic moment of the nanosized clusters of Er(3+) and Yb(3+) ion pairs is proposed to be responsible for the hysteresis behavior.

ChemInform Abstract: Synthesis and Luminescent Properties of LaInO3: RE3+ (RE: Sm, Pr and Tb) Nanocrystalline Phosphors for Field Emission Displays.

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