Nanocrystalline Phosphors Research Articles

Crystal structure and photoluminescence investigations of Y3Al5O12:Dy3+ nanocrystalline phosphors for WLEDs

Dy3+-doped Y3Al5O12 (YAG) nanophosphors have been effectively synthesized via facile solution technique at 600 °C using urea as a fuel and calcined at 950 and 1050 °C for further analysis. Powder X-ray diffraction (PXRD), Fourier transformation infrared (FTIR), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and photoluminescence (PL) studies were employed to confirm the synthesis, structural and luminescence properties of the phosphors. PXRD patterns of Y3Al5O12 and Y3Al5O12:Dy3+ corresponds to JCPDS card No. 73–1370 having cubic unit cell and Ia3̅d space group, which were also confirmed by Rietveld refinement analysis. TEM images display the homogeneously disseminated nanocrystals with smooth morphology and particle size in 50–85 nm range. Energy dispersive X-Ray analysis (EDAX) proposed the pure synthesis of materials with presence of only elements integrated within host lattice. The photoluminescence emission (PL) spectra of YAG:Dy3+ nanophosphors exhibit strong emission owing to 4F9/2→6F15/2 (480 nm) and 4F9/2→6F13/2 (577 nm) under 352 nm excitation. Critical distance confirmed that the energy transfer occurs via multipolar interaction mechanism. CIE chromaticity diagram displays the intense white light emission for the phosphors. These investigations suggest the potential applications of YAG:Dy3+ nanocrystals in white light emitting diodes (WLEDs).

Structural, optical and morphological features of combustion derived Ba3Y4O9: Dy3+ nanocrystalline phosphor with white light emission

A simple and highly efficient route of solution combustion process was utilized for preparing a series of white light emanating Ba3Y4(1-x)O9: 4xDy3+ (x = 0, 1.0, 3.0, 5.0, 10, 15 mol%) nanocrystalline phosphors. The resulted product was claimed to belonging to R3 space group (SG) with trigonal crystal system related to hexagonal crystal family utilizing powder X-ray diffraction (PXRD) technique and Rietveld refinement. The photoluminescence emission spectra comprised of two intense bands; one was yellow band (Y) in consequence of 4F9/2 → 6H13/2 transition positioned at 578 nm and another one was blue band owing to 4F9/2 → 6H15/2 transition sited at 489 nm. Ba3Y3.88Dy0.12O9 composition was considered as optimal composition for white light emission amongst the prepared nanophosphors owing to its maximal intense emission. The morphological aspects and elemental composition of the optimized sample were analyzed with the help of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDAX) studies. CIE (International Commission on Illumination) chromaticity coordinates confirmed the white light emission by all the synthesized nanophosphors that validate their potential applications in white light emitting diodes (WLEDs). Correlated color temperature (CCT) analysis substantiates their applications as cool white light source.

Development of dumbbell-shaped La2Si2O7:Eu3+ nanocrystalline phosphors for solid-state lighting applications

There are ongoing efforts to develop novel phosphors with the properties required to make an object look as natural as it does under sunlight. It has been proven that particle morphology influences the luminescent properties of a phosphor. The rare-earth pyrosilicates, such as La2Si2O7 (LSO) are currently regarded as novel promising scintillators and protective pigments because of their good chemical and thermal stabilities. However, to date, there have been no reports available on the definite morphology of the LSO matrix and the precise luminescent properties of the trivalent europium (Eu3+)-doped LSO phosphor. Herein, we report the development of Eu3+-activated dumbbell-shaped LSO nanocrystalline phosphors using a two-step solvothermal and hydrothermal approach. After regrowth, the amorphous natured LSO:Eu3+ precursor was transformed into well-defined LSO:Eu3+ dumbbell-shaped nanocrystalline phosphors (LSO-R:Eu3+) with a pure monoclinic phase. The phase purity was verified by Rietveld refinement analysis. The formation mechanism of the dumbbell-shaped particles was evaluated using surface morphological studies, and the involvement of the crystal splitting mechanism was established. The luminescence spectra revealed robust red emission at 614 nm under ultraviolet (UV) and near-UV excitations. The dipole-dipole interactions between the neighboring Eu3+ ions induced the concentration quenching in LSO host matrix. Chromaticity coordinates (0.645, 0.342) emerged in the reddish-orange region and were comparable to the standard-definition and high-definition digital television coordinates (0.640, 0.330). Therefore, these results indicate that the synthesized LSO-R:Eu3+ dumbbell-shaped nanocrystalline phosphor is a promising auxiliary red component for assembling desired white light-emitting diodes for indoor illuminations.

Synthesis and photoluminescence behavior of of SrMg2Al16O27:Eu2+ nanocrystalline phosphor

SrMg2Al16O27:Eu2+ nanocrystalline phosphor has been synthesized via rapid gel-combustion method at 600 °C in muffle furnace and detailed photoluminescence behavior is reported. The phase purity and structural properties have been examined via Powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and Fourier transformation infrared (FTIR) spectroscopy. PXRD measurements showed that prepared samples are in single phase having orthorhombic structure. TEM images demonstrate the non-agglomerated spherical particles of 40−60 nm size. The photoluminescence excitation spectrum shows a characteristic broad band at 338 nm and emission band maxima corrosponding to 4f65d1→4f 7electronic transition appears at 453 nm for SrMg2Al16O27:Eu2+ phosphor.

Broad-Band Excited and Tunable Luminescence of CaTbAl3O7:RE3+ (RE3+ = Ce3+ and/or Eu3+) Nanocrystalline Phosphors for Near-UV WLEDs.

Here, we demonstrate a novel donor-intermediate-receptor energy transfer model through a Ce3+ → Tb3+ → Eu3+ scheme in a CaTbAl3O7:Ce3+,Eu3+ nanocrystalline phosphor. A new type of CaTbAl3O7 and CaTbAl3O7:RE3+ (RE3+ = Ce3+ and/or Eu3+) nanocrystalline phosphors were prepared by a simple sol-gel method. There exist efficient energy transfers of Ce3+ → Tb3+, Tb3+ → Eu3+, and Ce3+ → Tb3+ → Eu3+ in CaTbAl3O7:RE 3+ (RE 3+ = Ce3+ and/or Eu3+) nanocrystalline phosphors. With near-UV or UV light excitation, the as-prepared CaTbAl3O7:RE 3+ (RE 3+ = Ce3+ and/or Eu3+) nanocrystalline phosphors' luminous color can be regulated from green to green-yellow, yellow, orange, and orange-red by adjusting the doping concentration, categories, and different proportions of codoping Ce3+ to Eu3+ ions in the CaTbAl3O7 matrix. The luminescence mechanism with respect to the CaTbAl3O7:RE3+ (RE3+ = Ce3+ and/or Eu3+) nanocrystalline phosphors has been tentatively proposed. Due to their excellent luminescence properties, the as-prepared CaTbAl3O7, CaTbAl3O7:Ce3+, CaTbAl3O7:Eu3+, and CaTbAl3O7:Ce3+,Eu3+ nanocrystalline phosphors exhibit bright prospects in NUV-LEDs and other photoelectric field.

Implementing Defects for Ratiometric Luminescence Thermometry.

In luminescence thermometry enabling temperature reading at a distance, an important challenge is to propose new solutions that open measuring and material possibilities. Responding to these needs, in the nanocrystalline phosphors of yttrium oxide Y2O3 and lutetium oxide Lu2O3, temperature-dependent emission of trivalent terbium Tb3+ dopant ions was recorded at the excitation wavelength 266 nm. The signal of intensity decreasing with temperature was monitored in the range corresponding to the 5D4 → 7F6 emission band. On the other hand, defect emission intensity obtained upon 543 nm excitation increases significantly at elevated temperatures. The opposite thermal monotonicity of these two signals in the same spectral range enabled development of the single band ratiometric luminescent thermometer of as high a relative sensitivity as 4.92%/°C and 2%/°C for Y2O3:Tb3+ and Lu2O3:Tb3+ nanocrystals, respectively. This study presents the first report on luminescent thermometry using defect emission in inorganic phosphors.

A new potential green-emitting erbium-activated α-Na3Y(VO4)2 nanocrystals for UV-excitable single-phase pc-WLED applications

A potential UV-excitable green-emitting erbium-doped α-Na3Y(VO4)2 nanocrystalline phosphor has been successfully synthesized via facile citrate-based solution combustion method. The structural characterization of the sample was carried out using X-ray diffraction, TEM, HRTEM, SAED, EDS and FTIR analysis. The phosphor exhibits green emission at 523 nm and 552 nm, which correspond to 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, respectively. The energy transfer mechanism among Er3+ ions and the concentration mechanism have been studied. The optimal doping concentration of Er3+ in the α-Na3Y(VO4)2 is found to be 0.025 mol. The non-radiative interaction among Er3+ is attributed to the dipole–quadrupole interaction. The quantum yield is found to be 7.25%, and chromaticity coordinates for the optimal concentration are found to be (0.2571, 0.6707) which are close to National Television System Committee standards for green (0.21, 0.71). The lifetime of erbium emission is found to be in microsecond range. The photoluminescence studies reveal that the newly synthesized Er3+-doped α-Na3Y(VO4)2 nanocrystals would be a good choice as green emitter to be combined with previously investigated blue- and red-emitting Tm3+- and Sm3+-doped α-Na3Y(VO4)2 nanocrystals for realizing RGB phosphor.

Facile combustion synthesis of Sm3+ activated orange-red light emanating Sr6Y2Al4O15 nanophosphor for photonic applications

A cost effective, environmentally benign and most convenient solution combustion scheme was utilized for preparing a series of Sr6Y2-2xSm2xAl4O15 (x = 0.5–10 mol%) crystalline nanophosphors. The morphological study and phase formation were examined via SEM (Scanning Electron Microscope) image, TEM (Transmission Electron Microscope) micrograph and measured PXRD (powder X-ray diffraction) patterns. The crystal structure information is gathered with the help of Rietveld refinement technique. Photoluminescence excitation-emission spectra revealed about the photometric characteristics of the synthesized down-conversion nanophosphors. The diffuse reflectance spectral measurements yield optical energy band gap of 5.96 eV and 5.88 eV for the host (Sr6Y2Al4O15) and optimized matrix (Sr6Y1.94Sm0.06Al4O15) respectively. The estimated CIE (international commission on illumination) chromaticity coordinates values were revealed to be located in orange-red section specifying their importance for solid state illumination. The optimum concentration of trivalent samarium ion as a dopant ion for this particular kind of matrix was observed to be 3.0 mol%, since, beyond this concentration, a fall in photoluminescence intensity occurred due to concentration quenching trend. The prominent feature of orange-red light emission by the prepared nanocrystalline phosphors upon excitation via near ultraviolet (NUV) light makes them potential contender for red emitting luminescent optical appliances.

Joint extended X-ray absorption fine structure and luminescence study of bulk and nanocrystalline Eu3+ and Bi3+ co-doped YVO4 phosphor

X-ray absorption fine structure measurements of all cation species and luminescence measurements were carried out in YVO4:Eu,Bi nanocrystalline (NC) and bulk phosphors, synthesized by liquid-phase synthesis and solid-state reaction, respectively. Structural disorders in the NC sample, especially for Bi3+ ions, were observed in the radial structure function (RSF) by comparing to the bulk sample. The peak height of the first shell in the RSF of NC becomes about half of that of the bulk, although no significant change in the first shell is found in the RSFs of Y, Eu and V ions, while the peaks beyond the first shell in the NC sample reveal significant decrease. The relative internal quantum efficiencies of NC normalized by bulk were evaluated as low as 35%, 25% and 55% in the VO, Bi3+ and Eu3+ absorption regions, respectively. These ratios correlate well with the disorders speculated by RSFs.

Synthesis and characterization of radio and thermoluminescence properties of Sm doped Gd2O3, Gd2O2S and Gd2O2SO4 nanocrystalline phosphors

In this study, nanocrystalline Gd2O3:Sm, Gd2O2S:Sm and Gd2O2SO4:Sm phosphors were successfully synthesized through the urea homogeneous precipitation method. Nanocrystalline Gd2O2S:Sm phosphors with two different concentrations of sulfur were prepared. In the first procedure, the weight of sulfur was five times more than Gd2O3 weight (was called sample Gd2O2S:Sm(1)) and in the second procedure, it was equal (was called sample Gd2O2S:Sm(2)). The nanocrystalline powders were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, radioluminescence and thermoluminescence. The XRD and FT-IR results confirmed that nanocrystalline Gd2O3:Sm, Gd2O2S:Sm and Gd2O2SO4:Sm powders had the cubic, hexagonal and orthorhombic structure in preparation temperatures of 620 °C and 900 °C. The SEM images showed that nanocrystalline Gd2O3:Sm, Gd2O2S:Sm and Gd2O2SO4:Sm powders had a semi-spherical shape with a uniform mean size of about 30 nm in diameter and confirmed the XRD results. The presence of all elements was confirmed in the synthesized samples using EDX spectroscopy. The results showed that the weight percent of sulfur during the preparation of Gd2O2S had an effect on the size, morphology and the luminescence intensity of nanocrystalline Gd2O2S powder. Under X-ray irradiation (40 kV and 40 mA), the emission spectra of nanocrystalline Gd2O3:Sm, Gd2O2S:Sm and Gd2O2SO4:Sm phosphors were studied. The results showed the orange-red emission peaks which were observed at 605 nm were the strongest emission peak for all nanocrystalline Gd2O3:Sm, Gd2O2S:Sm and Gd2O2SO4:Sm powders. This peak was corresponded to the 4G5/2-7H7/2 energy transition of Sm3+ ion. The TL response of Gd2O3:Sm, Gd2O2S:Sm and Gd2O2SO4:Sm after irradiation with different X-ray time exposure exhibited a peak shifting towards the lower temperatures. The glow curve deconvolution method was used to calculate trapping parameters and the obtained results were investigated in detail.

Photometric features and typical white light emanation via combustion derived trivalent dysprosium doped ternary aluminate oxide based nanophosphor for WLEDs

Trivalent dysprosium doped ternary aluminate oxide based SrGd2Al2O7 nanocrystalline phosphors were synthesized via an efficient single step solution combustion process. The estimated value of optical band gap energy for the optimized nanophosphor and host matrix were found to be 5.58 eV and 5.76 eV respectively. Diverse crystalline features were elucidated via analyzing PXRD patterns and Rietveld refined results of host matrix and optimal SrGd1.94Dy0.06Al2O7 nanophosphor. Diverse elements present in the optimized nanophosphor were found to be Sr, Gd, Dy, Al and O as revealed via energy diffraction X-ray analysis (EDAX). The surface morphological features and the particle size of the optimum nanosample were disclosed by scanning electron microscope (SEM) and transmission electron microscope (TEM) practices. The characteristic emission spectra recorded using excitation wavelength of 350 nm display two strong peaks at 484 nm and 578 nm that were allotted to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 respective transitions. The evaluated critical distance value was ensured to be much higher than 5 Å indicating no chance of occurrence of concentration quenching (CQ) as a result of exchange interactions amongst dopant ions. Dipole-dipole interactions in the midst of adjoining Dy3+ ions were observed to be an appropriate cause for following CQ trend. Based on outcomes of Correlated Color Temperature (CCT) and International Commission on Illumination (CIE), it was quite obvious that the prepared nanomaterial will prove to be absolutely beneficial for applications related to lightning and display systems. Thus, it can be concluded that a new window opens for white light emitting diode (WLED) applications using the synthesized nanophosphors.

Reddish-orange light emission via combustion synthesized Ba3Y4O9: Sm3+ nanocrystalline phosphor upon near ultraviolet excitation

A highly efficient solution combustion technique involving simple procedure and less energy consumption was adopted to prepare a homogeneous crystalline Ba3Y4O9:Sm3+ down-conversion nanophosphor emitting bright reddish orange light. The structural features of the synthesized nanophosphors were characterized by powder X-ray diffraction (PXRD), scanning electron microscope (SEM), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM), selected area energy diffraction (SAED) and energy-dispersive X-ray analysis (EDAX) techniques. Moreover, the photoluminescent features of the nanocrystalline phosphors were described by photoluminescence decay curves, excitation and emission spectra. Upon near-ultraviolet (NUV) excitation (410 nm), the emission spectra demonstrate the most prominent peak at 607 nm due to 4G5/2 → 6H7/2 transition. The optimum mol% of dopant (Sm3+) ion for doping in this particular host matrix was found to be 3; thus, a concentration quenching (CQ) phenomenon was observed beyond x = 0.03 in the synthesized Ba3Y4(1-x)O9: 4x Sm3+ nanophosphors. The most appropriate reason for CQ mechanism was reckoned to be radiative-reabsorption phenomenon and dipole-dipole (d-d) interactions. The CIE chromaticity coordinates for the optimum nanophosphor were found to lie in orange-red region that confirm its characteristic light emission. The results indicate the promising applications of the synthesized reddish-orange light emitting Ba3Y4O9: Sm3+ nanophosphor in white light emitting diodes (WLEDs).

Crystal chemistry and optical analysis of a novel perovskite type SrLa2Al2O7:Sm3+ nanophosphor for white LEDs

Abstract Perovskite orange-red SrLa2(1-x)Al2O7:2xSm3+ (x = 0.01–0.07) nanocrystalline phosphor series were successfully synthesized by means of solution combustion route for the first time. X-ray diffraction (XRD) measurements were performed to analyze the structural framework and it was concluded that the resulted nanophosphors were well-crystallized in tetragonal lattice with the space group I4/mmm (139). The lattice constants and refinement factors for SrLa2Al2O7 and SrLa1.92Sm0.08Al2O7 compositions were obtained by performing Rietveld refinement technique. From transmission electron micrographs, the spherical or semi-spherical nature of highly agglomerated particles (20–30 nm range) was noticed. Further, the diffuse reflectance (DR) studies yielded the band-gaps for SrLa2Al2O7 (5.03 eV) and SrLa1.92Sm0.08Al2O7 (4.88 eV) powder samples. The photoluminescence investigation of nanophosphors revealed the strong orange-red luminescence at 600 nm (most prominent) attributed to 4G5/2 → 6H7/2 emission of Sm3+ ions under near-ultraviolet (NUV) excitation (λex = 407 nm). The optimum composition of Sm3+ in SrLa2Al2O7 lattice was found to be 4 mol%. However, the double exponential decay behaviour confirmed the simultaneous occupancy of two crystallographic sites by activator ions in the host matrix. In addition, the critical distance for non-radiative energy migration amid adjacent activator centres was calculated to be 15.07 A, using Blasse's relation. A detailed analysis of non-radiative relaxation rates (131.9531 s−1), radiative life time (1.79 ms) and quantum efficiencies (81%) was carried out using Auzel's modal. Finally, the results highlight the importance of SrLa2Al2O7:Sm3+ orange-red nanophosphors in white LEDs under NUV excitation.

Crystal structure, synthesis and photoluminescent properties of a reddish-orange light emitting SrGdAlO4: Sm3+ nanophosphor

A series of reddish orange light emitting SrGdAlO4: Sm3+ nanophosphor was prepared by urea aided solution combustion process. Structural features of the prepared nanophosphor were described by the powder X-ray diffraction (PXRD) together with Rietveld refinement technique. The results authenticate the existence of single phased tetragonal lattice with space group of I4/mmm (139). The highly intense peak observed in the photoluminescence emission spectra of the synthesized nanocrystalline phosphors was observed at 600 nm owing to 4G5/2 → 6H7/2 transition upon excitation by 407 nm light. The estimated energy band gap of SrGd0.97Sm0.03AlO4 nanophosphor was found to be 5.45 eV. The critical distance value evaluated for non-radiative energy transfer comes around 17.52 Å. The main reason responsible for the observed concentration quenching phenomenon in emission intensity of the prepared nanophosphor beyond 3 mol % doping of Sm3+ was found to be dipole-dipole (d-d) type electric multipolar interaction. The refractive index of the optimized nanophosphor (SrGd0.97Sm0.03AlO4) comes around 1.682. The chromaticity coordinates were found to lie in the reddish-orange region of CIE (Commission Internationale De I'Eclairage) diagram. The vivid description of the photoluminescent and structural aspects validate the potential applications of synthesized nanophosphor as optical nanomaterials in phosphor converted white light emitting diodes (pc-WLEDs).

Host‐Sensitized Continuum‐Broad‐Band White‐Light Emitting Yb2O3:Er3+ Phosphor under CW‐NIR Light Absorption

AbstractSingle cubic phase Yb2O3:Er3+ phosphors have been synthesized via solid state reaction technique and characterized by X‐ray diffraction (XRD) and Field emission scanning electron microscopy (FE‐SEM) analysis. An unusual continuum broad band white colour upconversion (UC) emission has been observed under 980 nm continuous wave (CW) near infrared (NIR) laser diode excitation. Interestingly, the phosphor shows characteristic erbium ion transitions at low pump power, but at high pump power heat associated continuum broad band emission was realized. The continuum broad band emission was explained on the basis of energy level diagram, laser induced pump power dependence and blackbody radiation theory. The heat associated/incandescence‐like light is originated due to heating induced by the nonradiative relaxation followed by sensitization (Yb → Er) and Yb−Yb dimer formation. The synthesized phosphor emits effective broad band white colour emission under high pump power and colour co‐ordinates (white colour: 0.34, 0.32) were located in the chromaticity diagram. Intrinsic optical bistability (IOB) behavior shows the hysteresis‐like loop originated upon variation in the laser pump power. The observed high temperature at point of irradiation of the prepared nanocrystalline phosphor is about ∼ 2442 K at 1430 mW, which is high as compare to tungsten bulb (2400 K for 100 W). The developed phosphors can be useful in white light production, solar cell efficiency enhancement, switchable bistable and as light to heat converter.

Thermoluminescence studies of CaSO4:Dy nanophosphor for application in high dose measurements

Nanorods of CaSO4:Dy (having diameter ∼20 nm and length ∼200 nm) were prepared by chemical coprecipitation method and their thermoluminescence (TL) characteristics were studied after annealing at different temperatures in the range 400–1000 °C. Microcrystalline material was also prepared through acid recrystallization method. TL glow curves of the nanocrystalline material annealed at 700 °C showed a major peak at around 283 °C. The effect of annealing temperature on the phase structure/morphology of particles was studied. Phase change was observed when CaSO4:Dy was annealed at different temperatures as confirmed by XRD and TG-DTA. It was observed from the TEM images that the nanomaterial originally in the nanorods form broke into nanoparticles due to strain developed by phase change on annealing at higher temperatures. The combined effect resulted in changes in TL glow curve structure. For better clarity the TL glow curves were further deconvoluted by Computerized Glow Curve Deconvolution (CGCD) method and kinetic trapping parameters were determined. It was found that the values of kinetic parameters also changed for the nanocrystalline material annealed at different temperatures. These were also compared with the corresponding values of the microcrystalline material. It was also observed that the TL intensity saturates at about 100 Gy in case of microcrystalline phosphor, while that in case of nanocrystalline phosphor did not do saturate even up to 5 kGy.

Up-Conversion Photoluminescence of Sol–Gel Derived CaY2O4 Powders Under 980 nm Excitation

Up-conversion is an anti-stokes process that can convert near-infrared light into visible light. Besides the requirement of high optical conversion efficiency, the thermochemical stability of the host materials is critical for in practical applications, and a stable oxide host material is optimal. CaY₂O₄ follows the same ordered structure of CaFe₂O₄, which is composed of an (R₂O₄)2- (R = rare earth metal) framework of double octahedral moieties with rare earth ions residing within the framework. CaY₂O₄ is a promising host material due to its favorable characteristics such as high chemical and thermal stability, low-phonon energy, and environment friendliness. Rare earth ion (Er3+ and Yb3+)-doped nano-crystalline phosphors were prepared by a sol-gel process. The synthesized sample was characterized by X-ray diffraction analysis and the crystallite size was confirmed by Scherrer's formula and transmission electron microscopy. The surface morphology of the powders was determined by scanning electron microscopy. X-ray diffraction patterns confirmed their pure orthorhombic structure after annealing at 1,200 °C and the morphology of particles was found to be a nearly spherical shape with a diameter of the order of ~100 nm. Photoluminescence properties of the powders were measured by exciting the samples with a 980 nm diode laser at room temperature. Under the 980-nm laser excitation, the green and red up-conversion emissions were observed at around 520- 540 nm, 540-570 nm and 640-680 nm, which, are attributed to the transitions of ²H11/2 →⁴I15/2, ⁴S3/2 →⁴I15/2 and ⁴F9/2 →⁴I15/2 of Er3+ ions, respectively. The up-conversion intensity as a function of laser power shows that the up-conversion mechanism corresponding to green and red emissions occurs via a two-photon process.

Near-ultraviolet excited down-conversion Sm3+-doped Ba5Zn4Gd8O21 reddish-orange emitting nano-diametric rods for white LEDs

Down-conversion Ba5Zn4Gd8O21:Sm3+ reddish-orange nanocrystalline phosphors with high quantum efficiency were well-synthesized by solution combustion route. The structural and morphological characterizations were systematically done via X-ray diffractometry (XRD) and transmission electron microscopy (TEM). Refined atomic positions as well as unit cell parameters for optimum composition were calculated using refinement technique. The results revealed that the substitution of Gd3+ ions by Sm3+ ions in Ba5Zn4Gd7.76Sm0.24O21 nanophosphor did not induce any major structural change and confirmed its tetragonal framework with I4/m (87) space group. TEM studies indicated the rod-shaped morphology with uniform samarium ion distribution of combustion derived nanophosphor ranging from 40 to 90 nm. The energy band-gap values for Ba5Zn4Gd8O21 (5.1 eV) and Ba5Zn4Gd7.76Sm0.24O21 (4.88 eV) were estimated from diffuse reflectance (DR) measurements. Further, the photoluminescence studies upon 410 nm excitation yielded the reddish-orange luminescence (at 610 nm) attributed to 4G5/2→6H7/2 transition of Sm3+ ions. The luminescence intensity of samarium-activated phosphor reached the maximum at the Sm3+ content of 3 mol%, for which the critical separation (RSm-RSm) was ≈ 16.5 Å. In addition, the energy relaxation process in Ba5Zn4Gd8O21:Sm3+ was clearly observed, which was confirmed via electric dipole-quadrupole interaction pathway. Besides, the radiative lifetime (1.19 ms), non-radiative relaxation rate (544.31 s−1) and most importantly, the quantum efficiency of 4G5/2 electronic state (60%) were also analyzed in detail. The obtained CIE chromaticity coordinates favour the applications of samarium activated Ba5Zn4Gd8O21 phosphor in near-ultraviolet (NUV) excited tricolor phosphor based white light-emitting diodes (WLEDs).

Carbon quantum dot-sensitized and tunable luminescence of Ca19Mg2(PO4)14:Ln3+ (Ln3+ = Eu3+ and/or Tb3+) nanocrystalline phosphors with abundant colors via a sol–gel process

Efficient carbon quantum dot-sensitized luminescence of Ca19Mg2(PO4)14:Ln3+ (Ln3+ = Eu3+ and/or Tb3+) nanocrystalline phosphors with abundant colors.

Effect of annealing and excitation wavelength on the downconversion photoluminescence of Sm3+ doped Y2O3 nano-crystalline phosphor

This paper reports the downconversion photoluminescence in Sm3+ doped Y2O3 nano-crystalline phosphor synthesized through solution combustion method. The structural characterization of the phosphor has been carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, which reveals its nano-crystalline nature. The particles size of the phosphor increases on annealing it, which has been confirmed by SEM measurement. The energy dispersive spectroscopic (EDS) measurement verifies the presence of Y, Sm and O elements in the phosphor. The Fourier transform infrared (FTIR) measurements show the presence of vibrational bands due to different groups in the phosphor. The photoluminescence excitation spectra of the phosphor show large number of excitation bands due to CTS and 4f-4f electronic transitions of Sm3+ ion. The Sm3+ doped Y2O3 phosphor emits an intense reddish orange color centered at 606 nm due to 4G5/2 → 6H7/2 transition upon excitation with different wavelengths such as 238, 363, 407, 424 and 464 nm. The photoluminescence intensity is observed larger for 407 nm excitation. Interestingly, the peaks observed in the emission match well with those present in the excitation upon 238 and 363 nm excitations in lower wavelength side. The photoluminescence intensity of the phosphor sample is enhanced upto three times after annealing the as-synthesized phosphor. The improvement in the photoluminescence intensity is due to an increase in crystallinity, particles size and reduction in optical quenching centers. The lifetime of the 4G5/2 level is found to be increased in the annealed phosphor. Thus, the Sm3+ doped Y2O3 nano-crystalline phosphor may be a suitable candidate for displays and photonic devices.