Wide Excitation Band Research Articles

Yellow emitting Cd doped SnO2 nanophosphor for phosphor converted white LED applications

The optical properties and defect emission of Cd doped (10 and 20 mol%) SnO2 nanoparticles were investigated. The structure and phase purity of the nanoparticles were studied from powder XRD pattern and micro-Raman spectra. XRD pattern revealed the effective substitution of Cd2+ in the Sn4+ sites. The application of microwave for the combustion reaction causes the formation of homogeneous fine particles with crystallite size around 7.5, 6.1 and 7.1 nm for pure SnO2, 10 mol% Cd doped and 20 mol% Cd doped SnO2 nanoparticles, respectively. The influence of fine particle size and doping of Cd2+ ions reduces the optical band gap of SnO2 nanoparticles from 3.08 to 2.98 eV. The existence of surface defects, induced by the smaller size of the particles and the heterovalent doping, was confirmed by the micro-Raman spectral analysis. ESR spectra revealed that there is a significant concentration of singly ionized oxygen vacancies present in undoped as well as Cd doped SnO2 nanoparticles. High surface defects of the nanoparticles give rise to the broad emission band in the wavelength range of 450–625 nm. The surface defects, causes the visible emission, includes the oxygen deficiency produced by the doping of the cation (Cd2+) with low oxidation state than that of host cation (Sn4+), tin interstitials and dangling bonds. The excitation of pure and Cd doped SnO2 nanoparticles with 325 nm laser leads to the emission of yellow light and the emission intensity varied by Cd doping. A wide excitation band (300–400 nm) observed for the emission at 530 nm revealed that visible emission can be obtained by the pure and Cd doped SnO2 nanoparticles under the excitation of UV light. White emission with high CRI of 85.6 and CIE Coordinates of (0.32, 0.30) is achieved by exciting the mixture of blue phosphor with yellow emitting SnO2 nanoparticles using 375 nm LED chip.

Down-shifting emission by charge transfer band in porous silicon infiltrated with Eu3+ and Gd3+ ions

Down-shifting emission properties of europium (Eu3+) and gadolinium (Gd3+) ions incorporated in porous silicon (PS) layers are presented. Different samples were prepared by electrochemical etching of p+-type crystalline silicon wafers followed by simple impregnation with rare earth ions from nitrate solutions of europium and gadolinium and subsequent high temperature annealing in air. The samples exhibited room temperature photoluminescence (PL) and the excitation spectra present a wide excitation band around 233 nm which can be associated to the charge transfer band transitions. This band is attributed to O2−(2p)→Eu3+(4f) ligand-to-metal charge transfer. The down-shifting emission for Gd+3:Eu3+ was analyzed by a simple rate equation model to study the interaction between the O2− ions and Eu3+ ions. The exact solution of this model agrees very well with the experimental results.

Tunable emission color of Li2SrSiO4:Tb3+ due to cross‐relaxation process and optical thermometry investigation

AbstractA series of Li2SrSiO4:xTb3+ (0.2%, 0.4%, 0.6%, 0.8%, 2%, 4%, and 6%) phosphors were prepared by conventional solid‐state reaction. It was found that this silicate phosphor has a wide excitation band at near‐ultraviolet region (230‐300 nm) due to spin‐allowed 4f 8→4f75d1 transitions of Tb3+ ions, with the exact position dependent on the crystal field of the lattice. The cross‐relaxation process originating from 5D3→5D4 and 7F6→7F0 happened between different Tb3+ ions. It leads to the luminescence color of Li2SrSiO4: xTb3+ tuning from blue to green just by controlling Tb3+ concentrations. Furthermore, concentration quenching mechanism, energy migration type, cross‐relaxation rate and efficiency, are discussed in detail. Finally, optical thermometry properties were investigated via temperature‐dependent emission spectra. The results show that low‐concentration‐doped sample (Li2SrSiO4:0.4%Tb3+) shows remarkable optical thermometry based on fluorescence intensity ratio (FIR) between the blue and green emission of Tb3+ ions, whereas the high‐concentration‐doped sample (Li2SrSiO4:4%Tb3+) demonstrates small emission intensity loss. It illustrates that terbium‐doped silicate phosphor is a multifunctional material with potential application for display field and optical thermometry.

Site occupancy and photoluminescence properties of a novel deep-red-emitting phosphor NaMgGdTeO6:Mn4+ with perovskite structure for w-LEDs

A novel kind of deep red-emitting phosphor NaMgGdTeO6:Mn4+ prepared by a high-temperature solid-state reaction method is reported in this article. Rietveld refinement for the representative NaMgGdTeO6 host showed that its crystal structure is isostructural with that of the NaMgLaTeO6 compound. XRD measurements for the representative samples indicated a pure phase for the as-prepared phosphors and the successful introduction of Mn4+ into this matrix. Under UV or blue excitation, the NaMgGdTeO6:0.01Mn4+ shows a narrow emission band ranging from 12,500 cm−1 (800 nm) to 16,667 cm−1 (606 nm) centered at 14,347 cm−1 (697 nm), which has been attributed to the Mn4+ spin and parity-forbidden transition 2Eg→4A2g. Monitored at 697 nm, the phosphor exhibits a wide excitation band from 40,000 cm−1 (250 nm) to 16,667 cm−1 (600 nm) with four Gaussian peaks at 31,546 cm−1 (317 nm), 28,329 cm−1 (353 nm), 24,630 cm-1 (406 nm), and 20,833 cm-1 (480 nm), corresponding to the O2-→Mn4+ charge-transfer transition and the Mn4+ spin-allowed transitions 4A2g→4T1g, 4A2g→2T2g and 4A2g→4T2g, respectively, which can match well with commercial UV and blue chips. The optimal Mn4+-doping percentage for this phosphor was determined to be NaMgGdTeO6:0.01Mn4+ with a quantum yield of 41.19% upon 365 nm excitation. Concentration quenching took place with further Mn4+-concentration increase in NaMgGdTeO6:Mn4+, where the energy transfer mechanism was analyzed to be an electronic dipole-dipole interaction. Moreover, the luminescent decay times monitored at 697 nm showed a reasonable decrease with increasing Mn4+ concentration. The time-resolved emission spectra and decay times monitored at 680 and 720 nm of NaMgGdTeO6:0.01Mn4+ led to the distinguishing of two kinds of Mn4+ sites in the crystal structure. In addition, the temperature-dependent luminescence properties illustrated a good luminescence thermal stability. These results suggest that this kind of phosphor can be considered as a supplement of the red component for w-LEDs.

Growth of YAG:Ce3+-Al2O3 eutectic ceramic by HDS method and its application for white LEDs

The resin-free YAG:Ce3+-Al2O3 eutectic ceramic phosphor for white light emitting diodes (WLEDs) was successfully grown in vacuum by Horizontal Directional Solidification method (HDS). X-ray diffraction and scanning electron microscopy indicate that this material has a typical eutectic structure of interpenetrating sapphire and garnet phases. The excitation spectra, emission spectra and temperature characteristics of the eutectic show that it is characterized by a wide excitation band and it has good stability in high temperature. In X-ray photoelectron spectroscopy, annealing in an air atmosphere could eliminate the oxygen vacancies and didn’t change the Ce3+ valence in the eutectic. The YAG:Ce3+-Al2O3 eutectic ceramic with different thickness was fixed in COB (chip on board) element for researching the performance of the WLEDs with the phosphor. The electroluminescence characterization of the WLEDs show that the WLEDs with the eutectic ceramic are more excellent than the common commercial WLEDs.

Influence of Ce ion Concentration on Fluorescence Properties of Ce:TAG‐Al2O3 Eutectic Mixed Crystals

Combining a blue LED chip with phosphor, white light can be obtained. In traditional white LEDs, the phosphor was dispersed in resin in a form of particles. Particle‐resin composite has complex packaging and poor thermal stability. For high‐power white LEDs, this combination cannot provide long‐term stable work because of photo‐degradation of the organic epoxy resin under high irradiation and high temperatures. Here we develop resin‐free phosphor (Tb1‐nCen)3Al5O12‐Al2O3 eutectic mixed crystals (Ce:TAG‐Al2O3) to eliminate the photo‐degradation issue of the organic material. They were grown by optical floating zone method. X‐ray diffraction and scanning electron microscopy show that these materials have typical eutectic mixed crystal structure of sapphire and garnet phases, in which lamellar spacing is in an order of tens of microns. The doped Ce3+ ions do not influence basic structure of TAG crystal. Ce3+ ion replaces Tb3+ ion rather than Al3+ ion. Photoluminescence emission and excitation properties of the eutectic mixed crystals were investigated. They get to maximal value with 2.0% Ce3+, and have wide excitation band at 470 nm. Luminous efficiency of eutectic mixed crystal‐packaged LED is higher than that of particle‐resin composite packaged LED. These results show that Ce:TAG‐Al2O3 eutectic mixed crystals are promising phosphor for white LEDs.

Tunable colorimetric performance of Al2O3-YAG:Ce3+ eutectic crystal by Ce3+ concentration

Ce-doped Al2O3-YAG eutectics with different percentage of Ce were successfully grown by the optical floating zone technique. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the structure. The results show that they have typical eutectic structure of interpenetrating sapphire and garnet phases with the tens of microns lamella spacing. The photoluminescence spectra of the eutectics showed that they have wide excitation band, and samples with 1.6mol% Ce-doped has the highest emission intensity. The eutectic-packaged LED has high luminous efficiency and its color can be modulated by changing Ce concentration. The results reveal that Ce-doped Al2O3-YAG eutectic is a promising phosphor for white LED applications.

Synthesis and luminescent properties of terbium complex containing 4-benzoylbenzoic acid for application in NUV-based LED

A novel Tb(III) ternary complex Tb(p-BBA)3MAA was synthesized with 4-benzoylbenzoic acid (p-BBA) and methacrylic acid (MAA) as ligands. The complex was characterized by IR, UV-visible, thermogravimetric analysis and fluorescence spectroscopy. Monitored at 544 nm, the complex displayed wide and strong excitation band at 300-400 nm, which matched well with the 365 nm-emitting UV chip. The complex exhibited excellent green emission at 544 nm (5D4→7F5 transition of Tb3+) under an excitation at 365 nm. Besides, the complex showed high thermal stability. Its intramolecular energy transfer process was further discussed. Furthermore, the complex also had higher fluorescence lifetime (1.38 ms) and higher quantum yield (0.372). Finally, electroluminescent properties indicated that when used to fabricate LED with 365 nm UV chip (power efficiency is 18.6 lm/W), the complex remained its favorable optical performance. These results implied that Tb(p-BBA)3MAA could be used as a green phosphor for NUV-based white LED.

Luminescent properties of ZrO2:Tb nanoparticles for applications in neuroscience

In this paper a new generation of non-toxic nanoparticles based on the zirconium oxide doped with 0.5%Tb and co-doped by the range of 0–70% with Y was evaluated for the use as a fluorescent biomarker of neuronal trafficking. The ZrO2:Tb nanoparticles were created by microwave driven hydrothermal method. Influence of the yttrium content and thermal processing on the Tb3+ related luminescence emission was discussed. The higher intensities were achieved, when host was cubic and for the nanoparticles with 33nm. Presence of yttrium was associated with the energy coupling of the host and dopant, wide excitation band is present at 309 and 322nm before and after calcination respectively.For the experiment on living primary neurons, nanoparticles doped with 0.5%Tb and 7%Y were chosen based on their luminescence emission intensity. Recently transfer of the nanoparticles through the barriers in the organism including blood–brain barrier following their alimentary absorption was confirmed (Godlewski and Godlewski, 2012). This raised the possibility of the nanoparticle application as a tool in the neuroscience, and the question of potential mechanisms of nanoparticle turnover in neurons. Concentration of 0.001mg/ml of ZrO2:0.5%Tb 7%Y in growth medium was added to the primary murine culture medium, and the intracellular trafficking of nanoparticles was observed following 15min pre-incubation period. ZrO2:0.5%Tb 7%Y nanoparticles were dynamically absorbed by the neurons and the dynamic passage of transport vesicles containing ZrO2:0.5%Tb 7%Y nanoparticles was observed along the neuronal processes and in between two neighbouring neurons. Reassuming, the ZrO2:0.5%Tb 7%Y nanoparticles proved to be biocompatible and a valid tool to assess intracellular trafficking dynamics in the neurobiology.

Vacuum ultraviolet excitation luminescence spectroscopy of few-layered MoS2

We report on vacuum ultraviolet (VUV) excited photoluminescence (PL) spectra emitted from a chemical vapor deposited MoS2 few-layered film. The excitation spectrum was recorded by monitoring intensities of PL spectra at ~1.9 eV. A strong wide excitation band peaking at 7 eV was found in the excitation. The PL excitation band is most intensive at liquid helium temperature and completely quenched at 100 K. Through first-principles calculations of photoabsorption in MoS2, the excitation was explicated and attributed to transitions of electrons from p- and d- type states in the valence band to the d- and p-type states in the conduction band. The obtained photon-in/photon-out results clarify the excitation and emission behavior of the low dimensional MoS2 when interacting with the VUV light sources.

Near-Infrared Emission of Er3+ Sensitized by Mn4+ in Ca14Zn6Al10O35 Matrix

A novel near-infrared luminescent material Ca14Zn6Al10O35:Mn4+, Er3+ has been synthesized by a sol–gel method. The Ca14Zn6Al10O35:Mn4+, Er3+ phosphor exhibits strong near-infrared emission around 1540 nm with a wide excitation band extending from 250 to 550 nm. Efficient energy transfer from Mn4+ to Er3+ in Ca14Zn6Al10O35 is observed and gives about 13 times enhancement on the Er3+-1540 nm emission excited at 455 nm, which makes it convenient to obtain high-power emission by commercial GaN LED pumping. The energy transfer mechanism is discussed based on Dexter’s theory and Inokuti–Hirayama/Yokota–Tanimoto models. It is believed that a dipole–dipole interaction between Mn4+ and Er3+ ions is responsible for the large enhancement of 1540 nm emission.

Preparation of red nitride phosphor from powder mixture of metal nitrides using spark plasma sintering

A red nitride phosphor with a chemical formula: (Sr0.8Ca0.2)AlSiN3:Eu0.00752+ was successfully synthesized using spark plasma sintering at a temperature of 1680 °C from powder mixture of metal nitrides of Sr, Ca, Al, and Si and europium (III) oxide. The prepared phosphor showed a wide excitation band ranging from 300 to 550 nm and a broad emission band centered at a wavelength of about 618 nm, being suitable for use in down-conversion white LEDs. The effects of sintering temperature, holding time as well as uniaxial pressure on phosphor characteristics were investigated. Increasing the sintering temperature increased the crystallization and PL characteristics of the phosphor. The excitation and emission intensity of the phosphor was also affected by the holding time at the maximum temperature. When the synthesis was carried out at 1680 °C and 30 MPa, the optimal holding time was around 30 min. Increasing the uniaxial pressure resulted in the improvement of the excitation and emission intensity. The optical properties of the phosphor synthesized at an increased uniaxial pressure of 42 MPa (1680 °C for 30 min) was comparable to those of the commercial phosphor. The experimental results indicated that the spark plasma sintering method is a viable method to prepare nitride phosphors at relatively low temperatures in short synthesis time, compared to conventional methods.

Highly efficient reddish orange emission in Mn2+/Eu3+ co-doped phosphate glasses for greenhouse

A series of phosphate glasses doped with Mn2+/Eu3+ ions were prepared by high temperature melting method to explore reddish orange luminescent materials for greenhouse. The absorption, emission, and excitation spectra of the samples were studied in detail. A wide excitation band at 300-550 nm in the Mn2+/Eu3+ co-doped phosphate glasses was observed. Under the illumination of sunlight, the Mn2+/Eu3+ co-doped phosphate glasses can emit strong red emission. Our materials will be helpful to improve the utilization of solar for glass greenhouses.

Ce-doped Al2O3−YAG eutectic and its application for white LEDs

Ce-doped Al2O3−YAG (Y3Al5O12, yttrium aluminum garnet) eutectic, a resin-free phosphor for white light emitting diodes (WLEDs), was successfully grown by the Czochralski method. X-ray diffraction and scanning electron microscopy show that this material has a typical eutectic structure of interpenetrating sapphire and garnet phases, as well as lamellar spacing in the order of tens of microns. The eutectic has a higher Ce3+ segregation coefficient than YAG single crystal. The photoluminescence properties of this eutectic were also investigated. Results show that it is characterized by a wide excitation band, and that the luminous efficiency of a eutectic-packaged LED is higher than that of a phosphor powder-packaged LED. The findings indicate that the Al2O3−YAG eutectic is a promising phosphor for WLED applications.

Synthesis of CaWO4:Er3+@SiO2 and CaWO4:Tm3+@SiO2 nano-particles via a combustion pathway and study of their optical properties

Use of citric acid as a chelating agent and fuel, ammonium nitrate as fuel, boric acid as flux material and silica as supports, CaWO4:Ln3+@SiO2 (Ln = Er and Tm) nanoparticles were synthesized via a combustion reaction at 800 °C. Characterization of the samples was performed by X-ray diffractometer (XRD), reflectance UV–Vis spectrophotometer, fluorescence spectrophotometer (PL) and transmission electron microscope (TEM). XRD patterns showed that tetragonal crystalline structure of scheelite and silica supports were formed, and that the formation of a silica support could enhance the luminescence intensity of CaWO4:Ln3+. The reflectance UV–Vis and PL spectra indicated the broad absorption band of WO42− groups about 240 nm, the WO42− wide excitation band with maximum at 240 nm, a broad emission band of WO42− with maximum about 420 nm, and characteristic emissions of Ln3+ ions. According to the TEM analysis, CaWO4:Er3+@SiO2 and CaWO4:Tm3+@SiO2 nanoparticles have almost the same morphology with average particle sizes about 50 nm.

The Evolution of The Optical Spectra of (M,M′)TiO3: Pr3+ (M, M′: Ca2+, Zn2+, Sr2+)Phosphors

(M,M′)TiO3:Pr3+ (M, M′: Ca2+, Zn2+, Sr2+) phosphors were prepared by a solid-state reaction method, and the luminescent properties were investigated. The PLE and PL spectra of Ca1-xSrxTiO3:Pr3+ powders exhibited the strong dependence on the Sr content, resulting in the maximum emission intensity at x = 0.1. The PLE spectra of (Ca0.5Sr0.5-yZny)TiO3:Pr3+ showed wide excitation bands with increasing Zn content. The red emission intensity of (Ca0.5Sr0.35Zn0.15)TiO3:Pr3+ was higher than that of the pure CaTiO3:Pr3+.

Synthesis of Eu2+‐doped A‐site and oxygen‐deficient perovskite related host for photoluminescent materials

AbstractPerovskite‐related compounds have been investigated in many fields, because they have various unique properties due to their compositions and structures which are characterized by various combinations of the A and B cations. In addition, defect perovskite‐type compounds have also been investigated in many areas, especially as ion conductors. In this study, we used the A‐site and oxygen‐deficient perovskite, Sr3Al2O6, as a photoluminescent host material. This means that the defect sites were able to substitute several ions. We investigated the A3Al2O6:Eu2+ phosphor material (A = Mg, Ca, Sr, Ba) produced by a solid state reaction in several kinds of atmospheres. Based on our results, the phosphor showed wide excitation and emission bands of 300–400 nm and 450–600 nm, respectively. The emission color control depended on the A‐site substitution ratio. It is a very useful phosphor for white LEDs. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A novel europium(III)–imidazol–diketonate–phenanthroline complex as a red phosphor applied in LED

Abstract A β -diketone, 4-imidazol-4,4,4-trifluorobutane-1,3-dione (HIDTFBD) and the corresponding europium(III) complex Eu(IDTFBD) 3 phen were designed and synthesized, where phen was 1,10-phenanthroline. The complex was characterized by IR, UV–visible, thermogravimetric analysis (TGA) and photoluminescence (PL) spectroscopy. Monitored at 610 nm, the Eu(III) complex exhibited a strong and wide excitation band between 320 and 420 nm due to the formation of a big π -conjugated system in the complex molecule. Under near UV-light excitation, the complex showed strong red emission of Eu 3+ ion due to the 5 D 0 – 7 F J ( J = 0–4) transitions. The fluorescence lifetime and the luminescence quantum yield were also measured. Finally, a bright red light-emitting diode was fabricated by coating the europium ternary complex onto an ~ 395 nm-emitting InGaN chip, and the LED exhibited appropriate CIE chromaticity coordinates in red area. All these results suggest that the Eu(III) complex is a promising candidate as red component for fabrication of near UV-based white light-emitting diodes.

Preparation of CaWO 4:Ln 3+@SiO 2 (Ln=Tb, Dy and Ho) nanoparticles by a combustion reaction and their optical properties

The CaWO 4:Ln 3+@SiO 2 (Ln=Tb, Dy and Ho) nanoparticles were synthesized via a combustion process at 800 °C, using citric acid as chelating agent and fuel, ammonium nitrate as fuel, boric acid as flux material and silica as supports. The persistent phosphor nanoparticles were characterized by X-ray diffraction (XRD), reflectance UV–vis and fluorescence spectroscopy (PL) and transmission electron microscopy (TEM) techniques. XRD patterns indicated that crystalline calcium tungstate with scheelite structure was produced. The reflectance UV–vis spectra showed the broad absorption band of WO 4 2 − groups and the PL spectra showed the WO 4 2 − wide excitation band, broad emission band of WO 4 2 − and characteristic emissions of Ln 3+ ions. The average particle sizes were determined by TEM, which are about 50 nm.

Two-Photon Excitation Spectroscopy of Carotenoid-Containing and Carotenoid-Depleted LH2 Complexes from Purple Bacteria

We applied two-photon fluorescence excitation spectroscopy to LH2 complex from purple bacteria Allochromatium minutissimum and Rhodobacter sphaeroides . Bacteriochlorophyll fluorescence was measured under two-photon excitation of the samples within the 1200-1500 nm region. Spectra were obtained for both carotenoid-containing and -depleted complexes of each bacterium to allow their direct comparison. The depletion of carotenoids did not alter the two-photon excitation spectra of either bacteria. The spectra featured a wide excitation band around 1350 nm (2x675 nm, 14,800 cm(-1)) which strongly resembled two-photon fluorescence excitation spectra of similar complexes published by other authors. We consider obtained experimental data to be evidence of direct two-photon excitation of bacteriochlorophyll excitonic states in this spectral region.