Luminescence Decay Lifetimes Research Articles

One novel 3D tetranuclear cadmium coordination polymer based on 2,3′,4,5′-biphenyltetracarboxylic acid: Synthesis, crystal structure and luminescence

One novel 3D cadmium coordination polymer, namely, {[Cd2(bptc4−)(ox2−)0.5(H2O)2]·4.5H2O}n (1), has been hydrothermally synthesized through a reaction of 2,3′,4,5′-biphenyltetracarboxylic acid (H4bptc) with divalent cadmium salt in the presence of a second ligand (ox=oxalic acid). X-ray single crystal structure analysis reveals that tetranuclear cadmium clusters are formed by linking two Cd1 and two Cd2 ions with carboxyl groups. Each ox2− ligand, with a bidentate chelating mode, connects two Cd2 ions from two adjacent tetranuclear clusters to afford 1D zigzag cadmium cluster chains. Furthermore, 1D chains are connected by bptc4− ligands to extend a 3D framework with one-dimensional channels along the [001] direction. From a topology view, complex 1 exhibits an unprecedented 2-nodal 4, 10-connected net with the Schläfli symbol of {46}2{49. 512.622.72}. The luminescence analyses reveal that complex 1 shows an emission maximum at 437nm, which may be attributed to the intra-ligand transition. A red shift of 31nm compared with H4bptc ligand was observed and the luminescent decay lifetime is 2.74ns, which indicates that it might be a potential blue light-emitting candidate. In addition, it was also characterized by elemental, IR spectra, PXRD and TG analyses.

Spectroscopic investigation on tunable luminescence by energy transfer in Tb2−xSmx(MoO4)3 nanophosphors

New Sm3+ activated Tb2−xSmx(MoO4)3 nanophosphors were synthesized through sol–gel method. The structural and luminescence properties have been studied by XRD, TEM and photoluminescence measurements. The XRD pattern confirms that the Tb2−xSmx(MoO4)3 crystallizes in the same orthorhombic structure of Tb2(MoO4)3. The spectroscopic and laser parameters of Sm3+ ion in Tb2−x(MoO4)3 matrix were evaluated for the first time using Judd–Ofelt theoretical analysis. The higher value of stimulated emission cross-section of 4G5/2→6H7/2 transition of Sm3+ is favorable for low threshold and high gain to obtain continuous wave laser action. The photoluminescence excitation spectra suggest that this novel phosphor can be excited over a broad range from nUV to blue light (300–490nm). Under the excitation of UV, Tb2−xSmx(MoO4)3 nanophosphor exhibits the characteristic emissions of Tb3+ and Sm3+. By varying the doping concentration of Sm3+, the emission color of the phosphors can be tuned and white emission in a single composition can be obtained under host excitation, in which an energy transfer from MoO42−→Sm3+/ Tb3+ and Tb3+→Sm3+ was observed. The investigation of the luminescence decay curves and lifetime values implies the energy transfer between Tb3+→Sm3+ and confirms the absence of Sm3+→Tb3+ energy transfer. These phosphors might be a promising material for use in nUV LEDs and can exhibit tricolor luminescence under single excitation wavelength.

Novel luminescent lanthanide complexes assembling alumina/titania/silica hybrids through 2-phenylmalonic acid linkage

In this paper, a series of novel inorganic/organic rare earth (europium, terbium) hybrid materials through 2-phenylmalonic acid linkage were synthesized by the sol–gel process. The micro-structure and the luminescent properties of the complicated hybrids were studied in detail by Fourier transform infrared spectroscopy, wide angle x-ray diffraction, themogravimetric analysis, scanning electron microscope, and fluorescence spectra. The above research results indicate that the hybrids possess high thermal-stability, amorphous structure features and especially favorable luminescent performances such as long luminescent decay lifetime, and high quantum yield.

Low Temperature Synthesis of Luminescent RE2O3:Eu3+Nanomaterials Using Trimellitic Acid Precursors

[RE(TLA)·(H2O)n:Eu3+] (RE3+: Y, Gd and Lu; TLA: trimellitic acid) precursor complexes were synthesized by an one step aqueous co-precipitation method. After annealing for 1 h, RE2O3:Eu3+ nanophosphors were formed through the benzenetricarboxylate low temperature thermolysis method (500-1000 oC). The compounds were characterized by using different techniques [elemental analysis (CHN), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG/DTG), X-ray powder diffraction (XPD) and scanning electron microscope (SEM)]. The XPD data indicated that the Y2O3:Eu3+ materials have crystallite size range from 11 to 62 nm. The SEM and transmission electron microscopy (TEM) images show that the annealed materials keep morphological similarities with the precursor complexes. The photoluminescence properties were studied based on the excitation and emission spectra, and luminescence decay lifetimes of the 5D0 emitting level of the Eu3+ ion. The experimental intensity parameters (Ωλ), lifetimes (τ), as well as radiative (Arad) and non-radiative (Anrad) decay rates were calculated and discussed. The RE2O3:Eu3+ phosphors (RE: Y3+ and Lu3+) annealed at 500 to 1000 oC have emission quantum efficiency (intrinsic quantum yield) values from 60 to 82%, indicating that this material can be potentially used for optical markers applications.

Red Luminescence in calix[4]arene-europium Hybrid Material

Inorganic-organic hybrid material based on Eu3+ and calix[4]arene bearing four carboxylic acid groups has been synthesized via hydro(solvo)thermal method assisted by microwave irradiation. The obtained sample was characterized by FT-IR, PXRD, SEM and photoluminescence. The results showed the formation of coordination polymer with low symmetry around Eu3+ ion. The monoexponential luminescence decay lifetime reveals that all Eu3+ ions occupy the same coordination environment.

Luminescence of La0.2Y1.8O3 nanostructured scintillators.

For the first time, transparent La0.2Y1.8O3 nanostructured polycrystalline scintillators were fabricated by sintering nanoparticle powders at high temperatures and their scintillation properties are reported. La0.2Y1.8O3 is a host material that has never been investigated as scintillators for radiation detection. Our observations found that La0.2Y1.8O3 has an intense scintillation luminescence, a detection efficiency higher than that of YAG:Ce and a comparable energy resolution to NaI and CsI scintillators. In addition, La0.2Y1.8O3 is stable and has luminescence decay lifetime in the picosecond range which is favorable for radiation detection. The luminescence of La0.2Y1.8O3 has a large Stokes-shift and a large emission bandwidth, and the luminescence is highly temperature dependent. Different from most doped scintillators, the luminescence of La0.2Y1.8O3 is most likely from the self-trapped excitons. The discovery of La0.2Y1.8O3 scintillators opens a new door for the research of new materials for radiation detection.

Ortho-vanadates K3RE(VO4)2 (RE = La, Pr, Eu, Gd, Dy, Y) for near UV-converted phosphors

The orthovanadate poly-crystals K3RE(VO4)2 (RE = La, Pr, Eu, Gd, Dy, Y) were synthesized via the solid-state reaction route. The crystal phase formation was verified through X-ray diffraction (XRD) studies and was performed by structural refinements. The optical properties were also investigated in detail. K3RE(VO4)2 (RE = Eu, Dy, Gd, Pr, La, Y) phosphors present different luminescence behaviors: the profiles of excitation and emission spectra, the spectra shift, the luminescence decay lifetimes, the absolute quantum efficiency (QE), and the CIE color coordinates are very different. The luminescence of K3RE(VO4)2 (RE = La, Gd, Y, Pr) presents yellow or yellowish green color, while, K3Dy(VO4)2 and K3Eu(VO4)2 show white and red luminescence, respectively. This was discussed on the base of the different micro-structure, activator centers, and the charge transfer transitions from [VO4]3− groups in the lattices. K3Y(VO4)2 and K3Eu(VO4)2 show higher QE values of 47.0% and 45.0% at room temperature, respectively. All the phosphors have efficient absorption in the region of near-UV wavelengths or blue wavelength region. This can well match with the light from UV-LED (360–400 nm) or blue LED chips (450–480 nm) based on GaN semiconductor. K3RE(VO4)2 could be suggested to be a potential candidate to give further investigations for the application on near-UV excited white LEDs.

Luminescence properties and energy transfer investigations of tri-doped Sr3AlO4F:Ce3+,Tb3+,Yb3+ phosphors

Novel tri-doped Sr3AlO4F phosphors with Ce3+, Tb3+ and Yb3+ were synthesized by a conventional high temperature solid-state reaction. X-ray diffraction (XRD) was used to characterize the phase structure. The luminescence spectra and decay lifetime curves of co-doped Sr3AlO4F:Tb3+,Yb3+ have been measured to prove energy transfer (ET) from Tb3+ to Yb3+. From the photoluminescence excitation and emission spectra of tri-doped Sr3AlO4F:Ce3+,Tb3+,Yb3+, energy transfer processes from Ce3+→Tb3+→Yb3+ was observed in Sr3AlO4F:Ce3+,Tb3+,Yb3+ phosphors under the excitation of near ultraviolet light. The energy transfer efficiency from Ce3+→Tb3+→Yb3+ was calculated. The energy transfer mechanism was also discussed. The present Ce3+,Tb3+,Yb3+ tri-doped Sr3AlO4F phosphor may have ideally potential use in the down-conversion of c-Si solar cells.

Luminescent lanthanide-polyoxometalates assembling zirconia–alumina–titania hybrid xerogels through task-specified ionic liquid linkage

This paper is focused on the preparation and characterization of luminescent hybrid materials of zirconia (alumina, titania) xerogels encapsulated with high luminescent lanthanide polyoxometalates via a sol–gel process. Firstly, liquid compound 1-methyl-3-propionyloxy imidazolium bromide (IM+Br−) is assembled with lanthanide polyoxometalates (NaLnW10O36·32H2O, abbreviated as LnW10, Ln = Eu, Tb, Sm, Dy) through the ion exchange reaction, resulting in Ln-IL, Then Ln-IL is connected to zirconia (alumina/titania) by the chelating reaction between a propionyloxy group of the IM+ component and metallic alkoxides (Zr(OCH2CH2CH2CH3)4, Ti(OCH(CH3)2)4, Al(OCH(CH3)2)3) under mild conditions after hydrolysis and condensation. These hybrid materials are characterized by Fourier transform infrared spectroscopy, wide angle X-ray diffraction, thermogravimetric analysis, as well as luminescence. The above measurements indicate that they possess high thermal-stability, amorphous structure features and especially favorable luminescent performances such as long luminescent decay lifetime, high quantum yield. It is found that alumina and zirconia are superior matrices to titania for the luminescence of lanthanide polyoxometalates, and close white-light integration can be realized for hybrids of titania gels.

Luminescence Performance and Structural Characteristic of Eu2+-Activated Na2BaSi2O6 Phosphor

Green-emitting phosphor of Eu2+-doped sodium barium silicate Na2BaSi2O6 was prepared by chemical sol-gel method. The X-ray powder diffraction, SEM, photoluminescence pectra and time resolved emission spectra were measured. The excitation spectra can well match with the emission light of near UV-LED chips (360–400 nm). The luminescence was suggested to originate from two kinds of Eu2+ centers, which occupy both Ba2+ and Na(2) sites in Na2BaSi2O6 lattices. Energy transfer among the Eu2+-centers was discussed by analyzing the photoluminescence spectra, concentration-dependent luminescence intensity and decay lifetimes. The dependence of the luminescence intensity of Na2BaSi2O6:Eu2+ on temperatures was measured. The activation energy (ΔE) for thermal quenching was reported. The luminescence properties of Na2BaSi2O6:Eu2+ were compared with that of BaSiO3:Eu2+. The high luminescence of Na2BaSi2O6:Eu2+ were discussed on the base of luminescence properties and microstructure of Eu2+ ion.

Abnormal Reduction, Eu3+ → Eu2+, and Defect Centers in Eu3+-Doped Pollucite, CsAlSi2O6, Prepared in an Oxidizing Atmosphere

Eu-doped pollucite CsAlSi2O6 was synthesized by the sol-gel method and heated in an air atmosphere. The crystal structure and the microstructure of the phosphors were investigated by X-ray powder diffraction and SEM images, respectively. The photoluminescence spectra and temperature dependent decay curves were measured. An abnormal reduction phenomenon of Eu(3+) → Eu(2+) was reported when Eu(3+) ions were doped in alkaline metal cation sites in CsAlSi2O6 prepared in an oxidizing atmosphere. The abnormal mechanism was discussed on the basis of the charge compensation model and a rigid three-dimensional framework structure of CsAlSi2O6. The luminescence color centers were investigated by luminescence decay lifetimes and thermal stabilities of Eu(2+) ions. The defect complexes of [(Eu(3+)Cs)(••)-2VCs'] or [(Eu(3+)Cs)(••)-Oi″] induced by the substitution of Eu(3+) on Cs(+) were suggested in the lattices. Eu(2+) ions could be regarded as Eu(3+) ions combining with the released electrons from defects Oi″ or VCs' in close vicinity of Eu(3+) (Eu(3+) + e); the electrons cannot enter the atom track of Eu(2+) presenting luminescence of Eu(2+) ions. The results indicate that several defect traps can be attributed to the abnormal reduction mechanism of Eu(3+) to Eu(2+) ions in a matrix.

Preparation and luminescence characteristics of Eu-doped calcium chloride silicate Ca7Si2O8Cl6

Eu-activated (0.1–5.0 mol%) calcium chloride silicate Ca7Si2O8Cl6 phosphors were prepared by means of ceramic method, which were characterized by X-ray powder diffraction and SEM measurements. The luminescence properties were investigated by the photoluminescence excitation and emission spectra, the luminescence decay lifetimes, the color coordinates and the internal quantum efficiency. Ca7Si2O8Cl6:Eu contains two kinds of Eu-emission centers, i.e., Eu2+ and Eu3+ ions, even it was prepared in a reducing atmosphere. Eu2+ and Eu3+ ions which present the characteristic broad band (5d→4f) and narrow 4f→4f luminescent features, respectively. The luminescence mechanism was discussed by analyzing the spectra, concentration-dependent emission intensity and lifetimes. With increasing Eu-doping, the energy transfer happens from Eu2+ to Eu3+ centers. Upon an excitation with near UV light, the luminescence color can be tunable from green to orange with increasing the Eu-doping because of the changes of the luminescence components from Eu3+ and Eu2+ ions in Ca7Si2O8Cl6 host. This is a potential phosphor combining both luminescence effects of Eu2+ and Eu3+ ions.

Luminescence properties and energy transfer investigations of Sr3AlO4F:Ce3+, Tb3+ phosphor

Abstract Ce 3+ and Tb 3+ co-doped Sr 3 AlO 4 F phosphors were synthesized by the conventional high temperature solid-state method. X-ray diffraction (XRD) was used to characterize the phase structure. The luminescence spectra and decay lifetime curves of Ce 3+ have been measured to prove energy transfer (ET) from Ce 3+ to Tb 3+ . Under the excitation of near-ultraviolet light, Sr 3 AlO 4 F:Ce 3+ ,Tb 3+ phosphors exhibited blue emission corresponding to the f–d transition of Ce 3+ ions and green emission bands corresponding to the f–f transition of Tb 3+ ions, respectively. Effective energy transfer occurred from Ce 3+ to Tb 3+ in Sr 3 AlO 4 F host due to the observed spectra overlap between the emission spectrum of Ce 3+ ion and the excitation spectrum of Tb 3+ ion. The energy transfer efficiency from Ce 3+ ion to Tb 3+ ion was also calculated to be 51%. Furthermore, the energy transfer from Ce 3+ to Tb 3+ in Sr 3 AlO 4 F host was demonstrated to be resonant type via a dipole–dipole interaction mechanism. The present Ce 3+ and Tb 3+ co-doped Sr 3 AlO 4 F phosphor may have potential use in the technology of white light-emitting diodes.

Near-infrared luminescence and quantum cutting mechanism in CaWO4:Nd3+, Yb3+

An efficient near-infrared (NIR) quantum cutting (QC) by converting ultraviolet into NIR via cooperative downconversion (DC) for the (Nd3+, Yb3+) couple in CaWO4 is studied since efficient two-step energy transfer occurs from the 4G9/2 level of Nd3+ to two neighboring Yb3+:2F5/2 level. The authors have analyzed the measured luminescence spectra and decay lifetimes for different Yb3+ concentrations and proposed a mechanism to rationalize the DC effect. The maximum quantum efficiency (QE) of Nd3+ is up to 164.03 %. Application of the broadband NIR-QC phosphors might enhance the performance of silicon-based solar cells.

Band structure and near infrared quantum cutting investigation of GdF3:Yb3+, Ln3+ (Ln = Ho, Tm, Er, Pr, Tb) nanoparticles

A series of GdF3:Yb(3+), Ln(3+) (Ln = Ho, Tm, Er, Pr, Tb) nanoparticles were prepared by a simple and green hydrothermal method without any additives, which exhibited an ellipse-like shape with a diameter of 63 nm and a length of 101 nm on average. To prove the existence (or not) of near infrared quantum cutting for various lanthanide ion couples (Yb/Ho; Yb/Tm; Yb/Er; Yb/Pr; Yb/Tb) in one host lattice (GdF3), the measured luminescence spectra and decay lifetimes of these samples were analysed. Furthermore, the band structures and densities of state of GdF3 were also studied with the help of first-principles calculations, and the direct band gap of GdF3 was estimated to be 7.443 eV wide. Based on this, detailed processes and possible mechanisms of the luminescence phenomena are discussed. GdF3:Yb(3+), Ln(3+) nanoparticles may have potential applications in modifying the solar spectrum to enhance the efficiency of silicon solar cells.

Luminescence properties and efficient energy transfer in Ce3+ and Tb3+ co-doped Mg2La3[SiO4]2[PO4]O phosphor

A series of Ce 3+ , Tb 3+ singly doped and Ce 3+ / Tb 3+ -co-doped Mg 2 La 3 [SiO 4 ] 2 [PO 4 ]O phosphors with the apatite-like structure were prepared by conventional solid-state reaction. The X-ray diffraction (XRD), photoluminescence excitation and emission spectra, luminescence decay curves and lifetimes were applied to characterize the samples. The efficient excitation energy transfer from Ce 3+ to Tb 3+ was verified by the excitation and emission spectra together with the luminescence decay curves. The Ce 3+ /Tb 3+ co-doped phosphor shows an intense broad excitation band between 300 and 430 nm, which matches near-UV chip (350–420 nm). Under the excitation of near UV light, the Ce 3+ /Tb 3+ co-doped sample exhibits two distinct luminescence bands: a blue one centered at about 415 nm originating from Ce 3+ ions and a green-emitting at 543 nm from the 5 D 4 → 7 F 5 transition of Tb 3+ ions. The emission can be tuned from the blue to the green color by changing the doping concentration of Tb 3+ . The energy transfer from Ce 3+ to Tb 3+ has been demonstrated to be the dipole–quadrupole mechanism, and the critical distance is calculated to be 16.04 Å.

Efficient near-infrared quantum cutting in Yb3+ doped CaNb2O6 phosphor based on energy transfer

An efficient near-infrared (NIR) quantum cutting (QC) phosphor, CaNb2O6:Yb3+, has been developed to realize broadband downconversion from ultraviolet (UV) into NIR. Upon the excitation of UV light in the broad absorption band of niobate groups, an intense NIR emission of Yb3+ ion around 1μm, matching the maximum spectral response of Si-based solar cells very well, was obtained. Through the analyses of the measured luminescence spectra and decay lifetimes, a mechanism of energy transfer from niobate groups to Yb3+ ions was proposed to rationalize the downconversion process. This broadband NIR QC phosphor might have potential application in enhancing response of Si-based solar cells via spectral modification.

Synthesis, Optical and Structural Properties, and Charge Carrier Dynamics of Cu-Doped ZnSe Nanocrystals

Stable and luminescent Cu-doped ZnSe nanocrystals (NCs) were synthesized in organic solvents with octadecylamine (ODA) as the capping ligand and characterized using a combination of optical and structural characterization techniques. Successful doping was achieved by adding Cu during the growth phase of the NCs when their size was ∼4 nm. The appearance of red-shifted, intense photoluminescence (PL) peak with doping indicates the incorporation of Cu in the NCs, and stability of dopant emission infers the internal doping of NCs. Extended X-ray absorption fine structure (EXAFS) studies revealed that Cu is surrounded by four neighbors in the lattice but is very close to the NC surface and gets oxidized when NCs are precipitated from the solution. For the undoped sample, time-resolved PL studies using time-correlated single photon counting (TCSPC) reveal the luminescence decay lifetimes of about 1.1, 12, and 60 ns that we attribute to near-bandedge, shallow trap (ST) state, and deep trap (DT) state emissions, respectively. In addition to these decay components, the Cu-doped sample was found to have a long-lived component with a lifetime of 630 ns. Luminescence decay lifetimes of near-bandedge and ST state emissions were slightly shortened by doping (1 and 10 ns, respectively), suggesting that the Cu dopant competes with these states in trapping the charge carriers from the conduction band (CB) or near-bandedge states. However, the presence of Cu was found to increase the lifetime of DT states from 60 to 100 ns probably due to a decrease in coupling of electron and hole states involved in this emission upon Cu doping. Synthesis of such stable, doped samples along with a better understanding of charge carrier dynamics is significant for emerging optical applications of these NCs.

Measurement of transient acoustic fields using a single-shot pressure-sensitive paint system

A pressure-sensitive paint (PSP) system capable of measuring high-frequency acoustic fields with non-periodic, acoustic-level pressure changes is described. As an optical measurement technique, PSP provides the experimenter with a global distribution of pressure on a painted surface. To demonstrate frequency response and enhanced sensitivity to pressure changes, a PSP system consisting of a polymer∕ceramic matrix binder with platinum tetra(pentafluorophenyl) porphyrin (PtTFPP) as the oxygen probe was applied to a wall inside an acoustic resonance cavity excited at 1.3 kHz. A data acquisition technique based on the luminescent decay lifetime of the oxygen sensors excited by a single pulse of light afforded the ability to capture instantaneous pressure fields with no phase-averaging. Superimposed wave-like structures were observed with a wavelength corresponding to a 4.7% difference from the theoretical value for a sound wave emanating from the speaker. High sound pressure cases upwards of 145 dB (re 20 μPa) exhibited skewed nodal lines attributed to a nonlinear acoustic field. The lowest sound pressure level of 125.4 dB--corresponding to an amplitude of 52.7 Pa, or approximately 0.05% of standard sea-level atmospheric pressure--showed that the paint could resolve the spatial details of the mode shape at the given resonance condition.

Photoluminescence properties and analysis of spectral structure of R_2(MoO_4)_3:Eu^3+ (R=La,Gd) phosphors

The site-selective excitation and emission spectroscopy and luminescence decay (lifetime) have been investigated in the 5D0→7F0 region using a pulsed, tunable, and narrowband dye laser. We observed two crystallographic sites for Eu3+ in La2(MoO4)3 and one crystallographic site for Eu3+ in Gd2(MoO4)3. The stark energy levels for Eu3+ at different sites were assigned from site-selective emission spectra. The luminescent properties of the as-synthesized samples were also investigated, which showed strong lines at 395 nm in excitation spectra and intensive red emission, indicating that they could be used as red components for white LEDs.