Temperature-dependent Luminescence Properties Research Articles

An orange emitting phosphor Lu2−xCaMg2Si2.9Ti0.1O12:xCe with pure garnet phase for warm white LEDs

A new silicate garnet phosphor, Lu2−xCaMg2Si2.9Ti0.1O12:xCe was synthesized by a high temperature solid-state reaction under reductive atmosphere. X-ray diffraction (XRD) showed that the powder was pure garnet phase. The emission and excitation spectrum indicated that the Lu2−xCaMg2Si2.9Ti0.1O12:xCe phosphors could absorb blue light in the spectral range of 400–550nm efficiently and exhibit bright yellow–orange emission in the range of 520–750nm. With the increase of Ce3+ concentration, the emission band of Ce3+ showed a red shift. Interestingly, the concentration quenching occurred when the Ce3+ concentration exceeded 4mol%. The temperature-dependent luminescent properties of the phosphors were discussed and the Lu1.96CaMg2Si2.9Ti0.1O12:0.04Ce phosphors showed good performances in color temperature (2430K) and potential applications for warm white LEDs.

Temperature-dependent luminescence of GdTaO4 single crystal

Recently, we proposed GdTaO4 as a promising high-density scintillator but the scintillation mechanism was still unknown. In this paper, we investigated the temperature-dependent luminescent properties of GdTaO4 crystal. The emission spectra are characterized by a broad band composed of two Gaussian components located at ~2.2eV and ~2.7eV of 8–300K, which are both thermally quenched with the activation energy of 156meV and 175meV, respectively. These two bands are believed to be from different luminescent centers, judging from the excitation spectra, thermal activation energy and decay curves. Additionally, it was also found that the annealing atmosphere exerts little influence on the luminescence intensity. Based on the results, we tentatively assigned the high-energy band to self-trapped excitons (STE) localized in TaO43− groups and the low-energy band to relaxed excitons related to lattice imperfections.

Designed synthesis of large-pore crystalline polyimide covalent organic frameworks.

Covalent organic frameworks (COFs) are an emerging class of porous crystalline polymers with a wide variety of applications. They are currently synthesized through only a few chemical reactions, limiting the access and exploitation of new structures and properties. Here we report that the imidization reaction can be used to prepare a series of polyimide (PI) COFs with pore size as large as 42 × 53 Å(2), which is among the largest reported to date, and surface area as high as 2,346 m(2) g(-1), which exceeds that of all amorphous porous PIs and is among the highest reported for two-dimensional COFs. These PI COFs are thermally stable up to 530 °C. We also assemble a large dye molecule into a COF that shows sensitive temperature-dependent luminescent properties.

Luminescence properties of Lu3Al5O12:Tb3+ nano-garnet

Trivalent terbium-doped lutetium-aluminate nano-garnet (Lu3Al5O12:Tb3+) powder was prepared by using the Pechini sol-gel process. The structure and crystallinity of the Lu3Al5O12:Tb3+ nano-garnet were characterized by using X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy. The green emission of Tb3+ ions is observed at 545 nm corresponding to the 5D4 → 7F5 transition under the 271-nm excitation. The temperature dependent luminescence properties of the Lu3Al5O12:Tb3+ nano-garnet are investigated. The results show that the present garnet exhibits better thermal stability than the other green emitting phosphors, hence, the Lu3Al5O12:Tb3+ phosphor is a promising candidate for light-emitting devices.

Surface thermometry in combustion diagnostics by sputtered thin films of thermographic phosphors

The accuracy and robustness of the thermographic phosphors (TP) technique relies in the use of coatings with low thickness, high-intensity luminescent emission and high adhesion to the surfaces. Sputter deposition has been evaluated as an alternative for coating preparation of TPs for surface thermometry in combustion diagnostics. Thin films of \(\hbox {Gd}_{3}\hbox {Ga}_{5}\hbox {O}_{12}{:}\hbox {Cr}^{3+}\) have been deposited on fused silica and stainless steel substrates by radio frequency magnetron sputtering. Physical, chemical, and temperature-dependent luminescence properties of the phosphor films have been evaluated using X-ray diffraction, X-ray photoelectron spectroscopy and laser-induced luminescence, respectively. The results showed that the luminescence features of the thin films must be activated by heat treatment after sputter deposition. The \(\hbox {Gd}_{3}\hbox {Ga}_{5}\hbox {O}_{12}{:}\hbox {Cr}^{3+}\) films exhibited appropriate temperature sensitivity with adequate precision of the temperature determination, proving to be suitable for pointwise (0D) surface thermometry. An evaluation of the spatial homogeneity of the luminescence properties, which has not been yet addressed in the literature for thin films of TPs, revealed that thin \(\hbox {Gd}_3\hbox {Ga}_5\hbox {O}_{12}{:}\hbox {Cr}^{3+}\) films deposited on fused silica can be used for spatially resolved surface thermometry while those deposited on stainless steel require improvements to overcome spatial inhomogeneities of the luminescence lifetimes.

Average and Local Structural Origins of the Optical Properties of the Nitride Phosphor La3–xCexSi6N11 (0 < x ≤ 3)

Structural intricacies of the orange-red nitride phosphor system La(3-x)Ce(x)Si6N11 (0 < x ≤ 3) have been elucidated using a combination of state-of-the art tools, in order to understand the origins of the exceptional optical properties of this important solid-state lighting material. In addition, the optical properties of the end-member (x = 3) compound, Ce3Si6N11, are described for the first time. A combination of synchrotron powder X-ray diffraction and neutron scattering is employed to establish site preferences and the rigid nature of the structure, which is characterized by a high Debye temperature. The high Debye temperature is also corroborated from ab initio electronic structure calculations. Solid-state (29)Si nuclear magnetic resonance, including paramagnetic shifts of (29)Si spectra, are employed in conjunction with low-temperature electron spin resonance studies to probes of the local environments of Ce ions. Detailed wavelength-, time-, and temperature-dependent luminescence properties of the solid solution are presented. Temperature-dependent quantum yield measurements demonstrate the remarkable thermal robustness of luminescence of La2.82Ce0.18Si6N11, which shows little sign of thermal quenching, even at temperatures as high as 500 K. This robustness is attributed to the highly rigid lattice. Luminescence decay measurements indicate very short decay times (close to 40 ns). The fast decay is suggested to prevent strong self-quenching of luminescence, allowing even the end-member compound Ce3Si6N11 to display bright luminescence.

Carriers' localization and thermal redistribution in post growth voluntarily tuned quantum dashes' size/composition distribution

This paper treats the impact of post growth tuned InAs/InP quantum dashes' (QDas) size/composition distribution on carriers' localization and thermal redistribution. The spread of this distribution depends on the experimental conditions used for the phosphorus ion implantation enhanced intermixing process. Atypical temperature-dependent luminescence properties have been observed and found to be strongly dependent on the amount of QDas size/composition dispersion. The experimental results have been reproduced by a model that takes into account the width of the QDas localized states distribution and consequent thermally induced carriers' redistribution. This model gives critical temperature values marking the beginning and the end of carriers delocalization and thermal transfer processes via an intermixing induced carrier's transfer channel located below the wetting layer states.

Identification of the crystallographic sites of Eu2+ in Ca9NaMg(PO4)7: structure and luminescence properties study

The substituted activator ions Eu(2+) in the β-Ca3(PO4)2-type structure, Ca9NaMg(PO4)7 compound, have been studied as emission centers, as well as structural probes. The crystal structure and crystallographic sites of Eu(2+) in Ca9NaMg(PO4)7 have been identified by Rietveld refinements, electron paramagnetic resonance (EPR) spectroscopy analysis and combined photoluminescence spectral analysis, respectively. Ca9NaMg(PO4)7:Eu(2+) phosphor exhibits a broad emission band in the visible light region with three distinguished peaks centered at 415, 458, and 615 nm, which are ascribed to different Eu(2+) emission centers occupied in three different Ca crystallographic sites. The concentration quenching behavior, variation in lifetimes and the temperature dependent luminescence properties have been investigated as a function of different emission centers, 415, 458, and 625 nm for Ca9NaMg(PO4)7:Eu(2+) phosphor. Accordingly, the relationship among three different Eu(2+) luminescence centers originating from three different Ca(2+) crystallographic sites has been found and verified.

Temperature dependent luminescence and energy transfer properties of Na2SrMg(PO4)2:Eu2+, Mn2+ phosphors

Eu(2+) singly and Eu(2+)/Mn(2+) co-doped Na2SrMg(PO4)2 (NSMP) phosphors have been prepared via a high-temperature solid-state reaction process. Upon UV excitation of 260-360 nm, the NSMP:xEu(2+) phosphors exhibit a violet band located at 399 nm and a blue band centered at 445 nm, which originate from Eu(2+) ions occupying two different crystallographic sites: Eu(2+)(I) and Eu(2+)(II), respectively. Excitation wavelengths longer than 380 nm can selectively excite Eu(2+)(II) to emit blue light. Energy transfer processes in the Eu(2+)(I)-Eu(2+)(II) and Eu(2+)-Mn(2+) pairs have been observed and investigated by luminescence spectra and decay curves. The emission color of as-prepared samples can be tuned by changing the relative concentrations of Eu(2+) and Mn(2+) ions and adjusting the excitation wavelength. Under UV excitation of 323 nm, the absolute quantum yield of NSMP:0.005Eu(2+) is 91%, which is higher than most of the other Eu(2+)-doped phosphors reported previously. The temperature dependent luminescence properties and decay curves (4.3-450 K) of NSMP:Eu(2+) and NSMP:Eu(2+), Mn(2+) phosphors have been studied in detail. Thermal quenching of Eu(2+) has been observed while the emission band of Mn(2+) shows a blue-shift and an abnormal increase of intensity with increasing temperature. The unusual thermal quenching behavior indicates that the NSMP compound can serve as a good lattice host for Mn(2+) ions which can be used as a red-emitting phosphor. Additionally, the lifetimes for Eu(2+)(I) and Eu(2+)(II) increase with increasing temperatures.

Reactant ratio-modulated six new copper(i)–iodide coordination complexes based on diverse [CumIm] aggregates and biimidazole linkers: syntheses, structures and temperature-dependent luminescence properties

A total of six copper(I)–iodide coordination complexes with diverse [CumIm] aggregates and bidentate biimidazole bridging ligands, [Cu2I2(dmimb)]n (1), [Cu4I4(dmimb)]n (2), [Cu6I6(dmimb)3]n (3), [Cu4I4(dimb)]n (4), [Cu2I2(dimb)2]n (5), and [Cu4I4(dimb)3]n (6), (dmimb = 1,4-di(2-methyl-imidazol-1-yl)butane, dimb = 1,3-di(imidazol-1-yl)benzene), have been synthesized and structurally characterized by single-crystal X-ray diffraction analyses and further characterized by infrared spectra (IR), elemental analyses, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). Single-crystal X-ray diffraction analysis reveals that 1 is a double-chain based 1D ribbon incorporating a discrete [Cu4I4] stepped cubane aggregate. Complex 2 is a 2D layer based on the 1D infinite [Cu6I6]n column aggregate. Complex 3 is a triple-chain based 1D ribbon incorporating a discrete ladder-like [Cu6I6] aggregate. Interestingly, 1 and 3 are a pair of genuine supramolecular isomers controlled by the reaction ratio with an identical chemical composition. In the 2D layered structure of complex 4, the rare “S-shaped-double-bowl” Cu10I10 aggregate is observed and share the Cu2I2 rhomboids to form the 1D infinite [Cu10I10]n column. Complex 5 is a binuclear 18-membered metal–organic macrocycle with [CuI] monomeric units. Complex 6 is also a 2D layer constructed from a 1D infinite [Cu8I8]n aggregate which consists of a 1D infinite [Cu6I6]n column and a [Cu2I2]n ladder. In the overall view, the biimidazole linkers play an important role in the formation of diverse [CumIm] aggregates as well as the resulting structures and topologies. The structural changes of 1/2/3 and 4/5/6 are highly influenced due to the change of the reaction stoichiometry. For 1, 2, 5 and 6, they are non-emissive at both 298 K and 77 K, whereas 3 and 4 exhibit interesting temperature-dependent luminescence properties. The correlation between the luminescence thermochromism and the temperature-dependent variation of the Cu⋯Cu distance is also elucidated.

The effect of the lanthanide contraction on coordination with the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine (dpp)

Abstract Attempts to make homodinuclear lanthanide complexes using the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine (dpp) with Pr3 +, Nd3 +, Eu3 +, and Gd3 + are presented. The ionic radius of the Ln3 + ion plays a crucial role in complex formation, with the larger lanthanides forming mononuclear complexes, and the smaller lanthanides forming dinuclear complexes. The structural and temperature dependent luminescent properties of [Eu(tta)3]2dpp; where tta = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione, are reported. The β-diketonate ligand (tta) is used as a capping ligand in this bonding motif. Both Eu3 + ions are eight‐coordinate with six O atoms from the β-diketones and two N atoms from the polyazine bridging ligand. Repulsion of the pyridyl hydrogens causes a distortion in the bridging ligand. Excitation of the β-diketonate ligand tta at ca. 340 nm in methanol solutions results in the characteristic Eu3 + emission in the visible region of the spectrum. The emission intensity, associated with the Eu3 + centers, decreases as the temperature of the solution is increased. Evaluation of the data in comparison to data obtained from a monometallic Eu3 + analog reveals very similar photoluminescent properties. This suggests little electronic communication between Eu3 + ions via the polyazine dpp bridging ligand.

Thermally stable blue-emitting NaSrPO 4:Eu 2+ phosphor for near UV white LEDs

A novel blue light emitting NaSr 1 − x PO 4:Eu 2+ x ( x = 0.001 to 0.02) phosphors were prepared by solid-state reaction method to investigate its optical properties and thermal stability for its application in white light-emitting diodes (w-LEDs). The excitation and emission spectra of the prepared phosphor reveal a broad emission peak centered at 460 nm which arises due to 4f–5d transitions of Eu 2+ upon the near ultra-violet (n-UV) excitation wavelength at 380 nm. The effect of Eu 2+ doping concentration and sintering temperature on the emission intensity of NaSrPO 4:Eu 2+ was investigated along with its chromaticity coordinates. The temperature dependent luminescence properties of the prepared phosphor show better results than that of the commercial YAG:Ce 3+phosphor. Besides, their XRD, FT-IR, SEM, TG, and DTA profiles have also been analyzed to explore its structural details.

Phase structure and temperature dependent luminescence properties of Sr 2LiSiO 4F:Eu 2+ and Sr 2MgSi 2O 7:Eu 2+ phosphors

Green-emitting Sr 2LiSiO 4F:Eu 2+ and blue-emitting Sr 2MgSi 2O 7:Eu 2+ phosphors were synthesized by the conventional high temperature solid-state route, respectively. Their structures and photoluminescence properties were comparatively investigated. It was found that the mixture phases of Sr 2MgSi 2O 7 and SrF 2 were obtained when a part of Sr 2+ in Sr 2LiSiO 4F was replaced by some amount of Mg 2+ in order to design the possible SrMgLiSiO 4F:Eu 2+ phosphor. Based on the photoluminescence analysis, Sr 2LiSiO 4F:Eu 2+ phosphor exhibited a green broad emission band of main peak at 513 nm under the excitation of 365 nm, while the Sr 2MgSi 2O 7:Eu 2+ and SrMgLiSiO 4F:Eu 2+ phosphor showed blue emission centered at 467 nm. The temperature dependent photoluminescence properties and room temperature decay time for the three kinds of phosphors were also discussed in this paper.

Investigation on temperature dependence of photoluminescence in Sr 1.7Eu 0.3M xCeO 4.15+ x/2 (M = Li +, Na +, K +, x = 0, 0.3) red phosphors

The temperature-dependent luminescence of Sr 1.7Eu 0.3M x CeO 4.15+ x/2 (M = Li +, Na +, K +, x = 0, 0.3) samples was investigated and discussed in the temperature range from 303 to 573 K. It is found that the thermal quenching temperature of samples decreases with Li +-/Na +-doping but increases with the incorporation of K +. We suggest that these observations are resulted from two factors. One is that the incorporation of Li +/Na +/K + ions reduces the strength of potential field at the O 2− sites, and then results in a red-shift of the Eu–O charge transfer band. The other is that Δ r expands with Li +-/Na +-doping but shrinks with K +-doping. We consider that it is a feasible way to adjust the temperature-dependent luminescence properties of materials by introducing appropriate impurities.

Sol–gel deposition of multiply doped thermographic phosphor coatings Al 2O 3:(Cr 3+, M 3+) (M = Dy, Tm) for wide range surface temperature measurement application

A promising method of measuring surface temperatures in harsh environments is the use of thermographic phosphor coatings. There, the surface temperature is evaluated from the phosphorescence decay lifetime following a pulsed laser or flash lamp light excitation. Depending on the used dopant, single doped M 3+:α-Al 2O 3 (M = Cr, Dy, Tm) emit at 694 nm (Cr 3+), 488 nm (Dy 3+), 584 nm (Dy 3+), and 459 nm (Tm 3+), respectively. However, the accessible temperature range with a single dopant is limited: for the Cr 3+-transition from 293 K up to 900 K, and for the Dy 3+ and Tm 3+-transitions both from 1073 K up to 1473 K. In the present study a new approach is followed to extend these limitations by co-doping two dopants using the sol–gel method and dip coating of α-Al 2O 3 thin films. For that application (Dy 3+ + Cr 3+) co-doped thin α-Al 2O 3 films and (Tm 3+ + Cr 3+) co-doped α-Al 2O 3 films with thicknesses of 4–6 μm were prepared, and the temperature-dependent luminescence properties (emission spectra and lifetimes) were analysed after pulsed laser excitation in the UV (355 nm). The phosphorescence lifetime as a function of temperature were measured between 293 K and 1473 K. A considerably extended range for surface temperature evaluation was established following this new approach by combining different dopants on the molecular level.

Sol–gel-deposition of thin TiO 2:Eu 3+ thermographic phosphor films

A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this application, the temperature-dependent luminescence properties of europium(III)-doped anatase (TiO 2:Eu 3+) thin films were studied. The films were prepared by the sol–gel method using dip coating. The structures and the morphology of the films were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electron dispersive X-ray spectroscopy (EDX) was used to verify the europium concentration within the films. For using the films as temperature sensors the optical properties are the main concern. Therefore, the emission spectra of the films were measured after ultraviolet laser excitation (355 nm). They indicate that the red characteristic emission (617 nm) of TiO 2:Eu 3+ due to the 5 D 0 → 7 F 2 electric dipole transition is the strongest. The decay time constant of the exponential emission decay under UV excitation with a Nd:YAG laser (355 nm, f = 10 Hz) is strongly temperature dependent in the range from 200 °C up to 400 °C, making it useful for temperature evaluation. The temperature dependence was measured for the emission line at 617 nm; the results demonstrate that anatase doped europium(III) can be used as a thermographic phosphor.

Sol–gel-deposition of thin TiO 2:Eu 3+ thermographic phosphor films

A relatively new promising method for surface temperature measurement is the use of thermographic phosphors. For this application, the temperature-dependent luminescence properties of europium (III)-doped anatase (TiO 2:Eu 3+) thin films were studied. The films were prepared by the sol–gel method using dip coating. The structures and the morphology of the films were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Electron dispersive X-ray spectroscopy (EDX) was used to verify the europium concentration within the films. For using the films as temperature sensors the optical properties are the main concern. Therefore, the emission spectra of the films were measured after ultraviolet laser excitation (355 nm). They indicate that the red characteristic emission (617 nm) of TiO 2:Eu 3+ due to the 5 D 0 → 7 F 2 electric dipole transition is the strongest. The decay time constant of the exponential emission decay under UV excitation with a Nd:YAG laser (355 nm, f = 10 Hz) is strongly temperature dependent in the range from 200 °C up to 400 °C; making it useful for temperature evaluation. The temperature dependence was measured for the emission line at 617 nm; the results demonstrate that anatase doped europium (III) can be used as a thermographic phosphor.

Sol–gel deposition of multiply doped thermographic phosphor coatings Al 2O 3:(Cr 3+, M 3+) (M = Dy, Tm) for wide range surface temperature measurement application

A promising method of measuring surface temperatures in harsh environments is the use of thermographic phosphor coatings. There, the surface temperature is evaluated from the phosphorescence decay lifetime following a pulsed laser or flash lamp light excitation. Depending on the used dopant, single doped M 3+:α-Al 2O 3 (M = Cr, Dy, Tm) emit at 694 nm (Cr 3+), 488 nm (Dy 3+), 584 nm (Dy 3+), and 459 nm (Tm 3+), respectively. However, the accessible temperature range with a single dopant is limited: for the Cr 3+-transition from 293 K up to 900 K, and for the Dy 3+ and Tm 3+-transitions both from 1073 K up to 1473 K. In the present study a new approach is followed to extend these limitations by co-doping two dopants using the sol–gel method and dip coating of α-Al 2O 3 thin films. For that application (Dy 3+ + Cr 3+) co-doped thin α-Al 2O 3 films and (Tm 3+ + Cr 3+) co-doped α-Al 2O 3 films with thicknesses of 4–6 μm were prepared, and the temperature-dependent luminescence properties (emission spectra and lifetimes) were analysed after pulsed laser excitation in the UV (355 nm). The phosphorescence lifetime as a function of temperature were measured between 293 K and 1473 K. A considerably extended range for surface temperature evaluation was established following this new approach by combining different dopants on the molecular level.

X-ray structure and temperature dependent luminescent properties of two bimetallic europium complexes

The structural, luminescent and temperature dependent luminescent properties of two homodinuclear europium complexes bridged by 2,2′-bipyrimidine (bpm) are reported. β-Diketonate ligands 4,4,4-trifluoro-1-(2-furyl)-1,3-butanedione (tfa) and 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (tta) are used as capping ligands resulting in complexes of the form [Eu(tfa) 3] 2bpm ( 1) and [Eu(tta) 3] 2bpm ( 2). All Eu III ions are eight coordinate with six O atoms from the β-diketones and two N atoms from the polyazine bridging ligand. Excitation of the β-diketonate ligands tfa or tta at ca. 340 nm in toluene solutions results in the characteristic Eu III emission in the visible region of the spectrum. The emission intensity and lifetime associated with the Eu III centers decrease as the temperature of the solution is increased. Lifetime measurements are fit to a monoexponential while the temperature dependent lifetime data is fit to an Arrhenius-type equation. Evaluation of the data in comparison to data obtained from the monometallic Eu III analogs reveal very similar photoluminescent properties. This suggests little electronic communication between Eu III ions via the polyazine bpm bridging ligand.

Mesoporous Metal–Organic Framework with Rare etb Topology for Hydrogen Storage and Dye Assembly

More holes than Swiss cheese: A new MOF with the largest reported 1D hexagonal nanotube-like channels of 24.5 Å×27.9 Å and rare etb topology has been synthesized (see picture). It exhibits good fluorescence properties and substantial hydrogen uptake. For the first time, the compound was assembled with Rh6G dye molecules, and the product showed favorable temperature-dependent luminescent properties.