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Enhancement of Long-Persistence by Ce Co-Doping in CaS:Eu[sup 2+], Tm[sup 3+] Red Phosphor
Abstract
CaS:Eu2+, CaS:Ce3+, Na+, CaS:Eu2+, Ce3+, CaS:Eu2+, Tm3+, and CaS:Eu2+, Ce3+, Tm3+ phosphors samples were prepared using a solid state chemical reaction method. Emission and excitation spectra of CaS:Eu2+ (peak emission observed at 648 nm) and CaS:Ce3+ (peak emission observed at 515 nm) were investigated. Neither Eu2+ singly doped sample nor Ce3+ and Na+ codoped sample had a long afterglow. CaS:Eu2+, Ce3+ showed some afterglow with a short persistence. By incorporation of Ce3+, an efficient energy transfer from Ce3+ to Eu2+ was found at a low doping concentration 0.1%. Emission intensity of Eu2+ was enhanced by 20%. Enhancement of long persistence of Eu2+ afterglow emission was achieved by co-doping Ce3+ in CaS:Eu2+, Tm3+ as a durable sensitizer. For the triply doped CaS:Eu2+, Tm3+, Ce3+ it was found that the persistence time was 22% longer than that of CaS:Eu2+, Tm3+. (c) 2006 The Electrochemical Society.
... Ce 3+ ion activated long lasting phosphors have been reported by Dongdong Jia et al [11,12] and Kodama et al [13] . Ce 3+ is well known as an efficient sensitizer and Ce 3+ to Eu 2+ energy transfer is possible [14,15] . Dongdong Jia et al [14] found that through energy transfer from Ce 3+ to Eu 2+ , CaS:Eu 2+ ,Tm 3+ ,Ce 3+ has higher brightness and longer lasting time than CaS:Eu 2+ ,Tm 3+ All the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku diffractometer with Ni-filtered Cu Kα radiation. ...
... Ce 3+ is well known as an efficient sensitizer and Ce 3+ to Eu 2+ energy transfer is possible [14,15] . Dongdong Jia et al [14] found that through energy transfer from Ce 3+ to Eu 2+ , CaS:Eu 2+ ,Tm 3+ ,Ce 3+ has higher brightness and longer lasting time than CaS:Eu 2+ ,Tm 3+ All the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku diffractometer with Ni-filtered Cu Kα radiation. The photoluminescence spectra were measured by a FLS-920T fluorescence spectrophotometer with Xe 900 (450 W xenon arc lamp) as the light source. ...
... Spectral overlap has been observed in Ce 3+ emission and Eu 2+ excitation in Sr 2 MgSi 2 O 7 , which is shown in Figure 5. Energy transfer usually took place at high doping concentration [16] . But for Sr 1 Eu 2+ absorption was almost 100% ( Figure 5).Therefore, energy transfer from Ce 3+ to Eu 2+ was strong and Ce 3+ emission was totally quenched by Ce 3+ -Eu 2+ energy transfer, the similar phenomenon was reported by Jia et al [14] in CaS:Eu 2+ ,Ce 3+ system. Figure 6.The emission band centered at 466 nm is attributed to the typical 4f 6 5d 1 -4f 7 transition of Eu 2+ . ...
Sr2MgSi2O7:Ce3+, Sr2MgSi2O7:Eu2+, Sr2MgSi2O7:Eu2+, Ce3+, Sr2MgSi2O7:Eu2+,Dy3+ and Sr2MgSi2O7: Eu2+, Dy3+, Ce3+ were prepared under a weak reducing atmosphere by a solid state reaction. Ce3+ singly doped sample had no afterglow. Sr2MgSi2O7:Eu2+, Ce3+showed some afterglow with a short persistenence. By incorporation of Ce3+, an efficient energy transfer from Ce3+ to Eu2+ was found and emission intensity of Eu2+ was enhanced. The triply doped phosphor Sr2MgSi2O7: Eu2+, Dy3+, Ce3+ has higher brightness and longer lasting time than Sr2MgSi2O7:Eu2+, Dy3+.
... Twenty years after this discovery, many lanthanide ion doped aluminates and silicates such as CaAl 2 O 4 :Eu 2+ -Nd 3+ (λ em = 440 nm) [15,22], Sr 4 Al 14 O 25 :Eu 2+ -Dy 3+ (λ em = 490 nm) [23,24] and Sr 2 MgSi 2 O 7 :Eu 2+ -Dy 3+ (λ em = 470 nm) [25], etc. with emission colours mainly in blue and green regions, have achieved a big commercial success for civil applications, especially in emergency signage, safety indication, luminous paints and watch dials [13,14,17,19,26]. On the contrary, only very limited reddish orange and red afterglow phosphors such as Y 2 O 2 S:Eu 3+ , Mg 2+ , Ti 4+ [27][28][29][30][31] and (Ca 1-x Sr x )S:Eu 2+ [32][33][34] have gained the access to markets. The oxysulfides or sulfides rather than conventional oxides are chosen only because of the lack of really powerful candidates emitting in red, especially taking into account the insensitivity of human's visual perception in the red spectral region [4,35]. ...
... x Sr x )S:Eu 2+ [32][33][34]393]) persistent phosphors have gained the access to commercial markets. Note that the main shortcomings of sulfides, i.e., degradation by hydrolysis and instability against humidity, should not be equally translated to oxysulfides. ...
Great progress has been made in inorganic persistent phosphors, especially in the recent two decades, motivated by the discoveries of the SrAl2O4:Eu²⁺-Dy³⁺ in the green and Cr³⁺ doped spinel compounds in the deep-red to near-infrared (NIR) spectral regions. However, the physical mechanism behind this kind of “self-sustained” luminescence is still the subject of debate, and the improvement of known persistent phosphors and/or the development of new ones are still a matter of trial-and-error. In this review, starting from the introduction of longstanding histories of persistent luminescence (PersL), we provide comprehensive insights into its physical mechanism. Particular focus is put on the state-of-the-art of designing new persistent phosphors via “bandgap engineering” based on the knowledge about trapping-detrapping mechanisms of charge carriers and energy level locations of emitting/trapping centers. Recent significant works on PersL observed in organic molecules and phosphorescence observed in inorganic phosphors are also highlighted in order to give a clear distinction between these two long-lived luminescence phenomena. Key challenges, feasible improvements and perspectives of PersL working in the ultraviolent (UV), white, red and NIR (over 1000 nm) regions together with new charging concepts by NIR or visible-light lasers are also presented. It is hoped that this review could give new inspiration for the future development of PersL in emerging applications.
... However, Y2O2S:Eu 3+ ,Mg 2+ ,Ti 4+ can only be excited efficiently by UV or violet light because of the spinforbidden 4f-4f transition of Eu 3+ , which means that it can't be excited efficiently by room light, which will hinder its practical applications [41]. CaS:Eu 2+ ,Tm 3+ can be excited efficiently by room light but suffers from fast decay and poor water resistance [42]. SMS/CASN composite film possesses bright red persistent luminescence after blue light excitation, which can be seen for more than 5 h in the dark by naked eyes. ...
... However, Y 2 O 2 S:Eu 3+ ,Mg 2+ ,Ti 4+ can only be excited efficiently by UV or violet light because of the spin-forbidden 4f -4f transition of Eu 3+ , which means that it can't be excited efficiently by room light, which will hinder its practical applications [41]. CaS:Eu 2+ ,Tm 3+ can be excited efficiently by room light but suffers from fast decay and poor water resistance [42]. SMS/CASN composite film possesses bright red persistent luminescence after blue light excitation, which can be seen for more than 5 h in the dark by naked eyes. ...
Colorful spectra are important for the diverse applications of persistent phosphors. A color conversion concept is developed to obtain abundant persistent luminescence color by mining capacities of known persistent phosphors with the most efficient persistent properties. Here, SiO₂/Sr₂MgSi₂O₇:Eu,Dy nanoparticles are chosen as a blue persistent luminescence donor nanophosphor, while ultrafine CaAlSiN₃:Eu is utilized as a red conversion phosphor to tune the persistent luminescence spectra from blue to red. The red afterglow emission can persist for more than 5 h. The decay of the red afterglow follows nearly the same kinetics as that of the blue one. Continuous color tuning can be successfully obtained by simply changing the mass ratio of the donor/conversion phosphor pair. This color conversion strategy may be significant in indicating numerous persistent/conversion nanocomposites or nanostructures and advance the development of persistent phosphors in diverse fields which need colorful spectral properties.
... where H 2 was supplied by 5%H 2 /95%N 2 flow and considered as the reductant for Eu 3+ ions as well as an assistant for decomposition of ZnO. The excitation and emission spectra of the as-obtained CaS:Eu 2+ samples make no odds with those reported phosphors [2,8,26]. It indicates that the method in this paper can provide a new strategy to synthesize calcium sulfide based material. ...
... Firstly, the ultraviolet excitation bands in Fig. 3 reveal the biggest variance. CaS:Eu 2+ possesses a prominent UV excitation band which locates at about 327 nm attributed to 4f 7 ( 8 S 7/2 ) ? 4f 6 5d 1 (t 2g ) [2,26] transition of Eu 2+ ions, while the weak UV band of CaZnOS peaks at 378 nm attributed to host lattice (HL) absorption of CaZnOS. In addition, the main excitation band of CaS:Eu ranges from 400 to 630 nm. ...
A one-step surfactant-ligand co-assisted solvothermal technique was proposed for the preparation of CaS-based luminescent materials with ethylenediamine as solvent, and acetylacetone, 1-dodecanethiol and polyvinylpyrrolidone as capping agents. The morphologies of the obtained particles can be adjusted from an octahedron to a hexagonal prism or a cube by changing the solvent volume ratios of ethylenediamine and different capping agent(s), and the mean sizes of the crystals can be changed in a relatively wide range from tens of nanometers to a few micrometers. The growth process and dynamic mechanism were discussed. A series of CaS:xCe3+ nanocrystals were prepared and a small red shift of emission positions was observed from green to yellowish-green with increasing Ce3+ concentration.
... where H 2 was supplied by 5%H 2 /95%N 2 flow and considered as the reductant for Eu 3+ ions as well as an assistant for decomposition of ZnO. The excitation and emission spectra of the as-obtained CaS:Eu 2+ samples make no odds with those reported phosphors [2,8,26]. It indicates that the method in this paper can provide a new strategy to synthesize calcium sulfide based material. ...
... Firstly, the ultraviolet excitation bands in Fig. 3 reveal the biggest variance. CaS:Eu 2+ possesses a prominent UV excitation band which locates at about 327 nm attributed to 4f 7 ( 8 S 7/2 ) ? 4f 6 5d 1 (t 2g ) [2,26] transition of Eu 2+ ions, while the weak UV band of CaZnOS peaks at 378 nm attributed to host lattice (HL) absorption of CaZnOS. In addition, the main excitation band of CaS:Eu ranges from 400 to 630 nm. ...
The red-emitting phosphor CaZnOS:Eu2+ was synthesized from CaCO3, ZnS, Eu2O3 and CeCl3 by controlling the sintering condition. It was found that Ce3+ ions can play a role of reductant to contribute to the formation of Eu2+ in CaZnOS matrix under inert protective atmosphere. While the gas flow changed to H2/N2, the product turned to CaS easily. XRD, photoluminescence spectra, UV–vis and IR absorption spectra were evaluated to investigate the origin of the distinctions of the optical properties and stabilities between the two divalent europium ions doped phosphors CaZnOS:Eu2+ and CaS:Eu2+. The similarities and differences between them were analyzed.
... Persistent luminescence materials have drawn much attention due to their potential applications in many fields, such as road designs, safety indications, graphic arts, emergency lighting, interior decoration and billboards [1][2][3][4][5]. Up to date, most of the LLP phosphors are Eu 2+ -activated aluminate and silicate compounds, part of which have entered into the commercial market, such as SrAl 2 O 4 :Eu 2+ ,Dy 3+ , CaAl 2 O 4 :Eu 2+ ,Nd 3+ and Sr 2 MgSi 2 O 7 :-Eu 2+ ,Dy 3+ [6][7][8] owing to their superior properties in LLP brightness and persistent time as well as chemical and physical stabilities, compared to the traditional sulfide phosphors. Among those phosphors, the doped Eu 2+ ions act as luminescent centers and the phosphorescence is ascribed to its parity-allowed 4f 6 5d 1 -8 S 7/2 electronic transition, which is strongly influenced by the host lattice due to the crystal-filed effect. ...
... Among those phosphors, the doped Eu 2+ ions act as luminescent centers and the phosphorescence is ascribed to its parity-allowed 4f 6 5d 1 -8 S 7/2 electronic transition, which is strongly influenced by the host lattice due to the crystal-filed effect. Trivalent rare earth ions are usually incorporated into the host as auxiliary activators to modify the defects and improve the phosphorescence properties [1,[5][6][7][8][9][10]. In recent years, researchers focus their interests to develop new LLP materials based on different hosts with various activators, as well as to well understand the mechanisms of LLP [11][12][13][14]. ...
A new long-lasting phosphorescence phosphor Ca9Bi(PO4)7:Eu2+,Dy3+ was synthesized by solid state reaction and its long-lasting phosphorescence properties were investigated for the first time. The X-ray powder diffraction, photoluminescence, long-lasting phosphorescence spectra, decay curves and thermoluminescence curves were measured. The Ca9Bi(PO4)7:Eu2+,Dy3+ phosphor exhibits an asymmetric emission centered at 475 nm, which can be ascribed to the 4f65d1–4f7 electronic transition of Eu2+. For the optimized sample, the bright bluish green long-lasting phosphorescence could be observed for 5 h by naked eyes after the excitation source was removed. The long-lasting phosphorescence spectra show that the co-doping of Dy3+ ions greatly enhance the intensity of the long-lasting phosphorescence. Meanwhile, the long-lasting phosphorescence mechanism of this phosphor was discussed. Based on our study, Dy3+ ions are suggested to increase the density of electron or hole traps so as to improve the performance of the bluish green phosphorescence of Eu2+, including the intensity and persistent time.
... 33,40 The energy transfer mechanism via dipole-dipole interaction in SrS:Ce 3+ ,Eu 2+ is the same as that in CaS:Ce 3+ ,Eu 2+ . 41,42 As mentioned above, the single Eu 2+ -doped phosphors SrS:(Eu 2+ ) n show broad band absorption, which leads to a red body color under natural light (Fig. 6a) and favors its application for blue light-pumped LEDs. Notably, a long afterglow is observed by the naked eye and lasts for several seconds (Fig. 6a). ...
Although near-infrared phosphors have been widely reported, discovering efficient red-emitting phosphor materials with superior photophysical properties and high color quality is still a challenge for optimizing white light-emitting diodes (wLEDs). In this regard, a series of Eu²⁺ and Ce³⁺ doped SrS phosphors were synthesized via a one-step solid-state method at 1100 °C. The SrS:(Eu²⁺)n and SrS:(Ce³⁺)0.01,(Eu²⁺)n powders show substantial spectral broadening (∼80 nm) along with a significant thermal quenching resistance. The phosphor of SrS:(Eu²⁺)0.002 exhibits the most intense luminescence with a high color purity of 99.94% and an afterglow luminescence lasting several seconds. The luminescence intensity and peak position were modulated by the resonance-type energy transfer from Ce³⁺ to Eu²⁺ ions, which is proposed to follow the dipole–dipole interaction mechanism by Dexter's energy transfer theory and Reisfeld's approximation. The outstanding photoluminescence properties of Eu²⁺ are attributed to the nephelauxetic effect and crystal field splitting of rock-salt. The wLED device of SrS:(Eu²⁺)0.05 packaged with a blue chip and (Sr,Ba)2SiO4:Eu²⁺ shows a super-high color rendering index of 87.9, suggesting great potential for superior luminescence and promising wide applications.
... Only a handful number of stable and ideal red-emitting persistent phosphors have been identified so far, out of which sulfide-and oxysulfidebased phosphors, such as Y 2 O 2 S:Eu 3+ , exhibit appreciable red-emitting persistent luminescence. However, meager chemical stability of sulfide-based system makes their usage limited to only indoor usage [82,83]. ...
The current chapter presents a broad review of conventionally and recently developed oxide-based LA phosphors, which show unusual photon energy trapping property. These materials could be easily excited by sunlight and ambient lightings. Until a few years, the emission color of these LA phosphors was restricted to green (�520 nm) and yellow-green (�555 nm) regions, respectively. However, with the advancement in this field, other primary color emitting such as blue (�440 nm)and red (�610 nm)-emitting oxide phosphors is being developed even though their brightness and decay are comparatively less than that of established green-emitting LA phosphor. Exceptional emphasis has been laid in understanding the physics of materials and fundamental mechanism governing the afterglow luminescence property in oxide lattices. The article presents the classification of these phosphors concerning the year of development; their proposed afterglow mechanisms; and recently evolved exotic applications in the field of energy saving, medical, andforensic sciences. These multifunctional materials are impending candidates in the areas of fingerprint detection, marine mound application, sensors for structural damage, a biological marker, etc.
... Owing to this peculiarity, long afterglow phosphors have found wide applications in safety signage, decorations, optical memory, detection of high energy rays, night vision surveillance, drug carriers, and in vivo bio-imaging, and considerable research efforts have been invested in the development of new persistent phosphors in recent years [4][5][6][7]. Up to the present, many persistent phosphors with different emission colors have been developed, such as the green afterglow materials SrAl 2 O 4 :Eu 2+ ,Dy 3+ [8] and MgAl 2 O 4 :Mn 2+ [9]; the blue emitting CaAl 2 O 4 :Eu 2+ ,Nd 3+ [10] and SrMgSi 2 O 6 :Eu 2+ ,Dy 3+ [11]; the red and near infrared materials Y 2 O 2 S:Eu 3+ ,Mg 2+ ,Ti 4+ [12], CaS:Eu 2+ ,Tm 3+ ,Ce 3+ [13], ZnGa 2 O 4 :Cr 3+ [14], and Li 5 GaO 5 :Cr 3+ [15]. Among them, the commercialized SrAl 2 O 4 :Eu 2+ ,Dy 3+ and CaAl 2 O 4 :Eu 2+ ,Nd 3+ phosphors have already been widely used in daily life due to their more than 20 h afterglow. ...
... Europium doped alkaline earth sulfides have been known as good red emitting phosphors and can in general be synthesized at low cost [5][6][7]. Use rare earth ions as a dopant have been widely studied to modify the luminescence properties of CaS:Eu 2+ and the persistence or thermo-luminescence properties has become a hot spot of research [8][9][10][11][12]. A few issues, however, exist for this material system as red emitting phosphors for LED lighting applications. ...
The temperature dependence of the photoluminescence spectra of Ga³⁺-doped CaS:Eu²⁺ phosphors were investigated. The results showed that at room temperature, the peak position was blue shifted by increasing the Ga³⁺ concentration from 0 to 0.2 mol. The peak intensity showed bandwidth broadened and emission intensity decreased with the increase of temperature. With the increasing of Ga³⁺ dopant, the declining rate and dilated trend aggravated. Increasing FWHMs and decreasing emission intensities can be explained in terms of thermal quenching in the configuration coordinate diagram. The formula explanation is due to the dominant electron-phonon interaction for thermal decay and bandwidth broadening.
... Mainly due to their great research interests, the phosphors have been commercialized as night or dark environment vision materials for a wide range of applications such as security signs, emergency route signage, identification markers, or medical diagnostics [2]. The typical long-persistent phosphors are the commercialized primary color emitters, such as the red Y 2 O 2 S:Eu 3+ ,Mg 2+ ,Ti 4+ or CaS:Eu 2+ ,Tm 3+ ,Ce 3+ [3,4], the green SrAl 2 O 4 :Eu 2+ ,Dy 3+ or MgAl 2 O 4 :Mn 2+ [5,6], and the blue CaAl 2 O 4 :Eu 2+ ,Nd 3+ or SrMgSi 2 O 6 :Eu 2+ ,Dy 3+ [7,8] phosphors. Although many successes have been made in visible persistent phosphors, the investigation and development of persistent phosphors in the near infrared (NIR) region (~700-2500 nm) are insufficient. ...
Near infrared (NIR)-emitting persistent luminescent nanoparticles have been developed as potential agents for bioimaging. However, synthesizing uniform nanoparticles with long afterglow for long-term imaging is lacking. Here, we demonstrated the synthesis of spinel structured Zn3Ga2Ge2O10:Cr3+(ZGGO:Cr3+) and Zn3Ga2Ge2O10:Cr3+,Eu3+(ZGGO:Cr3+,Eu3+) nanoparticles by a sol-gel method in combination with a subsequent reducing atmosphere-free calcination. The samples were investigated via detailed characterizations by combined techniques of XRD, TEM, STEM, selected area electron diffraction, photoluminescence excitation (PLE)/photoluminescence (PL) spectroscopy, and temperature-dependent PL analysis. The single-crystalline nanoparticles are homogeneous solid solution, possessing uniform cubic shape and lateral size of ~ 80-100 nm. Upon UV excitation at 273 nm, ZGGO:Cr3+,Eu3+exhibited a NIR emission band at 697 nm (2E → 4A2transition of distorted Cr3+ions in gallogermanate), in the absence of Eu3+emission. NIR persistent luminescence of the sample can last longer than 7200 s and still hold intense intensity. Eu3+incorporation increased the persistent luminescence intensity and the afterglow time of ZGGO:Cr3+, but it did not significantly affect the thermal stability. The obtained ZGGO:Cr3+,Eu3+-NH2nanoparticles possessed an excellent imaging capacity for cells in vitro.
... In the early time, the sulfides activated by some rare earth ions such as MS: Eu 3+ (Eu 2+ ) (M = Ca, Sr, Zn) are the main red phosphorescence materials [26,27], but they are not very stable. Since Murazaki et al. reported a red LLP phosphor Y 2 O 2 S: Eu 3+ , Ti 4+ , Mg 2+ which afterglow lasted above 3 h [26], there have been many researches on red Eu 3+ activated oxysulfides LLP phosphors [28][29][30]. ...
A reddish orange emissive long afterglow phosphors Ca2−xSnO4:xSm³⁺ (x = 0.001–0.05) are prepared by solid-state reaction in air atmosphere. The synthesized phosphors are characterized and analyzed by X-ray diffraction, photoluminescence spectra, afterglow decay curves, and absorption spectra. Under excitation at 407 nm, three emission peaks locate at 565, 610 and 654 nm, respectively, which can be assigned to the ⁴G5/2→⁶HJ (J = 5/2, 7/2, 9/2) transitions of Sm³⁺ ion. The fluorescent intensity and the afterglow characteristic depend on the concentration of Sm³⁺. The optimal Sm³⁺ concentration is x = 0.01. The CIE 1931 chromaticity coordinates of the emission and afterglow are (x = 0.6103, y = 0.3891) and (x = 0.5668, y = 0.4325) located in the range of reddish orange light emission. The afterglow decay curves of the Ca2SnO4:Sm³⁺ phosphor indicate both fast and slow decay components. The striking difference of the afterglow luminescence intensities of the phosphor after irradiation under 254 and 365 nm UV is discussed in deeply with absorption spectrum.
... Since a classic deep red (peaking at 640 nm) persistent phosphor, CaS:Eu 2+ ,Tm 3+ ,Ce 3+ , can be efficiently excited by visible light that overlaps well with the efficient green upconversion of classical β-NaYF 4 :Yb,Er/ NaYF 4 nanoparticles ( Fig. S1 in the Electronic Supplementary Material (ESM)) [36,37], we hypothesize that efficient UCPL can be realized by integrating these two efficient PL and upconversion nanoparticles (UCNPs) (Scheme 1). Under 980 nm laser irradiation, ...
Persistent luminescence nanoparticles (PLNPs) and upconversion nanoparticles (UCNPs) are two special optical imaging nanoprobes. In this study, efficient upconverted persistent luminescence (UCPL) is realized by combining their unique features into polymethyl methacrylate, forming a film composed of both PLNPs and UCNPs. The red persistent luminescence (~640 nm) of the PLNPs (CaS:Eu,Tm,Ce) can be activated by upconverted green emission of UCNPs (β-NaYF 4 :Yb,Er@NaYF 4) excited by near-infrared light (NIR). Using this strategy, both the unique optical properties of PLNPs and UCNPs can be optimally synergized, thus generating efficient upconversion, photoluminescence, and UCPL simultaneously. The UCPL system has potential applications in in vivo bioimaging by simply monitoring the biocompatible low power density of NIR-light-excited persistent luminescence. Due to its simplicity, we anticipate that this method for the preparation of UCPL composite can be easily adjusted using other available upconversion and persistent phosphor pairs for a number of biophotonic and photonic applications.
... While blue or green afterglow is rather common in oxide hosts, it is much more difficult to find a suitable host material with sufficient red shift in order to obtain red (persistent) luminescence. Although there are a number of red-emitting Eu 2+ -doped persistent phosphors, such as CaS:Eu,Ln ( Jia, 2006a;Jia et al., 2000a,c) and Ca 2 Si 5 N 8 :Eu,Ln (Miyamoto et al., 2009;Van den Eeckhout et al., 2009), the choice is limited and the host lattices are chemically unstable or difficult to prepare. This is especially unfortunate, since red afterglow phosphors are strongly desired for applications in safety signage, paints, and as tracer particles for in vivo medical imaging (Section 10). ...
Persistent luminescence is the phenomenon whereby a material keeps emitting light for seconds to hours after the excitation has stopped. This chapter describes the history of this class of materials and how the discovery of a new family of very efficient persistent phosphors has given a boost to the development of both new materials and applications. Synthesis conditions and analytical techniques specific to persistent luminescent compounds are described, together with ways to evaluate their performance in terms of human eye perception. A state-of-the-art is presented for the materials—hosts and dopants—currently investigated for persistent luminescence, consistently referring to the original literature. Finally, in vivo medical imaging is shown to be a promising but challenging application of long-wavelength persistent luminescence and the relation between persistent luminescence and mechanoluminescence is described.
... In the past two decades, efforts have been made to either design or achieve long duration phosphors. By now blue CaAl 2 O 4 :Eu 2+ ,Nd 3+ [5], green SrAl 2 O 4 :Eu 2+ ,Dy 3+ [6], and red sulfides as Y 2 O 2 S: Eu 3+ ,Mg 2+ ,Ti 4+ [7] and CaS:Eu 2+ ,Tm 3+ , Ce 3+ [8] phosphors have been successfully commercialized. However, there exist challenges to develop red persistent emitters with better chemical stability and longer afterglow duration. ...
The impact of the addition of some amount of SiO2 in the ZnGa2O4:Cr3+ phosphors have been studied onto its persistent luminescent performance. The ZnGa2O4:Cr3+ phosphors with different Si4+ concentrations have been synthesized by using conventional solid-state reaction method. The X-ray diffractive patterns, photoluminescence (PL), thermoluminescence (TL) and afterglow decay have been measured and analyzed. The experimental results indicated that all the phosphors with different Si4+ codopant have the characteristic emission of Cr3+ and the co-doped Si4+ intensifies emission of N2 line and R lines. Furthermore the persistent luminescence was improved in both intensities and decay rates, in which the phosphor with 1mol% Si4+ has the best TL and the appropriate trap depth leading to the good persistent performance.
... For example, Jia et al. [21] have investigated trapping processes in CaS:Eu 2þ , Tm 3þ , and similar investigations have been reported in Refs. [22,23]. Furthermore, Kojima and co-workers [24] have studied the afterglow mechanism and thermoluminescence of CaS:Eu 2þ , Pr 3þ after irradiation with visible light. ...
In order to determine the suitability of Eu2+ doped CaS for phosphor-converted light emitting diodes, photoluminescence, cathodoluminescence mapping, thermal quenching of luminescence and thermoluminescence studies of this phosphor were carried out. Thermal quenching in CaS:Eu2+ is ascribed to energy transfer to impurities and to electrons activated to the conduction band states. The decay time of Eu2+ was partially influenced by oxygen impurities present in the phosphor. Persistent luminescence was observed, originating from trapping and detrapping of electrons by the sulfur or oxygen ion vacancies. The kinetic parameters of the persistent luminescence, namely activation energy (E) and order of kinetics of γ-irradiated CaS:Eu2+ were determined using initial rise and repeated dose methods, respectively.
... However, these studies have focused on the persistent or thermo-lumine scence properties. 8,9,[12][13][14] The luminescence properties of CaS:Eu2+ fabricated with the GaN LED are very limited. In the present study, we prepared silicon and gallium-coactivated CaS:Eu2+ phosphor by conventional method and investigated the incorporation effects of these ions on the luminescence properties. ...
The luminescence intensity of calcium sulfide codoped with Eu2+, Si4+ and Ga3+ was investigated as a function of the dopant concentration. An enhancement of the red luminescence resulted from the incorporation of Si4+ and Ga3+ into CaS:Eu2+. The non-codoped CaS:Eu2+ converted only 3.0% of the absorbed blue light into luminescence. As the Si4+ and Ga3+ were embedded into the host lattice, the luminescence intensity increased and reached a maximum of Q = 10.0% at optimized concentrations of the codopants in CaS. Optimized CaS:Eu2+,Si4+,Ga3+ phosphors were fabricated with blue GaN LED and the chromaticity index of the phosphor-formulated GaN LED was investigated as a function of the wt% of the optimized phosphor.
... The Zn 1+x Ga 2−2x Ge x O 4 :Cr 3+ samples may then be of great interest to replace the original ZnGa 2 O 4 :Cr 3+ material used in many applications for field emission display and electroluminescent devices. 12,13 Figure 3 shows the photoluminescence and long-lasting luminescence spectra of the ZnGa 2 O 4 :Cr 3+ (x = 0), Zn 1.1 Ga 1. 8 14c identified the Cr 3+neighboring antisite defect as positively charged and concluded that the long-lasting luminescence mechanism relies on the replacement of a Zn 2+ ion by a Ga 3+ ion in the tetrahedral site of the structure. The x = 0.1 Ge or Sn substituted sample exhibits rather similar photoluminescence spectra than those of the x = 0 sample. ...
The red long-lasting luminescence properties of the ZnGa2O4:Cr3+ spinel material are shown to be much improved when germanium or tin is substituted to the nominal composition. The resulting Zn1+xGa2–2x(Ge/Sn)xO4 (0 ≤ x ≤ 0.5) spinel solid solutions synthesized here by a classic solid state method have been structurally characterized by X-ray and neutron powder diffraction refinements coupled to 71Ga solid state NMR studies. In contrast to ZnGa2O4:Cr3+ for which long lasting luminescence properties have been reported to arise from tetrahedral positively charged defects resulting from the spinel inversion, our results show that a different mechanism occurs complementary for Zn1+xGa2–2x(Ge/Sn)xO4. Here, the great enhancement of the brightness and decay time of the long lasting luminescence properties is directly driven by the substitution mechanism which creates distorted octahedral sites surrounded by octahedral Ge and Sn positive substitutional defects which likely act as new efficient traps.
... It is quite common to obtain a blue or green afterglow using oxide hosts, but it is much more difficult to find a suitable host material with sufficient red shift, in order to obtain red (persistent) luminescence. Although there are a number of red emitting Eu 2+ -doped persistent phosphors, such as CaS:Eu789 and Ca 2 Si 5 N 8 :Eu [10,11], the choice is limited and the host lattices are chemically unstable or difficult to prepare. This is especially unfortunate, since red afterglow phosphors are strongly desired for several applications, such as safety signage, paints and, more recently, also, as tracer particles for in vivo medical imaging [3,121314. ...
During the past few decades, the research on persistent luminescent materials has focused mainly on Eu2+-doped compounds. However, the yearly number of publications on non-Eu2+-based materials has also increased steadily. By now, the number of known persistent phosphors has increased to over 200, of which over 80% are not based on Eu2+, but rather, on intrinsic host defects, transition metals (manganese, chromium, copper, etc.) or trivalent rare earths (cerium, terbium, dysprosium, etc.). In this review, we present an overview of these non-Eu2+-based persistent luminescent materials and their
afterglow properties. We also take a closer look at some remaining challenges, such as the excitability with visible light and the possibility of energy transfer between multiple luminescent centers. Finally, we summarize the necessary elements for a complete description of a persistent luminescent material, in order to allow a more objective comparison of these phosphors.
Deep-red emitting phosphors at 650-700 nm are highly required for chlorophyll a, chlorophyll b in photosynthesis process to promote plant growth. Here, single phased Sr4Al14O25 (SAO): x%Eu2+, y%Mn4+ (0.02 ≤...
Long persistent luminescence materials developed to commercial standards are primarily concentrated in the blue and green regions, with only a few in the red region. Red, as one of the three basic colors, can be mixed in various proportions with blue and green to yield various colors. The development of red persistent phosphors has a broader application potential but remains a challenge. A solid-state reaction method was used to synthesize new red persistent luminescent materials of Ba1-xSrxGa2O4:Sm³⁺ (x = 0–0.09). In BaGa2O4, both Sr²⁺ and Sm³⁺ preferentially occupy the Ba²⁺ site rather than the Ga³⁺ site. When exposed to UV light at 254 nm, the phosphors emit the characteristic red emission of Sm³⁺ at wavelengths ranging from 500 nm to 750 nm. After removing the UV light source, an intense red afterglow that lasted more than 1400 s was observed. The red afterglow signal reappears after a heating process. Doping Sr²⁺ reduces the trap depth and improves the red persistent luminescence significantly. Because the escaped electrons from traps compensate for the emission loss of Sm³⁺ during the heating process, the red phosphors have unimaginably luminescent thermal stability. Thus, the emission intensity at 200 °C is 1.6 times that at room temperature. The prepared red persistent phosphors show multimode luminescence, with the output signal being time and temperature sensitive, indicating that they are potential luminescent materials for anti-counterfeiting applications. Finally, a building-block strategy for advanced anti-counterfeiting applications of dynamic display information is proposed, with red persistent phosphors serving as an important component combined with upconversion phosphors of NaYF4:Yb³⁺, Tm³⁺, and green persistent phosphors of SrAl2O4:Eu²⁺, Dy³⁺.
Eu²⁺-doped CaAlSiN3‐type structure nitrides have been extensively studied as a red-emitting phosphors in white light emitting diodes (w-LEDs), but there are few reports of systematic research on the effect of solid solution substitution on their long persistent luminescent properties. In this work, a series of (M,Ca)AlSiN3:Eu²⁺ (M = Sr, Mg) phosphors have been successfully synthesized by efficient combustion synthesis. We investigated the effect of Sr and Mg solid solution on long persistent luminescent properties. Thermoluminescence (TL) measurements were performed in those persistent phosphors to obtain direct experimental results on the trap depth. The results show that the SrAlSiN3:0.2 at% Eu²⁺ phosphor exhibits the most intense red persistent luminescence with the persistent time of as long as 200 min at the threshold value of 0.32 mcd/m² due to the presence of more trap levels with suitable depths of 0.71 eV. Compared with the solid solution of Sr, the solid solution of Mg ions can increase the depth of trap levels in CaAlSiN3. Among them, Mg0.2Ca0.8AlSiN3:0.2 at% Eu²⁺ phosphor achieved a maximum trap depth of 0.98 eV, making it a promising candidate for optical information storage applications.
New photochromic film was developed toward the preparation of anti‐counterfeiting documents utilizing inorganic/organic nanocomposite enclosing a photoluminescent inorganic pigment and a polyacrylic binder polymer. To generate a translucent film from pigment/polyacrylic nanocomposite, the phosphorescent strontium aluminium oxide pigment should be well‐dispersed in the solution of the polyacrylic‐based binder without agglomeration. The photochromic nanocomposite was applied efficiently onto commercial cellulose paper documents utilizing the effective and economical spray‐coating technology followed with thermofixation. A homogeneous photochromic film was immobilized onto cellulose paper surface to introduce a transparent film changing to greenish‐yellow upon exposure to ultraviolet light as depicted by CIE coloration measurements. The photochromic effect was monitored at lowest pigment concentration (0.25 wt%). The spray‐coated paper documents exhibit two absorbance bands at 256 and 358 nm, and two fluorescence peaks at 433 and 511 nm. The morphologies of the spray‐coated documents were explored. The spray‐coated paper sheets showed a reversible photochromic effect without fatigue under ultraviolet irradiation. The rheology of the produced photochromic composites as well as the mechanical properties and photostability of the spray‐coated documents were studied.
Red / NIR long persistent phosphors have received extensive attentions in biomedical, food inspection, iris recognition, biological imaging, etc. Herein, a new phosphor, Li2ZnGe3O8:Cr3+, is reported with deep red persistent luminescence peaking at 708 nm. By adjusting the Cr3+ doping concentration, the competitive site occupation at [ZnO6] and [GeO6] polyhedral enables different traps behaviors including trap types, trap concentration and trap depth, which in turn leads to different afterglow duration time from 2 to 20 h. The persistent luminescence mechanisms originated from different trap models have been discussed and it is found that they can cooperate or inhibit each other, enabling different luminescence depending on time. The dynamic anti‐counterfeiting applications have been demonstrated, which provides a new way to rationally designing for multi‐functional luminescent materials.
Eu2+-activated Ca10M(PO4)7 (M = Li, Na, and K) phosphates have been explored extensively because of their tunable emission wavelengths and excellent luminescence performances. Herein, the persistent luminescence (PersL) properties of Eu2+-doped Ca10M(PO4)7 phosphors with a β-Ca3(PO4)2-type structure are reported. With the variation of alkali metal M from Li to Na and to K, the PersL color can be adjusted sequentially from yellow to white and to blue, and the persistent durations are prolonged significantly from about ∼61 s for Ca9.997Li(PO4)7:0.003Eu2+ to ∼1950 s for Ca9.999Na(PO4)7:0.001Eu2+ and to ∼7440 s for Ca9.9995K(PO4)7:0.0005Eu2+ at the threshold value (0.32 mcd/m2) after 254 nm irradiation. The trap depths are estimated according to the thermoluminescence glow curves with various heating rates. Comparing the thermoluminescence excitation and photoluminescence excitation spectra, it can be verified that there are two sources of ionized electrons in the charging process: one is excited from the valence band to the conduction band (CB) and the other is excited from the 4f ground state of Eu2+ to the higher 5d levels or directly to the CB. Finally, the PersL mechanism is proposed. This work is expected to motivate more research of Eu2+-doped phosphate-based PersL materials, as well as contributes to the understanding of the PersL mechanism of Eu2+-doped phosphors.
As a universal sunlight-conversion agent for agricultural shed films, the green-to-red phosphor CaS:Eu²⁺ suffers greatly from its poor moisture resistance. Unlike the conventional surface modifications upon application of postprocessing, this work proposes a one-step, two-additive strategy for synthesizing the CaS:Eu²⁺,CaBr2,CaF2 composite phosphor. For the first time, we realize remarkable improvements in both the chemical stability and the luminescence intensity of a CaS:Eu²⁺-based material synchronously. The dual halide-modified composite phosphor maintains a stable phase composition against moist air and water immersion. The red emission of the as-obtained CaS:Eu²⁺,CaBr2,CaF2 phosphor becomes 3 times as intense as that of nonmodified CaS:Eu²⁺. Moreover, the polymer film fabricated with the modified phosphor presents good weather resistance and brilliant light-conversion performance. Therefore, this one-step method gives the CaS:Eu²⁺,CaBr2,CaF2 phosphor a great advantage as a long life light-conversion material for agricultural shed films.
White light emission phosphors are widely researched for application in lighting and display fields. However, the poor thermal stability is a real problem for the known single-phased white phosphors, which limits their further application. In this paper, Ca19Na2Mg(PO4)14: xDy³⁺, yTm³⁺ (CNMP, 0 ≤ x ≤ 0.06, y = 0, 0.01) phosphors with adjustable emission and good thermal stability are synthesized. The X-ray diffraction and X-ray energy dispersive spectrometer measurement distinctly confirm the successful synthesis of CNMP: xDy³⁺, yTm³⁺ (CNMP, 0 ≤ x ≤ 0.06, y = 0, 0.01). The photoluminescence results reveal that CNMP: Dy³⁺ shows characteristic excitation peaks in the range of 350–450 nm, and mainly exhibits strong yellow emission around 575 nm ascribed to the ⁴F9/2-⁶H13/2 transitions of Dy³⁺. To compensate the deficiency of blue light emission of CNMP: Dy³⁺, the trivalent Tm³⁺ ion is co-doped owing to its characteristic blue emission at 450 nm due to its ¹D2-³F4 transitions. Therefore, the emission of CNMP: Dy³⁺, Tm³⁺ can be tuned from blue light region with CIE coordinates of (0.1649, 0.0387) to white light region with CIE coordinates of (0.3001, 0.3003) and finally move to yellow light region with CIE coordinates of (0.3732, 0.4493) through adjusting the doping ratio of Dy³⁺/Tm³⁺. The energy transfer efficiency and the energy transfer mechanism from Tm³⁺ to Dy³⁺ are further investigated. Moreover, CNMP: Dy³⁺, Tm³⁺ exhibites a high thermal stability and the emission intensity still keeps 84% of the initial intensity of Dy³⁺ at 230 °C. These outstanding properties show that Ca19Na2Mg(PO4)14: Dy³⁺, Tm³⁺ have great advantages and potentiality for applying in solid state lighting.
Warm-color persistent luminescent materials are strongly desired for mark signage and medical imaging in comparison with green or blue counterparts. Herein we report a novel yellow long-persistent phosphor, Nb-doped Sr3SiO5:Eu2+, with peak wavelength at ~580 nm and persistent time more than 14 hours at the 0.32 mcd/m2 threshold value after UV radiation. A combination of thermoluminescence (TL), thermoluminescence excitation (TLE), electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) calculations reveals that the persistent luminescence enhancement is attributed to a significant Nb-induced increase of oxygen vacancies that act as electron trapping centers with appropriate trap depths. Groups of time-dependent color-change images are realized with this material, which has potential applications as anti-counterfeit and indicator marks. This investigation also expands the application of transition metal (TM) ions to the field of persistent luminescence and would motivate more exploration of TM substitutions to design and improve silicate or aluminosilicate persistent phosphors with superior performance.
In this work, we designed and successfully synthesized a novel germanate-based Persistent luminescence (PersL) phosphor host, CaMgGe2O6, via a solid-state reaction. And we constructed an empirical energy level scheme of the CaMgGe2O6: Ln2+/Ln3+ phosphors according to Dorenbos model. Then we successfully theoretical predicting the choice of the optimal codopant of the CaMgGe2O6: Mn2+ phosphors by this empirical energy level scheme. As a result, after co-doping with Sm3+, the PersL intensity is increased to 16 times and the time duration are effectively improved to 52 times due to the high concentration of new traps. And the CaMgGe2O6: Mn2+,Sm3+ shows a red to near-infrared long persistent emission located at 675 nm, which can sustain about 60 minutes above the recognizable intensity level (≥0.32 mcd/m2), and the persistence duration of samples was three times that of the red commercial phosphor Y2O2S: Eu3+. The experimental results coincide well with the theoretical predictions. These results demonstrate that the strategy concepts of this work is feasible for the design of the Persistent luminescence materials. Moreover, a possible afterglow mechanism is studied and discussed with the assistance of thermoluminescence (TL) curves.
The Sr3SiO5:Eu2+ phosphor has attracted considerable attention for applications in white LEDs owing to its highly efficient yellow emission under violet-blue excitation. We report herein an enhancement of yellow persistent luminescence in Sr3SiO5:Eu2+ through Ge incorporation. The strongest persistent luminescence intensity is observed for Sr3(Si1- xGe x)O5:Eu2+ with x = 0.005 with a peak emission wavelength at ∼580 nm and a persistent time of ∼7000 s at the 0.32 mcd/m2 threshold value after UV radiation. A combination of thermoluminescence measurements and density functional theory (DFT) calculations reveals that the afterglow enhancement is due to a significant increase in the number of oxygen vacancies that act as electron trapping centers with appropriate trap depths. This investigation is anticipated to encourage more exploration of GeSi substitution to design and improve Si-containing persistent phosphors with superior functionalities.
A series of cyan emission (Ba, Li) (Si, Ge, P)2O5: Eu2+, Pr3+ long persistent phosphors (LPP) were designed by solid solution strategy and synthesized by solid state reaction, the crystal structure and photoluminescence of this long persistent phosphor have been analyzed systematically. Under 256nm light excitation, the as-prepared (Ba, Li) (Si, Ge, P)2O5: Eu2+, Pr3+ present a strong cyan-emitting located at 514 nm and the decay time of samples can be elongated to about 38h (BaSi2O5:0.008Eu2+, 0.01 Pr3+), 47h (BaSi1.5Ge0.5O5:0.008Eu2+, 0.01Pr3+) and 56h (Ba0.92Li0.08 Si1.92 P0.08O5: 0.008Eu2+,0.01Pr3+) after ceasing the excitation source, respectively. A number of the excitation duration(1s≤t≤30s), decay duration(10min≤t≤10h) and temperature dependent TL experiments of (Ba, Li)(Si, Ge, P)2O5:0.008Eu2+,0.01Pr3+ were conducted, revealing that the existence of shallow and deep traps caused by Eu2+ and Pr3+ ions in samples give rise to the excellent LPP property. According to the experimental results, a feasible mechanism on persistent luminescence of (Ba, Li) (Si, Ge, P)2O5:0.008Eu2+, 0.01Pr3+ was proposed and illustrated in detail.
Long persistence phosphors with high emitting intensity are promising materials for safety signage and energy storage applications. Herein, an improved persistent luminescence of Y3Al2Ga3O12 phosphor by co-doping Ce³⁺, Yb³⁺, and B³⁺ is achieved using conventional solid-state reaction. On one hand, the incorporation of H3BO3 can improve the crystallinity; on the other hand, B³⁺ can replace Al³⁺/Ga³⁺ in tetrahedral sites in the host lattice, causing lattice contraction and modifying the trap depth and density. It is found that adding B³⁺ forms a much deeper trap with ∼1.10 eV depth. In addition, the density of the electron trap can also be dramatically increased compared to the sample without B³⁺. The charging process for persistent luminescence is demonstrated by comparing the photoluminescence excitation spectrum with the thermoluminescence excitation spectrum. The persistence luminescence mechanism is given by a visual energy level diagram on the basis of the vacuum referred binding energy scheme of Y3Al2Ga3O12.
(Sr,Ca)AlSiN3:Eu²⁺ phosphors have been widely used in phosphor-converted white light emitting diodes. Herein, we reported the strong red persistent luminescence in (Sr,Ca)AlSiN3:Eu²⁺ under UV light excitation. The Sr0.8Ca0.2AlSiN3:0.15% Eu²⁺ shows the strongest red persistent luminescence with a peak emission wavelength at ~628 nm and a persistent time of ~9600 s at the 0.32 mcd/m² threshold value. A new persistent luminescence mechanism, which is different to that of SrAl2O4:Eu²⁺,Dy³⁺, was proposed by comparing the thermoluminescence excitation spectrum (TLES) and the photoluminescence excitation spectrum (PLES) of Sr0.8Ca0.2AlSiN3:0.15% Eu²⁺. The electrons are directly excited from 4f ground states to the conduction band or from valence band to conduction band in (Sr,Ca)AlSiN3:Eu²⁺; while, in SrAl2O4:Eu²⁺,Dy³⁺, they are first excited to 5d level and then stimulated thermal process to the conduction band. The effect of Eu²⁺ concentration on red persistent luminescence in (Sr,Ca)AlSiN3:Eu²⁺ were discussed. The proposed mechanism of persistent luminescence can help us to design and find new persistent luminescence materials.
Near-infrared (NIR) emitting persistent phosphors of Cr3+-doped zinc gallogermanate have emerged for in vivo bio-imaging with the advantages of no need for in situ excitation. However, it is challenging to synthesize well-dispersed and uniform spherical particles with high brightness, high resolution, and distinguished NIR long afterglow. In this work, Zn3Ga2Ge2O10:Cr3+ (ZGGC) monospheres were directly synthesized by a facile hydrothermal method with the assistance of citric anions (Cit3-), which emit a NIR emission at ~696 nm and exhibit excellent NIR persistent luminescence with rechargeability. Controlled experiments indicated that the shape evolution of ZGGC product is significantly affected by Cit3-, solution pH, and the duration and temperature of hydrothermal reaction. Furthermore, compositional influence on the crystal structure, bandgap, trap depth, and luminescence characteristics of ZnyGa2Ge2O10-δ:Cr3+ (y = 2.8, 3.0, 3.2) were investigated in details, which allows to construct an energy level diagram of the ZGGC host, Cr3+ ions, and electron traps. It was found that the bandgap and conduction-band minimum (CBM) are significantly affected by the Zn content, while the valence-band maximum (VBM) is not. The y = 3.0 sample exhibited the best persistent luminescence, owing to its deepest defects. The ZGGC-NH2 prepared through surface functionalization of ZGGC spheres showed distinguished NIR long afterglow, low toxicity, and great potential for in vitro cell imaging and in vivo bio-imaging in the absence of excitation. Moreover, the persistent-luminescence signal from the ZGGC-NH2 can be repeated in vivo through in situ recharge with external excitation of a red LED lamp, indicating that the ZGGC-NH2 is suitable for applications in long-term in vivo imaging.
Yttrium aluminum-gallium garnets with cerium doped is most widely used as green-yellow phosphor in solid state lighting. Extensive research has been performed on this material concerning the luminescent thermal quenching resistance and persistent luminescence. In this paper we find that a negative correlation exists between temperature-dependent luminescence and persistent luminescence with gallium content varying. The correlation originates from the electronic structures which influence both the thermal quenching of luminescence and persistent luminescence. A detailed crystal-field calculation has been performed to understand the peak shifts. In addition, theoretical calculations reveal that oxygen vacancies provide trap levels which implement the persistent luminescence. This material could be used as potential blue-light excited persistent luminescent material, with the after-glow time up to about 1. h with only cerium as the dopant, which is expected to be prolonged by co-doping other elements. This work may be helpful in guiding the discovery of other after-glow materials.
A red long-lasting phosphorescence phosphor Ca2Ge7O16:Sm3+ was synthesized by a high temperature solid state reaction. The persistent energy transfer process from Ca2Ge7O16 host to Sm3+ was confirmed to be much more efficient than that in the photoluminescence process. The color of the persistent luminescence could be adjusted from blue to red with increasing concentration of Sm3+ accordingly. Based on the analysis of the thermoluminescence curves, it was found that Sm3+ doped Ca2Ge7O16 also provided a possibility to be a kind of photo-storage phosphor. Detailed mechanism were studied and illustrated.
The fluorescence, phosphorescence and thermoluminescence properties of Eu3+ and Gd3+ single-doped and co-doped Ca2SnO4 phosphors sintered in air and in oxygen-deficient atmosphere (vacuum) were investigated. The red phosphorescence of phosphor Ca2SnO4: Gd3+, Eu3+ sintered in vacuum atmosphere can last for over 6480 sec at a recognizable intensity level, whereas the red phosphorescence of air-sintered phosphor Ca2SnO4: Eu3+ can only persist for 1080 sec. And the blue phosphorescence of phosphor Ca2SnO4: Gd3+ sintered in vacuum atmosphere can last for over 10200 sec, but the phosphorescence of air-sintered phosphor Ca2SnO4: Gd3+ can only persist for 5820 sec. A new efficient persistent energy transfer system from the self-trapped excitation of Gd3+ to Eu3+ was suggested based on the fact of obvious overlap between the emission band of Ca2SnO4: Gd3+ and the excitation band of Ca2SnO4: Eu3+. The oxygen vacancies were beneficial to the afterglow property of RE3+ (RE = Gd, Eu) doped Ca2SnO4. The results of this study suggested that the [2RF(Ca)(center dot) - 2V ''(Ca) - V-O(center dot center dot)] defects clusters were responsible for the afterglow of vacuum-sintered Ca2SnO4: RE3+.
Spectroscopic properties of Ba2Gd(BO3)2Cl: Dy3+ and Ba2Gd(BO3)2Cl: Dy3+, Tm3+ under vacuum ultraviolet (VUV) and ultraviolet (UV) light excitations were investigated. Dy3+ single-doped Ba2Gd(BO3)2Cl showed broad absorption band in the VUV region, and bright warm white light with chromaticity coordinates (CIE) of (0.340, 0.381) upon VUV excitation at 172 nm, demonstrating this phosphor's applicability in mercury free lamps. Upon direct excitation Tm3+ from its 6F6 level to 1D2 level, the decrease of emission intensity and lifetime of Tm3+1D2–3F4 emission with increasing concentration of Dy3+ in Ba2Gd(BO3)2Cl: Dy3+, Tm3+ confirmed the occurrence of energy transfer from Tm3+ to Dy3+. In addition, Ba2Gd(BO3)2Cl: Dy3+, Tm3+ could be efficiently excited by 358 nm UV light and its emission color could be tuned from blue to yellow by codoping Tm3+. When 1% Tm3+ and 5% Dy3+ were codoped in the Ba2Gd(BO3)2Cl, intensive white-emitting light with CIE of (0.352, 0.328) and correlated color temperature of 4589 K was achieved upon 358 nm excitation, revealing the potential application of Ba2Gd(BO3)2Cl: Dy3+, Tm3+ for white light-emitting diodes (LEDs).
Powder samples of KSrPO4 doped with Eu2+ and Ce3+ were prepared by combustion-assisted synthesis. Their structures and photoluminescence spectra were systemically studied. Energy transfer from Ce3+ to Eu2+ was observed by investigating the optical properties from photoluminescence spectra in Eu2+ single doped and Ce3+–Eu2+ co-doped KSrPO4. The enhancement of UV excitation is attributed to energy transfer from Ce3+ to Eu2+, and Ce3+ plays a role as a sensitizer. Ce3+–Eu2+ co-doped KrSrPO4 powders can possibly be applied as blue phosphors in the fields of lighting and display.
We presented the energy transfer from Ce3+ to Eu2+ in CaAl2Si2O8 host. The Ce3+-doped CaAl2Si2O8 phosphor had a strong emission band at 378 nm under the vacuum ultraviolet (VUV) light. This emission spectrum of Ce3+ well overlapped with the excitation spectrum of Eu2+ under the UV illumination. As a result, the energy transfer from Ce3+ to Eu2+ in CaAl2Si2O8 matrix was observed under VUV excitation, which resulted in a significant enhancement of the emission peak intensity at 446 nm. More details about the luminescent properties were presented.
We report on the controlled growth of novel BN-coated Ca1-xSrxS:Eu nanowires via a solid-liquid-solid process. The Ca1-xSrxS solid solution forms as one-dimensional nanowires and has been coated with homogeneous protective BN nanolayers. The structure and luminescence properties of this new nanocomposite have been systematically investigated. High-spatial-resolution cathodoluminescence investigations reveal that effective red color tuning has been achieved by tailoring the composition of the Ca1-xSrxS nanowires. Moreover, codoping of Ce(3+) and Eu(2+) in the CaS nanowire can induce energy transfer in the matrix and make it possible to obtain enhanced orange color in the nanowires. The BN-coated Ca1-xSrxS:Eu solid-solution nanowires are envisaged to be valuable red-emitting nanophosphors and useful in advanced nanodevices and white LEDs.
On the basis of structural information of its host material which shows excellent stability and absorption efficiency in ultra-violet (UV) region, a blue-emitting Sr2MgSi2O7:Eu2+ (SMS:Eu2+) phosphor was synthesized, and its photoluminescence (PL) performance was systematically optimized. In order to enhance its PL properties, Ce3+ was added as a sensitizer based on the energy transfer from the absorption energy of Ce3+ to Eu2+. It was due to the spectral overlap between the photoluminescence excitation spectrum of Ce3+ and the PL spectrum of Eu2+. Moreover, the energy transfer rate from Ce3+ to Eu2+ is generally faster than the emission rate of Ce3+ in the dipole-dipole interaction. Depending upon the amount of Ca2+ substituted into Sr site, their maximum wavelength was varied from -460 to -540 nm in terms of the crystal field effect confirmed by the structural analysis via Rietveld refinement method. Finally, the optimized blue-emitting SMS:Eu2+ and Ca(2+)-substituted yellowish green-emitting SMS:Eu2+ phosphors were applied with Eu(2+)-sensitized red-emitting Ca3Mg3(PO4)4:Mn2+ phosphor introduced in our previous research to UV light emitting diode (LED)-pumped white LEDs. The fabricated white LEDs showed a natural white light with the color coordinate of (0.3298, 0.3280) and the excellent color rendering index of 94.
Long-afterglow phosphors Ca1.998-x MgSi2O7:Eu 0.0022+, Cex3+(0 ≥ x ≥ 0.04) were prepared by a solid-state reaction under a weak reductive atmosphere. Spectroscopy measurements demonstrated that energy transferred from Ce 3+ to Eu2+. The energy-transfer efficiency from Ce3+ to Eu2+ was 61%. Thermoluminescence measurement revealed that Ce3+ co-doped samples created much appropriate traps leading to the enhancement of the afterglow. The afterglow emission spectra revealed that Ce3+ in the co-doped samples acted as sensitizers and Eu2+ in [CaO8] (535 nm) acted as activators. The duration of co-doped samples was prolonged nearly two times compared with that of the Eu2+ single-doped sample. A possible afterglow mechanism was presented and discussed.
Sr 2 MgSi 2 O 7 samples doped with Ce 3+ , Eu 2+ , (Eu 2+ , Ce 3+ ), (Eu 2+ , Dy 3+ ), and (Eu 2+ , Dy 3+ , Ce 3+ ) were prepared and studied. The Ce 3+ 5d-4f emissions from two Sr 2+ sites in Sr 2 MgSi 2 O 7 were observed. However, only one single emission band at 466 nm was found in Sr 2 MgSi 2 O 7 :Eu 2+ . The energy transfer from Ce 3+ to Eu 2+ in Sr 2 MgSi 2 O 7 was demonstrated to be a multipolar interaction. It was found that Sr 2 MgSi 2 O 7 :Ce 3+ showed no afterglow. Sr 2 MgSi 2 O 7 :Eu 2+ showed some afterglow with a short persistence, and its afterglow could be enhanced by the incorporation of Ce 3+ . The triply doped phosphor Sr 2 MgSi 2 O 7 :Eu 2+ , Dy 3+ , Ce 3+ exhibited higher brightness and longer lasting time than that of Sr 2 MgSi 2 O 7 :Eu 2+ , Dy 3+ , which could be ascribed to more traps formed by the incorporation of Ce 3+ .
Single-phased CaZrO3: Dy, Tm series have been successfully synthesized by solid-state reaction, and their luminescence properties were investigated. Under 355 nm excitation, CaZrO3: Dy series showed characteristic emission of Dy, which exhibited yellowish white color. By introducing Tm into the matrix, the emitted hue of the Dy-doped sample could be easily tailored to white, and simultaneously, energy transfer from Tm to Dy was observed. The color coordinates of the optimum white-emitting sample were (0.321, 0.323), which were very close to the data of the National Television Standard Committee (0.33, 0.33). The co-activated phosphors presented good match to ultraviolet light-emitting diodes (LEDs), which revealed that they could be novel promising phosphors utilized in white LED application.
An orange-emitting, long-persistent phosphor, Ca 2 Si 5 N 8 :Eu 2+ ,Tm 3+ , was developed. Afterglow of this phosphor decays more slowly than that of the conventional red-emitting, long-persistent phosphor, Y 2 O 2 S:Eu 3+ ,Ti,Mg. After 420 nm excitation, the afterglow luminance is initially lower, but, after 8 min, gets higher than luminance of Y 2 O 2 S:Eu 3 ',Ti,Mg. Long afterglow results from electron traps formed by Tm 3+ at the Ca 2+ site. The origin of two main thermoluminescence glow peaks at 220 and 350 K is discussed based on their dependence on Eu or Tm concentration and Ca/Si atomic ratio. Increased thermoluminescence intensity is observed by excitation in Eu 2+ 4f-5d transition at room temperature followed by the fundamental absorption of the host at low temperature.
Lanthanum gallogermanate co-doped with chromium (LaâGaâGeOââ:Cr{sup 3+}, M, where M = Li, Pb{sup 2+}, Zn{sup 2+}, Eu{sup 3+}, Tm{sup 3+}, and Dy{sup 3+}) samples have been prepared using a solid-state chemical reaction method. The phosphor with Dy{sup 3+} is observed to have a persistent IR emission for more than 8 h, which is recorded using a spectrometer. The wavelength of the major IR emission is in the range from 700 to 1100 nm. The intensity of the phosphorescence and persistent time can be modified by co-doping proper trapping centers.
phosphor samples were synthesized by a solid-state reaction method. The lattice parameter of the
phosphor samples was linearly decreased from 12.654 to
, and the main emission wavelength was shifted from 501 to
with increase of the
ion concentration
from 0.05 to 0.50. The relative photoluminescence (PL) intensity under
excitation was increased with increase of the
from 0.05 to 0.25 due to the excitation band extended to the longer wavelength. However, when
, the PL intensity was decreased due to nonradiative transitions among the
ions and the formation of
impurity phase. The Commission International de l’Eclairge chromaticity coordinates were also largely shifted from the bluish-green
emitting region (
,
) to the green emitting one (
= 2.414,
= 0.658) by increasing the
from 0.05 to 0.50. The energy transfer from the
ion to the
ion enhanced the PL intensity of the
ion emission under near ultraviolet and blue excitation. The PL intensity of the
phosphor sample was higher by 24% than that of the
phosphor sample under
excitation.
Mn 2+ activated long persistent phosphors, CaAl 2 O 4 :Mn 2+ , Ce 3+ , Ca 2 Al 2 SiO 7 :Mn 2+ , Ce 3+ and MgSiO 3 :Mn 2+ , Eu 2+ , Dy 3+ , were prepared. Long persistent phosphorescence in green, yellow, and red have been observed in the systems with persistence time over 10 h for the green (525 nm) and the yellow (550 nm) and 4 h for the red (660 nm). Emission and excitation spectra showed that Mn 2+ emission was dominant in these materials. Long persistent phosphorescence was also measured in the systems without Mn 2+ as a co-dopant. The emissions were much stronger due to 5d–4f transitions in Ce 3+ and Eu 2+ other than 3d–3d transitions in Mn 2+ . The disappearances of the long persistent afterglow in these systems when co-doped with Mn 2+ suggested the existence of a strong persistent energy transfer from Ce 3+ and Eu 2+ to Mn 2+ . r 2002 Elsevier Science B.V. All rights reserved.
This paper reports the detailed preparation and long lasting properties of Eu2+-activated Sr2ZnSi2O7 phosphor by sol–gel method. The preparation process of Sr2ZnSi2O7 was complicated and several intermediate phases were formed during the reaction process. The obtained phosphor showed two emission peaks at 385 and 457 nm, and they were due to the different Eu2+ luminescent centers in Sr2ZnSi2O7 host. Long afterglow was observed although the phosphor showed faint persistence. Investigation of thermoluminescence curve revealed one TL glow peak, which indicated the existence of the trap. The lifetime of the trap was calculated to be 212 s, which illustrated the faint afterglow.
A novel red long lasting phosphorescent materials β-Zn3(PO4)2:Mn2+,Sm3+ is firstly synthesized by high-temperature solid-state reaction. The influence of Sm3+ ions on luminescence and long lasting phosphorescence properties of Mn2+ in phosphor β-Zn3(PO4)2:Mn2+,Sm3+ are systematically investigated. It is found that the red phosphorescence (λ = 616 nm) performance of Mn2+ ion such as brightness and duration is largely improved when Sm3+ ion is co-doped into the matrix in which Mn2+ ion acts as luminescent center and Sm3+ ion plays an important role of electron trap. Thermoluminescence spectrums show that there exists one peak in β-Zn3(PO4)2:Mn2+,Sm3+, the depth of which is 0.33 eV, and that there are three peaks in β-Zn3(PO4)2:Mn2+, among which the depth of the lowest temperature peak in β-Zn3(PO4)2:Mn2+ is 0.37 eV. Such differences in the trap depth result in the improvement of red long lasting phosphorescence of Mn2+ in present matrix.
The luminescence of two newly developed blue-emitting long afterglow phosphors, Sr2−xCax MgSi2O7:Eu2+, Dy3+ (x=0x=0, 1), has been studied. The emission under VUV–UV excitation from a synchrotron radiation source, along with the long afterglow spectra have been measured at different temperatures ranging from 7 to 300 K. While the emission spectra under 170 nm excitation exhibit both the 5d–4f transitions of Eu2+ and 4f–4f transitions of Dy3+, only the blue band corresponding to the 5d–4f transitions of Eu2+ is observed in the long afterglow spectra. Emission of Sr2MgSi2O7:Eu2+, Dy3+ excited by an intense VUV laser source (157.6 nm) is also reported. The rich line structures in the laser-excited emission spectra partly result from 4f–4f transitions of Eu3+, indicating an efficient photon-induced process, which promotes Eu2+ to Eu3+. The thermoluminescence covering the temperature range from 50 to 450 K for both samples has been measured as well and the depth of the traps responsible for the long afterglow emission was estimated. The mechanism of the persistent luminescence and the origin of the traps are discussed in the light of these results.
Samples of CaAl4O7 doped with Tb3+, Ce3+, and Tb3+ and Ce3+, respectively are prepared by sintering. Microstructures are analyzed by X-ray diffraction and high-resolution transmission electron microscope experiments. The samples show single monoclinic phase. The blue Ce3+ emission at 440 nm is efficiently quenched in the sample of CaAl4O7 : Tb3+, Ce3+, in which the dominant emission is in the green at 545 nm, originating from the transition of 5D4 to 7F5. Excitation spectrum of the CaAl4O7 : Tb3+, Ce3+, monitoring at 545 nm of the green emission, consists of both contributions from Ce3+ and Tb3+. 95% of total energy of Tb3+ green emission is transferred from Ce3+.
CaS:Eu{sup 2+},Tm{sup 3+} is a persistent red phosphor. Thermoluminescence was measured under different excitation and thermal treatment conditions. The results reveal that the charge defects, created by substituting Tm{sup 3+} for Ca{sup 2+}, serve as hole traps for the afterglow at room temperature. Tm{sup 3+} plays the role of deep electron trapping centers, capturing electrons either through the conduction band or directly from the excited Eu{sup 2+} ions. These two processes, in which two different sites of Tm{sup 3+} are involved, correspond to two traps with different depths. (c) 2000 American Institute of Physics.
A very long persistent phosphor BaAl2O4:Ce3+ was prepared and studied. The Ce3+ 5d-4f emissions from two Ba2+ sites in the BaAl2O4 were observed at 450 and 402 nm. The lowest 4f-5d excitation peaks were recorded at 357 and 335 nm, respectively. The persistence times of the long afterglow emissions from Ce3+ at the two sites were found to be longer than 10 h. Site-selective thermoluminescence spectra of the sample were measured. Two sets of thermoluminescence peaks were detected at -43, -26 and 27 °C, and -53, -36 and 30 °C (heating rate 13.3 °C/min) corresponding to the two sites at 450 nm (site-1) and 402 nm (site-2), respectively. Dy3+, Ce3+ co-doped samples were also prepared, which introduce new defect-related traps at 18, 50, and 82 °C (heating rate 10 °C/min). These defect-related traps due to Dy3+ co-doping also contribute to the Ce3+ afterglow at the two sites.
Influences of excess Zn2+ ions and intrinsic defects on red (lambda=616nm) phosphorescence of beta-Zn3(PO4)2:Mn2+ are systematically investigated. It is clearly observed that red long lasting phosphorescence (LLP) properties of Mn2+, such as brightness and duration, are largely improved when excess Zn2+ ions are co-doped into the matrix. Photoluminescence (PL), LLP and thermoluminescence (TL) spectra indicate that Mn2+ ion acts as luminescent center whereas oxygen vacancy associated to Zn2+ ion plays a significant role in electron trap. The TL peak for oxygen vacancy is centered at 343K, the depth of which is suitable for improvement in LLP performance of Mn2+ at room temperature. The possible mechanism for this phenomenon of red LLP of Mn2+ in beta-Zn3(PO4)2:Mn2+ with excess of Zn2+ is explained by means of a competitively trapping model.
Eu3+-activated Y2O2S phosphors co-doped with Ti and Mg ions were prepared. The effect of various sintering atmospheric conditions on the crystalline phases and luminescent properties of the products were investigated in detail. Red and orange long afterglow was observed after phosphors were excited with 365nm ultraviolet light. The main emission peaks were ascribed to Eu3+ ions transition from 5DJ(J=0,1) to 7FJ(J=0,1,2,3,4), and the phosphorescence lasted for nearly 3h in the light perception of the dark-adapted human eye (0.32mcdm−2). The phosphorescence mechanism was also investigated.
Using rare earth coordination polymers with salicylic acid as precursors for the luminescence species YxGd2−xSiO5:Eu3+, composing the polyvinyl alcohol (PVA) as dispersing media, nanophosphors of YxGd2−xSiO5:Eu3+ (x=0.10,0.25,0.50,0.75,0.90) with different molar ratio of Y and Gd were synthesized by the sol-gel process. Both X-ray diffraction and scanning electronic microscope show that these materials have the nanometer size of 100–200nm. These nanometer materials exhibit red emission at 612nm. When x=0.5, Y0.5Gd1.5SiO5:Eu3+ shows the strongest emission intensity comparing with other molar ratio of Y to Gd.
Long persistent phosphor SrAl2O4:Eu2+, Dy3+ is prepared in ceramic form. Photo-charging curves are studied and found to be due to the trapping processes of the long persistent materials. A three-level model is used to explain the nature of the charging curve. The excitation spectra of the long persistent materials are found to depend on the intensity of the excitation source and the scanning speed of the measurement.
Luminescence, excitation spectra, and decay curves of afterglow of the phosphors of and with and without co‐doping of were investigated. It was found that co‐doping monovalent ion could efficiently reduce the afterglow and the persistent time in both samples. The results imply that the trapping centers responsible for afterglow are defect‐related in both samples. Based on these results, it was expected that co‐doping any trivalent ions, such as and , into to substitute could produce charge defects and create trapping centers. Acquisition of strong afterglow and long persistent time in and is a strong support for the conclusion. © 2000 The Electrochemical Society. All rights reserved.
A green emitting SrAl 2 O 4 :Eu 2+ ,Dy 3+ phosphor with very bright and long lasting phosphorescence has been newly developed. The incorporation of Dy 3+ ion into the SrAl 2 O 4 :Eu 2+ system as an auxiliary activator dominates the phosphorescence, thermoluminescence, and photoconductivity characteristics of the phosphor to a large extent. Evidence is presented for the mechanism that the phosphorescence is ascribed to the photoconductivity due to holes, and to the trapping and thermal release of the holes by Dy 3+ ions in the system. The incorporation of the Dy 3+ ion forms a highly dense trapping level located at a suitable depth in relation to the thermal release rate at room temperature, thus producing the very bright and long phosphorescence.
Data are presented on green-emitting CaS:Ce3+, red-emitting CaS:Eu2+, and doubly activated CaS:Eu2+, Ce3+, including spectra, efficiency, and decay characteristics. Green CaS:Ce3+ rivals the best green ZnS-type phosphors in efficiency, color, and brightness at low beam current and is appreciably better at high current. Red CaS:Eu2+, Ce3+ is better than YV04:Eu3+ in terms of efficiency but, because of the less favorable emission spectrum, only about equal to it in brightness. © 1971, by The Electrochemical Society, Inc. All rights reserved.
The luminescence of Rb2ZnBr4-Eu2+ is reported and discussed. Efficient energy transfer occurs between inequivalent Eu2+ ions (Rc ⋍ 35 Å). This phenomenon is compared with similar situations in other host lattices.
CaS formed from the CaO sorbent during desulfurization in coal gasifiers has to be converted to CaSO4 before disposal. CaS is mainly decomposed to CaO and SO2 by O2 and then CaO is converted to CaSO4 by SO2 and O2. The role of H2O in the oxidative decomposition of CaS with O2 was studied using reagent grade CaS and H218O. The following results were obtained: (1) there is a synergistic effect of H2O and O2 on the oxidative decomposition of CaS to CaO and SO2; (2) H2O reacts with CaS to form CaO, SO2 and H2 in the absence of O2; (3) the oxidative decomposition of CaS to CaO and SO2 occurs stepwise; (4) H2O directly reacts with CaS in the presence of O2; (5) H2O plays an important role in the oxidative decomposition of CaS even if the O2 concentration is high.
Ce3+ and Tb3+ singly doped and codoped Ca Al4 O7 and Y2 O3 single crystal fibers were grown using the laser heated pedestal growth method. Energy transfer from Ce3+ to Tb3+ was studied in these samples. No energy transfer between Ce3+ and Tb3+ was observed in codoped Y2 O3 . This is because electrons excited into the Ce3+ 5d states delocalize into the conduction band at a rate that is faster than the interionic energy transfer rate, whereas the converse is the case for Ca Al4 O7 doped with Ce3+ (1 at %) and Tb3+ (1 at %) . The energy transfer rate was determined to be on the order of 109 s-1 . The transfer occurs before the excited electrons can be thermally promoted into the conduction band. From these results, it can be inferred that the thermal ionization rate ( WTh ) from the lowest 5d excited Ce3+ state at 355 nm is less than 109 s-1 at room temperature, whereas the delocalization rate ( WD ) into the conduction band is faster than this rate. The location of the Tb3+ and Ce3+ states relative to the valence and conduction band of these two materials has been determined through photoconductivity measurements.
Pr3+:CaTiO3, which produces red emission at 612nm is an attractive phosphor. Its UV excitation spectrum consists of three bands at 279, 330 and 360nm, which are assigned to the Pr3+ 4f to 5d transition, the valence-to-conduction band transition Ti4+–O2− to Ti3+–O− and the charge transfer transition Pr3+–Ti4+ to Pr4+–Ti3+, respectively. 3P0 luminescence under UV excitation is completely quenched to 1D2 through a strong coupling with the charge transfer state at room temperature. However, quite strong 3P0 emission was observed at low temperatures when the 3P2 state was excited at 457.9nm. In addition, the phosphor showed a red afterglow, which can be enhanced by adding Al3+. The traps are associated with electron trapping, and are possibly related to Pr4+ and oxygen vacancies existing in the samples.
The energy positions relative to the host valence and conduction bands of the 5d excited states and 4f ground states of Ce3+ in CaS were determined from photoluminescence and photoconductivity measurements. The determination was based on the observation that the 5d electrons of Ce3+ are trapped after transport in the conduction band only when they were excited to the upper 5d-state (Eg), which, therefore, must lie above the minimum of the conduction band of CaS.
The intense and long-lasting phosphorescence of SrAl2O4:Eu2+, Dy3+ and CaAl2O4:Eu2+, Nd3+ is produced by holes thermally released from the trap levels formed by Dy3+ or Nd3+, which has the optimum trap depth. Photoconductivity measurements show that the holes are supplied from the Eu2+ centers during the 4f → 5d excitation with a low activation energy.
A series of ZnS, Se phosphor alloys activated with 1·5 × 10-3 Cu and 10-3 Cl was investigated for the wavelength dependence of the excitation and quenching of photoluminescence and the photoluminescence emission. Glow curves and the temperature dependence of the photoluminescence were also studied. The objective of this work is simply to show the position of the recombination and trapping levels that exist in these materials as a function of the varying base lattice composition and band edge. A modified Schön-Klasens(1,2) model is shown to accommodate the data with a satisfactory degree of consistency. The chemical nature of the centers that are actually responsible for the levels is not deduced.
In this work, CaS:Eu2+ and CaS:Eu2+, Cl- samples are prepared by solid chemical reaction. Excitation and emission spectra are investigated. The emission band is shifted to the red from 655 nm for CaS:Eu2+ to 670 nm for CaS:Eu2+, Cl-. The absorption bands of CaS:Eu2+ are resolved in the excitation spectrum, peaking at 280, 365, 460, and 570 nm. In contrast, the excitation (absorption) spectrum of CaS:Eu2+, Cl- becomes a very broad platform in the range of 250 to 640 nm. The changes in the optical spectra are ascribed to the additional field splitting of the 4f65d state of Eu2+ in the distorted lattice field by incorporation of Cl-, which replaces one of the six ligand ions of S2- and lowers the symmetry of the Eu site from Oh to C4v. The red shift is a resultant effect of field splitting and the Stokes shift of Eu2+ in the two lattice environments. In addition, an afterglow is observed in the sample of CaS:Eu2+, Cl-. The traps, which are responsible for the afterglow at room temperature, are related to the charge defects created by incorporation of the anion Cl-.
Ceramic samples of Tb3+ and Ce3+ singly doped and co-doped CaAl2O4 with 0.1 and 1 at. % doping concentrations have been prepared. The emission of the Ce3+ singly doped sample is at 413 nm. The persistence time of its afterglow is longer than 10 h. The strongest emission of the Tb3+ singly doped sample is at 543 nm and its afterglow persists for about 1 h. In the co-doped sample, the Ce3+ excitation is transferred to Tb3+ ion with an energy transfer rate of 109–1010 s−1 in the high concentration samples. The Ce3+ energy transfer occurs during its afterglow decay, so that the Tb3+ afterglow emission duration is also extended to about 10 h. Photoconductivity and thermoluminescence measurements have also been performed in these systems; the persistent sensitization of Tb3+ is also confirmed by these measurements. © 2003 American Institute of Physics.
A long afterglow phosphor,
was prepared and studied. It exhibits a broad-band emission in the deep blue with a persistence time longer than 10 h. The
ions occupy three distinct
sites in this material, and the emission and excitation spectra from
ions in these sites have been identified. The thermoluminescence spectra were obtained following irradiation at different
wavelengths. Five traps were found: two of the traps, with depths of 0.67 and 0.82 eV, are filled by electron transport though
the conduction band; the other three, with depths of 0.4, 0.46, and 0.6 eV, are filled by electrons tunneling from the 5d
levels of adjacent
ions. Photoconductivity measurements were also made which indicated that the lowest 5d levels of
lie below the conduction band and that the ground state of
is about 1.1-1.4 eV above the valence band of the host. © 2003 The Electrochemical Society. All rights reserved.
The photoemission properties of polycrystalline powder Tm3+-activated Y3Al5O12 and Tm3+-Li+ co-activated Y3Al5O12 were studied in the visible and near-IR ranges at 300 K. The polycrystalline materials were obtained through a novel combustion synthesis technique that yields chemically homogeneous and small particle size(<3.0 μm) powders. The emission properties of Tm3+-activated Y3Al5O12 showed that the intensity of the blue emission is weak compared to the red emission. With the addition of Li+ as a coactivator, the intensity of the blue emission increased by as much as 87%; however the red and IR emissions also increased. Efficiency measurements showed that the phosphor reached a maximum of 0.21 Im/W at a voltage of 11.7 kV and current of l μA/cm2. The optimal composition for maximum blue emission was found to be Y2.93Tm0.07Al5O12 doped with 1.0 at% Li.
A bright white‐light electroluminescence (EL) is obtained in a thin‐film EL device with Ce‐ and Eu‐doped SrS phosphors. The device shows a luminance level of 1500 cd/m<sup>2</sup> with 5 kHz drive (500 cd/m<sup>2</sup> at 1 kHz). The dominant excitation process of the Eu<sup>2</sup><sup>+</sup> centers is found to be due to an efficient nonradiative energy transfer from the Ce<sup>3</sup><sup>+</sup> to the Eu<sup>2</sup><sup>+</sup> centers.
We show that by comparing the optical spectra of these materials with complementary measurements such as photoconductivity (PC), the absolute energy level structure of the ions with respect to the intrinsic bands of the host can be determined. This positioning is crucial to the luminescence efficiency of RE-activated solids; the quenching of luminescence can arise from the conversion of energy into mobile charges. We believe that this process provides us with a sensitive way to probe the delocalization of optical energy in activated solids and gives additional insights on the nature of the high lying excitations in these materials.
From the time-resolved emissions of the Ti4+:Al2O3 charge-transfer transition, two types of Ti4+ are found. The temperature-dependent lifetimes of the excited Ti4+ are well explained by a three-level system with a lower triplet excited state and a higher singlet excited state. The trapping of charge carriers following the charge exchange between Ti and the host is shown by the thermal release of trapped electrons and holes. These are revealed by thermally stimulated conductivity and thermoluminescence. The possible identities of these traps are discussed.
The donor and acceptor charge-transfer transitions in YAlO3:Ti3+/Ti4+ were investigated through absorption, emission, excitation, one-photon and two-photon photoconductivities, excited-state absorption, and thermoluminescence. The results are contrasted to similar studies previously made on Al2O3:Ti3+/Ti4+. The dominant Ti3+ population has a photoionization threshold at 37 000 cm-1. The lowest energy Ti4+ charge-transfer transition has its zero-phonon energy near to 33 900 cm-1. The sum of these two quantities agrees with the reported band-gap energy of YAlO3. The position of the Ti3+/Ti4+ level in the band gap of YAlO3 is determined by these measurements. Stimulated emission from the 2E state of Ti3+ is limited by photoionization from this state.