Broadband Cr~(3+)-sensitized upconversion luminescence of LiScSi_2O_6:Cr~(3+)/Er~(3+)

Broadband sensitization is an effective strategy to enhance the upconversion luminescence(UCL) of lanthanide ions.Herein,novel UC materials LiScSi_2 O_6:Cr~(3+)/Er~(3+)(LSS:Cr~(3+)/Er~(3+)) were synthesized by high-temperature solid state reaction and their luminescent properties were investigated.LSS:Cr~(3+)/Er~(3+)has the broadband absorption in the spectral range of 600-800 nm,and meanwhile shows green UC emissions of Er~(3+)upon pumping Cr~(3+) by the 690 nm laser.The UCL of LSS:Cr~(3+)/Er~(3+)belongs to the twophoton process and is attributed to the energy transfer upconversion mechanism.The effects of the Cr~(3+)and Er~(3+)concentration as well as the Yb~(3+)introduction were also studied.LSS:Cr~(3+)/Yb~(3+)/Er~(3+) exhibits the interesting dual-mode UCL,capable of generating the UCL of Cr~(3+) upon pumping Yb~(3+)ions and the UCL of upon pumping Cr~(3+) ions.This research might promote the development of novel broadband Cr~(3+)-sensitized UC materials.     

Broadband near-infrared luminescence and energy transfer of Cr~(3+),Ce~(3+) co-doped Ca_2LuHf_2Al_3O_(12) phosphors

Broadband near-infrared phosphors are highly desirable for food testing.Targeted Ca₂LuHf₂Al₃O₁₂:Cr³⁺(CLHA:Cr³⁺) and Ca₂LuHf₂Al₃O₁₂:Ce³⁺,Cr³⁺(CLHA:Ce³⁺,Cr^(³⁺)) phosphors were synthesized by the conventional high-temperature solid state reaction.The CLHA:Cr³⁺phosphor,with a good thermal stability,shows a red shift owing to radiation reabsorption and non-radiative transition with increasing Cr³⁺content.For co-doped sample,the emission intensity of Cr³⁺can be enhanced by three times due to the energy transfer from Ce³⁺to Cr³⁺,which can be evidenced by a significant overlap between the PLE of Cr³⁺single-doped phosphor and the PL of Ce³⁺single-doped phosphor.In addition,the mechanism of energy transfer is identified as a quadrupole-quadrupole interaction according to decay Lifetime and Dexter’s energy transfer formula.The broadband NIR emission peaked at 775 nm of CLHA:Cr³⁺,Ce³⁺phosphor shows a bright prospect in nondestructive quality-control analysis systems for food.     

Luminescence tuning of Ce~(3+),Pr~(3+) activated(Y,Gd)AGG system by band gap engineering and energy transfer

Tuning of phosphor luminescence properties,including the emission energy/intensity and thermal stability,is an important way to develop superior luminescent materials for diverse applications.In this work,we discuss the effect of band gap engineering and energy transfer on the luminescence properties of Ce~(3+)or Pr~(3+) doped(Y,Gd)AGG systems,and analyze the underlying reasons for their different phenomena.By using VUV-UV excitation spectra and constructing VRBE schemes,the changes of host band structure,5 d excited level energies and emission thermal stability of Ce~(3+)and Pr~(3+) with the incorporation of Gd~(3+)ions were studied.In addition,the energy transfer dynamics was also investigated in terms of the luminescence decay curves.This work demonstrates a way to tune phosphor luminescence properties by combining band gap engineering and energy transfer tailoring and provides an inspiring discussion on the different results of Ce~(3+)doping on the Ce~(3+)and Pr~(3+)emissions.     

Enhancement of structure stability and luminescence intensity of LiYF_4:Ln~(3+) nanocrystals

LiYF_4 nanocrystals with tetragonal structure were adopted as the host materials for the phosphors and scintillators owing to the low phonon energy and high optical transparency. LiYF_4:Ln~(3+)(Ce~(3+),Eu~(3+)) nanocrystals were fabricated by solvothermal method. Under UV excitation, they could emit visible light. In order to improve the luminescence intensity, the method of co-doping LiYF_4 nanocrystals with Sc was adopted. Sc~(3+) ions could reduce the lattice expansion caused by the doping of Ce~(3+) or Eu~(3+) whose ionic radius was larger than Y~(3+). Crystal structure of Li(Y,Sc)F_4:Ln~(3+) kept much more stable and the luminescence intensity could be significantly enhanced when the concentration of Sc was a moderate value. Thermoluminescence was employed to analyze the electron traps in Li(Y,Sc)F_4:Ce~(3+). Results suggested that the suppression of the generation of electron traps with the co-doping of Sc contributed to the enhancement of luminescence intensity of LiYF_4:Ce~(3+).     

Cooperative energy acceptor of three Yb3+ ions

Our previous work first reported the cooperative sensitized luminescence from Cu~(2+) or Pb~(2+) by three clustered Yb~(3+) ions, in which three NIR photons can be converted into a high energy photon. Could a reverse process happen that a high energy photon is cut into three NIR photons? This work demonstrated an example of three-photon quantum cutting(QC) phosphor,CaF_2:Ce~(3+),Yb~(3+), in which three clustered Yb~(3+) ions(Yb~(3+)-trimer) cooperatively and indirectly received a 306 nm ultraviolet(UV) photon energy transferred from a Ce~(3+) ion in 5 d excited state and emitted three 975 nm near-infrared(NIR)photons. The cluster destruction experiments were designed to verify the necessity of the presence of Yb~(3+)-trimers for QC. The dynamical analysis on luminescence of Ce~(3+) ions confirmed the energy transfer from Ce~(3+) ions to Yb~(3+)-trimers. The doping concentration effect on luminescence was investigated.Furthermore, the maximum energy transfer(ET) efficiency and the corresponding QC efficiency were estimated to be 61% and 222%, respectively. Therefore, the reported three-photon QC phosphor has an attractive prospect in efficiently harvesting solar energy for silicon solar cells.     

Luminescence and energy transfer of color-tunable Lu_2MgAl_4SiO_(12):Eu~(2+),Ce~(3+),Mn~(2+) phosphors

Energy transfer among the co-doped activators is an efficient route to achieve color-tunable emission in inorganic phosphors.Herein,photoluminescence tuning from blue to cyan has been achieved in the Lu_2MgAl_4 SiO_(12);Eu~(2+),Ce~(3+)phosphors by varying the Ce~(3+) concentration with a fixed Eu~(2+)content.With the further introduction of a Mn~(2+)-Si4+couple into the host lattice,the emission color can be tuned to red through the energy transfer of Eu~(2+)and Mn~(2+).The luminescence properties and the energy transfer mechanism were studied in detail.The energy transfer from Eu~(2+)to Ce~(3+)is certified as a dipolequadrupole interaction with the energy transfer efficiency of 41.4% and Eu~(2+)to Mn~(2+)belongs to a dipole-dipole interaction with the energy transfer efficiency of 94.3%.The results imply that this singlephased Lu_2MgAl4 SiO_(12):Eu~(2+),Ce~(3+),Mn~(2+)phosphor has a potential prospect for application in near-UV chip pumped white light emitting diodes.     

Energy transfer in co-and tri-doped Y_3Al_5O_(12) phosphors

Single-doped(Ce~(3+),Tb~(3+)), co-doped(Ce~(3+)-Tb~(3+), Ce~(3+)-Yb~(3+), Tb~(3+)-Yb~(3+)) and tri-doped(Ce~(3+)-Tb~(3+)-Yb~(3+)) Y_3Al_5O_(12) phosphors were synthesized by a solid-state reaction method. The XRD, excitation and emission spectra and the fluorescence lifetime of the samples were measured. The energy transfer mechanism was also investigated. The results showed that the energy transfer efficiency from Tb~(3+) to Ce~(3+) was 51% and the energy transfer efficiency from Ce~(3+) to Yb~(3+) was 63.1%. Concomitantly, both were more efficient than that from Ce~(3+) to Tb~(3+)(7%) and from Tb~(3+) to Yb~(3+)(10.2%). Also, the Yb~(3+) ions received energy mainly from Ce~(3+) ions in Ce~(3+)-Tb~(3+)-Yb~(3+) tri-doped Y_3Al_5O_(12) phosphors. Among these materials, Ce~(3+)-Yb~(3+) co-doped YAG phosphors are a better choice than others as a down-conversion material due to their higher energy transfer efficiency.     

Crystal-field splitting of Ce~(3+) in narrow-band phosphor SrLiAl_3N_4

As a promising narrow-band phosphor,SrLiAl₃N₄has a seemingly ultra-small total crystal-field splitting of only 2400 cm^-1with Ce³⁺as dopant ions.This paper is devoted to unravel this anomalous phenomenon based on semi-quantitative crystal-field calculations.The results show that there may exist undetected excitation peaks immersed in the host excitation band,and the calibrated crystal-field splitting is 27000 cm^-1.comparable to those of other Ce³⁺doped phosphors.In the end the effect of polyhedral deformation on energy level is briefly discussed.     

Tunable luminescence,energy transfer and excellent thermal stability of SrMg_2(PO_4)_2:Ce~(3+),Tb~(3+) phosphors for LEDs

A series of novel SrMg_2(PO_4)_2:Ce~(3+),Tb~(3+)(SMP:Ce~(3+),Tb~(3+)) phosphors with tunable emission spectra were produced via high temperature solid phase method.XRD,fluorescence spectrum and fluorescence lifetime for SMP:Ce~(3+),Tb~(3+)were studied in detail.Under the excitation at 308 nm,SMP:Ce~(3+),Tb~(3+) samples can emit high efficiency tunable blue-green light by controlling the proportion of dopant concentration.Through the spectral overlap and the regular change of fluorescence lifetime,it is proved that there is a significant energy transfer between Ce~(3+) and Tb~(3+) in SMP matrix and the energy transfer mechanism is determined to be an electric dipole-dipole interaction with energy transmission efficiency of 55%.In additional,Commission International de L'Eclairage(CIE) color coordinates and thermal stability were studied.All above findings suggest that SMP:Ce~(3+),Tb~(3+)can be regarded as the potential bluish green phosphor for LED applications.     

Synthesis,optical properties,and energy transfer of Ce~(3+),Tb~(3+) doped KLu_2F_7

Ce~(3+), Tb~(3+) doped orthorhombic phases of KLu_2F_7 microcrystals with hexagonal disk morphology were synthesized by a hydrothermal method. Enhanced emissions of Tb~(3+) were observed in the Ce~(3+)/Tb~(3+) co-doped KLu_2F_7 sample compared to the Tb~(3+) single-doped KLu_2F_7. The energy transfer efficiency from Ce~(3+) to Tb~(3+) was calculated by the photoluminescence intensity in the Ce~(3+)/Tb~(3+) co-doped KLu_2F_7 samples. The average separations between Ce~(3+) and Tb~(3+) were calculated and the critical distance was 0.922 nm estimated by method of concentration quenching. The theoretical calculation proved the results. The theoretical analysis also suggested that the energy transfer from Ce~(3+) to Tb~(3+) in the KLu_2F_7 occurred predominantly via the dipole-quadrupole interaction. The KLu_2F_7 should be good host materials for emitters.     

Luminescence properties of Gd_2Si_2O_7:Ce and Gd_2Si_2O_7:La,Ce under vacuum ultraviolet irradiation

The luminescence properties of Ce- or Ce and La-doped gadolinium pyrosilicate(Gd_2Si_2O_7, GPS) were characterized using vacuum ultraviolet(VUV) excitation light. A prominent emission band was observed in the luminescence spectra with excitation at 60 nm and ascribed to 5d-4f transition of Ce~(3+). Because the excitation wavelength of 60 nm corresponded to the excitation in the host matrix, this result indicated that the excitation energy transfer occurred from the host matrix to Ce~(3+) ions. On the basis of the rise in the luminescence time profiles with excitation at 60 nm, the energy transfer occurred within 2 ns, which was much shorter than that of Ce-doped Gd_2SiO_5. For Ce-doped GPS, the decay rate was slower for the host excitation than that for direct excitation of Ce~(3+). In contrast, for Ce and La-doped GPS, no significant difference was observed for the host excitation and direct excitation of Ce~(3+). This result indicated that the energy transfer from the host to Ce~(3+) ions led to a different radiative decay process, and that La doping had an effect on the energy transfer and decay process.     

Effect of chloride on spectrum properties of Pr~(3+)/Ho~(3+) co-doped fluorozirconate glasses

An effective method of improving the luminescent properties of rare earth ions in fluoride glasses were reported.The Pr~(3+)/Ho~(3+)co-doped fluorochlorozirconate luminescent glasses were prepared,and the effects of chloride on the spectral properties and structure of the glasses were studied.According to the results,the glass stability is improved,and the luminescence intensity in the visible range is significantly enhanced with the introduction of chloride.By introducing 7.5 mol% BaCl_2,the luminescence intensity reaches the maximum and increases by three times,The mechanism of luminescence enhancement is explained by analyzing the correlation between the composition and the structure.The chloride ions disperse outside the glass network before the introduction of 7.5 mol% BaCl_2 and increased dispersity of Pr~(3+)and Ho~(3+)ions in the fluorozirconate glasses.     

ScF_3的水热法制备及掺Ce~(3+)光谱性质研究

首次用低温水热法制备了ScF_3晶体及其掺杂Ce~(3+)的粉体.通过X射线粉末衍射(XRD),扫描电镜(SEM),荧光光谱(FS)对样品进行了表征.结果表明,所用氟化物的种类和反应温度对ScF_3的制备至关重要.以NaF作为原料更容易得到纯的ScF_3.当水热温度为120℃时,产物为ScF_3晶体,而温度升高至180℃时则为NaScF4复合氟化物.所制备的ScF_3呈针尖棒状放射形.考察了ScF_3:Ce~(3+)粉体荧光光谱性质,讨论了其与同类型的稀土氟化物的荧光光谱的比较.     

Near-infrared luminescent Nd~(3+)/Yb~(3+)-codoped metal-organic framework for ratiometric temperature sensing in physiological range

Self-calibrating luminescent thermometry employing luminescence within the optical transparency windows provides a promising prospect for temperature measurement in the biological fields.In this work,a new Nd~(3+)/Yb~(3+)-codoped metal-organic framework Nd_(0.95)Yb_(0.05)BPTC showing threedimensional anionic network,obtained by reacting ligand [1,1'-biphenyl]-3,3',5,5'-tetracarboxylic acid(H_4BPTC) with Nd~(3+) and Yb~(3+) ions under solvothermal conditions,is reported.Upon 808 nm photoexcitation,Nd_(0.95)Yb_(0.05)BPTC simultaneously emits the characteristic near-infrared luminescence of Nd~(3+) and Yb~(3+) ions based on the efficient energy transfer from Nd~(3+) to Yb~(3+) ions.In addition,the emission intensity ratio of Yb~(3+) and Nd~(3+) shows good exponential-like response to temperature in the physiological range of 293-323 K.The feature properties of Nd_(0.95)Yb_(0.05)BPTC include near-infrared absorption and emission,favorable temperature sensitivity and accurate temperature uncertainty,as well as good chemical stability,making such system useful in biomedical applications.     

Temperature sensing behavior of Tm~(3+):~1G_4(a),~1G_4(b) in oxyfluoride glass ceramics containing BaYb_xY_(1-x)F_5 nanocrystals

Tm~(3+)-doped transparent oxyfluoride glass ceramics containing BaYb_xY((1-x))F_5 nanocrystals were prepared via high temperature solid phase melting method,of which up-conversion emission is achieved by the Yb~(3+)-mediated energy transfer process.The required photon number of Tm~(3+)ions emissions in BaYb_xY_((1-x))F_5 nanocrystals was calculated through the luminescence spectra,revealing the strong dependence of energy transfer mechanism on Yb~(3+)ions concentration.Meanwhile,based on the fluore scence intensity ratio technology,the effect of different energy transfer mechanism on the temperature sensitivity was investigated by the temperature-dependent luminescence intensity of thermally coupled energy levels of Tm~(3+):~1G_4(a),~1G_4(b).The obtained sensitivity decreases with the increase of Yb~(3+)ions content,which is mainly attributed to the changes in photon absorption process of Tm~(3+):~1G_4(b).     

Tunable luminescence and energy transfer in Y_2BaAl_4SiO_(12):Tb~(3+),Eu~(3+)phosphors for solid-state lighting

Single-phase Y₂BaAl₄SiO₁₂:Tb³⁺,Eu³⁺phosphors with adjustable luminescence were successfully prepared by high-temperature solid-state reaction method.The structural,luminescent properties and ene rgy transfer(ET) process of Y₂BaAl₄SiO₁₂:Tb^(³⁺),Eu³⁺phosphors were syste matically analyzed with the help of X-ray diffraction(XRD),scanning electron microscopy(SEM),excitation spectra,emission spectra and photoluminescence decay curves.Tunable luminescence ranging from green through yellow and definitively to red can be achieved by elevating amounts of Eu³⁺ions in Tb³⁺,Eu³⁺co-doped samples.Besides,the ET mechanism and efficiency were also analyzed and the maximum ET efficiency is 67%.All the results show that Y₂BaAl₄SiO₁₂:Tb³⁺,Eu³⁺phosphors can be used in solid-state lighting.     

Effect of RE~(3+) electronegativities on luminescence property of Ba_5Si_8O_(21):Eu~(2+) persistent phosphor

A series of Ba_5Si_8O_(21):0.02Eu~(2+),0.09RE~(3+) persistent phosphors were synthesized by the solid-state reaction method.The measurement results of photoluminescence(PL),phosphorescence and thermoluminescence(TL)were analysed and discussed.The XRD results showed that samples codoped with different RE~(3+) were Ba_5Si_8O_(21) single pure phase.Under the excitation,all samples exhibited a broad Eu~(2+) characteristic emission,and the La~(3+) co-doped sample emitted the brightest photoluminescence even though its persistent luminescence property was the worst because of the weakest electronegativity.However,Nd~(3+) electronegativity was suitable,thus after activation,the Ba_5Si_8O_(21):Eu~(2+),Nd~(3+) sample had the best persistent luminescence performance with the highest phosphorescence intensity and the persistent luminescence decay time beyond 8 h.The Nd~(3+) co-doped sample also had the largest thermoluminescence integral area which proved effectively it had longer persistent luminescence time.The luminescence mechanism was also proposed to study the photoluminescence and persistent luminescence process.These results showed that RE~(3+) electronegativities were distinctly important for persistent phosphors and choosing suitable electronegativity codopant was conducive to enhancing the phosphorescent performance.     

Preparation and upconversion luminescence modification of YbPO_4:Er~(3+) inverse opal heterostructure

Photonic crystal heterostructures composed of YbPO_4:Er~(3+) inverse opal and polystyrene opal were prepared via a template-assisted process, which exhibited two photonic band gaps. The microstructure,phase and optical properties of photonic crystal heterostructures were investigated by x-ray diffraction,scanning electron microscopy, fluorescence spectroscopy, absorption spectroscopy, fluorescence lifetime,etc. The upconversion emission suppression caused by single photonic band gap from the following YbPO_4:Er~(3+) inverse opal or the upper opal was observed. The upconversion luminescence was strongly suppressed due to the two photonic band gap overlapping effect caused by the following YbPO_4:Er~(3+) inverse opal or the upper opal. The modified mechanisms of upconversion luminescence were discussed by the upconversion luminescence lifetime of YbPO_4:Er~(3+) photonic crystal heterostructures. The results demonstrated the modified upconversion luminescence is attributed to the photon trapping caused by Bragg reflection of photonic crystal heterostructures.     

Luminescent and thermometric properties of dual emitting Eu~(2+)/Sm~(3+) co-doped Sr_4La(PO_4)_3O phosphor based on energy transfer

Eu²⁺/Sm³⁺co-doped dual-emitting Sr₄La(PO₄)₃O phosphors were synthesized through a convenient high temperature solid state reaction in reductive atmosphere.The structure,luminescence,energy transfer and temperature-dependent luminescence properties of Eu²⁺/Sm³⁺co-doped Sr₄La(PO₄)₃O phosphors were researched and analyzed in detail.The blue emission of Eu²⁺and the red emission of Sm³⁺can work together as FIR signals.Based on the different response characteristics of these two ion emissions to temperature,Sr₄La(PO₄)₃O:Eu²⁺/Sm^(³⁺)phosphor achieves the relative sensitivity of0.48384%/K and a wide range of temperature measurements from room temperature to 573 K.The results reveal that the Sr₄La(PO₄)₃O:Eu²⁺/Sm³⁺phosphor has application prospect in the field of high temperature optical thermometry.The energy transfer mechanism is proved to be the dipole-dipole interaction between Eu²⁺and Sm³⁺ions.     

Structure and luminescence properties of Sm~(3+) doped Y_2MoO_6 phosphor under near ultraviolet light excitation

A series of Sm~(3+) doped Y_2MoO_6 were prepared through high temperature solid state reaction technique. Their phase structures, morphologies and luminescence properties were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM) and photoluminescence spectrometry. The most intense absorption of Y_2MoO_6 host occurred at 367 nm. Energy transfer from host lattice to Sm~(3+) ions could be observed, and the schematic diagram of energy transfer was constructed. The critical energy transfer distance and energy transfer mechanism between Sm~(3+) ions were discussed in detail. Considering the high color purity and appropriate emission intensity, Sm~(3+) doped Y_2 MoO_6 could be a promising phosphor under near ultraviolet light excitation.