Electronic structure of the SrAl_2O_4:Eu^(2+) persistent luminescence material

The electronic structure of the strontium aluminate （SrAl2O4：Eu^2＋） materials was studied with a combined experimental and theo- retical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structure of the non-optimized and optimized crystal structure were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands as well as the band gap energy of the material together with the position of the Eu2＋ 4f7 85712 ground state were calculated. The calculated band gap energy （6.4 eV） agreed well with the experimental value of 6.6 eV. The valence band consisted mainly of oxygen states whereas the bottom of the conduction band of strontium states. In agreement with the experimental results, the calculated 4f7 8S7r2 ground state of Eu2＋ lies in the energy gap of the host. The position of the 4f7 ground state depended on the Coulomb repulsion strength. The position of the 4f7 ground state with respect to the valence and conduction bands was discussed using theoretical and experimental evidence available.

Synchrotron radiation investigations of the Sr_2MgSi_2O_7:Eu^(2+),R^(3+) persistent luminescence materials

The electronic and defect energy level structure of polyerystalline Sr2MgSi2OT：Eu^2＋,R^3＋ persistent luminescence materials were studied with thermoluminescence and different synchrotron radiation spectroscopies （UV-VUV emission and excitation, X-ray absorption near-edge spectroscopy （XANES） and extended X-ray absorption f＇me structure （EXAFS））. Special attention was paid on the effect of the R3＋ co-dopants on the persistent luminescence properties of the materials. Theoretical calculations using the density functional theory （DFT） were carried out simultaneously with the experimental work. The experimental band gap energy （Eg） value of ca. 7.1 eV agreed very well with the DFT value of 6.7 eV. The variation of the Eg value was attempted to relate with the trap structure as well as with the different properties of the R3＋ co-dopants. The trap level energy distribution depended strongly on the R3＋ co-dopant except for the shallowest trap energy above the room temperature remaining practically the same, however. The different processes in the mechanism of persistent luminescence from Sr2MgSi2OT：Eu^2＋,R^3＋ were assembled and their contributions discussed.

Roles of crystal defects in the persistent luminescence of Eu^(2+), Dy^(3+) co-doped strontium aluminate based phosphors

The roles of different point defects in persistent luminescence of SrAl2O4：Eu,Dy phosphors were investigated. The research results showed that Dyer plays an important role in the persistent luminescence of SrA1EO4：Eu, Dy phosphors. It can serve as the electron trap of suitable depth for persistent luminescence. V~ does not serve as the electron trap of suitable depth, but its existence can increase the depth of electron traps. There is interaction between the Dy^3＋（ DySr ） and the Eu^2＋（Eu^x Sr ）, and only if the distance between the Dy^3＋（DySr） and the Eu^2＋ （Eu^x Sr） is close enough, the Dyer can work as an effective electron trap. The point defect of V＂ Sr can be hole trap, but the change of its density in crystal matrix does not arouse the obvious change of persistent luminescence.

Crystal structure and optical performance analysis of a new type of persistent luminescence material with multi-functional application prospects

Persistent luminescence (PersL) materials,as environmentally friendly and energy-saving materials,have broad application prospects in many fields such as lighting,chemistry and even biomedicine.However,studies on the types,performances and mechanism of PersL materials are still insufficient,which significantly restricts their development and application.Under this consideration,we successfully synthesized a yellow PersL material CaSrGa_(4)O_(8)(CSG).The crystal structure was studied in detail through Rotation Electron Diffraction (RED) and Powder X-ray Diffraction (PXRD).What’s more,by co-doping Mn^(2+) and Yb^(3+),the afterglow brightness of CSG could be increased by nearly 20 times,and the afterglow duration could reach more than 6 h.It is worth mentioning that the samples also have excellent performances in mechanical luminescence (ML),photostimulated luminescence (PSL) and cathodoluminescence (CL),which was also investigated systematically.Finally,an anti-counterfeiting label was designed by the samples to reveal the potential of their application in anti-counterfeiting.The results showed that our research not only provided a new candidate PersL material for multifunctional applications,but also gave good help for studying the physical and chemical properties of CSG.

Roles of Eu^(2+), Dy^(3+) Ions in Persistent Luminescence of Strontium Aluminates Phosphors

The polycrystalline Eu^2＋ and Dy ^3＋ co-doped strontium aluminates SrAl2O4： Eu^2＋, Dy^3＋ with different compositions were prepared by solid state reactions. The UV-excited photoluminescence, persistent luminescence and thermo-luminescence were studied and compared. Results show that the doped Eu^2＋ ion in SrAl2O4： Eu^2＋, Dy^3＋ phosphors works as not only the UV-excited luminescent center but also the persistent luminescent center. The doped Dy^3＋ ion can hardly yield any luminescence under UV-excitation, but effectively enhance the persistent luminescence and thermo-luminescence of SrAl2O4： Eu^2＋. Dy^3＋ co-doping can help form electron traps with appropriate depth due to its suitable electro-negativity, and increase the density and depth of electron traps. Based on above observations, a persistent luminescence mechanism, electron transfer model, is proposed and illustrated.

Photoluminescence and persistent luminescence properties of non-doped and Ti^(4+)-doped Mg_2SnO_4 phosphors

Mg2Sn04 exhibits green photoluminescence and persistent luminescence, which originate from oxygen vacancies. When Ti4＋ ions were doped, an interesting Mg2SnO4：Ti4＋ phosphor with bluish white photoluminescence under ultraviolet irradiation and with green persistent luminescence was first obtained. Our investigation reveals that two emission centres exist in Mg2SnO4：Ti4＋. The centres responsible for the green emission are considered to be the F centres （oxygen vacancies） and the blue centres are the TiO6 complex. Trap clusters in the band gap with different [Snmg-oi],[snmg-VO], [SnMg-VO ] and mgsn,correspond to the components at 85 °C, 146 213 °C of the thermoluminescence curve.

X-ray-activated Bi^(3+)/Pr^(3+)co-doped LiYGeO_4 phosphor with UV and NIR dual-emissive persistent luminescence

X-ray-activated luminescence materials have broad application prospects in photodynamic therapy of deep tissue.Among them,X-ray-activated persistent luminescence materials(PLMs)exhibiting multiple emission peaks have drawn extensive attention for the ir capacity to achieve a combination of bioimaging and therapeutic functions.Here,we developed a novel PLM,LiYGeO_4:Bi^(3+),Pr^(3+),that simultaneously exhibits UV and NIR dual persistent luminescence(PersL)emissions after irradiation by X-ray.The material can be re peatedly excited by X-ray and emits similar lumine scence intensity every time,which shows good PersL stability.In addition,LiYGeO_4:Bi^(3+),Pr^(3+)exhibits photostimulated PersL properties by stimulation with a red light-emitting diode(LED)or NIR laser after long-term decay.This work provides a new choice of X-ray-excited PLMs with UV and NIR dual emission and the novel phosphor shows promise as a potential candidate for the integration of treatment and diagnosis of deep tumors.

X-ray excited Mn^(2+)-doped persistent luminescence materials with biological window emission for in vivo bioimaging

In recent years,persistent luminescence materials(PLMs)excited by X-rays and emitting in biological windows have received extensive attention in the field of high-sensitivity bioimaging.Transition metal Mn^(2+)is an ideal emission center,but few studies focus on Mn^(2+)-doped PLMs with X-ray excitation and biological window emission.Here,we report a Mn^(2+)-doped PLM,LiYGeO_(4):Mn^(2+)(LYGM),with excellent biological window persistent luminescence emission.After excitation by UV,LYGM produces a durable biological window of persistent luminescence emission at 660 nm for up to 20 h.More importantly.LYGM can be repeatedly excited by X-rays,resulting in long-term biological window persistent luminescence emission.In addition,we obtain LYGM around 200 nm in diameter by ball milling and centrifugation and improve its biocompatibility by surface modification to apply it to in vivo imaging in mice.After LYGM are injected into mice through the tail vein,in situ excitation of X-rays can be achieved.After the persistent luminescence decays,LYGM can be re-excited for repeated imaging.Therefore,LYGM shows potential prospects for in vivo deep tissue and long-term bioimaging.

Low-Dose X-Ray Activated Self-luminous Bone Cement with NIR-to-NIR Pseudo-upconverted Persistent Luminescence Sensitized by Nd

X-ray activated near-infrared(NIR)persistent luminescence has promising application in biomedical luminous imaging.However,there are strict medical restrictions on X-ray dosage to avoid radiation disease.There is a need to develop a simple strategy to improve persistent luminescence imaging quality using low-dose X-ray.NIR photo-stimulation can enhance the persistent luminescence intensity by transporting the stored energy from deep traps to shallow ones,so that enhanced upconversion-like NIR-to-NIR persistent luminescence imaging can be conducted after low-dose X-ray excitation.However,existing NIR persistent luminescence phosphors have limited absorption in the NIR.To realize optimized NIR photostimulation,Nd^(3+)is proposed as a potential sensitizer to enhance the absorption of persistent luminescence phosphor to 808 nm light.In this study,ZnGa_(2)O_(4):Sn_(0.1),Cr_(0.003),Nd_(0.01)(ZGSC-Nd)was synthesized with a persistent luminescence peak at~700 nm.After activation with 1 Gy of X-ray,its persistent luminescence attenuated to less than 5%of its original intensity after 2800 s.Interestingly,the nearly vanished persistent luminescence was enhanced 5 times after 808 nm photo-stimulation,which was assigned to Nd absorption.Thus,a pseudo-upconverted persistent luminescence from 808 to 700 nm was realized more efficiently.A Nd-sensitized NIR photo-stimulated persistent luminescence mechanism was proposed.This phenomenon was applied for improved luminous imaging of ZGSC-Nd labeled bone cement after a single low-dose X-ray imaging.This work provides an applicable strategy to realize better persistent luminescence imaging after low-dose X-ray activation and will be significant on designing X-ray activated persistent phosphors for biomedical application.

Room-Temperature Persistent Luminescence in Metal Halide Perovskite Nanocrystals for Solar-Driven CO_(2)Bioreduction

The rapid crystal growth of metal halide perovskite(MHP)nanocrystals inevitably leads to the generation of abundant crystal defects in the lattice.Here,defects-mediated long-lived charges and accompanying room-temperature persistent luminescence are demonstrated to be a general phenomenon in MHP nanocrystals.Density functional theory calculations suggest that the collaboration of Schottky and point defects enables upward cascading depletion for electron transfer in MHP nanocrystals,leading to the generation of long-lived photoexcited charges with lifetimes over 30 min.The excellent optical properties including the presence of long-lived charges,high charge separation efficiency,and broad absorption in the visible region make MHPs ideal candidates for both photocatalysis and photobiocatalysis.The MHPs were further integrated with enzymes to construct a light-driven biosynthetic system for the selective production of fine chemicals from CO_(2)with solar energy.The biosynthetic system can produce formate with a quantum yield of 3.24%,much higher than that of plants(∼0.2-1.6%).These findings will benefit the understanding of the optoelectronic properties of MHPs and further provide opportunities for the development of biosynthetic systems for solar-to-chemical synthesis.

New function of the Yb^(3+) ion as an efficient emitter of persistent luminescence in the short-wave infrared

The trivalent ytterbium(Yb^(3+))ion has been extensively used as an emitter in short-wave infrared(SWIR)lasers,a sensitizer to activate other lanthanide ions for up-conversion luminescence,and a spectral converter in Ln^(3+)-Yb^(3+)doubly doped quantum cutting phosphors.Here we report a new function of the Yb^(3+)ion—as an efficient emitting center for SWIR persistent luminescence.We have developed the first real SWIR persistent phosphor,MgGeO3:Yb^(3+),which exhibits very-long persistent luminescence at around 1000 nm for longer than 100 h.The MgGeO3:Yb^(3+)phosphor is spectrally transparent to visible/near-infrared light(~400–900 nm)and is a promising ultraviolet-to-SWIR spectral convertor.The MgGeO3:Yb^(3+)phosphor also exhibits a photostimulated persistent luminescence capability,where the SWIR persistent emission in an ultraviolet-light pre-irradiated sample can be rejuvenated by low-energy light(white or red light)stimulation.The MgGeO3:Yb^(3+)phosphor is expected to have promising applications in biomedical imaging,night-vision surveillance and photovoltaics.

Recent progress in synthesis of lanthanide-based persistent luminescence nanoparticles

Persistent luminescence nanoparticles(PLNPs)are a kind of phosphors that can remain luminescent for seconds to several days after the stoppage of excitation.Lanthanides show the special capability to largely broaden the emission range and enhance the luminescence intensity of PLNPs due to their dense energy structure and unique electronic configurations.In the past decades,various methods have been developed for the synthesis of lanthanide-based PLNPs with excellent pe rsistent luminescence propertie s,and the lanthanide-based PLNPs are widely studied in areas including biome dicine,energy,and information storage.In this review,we summarized the research progress in the synthe sis of lanthanidebased PLNPs and outline d several typical synthesis methods.We discussed the fundamental concepts of preparation methods as well as the advantages and drawbacks of the typical synthetic approache s.Moreove r,the current challenges and the potential solutions for the development of lanthanide-based PLNP s are also discussed in an attempt to provide strate gies to further improve the optical properties of lanthanide-based PLNPs.We hope this review can contribute to the design of lanthanide-based PLNPs with desired properties and further promote their applications in biomedicine,energy,and information science.

Direct low-temperature synthesis of ultralong persistent luminescence nanobelts based on a biphasic solution-chemical reaction

Here, we report the direct hydrothermal synthesis of 1 D-based Zn2GeO4：Mn2＋ persistent luminescent nanobelts （ZGO：Mn PLNBs）. The ZGO：Mn PLNBs exhibit rapid growth rate, and nanobelts can be obtained after 30 rain of hydrothermal treatment. The persistent luminescence performance can be fine-turned upon prolonging the hydrothermal time. Furthermore, the doping ratio of Mn2＋ exhibits influence on the persistent luminescence properties of ZGO：Mn PLNBs, and 2% doping of Mn2＋ shows superior persistent luminescence with decay time of longer than 20min. The developed 1D-based ZGO：Mn PLNBs can be simply prepared with the hydrothermal method and show tunable morphology and persistent luminescence. We believe that this solid-state-reaction-free chemical approach avoids the current key drawback in regard to PLNMs development, and thus will promote the broad use of these unique nanostructured PLNMs in developing optical device for imaging.

Thermally stable photoluminescence and long persistent luminescence of Ca3Ga4O9:Tb^3+/Zn^2+

A green long persistent luminescence（LPL） phosphor Ca3Ga4O9：Tb3＋/Zn2＋ was prepared. Ca3 Ga4 O9 matrix exhibits blue self-activated LPL due to the creation of intrinsic traps. When Tb3＋ is doped, the photoluminescence（PL） and LPL colors change from blue to green with their intensities significantly enhanced. The doping of Zn^（2＋） evidently improves the PL and LPL performances of the Ca3Ga4O9 matrix and Ca3Ga4O9：Tb^（3＋）. The thermoluminescence（TL） spectra show that a successive trap distribution is formed by multiple intrinsic traps with different depths in the Ca3 Ga4 O9 matrix, and the incorporation of Tb^（3＋） and Zn^（2＋） effectively increases the densities of these intrinsic traps. The existence of a successive trap distribution makes the Ca3 Ga4 O9：Tb^（3＋）/Zn^（2＋） phosphor exhibit thermally stable PL and LPL, It is indicated that this phosphor shows great promise for the application such as high-temperature LPL phosphor.

An outlook of rare-earth activated persistent luminescence mechanisms

By studying energy transfer and persistent energy transfer properties in Eu, Mn co-doped calcium aluminate CaAl_2O_4, it was found that traditional persistent luminescence mechanisms had difficulty in explaining persistent energy transfer property between Eu^（2＋） and Mn^（2＋） ions. Based on this question, a mechanism was suggested in which vibrational energy levels were considered as a Morse potential, and trap centers were distinguished into barriers and wells. Then, this phenomenon was explained by allowing energy transfer and persistent energy transfer could have different preference to different vibrational energy levels. Properties of trap centers were also discussed by using the concept of Madelung energy.

Fabrication and long persistent luminescence of Ce3+-Cr3+co-doped yttrium aluminum gallium garnet transparent ceramics

Pe rsistent luminescence(Pers L)materials are widely used in safety indication,traffic and transportation signs,architectural decoration and other fields.In this paper,(Y_(1-x)Ce_(x))(Al_(0.9995)Cr_(0.0005))_(2)Ga_(3)O_(12)(x=0.001,0.002,0.003,0.005)transparent ceramics were successfully prepared by solid-state reaction method in air followed by HIP post-treatment.With the increase of Cedoping concentration,the optical quality of the as-prepared ceramics is improved and the morphology is denser.Luminescent quenching occurs when the Ceconcentration is more than 0.2%.The as-prepared transparent ceramics we re annealed in different atmosphere s.From the PersL decay curve s,the transparent ce ramics after air annealing show the best PersL performance:luminance with 4424.0 mcd/m^(2)and PersL duration over865 min after ceasing 365 nm excitation,respectively.The effects of Cedoping concentration and annealing atmospheres are also discussed in detail.

Wavelength-tunable barium gallate persistent luminescence phosphors with enhanced luminescence

The near-infrared[NIR]emitting wavelength-tunable Cr^(3+)-doped barium gallate[BGO:Cr]persistent luminescence[PersL]phosphors with enhanced luminescence were reported.The emission wavelength of the BGO:Cr PersL phosphors was adjusted from 715 to 739 nm by varying the amount of Cr^(3+) and the ratio of Ga:Ba.Meanwhile,the luminescence intensity and afterglow of the BG0:Cr PersL phosphors were enhanced.BGO:Cr PersL phosphors exhibited UV excitation,LED light restimulation,PersL for more than 6 days,and excellent capability for information storage,which was expected to promote the development of cheap and wavelength-tunable PersL materials for practical applications.

Defect aggregates in the Sr_2MgSi_2O_7 persistent luminescence material

The crystal and electronic structure of the Eu^2＋ doped and defect containing Sr2MgSi2O7 persistent luminescence material were studied using the density functional theory （DFT）. The defects may act as energy storage or even luminescence quenching centres in these materials, however their role is very difficult to confirm experimentally. The probability of vacancy formation was studied using the total energy of the defect containing host. Significant structural modifications in the environment of the isolated defects, especially the strontium vacancy, as well as defect aggregates were found. The experimental band gap energy of Sr2MgSi2O7 was well reproduced by the calculations. The defect induced electron traps close to the host＇s conduction band were found to act as energy storage sites contributing to its efficient persistent luminescence. The interactions between the defects were found to modify both the Eu^2＋ 4f^7 ground state energy as well as the trap structure. The effect of charge compensation induced by the rare earth co-doping on the defect structure and energy storage properties of the persistent luminescence materials was discussed.

Cr^(3+)/Y^(3+)co-doped persistent luminescence nanoparticles with biological window activation for in vivo repeatable imaging

The near-infrared(NIR)persistent luminescence materials(PLMs)can remain long-lasting luminescence after removal of the excitation light,which permits bioimaging with high sensitivity owing to the absence of background fluorescence interference from in situ excitation.Recently,the NIR PLMs have aroused intensive research interest in bioimaging.However,the optimal excitation wavelength of current NIR PLMs is located in the ultraviolet region with shallow tissue penetration,making it difficult to activate effectively in vivo,and seriously hindering their further application in bioimaging.Herein,we report a novel kind of Cr^(3+)ions and Y^(3+)ions co-doped NIR PLM,Zn_(1.3)Ga_(1.4)Sn_(0.3)O_(4):Cr^(3+),Y^(3+)(ZGSCY),which emits NIR persistent luminescence at 696 nm.Compared with Zn_(1.3)Ga_(1.4)Sn_(0.3)O_(4):Cr^(3+)(ZGSC)excited by the light with a wavelength in the biological window(>650 nm),after being co-doped with Y^(3+)ions,the NIR persistent luminescence performance of ZGSCY is significantly improved because of the increase of trap concentration in the matrix.In addition,we synthesized ZGSCY nanoparticles(NPs)by the combustion method,which exhibit excellent optical properties after being excited by the light with a wavelength in the biological window.After surface modification with PEG,the ZGSCY NPs present low cytotoxicity.Notably,due to the co-doping of Y^(3+)ions,the signal-to-noise ratio(SNR)of ZGSCY NPs in vivo imaging is about 1.8 times higher than that of the ZGSC NPs.Furthermore,the rechargeable in vivo imaging and passive tumor-targeted imaging are successfully achieved by activating with a lightemitting diode(LED,659 nm)after intravenous injection of ZGSCY.Thus,this kind of NIR PLM with high excitation efficiency performance in the biological window is expected to promote its biomedical application in deep tissues.

A NIR to NIR rechargeable long persistent luminescence phosphor Ca_(2)Ga_(2)GeO_(7):Yb^(3+),Tb^(3+)

Near infrared to near infrared(NIR-NIR)photo-stimulated persistent luminescence(PSPL)has shown excellent potential in high-resolution bioimaging for deep tissues.However,the PSPL in NIR-Ⅱregion(900-1700 nm)is still lacking.In this work,Ca_(2)Ga_(2)GeO_(7):Yb^(3+),Tb^(3+)(CGGYT)phosphor with unique lowdimensional crystal structure was synthesized by high-temperature solid-state reaction.Thanks to the carriers transferring from deep traps to shallow ones induced by low energy light,the 978 nm PSPL originating from ^(2)F_(5/2) to ^(2)F_(7/2) transition of Yb^(3+)induced by multimode stimulating(980 nm or WLED)is successfully realized after pre-excited by UV lamp.The NIR PSPL of the specimen can be repeatedly stimulated after placed in dark for 12 h.Moreover,the results indicate that codoping with Tb^(3+)can significantly enhance the NIR-ⅡPSPL owing to the quantum cutting persistent energy transfer(QC PET)from Tb^(3+)to Yb^(3+).Our study points to a new direction for the future development of multimode PSPL materials for bioimaging or multimode optical storage applications.