Abstract：Ce3+/Eu2+-doped inorganic luminescence materials have been well developed as an indispensable component in fields of lighting， display， monitoring， tracing， anti-counterfeiting， and information storage. The f-d transitions from isolated luminescent centers Ce3+/Eu2+ are strongly influenced by the coordinated crystalline environment， under which energy levels of Ce3+/Eu2+ may be split and shifted due to covalency effect， crystal field splitting， and electron-phonon coupling， resulting in their highly tunable luminescence properties. To date， researchers have developed many high-performance Ce3+/Eu2+-doped luminescent materials， and a series of sound theories have been established to gain insight into the composition-structure-property correlations. Herein， we review the current progress in Ce3+/Eu2+-doped phosphors， focusing on composition-structure-property correlation and phenomenological theory. Particular attention is devoted to overviewing the effect of the local structure and electronic structure on the luminescence property. Finally， an outlook toward potential theory developments is proposed.
Abstract：Eu2+-activated A2CaPO4F（A=K，Rb）phosphors have attracted significant attention for their superior luminescence properties. However， the site occupation， the associated spectral assignment of dopant Eu2+， and hence the mechanism behind the site-regulated emission tuning， still remain elusive. Herein， we carried out systematic density functional theory calculations on defect formation energies and optical transitions of Eu2+ situated at different crystallographic sites with various local charge compensations. It shows that， for K2CaPO4F∶Eu2+， the ~660 nm emission is due to Eu2+ located on the K site with charge compensation by two coordinating OF substitutions plus one CaK near one of the two OF defects， while the ~480 nm emission comes from Eu2+ located on the K site with charge compensation by one coordinating OF substitution. For Rb2CaPO4F∶Eu2+， the ~480 nm emission originates from Eu2+ located at the Rb site with two coordinating OF defects plus one CaRb near both the OF defects. On this basis， we analyze Eu2+ local environments and electronic properties， and discuss their relationship with the relative spectral shift from Eu2+ in K2CaPO4F∶Eu2+ to Rb2CaPO4F. Our results can not only assist in the understanding of experimental observations but also provide a theoretical basis for further performance optimization of the phosphors.
Keywords：Eu2+ -doping;A2CaPO4F （A=K， Rb）;density functional theory calculations;site occupation;luminescence properties
Abstract：Tb3+-doped K3La（PO4）2 phosphors with monoclinic structure have been prepared via a high-temperature solid-state reaction method. The phase purity of samples is checked with X-ray diffraction （XRD） technique and the Rietveld refinement is performed using XRD data of the host compound. The temperature- and concentration- dependent luminescence properties of Tb3+-doped samples are further studied with spectral and decay measurements. Upon 373 nm excitation， Tb3+-doped K3La（PO4）2 phosphors display 5D3-7FJ（J=5， 4， 3， 2） and 5D4-7FJ′（J′=6， 5， 4， 3） transitions at room temperature（RT）. Temperature-dependent photoluminescence（PL） measurements for the sample with low Tb3+ doping concentration indicate that multi-phonon relaxation（MPR） has limited contribution to the quenching of 5D3 emission. As Tb3+ concentration increases， the emission of 5D4 is gradually enhanced while that of 5D3 weakened， leading to the color tunability from cyan to green， which is mainly due to the cross-relaxation（CR） among Tb3+ ions. The decay curves of 5D3 emission are further analyzed and fitted with Inokuti-Hirayama and generalized Yokota-Tanimoto models， manifesting that the main interaction type in CR process is electric dipole-quadrupole（EDQ） coupling with critical interaction distance of ~1.03 nm.
Abstract：The optimization and improvement of luminescent materials have long been a focus of related research fields. The carrier migration has close relationship with lattice defects which are formed during the crystal growth or preparation. Artificial defect control is an important method to regulate carrier migration and improve the luminescent properties， which is of great significance to the development and application of luminescent materials. In previous studies， by doping transition or rare earth metal ions in a specific rigid three-dimensional structure， the point defects caused by non-equivalent cationic substitution can not only effectively promote the self-reduction of activators， but also cooperate with the intrinsic defects to form functional trap levels. These energy levels can trap carriers and realize the dynamic balance of carrier storage and response to external excitation， which effectively improves the luminescence performance of the materials and even brings novel optical properties. This review systematically summarizes the internal coupling mechanism and control methods between point defects and luminescence properties， and provides valuable enlightenment for the subsequent development and exploration of novel optical-electronic functional materials.
HE Xian-guo,HUANG De-cai,SONG Li-ping,LIANG Si-si,ZHU Hao-miao
Abstract：Near-infrared phosphor-converted light-emitting diode（NIR pc-LED） light source has a great application prospect in medical imaging， food detection， sensing and other aspects. The spectral characteristic and photoelectric conversion efficiency of the pc-LEDs are highly dependent on the performance of NIR phosphors. Currently， the development of inexpensive， efficient， and thermally stable broadband NIR phosphors remains a great challenge. Among the potential broadband NIR phosphors， the Cr3+ activated NIR materials have attracted serious attentions due to their advantages of being able to be excited efficiently by blue light and tunable emission band. In this study， a broadband NIR phosphor NaAlP2O7∶Cr3+ was successfully synthesized by high temperature solid-state method. Under 450 nm blue light excitation， the phosphor shows a broadband emission covering 650-1 000 nm with a peak centered at ~780 nm and a full width at half-maximum（FWHM） of 1 580 cm-1. Particularly， at 423 K， the NaAlP2O7∶4%Cr3+ sample retained ~71% of the photoluminescence（PL） intensity at room temperature， exhibiting good PL thermal stability. Furthermore， the crystallographic site occupation and crystal field strength parameter of Cr3+ ions in NaAlP2O7 host were investigated based on the structural analysis and temperature-dependent（8-503 K） spectra of the phosphor. The zero-phonon lines of Cr3+ energy levels were determined by low temperature spectroscopy at 8 K and theoretical calculation. The PL thermal quenching mechanism of Cr3+ was discussed in combination with high temperature dependent spectra. In summary， this work presents a new broadband NIR phosphor for application in NIR pc-LEDs， and highlights some strategies to explore this class of materials.
Abstract：High-performance tunable luminescent materials activated by rare-earth ions， as well as studies on the involved physical mechanisms， are the emphasis and difficulties of design and development of novel luminescent materials and optoelectronic devices. Thereinto， fundamentally unveiling the energy transfer processes induced by the sophisticated interionic interaction has been a vital research problem for the rare-earth luminescent materials. Monte Carlo simulation is a statistical simulation method that relies on huge amounts of repeated random samples to acquire numerical results. In the research field of rare-earth luminescence， Monte Carlo method has become a critical tool to systematically study the interionic energy transfer mechanisms by fully considering the crystal structures， rare-earth-doping behaviors， decay dynamics of fluorescence， etc. In this paper， we first introduce the basic principles and modeling methods of Monte Carlo simulation. Then starting from the interaction mechanisms and geometrical factors of determining energy transfer processes， we generalize the research progresses using Monte Carlo method to fundamentally study the energy transfer modes. At last we give a brief summary as well as a short prospect on the applications of Monte Carlo simulation in investigating rare-earth luminescent materials.
Keywords：rare-earth ions;energy transfer;luminescent materials;Monte Carlo simulation;time-resolved spectrum
Abstract：In recent years， narrow-band emitting phosphors used to fabricate white LEDs play an important role in reducing energy consumption， improving light quality， and maintaining chromaticity stability， and have been widely paid attention to by researchers. The unique character of the 4f-5d transition of Eu2+ ions with host dependence provides the possibility of the design of Eu2+-activated narrow-band emitting phosphors. However， most of the works relies on trial and error， and theoretical understanding is lacking of the emission width of Eu2+ ions in solids. With this background， here we review the theoretical efforts on this topic in the past ten years. Based on configuration coordinate diagram， the effect of structure rigidity， local environment and phonon frequency on full-width at half-maximum（FWHM） of Eu2+ ions have been analyzed. The obtained conclusions are expected to give the design clue for the related experimentalist.
Keywords：Eu2+- doped phosphors;Narrow emission;Coordination environment;Stokes shift;Phonon frequency
Abstract：The definition， history， and developments of inorganic afterglow and storage phosphors will be shortly introduced. Three state-of-the-art phosphors of SrAl2O4∶Eu2+，Dy3+， BaFBr（Ⅰ）∶Eu2+， and Al2O3∶C chip will be first introduced. Based on the above three state-of-the-art phosphors， the issues to the rational design of afterglow and storage phosphors will be shortly analyzed. This work will demonstrate a strategy that how to rational design of inorganic afterglow and storage phosphors based on a so-called vacuum referred binding energy（VRBE） diagram. Firstly， we will shortly introduce what is the vacuum referred binding energy diagram and how it can be constructed by combining the VRBE model required parameters and experimental spectroscopy data. In a VRBE diagram for an inorganic compound including the level locations of lanthanides， the VRBE in the ground or excited states of Bi2+ and Bi3+ will be discussed and added. Secondly， based on the VRBE diagram of the model YPO4 compound， the definition and difference of an electron release model and a hole release model will be shortly introduced. Finally， how to rational design of electron or hole capturing centres and how to tailor their trapping depths will be demonstrated by utilizing the lanthanides and bismuth doped REPO4（RE=La， Y， Lu） and the NaYGeO4 family compounds. The constructed VRBE diagrams for different inorganic compounds will help us to identify and discuss charge carrier trapping and release processes， therefore promoting the development of inorganic afterglow and storage phosphors in a design way instead of by a trial-and-error method.
Keywords：vacuum referred binding energy（VRBE） diagram;lanthanides;Bi3+;Bi2+;afterglow phosphor;storage phosphor
Abstract：Crystal-field theory plays a fundamental role in understanding the energy levels of rare-earth and transition-metal elements in inorganic hosts. It has been fully developed for the octahedral field， both in analytical energy level expression and pictorial understanding. Nevertheless， the crystal field theory for other regular cubic polyhedra， including tetrahedron， cube and cuboctahedron， needs further investigation to show the similarities and differences with the octahedral field. In this work， the detailed crystal-field analysis of one d electron in those regular polyhedra is presented using the perturbation method. The exact crystal-field potentials are derived based on geometrical models with equivalent center-ligand bond length. The secular matrices are constructed and diagonalized to give the split energy levels. The crystal-field splittings for regular tetrahedron， octahedron， cube and cuboctahedron are calculated to be 40Dq/9， 90Dq/9， 80Dq/9 and 45Dq/9. In the end， the relative orientations between eg/t2g orbitals and the ligands are visualized to present a vivid understanding of the magnitude reverse.
Abstract：Deep ultraviolet（UV） luminescent materials with emission wavelength shorter than 320 nm have great potential for photochemistry and photomedicine because of the unique spectral features of UV light including high-energy photon and interference-free by indoor ambient light， which have aroused significant attention in the past few years. In this paper， we have synthesized deep UV emissive LiYSiO4∶Pr3+ and Li（Y，Gd）SiO4∶Pr3+ phosphors by doping Pr3+ and Pr3+-Gd3+ ion pairs into LiYSiO4 host. Upon 450 nm blue light（laser or LED） excitation， these phosphors can emit light in the ultraviolet C and narrowband ultraviolet B through photon upconversion. The deep UV upconversion luminescence properties were investigated in detail by varying the excitation power of 450 nm blue laser， indicating that two-photon upconversion luminescence process is responsible for the deep UV emission. Deep UV light source has been successfully created through a combination of LiYSiO4∶Pr3+ phosphor as luminescence converter and 450 nm LED as excitation source， which shows promising application in the optical locating and tracking field.
Keywords：ultraviolet upconversion luminescence;ultraviolet light source;Pr3+;Pr3+-Gd3+;optical locating and tracking
Abstract：The trivalent bismuth ions can produce， depending on the host， luminescence in deep ultraviolet， visible and even near-infrared wavelengths， which are often adopted as activators for various luminescent materials. The theoretical studies on energy level structures and the luminescence mechanism of Bi3+ dopants can support the design and performance improvement of new luminescent materials. Here， after providing a brief summary of the calculation methods developed and the formula derived to bridge the data from calculations based on density functional theory， we presented the theoretical results from the first-principles calculations on several prototype systems and provided an interpretation of the reported experimental results. The configurational coordinate diagrams with structures and energies from first-principles calculations played a central role in the analyses. Four different types of excited states of Bi3+ ions are covered： the internal excitation of 6s16p1 electron configuration， the ligand-dopant charge transfer excitation of 6s26p1 plus a localized hole， the metal to metal charge transfer excitation of 6s1 plus a bound electron， and the inter-valent Bi2+-Bi4+ state as an excited state of a Bi3+ pair. Great efforts have been done to provide detailed quantitative predictions on the spectroscopy of Bi3+ ions in solids. The results clearly show the role played by theoretical studies in designing and optimizing novel luminescent materials. In addition， the relationship of luminescent properties of Bi3+ ions with local coordination environment of dopant centers， band gap of hosts and defect levels was briefly discussed.
Keywords：first-principles;Bi3+ ions;photoluminescence;excited states
Abstract：Based on an up-do-date literature data， we consider an empirical trend between the energy of the spin-forbidden 3A2-1E transition of the octahedrally coordinated Ni2+ ions and a new nephelauxetic parameter （B， C （B0， C0） are the Racah parameters of Ni2+ ions in a crystal（free state）， respectively）. It is demonstrated that the energy of the Ni2+ 1E state is a linear function of the parameter. These findings prove importance of a simultaneous consideration of reduction of both Racah parameters B and C due to the nephelauxetic effect. Such an approach is more accurate in estimating the energy position of the 1E level. The commonly used nephelauxetic ratio ， which completely ignores the reduction in the values of the Racah parameter C， is not accurate enough for this purpose. The collected in the present paper experimental data and their analysis can be useful for researchers working with the crystalline materials doped with Ni2+ ions.