Li4 Zn(PO4 )2 :Mn2+ thermosensitive phosphor with dual luminescent centres for optical thermometry.

An optical thermometry strategy based on Mn2+ -doped dual-wavelength emission phosphor has been reported. Samples with different doping content were synthesized through a high-temperature solid-phase method under an air atmosphere. The electronic structure of Li4 Zn(PO4 )2 was calculated using density functional theory, revealing it to be a direct band gap material with an energy gap of 4.708 eV. Moreover, the emitting bands of Mn2+ at 530 and 640 nm can be simultaneously observed when using 417 nm as the exciting wavelength. This is due to the occupation of Mn2+ at the Zn2+ site and the interstitial site. Further analysis was conducted on the temperature-dependent emission characteristics of the sample in the range 293-483 K. Mn2+ has different responses to temperature at different doping sites in Li4 Zn(PO4 )2 . Based on the calculations using the fluorescence intensity ratio technique, the maximum relative sensitivity at a temperature of 483 K was determined to be 1.69% K-1 , while the absolute sensitivity was found to be 0.12% K-1 . The results showed that the Li4 Zn(PO4 )2 :Mn2+ phosphor has potential application in optical thermometry.

Broadband La2LiSbO6: Cr3+ Phosphors with Double Luminescent Centers for NIR pc-LEDs.

The La2LiSbO6: xCr3+ phosphors were synthesized by means of a high-temperature solid-phase method. Based on the differences in ionic radius, valence state, and formation energy, the substitution sites of Cr3+ ions are discussed in detail. The optimized doping concentration of Cr3+ is determined to be 0.01. Under 517 nm excitation, the La2LiSbO6: 0.01Cr3+ phosphor presents a wide emission band (from 700 to 1350 nm) with a peak centered at 952 nm. Additionally, its corresponding full width at half-maximum is 155 nm, and the internal quantum efficiency reaches 62.4%. Meanwhile, the emission intensity of the La2LiSbO6: 0.01Cr3+ phosphor at 373 K is about 63.7% of that at room temperature, exhibiting good thermal stability. Aiming to fabricate a near-infrared phosphor-converted light-emitting diode device, the La2LiSbO6: 0.01Cr3+ phosphor is mixed with epoxy adhesive and cured on a green light-emitting diode chip. Under the irradiation of the fabricated light-emitting diode device, fruits and writing in the dark environment can be captured by a near-infrared camera. Hence, the La2LiSbO6: 0.01Cr3+ phosphor is promising for night vision.

Dual-emitting temperature dependent material based on Bi3+/Eu3+ co-activated Ba2Y2Si4O13 phosphor with multicolor anti-counterfeiting.

Bi3+, Eu3+ co-doped Ba2Y2Si4O13 phosphors with multi-color luminescence properties were prepared by high temperature solid state method. The structure, luminescent properties and temperature characteristics were studied by X-ray diffraction, scanning electron microscope, fluorescence spectrum and temperature-dependence of emission spectrum. Ba2Y2Si4O13: Bi3+, Eu3+ phosphors can emit color from cyan to red when the excitation wavelength was changed from 340 nm to 390 nm, which is attributed to that there are two Bi3+ ion emission centers, and their emission intensity will change with the change of excitation wavelength. Moreover, the emission of the phosphor has good temperature sensing characteristics, based on the fluorescence intensity ratio of the two blue emission bands of Bi3+ and the red emission peak of Eu3+, a multimode thermometer with high temperature sensitivity was constructed. At the same time, based on the dynamic luminescence characteristics of the phosphor, the dynamic anti-counterfeiting experiments are designed. Therefore, the results show that the material has a bright prospect in the field of temperature sensing and anti-counterfeiting.

[Zn2+-Ge4+] co-substitutes [Ga3+-Ga3+] to coordinately broaden the near-infrared emission of Cr3+ in Ga2O3 phosphors.

Here, a "chemical unit co-substitution" method is used to improve the near-infrared (NIR) emission of phosphors, using [Zn2+-Ge4+] to co-substitute [Ga3+-Ga3+] sites to reduce crystal field splitting to affect the structure of gallium oxide. A series of broadband NIR phosphors are synthesized by a high-temperature solid-phase method, and their phase structures, crystal structures, morphologies, diffuse reflectance spectra, and luminescence lifetimes are investigated. The Ga1.68(Zn-Ge)0.3O3:0.02Cr3+ (GZGOC) phosphor exhibits NIR wide-band emission, with a peak wavelength of 766 nm and a half-width of 138 nm. Meanwhile, the quantum yield of photoluminescence can reach 81.2%. The phosphor has good thermal stability. When the temperature reaches 373 K, its emission intensity still remains at 73.4% of that at room temperature. A 460 nm LED chip and this phosphor are used to fabricate a phosphor-converted light emitting diode (pc-LED) device which can be used as a NIR light source. All these results show the application potential of the as-prepared phosphor in NIR imaging.

Manipulation of time-dependent multicolour evolution of X-ray excited afterglow in lanthanide-doped fluoride nanoparticles.

External manipulation of emission colour is of significance for scientific research and applications, however, the general stimulus-responsive colour modulation method requires both stringent control of microstructures and continously adjustment of particular stimuli conditions. Here, we introduce pathways to manipulate the kinetics of time evolution of both intensity and spectral characteristics of X-ray excited afterglow (XEA) by regioselective doping of lanthanide activators in core-shell nanostructures. Our work reported here reveals the following phenomena: 1. The XEA intensities of multiple lanthanide activators are significantly enhanced via incorporating interstitial Na+ ions inside the nanocrystal structure. 2. The XEA intensities of activators exhibit diverse decay rates in the core and the shell and can largely be tuned separately, which enables us to realize a series of core@shell NPs featuring distinct time-dependent afterglow colour evolution. 3. A core/multi-shell NP structure can be designed to simultaneously generate afterglow, upconversion and downshifting to realize multimode time-dependent multicolour evolutions. These findings can promote the development of superior XEA and plentiful spectral manipulation, opening up a broad range of applications ranging from multiplexed biosensing, to high-capacity information encryption, to multidimensional displays and to multifunctional optoelectronic devices.

Improved relative temperature sensitivity of over 10% K-1 in fluoride nanocrystals via engineering the interfacial layer.

Real-time in situ temperature sensing is of significance in the bio-medical field; however, the low relative temperature sensitivity Sr is one of the major obstacles in the development of nanothermometers. Herein, we provide an effective route that engineers the interfacial layer in a core/shell/shell nanostructure to enlarge the temperature-dependent luminescence intensity ratio (LIR) variations followed by an improved Sr. The CaF2 interlayer is employed to inhibit the interaction between the core and outer shell, and increase the interfacial phonon energy to enhance the negative thermal quenching effect (TQE) of Nd3+ ions in the outer shell and positive TQE of Er3+ ions in the core layer. Based on the temperature-dependent LIR variations of Er (650 nm) to Nd (800 nm), the maximum Sr of 10.01% K-1 and minimum Sr of % 2.56% K-1 are achieved.

Designing Optical Thermometers Using Down/Upconversion Ca14Al10Zn6O35: Ti4+, Eu3+/Yb3+, Er3+ Thermosensitive Phosphors.

Down/upconversion Ca14Al10Zn6O35 inorganic phosphors codoped with Ti4+/Eu3+ or Yb3+/Er3+ were prepared. The crystal structure and downconversion luminescence properties of Ca14Al10Zn6O35:Ti4+, Eu3+ phosphors were studied in detail. Ti4+ and Eu3+ occupied Al3+ and Ca2+ sites in the host lattice, respectively. Under the excitation of 273 nm, the emission peak in the 300-570 nm band originated from the 2T2 → O2- transition of Ti4+. The f-f transition of Eu3+ ions generated multiple peaks in the 570-800 nm range. The emission intensity of Ti4+ and Eu3+ ions can be used as a fluorescence intensity ratio (FIR) signal. Based on the FIR technology, the maximum relative sensitivity (Sr) and the minimum temperature uncertainty (δT) reached 1.41% K-1 and 0.07 K, respectively. Meanwhile, the temperature-sensing behaviors were explored by the temperature-dependent upconversion spectra of Er3+- and Yb3+-codoped Ca14Al10Zn6O35 phosphors. Based on the fluorescence intensity ratio of thermal coupling levels (Er3+:2H11/2/4S3/2), the maximum Sr and minimum δT of upconversion phosphors reached 1.28% K-1 and 0.08 K in the temperature range of 293-473 K, respectively. Ca14Al10Zn6O35:Ti4+/Eu3+ (Yb3+/Er3+) phosphors realize temperature sensors with higher relative sensitivity, and it is a good candidate material for optical temperature measurement.

Dual-mode optical thermometry design in K3YSi2O7:Eu multi-site phosphor.

In this study, a dual-mode optical thermometer is designed based on radiative transitions from Eu3+ and Eu2+ ions at different K3YSi2O7 lattice sites. In the luminescence-intensity-ratio strategy, a ratiometric signal composed of Eu3+:5D0→7F1 and Eu3+:5D0→7F2 emissions at 593 and 616 nm, respectively, is employed. Meanwhile, the intensity ratio of the 593-nm emission under O2-→Eu3+ charge transfer excitation (λex = 249 nm) to that upon Eu2+:4f7→4f65d1 excitation (λex = 349 nm) is selected as a thermometric parameter in the single-band-ratio approach. The study findings show that combining the two strategies is conducive to the improvements in sensing-sensitive and anti-interference performance.

A Dual-Mode Optical Thermometer with High Sensitivity Based on BaAl12O19:Sm2+/SrAl12O19:Sm3+ Solid Solution Phosphors.

A series of BaAl12O19:Sm2+/SrAl12O19:Sm3+ mixed-phase phosphors were produced in one step using the traditional high-temperature solid-phase process. Because Sm is divalent in BaAl12O19 and trivalent in SrAl12O19, the coexistence of Sm2+ and Sm3+ is realized in the mixed-phase host. Since the temperature sensitivity of Sm2+ and Sm3+ in the solid solution host is significantly different, this makes it possible for the sample to measure temperature based on the fluorescence intensity ratio (FIR). The crystal model, ion emission spectrum, and temperature sensitivity of these phosphors are studied in detail. Under the co-excitation of a 410 nm excitation source, this sample has excellent temperature measurement performance in the range of 313-513 K. Based on the FIR method, the maximum absolute temperature sensitivity (Sa) is 0.55 K-1 at 513 K, and the maximum relative temperature sensitivity (Sr) is 2.47%K-1 at 453 K. Moreover, based on the photoluminescence lifetime temperature measurement mode, the largest value of Sa at 413 K is 0.046 K-1, and the maximum value of Sr at 473 K is 3.10%K-1. In short, the BaAl12O19:Sm2+/SrAl12O19:Sm3+ solid solution is a kind of phosphor with nice temperature measurement ability, and it has very strong potential in the application of noncontact optical thermometers.

Tunable emission of Li4SrCaSi2O4-yN2y/3:Eu2+ phosphors based on anion substitution induction for WLEDs and optical thermometry.

Polychromatic emission can be achieved by controlling the distribution of the rare earth activator in multi-cation lattices, which can be used in the fields of white light LED and fluorescence temperature sensing. However, it is still a challenge to control their distribution and location of the target site in a given host material because the distribution of the rare earth activator is uncertain. In this paper, we have chosen Li4SrCa(SiO4)2 as the multi-cation site host and induced the distribution of Eu2+ ions between different cation sites through anion substitution, for the first time, to regulate the luminescence characteristics of a series of Li4SrCaSi2O8-yN2y/3:Eu2+ phosphors. In Li4SrCa(SiO4)2:Eu2+ phosphors, the substitution of O2- by N3- triggered a distinct ordered to disordered structure transition of the SiO4 tetrahedron and induced the remote distribution of the Eu2+ activator, which was verified through the analysis of the XRD, EPR, FT-IR and fluorescence spectra. Due to the location of Eu2+ ions in different cation sites (Eu2+Sr and Eu2+Ca), two distinguishable emission peaks with tunable color emissions and different responses to temperature were realized. The white LED that utilized blue-orange-emitting Li4SrCaSi2O4N8/3:Eu2+ and green-emitting BaSi2O2N2:Eu2+ (500 nm) displayed an outstanding color rendering index (Ra) of 85.1. Based on the fluorescence intensity ratio (FIR) technique, an optical temperature measurement mechanism was hypothesized and studied in the temperature range of 293-473 K. The highest Sa of the material was 0.086 K-1, and Sr was 1.76% K-1 based on the FIR detection technology, revealing obviously better than most inorganic optical temperature-measuring materials reported before. Our work indicates that Li4SrCaSi2O8-2yN4y/3:Eu2+ is a promising material for application in White LEDs and optical thermometers.

Thermal enhancement of upconversion in lanthanide-doped Gd2Ti2O7 crystals via fast evaporation in a sol-gel procedure.

A thermal quenching effect caused by the increased multi-phonon assisted non-radiative relaxation possibility greatly restricts the application of luminescent materials. Herein, a modified sol-gel method where the gels are achieved by an ultra-fast evaporation process is employed to prepare lanthanide doped Gd2Ti2O7 crystals, which leads to lattice contraction owing to the formation of vacancy defects. Upon increasing the temperature from 293 to 573 K, the red UC intensities of Er3+ and Ho3+ ions enhance about 5.6 and 9.6 times, respectively, which is attributed to the reduced energy migration from activators to vacancy defects at higher temperatures. These processes are fully reversible. Upon integrating the Gd2Ti2O7:Yb/Er crystals with a negative thermal quenching effect and the NaGdF4:Yb/Tm@NaYF4 nanocrystals with a positive thermal quenching effect, a high relative temperature sensitivity of 8.73% K-1 at 293 K is achieved. These crystals have potential applications not only in highly sensitive temperature sensing, but also in anti-counterfeiting with high security levels.

Highly Efficient NaGdF4:Ce/Tb Nanoscintillator with Reduced Afterglow and Light Scattering for High-Resolution X-ray Imaging.

Scintillation-based X-ray excited optical luminescence (XEOL) imaging shows great potential applications in the fields of industrial security inspection and medical diagnosis. It is still a great challenge to achieve scintillators simultaneously with low toxicity, high stability, strong XEOL intensity, and weak afterglow as well as simple device processibility with weak light scattering. Herein, we introduce ethylenediaminetetraacetate (EDTA)-capped NaGdF4:10Ce/18Tb nanoparticles (NPs) as a highly sensitive nanoscintillator, which meets all of the abovementioned challenges. These NPs show comparable XEOL intensity to the commercial CsI (Tl) single crystal in the green region. We propose a mechanism that involves a new electron-captured path by Ce3+ ions and the promotion of energy migration from a trap center to surface quenchers via a Gd3+ sublattice, which greatly reduces the population in traps to produce significant reduction of afterglow. Moreover, by employing an ultrathin transparent NaGdF4:10Ce/18Tb film (0.045 mm) as a nanoscintillator screen for XEOL imaging, a high spatial resolution of 18.6 lp mm-1 is realized owing to the greatly limited optical scattering, which is superior to the commercial CsI (TI) scintillator and most reported lead halide perovskites. We demonstrate that doping Ce3+ ions can greatly limit X-ray-activated afterglow, enabling to use an ultrathin transparent fluoride NP-based nanoscintillator screen for high-quality XEOL imaging of various objects such as an electronics chip and biological tissue.

Integrating Positive and Negative Thermal Quenching Effect for Ultrasensitive Ratiometric Temperature Sensing and Anti-counterfeiting.

Fluorescence intensity ratio-based temperature sensing with a self-referencing characteristic is highly demanded for reliable and accurate sensing. Although enormous efforts have been devoted to explore high-performance luminescent temperature probes, it remains a daunting challenge to achieve highly relative sensitivity which determines temperature resolution. Herein, we employ a novel strategy to achieve temperature probes with ultrahigh relative sensitivity through integrating both positive and negative thermal quenching effect into a hydrogel. Specifically, Er3+ ions show evidently a positive thermal quenching effect in Yb/Er:NaYF4@NaYF4 nanocrystals while Nd3+ and Tm3+ ions in a Yb2W3O12 bulk exhibit prominently a negative thermal quenching effect. With elevating temperature from 313 to 553 K, the fluorescence intensity ratio of Er (540 nm) to Nd (799 nm) and Tm (796 nm) to Er (540 nm) is significantly decreased about 1654 times and increased about 14,422 times, respectively. The maximum relative sensitivity of 15.3% K-1 at 553 K and 23.84% K-1 at 380 K are achieved. The strategy developed in this work sheds light on highly sensitive probes using lanthanide ion-doped materials.

Ln3+ doped phosphor-in-glass: A new choice of color filter for wide-color gamut white light-emitting diodes.

Phosphor-in-glass (PiG) is a novel fluorescent color conversion material that is an excellent choice for preparing wide-color gamut white LEDs due to its excellent thermal stability, high efficiency and facile preparation process. To the best of our knowledge, this paper is the first to widen the color gamut of the white LED from 77.33% to 92.02% by doping the PiG substrate with two kinds of lanthanide ions: Er3+ and Nd3+. The low sintering temperature and suitable preparation process has ensured the phosphor and glass become compounded independently together while still exhibiting good luminescence performance; this has been demonstrated via X-ray diffraction and field emission scanning electron microscopy. The influence of doping the host glass with Ln3+ ions has been explored, specifically considering the effects on the color gamut, the color coordination, the correlated color temperature and the color rendering index. The results presented herein have demonstrated that the Er3+/Nd3+-doped PiG is a promising candidate as a color filter in the domain of wide-color gamut white LED.

Extended bidirectional reflectance distribution function for polarized light scattering from subsurface defects under a smooth surface.

By introducing the scattering probability of a subsurface defect (SSD) and statistical distribution functions of SSD radius, refractive index, and position, we derive an extended bidirectional reflectance distribution function (BRDF) from the Jones scattering matrix. This function is applicable to the calculation for comparison with measurement of polarized light-scattering resulting from a SSD. A numerical calculation of the extended BRDF for the case of p-polarized incident light was performed by means of the Monte Carlo method. Our numerical results indicate that the extended BRDF strongly depends on the light incidence angle, the light scattering angle, and the out-of-plane azimuth angle. We observe a 180 degrees symmetry with respect to the azimuth angle. We further investigate the influence of the SSD density, the substrate refractive index, and the statistical distributions of the SSD radius and refractive index on the extended BRDF. For transparent substrates, we also find the dependence of the extended BRDF on the SSD positions.

Propagation properties of a hard-edged diffracted beam generated by a Gaussian mirror resonator.

Based on the scalar diffraction theory, the propagation and focusing properties of a hard-edged diffracted beam generated by a Gaussian mirror resonator were investigated. Explicit expressions for the field distribution of the truncated beam that propagates through a paraxial optical ABCD system were derived in detail. Numerical examples are given to illustrate our analytical results.

Electric field enhancement in guided-mode resonance filters.

Electric fields inside guided-mode resonance filters (GMRFs) may be intensified by resonance effects. The electric field enhancement is investigated in two GMRFs: one is resonant at normal incidence, the other at oblique incidence. It is shown that the two GMRFs exhibit different behaviors in their electric enhancement. Differences between the electric field distributions of the two GMRFs arise because coupling between counterpropagating modes occurs in the first case. It is also shown that the order of the electric field of maximum amplitude can be controlled by modulation of the dielectric constant of the grating.

Far-field intensity distribution and M2 factor of hollow Gaussian beams.

The far-field intensity distribution of hollow Gaussian beams was investigated based on scalar diffraction theory. An analytical expression of the M2 factor of the beams was derived on the basis of the second-order moments. Moreover, numerical examples to illustrate our analytical results are given.

Far-field intensity distribution of a beam generated by a resonator with a phase-unifying mirror.

The far-field intensity distribution (FFID) of a beam generated by a phase-unifying mirror resonator was investigated based on scalar diffraction theory. Attention was paid to the parameters, such as obscuration ratio and reflectivity of the phase-unifying mirror, that determine the FFID. All analyses were limited to the TEM00 fundamental mode.