Cross Relaxation Research Articles

Numerical investigation of in-band pumped holmium-doped fluoride fiber laser operating at 2.1 µm

Abstract In this study, the theoretical model of in-band pumped holmium-doped fluoride fiber laser was numerically investigated. A holmium-doped fluoride fiber is promoted for laser operating at 2.1 µm. The proposed model is based on the fundamental properties of a holmium doped fluoroindate glasses and parameters obtained from the previously published results. The model was simulated via a cladding pumped system at 2.051 µm pumping wavelength. Good validation between the suggested model with the previously published experimental measurements was achieved with considering the main transfer processes such as energy up-conversion transition and cross relaxation process. The numerical analysis indicated that the energy transfer up-conversion has a negative effect on the laser performance while the cross relaxation process causes significant enhancement on the performance of in-band pumped holmium doped fluoride fiber laser.

Calcination-assisted hydrothermal crystallization and comparative up-/down-conversion luminescence of RE3+ (RE=Ho, Er and Tm)-doped YbNbO4 and Yb3NbO7

The RE3+ doped YbNbO4 and Yb3NbO7 nanophosphors (RE=Ho, Er and Tm) were synthesized via a calcination-assisted hydrothermal procedure. YbNbO4 intensified almost the whole up-conversion (UC)/down-conversion (DC) emissions of RE3+ by ∼2 times when compared with Yb3NbO7 components, indicating that a lower symmetry environment in the former case is more favorable for photoluminescence. UC luminescence in each case was dominated by a green band at ∼539 nm for Ho3+ and a NIR band at ∼803 nm for Tm3+, while doping YbNbO4 and Yb3NbO7 with Er3+ separately yielded the slightly stronger green 556 nm and red 658 nm emissions. The UC/DC luminescence mechanisms were elaborated via Yb3+ → RE3+ forward/backward energy transfer and cross relaxation.

Structural determination, energy transfer and color-tunable photoluminescence of stable LiYSiO4: Ce/Tb/Sm Phosphors toward nUV pumped wLEDs application

Study on photon emission of Ce3+-doped phosphors is essential for advanced optoelectronic application. However, it remains quite difficult to design near-ultraviolet excitable Ce3+-activated silicate blue/cyan-emitting phosphors because of the combined influence of iconicity of Ce3+, crystal field strength, and nephelauxetic effect. Herein, Ce3+ incorporated into a proper host LiYSiO4 (LYS) phosphor is mechanistically investigated and shown efficient blue/cyan photoluminescence (PL) owing to Ce3+ 5d1 electronic state experiencing appropriate crystal field splitting. The comprehensive analysis based on structural refinement, dynamic/static spectra, and density functional theory calculations indicates that Ce3+ ions are energetically favored in octahedral Y site as compared with Li site in the host, resulting in attractive PL properties. According to the principle of color superposition and via cascading energy transfer (ET) from Ce3+ → Tb3+ → Sm3+, systematic PL tuning in a large color gamut is realized by further codoping with the green- and red-emissions of Tb3+ and Sm3+ in the phosphor LYS: Ce3+/Tb3+/Sm3+. The direct ET from Ce3+ → Sm3+ is difficult to occur due to the metal-metal charge transfer effect. Of note, the PL thermal-quenching of Ce3+ is mainly associated with multiphonon relaxation, while the quenching of Tb3+/ Sm3+ is primarily due to the cross relaxation (CR) process via dipole-dipole (d-d) interaction. The appropriate distance between the two nearest adjacent Y sites in the host together with the ET are conducive to weaken the CR process of Tb3+/Sm3+, leading to high PL efficiency. Finally, a prototype pc-wLED assembled via a remote ‘capping’ packaging strategy and by using only the stably composition-optimized white-emitting LYS: Ce3+/Tb3+/Sm3+ demonstrates that the phosphor is promising for high-quality of light.

Tuning the emission color of SrLaAlO4:Er,Yb upconversion phosphors by decorating their surface with CsPbBr3-xIx quantum dots

In this work, SrLaAlO4:Er3+ (2 mol%),Yb3+ (4 mol%) upconversion phosphor was synthesized by using a combustion method. Later, CsPbBr3-xIx (x = 0, 1.5 and 0.75) perovskite quantum dots (QDs) were synthesized by using hot injection method. According the analysis by X-ray diffraction, the SrLaAlO4:Er,Yb (SL:Er,Yb) phosphors and QDs had tetragonal and cubic phases, respectively. Moreover, the analysis by microscopy revealed that the SL:Er,Yb phosphors are composed of micro-grains with irregular morphology, while the CsPbBr3-xIx QDs had a morphology of cubes. The surface of the SL:Er,Yb phosphors was decorated with CsPbBr3-xIx QDs and obtained three composite powders: CsPbBr3+SL:Er,Yb, CsPbBr1.5I1.5+SL:Er,Yb and CsPbBr2.25I0.75+SL:Er,Yb. Those composites were firstly excited with UV light (380 nm) and produced emissions in the green and orange-red regions by downconversion. Interestingly, the emission intensity of the composite powders was 45–75 % higher than that for the individual CsPbBr3-xIx QDs or SL:Er,Yb phosphors. Later, the same composite powders were excited with NIR light (980 nm), in consequence, intense green and yellow emissions were produced by upconversion. In particular, the CsPbBr1.5I1.5+ SL:Er,Yb composite produced strong red emission by upconversion because the presence of the QDs on the SL:Er,Yb surface promoted the following cross relaxation process: 4I11/2 (Er3+)+ 4F7/2 (Er3+)→4F9/2 (Er3+)+ 4F9/2 (Er3+). In general, the quantum dots deposited on the SL:Er,Yb surface not only enhanced the red upconversion emission, but also provoked a color tuning when they are excited with NIR or UV light. Those last effects have not been reported in the literature previously. Thus, the results of this investigation demonstrate that combining perovskite quantum dots and the SL:Er,Yb forms a luminescent material able to tune its emission by upconversion or downconversion. This last property can be utilized for the design of new LED based lamps, which are employed for general illumination in houses and buildings.

Microbial green synthesis of luminescent terbium sulfide nanoparticles using E. Coli: a rare earth element detoxification mechanism

BackgroundRare-earth sulfide nanoparticles (NPs) could harness the optical and magnetic features of rare-earth ions for applications in nanotechnology. However, reports of their synthesis are scarce and typically require high temperatures and long synthesis times.ResultsHere we present a biosynthesis of terbium sulfide (TbS) NPs using microorganisms, identifying conditions that allow Escherichia coli to extracellularly produce TbS NPs in aqueous media at 37 °C by controlling cellular sulfur metabolism to produce a high concentration of sulfide ions. Electron microscopy revealed ultrasmall spherical NPs with a mean diameter of 4.1 ± 1.3 nm. Electron diffraction indicated a high degree of crystallinity, while elemental mapping confirmed colocalization of terbium and sulfur. The NPs exhibit characteristic absorbance and luminescence of terbium, with downshifting quantum yield (QY) reaching 28.3% and an emission lifetime of ~ 2 ms.ConclusionsThis high QY and long emission lifetime is unusual in a neat rare-earth compound; it is typically associated with rare-earth ions doped into another crystalline lattice to avoid non-radiative cross relaxation. This suggests a reduced role of nonradiative processes in these terbium-based NPs. This is, to our knowledge, the first report revealing the advantage of biosynthesis over chemical synthesis for Rare Earth Element (REE) based NPs, opening routes to new REE-based nanocrystals.

Emission color tuning and enhancement of the upconversion emission in NaBiF4:Yb3+,Er3+ phosphors by controlling the Na/F ratio

NaBiF4:Yb3+(20 mol%),Er3+(2mol%) phosphors (NBF:Yb,Er) were synthesized by a simple precipitation method. Several NBF:Yb,Er samples were made with different Na/F ratios (from 2/16 to 6/16 and 6/24). XRD and SEM measurements indicated that all the NBF:Yb,Er phosphors presented an hexagonal phase and morphology of microbars with sizes of 1–2 μm, respectively. According to the photoluminescence measurements, these phosphors emitted red/green emission bands at 541/655 nm and their emission intensity increased ⁓10.5 and 21 times for the green and red bands, respectively, after increasing the Na/F ratio from 2/16 to 4/16. In addition, the emission color was tuned from deep-green to yellow. Moreover, the Na/F ratio was decreased from 6/16 to 6/24, this in turn, changed the color emission from deep-green to orange. This last change of color occurred because the red emission was mainly favored by the following two cross relaxation (CR) processes CR1: 4I11/2 (Er3+)+ 4F7/2 (Er3+)→4F9/2 (Er3+)+ 4F9/2 (Er3+) and CR2: 4S3/2 (Er3+)+ 2F7/2 (Yb3+)→4I13/2 (Er3+)+ 2F5/2 (Yb3+). The second CR2 process also promoted the NIR emission at 1527 nm. Additionally, the lifetimes for the green and red bands increased from 393 to 420 μs and from 428 to 518 μs with the Na/F ratio, respectively. Thus, we presented here a new strategy to enhance the photoluminescence intensity and to control the emission-color/lifetimes without using extra co-dopants (as previously reported in literature).

A bright Ce-based downshifting luminescence nanoprobe for NIR-IIb vessel imaging

Nanomaterials with unique emission in the second near-infrared window (NIR-II, 1000–1700 nm) are regarded as the promising probes for visualization of tumor vessels and cerebrovascular architecture, because of their superior features such as deep tissue penetration and large imaging resolution. Herein, the new Ce-based fluoride nanoparticles (NPs) with high-efficiency NIR-IIb emission were explored for high-resolution vascular imaging and tumor associated vessel detection. Under the same synthesis conditions, the Ce-based fluoride present the preferential formation CeF3 host and low upconversion (UC) emission, but possess more strong (1.45 times) NIR-IIb downconversion (DC) emission centered at 1525 nm under the irradiation of a 980 nm laser, which is significantly different from the traditional NaYF4:Yb/Er host. Through the efficient cross relaxation between Ce3+ and Er3+, intense NIR-IIb luminescence from Er3+ with high quantum yields were achieved, which make sure the entire mouse body and brain blood vessel imaging with supernal resolution (47.6 μm) were demonstrated. Moreover, the optical imaging in NIR-IIb region guided tumorous vascular visualization was successfully achieved. Therefore, the explored CeF3 NPs with boosting NIR-IIb emission can be employed as new diagnostic bio-probes used in high spatial resolution tumorous vessel detection.

Synthesis and Emission Dynamics of Sub‐3 nm Upconversion Nanoparticles

AbstractReducing the size of upconversion nanoparticles (UCNPs) down to a few nm yields luminescent materials containing a very small number of emitters. Considering the bottom limit of one activator per particle ultrasmall UCNPs offer an unprecedented platform to study the contributions of the different energy transfers at play in upconversion luminescence. Maintaining detectable emission despite the limited number of emitting ions and the high surface‐to‐volume ratio requires suitable particle architectures. Na(Gd‐Yb)F4:Tm3+ emissive sub‐3 nm diameter β‐phase UCNPs are prepared using a gadolinium‐rich composition in situ mixing of the precursors and a microwave high‐temperature cycling sequence allowing precise control of the particle size and dispersity. These cores are coated with a NaGdF4 inert shell to minimize the deleterious influence of surface quenching (SQ). Time‐resolved luminescence measurements combining standard NIR excitation of the Yb3+ sensitizer and direct UV excitation of the Tm3+ activator are performed to quantify cross relaxation and surface quenching processes. The fine tuning of the number of activators per particle via an optimized synthesis pathway along with the use of an appropriate excitation scheme enabled to provide an accurate analysis of the different mechanisms at play in these model nanoparticles and to characterize the structure of the core‐shell architecture.

Study on temperature-sensitive factors relevant to thermographic features of Tb3+-doped phosphate glasses

Intensities of the 5D4 luminescence with the 5D3 level populated by GSA and ESA transitions of Tb3+ in ternary phosphate glass were measured at different temperature between 300 K and 675 K. Acquired 5D4 luminescence intensities related to GSA and ESA excitation transitions from thermally coupled 7F6 and 7F5 levels were treated to assess luminescence intensity ratios (LIR) as a function of temperature and resulting values of relative sensitivity SR. To understand origin of marked dissimilarity of SR values observed in some few Tb3+-doped hosts reported thus far an in-depth analysis of the excitation energy flow in systems under study is made. Spectra of absorption cross section and temperature dependent absorption coefficient for ESA transition were evaluated. Rates of temperature dependent cross relaxation process governing energy transfer between 5D3 and 5D4 were calculated. It was supposed that temperature-dependent absorption of colour centres in the blue region may affect adversely relative thermal sensitivity.

S+E+O-band luminescence and enhancement in Nd3+/Tm3+/Ce3+ doped tellurite glass

A new, to our knowledge, doped combination of Nd3+, Tm3+, and Ce3+ ions was developed in tellurite glass with a fundamental composition of TeO2-ZnO-WO3-Bi2O3, and the structural, thermal, and especially near-infrared (NIR) luminescent properties in O-, E-, and S-bands were explored. The XRD pattern confirmed the amorphous nature of synthesized tellurite glass, the Raman spectrum disclosed different structural units that make up the glass network, the DSC curve confirmed good host thermal stability, while XPS measurement revealed that cerium mainly exists in the trivalent form in a glass network. Upon being pumped with an 808 nm laser, a 1.34 µm band fluorescence from Nd3+:4F3/2level→4I13/2 level transition and a 1.46 µm band fluorescence from Tm3+:3H4level→3F4 level transition were observed, which in turn formed a wide luminescence band from 1260 to 1560 nm in the tellurite glass containing both Nd3+ and Tm3+ ions owing to their spectral overlapping. The measured full width at half maximum (FWHM) for the aforementioned wideband luminescence was about 203 nm with Nd2O3 and Tm2O3 concentrations of 0.015 mol. % and 0.5 mol. %, respectively, and exhibited a further ∼60% increase in peak intensity after the incorporation of 0.4 mol. % amount of CeO2, which arises from the phenomena of cross relaxation and energy transfer among the three trivalent rare earth ions. The experimental evidence presented in this work shows that tellurite glass containing Nd3+, Tm3+, and Ce3+ has potential as an active material for S-, E-, and O-band wideband luminescence.

Dual‐Mode Thermal Responses in Red and Near‐Infrared Emission Channels via Manipulating Cross Relaxations

AbstractLead‐free double perovskites with multi‐mode emission characters are highly desired for optoelectronic devices. Herein, dual‐mode thermal responses with low thermal quenching in the red emission channel and high thermal quenching in the near‐infrared (NIR) channel are achieved in a holmium‐based double perovskite (Cs2NaHoCl6). The temperature‐dependent emission is modulated by manipulating three cross relaxations of CR1 (5F4, 5I7) → (5F5, 5I6), CR2 (5S2, 5I8) → (5F5, 5I7), and CR3 (5F5, 5F5) → (5I7, 5G4) through adjusting the concentration of Ho3+. The difference in thermal response between the red and NIR emissions becomes more pronounced with a higher concentration of Ho3+ up to 100%, while the two emissions show almost the same thermal quenching behavior when the Ho3+ concentration is below 20%. To demonstrate their applications, a thermal‐responsive dual‐mode information encryption system and a NIR emitting LED with night vision capacity are fabricated using the prepared samples. This study provides a new approach for thermal quenching regulation in rare‐earth‐based lead‐free perovskites and illustrates their potential in lighting, anti‐counterfeiting, and sensing applications.

Nd3+-sensitized multilayered rare-earth nanocrystals with enhanced NIR-IIb luminescence for high resolution optical imaging

Luminescence imaging in the second near-infrared (NIR-II) window, particularly within the NIR-IIb (1500–1700 nm) sub-window, has been utilized for both fundamental research and clinical applications due to its exceptional spatiotemporal resolution. Nevertheless, efficient contrast agents designed for NIR-IIb imaging are still scarce. Herein, a multilayered rare-earth nanocrystal (NaYF4:Yb,Er,Ce@NaYF4:Yb,Nd@NaYF4) with strong NIR-IIb luminescence was designed and synthesized. The doping of Ce3+ ions in the core effectively suppresses the upconversion and boosts the downconversion luminescence by populating the Er 4I13/2 level. Additionally, by separating the sensitizer (Nd3+) and the activator (Er3+) in different layers, we are able to maximize the light absorption around 800 nm and reduce the cross relaxations between rare-earth elements, which leads to a strong downconversion emission around 1530 nm under 808 nm excitation. After passivation with NaYF4 shell and phase transfer with highly PEGylated mesoporous silica layer, the nanocomposites with excellent colloidal stability and bright NIR-IIb emission were obtained. With long blood circulation time in vivo, the fabricated nanocomposites enable high resolution and contrast luminescence imaging of the blood vessels and tumors in living mice. Moreover, the rare-earth nanocrystals with superior optical properties also hold great promise for various applications in optical sensing and biomedical imaging.

Infrared luminescence properties and energy transfer mechanism in NaYGeO4:Tm3+ powders

Two series of NaY1-xTmxGeO4 olivines with x = 0.005–0.04 and x = 0.0–0.2 have been prepared by the solid state reaction method and citrate technique, respectively. These germanates crystallize in olivine structure and have the orthorhombic lattice, space group Pnma, Z = 4. The diffuse reflectance spectra have been measured and the optical band gap has been estimated. Under 808 nm laser diode excitation, the NaY1-xTmxGeO4 samples yield luminescence in the range of 1300–1600 nm (3H4 → 3F4) and 1600–2200 nm (3F4 → 3H6). The maximum of luminescence integrated intensity has been achieved for the composition NaY0·85Tm0.15GeO4. The luminescence decay kinetics has been explained using migration. The parameters of the cross–relaxation process: 3H4 + 3H6 → 3F4 + 3F4 were obtained taking into account the energy transfer rate between thulium ions. It is shown that the presented materials are characterized by an effective migration-accelerated cross-relaxation process, which occurs as a result of dipole-dipole interactions.

Energy Migration Layer Modulated Lanthanide Luminescent Nanoparticles Toward Multimode Ratio Fluorescence Thermometers

AbstractFluorescent temperature sensing is considered as a research hotspot in the fields of life sciences and medicine. Despite the existence of numerous materials, the low sensitivity still limits their broader application. Herein, a core‐shell‐shell structure of lanthanide‐doped nanoparticles is designed, which can adjust the energy transfer process by controlling the Yb3+ concentration in the energy migration layer, so as to generate different but regular visible upconversion and near‐infrared downshifting emission modes depending on 808 or 980 nm excitation wavelengths. And the energy migration, cross relaxation, and energy cycling processes involved in energy migration ions are analyzed and summarized. Subsequently, unlike traditional thermal coupled fluorescence thermometers, the visible and near‐infrared emission under different excitations is selected as the fluorescence intensity ratio by combining both thermal quenching and enhancement. Thus, a new type of fluorescence thermometer with deep penetration capability and high sensitivity within the biological temperature range is obtained, which can inspire a new design strategy for the future biosensing field.

Enhancing NIR-II Upconversion Monochromatic Emission for Temperature Sensing.

The upconversion luminescence (UCL) in the second near-infrared window (NIR-II) is highly attractive due to its excellent performance in high-resolution bioimaging, anticounterfeiting, and temperature sensing. However, upconvertion nanoparticles (UCNPs) are normally emitted in visible light, potentially impacting the imaging quality. Here, a monochromatic Er3+-rich (NaErF4:x%Yb@NaYF4) nanoparticles with excitation at 1532nm and emission at 978nm is proposed, both situated in the NIR-II region. The proper proportion of Yb3+ ions doping has a positive effect on the NIR-II emission, by enhancing the cross relaxation efficiency and accelerating the energy transfer rate. Owing to the interaction between the Er3+ and Yb3+ is inhibited at low temperatures, the UCL emission intensities at visible and NIR-II regions show opposite trend with temperature changing, which establishes a fitting formula to derive temperature from the luminous intensity ratio, promoting the potential application of UCL in NIR-II regions for the temperature sensing.

The use of energy looping between Tm3+ and Er3+ ions to obtain an intense upconversion under the 1208 nm radiation and its use in temperature sensing.

The upconversion phenomenon allows for the emission of nanoparticles (NPs) under excitation with near-infrared (NIR) light. Such property is demanded in biology and medicine to detect or treat diseases such as tumours. The transparency of biological systems for NIR light is limited to three spectral ranges, called biological windows. However, the most frequently used excitation laser to obtain upconversion is out of these ranges, with a wavelength of around 975 nm. In this article, we show an alternative - Tm3+/Er3+-doped NPs that can convert 1208 nm excitation radiation, which is in the range of the 2nd biological window, to visible light within the 1st biological window. The spectroscopic properties of the core@shell NaYF4:Tm3+@NaYF4 and NaYF4:Er3+,Tm3+@NaYF4 NPs revealed a complex mechanism responsible for the observed upconversion. To explain emission in the studied NPs, we propose an energy looping mechanism: a sequence of ground state absorption, energy transfers and cross-relaxation (CR) processes between Tm3+ ions. Next, the excited Tm3+ ions transfer the absorbed energy to Er3+ ions, which results in green, red and NIR emission at 526, 546, 660, 698, 802 and 982 nm. The ratio between these bands is temperature-dependent and can be used in remote optical thermometers with high relative temperature sensitivity, up to 2.37%/°C at 57 °C. The excitation and emission properties of the studied NPs fall within 1st and 2nd biological windows, making them promising candidates for studies in biological systems.

Constructing Efficient and Thermostable Red‐NIR Emitter via Cross Relaxation and Crystal‐Field Engineering of Holmium‐Based Perovskite‐Type Half Metal

AbstractLanthanide ions, as dopants, have evoked widespread research interest owing to the rich optical, magnetic, and electrical properties, but their luminescent intensity is always limited by typical concentration quenching. Herein, a holmium‐based double perovskite of Cs2NaHoCl6 is reported, which surprisingly breaks through this barrier and achieves efficient red‐NIR emission by virtue of the large unit cell, low phonon energy, high content of activators, and cross relaxation phenomenon between Ho3+. The heavy Ho3+ also endows the intriguing half‐metallic nature with a down‐spin conducting band and an up‐spin insulating band. After performing ion doping on crystallographic sites of Na+ and Ho3+, the photoluminescence quantum yield of such red‐NIR emitter under 450 nm excitation is dramatically promoted to 82.3%, benefiting from the improved crystal field environment that alleviates the parity forbidden rule and suppresses non‐radiative recombination loss. Furthermore, the heat‐favorable phonon‐assisted population processes enable the robust photostability against thermal quenching. By combining a 450 nm chip, the red‐NIR light‐emitting diodes are fabricated, in which the wide‐coverage NIR emissions are ideally suited for medical light source, night vision, nondestructive examination, and transmission imaging. It is believed that this work will open an avenue for enhancing the fluorescence of lanthanide ions and developing advanced spintronic materials.

Cross Relaxation Enables Spatiotemporal Color-Switchable Upconversion in a Single Sandwich Nanoparticle for Information Security.

Smart control of ionic interaction dynamics offers new possibilities for tuning and editing luminescence properties of lanthanide-based materials. However, it remains a daunting challenge to achieve the dynamic control of cross relaxation mediated photon upconversion, and in particular the involved intrinsic photophysics is still unclear. Herein, this work reports a conceptual model to realize the color-switchable upconversion of Tm3+ through spatiotemporal control of cross relaxation in the design of NaYF4:Gd@NaYbF4:Tm@NaYF4 sandwich nanostructure. It shows that cross relaxation plays a key role in modulating upconversion dynamics and tuning emission colors of Tm3+. Interestingly, it is found that there is a short temporal delay for the occurrence of cross relaxation in contrast to the spontaneous emission as a result of the slight energy mismatch between relevant energy levels. This further enables a fine emission color tuning upon non-steady state excitation. Moreover, a characteristic quenching time is proposed to describe the temporal evolution of cross relaxation quantitatively. These findings present a deep insight into the physics of ionic interactions in heavy doping systems, and also show great promise in frontier applications including information security, anti-counterfeiting and nanophotonics.

High-Efficiency Red-Emitting Phosphor of Li0.5La0.5SrMgWO6: Mn4+, Pr3+

The 400–500 nm blue light and 630–750 nm red light play crucial roles in plant cultivation. In this work, a high efficiency red-emitting phosphor, Li0.5La0.5SrMgWO6:Mn4+, Pr3+, was synthesized through the solid-state reaction technique. The absorption and excitation spectra were measured, indicating that 467 nm was an suitable excitation wavelength for this material. Upon excitation with a 467 nm LED, the fluorescence characteristics were evaluated, revealing dual-band red emission peaks located at 621–654 nm and 695–710 nm. The quantum efficiency and pumping slope efficiency were measured to be 63% and 35%, respectively. Based on energy level theory and Tanabe-Sugano theory, the dual-band fluorescence mechanism was analyzed, revealing that cross relaxation phenomenon contributed to the high quantum efficiency. The temperature sensing property was also demonstrated. The relative sensitivity (Sr) exhibited a significant change from 0.05% K−1 to 0.85% K−1 with different temperatures. The phosphor’s high efficiency and exceptional temperature property confer it with extensive potential applications in the fields of plant cultivation and display technology.

Dual-mode photoluminescent microspheres with silica core, separator layer and envelope for anti-counterfeiting ink

The microsphere is concentrically constructed on the basis of SiO2 nanocore on which Gd2O3:Yb,Er upconversion luminescent layer, SiO2 separator, Gd2O3:Ln@SiO2 (Ln3+ = Tb3+, Eu3+) downconversion luminescent layer, and SiO2 envelope are successively deposited by means of wet chemistry method. Under 980 nm near-infrared laser excitation, the microspheres emit bright upconverted green light due to Gd2O3:Yb,Er luminescent layer. While, under 254 nm portable ultraviolet lamp irradiation, they produce red and green downconversion fluorescence because of Gd2O3:Ln (Ln3+ = Tb3+ or Eu3+) luminescent layer. The silica core fixes the luminescence center, reduces non radiative transitions caused by molecular vibration, enhances fluorescence intensity, and saves rare earth resources. Impressively, the silica separator layer isolates the upconversion luminescence of Er3+ and downconversion of Eu3+, preventing cross relaxation and quenching of Er3+ fluorescence. The silica envelope plays a crucial role in the photoluminescence of the prepared microspheres. The photoluminescence quantum yield of the microspheres initially increases from 90.4 % to 95.9 % and then decreases to 82.9 % as the thickness of SiO2 layer on the ∼3 nm-thick Gd2O3:Ln shell rises from 3 nm to 16 nm. The absorption of photons reaches its maximum when the SiO2 layer is 8 nm thick. This thickness corresponds to the lowest rate constant for non-radiative transitions. The proto inks prepared from the dual-mode microspheres exhibit good performance including good luminescence and high dispersibility for practical anti-counterfeiting applications.