Tuning size and up-conversion luminescence of CeO2:Yb3+/Er3+ nanoparticles through lanthanide doping

Particle size is a critical parameter in up-conversion luminescence tuning and application research. In this study, CeO₂:Yb³⁺/Er³⁺ nanospheres were synthesized via coprecipitation. The average size of these nanospheres gradually decreased as the Yb³⁺ doping concentration increased, which might be attributed to the influence of Yb³⁺ doping on the growth rate of nanospheres by surface charge repulsion. Upon exciting these nanospheres using a 980-nm laser, the corresponding up-conversion red-green emission intensity ratio gradually increased as the Yb³⁺ doping concentration increased, which might be ascribed to two reasons: the strengthened ⁴S_3/2 → ⁴F_9/2 nonradiative relaxation process and the enhanced Er³⁺ → Yb³⁺ energy back-transfer process. To assess the influence of the nonradiative relaxation process on the up-conversion emission red-green ratios, the down-conversion emission spectra and decay curves of CeO₂:x%Yb³⁺/2%Er³⁺ nanospheres that were excited by a 520 nm laser were investigated. To validate how the particle size affects the up-conversion emission, CeO₂:x%Lu³⁺/2%Yb³⁺/2%Er³⁺ nanospheres of various sizes were synthesized by substituting optically active Yb³⁺ using optically inert Lu³⁺. The corresponding up-conversion emission spectra and decay curves were investigated. The experimental results enhanced our understanding of how lanthanide doping affects the up-conversion luminescence tuning of Er³⁺, offering great potential to regulate the morphology and optical properties of the up-conversion luminescence nanoparticles.     

Phase evolution in Bi2O3:Yb3+/Er3+ nanoparticles with nearly single-band red upconversion luminescence

Bi₂O₃:Yb³⁺/Er³⁺ nanoparticles with flower-like morphology were easily synthesized by urea-assisted coprecipitation reactions. The influences of calcination temperature and doping concentration on the crystal phase structures of Bi₂O₃ and Bi₂O₃:x%Yb³⁺/2%Er³⁺ (x = 0–30) were systematically investigated by XRD analysis. The experimental results revealed that lanthanide doping could effectively improve the thermal stability of Bi₂O₂CO₃ samples, and the monoclinic-to-tetragonal-to-cubic phase transitions of Bi₂O₃ were implemented by controlling calcination temperatures and introducing smaller lanthanide ions (Ln³⁺) into the host lattices. Based on the analysis of TG and DSC curves, we found that the fundamental reason for this phase transition was the different stabilities of each crystalline phase under different doping conditions. Upon 980 nm laser excitation, Bi₂O₃:x%Yb³⁺/2%Er³⁺ samples presented near single-band red upconversion emission owing to the efficient energy reabsorption of the Bi₂O₃ host to Er³⁺ emission.     

Yb3+/Er3+ co-doped Gd2Te4O11 nanosheets with intrinsic polarity: One-step hydrothermal synthesis and upconverted optical temperature measuring ability

The digadolinium tellurite phosphors of Gd₂Te₄O₁₁(GTO):Yb³⁺/Er³⁺ have been successfully synthesized as upconversion luminescence (UCL) materials via one-step hydrothermal method. The crystal structure, morphology, and upconversion luminescence property were systematically characterize by XRD, SEM, and spectroscopy techniques. The Rietveld refinements of crystal structure were carried out on the XRD patterns and the feature of crystal structure was analyzed. Under the 980 nm NIR excitation, these materials showed very bright upconverted emissions. The concentrations of Yb³⁺ and Er³⁺ were optimized and the strongest upconverted emissions were achieved in the GTO:15%Yb³⁺/1%Er³⁺. The possible energy transfer mechanism of UCL was proposed based upon the analysis of power-dependent UCL and fluorescence kinetics. Furthermore, the fluorescence intensity ratio (FIR) derive from the two thermally coupled energy levels (²H_11/2 and ⁴S_3/2) of Er³⁺ was employed as indicator for temperature measurement. The maximum absolute sensitivity can be achieved to be 7.34 × 10^?3 K^?1 at 501 K. This material exhibited good reliability and repeatability in optical temperature measurement, which could be a novel promising candidate for noncontact temperature sensors.     

Preparation and infrared to visible upconversion luminescence of Yb2O3:Ho3+ nanocrystalline powders

Yb₂O₃:Ho³⁺ nanocrystalline powders were synthesized through a solid state reaction method. X-ray diffraction analysis and field emission scanning electron microscopy were used to analyze the phase composition and morphology of the powders. Then under the 980 nm excitation of laser diode, the fluorescence of the crystals was studied via a fluorescence spectrometer. The green and red emissions centered on 551 and 668 nm were observed, and the green band dominated the emission spectrum. The effect of the concentration of Ho³⁺ on the upconversion luminescence intensity was discussed and the possible upconversion emission mechanism was explained. It indicates that like other metal oxide nanoparticles, Yb₂O₃ could also be a potential host material for doping to prepare the upconversion phosphor.     

Y4.67Si3O13-based phosphors: Structure,morphology and upconversion luminescence for optical thermometry

How to improve the sensitivity of the temperature-sensing luminescent materials is one of the most important objects currently. In this work, to obtain high sensitivity and learn the corresponding mechanism, the rare earth (RE) ions doped Y_4.67Si₃O₁₃ (YS) phosphors were developed by solid-state reaction. The phase purity, structure, morphology and luminescence characteristics were evaluated by XRD, TEM, emission spectra, etc. The change of the optical bandgaps between the host and RE-doped phosphors was found, agreeing with the calculation results based on density-functional theory. The temperature-dependence of the upconversion (UC) luminescence revealed that a linear relationship exists between the fluorescence intensity ratio of Ho³⁺ and temperature. The theoretical resolution was evaluated. High absolute (0.083 K⁻¹) and relative (3.53% K⁻¹ at 293 K) sensitivities have been gained in the YS:1%Ho³⁺, 10%Yb³⁺. The effect of the Yb³⁺ doping concentration and pump power on the sensitivities was discussed. The pump-power–dependence of the UC luminescence indicated the main mechanism for high sensitivities in the YS:1%Ho³⁺, 10%Yb³⁺. Moreover, the decay-lifetime based temperature sensing was also evaluated. The above results imply that the present phosphors could be promising candidates for temperature sensors, and the proposed strategies are instructive in exploring other new temperature sensing luminescent materials.     

Influence of the processing way for La3+-doping on crystal structure,microstructure, and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3 ceramics

Perovskite ceramics with a formula of Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) were produced through a conventional solid-state reaction procedure following three different La³⁺-doping methods using powders of La₂O₃, or La₂O₃/Al₂O₃ powder mixture, or LaAlO₃. La³⁺ doping favored grain growth and densification, affected the grain size distribution, and improved the dielectric properties of the produced sintered CTLA ceramics. The doping methods had a strong influence on these properties. More specifically, doping with La₂O₃ and La₂O₃/Al₂O₃ resulted in formation of solid solution, while a secondary phase formed in the CTLA ceramics doped with LaAlO₃, which caused a coarsening of the microstructure and lowered the La³⁺ doping effects on the dielectric properties. The experimental results suggest that La³⁺ doping improves the dielectric properties of the sintered CTLA perovskite ceramics, which are further enhanced by doping with Al³⁺ ions in small amounts. However, further increase of Al³⁺ ions content jeopardizes them.     

Effect of Ce dopant on upconversion and temperature sensing performances in homogeneous ultrasmall Y2O3:Yb3+/Ho3+ nanoparticles through flame aerosol synthesis

Flame aerosol synthesis (FAS) is an excellent strategy for continuous, fast, and mass production of small-size upconversion nanoparticles (UCNPs), which have high potential applications in fields like biological imaging, colour display and optical temperature sensing. However, flame-made UCNPs have received less attention, and relevant studies are limited. Herein, for the first time, we successfully fabricated cerium (Ce)-doped homogeneous ultrasmall Y₂O₃:Yb³⁺/Ho³⁺ UCNPs using a liquid-fed FAS method. Ce was doped to improve the upconversion luminescence (UCL) of the Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. The overall UCL intensity was enhanced ～77.9-fold for an optimal concentration of 20 mol% Ce-doped UCNPs, compared with the UCNPs without Ce doping with a relatively homogeneous ultrasmall size of 8–10 nm. Further studies confirmed that both trivalent (Ce³⁺) and tetravalent (Ce⁴⁺) simultaneously exist in the Y₂O₃ hosts and are critical in enhancing the UCL properties. In addition, the fluorescence intensity ratio (FIR) method was used to evaluate the thermal properties of the fabricated UCNPs. Ce doping significantly improved the thermal sensitivity of Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. An excellent relative sensitivity (S_R) of 0.622% K^?1 at 598 K was obtained for flame-made UCNPs doped with 20 mol% Ce.     

Manipulation of upconversion property and enhancement of sensitivity by tailoring the local structure

Tailoring the local crystal environment around the activators is one of an important way to enhance the upconversion (UC) luminescence intensity. Herein, we substitute the Y³⁺ lattice with La³⁺ ion gradually in Y₂Ti₂O₇:Yb³⁺, Er³⁺ phosphor and investigate the effect of La³⁺ concentration on the UC luminescent properties as well as the temperature sensing behaviors. During the phase transformation process from cubic Y₂Ti₂O₇ to monoclinic La₂Ti₂O₇, the La³⁺ ions also play an important role on the adjustment of size and morphology of particles. Furthermore, the cooperation of La³⁺ ion is in favor of the crystal growth toward the easy growth directions. The UC luminescence intensities can be enhanced efficiently with the increasing of La³⁺ concentration. The sensitivity for temperature sensing can be improved by the increasing of La³⁺ concentration in the Y₂Ti₂O₇ and La₂Ti₂O₇ two-phase coexistence system and the maximum S_A is 45.3 × 10⁻⁴ K⁻¹ at 410K when the La³⁺ concentration is 80%. The maximum S_A and corresponding temperature can be adjusted by controlling the La³⁺ concentration, which means that (Y_0.94-xLa_x)₂Ti₂O₇: Er³⁺/Yb³⁺ phosphors may be applicable to different working environment.     

Fabrication and Upconversion Luminescent Properties of Er3+‐Doped and Er3+/Yb3+ Codoped La2O2CN2 Nanofibers

La₂O₂CN₂:Er³⁺and La₂O₂CN₂:Er³⁺/Yb³⁺ upconversion (UC) luminescence nanofibers were successfully fabricated via cyanamidation of the respective relevant La₂O₃:Er³⁺ and La₂O₃:Er³⁺/Yb³⁺ nanofibers which were obtained by calcining the electrospun composite nanofibers. The morphologies, structures, and properties of the nanofibers are investigated. The mean diameters of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are 179.46 ± 12.58 nm and 198.85 ± 17.07 nm, respectively. It is found that intense green and weak red emissions around 524, 542, and 658 nm corresponding to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_l5/2 energy levels transitions of Er³⁺ ions are observed for La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers under the excitation of a 980‐nm diode laser. Moreover, the emitting colors of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are all located in the green region. The upconversion luminescent mechanism and formation mechanism of the nanofibers are also proposed.     

Gd2O3:Pr3+ nanospheres as bi-functional contrast agents for optical and magnetic resonance imaging properties

Gadolinium based luminescent materials have been researched due to their unique optical and magnetic properties with excellent chemical stability, which are useful to apply in both magnetic resonance imaging (MRI) and fluorescence imaging (FI). In this work, red emitting Gd₂O₃:Pr³⁺ nanospheres were successfully fabricated by a facile co-precipitation method. The change of reaction time influenced the size of Gd₂O₃:Pr³⁺ nanospheres and exhibited the sizes between 110 and 250?nm. The size of the Gd₂O₃:Pr³⁺ nanospheres influenced the luminescence and MR imaging signal intensity. Therefore, it can be applied for bi-functional contrast agents.     

Structure and Luminescence Characteristics of Eu3+‐Activated Trigonal Layered Perovskite Ba2La2ZnW2O12

Eu³⁺‐doped tungstate Ba₂La₂ZnW₂O₁₂ phosphors with perovskite‐structure were prepared by the high temperature solid‐state reaction. The X‐ray powder diffraction (XRD) patterns and structure refinements indicate that the phosphors crystalized in the trigonal layer‐perovskite. The luminescence properties of the phosphors were investigated such as photoluminescence (PL) excitation and emission spectra, decay lifetimes, and color coordinates. It was found that the pure host shows self‐activated emission excited by the UV light. Moreover, Ba₂La₂ZnW₂O₁₂ also shows scintillation characteristics under the X‐ray irradiation. The near‐UV and blue light can efficiently excite Eu³⁺‐doped Ba₂La₂ZnW₂O₁₂ phosphors inducing the strong orange–red luminescence. The optimal Eu³⁺ doping concentration in this host is 40 mol%. The luminescence spectra and the luminescence color of the phosphors strongly depend on the doping levels and excitation wavelength. The different luminescence features were discussed on the base of crystal structure. Eu³⁺ ions have two possible substitutions on A or B sites in this trigonal layered perovskite. The phosphor could act as a candidate for the potential application in near‐UV excited white‐LEDs lighting.     

Upconversion luminescence enhancement of NaYF4:Yb3+,Er3+ nanocrystals induced by the surface plasmon resonance of nonstoichiometric WO2.72 semiconductor

Upconversion luminescence of rare‐earth ions doped nanoparticles can be enhanced by the localized surface plasmon resonance (LSPR) of noble metals nanoparticles, which was extensively investigated. The semiconductor nanomaterials such as the WO_2.72 exhibited the tunable LSPR, which provide the possibility for the luminescence enhancement of upconversion nanoparticles. In this work, the urchin‐like WO_2.72 was successfully prepared by solvothermal method, exhibiting the LSPR in the near infrared region. The influence of LSPR of WO_2.72 on the upconversion luminescence of NaYF₄:Yb³⁺,Er³⁺ nanoparticles was investigated firstly. The 525, 542, and 660 nm upconversion luminescence of NaYF₄: Yb³⁺,Er³⁺ nanoparticles was increased by the 10, 8, and 12 factors, respectively, which was from the enhanced excitation field induced by the WO_2.72 film.     

Study on luminescence properties of co-doped nanoparticles Gd2O3:Tb3+, Eu3+ synthesized by polyol route

Spherical-like Tb³⁺ and Eu³⁺ co-doped Gd₂O₃ nanoparticles with a particle size around 5.5 nm were synthesized by a polyol route. The optimized luminescence property was obtained when 5 mol% Tb³⁺ and 2 mol% Eu³⁺ were co-doped. The influence of different polyalcohol solvents (DEG/PEG) on particle size and luminescence properties was investigated. The results show that the nanoparticles Gd₂O₃:5%Tb³⁺ prepared in PEG presented greater particle size (around 79 nm) and higher luminescence intensity.     

Near-infrared photo-responsive Er3+-K+ co-doped Y2O3 phosphors with enhanced UC luminescence

Y₂O₃: x% Er³⁺ (x=5, 7, 10, 12, 15) and Y₂O₃: 10% Er³⁺，x% K⁺ (x=0, 1, 3, 5, 7, 10, 15) phosphors were successfully prepared by a low-temperature combustion method. The structure as well as the absorption/emission spectra of phosphors were investigated. The effect of doping concentration of K⁺ ions on the upconversion (UC) luminescence of Y₂O₃: 10% Er³⁺ phosphor was examined and the possible optical transitions were discussed. The results showed that K⁺ ion doping not only changed the microstructure and crystallinity of the phosphors, but also enhanced its UC luminescence intensity. The Y₂O₃: 10% Er³⁺, 7% K⁺ phosphor exhibit the strongest UC emission intensity. Compared with the Y₂O₃: 10% Er³⁺ phosphor, the UC luminescence intensity at 563 nm and 661 nm was enhanced by 67.8 and 27.3 times for the K-codoped samples, respectively. The phosphor with the optimal doping concentration was mixed with a polymer to form a composite film, which was employed for the fabrication of near-infrared (NIR) photo-responsive detection devices. The device exhibited strong photo-current response to NIR light at 980 nm, implying that our work could inspire new design strategy for the development of NIR photo-detection devices.     

Simultaneously controlling phase,morphology and up-conversion luminescence via lanthanide doping in Bi2O3: Yb3+/Tm3+ nanoparticles

Bi₂O₃: Yb³⁺/Tm³⁺ nanoparticles with varied phase structures and morphologies were synthesized via the co-precipitation method assisted with a subsequent high-temperature calcination process. The influences of experimental parameters including dosage of urea, dopant concentration, reaction temperature and time on the crystal phase structures and morphologies of the as-prepared nanoparticles were investigated. Results revealed that the morphology, the crystal phase and the up-conversion emission of Bi₂O₃: Yb³⁺/Tm³⁺ could be simultaneously controlled by varying Yb³⁺ doping concentration. With increasing Yb³⁺ doping concentration (0-40%), the dramatic morphological evolution (nano-sheet to nano-flower to nano-sphere) and phase transition (monoclinic to tetragonal to cubic phase) were observed. Under the excitation of CW and pulse 980 nm lasers, Bi₂O₃: Yb³⁺/Tm³⁺ samples exhibited red (693 nm) and NIR (795 nm) double-band up-conversion emissions, where the increase of Yb³⁺ concentration leaded to the decreased emission intensity ratio of NIR to red (I₇₉₅/I₆₉₃) and the suppressed pulse-width-dependent up-conversion output.     

Embedding Ag or Au nanoparticles within the nano-sized wall of YbPO4:Er3+ inverse opals and resulting enhanced upconversion luminescence

Metal nanoparticles preparation in the interior of nanoscale skeleton of inverse opals made up of crystallized matrix is more difficult than the preparation of pure inverse opals. In the present work, the Ag or Au nanoparticles embedded YbPO₄:Er³⁺ inverse opals were prepared by a simple approach, which involved the infiltration of opal template by using the transparent YbPO₄:Er³⁺ sol including silver nitrate or chloroauric acid and the sintering at high temperature. The 20–30?nm Au or 5–10?nm Ag nanoparticles were formed in the interior of nanoscale skeleton in the YbPO₄:Er³⁺ inverse opals, and the Ag or Au nanoparticles embedded YbPO₄:Er³⁺ inverse opals were prepared. The influence of Ag or Au nanoparticles on the upconversion photoluminescence of YbPO₄:Er³⁺ inverse opal was studied, and the upconversion luminescence enhancement induced by the Ag or Au nanoparticles was observed. The mechanisms of upconversion luminescence enhancement of YbPO₄:Er³⁺ inverse opals induced by Ag or Au nanoparticles were discussed. The enhancement of upconversion luminescence induced by Ag nanoparticles was attributed to the enhancement of the excitation field, and the enhancement of upconversion emission induced by Au nanoparticles was related to the increasing of the radiation decay rate of Er³⁺.     

Multifunctional α-NaYbF4:Tm3+ nanocrystals with intense ultraviolet self-sensitized upconversion luminescence and highly efficient optical heating

Lanthanide-doped upconversion photoluminescent nanoparticles with unique anti-Stokes spectroscopic properties excel in many fields of application. Ytterbium-based self-sensitized fluorides with rich Yb³⁺ possess higher absorption efficiency of incident near infrared laser, and are more favorable for photoluminescence or optical heating applications. In this work, α-NaYbF₄:Tm³⁺ crystalline nanoparticles are synthesized, which exhibit intense ultraviolet self-sensitized upconversion photoluminescence and highly efficient optical heating capability under 980 nm laser excitation. NaYbF₄:Tm³⁺ nanocrystals emit multi-band luminescence with emission peaks located in the ultraviolet, blue and red spectral regions. The energy transfer mechanism and electronic transition pathways for the Upconversion luminescence are investigated based on the energy level scheme, and are further confirmed by luminescent dynamic analysis. Due to cross-relaxations between Tm³⁺ and energy back transfer from Tm³⁺ to Yb³⁺ processes, the NaYbF₄: 1 mol% Tm³⁺ nanoparticles possess the highest luminescence intensity. The luminescent dynamic characteristics, such as decay time and rise time, vary with Tm³⁺ doping concentrations. Highly efficient optical heating effect is observed in the NaYbF₄:Tm³⁺ nanoparticles with slope efficiency of photothermal conversion for 10 s laser irradiation is as high as 100.48 °C/W.     

Upconversion Luminescence and Energy‐Transfer Mechanism of NaGd(MoO4)2: Yb3+/Er3+ Microcrystals

Uniform and well‐crystallized NaGd(MoO₄)₂: Yb³⁺/Er^{3 +} microcrystals with tetragonal plate morphology were synthesized by a facile hydrothermal method. The structure and phase purity of the samples were identified by powder XRD analysis. The steady‐state and transient luminescence spectra were measured and analyzed. Under 980 nm excitation, intense green luminescence at 531 and 553 nm, and red luminescence at 657 and 670 nm were observed. The optimum doping concentrations for Yb³⁺ and Er³⁺ are determined to be 20% and 1% in NaGd(MoO₄)₂ tetragonal plate microcrystals. With increasing Yb³⁺ doping concentrations, the total integral emission intensities increase first and then decrease. The red/green intensity ratio of NaGd(MoO₄)₂: Yb³⁺/Er³⁺ microcrystals increases from 0.4 to 1.0 with the increase in Yb³⁺ concentrations. Based on the energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the double logarithmic plot of upconversion intensities versus pump powers. The energy‐transfer mechanisms for green and red upconversion luminescence are ascribed to two‐photon processes at lower Yb³⁺ concentrations, and involve high‐Yb³⁺‐induced one‐photon processes at higher Yb³⁺ concentrations. For the red upconversion luminescence, energy back‐transfer process, that is, ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺), is dominant at higher Yb³⁺ concentrations. Theoretical model of the energy‐transfer mechanisms based on rate equations is established, which agrees well with the experimental results.     

Realization of pure red emission from energy transfer in the Er3+–Tm3+ system under 1550 nm excitation

NaY(WO₄)₂ is one of the excellent host materials for high-efficient upconversion luminescence, but it is still challenging to obtain red emission via lanthanide doping. In this work, pure red emission was achieved in the heavily-Er³⁺-doped NaY(WO₄)₂ by using a 1550 nm laser diode and introducing mediator Tm³⁺ ions in the lattice. On basis of the analysis of steady-state and transient-state luminescence properties related to dopant concentration and excitation wavelength, all possible red emission mechanisms were discussed. Finally, it has been demonstrated that the high-purity red emission was due to the several energy transfer processes between Er³⁺ and Tm³⁺. Our results provide a convenient pathway to investigate the upconversion luminescence mechanisms.     

Solvothermal synthesis of columnar Gd2O2S:Eu3+ and a comparative study with columnar Gd2O3:Eu3+

Columnar Gd₂O₂S:Eu³⁺ nanophosphors have been synthesized through solvothermal reaction and subsequent calcination process. The structure, morphology, and luminescence properties of samples were investigated via X-ray diffraction (XRD), Fourier transform infra-red (FT-IR) spectra, UV-vis absorption spectra, field emission scanning electron microscope (FE-SEM), and photoluminescence (PL) spectra. The Judd-Ofelt (J-O) parameters of Gd₂O₂S:Eu³⁺ nanophosphors with different doping concentration were calculated by emission spectra to understand the symmetry, coordination environment, and luminescence behavior of Eu³⁺ ions in Gd₂O₂S matrix. In addition, the luminescence properties of columnar Gd₂O₂S:Eu³⁺ and Gd₂O₃:Eu³⁺ were compared in detail. The comparison results show that there are subtle differences in luminescence intensity, main peak position, luminescence color, and fluorescence lifetime between them, which are closely related to their different substrate's structure, including the band-gap energy, crystallinity, and symmetry, etc.