Power dependent upconversion in Er3+/Yb3+ co-doped BiNbO4 phosphors

In the present article, optical properties and energy upconversion in Er³⁺/Yb³⁺ co-doped BiNbO₄ matrix were investigated. The BiNbO₄ matrix was prepared using the solid-state reaction method. X-ray diffraction of the matrix shows that the crystal structure is consistent with ICSD code 74338. The grain distribution and the behavior of doping with Er³⁺ and Yb³⁺ on the sample surface were obtained by scanning electron microscope. Raman spectral characterization was carried out to examine the behavior of the vibrational modes of the samples. Upconversion emissions in the visible region at 484.5, 522, 541.5 and 670.5 nm in the matrices BiNbO₄:Er,Yb and BiNbO₄:Er were observed and analyzed as a function of 980 nm laser excitation power and rare-earth doping concentration. The results show that BiNbO₄ is a promising host material for efficient upconversion phosphors.     

Preparation of Er3+/Yb3+ co-doped zeolite-derived silica glass and its upconversion luminescence property

A novel upconversion luminescence transparent glass has been successfully synthesized from Er³⁺/Yb³⁺ co-doped zeolite powder by Spark Plasma Sintering (SPS) method through the order–disorder transition process. XRD was used to detect the order–disorder transition process of each phase after SPS. These zeolite-derived silica glasses showed enhanced upconversion luminescence under the excitation of 980 nm diode laser, which was caused by the change of phonon energy according to the results of Raman spectrum, and the corresponding energy transfer mechanism was also discussed in detail.     

Concentration effects on the upconversion luminescence in Ho3+/Yb3+ co-doped NaGdTiO4 phosphor

Ho³⁺/Yb³⁺ co-doped NaGdTiO₄ phosphors were synthesized by a solid-state reaction method. The upconversion (UC) luminescence characteristics excited by 980 nm laser diode were systematically investigated. Bright green UC emission centered at 551 nm accompanied with weak red and near infrared (NIR) UC emissions centered at 652 and 761 nm were observed. The dependence of UC emission intensity on excitation power density showed that all of green, red and NIR UC emissions are involved in two-photon process. The UC emission mechanisms were discussed in detail. Concentration dependence studies indicated that Ho³⁺ and Yb³⁺ concentrations had significant influences on UC luminescence intensity and the intensity ratio of the red UC emission to that of the green one. Rate equations were established based on the possible UC mechanisms and a theoretical formula was proposed to describe the concentration dependent UC emission. The UC luminescence properties of the presented material was evaluated by comparing with commercial NaYF₄:Er³⁺, Yb³⁺ phosphor, and our sample showed a high luminescence efficiency and good color performance, implying potential applications in a variety of fields.     

Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

ZnO-TiO₂ composites co-doped with Er³⁺ and Yb³⁺ ions were successfully synthesized by powder-solution mixing method and their upconversion (UC) luminescence was evaluated. The effect of firing temperature, ZnO/TiO₂ mixing ratio, and dopant concentration ranges on structural and UC luminescence properties was investigated. The crystal structure of the product was studied and calculated in detail by means of X-ray diffraction (XRD). Also, the site preference of Er³⁺ and Yb³⁺ ions in the host material was considered and analyzed based on XRD results and UC luminescence characteristics. Brightest UC luminescence was observed in the ZnO-TiO₂:Er³⁺,Yb³⁺ phosphor fired at 1300 °C in which the system consisted of mixed phases; Zn₂TiO₄, TiO₂, RE₂Ti₂O₇ and RE₂TiO₅ (RE = Er³⁺ and/or Yb³⁺). Under the excitation of a 980 nm laser, the two emission bands were detected in the UC emission spectrum, weak green band centered at 544 and 559 nm, and strong red band centered at 657 and 675 nm wavelengths in accordance with ²H_11/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The simple chemical formula equations, for explaining the site preference of Er³⁺ and Yb³⁺ ions in host crystal matrix, were generated by considering the Zn₂TiO₄ crystal structure, its crystal properties, and the effect of Er³⁺ and Yb³⁺ ions to the host crystal matrix. The UC emission intensity of the products was changed by varying ZnO/TiO₂ mixing ratios, and Er³⁺ and Yb³⁺ concentrations. The best suitable condition for emitting the brightest UC emission was 1ZnO:1TiO₂ doped with 3 mol% Er³⁺, 9 mol% Yb³⁺ fired at 1300 °C for 1 h.     

Effect of crystalline fraction on upconversion luminescence in Er3+/Yb3+ Co-doped NaYF4 oxyfluoride glass-ceramics

The crystalline fraction were adjusted MgO concentration and the corresponding effect on upconversion (UC) luminescence in Er³⁺/Yb³⁺ co-doped NaYF₄ oxyfluorode glass-ceramics was investigated. With increase of MgO and the content of Na₂O reduced, the internal network structure of the glass became compact, which made the size of NaYF₄ nanocrystals unchanged, while the average distance between the nanocrystals increased significantly. Crystal growth is limited with the glass network, keeping the crystal size not changed. SNM-1 glass ceramics samples show a predominant red up-conversion emission under near infrared excitation at 980 nm, while a predominant green emission is observed in the SNM-3 samples. In this paper, it was indicated that it changed the effect of glass network modifier MgO in the glass structure. The possible mechanism responsible for the color variation of UC in Er³⁺/Yb³⁺ co-doped was discussed.     

Er3+/Yb3+ co-doped bioactive glasses with up-conversion luminescence prepared by containerless processing

Er³⁺/Yb³⁺ co-doped bioactive glasses were prepared via containerless processing in an aerodynamic levitation furnace. The as-prepared glasses were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) equipped with energy dispersive X-Ray spectroscopy (EDX). The up-conversion luminescence of as-prepared glasses was measured using an Omni- 3007 spectrometer. Furthermore, the in vitro bioactivity was evaluated by soaking the materials in simulated body fluid, and the biocompatibility was evaluated in MC3T3-E1 cell culture.The results show that containerless processing is a unique method to prepare homogeneous rare earth doped bioactive glasses. The obtained Er³⁺/Yb³⁺ co-doped glasses show green and red up-conversion luminescence at the excitation of 980 nm laser. The XRD analysis confirmed that calcium silicate powders, as starting materials, were completely transformed from the original multi-crystalline phase (CS-P) into the amorphous-glassy phase (CS-G, EYS, LCS) via containerless processing. The SEM observation combined with EDX and FTIR analyses showed that the as-prepared glasses were bioactive. The cell proliferation assay also revealed that the as-prepared glasses were biocompatible and nontoxic to MC3T3-E1 cells. This study suggests that the luminescent bioactive glasses prepared by containerless processing could be used for studying biodegradation of bone implantation materials.     

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.     

Enhanced upconversion emission in Yb3+ and Er3+ codoped NaGdF4 nanocrystals by introducing Li+ ions

β-NaGdF(4)?:?Yb(3+)/Er(3+) nanoparticles (NPs) codoped with Li(+) ions were prepared for the first time via a thermal decomposition reaction of trifluoroacetates in oleylamine. The influence of site occupancy of Li(+) on the upconversion emission of β-NaGdF(4)?:?Yb(3+)/Er(3+) NPs was investigated in detail. The upconversion emission intensity was significantly enhanced by introducing different concentrations of Li(+) ions. In contrast to lithium-free β-NaGdF(4)?:?Yb(3+)/Er(3+), the green and red UC emission intensities of the NPs codoped with 7 mol% Li(+) ions were enhanced by about 47 and 23 times, respectively. The luminescence enhancement should be attributed to the distortion of the local asymmetry around Er(3+) ions. The mechanisms for the enhancement of upconversion emission were discussed. In addition, it was found in our research work that β-NaGdF(4)?:?Yb(3+)/Er(3+) NPs exhibited paramagnetic features at room temperature and the magnetization was slightly increased by introducing Li(+) ions.     

Effect of glass-ceramics network intermediate Al2O3 content on up-conversion luminescence in Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

Effect of alumina as a glass network intermediate on the up-conversion luminescence (UCL) in NaYF₄:Er³⁺/Yb³⁺ co-doped oxy-fluoride glass-ceramics (GCs) was investigated. Combinations of smaller NaYF₄ nanocrystals (10 and 13 nm) and lower Al₂O₃ contents (5% and 10%), as well as larger NaYF₄ nanocrystals (26 and 40 nm) and higher lower Al₂O₃ contents (15% and 20%) were prepared after heat-treatment, respectively. The glass network of intermediate partial replacement of SiO₂ with Al₂O₃ was investigated, and the consequence on the response to the up-conversion of the lanthanide ions was also studied. The UCL properties of Er³⁺ ions were changed in accordance with the addition of Al₂O₃, the red UCL intensity decreased with an increased Al₂O₃ concentration, while the green emission intensity showed opposite tendency. Our results showed that adding Al₂O₃ to 20 mol% is an effective strategy to simultaneous control of the magnitude and luminescence properties of lanthanide ion doped GCs.     

Enhancing upconversion luminescence and thermal sensing properties of Er/Yb co-doped oxysulfide core-shell nanocrystals

The development of noncontact thermal probe based on stable inorganic materials of trivalent lanthanide (Ln³⁺) doped phosphors with nontoxicity is of vital importance for their promising applications in bio-medical fields. Here we explore the upconversion luminescence and thermal sensing properties of Er³⁺, Yb³⁺ co-doped oxysulfide in a broad temperature range of 300-583 K. It was found that constructing an active shell with an optimum concentration of sensitizers is an efficient way to improve both the luminescent intensities and thermal sensitivity. Compared with the core-only sample, the luminescent intensity of the Y₂O₂S: Er³⁺, Yb³⁺@ Y₂O₂S: 5%Yb³⁺ sample is significantly enhanced by 12-fold at excitation of 980 nm. While further increasing the Yb³⁺ concentration in the shell activates new quenching pathways of Er³⁺ → Yb³⁺ → quencher from the core to the shell. Similar quenching mechanisms are also observed at excitation of 1550 nm. These energy transfer processes and luminescence mechanisms are verified in the fluorescence decay measurements. Furthermore, coating the core sample with an active shell doped by 10% Yb³⁺ enhances the thermal sensitivity by 30%, holding a high and stable sensitivity more than 50 × 10⁻⁴ K⁻¹ in a broad temperature range of 423-573 K at 980 nm excitation. In addition, at the much safer excitation wavelength of 1550 nm, this sample achieved the maximum sensitivity of 45 × 10⁻⁴ K⁻¹ at 503 K. Our work contributes a feasible and versatile way to promote the luminescence and thermal sensing properties of Ln³⁺-based materials, combining with the nontoxic oxysulfide host, indicating their potential applications as safe fluorescent and temperature nano-probes in bio-field.     

Bioactivity and up-conversion luminescence characteristics of Yb3+/Tb3+ co-doped bioglass system

The present study elaborates the bioactive and optical features of Yb³⁺/Tb³⁺ co-doped bio-active glass. Three different combinations with assorted elemental concentrations were investigated. The amorphous nature of bioglass devoid of crystalline phases until 700 °C has been confirmed through XRD analysis. The results from Raman and FT-IR accomplished the presence of Si–O–Si broad vibrational modes typical of amorphous glass and further the characteristic PO₄ vibrations were also confirmed. The up-conversion luminescence studies revealed the typical Tb³⁺ emission excited at 976 nm. The mechanism that involved in the up-conversion and energy transfer emission phenomena were discussed. Morphological features of SBF immersed specimen affirm the formation of apatite layer by the spherical spongy agglomerates observed over the surface.     

Enhanced up-conversion luminescence and optical thermometry characteristics of Er3+/Yb3+ co-doped Sr10(PO4)6O transparent glass-ceramics

In this paper, the Yb³⁺/Er³⁺ co-doped parent glass (PG) with composition (in mol%) of 30P₂O₅-10B₂O₃-38SrO-22K₂O and transparent glass-ceramics (GCs) containing hexagonal Sr₁₀(PO₄)₆O nanocrystals (NCs) were synthesized for the first time by melt-quenching method and subsequent heating treatment in air. Under 980 nm laser prompting, the GCs samples showed intense red and green up-conversion emissions compared to those characteristics for the PG sample. The emission intensities varied with Er³⁺ concentration and heat treatment conditions. Furthermore, in Yb³⁺/Er³⁺ co-doped GCs specimens, the optical thermometry was researched by means of fluorescence intensity ratio (FIR) of ⁴S_3/2 and ²H_11/2 levels. The GC sample heated at 620°C for 5 hours possessed a high relative temperature sensitivity (S_r) of 0.769% K⁻¹ at 303 K and the maximal absolute temperature sensitivity (S_a) of 5.951 × 10⁻³ K⁻¹ at 663 K, respectively. It is expected that the as-fabricated GC materials with Sr₁₀(PO₄)₆O NCs are promising efficient up-conversion materials for optical temperature sensor.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Effects of Er3+ concentration on down-/up-conversion luminescence and temperature sensing properties in NaGdTiO4: Er3+/Yb3+ phosphors

Usually, Er³⁺ doping concentration effect on the temperature sensing properties of Er³⁺ containing materials is ignored. In this work, we demonstrated the influence of Er³⁺ concentration and excitation path on the spectral and temperature sensing properties in Er³⁺, Yb³⁺ co-doped NaGdTiO₄ system. The NaGdTiO₄: Er³⁺/Yb³⁺ phosphors were prepared by a high temperature solid state reaction method. Different spectral patterns for down- and up-conversion processes were observed and ascribed to the different excitation and population routes. The concentration quenching behaviors for down- and up-conversion processes were explained via cross relaxations between Er³⁺ ions. Most importantly, the Er³⁺ concentration dependent optical temperature sensing performance was observed and experimentally explained as a fact that the optical transition rate of Er³⁺ in different samples was changed with various Er³⁺ doping concentration.     

Near-infrared luminescence enhancement in Yb3+/Ho3+ co-doped bismuth-tellurite glass by tailoring glass network

In this work, the glass network has been tailored by introducing the modifier WO₃ when a Ho³⁺/Yb³⁺ co-doped bismuth-tellurite glass composition was designed. The physical, absorption, emission, structure, and gain characteristics of the glasses with the different contents of WO₃ were investigated comprehensively based on various tests and analytical methods such as absorption spectra, fluorescence spectra, Raman spectra, and J-O theory. The results show that both the optical band gap and the bridge oxygen content are enhanced remarkably while the substitution of TeO₂ by WO₃, accelerating the transition from [TeO₃] to [TeO₄] units. Simultaneously, the value of Ω₆ reached the maximum of 2.26×10^-20 cm² when the WO₃ is equal to 10 mol%, indicating that the Stark splitting of Ho³⁺: ⁵I₇ energy level is the weakest. The optimal FWHM and the gain coefficient are 143.3 nm and 2.22 cm^-1, respectively. Furthermore, a blue shift of the central wavelength in absorption and emission peaks can be observed at the 2 μm band. As mentioned above, the bismuth-tellurite glass prepared is an ideal gain material that can realize a wider spectra span.     

Tunable upconversion luminescence in new Ho3+/Yb3+-doped SrBi4Ti4O15 photochromic ceramics for switching application

Upconversion (UC) luminescence modulation is quite important in controlling and processing light for active components of light sources, photoswitches, optical memories, and optical sensing devices. In this work, we reported one kind of novel phosphor, Ho³⁺/Yb³⁺-doped SrBi₄Ti₄O₁₅ ceramics, which displayed both strong UC luminescence and obvious photochromic (PC) reaction. The UC luminescence, PC effect, and the modulation of UC performance based on PC behavior were investigated in detail. By alternating visible light irradiation and thermal stimulus, the UC luminescence could be reversibly regulated. Meanwhile, the modulation was unveiled to tightly rely on the irradiation time and thermal treatment processes. Excellent reproducibility was also achieved. In addition, as an alternative method to thermal treatment, the manipulation of luminescence by electric field was also explored. Finally, the mechanism related to the UC luminescence manipulation was illustrated. The results indicate that these samples could be potentially utilized in optical data storage and anti-counterfeiting security fields.     

Synthesis of NaYF4:Yb3+, Er3+ upconversion nanoparticles in normal microemulsions

An interface-controlled reaction in normal microemulsions (water/ethanol/sodium oleate/oleic acid/n-hexane) was designed to prepare NaYF4:Yb3+, Er3+ upconversion nanoparticles. The phase diagram of the system was first studied to obtain the appropriate oil-in-water microemulsions. Transmission electron microscopy and X-ray powder diffractometer measurements revealed that the as-prepared nanoparticles were spherical, monodisperse with a uniform size of 20 nm, and of cubic phase with good crystallinity. Furthermore, these nanoparticles have good dispersibility in nonpolar organic solvents and exhibit visible upconversion luminescence of orange color under continuous excitation at 980 nm. Then, a thermal treatment for the products was found to enhance the luminescence intensity. In addition, because of its inherent merit in high yielding and being economical, this synthetic method could be utilized for preparation of the UCNPs on a large scale.     

Stannum-(II) dopant effects on morphology evolution and upconversion performance of Yb3+/Er3+:NaGdF4 crystals

Hexagonal Yb³⁺/Er³⁺:NaGdF₄ nanocrystals codoped with Sn²⁺ ions were prepared via a modulated solvothermal method. Upon introducing 25mol% Sn²⁺ ions into the host lattice by substituting Gd³⁺ ions, a portion of Yb³⁺/Er³⁺:NaGdF₄ nanocrystals was converted into nanorods. Meanwhile, the upconversion (UC) luminescence intensity of 542 nm and 652 nm were intensified by 24 and 33 times respectively, when compared with samples without Sn²⁺ ions doping. The effect of Sn²⁺ ions doping content on the morphology and UC emission performances of Yb³⁺/Er³⁺:NaGdF₄ nanocrystals were discussed in detail. The enhancement of UC luminescence intensity could be attributed to the growth of UC nanocrystals and the low crystal local symmetry around Yb³⁺/Er³⁺ ion pairs. This study may be beneficial for fabricating high-performance UC materials and realizing their practical applications.     

Temperature self-monitoring photothermal nano-particles of Er3+/Yb3+ Co-doped zircon-tetragonal BiVO4

Self-monitored photo-thermal therapy (PTT) still faces huge challenge in cancer treatment, which aims to realize the real-time temperature reading during the course of optical heating. Exploiting new-type photo-thermal therapeutic agent (PTA) with thermometric function is considered to be one of effective methods to fulfill self-monitored PTT. In this work, spindle-like zircon-tetragonal (z-t) phase BiVO₄:Yb³⁺/Er³⁺ up-conversion (UC) nano-particles as self-monitored PTAs were prepared through the combination of co-precipitation and hydrothermal method. Under 980 nm laser diode excitation, real-time thermometry was accomplished by monitoring thermo-responsive emission intensity ratio of Er³⁺ (²H_11/2/⁴S_3/2 → ⁴I_15/2) transitions. Meanwhile, the photo-thermal conversion effect associated with UC process was trigged via the non-radiative transition channels. Considering the balance between UC emission intensity and heat generation, the optimal sample composition was determined as BiVO₄:20%Yb³⁺/3%Er³⁺. Their maximum absolute sensitivity (S_a) reached 0.0125 K^-1 at 460 K as the thermometer and the ability of photo-thermal conversion up to 3.32 K cm²/W as PTAs. Their potential applications in controlled subcutaneous photo-thermal treatment were estimated through ex vivo experiments. Results provided a new choice for nano-materials to realize real-time temperature feedback in the single host material (z-t BiVO₄) during the course of PTT.     

Optical temperature sensor based on upconversion luminescence of Er3+ doped GdTaO4 phosphors

For the development of optical temperature sensor, a series of GdTaO₄ phosphors with various Er³⁺-doping concentrations (0, 1, 5, 10, 25, 35, 50 mol%) were synthesized by a solid-state reaction method. The monoclinic crystalline structure of the prepared samples was determined by X-ray diffraction (XRD). Under excitations of 980 and 1550 nm lasers, the multi-photon-excited green and red upconversion (UC) luminescence emissions of Er³⁺ were studied, and the critical quenching concentration of Er³⁺-doped GdTaO₄ phosphor was derived to be 25 mol%. By changing the pump power of laser, it was found that the two-photon and three-photon population processes happened for the UC emissions of Er³⁺-doped GdTaO₄ phosphors excited by 980 and 1550 nm lasers, respectively. Furthermore, based on the change of thermo-responsive green UC luminescence intensity corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ with temperature, the optical temperature sensing properties of Er³⁺-doped GdTaO₄ phosphor were investigated under excitations of 980 and 1550 nm lasers by using the fluorescence intensity ratio (FIR) technique. It was obtained that the maximum absolute sensitivity (S_A) and relative sensitivity (S_R) of Er³⁺-doped GdTaO₄ phosphors are as high as 0.0041 K⁻¹ at 475 K and 0.0112 K⁻¹ at 293 K, respectively. These significant results suggest that the Er³⁺-doped GdTaO₄ phosphors are a promising candidate for optical temperature sensor.