Intense Red Upconversion Emission Research Articles

Multi-shell structured nanomaterials with strong red upconversion emission for trimodal biomedical imaging

Upconversion nanomaterials show promising applications in multimodal imaging due to their advantages such as sharp emission bands and non-autofluorescence in tissue. Obtaining intense red upconversion emission is crucial for fluorescence imaging. Herein, we designed a multi-shell structured NaErF4:0.5%Tm3+@NaYF4:20%Yb3+ @NaLuF4 upconversion nanomaterial exhibiting strong red emission and high red-to-green ratio under 980 nm irradiation. The main mechanism of the strong emission is attributed to the inert NaLuF4 shell which serves to block the energy transfer from Yb3+ to surface defects. In addition, a NaGdF4 shell is further cladded considering the property of Gd3+ of facilitating longitudinal relaxation rate of water. The properties of X-ray attenuation, longitudinal relaxation rate, and the upconversion emission of the measured sample suggest that our designed nanomaterial possesses potential for X-ray computed tomography (CT), magnetic resonance imaging (MRI), and fluorescence trimodal imaging.

Enhancing red upconversion emission in NaErF4@NaYF4 core-shell nanoparticles by introducing Yb3+ ions as energy trapping centers

Rare-earth-doped red upconversion nanocrystals (UCNPs) have shown excellent prospects for application in biological imaging, labeling and other fields. In this work, NaErF4@NaYF4 core-shell (CS) UCNPs were successfully constructed to enhance red emission by introducing Yb3+ ions as energy trapping centers under different near-infrared excitation wavelengths. The experimental results showed that the red upconversion emission intensity of NaErF4@ NaYF4 CS UCNPs doped with 15%Yb3+ ions in the core is nearly 202 times higher than that of NaErF4 UCNPs under 980 nm excitation wavelength, with a red/green emission intensity ratio of 18.7. When Yb3+ ions were introduced into the core and shell of NaErF4@NaYF4 CS UCNPs, NaErF4:15%Yb3+@NaYF4:10%Yb3+ CS UCNPs showed the best red emission due to the energy network characteristics of the Yb3+ ions. Furthermore, the enhancement mechanism of red emission in NaErF4@NaYF4 CS UCNPs with different Yb3+ ion doping concentrations were discussed, based on the spectral characteristics and lifetime. It was found that doping of Yb3+ ions in NaErF4@ NaYF4 CS UCNPs can significantly improve the red emission intensity of Er3+ ions through controlling energy migration (EM) and energy back transfer (EBT) processes, which has considerable potential for applications in bioimaging and anti-counterfeiting.

Anomalous upconversion behavior and high-temperature spectral properties of Yb/Ho-SiAlON ceramics

Traditionally, NaYF4 and glass-based materials are considered the most efficient upconversion materials. However, those materials may show signs of thermal warping or risks of fracture during long-term service life in extreme environments. In contrast, SiAlON ceramics preserve their spectral and physical integrity during continuous use due to their ultra-low thermal expansion coefficient (∼3.0 × 10−6/K) even at extreme temperatures. Here, we investigate the upconversion photoluminescence properties of Ho and Yb co-doped SiAlON (Ho/Yb-SiAlON) ceramics, prepared by hot-press method, at room temperature and at high temperature (298–1023 K). At room temperature, Ho/Yb-SiAlON ceramics show intense red upconversion emission under 980 nm excitation. At the high-temperature range, the spectral shift is absent in Ho/Yb-SiAlON indicating that the SiAlON ceramics have high-temperature spectral stability, which may be attributed to the low thermal expansion coefficient of SiAlON ceramics. Ho and Yb concentration-dependent spectra show an anomalous upconversion emission behavior. It is found that a higher sensitizer and low activator concentration combination is not always an ideal choice for sensitized luminescence in SiAlON ceramics which is in stark contrast to the common understanding of sensitized luminescence. The mechanism involving the dominant cross-relaxation process has been proposed to explain the observed anomalous upconversion behavior.

Enhanced red up-conversion of β-NaYF4:Er3+,Tm3+ microcrystals for bio-imaging applications

Pure red fluorescence generated by near infrared (NIR) excitation is desired for bio-imaging with high biological penetration, but without cell damage or multi-photon photo-toxicity. In this work, β-NaYF4:Er3+ and β-NaYF4:Er3+,Tm3+ microcrystals were synthesized using a novel hydrothermal method. β-NaYF4:Er3+ microcrystals exhibited intense green and red up-conversion emissions under 976 nm laser diode excitation. The introduction of Tm3+ ions into β-NaYF4:Er3+ resulted in strong inhibition of green up-conversion emission (Er3+:2H11/2 /4S3/2→4I15/2) and, consequently, red up-conversion emission (Er3+:4F9/2→4I15/2) was significantly promoted. To simultaneously restrain green up-conversion emissions and enhance red up-conversion emissions, the optimal concentrations of Er3+ and Tm3+ ions were determined to be 20 mol% and 2.0 mol%, respectively. The red/green up-conversion emission intensity ratio increased from ~1.80 (β-NaYF4:20Er3+, mol%) to ~78 (β-NaYF4:20Er3+,2Tm3+, mol%) after the addition of 2 mol% Tm3+ ions to β-NaYF4:20Er3+ (mol%). The red up-conversion emission intensity of β-NaYF4:20Er3+,2Tm3+ was stronger than that for commercial red up-conversion phosphors (NaYF4 and ZnF2) under the same experimental conditions. Based on the results of the emission spectra, luminescence rise, and decay profiles, the mechanism of enhanced red up-conversion emission by Tm3+ ion co-doping in β-NaYF4:20Er3+,2Tm3+ (mol%) was proposed. The bio-fluorescence experiment under NIR excitation proves that β-NaYF4:Er3+,Tm3+ microcrystals have potential applications in bio-imaging.

Intense green upconversion emission of rare-earth-doped Sr3(PO4)2/Sr2P2O7 powder: Effect of annealing temperature and temperature-sensor properties

A series of Mo6+-doped Sr3(PO4)2/Sr2P2O7: Er3+-Yb3+ was synthesized by coprecipitation followed by heat treatment. X-ray diffraction and scanning electron microscopy analyses confirmed that the annealing temperature strongly influenced the morphology and structure of the material. Under laser-diode excitation wavelength of 975 nm, the green and red upconversion emission intensity as a function of annealing temperature reached its maximum value at 1000℃. The green emission intensity of the 3 mol% Mo6+-doped sample sharply increased, becoming about ~143 times higher than that of the undoped samples. Furthermore, the obtained phosphor presented a high temperature sensitivity of Sa and Sr with a maximum value of 1.05% K−1 at 363K and 0.8% K−1 at 303K, respectively. These results indicated that the Sr3(PO4)2/Sr2P2O7: Er−Yb−Mo phosphor was an efficient upconversion luminescent material suitable for thermal-sensor applications. The selection and control of annealing temperature played a key role in the success of the synthesis and acquisition of the phosphor’s desired physical property. Finally, several explorations in this work that enhanced UC luminescence by using the transition-metal ion Mo6+ provided a novel idea to design strong UC emission biomaterials for their medical applications.

Tailoring the upconversion emission and magnetic properties of NaGdF4:Yb, Er by Mg2+ or Fe3+ doping and optical trapping of individual magnetic nanoparticle at NIR 980 nm

This paper presents the upconversion emission and magnetic properties of NaGdF4:Yb, Er nanoparticles separately doped with Fe and Mg in the 10–40% concentration. The XRD analysis validates the structural properties of NaGdF4:Yb, Er nanoparticles with different concentrations of the dopants. The XPS spectra of Fe-doped NaGdF4:Yb, Er and Mg-doped NaGdF4:Yb, Er nanoparticles reveal the presence of the dopants and their valency. The 30%Mg doping resulted in an intense red upconversion emission at 650 nm compared to the Fe-doped NaGdF4:Yb, Er due to the high lifetime of 254 μs for the 4F9/2 → 4I15/2 (660 nm) energy level transitions of Er3+ ion. The Mg-doped nanoparticles showed a 2.7 red-to-green emission intensity ratio, 5.7 for Fe doping at the same 30% concentration and above this doping level, upconversion emission quenching resulted. The room temperature VSM study demonstrated paramagnetic property in the Mg-doped NaGdF4:Yb, Er nanoparticles characterized by negligible coercivity and retentivity with low saturation magnetization. Similar behaviour is also observed for the Fe-doped nanoparticles up to 20% concentration, and it became weak ferromagnetic at a high 30 and 40% dopant. In addition, the present work demonstrated the optical trapping of individual magnetic nanoparticles of 30%Fe-doped NaGdF4:Yb, Er by using a single beam optical tweezer technique under the tightly focused 980 nm diode laser interaction and the estimated trap stiffnesses is 1.85 pN μm−1 W−1. The multifunctional upconversion red emission, magnetic and optical trapping properties obtained in NaGdF4:Yb,Er,30%Fe nanoparticles can be used as a single particle probe in the theragnostic application.

Enhancement of Red Upconversion Emission of ZnS: Er3+/Yb3+ Powders Synthesized via Solid Phase Reaction Method by Introducing Metal Fluoride

AbstractZnS: Er3+/Yb3+ powders were synthesized by a common solid phase reaction method with NaF, CaF2, LiF, AlF3 and MgF2 as dopant. The enhancement mechanisms of different metal fluorides (NaF, CaF2, LiF, AlF3 and MgF2) and concentrations (0 wt% ∼9 wt %) on structure, morphology and optical properties of ZnS: Er3+/Yb3+ powders were investigated systematically. The results revealed that as synthesized ZnS: Er3+/Yb3+ powders are main exhibited as wurtzite phase, the lattice constant, unit cell volume and lattice strain increase with metal fluoride codoping, which indicates the increase of resultant distortion. As synthesized ZnS: Er3+/Yb3+ powders have an intense red upconversion emission centered at 663 nm and a weak green upconversion emission centered at 567 nm under the excitation of a 980 nm laser diode. Compared with ZnS: Er3+/Yb3+ powders synthesized without metal fluoride, the upconversion emission intensities of ZnS: Er3+/Yb3+ powders synthesized with LiF, NaF, MgF2, AlF3 and CaF enhance ∼12 times, ∼9 times, ∼5 times, ∼3 times and ∼3 times, respectively. The strong red upconversion properties make the ZnS: Er3+/Yb3+ powders synthesized with metal fluoride a potential application in the field of light emitting diodes.

Enhancement of red upconversion emission intensity of Ho3+ ions in NaLuF4:Yb3+/Ho3+/Ce3+@NaLuF4 core–shell nanoparticles

The red upconversion emission of Ho3+ ions, in the optical window of biological tissue, exhibits excellent prospects in biological applications. This study aims to enhance the red upconversion emission intensity of Ho3+ ions in NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+ nanoparticles through building different core–shell structures with different excitation wavelengths. A significantly enhanced red upconversion emission with a higher red-to-green ratio was successfully obtained in NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+@NaLuF4 core–shell nanoparticles by introducing the Yb3+ and Yb3+/Nd3+ ions into the NaLuF4 shell, with enhancement of the red emission occurring when Yb3+ and Nd3+ ions in the shell transfer more excitation energy to the Ho3+ ions. Investigation of the red emission enhancement mechanism is based on spectral characteristics and lifetimes. We examined the synergistic effect of dual-wavelength co-excitation NaLuF4:20%Yb3+/2%Ho3+/12%Ce3+ @NaLuF4:10%Yb3+/15%Nd3+ core–shell nanoparticles to establish optimal excitation conditions. It is hoped that this method, using red upconversion emission core–shell nanoparticles with multi-mode excitation, can provide new ways to expand the applications of rare-earth luminescent materials in biomedicine and anti-counterfeiting.

Enhancing red upconversion emission of Ho3+ in a single NaYbF4:Ho3+ microdisk through building different core-shell structures

The construction of different core-shell structures has been widely used to enhance and regulate the luminescent properties of rare-earth-doped micro/nano-materials. This study aimed to enhance the red upconversion emission intensity of Ho3+ in NaYbF4 microdisks through coating with NaYF4 inert and NaYbF4 active shells. The red upconversion luminescence characteristics of a single NaYbF4:2%Ho3+ microdisk with different core-shell structures under near-infrared 980 nm excitation were studied using a confocal microscope. The experimental results showed that the red emission intensity of the NaYbF4:2%Ho3+@NaYbF4 core-shell microdisk was enhanced about 2.4 times compared with the single NaYbF4:2%Ho3+ core microdisk. Meanwhile, the NaYbF4:2%Ho3+@NaYbF4@NaYF4 core-shell-shell microdisk was constructed to further enhance the red emission intensity, which produced much higher red emission intensity and R/G ratio compared with the single NaYbF4:2%Ho3+ core microdisk (about 3.5 times and 1.2 times, respectively). The enhancement mechanisms of red emission were studied based on spectral characteristics and lifetime. The results indicated that the luminescent properties of microcrystals can also further improved through building different core-shell structures, and it is very helpful to expand the application of display and optoelectronic devices.

Enhanced red emission of Yb, Ho: NaYGd(WO4)2 phosphors by codoping Ce3+

In this work, Yb, Ho, Gd3+/Gd3+, Ce3+: NaY(WO4)2 phosphors doped with various concentrations of Gd3+ and Ce3+ ions were prepared via high-temperature solid-state method. Under the excitation of 980 nm near-infrared light, the doping of Gd3+ ions enhances the red and green emission of Yb, Ho: NaY(WO4)2, and on this basis, the Y3+ ion is replaced by Ce3+ ion doping, the red upconversion emission intensity was enhanced with Ce3+ concentration increasing. It was found that two efficient cross-relaxation processes between Ho3+ and Ce3+ ions had been employed to enhance red emission and suppress green emission. The possible upconversion mechanisms between Ho3+ and Ce3+ ions were investigated in detail. Yb3+, Ho3+, Gd3+, Ce3+ co-doped NaY(WO4)2 phosphors have broad application prospects in the fields of anti-counterfeiting and display, magnetic resonance imaging and optical temperature sensors.

Enhanced red upconversion emission and energy transfer of Tm3+/Cr3+/Yb3+ tri-doped transparent fluorosilicate glass-ceramics

In this paper, the upconversion (UC) emission of Tm3+/Cr3+/Yb3+ tri-doped in SiO2–AlF3–BaF2–La2O3 transparent fluorosilicate glass-ceramics (SGC) containing Ba2LaF7 nanocrystals were investigated. The enhancement of red UC emission of Tm3+/Cr3+/Yb3+ tri-doped in SiO2–AlF3–BaF2–La2O3 transparent SGC were also investigated. Through the energy transfer processes between Cr3+ and Tm3+, and the combination of Cr3+-Tm3+ led to the red UC emission intensity of Tm3+/Cr3+/Yb3+ tri-doped transparent SGC bands centered at ~806 and ~858 nm has been significantly enhanced. The CIE chromaticity coordinates for UC emission of Tm3+/Cr3+/Yb3+ tri-doped in SGC-0.2Tm1Cr2Yb transparent SGC sample is calculated (x = 0.704; y = 0.296) and the UC emission of Tm3+/Cr3+/Yb3+ tri-doped in SiO2–AlF3–BaF2–La2O3 transparent SGC has been identified in red color region of CIE chromaticity coordinates. In addition, the ET processes between Cr3+ and Tm3+ are also proposed and discussed.

Building vertical gold nanorod arrays to enhance upconversion luminescence of <i>β</i>-NaYF<sub>4</sub>: Yb<sup>3+</sup>/Er<sup>3+</sup> nanocrystals

The plasmon resonance effect is one of the effective ways to enhance the upconversion (UC) luminescence, which is realized by enhancing the electromagnetic field from incident light interacting with free electrons of AuNRs surface. In this work, a series of GVA@SiO<sub>2</sub>@NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> composite structures with different thickness values of SiO<sub>2</sub> isolation layer is successfully built from self-assembled gold nanorods, steamed SiO<sub>2</sub>, and spin-coating rare-earth nanocrystals. The results of the SEM indicate that the size of gold-nanorods is approximately 22 nm in diameter and 65 nm in length. The X-ray diffraction and transmission electron microscope results demonstrate that the NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> nanocrystals possess hexagonal-phase structure with a size of about 20 nm. Under 980 nm near-infrared (NIR) excitation, the UC emission characteristics of GVA@SiO<sub>2</sub>@NaYF<sub>4</sub>:Yb<sup>3+</sup>/Er<sup>3+</sup> composite structure are studied by using a confocal microscope spectroscopic test system, and regulated by changing the thickness of SiO<sub>2</sub> isolation layer. The results indicate that the UC emission intensity of NaYF<sub>4</sub>:20%Yb<sup>3+</sup>/2%Er<sup>3+</sup> nanocrystals is enhanced by about 8.8 times, and the enhancement factor of red UC emission intensity is about 16.2. In order to further prove the enhancement effect of the red UC emission, the GVA@SiO<sub>2</sub>@NaYF<sub>4</sub>:40%Yb<sup>3+</sup>/20%Er<sup>3+ </sup>composite structure with red UC emission is constructed in the same way. It can be found that the UC emission intensity of NaYF<sub>4</sub>:40%Yb<sup>3+</sup>/20%Er<sup>3+</sup> nanocrystals is enhanced by 8.7 times and the red UC emission intensity is raised by about 9.7 times under the 980 nm NIR excitation. The corresponding excitation enhancement mechanism is simulated according to the power excitation dependence. And it is found that the rate of UC emission decreases and the R/G ratio also decreases with the excitation pump power increasing. The analysis of the above results shows that the excitation enhancement plays a leading role and is accompanied by emission enhancement. Meanwhile, the study of Er<sup>3+</sup> ion dynamic process indicates that the Er<sup>3+</sup> ion transition rate is accelerated due to the coupling from UC emission peaks and gold nanorod absorption peaks in GVA@SiO<sub>2</sub>@NaYF<sub>4</sub>:40%Yb<sup>3+</sup>/20%Er<sup>3+ </sup>composite structure. The enhancement mechanism of UC emission is also simulated, which further proves that the excitation enhancement is dominant. This kind of composite structure can not only help us to further understand the physics mechanism of the plasmon-enhanced UC luminescence but also promote the applications of rare-earth materials in medical imaging and fingerprint recognition.

EDTA functionalization of SrF2:Yb,Er nanoparticles by hydrothermal synthesis: Intense red upconversion, NIR-to-NIR emission and luminescence nanothermometry characteristics

SrF2 based upconversion nanoparticles (UCNPs) were synthesized by hydrothermal method using ethylenediaminetetraacetic acid (EDTA) as surfactant, by doping with single-lanthanide-activator Er ion and sensitizer-activator Yb3+−Er3+ ion-pair. The excited state absorption (ESA) and energy transfer upconversion (ETU) properties were investigated respectively by synthesizing SrF2:2%Er and SrF2:20%Yb,2%Er nanoparticles. EDTA aided hydrothermal synthesis resulted in nano-rod, nano-plate and nano-sphere morphological particles. The hydrothermally grown SrF2:Yb,Er UCNPs are formed as nano-spheres in size ~ 30 to 60 nm. It gives a high specific surface area of 47.17 to 23.58 m2 g−1. The nano-spherical SrF2:Yb,Er resulted an intense red upconversion emission at 660 nm, under the near-infrared (NIR) 980 nm excitation, due to its high specific surface area and ETU mechanism. The post-calcined SrF2:Yb3+,Er3+ gives better red-to-green emission intensity ratio compared to the as-prepared counterparts. SrF2:Er3+ gives very weak red emission due to ESA mechanism. The optical energy gap of SrF2:Yb,Er UCNP was calculated using the barycentre of green emission at 524 and 542 nm from the thermally coupled energy levels of 2H11/2 and 4S3/2. It gives an optical energy gap of 766 cm−1 and the estimated thermal sensitivity is 1.22% K−1 at 300 K. The result is in agreement with other published reports. We also demonstrated the NIR-to-NIR (980-to-850 nm) upconversion emission in the SrF2:Yb3+, Er3+ nanoparticle.

Thermally stable red emitting xenotime phosphate nanophosphors for displays

The synthesis, structural and optical properties of the Ho3+/Tm3+/Yb3+ codoped xenotime YPO4 nanophosphors have been reported. The developed phosphors have been structurally characterized through XRD, FESEM, HRTEM, EDAX and FTIR analysis. The diffuse reflectance and frequency upconversion (UC) emission study have been performed. In the developed nanophosphors, the red UC emission intensity remains always five times stronger than the green emission of Ho3+ ion irrespective of the concentration of Ho3+ ion. The pump power dependence UC luminescence intensity and the photon numbers involved have been also studied. The energy transfer efficiency has been calculated from the decay curve analysis. The developed nanophosphors emit intense red colour having (0.60, 0.32) CIE co-ordinate with 78.4% colour purity. The nanophosphors exhibit good thermal stability having 0.22 eV activation energy.

Intense red upconversion luminescence in Er3+-sensitized particles through confining the 1532 nm excitation energy

Lanthanide-doped upconversion (UC) materials are commonly excited by 700–1000 nm laser because of Yb3+ (980 nm excitation) and Nd3+ (808 nm excitation) are typically used as sensitizers. However, the excitation band located in the second near-infrared window (NIR-II, 1000–1700 nm) demonstrates the deeper penetration in biological tissue with minimum tissue scattering and autofluorescence. Herein, intense red UC emission in Er3+-sensitized NaLuF4 particles under 1532 nm NIR-II excitation is presented. Red UC luminescence is significantly enhanced and purified through confining the 1532 nm excitation energy by Ho3+ and Tm3+ mediators, via an energy back transfer pathway (Er3+ → Ho3+ → Er3+ and Er3+ → Tm3+ → Er3+). Furthermore, when the particles injected into the fresh beef, red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation, implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission. Our results suggest that bright red UC luminescence from Er3+-sensitized NaLuF4 crystals with 1532 nm NIR-II excitation may be suitable for deep optical imaging.

Upconversion luminescence, temperature sensing and internal heating behaviors of Er3+/Yb3+/Fe3+-tridoped NaBiF4 nanoparticles

Series of Er3+/Yb3+/Fe3+-tridoped NaBiF4 nanoparticles were synthesized through a simple precipitation reaction method at room temperature. The effect of the Fe3+ ions on the microstructure and upconversion (UC) luminescence properties of the developed samples were studied in detail. Through the Fe3+ ions doping engineering, the enhancement in both green and red UC emission intensities were observed and the optimal doping concentration for Fe3+ ions was revealed to be 30 mol%. Owing to the fluorescence intensity ratio technology, the optical thermometric performances of the resultant products in the temperature range of 303–543 K were investigated and the maximum sensor sensitivity of the studied nanoparticles was 0.0053 K−1 at 543 K. Moreover, the internal heating behaviors of the obtained nanoparticles were investigated and the product temperature was improved to 399 K when the pump power was set at 260 mW. Our systematic and complementary studies suggested that the Er3+/Yb3+/Fe3+-tridoped NaBiF4 nanoparticles with splendid UC behaviors were poised as next-generation promising candidates for nanothermometer and optical heaters.

Sol-gel synthesis and enhanced 1.54 μm emission in Y2O3:Yb3+, Er3+ nanophosphors co-doped with Ce3+ ions

Nano-sized Y2O3:Yb3+, Er3+ phosphors were synthesized by sol-gel method and characterized by X-ray diffraction, scanning electron microscope, near-infrared and upconversion fluorescence spectroscopy. The phosphors calcined at 1100 °C for 2 h were confirmed to be cubic Y2O3 phase with well-crystallized particles. With the introduction of Ce3+ ions at proper doping concentration, the 1.534 μm emission of Er3+ was found to be greatly enhanced, while the green and red upconversion emission intensity obviously decreased. The enhancement of 1.534 μm emission could be attributed to the phonon-assisted energy transfer process between Ce3+ and Er3+ ions.

Enhanced red upconversion emission of Ho3+ in NaYF4 nanocrystals

Hexagonal-phase NaYF4:Yb3+/Ho3+ nanocrystals (with size of 20 nm) were successfully prepared by a modified solution-based method. Under 980 nm near- infrared (NIR) excitation, the upconversion (UC) emission was tuned from green to red in NaYF4:Yb3+/Ho3+ nanocrystals by introducing Ce3+ ions. The corresponding red to green (R/G) emission ratio of Ho3+ increased by approximately 10-fold as the Ce3+ concentration increased to 14%. However, the overall UC emission intensity gradually decreased. A series of core–shell structures for NaYF4:Yb3+/Ho3+/Ce3+@NaYF4:Yb3+ nanocrystals codoped with different Yb3+ ion concentrations in the shell was successfully fabricated by a sequential growth process to further enhance the red UC emission intensity. When the Yb3+ concentration from the shell of NaYF4:Yb3+ was increased from 0% to 20%, the UC emission intensity of the core–shell nanocrystals first increased and then decreased. Moreover, the highest UC emission intensity was obtained in the core–shell nanocrystals with 10% Yb3+. Experimental results indicated red UC was enhanced due to the resonance energy transfer between adjacent Ho3+ and Ce3+ and Yb3+ and Ho3+ ions. These nanocrystals with red UC emission have great potential in the biological field and multicolor applications.

Improved piezoelectricity and luminescence of Er3+/Yb3+ co-doped (K, Na)NbO3 ceramics by microwave sintering

ABSTRACTYb2O3 doped (K,Na)NbO3-0.5% Er2O3 (KNNEr) ceramics with the composition around the morphotropic phase boundary was synthesized by microwave sintering. KNN co-doped Yb2O3/Er2O3 ceramics exhibited enhanced ferroelectric, piezoelectric properties owing to the donor doping effect. The optimal performance of d33 = 115 pC/N, kp = 0.37, ϵr = 620 and tanδ = 4.9% were obtained for the ceramics with the addition of 0.25 mol% Yb2O3. And the green and red up-conversion emission intensities of these ceramics excited under 980 nm increased with the increasing content of Yb2O3. The results suggest that the KNNEr-Yb2O3 ceramics have potential application as a multifunctional device by integrating its excellent electrical and luminescence properties.

Highly efficient upconversion luminescence in hexagonal NaYF4:Yb3+, Er3+ nanocrystals synthesized by a novel reverse microemulsion method

A new reverse microemulsion system is proposed for the first time to synthesize NaYF4:Yb,Er nanocrystals, which demonstrated high upconversion emission in 550 and 662 nm at 980 nm diode laser excitation. The reverse microemulsion (μEs) system is comprised of CTAB and oleic acid as surfactant and 1-butanol co-surfactant and isooctane oil phase. The surfactant to water ratio is able to tune the microemulsion droplet size from 14 to 220 nm, which eventually controls the crystallinity and particulate morphology of NaYF4:Yb,Er. Also, the microemulsion precursor and calcination temperature plays certain role in transforming the cubic NaYF4:Yb,Er to highly luminescent hexagonal crystal structured upconversion material. Single phase hexagonal NaYF4:YbEr nanorod prepared by water-in-oil reverse microemulsion (μEs) gives intense red upconversion emission. Both nanosphere and nanorod shaped NaYF4:Yb,Er was obtained, but nanorod morphology resulted an enhanced upconversion luminescence. The structural, morphological, thermal and optical luminescence properties of the NaYF4:Yb,Er nanoparticles are discussed in detail by employing powder X-ray diffraction, dynamic light scattering, high resolution electron microscopy, TGA-DTA, UV-DRS, FTIR and photoluminescence spectroscopy. Intense upconversion emission achieved in the microemulsion synthesized NaYF4:Yb3+,Er3+ nanocrystal can make it as useful optical phosphor for solar cell applications.