Hexagonal NaYbF4 Research Articles

Manipulating concentration quenching and thermal stability of Eu3+-activated NaYbF4 nanoparticles via phase transition strategy toward diversified applications

Red-emitting phosphors play a vital role in reinforcing the quality of phosphor-converted white light-emitting diodes. Unfortunately, concentration quenching and unsatisfied thermal stability are still the inevitable shortcomings of the red-emitting phosphors. Herein, we utilized phase transition to simultaneously tailor the concentration quenching and thermal stability of Eu3+-activated NaYbF4 (NaYbF4:xEu3+) nanoparticles, in order to overcome these limitations. With the increasing doping level, the resultant samples undergo a phase transition from cubic to hexagonal phase. Notably, the optimal doping content for Eu3+ in the NaYbF4 nanoparticles with cubic phase is 30 mol%. However, when the phase transition occurs, the concentration quenching takes place at a much higher doping concentration, i.e. the optimum content of Eu3+ in the structure is increased to about 80 mol%. Moreover, with elevating the Eu3+ content, the local coordination environment of Eu3+ in the NaYbF4 host lattice is changed from inversion (centrosymmetric) to non-inversion symmetry (non-centrosymmetric), which is further confirmed by the theoretical calculation results via Judd-Oflet theory. Furthermore, compared with cubic samples, the hexagonal NaYbF4:0.8Eu3+ nanoparticles show improved thermal stability. Importantly, utilizing hexagonal NaYbF4:0.8Eu3+ nanoparticles, a designed white light-emitting diode emits warm white light, with desired color coordinates (0.398,0.374), high color rendering index (86.3), and low correlated color temperature (3537 K). Besides, NaYbF4:xEu3+ nanoparticles and their flexible polydimethylsiloxane films exhibit potential applications in high-temperature optical anti-counterfeiting. This work may support a new perspective for tailoring the concentration quenching and thermal stability of various phosphors.

Synthesis of small-sized hexagonal NaREF4 (RE = Yb, Lu) nanocrystals through accelerating phase transformation

Heavy lanthanide element-based NaREF4 nanocrystals have recently been proposed as a promising replacement of classical hexagonal NaYF4 for their efficient luminescence and excellent X-ray attenuation property. However, synthetic control over their particle size remains challenging due to the fast growth and slow nucleation. Herein, a simple and robust approach for the controlled synthesis of small NaYbF4 and NaLuF4 nanocrystals is presented as well as a mechanistic investigation of the nanocrystal growth process. The injection order of NaOH and NH4F plays critical role in controlling the nanocrystal growth by accelerating phase transformation from cubic phase to hexagonal phase. We also verify experimentally that control of the hexagonal-phase nucleation rather than growth process is crucial for controlling the final particle size of NaYbF4 and NaLuF4. After coating an inert shell, we found enhanced downshifting emission of NaYbF4:Er nanocrystals compared with less Yb3+ doping. The results shown here provide a reliable method for preparing small-sized hexagonal NaYbF4 nanocrystals with enhanced downshifting emission, which favors their further applications in bioscience.

Yttrium-dopants-induced phase-controllable and luminescence-tunable lanthanide-doped α/β-NaYbF4 nanocrystals in glass for laser-driven upconverted lighting

In-situ glass crystallization is an effective way to integrate lanthanide-doped upconversion nanocrystals into amorphous glass, producing bulky nanocomposites for specific applications. However, the precipitation of hexagonal NaYbF4 nanocrystals from glass is rarely reported owing to the preferred crystallization of cubic phase. Herein, phase-controllable in-situ crystallization of cubic/hexagonal NaYbF4 nanocrystals in glass is achieved via Y3+ doping. A series of structural and spectroscopic characterizations evidence the formation of Na(Yb/Y)F4 solid-solution nanocrystals in glass and the incorporation of lanthanide emitting centers (Er3+, Tm3+, Eu3+) into the crystalline lattice. Introducing high-content Y3+ dopants is beneficial to induce the crystallization of hexagonal NaYbF4 phase in glass and enables elaborated tunability of Er3+ upconversion emissive colors. Finally, a prototype upconverted solid-state-lighting device is constructed by coupling 980 nm laser with the as-prepared Er/Tm doped monolithic glasses containing Na(Yb/Y)F4 nanocrystals, producing bright upconversion white-light with luminous efficiency of 1 lm/W and energy efficiency of 0.5%.

NaYbF4:Tb/Eu modified with organic antenna for improving performance of polymer solar cells

Inorganic-organic nanocomposites are synthesized and applied in polymer solar cells (PSCs) for improving power conversion efficiency (PEC). The hexagonal NaYbF4:Tb/Eu nanocrystals are synthesized, and then two organic molecules, phenanthroline (Phen) and thenoyltrifluroacetone (TTA), are introduced to modify the surface of the NaYbF4:Tb/Eu. As organic antennas, Phen and TTA can absorb UV and then transfer energy to inorganic nanocrystals. The prepared nanocomposites combine upconversion (UC) and downshift (DS) process under excitation, which can convert them into visible light. Thus, the inorganic-organic nanocomposites display photoresponse both in UV and NIR region. Two organic ligands and Tb3+ play an indispensable role during the energy transfer process. The enhanced UC and tunable DS emission match well with absorption of active layer of PSC, which can enhance the performance of PSC. The open-circuit voltage (Voc) and short-circuit current density (Jsc) of the devices are about 0.77 V and 16.48 mA cm−2, respectively. And the PEC of PSC can be increased 7.85%, which is about 10.2% higher than that of the reference device without UC and DS.

Nano-sized NaF inspired intrinsic solvothermal growth mechanism of rare-earth nanocrystals for facile control synthesis of high-quality and small-sized hexagonal NaYbF4:Er

Facile solvothermal control synthesis of hexagonal NaYbF4:Er was achieved by simultaneous regulation of nano-sized NaF and HF levels based on the findings of NaOH to nano-sized NaF conversion via reacting with NH4F and decomposition of NH4F to HF.

Tuning hexagonal NaYbF4 nanocrystals down to sub-10 nm for enhanced photon upconversion.

Enhancing upconversion emission is critical for small-sized lanthanide doped upconversion nanocrystals. A promising way is increasing the doping concentration of excitation energy absorbers, the Yb3+ sensitizer. However, it is still a challenge to obtain small-sized hexagonal NaLnF4 (Ln: lanthanide) upconversion nanocrystals with a high Yb3+ concentration due to the fast growth of NaYbF4 crystals, which hinders their applications particularly in biology. We here demonstrate a highly repeatable and controllable method for tuning the size of hexagonal NaYbF4 nanocrystals, down to ∼7 nm, without the assistance of additional impurity doping. By monitoring the reaction process, we found that ultrasmall hexagonal NaYbF4 nanocrystals were formed through an in situ transformation of their cubic counterparts. We observed an enhanced upconversion emission of NaYbF4:Tm nanocrystals when compared to that of NaYbF4:Y/Tm nanocrystals with less Yb3+ doping. After coating a thin layer of a NaYF4 shell on NaYbF4:Tm nanocrystals, a ∼100 times upconversion emission enhancement with over 800 times stronger emission in the ultraviolet and blue ranges was observed. This versatile method, together with the strong upconversion emission of the as-prepared ultrasmall nanocrystals, should facilitate the future applications of upconversion nanocrystals.

Water soluble cubic K2NaScF6nanocrystals: crystal structure evolution and luminescence properties

Studies on Sc3+-based nanomaterials have been reported in recent years. However, most of the as-prepared nanomaterials are hydrophobic, thus complicated surface encapsulation renders them hydrophilic for biological applications. In our work, we used water soluble polyvinylpyrrolidone (PVP) as a chelating agent and a classic hydrothermal method to prepare water soluble cubic K2NaScF6 nanocrystals (NCs) for the first time. Different ratios of NH4F/RE (3, 4, 5, 6) have been studied in the crystalline phase transformation from KSc2F7 to K2NaScF6 NCs. When the ratio is over 6, an unusual hexagonal NaYbF4 phase separation phenomenon can be observed. In addition, Yb3+/Er3+ ion pairs have been doped into the K2NaScF6 NCs to verify that they are better up-conversion luminescence matrix materials than KSc2F7 NCs.

Upconversion improvement by the reduction of Na⁺-vacancies in Mn²⁺ doped hexagonal NaYbF₄:Er³⁺ nanoparticles.

Hexagonal-phase NaYbF4:Er(3+) upconversion nanoparticles (UCNPs) have been synthesized via a co-precipitation method in high-boiling-point solvents, and remarkably enhanced upconversion luminescence, particularly in red emission bands (650-670 nm) in NaYbF4:Er(3+) UCNPs, has been achieved by Mn(2+) doping. The underlying reason for luminescence enhancement by Mn(2+) doping is explored by a series of controlled experiments, and a mechanism of enhancement based on the decrease of Na(+)-vacancies and organic adsorption is proposed. The Mn(2+) substitution disturbs the equilibrium of the charge and crystal lattice in the hexagonal-phase NaYbF4:Er(3+) UCNPs, which makes the Na(+)-vacancies that quenched luminescence become filled with Na(+) or Mn(2+) to offset the imbalance of the charge and electron cloud distortion. In addition, the Mn(2+) doping at the surface of UCNPs could reduce the organic adsorption on the surface of the UCNPs by an extra F(-) ion on the grain surface resulting in luminescence enhancement. Therefore, the Mn(2+)-doping approach provides a facile strategy for improvement of luminescence, which will impact on the field of bioimaging based on UCNP nanoprobes.

NaF-mediated controlled-synthesis of multicolor Na(x)ScF(3+x):Yb/Er upconversion nanocrystals.

Synthesis of lanthanide-doped upconversion nanocrystals (LDUNs) with controlled morphology and luminescence has long been desired in order to fulfill various application requirements. In this work, we have investigated the effect of the NaF : Ln(3+) molar ratio, in the range of 1 to 20, on the morphology, crystal structure, and upconversion properties of NaxScF(3+x):Yb/Er nanocrystals that are reported to possess different upconversion properties from those of NaYF4:Yb/Er nanocrystals. The experimental results prove that the NaF : Ln(3+) molar ratio influences significantly the growth process of the nanocrystals, i.e. a low NaF : Ln(3+) molar ratio results in hexagonal NaScF4 nanocrystals, while a high NaF : Ln(3+) molar ratio favors monoclinic Na3ScF6 nanocrystals. When the NaF : Ln(3+) molar ratio is as high as 6 or above, phase separation is found and hexagonal NaYbF4 nanocrystals showed up for the first time. Simply by adjusting the NaF : Ln(3+) molar ratio, we have successfully achieved the simultaneous control of the shape, size, as well as the crystallographic phase of the NaxScF(3+x):Yb/Er nanocrystals, which give different red to green (R/G) ratios (integral area), leading to a multicolor upconversion luminescence from orange-red to green. This study provides a vivid example to track and interpret the formation mechanisms and growth processes of NaxScF(3+x):Yb/Er nanocrystals, which shall be instructive for guiding the controlled synthesis of other LDUNs and extending their according applications in optical communication, color display, anti-counterfeiting, bioimaging, and so on.

Size-tunable and monodisperse Tm³⁺/Gd³⁺-doped hexagonal NaYbF₄ nanoparticles with engineered efficient near infrared-to-near infrared upconversion for in vivo imaging.

Hexagonal NaYbF4:Tm3+ upconversion nanoparticles hold promise for use in high contrast near-infrared-to-near-infrared (NIR-to-NIR) in vitro and in vivo bioimaging. However, significant hurdles remain in their preparation and control of their morphology and size, as well as in enhancement of their upconversion efficiency. Here, we describe a systematic approach to produce highly controlled hexagonal NaYbF4:Tm3+ nanoparticles with superior upconversion. We found that doping appropriate concentrations of trivalent gadolinium (Gd3+) can convert NaYbF4:Tm3+ 0.5% nanoparticles with cubic phase and irregular shape into highly monodisperse NaYbF4:Tm3+ 0.5% nanoplates or nanospheres in a pure hexagonal-phase and of tunable size. The intensity and the lifetime of the upconverted NIR luminescence at 800 nm exhibit a direct dependence on the size distribution of the resulting nanoparticles, being ascribed to the varied surface-to-volume ratios determined by the different nanoparticle size. Epitaxial growth of a thin NaYF4 shell layer of ∼2 nm on the ∼22 nm core of hexagonal NaYbF4:Gd3+ 30%/Tm3+ 0.5% nanoparticles resulted in a dramatic 350 fold NIR upconversion efficiency enhancement, because of effective suppression of surface-related quenching mechanisms. In vivo NIR-to-NIR upconversion imaging was demonstrated using a dispersion of phospholipid-polyethylene glycol (DSPE-PEG)-coated core/shell nanoparticles in phosphate buffered saline.

Controlled synthesis of ultrasmall hexagonal NaTm0.02Lu0.98−xYbxF4 nanocrystals with enhanced upconversion luminescence

Ultrasmall upconversion (UC) nanoparticles with intense luminescence are quite desirable for in vitro and in vivo bioimaging. Herein, we report the successful synthesis of sub-10 nm hexagonal NaTm0.02Lu0.98−xYbxF4 nanocrystals via a high boiling solvent process. The sizes and phases of the crystals can be finely tailored by tuning the amounts of the ligands used (octadecylamine and sodium oleate) and the temperature. The intensity of the UC emission of the nanocrystals increases as the the relative content of Yb3+ ions increases from 20% to 98%, owing to improved near-infrared absorption and energy-transfer from Yb3+ to Tm3+. The experimental results indicate that hexagonal NaYbF4 is the best host for ultrasmall Tm3+-doped UC nanoparticles and shows great potential in biomedical applications.

Influence of Ce3+ and Gd3+ co-doping on the structure and upconversion emission in hexagonal Ho3+ doped NaYbF4 phosphors

Hexagonal Ho3+ doped NaYbF4 phosphors are synthesized via a hydrothermal method. The influence of Gd3+ and Ce3+ content on the phase structure and upconversion (UC) emission of NaYbF4 phosphors is investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. The results of XRD and TEM indicate that the solubility of Ce3+ in hexagonal NaYbF4 is low due to the large difference of ionic radius between Ce3+ and Yb3+. With help of Gd3+ co-doping (15mol%), pure hexagonal NaYbF4 phosphors with high doping concentration of Ce3+ (15mol%) and small crystal size are obtained. When excited by a 980nm laser diode, Ho3+ doped hexagonal NaYb0.85Gd0.15F4 phosphors exhibit strong green UC emission at 540nm and weak red one at 646nm. UC luminescence tuning from green emission to red emission is observed in hexagonal Ho3+ doped NaYb0.85Gd0.15F4 phosphors by co-doping with Ce3+ ions. The UC luminescence tuning phenomenon is attributed to two resonant energy transfer processes of 5S2/5F4(Ho3+)+2F5/2(Ce3+)→5F5(Ho3+)+5F7/2(Ce3+) and 5I6(Ho3+)+2F5/2(Ce3+)→5I7(Ho3+)+5F7/2(Ce3+) between Ho3+ and Ce3+, which suppress the green emission at 540nm, while promote the red one at 646nm.

Highly efficient cooperative up-conversion of Yb3+ in NaYF4

Strong blue cooperative up-conversion emission around 475 nm has been observed in Yb3+-doped hexagonal NaYF4. The influence of concentration of the Yb3+ ion on the luminescence intensity is investigated. It is found that the sample shows the strongest cooperative luminescence when the Yb3+ ion concentration is 75%. The investigation shows that hexagonal NaYbF4 is an efficient blue up-converted phosphor, which might be potentially applicable in three-dimensional solid-state fluorescence display.