Colloidal β-KYF₄:Yb³⁺,Er³⁺/Tm³⁺ nanocrystals: tunable multicolor up-conversion luminescence from UV to NIR regions

Abstract

Pure β-KYF₄ nanocrystals codoped with Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ were successfully synthesized via the thermal decomposition of trifluoroacetate precursors using oleic acid and octadecylene as coordinating solvents. The up-conversion optical properties of the β-KYF₄ nanocrystals with different lanthanide (Ln)-doped ions (Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺) were investigated. It is found that the colloidal dispersion of β-KYF₄:Yb,Er/Tm nanocrystals display strong multiple up-conversion emission spanning from the deep UV-to-NIR regions under 980 nm excitation. Pure β-KYF₄ nanocrystals codoped with Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ can emit bright blue, green, yellow, and red emissions visible to the naked eye by adjusting the concentrations of Yb³⁺ sensitizer ion and Er³⁺ or Tm³⁺ activator ions. Moreover, the UC luminescent colors can be tuned for the four colors from blue to green to yellow and finally to red emission in the β-KYF₄:X%Yb³⁺/(0.2%–2%)Er³⁺ and β-KYF₄:25%Yb³⁺/0.2%Tm³⁺ colloidal samples. This work substantially expands our understanding of this category of KYF₄ upconversion nanocrystals.

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1. Introduction

Lanthanide (Ln)-doped up-conversion nanocrystals (UCNCs) have attracted considerable attention because of their potential applications in many fields such as solid-state lasers, solar cells, three-dimensional displays, infrared radiation detectors, and biomedical imaging.^1–4 Compared with traditional fluorescent materials, UCNCs exhibit excellent photostability, continuous emission capability, and sharp multi-peak line emission.^5–8 In particular, lanthanide-doped UCNCs have the remarkable ability to transfer the excited energy states of lanthanide ions (sensitizer ion) to a neighboring ion (activator ion) to emit efficient emissions by the anti-stokes process.^1,9–11 Anti-stokes process of upconverted luminescence is a nonlinear process in which low energy excited-light (near-infrared (NIR) or infrared) is converted to higher energy emitted-light (ultraviolet (UV) or visible (VIS)) through multiple sequential absorption of two or more low energy photons.^5,12,13 The anti-stokes process emerges largely due to unique intra 4f transitions and substantial energies states of the lanthanide ions, which are less influenced by the ligand field (due to the shielding of the 4f orbitals by the outer 5s and 5p orbitals) leading to sharp emission transitions.^14,15

Yb³⁺-sensitized fluoride NCs have been believed to be one of the most efficient upconverters due to low energy lattice phonons which can minimize the quenching effect of the excited state of lanthanide ions.^16–18 Moreover, multiple emissions in the visible region have been produced by doping multiple Ln³⁺ ions (Er³⁺/Yb³⁺/Tm³⁺, Er³⁺/Yb³⁺ or Yb³⁺/Tm³⁺),^19,20 since these materials display a very strong effect of NIR-to-VIS photon up-conversion. Green and blue up-conversion emissions are achieved by codoping Er³⁺ and Tm³⁺, respectively, with Yb³⁺ (as a sensitizer) in sodium host fluorides. Several research groups have recently reported up-conversion from colloids of either cubic or hexagonal NaYF₄ NCs.^21–23 For example, Krämer et al.²⁴ have identified micrometer-sized hexagonal NaYF₄ co-doped with Er³⁺/Yb³⁺ or Tm³⁺/Yb³⁺ with the highest up-conversion efficiencies, as well as hexagonal NaGdF₄ (ref. 25) or NaLuF₄ (ref. 5) NCs. Ln³⁺-doped KYF₄ NCs also have been reported as one of the most efficient Ln³⁺ host for infrared (IR) to VIS UC processes. Several groups also have investigated on Er³⁺/Yb³⁺ or Tm³⁺/Yb³⁺ co-doped KYF₄ NCs.^26–29 The cubic phase KYF₄:Yb,Er nanocrystals were reported by Schäfer et al.²⁶ Intense white luminescence has also been obtained in cubic α-KYF₄:Yb³⁺-Er³⁺-Tm³⁺ nanocrystals by J. Méndez-Ramos et al.²⁷ Further, UV-VIS UC emissions containing in cubic α-KYF₄-SiO₂ doped with Eu³⁺ and co-doped with Yb³⁺ and Tm³⁺ ions have been reported by Yanes et al.²⁸ It can be found in the literature a few works concerning synthesis and optical properties of Ln³⁺-doped KYF₄ cubic nanocrystals. However, the quantum yield of cubic phase nanocrystals is generally lower an order of magnitude than that of hexagonal phase. There are no further literature reports on the synthesis and optical properties of β-KYF₄:Yb,Er/Tm hexagonal nanocrystals.

The optical properties of Ln³⁺-doped bulk KYF₄ are widely studied. However, there are few reports on the monodispersed multicolor functional KYF₄ nanocrystals with finely tuned UC emissions. It is well known that the realization of multicolor UC emissions requires the generation and intensity control of the three fundamental blue, green, and red colors. Tuning the visible color output such as three fundamental blue, green, and red colors has been reported by tuning host lattice/dopant combinations and concentration.³⁰ For example, NaYF₄ and NaLuF₄ NCs doped with different lanthanide activators (Er³⁺, Ho³⁺, and Tm³⁺) demonstrate tunable spectra spanning visible and near-infrared regions.^31,32 However, there is no established approach to fine-tuning up-conversion emission for the UV-NIR regions by single-wavelength excitation.

Considerable efforts have been directed to the development and optimization of synthetic approaches to up-conversion nanoparticles. Schäfer et al.³³ reported a solvent-free room-temperature synthesis for hexagonal NaYF₄ nanocrystalline. However, the solvent-free approach to small particles from bulk specimens using a readily available technique has faced challenges. Yi et al.³⁴ have reported the synthesis of (Yb–Er)- and (Yb–Tm)-doped hexagonal-phase NaYF₄ nanoparticles by decomposition of multiprecursors dissolved in oleylamine at 330 °C. Herein, pure β-KYF₄ nanocrystals codoped with Er³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ were synthesized via the thermal decomposition of trifluoroacetate precursors using oleic acid and octadecylene as coordinate solvents. The as-synthesized β-KYF₄ NCs were dispersed readily into solvent to stable colloidal solution for up-conversion emission.

In this paper, we studied the tunable multicolor up-conversion (UC) emissions using potassium host instead of traditionally used sodium host. Lanthanide (Ln)-doped up-conversion optical properties of the β-KYF₄ doped with different lanthanide doping ions (Yb³⁺/Er³⁺, Yb³⁺/Tm³⁺) were investigated. Moreover, UC mechanism and multicolor emissions control of Yb³⁺/Er³⁺ co-doped β-KYF₄ NCs were also discussed. It is found that the colloidal dispersion of β-KYF₄:Yb,Er/Tm NCs display strong multiple up-conversion emission spanning from the deep UV-to-NIR region under 980 nm excitation. In addition, β-KYF₄ NCs could emit the bright eye-visible blue, green, yellow, and red emissions by adjusting concentration of Yb³⁺ and activator ions (Er³⁺ or Tm³⁺). This work substantially expands our understanding of this category of KYF₄ upconversion nanocrystals.

2. Experimental section

2.1 Materials

Yttrium(III) oxide (Y₂O₃, 99.99%), ytterbium(III) oxide (Yb₂O₃, 99.99%), erbium(III) oxide (Er₂O₃, 99.99%), and thulium(III) oxide (Tm₂O₃, 99.99%), potassium trifluoroacetate (CF₃COOK, ≥98%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), trifluoroacetic acid (CF₃COOH, 99.0%), hexane (C₆H₁₄, 97%), and ethanol (CH₃CH₂OH, 99.7%). All chemicals were purchases from Aladdin Inc. and used without any further purification. Lanthanide trifluoroacetates ((CF₃COO)₃Ln) were prepared by dissolving the respective lanthanide oxides in trifluoroacetic acid (CF₃COOH) into a 50 mL three-neck flask on a heating mantle.

2.2 Synthesis of β-KYF₄:Yb³⁺,Er³⁺/Tm³⁺ NCs

β-KYF₄:X%Yb³⁺/2%Er³⁺, 20%Yb³⁺/0.2%Er³⁺, 25%Yb³⁺/0.2%Tm³⁺ NCs were synthesized by thermal decomposition of trifluoroacetate using oleic acid and octadecylene as coordinate solvents,³⁵ including Yb³⁺ by X = 20, 35, 50, and 78 ratio instead of the host element Y³⁺, the total mole number with lanthanide ions of each sample is 1 mmol. In a typical synthesis, the (CF₃COO)₃Ln (Ln = Y, Yb, Er, and Tm), CF₃COOK (2 mmol), oleic acid (6 mL) and 1-octadecene (15 mL) were added to a 50 mL three-neck flask and stirred. The flask of oxygen and water were purged by argon bubbling at 130 °C for 30 min. Next, the temperature of the mixture was slowly raised to 330 °C at a ramping rate of 5°C min⁻¹. The reaction was held at 330 °C for 1 h with continuous vigorous stirring. After reaction, this stock solution was cooled to ambient temperature prior to precipitation with absolute ethanol. The NCs were isolated by precipitation with addition of 10 mL of hexane and 15 mL of ethanol followed by centrifugation at 11 000 rpm for 5 min. The supernatant containing byproducts was discarded. The white precipitate was collected by centrifugation and washed with hexane and ethanol two times. Different lanthanide-doped KYF₄ NCs were dried at 60 °C under the condition of vacuum.

2.3 X-ray powder diffraction (XRD)

The X-ray powder diffraction patterns were measured by using the Bruker Discover D8 high-resolution diffractometer with CuKα radiation (λ = 1.5418 Å). The samples were measured at a scanning rate of 15° min⁻¹ in the over the range of 2θ = 10–70°.

2.4 Element composition analysis

The energy dispersive X-ray spectroscopy (EDS) for elemental analysis was carried out with a Hitachi S-4800 field emission scanning electron microscope (FE-SEM). The powder samples were put on a carbon fiber copper wire mesh.

2.5 Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM)

The morphology and microstructure of as-synthesized sample NCs were measured using a transmission electron microscope and high-resolution transmission electron microscope (model JEM 2100F) equipped with a field emission gun operating at 200 KV (0.23 nm point-to-point). The colloidal NCs for TEM and HRTEM images were dispersed in an ethanol solution, subsequently one drop of the colloidal suspension was placed on carbon-coated copper grid.

2.6 Up-conversion spectra analysis

The UC emission spectra were obtained using a 980 nm CW Ti:Sapphire laser (3900S; Spectra Physics) with an excitation power density of 0–2 W cm⁻² adjustable as the excited source from a continuous-wave xenon lamp with a slit width defining spectral resolution of 1 nm. As-synthesized KYF₄ samples were uniformly dispersed in ethanol (1 wt%), and the obtained colloidal NCs were placed in 1 cm path-length quartz cuvettes (Hellma, QS). The emission signal was collected by a half-meter monochromator (HR460, Jobin Yvon) equipped with a 1200 lines per mm and detected with a CCD detector (Spectrum One, Jobin Yvon) from 300 to 850 nm. UC luminescence images of the colloidal NCs were acquired with a Nikon multiple camera (model D7100) without adding any filter. All the measurements were performed at room temperature.

3. Results and discussion

3.1 Crystal phase, morphology and microstructure, and element compositions of β-KYF₄:Yb,Er/Tm NCs

Fig. 1 shows the typical XRD patterns of as-prepared KYF₄ NCs co-doped with 20% Yb³⁺/2% Er³⁺ and 25% Yb³⁺/0.2% Tm³⁺.

Fig. 1 XRD patterns of as-synthesized KYF₄:20%Yb³⁺,2%Er³⁺ and KYF₄:25%Yb³⁺, 0.2%Tm³⁺ NCs with a hexagonal crystal structure (JCPDS: 79-1688).

As demonstrated in Fig. 1, the XRD patterns of KYF₄:Yb,Er and KYF₄:Yb,Tm are provided with the similar diffraction peaks, which both belong to a P31(144) space group. All of the intense diffraction peaks that located at 2θ = 15.5 °C, 21.8 °C, 27.0 °C, 28.3 °C, 31.2 °C, 44.5 °C and 52.6 °C can easily be specified as the diffraction peaks of standard β-KYF₄ XRD patterns (JCPDS79-1688). No other miscellaneous peak is detected, which indicates the pure hexagonal KYF₄ NCs with a high crystallinity can be readily synthesized using thermal decomposition of trifluoroacetates metal-precursor method. The average nanoparticle size of KYF₄:Yb,Er and KYF₄:Yb,Tm NCs are calculated to be around 18 and 20 nm according to the Scherrer's formula D = 0.89λ/(β cos θ), where D, λ, θ, and β are the average particle size, X-ray wavelength (0.15405 nm), the diffraction angle and full-width at half-maximum, respectively.

The morphology and size of Yb³⁺, Er³⁺/Tm³⁺ codoped KYF₄ NCs are analyzed by TEM and HRTEM. Fig. 2 shows the TEM images of KYF₄:20%Yb³⁺/2%Er³⁺ and KYF₄:25%Yb³⁺/0.2%Tm³⁺ NCs, respectively. It is shown that as-synthesized KYF₄:Yb³⁺,Er³⁺/Tm³⁺ NCs contain smaller hexagonal nanorods particles and nanoplates with good monodispersity. From Fig. 2a and b the average grain size for KYF₄:20%Yb/2%Er and KYF₄:25%Yb/0.2%Tm NCs are calculated from random 100 nanoparticle to be 16 nm, 18 nm, respectively, which are in good consistent to XRD analysis result calculated by Scherrer's formula. The lattice fringes are obviously observed in HR-TEM images of KYF₄:Yb,Er/Tm (Fig. 2c and d), which indicated that as-synthesized KYF₄ NCs possessed highly crystallinity. The HRTEM images (Fig. 2c) of as-synthesized nanocrystals showed clear lattice fringes with a interplanar spacing of d = 0.33 nm, which corresponds to the (221) lattice plane of the hexagonal phase KYF₄. In addition, the interplanar distances of 0.21 nm was also observed by HR-TEM images in Fig. 2d, corresponding to the (600) lattice planes of the hexagonal phase KYF₄, which was consistent with the XRD analysis. The corresponding elemental composition of KYF₄ codoped with 20% Yb³⁺/2% Er³⁺ and 25% Yb³⁺/0.2% Tm³⁺ NCs were obtained by the EDS analysis, as shown in Fig. 2f. The EDS result for KYF₄:20%Yb/2%Er (Fig. 2e) demonstrates that as-synthesized NCs are mostly consist of K, Y, Yb, Er and F. The EDS result for KYF₄:25%Yb/0.2%Tm as-synthesized NCs are mostly consist of K, Y, Yb, Tm and F. Based on the XRD and TEM analyses, it is concluded that the KYF₄ NCs with good monodispersity and small particle size can be readily controlled via the thermal decomposition of trifluoroacetate precursors.

Fig. 2 TEM and HRTEM images of the as-synthesized KYF₄ NCs: (a) KYF₄:20%Yb³⁺,2%Er³⁺, (b) KYF₄:25%Yb³⁺,0.2%Tm³⁺, (c) HRTEM image of KYF₄:20%Yb³⁺,2%Er³⁺ NCs, (d) HRTEM image of KYF₄:25%Yb³⁺,0.2%Tm³⁺ NCs, (e) EDS of KYF₄:20%Yb³⁺,2%Er³⁺ NCs, (f) EDS of KYF₄:25%Yb³⁺,0.2%Tm³⁺ NCs.

3.2 Tunability of multicolor up-conversion emissions of the β-KYF₄:Yb,Er/Tm NCs

Under a 980 nm laser excitation, Yb³⁺ ions are excited from the ²F_7/2 level to the ²F_5/2 level, and then transfer their energies to the nearby Er³⁺ or Tm³⁺ ions. On the basis of energy-matching conditions, the possible UC mechanisms for the β-KYF₄:Yb,Er/Tm are illustrated in Fig. 3.

Fig. 3 The simplified UC emission mechanisms of β-KYF₄:Yb,Er/Tm.

Fig. 4 shows UC luminescence spectra of the different concentration Yb,Er/Tm-codoped KYF₄ NCs. As can be observed in Fig. 4A, six dominant emission peaks of KYF₄ doped with different Yb and Er ion concentration centered around 668, 650, 542, 522, 410, and 380 nm, respectively. These emission peaks occurred as a result of Yb³⁺ sensitization, leading to the photon energy transfer to the nearby Er³⁺ ions. According to the simplified energy level diagram shown in Fig. 3, these obvious emission peaks corresponding to 668/650, 542 and 522 nm are attributed to the electronic transition of ⁴F_9/2 → ⁴I_15/2, ⁴S_3/2, ²H_11/2 → ⁴I_15/2 of the Er³⁺ ions, respectively, while emission peaks center at 410 and 380 nm correspond to electronic transitions of ⁴G_11/2, ²H_9/2 → ⁴I_15/2 of the Er ions, respectively. The relative emission peaks intensity of the three KYF₄:Yb,Er samples affected the final luminescence performance. The intense green (542 nm) and weak red emissions (650 nm) were observed in the KYF₄:20%Yb,0.2%Er, which corresponding to the bright eye-visible green luminescence in Fig. 4C. The weak green and intense red emissions centered at 542 nm and 650 nm were observed in the KYF₄:20%Yb, 2%Er, which corresponding to the bright eye-visible yellow luminescence solution, its transparency was shown in Fig. 4D. Compared with emission for KYF₄:20%Yb,2%Er, the intense red emissions for KYF₄:78%Yb/2%Er changed from 650 nm to 680 nm with a little red shift, which corresponding to the bright eye-visible red luminescence, as shown in Fig. 4E.

Fig. 4 UC luminescence spectra of the Yb,Er/Tm-codoped KYF₄ NCs: (A)KYF₄:20%Yb,0.2%Er, KYF₄:20%Yb,2%Er, KYF₄:78%Yb,2%Er, and KYF₄:25%Yb,0.2%Tm. (B)The photographs show the corresponding 1 wt% ethanol solutions of (C)KYF₄:20%Yb,0.2%Er, (D)KYF₄:20%Yb,2%Er, (E)KYF₄:78%Yb,2%Er, and (F)KYF₄:25%Yb,0.2%Tm. The colloidal NCs were excited with 980 nm laser under power density of 2 W cm⁻².

Fig. 4B shows UC luminescence spectra of Yb,Tm-codoped KYF₄ NCs. Under the 980 nm near-infrared excitation, the Tm³⁺/Yb³⁺-codoped KYF₄ colloidal NCs dispersed in the 1 wt% ethanol display unexpected up-conversion emission spanning the UV-to-NIR regions, which emerge from all the energy transfer of Tm³⁺ ions. Interestingly, two intense emission peaks and one weak peak at 350, 360, and 382 nm occurred in the UV emission region, which are generated from the electronic transition of ¹I₆ → ³F_4, ¹D₂ → ³H_6, and ¹I₆ → ³H₅, respectively. In the visible wavelength range, two strong blue peaks located at 456 and 476 nm, and a red peak located at 652 nm, which were attributed to electronic level transitions of ¹D₂ → ³F₄, ¹G₄ → ³H₆/³F₄ of the Tm³⁺ ions, respectively. Because of the population of ¹D₂ level of Tm³⁺ ions is not restrained, the ¹D₂ level of Tm³⁺ is unable to transfer the fourth photon from the energy level of Yb³⁺ to the energy of ¹G₄ level. Therefore, the cross relaxation (CR) process of ³F₂ + ³H₄ → ³H₆ + ¹D₂ between Tm³⁺ ions may alternatively play an important role in populating the ¹D₂ level.³⁶ In the NIR region, one emission at 800 nm is again observed, corresponding to electronic level transitions of ³H₄ → ³H₆ of the Tm ions. As shown in Fig. 4F. KYF₄:25%Yb, 0.2%Tm solution emits the strong blue up-conversion luminescence, which can be explained by the energy transfer mechanism of Tm³⁺ ions derived in the Fig. 3.

From what has been discussed above, the Yb³⁺ and Er³⁺ doped ion concentration caused little change in the position of the characteristic emission peak, which is determined by Er³⁺ photons number. Therefore, it is necessary to study the relationship between the pumping power density (I_IR) and the UC emission intensities (I_UC) to determine the number of photons involved in the UC process. In general, the UC emission intensity (I_UC) is expected to be directly proportional to the nth power of the excitation power (I_IR): I_UC ∼ I_IRⁿ, where n is the absorbed photon numbers (>1) per visible photon emitted, and its value can be obtained from the slope of the fitted line of the plot of log(I_UC) versus log(I_IR).³²

Fig. 5A and B show the UC spectra and the corresponding to luminescence photographs of KYF₄:20%Yb,0.2%Er NCs excited with a 980 nm diode laser with power density of 100–600 mW cm⁻². Fig. 5C shows the log(I_UC)–log(I_IR) plots of emission intensity and excitation power. The slopes of the linear fits of log(I_UC) versus log(I_IR) for the UV, blue, green, and red emissions at 380, 410, 522, 542, and 650 nm in the KYF₄:20%Yb,0.2%Er sample are 2.24, 2.18, 1.94, 1.83, and 1.67, respectively. These results indicate that emission peaks at 650, 542, and 522 nm are attributed to 2-photon up-conversion process, while these peaks at the spectrum of 410 and 380 nm emerge via 3-photon up-conversion process, which are consistent with previous reports on NaYF₄,²⁴ RbGdF₄,³⁷ NaGdF₄ (ref. 38) NCs. As shown in Fig. 3, the potential upconversion energy-mechanisms from the Yb³⁺/Er³⁺ co-doped KYF₄ NCs are illustrated by simplified energy level diagrams. Fig. 5D clearly shows the calculated color coordinates and the CIE 1931 chromaticity diagram of the Yb³⁺/Er³⁺ codoped KYF₄ NCs for the UC emissions at various powers. When the laser power changed from 100 to 600 mW, the CIE color coordinates of the UC emission of Yb³⁺/Er³⁺ co-doped KYF₄ NCs shifted from whitish green to the bright green region.

Fig. 5 (A) UC luminescence spectra of the KYF₄:20%Yb,0.2%Er NCs excited with 980 nm diode laser with power density of 100 to 600 mW cm⁻², (B) luminescence photographs of the corresponding KYF₄:20%Yb³⁺,0.2%Er³⁺ samples dispersed in 1 wt% ethanol colloidal solutions under 980 nm laser excitation with power density of 100 to 600 mW cm⁻², (C)log–log power dependence of the UC emissions of KYF₄:20%Yb,0.2%Er NCs, and (D) CIE(X,Y) coordinate diagram showing the chromaticity points calculated from the UC emission spectra of KYF₄:20%Yb,0.2%Er NCs excited under a 980 nm laser.

Transition between yellow and red emissions by the host element Yb³⁺ instead of Y³⁺ also observed in the Fig. 4A, indicating that the change of Yb³⁺ concentrations can affect color output. Therefore, it is necessary to further study influence of Yb³⁺ doped concentration on light-emitting properties of β-KYF₄ nanocrystals.

Fig. 6A and B show the UC spectra and the luminescence photographs of KYF₄:X%Yb³⁺,2%Er³⁺ with different concentrations of Yb³⁺ ion. As can be seen from Fig. 6A, the position of band peaks have no obvious change among the four colloidal samples dispersed in ethanol (1 wt%) solution with the increase of Yb³⁺ concentration. The content of Yb³⁺ increase resulted in the decrease of relative peak intensity at 380 nm UV (⁴G_11/2 → ⁴I_15/2), 410 nm blue (²H_9/2 → ⁴I_15/2) and 522, 542 nm green (²H_11/2,⁴S_3/2 → ⁴I_15/2) light emissions. When the doped Yb³⁺ concentration changed from 20% to 50%, they both finally emitted the bright yellow light, as shown in Fig. 6B. Interestingly, the intensity of the 668 nm red emissions was slightly enhanced by increasing the Yb³⁺ concentration. When the doped Yb³⁺ concentration increased to 78%, the red emission become the primary colors, which is quite similar to previous reports of Yb³⁺/Er³⁺ codoped NaYF₄ (ref. 21) and NaLuF₄.²⁶

Fig. 6 (A) UC luminescence spectra of the KYF₄:X%Yb,0.2%Er NCs (X = 20, 35, 50, and 78) excited with 980 nm diode laser. (B) Luminescence photographs of the corresponding KYF₄:20%Yb³⁺,0.2%Er³⁺ samples dispersed in 1 wt% ethanol colloidal solutions under 980 nm laser excitation with a power density of 2 W cm⁻². (C) log–log power dependence of the UC emissions of KYF₄:X%Yb,0.2%Er NCs, and (D) CIE (X,Y) coordinate diagram showing the chromaticity points calculated from the UC emission spectra of KYF₄:X%Yb,0.2%Er NCs excited under a 980 nm laser.

In order to deeply investigate the involved UC mechanism in KYF₄:X%Yb³⁺,2%Er³⁺, the log(I_UC)−log(I_IR) plots of emission intensity and excitation power are shown in Fig. 6C. The slope of the fitted line of the plot of log(I_UC) versus log(I_IR) for the UV, blue, and green emission intensity in KYF₄:X%Yb³⁺,2%Er³⁺ samples are negative, while red emission intensity gradually increase with the content of Yb³⁺ increases. As reported UC mechanism in KYF₄:Yb³⁺/Er³⁺, only 2-photon emission process involves the production of green and red UC, UC and 3-photon processes is the need for UV and blue emission. Based on above discusses, we explain UC mechanism in KYF₄:Yb³⁺,Er³⁺ with different Yb³⁺ concentration. With the amount of Yb³⁺ dopants increase in the KYF₄ host lattice, the Yb–Er inter-atomic distance will be decreased to facilitate back-energy-transfer from Er³⁺ to Yb³⁺, which will subsequently suppress the population in excited levels of ⁴G_11/2, ²H_9/2, ²H_11/2 and ⁴S_3/2 and result in the decrease of UV (⁴G_11/2 → ⁴I_15/2),blue (²H_9/2 → ⁴I_15/2), and green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) emissions. In addition, the energy-transfer from Yb³⁺ to Er³⁺ results in the saturation of the ⁴I_13/2 (Er³⁺) state, and then energy of the excited Yb³⁺ ions transfer to Er³⁺ ions through the anti-stokes emission process ²F_5/2(Yb³⁺) + ⁴I_13/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴F_9/2(Er³⁺), which can be directly filled in the ⁴F_9/2 level, resulting in the enhancement of red emission (⁴F_9/2 → ⁴I_15/2). The chromaticity coordinates shift slowly to red region with the increasing of Yb³⁺ concentration, indicating that luminescent colors can be adjusted in a wider range. Fig. 6D shows the calculated color coordinates and CIE 1931 chromaticity diagram of KYF₄:X%Yb³⁺,2%Er³⁺ (X = 20, 35, 50, and 78) under 980 nm laser excitation, which vividly describes the change in crystal color trends by adjusting Yb³⁺ concentrations. In short, the upconversion luminescent output colors of KYF₄:Yb³⁺,Er³⁺ can be changed from yellow to red emission with the increasing of Yb³⁺ concentration.

4. Conclusions

In conclusion, we have successfully synthesized pure hexagonal phase β-KYF₄ NCs with a good monodispersity by a trifluoroacetates thermolysis method using oleic acid and octadecylene as a coordinate solvent. The microstructure and nanoparticle size of KYF₄:Yb,Er/Tm NCs were characterized using XRD and TEM. The average particle size was 18 nm for the β-KYF₄:20%Yb,2%Er NCs and 20 nm for the β-KYF₄:20%Yb,0.2%Tm NCs. The optical properties of the β-KYF₄ doped with various lanthanide ions (Ln³⁺ = Yb³⁺,Er³⁺, and Tm³⁺) were investigated under 980 nm excitation. The colloidal KYF₄:Yb, Er/Tm NCs exhibit strong multiple up-conversion emission spanning from the deep UV-to-NIR region (300–850 nm) with 980 nm laser excitation. It is shown that the as-synthsized β-KYF₄ NCs can emit the bright eye-visible blue, green, yellow, and red emissions by adjusting concentration of Yb³⁺ and activator ions (Er³⁺ or Tm³⁺). Moreover, the UC luminescent colors can be tuned for the four basic colors from blue to green to yellow and finally to red emission in the β-KYF₄:X%Yb³⁺/(0.2%–2%)Er³⁺ and β-KYF₄:25%Yb³⁺/0.2%Tm³⁺ colloidal samples. This work substantially expands our understanding of this category of KYF₄ upconversion nanocrystals.

Acknowledgements

This work was supported by National College Students' innovation and entrepreneurship training plan of China (20141049701020), self-determined and innovative research funds from Wuhan University of Technology(grant number 146801006) and Wuhan Municipal Science and Technology Bureau, China (grant No. 2015010101010006).

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