1. Introduction

In the past tens years, many scientific researchers have garnered a tremendous amount of attention to ongoing research of upconversion (UC) nano-/micro-materials. These materials can absorb two or more long-wavelength (i.e. low-energy) photons and convert into short-wavelength (i.e. high-energy) radiation. Compared to other materials, they exhibit unique optical characteristic, such as large Stokes shift, sharp emission band, high quantum yield, long lifetime, weak background luminescence, and good resistance to photo-bleaching, blinking and photochemical degradation [1,2]. Thereby, these UC materials have diverse potential applications in photovoltaic devices [3], advanced lighting and displays [4,5], sensors [6,7], and biological labels [8].

It is well known that Er³⁺ is an excellent candidate doped ions to produce short wavelength emission, i.e. 530 nm green light (due to the ²H_11/2 + ⁴S_3/2→⁴I_15/2 transition) and 650 nm red light (due to the ⁴F_9/2→⁴I_15/2 transition). Unfortunately, Er³⁺ exhibits the low absorption cross section in the NIR range (compared with Yb³⁺). Yb³⁺ is often used as a sensitizer for Er³⁺ to increase the absorption cross section to enhance the UC emission efficiency, since Yb³⁺ exhibits the large absorption cross section and broad absorption band at around 980 nm matching with the emission wavelength of powerful commercial laser diodes, and its excited energy level ²F_5/2 matches with the excited energy level ⁴I_15/2 of Er³⁺.

Among various UC host materials, the alkali metal–rare earth– fluoride compounds with the formula AREF₄ (A = alkali metal, RE = rare earth) have been paid more and more attention due to low phonon energy, broad transparency range, and high UC emission efficiency, thereby, they have served as very efficient UC matrixes from the UV to NIR region [9–18]. LiYF₄ is one member of AREF₄ family and rare-earth-doped LiYF₄ bulk crystals can generate comparable emission intensity and additional emission lines in comparison to NaYF₄ hosts [11–20]. Thus, LiYF₄ can provide us with a complementary host to NaYF₄ for technical applications such as scintillation and tunable UC lasers [19]. Recently, rare-earth doped LiYF₄ nano/micro-crystals have been fabricated via hydrothermal method [11,12], thermal decomposition [13–16], high temperature solid state [17] and molten salt [20]. To the best of our knowledge, most of rare earth doped LiYF₄ nano-/micromaterials to date have been synthesized by thermal decomposition method [13–17]. Although this method can be used to fabricate monodisperse nanoparticles with high dispersability in organic solvents, it suffers from some inherent disadvantages of low yield, large total costs, toxic rare-earth trifluoroacetate precursors and byproducts, and substantial environmental loads, thereby limiting its use for commercial purposes. The synthetic route based on molten salt method has been proved to an economical mass, convenient, effective and facile and environmentally friendly approach. It is very easy to obtain the materials with clean surface, chemical purification, and few residual impurities below the melting point of the as-grown crystals. This method has been adopted to fabricate abundant inorganic functional materials, such as complex oxide, hydroxides, sulfides and fluorides [21, 22]. The key of this synthesis technique is to select a suitable flux. For example, the 50 mol% NaNO₃/25 mol% KNO₃ flux has been applied to prepare RE³⁺/Yb³⁺-doped LiYF₄ microcrystals [20]. Unfortunately, its synthesis temperature still was very high (400 °C) and it was very difficult to separate the NaNO₃/KNO₃ and LiYF₄. Since ammonium nitrate (NH₄NO₃) exhibits many advantages, including abundance in nature, low melting point, and easy deliquescence in water/alcohol causing to easily separate from the flux and the target compound [23], NH₄NO₃ can act as a flux to synthesize LiYF₄ materials. Here, we mainly concentrate upon the effects of the usage amount of reaction materials and NH₄NO₃ flux, the temperature and the time on the structure, morphology and size of the as-obtained products. Finally, the UC properties of the as-obtained products have also been investigated and discussed in detail.

2. Experimental method

2.1 Sample preparation

Reagent-grade LiNO₃, NH₄NO₃, Y(NO₃)₃, Yb(NO₃)₃, Er(NO₃)₃, and NH₄F powders were used to synthesis Er³⁺,Yb³⁺-codoped LiYF₄ crystals. All reagent-grade powders and NH₄NO₃ used as a flux to prepare Er³⁺:Yb³⁺:LiYF₄ crystals were mixed well. The mixtures were put into 15 ml capacity crucibles. After the lids were tightly closed, the crucibles were placed in an oven. The sealed tank was heated to 160-250 °C, and held for different times in an oven, and then cooled to room temperature naturally. After being washed several times with deionized water and ethanol, the precipitates were dried at 70°C for 12 hours in vacuum.

2.2 Characterization

The samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), UV-vis spectroscopy, and photoluminescence (PL). XRD analyses were carried out on a Bruker D8-Advance diffractometer with graphite-monochromatized Cu Kα radiation (40 KV/60 mA, graphite monochromator, λ = 0.1541 nm). The size, morphology and chemical compositions of products were determined by a transmission electron microscopy (TEM, JEOL2010) operating at 200 kV and a JSM 6700F scanning electron microscope (SEM) equipped with the energy dispersive X-ray spectrum (EDS). Structural information of the nanocrystals was measured by a high-resolution transmission electron microscopy (HRTEM).

Emission and excitation spectra and fluorescence decays were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) and pulsed Xe lamps. For the low-temperature measurements, samples were mounted on a closed-cycle cryostat (10-350 K, DE202, Advanced Research Systems). The best wavelength resolution is 0.05 nm. The line intensities and positions of the measured spectra were calibrated according to the FLS920 correction curve and standard mercury lamp.

3. Results and discussion

3.1 Crystalline structure and morphology characterization

Since fluoride source NH₄F has the same cation with the flux NH₄NO₃, NH₄F was selected as the fluoride source to synthesize LiYF₄:Er³⁺/Yb³⁺ crystals. The reaction time and the reaction temperature were fixed to be 24 hours and 250 °C, respectively, the molar ratio of reaction materials of LiNO₃, Ln(NO₃)₃ (Ln represents the total amount of Y, Yb and Er) and the NH₄F amount were selected to study the crystal phase, size, and morphologies of the as-obtained products. The XRD patterns of the as-prepared products, as shown in Fig. 1(a), indicates that the crystal phase of the products is decided by the molar ratio of Ln(NO₃)₃-to-NH₄F. When the usage amount of Ln(NO₃)₃ and NH₄F was 1, 1 and 2 mmol, i.e. the amount of NH₄F was half of the stoichiometric proportion, and usage amount of flux NH₄NO₃ was 5 mmol, the diffraction peaks matched well with that of the orthorhombic phase YF₃ (JCPDS No. 74-0911), not with the hexagonal LiYF₄ (JCPDS No. 85-0806) and the cubic YF₃ (JCPDS No. 72-0579), thereby the product (S1) was pure orthorhombic YF₃. When the NH₄F usage amount increase to was 4.0 mmol, i.e. the stoichiometric proportion, in the XRD pattern of the product (S2) the intensity of the XRD peaks originating from orthorhombic YF₃ became weak; howbeit the XRD peaks belonging to tetragonal LiYF₄ appeared and took a role, implying that the as-obtained product was made of LiYF₄ and orthorhombic YF₃. As the NH₄F usage amount further increased to 6.0 mmol, the diffraction peaks of the product (S3) due to the orthorhombic YF₃ disappeared, and all the diffraction peaks were in agreement with those of pure tetragonal LiYF₄, which implies that the LiYF₄:Er³⁺/Yb³⁺ crystals obtained have the same crystallographic structure as tetragonal LiYF₄ crystal. Therefore, the tetragonal LiYF₄:Er³⁺/Yb³⁺ materials can be synthesized in the condition that NH₄F amount exceeded the stoichiometric proportion, and the NH₄NO₃ molten solution system can also be developed to prepare the tetragonal LiYF₄ materials at low temperature.

 

Fig. 1 The XRD patterns of the samples (a)for different NH₄F usage amounts (reaction temperature: 250 °C, reaction time: 24 hours, X-Y-Z: molar ratio of LiNO₃, Ln(NO₃)₃ and NH₄F); (b) at reaction temperatures of 160, 200 and 250 °C (the molar ratio of LiNO₃, Ln(NO₃)₃ and NH₄F: 1:1:6); (c) for reaction times of 2, 6, 12 and 24 hours (the molar ratio of LiNO₃, Ln(NO₃)₃ and NH₄F: 1:1:6).

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To make an investigation of the effect of reaction temperature on the crystal structure, and shapes and sizes, the reaction time, the ratios of Li-Ln-F and Ln-NH₄NO₃ maintained to be 24 hours, 1:1:6 and 1:5, respectively, the reaction temperature decreased from 250 through 200 to 160 °C. When the temperature decreased to 200 °C, the diffraction peaks of the product (S4) due to the tetragonal LiYF₄ disappeared, and all the XRD peaks were in agreement with those of the cubic NH₄Y₂F₇ (JCPDS No. 01-074-0911) (see Fig. 1(b)), which implies that the as-obtained product transformed from the tetragonal LiYF₄ to the cubic NH₄Y₂F₇ and made up of pure cubic NH₄Y₂F₇. Further decreasing to 160 °C, the structure of the product (S5) kept unchanged, but the relative intensity of XRD peaks became weak and the relative intensities between each peak varied, suggesting that the crystalline decreased and the preferential growth orientation changed. From the above analysis, high temperature is favorable to transform from cubic NH₄Y₂F₇ to tetragonal LiYF₄ and improve the crystalline. As known, the phase transformation requires enough energy to overpass the potential barrier and the thermodynamic growth regime is driven by a sufficient supply of thermal energy, thereby, the most stable crystal structure is preferred. The tetragonal LiYF₄ is more thermodynamically than the cubic NH₄Y₂F₇, thereby, the cubic NH₄Y₂F₇ is susceptible to transform to the tetragonal LiYF₄. Since the energy barrier hinders to form tetragonal LiYF₄, a sufficient supply of thermal energy must be required to overcome the energy barrier in order to modify the environment of Y³⁺ and Li⁺ occupation sites, including coordination number [24]. Thus, the high temperature can provide more sufficient supply of thermal energy and favors to form the tetragonal LiYF₄.

To get insight into the effect of the reaction time on the crystal structure and morphological evolution, the XRD analysis, TEM and SEM observation for different reaction time, and the SAED pattern and HRTEM image study of the as-prepared products were carried out. Take Li-Ln-F molar ratio of 1-1-6 as an example, the detailed time-dependent experiments were carried out under the similar reaction conditions. The reaction temperature and the molar ratio of Ln-NH₄NO₃ were 250 °C and 1:5, respectively, different reaction times of 2, 6, 12 and 24 hours were selected to investigate the process of the crystal growth process. The change of XRD patterns during different growth periods, as shown in Fig. 1(c), can further provide evident for the crystal structure and shape evolution. When the reaction time was 2 hours, in the XRD pattern of the product (S6) the intensity of the XRD peaks originating from tetragonal LiYF₄ phase was very weak; howbeit the XRD peaks belonging to the cubic NH₄Y₂F₇ were strong and took a predominant role, implying that the as-obtained product was made of predominant cubic NH₄Y₂F₇ and minute tetragonal LiYF₄. As the reaction time further extended to 6 hours, the chemical composition and crystal structure of the product (S7) maintained nearly constant, while the intensities of the XRD peaks belonging to cubic NH₄Y₂F₇ became strong, implying that crystalline of this product was improved. When the reaction time elongated to 12 hours, the XRD peaks due to cubic NH₄Y₂F₇ disappeared, and all XRD peaks matched with the characteristic peak of pure tetragonal LiYF₄ and there existed no additional diffractions, implying that the product S8 consists of pure tetragonal LiYF₄ materials and they can be synthesized for over 12 hours at 250 °C. Moreover, the relative intensities of (101) and (112) diffraction peaks exhibited a large difference between S8 and S3, which indicates different preferential growth orientation, and (101) XRD peak becomes strong and sharp with the extension of the reaction time, revealing that the crystal preferably grow along [101] direction.

What’s more, the reaction time also affected the shape and size of the as-prepared products. The corresponding SEM and TEM images of the intermediates at different growth stages are shown in Fig. 2 and 3.The reaction treatment for 2 hours leads to the formation of ellipsoids with the diameter of ca. 100 nm and minute rods (see Fig. 2(a) and Fig. 3(a), 3(b)). When the reaction time increases to 6 hours, the morphology of the as-prepared sample S7 transforms from ellipsoid to irregular rods. These rods exhibit the length of 4-5 μm and the diameter of ca. 500 nm. And some irregular nanoparticles are adhered on their surface (see Fig. 2(b)). As the reaction time elongates to be 12 hours, the morphology of the product is polyhedral (see Fig. 2(c)). The spacings between two adjacent lattice fringes are 0.256 nm (Fig. 3(c)) and much closer to the d-spacing of 0.253 nm for the (200) plane. The EDS pattern spectra, as shown in Fig. 3(d) shows that a characteristic intensity profile of Er, Yb, Y and F elements (Li element can’t be detected). When the reaction time reached to 24 hours, the irregular nanoparticles disappear (see Fig. 2(d)). The dissolution - recrystallization process took place in order to minimize the surface energy of the system, and the stable LiYF₄ polyhedrals grows at the consumption of smaller nanoparticles by Ostwald ripening. The morphology of the sample S3 evolves to tetragonal bipyramid with average sizes of 8-10 μm in length and 4μm in diameter. There exist some broken holes on their faces (see Fig. 3(e)), which can be resulted from the preferential 2 D-nucleation at the border of the growing face [20].

 

Fig. 2 The morphology evolution of the as-obtained samples at reaction times (a: 2 hours, b: 6 hours, c:12 hours, d: 24 hours (the molar ratio of LiNO₃, Ln(NO₃)₃ and NH₄F: 1:1:6, reaction temperature: 250 °C)

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Fig. 3 The magnified TEM image (a, b) of S6, SAED image (c) and EDS spectrum (d) of S7, the magnified SEM image (e) of S3.

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3.2 Upconversion properties

To study the effects of LiYF₄:2mol.% Er³⁺/20 mol.%Yb³⁺ products with different morphologies on their UC emission characteristics, the UC emission intensities were recorded under the same experimental condition. The excitation wavelength of all the samples was 979 nm. Figure 4 depicts the room temperature UC emission spectra of LiYF₄:Er³⁺/Yb³⁺ samples. The strong green UC emission bands were observed in the region of 512-573 nm, attributed to the ²H_11/2,⁴S_3/2→⁴I_15/2 transitions of Er³⁺ ion [25], respectively. The strong red UC emission between 630 and 700 nm belongs to ⁴F_9/2→⁴I_15/2 transition of Er³⁺ ion (see Fig. 4(a)) [25]. Except the above strong green and red UC emissions, four weak UC emission bands also were observed at room temperature in the region between 300 and 505 nm (see Fig. 4(b)), centering at 370-390, 400-428, 445-460 and 485-505 nm, stemming from ⁴G_11/2, ²H_9/2, ⁴F_3/2 + ⁴F_3/2 and ⁴F_7/2 excited state to the ground state ⁴I_15/2, respectively [25]. From Fig. 4, it can be seen that Er³⁺Yb³⁺:LiYF₄ samples with different morphologies exhibited the differences of the relative intensity of each UC emission peak. The UC emission band from 455 nm to 670 nm for the tetragonal bipyramid-like sample is much stronger than that for the polyhedra-like sample and the relative intensity of the former is four times as large as the latter. To our surprise, the emission intensity at 370- 430 nm for the tetragonal bipyramid-like sample is much lower than that for the polyhedra-like sample and the relative intensity of the former is one-third as large as that of the latter. According to possible UC process reported before, the UC emission band from 455 to 670 nm with the involvement of 2 NIR photons for the sample with tetragonal bipyramid is much stronger than that for one with polyhedra shape and exhibits low polarization degree, whereas that at 370-430 nm involving in 2 NIR photons for the former is much weaker than that for the latter and possesses high polarization degree. To our knowledge, this abnormal phenomenon is observed for the first time. This further approves this phenomenon that the stronger the relative UC emission related to 2 NIR photons, the weaker that dealt with 3 NIR photons.

 

Fig. 4 The UC emission spectra of the samples S8 and S3.

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As well known, the UC emission intensity is decided by several aspects inherited in UC particles ensembles, e.g. crystalline phase, dopant concentrations, shapes, sizes and surface properties [26–28]. The dopant concentration is constant, the solvent of crystal growth is NH₄NO₃ flux and the host matrix is tetragonal LiYF₄, thus, the effect of the host matrix, crystalline phases, ligands and dopant concentrations on UC emissions can be excludes as the most important factor. Here we will discuss the effect of particle shapes and sizes on the UC emission intensities. Each emission peak is dependent on optical emissions contributed from the dopant ions at the surface and in the interior of the particles. Compared to the interior dopant ions, the surface dopant ions could exhibit weak emission peaks owing to the quenching of the excitation energy by surface defects and impurities. Firstly, the mean distance between Y atoms along [001] crystal axis is smaller than that perpendicular to [001] crystal axis in LiYF₄ structure [18]. Thereby, the energy transfer between Yb³⁺ and Er³⁺ became more effective along the c-axis. If the distance between the active and sensitized ions is small, the energy transfer is effective. Secondly, the small particles have a large fraction of the surface defects as nonradiative recommendation centers, thus, exhibit the nonradiative relaxation probabilities. Thirdly, the intermediate levels of green and red UC emission are ⁴I_11/2 and ⁴I_13/2 and the populations of the ⁴I_11/2 and the ⁴I_13/2 levels decide the emission intensities of red and green light [29–31], respectively. According to the energy gap law [32], the nonradiative rate depends mainly on the energy gap between the excited state and the next lower level and the highest phonon energy of the host crystal. The smaller the energy gaps, the higher nonradiative transition rates. The energy gap between the ⁴I_13/2 and the ⁴I_11/2 (3600 cm⁻¹) is much smaller than between the ⁴I_13/2 and the ⁴I_15/2 (approximately 7000 cm⁻¹) [33]. Thereby, the nonradiative transition rate of ⁴I_11/2→ ⁴I_13/2 is larger than that of ⁴I_13/2→ ⁴I_15/2. As a consequence, the linear decay is dominant in the ⁴I_11/2 level and the total linear decay rate of ⁴I_11/2 is larger than UC rate. For ⁴I_13/2 level, the UC is the dominant depletion mechanism. The increase of sizes results in the decrease of the population ratio of I_11/2 to ⁴I_13/2 and the suppression the intensity of red light and enhance the green UC emission simultaneously.

Of the Er³⁺ ions excited to the ⁴F_7/2 state, some relax to the ground states ⁴I_15/2 and generate the red light. And some ⁴F_7/2 Er³⁺ ions are excited to the ⁴G_11/2 or ²H_9/2 energy level by the following energy transfer process: ⁴F_9/2(Er) + ²F_5/2(Yb)→⁴G_11/2/²H_9/2(Er) + ²F_7/2(Yb) and ⁴F_9/2(Er1) + ²I_11/2(Er2) → ⁴G_11/2/²H_9/2 (Er1) + ²I_15/2(Er2) [34,35]. Some ⁴G_11/2 or ²H_9/2 Er³⁺ ions relax to the lower energy levels ⁴F_5/2 and ⁴F_3/2 by multi-phonon nonradiative process. The materials with the small size have larger specific surface area and more surface defects, and thus the larger nonradiative rates, thus, more population on the ⁴G_11/2 energy level accumulate in the same experimental condition, accordingly, the relative intensity of UC emission belong to the transitions from the excited states ⁴G_11/2, ²H_9/2, ⁴F_5/2 and ⁴F_3/2 to the ground state ⁴I_15/2 becomes strong.

Polarized UC luminescence in rare-earth ions doped crystals has been paid considerable attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, biological imaging, and so forth since polarized light sources would largely enhance the efficiency and greatly improve contrast in general illumination due to the reduction in indirect glare [36,37]. To probe the polarization dependent UC phenomena, a half-wave-plate was placed before emission collection in the system. By rotating the half-wave-plate before the polarizer, polarized UC emission was checked as a function of the rotation angle. The excitation wavelength is 979 nm. Figure 5 depicted the polarized UC emission spectra of the sample S3 at room temperature. From this figure, it can be seen that the UC emission intensity varied with the change of the rotation angle. In the range of rotation angle from 0° to 90°, the UC emission intensity increases successively at 500-720 nm with the rotation angle from 35°, through 0° and 55°, to 90°, whereas that at 350-500 nm is on the contrary. The polarization degree ρ can be calculated by ρ = (I_max−I_min)/(I_max + I_min), where I_max and I_min present the maximum and minimum emission intensity. The polarization degrees of the transitions from ⁴G_11/2, ²H_9/2, ⁴F_3/2 + ⁴F_3/2, ⁴S_3/2,²H_11/2 and ⁴F_7/2 to the ground state ⁴I_15/2 were calculated to be 0.81, 0.75, 0.34, 0.04, 0.08 and 0.44, respectively, i.e. different energy transitions have different polarization degrees. What’s surprising, the polarization degree of ⁴G_11/2→⁴I_15/2, ²H_9/2→⁴I_15/2 transitions approaches to be 0.8 and that of ⁴S_3/2,²H_11/2→⁴I_15/2 is close to 0, indicating that the UC emission band of Er³⁺ from 455 to 670 nm with the involvement of 2 NIR photons exhibits low polarization degree, whereas that at 370-430 nm involving in 2 NIR photons possesses high polarization degree.

 

Fig. 5 The polarized UC emission spectra of the sample S3 at room temperatures (the excitation wavelength is 979 nm)

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Figure 6(a) and 6(b) depicts The UC emission spectra of the sample S3 at different temperatures under the excitation wavelength of 979 nm. The intensity of the UC emission band G1 attributed to the ²H_11/2→⁴I_15/2 Er³⁺ transition decreases monotonically as the temperature decreases and almost disappears at 10 K, which can be originated from detuning of the ground state absorption and resonant energy transfer ⁴I_13/2→⁴F_9/2 of Er³⁺ ions. However, the emission green UC band (G2) due to ⁴S_3/2→⁴I_15/2 of Er³⁺ transition and the red UC band (R) near 650 nm belonging to ⁴F_9/2→⁴I_15/2 of Er³⁺ transition exhibit different temperature dependent behaviors. With decreasing temperature the intensities of G2 and R bands increase initially, reach a maximum at 200 K and 158 K, respectively, and then decrease. Except for the above strong red and green UC peaks, as the temperature decreases from 300 K to 10 K, the UC emission bands in the region of 350-500 nm exhibit temperature dependence characteristics. When the temperature reduces to 200 K, the UC peaks corresponding to F_3/2,5/2→⁴I_15/2 transitions of Er³⁺ disappeared, the UC emission band attributed ⁴F_7/2→⁴I_15/2 transition becomes weak, whereas the UC emission bands due to ⁴G_11/2/²H_9/2→⁴I_15/2 transition of Er³⁺ reach maxima, similar to ⁴S_3/2→⁴I_15/2 transition of Er³⁺. Further decreasing the temperature to 158 K, the UC emission peaks due to ⁴G_11/2/ ⁴F_7/2→⁴I_15/2 transitions of Er³⁺ also are nearly not observed and the intensity of ²H_9/2→⁴I_15/2 transition of Er³⁺ becomes weak. Further increasing temperature, the UC emission intensity of ²H_9/2→⁴I_15/2 transition of Er³⁺ is weaker and weaker. A similar dependence was found in Er³⁺:YF₃ crystal [35], Er³⁺/Yb³⁺:NaYF₄ nanocrystals [38], and Er³⁺/Yb³⁺:Y₂O₃ nanocrystals [39].

 

Fig. 6 The UC emission spectra of the product S3 at different temperatures (the excitation wavelength is 979 nm).

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It should be noted that the transition from the ⁴F_9/2 level to either ⁴G_11/2 or ²H_9/2 level are nonresonant via excited state absorption or energy transfer from the neighboring excited state ⁴I_11/2 Er³⁺ or ²F_5/2 Yb³⁺ and must be mediated by phonons [35,39]. The mechanism that deals with ⁴F_9/2→⁴G_11/2 transition requires at least 1000 cm⁻¹ of energy from lattice to bridge this gap, while the other involving in the ⁴F_9/2→²H_9/2 transition requires approximately 1000 cm⁻¹ of energy to become dissipated. Therefore, the relative intensity of ⁴G_11/2→⁴I_15/2 transition is much weaker than that of ²H_9/2→⁴I_15/2 transition at room temperature and is not detected when the temperature decrease to 158 K..

The overall ingredient intensity of green and red UC emissions reduces slightly with the decreasing temperature, which can be due to the temperature-induced detuning the resonant levels of the erbium ions (⁴I_11/2) and the ytterbium ion (²F_7/2) [35]. The ingredient intensity ratio f(G/R) of green to red UC emission, corresponding to ²H_11/2/⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transition of Er³⁺, respectively, as shown in Fig. 6(c), varied from 0.02 to 1.7 in the studied temperature range . It increased very slowly at 10-150 K with increasing temperature, while the temperature increased to 300 K, it increased sharply and the maximum was not observed in the studied temperature range. As well known, the intensity ratio f(G/R) is a parameter for the correlation of these nonradiative processes in the Er³⁺-Yb³⁺doped system. The reduction in the emission below 158 K can be resulted from the decrease in resonant transfer because of the thermal depopulation of the higher-energy ⁴I_11/2 levels, which has a resonant match with the ⁴F_7/2 level. As the temperature increases from 158 to 300 K, the emission intensity decrease above 158 K can be originated from the temperature dependence of the multi-phonon de-excitation of the ⁴S_3/2(²H_11/2) and ⁴F_9/2 level, which increases significantly above 158 K. Since the transfer of the excitation to a single ion is most likely for a relatively tightly coupled pair and such pairs has the probability to relax back to ⁴I_J levels again, the quenching of the emission happens [35,39]. As the temperature increases, the quenching of the emission becomes more effective than there occurs for the excitation into ⁴F_7/2 of Er³⁺_, aggravating the quenching of the emission. For ⁴F_7/2, the emission is mainly from relatively isolated ions resulting in the reduction of the rate of ion-pair relaxation and the decrease of the UC emission [35,39].

4. Conclusions

The tetragonal LiYF₄:Er³⁺/Yb³⁺ micro-crystals were synthesized via the molten salt method in the NH₄NO₃ molten solution at 250 °C. The influences of the reaction temperature, the reaction time, the usage amounts of NH₄F on the crystal phase and shape evolution of the as-obtained products were studied and discussed at length. The long time and the high temperature are beneficial to transform from cubic NH₄Y₂F₇ to tetragonal LiYF₄. The UC emission band at 455- 670 nm for micro-sized bipyramids is much stronger than that at 370-430 nm for polyhedrals, whereas that is on the contrary. At the same time, the former and the latter exhibit different polarization degrees, i.e. low and high polarization degree, respectively. That is to say, the stronger the UC band with the involvement of 2 NIR photons emission, the weaker that involving in 2 NIR photons emission. When the temperature decreases from 300 to 10K, the UC emission intensities of the ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions for the micro-sized tetragonal bipyramids reach a maximum at 200 K and 158 K, respectively. This work is significant not only to synthesis other materials via molten salt method in NH₄NO₃ fluxes at low temperature (250°C) but also to understand the abnormal upconversion emission phenomenon.