The NH4NO3 molten solution system was developed to synthesize LiYF4:Er3+/Yb3+ micromaterials. The influences of the reaction temperature and time on the crystal phase and shape evolution were investigated in detail. The morphology, rotation angle and temperature dependences of the visible emission of Er3+ in LiYF4:Er3+/Yb3+ were measured under the excitation of 979 nm. The UC emission band from 455 to 670 nm with the involvement of 2 NIR photons for the sample with tetragonal bipyramid shape 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. Finally, the temperature sensitive visible upconversion fluorescence characteristics of Er3+ in tetragonal bipyramid-like LiYF4:Er3+/Yb3+ micromaterials at temperatures between 300 and 10 K are investigated and discussed in detail.
© 2014 Optical Society of America
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 , advanced lighting and displays [4,5], sensors [6,7], and biological labels .
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 2H11/2 + 4S3/2→4I15/2 transition) and 650 nm red light (due to the 4F9/2→4I15/2 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 2F5/2 matches with the excited energy level 4I15/2 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 . Recently, rare-earth doped LiYF4 nano/micro-crystals have been fabricated via hydrothermal method [11,12], thermal decomposition [13–16], high temperature solid state  and molten salt . 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 . 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 , 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.
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.
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 . 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  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 .
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 2H11/2,4S3/2→4I15/2 transitions of Er3+ ion , respectively. The strong red UC emission between 630 and 700 nm belongs to 4F9/2→4I15/2 transition of Er3+ ion (see Fig. 4(a)) . 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 4G11/2, 2H9/2, 4F3/2 + 4F3/2 and 4F7/2 excited state to the ground state 4I15/2, respectively . 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.
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  crystal axis is smaller than that perpendicular to  crystal axis in LiYF4 structure . 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 4I11/2 and 4I13/2 and the populations of the 4I11/2 and the 4I13/2 levels decide the emission intensities of red and green light [29–31], respectively. According to the energy gap law , 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 4I13/2 and the 4I11/2 (3600 cm−1) is much smaller than between the 4I13/2 and the 4I15/2 (approximately 7000 cm−1) . Thereby, the nonradiative transition rate of 4I11/2→ 4I13/2 is larger than that of 4I13/2→ 4I15/2. As a consequence, the linear decay is dominant in the 4I11/2 level and the total linear decay rate of 4I11/2 is larger than UC rate. For 4I13/2 level, the UC is the dominant depletion mechanism. The increase of sizes results in the decrease of the population ratio of I11/2 to 4I13/2 and the suppression the intensity of red light and enhance the green UC emission simultaneously.
Of the Er3+ ions excited to the 4F7/2 state, some relax to the ground states 4I15/2 and generate the red light. And some 4F7/2 Er3+ ions are excited to the 4G11/2 or 2H9/2 energy level by the following energy transfer process: 4F9/2(Er) + 2F5/2(Yb)→4G11/2/2H9/2(Er) + 2F7/2(Yb) and 4F9/2(Er1) + 2I11/2(Er2) → 4G11/2/2H9/2 (Er1) + 2I15/2(Er2) [34,35]. Some 4G11/2 or 2H9/2 Er3+ ions relax to the lower energy levels 4F5/2 and 4F3/2 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 4G11/2 energy level accumulate in the same experimental condition, accordingly, the relative intensity of UC emission belong to the transitions from the excited states 4G11/2, 2H9/2, 4F5/2 and 4F3/2 to the ground state 4I15/2 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 = (Imax−Imin)/(Imax + Imin), where Imax and Imin present the maximum and minimum emission intensity. The polarization degrees of the transitions from 4G11/2, 2H9/2, 4F3/2 + 4F3/2, 4S3/2,2H11/2 and 4F7/2 to the ground state 4I15/2 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 4G11/2→4I15/2, 2H9/2→4I15/2 transitions approaches to be 0.8 and that of 4S3/2,2H11/2→4I15/2 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.
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 2H11/2→4I15/2 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 4I13/2→4F9/2 of Er3+ ions. However, the emission green UC band (G2) due to 4S3/2→4I15/2 of Er3+ transition and the red UC band (R) near 650 nm belonging to 4F9/2→4I15/2 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 F3/2,5/2→4I15/2 transitions of Er3+ disappeared, the UC emission band attributed 4F7/2→4I15/2 transition becomes weak, whereas the UC emission bands due to 4G11/2/2H9/2→4I15/2 transition of Er3+ reach maxima, similar to 4S3/2→4I15/2 transition of Er3+. Further decreasing the temperature to 158 K, the UC emission peaks due to 4G11/2/ 4F7/2→4I15/2 transitions of Er3+ also are nearly not observed and the intensity of 2H9/2→4I15/2 transition of Er3+ becomes weak. Further increasing temperature, the UC emission intensity of 2H9/2→4I15/2 transition of Er3+ is weaker and weaker. A similar dependence was found in Er3+:YF3 crystal , Er3+/Yb3+:NaYF4 nanocrystals , and Er3+/Yb3+:Y2O3 nanocrystals .
It should be noted that the transition from the 4F9/2 level to either 4G11/2 or 2H9/2 level are nonresonant via excited state absorption or energy transfer from the neighboring excited state 4I11/2 Er3+ or 2F5/2 Yb3+ and must be mediated by phonons [35,39]. The mechanism that deals with 4F9/2→4G11/2 transition requires at least 1000 cm−1 of energy from lattice to bridge this gap, while the other involving in the 4F9/2→2H9/2 transition requires approximately 1000 cm−1 of energy to become dissipated. Therefore, the relative intensity of 4G11/2→4I15/2 transition is much weaker than that of 2H9/2→4I15/2 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 (4I11/2) and the ytterbium ion (2F7/2) . The ingredient intensity ratio f(G/R) of green to red UC emission, corresponding to 2H11/2/4S3/2→4I15/2 and 4F9/2→4I15/2 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 4I11/2 levels, which has a resonant match with the 4F7/2 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 4S3/2(2H11/2) and 4F9/2 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 4IJ 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 4F7/2 of Er3+, aggravating the quenching of the emission. For 4F7/2, 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].
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 4S3/2→4I15/2 and 4F9/2→4I15/2 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.
This work was supported by the National Natural Science Foundation of China (No.60808033 and 61204003), Train Object Program of Jiangxi Province Young Scientists, Natural Science Foundation of Jiangxi Province (No. 2011BAB206029), Luodi program of Jiangxi Province (No. KJLD13030), and the excellent young academic talent program of Jiangxi University of Finance and Economics, respectively.
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