1. Introduction

Upconversion is the conversion of low energy photons to higher energy photons [1]. In recent years upconversion luminescence materials have attracted more and more attention because of being well used for three-dimensional solid-state multicolor display, photonics applications and solar cells [2–7]. Matrix material plays an important role in the rear earth (RE) ions doped upconversion materials, and the main researches are focus on fluoride and oxide in the past few years. Fluoride has low phonon energy, great optical transmission from ultraviolet to infrared, and good upconversion efficiency. But it also has some obvious disadvantages such as high cost, poor chemical stability, complicated preparing processes and pollutions to the environment. Compared with fluoride, oxide has great mechanical strength, better physical and chemical stability. In addition, the costs of manufacturing and raw materials are usually low [8–12]. Rare-earth (RE) ions are suitable for the upconversion emissions because of their long lifetime excited states, abundant energy levels, and excellent chemical durability. In recent years, many RE ions codoped systems such as Pr³⁺/Er³⁺/Yb³⁺, Er³⁺/Yb³⁺/Tm³⁺, and Ho³⁺/Yb³⁺/Tm³⁺ have been used for realizing the white-light upconversion emissions [13,14].

12CaO·7Al₂O₃ (C12A7) is a kind of new transparent material with wide band gap. The unit cell containing two molecules of C12A7 is represented as [Ca₂₄Al₂₈O₆₄]⁴⁺(O^2-)₂. The first part denotes a positively charged framework constructed by 12 crystallographic cages with an inner diameter of 0.4nm, and the second part represents two extra framework oxygen ions entrapped in two different cages as counteranions. They can be replaced with other anions such as OH^- and F^-.The micro cage structures make the lattice be able to contain many outer impurities which apply great opportunities on the doping modification of C12A7. After reduction treatments with Ti metal, the superconducting transition (Tc) has been clearly observed. The value of Tc varies in the range of 0.14-0.4K with the concentration of anionic electrons(Ne.) [15–20]. The highest phonon energy of C12A7 is about 787cm⁻¹ which is lower than that of LiNbO₃(880cm⁻¹) [21]. It suggests that C12A7 may be more suitable for realizing upconversion luminescence. Unique optical and electrical properties indicate that C12A7 have broad application prospects in many applications such as multicolor displays and other instrument integration.

2. Experimental

C12A7 single crystal with concentrations (mol%) of 0.4Ho³⁺/2.0Yb³⁺/0.5Tm³⁺ is grown by Czochralski method. Starting materials of CaCO₃(99.99%), Al₂O₃(99.99%), Ho₂O₃(99.99%), Yb₂O₃ (99.99%) and Tm₂O₃(99.99%) are fully ground for 6 h. After that the well mixed powders are pressed into a disk under 18Mpa, then sintered 10h at 1300°C in a corundum crucible. The sintered powders are melted in an Ir crucible under the situation of 1450°C and a 2.5% oxygen-containing nitrogen atmosphere. The crystal is very slowly pulled up from the melt at a rate of 0.5-0.8mm/h with the rotation rate of 1.0-2.5.0min⁻¹.

The prepared single crystal was checked by transmission electron microscopy (TEM). One piece of the crystal, about 1 mm thick, was cut off from the bulk. Then it was polished with abrasive paper carefully up to 50 micron thick. Double sides were dimpled before ion milling with high voltage of 4 kV. TEM observation was performed in the TEM (FEI Tecnai F30) operating at 300 kV. X-ray Diffraction (XRD) spectra of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺ codoped C12A7 single crystal were measured by an XRD-6000 diffractometer with Cu Kα radiation source. The upconversion luminescence spectra were recorded by a SPEX1000M spectrometer with a photomultiplier tube under 980nm excitation, and the spectrometer was calibrated on spectral sensitivity by a standard lamp. The luminescence decay curves are measured by the 980nm diode laser modulated through square-wave electric current and recorded by Tektronix DOP 4140 oscilloscope. When measuring the luminescence decay curves for the infrared emissions, the light signal was detected by InGaAs photodiode. CIE chromaticity coordinate for the upconversion fluorescence of Ho³⁺/Yb³⁺/Tm³⁺/C12A7 single crystal was calculated based on the 1931 CIE standard. All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows one piece of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 crystal and the selected area electron diffraction patterns along [113]. The sample is transparent, and the color is orange. It is because that the Ir crucible is oxidized during the crystal growth process, the color is from the incorporation of Ir⁴⁺ which will occupy the Ca²⁺ sites in the crystal [22]. The regular points array in the electron diffraction patterns show clearly that the specimen is absolutely single crystal.

 

Fig. 1 Digital photo of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 crystal and the election diffraction patterns of the crystal along[113].

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One sample of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal is fully ground to powder for about 4 hours. Figure 2 shows the X-ray diffraction patterns of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 powder and the JCPDS Card (09-0413). Compared with the JCPDS Card, there is hardly any impure peak on the XRD pattern of C12A7 doped with rare earth. The characteristic diffraction peaks are consistent with those on the JCPDS Card. No new phase appears, it suggests that the doping Ho³⁺, Yb³⁺ and Tm³⁺do not change the structure of C12A7. Moreover, considering the radius of doping rare earth ions(~0.1nm) which is greater than that of Al³⁺(~0.054nm) and equals to that of Ca²⁺(~0.1nm) [23], Ca²⁺ lattice sites in C12A7 nanocage should be occupied by the doping rare earth ions [24,25].

 

Fig. 2 X-ray diffraction patterns of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 powder and the JCPDS Card of C12A7(09-0413).

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Figure 3 shows the upconversion emission spectra of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/ C12A7 single crystal under 980nm excitation of 600mW, 700mW and 800mW respectively. As shown in the figure, there is a blue upconversion emission at 475nm which corresponds to the transition: ¹G₄→³H₆ of Tm³⁺. The green emission centered at 545nm is contributed to the transition: ⁵F₄/⁵S₂→⁵I₈ of Ho³⁺. The 650nm red emission is from the Tm³⁺:¹G₄→³F₄ transition, and the 660nm red light is corresponding to the transition of ⁵F₅→⁵I₈ of Ho³⁺. It also shows that the intensities of blue, green and red emissions are all enhanced with the increase of excitation power. But the enhancement extents of green and red emissions are less than the blue one, it is because of the different photons processes during the upconversion emissions which will be analyzed in the following parts.

 

Fig. 3 Upconversion emission spectra of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal under 980nm excitation of 600mW, 700mW and 800mW at room temperature.

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In order to show the true color of the upconversion luminescence, the CIE color coordinates for the upconversion emissions of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal under 980nm excitation of different pump powers are calculated. These calculations are using the following formulas:

where X, Y and Z are the three tristimulus values. The tristimulus values for a color with a spectral power distribution $P(\lambda )$are given by

where$\lambda$is the wavelength of the equivalent monochromatic light, and$x^{\prime }(\lambda )$, $y^{\prime }(\lambda )$and $z^{\prime }(\lambda )$are three color-matching functions [26].

The color coordinates (x,y) of multicolor upconversion emissions in 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 sample are shown in Fig. 4. They are about (0.40, 0.39), (0.39, 0.38), (0.38, 0.34), (0.35, 0.33), (0.34, 0.30) and (0.31, 0.28), which correspond to the laser pump powers of 650mW, 700mW, 750mW, 800mW, 850mW, and 900mW, respectively. The color coordinate at 800mW laser pump power is (0.35, 0.33) which is very close to the white center (0.33, 0.33), and bright white-light emission can be easily observed by naked eyes during the test. As the pump powers increasing, the CIE coordinates have the trend of moving from the white-light region to the blue-light region, it is because the blue emission is a three-photons process, which is higher than that of green and red emissions. By simply adjusting the excitation power, the color of upconversion light can be changed. With this power dependent color tuning property, Ho³⁺/Yb³⁺/Tm³⁺/C12A7 single crystal is expected to have broad application prospects in multicolor displays.

 

Fig. 4 Calculated color coordinates for the uponversion emissions of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal under 980nm excitation at various pump power

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Figure 5 shows the pump power dependence of blue, green and red upconversion emissions of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal. It is well known that the upconversion emission intensity depends on the excitation power according to the following formula:

 

Fig. 5 Dependence of upconversion intensities on excitation powers for 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal under 980nm excitation

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Figure 6 shows the luminescence decay dynamics of ³F₄→³H₆ (Tm³⁺) in 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal at room temperature. The decay data are fitted as double exponential equation as follows [28]:

 

Fig. 6 Decay dynamics of the ³F₄ level of Tm³⁺ ions in 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal.

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Where I(t)is the luminescence intensity at t moment,${\tau }_s$,${\tau }_f$ are slow and fast components of fluorescence lifetime. A_s, A_f stand for the weight factor of the two components respectively, and I₀ is luminescence intensity of background. The exponential lifetime is calculated by the following equation:

The exponential lifetime is 2.1ms for the ³F₄→³H₆ of Tm³⁺ ($\lambda$ = 1800nm).

In order to explain the luminescence dynamics better, the rate equations of Tm³⁺-Yb³⁺ system under steady-state excitation are set up as follows:

Where N_b, and W stand for the population density of excited state Yb³⁺ at any time, and the energy transition rate from Yb³⁺ to Tm³⁺, respectively. N_i (i = 0-3) is the number of Tm³⁺ on level i. R_i (i = 1-3) represents the total decay rate of Tm³⁺ on level i.

If the lifetime of ³F₄ (Tm³⁺) is long enough, the linear decay term R₁ can be neglected. Then from the Eq. (6)-(9), $N_1\approx N_0\propto P^0$,$N_2\propto P^1$ and $N_3\propto P^2$. Therefore the slopes of blue and red emissions from Tm³⁺ should be close to 2. If the lifetime of ³F₄ (Tm³⁺) is short, R₁ cannot be neglected. Then $N_1\propto P^1$,$N_2\propto P^2$ and $N_3\propto P^3$ which result in that the slopes of blue and red emissions from Tm³⁺ should be close to 3. The exponential lifetime of Tm³⁺ at ³F₄ state is 2.1ms which is not long, it must have some influences on the linear decay term R, so the slopes of blue and red emissions from Tm³⁺ should be between 2 and 3.

Figure 7 shows the luminescence decay dynamics of ⁵I₆→⁵I₈ (Ho³⁺) and ⁵I₇→⁵I₈ (Ho³⁺) in 0.4Ho³⁺/2.0Yb³⁺/0.5Tm³⁺/C12A7 single crystal at room temperature. The exponential lifetime are deduced to be 0.094ms for the ⁵I₆→⁵I₈ of Ho³⁺ ions ($\lambda$ = 1150nm) and 2.76ms for the ⁵I₇→⁵I₈ of Ho³⁺ ions ($\lambda$ = 2000nm) .

 

Fig. 7 Decay dynamics of the ⁵I₆ level of Ho³⁺ ions (a) and ⁵I₇ level of Ho³⁺ ions (b) in 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal.

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The rate equations of Ho³⁺-Yb³⁺ system under steady-state excitation are set up as follows:

where N_b denotes the population density of excited state Yb³⁺ at any time, W is the energy transition rate from Yb³⁺ to Ho³⁺ and β_ij stands for the nonradiative relaxation rate from level i to j. N_i (i = 0-4) represents the population of Ho³⁺ on level i. R_i(i = 1-4) is the total decay rate of Ho³⁺ on level i.

The green emission has only one upconversion path. Based on the Eq. (10),(12),(14) and the upconversion decay dynamic of the ⁵I₆→⁵I₈ of Ho³⁺ ions, the lifetime of ⁵I₆(Ho³⁺) level is 0.094ms which is so short that the linear decay term or the upconversion term in Eq. (12) cannot be neglected. If the upconversion process (⁵I₆→⁵F₄/⁵S₂) is the dominant depletion mechanism of level${N^{\prime }}_2$, the linear decay term in Eq. (12) can be neglected which results in ${N^{\prime }}_2\propto P^0$and ${N^{\prime }}_4\propto P^{{}_{{}^1}}$. So the slopes of green emissions from Ho³⁺ should be approach to 1. In contrast, if the (⁵I₆→⁵I₈) process is the dominant depletion mechanism of level ${N^{\prime }}_2$, the upconversion term in Eq. (12) can be neglected. Then from Eq. (10) and (12), we know that ${N^{\prime }}_2\propto P^1$, and from Eq. (14) that ${N^{\prime }}_4\propto P^2$. So the slopes of green emissions from Ho³⁺ should be approach to 2 as a consequence. Considering the two consequences, the slopes of green emission for Ho³⁺ are between 1 and 2.

The red light is the combination of Tm³⁺:¹G₄→³F₄ transition and Ho³⁺:⁵F₅→⁵I₈ transition. According to the same analysis method, the slope of red emission should be between 2 and 3 from Tm³⁺, and should be between 1 and 2 from Ho³⁺. So the red emissions are a combination of two and three photons processes.

Figure 8 shows the upconversion mechanisms of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal under 980nm excitation. The Yb³⁺ is the sensitizers to absorb laser photons in the system of Ho³⁺/Yb³⁺/Tm³⁺. For the blue emission, Yb³⁺ absorbs a 980nm photon by Ground State Absorption(GSA) to finish the transition from state ²F_7/2 (N_a)to ²F_5/2(N_b), the Tm³⁺ in the ground state is excited to the ³H₅ level by energy transition of neighboring Yb³⁺. Tm³⁺ in ³H₅ state relaxes non-radioactively to ³F₄ level. Then Tm³⁺ is excited to ³F_2/3 level by photon assisted energy transition, and relaxes non-radioactively again to ³H₄ level. The Tm³⁺ in ³H₄ level is excited to ¹G₄ level by photon assisted energy transition, and finally relaxes to the ground state ³H₆ which generates blue emission at 475nm.

 

Fig. 8 The upconversion mechanisms of 0.4%Ho³⁺/2.0%Yb³⁺/0.5%Tm³⁺/C12A7 single crystal under 980nm excitation.

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For the green emissions, the Ho³⁺ in the ground state is excited to the ⁵I₆ level by the energy transition of neighboring Yb³⁺. Ho³⁺ in ⁵I₆ level is excited to ⁵F₄/⁵S₂ by absorbing energy from one photon or another Yb³⁺ ion in ²F_5/2 level, and finally the transition of ⁵F₄/⁵S₂ to ⁵I₈ of Ho³⁺ emits green light of 545nm.

The red emissions contain two parts. For the Tm³⁺/Yb³⁺ system, Tm³⁺ in ¹G₄ level relaxes to ³F₄ level and emits red light at 650nm. For the Ho³⁺/Yb³⁺ system, there are two possible ways. The first possibility is Ho³⁺ in ⁵I₆ state relaxes non-radioactively to ⁵I₇ state, then Ho³⁺ in ⁵I₇ state is excited to ⁵F₅ by absorbing energy from one photon or another Yb³⁺ ion in ²F_5/2 state, then the transition of ⁵F₅ to ⁵I₈ of Ho³⁺ emits red light of 660nm. The second way is as following, after two energy transition processes, Ho³⁺ is excited to the ⁵F₄/⁵S₂ state, Ho³⁺ in the ⁵F₄/⁵S₂ state relaxes non-radioactively to ⁵F₅ state, finally the transition of ⁵F₅ to ⁵I₈ of Ho³⁺ emits red light of 660nm.

4. Conclusions

Ho³⁺/Yb³⁺/Tm³⁺ codoped 12CaO·7Al₂O₃ single crystal exhibiting upconversion white light under 980nm excitation have been successfully prepared by Czochralski method. CIE coordinate of 0.4mol%Ho³⁺/2.0mol%Yb³⁺/0.5mol%Tm³⁺/C12A7 single crystal under 980nm excitation of 800mW is (0.35, 0.33), which is very close to the white emission center (0.33, 0.33). 0.4mol%Ho³⁺/2.0mol%Yb³⁺/0.5mol%Tm³⁺/C12A7 single crystal has a power-dependent color tuning property which can be achieved by simply changing the excitation power. The red, blue and green emissions can be ascribed to Tm³⁺:¹G₄→³F₄, Ho³⁺: ⁵F₅→⁵I₈; Tm³⁺:¹G₄→³H₆ and Ho³⁺:⁵F₄/⁵S₂→⁵I₈, respectively. The pump dependence and luminescent decay dynamics suggest that the blue upconversion emissions are populated by a three-photon process, the green emissions are a two-photon process, and the red emissions are a combination of two and three photons processes. The upconversion mechanism is analyzed in detail. C12A7 single crystal with white-light upconversion emissions has broad application prospects in solid-state multicolor display and instrument integration.