Enhanced and Stable Upconverted White-light Emission in Ho3+/Yb3+/Tm3+-doped LiNbO3 Single Crystal via Mg2+ Ion Doping

A strategy to enhance the upconversion white-light intensity via Mg2+ ion doping was demonstrated in Ho3+/Yb3+/Tm3+/LiNbO3 single crystal. It is found Mg2+ ion doping affects the crystal field symmetry around RE3+ ions and enhance the upconversion emission intensity. Bright white-light is obtained when the Mg2+ ion concentration is 0.5 mol% in the melt. And the CIE coordinates are hardly changed with Mg2+ ion doping. In addition, the upconversion mechanism is discussed in detail. It is observed the longer lifetimes of intermediate levels result in the lower upconversion photon numbers, which are beneficial to the upconversion process. Therefore, Mg2+ ion doped Ho3+/Yb3+/Tm3+/LiNbO3 single crystals would have potential applications in stable white-light devices and photoelectric instruments.

In this article, LiNbO 3 single crystal was used as host material, its lower phonon energy guarantees the higher upconversion efficiency. Mg 2+ ion was introduced into Ho 3+ /Yb 3+ /Tm 3+ tri-doped LiNbO 3 single crystals due to its small ionic radius. Here, we represent a new strategy to improve the properties of upconversion white-light emission. Under 980 nm excitation, the influences of Mg 2+ ion on the intensity and color tunability of upconversion white-light emission were demonstrated and the rational explanation was given. Preferably, the multi-function of LiNbO 3 single crystal will create sufficient conditions for opening up new perspectives to the studies of integration and tiny devices. Table 1 presents the molar compositions of cations in the melt or crystal for Mg 2+ doped Ho 3+ /Yb 3+ /Tm 3+ / LiNbO 3 crystals. It can be seen that the Mg 2+ and RE 3+ ions are introduced into the crystals successfully. With increasing Mg 2+ ion concentrations in the melt, Mg 2+ ion concentrations in the crystals are increased evidently. However, the total concentrations of RE 3+ ions in the crystals are decreased slightly with increasing Mg 2+ ion concentrations, which could be considered as unchanged. In addition, Fig. 1(a) shows the powder XRD patterns of pure LiNbO 3 and Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystals doped with different Mg 2+ ion concentrations. As shown, all the diffraction peaks of the samples can be well indexed to the standard LiNbO 3 phase (JCPDS file no. 20-0631), no secondary phases were identified. It can be concluded that the doping ions do not alter the phase structure of host material, and the ionic radius differences between doping ions and host ions result in the variations of diffraction peak intensities. To further investigate the effect of Mg 2+ ions on the structure of Ho 3+ /Yb 3+ / Tm 3+ /LiNbO 3 single crystal, the main diffraction peak is amplified, as shown in Fig. 1(b). In general, the RE 3+ ions with larger ionic radius (r Ho 3+ = 0.89 Å, r Yb 3+ = 0.86 Å, r Tm 3+ = 0.87 Å, r Li + = 0.68 Å, r Nb 5+ = 0.69 Å) will enter into the LiNbO 3 crystals in the form of lattice substitution. So the host lattice is expanding, which could lead to the shift of the main diffraction peak towards smaller angle, as shown in Fig. 1(b). However, Mg 2+ ions with smaller ionic radius (r Mg 2+ = 0.66 Å) may enter into the crystals in the form of lattice substitution or interstitial. For lattice substitution, the host lattice shrinking is induced since the ionic radius of Mg 2+ ion is smaller than that of Li + or Nb 5+ ions, corresponding to the shift of the main diffraction peak towards larger angle. By contrast, for the interstitial, the host lattice expanding occurs, resulting in the shift of the main diffraction peak towards smaller angle. Based on the above mentioned, the site occupancy of Mg 2+ ion is mainly indentified by the shift of the main diffraction peak. It can be seen from Fig. 1(b) that with increasing Mg 2+ ion concentrations, the main diffraction peak shifts gradually towards smaller angles, which means the lattice is expanding and the Mg 2+ ions enter into the crystals in the form of interstitial. As a result, the occupation of interstitial site for Mg 2+ ion can tailor the local crystal field around RE 3+ ions in the host lattice, which will affect its luminescence properties.

Results and Discussion
The upconversion emission spectra of Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystals doped with various Mg 2+ ion concentrations under 980 nm excitation are shown in Fig. 1(c). As shown, the blue emission has a luminescence peak at 477 nm that corresponds to Tm 3+ : 1 G 4 → 3 H 6 transition; the green emission band centered around 549 nm is contributed to Ho 3+ : 5 S 2 , 5 F 4 → 5 I 8 transition; and the red emission has luminescence peaks at 652 nm and 665 nm originating from Tm 3+ : 1 G 4 → 3 F 4 transition and Ho 3+ : 5 F 5 → 5 I 8 transition, respectively. From Fig. 1(c), it is observed that the intensities of blue, green and red emissions increase first and then decrease with increasing the Mg 2+ ion concentrations. The optimum Mg 2+ ion concentration is 0.5 mol% in the melt, corresponding to 0.66 mol% in the single crystal. In our case, we argue that Mg 2+ ion with small ionic radius can be doped into the host lattice easily in the form of interstitial according to the XRD results, and this will break the symmetry of the crystal field around the rare earth ions. If the rare earth ions are placed at a low symmetry site, the forbidden transitions will be weakened, leading to the enhancement of upconversion emission. The similar phenomena are also found in the research reports, such as refs 22,23 . But when the Mg 2+ ion concentration is above optimum concentration, the doped ions may cause the lattice distortion around the rare earth ions, resulting in the quenching of the upconversion emission. To investigate the color tunability, Fig. 1(d) shows the CIE coordinates of Ho 3+ / Yb 3+ /Tm 3+ /LiNbO 3 single crystals doped with various Mg 2+ ion concentrations. It can be seen that the CIE coordinates of samples undoped and doped Mg 2+ ion are located in the white-light region basically. Moreover, the CIE coordinates have the trend of shift towards green/red region first and then tend to shift towards blue region with increasing the Mg 2+ ion concentrations. But it should be noted that it shows little color tunability under Mg 2+ ion doping. So the Mg 2+ ion doped Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystals may be suitable for making the non-tunable white-light display devices.
To analyze the possible white-light upconversion mechanism in Mg 2+ ions doped Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystal, the dependences of upconversion emission intensities on pump powers are measured under 980 nm excitation, as shown in Fig. 2 Yb 3+ /Tm 3+ /LiNbO 3 single crystal are 2.30, 1.78, and 1.63, respectively. It can be obtained that the blue emission is a three-photon process, the green and red emission are two-photon processes. When the Mg 2+ ion concentration in the melt is 0.5 mol%, the slopes of blue, green, and red emissions are 1.52, 1.02, and 0.96, respectively. And when the Mg 2+ ion concentration in the melt reaches up to 4.0 mol%, the above values are 1.65, 1.14, and 1.01, respectively. As known, the slopes deviating from the integer values (3 or 2 or 1) are attributed to the competition between the linear decay and the upconversion processes for the depletion of the intermediate excited states and the local thermal effect 24,25 . These results indicate that the blue emission is a two-photon process, the green and red emission are one-photon processes with Mg 2+ ion doping. As a supplement, the luminescence decay behaviors of Ho 3+ : 5 I 6 → 5 I 8 (λ em = 1150 nm), Ho 3+ : 5 I 7 → 5 I 8 (λ em = 2000 nm), and Tm 3+ : 3 F 4 → 3 H 6 (λ em = 1800 nm) are investigated, as shown in Fig. 3. It can be easily seen that all the emission intensities decay exponentially. The double-exponential is adopted to fit the experiment data using the equation (1)  The obtained lifetime values are shown in Table 2. It can be observed that the lifetime values of intermediate levels are increased with Mg 2+ ion doping, resulting in the decrease of upconversion photon numbers. We believe that the longer lifetime of intermediate level is beneficial to the upconversion process, leading to the stronger upconversion emission intensity. But when the Mg 2+ ion concentration in the melt is too high (4.0 mol% in this article), the lattice distortion plays an important role in upconversion process, so the upconversion luminescence is not emitted effectively though the lifetime of its intermediate level is long, the upconversion emission intensity is decreased consequently. Furthermore, the decay curve analysis of the excited levels involved in the following upconversion emission processes Tm 3+ : 1 G 4 → 3 H 6 (λ em = 477 nm), Ho 3+ : 5 F 4 , 5 S 2 → 5 I 8 (λ em = 550 nm), and Ho 3+ : 5 F 5 → 5 I 8 (λ em = 665 nm) are performed and calculated, the obtained lifetime values are shown in Table 1. It is suggested that the proper Mg 2+ ion incorporation modifies the crystal field and results in the fast emitting of upconversion luminescence, and hence the upconversion emission intensity is enhanced. But the excessive Mg 2+ ion concentration is detrimental to the enhancement of upconversion emission intensity.
The Schematics of populating and upconversion luminescence processes for the blue, green and red emissions in the Mg 2+ ions doped Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 system under 980 nm excitation have been described in Fig. 4. From Fig. 4, it can be seen that Yb 3+ ions act as sensitizers to absorb laser photons and transfer their energy to Ho 3+ and Tm 3+ ions effectively. Through two successive energy transfer processes from Yb 3+ ions to Ho 3+ ions, the 5 F 4 , 5 S 2 levels and 5 F 5 levels of Ho 3+ ions are populated, which generate the upconversion green and red emissions, respectively. Similarly, through three successive energy transfer processes from Yb 3+ ions to Tm 3+ ions, the upconversion blue emissions and weak red emissions are obtained originating from the 1 G 4 → 3 H 6 and 1 G 4 → 3 F 4 transitions of Tm 3+ ions. In the upconversion processes, Mg 2+ ion is not the luminescent ion. The doping Mg 2+ ions can impact the lifetimes of excited levels, and further influence the upconversion emission intensity.

Conclusion
Mg 2+ -doped Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystals have been successfully prepared by Czochralski method. Bright upconversion white-light emission is achieved under 980 nm excitation at room temperature. It is found that Mg 2+ and RE 3+ ions could not alter the phase structure of host material, Mg 2+ ions enter into the single crystals in the form of interstitial. The intensities of upconversion emissions are increased firstly and decreased subsequently with increasing Mg 2+ ion concentrations. The optimum Mg 2+ ion concentration is 0.5 mol% in the melt. The red, green and blue emissions in this system can be ascribed to Ho 3+ : 5 F 5 → 5 I 8 , Tm 3+ : 1 G 4 → 3 F 4 ; Ho 3+ : 4 S 2 , 5 F 4 → 5 I 8 and Tm 3+ : 1 G 4 → 3 H 6 transitions, respectively. The research results indicate that Mg 2+ ion doping could     To grow crystals with good quality, the following optimum growth conditions were selected: the temperature gradient above the melt was 25 °C/mm, the pulling rate was 0.2 mm/h, and the seed rotation rate was 28 rpm. After growth, the crystals were cooled down to room temperature at a speed of 30 °C/h. For phase structure analyses, the samples were grinded into powder using an agate mortar. And for optical tests, Y-cut plates of the samples were cut and polished.
Data availability statement. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
Characterization. The Inductively Coupled Plasma Mass Spectrometry (ICP-MS) with Optima-7500 type was used to measure the mass fractions of Mg 2+ ions and rare earth ions (RE 3+ ) in the single crystals. To identify the crystallization phase, X-ray diffraction spectra of Mg 2+ ion doped Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystals were measured by an XRD-6000 diffractometer using a copper Kα radiation source. The upconversion luminescence spectra were recorded by Zolix-SBP300 grating spectrometer equipped with CR131 photomultiplier under 980 nm excitation. In the measurement of luminescence decay dynamics, the continuous wave from 980 nm laser diode was tuned into pulsing by a signal generator, and the luminescence decay curves were measured by a digital phosphor oscilloscope (Tektronix DPO 4140). The CIE chromaticity coordinate for the upconversion fluorescence of Mg 2+ -doped Ho 3+ /Yb 3+ /Tm 3+ /LiNbO 3 single crystal was calculated based on the 1931 CIE standard and marked in the CIE standard chromaticity diagram.