Simultaneous enhancement of upconversion luminescence and photoelectric effect in NaYF4:Yb3+/Tm3+/Zr4+ tri-doped microcrystals

β-NaYF4: Yb3+/Tm3+/Zr4+ microcrystals are synthesized by a facile hydrothermal route. The up-conversion (UC) luminescence and photoelectric effects are enhanced simultaneously with the introduction of Zr4+ ions under IR excitation. With the Tm3+ ions fluorescence spectra under 980 nm excitation, the blue and red emissions reached the highest intensity in the case of 4 mol% Zr4+ doping. Taking the advantage of Zr4+ stimulates the free electrons transfer from the valence band (VB) to the conduction band (CB) in β-NaYF4:Yb3+/Tm3+/Zr4+ microcrystals, the photocurrent reached the highest intensity in the case of 4 mol% Zr4+ doping at 780 nm excitation. The β-NaYF4: Yb3+/Tm3+/Zr4+ microcrystals with both enhanced UC luminescence and photoelectric properties are highly potential to contribute in photovoltaic and infrared sensors application.


Introduction
In the last decade, upconversion (UC) photoluminescence (PL) materials have gained increasing attention for their ability to absorb multiple low-energy long-wave radiations, to achieve the infrared to visible light conversion [1][2][3][4][5]. Among the reported materials, rare earth doped UC compounds have the advantages of high luminescence efficiency. Furthermore, the emission wavelength by rare earth materials is tunable and reliable. These characteristics have been utilized in bioimaging, multidimensional displays, anti-counterfeit sensors and biomedical diagnostics applications [6][7][8][9][10][11]. As a result, the enhancement of UC emission is one of the main research focuses in the photonics industry. Some proven techniques of UC emission enhancement including internal microstructure or changing the outside environment, combination of non-lanthanide ions, controlling the content of sensitizers and activators, and the creation of core-shell architectures [12][13][14][15][16].
In year 2012, Professor Cheng's group successfully increased the red and green light intensity of Er 3+ ions by 47 times and 23 times respectively through the thermal decomposition method [17]. Two year later, Sun's team has demonstrated the incorporation of modulating additives and employment of a shell layer to prevent energy loss during UC, and achieved the tunable excitation and emission wavelengths simultaneously [18].
In addition to the remarkable PL, it is worth mentioning that such UC materials may exhibit exceptional photoelectric properties. For instance, Zhang's group has improved the intensity of both UC green emissions of Er 3+ ions and photoelectric properties by introducing non-lanthanide doping in the NaGdF 4 matrix [19]. Besides, Bu's group has successfully controlled multiple color luminescence and photoelectric effect simultaneously by modifying morphological shapes of Na 1.23 Ca 0.12 Y 1.28 Er 0.24 F 6 phosphors. In addition, they have also discussed the mechanism of photoelectric effect [20]. Furthermore, Yan's group successfully discovered that the internal photoelectric effect of NaYbF 4 @TiO 2 core-shell structure under 980 nm excitation, which is attributed to excitation energy migration from the NaYbF 4 : Tm 3+ core to the TiO 2 shell [21]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
In this study, we successfully doped non-lanthanide Zr 4+ ions into NaYF 4 matrix and synthesized Tm 3+ /Yb 3+ co-doped NaYF 4 microcrystals using a hydrothermal method. Under 980 nm excitation, and the Zr 4+ doping concentration of 4 mol%, the highest luminescence intensity is achieved in ultraviolet, blue bands, and near infrared (NIR) region. Meanwhile, the photocurrent reached the highest intensity with 4 mol% Zr 4+ doping at 780 nm excitation. These dual-function NaYF 4 microcrystals play an important role in further studies of UC luminescence materials doped with non-lanthanide Zr 4+ ions.

Experimental details 2.1. Materials
The chemicals used in the experiments were of analytical grade and were not further purified. The microcrystals NaYF 4 : Yb/Tm/Zr (20/0.5/x mol%, x = 0, 2, 4, 6, 8, 10) were synthesized by a hydrothermal method. Firstly, mixed 0.1461 g EDTA with 8 ml deionized water and agitate for 30 min Next, 0.02, 0.04, 0.06, 0.08, 0.1) were added to the EDTA liquid. After 30 min of vigorous stirring, 0.1680g NaF and 8 ml deionized water were slowly dropped into the mixed solution followed 30 min stirring. The semi-clear solution was then reheated in a 30 ml teflon-lined stainless steel autoclave at 180°for 12 h. Finally, the desired microcrystals were obtained by precipitation and centrifugation.

Characterization
To determine the crystallization phase of the samples, XRD measurements were performed with a powder diffractometer (Model Rig-aku RU-200b), using Ni-filtered Cu Ka radiation (λ = 1.5406 Å). The size and shape of the samples were checked using a SEM (KYKY1000B). Power-adjustable laser diode (CW 980 nm) was used as the UC source of pumps. The emission spectra of UC luminescence were recorded by Hitachi F-7000 fluorescence spectrometer. Measurements of impedance spectra and photocurrents of the prepared samples were performed on an SP300 electrochemical workstation (Gamry biologic). The workstation for measuring the photoelectric effect is a three-electrode workstation where a Pt foil and an Ag/AgCl saturation are used as working and comparison electrodes. The measurements were carried out with an oscillation of 0.005V under 780 nm light source.

Results and discussion
The XRD patterns of NaYF 4 : 20 mol% Yb 3+ , 0.5 mol% Tm 3+ , x mol% Zr 4+ (x = 0, 2, 4, 6, 8, 10) microcrystals are shown in figure 1. The diffraction peaks from the samples are all consistent with the standard card of hexagonal NaYF 4 (JCPDS 00-028-1192) without impurity peaks. According to the diffraction peak amplification region observed in figure 1, the positions of the diffraction peaks shift slightly with the Zr 4+ ions doping concentration increasing. Based on the Bragg's law, the larger the lattice constant, the smaller the diffraction angle, and vice versa [22]. Because the radius of the Zr 4+ (0.084 nm) is less than the radius of the Y 3+ ions (0.101 nm), when Y 3+ ions is replaced by smaller Zr 4+ , inter-planar distance decreases, at the same time, diffraction angle increases. When the doping concentration of Zr 4+ ions increases from 0 mol% to 4 mol%, this diffraction peak moves toward a larger angle. The doping concentration of Zr 4+ ions continues to increase and the possibility of Zr 4+ ions occupying the interstitial sites increases, causing the main lattice to expand, which will counteract some of the contraction, and then moves backward when the Zr 4+ ions concentration continues to increase to 6 mol% [23].
To determine the element composition and elemental contents of NaYF 4 : 20 mol% Yb 3+ , 0.5 mol% Tm 3+ , x mol% Zr 4+ (x = 0, 2, 4, 6, 8, 10) microcrystals, elemental mapping and EDS spectra were used to confirm the existence of Na, Y, F, Yb, Tm and Zr elements in NaYF 4 matrix. According to figures 3(a)-(f), all the elements are shown in NaYF 4 : 20 mol% Yb 3+ , 0.5 mol% Tm 3+ , 4 mol% Zr 4+ microcrystals. However, due to the different concentrations of the added elements, the distribution densities of F, Y and Na are higher than those of Yb, Tm and Zr. It was further demonstrated that the Zr 4+ ions were successfully doped into the NaYF 4 matrix.
The resistance curves of doped Zr0 and Zr0.04 are shown in figure 8. Alternating current impedance spectroscopy has the advantages of wide frequency range and low perturbation to the system, which is a fast and non-destructive measurement method [26]. The reduced semicircle shows the reduction of the operating electrode resistance, indicating that the resistance of Zr0.04 is lower than that of Zr0. Correspondingly, the NaYF 4 : Yb/Tm/Zr microcrystals with 4 mol% Zr 4+ ions doping achieved the highest photocurrent value.

Conclusions
In summary, we have confirmed that the UC luminescence intensity and photoelectric effect of β-NaYF 4 :Yb 3+ /Tm 3+ /Zr 4+ microcrystals were enhanced simultaneously after the introduction of Zr 4+ ions. The formation of homogeneous β-NaYF 4 : Yb 3+ /Tm 3+ /Zr 4+ microcrystals has been verified based on XRD analysis and SEM observation. The red and blue emission intensities and optoelectronic properties of the 4 mol% Zr 4+ doped NaYF 4 microcrystals reached the highest under IR light excitation. The prepared β-NaYF 4 : Yb 3+ /Tm 3+ /Zr 4+ microcrystals are important reference for further studies of UC materials doped with nonlanthanide Zr 4+ ions.

Acknowledgments
This work was supported by the Jilin Province Education (JJKH20220665KJ).

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Notes
The authors declare no competing financial interest.