Enhanced Upconversion Photoluminescence of LiYF₄: Yb³⁺/Ho³⁺ Crystals by Introducing Mg²⁺ Ions for Anti-Counterfeiting Recognition

Abstract

By doping appropriate lanthanide ions, LiYF₄ as a host luminescent material can simultaneously exhibit bright visible-light emission. A series of LiYF₄:Yb³⁺/Ho³⁺ microparticles with different Mg²⁺ doping concentrations were synthesized and investigated. The crystal structure of the synthesized microparticles was tested by X-ray diffraction (XRD). Notably, a significant increase in the upconversion photoluminescence intensity of upconversion microparticles (UCMPs) was obtained by introducing Mg2+ ions under 980 nm laser excitation, and achieved a maximum level when the concentration of Mg²⁺ ions was 8 mol%. Additionally, the practicality of the resultant UCMPs used as the raw material of anti-counterfeiting ink was systematically investigated. These results prove that the Mg²⁺-doped LiYF₄:Yb³⁺/Ho³⁺ are very promising as screen-printing materials for anti-counterfeiting recognition labels.

1. Introduction

Compared with other luminescent materials (e.g., organic dye, quantum dots, etc.), rare-earth-doped luminescent materials have many advantages, such as sharp emission and long lifetime, good photochemical and thermal stability, etc. [1]. The luminescent properties of lanthanide-doped fluoride materials are mainly subject to the host matrix, sensitizers, and activators [2,3,4,5]. Hence, optimal host–dopant combinations are necessary for an efficient UC luminescence process. It is universally known that the UC efficiency of rare-earth ions depends largely on the phonon energy of the matrix. LiYF₄ is also considered to be an ideal alternative method, which is likely to inhibit non-radiative relaxation due to its low phonon energy [6]. In addition, the LiYF₄ host can simultaneously exhibit bright visible-light emission by doping with appropriate lanthanides. Herein, LiYF₄ was selected as the host material, and the combination Yb³⁺/Ho³⁺ was used as a sensitizer/activator to increase the excitation of the photons.

The doping of non-rare-earth ions is regarded as a promising approach for the improvement of UC [6,7,8,9,10]. Zhou et al. reported the enhancement of green upconversion luminescence in tetrahedral LiYF₄:Yb/Er nanoparticles by doping with Mn²⁺ ions [6]. Li et al. synthesized NaLnF₄ UCNPs via a simple M²⁺ (Mg²⁺, Co²⁺) doping method [7]. Yang et al. synthesized Mg²⁺-doped hexagonal-phase NaGdF₄:Yb³⁺, Er³⁺ nanocrystals; the results show that the intensity of UC emission increased significantly as a result of Mg²⁺ doping [8]. The results show that overall UCL is enormously enhanced after doping with Mg²⁺ (≈27.5-fold). Ding et al. synthesized Li⁺-doped β-NaGdF₄:Ln³⁺ nanocrystals, and a significant enhancement of UC emission in the β-NaGdF₄:Ln³⁺ nanocrystals was observed [9]. Ding et al. synthesized a Mn⁺-doped core–shell–shell nanoarchitecture, and the NIR-to-red upconversion emission was efficiently promoted [10]. Dopant-enhanced upconversion nanoparticles (UCNPs) have attracted great interest for anti-counterfeiting for their specific UC luminescence property of emitting higher-efficiency shorter wavelengths via near-infrared (NIR) excitation [11,12,13,14,15].

In this work, a series of LiYF₄:Yb³⁺/Ho³⁺ UCMPs with different Mg²⁺ doping concentrations were successfully prepared via hydrothermal synthesis methods. In LiYF₄ crystals, because the radius of Mg²⁺ (0.72 Å) ions is less than that of Y³⁺ (0.90 Å), the incorporation of Mg²⁺ ions replaces some of the Y³⁺ ions or fills the crystal gap, resulting in the shrinkage and expansion of the main lattice and the asymmetry of local crystal field. The symmetry of the LiYF₄ local crystal field is changed by the doping with Mg²⁺ ions, so as to affect the UC luminescence efficiency of the product [16,17]. The crystal structure and photoluminescence intensity of LiYF₄:Yb³⁺/Ho³⁺ UCMPs were characterized. Finally, the fluorescence imaging experiment was carried out. The imaging results prove that the prepared UCMPs can be applied to screen printing.

2. Experimental Section

Mg²⁺-ion-doped LiYF₄:Yb³⁺/Ho³⁺ micron crystals were prepared via hydrothermal synthesis. The specific preparation process was as follows: First, 1 mmol ethylenediaminetetraacetic acid (EDTA) powder was dissolved in 20 mL of deionized water in a beaker and stirred continuously under the auxiliary condition of constant-temperature heating at 60 °C to form solution A. Secondly, 1 mmol rare-earth nitrate was dissolved in 10 mL of deionized water and subjected to ultrasonic treatment for 10 min to form a mixed solution B. The rare-earth nitrate included Y(NO₃)₃·6H₂O, Mg(NO₃)₂·6H₂O, Yb(NO₃)₃·5H₂O, and Ho(NO₃)₃·5H₂O, with the proportions of 0.79-x, x, 0.2, and 0.01, respectively. X is the doping amount of Mg²⁺. Solution A and solution B were mixed and stirred well. A mixed solution of 20 mmol LiF and 5 mmol NH₄F was dissolved in water to form solution C (5 mL). Finally, solution C was dropped slowly into the mixed solution and continuously stirred for about 30 min to form a milky white liquid. Finally, the mixture was put into the autoclave, and maintained for 48 h at 230 °C. The products were washed with a mixture of deionized water and ethanol (volume, 1:1) and centrifuged several times. Then, they were dried in a vacuum-drying oven at 60 °C for 12 h and, finally, the samples were ground and collected.

The X-ray diffraction (XRD) patterns of the products demonstrated their crystal structure and phase identification, and were recorded using a BRUKER D8 ADVANCE type X-ray diffractometer with Cu Kα radiation (λ is 0.1546 nm). The elemental content of the sample was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5110), see Table S1. A power-adjustable 980 nm laser diode (0 to 2W) was selected as a pump light source. The photoluminescence spectra were obtained using a Hitachi F-4500 fluorescence spectrophotometer (1.0 nm for spectral resolution (FWHM) of the spectrophotometer and 400 V for PMT voltage) at room temperature. The camera employed for taking photographs in the imaging experiments was a high-performance CCD camera produced by Andorra, UK.

3. Results and Discussion

The XRD results of LiYF₄:Yb³⁺/Ho³⁺ UCMPs with different Mg²⁺ doping concentrations are shown in Figure 1. The concentrations of LiYF₄:Yb³⁺/Ho³⁺ UCMPs with Mg²⁺ ion doping were 0 mol%, 4 mol%, 6 mol%, 8 mol%, 10 mol%, and 12 mol%. The main diffraction peaks were 100, 110, 101, 200, 111, 201, 210, 002, 300, 211, 102, 112, 220, 311, 312, and 302. The diffraction peaks of the LiYF₄:Yb³⁺/Ho³⁺ UCMPs generally coincided with the PDF standard card (PDF: 17-0874).There were no other miscellaneous peaks, even with high-concentration Mg²⁺ ion doping, indicating that pure UCMPs were prepared.

As a result, Figure 1a shows the XRD pattern of Mg²⁺-doped LiYF₄:Yb³⁺/Ho³⁺ around 19°. As seen from the local magnification of the XRD images, the diffraction peak shifts to the right with the increase in Mg²⁺ concentration. The angular movement of the main diffraction peak reaches the limit when the Mg²⁺ doping concentration is 8 mol%. When the concentration of Mg²⁺ ions exceeds 8 mol%, the diffraction peak of the nanocrystal starts to move to the left again.

In LiYF₄ crystal, because the radius of Mg²⁺ (0.72 Å) ions is less than that of Y³⁺ (0.90 Å), the incorporation of Mg²⁺ ions replaces some of the Y³⁺ ions or fills the crystal gap, resulting in the shrinkage and expansion of the main lattice and the asymmetry of the local crystal field. When the doping concentration of Mg²⁺ ions is 0–8 mol%, the crystal shrinks, and the main diffraction peak of XRD moves to the right. When the doping concentration of Mg²⁺ ions is greater than 8 mol%, the lattice structure expands, and the main diffraction peak begins to move to the left. The symmetry of the LiYF₄ local crystal field is changed by the doping of Mg²⁺ ions, so as to affect the UC luminescence efficiency of the product [16,17].

2dsinθ = nλ

(1)

In the Bragg’s law formula, λ, d, and θ are the diffraction wavelength, the distance between planes, and the diffraction angle, respectively. According to Formula (1), when a certain amount of Mg²⁺ ions are added, some Mg²⁺ ions will enter the lattice gap of LiYF₄, increasing the distance d between planes and reducing the diffraction angle θ. As the concentration of Mg²⁺ ions decreases, the LiYF₄:Yb³⁺/Ho³⁺ lattice expands, so the XRD moves to a low angle.

As shown in Figure 2, the LiYF₄:Yb³⁺/Ho³⁺ UCMPs with different Mg²⁺ impurity concentrations were tested by SEM. In Figure 2, the samples have good crystallinity, and the morphology of the samples is similar to a biconical octahedron structure. The morphology is regular and the dispersion is mainly uniform. The average size of the microcrystals is about 55 μm in length and 30 μm in width under different Mg²⁺ ion doping concentrations.

The photoluminescence was investigated as a function of the change in Mg²⁺ doping concentration. The LiYF₄:Yb³⁺/Ho³⁺ UCMPs with Mg²⁺ doping concentrations of 0%, 4%, 6%, 8%, 10%, and 12% were selected for fluorescence spectroscopy tests.

Figure 3 shows the photoluminescence spectra of different Mg²⁺ doping concentrations of LiYF₄:Yb³⁺/Ho³⁺UCMPs under 980 nm laser excitation. As shown in Figure 3, the photoluminescence intensity increased continuously as the Mg²⁺ doping concentration changed from 0 mol% to 8 mol%. The photoluminescence intensity reached the maximum as the Mg²⁺ doping concentration reached 8 mol%. However, when the Mg²⁺ doping concentration was greater than 8 mol%, the photoluminescence intensity gradually weakened with the increase in Mg²⁺ doping concentration. Compared with the samples without Mg²⁺ doping, the UC luminescence intensity of LiYF₄:Yb³⁺/Ho³⁺ UCMPs with a doping concentration of 8 mol% was increased by about fourfold. It was thus proven that the adding Mg²⁺ ions into LiYF₄:Yb³⁺/Ho³⁺ micron crystals can enhance the upconversion luminescence intensity effectively. Because the incorporation of Mg²⁺ ions changes the symmetry of the crystal field, the crystal field distortion also affects the UC luminescence efficiency of Ho³⁺ ions, improving the UC luminescence performance of the LiYF₄:Yb³⁺/Ho³⁺ crystals [7,8].

The energy level transition diagram of Mg²⁺-doped LiYF₄:Yb³⁺/Ho³⁺ under the irradiation of 980 nm lasers is shown in Figure 4. After the Yb³⁺ ions absorb laser energy, they first transition from the ²F_7/2 energy level to the ²F_5/2 energy level, followed by cross-relaxation and energy transfer with Ho³⁺ions. Then, a series of radiative transitions of Ho³⁺ ions are formed through the energy transfer (ET₁) process, and through energy transfer (ET₂, ET₃) caused by cross-relaxation processes (CR₁ and CR₂) between two close Yb³⁺ and Ho³⁺ ions [18]. This results in the green UC emission of Ho³⁺ ions at 535 nm (⁵F₄→⁵I₈) and 543 nm (⁵S₂→⁵I₈), the red UC emission of Ho³⁺ at 638 nm (³K₈, ⁵F₂, ⁵F₃→⁵I₇) and 657 nm (⁵F₅→⁵I₈), and near-infrared UC emission of Ho³⁺ ions at 750 nm (⁵F₄, ⁵S₂→⁵I₇).

The red–green light intensity ratio is an essential standard to evaluate the luminescence performance of luminescent materials. Figure 5a shows the changes in the upconversion red–green light intensity ratio (R/G = I_657nm/I_543nm) of LiYF₄:Yb³⁺/Ho³⁺ UCMPs doped with different Mg²⁺ ion concentrations. With the change in Mg²⁺ ion concentration, the conversion of the ratio of red light to green light of UC luminescence is only about 10%, indicating that the UC luminescence of the sample has good monochromaticity, and is affected only slightly by the concentration of Mg²⁺ ion doping. In Figure 5b, the visible light colors of LiYF₄:Yb³⁺/Ho³⁺ with different concentrations of Mg²⁺ ion doping are exhibited in the CIE color coordinates (without interference by UV/IR light). The coordinates of LiYF₄:Yb³⁺/Ho³⁺ UCMPs with different concentrations Mg²⁺ ion doping were (0.2653, 0.5731), (0.2809, 0.5792), (0.2977, 0.5563), (0.2844, 0.6024), (0.2990, 0.6127), and (0.3151, 0.6204), respectively.

In order to further study the UC luminescence enhancement effect of Mg²⁺ ions on LiYF₄:Yb³⁺/Ho³⁺ UCMPs, the UC luminescence lifetimes at 543 nm and 657 nm of LiYF₄:Yb³⁺/Ho³⁺ UCMPs doped with and without Mg²⁺ ions were measured under 980 nm laser excitation. Then, the corresponding fluorescence lifetimes were calculated by fitting the test results. As shown in Figure 6a,b, the UC fluorescence lifetime of LiYF₄:Yb³⁺/Ho³⁺ UCMPs without Mg²⁺ ion doping at 543 nm was 551.6 μs, and the UC fluorescence lifetime at 657 nm was 361.438 μs. As shown in Figure 6c,d, the UC fluorescence lifetime of 8 mol% Mg²⁺-ion-doped LiYF₄:Yb³⁺/Ho³⁺ UCMPs at 543 nm was 662.824 μs, and the UC fluorescence lifetime at 657 nm was 639.58 μs. This shows that the UC fluorescence lifetime at 543 nm and 657 nm is increased by about 20% and 70%, respectively, due to doping with Mg²⁺ ions. These results demonstrate that doping with Mg²⁺ ions can significantly enhance the fluorescence lifetime of the samples.

Figure 7a shows the screen-printing process of making anti-counterfeiting characters. Firstly, LiYF₄:Yb³⁺/Ho³⁺ micron crystal particles doped with a Mg²⁺ ion concentration of 8 mol% were selected as the raw material for anti-counterfeiting ink. The ground LiYF₄:Yb³⁺/Ho³⁺/Mg²⁺ micron crystal powder was evenly mixed with mesh metal ink to make anti-counterfeiting ink, which was prepared at a ratio of 5:1 (samples:ink). Finally, the prepared anti-counterfeiting ink was poured onto the screen-printing template, and then brushed with a scraper. The specific anti-counterfeiting characters were successfully printed on the base material below the screen-printing template, and the upconversion luminous anti-counterfeiting characters were obtained.

Screen printings were made on various materials, including paper, plastic sheet, and glass sheet. The photographs of the imaging experiments are shown in Figure 7b. All characters’ patterns on different materials were almost invisible under daylight conditions. However, the as-printed characters on various materials became green and clearly visible under the excitation of the 980 nm laser. This shows that the LiYF₄:Yb³⁺/Ho³⁺ UCMPs with different Mg²⁺ ion doping have good application potential in anti-counterfeiting recognition.

4. Conclusions

In summary, LiYF₄:Yb³⁺/Ho³⁺ UCMPs with different Mg²⁺ ion doping concentrations were synthesized and investigated. The emission peaks of Ho³⁺ ions were 480 nm, 535 nm, 543 nm, 638 nm, 657 nm, and 750 nm under the excitation of a 980 nm laser when different concentrations of Mg²⁺ ions were incorporated. The photoluminescence intensity reached its maximum when the Mg²⁺ ion doping concentration was 8 mol%. The prepared UCMPs were incorporated into a screen-printing agent. Characters were printed on different materials using the engraved screen-printing template. The printed characters were clearly visible under the irradiation of a 980 nm laser. The synthesized LiYF₄:Yb³⁺/Ho³⁺/Mg²⁺ UCMPs have potential application prospects in anti-counterfeiting recognition.