The effect of Er3+ concentration on the kinetics of multiband upconversion in NaYF4:Yb/Er microcrystals

In Yb-Er co-doped upconversion (UC) nanomaterials, upconversion luminescence (UCL) can be modulated to generate multiband UCL emissions by changing the concentration of activator Er³⁺. Nonetheless, the effect of the Er³⁺ concentrations on the kinetics of these emissions is still unknown. We here study the single β-NaYF₄:Yb³⁺/Er³⁺ microcrystal (MC) doped with different Er³⁺ concentrations by nanosecond time-resolved spectroscopy. Interestingly, different Er³⁺ doping concentrations exhibit different UCL emission bands and UCL response rates. At low Er³⁺ doping concentrations (1 mol%), multiband emission in β-NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%) MCs could not be observed and the response rate of UCL was slow (5–10 μs) in β-NaYF₄:Yb³⁺/Er³⁺. Increasing the Er³⁺ doping concentration to 10 mol% can shorten the distance between Yb³⁺ ions and Er³⁺ ions, which promotes the energy transfer between them. β-NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%) can achieve obvious multiband UCL and a quick response rate (0.3 µs). However, a further increase in the Er doping concentration (80 mol%) makes MCs limited by the CR process and cannot achieve the four-photon UC process (⁴F_5/2 → ²K_13/2 and ²H_9/2 → ²D_5/2). Therefore, the result shows that changing the Er³⁺ doping concentration could control the energy flow between the different energy levels in Er³⁺, which could affect the response time and UCL emission of the Yb/Er doped rare earth materials. Our work can facilitate the development of fast-response optoelectronics, optical-sensing, and display industries.

1 Introduction

Lanthanide-doped nanomaterials have been widely studied in optics and biology due to luminescence multi-modulation at the micro-nano scale. Lanthanide-based upconversion (UC) microcrystals (MCs) transform infrared photons with low energy into high-energy photons (ultraviolet or visible), and have thus been increasingly used in laser (Chen et al., 2016), photovoltaics (Liang et al., 2013), storage (Zheng et al., 2018), and anticounterfeiting (Li et al., 2022). β-NaYF₄ is the most efficient UC material lattice because of low phonon energy. Co-doping typical sensitizer Yb³⁺ and activator Er³⁺ in NaYF₄ achieves multi-photons UC and exhibits multiband emission under saturated excitation (Yuan et al., 2018; Frenzel et al., 2021); this has potential application in super-resolution (Liu et al., 2017), optical multiplexing (Lu et al., 2013), and display (Zhang et al., 2015; Gao et al., 2020). Recent studies have demonstrated that the luminescence properties of upconversion nanoparticles (UCNPs) can be effectively enhanced (Han et al., 2014) or modulated by changing the structure (Wang et al., 2014), doping concentration (Wen et al., 2018), and surface modification (Zhou B et al., 2020), thus extending the application of UCNPs. In particular, the increasing study of highly doped Er³⁺ ions has shown that a high degree of energy migration between them occurs to suppress the effect of concentration quenching upon surface coating (Chen et al., 2017). The self-sensitization of Er ions has also been reported to directly achieve the modulation of energy transfer within UCNPs (Zhou J et al., 2020). These suggest that highly doped UCNPs are important for luminescence modulation.

High irradiation power density conditions allow the highly doped β-NaYF₄:Yb³⁺/Er³⁺ MCs to exhibit stronger emissions. Huang et al. achieved nearly white luminescence in a single β-NaYbF₄: Er³⁺ (2 mol%) MC; they studied the effect of Yb³⁺ ion doping concentration on UC kinetics (Huang et al., 2022b). For the UC system, the Yb³⁺ ion is the absorption antenna of the photon, and the Er³⁺ ion is the luminescence center of the MCs. The doping concentration of the Er³⁺ ion will directly affect the luminescence characteristics of the MCs. However, the effect of Er³⁺ concentration on UC kinetics still seems unknown. Moreover, an advanced setup is needed to better understand the kinetics of β-NaYF₄:Yb³⁺/Er³⁺ systems with different Er³⁺ doping concentrations at nanosecond timescale. Optical trapping time-resolved photoluminescence spectroscopy (OT-TRPLS) combines a nanosecond pulsed laser (976 nm), an optical tweezer (with 1342 nm laser), and an advanced time-resolved photoluminescence device (Huang et al., 2022a). It has the ability to measure the nanosecond timescale transient spectra by trapping a single MC to deter the effect resulting from the MC movement and its interactions with other surrounding MCs.

In this paper, we investigate the effect of Er³⁺ concentration on the multiband emissions in the β-NaYF₄:Yb³⁺/Er³⁺ MCs which are optically trapped by our OT-TRPLS platform. The kinetics were quantificationally resolved on the basis of the time-resolved spectra. Interestingly, under the 20 mol% condition of Yb³⁺ doping concentration, different Er³⁺ doping concentrations modulated MCs to exhibit different kinetic processes and response rates of upconversion luminescence (UCL). NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%) populated the characteristic ²P_9/2 energy level of Er³⁺ ion (the energy level corresponding to a four-photon absorption) as fast as ∼0.1 μs after the excitation. In contrast, β-NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%) needed a timescale of 5–10 m after the excitation. However, further increasing the doping Er³⁺ concentration in β-NaYF₄:Yb³⁺/Er³⁺ MCs, MCs cannot achieve the four-photon UC process. The time-evolution spectra of NaYF₄:Er 10 and 80 mol% MCs show that single doping Er³⁺ ions cannot significantly improve the response rate of the UCL due to the smaller absorption cross section of Er³⁺ near 976 nm. The result indicates that increasing the doping concentration of Er³⁺ in β-NaYF₄:Yb³⁺/Er³⁺ shortens the distance between Yb³⁺ and Er³⁺, thus changing the rate at which Yb ions transfer energy to Er ions. In addition, the cross-relaxation (CR) of highly doping Er ions will make the MCs unable to achieve the four-photon UC process.

2 Experimental

2.1 Materials

Yb³⁺, Er³⁺, and Y³⁺ were provided by ytterbium (III) chloride (YbCl₃‧6H₂O, 99.99%), erbium (III) chloride (ErCl₃‧6H₂O, 99.99%), and yttrium (III) chloride (YCl₃‧6H₂O, 99.99%) from Aladdin Industrial Corporation. Sodium fluoride (NaF), Na₂-ethylenedia-metracetic acid (EDTA-2Na), and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2 Preparation of β-NaYF₄:Yb³⁺/Er³⁺ MCs

NaYF₄:Yb³⁺/Er³⁺ MCs were prepared by the hydrothermal method. To ensure a total rare-earth-ion content of exactly 1 mmol, YbCl₃‧6H₂O, ErCl₃‧6H₂O, and YCl₃‧6H₂O of different qualities from the specific doping ratio were dissolved in the deionized water (22 mL). Thereafter, ultrasonic stirring occurred until the solution became transparent; 1 mmol EDTA-2Na was then added and stirred into the mixed solution (30 min). Then, 10 mmol NaF was added and stirred for another 30 min until the solution became colloidal. Subsequently, the mixtures were transferred to a hydrothermal reactor for 24 h annealing at 160°C and then cooled. The final product was thrice washed with ethanol and deionized water.

2.3 Sample characterization

NaYF₄:Yb³⁺/Er³⁺ MCs of the β crystalline phase were characterized by scanning electron microscopy (SEM). The hexagonal MCs were about 2 μm in length (Figures 1A–E). An X-ray diffractometer with Cu K radiation was employed at 40 kV and 200 mA (Rigaku) to record the X-ray diffraction (XRD) patterns of the MCs. The XRD data in the 2θ range from 10° to 70° were collected at a 10° min⁻¹ scanning rate. Based on the Joint Committee on Powder Diffraction Standards (JCPDS), it was demonstrated that the prepared NaYF₄ MCs are β-phase crystalline (Figure 1F).

FIGURE 1

FIGURE 1. (A–E) SEM micrographs of β-NaYF₄:Yb/Er MCs. (F) XRD patterns of the measured β-NaYF₄:Yb³⁺/Er³⁺ MCs compared to the standard JCPDS file no.16-0334.

2.4 Photoluminescence measurements

We used OT-TRPLS to measure the time evolution spectra. As previously mentioned, optical tweezers (OT) ensured the stability and isolation of the sample as well as avoiding any undesired interaction by the sample with its surroundings. The 1342 nm excitation laser works as OT for the low absorption coefficients of both Er³⁺ and Yb³⁺ ions at this wavelength. The optical trapping process was observed using a color-complementary metal oxide semiconductor (CMOS). The peak excitation power density was ∼0.32 GW cm⁻², and the pulse width of the laser was 15 ns. The time evolution spectra were captured using an ICCD camera. A 976 nm ns-pulsed laser source was utilized in OT-TRPLS under the control of digital delay generator (DDG) at a repetition rate of 37 Hz (see Figure 2).

FIGURE 2

FIGURE 2. Setups for UCL detection with the self-built OT-TRPLS device. M1, planar reflective silver mirror; M2, polarization beam-combining mirror (for a 976 nm laser); M3, beam-combining mirror (for a 976 nm laser and 1342 nm laser); M4, dichroic mirror for short-wave pass and the cutoff wavelength is 900 nm; M5, 50:50 beam-splitter. Objective 1 is an oil lens (1.3 NA, 100x). Objective 2 has 10 × magnification with .25 NA for lighting focusing. All lasers were coaxial.

3 Results and discussion

We captured the time evolution spectra of optically trapped NaYF₄:Yb³⁺/Er³⁺ MCs after the 976 nm ns-pulsed laser excitation (see Figure 3). Our results indicate that NaYF₄:Yb³⁺/Er³⁺ MCs show sequent UCL emissions at different wavelengths and that different Er³⁺ doping concentrations can modulate UCL emissions. The main emissions are 522 nm, 542 nm, 410 nm, 558 nm, and 654 nm for NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%) MCs within 1 μs after excitation (see Figure 3A). In the timescale from 5 to 10 µs, NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%) MCs show weak 402 nm and 470 nm emissions (see Figure 3A). Increasing the Er³⁺ doping concentration to 10 mol%, NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%) MCs show multiband UCL emissions within 0.3 µs, such as 522 nm, 542 nm, 410 nm, 558 nm, 654 nm, 505 nm, 384 nm, 496 nm, 430 nm, 482 nm, 617 nm, 654 nm, 456 nm, 443 nm, 575 nm, 585 nm, and 640 nm. However, when the concentration of Er³⁺ ions is increased to 80 mol%, NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%) MCs only show partly multiband UCL emissions. The concentration of doping Er³⁺ could effectively change the UCL emission under 976 nm ns-pulsed excitation.

FIGURE 3

FIGURE 3. NaYF₄:Yb³⁺/Er³⁺ evolution spectrum of Yb³⁺/Er³⁺(20/x mol%) under 976 nm pulse excitation. (A, B) x = 1, (C) x = 10, and (D) x = 80. The 976 nm ns-pulsed excitation starts from the 0 moment.

Next, we focused on the major UCL peaks; previous studies (Cheng et al., 2002; Sardar et al., 2003; Wegh et al., 2003; Chen et al., 2007; O'Shea et al., 2007; Yuan et al., 2018; Frenzel et al., 2021) indicate energy levels of Er³⁺ ions and their corresponding emission wavelengths (Figure 4. Pathway A (⁴I_15/2 → ⁴I_11/2 → ⁴F_7/2 → ²H_11/2 →⁴S_3/2 → ²G_7/2 → ⁴G_11/2 → ²H_9/2 → ⁴F_5/2 → ²K_13/2) and pathway B (⁴I_15/2 → ⁴I_11/2 → ⁴I_13/2 → ⁴F_9/2 → ²H_9/2 → ²D_5/2) are two main channels where energy levels are populated to achieve UCL emission. (Anderson et al., 2014; Jung et al., 2015; Yuan et al., 2018; Frenzel et al., 2021).

FIGURE 4

FIGURE 4. Transition mechanism of NaYF₄:Yb³⁺/Er³⁺ (20/x mol%) with different Er³⁺ doping concentrations under 976 nm pulse excitation. The 976 nm ns-pulsed excitation starts from the 0 moment. Green part is the populating process. Red part is the radiative transition process at x = 1. Blue part is the radiative transition process at x = 10. Purple part is the radiative transition process at x = 80.

For NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%), the UCL at 542 nm (⁴S_3/2→⁴I_15/2) and 522 nm (²H_11/2→⁴I_15/2) emits within 0.2 μs of the 976 nm ns-pulsed excitation. The aforementioned emissions originated from ⁴I_15/2 → ⁴I_11/2 → ⁴F_7/2 → ²H_11/2 →⁴S_3/2 of pathway A. In the timescale from 0.2 μs to 0.4 μs after excitation, the emergence of 505 nm (⁴G_11/2 → ⁴I_13/2) and 384 nm (⁴G_11/2 → ⁴I_15/2) demonstrates that ⁴G_11/2 has accumulated electron population by ⁴I_15/2 → ⁴I_11/2 → ⁴F_7/2 → ²H_11/2 →⁴S_3/2 → ²G_7/2 → ⁴G_11/2 from pathway A. Then, in the timescale from 0.4 μs to 0.7 μs after excitation, the emergence of 410 nm (²H_9/2 → ⁴I_15/2) and 558 nm (²H_9/2 → ⁴I_13/2) demonstrates that ²H_9/2 has accumulated an electron population by ⁴G_11/2 → ²H_9/2. Although the ²H_9/2 energy level could be populated by ⁴I_15/2 → ⁴I_11/2 → ⁴I_13/2 → ⁴F_9/2 → ²H_9/2 of pathway B, no obvious electron population accumulated at the ⁴F_9/2 energy level. The 654 nm (⁴F_9/2 → ⁴I_15/2) is not observed at this time. In the timescale from 0.7 μs to 1 μs after excitation, the emergency of 654 nm emission indicates that an ⁴F_9/2 energy level has accumulated an electron population. However, in the timescale from 5 μs to 10 μs after excitation, the emergence of 402 nm (²P_3/2 → ⁴I_13/2) and 470 nm (²P_3/2 → ⁴I_11/2) emissions demonstrates the occurrence of the four-photon process from pathway A (⁴I_15/2 → ⁴I_11/2 → ⁴F_7/2 → ²H_11/2 →⁴S_3/2 → ²G_7/2 → ⁴G_11/2 → ²H_9/2 → ⁴F_5/2 → ²K_13/2). The four-photon process from pathway B is not observed by UCL, such as 430 nm (⁴G_7/2 → ⁴I_11/2). It can be concluded through data and analysis that pathway A is more effective than B in NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%).

For NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%), the UCL at 542 nm (⁴S_3/2→⁴I_15/2), 522 nm (²H_11/2→⁴I_15/2), 496 nm (⁴F_7/2 → ⁴I_15/2), 505 nm (⁴G_11/2 → ⁴I_13/2), and 384 nm (⁴G_11/2 → ⁴I_15/2) emits within 0.1 μs after 976 nm ns-pulsed excitation, which originated from pathway A. In the timescale from 0.1 μs to 0.2 μs after excitation, the emergence of 410 nm (²H_9/2 → ⁴I_15/2) 558 nm (²H_9/2 → ⁴I_13/2), 402 nm (²P_3/2 → ⁴I_13/2), 470 nm (²P_3/2 → ⁴I_11/2) 654 nm (⁴F_9/2 → ⁴I_15/2), and 430 nm (⁴G_7/2 → ⁴I_11/2) demonstrates that ⁴F_9/2, ⁴G_11/2, ²H_9/2, ²P_3/2, and ⁴G_7/2 energy levels have accumulated electron population by four-photon pathway A and four-photon pathway B. Moreover, 482 nm (²K_15/2 → ⁴I_13/2), 575 nm (²G_7/2 → ⁴I_11/2), 585 nm (⁴G_9/2 → ⁴I_11/2), 456 nm (⁴F_5/2 → ⁴I_15/2), and 443 nm (⁴F_3/2 → ⁴I_15/2) emissions indicate that the ²K_15/2, ²G_7/2, ⁴G_9/2, ⁴F_3/2, and ⁴F_5/2 levels can accumulate electron populations. Compared with NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%) MCs, NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%) exhibited more UCL emissions and a faster response rate of UCL. This indicates that increasing the Er³⁺ ion-doping concentration can effectively enhance the UC process of NaYF₄:Yb³⁺/Er³⁺ MCs in specific concentration ranges of Er³⁺.

Further increasing the doping concentration of Er³⁺, for NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%), the UCL at 542 nm (⁴S_3/2→⁴I_15/2), 522 nm (²H_11/2→⁴I_15/2), 496 nm (⁴F_7/2 → ⁴I_15/2), 505 nm (⁴G_11/2 → ⁴I_13/2), and 384 nm (⁴G_11/2 → ⁴I_15/2) emits within .1 μs after 976 nm ns-pulsed excitation, which originated from pathway A. However, in the timescale from .1 μs to .2 μs after excitation, only 410 nm (²H_9/2 → ⁴I_15/2), 558 nm (²H_9/2 → ⁴I_13/2), 654 nm (⁴F_9/2 → ⁴I_15/2), 456 nm (⁴F_5/2 → ⁴I_15/2), and 443 nm (⁴F_3/2 → ⁴I_15/2) emissions could be observed, demonstrating that ²H_9/2, ⁴F_9/2, ⁴F_5/2, and ⁴F_3/2 energy levels have accumulated electron populations. The UCL emissions from four-photon pathways are not observed. This indicates that further increasing the Er³⁺ doping concentration can increase the response rate of the UCL, but the kinds of emissions are reduced and the UCL emissions from the four-photon UC process could not emit. For NaYF₄:Yb/Er (20/X mol%) MCs, 80 mol% Er doping concentration takes 0.2 μs to achieve the A-1, A-2, A-3, B-1, B-2, and B-3 ET processes, while 1 mol% Er doping concentration takes 1 μs. This indicates that for NaYF₄:Yb/Er (20/80 mol%) MCs, the two- and three-photon UCLs maintain a high response rate even if all Y³⁺ is replaced with Er³⁺. However, for high Er³⁺ ion doping concentration (80 mol%), the CR processes of Er³⁺ prevents the energy levels of the four-photon process from accumulating effective populations, thus preventing the emission of typical four-photon UCLs such as 402 and 430 nm.

To further study the influence of Er³⁺ on the kinetics of multiband UCL, we measured and analyzed the time evolution spectra of NaYF₄:Er³⁺ MCs with different Er³⁺ doping concentrations (Figures 5A, B. According to the result of the evolution spectrum, both NaYF₄:Er³⁺ (10 mol%) and NaYF₄:Er³⁺ (80 mol%) can achieve the UC process by Er³⁺ ion self-sensitization. Nonetheless, without Yb³⁺ ion sensitization, the UCL of NaYF₄:Er³⁺ MCs was weakened and the UCL response rate slowed. According to the time evolution spectrum and a previous report on Er³⁺ self-sensitization (Lu et al., 2014), the schematics of the transition kinetics are presented in Figure 5C. NaYF₄:Er³⁺ (10 mol%) cannot emit UCL from a four-photon ET process. Moreover, the UCL response rate is relatively slow, and it takes 4 μs to perform three-photon pathways A and B. Increasing Er³⁺ doping concentration in NaYF₄, NaYF₄:Er³⁺ (80 mol%) could absorb more photons by the self-sensitization of Er³⁺ ions. However, NaYF₄:Er³⁺ (80 mol%) cannot emit UCL by a four-photon UC process. In addition, the UCL response rate in NaYF₄:Er³⁺ (80 mol%) is at the timescale of ∼1 μs for achieving three-photon pathways A and B. In contrast, for NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%) and NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%), the UCL response rates are at the timescale of ∼0.2 μs to achieve three-photon pathways A and B. This suggests that the self-sensitization of Er³⁺ is not the main factor for the changing response rate in NaYF₄:Yb³⁺/Er³⁺.

FIGURE 5

FIGURE 5. NaYF₄:Er³⁺ evolution spectrum under 976 nm pulse excitation: (A) Er 10 mol% and (B) Er 80 mol%. (C) Schematic representation of the transition kinetics in NaYF₄:Yb³⁺/Er³⁺ (00/x mol%) with different Er³⁺ doping concentrations under 976 nm pulse excitation. Green part is the populating process. Red part is the radiative transition process at x = 10. Blue part is the radiative transition process at x = 80. Purple part is the CR process. The 976 nm ns-pulsed excitation starts from the 0 moment.

More interestingly, in studying the influence of the Er³⁺ doping concentration on the distribution of doped ions in NaYF₄:Yb³⁺/Er³⁺ (20/x mol%) MCs (see Figure 6), increasing the Er³⁺ doping concentration can significantly shorten the distance between Yb³⁺ ions and Er³⁺ ions, which could improve the energy transfer between them (Dexter, 1953). When Er³⁺ doping concentration is low, such as NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%), the distance between Yb³⁺ ions and Er ions is great. Yb³⁺ ions transfer little energy to Er³⁺, leading to a slow UCL response rate of MCs; these can only show part of the four-photon UC process (A-4). Increasing the concentration of Er³⁺ doping can shorten the distance between Yb³⁺ and Er³⁺ ions such as NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%) MCs, which facilitate the energy transfer between Yb³⁺ ions and Er³⁺ ions. Hence, NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%) MCs show obvious multiband UCL and a fast UCL response rate. When further increasing the Er³⁺ doping concentration, such as NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%), the complex CR process reduces the electron population of Er³⁺, and NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%) cannot achieve a four-photon UC process. However, the shorter Yb³⁺–Er³⁺ distance makes the NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%) UCL response rate faster.

FIGURE 6

FIGURE 6. Schematic diagram of the influence of Er³⁺ doping concentration on the UCL emission of NaYF₄:Yb³⁺/Er³⁺ MCs under the excitation of 976 nm laser.

4 Conclusion

In summary, we utilized the OT-TRPLS to trap single β-NaYF₄:Yb³⁺/Er³⁺ MC varying Er³⁺ activator doping concentration and measured the time evolution spectrum. The result shows that varying the Er³⁺ doping concentration can change the UCL emitting waveband and response rate, which indicates different UC kinetics. The time-evolution spectra of β-NaYF₄:Yb³⁺/Er³⁺ MCs by single-doped Er show that the self-sensitization of Er³⁺ ions is not a major factor in the change of the UCL kinetics of MCs. The varying Er³⁺ doping concentration changes the Yb³⁺–Er³⁺ distance, which leads to a different energy transfer between Er and Yb ions. An appropriate increase in the Er³⁺ doping concentration can promote the multiband emission and UCL response rate under strong ns-pulse excitation, such as NaYF₄:Yb³⁺/Er³⁺ (20/1 mol%) and NaYF₄:Yb³⁺/Er³⁺ (20/10 mol%). However, the CR process inhibits the population of electrons at the higher energy levels of Er³⁺ ions, leading to the inability of the four-photon UC process to be realized in NaYF₄:Yb³⁺/Er³⁺ (20/80 mol%) MCs. Our research offers a new understanding of the influence of Er³⁺ concentration on UC, which is promising in fast-response optoelectronics, optical-sensing, and display industries.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

Author contributions

HH and KH conceived the work. ZX, CG, KH, and GZ provided guidance and supervision. ML, YZ, and WC characterized the upconversion microcrystals by SEM and XRD. HH and CG built the photoluminescence platform. HH and YZ performed the spectroscopy experiment. HH, ML, TY, and YZ analyzed the data and wrote the manuscript. All authors participated in the discussion and finalization of the manuscript.

Funding

This work was supported financially by the Science Fund for Excellent Young Scholars (2017X016), the Innovation Fund for Young Scholars (19QNCXJ), and the State Key Laboratory of Laser Interaction with Matter Foundation (SKLLIM1708).

Acknowledgments

The authors thank Xi’an ruixi Biological Technology Co., Ltd. for providing the β-NaYF₄ microcrystals.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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