Yb³⁺/Er³⁺ co-doped NaYF₄ (NaYF₄: Yb³⁺, Er³⁺) hexagonal nanoplates were synthesized by a tetrasodium iminodisuccinate assisted hydrothermal synthesis method for the first time. The crystal structure and morphology was investigated by the X-ray diffraction and scanning electron microscope analysis. The upconversion luminescent property of the synthesized NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates was discussed as a function of Yb³⁺ and Er³⁺ concentration. Under 980 nm excitation, NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates exhibited strong red emissions near 660 nm and green upconversion emissions at 530 and 550 nm corresponding to the intra 4f transitions of Er³⁺ (⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺ (⁴I_15/2). The Yb³⁺ 5 mol% and Er³⁺ 0.5 mol% co-doped NaYF₄ sample exhibited strongly shiny upconversion emission, which was considered to be the optimum doping concentration. A possible upconversion mechanism for NaYF₄: Yb³⁺, Er³⁺ depending was discussed in detail.

Keywords: hydrothermal, NaYF₄, upconversion, IDS

In the past decades, upconversion luminescent (UCL) nanomaterials doped with rare earth ions have become one of the research focuses in the field of materials and luminescence [1-12]. UCL nanomaterials have potential promising applications in many fields, such as optical communication, luminescence display, high density storage, infrared detection and biomedical imaging [13-15]. Up to now, rare earth ions have been doped in many matrix materials and UCL has been successfully realized [16]. The transition of electrons in the 4f shell of rare earth ions induces narrow band luminescence, which are very sensitive to the composition and crystal structure of the matrix materials [1, 10, 16]. Er³⁺ has been considered to be an ideal doping ion to realize infrared to visible UCL owing to its good matching energy level and long excited state lifetime [17]. Compared with Er³⁺, Yb³⁺ has a larger optical absorption cross section in the infrared wavelength, which can effectively transfer the absorbed light energy to Er³⁺ [18, 19]. Therefore, as a sensitizer, Yb³⁺ can effectively increase the emission efficiency of the luminescence center [20-24]. Rare earth ions are easy to cause multiphonon relaxation due to their abundant energy levels and small energy gap between energy levels [25]. Therefore, materials with relatively low phonon energy being used as the matrix can effectively reduce the luminescence loss caused by the non-radiative process [9]. This is particularly important for the upconversion process, which is related to the fluorescence quenching because of high energy vibration. The phonon energy of fluoride is much lower than that of oxide. The multiphonon relaxation of rare earth ions can be reduced by using fluoride as matrix material [9]. Among them, the rare earth ion UCL materials based on NaYF₄ are found to be of high efficiency in recent years [1, 25, 26].
In the past years, there have been many reports on upconversion materials based on NaYF₄ [9]. In order to effectively control the nucleation and growth process of nanocrystals by wet chemical method, it is often necessary to introduce organic reagents as ligands or chelators in the synthesis process [27, 28]. In this paper, NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates with UCL were prepared by coprecipitation and hydrothermal method using tetrasodium iminodisuccinate (IDS Na₄) as chelating agent and NaF as precipitant. The influence of Yb³⁺ and Er³⁺ concentration on the upconversion luminescence of NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates was studied in detail. A possible upconversion mechanism for NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates was proposed. The results provide new insights for the research of other upconversion luminescent materials.

Firstly, the rare earth nitrate mixed aqueous solution (molar ratio, Y: Yb: Er = 89 : 10 : 1, 10 mL, 0.1 mol/L) was added into an IDS Na₄ aqueous solution (10 mL, 0.1 mol/L) under magnetic stirring, and the reaction lasted for half an hour to obtain the chelate complex of rare earth ions and IDS Na₄. Then the NaF aqueous solution (20 mL, 0.4 mol/L) was injected into the above reaction system, and the reaction lasted for one hour to obtain the precursor solution. After that, the reactant solution was transferred to a hydrothermal reactor, and the hydrothermal reaction was performed at 180 ^oC for 15 h. The obtained samples were centrifuged at 3,500 rpm for 15 min, and the precipitates were retained. Finally, the samples were dried in an oven at 60 ºC for 24 h to obtain the final products. The synthesis process of other doping concentrations is similar.
The crystal structure was characterized by X-ray diffraction (XRD, Y-4Q, CuKα, λ = 1.5418 Å). The morphology and particle size of the products were observed by scanning electron microscope (SEM, FEI Quanta 250 FEG). UCL spectra were measured in the range of 500-800 nm using a photoluminescence spectrophotometer (PerkinElmer, LS55) with a semiconductor laser diode (980 nm, 3W) as an external excitation source.

The synthesis process of NaYF₄:Yb, Er phosphors is illustrated in Fig. 1. In our case, the chelate complex of rare earth ions and IDS Na₄ (Ln-IDS) was formed at the beginning. After the addition of NaF aqueous solution, the F⁻ anion must compete with the chelating agent to transform the Ln-IDS complex into the NaYF₄ lattice. Fig. 2 shows the representative XRD pattern of NaYF₄: Yb³⁺, Er³⁺ phosphors. As shown in Fig. 2, the XRD peaks were in accordance with that of hexagonal β-NaYF₄ (JCPDF card 28-1192) [17, 28]. There are no diffraction peaks corresponding to impurities of α-NaYF4 (JCPDS card 77-2042), indicating the high purity of the as-obtained products. The XRD pattern of the sample shows clear and sharp peaks, which indicate the high crystallinity of the obtained product.
Fig. 3 shows the representative SEM images of NaYF₄: Yb³⁺, Er³⁺ phosphors with different magnifications. It is observed that the obtained NaYF₄: Yb³⁺, Er³⁺ phosphors are uniform and monodisperse hexagonal nanoplates. The size is about 500 nm × 220 nm (side length × thickness). In our previous work, we prepared similar NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates with trisodium citrate as chelating agent [28]. It is one of the basic deposition processes that chelating agent (e.g. ethylene diamine tetraacetic acid disodium salt and trisodium citrate) controls the nucleation and growth of inorganic structures [27, 28]. In our present case, we used IDS Na₄ as chelating agent and obtained similar results. IDS Na₄ is a biodegradable chelating agent, and its chelating capacity can comparable with that of ethylene diamine tetraacetic acid. Chelating agents coordinate with metal ions in the synthesis process through amine, carboxyl group, hydroxyl and other functional groups, and passivate the surface of nano particles after synthesis, which are effective ligands of the synthesis of inorganic nanomaterials [27, 28]. In this study, IDS Na₄ and rare earth nitrate were incubated under environmental conditions to form the coordination between Ln³⁺ and IDS Na₄ and promote heterogeneous nucleation. Then, the freshly prepared NaF solution was injected into the mixed solution with vigorous stirring. The mixture underwent hydrothermal crystallization, and finally NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates were obtained. In addition, IDS Na₄ molecules were coated on NaYF₄: Yb³⁺, Er³⁺ to make the products water-soluble and biocompatible.
Fig. 4(a) shows the UCL spectra of representative NaYF₄: Yb³⁺ 5%, Er³⁺ 0.5% hexagonal nanoplates excited by a 980 nm near-infrared laser. The NaYF₄: Yb³⁺ 5%, Er³⁺ 0.5% hexagonal nanoplates exhibit UCL from infrared to visible emission. The visible emission bands are around at 530 nm and 550 nm, which originate from the transition of electrons from ²H_11/2 and ⁴S_3/2 to ⁴I_15/2 in Er³⁺ ions. The emission band near 660 nm is attributed to the transition of ⁴F_9/2 level to ⁴I_15/2 level in Er³⁺ ions. The emission intensities of green and red bands of the NaYF₄: Yb³⁺ 5%, Er³⁺ 0.5% hexagonal nanoplates increase gradually with the increase of the laser excitation power. To understand the UCL process and the population of ²H_11/2, ⁴S_3/2 and ⁴F_9/2 excited states under infrared wavelength excitation, it is necessary to study the relationship between laser excitation power and UCL intensity. The upconversion process involves multiphoton absorption, and there is a relationship between the intensity of the output visible light and the excitation power:
 


where I_VIS is the intensity of visible upconversion emission, I_IR is the excitation power, and n is the number of infrared photons needed to be absorbed by the sample to emit a visible photon. Fig. 4(b) shows the excitation power relationship of the UCL spectra of the NaYF₄: Yb³⁺ 5%, Er³⁺ 0.5% hexagonal nanoplates. The emission intensity increases with the increase of excitation power density. The slopes of (²H_11/2, ⁴S_3/2) →⁴I_15/2 and ⁴F_9/2→⁴I_15/2 transitions are 1.57 and 1.50, respectively. It is shown that UCL involves a two-photon absorption process (Fig. 5) [28, 29]. The slopes deviate from 2 because of the competition between different decay channels in the intermediate state. These decay channels include multiphonon relaxation of low energy state, radiation decay of ground state, upconversion of intermediate state and non-radiation capture [30].
To study the effect of Er³⁺ concentration on UCL in NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates and to explore the optimum doping concentration of Er³⁺, a series of powder samples of NaYF₄: Yb³⁺ 10%, Er³⁺ x% doped with Er³⁺ of different concentrations were made and their UCL spectra were measured. Generally speaking, the intensity of green emission is high, and the intensity of red emission is relatively weak (Fig. 6). It should be noted that with the increase of Er³⁺ concentration, the green and red emission intensities of the samples decrease gradually. In our case, when the Er³⁺ ion doping concentration is 0.5 mol%, the green and red UCL intensity reached the maximum (Fig. 7(a)). The UCL intensity decreases with the increase of Er³⁺ concentration, which may be due to the decrease of the distance between two adjacent Er³⁺ ions and the increase of the interaction between the two ions when the concentration of Er³⁺ ion is high, which leads to the concentration quenching of Er³⁺ ion and the weakening of UCL intensity. It is also found from Figure 7b that the ratio of green to red emission intensity increases with the increase of Er³⁺ concentration. The main reason is due to the following cross relaxation (CR) between two adjacent Er³⁺ ions (Fig. 5):
 

 
With the increase of Er³⁺ doping concentration, the cross relaxation becomes stronger and stronger, resulting in higher green/red emission intensity ratio.
In order to reveal the relationship between the UCL mechanism of NaYF₄: Yb³⁺, Er³⁺ and Yb³⁺ doping concentrations, samples doped with Yb³⁺ concentration (from 0 mol% to 20 mol%) were prepared by controlling the concentration of Er³⁺ at 0.5 mol%. Fig. 8 exhibits the UCL spectra of NaYF₄: Yb³⁺ x%, Er³⁺ 0.5% hexagonal nanoplates with different Yb³⁺ ion doping concentrations under 980 nm laser excitation. The UCL intensity of the samples varies greatly by increasing Yb³⁺ doping concentrations (Fig. 8). With the increase of Yb³⁺ concentration, the UCL intensity of the samples gradually increases to the strongest and then decreases, while the ratio of green to red emission intensity decreases all the time (Fig. 9). When the Yb³⁺ ion doping concentration is 5.0 mol%, the intensity of red and green emission is the highest. When the concentration of Yb³⁺ exceeds 5.0%, the UCL of the sample is obviously suppressed. The concentration quenching and reverse energy transfer can be occurred when the concentration of sensitizer is too high. Therefore, there is an optimal concentration of Yb³⁺ in NaYF₄: Yb³⁺, Er³⁺. Yb³⁺ ions can be doped as sensitizer because the first excited states absorption energy of Yb³⁺ ion and Er³⁺ ion are consistent, and the near-infrared absorption cross section of Yb³⁺ ion is much higher than that of Er³⁺. Yb³⁺ ions can transfer the energy absorbed by themself to their neighboring Er³⁺ ions. When the doping concentration of Yb³⁺ ions in the sample is high, the energy transfer probability between Yb³⁺ and Er³⁺ will be higher and there will be more electrons in the energy levels of ⁴I_11/2 and ⁴I_13/2 in Er³⁺ ions. Therefore, with the increase of Yb³⁺ ion concentration, the following CR process will occur [31]:



As a result, the intensity of UCL of red emission becomes stronger.

NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates were synthesized by an IDS Na₄ assisted hydrothermal synthesis method for the first time. Under NIR excitation (980 nm), NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates exhibited obviously strong green UCL at 530 and 550 nm and red UCL at 660 nm. The UCL intensity depends on the concentration of Yb³⁺ ion, which acts as a sensitizer ion to improve the absorption around 980 nm, while Er³⁺ acts as an activator ion for UCL in the NaYF₄ matrix. The optimum doping concentrations of Er³⁺ and Yb³⁺ for strong UCL luminescence were 0.5 and 5.0 mol%, respectively. The possible CR process in the NaYF₄: Yb³⁺, Er³⁺ hexagonal nanoplates was discussed in detail, which is the main reason for the intensity of UCL related to doping concentration.

This work was supported by the Opening Foundation of Henan Key Laboratory of Special Protective Materials (Grant No. SZKFJJ201903).