1. Introduction

Rare earth-doped upconversion nanoparticles (UCNPs) can absorb over two low-energy photons and emit one high-energy photon via energy transfer. This optical property enables them to emit ultraviolet or visible light when excited by near-infrared light. These distinctive luminescent characteristics provide them with enormous potential for high-resolution bio-imaging, temperature sensing, photothermal therapy, security and anti-counterfeiting, drug delivery, and photovoltaic applications [1–17]. Hexagonal-phase NaREF₄ (β-NaREF₄) nanocrystals have recently emerged as effective substrate materials for upconversion luminescence. This is primarily because of their exceptional chemical stability and low phonon energy [18].

Achieving upconversion luminescence relies mainly on dopant ions. Since the doping concentrations of both the sensitizer and activator ions are usually relatively low, the luminescence performance of UCNPs often falls short of practical application requirements. However, increasing the dopant concentration directly decreases the distance between the dopant ions, leading to intensified cross-relaxation. Additionally, enhancing the dopant ion concentration facilitates energy migration to quenching sites, such as surface ligands and defects that cause concentration quenching in the system [19–22]. Moreover, the considerable surface area of upconversion nanocrystals increases their vulnerability to the surface quenching effect, leading to a reduction in the luminescence efficiency of the nanocrystals. In contrast to identical materials on a single crystal or microcrystalline scale, the luminescence quantum yield of upconversion nanocrystals is 100-10000 times weaker under equivalent test conditions [23,24].

In recent years, the construction of core-shell UCNPs with luminescent core-sensitizing shell-passivated shell structures have emerged as one of the most effective and inclusive methods for amplifying the upconversion luminescence of lanthanide UCNPs. First, various ions can be doped into separate layers by sensitizing shell encapsulated. This inhibits surface quenching and suppresses cross-relaxation, significantly enhancing the luminescence efficiency of the nanoparticles and the doping concentration of lanthanide ions. The specific surface area of UCNPs can be further reduced through an epitaxially grown inert shell. This layer can effectively passivate the surface ligands and defects of the nanoparticles and isolate the core from the surroundings. Consequently, luminescence weakening due to the environment can be effectively mitigated, and the energy loss due to non-radiative leaps in the energy transfer process can be decreased. This approach enhances the efficiency of upconversion luminescence by suppressing both the surface and concentration quenching. Additionally, it maintains the initial properties of the nanocrystalline cores and does not affect the combination of UCNPs with other materials [25–29]. Numerous investigations have examined core-shell-structured UCNPs. Liu et al. achieved a two-order-of-magnitude advancement in the overall luminescence of UCNPs by synthesizing NaYF₄@NaYbF₄:Er@NaYF₄ multilayered structures, with the detection of single-particle luminescence at a power of 8 W/cm² [25]. Zhou et al. constructed multilayered structures of NaYF₄:Er@NaYbF₄@NaYF₄, with quantum yields of 4.5 W/cm² excitation power density of 6.34% [26]. Zhang et al. achieved the regulation of energy migration direction by modulating the spatial distribution between core-shells and found that the UCNPs with the structure of luminescent core @ sensitizing shell @ passivated shells, which have the energy from the outward to the inward direction, can produce the maximum upconversion luminescence, which is 6.34% brighter than that of the activator core @ sensitizing shell @ inert shell at the single-particle level. Outside-in luminescence is approximately 6 times brighter at the single-particle level than inside-out luminescence [30].

Now, with the increasing demand for applications, it not only has higher requirements for the overall luminescence intensity of lanthanide UCNPs, but also for the luminescence lifetime, thermal stability, and emission intensity in different bands of UCNPs. For example, in the fields of photodynamic therapy and photogenetics, there is an increasing demand for the emission of high-energy multiphoton upconversion bands, whereas deep tissue biological imaging has a greater demand for the emission of near-infrared bands. In previous reports, the optimization of luminescence intensity had been focused, while there have been few reports on the luminescence process and related performance. Therefore, while enhancing the luminescence intensity of UCNPs, further exploration of their effects on luminescence lifetime, thermal stability, and energy level transitions leading to different wavelengths of luminescence is key to their practical applications.

In addition, upconversion luminescent films have attracted much attention because they can be easily combined with other films in a compatible manner so that they can be used in devices such as solar cells, and the use of the surface plasmon resonance effect to enhance the luminescence of upconversion films has become a research hotspot. The local field enhancement effect of the metal plasma resonance can significantly improve the upconversion efficiency by enhancing the local excitation energy density [31,32]. Therefore, the plasma resonance strategy can be combined with core-sensitizing shell-passivated shell structures to further enhance the upconversion luminescence performance of UCNPs. UCNPs have significant potential for application in the field of anti-counterfeiting owing to their unique luminescent properties [33]. The application of UCNPs with optimized upconversion luminescence performance in the field of anti-counterfeiting is also worth of further research.

In this study, mononuclear β-Na(Y_1-xEr_x)F₄ (x = 0.04, 0.07, 0.1, and 0.13), sensitizing shell-encapsulated β-Na(Y_1-xEr_x)F₄@NaYbF₄, and multilayer-structured β-Na(Y_1-xEr_x)F₄:@NaYbF₄@NaYF₄. In a previous study, we explored mononuclear UCNPs with the best upconversion performance: β-Na(Y_0.78Yb_0.18Er_0.04)F₄ (relevant work is being published). With shell layer encapsulation, the luminescence quenching of UCNPs was substantially suppressed and the luminescence intensity was significantly enhanced. Compared with mononuclear UCNPs, the luminescence intensity of UCNPs with multilayer structure was enhanced by 346-fold, 22-fold and 54-fold in the three main emission bands, namely, blue, green, and red light, respectively. The fluorescence decay time was extended in all bands. Meanwhile, the weakening of luminescence by warming is suppressed by the presence of the shell layer that isolates the core from the external environment, and the thermal stability of upconversion luminescence is significantly improved. In addition, the utility of excitation light by UCNPs was significantly improved with shells coating, and the saturation of the energy level layout was delayed.

On this basis, we prepared films with the best upconversion performance of UCNPs and compounded them with Ag films. Under the plasmon resonance effect, the luminescence intensity of the composite film was significantly enhanced compared with that of the UCNPs film. In addition, owing to the large anti-Stokes shift and long luminescence lifetime of rare earth-doped upconversion luminescent materials, they have important applications in optical data storage, document security, anti-counterfeiting, and bio-imaging. We explored the application of upconversion anti-counterfeiting by combining the prepared upconversion materials with information coding.

2. Experimentation

The reagents and materials used in the experiment are as follows, yttrium chloride (YCl₃, 99.9%), ytterbium chloride (YbCl₃, 99.9%), erbium chloride (ErCl₃, 99.9%), ethylene glycol (EG, AR), sodium fluoride (NaF, AR), polyvinyl pyrrolidone (PVP, Mw∼40,000), sodium hydroxide (NaOH, AR), glacial acetic acid (CH₃COOH, AR), isopropyl alcohol ((CH₃)₂CHOH, AR), trifluoroacetic acid (CF₃COOH, AR), silver nitrate (AgNO₃, AR), anhydrous ethanol and Indium-Tin Oxide conductive film glass (ITO). All reagents and materials were purchased from Titan Technology Exploration Platform. None of the reagents were purified further.

2.1 Preparation of UCNPs

The beaker A contained 0.78 mmol of YCl₃, 0.18 mmol of YbCl₃, 0.04 mmol of ErCl_3, 1 g of PVP and 40 ml of EG, was placed on a magnetic stirrer and stirred at a speed of 800 r/min in a 40 °C water bath. Then, 11 mmol of NaF and 20 ml of deionized water were added to beaker B and ultrasonically shaken for 30 min until the NaF was fully dissolved, and then the mixed solution was slowly added to beaker A. The stirring was continued for 2 h by maintaining the 40 °C water bath and the original rotational speed. Subsequently, the mixed solution was added to a hydrothermal reactor with a polytetrafluoroethylene liner and held in a drying oven at 200 °C for 14 h. Subsequently, the mixed solution was centrifuged at 8000 r/min and the precipitate was obtained. The precipitate was washed three times with deionized water and anhydrous ethanol and then held in a 70 °C drying oven for 12 h. Subsequently, the dried sample was removed and fully ground in an agate mortar to obtain powdered β-Na(Y_0.78Yb_0.18Er_0.04)F₄.

β-Na(Y_1-xEr_x)F₄ was prepared in the same way as β-Na(Y_0.78Yb_0.18Er_0.04)F₄, with the difference that YbCl₃ was no longer added when designing the components, and the dosage of ErCl₃ and YCl₃ was adjusted to x mmol and (1-x) mmol respectively.

The solvothermal method was used to prepare mononuclear UCNPs. The sensitizing layers and inert layers epitaxial growth technique used was the sol-gel method. The nanocrystalline seeds and precursor components required for layer growth were added to the reaction solvent. The formed crystal planes could absorb the growth components into the solvent and directly epitaxially grow nanocrystalline shells. This method allows for the simple and efficient design and synthesis of nanocrystals with different core-shell molar ratios. The specific experimental procedures for the shell coating were as follows:

β-Na(Y_1-xEr_x)F₄@β-NaYbF₄ was prepared by adding 5 mmol of prepared β-Na(Y_1-xEr_x)F₄, a certain amount of YCl₃, YbCl₃ and 10 ml of glacial acetic acid to beaker A, and this beaker was stirred at 400 r/min at room temperature. For beaker B, 30 mmol of NaOH and 5 ml of deionized water were added and ultrasonically shaken for 30 min, and when the NaOH was fully dissolved, it was added to beaker A and stirring was continued for 30 min. Then, 10 ml of isopropanol and 2.5 ml of trifluoroacetic acid were added to the solution, and stirring was continued for 2 h at 50 °C maintaining 400 r/min until the mixed solution became soluble. Subsequently, the beaker was transferred to a blast-drying oven set at 100 °C and held for 24 h. Subsequently, the samples were transferred into an alumina crucible and placed in a muffle furnace for calcination. The calcination was heated to 550 °C at a ramping rate of 5 °C/min and held at this temperature for one hour before cooling naturally in the furnace. The samples were removed and fully ground in an agate mortar to obtain a β-Na(Y_1-xEr_x)F₄ @β-NaYbF₄ upconversion powder.

β-Na(Y_1-xEr_x)F₄ @β-NaYbF₄@β-NaYF₄ was prepared in the same way as β-Na(Y_1-xEr_x)F₄@β-NaYbF₄, with the difference that β-Na(Y_1-xEr_x)F₄@β-NaYbF₄ was used instead of β-Na(Y_1-xEr_x)F₄ as the seed crystal.

2.2 Preparation of Ag/UCNPs composite film

First, Ag particles were prepared. We added 10 ml of EG, 2 mmol of PVP, 0.2 mmol of silver nitrate solution into a beaker and stirred at 500 r/min for 30 min, followed by raising the temperature to 120 °C and continued stirring for 8 h until the solvent was completely evaporated to obtain silver particles. The Ag particles in the beaker were subsequently dispersed in anhydrous ethanol and set aside. Then, an upconversion film was prepared. 0.5 mmol of NaYF₄@NaYbF₄@NaYF₄ with the best upconversion luminescence performance was dispersed in 10 ml of anhydrous ethanol, and the mixed solution was loaded into a hydrothermal reactor equipped with a polytetrafluoroethylene liner. Indium-Tin Oxide conductive film glass (ITO) was immersed in this solution. Subsequently, the reactor was placed in a drying oven at 80 °C for 6 h. Subsequently, ITO was removed and dried at 60 °C for 12 h to obtain an upconversion film deposited on the ITO surface.

Silver thin films were prepared in the same manner as the upconversion films. The difference is that 0.5 mmol of upconversion particles dispersed in 10 ml anhydrous ethanol were changed to 0.5 mmol of Ag particles.

Subsequently, the Ag/UCNPs composite film was prepared. 0.5 mmol of upconverted particles was dispersed in 10 ml of anhydrous ethanol and the mixed solution was loaded into a hydrothermal reactor equipped with a polytetrafluoroethylene lining. ITO with the Ag film deposited on the surface was immersed in the solution. Subsequently, the reactor was placed in a drying oven at 80 °C for 6 h. Subsequently, ITO was removed and dried at 60 °C for 12 h to obtain a composite film of Ag/UCNPs deposited on the surface of ITO.

2.3 Characterization

Scanning electron microscope was used to observe the microscopic morphology of the samples (SEM, JSM-IT500HR) and elemental analysis of the materials was carried out by spectral line scanning using an energy dispersive X-ray spectrometer (EDS, JEM-F200, URP) operating at 10 mA and accelerating at 5 kV, the samples were treated with gold spraying prior to testing. The lattice fringes of the samples were observed using a transmission electron microscope (TEM, FEI Tecnai F20), and the samples were homogeneously dispersed in anhydrous ethanol prior to testing, dropped onto a copper grid and left to dry for analysis. The samples were analyzed for the physical phase using an X-ray diffractometer equipped with Cu-Kα radiation (XRD, Bruker D8 Advance X), with a scanning range of 10°-60° and a scanning speed of 2°/min. The crystallinity of the samples can be obtained from their XRD data [34–37]. The upconversion luminescence test was carried out using a fluorescence spectrometer (Thermo Fisher IN10) with a test wavelength of 350 nm-750 nm, and scanning speed of 2000nm/min. The excitation and emission bandwidths were both 5 nm. Upconversion fluorescence lifetime tests were conducted using a steady/transient fluorescence spectrometer (Edinburgh FLS980), with fluorescence decay times measured at three positions: 408 nm, 539 nm, and 657 nm. The time dependence I (t) of upconversion luminescence emission intensity can be expressed as:

3. Results and discussion

First, we performed the sensitizing layers coating on the activator cores by sol-gel method. Figure 1(a-c) shows the SEM images of the sensitizing layer with core molar ratios of 1:1, 3:1 and 5:1, respectively. It could be found that the synthesized granular UCNPs are morphologically more regular. XRD tests were carried out on this group of samples and it can be seen from Fig. 1(d) that shell capping has little effect on the crystalline pattern of the samples. The XRD peaks were assigned to the mixed phase of hexagonal NaYF₄ (JCPDS no. 16-0334) and hexagonal NaYbF₄ (JCPDS no. 27-1427). As shown in Fig. 1(e and f), the relative intensities of the three elements (Yb, Y and Er) were consistent with the initial additions, and only Yb was detected at the edge of the sample (β-Na(Y_0.9Er_0.1)F₄@3β-NaYbF₄), whereas Er and Y only appeared in the middle region, which indicated the stability of the structure during the high-temperature crystalline phase transition process. The coating process did not change the chemical composition of the core-shell UCNPs, and also indicated that the coating process was a typical epitaxial growth, and the particles were completely encapsulated with the sensitizing layer. Because both the sensitizing layer and the core belong to NaREF₄, they have the same hexagonal phase crystalline form. Moreover, the layer encapsulation reaction occurs in a liquid-phase environment. Therefore, a certain degree of intercalation between the layer and core occurs during the growth of the cladding. As a result, the overlap of the three rare earth elements occurs while the detection position transitions to the center. The upconversion luminescence testing is shown in Fig. 1(g), which shows the best luminescence performance at a sensitizing layer to core molar ratio of 3:1. In addition, all three samples exhibited distinct emission peaks at 408 nm, 523 nm, 539 nm, and 657 nm, which correspond to the energy level jumps of ²H_9/2-⁴I_15/2, ²H_11/2-⁴I_15/2, ⁴S_3/2-⁴I_15/2, and ⁴F_9/2-⁴I_15/2 of Er³⁺.

 

Fig. 1. Optimization of upconversion luminescence performance due to sensitizing layers encapsulated: SEM images (a-c) and XRD patterns for sensitizing layer to core molar ratios of 1:1, 3:1 and 5:1 (d), respectively; EDS images (e and g) and upconversion spectra (g) of molar ratio at 3:1 of sensitizing layer to core

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NaYbF₄ as the sensitizing layers can spatially separate Er³⁺ and Yb³⁺ in the nucleus and adjacent layers, and inhibit Yb³⁺-Er³⁺ cross-relaxation. Moreover, the spatially adjacent positions of Yb³⁺ and Er³⁺ ensure efficient energy transfer at the core-shell interface. Therefore, when the sensitizing layer is thin (β-Na(Y_0.9Er_0.1)F₄@β-NaYbF₄), the activator in the core and the sensitizer in the sensitizing layer may not be effectively separated because of the mutual fusion between the edge of the shell layer and the core, which results in concentration quenching and cross-relaxation not being effectively suppressed. When the sensitization layer is thick (β-Na(Y_0.9Er_0.1)F₄@5β-NaYbF₄), the Yb³⁺ on the surface is far from the core, making it difficult for effective energy transfer to occur between the activator and the activator. At the same time, a thick sensitization layer exacerbates the cross-relaxation between Yb³⁺, thus causing luminescence quenching. To analyze the effect of different core-shell molar ratios on luminescence, crystallinity calculations were carried out for the three samples and the results are shown in Table 1. It can be observed that the sample with molar ratio of 3:1 exhibited the highest crystallinity (86.17%) and the change in crystallinity of the three samples likewise correspondeds to the change in the respective upconversion intensities (85.44% and 84.92%). This result indicates that the crystallinity and core-shell molar ratio have a significant impact on luminescence performance.

Table 1. The effect of changes in the molar ratio of the activator core, sensitizing layer, and inert layer on the crystallinity of UCNPs

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As shown in Fig. 2(a), on the basis of sensitizing layer coating, we varied the rare earth elements in the outermost inert layers and prepared NaYF₄, NaGdF₄ and NaLuF₄ shells using the sol-gel method, which are common upconversion matrix materials. UCNPs with NaYF₄ as the inert layer exhibited the best luminescent properties and the weakest surface quenching. This may be due to the small lattice mismatch of NaYF₄ with NaYbF₄, which is 1.7% in the (100)/(010) direction and 0.52% in the (001) direction. In contrast, the lattice mismatches of NaGdF₄ with NaYbF₄ were 3.75% and 1.53% in these two directions, respectively. When the lattice constants of the two materials differ significantly, it is easy to generate stress at the growth interface, forming defects, that are misfit dislocations. The size of the lattice mismatch affects whether the two materials grow together. It is generally believed that when the lattice mismatch exceeds 5%, it is difficult for material to achieve heteroepitaxial growth [38–40]. The lattice mismatch degree of NaLuF₄ to NaYbF₄ in the (100)/(010) direction is -0.52%, and in the (001) direction is -0.47%. However, because NaYF₄ is the matrix material of the luminescent core and is isomorphic with the nucleus, it facilitates filling of the surface defects of the nuclear material and reduces the energy-exposed annihilation of Yb³⁺, which facilitates the transfer of energy from the surface of the nucleus to the interior, thereby enhancing the energy transfer efficiency.

 

Fig. 2. Optimization of upconversion luminescence performance due to inert layers encapsulated: effect of inert shell rare earth elements on upconversion luminescence (a); SEM images (b-d); XRD images (e) and Upconversion spectra (i)for the UCNPs with activator core, sensitizing layer to inert layer (NaYF₄) molar ratios of 1:3:1, 1:3:3 and 1:3:5, respectively; EDS images (f and g), corresponding selected area electron diffraction plots (h) for the UCNPs with activator core, sensitizing layer to inert layer molar ratio of 1:3:3

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Figure 2(b-d) shows the SEM images of the activator core, sensitizing layer, and inert layer with molar ratios of 1:3:1, 1:3:3 and 1:3:5, respectively. It can be seen that the double-shell-encapsulated UCNPs showed a large change in morphology, and when the molar ratio was 1:3:1 and 1:3:3, the samples basically retained the granular structure, and the surface of UCNPs with molar ratio of 1:3:3 was smoother relative to the other two samples. When the molar ratio was 1:3:5, the sample remained granular, but the surface became rougher, as shown in Fig. 2(e). From the XRD results, it can be seen that the inert shell layer coating did not affect the crystalline pattern of the samples, all these UCNPs were determined to have a conventional hexagonal phase. As shown in Fig. 2(f), energy spectrum analysis along the dashed line position was performed for the EDS test, and the results are shown in Fig. 2(g). The relative intensities of the three rare earth elements were in accordance with the initial additions. The distribution of Y was throughout the entire particle, which showed the integrity and excellent homogeneity of the core-shell structured particles. Yb and Er were detected as the detection position moved towards the center. The large number of overlapping regions of the elements, as well as the absence of obvious interfacial delamination, which are shown in Fig. 2(g), was also due to the intergrowth of NaREF₄ in the same crystalline form under liquid-phase reaction conditions. In Fig. 2(h) is the HRTEM image of the double-shell-encapsulated UCNPs, the sample indicated a lattice stripe spacing of 0.51 nm, which corresponds to the crystallographic spacing of the (100) crystalline plane of the outer capped β-NaYF₄. The upconversion luminescence analysis of the three samples is shown in Fig. 2(i). The sample with a molar ratio of the core, sensitizing layer and inert shell layer of 1:3:3 has the best luminescence intensity.

UCNPs have a high specific surface area owing to their small size, and there are a large number of sites and defects on their surfaces that are prone to fluorescence quenching. When receiving the excitation energy, the quenched sites and defects on the surface first absorb part of the energy, which reduces the energy transferred to the sensitizer, leading to a decrease in the upconversion luminescence efficiency. Yb³⁺ in the sensitizing shell layer has two main energy migration paths after absorbing the excitation light energy: one is to Er³⁺ in the core, and the other is to migrate to the defect centers on the surface of the nanoparticles. Meanwhile, Yb³⁺ in the sensitizing layer is exposed to the nanocrystalline surface, therefore, its photon energy is dissipated at the surface, which also weakens the upconversion luminescence. Therefore, the activator ions and sensitizer ions can be isolated from surface defects and other groups that quench luminescence by epitaxial growth of optically inert layers, which can promote the reverse energy transfer process from Yb³⁺ to the inner core Er³⁺ by inhibiting the energy transfer process between the excited states Yb³⁺ and Er³⁺ and the surface defects of the nanoparticles and avoiding energy loss due to migration to the surface. Moreover, surface modification of the inert shell layer can reduce the activity of the surface defects of UCNPs, passivating the surface and weakening the luminescence quenching phenomenon, which can effectively improve the upconversion luminescence intensity. However, when the inert layer is thin (β-Na(Y_0.9Er_0.1)F₄@3β-NaYbF₄@β-NaYF₄), Yb³⁺, which was originally located inside the sensitizing layer, enters the inert shell layer through cation exchange, thus, a thicker inert shell is required to completely suppress its energetic coupling with the surface vibrational state. As the thickness of the inert layer increased, the modification inhibition of surface defects and quenching sites was enhanced, resulting in an increase in the upconversion luminescence intensity. However, a thick layer prolongs the photon emission path and affects the emission of upconversion photons, and also leads to a gradual weakening of its inhibitory effect on the quenching site, which diminishes the luminescence intensity enhancement effect (β-Na(Y_0.9Er_0.1)F₄@3β-NaYbF₄@5β-NaYF₄).To analyze the effect of different core-shell molar ratios on luminescence, crystallinity calculations were carried out for the three samples and the results are shown in Table 1. The results of the change in crystallinity have a clear correlation with the results of the upconversion luminescence test, which is the same as that after the sensitizing layer coating, UCNPs with the highest crystallinity (86.34%) exhibited the best up-conversion performance.

As shown in Fig. 3(a), the Er³⁺ content in the activator core was adjusted, and the inhibition effect of the sensitizing and insert layer cladding on concentration quenching was investigated. The doping amount of Er³⁺ was set to 4%, 7%, 10% and 13%, and performed upconversion tests on this group of samples. It was found that the doping amount with the best luminescence performance (4%) with the mononuclear structure showed the weakest luminescence intensity when the layers were coated. In contrast, the samples exhibited the best luminescence intensity when the core Er³⁺ doping was 10%. Generally, a higher doping concentration of rare earth ions shortens the distance between them, increasing the chance of non-radiative leaps between the ions. Simultaneously, a large amount of energy is transferred to the crystal surface and is quenched by surface defects, at which time UCNPs undergo severe fluorescence concentration quenching. The increase in the activator ions concentration indicates that shell capping effectively reduces the energy loss due to surface quenching, and the energy transfer efficiency from the sensitizer to the activator is improved, which in turn suppresses the concentration quenching and allows more activator ions to be doped, thus enhancing the upconversion luminescence.

 

Fig. 3. Performance optimisation and upconversion luminescence mechanism of multilayer shell-structured particles: effect of activator Er³⁺ concentration on upconversion luminescence (a); comparison of upconversion intensity of mononuclear, single-shell-encapsulated and double-shell-encapsulated UCNPs (b and c); the change of luminescence intensity at the position of main peaks for mononuclear, single-shell-encapsulated and double-shell-encapsulated particles (c); light color contrast (d) of different UCNPs; upconversion luminescence mechanism diagram (e); (C, C-S, C-S-S in Fig. 3(c and d) means β-Na(Y_0.78Yb_0.18Er_0.04)F_4, β-Na(Y_0.9Er_0.1)F₄@3β-NaYbF₄ and β-Na(Y_0.9Er_0.1)F₄@3β-NaYbF₄@3β-NaYF₄, respectively)

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Subsequently, upconversion tests were conducted on the three samples with the best luminescence (Fig. 3(b)), and it was found that as the layers were constructed, the luminescence intensity of the UCNPs significantly improved, and the double-shell-encapsulated UCNPs exhibited the best upconversion performance (β-Na(Y_0.9Er_0.1)F₄@3β-NaYbF₄@3β-NaYF₄). This phenomenon indicates that a more effective energy transfer upconversion occurs in the core-shell-shell structure. In addition, it can be found that if the sensitizing layer is not encapsulated by an inert shell, a significant luminescence quenching phenomenon will also occur.

As shown in Fig. 3(c), the luminescence intensities of the three main luminescence peaks were extracted and analyzed (408 nm, 539 nm and 657 nm). It was found that at 408 nm, after the sensitization layer was encapsulated, the luminescence intensity was 132 times higher than that of the mononuclear sample, and after coating the insert layer was encapsulated, the luminescence intensity was 346 times higher than that of the mononuclear sample. At 539 nm, the upconversion luminescence intensity increased by 9 times and 22 times, respectively. At 657 nm, the luminescence intensity increased by 43 times and 54 times, respectively.

The luminescence chromaticity diagrams of three UCNPs with different structures are shown in Fig. 3(d). It can be observed that owing to the differences in the relative intensity of each UCNPs in the emission wavelength, there are significant changes in the upconversion light color. The color of the UCNPs with a mononuclear structure was green (0.27,0.71). When the sensitizing layer was encapsulated, the light color turned red (0.47,0.51), and then turned yellow when the insert layer was encapsulated (0.43,0.54).

Figure 3(f) shows the upconversion luminescence process of the sample with a double-shell structure after receiving 980 nm excitation light. The luminescence of the sample in the visible light band was mainly derived from the ²H_9/2-⁴I_15/2,⁴S_3/2-⁴I_15/2 and ⁴F_9/2-⁴I_15/2 energy level transitions of Er³⁺, corresponding to the luminescence bands at 408 nm (blue), 539 nm (green) and 657 nm (red), respectively.

Lanthanide ions usually have low molar extinction coefficients and long luminescence lifetime because of the Laporte forbidden property of f-f electron transitions [41–43].To test the luminescence stability of the mononuclear, shell-encapsulated and double-shells encapsulated UCNPs, fluorescence decay tests were carried out on three main emission peaks at 408 nm, 539 nm and 657 nm (Fig. 4(a-c)). Each curve shows a rise and decay process, corresponding to a typical upconversion luminescence energy transfer process. With the layer encapsulated, the fluorescence decay time of each wavelength gradually increased (Fig. 4(d)). The luminescence lifetimes of mononuclear UCNPs at 408 nm, 539 nm and 657 nm were 2.91 ms, 3.89 ms and 4.44 ms, respectively. After the sensitizing layer was encapsulated, the luminescence lifetime was increased to 3.04 ms, 4.33 ms and 4.91 ms. After the inert layer was encapsulated, the luminescence lifetime was further increased to 4.99 ms, 6.67 ms and 5.19 ms. Compared with the mononuclear, the luminescence lifetimes of the multilayer shell-encapsulated UCNPs at main wavelengths were increased by 71.48%, 71.47% and 15.99%, respectively.

 

Fig. 4. Optimization of shell coating on luminescence stability of UCNPs with different structure: the fluorescence decay curves (a-c) of mononuclear, single-shell encapsulated and double-shell encapsulated particles at 408 nm, 539 nm and 657 nm, respectively; comparison of the luminescence lifetime of the three UCNPs (d)

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The lifetime (τ) of the excited state energy level mainly depends on the radiative transition rate (W_R), non-radiative transition rate (W_NR) and energy transfer rate (W_ET), which can be expressed as:

Compared with mononuclear UCNPs, the separation and distribution of Yb³⁺ and Er³⁺ in NaYF₄@NaYbF₄ slowed the energy transfer from Yb³⁺ to Er³⁺ and prolonged the decay time of Yb³⁺ (²F_5/2 energy level) after being encapsulated with the sensitizing layer. For the insert layer, the large number of surface quenching sites generated by the direct contact of the active shell with the environment after the epitaxial growth of the inert layer on the nanocrystals, which inhibits the transfer of excitation energy to the activator Er³⁺, resulting in a decrease in the upconversion luminescence efficiency of the nanocrystals. Surface quenching is inhibited by the inert layer encapsulated, which weakens this phenomenon and prolongs luminescence lifetime. In addition, the epitaxial growth of the inert layer inhibits the non-radiative relaxation of the sensitizer Yb³⁺ when it transfers energy to the activator Er³⁺, thereby transferring more energy to Er³⁺. At the same time, it also inhibits the non-radiative relaxation phenomenon when the activator Er³⁺ in the excited state releases energy, so that more energy is emitted in the form of light. The combined effect of these factors leads to a significant increase in the upconversion luminescence intensity of UCNPs.

The luminescence intensity of UCNPs is determined by the competition between radiative and non-radiative transition in the luminescence center. The non-radiative transition rate of the radiative energy level can be expressed as:

The upconversion luminescence properties of the mononuclear were tested (Fig. 5(a)), the sensitizing layer-encapsulated samples (Fig. 5(b)), and the double-shell layer-encapsulated UCNPs (Fig. 5(c)) with temperature cycling were tested, and six different wavelengths were selected within the upconversion emission range (521 nm, 539 nm, 550 nm, 657 nm, 671 nm, 810 nm), and their upconversion luminescence intensities at different temperatures were extracted and plotted as curve (Fig. 5(d–f)). Starting from room temperature (298 K), the three UCNPs were gradually hated to 473 K and subsequently cooled to room temperature (298 K). It could be seen that the upconversion luminescence intensity of the different UCNPs decreased when the temperature was increased from 298 K to 498 K. When the temperature was gradually lowered from 498 K to 298 K, the luminescence of all the UCNPs was gradually enhanced owing to the weakening of thermal quenching. Furthermore, the luminescence properties of the UCNPs also returned to the initial intensity when the temperature was lowered to room temperature, and the luminescence intensity exceeded that of the pristine state for the double-shell-encapsulated UCNPs (Fig. 5(a-f)). In addition, for the mononuclear UCNPs, the wavelength located in the infrared light (810 nm) showed a significant luminescence enhancement owing to the thermal energy elevation as the temperature increased (Fig. 5(a and d)). For the shell-encapsulated UCNPs, the temperature elevation had almost no effect on the luminescence intensity of the UCNPs in the infrared wavelength range, which suggests that the shell-encapsulated effectively suppressed the transfer of thermal energy to the core and improved the thermal stability of the luminescence of UCNPs. For the luminescence enhancement near 360 K in Fig. 5(e and f), it could be attributed to the phonon-assisted energy transfer upconversion phenomenon [45], which was capable of emitting photons of higher energy than excitation light by absorbing phonons (lattice vibrations). The electron absorbs the incident photon energy and one or more phonon energies, is transferred to a real electronic state energy level, and then produces high-energy photon emission. At high temperatures, the lattice vibrations are intensified and the phonon density is elevated, boosting the probability of this process. Therefore, the upconversion luminescence intensity is enhanced to some extent. However, as the temperature continued to increase, the non-radiative energy relaxation intensified, allowing the luminescence intensity to continue to weaken in general.

 

Fig. 5. Optimization effect of shell coating on temperature stability of UCNPs with different structure: the upconversion luminescence images of mononuclear, single-shell encapsulated and double-shell encapsulated UCNPs were heated to 473 K and cooled to the initial temperature of 298 K, respectively (a-c); the emission peak intensity curve caused by temperature change (d-f) and the light color change of upconversion luminescence caused by temperature change (g-i)

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As shown in Figs. 5(g-i), the temperature changes also had a significant effect on the luminescence color of the UCNPs. When the temperature gradually increased, the luminescence color of all the UCNPs gradually moved towards the red region, which can be found in combination with Figs. 5(d-f), the dominant role of the electron leaped in the luminescence bands located in the red-light region of ⁴F_9/2-⁴I_15/2 and ⁴I_9/2-⁴I_15/2 was even more significant. This was due to the fact that, at room temperature, the upconversion process was dominated by two-photon leaps, which were manifested as green-dominant red-green mixed light. However, as the ambient temperature increases, Er³⁺ is excited to higher energy levels through a three-photon process, and after energy back propagation, more excited state electrons are populated to the red energy level, resulting in red luminescence emission.

The upconversion luminescence performance of the three UCNPs were tested by adjusting the excitation power, as shown in Fig. 6(a-c). The UCNPs exhibited a clear power dependence, and the luminescence intensity of all three samples appeared to be significantly enhanced with a gradual increase in the excitation power. The emission intensities of each major luminescence wavelengths of the three UCNPs were extracted under different excitation powers and the values of intensity were fitted by the empirical formula:

 

Fig. 6. Effect of excitation power on upconversion luminescence performance of UCNPs with different structure: excitation power dependent upconversion spectra (a-c) and upconversion photon number fitting curves (d-f) of mononuclear, single-shell encapsulated and double-shell encapsulated UCNPs

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For UCNPs with single-wavelength excitation, the upconversion process can be affected by changing the power density of the excitation light. From the variation in the fitted photon number, it can be seen that the upconversion emission in the blue emission (400 nm-500 nm), green emission (500 nm-600 nm), and red emission (600 nm-750 nm) wavelengths all exhibited the dominance of the three-photon transition process to the mixed role of two-photon and three-photon transition processes. This indicates that under the same excitation power conditions, the electron carrying capacity of the energy levels involved in the two-photon process was gradually increased with the layers encapsulated, and the saturation phenomenon of the energy level population was delayed, which improved the utilization of rare-earth ions for the excitation light and enhanced the upconversion efficiencies of the core–shell–shell structured UCNPs. Usually, at higher excitation power, owing to the large amount of Yb³⁺ in the sensitizing layer, after receiving the excitation energy, the excited Yb³⁺ undergoes resonance energy transfer, causing the adjacent Er³⁺ to reach the ⁴F_7/2 energy level. At this time, this part of Er³⁺ continues to absorb the energy transferred from another excited state Yb³⁺ and transfers to the ²K_15/2 and ²G_9/2 energy levels, after non-radiative relaxation, Er³⁺ reaches more stable ⁴G_11/2 and ²H_9/2 energy levels. Because the energy difference between ⁴G_11/2-⁴F_9/2 and ²H_9/2-⁴F_9/2 has a high degree of matching with that of the ²F_7/2-²F_5/2 leap of Yb³⁺, the excited state electrons of Er³⁺ will be bibbed to the ⁴F_9/2 energy level and then fall back to the ground state after the process of energy transfer back, at this time, the energy level transition ²H_11/2-⁴I_15/2 and ⁴S_3/2-⁴I_15/2, which are dominated by two-photon leaps, are not dominant. Instead, the utility of the excitation light and the energy-level decay time of the UCNPs are enhanced owing to the shells encapsulated. After receiving energy, the two-photon energy level preferentially accumulates a large number of excited-state electrons compared with the three-photon energy level, and the electrons are more inclined to accumulate rather than continue to receive energy and leap to higher energy levels before the energy level decays. Consequently, more electrons are transferred back to the ground state ⁴I_15/2 from the two-photon energy levels ²H_11/2 and ⁴S_3/2, leading to an increase in the proportion of the two-photon jump process in the entire luminescence process.

The improvement in upconversion luminescence performance provides more opportunities for luminescent films for various applications. As shown in Fig. 7(a), the Ag particles were rod-like and smooth-surfaced by reduction with silver nitrate. The UCNPs films, Ag films, and composite films of the two were then deposited on ITO glass using the solvothermal method. Figure 7(b) shows the SEM image of the Ag/UCNPs composite film. It can be observed that the particle size of the UCNPs is large, and the rod-like Ag particles are uniformly dispersed near them, which makes the structure more convenient for resonance energy transfer. Figure 7(c) shows the XRD patterns of the composite films. It can be found that the composite film has obvious diffraction peaks of NaYF₄@NaYbF₄@NaYF₄ and the diffraction peaks of the cubic phase Ag were observed at 2θ = 38.1° and 44.3°, corresponding to the (111) and (200) crystal planes of Ag, respectively. To determine the resonance energy absorption range of Ag, absorption spectra of the fabricated Ag particles were measured (Fig. 7(d)). It can be found that Ag has a significant absorption peak near 410 nm, which exactly corresponds to the blue light range of the upconversion emission of UCNPs. In order to investigate the upconversion enhancement effect of Ag particles on UCNPs, we constructed Ag/UCNPs composite films (the UCNPs film was above the Ag film), UCNPs/Ag composite films (the Ag film was above the UCNPs films) and UCNPs films and performed upconversion tests on all three films, and the results are shown in Fig. 7(e). It was found that the composite films have better upconversion performance than the UCNPs films. And the Ag/UCNPs composite films also gained a significant upconversion effect compared to the UCNPs/Ag composite films, which may due to the fact that Ag nanoparticles are more prone to photothermal effects leading to upconversion luminescence quenching when they are in direct contact with the excitation light.

 

Fig. 7. Optimisation effect of luminescent properties of silver on upconversion films: SEM images of Ag particles (a) and Ag/UCNPs films (b); XRD image of Ag/UCNPs films (c); UV-vis absorption spectra of Ag particles (d); and upconversion emission maps of UCNPs films; UCNPs/Ag films, and Ag/UCNPs films (e)

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Fig. 8. Application of upconversion luminescence in anti-counterfeiting: photographs of luminescence of powder (a) and solution (b) UCNPs with mononuclear, single-shell encapsulated and double-shell encapsulated structures under 980 nm excitation; upconversion anti-counterfeiting schematic diagrams of double-shell encapsulated UCNPs (c and d)

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The enhancement of upconversion luminescence by Ag coating is primarily achieved through the combined effect of localized surface plasmon resonance (SPR) and local field-enhanced excitation power density. Essentially, Ag nanoparticles act as optical nanoantennas, broadening the absorption cross-section or optimizing the radiative efficiency for rare earth ions nearby by modifying the electronic transitions between the ground and excited states. Ag nanoparticles are precious metal nanomaterials with strong surface plasmon resonance effect, and their free electrons are distributed throughout the metal lattice, which is shared by the entire metal lattice. When light is incident on the surface of Ag nanomaterials, the free electrons move in a potential field composed of positive ions and other electrons on the lattice and continuously exchange energy with the potential field, forming a plasmonic resonance phenomenon and generating a strong local electric field near the surface [46–48]. At the resonance frequency, a portion of the incident photons can be absorbed and subsequently converted into phonons leading to lattice vibrations, while another portion of the photons is scattered in all directions at the incident frequency and captured and utilized by the nearby UCNPs. The high concentration of sensitizer ions in the excited state leads to the excitation of the surrounding activator ions to high energy levels through energy transfer, which rapidly enhances the upconversion filling process and improves upconversion emission efficiency. Simultaneously, the surface plasma exciton field-coupled emission enhances the radiative decay rate, thereby increasing the upconversion emission efficiency [49–51]. Thus, the enhancement of the local field effect indirectly increases the pump power density of the excitation light and improves the upconversion excitation efficiency. Therefore, under the influence of the plasmon resonance effect, Ag nanoparticles increase the upconversion excitation rate by local field amplification, and the coupling of the emitted light with the surface plasmon excitations significantly increases the radiative excitation rate of the upconverted nanoparticles, which leads to a substantial enhancement of the upconversion fluorescence intensity.

In order to investigate the role of UCNPs in the field of anti-counterfeiting, luminescence photos of different powder samples of 0.5 mmol at 980 nm were taken (Fig. 8 (a)). Subsequently, the three UCNPs with different structures were dissolved in cyclohexane as solution with a concentration of 0.1 mol/L, and the luminescence photos of the three solutions were taken under the excitation of near-infrared light at 980 nm (Fig. 8 (b)). It can be found that the luminescence intensity of the UCNPs gradually increased with the shells encapsulated and underwent a green-red-yellow luminescence color change, consistent with the previous description. Double-shell encapsulated UCNPs with the highest luminescence intensity were chosen for subsequent anti-counterfeiting applications. We drew the pattern of ‘USST’ with NaF powder on dry-abrasive paper and the letters ‘S’, ‘S’, and ‘T’ were partially covered with a double-shell encapsulated UCNPs, the pattern did not change under natural light (Fig. 8 (c)), while when the pattern was irradiated using 980 nm near-infrared light, significant luminescence appeared at the locations covered with the UCNPs, while the other areas not covered with UCNPs did not show any change under natural light (Fig. 8 (d)). This demonstrates that UCNPs can realize information encoding and effective anti-counterfeiting applications.

4. Conclusion

A stable and excellent luminescence performance is key for the in-depth application of multilayer structured UCNPs in the future. Using β-NaREF₄, which is not susceptible to ion migration, as the matrix, we identified the activator core–sensitizing layer–inert layer UCNPs with the best upconversion performance by designing and synthesizing different molar ratios of the core to the layers and adjusting the activator concentration. The activator Er³⁺ and sensitizer Yb³⁺ were separated into different layers using the β-Na(Y_0.9Er_0.1)F₄@NaYbF₄@NaYF₄ multilayered structure, which significantly inhibited the Yb³⁺-Er³⁺ cross-relaxation and concentration quenching of activator ions and achieved efficient energy transfer upconversion at the interface. The upconversion luminescence intensity substantially enhances the main emission wavelengths. In terms of luminescence stability, UCNPs with core–shell–shell structures have longer fluorescence lifetimes and excellent thermal stability. In addition, the layers encapsulated effectively enhances the electron-carrying capacity of the energy levels involved in the two-photon emission process, which delays the saturation population phenomenon of the energy level, and improves the utility of the excitation light. This strategy of layers cladding provides a simple and effective method for the design and synthesis of lanthanide-doped UCNPs with excellent performance and versatility for various applications. In addition, research on the application of UCNPs has been conducted. Ag/UCNPs composite film was prepared, and the luminescence of the films was significantly enhanced under the effect of plasma resonance. A feasible application strategy for upconversion anti-counterfeiting was designed by combining UCNPs with information coding.