Improved Photothermal Heating of NaNdF₄ Microcrystals via Low-Level Doping of Sm³⁺ for Thermal-Responsive Upconversion Luminescence Anti-Counterfeiting

Abstract

This work reports the light-to-heat conversion (LHC) behavior of NaNdF₄ doped with Sm³⁺. Due to the cross-relaxation between Nd³⁺ and Sm³⁺, the improved LHC is obtainable by introducing 5% Sm³⁺. When the laser power density is only 1.72 W/cm², the spot temperature of NaNdF₄:5%Sm³⁺ powder reaches as high as 138.7 ± 4.04 °C. More importantly, the photoheating response to the pump laser has favorable linear characteristics within a specific power range. A simple physical model is applied to analyze the relationship between photothermal heating and pump power. Finally, the temperature-responsive luminescence anti-counterfeiting is designed by combining the LHC material with the NaYF₄:Yb³⁺/Ho³⁺/Ce³⁺ microcrystals. This novel strategy only requires two laser beams, and thus is more convenient to apply.

1. Introduction

Photothermal heating (i.e., electron−phonon coupling under radiation excitation) has been considered harmful to luminescence [1]. However, it has recently attracted increasing attention because of its potential applicability in tumor photothermal therapy [2,3,4,5].

In principle, the ion centers for light-to-heat conversion (LHC) should have a high state density to allow for the fast multi-phonon relaxation processes. Moreover, it is significant to incorporate a high concentration of LHC ions into the host materials since the cross-relaxation between LHC ions is another effective channel to quench luminescence. Due to meager quantum yield, Yb³⁺-sensitized upconversion luminescence (UCL) materials have been reported for optical heaters [6,7,8]. However, this strategy is not particularly ideal because, on the one hand, the Yb³⁺ ions produce strong ~1000 nm emission [9], while on the other hand, the energy gap between some levels of Er³⁺, Tm³⁺, and Ho³⁺ is relatively large, which makes it difficult for the multi-phonon relaxation to occur. To solve the issues, Chen et al. selected Dy³⁺ and Sm³⁺, with a high state density in the energy range of 0–10,000 cm⁻¹, as LHC ions, thus obtaining favorable photothermal heating properties [10,11]. Compared to Yb³⁺, Nd³⁺ possesses a larger absorbing cross-section, and its relatively abundant energy levels allow it to be a self-sensitized LHC ion [12,13]. Notably, the pure Nd³⁺-based materials (e.g., NaNdF₄) emit efficient near-infrared light when excited at ~800 nm radiation [14,15], which means the LHC is improvable via quenching the luminescence. Recently, Yu et al. reported that designing Prussian blue-coated NaNdF₄ nanoparticles is an effective strategy [16]. In contrast, Nd³⁺/Sm³⁺ codoping is much simpler in the preparation procedure. Nevertheless, the previous reports doped relatively low concentrations of Nd³⁺ or high concentrations of Sm³⁺, resulting in low radiation absorption at ~800 nm [12,17]. A more reasonable doping should be to fully quench the luminescence of Nd³⁺ ions without significantly affecting the light absorption. That is, the content of Nd³⁺ should be high, while that of Sm³⁺ is relatively low.

On the other hand, some studies have shown that the thermal-responsive UCL color tuning is potentially applicable in advanced luminescence anti-counterfeiting [18,19,20,21]. Nevertheless, external heating prevents its real applications. Applying Nd³⁺-sensitized LHC materials to tune the upconversion luminescence seems a promising strategy. But as far as we know, there have been no relevant reports.

In this work, 5% Sm³⁺ is introduced into the NaNdF₄ lattice to quench the near-infrared emission of Nd³⁺ under 808 nm excitation. As a result, the improved LHC property is obtainable. Mechanism studies indicate that the absorbed excitation energy is completely converted into thermal energy through the self-quench of Nd³⁺ and the cross-relaxation between Nd³⁺ and Sm³⁺. Impressively, when the change of dissipation temperature with power is ignorable, the spot temperature of NaNdF₄:5%Sm³⁺ has a favorable linear relationship with the pump power density. Finally, this LHC material is applied to regulate the UCL color of NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺ under 980 nm excitation. Compared to the external temperature control, this novel strategy only requires two laser beams and thus is more convenient to use.

2. Results and Discussion

Figure 1a,b shows the Rietveld refinement of XRD data for the samples obtained by adjusting the ratio of Ln³⁺ and F⁻. R_wp and R_p below 10% suggest that the phase purity is high. More interestingly, the phase structure of fluorides is controllable. When the molar ratio of Ln³⁺ to F⁻ is 4:1 and 12:1, the as-prepared samples are indexed to hexagonal NdF₃ (JCPDS No. 09-0416) and NaNdF₄ (JCPDS No. 72-1532), respectively. After introducing 5% Sm³⁺ into the host, no impurities are observable, which suggests that the introduction of Sm³⁺ ions does not destroy the crystal structure. Based on Hume-Rothery rules for atomic substitution [22], it can be inferred that the doped Sm³⁺ ions occupy the Nd³⁺ sites. SEM images, shown in Figure 1c,d, reveal that NdF₃:5%Sm³⁺ and NaNdF₄:5%Sm³⁺ present different preferred orientation growth, which can be attributed to the difference in surface density of the lattice [18]. Furthermore, EDX mappings show that the Sm³⁺ ions are uniformly distributed in the lattice of the NaNdF₄:5%Sm³⁺ sample, as shown in Figure 1e–i. The concentration ratio of Nd to Sm is estimated to be about 19.8, which is very close to the designed molar ratio.

Figure 2 shows the heating characteristics of NdF₃:5%Sm³⁺ and NaNdF₄:5%Sm³⁺ samples under 808 nm laser irradiation. Both samples show good LHC behavior, but the latter is better due to its stronger light absorption. When the laser power density is 1.72 W/cm², the spot temperature of NaNdF₄:5%Sm³⁺ powder reaches 138.7 ± 4.04 °C, higher than NaGdF₄:Yb³⁺/Er³⁺ [6] and NaNdF₄ (~128.4°C, in this work) under the same excitation conditions. More interestingly, the photoheating response of NaNdF₄:5%Sm³⁺ to the pump laser has favorable linear characteristics within a specific power range. Still, overall, the responsiveness (i.e., the slope of the linear fit) is on a downward trend. In addition, due to high energy dissipation, increased measurement errors are observed when the pump power density is beyond 6 W/cm².

Nd³⁺ and Sm³⁺ are both excellent luminescence centers [23,24,25], especially, the proportion between an electric dipole to magnetic dipole transition (i.e., symmetry ratio) of Sm³⁺ ions can be used to examine its local environment [26]. Herein, to understand the LHC behavior of NdF₃:5%Sm³⁺ and NaNdF₄:5%Sm³⁺, we tested the photoluminescence spectra of the samples with and without Sm³⁺ ions under 808 nm excitation. As shown in Figure 3a, NdF₃ and NaNdF₄ produce typical near-infrared emission in the range of 850–910 nm, corresponding to the ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 transitions of Nd³⁺, respectively. However, the luminescence quenching occurs after being doped with Sm³⁺. When the Sm³⁺ concentration is 5%, the emission peaks are completely quenched. Continuing to increase the concentration of Sm³⁺ (e.g., 40%), would result in little observable change in the spectrum.

No emissions belonging to Sm³⁺ ions are observed in the Sm³⁺-doped sample, indicating the doped Sm³⁺ acts only as the quenching center of luminescence. Direct evidence can be found in Figure 3b, in which the emission lifetime of Nd³⁺ decreases with increasing the Sm³⁺ concentration. Taken together, Figure 3c illustrates the proposed mechanism of photon absorption, radiation transition, and non-radiative transition. When excited at 808 nm, Nd³⁺ first transitions from the ⁴I_9/2 level to the ⁴F_5/2 state and then rapidly decays to the ⁴F_3/2 level via the multi-phonon relaxation. Since the energy gap (~5380 cm⁻¹) from this level to the ⁴I_15/2 level far exceeds the phonon energy of NdF₃ and NaNdF₄ [15,27,28], the electrons at the ⁴I_15/2 level mainly return to ⁴I_11/2 and ⁴I_13/2 levels through radiation transition. However, after introducing Sm³⁺, the following cross-relaxation processes occur and quench the luminescence of Nd³⁺.

CR1: ⁴F_3/2(Nd³⁺) + ⁶H_5/2(Sm³⁺)→⁴I_15/2(Nd³⁺) + ⁶H_13/2(Sm³⁺);

CR2: ⁴F_3/2(Nd³⁺) + ⁶H_5/2(Sm³⁺)→⁴I_13/2(Nd³⁺) + ⁶F_5/2(Sm³⁺);

CR3: ⁴F_3/2(Nd³⁺) + ⁶H_5/2(Sm³⁺)→⁴I_11/2(Nd³⁺) + ⁶F_9/2(Sm³⁺);

CR4: ⁴F_3/2(Nd³⁺) + ⁶H_5/2(Sm³⁺)→⁴I_9/2(Nd³⁺) + ⁶F_11/2(Sm³⁺).

In summary, herein the absorbed excitation energy is completely converted into thermal energy through the self-quench of Nd³⁺ [12,13] and the cross-relaxation between Nd³⁺ and Sm³⁺.

The inset in Figure 2 shows a simple model for analyzing the relationship between LHC and pump laser power. As shown, Q_a, Q_d, and Q_T represent the absorbed excitation energy, dissipated thermal energy, and residual thermal energy, respectively. According to the principle of conservation, Q_T = Q_a − Q_d. Therefore, the temperature function is written as

$$T_s={\displaystyle \frac{\rho \sigma N_0}{C}}-{\displaystyle \frac{Q_d}{C}}+T_0$$

(1)

where T_s is the radiation spot temperature, ρ represents the 808 nm laser power density, C represents the heat capacity of the material, σ represents the transition cross-section from the Nd³⁺ ground state to ⁴F_5/2, N₀ represents the population of Nd³⁺ ground state, and T₀ is the ambient temperature. Furthermore, according to the dimensions of physical quantities, the first and second terms on the right side of Equation (1) can be defined as the light absorption temperature (T_a) and the heat dissipation temperature (T_d), respectively. In this case, Equation (1) is rewritten as the following simplified form:

$$T_s=T_a-T_d+T_0$$

(2)

It is evident that T_a and T_d are increasing functions of pump power. However, T_a is always greater than T_d, and thus T_s increases monotonically with the increase in pump power. Impressively, when T_d does not change much with power, T_s can be approximately regarded as a linear function of pump power. The analysis is consistent with the experimental results in Figure 2, indicating that the physical model we proposed is reasonable.

As a proof of concept, the LHC of NaNdF₄:5%Sm³⁺ will be applied to regulate the UCL color of NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺ microcrystals. Figure 4a,b shows the XRD pattern and SEM image of NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺, respectively. It is clear that the as-prepared UCL material belongs to the hexagonal structure, and its particles appear as short rods. Furthermore, the temperature-dependent UCL properties, shown in Figure 4c, indicate that the NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺ is applicable in thermal-responsive UCL color regulation. The possible mechanism is given in Figure 4d. Since heating improves the phonon-assisted CR1 and CR2 processes, the UCL color tuning from green to red is observed [29]. As shown in Figure 4e, a luminescence anti-counterfeiting is designed by using NaNdF₄:5%Sm³⁺ and NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺, in which the Chinese character “Light” is printed on a transparent glass substrate, and NaNdF₄:5%Sm³⁺ and silica gel are mixed and evenly coated on the Chinese character to form a dense heating film (referred to as NNS film). As expected, when irradiated from the glass substrate side with a 980 nm laser, clear green fonts can be observed (Figure 4f), followed by the spot temperature of 35 °C. However, when 808 nm is used to excite the NNS film, the color of the font quickly turns to red, corresponding to the spot temperature as high as 180 °C. Keeping the 808 nm radiation but turning off the 980 nm laser, the temperature of the irradiated area remains almost unchanged, but the luminous font is no longer visible. Notably, herein 808 nm and 980 nm lasers are used as a thermal- and optical-switch, respectively, and thus the external temperature-control device is abandoned.

3. Experimental Section

3.1. Preparation of Sm³⁺-Doped NdF₃ and NaNdF₄ Microcrystals

A hydrothermal method was used to prepare the NdF₃ and NaNdF₄ microcrystals doped with different concentrations of Sm³⁺. Typically, LnNO₃ (Ln³⁺ = 95 mol% Nd³⁺ + 5 mol% Sm³⁺) solution (20 mL) was obtained by dissolving Nd(NO₃)₃·6H₂O (99.9%, Aladdin Scientific Corp., Riverside, CA, USA) and Sm(NO₃)₃·6H₂O (99.9%, Aladdin Scientific Corp.) with deionized water, and then 1.7295 g C₆H₈O₇∙H₂O (A.C.S. grade, Aladdin Scientific Corp.) (mole ratio, citrate/Ln³⁺ = 2/1) was added under vigorous stirring. Subsequently, another 20 mL aqueous solution containing 2.0741 g NaF (A.C.S. grade, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China)was added slowly drop by drop (mole ratio, F^-/Ln³⁺ = 12/1). After 20 min, the resulting precursor solution was transferred to a 50 mL autoclave. The autoclave was heated at 200 °C for 12 h and then allowed to cool down to room temperature naturally. The product was separated from the reaction media by centrifugation and then washed three times with deionized water. After being dried at 50 °C for 12 h, the white phosphors were obtained.

3.2. Preparation of NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺ Upconversion Materials

A similar procedure was used to prepare NaYF₄:20%Yb³⁺/2%Ho³⁺/5%Ce³⁺ upconversion microcrystals, in which Ln = 73 mol% Y + 20 mol% Yb + 2 mol% Ho + 5 mol% Ce, citrate/Ln³⁺ = 2/1, and F⁻/Ln³⁺ = 12/1.

3.3. Characterization

X-ray diffraction (XRD) analysis was performed at 40 kV and 15 mA using a Miniflex600 X-ray generator (Rigaku Corp., Tokyo, Japan) with Cu K_α radiation (λ = 1.5406 Å). The 2θ scan range was 10–90° with a step size of 0.02°. Sample morphology was determined using a JSM-7800F (JEOL Ltd., Tokyo, Japan) scanning electric microscope (SEM). Energy dispersive X-ray (EDX) mapping was determined using a Phenom Pharos G2 (Phenom-World B.V., Eindhoven, The Netherlands) desktop scanning electron microscope equipped with elemental mapping. Photoluminescence spectra were collected by a portable spectrometer (Maya2000Pro, Ocean Optics Co., Hong Kong) using a continuous 980 nm diode laser as the excitation source. The photothermal heating was evaluated by a Fotric 288 infrared thermal imaging camera (Feichuke Intelligent Technology Co., Ltd., Shanghai, China).

4. Conclusions

The light-to-heat conversion (LHC) behavior of NaNdF₄ doped with Sm³⁺ is reported. Due to the cross-relaxation between Nd³⁺ and Sm³⁺, the improved LHC is obtained after introducing the 5% Sm³⁺. The principle of conservation is applied to understand the relationship between photothermal heating and pump laser power. Results show that when the change of dissipation temperature with power is ignorable, the spot temperature of NaNdF₄:5%Sm³⁺ has a favorable linear relationship with the pump power density. Combining the NaNdF₄:5%Sm³⁺ with the classic NaYF₄:Yb³⁺/Ho³⁺/Ce³⁺, dynamic luminescence anti-counterfeiting is developed. This novel strategy only requires two laser beams and thus, is more convenient to apply compared to the external temperature control.