Reduced photon quenching in Ce-doped NaYF<sub>4</sub>:Yb/Ho upconversion nanoparticles with core/shell structure

Shuai Ye; Jun Song; Dong Wang; Yuliang Tian; Junle Qu; Hanben Niu

doi:10.3788/COL201614.021601

Due to their interesting properties, including absence of autofluorescence, low photobleaching, strong penetration abilities, low toxicity, etc., rare-earth-doped upconversion nanoparticles (UCNPs) have attracted increasing interest^[1–5]. These special features provide UCNPs with a great potential for applications in several fields, such as solar cells, solid-state lasers, boilables, and imaging^[6–10]. In particular, among the various applications, in vivo imaging based on UCNPs is expected to be a promising photoluminescence imaging technique, as it provides high sensitivity and spatial resolution, leading to predictive models for potential clinical applications^[11,12]. For in vivo imaging, long-wavelength light (above 600 nm) is necessary as the luminescent probe, as the tissue strongly absorbs light with a short wavelength (below 600 nm)^[13]. However, three of the most important types of UCNPs, i.e., Yb/Er-, Yb/Tm-, and Yb/Ho-codoped

{NaYF}_{4}

nanoparticles, radiate green, blue, and green light, respectively^[14–18].

Presently, most of the research on red-emission UCNPs mainly focuses on the Yb/Er-codoped ${NaYF}_{4}$ nanoparticles, as they show a strong green emission ( $\sim 550 nm$ ) along with a weak dark-red emission ( $\sim 660 nm$ ). The color can be tuned from green to red using different methods: controlling the particle’s size or the Yb concentration^[19–22], changing the surface ligands^[23,24], partially replacing the Y site of the lattice by ${Mn}^{2 +}$ or ${Zr}^{4 +}$ ions^[25–30], etc. Similar to the Yb/Er-codoped ${NaYF}_{4}$ nanoparticles, the Yb/Ho-codoped ${NaYF}_{4}$ nanoparticles also show a strong green emission and a weak red emission, and it is possible to tune the emission color from green to red using different methods. Unfortunately, the tuning seems very hard to achieve. Until now, only one method has been reported to successfully tune the emission color from green to red in Yb/Ho-codoped ${NaYF}_{4}$ nanoparticles by incorporating ${Ce}^{3 +}$ ions and inducing a cross-relaxation process between the ${Ho}^{3 +}$ and ${Ce}^{3 +}$ ions^[31–33]. However, a significant quenching effect due to the Ce incorporation was simultaneously produced, and the upconversion (UC) photoluminescence intensity of the ${NaYF}_{4} : {Yb}^{3 +}$ , ${Ho}^{3 +}$ 1%, ${Ce}^{3 +}$ 15% nanoparticles was several orders of magnitude lower than that of the ${NaYF}_{4} : {Yb}^{3 +}$ , ${Ho}^{3 +}$ 1% nanoparticles.

In our previous research, core/shell structured ${NaYF}_{4} : {Yb}^{3 +}$ at ${NaYF}_{4} : {Ho}^{3 +} 1 %$ nanoparticles were proven to enhance the red emission by increasing the ${Yb}^{3 +}$ concentration in the core^[34]. Here, the ${Ce}^{3 +}$ ions were incorporated into the core/shell structured ${NaYF}_{4} : {Yb}^{3 +}$ @ ${NaYF}_{4} : {Ho}^{3 +}$ nanoparticles to further enhance the red emission; in addition, a method to eliminate the quenching effect is proposed.

Two types of ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles with incorporated ${Ce}^{3 +}$ ions were prepared to investigate the influence of the ${Ce}^{3 +}$ ions. In addition, ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 0.5% and ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 0.5%, ${Ce}^{3 +}$ 20% nanoparticles were also prepared as the contrastive samples. All samples were prepared by the procedure descripted in our previous work^[34]. Figure 1 shows the typical transmission electron microscopy morphologies of such nanoparticles. The mono-dispersed ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 0.5%, ${Ce}^{3 +}$ 20% nanospheres exhibited an average size of $\sim 24 nm$ , similar to that of the conventional ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 0.5% nanospheres, revealing that the ${Ce}^{3 +}$ incorporation had a minimal effect on the nanoparticle morphology and size. The ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles exhibited a hexagonal shape with an average size of $\sim 55 nm$ . The larger size of these nanoparticles was mainly due to the high ${Yb}^{3 +}$ concentration in the core. The two types of ${Ce}^{3 +}$ -incorporated ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles were also hexagonal and showed a similar size to the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles. The selected-area electron diffraction (SAED) patterns of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles confirmed their hexagonal shape with crystallographic phases belonging to the standard hexagonal ${NaYF}_{4}$ host lattice (JCPDS 28-1192). All the results indicated that the ${Ce}^{3 +}$ incorporation in the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles did not affect the microstructure, morphology, or size, as in the case of the ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 1% nanoparticles.

Figure 1.Transmission electron microscopy morphologies of (a) ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 1%, (b) ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 0.5%, ${Ce}^{3 +}$ 20%, (c) ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , (d) ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ @ ${NaYF}_{4} : {Ce}^{3 +} 20 %$ , (e) ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles (the insets show the size distribution of the corresponding nanoparticles), and (f) SAED pattern of ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 1 % : {Ce}^{3 +} 20 %$ nanoparticles.

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The photoluminescence spectra of the nanoparticles, with and without incorporated ${Ce}^{3 +}$ , were measured using a 980 nm laser excitation, as shown in Fig. 2. The photoluminescence intensity of ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 1%, ${Ce}^{3 +}$ 20% was reduced by 97%, although the $I_{Red} / I_{Green}$ ratio was enhanced from 0.9 to 7 compared with that of the ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 1% nanoparticles (Fig. 2(a)). This result revealed a strong quenching effect in the nanoparticles due to the ${Ce}^{3 +}$ incorporation. However, a slight effect of the ${Ce}^{3 +}$ incorporation on the emission intensity was observed in the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles, as shown in Fig. 2(b). Compared with the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles without incorporated ${Ce}^{3 +}$ , the photoluminescence intensity of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles shelled by ${NaYF}_{4} : {Ce}^{3 +}$ 20% was reduced by $\sim 8 %$ , and the $I_{Red} / I_{Green}$ ratio was approximately equal to 7, slightly higher than that of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles. This result indicated that an enhancement of the red emission of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles was not achievable by only coating them with a ${Ce}^{3 +}$ -contained shell layer. When the ${Ce}^{3 +}$ ions were incorporated into the shell of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles, the $I_{Red} / I_{Green}$ ratio was enhanced to 11, nearly twice that of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles, although the photoluminescence intensity was also reduced by $\sim 22 %$ . However, the emission intensity of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles was enhanced by a factor of more than 120 compared with that of the ${NaYF}_{4} : {Yb}^{3 +}$ 50%, ${Ho}^{3 +}$ 1%, ${Ce}^{3 +}$ 20% nanoparticles. These results demonstrated the weak quenching effect that exists in the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles.

Figure 2.Photoluminescence spectra of different types of ${Ce}^{3 +}$ ion incorporations: (a) ${NaYF}_{4} : {Yb}^{3 +}$ 20%, ${Ho}^{3 +}$ 1% and (b) ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles. (c) Photoluminescence intensity of the green and red emission bands. (d) The intensity ratios from the nanoparticles in (a) and (b).

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The dependence of the intensities of the green and the red UC emission band on the pump power for the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ UCNPs was measured, as shown in Fig. 3. In general, the photoluminescence intensity increased on the pump laser power and obeyed the rule of $I_{UCL \propto} p^{n}$ , where $I_{UCL}$ is the photoluminescence intensity, $p$ is the pump laser power, and $n$ is the number of laser photons required. The slope values for the green and the red emission bands of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ UCNPs approached 2, indicating that both the green and the red emissions involve a two-photon process for their generations. The slope values were in good agreement with previous results on ${Yb}^{3 +} / {Ho}^{3 +}$ -codoped UCNPs^[32,34].

Figure 3.Logarithmic plot of the dependence of the intensities of the green and the red UC emission bands on the pump power for the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 1 %$ , ${Ce}^{3 +} 20 %$ UCNPs.

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High emission intensity was observed in the core/shell nanoparticles consisting of a core containing ${Yb}^{3 +}$ ions and a shell containing ${Ho}^{3 +}$ and ${Ce}^{3 +}$ ions. This result implied that the strong quenching effect in the Yb/Ho/Ce-tridoped ${NaYF}_{4}$ nanoparticles may be related to the interaction between the ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions. To verify and provide a theoretical background to this hypothesis, the steady-state rate equations were used. In this physical model, $N_{Yb 0}$ and $N_{Yb 1}$ are the population densities of the ${Yb}^{3 +}$ ions in the ground and the excited states, respectively; $N_{0}$ , $N_{1}$ , $N_{2}$ , $N_{3}$ , and $N_{4}$ are the population densities of the ${}^{5}{I_{8}}$ , ${}^{5}{I_{7}}$ , ${}^{5}{I_{6}}$ , ${}^{5}{F_{5}}$ , and ${}^{5}{S_{2}} / {{}^{5}F}_{4}$ states, respectively, of the ${Ho}^{3 +}$ ions; and $N_{Ce 0}$ and $N_{Ce 1}$ are the population densities of the ${Ce}^{3 +}$ ions in the ground and the excited states, respectively. In addition, $W_{0}$ , $W_{1}$ , and $W_{2}$ are the energy transfer rates from the excited ${Yb}^{3 +}$ ions to the ${Ho}^{3 +}$ ions, and $R_{1}$ , $R_{2}$ , $R_{3}$ , and $R_{4}$ are the radiation rates of the energy states of the ${Ho}^{3 +}$ ions. $β_{1}$ and $β_{2}$ are the phonon-assisted nonradiative relaxation rates from the ${}^{5}{I_{6}}$ to ${}^{5}{I_{7}}$ and from the ${}^{5}{S_{2}} / {{}^{5}F}_{4}$ to ${}^{5}{F_{5}}$ states, respectively, of the ${Ho}^{3 +}$ ions; $C_{2}$ and $C_{4}$ are the coefficients of the cross relaxations between ${Ho}^{3 +}$ and ${Ce}^{3 +}$ ions in the ${}^{5}{I_{6}}$ and ${}^{5}{S_{2}} / {{}^{5}F}_{4}$ states, respectively; and $M$ is the coefficient of the cross relaxation between the ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions. $I$ is the laser intensity at 980 nm, $ν$ is the laser frequency, $σ_{Yb}$ is the absorption cross section of the ${Yb}^{3 +}$ ion, $R$ is the radiation rate of the excited state of ${Yb}^{3 +}$ , and $R_{Ce}$ is the radiation rate of the excited state of ${Ce}^{3 +}$ . The steady-state rate equations for the discussed system can be described as follows: After solving Eqs. (1) and (2), we obtained the following expressions: $N_{1} = [\frac{(β_{1} + C_{2} N_{Ce 0}) W_{0} N_{0}}{R_{1} (β_{1} + C_{2} N_{Ce 0} + R_{2})} - \frac{W_{1}}{R_{1}}] N_{Yb 1},$ (3) $N_{2} = \frac{W_{0} N_{0}}{β_{1} + C_{2} N_{Ce 0} + R_{2} + W_{2} N_{Yb 1}} N_{Yb 1},$ (4) $N_{3} = [\frac{(β_{1} + C_{2} N_{Ce 0}) W_{1}}{R_{1} + W_{1} N_{Yb 1}} + \frac{(β_{2} + C_{4} N_{Ce 0}) W_{2}}{R_{4} + C_{4} N_{Ce 0} + β_{2}}] \frac{W_{0} N_{0} {(N_{Yb 1})}^{2}}{(R_{2} + β_{1} + C_{2} N_{Ce 0} + W_{2} N_{Yb 1}) R_{3}},$ (5) $N_{4} = \frac{W_{0} W_{2} N_{0} {(N_{Yb 1})}^{2}}{(R_{4} + C_{4} N_{Ce 0} + β_{2}) (R_{2} + C_{2} N_{Ce 0} + β_{1} + W_{2} N_{Yb 1})},$ (6) $N_{Yb 1} = \frac{I σ_{Yb} N_{Yb 0}}{h ν (R_{Yb} + W_{0} N_{0} + W_{1} N_{1} + W_{2} N_{2} + M N_{Ce 0})},$ (7) $N_{Ce 0} = \frac{R_{Ce} N_{Ce 1}}{C_{2} N_{2} + C_{4} N_{4} + M N_{Yb 1}} .$ (8)Then, we can obtain the intensities of the red and green lights by applying the following equations: $I_{r} = N_{3} h ν_{r} R_{3},$ (9) $I_{g} = N_{4} h ν_{g} R_{4},$ (10)where $ν_{r}$ and $ν_{g}$ are the frequencies of the red light and green light, respectively. Furthermore, by combining Eqs. (5), (6), (9), and (10), we can derive the following expressions: $I_{r} = [\frac{(β_{1} + C_{2} N_{Ce 0}) W_{1}}{R_{1} + W_{1} N_{Yb 1}} + \frac{(β_{2} + C_{4} N_{Ce 0}) W_{2}}{R_{4} + C_{4} N_{Ce 0} + β_{2}}] \frac{W_{0} N_{0} {(N_{Yb 1})}^{2}}{(R_{2} + β_{1} + C_{2} N_{Ce 0} + W_{2} N_{Yb 1})} h ν_{r},$ (11) $I_{g} = \frac{R_{4} W_{0} W_{2} N_{0} {(N_{Yb 1})}^{2}}{(R_{4} + C_{4} N_{Ce 0} + β_{2}) (R_{2} + C_{2} N_{Ce 0} + β_{1} + W_{2} N_{Yb 1})} h ν_{g} .$ (12)

According to Eqs. (7) and (8), we know that $N_{Yb 1}, N_{Ce 0} \propto \frac{1}{1 + M} .$ (13)

Moreover, considering Eqs. (11) and (12), we can also obtain the following expressions: $I_{r} \propto \frac{1}{{(1 + M)}^{2} [R_{2} + β_{1} + C_{2} (1 + M) + W_{2} (1 + M)]},$ (14) $I_{g} \propto \frac{1}{[(R_{4} + β_{2}) (1 + M) + C_{4}] [(R_{2} + β_{1}) (1 + M) + C_{2} + W_{2}]} .$ (15)

Hence, from Eqs. (14) and (15), it was demonstrated that both the intensities of the red and green lights decrease by the coefficient $M$ of the cross relaxation between ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions, although the two cross relaxations between ${Ho}^{3 +}$ and ${Ce}^{3 +}$ ions can lead to an enhancement of the red light radiation by reducing the green light radiation, as shown in Fig. 4(b). This model is able to explain the low emission intensity of the Yb/Ho/Ce-tridoped ${NaYF}_{4}$ nanoparticles. When the ${Ho}^{3 +}$ and ${Ce}^{3 +}$ ions were moved to the shell layer, as shown in Fig. 4(c), the cross relaxation between the ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions could be reduced drastically as a result of the extended distance between the ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions, while the cross relaxation between the ${Ho}^{3 +}$ and ${Ce}^{3 +}$ ions was kept constant. Thus, high-intensity red light radiation was obtained for the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles. The $\sim 22 %$ reduction of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ , ${Ce}^{3 +} 20 %$ nanoparticles, compared with that of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles (Fig. 2), mainly resulted from the cross relaxation between the ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions at the core/shell interface. The $\sim 8 %$ reduction of the ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ @ ${NaYF}_{4} : {Ce}^{3 +} 20 %$ nanoparticles indicated that the quenching effect was further reduced by extending the distance between the ${Yb}^{3 +}$ and ${Ce}^{3 +}$ ions.

Figure 4.(a) Energy level diagrams of ${Ho}^{3 +}$ , ${Yb}^{3 +}$ , and ${Ce}^{3 +}$ ions and proposed UC mechanisms. (b) Schematic illustration of the proposed energy-transfer mechanisms in ${NaYF}_{4} : Yb / Ho / Ce$ nanoparticles. (c) Schematic illustration of the proposed energy-transfer mechanisms in core/shell ${NaYF}_{4} : Yb$ @ ${NaYF}_{4} : Ho / Ce$ nanoparticles.

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In conclusion, the ${Ce}^{3 +}$ ions are incorporated into the Yb/Ho-codoped ${NaYF}_{4}$ nanoparticles to enhance the red emission. The ${Ce}^{3 +}$ incorporation enhances the intensity ratio between the red emission and the green emission of the ${NaYF}_{4} : Yb / Ho$ nanoparticles, but largely reduces the total photoluminescence intensity. However, when the ${Ce}^{3 +}$ ions are incorporated into the shell of the core/shell ${NaYF}_{4} : {Yb}^{3 +} 50 %$ @ ${NaYF}_{4} : {Ho}^{3 +} 0.5 %$ nanoparticles, the emission intensity is also enhanced by a factor of more than 120 compared with that of the ${NaYF}_{4} : Yb / Ho / Ce$ nanoparticles. This result indicates that the ${Ce}^{3 +}$ incorporation into the ${NaYF}_{4}; Yb / Ho$ nanoparticles promotes a strong quenching effect that reduces the emission intensity; the quenching effect is significantly reduced by incorporating the ${Ce}^{3 +}$ ions into the core/shell structured Yb/Ho codoped ${NaYF}_{4}$ nanoparticles. A theoretical model is proposed to explain the quenching effect existing in the ${NaYF}_{4} : Yb / Ho / Ce$ nanoparticles, revealing that the quenching is mainly related to the interaction between the ${Yb}^{3 +}$ ions and the ${Ce}^{3 +}$ ions.

Category: Materials

Received: Oct. 12, 2015

Accepted: Dec. 11, 2015

Published Online: Sep. 21, 2018

The Author Email: Jun Song (songjun@szu.edu.cn), Junle Qu (jlqu@szu.edu.cn)

DOI:10.3788/COL201614.021601