The Influence of SiO₂ Shell on Fluorescent Properties of LaF₃:Nd³⁺ /SiO₂ Core/Shell Nanoparticles

Abstract

Distinct effects of the SiO₂ shell on fluorescence properties of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles were demonstrated by annealling the nanoparticles at different temperatures. Emission spectra, excitation spectra, and decay curves of the nanoparticles were measured. A significant improvement of fluorescence intensity was observed for LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles annealed at 900^∘C. The phenomenon is ascribed to the change of environment of LaF₃:Nd³⁺ core which is imposed by SiO₂ shell. And the change is confirmed by the excitation spectra. It provides a useful way to improve fluorescent intensity of the SiO₂-coated LaF₃:Nd³⁺ nanoparticles. The lifetime for nanoparticles annealed at 900^∘C shows a slight decrease contrast with nanoparticles annealed at 400 and 600^∘C. This is caused by the higher phonon energy of SiO₂ than that of LaF₃.

1. Introduction

In the past decade, the synthesis of lanthanide-doped nanoparticles has attracted a great deal of attention, since the materials are considered as potentially useful phosphors in lamps, display devices [1], components in optical telecommunication [2], new optoelectronic devices [3],and probes in biomedical imaging and detection [4]. LaF₃ possessing low phonon energy, adequate thermal and environmental stability, is regarded as excellent host matrixes for performing luminescence [5, 6]. Nanoparticles of LaF₃ doped with lanthanide ions have been studied for years for their luminescence properties [7, 8]. However, the water and organic molecules absorbed on nanoparticles noticeably hampered their optical efficiency, when the nanoparticles are dispersed into aqueous and organic environment. The O-H and C-H groups have a high vibration frequency and can efficiently quench the luminescence of lanthanide ions [9, 10]. This is in particular true for the lanthanide ions emitted in the near-infrared region, like Nd³⁺, Yb³⁺, and Er³⁺ because the energy gap between excited state and ground state is small [11]. Fortunately, these problems can be overcome when an appropriate shell is grown around the lanthanide ions doped LaF₃ core, and silica is usually used as a coating layer due to its high chemical stability, optical transparency, and biocompatibility [12]. LaF₃ nanoparticles with different thickness of SiO₂ shell were synthesized and the LaF₃:Nd³⁺/SiO₂ core/shell nanomaterials used for biological NIR probes has been reported [13, 14]. However, except the protection effect of SiO₂ layer, the influence of SiO₂ shell on fluorescent properties of the Lanthanide-doped LaF₃ core has seldom been discussed.

In this work, To investigate the interactions between SiO₂ shell and lanthanide ions doped LaF₃ core, a series of Nd³⁺-doped LaF₃ nanoparticles capped with SiO₂ shell were synthesized and annealed at different temperatures. When the anneal temperature is 900^∘C, spectroscopic evidence for the change of LaF₃ environment created by SiO₂ shell was observed. And the change of environment leads to a significant improvement of the fluorescent intensity of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles. This provides a simple and useful way to improve the fluorescent properties of lanthanide-doped LaF₃/SiO₂ core/shell nanomaterials.

2. Experimental

The SiO₂-coated LaF₃:Nd³⁺ nanoparticles were synthesized as follows. NH₄F (0.44 g, 12 mmol) was dissolved in methanol (20 mL), and then the solution was heated to 65^∘C. Another solution of La(NO)₃⋅6H₂O (1.694 g, 3.89 mmol), Nd(NO)₃⋅6H₂O (0.052 g, 0.11 mmol) in methanol (10 mL) was added dropwise to the NH₄F solution. The resulting solution was stirred at 65^∘C for 2 h. And the LaF₃:Nd³⁺ nanoparticles were collected by centrifugation. Stöber method was adopted for the SiO₂ coating process. LaF₃:Nd³⁺ nanopartilces (0.5 g) were dispersed in ethanol (100 mL). Then tetraethyl orthosilicate (TEOS) (0.2 mL, 1 mmol) was added dropwise to the solution. After mixing for 1 min, NH₄OH (25%) (3 mL) were added in the mixture under stirring. The mixture was stirred for 3 h to get the as-grown sample. For better crystallinity and enhanced luminescence, the as-grown sample was annealed in the air for 4 h to get the final product.

The Inductively coupled plasma (ICP) analyses were carried out on a Hitachi P-4010 inductively coupled plasma emission spectrometer. X-ray diffraction (XRD) patterns were measured on a Rigaku D/max-2400 X-ray powder diffractometer. The size and morphology of nanoparticles were determined at 300 kV by a JEOL JEM-3010 transmission electron microscope (TEM) and XL30 field-emission scanning electron microscope (SEM). Photoluminescence emission spectra were recorded on a Zolix Omini-k 300 spectrophotometer pumped by a laser diode at 800 nm. The Fourier transform infrared (FTIR) spectra were made with a Shimadzu FT-IR 8900 spectrometer. The excitation spectra were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter.

3. Results and Discussion

The concentrations of Nd, La, and Si in LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles were determined to be 1.79, 57.1, and 7.93% by ICP. The XRD patterns of samples annealed at different temperatures are shown in Figure 1. When the as-prepared sample was heated at 900^∘C for 4 h, well-defined diffraction peaks were obtained. All the peaks can be well indexed to the hexagonal LaF₃ crystal structure, and no trace of other characteristic peaks were observed. The TEM and SEM images provide direct information about the sizes and typical shapes of the nanoparticles. Figure 2 illustrates the representative TEM and SEM images of the LaF₃:Nd³⁺ nanoparticles and those coated of SiO₂ shells. The bare LaF₃:Nd³⁺ sample contains nanoparticles with an average size of 8 nm. After coating the SiO₂ shell, it is clearly observed that the particles have a core-shell structure and the silica shell thickness is about 5–7 nm. When the LaF₃:Nd³⁺/SiO₂ nanoparticles were annealed at 900^∘C for 4 h, the morphology of the sample aggregates with a size from 30–60 nm, and the SEM image (Figure 2(d)) showed that the annealed LaF₃:Nd³⁺/SiO₂ nanoparticles consists of spherical particles with a size between 50–150 nm.

Figure 1 

XRD patterns of LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at different temperatures.

(a)

(b)

(c)

(d)

Figure 2 

TEM images of (a) LaF3:Nd3+, (b) LaF3:Nd3+/SiO2, (c) LaF3:Nd3+/SiO2 annealed at 900∘C, SEM image of (d) LaF3:Nd3+/SiO2 annealed at 900∘C.

The FTIR spectra of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles are presented in Figure 3. Strong vibrational absorption bands at 3400–3600 and 1350–1600 cm⁻¹ were observed in as-prepared sample, which correspond to O-H mode. So the physically adsorbed solvent and O-H groups on the as-prepared nanoparticles are still not removed. Whereas for the nanoparticles annealed at 400^∘C, the former absorption peaks show a great decrease. When the anneal temperature raised to 900^∘C, the absorption peaks of O-H mode completely disappeared. Thus, the nonradiative vibrational excitation of Nd³⁺ in the nanoparticles created by O-H and C-H groups can be excluded.

Figure 3 

FTIR spectra of LaF3:Nd3+/SiO2 core/shell nanoparticles annealed at different temperatures.

Figure 4 shows the room temperature emission spectra of LaF₃:Nd³⁺/SiO₂ core/shell and LaF₃:Nd³⁺ nanoparticles under excitation at 808 nm. The emission lines centered at 880, 1060, and 1330 nm correspond to the transitions from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2, respectively [15]. For LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles as-prepared and annealed at 400 and 600^∘C, the emission pattern is similar with that of LaF₃:Nd³⁺ nanoparticles in both the peak positions and shapes, which means that the SiO₂ shell have minimal effect on LaF₃:Nd³⁺ core. However, when the annealed temperature is 900^∘C, emission spectra of the LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles show a very unusual manner. (1) Their fluorescence intensity show a great increase compared with that of the LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles annealed at 600^∘C. (2) The strongest emission line for ⁴F_3/2→⁴I_11/2 transition peaking at 1073 nm are redshifted by about 17 nm contrasted with that of nanoparticles annealed at 600^∘C (1056 nm). (3) The ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I_13/2 emission of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles annealed at 900^∘C have remarkable Stark splittings which have not been found in other samples. In contrast, emission spectrum of 900^∘C annealed LaF₃:Nd³⁺ nanoparticles which have no SiO₂ shell has also been recorded, and is shown in Figure 4(b). The fluorescence intensity is only about 1/5 compared with that of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles annealed at 900^∘C, the ⁴F_3/2→⁴I_11/2 emission is peaking at about 1058 nm, and no emission lines have Stark splittings. The results mean, for LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles, that the influence of SiO₂ shell has become clearly when the anneal temperature is 900^∘C, and the remarkable improvement of fluorescence intensity is caused by the SiO₂ shell.

(a)

(b)

Figure 4 

Emission spectra of nanoparticles annealed at different temperatures: (a) LaF3:Nd3+/SiO2, (b) LaF3:Nd3+.

To get more information on the functions of SiO₂ shell, the excitation spectra by monitoring the ⁴F_3/2→⁴I_11/2 emission in LaF₃:Nd³⁺/SiO₂ core/shell and LaF₃:Nd³⁺ nanoparticles annealed at 900^∘C are compared in Figure 5. The spectra displays well-resolved lines, centered at 328, 352, 430, 472, 519, 578.9, 737, 801, and 881 nm corresponding to the direct excitation of Nd³⁺ from ⁴I_9/2 to the higher excited states: ²D_5/2, ²P_1/2, ⁴G_11/2, ²K_15/2 + ²D_3/2 + ²G_9/2, ²K_13/2 + ⁴G_7/2 + ⁴G_9/2, ²G_7/2 + ²G_5/2, ²H_11/2, ⁴F_9/2, ⁴S_3/2 +⁴F_7/2, ⁴F_5/2 + ²H_9/2, and ⁴F_3/2. Interestingly, remarkable differences are observed in excitation spectra of the two samples. As shown in the inset of Figure 5, the shape of peaks for ⁴I_9/2→²P_1/2 transition are different between the two nanoparticles, and the lines for ⁴I_9/2→⁴S_3/2 + ⁴F_7/2, and ⁴F_5/2 + ²H_9/2 transitions of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles are much broader compared with that of LaF₃:Nd³⁺ nanoparticles. The changes of excitation spectra mean that the environment of LaF₃:Nd³⁺ is different for the two nanoparticles. The Stark splittings of ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I_13/2 emission for LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles annealed at 900^∘C also confirmed the change. In previous reports, the phenomena were often caused by the host matrixes or surfactants [15]. In this work, for SiO₂ shell and LaF₃ core have not the same lattice structures, the shell could bring a noncentrosymmetric environment of the LaF₃:Nd³⁺ core [16]. And the new noncentrosymmetric environment causes a significant improvement of fluorescent intensity of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles [17].

(a)

(b)

Figure 5 

Excitation spectra of nanoparticles annealed at 900∘C: (a) LaF3:Nd3+/SiO2, (b) LaF3:Nd3+.

To have a further investigation of influence of the SiO₂ shell on LaF₃:Nd³⁺ core, fluorescent decay curves of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles were measured by monitoring the ⁴F_3/2→⁴I_11/2 emission at about 1060 nm. As shown in Table 1. For as-prepared nanoparticles, the lifetime is very difficult to be detected due to the presence of a large amount of O-H groups. When the nanoparticles were annealed at 400^∘C, the lifetime is 159 𝜇s. Surprisingly, with a further rising of the anneal temperature, the lifetimes show a slight decrease. When annealed at 600 and 900^∘C, the lifetimes are 156 𝜇s and 148 𝜇s, respectively. However, for 900^∘C annealed LaF₃:Nd³⁺ nanoparticles which have no SiO₂ shell, the lifetime is 196 𝜇s. It can be deduced that the lifetimes decrease in LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles is not caused by the LaF₃:Nd³⁺ core itself. In general, there are two factors that could affect radiative lifetime of Nd³⁺ in nanoparticles. (1) The existence of O-H groups and impurity on nanoparticles surfaces which could effectively reduce the lifetime of Nd³⁺; (2) vibrational energies of host matrix. For our samples, the difference is just the anneal temperature. For LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles annealed at 900^∘C, the effect of O-H groups can be obviated (Figure 3) and the LaF₃ host has a good crystallinity (Figure 1).

Table 1 

Lifetimes of nanoparticles annealed at different temperatures.

The LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles were synthesized in two steps. In as-prepared nanoparticles, SiO₂ shell is just coated on LaF₃:Nd³⁺ core and interaction between the two parts is very weak. When the anneal temperature is 400^∘C, most solvent remained on nanoparticles is removed. Thus its lifetime is of 159 𝜇s. When the anneal temperature was raised to 900^∘C, the interaction between SiO₂ shell and LaF₃:Nd³⁺ core increases. Since the phonon energy of SiO₂ (1100 cm^-1) is higher than that of LaF₃ (350 cm^-1) and the nanoparticles have a larger surface-to-core ratio [16], lifetime of Nd³⁺ near or on the surfaces of nanoparticles is slightly shorter than that of Nd³⁺ within the core structures and the measured lifetime of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles is decreased.

4. Conclusions

To summarize, we have shown that, for the SiO₂-coated LaF₃:Nd³⁺ nanoparticles, the SiO₂ shell has an interesting influence on their LaF₃ core. When the anneal temperature is 900^∘C, the change of environment of LaF₃:Nd³⁺ core caused by SiO₂ shell has become obvious. As a result, Stark splittings of ⁴F_3/2→⁴I_9/2 and ⁴F_3/2→⁴I_13/2 transition peaks and the redshift of ⁴F_3/2→⁴I_11/2 transition peak were observed in their emission spectrum. The fluorescent intensity of LaF₃:Nd³⁺/SiO₂ core/shell nanoparticles also has a great improvement, which can be very beneficial for applications.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (no. 10876009) and one Hundred Talents Programs of the Chinese Academy of Sciences.