Open Access
Add to BibSonomy
Abstract
The rare earth ions doped up-converting inverse opals exhibited application potential in the fields of biosensors and optic–electronic devices. However, there are two major factors which have limited their applications. One is the parity forbidden characters of 4f-f transitions of rare earth ions. Another factor is their low upconversion luminescence efficiency caused by the surface defects. In the present work, the SiO2 shell was coated on the skeleton surface of YbPO4:Er3+ inverse opals to protect the luminescent ions, and about a 10-fold enhancement of upconversion luminescence was obtained. Additionally, the localized surface plasmon resonance from Ag nanoparticles was used to enhance the upconversion luminescence of SiO2 coated YbPO4:Er3+ inverse opals. The green and red upconversion luminescence of inverse opal was enhanced selectively. Finally, the green and red upconversion luminescence are enhanced by about 50 and 30 folds by coupling effect of SiO2 coating and Ag nanoparticles, respectively. The selective enhancement mechanism of upconversion luminescence was investigated.
© 2017 Optical Society of America
1. Introduction
The rare earth (RE) ions can emit a high-energy photon by absorbing two or more low energy photons. This process was called as the upconversion luminescence (UCL). In recent years, more and more attention has been paid to the RE ions doped UCL materials because of their potential applications ranging from displays and sensors to photodynamic therapy and biological imaging agents [1–4]. Inverse opals (IOs) represent a new kind of photonic material of highly ordered structures with a periodically modulated refractive index [5]. The combination of the RE ions with the IOs can get many novel applications in the fields of biosensors and optic–electronic devices, etc [6–9]. For example, the combination of NaYF4:Yb,Tm up-converting nano-crystals with TiO2 IOs have been used as sensitive sensors for the avidin detection [6]. However, there are two main problems which limit the applications of the RE ions doped IOs: (1) the absorption and emission cross-sections of RE ions are small because of the parity forbidden characters of 4f-f transitions; (2) the defects such as surface adsorbents, lattice distortion and broken bonds on the surface of nano-sized skeleton of RE ions doped IOs. Both of them can quench the luminescence of RE ions. Therefore, the preparation of RE ions doped IOs with high luminescence efficiency is very important for considering their practical applications. Based on the influence factors of UCL of RE ions doped IOs, decreasing surface defects of RE ions doped IOs as well as improving absorption and emission cross-sections of RE ions are two available approaches to obtain the RE doped IOs with high luminescence efficiency.
Traditionally, the preparation of core–shell structure was used to enhance the UCL of nanoparticles (NPs) doped with RE ions. The shell around the NPs surface can increase the UCL intensity of RE ions doped NPs by protecting the luminescent RE ions at the NPs surface [10–14]. Additionally, the metal NPs exhibited unique electronic and optical properties, and they have shown potential application in sensors, surface-enhanced Raman scattering and solar cells [15–17]. Localized surface plasmon (LSP) is attributed to the free electrons oscillate of the metal nanoparticles, which has also been used to improve the UCL properties of RE ions [18–20]. According to the above introduction, the combined application of the core–shell structures and metal NPs can provide another avenue for enhancing the emission efficiency of RE ions doped materials [21–23]. Very recently, we observed the 7-fold common photoluminescence enhancement of Eu3+ doped IOs by the SiO2 shell and LSPR effect of metal nanoparticles [24]. However, there is no report on the UCL enhancement of the RE ions doped IOs. In the present work, the YbPO4:Er3+ IOs have been prepared via self-assembly approach combined with sol-gel method. The SiO2 has been coated at the surface of three-dimensionally skeleton of the YbPO4:Er3+ IOs, which was filled with the various concentrations Ag NPs. The influence of SiO2 shell and Ag NPs on the UCL properties of the YbPO4:Er3+ IOs were investigated. The green and red UCL are enhanced by about 50 and 30 folds by the combined effect of SiO2 coating and LSPR effect of Ag NPs, respectively.
2. Experimental
In this paper, the self-assembled ordered opals were obtained on the quartz substrates by using 300 nm polystyrene (PS) microspheres, which was used as a template to prepare the YbPO4:Er3+ IOs. For the YbPO4:Er3+ IOs preparation, the Yb(NO3)3 and Er(NO3)3 were prepared by dissolving the corresponding Yb2O3 and Er2O3 into hot nitric acid, respectively. Then the P2O5, Yb(NO3)3 and Er(NO3)3 dissolved separately in absolute ethyl alcohol were mixed to form a homogeneous solution. The YbPO4: Er3+ IOs were prepared by sintering PS templates infiltrating with the YbPO4: Er3+ sol at 950 °C for 5h.
The SiO2 sols with the concentrations of 0.1, 0.3, 0.5, 0.7 and 0.9M were prepared by diluting various amounts of tetraethyl orthosilicate into the absolute ethyl alcohol, respectively. Subsequently, the YbPO4: Er3+ IOs with the infiltration of different concentrations SiO2 sols were sintered at 650 °C for 5h to form the SiO2 shell around the surface of YbPO4: Er3+ IOs. The YbPO4: Er3+ IOs coated with 0.1, 0.3, 0.5, 0.7 and 0.9 M SiO2 solutions were marked as the IO-0.1, IO-0.3, IO-0.5, IO-0.7 and IO-0.9. To eliminate the influence of second sintering on UCL of YbPO4:Er3+ IOs, the second sintering of YbPO4:Er3+ IOs without the SiO2 sol infiltration was carried out at 650 °C for 5h, which was marked as the reference sample.
For the preparation of Ag NPs, the 36 mg AgNO3 was introduced into 200 mL distilled water in the flask placed at oil bath with the temperature of 105°C accompanied by vigorous agitation, in which 4 mL 4% sodium citrate was introduced. Under the reduction of sodium citrate, the AgNO3 was transferred to the Ag NPs. After infiltrating the SiO2 coated YbPO4:Er3+ IOs with the diluted Ag NPs, the Ag NPs embedded YbPO4:Er3+ IOs was prepared. The preparation process of SiO2 coated YbPO4: Er3+ IOs with Ag NPs was exhibited in the Fig. 1.
The UCL measurements of YbPO4: Er3+ IOs were carried out on a Hitachi F-7000 spectrophotometer upon the 980 nm excitation. The absorption spectra of Ag NPs and transmittance spectra of YbPO4:Er3+ IOs were recorded by the Hitachi U-4100 spectrophotometer, respectively. The morphologies and structures of ordered PS templates and YbPO4:Er3+ IOs were detected by using a scanning electron microscope (SEM). The phase purity and crystallinity of YbPO4:Er3+ IOs were examined by powder X-ray diffraction (XRD) diffractmeter. The average diameter of Ag NPs has been observed by using the JEOL 2100 transmission electron microscope(TEM). The UCL decay curves of the samples were measured by the FLSP-980 spectrophotometer (Edinburgh, UK).
3. Results and discussions
The morphology of as-prepared Ag NPs was detected by the TEM, as shown in the Fig. 2(a), and it exhibits that the Ag NPs with the average diameter of 5~15 nm has been successfully fabricated. In order to obtain the Ag NPs with various concentrations, the raw 200, 400, 600, 800 and 1000 L as-prepared Ag NPs solution were introduced into the 10 mL ethanol, respectively, and the corresponding absorbed spectra of diluted Ag NPs were measured, as exhibited in Fig. 2(b). A broad absorption band with maximum intensity at the 450 nm ranging from 350 to 650 nm was observed, corresponding the LSPR absorption of Ag NPs. It has been proved that the LSPR effect of Ag NPs can be simulated by the FDTD method and the present experimental results show a great agreement with the calculated spectra [25]. The increased absorption intensity of Ag NPs was exhibited with the increasing of Ag NPs concentration. The specific Ag NPs concentration (c) was determined by the following equation [26]:
Where the T, A, k and d are the ratio of transmitted light (I) to incident (I0) light intensity, absorption intensity of Ag NPs, molar absorption coefficient (1.3 × 104 M−1 × cm−1) and light path (1cm), respectively [27]. The diluted Ag NPs concentration (c) calculated by the Eq. (1) and absorption spectra are 2.35 × 10−5, 2.96 × 10−5, 4.11 × 10−5, 4.36 × 10−5 and 5.37 × 10−5 M, respectively. It should be noted that the absorption peak shows a small red-shift with the increase of the concentration. This effect is due to the near-field and far-field coupling of Ag NPs, which has been proved by FDTD simulation [28].
The SEM image of opal template is shown Fig. 3(a), which demonstrates a highly ordered face-centered cubic arrangement. The structure and quality of the original YbPO4: Er3+ IOs, SiO2 coated YbPO4: Er3+ IOs and SiO2 coated YbPO4: Er3+ IOs filled with Ag NPs were analyzed by the SEM images. As shown in the Fig. 3(b), the organic PS microspheres volatilized after being sintered at 950 °C with the formation of the long-range ordered YbPO4: Er3+ nanosized skeleton. In this work, the SiO2 sols with the different concentrations were employed to form the SiO2 shells on the surface of skeleton of YbPO4: Er3+ IOs. The SEM image of IO-0.5 sample was shown in Fig. 3(c). It can be seen that the ordered structure of IOs had been completely maintained after the formation of SiO2 shell, which exhibits the good thermal and mechanical stability. In comparison with the original IOs without the SiO2, the SiO2 coated IOs showed a smoother surface morphology. Figure 3(d) shows the SEM image of YbPO4:Er3+ IOs with the concentration of 4.11 × 10−5 M Ag NPs, exhibiting no Ag NPs because of its low concentration as well as small size. From this image, it can be obtained that the YbPO4:Er3+ IOs structure does not change, indicating that there’s no influence of Ag NPs addition on the order property of YbPO4:Er3+ IOs.
Fig. 3 (a) SEM image of PS opal template; SEM image of original YbPO4:Er3+ inverse opal (b), 0.5M SiO2 coated YbPO4:Er3+ inverse opal (c) and 0.5M SiO2 coated YbPO4:Er3+ inverse opal filled with 4.11 × 10−5M Ag NPs (d).
Figure 4 exhibited the XRD patterns of the original YbPO4:Er3+ IOs, 0.5M SiO2 coated YbPO4:Er3+ IOs and 0.5M SiO2 coated YbPO4:Er3+ IOs filled with the 4.11 × 10−5 M Ag NPs. The YbPO4: Er3+ IOs with tetragonal phase was prepared based on the JCPDS card (No.000450530). Form the XRD pattern of SiO2 coated YbPO4: Er3+ IOs, the diffraction peaks from SiO2 shell could not be found due to its amorphous phase. The XRD pattern of SiO2 coated IOs filled with the Ag NPs was exhibited in Fig. 4(c). It is noted that the XRD pattern of Ag NPs was not found due to its low concentration.
Fig. 4 XRD patterns of original YbPO4:Er3+ inverse opal (a), 0.5M SiO2 coated YbPO4:Er3+ inverse opal (b) and 0.5 M SiO2 coated YbPO4:Er3+ inverse opal filled with the 4.11 × 10−5 M Ag NPs (c).
In order to demonstrate the formation of SiO2 shell, the SiO2 coated YbPO4:Er3+ IOs prepared by the 0.5 M SiO2 sol concentration was scraped from the quartz substrate and the TEM images of the corresponding powder were measured, as shown in Fig. 5. The wall of the YbPO4: Er3+ IOs consisted a large number of small NPs with the diameter of 75 nm, as shown in Fig. 5(a). The high magnification TEM image of this sample was shown in Fig. 5(b). It can be seen clearly that the gray region adhering to the black layer of YbPO4: Er3+ IOs was the amorphous SiO2 shell with the thickness of about 14 nm. To demonstrate the infiltration of Ag NPs, the morphology of powder ground by this silica coated YbPO4:Er3+ IOs filled with 4.11 × 10−5 M Ag NPs was observed, as shown in the Fig. 5(c). The successful adhering of Ag NPs on silica coated YbPO4: Er3+ IOs surface was observed. Figure 5(d) shows the EDS spectrum of powder from the Ag NPs embedded SiO2 coated YbPO4:Er3+ IOs, exhibiting the signals of Si, O, Yb, P, O and Er elements. The signal presence of Ag element demonstrated that the Ag NPs was infiltrated into the IOs voids.
Fig. 5 Low (a) and high (b) magnification TEM images of SiO2 coated YbPO4:Er3+ inverse opal prepared by 0.5 M SiO2 sol; TEM image (c) and EDS spectrum (d) of 0.5 M SiO2 coated inverse opal filled with the 4.11 × 10−5 M Ag NPs
To further investigate ordered structure of YbPO4: Er3+ IOs, the UV-visible transmission spectra of YbPO4: Er3+ IOs before and after the SiO2 shell formation with the 0.5 M SiO2 concentration sol were recorded on a Hitachi U-4000 spectrophotometer, as shown in Fig. 6. The transmission spectrum exhibited an obvious dip from the Bragg diffraction of ordered periodic structure in the YbPO4:Er3+ IOs. The highly ordered YbPO4:Er3+ IOs were fabricated. The transmittance spectrum of YbPO4:Er3+ inverse opals after the SiO2 shell formation exhibited a little changing, which indicated that ordered structures of the YbPO4: Er3+ IOs was maintained. The transmission spectrum of 0.5 M SiO2-coated YbPO4: Er3+ IOs after the infiltration of Ag NPs with the concentration of 4.11 × 10−5 M was shown in Fig. 6. The observable dip in the SiO2-coated YbPO4:Er3+ IOs after the addition of Ag NPs demonstrated that the Ag NPs infiltration has no effect on their ordered structure.
Fig. 6 Transmittance spectra of YbPO4:Er3+ inverse opals before (black line) and after (red line) coating with the 0.5 M SiO2; Transmittance spectra of 0.5 M SiO2 coated YbPO4:Er3+ inverse opals after the infiltration of 4.11 × 10−5 M Ag NPs (blue line).
In order to obtain the coating of SiO2, the YbPO4:Er3+ IOs were required to have a double-sintering process. The influence of double-sintering process on the UCL of YbPO4: Er3+ IOs was investigated. The UCL spectra of the YbPO4:Er3+ IOs before and after the double-sintering under the excitation of 980 nm exhibit the typical 525 (2H11/2→4I15/2), 550 (4S3/2→4I15/2) and 660 nm(4F9/2→4I15/2) UCL bands of Er3+, as shown in Fig. 7(a). It can be seen that the intensity of UCL from the Er3+ ions does not change significantly, which indicates that the influence of double sintering process on the UCL should not be considered in the YbPO4:Er3+ IOs. To obtain the influence of SiO2 shell on the UCL of YbPO4: Er3+ IOs, the UCL of YbPO4: Er3+ IOs with and without the SiO2 shell was measured, as shown in Fig. 7(b) and 7(c). The same UCL intensity of five YbPO4: Er3+ IOs was observable before the formation of SiO2 shell, as shown in Fig. 6 (b on the left). After the formation of the SiO2 shell, the UCL intensity of five YbPO4: Er3+ IOs was increased at beginning and then decreased with the increasing of concentration of SiO2 solution in comparison with that of YbPO4: Er3+ IOs without the SiO2 shell. When the concentration of SiO2 is 0.5 M, the UCL of YbPO4 IOs exhibited the biggest enhancement, having about 10-fold enhancement. This enhancement was produced by suppressing the surface quenching effect via SiO2 shells. The whole UCL intensity of YbPO4 IOs is dependent on the doping Er3+ ions located at the surface or interior of IOs wall. It has been known from the previous work that the defects such as surface adsorbents, lattice distortion and broken bonds on the nano-sized skeleton surface of YbPO4 IOs can quench the excitation energy, resulting in the decreasing of UCL from the surface Er3+ions [29,30]. The forming SiO2 shell around the wall of YbPO4 IOs eliminated its surface defects, suppressing the surface quenching effect [29, 30]. Therefore, the UCL enhancement occurred after forming SiO2 shell. When the concentration of SiO2 is higher than 0.5mol/L, the UCL of the YbPO4: Er3+ IOs decreased. The thicker SiO2 shell formed at the surface YbPO4: Er3+ IOs at the higher concentration of SiO2 may inhibit excitation light to excite YbPO4: Er3+ IOs, decreasing the UCL of the YbPO4: Er3+ IOs.
Fig. 7 (a) UCL spectra of YbPO4:Er3+ inverse opal before (black line) and after (red line) double sintering ; UCL spectra of YbPO4:Er3+ inverse opals before (b, left) and after SiO2 coated inverse opal (c, right); the Log-Log plots of pump power dependence of the UCL in the 0.5 M SiO2 coated inverse opal (d); the mechanism of UCL (e).
The mechanism of UCL process of 0.5M SiO2 coated YbPO4:Er3+ IOs was investigated. The equation(I = Pn) between UCL intensity (I) and the excitation light power (P) can use to determine the number(n) of photons required to realize UCL. The measured UCL intensity of 0.5 M SiO2 coated YbPO4:Er3+ IOs was presented in Fig. 7(d) under the various excitation light power. The 1.81, 2.24 and 2.11 slope value were obtained for the 660, 550 and 525 nm UCL of Er3+ ions, respectively, which indicate that the UCL mechanisms in YbPO4:Er3+ IOs is considered as two photons processes regarding of the red and green UCL. Figure 7(e) shows the UCL mechanisms of YbPO4:Er3+ IOs under the excitation of 980 nm. The 2F5/2 state of Yb3+ was populated under the excitation of 980 nm. The energy transfer (ET) from Yb3+ to the Er3+ ions populated the 4I11/2 state Er3+ ions, and then it relaxed nonradiatively to the 4I13/2 state. The ET from Yb3+ to the Er3+ ions at 4I13/2 state populated its 4F9/2 state, generating the red UC emission by from the 4F9/2 to 4I15/2 transition. In addition, The ET from Yb3+ to the Er3+ ions at 4I11/2 state populated the 4F7/2 state. Then the 4F7/2 state electrons transited nonradiatively to the 2H11/2 and 4S3/2 states, causing the green UCL due to the transitions from the 2H11/2 and 4S3/2 to 4I15/2.
The decay curves of the 550 and 660 nm UCL of YbPO4:Er3+ IOs before and after the formation of SiO2 shell prepared with the 0.5 M SiO2 have been recorded by a FLSP-980 spectroscopy, as shown in Fig. 8. The lifetime of 550 nm UCL for the YbPO4:Er3+ IOs before and after the formation of SiO2 shell is 138 and 89 μs, respectively. The lifetime of 660 nm UCL for the original YbPO4:Er3+ IOs is 369 μs. After being coated by the 0.5 M SiO2, it decreased to 238 μs. Generally speaking, eliminating the structural defects might increase the lifetime since the suppression of surface quenching effect can decrease the non-radiative rate. Another reason for increasing the plasmon lifetime is decreasing electron resistive loss. In the present work, the decay lifetime of rare earth ions was decreased after the formation of SiO2 shell, which could be explained by the following virtual-cavity model [31]:
Fig. 8 Decay spectra of 554 (a) and 660 nm (b) UCL of YbPO4:Er3+ inverse opals before and after the SiO2 coating
In the above equation, the f(ED), neff and λ0 denoted as the strength of electric dipole from luminescent centers, average refractive index of YbPO4:Er3+ IOs and the vacuum wavelength, respectively. The strength of electric dipole from luminescent centers and average refractive index of YbPO4:Er3+ IOs governed the decay lifetime of Er3+. The lifetime of Er3+ will decrease with the increasing of the f(ED)from luminescent centers and neff of YbPO4:Er3+ IOs. The f(ED) should be the same in the YbPO4:Er3+ IOs before and after the formation of SiO2 shell. Therefore, the decay lifetime was determined by the average refractive index of YbPO4:Er3+ IOs. The SiO2 shell around the wall of YbPO4:Er3+ IOs replaces the air of raw YbPO4:Er3+ IOs voids, which lead to the increasing of average refractive index of YbPO4:Er3+ IOs in comparison with that of original YbPO4:Er3+ IOs, thus the decay lifetime of YbPO4:Er3+ IOs became short.
When the concentration of SiO2 solution was determined to be the 0.5 M, the most intensive UCL was obtained. The Ag NPs with different concentration was infiltrated into the voids of YbPO4:Er3+ IOs coated with the 0.5 M SiO2 sol. Figure 9(a) and 9(b) show the UCL spectra of SiO2 coated YbPO4: Er3+ IOs before and after the addition of Ag NPs with the different concentrations, presenting the typical UCL bands of Er3+. The pump power dependence of UCL of SiO2 coated YbPO4: Er3+ IOs with 4.11 × 10−5 M Ag NPs was measured, as shown in Fig. 9(c), which exhibited the two photons processes for the red and green emissions for the SiO2 coated YbPO4: Er3+ IOs after addition of 4.11 × 10−5 M Ag NPs. The UCL mechanism is similar with that of SiO2 coated YbPO4: Er3+ IOs without the Ag NPs. It is obvious that the UCL intensity of Er3+ is increased at first and then decreased with the increasing of Ag NPs concentration. The UCL enhancement with 50 and 30 folds in the SiO2 coated YbPO4: Er3+ IOs was observed for the green and red UCL, respectively, after introducing 4.11 × 10−5 M Ag NPs in the YbPO4: Er3+ IOs, which is attributed to the LSPR effect of Ag NPs.
Fig. 9 UCL spectra of SiO2 coated YbPO4:Er3+ inverse opals before (a, left) and after (b, right) infiltration of Ag nanoparticles. The pump power dependence of the UCL of SiO2 coated inverse opal with 4.11 × 10−5 M Ag NPs (c)
It has been widely known that Ag NPs could induce a local field enhancement, which may affect the UCL of rare earth ions. Thus, the following rate equations were employed to calculate the populations of the excited energy levels of Er3+.
Here, I and ρ stand for the excitation field intensity as well as the absorption section of Yb3+. The population of 2F5/2 and 2F7/2 states of Yb3+ was marked as Ny0 and Ny1 respectively. As shown in the Fig. 7(e), the population of each levels of Er3+ was marked as n0, n1, n2, n3 and n4, respectively, and the three energy transfer processes from the Yb3+ to Er3+ was marked as the E1, E2 and E3, respectively. The 4S3/2→4F9/2 and 4I11/2→4I13/2 nonradiative transitions of Er3+ were denoted as Wr1 and Wr2, respectively. The W1, W2, W3 and Wro are the radiative transitions processes, respectively, as shown in the Fig. 7(e). The population of luminescent n2 and n3 levels of Er3+ ions could be obtained according to the above rate equations:
It is noted from the Eqs. (8) and (9) that the intensity of excitation field affects the population number of luminescent n2 and n3 levels of Er3+. The LSPR effect of Ag NPs enhances the intensity of excitation field, increasing the population number of luminescent n2 and n3 levels. Thus, the UCL intensity of Er3+ could be enhanced in the SiO2 coated YbPO4:Er3+ IOs after the infiltration of Ag NPs. The intensities of electric field from the Ag NPs embedded SiO2 coated YbPO4 rings was simulated by the FDTD method, as shown in Fig. 10(a) and 10(b). Figure 10 a) shows the scheme of simulation. The red bright rings marked in the figures is the silicon wall while the black spots stand for the Ag NPs. The main structure of YbPO4:Er IOs is also being marked in this figure. Figure 10(b) shows the final result of FDTD simulations. The color distribution shows the field intensity distribution of electronic field. The enhanced electric field intensity upon the 980nm excitation was obtained. Therefore, the enhanced excitation field induced by the Ag NPs is responsible for the enhanced UCL.
Fig. 10 (a) the scheme of FDTD simulation and (b) the electric field intensity simulated by the FDTD solution.
It is noted that the green UCL of Er3+ could get a stronger enhancement in contrast to the red UCL, as shown in the Fig. 9. The UCL enhanced factor (EF) is marked as the ratio of UCL intensity of SiO2 coated YbPO4:Er3+ IOs with Ag NPs to the YbPO4:Er3+ IOs without the Ag NPs. The SiO2 coated YbPO4:Er3+ IOs filled with the 2.35 × 10−5 M, 2.96 × 10−5 M, 4.11 × 10−5 M and 4.36 × 10−5 M Ag NPs were marked as the IO-I, IO-II, IO-III and IO-IV. Figure 11(a) exhibited the UCL enhanced factor (EF). It can be seen obviously that the EF of UCL at 550 nm is bigger than 660 nm UCL, which demonstrated that the 550 nm UCL of Er3+ could get a stronger enhancement. To investigate the mechanism of this enhanced phenomenon, the decay curves of the UCL of 0.5M SiO2 coated YbPO4:Er3+ IOs before and after the addition of Ag NPs were recorded by a FLSP-980 spectroscopy, as shown in Fig. 11(b) and 11(c). The 550 nm UCL lifetime for the SiO2 coated YbPO4:Er3+ inverse opals is 89 μs, and it decreased to 67 μs after the infiltration with the 4.11 × 10−5 M Ag NPs. Additionally, the 660 nm UCL lifetime is about 238 μs before and after the Ag NPs, no lifetime change was observed. The decreasing of 550 nm UCL lifetime is due to the better overlapping between the 550 nm UCL and the LSPR peaks of Ag NPs in comparison with the 660 nm UCL lifetime [32].
Fig. 11 (a) the EF image of IO-I, IO-II, IO-III and IO-IV; decay curves of 550nm and 650 nm of 0.5 M SiO2 coated samples before (b) and after (c) the addition of Ag NPs.
It has been widely known that the below equations can be used to express the UCL quantum efficiency (η) of luminescent ions [33,34]:
where the average lifetime of Er3+, radiative and nonradiative rate was marked as τ, Ar and Anr, respectively. Thus, the η will increase with either the Ar increasing or the Anr decreasing. The 550 nm UCL lifetime corresponding to the 4S3/2→4I15/2 transition showed evident decreasing after the addition of Ag NPs, which means that the selective enhancement of green UCL of Er3+ comes from the increasing of radiative rate.
In order to get the efficient influence of SiO2 shell on the UCL, the 4.11 × 10−5 M Ag NPs were added into the YbPO4:Er3+ IOs voids without the SiO2 shell and the corresponding UCL spectrum were measured, as shown in the Fig. 12, exhibiting the quenching of the UCL in the YbPO4:Er3+ IOs without the SiO2 shell. The distance between active centers and Ag NPs could affect the UCL [35]. The touching between the luminescent centers and metal NPs can result in the UCL quenching, which was eliminated by forming SiO2 shell on the YbPO4:Er3+ IOs surface in the present work.
Fig. 12 UCL spectra of YbPO4:Er3+ inverse opal without the SiO2 shell after and before the addition of 4.11 × 10−5 M Ag NPs
4. Conclusion
In this article, the sol-gel method combined with a self-assembly process was used to prepare the YbPO4:Er3+ inverse opal and YbPO4:Er3+ inverse opal with the SiO2 shell. The influence of SiO2 shell on the upconversion emission of YbPO4:Er3+ inverse opal was observed, and about 10-fold enhancement of upconversion luminescence was obtained due to the suppression of surface quenching effect via SiO2 shells. Additionally, the localized surface plasmon resonance from Ag nanoparticles was used to enhance the upconversion luminescence of SiO2 coated YbPO4:Er3+ inverse opals. The green and red upconversion luminescence are enhanced by about 50 and 30 folds, respectively, due to the combination effect of SiO2 shell and localized surface plasmon of Ag NPs. The selective enhancement of green upconversion luminescence was attributed to the increasing of radiative rate.
Funding
National Natural Science Foundation of China (11674137); Applied basic research program of Yunnan Province (2014FB127); Reserve talents project of Yunnan Province (2012HB068).
References and links
1. M.-H. Chan, C.-W. Chen, I.-J. Lee, Y.-C. Chan, D. Tu, M. Hsiao, C.-H. Chen, X. Chen, and R.-S. Liu, “Near-infrared light-mediated photodynamic therapy nanoplatform by the electrostatic assembly of upconversion nanoparticles with graphitic carbon nitride quantum dots,” Inorg. Chem. 55(20), 10267–10277 (2016). [CrossRef] [PubMed]
2. L. Liang, A. Care, R. Zhang, Y. Lu, N. H. Packer, A. Sunna, Y. Qian, and A. V. Zvyagin, “Facile assembly of functional upconversion nanoparticles for targeted cancer imaging and photodynamic therapy,” ACS Appl. Mater. Interfaces 8(19), 11945–11953 (2016). [CrossRef] [PubMed]
3. H. N. Luitel, R. Chand, H. Hamajima, Y. R. Gaihre, T. Shingae, T. Yanagita, and T. Watari, “Highly efficient NIR to NIR upconversion of ZnMoO4: Tm 3+, Yb 3+ phosphors and their application in biological imaging of deep tumors,” J. Mater. Chem. B Mater. Biol. Med. 4(37), 6192–6199 (2016). [CrossRef]
4. M. Mousavi, H. Söderlund, H. Liu, and S. Andersson-Engels, “Improving penetration depth in biological imaging using Nd3+/Yb3+/Er3+-doped Upconverting Nanoparticles,” in Optical Tomography and Spectroscopy, (Optical Society of America, 2016), OTu2C. 5.
5. G. I. Waterhouse and M. R. Waterland, “Opal and inverse opal photonic crystals: fabrication and characterization,” Polyhedron 26(2), 356–368 (2007). [CrossRef]
6. S. Xu, W. Xu, Y. Wang, S. Zhang, Y. Zhu, L. Tao, L. Xia, P. Zhou, and H. Song, “NaYF4:Yb,Tm nanocrystals and TiO2 inverse opal composite films: a novel device for upconversion enhancement and solid-based sensing of avidin,” Nanoscale 6(11), 5859–5870 (2014). [CrossRef] [PubMed]
7. Y. Zhu, S. Cui, Y. Wang, M. Liu, C. Lu, A. Mishra, and W. Xu, “Enhanced rare earth photoluminescence in inverse opal photonic crystals and its application for pH sensing,” Nanotechnology 27(40), 405202 (2016). [CrossRef] [PubMed]
8. L. D. DeLoach, S. A. Payne, L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Evaluation of absorption and emission properties of Yb3+ doped crystals for laser applications,” IEEE J. Quantum Electron. 29(4), 1179–1191 (1993). [CrossRef]
9. C. M. Johnson, P. J. Reece, and G. J. Conibeer, “Slow-light-enhanced upconversion for photovoltaic applications in one-dimensional photonic crystals,” Opt. Lett. 36(20), 3990–3992 (2011). [CrossRef] [PubMed]
10. G.-S. Yi and G.-M. Chow, “Water-soluble NaYF4: Yb, Er (Tm)/NaYF4/polymer core/shell/shell nanoparticles with significant enhancement of upconversion fluorescence,” Chem. Mater. 19(3), 341–343 (2007). [CrossRef]
11. D. Jiang, L. Cao, W. Liu, G. Su, H. Qu, Y. Sun, and B. Dong, “Synthesis and luminescence properties of core/shell ZnS: Mn/ZnO nanoparticles,” Nanoscale Res. Lett. 4(1), 78–83 (2009). [CrossRef] [PubMed]
12. W. Zhang, J. Long, P. Zhang, J. Li, and K. Qiu, “Luminescence enhancement of ZnS: Cu nanocrystals by zinc sulfide coating with core/shell sstructure,” Integr. Ferroelectr. 154(1), 110–119 (2014). [CrossRef]
13. K. Prorok, A. Bednarkiewicz, B. Cichy, A. Gnach, M. Misiak, M. Sobczyk, and W. Strek, “The impact of shell host (NaYF4/CaF2) and shell deposition methods on the up-conversion enhancement in Tb3+, Yb3+ codoped colloidal α-NaYF 4 core-shell nanoparticles,” Nanoscale 6(3), 1855–1864 (2014). [CrossRef] [PubMed]
14. Y. Zhang, W. Feng, K. Huang, L. Yuan, Y. Du, X. Wu, and S. Feng, “Luminescence enhancement of Lu3TaO7: Eu3+@ Lu3TaO7 red‐emitting nanophosphors,” Eur. J. Inorg. Chem. 2015(4), 690–695 (2015). [CrossRef]
15. P. K. Jain, X. Huang, I. H. El-Sayed, and M. A. El-Sayed, “Noble metals on the nanoscale: optical and photothermal properties and some applications in imaging, sensing, biology, and medicine,” Acc. Chem. Res. 41(12), 1578–1586 (2008). [CrossRef] [PubMed]
16. C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars, S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, “Surface-enhanced Raman scattering from individual Au nanoparticles and nanoparticle dimer substrates,” Nano Lett. 5(8), 1569–1574 (2005). [CrossRef] [PubMed]
17. S.-S. Kim, S.-I. Na, J. Jo, D.-Y. Kim, and Y.-C. Nah, “Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles,” Appl. Phys. Lett. 93(7), 073307 (2008). [CrossRef]
18. Y. Zhu, W. Xu, G. Li, S. Cui, X. Liu, and H. Song, “Plasmonic enhancement of the upconversion fluorescence in YVO4:Yb3+, Er3+ nanocrystals based on the porous Ag film,” Nanotechnology 26(14), 145602 (2015). [CrossRef] [PubMed]
19. Z. Yin, X. Zhang, D. Zhou, H. Wang, W. Xu, X. Chen, T. Zhang, and H. Song, “Enhanced upconversion luminescence on the plasmonic architecture of Au–Ag nanocages,” RSC Advances 6(89), 86297–86300 (2016). [CrossRef]
20. P. Wang, Z. Li, W. J. Salcedo, Z. Sun, S. Huang, and A. G. Brolo, “Surface plasmon enhanced up-conversion from NaYF4:Yb/Er/Gd nano-rods,” Phys. Chem. Chem. Phys. 17(24), 16170–16177 (2015). [CrossRef] [PubMed]
21. Z. Wang, W. Gao, R. Wang, J. Shao, Q. Han, C. Wang, J. Zhang, T. Zhang, J. Dong, and H. Zheng, “Influence of SiO2 layer on the plasmon quenched upconversion luminescence emission of core-shell NaYF4: Yb, Er@ SiO2@ Ag nanocomposites,” Mater. Res. Bull. 83, 515–521 (2016). [CrossRef]
22. Y. Qin, Z. Dong, D. Zhou, Y. Yang, X. Xu, and J. Qiu, “Modification on populating paths of β-NaYF4: Nd/Yb/Ho@ SiO2@ Ag core/double-shell nanocomposites with plasmon enhanced upconversion emission,” Opt. Mater. Express 6(6), 1942–1955 (2016). [CrossRef]
23. Q. Han, Y. Zhang, Z. Ren, Z. Wang, W. Gao, E. He, and H. Zheng, “Ag@SiO2/LaF3:Eu3+ Composite Nanostructure and Its Surface Enhanced Luminescence Effect,” J. Nanosci. Nanotechnol. 16(4), 3759–3762 (2016). [CrossRef] [PubMed]
24. J. Li, Z. Yang, B. Shao, J. Yang, Y. Wang, J. Qiu, and Z. Song, “Photoluminescence Enhancement of SiO2‐Coated LaPO4: Eu3+ Inverse Opals by Surface Plasmon Resonance of Ag Nanoparticles,” J. Am. Ceram. Soc. 99(10), 3330–3335 (2016). [CrossRef]
25. J. M. McMahon, Y. Wang, L. J. Sherry, R. P. Van Duyne, L. D. Marks, S. K. Gray, and G. C. Schatz, “Correlating the structure, optical spectra, and electrodynamics of single silver nanocubes,” J. Phys. Chem. C 113(7), 2731–2735 (2009). [CrossRef]
26. I. Botev, “A new conception of Bouguer-Lambert-Beer’s law,” Fresenius Z. Anal. Chem. 297(5), 419 (1979). [CrossRef]
27. S. C. Yang, C. Tang, S. Dong, and P. J. Li, “Determination of cyanide on surface plasma resonance spectrum of silver nanoparticles,” Anal. Lab. 1, 018 (2005).
28. M. Rycenga, C. M. Cobley, J. Zeng, W. Li, C. H. Moran, Q. Zhang, D. Qin, and Y. Xia, “Controlling the synthesis and assembly of silver nanostructures for plasmonic applications,” Chem. Rev. 111(6), 3669–3712 (2011). [CrossRef] [PubMed]
29. Q. Lü, A. Li, F. Guo, L. Sun, and L. Zhao, “The two-photon excitation of SiO2-coated Y2O3:Eu3+ nanoparticles by a near-infrared femtosecond laser,” Nanotechnology 19(20), 205704 (2008). [CrossRef] [PubMed]
30. A. Lukowiak, R. J. Wiglusz, A. Chiappini, C. Armellini, I. K. Battisha, G. C. Righini, and M. Ferrari, “Structural and spectroscopic properties of Eu3+-activated nanocrystalline tetraphosphates loaded in silica–hafnia thin film,” J. Non-Cryst. Solids 401, 32–35 (2014). [CrossRef]
31. R. Meltzer, S. Feofilov, B. Tissue, and H. Yuan, “Dependence of fluorescence lifetimes of Y2O3: Eu3+ nanoparticles on the surrounding medium,” Phys. Rev. B 60(20), R14012 (1999). [CrossRef]
32. N. Liu, W. Qin, G. Qin, T. Jiang, and D. Zhao, “Highly plasmon-enhanced upconversion emissions from Au@β-NaYF4:Yb,Tm hybrid nanostructures,” Chem. Commun. (Camb.) 47(27), 7671–7673 (2011). [CrossRef] [PubMed]
33. K. K. Haldar and A. Patra, “Fluorescence enhancement and quenching of Eu3+ ions by Au-ZnO core-shell and Au nanoparticles,” Appl. Phys. Lett. 95(6), 063103 (2009). [CrossRef]
34. J. R. Lakowicz, Y. Shen, S. D’Auria, J. Malicka, J. Fang, Z. Gryczynski, and I. Gryczynski, “Radiative decay engineering. 2. Effects of silver island films on fluorescence intensity, lifetimes, and resonance energy transfer,” Anal. Biochem. 301(2), 261–277 (2002). [CrossRef] [PubMed]
35. A. Campion, A. Gallo, C. Harris, H. Robota, and P. Whitmore, “Electronic energy transfer to metal surfaces: a test of classical image dipole theory at short distances,” Chem. Phys. Lett. 73(3), 447–450 (1980). [CrossRef]
