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Coupling of Ag nanoparticle with inverse opal photonic crystals as a novel strategy for upconversion emission enhancement of NaYF4: Yb3+, Er3+ nanoparticles Bo Shao, Zhengwen Yang, Yida Wang, Jun Li, Jianzhi Yang, Jianbei Qiu, and Zhiguo Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06817 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Applied Materials & Interfaces

Coupling of Ag Nanoparticle with Inverse Opal Photonic Crystals as a Novel Strategy for Upconversion Emission Enhancement of NaYF4: Yb3+, Er3+ Nanoparticles Bo Shao1, Zhengwen Yang1*, Yida Wang1, Jun Li 1, Jianzhi Yang1, Jianbei Qiu1, Zhiguo Song1 1

College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093,China. Corresponding author: Zhengwen Yang, E-mails: [email protected]

Abstract: Rare earth ions doped upconversion (UC) nanoparticles have shown considerable interesting due to their potential application in solar cell, biological labeling, therapeutics and imaging. However, the applications of UC nanoparticles were still limited due to their low emission efficiency. Photonic crystals and noble metal nanoparticles are applied extensively to enhance the UC emission of rare earth ions, respectively. In the present work, a novel substrate consisted of inverse opal photonic crystals and Ag nanoparticles were prepared by the template assisted method, which were used to enhance the UC emission of NaYF4: Yb3+, Er3+ nanoparticles. The red or green UC emissions of NaYF4: Yb3+, Er3+ nanoparticles were selectively enhanced on the inverse opal substrates due to the Bragg reflection of photonic band gap. Additionally, the UC emission enhancement of NaYF4: Yb3+, Er3+ nanoparticles induced by the coupling of metal nanoparticle plasmons and photonic crystal effects was realized on the Ag nanoparticles including inverse opal substrate. The present results demonstrated that coupling of Ag nanoparticle with inverse opal photonic crystals provides a useful strategy to enhance UC emission of rare earth ions doped nanoparticles. Keywords: NaYF4: Yb3+, Er3+ nanoparticles; SiO2 inverse opal including Ag nanoparticles; Upconversion emission; Enhancement; Surface plasmon resonance.

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Introduction Rare earth doped upconversion (UC) nanoparticles can convert low-energy near infrared or infrared light to higher energy visible photons, which have attracted increasing attention due to their potential application in biological labeling, therapeutics, imaging and solar cell1-12. Unfortunately, the application of rare earth doped UC nanoparticles has been constrained by their low emission efficiencies, which is largely attributed to the very small near-infrared or infrared absorption section of rare earth ions. At present, major investigations on the UC nanoparticles are developed various methods including the host selection, surface coating, and broadband dye sensitization to improve their UC emission efficiency13-16. Photonic crystals (PC) can influence the propagation of photons by their periodic variation of refractive index, which has also been used as an effective way to enhance the UC emission of rare earth doped nanoparticles17-18. When the rare earth doped nanoparticles were deposited on the surface of PC, their UC emission can be significantly enhanced by the surface effect of PC. For example, our group presented enhanced UC emission of NaYF4: Yb3+, Er3+ / Tm3+ nanoparticles deposited on an opal photonic crystal. The UC emission enhancement of NaYF4: Yb3+, Er3+ / Tm3+ nanoparticles on the opal photonic crystal is attributed to the Bragg reflection of photonic band gap effects19. Additionally, the coupling of surface plasmon resonance (SPR) of metal nanostructures with UC emission of nanoparticles allows for further enhancement in the UC emission intensity. The UC emission enhancement induced by the SPR is attributed to enhancing both absorption and emission processes by tailoring


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the electric fields around UC nanoparticles20-24. Previous results demonstrated that the PC effects or SPR effects of metal nanostructures are two kinds of important strategies to improve UC emission of rare earth ions doped nanoparticles. Commonly, the PC and metal nanostructures are separately used for the enhancement of UC emissions. In fact, the PC and metal nanoparticles can be coupled together to enhance the luminescence properties of the UC nanocrystals. For example, our recent work demonstrated that the UC emission from NaYF4: Yb3+, Er3+ nanoparticles deposited on an opal covered with Ag nanoparticles was significantly enhanced due to the coupling of photonic band gap of the opal and SPR effect of the Ag nanoparticles25. However, in this works, the UC nanoparticles were directly deposited on the surface of Ag nanoparticles, which may result in the quenching of UC emission26. To avoid this problem, inverse opal photonic crystal embedded in Ag nanoparticles were prepared by the template assisted method in the present work, which were used to enhance the UC emission of NaYF4: Yb3+, Er3+ nanoparticles. The inverse opal including Ag nanoparticles has several advantages in contrast to opal/Ag substrate. Firstly, the Ag nanoparticles were embedded into the interior of inverse opal, which avoids the direct touch between the Ag nanoparticles and NaYF4: Yb3+, Er3+ nanoparticles. Secondly, this substrate can also realize the coupling of SPR effects of Ag nanoparticle and photonic band gap effects for enhancing the UC luminescence of NaYF4: Yb3+, Er3+ nanoparticles. Experimental methods Polystyrene (PS) microsphere suspensions with a diameter of 335, 410 and 500



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nm were used to fabricate face-centered cubic opal photonic crystal on quartz substrates by vertical deposition method, as described in our previous works27-28. The opal templates were used to fabricate the SiO2 inverse opal photonic crystals including Ag nanoparticles. The tetraethyl orthosilicate [Si(CH3CH2O)4, TEOS] and AgNO3 were dissolved in ethanol, respectively. After mixing and stirring, the SiO2: 0.6 mol % Ag sol was obtained. The opal templates infiltrated with SiO2: Ag sols were sintered in an air furnace at temperature of 750oC to obtain the SiO2 inverse opal including Ag nanoparticles. The SiO2 inverse opal including Ag nanoparticles made of 335, 410 and 500 nm opal templates were denoted as Ag-IPO-I, Ag-IPO-II, Ag-IPO-III, respectively. For comparison, the SiO2 inverse opals without Ag nanoparticles made of 335, 410 and 500 nm opal templates were prepared, which were denoted as IPO-I, IPO-II, IPO-III, respectively. High purity (99.99%) Y2O3, Yb2O3, Er2O3 and analytical regent grade NaF and NaOH were used to synthetize NaYF4: Yb3+, Er3+nanoparticles without further purification. The NaYF4: 20% Yb3+/ 2% Er3+ nanoparticles were synthetized via a hydrothermal synthesis method according to literature29-32. The NaYF4: Yb3+, Er3+ nanoparticles dispersed in cyclohexane were coated on the surface of SiO2 inverse opal with and without Ag nanoparticles by spin-coating process. Scheme 1 showed formation process of the composites consisted of Ag nanoparticles including SiO2 inverse opal and NaYF4: Yb3+, Er3+ nanoparticles.


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Scheme 1. Formation process of Ag nanoparticles including SiO2 inverse opal photonic/NaYF4: Yb3+, Er3+ composites. The UC emission spectra of samples were collected with a HITACHI F-7000 spectrophotometer at room temperature, and a 980 nm diode laser was used as the excitation source. The UC emission dates were collected along the normal direction of the surfaces of SiO2 inverse opal photonic/NaYF4: Yb3+, Er3+ composites, and the incident 980 nm excited light were focused on the centers of samples, as shown in Scheme 2. Absorption spectra of samples were collected by a HITACHI U-4100 spectrophotometer. The scanning electron microscopy (SEM) images of samples were characterized with a field-emission scanning electron microscope (QUANTA 650). Transmission electron microscope (TEM) images of SiO2 inverse opal including Ag nanoparticles were obtained with a JEOL 2100 transmission electron microscope. The SiO2 inverse opal including Ag nanoparticles was scraped from the quartz substrate and dispersed in ethanol, then dropped onto a copper grid for TEM measurements.

Scheme 2. The schematic illustration of light path of UC emission measurement.



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The elemental compositions of SiO2 inverse opal including Ag nanoparticles were analyzed by an energy dispersive X-ray (EDX) analyzer attached to a JEOL 2100 transmission electron microscope. The elemental compositions of NaYF4: Yb3+, Er3+ nanoparticles were characterized by XRF (ZSX100e). The decay lifetime of NaYF4: Yb3+, Er3+ UC nanoparticles were investigated under excitation of a 980 nm diode laser with a power density of 10 W/cm2. The electric field intensities and distributions of SiO2 inverse opal photonic crystals containing Ag nanoparticles were simulated FDTD method. The 980 nm incidence light was used for the simulation. The size of the SiO2 rings and Ag nanoparticles were determined for the simulation of electric field intensities and distributions according to the SEM and TEM figures.

Results and discussions The morphology and microstructure of the SiO2 inverse opal with and without Ag nanoparticles were characterized by the SEM. Fig. 1 (a) and (b) show the SEM images of IPO-II and Ag-IPO-II. The SEM images of IPO-II and Ag-IPO-II exhibited a typical inverse opal structures with long-range ordered hexagonal arrangement. The center to center distance between the air spheres of the IPO-II and Ag-IPO-II was calculated based on the SEM images, which was about 330 nm. The center to center distance between the air spheres of the IPO-II and Ag-IPO-II is smaller than the diameter of the PS microspheres used to form the opal template, which suggested that considerable shrinkage occurred during sintering process. Three dark regions inside each air spheres in the SEM images of the IPO-II and Ag-IPO-II can be seen, which correspond to the air spheres in the underlying layer. The size of each dark region


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inside air spheres were about 40 nm. The size of dark region inside air spheres is very small, indicated that NaYF4: Yb3+, Er3+ nanoparticles deposited by spin-coating method is mainly located at the surface of inverse opal. The SEM images of inverse opals with a low magnification were shown in Fig. 1 (f), which exhibiting that the domain sizes for uniform structures are about one hundred square millimeters. Generally, the shrinkage of the samples after removal of the opal template by calcination occurred for the preparation of inverse opals, which may result in some cracking and defects formation in the inverse opals. The presence of cracks and defects may influence the enhancement of UC emission intensity. Therefore, the preparation of inverse opals without cracks and defects may be advantages to the enhancement of UC emission. The SiO2 inverse opal including Ag nanoparticles was scraped from the quartz substrate, and the TEM measurement of corresponding powder was carried out in order to demonstrate the formation of Ag nanoparticles in the SiO2 inverse opal. As shown in Fig. 1 (c), some small Ag nanoparticles were observed in the powder scraped from Ag-IPO-II, and the size of Ag nanoparticles is about 6–10 nm. In order to demonstrate the presence of Ag nanoparticles, high-resolution TEM of black dot was measured in the TEM image, as shown in Fig. 1 (d). It exhibits clear lattice fringes, and the lattice distance was about 0.238 nm, which is corresponded to the (111) lattice planes of the cubic phase Ag. The EDX was used to analyze the chemical elemental compositions of the SiO2 inverse opal including Ag nanoparticles. It can be seen clearly from Fig. 1 (e) that the signal of Ag element was observed besides Si, Cu, and O.



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Fig. 1. The SEM images of the IPO-II (a) and Ag-IPO-II (b); TEM (c) and high-resolution TEM (d) image of the Ag nanoparticles from Ag-IPO-II; EDS spectrum of the Ag-IPO-II (e); Low magnification SEM image of inverse opals (f). In order to further demonstrate the formation of Ag nanoparticles, the absorption spectra of IPO-I~III and Ag-IPO-I~III were measured, as shown in Fig. 2. It can be clearly seen from Fig. 2 (a) that the Bragg diffraction peaks of light caused by the ordered structures in the IPO-I~III were centered at 545, 650 and 760 nm, respectively. The positions of photonic band gaps of the SiO2 inverse opal could be controlled by the PS microsphere size of opal templates. The light propagation in the region of photonic band gap can be inhibited in the photonic crystals due to the Bragg diffraction, which may enhance emission properties of materials. In order to obtain the selective enhancement of UC emission from NaYF4: Yb3+, Er3+ nanoparticles, the spectral overlapping between the photonic band bap and UC emission band is required. Theoretically, the sizes of inverse opal photonic crystals could be determined by Bragg’s law: λ=1.633D ( n2eff

－

sin2θ)1/2. In this equation, λ, D, neff and

θ denote the position of the photonic band-gap, center to center distance between the


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air spheres of inverse opal, the refractive indexes of the inverse opal, and the angle between the incidence light and the normal line of inverse opal. The neff can be obtained by n2eff = n + n2air (1－ ), where the , nair (1) and (1.544) is the volume fraction of SiO2 in the inverse opal, refractive indexes of the air and SiO2 host, respectively. The volume fraction of SiO2 is 26 % for the inverse opal. Based on the Bragg’s law, the calculated center to center distance between the air spheres of inverse opal are 285, 340 and 400 nm for inverse opal with 545, 650 and 760 nm photonic band gap, respectively. As seen from Fig. 2 (b), another absorption peak located at about 410 nm in the Ag-IPO-I~III was observed in contrast to these of the IPO-I~III, which was from SPR absorption of Ag nanoparticles. The spin-coating process was used to deposit the NaYF4: Yb3+, Er3+ nanoparticles on the surface of the IPO-I~III and Ag-IPO-I~III. Fig. 2 (c) and (d) exhibited the absorption spectra of the IPO-I~III and Ag-IPO-I~III after the deposition of NaYF4: Yb3+, Er3+ nanoparticles, respectively. The position of photonic band gap of inverse opal and SPR absorption of Ag nanoparticles were not suffered from effect by the coating of NaYF4: Yb3+, Er3+ nanoparticles. No obvious shift was observed in the absorption spectra of IPO-I~III and Ag-IPO-I~III after the coating of NaYF4: Yb3+, Er3+ nanoparticles, suggested that the NaYF4: Yb3+, Er3+ nanoparticles were mainly located at the surface of IPO-I~III and Ag-IPO-I~III.



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Fig. 2. Absorption spectra of IPO-I~III before (a) and after (c) the deposition of NaYF4: Yb3+, Er3+ nanoparticles; Absorption spectra of Ag-IPO-I~III before (b) and after (d) the deposition of NaYF4: Yb3+, Er3+ nanoparticles. The size distribution and morphology of NaYF4: Yb3+, Er3+ nanoparticles synthesized by a hydrothermal method were observed by TEM, as shown in Fig. 3 (a). The size distribution of NaYF4: Yb3+, Er3+ nanoparticles is relative uniform, and the size of nanoparticles was confirmed to be about 9 nm. Fig. 3 (b) exhibited the high resolution TEM image of as-synthesized NaYF4: Yb3+, Er3+ nanoparticle. The lattice fringe can be distinguished, and the crystallized NaYF4: Yb3+, Er3+ nanoparticles were obtained. The inter-planar spacing of lattice fringes of NaYF4: Yb3+, Er3+ nanoparticle is about 0.32 nm, which matches with that of the (111) planes of NaYF4 in cubic phase. The crystal structure of NaYF4: Yb3+, Er3+ nanoparticles were characterized by the XRD. As seen from the Fig. 3 (c), the XRD peaks can be indexed by standard pattern (JCPDS: No. 00-077-2042), and the NaYF4: Yb3+, Er3+ nanoparticles is pure cubic in the phase. The Scherrer equation (D=k λ / β cosθ) was used to determine size


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of NaYF4: Yb3+, Er3+ nanoparticles, and their sizes were estimated to be 5 nm. The Yb and Er amounts were measured by XRF, which are 14% mol and 2% mol, respectively. The concentration of Yb3+ in the NaYF4: Yb3+, Er3+ nanoparticles is lower than that in raw solution, indicated that only part of Yb3+ were doped into NaYF4 host. The NaYF4: Yb3+, Er3+ nanoparticles distribution on the surface of inverse opals was observed. Fig. 3 (d) showed that field emission SEM of Ag-IPO-II coated by NaYF4: Yb3+, Er3+ nanoparticles. It can be seen that a dense layer of NaYF4: Yb3+, Er3+ nanoparticles were formed on the surface of SiO2 inverse opal including Ag nanoparticles.

Fig. 3. TEM (a) and high-resolution (b) image of the NaYF4 nanoparticles; XRD pattern of the NaYF4: Yb3+, Er3+ nanoparticles (c). SEM (d) of Ag-IPO-II coated by NaYF4: Yb3+, Er3+ nanoparticles. The UC emission spectra of NaYF4: Yb3+, Er3+ on the surface of IPO-I~III and Ag-IPO-I~III were measured, as shown in Fig. 4. In all the samples, sharp UC emission lines were located at 520, 540, 550, 655 and 670 nm, which arise from the 2

H11/2→4I15/2 (520),4S3/2→4I15/2 (540), 4S3/2→4I15/2 (550), 4F9/2→4I15/2 (655) and 4F9/2→

4

I15/2 (670) inner shell transitions of Er3+ ions, respectively. The integrated intensity of



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UC emission from the 2H11/2→4I15/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 as a function of the pump power can be used to investigate the UC emission mechanism. The power dependent UC emission properties of NaYF4: Yb3+, Er3+ on the surface of IPO-I~III and Ag-IPO-I~III were investigated and compared under the excitation of 980 nm. As shown in Fig. 5, the n value of UC emission corresponding to the 2H11/2→4I15/2, 4S3/2 →4I15/2 and 4F9/2→4I15/2 are 1.543, 1.444 and 1.277 for the NaYF4: Yb3+, Er3+ on the surface of IPO-I~III, respectively. For the NaYF4: Yb3+, Er3+ on the surface of Ag-IPO-I~III, the n value of UC emission corresponding to the 2H11/2→4I15/2, 4S3/2→ 4

I15/2 and 4F9/2→4I15/2 are 1.081, 1.086 and 1.057, respectively. In general, the n value

is about 2 for the red or green UC emission of Er3+, suggested the two-photon process was involved. The n values of UC emission of NaYF4: Yb3+, Er3+ on the surface of IPO-I~III and Ag-IPO-I~III are lower than 2, which could be due to saturation effect33-34. The UC emission mechanisms of NaYF4: Yb3+, Er3+ on the surface of inverse opal are illustrated in Fig. 6. The Yb3+ ions with bigger absorption section can transfer its absorption energy to the Er3+ ions with the small absorption section in the co-doping NaYF4 nanoparticles. As shown in Fig. 6, two energy transfer (ET-1, ET-2 and ET-3) processes could promote the ground state Er3+ ions to the 4F7/2 or 4F9/2 state in a UC emission processes. Then, the excited 4F7/2 state Er3+ ions relax nonradiatively to the lower 2H11/2 / 4S3/2 and 4F9/2 states, and radiative transitions from 2H11/2 / 4S3/2 and 4

F9/2 state to the ground state result in the green and red UC emission, respectively.


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Fig. 4. The UC emission spectra of NaYF4:Yb3+, Er3+ on the surface IPO-I, IPO-III, Ag-IPO-I and Ag-IPO-III (a) and IPO-II, IPO-III, Ag-IPO-II and Ag-IPO-III (b).

Fig. 5. Dependence of excitation light on UC emission intensity of the NaYF4: Yb3+, Er3+ nanoparticles on IPO-III (a) and Ag-IPO-III (b).

Fig. 6. The UC emission mechanisms of NaYF4: Yb3+, Er3+ nanoparticles.



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In contrast to the NaYF4: Yb3+, Er3+ on the surface of IPO-III, the UC emission intensities of NaYF4: Yb3+, Er3+ on the surfaces of IPO-I~II are enhanced to different extents. The relative thickness of the NaYF4: Yb3+, Er3+ nanoparticles on various inverse opals has influence on its UC emission intensity. In order to avoid the problem, the UC emission spectra of the NaYF4: Yb3+, Er3+ on the surface of IPO-I~III without Ag nanoparticles were normalized at their UC emission band far away from the photonic band gap, as shown in the Fig. S1 of supporting information. As seen from the Fig. S1, the selective enhancement of green or red UC emission intensity of NaYF4: Yb3+, Er3+ were obtained on the surface of IPO-I and IPO-II, respectively. The ratio of the UC emission intensity of NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-I or IPO-II to that of the NaYF4: Yb3+, Er3+ on the surface of IPO-III was defined as the enhancement factor (EF), as shown in the Fig. 7 (a). For the green UC emission of NaYF4: Yb3+, Er3+ on the surface of IPO-I, the EF is 1.5 factor, while the EF of red UC emission of NaYF4: Yb3+, Er3+ on the surface of the IPO-II is about 1.3 factor. For the red UC emissions of NaYF4: Yb3+, Er3+ on the surface of IPO-I and the green UC emissions of NaYF4: Yb3+, Er3+ on the surface of IPO-II, no enhancement was obtained in comparison with these of the NaYF4: Yb3+, Er3+ on the surface of IPO-III. The selective enhancement of green or red UC emission intensity of NaYF4: Yb3+, Er3+ on the surface of inverse opal was attributed to the photonic band gap effect. The spontaneous emission suppression or enhancement of active centers inside the photonic crystals were be demonstrated due to the variety of density of states caused by the photonic band gap effect19, 35. In


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addition, the periodic change of refractive index of the PC can control the flow of light due to the presence of photonic band gap. Light wavelength in the region of photonic band gap cannot propagate the PC due to the Bragg reflection effect of photonic band gap. In the present work, the NaYF4: Yb3+, Er3+ were distributed on the surface of inverse opal. The green UC emission light of NaYF4: Yb3+, Er3+ on the surface of IPO-I and the red UC emission light of NaYF4: Yb3+, Er3+ on the surface of IPO-II was effectively reflected due to the appropriate spectral overlapping between the photonic band gap and UC emission light. As a consequence, the selective enhancement of green or red UC emission intensity of NaYF4: Yb3+, Er3+ were observed. It is noted that the UC emission intensities of the NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-I~III without Ag nanoparticles were different from that of the NaYF4: Yb3+, Er3+ on the surface of Ag-IPO-I~III. The UC emission intensity may suffer from the influence of relative thickness of the NaYF4: Yb3+, Er3+ nanoparticles on the surface of inverse opals without and with Ag nanoparticles. The Ag nanoparticles embedded in the inverse opals may modify the whole UC emission intensity of the NaYF4: Yb3+, Er3+ nanoparticles, therefore, the normalization of UC emission spectra is not appropriate for the comparison of UC emission enhancement. In order to remove the influence of the thickness of NaYF4: Yb3+, Er3+ nanoparticles in the surface of inverse opals without and with Ag nanoparticles on the UC emission, the Lambert–Beer law was used to determine the relative thickness of the NaYF4: Yb3+, Er3+ nanoparticles on the surface of various inverse opals. According to the



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Lambert–Beer law: A

= lg = lg ∝ kd

where A and T are the absorption and transmittance of sample, respectively. The d and k denote the thickness of the NaYF4: Yb3+, Er3+ nanoparticles and molar absorption coefficient, respectively. The molar absorption coefficient is same for the NaYF4: Yb3+, Er3+ nanoparticles on the surface of various inverse opals. Therefore, the absorption form NaYF4: Yb3+, Er3+ nanoparticles on the various inverse opals is linearly related to its thickness, and the relative thickness of NaYF4: Yb3+, Er3+ nanoparticles on the surface of various inverse opals can be obtained by Lambert– Beer law. The thickness ratio (D) of NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-III inverse opal to that of Ag-IPO-I~III with Ag nanoparticles can be expressed as: D=A1/A2=d1/d2, where the d1 denoted the thickness of NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-III inverse opal without Ag nanoparticles, and the d2 is the thickness of NaYF4: Yb3+, Er3+ nanoparticles on the surface of Ag-IPO-I~III with Ag nanoparticles. The A1 is the absorption of the composites consisting of NaYF4: Yb3+, Er3+ nanoparticles and IPO-III inverse opal. The A2 is the absorption of composites consisting of the NaYF4: Yb3+, Er3+ nanoparticles and inverse opals with Ag nanoparticles. Based on the absorption spectra of the NaYF4: Yb3+, Er3+ nanoparticles on the various inverse opals presented in the Fig. S2 of supporting information, the thickness ratio (D) of NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-III inverse opal to that of Ag-IPO-I~III with Ag nanoparticles were calculated and listed in Table 1. As shown in Table 1, the relative thickness of NaYF4: Yb3+, Er3+


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nanoparticles on the surface of various inverse opals has slight difference and the thickness ratio (D) of NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-III inverse opal to that of the Ag-IPO-I~III with Ag nanoparticles is from 0.971 to 1.012, suggesting that the influence of the thickness of NaYF4: Yb3+, Er3+ nanoparticles should be considered for the enhancement of UC emission. Therefore, the enhancement factor (EF) of NaYF4: Yb3+, Er3+ nanoparticles corrected by the relative thickness should be expressed as:

EF =

× = ×

where the I2 and I1 represent the UC emission intensity of NaYF4: Yb3+, Er3+ nanoparticles on the surface of Ag-IPO-I~III and IPO-III, respectively. Fig. 7 (b) presented the UC emission EF of NaYF4: Yb3+, Er3+ nanoparticles on the surface of Ag-IPO-I~III. In contrast to the NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-III, the UC emission intensities of NaYF4: Yb3+, Er3+ on the surface of Ag-IPO-I~III are significant enhanced, and the overall UC intensity of NaYF4: Yb3+, Er3+ increases by about 4 fold. The UC emission enhancement of NaYF4: Yb3+, Er3+ nanoparticles on the surface of Ag-IPO-I~III was from the SPR effect of Ag nanoparticles. To probe the influence of Ag nanoparticles on the UC emission of NaYF4: Yb3+, Er3+ nanoparticles, and decay lifetime measurements of the NaYF4: Yb3+, Er3+ nanoparticles are particularly useful. The decay curves of 540 nm UC emission of NaYF4: Yb3+, Er3+ nanoparticles on the surface of IPO-III and Ag-IPO-III were measured, as shown in Fig. 8. The 540 nm lifetimes of NaYF4: Yb3+, Er3+ nanoparticles at the surfaces of IPO-III and Ag-IPO-III samples are about 276 and



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Page 18 of 25

264 µs, respectively. The luminescence quantum yield (Qy) is related to the decay lifetime (τm) of luminescent material as following 36:

=

（1）

τ = （ + " ）

#

（ 2）

Where the τ, knr and kr stand for the luminescence lifetime, the rate of non-radiative and radiative decay, respectively. As shown in above equation, either the increase of kr or the decrease of knr will increase the luminescence quantum yield, causing luminescence enhancement. Additionally, the decrease of knr can increase lifetime of active centers, while the increasing of kr can cause the decreasing of lifetime. In the present work, the decrease in the lifetime of NaYF4: Yb3+, Er3+ nanoparticles indicated that one of the mechanisms of the UC emission enhancement of NaYF4: Yb3+, Er3+ nanoparticles is attributed to the increasing of radiative rate caused by Ag nanoparticles26. In addition, the Ag nanoparticles can result in the enhancement of absorption processes by tailoring local electric field, which has influence on the UC emission level population from Er3+. For well understanding the population of the excited level of Er3+, the rate equations were considered by the energy level diagram as shown in Fig. 6. $%&' ()

= *+,- − - / +, − / +, − 0 /1 +, − 2 3 +, = 0

(1)

$"'

= - / +, − 2 0 − / +, = 0

(2)

$"

= 0 /1 +, − 21 + 5 261 = 0

(3)

$"7

= / +, − 1 2 − 1 2 = 0

(4)

$"8

= 2 0 − 0 /1 +, = 0

(5)

() () () ()


Page 19 of 25

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$"9

= 1 2 − : 2 = 0

(6)

$";

= 1 2 − 5 2 1 − 5 2 = 0

(7)

() ()

Where the +,- and +, is the population of ground state and excitation state of Yb3+. The E1, E2, and E3 are the energy transfer from the Yb3+ to the Er3+, respectively, as shown in Fig. 6. The n0, n1 n2, n3, n4, n5 and n6 denoted the population of ground state and excitation state (4I15/2, 4I11/2, 4F9/2, 4F7/2, 4I13/2, 2H11/2, 4S3/2) of Er3+, respectively. The Wr1, Wr2, Wr3 and Wr4 indicates the nonoradiative transition from upper energy level(4F7/2, 4F7/2, 4S3/2, 4I11/2) to lower energy lever (2H11/2, 4S3/2, 4F9/2, 4I13/2) of Er3+, respectively. The W1, W2 and W3 are the radiative transition from the luminescent state (2H11/2, 4S3/2, 4F9/2) to ground state (4I15/2) of Er3+, respectively. The I and ρ are the excitation light power and the absorption section, respectively. The Wr0 is the radiative transition from excitation state (2F5/2) to ground state (2F7/2) of Yb3+. The Wr0 can be neglected due to the absence of direct luminescence. By resolving the rate equation, the population of the luminescent levels of Er3+ ions can be expressed as:

= 0>

' " =7 =8 (=' = )(=7 = )

: =

B

+

=7 (=' = )(=7 = )

Coupling of Ag Nanoparticle with Inverse Opal ... - ACS Publications

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