1. Introduction

Rare-earth-ion-doped inorganic nanoparticles with narrow emissions have potential applications in many optoelectronic devices such as lasers, displays and optical communication [1–4]. Some rare-earth-ion-doped nanoparticles can also emit visible and near-infrared (NIR) light when excited with NIR light (called as upconversion emission), which exhibited considerable interesting in biological labeling, therapeutics and imaging [5, 6]. In particular, in contrast to conventional quantum dots and dyes used in two photon imaging, the upconversion (UC) luminescence nanoparticles have many advantages such as low cytotoxicity, low auto-fluorescence, good photostability, large stock shift and high penetration depth of NIR excitation light [7]. Among the UC emission nanomaterials, the Yb³⁺, Er³⁺ co-doped NaYF₄ nanoparticles have been proved to have the highest efficiency of UC emission [8–14]. However, the UC emission efficiency of Yb³⁺, Er³⁺ co-doped NaYF₄ nanomaterials remains low because of their small absorption cross-section attributed to forbidden f-level transitions of rare earth ions, which limit their practical applications. Therefore, it is very necessary to develop some approaches to enhance UC luminescence of Yb³⁺, Er³⁺ co-doped NaYF₄ nanomaterials. At present, various methods such as amorphous shell coating on the surface of UC nanocrystals and tailoring the local crystal field of the rare earth ions caused by co-doping other ions have been developed to enhance the UC efficiency of Yb³⁺, Er³⁺ co-doped NaYF₄ nanomaterials [15–20].

It is well known that another efficient approach to improve UC emission is the coupling of the UC nanocrystals with noble metal Ag nanostructures. In the vicinity of Ag nanostructures, the electromagnetic field distribution around the rare earth ions can be changed due to the excited light interaction with free electrons of the Ag nanostructures. As a result, the emission rate of the rare earth ions and the excitation field acting on the rare earth ions may be enhanced by the surface plasmonic resonance effect of Ag nanostructures, resulting in the enhancement of UC emission. Recently, the enhancement of UC emission caused by surface plasmonic resonance effect of Ag nanostructures was extensively reported [21, 22]. These previous investigations have demonstrated that the surface plasmonic absorption peaks of Ag nanostructures are related to the size and shape of the metal nanostructures. At present, the Ag nanostructures with various geometric configurations including Ag nanoparticles and films were designed and fabricated [23, 24]. But in previous Ag nanostructures, the surface plasmonic absorption peaks from the Ag nanostructures are commonly located at the visible region, which cannot couple effectively with the NIR excitation light. Thus the enhancement factor of UC emission from rare-earth-ion-doped nanoparticles is limited. In addition, it is well-known that the established guiding principle for the UC emission enhancement caused by Ag nanostructures is to adjust the surface plasmon absorption peak to the region of 980 nm excitation light or UC emission wavelength of the nanoparticles [25]. Therefore, the Ag nanostructures with tunable and broad surface plasmonic absorption are required to the enhancement of UC emission. However, there is still a lack of effective strategy on extending and adjusting the surface plasmonic absorption of Ag nanostructures to overlap with the 980 nm excitation light or UC emission wavelength of the nanoparticles, and the investigations of influence of tunable plasmonic absorption on the UC emission have few reported. In this work, we present a facile method to fabricate the porous Ag films with tunable and broad surface plasmonic absorption by using ordered polystyrene microspheres as a hard template, and then investigated tunable and broad surface plasmonic absorption of porous Ag films on the UC emission of Yb³⁺, Er³⁺ co-doped NaYF₄ nanoparticles. The efficient coupling between the Ag film with ultra-broad surface plasmonic absorption and the 980 nm near infrared excitation light and UC emission were obtained in contrast to the Ag film with narrow plasmonic absorption, which result in the larger UC emission enhancement.

2. Experimental methods

The 230 nm polystyrene (PS) microspheres were used to fabricate the ordered hard template with face-centered cubic structure on quartz substrates. The preparation method of ordered hard template was similar to that presented in our previous works [26, 27]. The porous Ag films with broad surface plasmonic absorption were prepared by using ordered PS microspheres as a template. The 0.1 and 0.3 M AgNO₃ ethanol solution were prepared by dissolving analytical reagent grade AgNO₃ in ethanol solution. The 0.1 and 0.3 M AgNO₃ ethanol solution was infiltrated into the voids of the ordered PS template, respectively. Subsequently, the PS microspheres were removed by sintering the samples at 450 °C and the porous Ag films with tunable and broad surface plasmonic resonance peaks were obtained. When the AgNO₃ solution concentrations were the 0.1 M and 0.3 M, the Ag films prepared by 230nm opal templates were denoted as Ag-230-0.1 and Ag-230-0.3, respectively.

The Yb³⁺ (20 mol%)/Er³⁺ (2 mol%) co-doped NaYF₄ nanoparticles were prepared by the hydrothermal method as originally proposed by Li et al [28–30]. High purity (99.99%) Y₂O₃, Yb₂O₃, Er₂O₃ and analytical regent grade NaF and NaOH were directly used to prepare NaYF₄: Yb³⁺, Er³⁺ nanoparticles without further purification. The RE₂O₃ (RE = Y, Yb and Er) were dissolving in hot HNO₃ solution to prepare corresponding RE(NO₃)₃. After that, RE(NO₃)₃ was obtained through evaporation. In a typical synthesis, the RE(NO₃)₃ were dissolved in 80 mL of a 1:3 (v/v) mixture of oleic acid and ethanol under vigorous stirring. Then, the NaF dissolved in the water was dropped into the above solution. After agitation for 0.5h, the mixture was transferred into a Teflon-lined autoclave Teflon vessel (100 mL), then the resulting solution was heated at 160 °C and kept at that temperature for 8 h. The autoclave was cooled naturally to room temperature. The NaYF₄: Yb³⁺, Er³⁺ nanoparticles were separated by centrifugation and collected after washing with ethanol and deionized water for three times in sequence. The washed nanoparticles were dispersed in cyclohexane for further experiments. In the preparation of porous Ag/NaYF₄: Yb³⁺, Er³⁺ hybrids, the prepared NaYF₄: Yb³⁺, Er³⁺ nanoparticles dispersed in cyclohexane were spin-coated onto the surface of the different porous Ag films by spin coater(MTI-VCT 100).

The XRD patterns of porous Ag films, NaYF₄: Yb³⁺, Er³⁺ and porous Ag/NaYF₄: Yb³⁺, Er³⁺ hybrids were recorded using a D8 ADVANCE diffractmeter with the Cu Kα radiation. The SEM images of the porous Ag films were characterized with a scanning electron microscope (QUANTA 200). The SEM images of the porous Ag/NaYF₄: Yb³⁺, Er³⁺ hybrids were observed by a field-emission scanning electron microscope (QUANTA 650). The TEM images of NaYF₄: Yb³⁺, Er³⁺ nanoparticles were taken by JEOL 2100 transmission electron microscope. The absorbance and reflectance spectra of the porous Ag films were measured by HITACHIU-4100 spectrophotometer. The emission measurements of the UC spectra of the porous Ag/NaYF₄: Yb³⁺, Er³⁺ hybrids were determined by using HITACHIU-F7000 at room temperature under the excitation of 980 nm. The decay curves of UC emission of NaYF₄: Yb³⁺, Er³⁺ nanoparticles were determined under excitation of a 980 nm diode laser with a power density of 10 W cm⁻². The electric field intensities and distributions of single and two 500 nm silver nanoparticles were simulated by FDTD Solution software. The size of Ag nanoparticles were according to the SEM and TEM figures. The simulation region is set to 2000 × 2000 nm with a mesh accuracy 2, and the boundary conditions were set to PML for X and Y. The simulation time and mesh override region was set to 1000 fs and 1, respectively. The 980 nm incidence light was used for simulation.

3. Results and discussions

The typical morphologies of the Ag-230-0.1 and Ag-230-0.3 films were observed by the SEM. The SEM images of the Ag-230-0.1 and Ag-230-0.3 films presented in the Fig. 1 exhibited clearly that the morphologies of Ag films depend on the AgNO₃ solution concentrations infiltrated into the PS templates. It can be clearly seen that the Ag-230-0.1 film was composed of the random and irregular Ag particles with sizes from 400 to 2000 nm, and the sizes of most Ag particles were about 500 nm. The continuous porous Ag films composed of the 500 nm Ag particles were formed for Ag-230-0.3 sample, and lots of pores were formed in the continuous Ag-230-0.3 films. The absorption spectra of Ag-230-0.1 and Ag-230-0.3 films were investigated, as shown in Fig. 1(c). The Ag-230-0.1 films exhibited an absorption band located at about 430 nm, which is attributed to the plasmon resonance absorption band from the single silver particle. It is interesting that the ultra-broad absorption band ranging from 350 to1400 nm was observed in the Ag-230-0.3 films, which is attributed to the efficient plasmon coupling among the neighboring Ag particle.

 

Fig. 1 SEM images of the porous Ag films prepared by 0.1 (a) and 0.3 (b) concentrations AgNO₃ solution; absorption spectra of the porous Ag films (c); TEM (d) and high-resolution (e) TEM image of the NaYF₄ nanoparticles; SEM image of NaYF₄: Yb³⁺, Er³⁺/porous Ag film composites (f).

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The morphologies of the prepared Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles have been examined by the TEM, as shown in Fig. 1(d). It can be seen that the NaYF₄: Yb³⁺, Er³⁺ appear uniform and well-dispersed nanoparticles with a mean particle size of 10 nm. The lattice fringe of the NaYF₄: Yb³⁺, Er³⁺ nanoparticle is obviously distinguished in the high-resolution TEM image as shown in Fig. 1(e), indicating that the highly crystallized NaYF₄: Yb³⁺, Er³⁺ nanoparticles were obtained. The lattice fringes distances of NaYF₄:Yb³⁺/ Er³⁺ nanoparticle was about 0.32 nm, which corresponds to the spacing of the (111) lattice planes of the cubic NaYF₄. The SEM image of NaYF₄: Yb³⁺, Er³⁺/porous Ag-230-0.3 Ag film composites was given in the Fig. 1(f). Dense NaYF₄: Yb³⁺, Er³⁺ layer was formed on the porous Ag-230-0.3 Ag films.

The X-ray diffraction patterns of the as-prepared Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles were presented in the Fig. 2. The Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles exhibit diffraction peak positions and intensities that can be well indexed in accord with the cubic-phase NaYF₄ crystals, suggesting that single phase Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles with high crystallinity was obtained. The average size of the Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles was calculated according to the Scherrer equation [31]: D = k λ / β cosθ. In this equation, K is a constant depends on the particle shape (K = 0.89), D is the size of nanoparticles, λ is the wavelength of the X-ray produced by the Cu Kα radiation (λ = 1.5406 Å), β is the half width of the main diffraction peak (28°), and θ is the Bragg angle of the diffraction peak. The average size of the Yb³⁺/Er³⁺ co-doped nanoparticles was calculated to be about 8 nm by the Scherrer equation, which is consistent with that observed by the TEM. The XRD patterns of the Ag-230-0.1 Ag films and Yb³⁺/Er³⁺ co-doped NaYF₄ nanoparticles deposited on the surfaces of Ag-230-0.1 Ag film was also presented in the Fig. 3. It is noted that the diffraction peak positions and intensities from the porous Ag film were in agreement with the corresponding standard cards of cubic silver, indicating the cubic silver were prepared by template-assisted method. For the porous Ag/NaYF₄: Yb³⁺, Er³⁺ sample, the diffraction peaks located at 2θ = 28 ° from cubic NaYF₄: Yb³⁺, Er³⁺ and characteristic diffraction peak at 2θ = 38 ° from cubic Ag was observed, implying the formation of porous Ag/NaYF₄: Yb³⁺, Er³⁺ nanocomposites.

 

Fig. 2 XRD pattern of the NaYF₄: Yb³⁺, Er³⁺ nanoparticles, the Ag-230-0.1 film and the Ag-230-0.1/NaYF₄: Yb³⁺, Er³⁺ hybrids.

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Fig. 3 UC emission spectra of NaYF₄: Er³⁺ hybrid deposited on various substrates (a), dependence of excitation light on UC emission intensity of the NaYF₄: Er³⁺ nanoparticles on the surface of RS (b), Ag-230-0.1(c) and Ag-230-0.3(d).

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The prepared nanoparticles were directly spin-coated onto the surface of the quartz substrate as the reference sample (RS). The UC emission spectra of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the RS, Ag-230-0.1 and Ag-230-0.3 surfaces were measured and compared under the excitation of a 980 nm laser diode with same power, as shown in Fig. 3(a). The UC emission peaks are assigned to ²H_11/2→ ⁴I_15/2 (525 nm), ⁴S_3/2→⁴I_15/2 (546 nm) and ⁴F_9/2→⁴I_15/2 (650 nm) transitions of Er³⁺. In the UC emission process, the UC luminescence intensity (Iuc) is proportional to the n^th of the pump power (P). That n is the number of absorbed infrared photons required for emitting one visible photon. The pump power dependence of UC emission intensity of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on RS, Ag-230-0.1 and Ag-230-0.3 surfaces was measured. A straight line with slope n is yield by a plot of log. Iuc vs. log. Pⁿ, as shown in Fig. 3(b)-(d). From the Fig. 3(b)-(d), the n values for ²H_11/2→⁴I_15/2(525 nm), ⁴S_3/2→⁴I_15/2(546 nm), ⁴F_9/2→⁴I_15/2(650 nm) transitions of Er³⁺ ions in the all samples are smaller than the required photon number 2, which are mainly attributed to the saturation effect [32, 33]. The saturation effect is related to the competition between UC emission and linear decay for the intermediate excited state depletion of rare earth ions. If the UC emission process is dominant, this will result in that the slope value is smaller than the theoretical value [34, 35]. In addition, the slope value of NaYF₄: Yb³⁺, Er³⁺ nanoparticles deposited on the Ag films are smaller than those of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the quartz substrate, which is contributed to the local thermal effect caused by Ag film. It is reported that the slopes of the NaYF₄: Yb³⁺, Er³⁺ nanoparticles depend on the local thermal effect caused by metal nanoparticles. The presence of thermal effect caused by metal nanoparticles will lead to the smaller slope value in the UC emission processes [36, 37]. Figure 4 shows the mechanisms of UC emissions. The absorption section of Er³⁺ ions is smaller in comparison with that of Yb³⁺ ions. Therefore, the co-doping of Yb³⁺ ions is order to transfers its energy to Er³⁺ ions. In the UC emission processes, the ground state Er³⁺ ions can be promoted to the ⁴F_7/2 or ⁴F_9/2 state by three energy transfers (ET-1, ET-2 and ET-3) from Yb³⁺ to Er³⁺ ions, as shown in Fig. 4. Subsequently, the excited ⁴F_7/2 state Er³⁺ ions relax nonradiatively to the lower ²H_11/2/⁴S_3/2 and ⁴F_9/2 states. The radiative transitions from the ²H_11/2/⁴S_3/2 and ⁴F_9/2 state to the ground state result in the green and red UC emission, respectively.

 

Fig. 4 UC emission mechanisms of NaYF₄: Yb³⁺, Er³⁺ nanoparticles.

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The UC emission of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the surfaces of the Ag-230-0.1, Ag-230-0.3 were enhanced in contrast to that of RS, respectively, which is attributed to surface plasmon resonance (SPR) effect of the porous Ag films. The SPR stems from the collective oscillations of the free electrons exhibited by metal nanostructure, which could produce very giant localized electric fields around the metal. Thus, the UC luminescence of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the surfaces of the Ag-230-0.1 and Ag-230-0.3 was enhanced due to generation of intense electric fields, as shown in the next section. It is noted that UC enhancement factor is dependent strongly on the structure of the porous Ag film. UC emission enhancement factor (EF) is defined as the ratio of the UC emission intensity of NaYF₄: Yb³⁺, Er³⁺ nanoparticles at the surfaces of the Ag-230-0.1 or Ag-230-0.3 to that from NaYF₄:Yb³⁺, Er³⁺ nanoparticles on the RS bald quartz surface without Ag, as shown in Fig. 5. The EF of the NaYF₄: Yb³⁺, Er³⁺ on the surfaces of Ag-230-0.1 and Ag-230-0.3 are 10 and 60 fold, respectively. It is interesting that the EF in the Ag-230-0.3 sample with broad absorption band is larger than that the Ag-230-0.1 sample with narrow absorption band. This is because the absorption band ranging from 350 to 1400 nm of Ag-230-0.3 could overlap with the excited light of 980 nm and UC emission, resulting in well coupling together between the SPR of Ag-230-0.3 and the excitation light and UC emission.

 

Fig. 5 Enhancement factor (EF) of UC emission of NaYF₄: Yb³⁺, Er³⁺ on the surfaces of the various substrates.

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The UC emission enhancement of NaYF₄: Yb³⁺, Er³⁺ nanoparticles caused by Ag films may be attributed to three possible mechanisms. One is the increasing of radiative decay rate of NaYF₄: Yb³⁺, Er³⁺ nanoparticles induced by Ag films. The increasing of radiative decay rate may result in the decreasing in UC emission lifetime of NaYF₄: Yb³⁺, Er³⁺ nanoparticles. The UC emission lifetime of NaYF₄: Yb³⁺, Er³⁺ nanoparticles were measured, as shown in Fig. 6. The UC luminescence lifetimes at 540 nm of NaYF₄: Yb³⁺, Er³⁺ nanoparticles at the surfaces of RS, Ag-230-0.1 and Ag-230-0.3 samples are about 304, 267 and 260 μs, respectively. The UC luminescence lifetimes at the 670 nm of NaYF₄: Yb³⁺, Er³⁺ nanoparticles at the surfaces of RS, Ag-230-0.1 and Ag-230-0.3 samples are about 347, 330 and 320 μs, respectively. The decreasing of the 540 and 670 nm UC emissions suggested that the increasing of radiative rate is one of the mechanisms of the UC emission enhancement of NaYF₄: Yb³⁺, Er³⁺ nanoparticles. It is interesting that the modification extent of 540 and 670 nm UC emissions are associated to the surface plasmonic absorption positions of Ag films. The 540 nm UC emission lifetimes of NaYF₄: Yb³⁺, Er³⁺ nanoparticles at the surfaces of Ag-230-0.1 and Ag-230-0.3 samples decreased 37 and 44 μs in contrast to that of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the RS, respectively, while the 670 nm UC emission lifetimes of NaYF₄: Yb³⁺, Er³⁺ nanoparticles at the surfaces of Ag-230-0.1 and Ag-230-0.3 decreased 17 and 27 μs. The radiative lifetime of 540 nm UC emission from NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the Ag-230-0.1 and Ag-230-0.3 thin films was strongly modified in contrast to that of 670 nm UC emission, which is attributed to the well overlapping between the surface plasmon absorption of Ag-230-0.1 and Ag-230-0.3 and the 540 nm UC emission wavelength of the NaYF₄: Yb³⁺, Er³⁺ nanoparticles [38], as shown in Fig. 1(c). In addition, the larger modification of radiative lifetime of 540 and 670 nm UC emission from NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the Ag-230-0.3 thin film was obtained in contrast to that of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the Ag-230-0.1, which demonstrated that larger UC enhancement of NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the Ag-230-0.3 thin film is attributed to the efficient coupling between of the broad plasmonic resonance of the Ag-230-0.3 film and UC emission. This result indicated Ag film with ultra-broad plasmonic absorption is more appropriate as substrate for the enhancement of UC emission in contrast to the Ag film with narrow plasmonic absorption. Another enhancement mechanism of UC emission is the influence of scattering and reflectance component of UC emission and excitation light caused by the Ag films.

 

Fig. 6 The 540 nm (a) and 670 nm (b) UC emission decay curves of NaYF₄: Yb³⁺, Er³⁺ nanoparticles.

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Figure 7 showed the diffuse reflection spectra of Ag-230-0.1 and Ag-230-0.3 films. The diffuse reflections of Ag-230-0.3 and Ag-230-0.1 films in the region of 545-700 nm are about 40 and 15%, respectively. In addition, the 980 nm reflections of Ag-230-0.3 and Ag-230-0.1 films are about 55 and 35%, respectively. Therefore, the enhancement of scattering and reflection component of UC emission and excitation light on the Ag-230-0.3 and Ag-230-0.1 films can result in enhancement of the UC emission of the NaYF₄: Yb³⁺, Er³⁺ nanoparticles. The third mechanisms of UC emission enhancement is attributed to local field enhancement induced by Ag film. In Alexander E. Krasnok’s work they use the Purcell effect to describe the environment changed. They calculate the Purcell factors around the mental nanoparticles, which also shown the local field could be used for the enhancement [39].

 

Fig. 7 Diffuse reflection spectra of the porous Ag films.

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The SPR effect of Ag nanoparticles can cause the enhancement of electric field, leading to the enhancement of the excitation field [40, 41]. Thus, the population of luminescent levels of Er³⁺ ions increased, causing the UC enhancement of Er³⁺. In order to demonstrate this mechanism, the electric field intensities and distributions around single and two 500 nm silver particles based on the SEM images were simulated by the FDTD solution software, respectively. The simulated electric field intensity distribution around single and two 500 nm silver particles was shown in the Fig. 8. It is clearly seen that the electric field intensities around single and two 500 nm silver particles can be improved, which may lead to the UC emission enhancement of the NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the Ag-230-0.1 and Ag-230-0.3 film. It is interesting that the electric field intensities around two 500 nm silver particles coupled each other is stronger than that of single 500 nm silver particles, thus significant excitation enhancement effect was obtained due to effective coupling between the SPR of Ag-230-0.3 and the 980 nm excitation light, resulting in significant enhancement of UC emission of the NaYF₄: Yb³⁺, Er³⁺ nanoparticles on the surface of Ag-230-0.3 films, which further demonstrated that Ag film with ultra-broad plasmonic absorption is more appropriate as substrate for the enhancement of UC emission in comparison with the narrow plasmonic absorption Ag film.

 

Fig. 8 FDTD simulated near-field intensity distributions of single 500 nm silver particle (a) and two coupling Ag particles (b).

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4. Conclusions

In this work, we prepared porous silver film with ultra-broad absorption band by a simple PS template method, and the influence of ultra-broad plasmon absorption on the upconversion emission of NaYF₄: Yb³⁺, Er³⁺ nanoparticles were investigated. The UC emission of NaYF₄: Yb³⁺, Er³⁺ enhanced by 10 fold at the surface of porous Ag film surface with narrow plasmonic resonance absorption. While about than 60 fold enhancement of UC emission were obtained on the surface of porous Ag film surface with ultra-broad plasmonic resonance absorption, which is attributed to effective coupling between the ultra-broad surface plasmonic resonance and the 980 nm near infrared excitation light and UC emission. The significant enhancement of the upconversion emission caused by ultra-broad surface plasmonic absorption may be extended to other phosphors, resulting in the generation of new lighting devices.