1. Introduction

With the advantage of requiring no fossil fuels and producing no emissions, the application of solar cells is of great significance. The conversion from sunlight to electricity using solar cell devices represents a promising approach to green and renewable energy. Solar cells present an intrinsic limitation for the efficient conversion of sunlight radiation into electricity and the theoretical maximum conversion efficiency is ~30% in classical Si-based solar cells [1, 2]. The major part of the losses is related to the spectral mismatch and only the photons with energy close to the semiconductor’s band-gap energy can be profitably absorbed by a solar cell [3]. Luminescent materials and antireflection (AR) coatings used to enhance the efficiency of the commercial crystalline Si (c-Si) solar cells arose much attention in recent years. The efficiency of the c-Si is estimated to be improved up to 40% by downconversion (DC) luminescent materials [2]. A quantum cutting (QC) phosphor which converts one high-energy photon into two low energy photons have an ideal quantum efficiency (QE) of up to 200%, which makes it a candidate for improving the efficiency of c-Si solar cells. Owing to the abundant energy levels and narrow emission spectra lines, rare earth (RE) ions play a great role in the QC process. Yb³⁺ ion has been extensively applied for its near-infrared (NIR) emission around 980 nm (²F_5/2→²F_7/2), which is just above the band-gap of the c-Si solar cells. Therefore, the phosphor based on Yb: rare-earth couples could be realized as a QC convertor coating on the solar cell panels [4]. At present, many efforts have been made to develop Tb³⁺-Yb³⁺ co-activated phosphors [5–11], which of the energy transfer (ET) from the donor (Tb³⁺) to the acceptor (Yb³⁺) is involved with the NIR QC process. Unfortunately, due to its parity forbidden of 4f-4f transitions,the narrow and low absorption efficiency of Tb³⁺ ion leads to the relatively low NIR emission although the physics of the ET process allows for efficient QC.

To overcome the limitations, introduction of metal ions to improve the absorption efficiency has been reported recently [12–15]. RE ions and silver co-doped dielectric host have attracted persistent interest for the development of optoelectronic devices. The research was focused on the effect of silver species on the luminescent properties of RE ions [12–24]. Large enhancement of the RE ions emission was observed by cooperation with Ag ions. Two effects of silver species on RE ions emission were observed: the enhancement by surface plasmon resonance (SPR) of Ag NPs [12–15] and ET from charged silver ionic species such as Ag⁺, (Ag₂)⁺, (Ag₄)²⁺ to RE ions [16–24]. Therefore, the silver ionic species with spin-allowed broad absorption in the UV-blue region was believed to be an excellent sensitizer for certain rare ions. At present, many literatures have reported that the Tb emission was greatly enhanced due to the energy transfer between Ag nanoclusters and Tb³⁺. Therefore, it is possible that the Yb³⁺ can be indirectly sensitized by Ag nanoclusters, while the Tb ions is a bridge between Ag nanoclusters and Yb³⁺. And then the NIR QC emission of Yb ions may be enhanced.

Nevertheless, although the RE ions luminescence may be enhanced (or sensitized) by silver species, however, even in the highest value before the concentration quenching threshold of Yb ion, the the visible emission of the absorption center ions always remains strong and the NIR luminescence of Yb³⁺ is still inevitably weak. This phenomenon is caused by the following reasons. First, the Yb ion light-emitting intensity achieve maximum with low Yb ion concentration which lead to the actual ET efficiency is not high. The second, the upconversion (UC) process of Tb³⁺ and Yb³⁺ co-doped system excited by ET from Yb³⁺, which is so-called back ET, is more efficient than the QC process [10, 25]. The issues above maybe meliorated from the view of samples structure . Very recently, a few papers reported that spontaneous emission of RE ions could be modified by photonic band gap (PBG) effects [26–28]. A decrease in the spontaneous emission of rare earth ions in the range of their band gap, and enhancement at the band gap edge have been obtained in photonic crystals [26]. Photonic crystal is a three-dimensional orderly inverse opals structure, which can adjust the luminescence properties by controlling energy transitions of doped activators. Enhancement of UC emission was realized by ET process in this inverse opals structure [28]. Consequently, ordering the structure of QC materials to photonic crystals is perhaps a promising way to weaken spontaneous radiation of sensitizer ions and improve the ET efficiency of activators.

In this work, the visible-to-NIR light emitting thin films with silver (SiO₂: Tb, Yb, Ag) were spin coated on quartz substrates by using SiO₂ precursor prepared simply through the sol-gel method. Compared to the phosphors, AR coating thin films is applied to reducing reflectance on both c-Si and baffle glass in order to enhance the performance of the solar cells. SiO₂ is chosen as a host material owing to the low refractive index, good durability, and environmental resistance which is being a better candidate to construct AR coatings [29]. Then the SiO₂: Tb, Yb, Ag inverse opal photonic crystals was fabricated by a self-assembly technique in combination with a sol-gel method. The experimental results showed that the different structure of the film have different effects on the state of the silver. Different silver species, which represent Ag NPs, silver aggregates of (Ag₂)⁺ and (Ag₄)²⁺, was observed in those thin films. In general, the formation of the Ag nanoclusters requires a matrix to provide a nucleating agent. For example, J. J. Velázquez [24] found that the F^- vacancies are required for nucleation and charge compensation of the (Ag₄)²⁺ in oxyfluoride glass hosts. Herein, we demonstrate that Tb is a necessary condition for the formation of (Ag₄)²⁺ in the system. The enhancement of the visible-to-NIR emission of Tb and Yb ions by Ag species were investigated in those thin films. The possible mechanism of visible-to-NIR QC emission and the effect induced by Ag species and PBG are discussed.

2. Experimental

The SiO₂: Tb, Yb, Ag sol was prepared by using tetraethyl orthosilicate [Si(CH₃CH₂O)₄, TEOS], Tb₄O₇, Yb₂O₃, ethanol, HNO₃ and AgNO₃ as raw materials. The dried Tb(NO₃)₃ and Yb(NO₃)₃ were obtained by dissolving Tb₄O₇ and Yb₂O₃ into hot HNO₃ followed by evaporation of water. The Tb(NO₃)₃, AgNO₃ and Si(CH₃CH₂O)₄ were dissolved in ethanol, respectively. Then the above solutions were mixed together. After moderate stirring, homogeneous SiO₂: Tb, Yb, Ag sol was obtained. The resultant solution was highly transparent and remained very stable for a period of two months under sealed condition. These films were first baked at 80 °C for 1h and then sintered in an air furnace at temperature of 750 °C for 5h. Finally, the SiO₂: Tb³⁺, Yb³⁺ composites thin films including Ag NPs were obtained. The final annealed film thickness ranges from 0.5 to 0.7 µm.

The SiO₂:Tb, Yb, Ag inverse opal photonic crystals have been synthesized by colloidal crystal template method. Using commercially available suspensions of polystyrene (PS) microspheres with a diameter of 360 nm, 400 nm or 500 nm and about 10% PS microspheres was in the suspension. The ordered colloidal crystal templates on quartz substrates were fabricated by self-assembly technique, as reported in our previous works [30]. The PS templates filled with SiO₂: Tb, Yb, Ag sol was performed at room temperature. After infiltration, the opals were hydrolyzed and condensed in the air for 24 h. Then the filled ordered colloidal crystal templates were sintered in an air furnace at temperature of 750 °C to remove the PS microspheres. Finally, the SiO₂: Tb³⁺, Yb³⁺ composites inverse opals including Ag NPs were obtained. The SiO₂: Tb, Yb, Ag inverse opals prepared by unitary opal templates constructed with single size microspheres 360 nm, 400 nm or 500 nm in diameter were denoted as IPC-485-Ag, IPC-540-Ag, and IPC-663-Ag, respectively.

The QC emission measurements and luminescent decay curves of the inverse opals were carried out on FLSP-980 spectrophotometer under 250 nm, 330 nm, 377 nm, and 467 nm light excitation, respectively. Absorption spectra were measured by a HITACHIU-4100 spectrophotometer in the 300-800 nm regions. The SEM images were obtained with a field-emission scanning electron microscope (QUANTA200). An energy dispersive X-ray spectroscopy (EDS) attached to a JEOL 2100 transmission electron microscope was used to analysis the chemical composition. Transmission electron microscope (TEM) images were taken using a JEOL 2100 transmission electron microscope operating at an acceleration voltage of 200 kV.

3. Results and discussions

3.1 Near-infrared quantum cutting emission in Tb-Yb co-doped SiO₂ thin films

A series of SiO₂: 1 mol% Tb, x mol% Yb thin films were first prepared to study the mechanism of the near-infrared emission. The Figs. 1(a) shows the excitation spectrum of the SiO₂: 1% Tb, 4% Yb thin film by monitoring the 543 nm emission and the 977 nm emission. An intense band observed at 250 nm, which was attributed to the defects of SiO₂ matrix [31]. In addition, several very weak sharp excitation peaks at 316, 339, 378, 480 nm were observed by monitoring 543 nm emission, which are originated from the ⁷F₆→⁵D₀, ⁷F₆→⁵L₉, ⁷F₆→⁵L₁₀, and ⁷F₆→⁵D₄ of Tb³⁺, respectively. We demonstrated that the ET between silica matrix and Tb³⁺ occurred [31]. Figures 1(b) shows the visible-NIR emission spectrum of the samples under 250nm light excitation. As can be seen, the visible emission spectra of all samples consists of four emission bands located at 489, 543, 587 and 623 nm, which are attributed to the ⁵D₄→⁷F_j (j = 6, 5, 4, 3₎ transition of Tb³⁺, respectively. Meanwhile, an emission centered at 977 nm along with a shoulder at 1015 nm has also been observed and it corresponds to the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. With increase of Yb³⁺ concentration, the intensity of Tb³⁺ luminescence gradually decreased while the NIR emission of Yb³⁺ was significantly enhanced. The results indicate the possible ET from Tb³⁺ to Yb³⁺ ions.

 

Fig. 1 (a) excitation spectra by monitoring the Tb³⁺: ⁵D₄→⁷F₅ emission (543 nm), and the Yb³⁺: ²F_5/2→⁷F_7/2 emission (977 nm) of SiO₂: 1% Tb³⁺, 4% Yb³⁺ thin films, and (b) visible-NIR emission spectrum under 250 nm excitation of SiO₂: 1% Tb³⁺, x% Yb³⁺ thin films.

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Further investigation was made by the luminescence decay curves monitoring the ⁵D₄→⁷F₅ transition of Tb³⁺ at 543 nm in Fig. 2. The curve of the singly doped SiO₂:1% Tb³⁺ thin film exhibits a nearly single exponential decay with a decay time of 2.64 ms. As expected, the lifetimes decreased rapidly from 2.64 to 1.58 ms with increase of Yb³⁺ concentration from 0 to 10 mol%. The large decay time difference between SiO₂:Tb³⁺, 0mol%Yb³⁺ sample and SiO₂: Tb³⁺, 10mol%Yb³⁺ identified the efficient ET from Tb³⁺ to Yb³⁺ ions. If ET by a second-order process is responsible for the up converted luminescence, the second-order down conversion should occur as well [11]. In the Tb-Yb system, the blue emission (489nm) and green emission (543nm) of Tb³⁺ have been studied as a classical two photon upconversion process, where Yb³⁺ ions act as absorption centers [32]. Therefore, in this system, we considered that QC occurs upon excitation at the 250nm and the energy of ⁵D₄ level of a Tb³⁺ ion is transferred to two different Yb³⁺ ions. The ET efficiency is calculated to be 40.15% for 10 mol% Yb³⁺ doping by the equation η_T = 1-τ_{x% Yb}/τ₀ [33]. There, the τ ₀ is the decay lifetime of Tb³⁺: ⁵D₄ in the singly doped samples. The quantum efficiency (QE) is calculated to be 140.15% for SiO₂: 1% Tb³⁺, 10% Yb³⁺ thin film. The relation between the ET efficiency and the total QE is defined as [6] η_QE = η_Tb(1−η_{x% Yb}) + 2η_{x% Yb}, where QE for the Tb³⁺ ions, η_Tb, is set to 1.

 

Fig. 2 The luminescence decay curves of Tb³⁺ at 543 nm with different Yb³⁺ concentrations excited at 250 nm of SiO₂: 1% Tb³⁺, x% Yb³⁺ thin films.

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3.2 Enhancement of the quantum cutting luminescence of Tb-Yb co-doped SiO₂ thin films by Ag species

The existing states of silver in the SiO₂ thin films materials were investigated by the absorption spectra, as shown in Fig. 3. The plasmon resonance absorption peak located at about 410 nm from silver NPs was observed, which demonstrated the formation of Ag NPs in the SiO₂ thin films triple added with Tb, Yb, and Ag. The plasmon resonance absorption peak from silver NPs increased with increasing Ag concentration, suggested that the more silver NPs were obtained in the SiO₂: Tb, Yb, Ag thin films. The plasmon resonance absorption peak from silver NPs had not appeared in the SiO₂: Tb³⁺,Yb³⁺ films.

 

Fig. 3 The absorption spectra of SiO₂: 1% Tb³⁺, 4% Yb³⁺ thin films with various Ag concentrations.

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In order to verify the formation of Ag NPs in the SiO₂: Tb, Yb thin films materials, the SiO₂: 1 mol% Tb, 4 mol% Yb thin films with 0.3 mol% Ag was scraped from quartz substrate and the TEM image of the corresponding SiO₂: 1% Tb³⁺, 4% Yb³⁺, 0.3% Ag powder was measured, as shown in Figs. 4(a) and 4(b). It can be clearly seen that silver NPs were observed in the SiO₂: 1% Tb³⁺, 4% Yb³⁺,0.3% Ag thin film sintered at 750 °C. At high magnification, the Ag NPs which we investigate in Figs. 4(b) are composed of many small single crystal Ag NPs. There are also a small number of single crystal Ag NPs dispersed in SiO₂ substrate, which do not aggregate, as shown in Figs. 4(c). By the high resolution TEM technology in inset of Figs. 4(d), we can clearly observe that the Ag NPs has perfect lattice planes identified to be (200) face of crystalline metal Ag.

 

Fig. 4 The TEM image of SiO₂: 1% Tb³⁺, 4% Yb³⁺ thin films with 0.3 mol% Ag (a), (b), (c), and (d).

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Figure 5 shows the visible emission and the NIR emission spectra of the SiO2: Tb, Yb, Ag thin films under the excitation of 250 nm. All samples show typical Tb³⁺ or Yb³⁺ emission bands. The emission intensity of Tb³⁺ and Yb³⁺ increase with increasing Ag concentration from 0.0 mol% to 0.3 mol% and then decreases with the further increase of Ag concentration, which indicates QC luminescence enhancement was achieved. The origin of silver NPs enhanced luminescence in rare earth doped materials is attributed to the SPR effect of Ag NPs. In the previous studies, two luminescence enhancement mechanisms due to the SPR effect of Ag NPs have been reported. One is an increase in the effective excitation induced by local field enhancement (LFE) of metal NPs. Another is the increase of the radiative decay rate of active centers [34]. For the sake of understanding the luminescence enhancement mechanisms of Tb³⁺ and Yb³⁺ in the those Ag doped thin films, the luminescence lifetimes of the Tb³⁺: ⁵D₄→⁷F₅ emission at 543 nm and Yb³⁺: ²F_5/2→²F_7/2 emission at 977 nm were measured, as shown in Figs. 6(a) and 6(b). It is noted that the lifetime of Tb³⁺ and Yb³⁺ decreased obviously with the increasing of Ag concentration from 0 to 0.3 mol%, and than increase with the more Ag NPs. According to previous studies, the decreased lifetime is due to electromagnetic coupling between the RE³⁺ and the plasmon resonances of the silver NPs [34]. The effects of the Ag NPs depended on the surrounding density of the fluorophores. There were only moderate changes in the intensity induced by the islands. If a fluorophore already has a quantum yield near unity, the Ag NPs surface cannot make the quantum yield exceed unity. In such cases the dominant effect is quenching by the Ag NPs surface. In contrast, the Tb and Yb emission was slightly increased. This enhancement is the result of an increase in the radiative decay rate of Tb and Yb which increases the quantum yield and decreases the lifetime. These results are those expected for an increase in the radiative rate of high and low quantum yield fluorophores. The luminescence intensity decrease of Tb and Yb may be due to the concentration quenching when the concentration of Ag is about 0.3 mol%.

 

Fig. 5 Visible and NIR emission spectrum under 250 nm excitation of SiO₂: 1% Tb³⁺, 4% Yb³⁺ thin films with various Ag concentrations.

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Fig. 6 The luminescence lifetimes of the 543 nm emission of Tb³⁺ (a) and the 977 nm emission of Yb³⁺ (b) excited at 250 nm.

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In Figs. 7(a), by monitoring the 543 nm emission, compared with the excitation spectrum SiO₂: Tb, Yb thin film without Ag, as the Ag concentration gradually increased, the peak position remained almost unchanged. The excitation peaks at 377 nm was ascribed to Ag nanoclusters of (Ag₂)⁺ due to the excitation intensity of 377 nm decreased with 0.3 mol% Ag. The details of this mechanism will be discussed in the next section. However, As shown in Figs. 7(b), by monitoring the 977 nm emission, with the addition of Ag, two new excitation peaks appeared, which located in 330 nm and 467 nm, respectively. Those two excitation peaks intensity obviously increased with the increasing concentration of Ag. Excited at 330 nm, as shown in Figs. 8(a), an emission band located at 474 nm was observed in the Ag co-doped samples, which may be attributed to Ag nanoclusters. We considered that the Ag nanoclusters were supposed as (Ag₄)²⁺ tetramers, which were similar to the previous studies [19].

 

Fig. 7 The excitation spectrum of SiO₂: 1% Tb³⁺, 4% Yb³⁺ thin films with various Ag concentrations by monitoring both the 543 nm emission (a) and the 977 nm emission (b), respectively.

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Fig. 8 The visible emission spectrum excited at 330 nm of SiO₂: 1% Tb³⁺, 4% Yb³⁺ thin films with various Ag concentrations (a), Inset shows the excitation spectrum of SiO₂: 1% Tb, 4% Yb, 0.3% by monitoring the 474 nm emission. The NIR emission spectrum of SiO₂: 1% Tb, 4% Yb, 0.3% Ag thin film under various excitation (b).

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In order to verify this supposition, we prepared SiO₂: 4 mol% Yb, 0.3 mol% Ag thin film without Tb. The absorption spectra is shown in inset of Figs. 9(a), and the absorption peak of silver NPs was observed. The excitation and emission excitation spectrum were exhibited in Figs. 9(a) and 9(b). A clearly visible emission at 395nm (In the SiO₂: 1 mol% Tb³⁺, 4 mol% Yb³⁺, x mol% Ag thin films, we did not investigate an obviously 395nm emission) and a weak NIR emission at 977 nm was obtained under the excitation of 250 nm. By monitoring the 395nm and 977 nm emission, we only observed a excitation peak located in 250 nm. That is to say, the (Ag₄)²⁺ cannot formed in this sample. Since the Tb ions has a trend from Tb³⁺ to Tb⁴⁺, an electron will be given to compensate for the positive charge of the (Ag₄)²⁺. We considered that the Tb ions are required for nucleation and charge compensation of the (Ag₄)²⁺ in SiO₂ glass hosts. The visible emission peaked at 395 nm in the Figs. 9(a) is attributed to Ag⁺. It is supposed that the ET from Ag⁺ to Yb³⁺ occured, where the ET process is not very effective.

 

Fig. 9 The visible-NIR emission spectrum excited at 250 nm of SiO₂: 4% Yb³⁺,0.3% Ag thin film (a), and the excitation spectra by monitoring both 395 nm emission and 977 nm emission of SiO₂: 4% Yb³⁺,0.3% Ag thin film (b).

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The inset of Figs. 8(a) shows excitation spectrum by monitoring 474 nm emission, an intense band observed at around 330 nm. The excitation spectrum (Figs. 7(a)) indicates that the ET between (Ag₄)²⁺ with Tb³⁺ has not occuerd. The increasing Tb³⁺ emission intensity may be due to the LEF enhancement around the Ag NPs. By contrast, it is worthy to note that the strongest emission of Yb³⁺ was observed by the excitation 330 nm, as shown in Figs. 8(b). The excitation spectrum and emission spectrum suggested that ET processes from the (Ag₄)²⁺ to the Yb³⁺ took place. The ET between (Ag₄)²⁺ to the Yb³⁺ was suggested a large amount of QC processes by excited 330nm [19]. However, under the excitation of 467 nm, an intense NIR emission of Yb³⁺ was investigated while no obvious emission peak was obtained in the visible region, as shown in Figs. 8(a). As the 467 nm and 330 nm excitation peaks always appeared at the same time, and the changing rule was also identical, so we believed that the 467 nm excitation peak was attributed to (Ag₄)²⁺ too. We considered that the formation of (Ag₄)²⁺ depends on the quantity and total surface area of the Ag NPs in the SiO₂ substrate. With the increasing of Ag⁺ content, the precipitation of Ag NPs increases, and the total surface area of the Ag NPs increases, which leads to increase intensity of the excitation peak. According to the TEM, due to the Ag NPs in those thin films gathered together easily, then the surface area and quantity of Ag NPs reduced. When the number of Ag NPs further increased, the cluster function played a leading role. The surface area and number of silver NPs were reduced, which hindered the formation of (Ag₄)²⁺.

3.3 Visible-NIR quantum cutting photoluminescence enhancement in Tb-Yb-Ag triply doped SiO₂ inverse opal photonic crystals

The QC light emitting PBG materials (SiO₂: Tb, Yb, Ag) with inverse opal structure were prepared by a self-assembly technique in combination with a sol-gel method. The SEM image of the IPC-485-Ag was shown in Figs. 10(a). The IPC-485-Ag sample exhibit a ordered hexagonal arrangement. The center to center distance of macroporous in the IPC-485-Ag is 330 nm. The microstructure characteristics of the IPC-540-Ag and IPC-663-Ag are similar to that of the IPC-485-Ag, which are not shown herein. Figures 10(c) shows the EDS spectrum of the IPC-485-Ag. It can be clearly seen that the Si, O, Tb, Yb, and Ag elements were observed in this sample. In order to verify the formation of Ag NPs in the inverse opals, the IPC-485-Ag was scraped from quartz substrate and the TEM image of the corresponding IPC-485-Ag powder was measured, as shown in Figs. 10(b). We can clearly observe that silver NPs take shape in the IPC-485-Ag sintered at 750 °C, and the size of Ag NPs is about 10 nm. It is suggested that the inverse opal structure is conducive to the formation of the small size of Ag NPs, and hinder the interaction between Ag NPs then inhibit the clusters. Figures 10(d) shows the absorption spectra of IPC-485-Ag, IPC-540-Ag, and IPC-663-Ag. The plasmon resonance absorption peak located at about 400 nm from silver NPs was observed, which further demonstrated the formation of Ag NPs in the inverse opals. According to the absorption spectra, the photonic band gaps of IPC-485-Ag, IPC-540-Ag, and IPC-663-Ag locate in 485 nm, 540 nm, and 663 nm, respectively.

 

Fig. 10 SEM image of the IPC-485-Ag (a); the TEM image of the IPC-485-Ag (b); the EDS of the IPC-485-Ag (c); the absorption spectra of the IPC-485-Ag, IPC-540-Ag and IPC-663-Ag.

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Figures 11(a) and 11(b) shows the visible emission spectra of IPC-485-Ag, IPC-540-Ag and IPC-663-Ag under the excitation of 251 nm. In the IPC-663-Ag, the PBG at 663nm is located far from the emission bands of Tb³⁺. Therefore, the photonic band gaps do not affect the emission intensity of the embedding Tb³⁺. As shown in Figs. 11(a) and 11(b), the Tb³⁺ transition at 488 and 543 nm is affected due to the band gap of the IPC-485-Ag or IPC-540-Ag. When the visible emission intensity of the two emission spectra shown in Figs. 11(a) is normalized at 543 nm, the visible emission intensities of the bands at 488 nm are decreased. For IPC-540-Ag, with a band gap at 540 nm, the 543 nm visible emission of Tb³⁺ is declined in intensity when normalized at 488 nm, as shown in Figs. 11(b). The emission intensity is significantly reduced in the spectral region of the PBG [35]. When the emission peak of the Tb³⁺ overlaps with the PBG, suppression of Tb³⁺ emission occurs due to the photon trapping caused by the Bragg reflection of lattice planes. While the visible emission located at 488 nm or 543 nm is reduced, and the NIR emission around 976 nm is enhanced in the IPC-485-Ag or IPC-540-Ag, compared with those of the IPC-663-Ag, as shown in Figs. 11(c).

 

Fig. 11 Visible emission spectrum of (a) the IPC-485-Ag and IPC-663-Ag, and (b) the IPC-540-Ag and IPC-663-Ag; the QC emission spectra of the IPC-485-Ag, IPC-540-Ag and IPC-663-Ag under 251nm excitation.

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To investigate the influence of the PBG on the QC emission properties, the luminescence decay curves of Tb³⁺: ⁵D₄→⁷F₅ emission at 543 nm was also observed to explain the changing of the emission spectra, as shown in Figs. 14(a). It can be clearly seen that the luminescence lifetimes at 543 nm for IPC-485-Ag, IPC-540-Ag and IPC-663-Ag are 1.68, 1.45 and 1.19 ms under the excitation of 251 nm, respectively. Compared with the decay curves of IPC-485-Ag and IPC-663-Ag, the increase with the IPC-485-Ag revealed that the radiative transition of Tb³⁺ was weakened. The luminescence lifetimes at 543 nm for IPC-540-Ag also increases in contrast to the IPC-663-Ag. In the photonic crystals, spontaneous emission will be inhibited because the optical electromagnetic modes do not exist within the PBG frequency range. In comparison with that of IPC-663-Ag, the NIR QC emission intensity was increased in IPC-485-Ag and IPC-540-Ag, which may be due to energy transfer enhancement. The ⁵D₄ excited energy of Tb³⁺ ions can loss by directly spontaneous emission or the ET by the route (Tb³⁺: ⁵D₄→⁷F₆) → 2(Yb³⁺, ²F_7/2 → ²F_5/2). The inhibition of blue or green emission of Tb³⁺ in inverse opal leads to increasing luminescence decay time of Tb³⁺: ⁵D₄, thus electrons population of ⁵D₄ level of Tb³⁺ improves. We demonstrated that a strong ET from Tb³⁺: ⁵D₄ to Yb³⁺ occurs in the upper section. With a band gap at 485 nm, the ⁵D₄ excited state of Tb³⁺ returns to the ground state by (Tb³⁺: ⁵D₄→⁷F₆) → 2(Yb³⁺, ²F_7/2 → ²F_5/2) ET much more readily. Therefore, the ET between Tb³⁺ and Yb³⁺ is greatly enhanced, and thus the QC emission from Yb³⁺ is considerably improved in the IPC-485-Ag. With a band gap at 540 nm, the electrons on the ⁵D₄ level cannot transit to the ⁷F₆ level. Based on the energy matching conditions, the energy transfer between Tb³⁺ and Yb³⁺ through the route (Tb³⁺: ⁵D₄→⁷F₅) → (Yb³⁺, ²F_7/2 → ²F_5/2) is not allowed. We supposed that the Tb³⁺: ⁵D₄→⁷F₆ transition was improving by suppressing the green emission of Tb³⁺, which resulting in the (Tb³⁺: ⁵D₄→⁷F₅) → 2(Yb³⁺, ²F_7/2 → ²F_5/2) ET route enhanced. When the PBG is located in the 485 nm, the NIR emission enhancement effect is more distinct, which further demonstrates the above conclusion.

Meanwhile, the excitation spectrum of those SiO₂: Tb, Yb, Ag inverse opal photonic crystals by monitoring both the 543 nm emission and the 976 nm emission were detected, respectively, as shown in Figs. 12(a) and 12(b). The 251 nm excitation peak by monitoring the 543 nm emission was attributed to the SiO₂ matrix. The 330 nm and 467 nm excitation peak by monitoring the 976 nm emission were ascribed to (Ag₄)²⁺, which have been discussed in the upper section. However, as shown in Figs. 12(a), a novel intense excitation peak appeared, which located in 377 nm. In order to explore the new excitation peak belonging, the SiO₂: 1 mol% Tb, 4 mol% Yb inverse opal without Ag were prepared. As shown in the inset of Figs. 12(a), a very weak sharp excitation peaks at 375 nm was observed by monitoring 543 nm emission, which are originated from the ⁷F₆→⁵L₁₀ of Tb³⁺. In Figs. 13(a), under 377 nm excitation, a new emissions band at 467 nm was observed. From the above results, we considered that the excitation peaks at 377 nm and the emission at 467 nm were ascribed to (Ag₂)⁺, which reported in previous works [18]. According to TEM in the Figs. 10(b), the Ag NPs did not reunited together to form polycrystal in the inverse opal, and the size of the Ag NPs was around 10 nm. We supposed that the formation of the (Ag₂)⁺ is related to the size of the Ag NPs. Only the Ag NPs is small enough so that stable (Ag₂)⁺ can exist. In the case of 377 nm excitation, it is accepted that one part of the absorption energy of (Ag₂)⁺ has been released in the form of 467 nm emission; the other part is contributed by the ET process from the silver aggregates to the Tb³⁺. Since the value of energy level of excited state of (Ag₂)⁺ is close to the ⁵D₃ level of Tb³⁺, it is highly possible that ET from (Ag₂)⁺ to ⁵D₃ level of Tb³⁺, then the excited Tb³⁺ relaxes to the ⁵D₄ levels via cross relaxation. The NIR emission of Yb³⁺ of IPC-485-Ag was obtained under various excitation band, as shown in Fig. 13(b). And the energy level diagrams of SiO₂ matrix, silver aggregates of (Ag₂)⁺, Tb³⁺, Yb³⁺ ions and the QC emission mechanism is shown in Figs. 14(b).

 

Fig. 12 The excitation spectrum of the IPC-485-Ag, IPC-540-Ag and IPC-663-Ag by monitoring both the 543 nm emission (a) and the 976 nm emission (b), respectively.

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Fig. 13 Visible emission spectrum of the IPC-485-Ag, IPC-540-Ag and IPC-663-Ag excited at 377 nm (a); the NIR emission spectrum of the IPC-485-Ag, IPC-540-Ag and IPC-663-Ag under various excitation (b).

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Fig. 14 The luminescence lifetimes of the 543 nm emission of Tb³⁺ of the IPC-485-Ag, IPC-540-Ag and IPC-663-Ag under 251 nm excitation (a); the energy level diagrams of SiO₂ matrix, silver aggregates of (Ag₂)⁺, Tb³⁺, Yb³⁺ ions and the QC emission mechanism (b).

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4. Conclusions

(1) In this summary, we have successfully fabricated the QC thin film or inverse opals of SiO₂: Tb, Yb, Ag. In this system, the visible-NIR QC upon excitation at the 250 nm were investigated and the energy of ⁵D₄ level of a Tb³⁺ ion is transferred to two different Yb³⁺ ions.

(2) The introduction of silver into the SiO₂: Tb, Yb thin films leads to the formation of different silver species, which represent Ag NPs, silver aggregates of (Ag₂)⁺ and (Ag₄)²⁺, with characteristic luminescence spectra. Under 330 nm excitation, effective enhancement of Yb³⁺ luminescence is ascribed to ET from silver aggregates of (Ag₄)²⁺ to Yb³⁺. Meanwhile, it was found that the Ag NPs lead to a significant enhancement in spontaneous emission of Tb³⁺ and the QC emission of Yb³⁺ excited at 250 nm, which is ascribed to the increase of the radiative decay rate of active centers induced by SPR of Ag NPs.

(3) In the SiO₂: Tb, Yb, Ag inverse opals, the effect of the PBG on QC emission of rare earth ions was investigated. The size of the Ag NPs can be controlled by inverse opal structure, thus the types of Ag nanoclusters will be affected. With the inhibition of Tb³⁺ emission by PBG in the inverse opals, the Yb³⁺ NIR emission was enhanced at the same time, and the fluorescence decay of Tb³⁺ was increased. The ionic silver species represent (Ag₂)⁺ and (Ag₄)²⁺ with the inverse opal structure. And the ET from (Ag₂)⁺ to Tb³⁺ was investigated. Additionally, the mechanisms for the influence of the PBG and silver aggregates of (Ag₂)⁺ on ET process of the SiO₂: Tb, Yb, Ag inverse opals are discussed.

(4) Tb ions is a necessary condition for the formation of (Ag₄)²⁺, and Tb³⁺ is a bridge between (Ag₂)⁺ and Yb³⁺. Both (Ag₄)²⁺ and (Ag₂)⁺ are favorable for the NIR QC emission of Yb³⁺. In particular, the ultra wide band excitation peak of 200-500 nm in those two kind of films have significant potential application value for the Si-based solar cells.