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Abstract
Large-area high-ordered Ag nanopillar arrays with tunable nanogaps are fabricated by an electroless deposition method. Then, β-NaYF4:Pr3+ nanoparticles (NPs) are spin coated on the Ag nanopillar arrays. Plasmon enhanced near-infrared (NIR) downconversion (DC) luminescence involving the 876 nm (1D2→3F2) and 1017 nm (1D2→3F4,3), as well as the two-step sequential transitions at 915 nm (3P0→1G4) and 990 nm (1G4→3H4), are achieved by Ag nanopillar arrays under the excitation of 444 nm (3H4→3P2) of Pr3+ ions. The influence of different nanogaps between Ag nanopillar arrays on NIR DC luminescence is investigated, and the results show that the optimal nanogap size is 25 nm with a maximum enhancement factor about 3.98. Furthermore, 3D finite-difference time-domain (FDTD) simulation is performed to analyze the enhancement mechanism of NIR DC luminescence. Our study may have potential application in the field of silicon-based solar cells.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solar cells, with the advantage of requiring no fossil fuels and producing no pollutions, present a promising approach to green and renewable energy [1,2]. However, solar cells have an intrinsic photovoltaic conversion efficiency limitation attributed to spectral mismatch between the solar spectrum and the band gap of solar cells [3]. Photons with energy less than the band gap will not be absorbed, and the ultraviolet−visible (UV−vis) photons with energy higher than the band gap are lost as heat by electronic recombination and thermal relaxation [4]. An improvement of photovoltaic conversion efficiency is expected if this part of the incoming spectrum can be modified to the wavelength that is in the band gap of solar cells [5]. Fortunately, downconversion (DC) is a process where a high energy photon is converted into two or more photons with lower energy [6,7], which is possible to use a wider portion of the solar spectrum raising the photovoltaic conversion efficiency of solar cells [8].
Recently, noble metal nanoparticles (NPs), such as Ag and Au, have been reported to enhance upconversion luminescence when the spectral matching between surface plasmon resonance (SPR) and excitation or emission of rare-earth (RE) ions [9–15]. In the plasmon-enhanced luminescence process, the observed luminescence intensity I can be described by I = γex·ηf·εcoll, where γex is the excitation efficiency, ηf is the quantum yield and εcoll is the light collection efficiency of the optical measurement system [16]. Herein, two major enhancement mechanisms, including excitation enhancement and emission enhancement, have been reported [17,18], where the excitation efficiency can be enhanced and the radiative decay rate can be increased by the local electric field. However, few works are focused on plasmon-enhanced DC luminescence [19]. Despite our research team firstly realize the plasmon-enhanced DC luminescence in Tb3+-Yb3+ co-doped KYF4/NaYF4 phosphor by Ag NPs, it is still not an efficient enhanced DC luminescence [19,20]. On one hand, the distribution of Ag NPs on KYF4/NaYF4 phosphor is random, and the inter-nanoparticle distance between adjacent Ag NPs is large and uncontrollable, which leads to lower local electric field intensity around Ag NPs and weaker plasmon coupling between adjacent Ag NPs. On the other hand, the energy transfer (ET) mechanism of Tb3+-Yb3+ ion pair is second-order cooperative ET [21]. It is well known that the probability of second-order cooperative ET is 103 times lower than the probability of first-order resonant ET [22]. To overcome these shortcomings, large-area high-ordered noble metal arrays with tunable nanogaps is desired [11]. Moreover, RE ions, with first-order resonant ET mechanism, is also preferred [22].
In this paper, large-area high-ordered Ag nanopillar arrays with tunable nanogaps are fabricated by electroless deposition method based on the anodic aluminium oxide (AAO) templates. Subsequently, single-doped β-NaYF4:Pr3+ NPs are synthesized and spin coated on the Ag nanopillar arrays. Plasmon-enhanced NIR DC luminescence involving the 876 nm and 1017 nm, as well as the two-step sequential transitions at 915 nm and 990 nm, are realized by Ag nanopillar arrays, and the influence of different nanogaps between adjacent Ag nanopillars on NIR DC luminescence is investigated. Furthermore, the enhancement mechanism of NIR DC luminescence, boosting the excitation efficiency of Pr3+ ion, is demonstrated through 3D finite-difference time-domain (FDTD) method. It is notable that plasmon-enhanced NIR DC luminescence of β-NaYF4:Pr3+ NPs by large-area high-ordered Ag nanopillar arrays with tunable nanogaps have not been reported before. Our study will provide a potential application for silicon-based solar cells to improve the photovoltaic conversion efficiency.
2. Experimental
2 mmol β-NaYF4:2%Pr3+ NPs were synthesized by coprecipitation method, which were introduced in detail in the literature [23], and dispersed in 3 ml cyclohexane. Then, the large-area high-ordered porous AAO templates were prepared by a two-step anodization process [24]. After the second-step anodization, the porous AAO templates were immersed in 1 M sodium hydroxide (NaOH) solution to etch and widen the pore size of AAO templates. The etching duration was 0 s, 20 s, 35 s and 50 s, and the diameter of AAO pore were tuned to 40 nm, 60 nm, 75 nm and 90 nm, with the thickness of AAO pore-wall reducing to 60 nm, 40 nm, 25 nm, and 10 nm, respectively. Subsequently, Ag nanopillar arrays were grown in the pore channels of AAO templates using electroless depositon method [25]. In brief, the as-prepared AAO templates were immersed in 1 M silver nitrate aqueous solution on 50 ml Teflon autoclave and treated with 60 °C for 12 h. By this way, Ag nanopillar arrays with different inter-nanopillar gaps (60 nm, 40 nm, 25 nm, and 10 nm) were obtained. Finally, 100 μl β-NaYF4: Pr3+ NPs solution, with the number of β-NaYF4: Pr3+ NPs about 2.56 × 106, were spin coated on the Ag nanopillar arrays through spin coater at 2000 r.p.m for 1 min to obtain the plasmon-enhanced NIR DC luminescence. For comparison, 100 μl β-NaYF4:Pr3+ NPs solution were also spin coated on the blank aluminum sheet. The schematic illustration of the synthesis of β-NaYF4: Pr3+ NPs coated Ag nanopillar arrays is shown in Fig. 1.
As-prepared samples were characterized by X-ray diffraction (XRD, MiniFlex II, Rigaku), fluorescence spectra (Fluorolog 3-22 spectrofluorometer, Horiba Jobin Yvon), scanning electron microscope (SEM, SU8010, Hitachi), and absorption spectra (Lambda 950, PerkinElmer). It is noted that the fluorescence spectra in the same figure were measured by the same spectrofluorometer in one experiment under the same measuring conditions (temperature, slit width, placement of samples, optical path, etc). Therefore, the intensity of these spectra in one figure is comparable.
3. Results and discussion
3.1 Characterization of high-ordered Ag nanopillar arrays
The SEM images of the as-prepared AAO templates under different NaOH etching duration are shown in Fig. 2(a), 2(b), 2(c) and 2(d). It is obvious that AAO templates have a hexagonal pore arrangement with a uniform pore size (see the inset of Fig. 2(b)). When the etching duration is 0 s, 20 s, 35 s, and 50 s, the diameter of AAO pore is tuned to 40 nm, 60 nm, 75 nm, and 90 nm, with the thickness of AAO pore-wall reducing to 60 nm, 40 nm, 25 nm, and 10 nm, respectively.
Fig. 2 SEM images of the as-prepared AAO templates with different NaOH etching duration (a) 0 s, (b) 20 s; Inset: Enlarge SEM image of AAO template, (c) 35 s, (d) 50 s.
To analyze the morphology and SPR property of Ag nanopillar arrays, the SEM image, EDX spectrum and extinction spectrum of the Ag nanopillar arrays are measured and shown in Fig. 3. From the SEM images shown in Fig. 3(a), 3(b), 3(c) and 3(d), it can be seen that Ag nanopillar arrays have a hexagonal arrangement with a uniform Ag nanopillar size. The inter-nanopillar gaps between adjacent Ag nanopillars are 60 nm, 40 nm, 25 nm, and 10 nm, respectively, which is in agreement with the thickness of as-prepared AAO pore-wall. In addition, according to the inset of Fig. 3(c), the average height of Ag nanopillar arrays is about 90 nm. The EDX spectrum in Fig. 3(e) indicates the presence of Ag, Al, and O elements. According to the inset of Fig. 3(e), the percentage of Ag, Al, and O elements in Ag nanopillar arrays are 7.68 wt%, 58.30 wt%, and 34.02 wt%, respectively. The Ag element originates from Ag nanopillar, while the Al and O elements belong to the AAO templates.
Fig. 3 SEM images of Ag nanopillar arrays with different nanogaps (a) 60 nm, (b) 40 nm, (c) 25 nm; Inset: The corresponding oblique view image, (d) 10 nm. (e) EDX analysis; Inset: The percentage of elements of Ag nanopillar arrays, and (f) Normalized extinction spectrum of Ag nanopillar arrays with the nanogap of 25 nm.
Furthermore, the extinction spectrum of Ag nanopillar arrays is shown in Fig. 3(f). There is an obvious peak located at 450 nm, which is attributed to the SPR of Ag nanopillar, while the broad extinction band from 300 nm to 1050 nm is originated from the gap-induced plasmon coupling between adjacent Ag nanopillars [13–15]. Notably, because the single-doped β-NaYF4:Pr3+ NPs have a low quantum yield [2], the SPR of Ag nanopillar arrays have little influence on quantum yield of Pr3+ ions. Therefore, Ag nanopillar arrays could be applied to enhance the NIR DC luminescence of β-NaYF4:Pr3+ NPs by boosting the excitation efficiency of Pr3+ ions.
3.2 Plasmon-enhanced NIR DC luminescence
Figure 4(a) illustrates the XRD pattern of β-NaYF4:Pr3+ NPs. All major diffraction peaks match well with the standard data (JCPDS 16-0334), implying that β-NaYF4:Pr3+ NPs is well crystallized and Pr3+ ions can substitute Y3+ ions without disturbing the crystal lattice. The corresponding SEM image of β-NaYF4:Pr3+ NPs is presented in Fig. 4(b). It can be observed that the β-NaYF4:Pr3+ NPs are well-dispersed and uniform with an average diameter of about 20 nm, excluding the influence of β-NaYF4:Pr3+ NPs size on the NIR DC luminescence properties.
Fig. 4 (a) XRD patterns of β-NaYF4:2%Pr3+ NPs; (b) SEM image of β-NaYF4:2%Pr3+ NPs; (c) Excitation spectrum (λem = 1017 nm) and (d) NIR emission spectrum (λex = 444 nm) of β-NaYF4:2%Pr3+ NPs spin-coated on aluminum sheet; In Fig. 4(d), solid curve represents NIR emission centered at ~1017 nm in 950-1050 nm, sphere is Gaussian fitting of NIR emission in 950-1050 nm, and dash curves are Gaussian fitting peaks; (e) Schematic diagram of the energy levels of Pr3+ ion in NIR DC luminescence.
Figure 4(c) demonstrates the excitation spectrum of β-NaYF4:Pr3+ NPs monitored at 1017 nm. The major excitation peaks are located at 444 nm, 466 nm, and 481 nm, attributed to 3H4 →3P2, 3H4→3P1, 1I6, and 3H4→3P0 transitions of Pr3+ ions, respectively [26]. Figure 4(d) illustrates the NIR emission spectrum of β-NaYF4:Pr3+ NPs under the excitation of 444 nm. There are several NIR emission bands that overlap with each other in the range of 950-1050 nm. Through Gaussian multi-peaks fitting, the overlapped emission bands can be appropriately fitted by a sum of two Gaussian peaks located at 990 nm and 1017 nm (Goodness of fit value, R2, ~0.9975), and the fitted curve is well consistent with the experimental curve. Therefore, four NIR emission peaks of Pr3+ ion, located at 876 nm (1D2→3F2), 915 nm (3P0→1G4), 990 nm (1G4→3H4), and 1017 nm (1D2→3F4,3) [26], are obtained under the excitation of 444 nm.
To illustrate the NIR DC luminescence process of β-NaYF4:Pr3+ NPs under the excitation of 444 nm, the schematic diagram of the energy levels of Pr3+ ion is plotted as Fig. 4(e). Firstly, when Pr3+ ion is pumped to excited 3P2 state from ground 3H4 state under the excitation of 444 nm, nonradiative relaxation from 3P2 state to 3P1 state occurs. Subsequently, Pr3+ ion on 3P1 state undergo two main processes: cross-relaxation (CR1:3P1 + 3H4→1D2 + 3H6),and nonradiative relaxation (3P1→3P0) leading to the two-step sequential transitions of 915 nm (3P0→1G4) and 990 nm (1G4→3H4). Similarly, Pr3+ ion on 1D2 state also experience two main process: radiative transitions at 876 nm (1D2→3F2) and 1017 nm (1D2→3F4,3), and cross-relaxation (CR2:1D2 + 3H4→1G4 + 3F4,3). Here, a two-step sequential transitions from Pr3+:3P0 excited state with 1G4 intermediate level: the first-step 3P0→1G4 at 915 nm and the second-step 1G4→3H4 at 990 nm, is known as a quantum cutting process [26]. By this way, the NIR DC luminescence involving two-step sequential transitions at 915 nm (3P0→1G4) and 990 nm (1G4→3H4), as well as the 876 nm (1D2→3F2) and 1017 nm (1D2→3F4,3) is realized under the excitation of 444 nm (3H4→3P2).
Figure 5 is the SEM image of Ag nanopillar arrays coated with β-NaYF4:Pr3+ NPs. It can be seen that the nanogaps between adjacent Ag nanopillars are filled with β-NaYF4:Pr3+ NPs uniformly, indicating the NIR DC luminescence of β-NaYF4:Pr3+ NPs situated in the nanogaps would be enhanced by the SPR of Ag nanopillar and the gap-induced plasmon coupling between adjacent Ag nanopillars. Moreover, the relative number of β-NaYF4:Pr3+ NPs on different nanogaps around single Ag nanopillar are calculated as 0 (G = 10 nm), 295 (G = 25 nm), 432 (G = 40 nm), 567 (G = 60 nm), respectively. When the size of nanogaps between adjacent Ag nanopillars is smaller than the size of β-NaYF4:Pr3+ NPs, the β-NaYF4:Pr3+ NPs will not get access to nanogaps.
To investigate the influence of different nanogaps between adjacent Ag nanopillars on the NIR DC luminescence, the excitation spectra monitored at 1017 nm and NIR emission spectra excited at 444 nm of β-NaYF4:Pr3+ NPs spin-coated on Ag nanopillar arrays with different nanogaps are measured. According to the excitation spectra in Fig. 6(a), the excitation intensity first increases with increasing of nanogaps size from 10 nm to 25 nm, and then decreases with the further increase of nanogaps size. This phenomenon mainly attributes to that β-NaYF4:Pr3+ NPs with a diameter of 20 nm cannot get access to the region of large local electric filed intensity when the nanogaps size is smaller than the diameter of β-NaYF4:Pr3+ NPs, while for the nanogap size larger than 25 nm, gap-induced plasmon coupling between adjacent Ag nanopillars become weaker and local electric field intensity around Ag nanopillar is lower. The excitation enhancement factor reached a maximum as 4.04 when the optimal nanogap size is 25 nm.
Fig. 6 (a) Excitation spectra (λem = 1017 nm) and (b) NIR emission spectra (λex = 444 nm) of β-NaYF4:2%Pr3+ NPs spin-coated on blank aluminum sheet and Ag nanopillar arrays with different nanogaps (G = 10 nm, 25 nm, 40 nm, 60 nm); (c) NIR emission spectra (λex = 444 nm) of β-NaYF4:Pr3+ NPs spin coated on Ag nanopillar arrays (G = 25 nm) from five different areas; (d) NIR emission spectra (λex = 444 nm) of β-NaYF4:Pr3+ NPs spin-coated on Ag nanopillar arrays (G = 25 nm) from same area under the different polarization of incident light. Inset: The polarization direction of the incident light.
Figure 6(b) demonstrates the NIR emission spectra of β-NaYF4:Pr3+ NPs spin-coated on Ag nanopillar arrays with different nanogaps under the excitation of 444 nm. From the NIR emission spectra, the emission intensity increases with increasing of nanogaps size until it reaches 25 nm, then decreases. The maximum enhancement factor of NIR DC luminescence is about 3.98 when the nanogap size is 25 nm, indicating that plasmon-enhanced NIR DC luminescence of β-NaYF4:Pr3+ NPs is achieved. The corresponding enhancement mechanism can be described as follows: the excitation peak of Pr3+ ion (~444 nm) matches well with the SPR of Ag nanopillar arrays (~450 nm), which will cause the SPR of Ag nanopillar and the gap-induced plasmon coupling between adjacent Ag nanopillars. Subsequently, β-NaYF4:Pr3+ NPs situated in the local field-enhancement regions essentially feels an enhanced excitation light intensity, and the excitation efficiency from Pr3+:3H4 state to Pr3+:3P2 state is boosted, resulting in the enhancement of NIR DC luminescence [17,18]. In addition, the energy transfer from Ag nanopillar arrays to Pr3+ ions does not exist, because the relaxation time in the Ag nanopillar arrays is few tens of fs, which is much smaller than the relaxation time of Pr3+ ions.
To add the statistical significance to our experimental results, the plasmon enhanced NIR DC luminescence from five different areas of β-NaYF4:Pr3+ NPs spin-coated on Ag nanopillar arrays are measured, as shown in Fig. 6(c). It can be seen that the NIR DC luminescence intensities on five areas exhibit no change, further indicating that the distribution of β-NaYF4:Pr3+ NPs on Ag nanopillar arrays is uniform. In addition, in order to investigate the dependence of the enhancement process upon the polarization of the excitation light, the NIR DC luminescence spectra of β-NaYF4:Pr3+ NPs spin-coated on Ag nanopillar arrays under the excitation of different polarization light are measured in Fig. 6(d). The inset of Fig. 6(d) shows the polarization direction of incident light. It is obvious that the NIR DC luminescence intensities almost keep unchanged, attributed to that Ag nanopillar arrays are isotropic. Therefore, the polarization of the excitation light has no influence on the plasmon enhanced NIR DC luminescence of β-NaYF4:Pr3+ NPs on Ag nanopillar arrays.
To further confirm that the enhancement of NIR DC luminescence stems from the SPR of Ag nanopillar and the gap-induced plasmon coupling between adjacent Ag nanopillars, the intensities and distributions of local electric field in Ag nanopillar arrays with different nanogaps are simulated through FDTD method (FDTD Solutions 8.15, Lumerical Solutions, Inc.). During the FDTD simulation, a numerical model of hexagonal arrangement Ag nanopillar arrays with different nanogaps was applied based on our experimental results, and an electromagnetic pulse with a central wavelength at 444 nm was used. The Ag nanopillar arrays and its surrounding space were divided into 1 nm meshes and the boundary condition was perfectly-matched-layer (PML). According to the simulation results shown in Fig. 7(a), 7(b), 7(c) and 7(d), it is obvious that large local electric field intensities are generated in the nanogaps between adjacent Ag nanopillars, which will enhance NIR DC luminescence via effective coupling between the large local electric field and excitation efficiency of Pr3+ ion [9]. Notably, the intensities of local electric field decrease with the further increase of nanogaps size, attributed to that the gap-induced plasmon coupling between adjacent Ag nanopillars become weaker, which is in agreement with our experimental results.
Fig. 7 Contour of simulated local electric field distributions in the Ag nanopillar arrays with different nanogaps excited at 444 nm; (a) G = 10 nm, (b) G = 25 nm, (c) G = 40 nm, (d) G = 60 nm.
4. Conclusion
In this paper, large-area high-ordered Ag nanopillar arrays with tunable nanogaps are fabricated by electroless deposition method based on the AAO templates. Subsequently, single-doped β-NaYF4:Pr3+ NPs are synthesized and spin coated on the Ag nanopillar arrays. Plasmon-enhanced NIR DC luminescence involving the 876 nm and 1017 nm, as well as the two-step sequential transitions at 915 nm and 990 nm, are realized by the SPR of Ag nanopillar and the gap-induced plasmon coupling between adjacent Ag nanopillars. The influence of different nanogaps between Ag nanopillar arrays on NIR DC luminescence is investigated, and the results show that the enhancement factor reached a maximum as 3.98 with the optimal nanogap size of 25 nm. Furthermore, the enhancement mechanism of NIR DC luminescence, boosting the excitation efficiency of Pr3+ ions, is demonstrated by FDTD method. Our study may provide a promising DC layer for silicon-based solar cells to improve the photovoltaic conversion efficiency.
Funding
National Natural Science Foundation of China (NFSC) (11204039, 51202033); Natural Science Foundation of Fujian Province of China (2016J01213, 2017J01399).
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