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Plasmonic enhancement of Eu:Y2O3 luminescence by Al percolated layer
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 095701 (http://iopscience.iop.org/0957-4484/26/9/095701) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 095701 (8pp)
doi:10.1088/0957-4484/26/9/095701
Plasmonic enhancement of Eu:Y2O3 luminescence by Al percolated layer N Abdellaoui, A Pereira, A Berthelot, B Moine, N P Blanchard and A Pillonnet ILM- Université de Lyon, Université Claude Bernard Lyon 1, CNRS UMR5306, Villeurbanne F-69622, France E-mail: [email protected] Received 21 October 2014, revised 9 December 2014 Accepted for publication 12 January 2015 Published 11 February 2015 Abstract
The coupling between Eu3+ rare earth emitters and Al has been investigated in multilayer structures, which consist of an Eu:Y2O3 phosphor film deposited between percolated and continuous Al films. Passive buffer Y2O3 layers were deposited between phosphor and Al films with different thicknesses to analyze the role of the Eu–Al distance on the nanostructuration and emission of the Eu:Y2O3 film. By using Eu3+ emitters as local structural probes completed by transmission electron microscopy analyses, we show that the deposition on Al promotes the growth of the cubic crystallites. A fluorescence analysis allows us to evaluate the presence of a perturbed structural shell around the cubic core of the crystallites. Moreover, the enhancement observed at short distances is attributed to the localized plasmon resonance of the percolated upper Al film. S Online supplementary data available from stacks.iop.org/nano/26/095701/mmedia Keywords: luminescence, aluminum nanostructures, surface plasmon resonance, thin films (Some figures may appear in colour only in the online journal) 1. Introduction
and emitting in the visible or higher wavelenghts. However, many widely used phosphors, such as biomolecules, proteins, or luminescent ions, absorb or emit at UV wavelengths. Despite the difficulties of its fabrication, an Al-supporting UV plasmon has been used to design UV-blue optical devices such as a biosensing captor, organic light-emitting diode (OLED), and solar cells [10, 11]. Akimov et al concluded that Al nanoparticles are preferable for the fabrication of broadband light trapping in thin film solar cells due to their superior stability in contact with air or moisture [10]. In this paper, we study the ability of Al nanostructures to enhance rare earth (RE) absorption in the UV range. Only a few papers about plasmonic phosphor molecule coupling have focused on (REs) due to their low absorption coefficients, their generally high quantum yields, and their long emission lifetimes. As far as we know, the coupling between an Al structure and RE ions has never been reported. Among the different plasmonic systems, the architectures based on films have shown promising results. Papers have reported that the emission of molecule films deposited on an Al nanostructured layer is
There has been sustained interest in the plasmonic properties and sensing capacities of aluminum over the past 20 years. Aluminum has been used in plasmonic systems in the ultraviolet-blue spectral region to study localized surface plasmon resonances (LSPRs) [1, 2], surface plasmon polariton propagation [3], surface-enhanced fluorescence [4, 5], and Raman spectroscopy [6, 7]. Aluminum has been shown to be a better plasmonic material than gold or silver in the ultraviolet (UV) range due to the negative real part and the low imaginary part of its permittivity at wavelengths smaller than 200 nm [1, 4, 8]. Nevertheless, aluminum is easily oxidized and forms a native aluminum oxide (Al2O3) layer under atmospheric conditions, making device fabrication with Al difficult. The presence of this oxide layer results in a red shift in the LSPR peak position [2] and a lower extinction efficiency [9]. There are only a few reports in the literature describing the Al plasmon resonance compared to those for silver and gold, thereby limiting the selection of phosphors to those absorbing 0957-4484/15/095701+08$33.00
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emission that is widely used for different applications (lighting, biomarker, etc). When Eu3+ ions are incorporated in oxides with a broad band gap, the most effective excitation is within the UV-blue spectral range corresponding to the Eu-O charge transfer (CT) band, which could be potentially exalted by an Al UV plasmon. Otherwise, Eu3+ ions are excellent structural probes, because their fluorescence properties are strongly related to their local environment, particularly in a sesquioxide structure. In the first step of this study, we use the properties of Eu3+ to monitor the growth of an Eu:Y2O3 phosphor film inside the different multilayer architectures, and to distinguish between the structural effects and potentially plasmonic effects. In the second step, we present the influence of the multilayer architecture and the coupling distance, d, on the luminescence properties of Eu:Y2O3.
Al d
d
Passive spacer
2. Experimental section The different multilayers presented in figure 1 were formed by pulsed laser deposition (PLD). This technique allows a single-step deposition, guarantees a high quality of the interfaces, and prevents any reactivity between the embedded metallic structures and the ambient atmosphere. The same method was recently used to study the coupling distance between RE and silver nanoparticles [19]. The multilayer structures were elaborated in a high-vaccum deposition chamber (P = 10−7 mbar) by successive ablation of Al, Y2O3, and 1% Eu:Y2O3 targets with a Lambda-Physik LPX-100 KrF excimer laser (λ = 248 nm, τ = 17 ns) at 10 Hz. The laser beam was focused on the targets at an incident angle of 45° with respect to the target normal. The substrate–target distance was kept constant at 4 cm. Previously, the oxide targets, consisting of Y2O3 or Eu:Y2O3 powders with a 1% Eu/Y atomic ratio, were pressed and sintered at 1400° C for 12 h in air. The cubic phase purity of the targets was controlled by xray diffraction [JCPDS card 41-1105]. The Al target was obtained from Neyco with 99.999% purity. The films were deposited on fused-silica substrates (Suprasil). The Al films were deposited under vacuum with a laser fluence of 2.6 J cm−2. A deposition rate of around 0.02 Å/ pulse of the Al layer was determined from thickness measurements taken by profilometry. For the different multilayer architectures with the Eu:Y2O3 emitting layer illustrated in figure 1, the thickness of the Al layer was fixed at 1.5 +/−0.8 nm. Thin oxide layers of Y2O3 and Eu:Y2O3 were deposited under an oxygen atmosphere (10−3 mbar) with a laser fluence of 2.3 J cm−2. All deposits were realized at room temperature to avoid ionic diffusion between the layers. The thickness of the emitting Eu:Y2O3 layer was kept constant and equal to 10 nm, whereas the thickness of the Y2O3 spacer layers, denoted by d, was varied from 0 to 60 nm. For each material, the deposition rate was first determined by means of the mline spectroscopy at 543.5 nm using an LaSF35 prism, assuming a step index profile [20]. The number of pulses was then adjusted to achieve the selected thickness of the Eu:Y2O3 and Y2O3 layers. Note that on a silica substrate, deposited Al
Al Figure 1. Sketch of the multilayer structure: an Eu:Y2O3 phosphor layer deposited on an Al film covered with a percolated Al film. A Y2O3 passive layer is inserted between the phosphor and the Al layers to study the influence of the coupling distance d.
higher than the emission of the same film deposited on an insulator substrate [4, 5]. The gain depends on the thickness of the Al layer [5], its orientation [12], and the distance between the metal and the emitter, as reported for other metals [13]. A few papers have reported the enhancement of an emitting film placed between two metallic layers, which can be continuous [14] or made of nanoparticles [13, 15]. In the case of a nanometer-sized gap between nanostructured metal surfaces, field amplification is likewise observed [15]. Polemi et al compared the results for these multilayer systems to those modeled on nanoparticle dimers [16]. Finite-difference time domain calculations predict that the electric field enhancement in the UV-blue range is higher inside a dimer system compared to Al isolated nanoparticles. These theoretical results have been demonstrated experimentally for other metals such as Ag [17] and Au [18]. Our initial goal was to study the electric field enhancement in the UV range between two nanostructured aluminum layers, to generate effects comparable to those observed in dimers. The adjustment of the growth parameters led us to develop several types of nanostructured Al layers, and we found that the multilayer system, consisting of an emitting layer between a percolated and a continuous Al nanometer-thick layer, yields on enhancement. This paper presents these original results. We investigate the effect of the distance, d, between Al and RE on its emission by incorporating passive spacer layers, as shown in figure 1. In this paper, Eu3+ ions were chosen from among the RE elements because these ions, incorporated as dopants in a sesquioxide matrix (especially Y2O3), produce the intense red 2
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CT 35000
Level energy (cm-1)
30000 25000
5D 5G4 5L 2 5D6 5D3 2 5D 5D 1 0
20000
UV
15000 10000 5000
7
FJ =2...7 1 0
0
Figure 2. (a) Diagram of the f–f intraconfigurational energy levels of Eu3+ transitions and Eu-O CT band with the characteristic splitting of
7 F0,1,2 levels by crystal fields in symmetry C2 corresponding to the symmetry of sites occupied by Eu3+ ions in cubic Y2O3 crystal. (b) Excitation spectra of bulk (target) Eu:Y2O3 and Eu:Y2O3 nanofilms deposited on SiO2 substrate.
spontaneously self-arranges to form a very thin layer, while on doped or undoped Y2O3, deposited Al forms a percolated layer. More details on Al growth by PLD have been studied and will be presented in the near future. The film morphology and structure was analyzed by absorption spectroscopy (Lambda900 from Perkin Elmer), by atomic force microscopy (AFM, Asylum), and by transmission electron microscopy (TEM, JEOL 2100 F and TOPCON EM0026). For the TEM analysis, a 10-nm-thick Eu:Y2O3 film was deposited onto a 15-nm-thick SiO2 membrane TEM grid as a reference sample. A second membrane grid was prepared by first depositing 5 nm of Al onto the SiO2, followed by a 10-nm layer of Eu:Y2O3. The emission and excitation spectra were collected using a 450 W Xe lamp followed by a GEMINI Jobin-Yvon double monochromator with an 8-nm bandwidth as an excitation source. The fluorescence was collected through an optical fiber, analyzed by a Triax 190 Jobin-Yvon monochromator, and detected by a cooled charge-coupled device (CCD) detector or a photomultiplier. The emission spectra were recorded with a resolution of 0.4 nm. The excitation spectra were registered by selecting the emission wavelength with a 6-nm band pass, and were corrected from the experimental setup response.
mainly located in the sites of the C2 symmetry that result from a splitting of the ground 7FJ levels in C2 symmetry [22, 23]. Some Eu3+ ions are also located in S6 symmetry sites, but their emission lines are much weaker [24]. The emission corresponding to the 5D0 → 7FJ transitions of Eu3+ can be excited by either f–f intra-configurational transitions or by the Eu–O CT band. The latter is more efficient because it involves an allowed dipolar electric transition. This band originates from the interaction between Eu3+ and O2− ions and its position varies depending on the composition and structure of the matrix. In oxides, it remains in the UVblue range. For micrometric-size cubic Y2O3 crystals, it is located at 239 nm [25]. Figure 2(b) shows the excitation spectra of the Eu:Y2O3 target and of a reference Eu:Y2O3 nanofilm deposited on a silica substrate that consists of crystallites with a mean diameter of 69 nm +/−5 nm and around 2 nm, respectively. As the size of the cubic crystallites decreases, a red shift of the CT band is observed, which increases with longer emission wavelengths. The red shift has been explained by a local disorder and a volume expansion that appears on nanosized Eu:Y2O3 crystallites [26]. This combined effect leads to a change in the energy bands with a decrease in the energy gap of the nanocrystallites. This shift is also in agreement with those reported for Eu:Y2O3 nanocrystallized powders prepared by the combustion method [25]. The red shift of the CT band was, in that case, attributed to an increase in the coordination number from 6 to 8 and an increase in the Eu-O bond length for ions located at the nanocrystallites’ surface [27]. A red shift of the CT band is also observed for a monoclinic film compared to the cubic film with a similar crystallite size; this could be attributed to an increase in the coordination number from the cubic to the monoclinic phase [28]. Due to these excitation spectra results, it is possible to choose the appropriate excitation wavelengths within the CT band to quasiselectively excite ions located in different environments,
3. Growth monitoring of Eu:Y2O3 phosphor layer using ions as local structural probes Eu3+ ions are excellent probes of their structural environment. The position and the intensity of their excitation and emission transitions depend strongly on the symmetry of the local crystal field of the site occupied by the ions. Figure 2(a) shows the energy level diagram of Eu3+ in cubic Y2O3. The values of the energy levels are deduced from previous work on PLD of Eu:Y2O3 film [21]. In cubic Y2O3, Eu3+ ions are 3
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Figure 3. Excitation spectra of the most intense emission at 612 nm of Eu:Y2O3 nanofilm deposited on SiO2 substrate (reference) and on Al layer, both without (d = 0 nm) and with a spacer (d = 20 nm).
either in the regular sites of the cubic crystallites (at 240-nm excitation wavelength) or in the perturbed sites of smaller nanometric cubic crystallites (at 270-nm excitation wavelength). Figure 3 shows the excitation spectra of a 10-nm-thick Eu:Y2O3 layer deposited on silica (reference) and on an Al layer without a spacer (d = 0 nm), or with a spacer of 20 nm (d = 20 nm). The figure shows that as the distance, d, decreases below 10 nm, a blue shift and narrowing are observed. This suggests a UV plasmon enhancement of the excitation band or an increase in the size of the Eu: Y2O3 crystallites as the Eu:Y2O3 grows directly on the Al surface [26]. This evolution is confirmed by the emission spectra, which were recorded for different excitation wavelengths in the CT band range. Figure 4 shows the emission spectra in the 5 D0 → 7FJ transition range, restricted to the lowest multiplets, J = 0, 1, 2, for the Eu:Y2O3 layer deposited on silica (reference) and on the Al layer either without a spacer (d = 0 nm) or with a spacer (d = 20 nm and d = 60 nm). Under UV excitation at 240 nm (figures 4(a) and (c)), all the cubic samples show the same emission lines, which are mainly due to Eu3+ ions located in the sites of C2 symmetry [22, 23]. Under excitation at 270 nm (figures 4(b) and d)), the spectra of the reference sample or of the multilayer including a Y2O3 spacer layer are broadened and show additional features compared to Eu:Y2O3 deposited directly on Al; the 5D0 →7F0,1 emission lines present a duplication of 5D0 →7F0 and a clear broadening of 5D0 → 7F1. These emission changes at a lower excitation energy suggest the presence of two kinds of sites characteristic of small crystallites [25]. These observations are in agreement with the emission spectra corresponding to the 5D0 →7F2 transition (figure 4(d)). For the reference sample, and as the distance, d, increases, a shoulder located at about 613 nm broadens toward longer wavelengths, and a broad line centered at 624 nm appears. These additional features are a clear indication that Eu3+ ions are distributed among sites of different crystal field symmetries and strengths. By analogy, the conclusions of spectroscopic and EXAFS studies on nanocrystalline powders prepared by the combustion method [25, 27], are attributed to Eu3+ ions
Figure 4. Emission spectra corresponding to 5D0 → 7F0,1,2 transitions under excitation at (a), (c) 240 nm, and (b), (d) 270 nm of Eu:Y2O3 nanofilm deposited on Al with different spacer distances, d.
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Figure 6. Distribution of the diameter of the particles for Y2O3:Eu nanocrystallites deposited on either SiO2 or on an Al continuous layer.
Figure 5. Evolution of the percentage of volume of Eu:Y2O3
crystallite sites versus the thickness, d, extracted from the fit of 7 F2 (black squares) and 7 F0 (red crosses) transitions. The line shows the reference.
normal distribution. The average crystallite size is 1.7 nm +/−0.3 nm. As expected, Eu:Y2O3 on Al shows different growth, with bigger crystallites and an inhomogeneous size distribution ranging from 1.8 to 20.7 nm. The density of the Eu:Y2O3 crystallites is ≈60 times lower on Al. Thanks to the fluorescence and TEM data, we were able to estimate the thickness, δ, of the perturbed shell around the cubic core of the Eu:Y2O3. For the Eu:Y2O3 film deposited at a distance, d ⩾ 10 nm on Al or directly on silica (reference), the average diameter of the crystallites is estimated at 1.7 nm and the volume site percentage at 8% (figure 5). Assuming that the Eu:Y2O3 crystallites are spherical, δ could be evaluated at 0.7 nm using the equation
located in a perturbed environment at the crystallite surfaces. In the case of Eu:Y2O3 directly grown on aluminum (d = 0 nm), no emission surface sites appear. This is partly the result of a decrease in the ratio of the surface area to the volume when the crystallites’ size increases. The observed evolution of the excitation and emission spectra suggest that the Eu: Y2O3 grown on Al promotes bigger cubic crystallites. However, the additional excitation and emission lines are close to those of the Eu3+ in the B or C sites of the monoclinic Y2O3 phase, which are characterized by a higher coordination number (7 instead of 6 for cubic sites) and a lower site symmetry [29]. This suggests that perturbed sites in the crystallites’ surface have a crystallographic symmetry close to the monoclinic phase, as previously reported in the literature [27, 30]. Moreover, the fluorescence modification depends not only on the crystallites’ size, but also on the synthesis route and post-treatment [31], as shown by the fluorescence difference observed for nanopowders prepared by sol-liophilization [32], colloidal precipitation [33], and low energy cluster beam deposition [34], which emphasizes the crucial role of the surface properties. An adjustment of the different emission spectra has been performed versus the distance, d, to calculate the ratio of the volume site number, Nvolume, relative to the total number, Ntotal, of sites occupied by ions (figure 5). More details on the fitting procedure are included in the supplementary material. The results obtained from fitting the 5D0 → 7F2 and 5D0 → 7F0 transitions, which are less intense but only split into two emission lines, are coherent (figure 5). As the distance between the Al and the Eu increases, the proportion of the volume sites decreases to reach the reference value. This result is coherent with the evolution of the excitation spectrum discussed previously (figure 3), and it confirms our hypothesis of the growing of bigger Eu:Y2O3 crystallites when deposited on Al, compared to SiO2. This spectroscopic result is confirmed by TEM measurements. The crystallite size distribution extracted from TEM measurement of Eu:Y2O3 grown on either SiO2 or Al is presented in figure 6. For Eu:Y2O3 on SiO2, the size distribution for the nanoparticles is fitted with a logarithmic
4 3 × πr 3 − 4 3 × π (r − δ )3 = 1 − % volume site. 4 3 × πr 3
This value is consistent with the literature [35]. For Eu: Y2O3 on Al, the size distribution is too broad to evaluate the average diameter of the crystallites. The growth process of an Eu:Y2O3 phosphor layer depends on the distance to the bottom Al layer. Therefore, we will focus on Eu3+ in the cubic core sites always present in the Eu:Y2O3 film to avoid any mixing of crystallization and of potential plasmonic effects on emission.
4. Influence of coupling distance, d, and the multilayer architecture on emission The coupling between Eu:Y2O3 and Al has been analyzed inside the multilayer structures illustrated in figure 1: Al(percolated)/Y2O3buffer/Eu:Y2O3 (10 nm)/Y2O3 buffer/Al(continuous). A panel of multilayers has been realized with different thicknesses, d, of the buffer layer. Figure 7 shows the AFM images of the Al (a) continuous and (b) percolated films deposited with the same number of laser pulses corresponding to an effective thickness of 1.5 nm. In fact, as the Al film is deposited on a silica substrate, the film is continuous, with a very low roughness estimated at 100 pm. This effect is due to the high wettability of Al on silica. Otherwise, the upper Al film grown on Y2O3 is 5
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Figure 7. AFM images of Al thin films deposited by PLD onto (a) SiO2 and (b) Y2O3 with a nominal film thickness of 1.5 nm.
centered around 300 nm. A broad UV band is likewise observed for the Al rough film, while the absorption of isolated nanoparticles presents a narrower band whose position depends on their diameter and shape [1, 2, 36]. The absorption band overlaps the CT band of Eu:Y2O3 (figure 3), suggesting a potential coupling effect. We investigated the fluorescence properties of the Eu: Y2O3 film inside the multilayers versus the distance, d. The results are compared to those of a reference film, namely a single Eu:Y2O3 film deposited on a silica substrate with the same thickness (10 nm). Based on the study reported in the first section of this paper, the excitation wavelength was selected at 240 nm to excite ions located in the Eu:Y2O3 core sites, which are present in the film whatever the distance, d. Figure 9(a) shows the intensity of the most intense emission line at 612 nm of Eu3+ versus the distance, d. The intensities of the different emissions are normalized with respect to the reference sample. In figure 9(a), we distinguish two optimal distances for luminescence enhancement. The first one appears at a short distance (without the introduction of a buffer layer) and the second one at a longer distance, d ≈ 20 nm. The enhancement at a short distance is due to a coupling with the localized surface plasmon supported by the percolated Aluminum layer. The nanostructuration of the metallic percolated film promotes a local field enhancement close to the metallic nanoparticles [37], especially for a high-density Al layer. To verify that this effect is due to the percolated layer and not to the size of the crystallites, we deposited Eu: Y2O3 on an Al continuous layer without the percolated layer. In this case, no enhancement was observed. A similar effect has been observed with other Al–phosphor structures for metal enhanced fluorophore applications [4, 5]. Note that in our case, we do not observe a luminescence quenching for d = 0 nm, suggesting a strong plasmonic effect due to the upper Al layer. To highlight this assumption, we looked at the structure without the upper Al layer. In this case, the luminescence was equivalent to that of the reference films deposited on silica. This confirms that the enhancement at a short distance is due to the presence of the upper Al percolated layer. The no-quenching may be explained by the fact
Figure 8. Absorption spectrum of Al percolated film on Y2O3 film (a) and, excitation at λ emission = 612 nm (b) and emission at λ excitation = 240 nm (c) photoluminescence spectra of Eu:Y2O3 reference sample.
discontinuous, consisting of wide holes with an average diameter of 62 +/−12 nm, and heights of 2.0 +/−0.5 nm. The Al filling factor is estimated at 69%. We estimate the roughness to be 814 pm, which is ≈8 times higher than that of an Al film on SiO2. This growth difference can be explained by the difference of the surface energy between SiO2 and Y2O3. To characterize the plasmon resonance, absorption measurements have been realized on an Al percolated layer with an effective thickness ranging from 1.5 to 15 nm. Below 5 nm, the signal-to-noise ratio was too low, due to the low intensity of the plasmon resonance. This measurement limitation for the nanometric Al layer was previously observed in the literature [5]. One explanation stems from the oxidation layer of the Al film in an ambient atmosphere. As the effective thickness decreases, the thickness ratio between the Al native oxide and the Al layer increases. This oxidation induces a decrease in the absorption intensity and a small red shift of the plasmon resonance as it is calculated in the literature for Al nanodisks [9]. Figure 8 presents the absorption spectrum of a 5 nm Al percolated, which overcomes these drawbacks and still gives us good information on the real plasmonic properties. For 5-nm-thick a broad absorption band attributed to localized plasmon resonance is measured and is 6
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Figure 9. Variation of the most intense emission intensity at 612 nm of the Eu:Y2O3 layer under excitation at 240 nm versus the coupling distance, d (a), and the corresponding decay (b).
that the Eu:Y2O3 layer is 10 nm thick, so the Eu3+ ions are not directly in contact with the Al. In addition, the thickness of the Al films is too low to promote efficient nonradiative transfer into the metal. Another factor which could contribute to the enhancement at a short distance is the coupling of the two metallic layers. We can expect that the coupling between these two Al layers promotes a strong electromagnetic coupling, which overcomes the quenching process at short distances. The coupling between the percolated and continuous Al layers can be compared with the modeled effect of an Al dimer [4, 38]. Other multilayer system studies are underway to investigate the impact of the coupling of the two Aluminum layers and of the incident beam polarization. When the distance, d, increases from 0 to 10 nm, the fluorescence enhancement decreases to 1. This suggests that for d ⩾ 10 nm, the Eu:Y2O3 film is unaffected by the localized plasmon, and/or that there are no coupling effects between the two Al layers. The second mechanism of luminescence enhancement for the longer distance could be due to the collective effect of the Al nanoparticles already observed in the coupling between Ag nanoparticles and an Eu:Y2O3 film [19]. In fact, the Al layers can be viewed as a low-quality mirror [39]. Figure 9(b) shows the evolution of the luminescence decay versus d normalized to the reference. The intensity decay of the Eu:Y2O3 can be fitted with an average lifetime of τ ≈ 0.80 ms. The multiexponential character of the luminescence decay of Eu:Y2O3 coupled with Al is emphasized for d ⩽ 15 nm. This accentuation is due to a distance distribution of the emitter from the metal surface. The Eu3+ ions are randomly distributed inside the 10-nm-thick Eu:Y2O3 layer, so the emission of Eu3+ closer to the metal should be affected by nonradiative transfer to the metal, resulting in a reduction of the lifetime [40] and the lifetime distribution. At a greater distance, this effect is normally suppressed. The long lifetime component is stable whatever the distance, indicating that the emission enhancement comes from an increasing CT band absorption, thanks to the Al layers.
5. Conclusion The coupling between Eu3+ RE emitters and Al has been investigated in multilayer structures, which consist of an Eu: Y2O3 film deposited between two Al films: one percolated and one continuous. The percolated Al layer consists of broad holes of 60nm diameter with a thickness of 2 nm. The introduction of a passive buffer Y2O3 layer with different thicknesses, d, allowed us to investigate the role of the distance, d, on the nanostructuration and emission of the Eu: Y2O3. In the first part of the paper, the growth of the Eu: Y2O3 film versus the distance, d, is analyzed by both using the emitters as local structural probes and by TEM analysis. We show that two types of sites, one corresponding to the cubic cores and the other to the perturbed shell of the Eu: Y2O3 crystallites, coexist in the film. For d lower than 10 nm, the emission of the core sites increases, which is consistent with the growth of the crystallites observed in TEM measurements. Thanks to the analysis, we selected an excitation both to observe only the Eu3+ in the core sites, which are always present whatever the distance, d, and to investigate the plasmonic effect of Al on the Eu3+ emission. An enhancement is observed at a short distance, and also at a distance of 20 nm. The enhancement at the short distance is attributed to the localized plasmon resonance of the percolated upper Al film, while the enhancement for the larger d is attributed to the collective effect of the Al nanoparticles. These multilayer structures are very promising as amplifiers for all kinds of optical devices developed for absorption/emission of light, and/or wavelength conversion (solar-cell, LED, etc).
Acknowledgments The authors wish to thank Professor G Colas des Francs (ICB-UMR 5209 CNRS Université de Bourgogne, France), for fruitful discussions. The TEM images were taken at the Centre Technologiques des Microstructures platform (Faculté
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des Sciences et Technologies de l’Université Lyon 1). The quality of the English has been checked by the proofreadingservice.
[18] [19] [20] [21] [22] [23]
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Plasmonic enhancement of Eu:Y2O3 luminescence by Al percolated layer
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 095701 (http://iopscience.iop.org/0957-4484/26/9/095701) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 095701 (8pp)
doi:10.1088/0957-4484/26/9/095701
Plasmonic enhancement of Eu:Y2O3 luminescence by Al percolated layer N Abdellaoui, A Pereira, A Berthelot, B Moine, N P Blanchard and A Pillonnet ILM- Université de Lyon, Université Claude Bernard Lyon 1, CNRS UMR5306, Villeurbanne F-69622, France E-mail: [email protected] Received 21 October 2014, revised 9 December 2014 Accepted for publication 12 January 2015 Published 11 February 2015 Abstract
The coupling between Eu3+ rare earth emitters and Al has been investigated in multilayer structures, which consist of an Eu:Y2O3 phosphor film deposited between percolated and continuous Al films. Passive buffer Y2O3 layers were deposited between phosphor and Al films with different thicknesses to analyze the role of the Eu–Al distance on the nanostructuration and emission of the Eu:Y2O3 film. By using Eu3+ emitters as local structural probes completed by transmission electron microscopy analyses, we show that the deposition on Al promotes the growth of the cubic crystallites. A fluorescence analysis allows us to evaluate the presence of a perturbed structural shell around the cubic core of the crystallites. Moreover, the enhancement observed at short distances is attributed to the localized plasmon resonance of the percolated upper Al film. S Online supplementary data available from stacks.iop.org/nano/26/095701/mmedia Keywords: luminescence, aluminum nanostructures, surface plasmon resonance, thin films (Some figures may appear in colour only in the online journal) 1. Introduction
and emitting in the visible or higher wavelenghts. However, many widely used phosphors, such as biomolecules, proteins, or luminescent ions, absorb or emit at UV wavelengths. Despite the difficulties of its fabrication, an Al-supporting UV plasmon has been used to design UV-blue optical devices such as a biosensing captor, organic light-emitting diode (OLED), and solar cells [10, 11]. Akimov et al concluded that Al nanoparticles are preferable for the fabrication of broadband light trapping in thin film solar cells due to their superior stability in contact with air or moisture [10]. In this paper, we study the ability of Al nanostructures to enhance rare earth (RE) absorption in the UV range. Only a few papers about plasmonic phosphor molecule coupling have focused on (REs) due to their low absorption coefficients, their generally high quantum yields, and their long emission lifetimes. As far as we know, the coupling between an Al structure and RE ions has never been reported. Among the different plasmonic systems, the architectures based on films have shown promising results. Papers have reported that the emission of molecule films deposited on an Al nanostructured layer is
There has been sustained interest in the plasmonic properties and sensing capacities of aluminum over the past 20 years. Aluminum has been used in plasmonic systems in the ultraviolet-blue spectral region to study localized surface plasmon resonances (LSPRs) [1, 2], surface plasmon polariton propagation [3], surface-enhanced fluorescence [4, 5], and Raman spectroscopy [6, 7]. Aluminum has been shown to be a better plasmonic material than gold or silver in the ultraviolet (UV) range due to the negative real part and the low imaginary part of its permittivity at wavelengths smaller than 200 nm [1, 4, 8]. Nevertheless, aluminum is easily oxidized and forms a native aluminum oxide (Al2O3) layer under atmospheric conditions, making device fabrication with Al difficult. The presence of this oxide layer results in a red shift in the LSPR peak position [2] and a lower extinction efficiency [9]. There are only a few reports in the literature describing the Al plasmon resonance compared to those for silver and gold, thereby limiting the selection of phosphors to those absorbing 0957-4484/15/095701+08$33.00
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emission that is widely used for different applications (lighting, biomarker, etc). When Eu3+ ions are incorporated in oxides with a broad band gap, the most effective excitation is within the UV-blue spectral range corresponding to the Eu-O charge transfer (CT) band, which could be potentially exalted by an Al UV plasmon. Otherwise, Eu3+ ions are excellent structural probes, because their fluorescence properties are strongly related to their local environment, particularly in a sesquioxide structure. In the first step of this study, we use the properties of Eu3+ to monitor the growth of an Eu:Y2O3 phosphor film inside the different multilayer architectures, and to distinguish between the structural effects and potentially plasmonic effects. In the second step, we present the influence of the multilayer architecture and the coupling distance, d, on the luminescence properties of Eu:Y2O3.
Al d
d
Passive spacer
2. Experimental section The different multilayers presented in figure 1 were formed by pulsed laser deposition (PLD). This technique allows a single-step deposition, guarantees a high quality of the interfaces, and prevents any reactivity between the embedded metallic structures and the ambient atmosphere. The same method was recently used to study the coupling distance between RE and silver nanoparticles [19]. The multilayer structures were elaborated in a high-vaccum deposition chamber (P = 10−7 mbar) by successive ablation of Al, Y2O3, and 1% Eu:Y2O3 targets with a Lambda-Physik LPX-100 KrF excimer laser (λ = 248 nm, τ = 17 ns) at 10 Hz. The laser beam was focused on the targets at an incident angle of 45° with respect to the target normal. The substrate–target distance was kept constant at 4 cm. Previously, the oxide targets, consisting of Y2O3 or Eu:Y2O3 powders with a 1% Eu/Y atomic ratio, were pressed and sintered at 1400° C for 12 h in air. The cubic phase purity of the targets was controlled by xray diffraction [JCPDS card 41-1105]. The Al target was obtained from Neyco with 99.999% purity. The films were deposited on fused-silica substrates (Suprasil). The Al films were deposited under vacuum with a laser fluence of 2.6 J cm−2. A deposition rate of around 0.02 Å/ pulse of the Al layer was determined from thickness measurements taken by profilometry. For the different multilayer architectures with the Eu:Y2O3 emitting layer illustrated in figure 1, the thickness of the Al layer was fixed at 1.5 +/−0.8 nm. Thin oxide layers of Y2O3 and Eu:Y2O3 were deposited under an oxygen atmosphere (10−3 mbar) with a laser fluence of 2.3 J cm−2. All deposits were realized at room temperature to avoid ionic diffusion between the layers. The thickness of the emitting Eu:Y2O3 layer was kept constant and equal to 10 nm, whereas the thickness of the Y2O3 spacer layers, denoted by d, was varied from 0 to 60 nm. For each material, the deposition rate was first determined by means of the mline spectroscopy at 543.5 nm using an LaSF35 prism, assuming a step index profile [20]. The number of pulses was then adjusted to achieve the selected thickness of the Eu:Y2O3 and Y2O3 layers. Note that on a silica substrate, deposited Al
Al Figure 1. Sketch of the multilayer structure: an Eu:Y2O3 phosphor layer deposited on an Al film covered with a percolated Al film. A Y2O3 passive layer is inserted between the phosphor and the Al layers to study the influence of the coupling distance d.
higher than the emission of the same film deposited on an insulator substrate [4, 5]. The gain depends on the thickness of the Al layer [5], its orientation [12], and the distance between the metal and the emitter, as reported for other metals [13]. A few papers have reported the enhancement of an emitting film placed between two metallic layers, which can be continuous [14] or made of nanoparticles [13, 15]. In the case of a nanometer-sized gap between nanostructured metal surfaces, field amplification is likewise observed [15]. Polemi et al compared the results for these multilayer systems to those modeled on nanoparticle dimers [16]. Finite-difference time domain calculations predict that the electric field enhancement in the UV-blue range is higher inside a dimer system compared to Al isolated nanoparticles. These theoretical results have been demonstrated experimentally for other metals such as Ag [17] and Au [18]. Our initial goal was to study the electric field enhancement in the UV range between two nanostructured aluminum layers, to generate effects comparable to those observed in dimers. The adjustment of the growth parameters led us to develop several types of nanostructured Al layers, and we found that the multilayer system, consisting of an emitting layer between a percolated and a continuous Al nanometer-thick layer, yields on enhancement. This paper presents these original results. We investigate the effect of the distance, d, between Al and RE on its emission by incorporating passive spacer layers, as shown in figure 1. In this paper, Eu3+ ions were chosen from among the RE elements because these ions, incorporated as dopants in a sesquioxide matrix (especially Y2O3), produce the intense red 2
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CT 35000
Level energy (cm-1)
30000 25000
5D 5G4 5L 2 5D6 5D3 2 5D 5D 1 0
20000
UV
15000 10000 5000
7
FJ =2...7 1 0
0
Figure 2. (a) Diagram of the f–f intraconfigurational energy levels of Eu3+ transitions and Eu-O CT band with the characteristic splitting of
7 F0,1,2 levels by crystal fields in symmetry C2 corresponding to the symmetry of sites occupied by Eu3+ ions in cubic Y2O3 crystal. (b) Excitation spectra of bulk (target) Eu:Y2O3 and Eu:Y2O3 nanofilms deposited on SiO2 substrate.
spontaneously self-arranges to form a very thin layer, while on doped or undoped Y2O3, deposited Al forms a percolated layer. More details on Al growth by PLD have been studied and will be presented in the near future. The film morphology and structure was analyzed by absorption spectroscopy (Lambda900 from Perkin Elmer), by atomic force microscopy (AFM, Asylum), and by transmission electron microscopy (TEM, JEOL 2100 F and TOPCON EM0026). For the TEM analysis, a 10-nm-thick Eu:Y2O3 film was deposited onto a 15-nm-thick SiO2 membrane TEM grid as a reference sample. A second membrane grid was prepared by first depositing 5 nm of Al onto the SiO2, followed by a 10-nm layer of Eu:Y2O3. The emission and excitation spectra were collected using a 450 W Xe lamp followed by a GEMINI Jobin-Yvon double monochromator with an 8-nm bandwidth as an excitation source. The fluorescence was collected through an optical fiber, analyzed by a Triax 190 Jobin-Yvon monochromator, and detected by a cooled charge-coupled device (CCD) detector or a photomultiplier. The emission spectra were recorded with a resolution of 0.4 nm. The excitation spectra were registered by selecting the emission wavelength with a 6-nm band pass, and were corrected from the experimental setup response.
mainly located in the sites of the C2 symmetry that result from a splitting of the ground 7FJ levels in C2 symmetry [22, 23]. Some Eu3+ ions are also located in S6 symmetry sites, but their emission lines are much weaker [24]. The emission corresponding to the 5D0 → 7FJ transitions of Eu3+ can be excited by either f–f intra-configurational transitions or by the Eu–O CT band. The latter is more efficient because it involves an allowed dipolar electric transition. This band originates from the interaction between Eu3+ and O2− ions and its position varies depending on the composition and structure of the matrix. In oxides, it remains in the UVblue range. For micrometric-size cubic Y2O3 crystals, it is located at 239 nm [25]. Figure 2(b) shows the excitation spectra of the Eu:Y2O3 target and of a reference Eu:Y2O3 nanofilm deposited on a silica substrate that consists of crystallites with a mean diameter of 69 nm +/−5 nm and around 2 nm, respectively. As the size of the cubic crystallites decreases, a red shift of the CT band is observed, which increases with longer emission wavelengths. The red shift has been explained by a local disorder and a volume expansion that appears on nanosized Eu:Y2O3 crystallites [26]. This combined effect leads to a change in the energy bands with a decrease in the energy gap of the nanocrystallites. This shift is also in agreement with those reported for Eu:Y2O3 nanocrystallized powders prepared by the combustion method [25]. The red shift of the CT band was, in that case, attributed to an increase in the coordination number from 6 to 8 and an increase in the Eu-O bond length for ions located at the nanocrystallites’ surface [27]. A red shift of the CT band is also observed for a monoclinic film compared to the cubic film with a similar crystallite size; this could be attributed to an increase in the coordination number from the cubic to the monoclinic phase [28]. Due to these excitation spectra results, it is possible to choose the appropriate excitation wavelengths within the CT band to quasiselectively excite ions located in different environments,
3. Growth monitoring of Eu:Y2O3 phosphor layer using ions as local structural probes Eu3+ ions are excellent probes of their structural environment. The position and the intensity of their excitation and emission transitions depend strongly on the symmetry of the local crystal field of the site occupied by the ions. Figure 2(a) shows the energy level diagram of Eu3+ in cubic Y2O3. The values of the energy levels are deduced from previous work on PLD of Eu:Y2O3 film [21]. In cubic Y2O3, Eu3+ ions are 3
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Figure 3. Excitation spectra of the most intense emission at 612 nm of Eu:Y2O3 nanofilm deposited on SiO2 substrate (reference) and on Al layer, both without (d = 0 nm) and with a spacer (d = 20 nm).
either in the regular sites of the cubic crystallites (at 240-nm excitation wavelength) or in the perturbed sites of smaller nanometric cubic crystallites (at 270-nm excitation wavelength). Figure 3 shows the excitation spectra of a 10-nm-thick Eu:Y2O3 layer deposited on silica (reference) and on an Al layer without a spacer (d = 0 nm), or with a spacer of 20 nm (d = 20 nm). The figure shows that as the distance, d, decreases below 10 nm, a blue shift and narrowing are observed. This suggests a UV plasmon enhancement of the excitation band or an increase in the size of the Eu: Y2O3 crystallites as the Eu:Y2O3 grows directly on the Al surface [26]. This evolution is confirmed by the emission spectra, which were recorded for different excitation wavelengths in the CT band range. Figure 4 shows the emission spectra in the 5 D0 → 7FJ transition range, restricted to the lowest multiplets, J = 0, 1, 2, for the Eu:Y2O3 layer deposited on silica (reference) and on the Al layer either without a spacer (d = 0 nm) or with a spacer (d = 20 nm and d = 60 nm). Under UV excitation at 240 nm (figures 4(a) and (c)), all the cubic samples show the same emission lines, which are mainly due to Eu3+ ions located in the sites of C2 symmetry [22, 23]. Under excitation at 270 nm (figures 4(b) and d)), the spectra of the reference sample or of the multilayer including a Y2O3 spacer layer are broadened and show additional features compared to Eu:Y2O3 deposited directly on Al; the 5D0 →7F0,1 emission lines present a duplication of 5D0 →7F0 and a clear broadening of 5D0 → 7F1. These emission changes at a lower excitation energy suggest the presence of two kinds of sites characteristic of small crystallites [25]. These observations are in agreement with the emission spectra corresponding to the 5D0 →7F2 transition (figure 4(d)). For the reference sample, and as the distance, d, increases, a shoulder located at about 613 nm broadens toward longer wavelengths, and a broad line centered at 624 nm appears. These additional features are a clear indication that Eu3+ ions are distributed among sites of different crystal field symmetries and strengths. By analogy, the conclusions of spectroscopic and EXAFS studies on nanocrystalline powders prepared by the combustion method [25, 27], are attributed to Eu3+ ions
Figure 4. Emission spectra corresponding to 5D0 → 7F0,1,2 transitions under excitation at (a), (c) 240 nm, and (b), (d) 270 nm of Eu:Y2O3 nanofilm deposited on Al with different spacer distances, d.
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Figure 6. Distribution of the diameter of the particles for Y2O3:Eu nanocrystallites deposited on either SiO2 or on an Al continuous layer.
Figure 5. Evolution of the percentage of volume of Eu:Y2O3
crystallite sites versus the thickness, d, extracted from the fit of 7 F2 (black squares) and 7 F0 (red crosses) transitions. The line shows the reference.
normal distribution. The average crystallite size is 1.7 nm +/−0.3 nm. As expected, Eu:Y2O3 on Al shows different growth, with bigger crystallites and an inhomogeneous size distribution ranging from 1.8 to 20.7 nm. The density of the Eu:Y2O3 crystallites is ≈60 times lower on Al. Thanks to the fluorescence and TEM data, we were able to estimate the thickness, δ, of the perturbed shell around the cubic core of the Eu:Y2O3. For the Eu:Y2O3 film deposited at a distance, d ⩾ 10 nm on Al or directly on silica (reference), the average diameter of the crystallites is estimated at 1.7 nm and the volume site percentage at 8% (figure 5). Assuming that the Eu:Y2O3 crystallites are spherical, δ could be evaluated at 0.7 nm using the equation
located in a perturbed environment at the crystallite surfaces. In the case of Eu:Y2O3 directly grown on aluminum (d = 0 nm), no emission surface sites appear. This is partly the result of a decrease in the ratio of the surface area to the volume when the crystallites’ size increases. The observed evolution of the excitation and emission spectra suggest that the Eu: Y2O3 grown on Al promotes bigger cubic crystallites. However, the additional excitation and emission lines are close to those of the Eu3+ in the B or C sites of the monoclinic Y2O3 phase, which are characterized by a higher coordination number (7 instead of 6 for cubic sites) and a lower site symmetry [29]. This suggests that perturbed sites in the crystallites’ surface have a crystallographic symmetry close to the monoclinic phase, as previously reported in the literature [27, 30]. Moreover, the fluorescence modification depends not only on the crystallites’ size, but also on the synthesis route and post-treatment [31], as shown by the fluorescence difference observed for nanopowders prepared by sol-liophilization [32], colloidal precipitation [33], and low energy cluster beam deposition [34], which emphasizes the crucial role of the surface properties. An adjustment of the different emission spectra has been performed versus the distance, d, to calculate the ratio of the volume site number, Nvolume, relative to the total number, Ntotal, of sites occupied by ions (figure 5). More details on the fitting procedure are included in the supplementary material. The results obtained from fitting the 5D0 → 7F2 and 5D0 → 7F0 transitions, which are less intense but only split into two emission lines, are coherent (figure 5). As the distance between the Al and the Eu increases, the proportion of the volume sites decreases to reach the reference value. This result is coherent with the evolution of the excitation spectrum discussed previously (figure 3), and it confirms our hypothesis of the growing of bigger Eu:Y2O3 crystallites when deposited on Al, compared to SiO2. This spectroscopic result is confirmed by TEM measurements. The crystallite size distribution extracted from TEM measurement of Eu:Y2O3 grown on either SiO2 or Al is presented in figure 6. For Eu:Y2O3 on SiO2, the size distribution for the nanoparticles is fitted with a logarithmic
4 3 × πr 3 − 4 3 × π (r − δ )3 = 1 − % volume site. 4 3 × πr 3
This value is consistent with the literature [35]. For Eu: Y2O3 on Al, the size distribution is too broad to evaluate the average diameter of the crystallites. The growth process of an Eu:Y2O3 phosphor layer depends on the distance to the bottom Al layer. Therefore, we will focus on Eu3+ in the cubic core sites always present in the Eu:Y2O3 film to avoid any mixing of crystallization and of potential plasmonic effects on emission.
4. Influence of coupling distance, d, and the multilayer architecture on emission The coupling between Eu:Y2O3 and Al has been analyzed inside the multilayer structures illustrated in figure 1: Al(percolated)/Y2O3buffer/Eu:Y2O3 (10 nm)/Y2O3 buffer/Al(continuous). A panel of multilayers has been realized with different thicknesses, d, of the buffer layer. Figure 7 shows the AFM images of the Al (a) continuous and (b) percolated films deposited with the same number of laser pulses corresponding to an effective thickness of 1.5 nm. In fact, as the Al film is deposited on a silica substrate, the film is continuous, with a very low roughness estimated at 100 pm. This effect is due to the high wettability of Al on silica. Otherwise, the upper Al film grown on Y2O3 is 5
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Figure 7. AFM images of Al thin films deposited by PLD onto (a) SiO2 and (b) Y2O3 with a nominal film thickness of 1.5 nm.
centered around 300 nm. A broad UV band is likewise observed for the Al rough film, while the absorption of isolated nanoparticles presents a narrower band whose position depends on their diameter and shape [1, 2, 36]. The absorption band overlaps the CT band of Eu:Y2O3 (figure 3), suggesting a potential coupling effect. We investigated the fluorescence properties of the Eu: Y2O3 film inside the multilayers versus the distance, d. The results are compared to those of a reference film, namely a single Eu:Y2O3 film deposited on a silica substrate with the same thickness (10 nm). Based on the study reported in the first section of this paper, the excitation wavelength was selected at 240 nm to excite ions located in the Eu:Y2O3 core sites, which are present in the film whatever the distance, d. Figure 9(a) shows the intensity of the most intense emission line at 612 nm of Eu3+ versus the distance, d. The intensities of the different emissions are normalized with respect to the reference sample. In figure 9(a), we distinguish two optimal distances for luminescence enhancement. The first one appears at a short distance (without the introduction of a buffer layer) and the second one at a longer distance, d ≈ 20 nm. The enhancement at a short distance is due to a coupling with the localized surface plasmon supported by the percolated Aluminum layer. The nanostructuration of the metallic percolated film promotes a local field enhancement close to the metallic nanoparticles [37], especially for a high-density Al layer. To verify that this effect is due to the percolated layer and not to the size of the crystallites, we deposited Eu: Y2O3 on an Al continuous layer without the percolated layer. In this case, no enhancement was observed. A similar effect has been observed with other Al–phosphor structures for metal enhanced fluorophore applications [4, 5]. Note that in our case, we do not observe a luminescence quenching for d = 0 nm, suggesting a strong plasmonic effect due to the upper Al layer. To highlight this assumption, we looked at the structure without the upper Al layer. In this case, the luminescence was equivalent to that of the reference films deposited on silica. This confirms that the enhancement at a short distance is due to the presence of the upper Al percolated layer. The no-quenching may be explained by the fact
Figure 8. Absorption spectrum of Al percolated film on Y2O3 film (a) and, excitation at λ emission = 612 nm (b) and emission at λ excitation = 240 nm (c) photoluminescence spectra of Eu:Y2O3 reference sample.
discontinuous, consisting of wide holes with an average diameter of 62 +/−12 nm, and heights of 2.0 +/−0.5 nm. The Al filling factor is estimated at 69%. We estimate the roughness to be 814 pm, which is ≈8 times higher than that of an Al film on SiO2. This growth difference can be explained by the difference of the surface energy between SiO2 and Y2O3. To characterize the plasmon resonance, absorption measurements have been realized on an Al percolated layer with an effective thickness ranging from 1.5 to 15 nm. Below 5 nm, the signal-to-noise ratio was too low, due to the low intensity of the plasmon resonance. This measurement limitation for the nanometric Al layer was previously observed in the literature [5]. One explanation stems from the oxidation layer of the Al film in an ambient atmosphere. As the effective thickness decreases, the thickness ratio between the Al native oxide and the Al layer increases. This oxidation induces a decrease in the absorption intensity and a small red shift of the plasmon resonance as it is calculated in the literature for Al nanodisks [9]. Figure 8 presents the absorption spectrum of a 5 nm Al percolated, which overcomes these drawbacks and still gives us good information on the real plasmonic properties. For 5-nm-thick a broad absorption band attributed to localized plasmon resonance is measured and is 6
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Figure 9. Variation of the most intense emission intensity at 612 nm of the Eu:Y2O3 layer under excitation at 240 nm versus the coupling distance, d (a), and the corresponding decay (b).
that the Eu:Y2O3 layer is 10 nm thick, so the Eu3+ ions are not directly in contact with the Al. In addition, the thickness of the Al films is too low to promote efficient nonradiative transfer into the metal. Another factor which could contribute to the enhancement at a short distance is the coupling of the two metallic layers. We can expect that the coupling between these two Al layers promotes a strong electromagnetic coupling, which overcomes the quenching process at short distances. The coupling between the percolated and continuous Al layers can be compared with the modeled effect of an Al dimer [4, 38]. Other multilayer system studies are underway to investigate the impact of the coupling of the two Aluminum layers and of the incident beam polarization. When the distance, d, increases from 0 to 10 nm, the fluorescence enhancement decreases to 1. This suggests that for d ⩾ 10 nm, the Eu:Y2O3 film is unaffected by the localized plasmon, and/or that there are no coupling effects between the two Al layers. The second mechanism of luminescence enhancement for the longer distance could be due to the collective effect of the Al nanoparticles already observed in the coupling between Ag nanoparticles and an Eu:Y2O3 film [19]. In fact, the Al layers can be viewed as a low-quality mirror [39]. Figure 9(b) shows the evolution of the luminescence decay versus d normalized to the reference. The intensity decay of the Eu:Y2O3 can be fitted with an average lifetime of τ ≈ 0.80 ms. The multiexponential character of the luminescence decay of Eu:Y2O3 coupled with Al is emphasized for d ⩽ 15 nm. This accentuation is due to a distance distribution of the emitter from the metal surface. The Eu3+ ions are randomly distributed inside the 10-nm-thick Eu:Y2O3 layer, so the emission of Eu3+ closer to the metal should be affected by nonradiative transfer to the metal, resulting in a reduction of the lifetime [40] and the lifetime distribution. At a greater distance, this effect is normally suppressed. The long lifetime component is stable whatever the distance, indicating that the emission enhancement comes from an increasing CT band absorption, thanks to the Al layers.
5. Conclusion The coupling between Eu3+ RE emitters and Al has been investigated in multilayer structures, which consist of an Eu: Y2O3 film deposited between two Al films: one percolated and one continuous. The percolated Al layer consists of broad holes of 60nm diameter with a thickness of 2 nm. The introduction of a passive buffer Y2O3 layer with different thicknesses, d, allowed us to investigate the role of the distance, d, on the nanostructuration and emission of the Eu: Y2O3. In the first part of the paper, the growth of the Eu: Y2O3 film versus the distance, d, is analyzed by both using the emitters as local structural probes and by TEM analysis. We show that two types of sites, one corresponding to the cubic cores and the other to the perturbed shell of the Eu: Y2O3 crystallites, coexist in the film. For d lower than 10 nm, the emission of the core sites increases, which is consistent with the growth of the crystallites observed in TEM measurements. Thanks to the analysis, we selected an excitation both to observe only the Eu3+ in the core sites, which are always present whatever the distance, d, and to investigate the plasmonic effect of Al on the Eu3+ emission. An enhancement is observed at a short distance, and also at a distance of 20 nm. The enhancement at the short distance is attributed to the localized plasmon resonance of the percolated upper Al film, while the enhancement for the larger d is attributed to the collective effect of the Al nanoparticles. These multilayer structures are very promising as amplifiers for all kinds of optical devices developed for absorption/emission of light, and/or wavelength conversion (solar-cell, LED, etc).
Acknowledgments The authors wish to thank Professor G Colas des Francs (ICB-UMR 5209 CNRS Université de Bourgogne, France), for fruitful discussions. The TEM images were taken at the Centre Technologiques des Microstructures platform (Faculté
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des Sciences et Technologies de l’Université Lyon 1). The quality of the English has been checked by the proofreadingservice.
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