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Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 115604 (http://iopscience.iop.org/0957-4484/26/11/115604) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 115604 (7pp)
doi:10.1088/0957-4484/26/11/115604
Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement N Abdellaoui1, A Pillonnet1, J Berndt2, C Boulmer-Leborgne2, E Kovacevic2, B Moine1, J Penuelas3 and A Pereira1 1
ILM- Université de Lyon, Université Lyon 1, CNRS UMR5306, Villeurbanne F-69622, France GREMI, UMR7344 CNRS and Université d’Orléans, 14 rue d’Issoudun BP6744-45067, Orléans Cedex 2, France 3 Institut des Nanotechnologies de Lyon—Université de Lyon, UMR 5270—CNRS, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, F-69134 Ecully cedex, France 2
E-mail: [email protected] Received 24 November 2014, revised 13 January 2015 Accepted for publication 30 January 2015 Published 25 February 2015 Abstract
Pulsed laser deposition was used to deposit aluminum thin films of various thicknesses (tAl) ranging from 5 to 40 nm and to investigate their growth process when they are deposited onto SiO2 and Y2O3. Atomic force microscopy and x-ray reflectivity measurements show that the structure of the Al films are related to the wettability properties of the underlaying layer. Onto SiO2, ultra-smooth layers of aluminum are obtained, due to a perfect wetting of SiO2 by Al. In contrast when deposited onto Y2O3, percolated Al layers are observed with apparent pore size decreasing from 200 to 82 nm as tAl is increased from 5 to 40 nm, respectively. This particular morphology is related to partial dewetting of Al on Y2O3. These two different growth mechanisms of aluminum depend therefore on the surface properties of SiO2 and Y2O3. The plasmon resonance of such Al nanostructures in the UV region was then analyzed by studying the coupling between Eu3+ rare earth emitters and Al. Keywords: pulsed laser deposition, aluminum nanostructures, fluorescence enhancement, rare earth (Some figures may appear in colour only in the online journal) 1. Introduction
methods have been explored to synthetize nanostructured aluminum and control it in a desired fashion. For example, evaporation [7, 8, 11, 14], sputtering [15, 16] and molecular beam epitaxy [17] are certainly the most conventional techniques used to obtain Al thin films. Such techniques have also been used in connection with functionalized or patterned substrates to obtain a large variety of Al nanostructures [18– 20]. Other approaches, based on lithography techniques, have been investigated and developed as high-resolution techniques to pattern Al films [6, 9, 21, 22]. This work presents investigations concerning the use of pulsed laser deposition (PLD) for the growth of nanosized aluminum thin films. Indeed, PLD is a physical method that has emerged as an effective method for the growth of highpurity thin films and nanostructured materials [23]. By
The development of aluminum nanostructures has received considerable attention because of their plasmonic properties in the UV range [1, 2] making them interesting for many potential applications, e.g. photovoltaics [3–6], biosensing and biological imaging [7–9], and tip or surface-enhanced Raman scattering [10–12]. Different types of Al nanostructures have been actively investigated over the past years. For example, Al nanoparticles can be used to enhance the radiative rate of a fluorophore due to plasmonic-enhanced near fields [13]. Another prominent example relates to the use of rough Al thin films as substrates for metal enhanced fluorescence (MEF) [7, 8] or surface enhanced Raman spectroscopy [11] in the UV/blue range. In this context, several 0957-4484/15/115604+07$33.00
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Nanotechnology 26 (2015) 115604
Figure 1. XRR curves of Al thin films deposited by PLD onto SiO2 (a) and Y2O3 (b) for different values of nominal thickness of Al ( tAl ). The 4π scattering vector q is expressed as q = λ sin θ. The red solid line is the result of the fit.
adjusting the deposition parameters, this technique offers the possibility to deposit and control the morphology of a large variety of materials from small metallic nanoparticles, to semi-continuous or continuous ultra-thin layered or multilayered films [24–31]. Most studies dealing with the deposition of metals by PLD concern noble metals for catalytic and optical applications. There are only a few reports in the literature describing the deposition of Al by PLD [19, 32–34]. One reason is the high reflectivity of Al in the first steps of the ablation process (∼92% in the UV range), leading to a low ablation rate. In addition, plume deflection occurring as a result of Al target degradation under UV irradiation can also affect the deposition rate and the quality of the deposited layer [34]. For these reasons, it can be challenging to obtain thick aluminum films (>100 nm) by PLD. However, in the case of ultra-thin films these drawbacks should be limited. We report here on the growth of aluminum thin films onto SiO2 and Y2O3 with thicknesses ranging from 5 to 40 nm. To understand the Al growth process, the synthesized films are characterized by atomic force microscopy (AFM) and x-ray reflectivity (XRR). We comment the effect of the surface wettability on the change of the Al thin structure. At the end, we also outline the use of these nanostructured Al thin films which support UV plasmon for enhancing the luminescence of Eu3+ ions.
was varied by increasing the number of laser pulses taking into account the deposition rate (0.01 nm s−1) previously determined by profilometric measurements (Tencor Alpha Step 100). For luminescence investigations, composite films were prepared by alternating the ablation of three targets of Al, passive Y2O3, and emitting Y2O3:Eu, as described elsewhere [31]. For each material, the deposition rate was first determined by means of a profilometer for Al and m-lines spectroscopy measurements for Y2O3. The number of laser pulses was then adjusted to achieve the desired thickness. Al thin films were characterized by AFM and XRR. AFM (Asylum Research) in tapping mode with a scan rate of 2 Hz was used to investigate the morphology of the Al films. Silicon AFM tips (Tap190Al-G, Budgetsensors) were used to scan the Al films. Three different areas on the sample (5 × 5 μm) were scanned to obtain an average value of the root mean square roughness (rms). XRR analysis was used to investigate the thickness, density, surface and interface roughness of Al and Al/Y2O3 films. XRR spectra were recorded using a Rigaku SmartLab diffractometer. The x-ray source is a rotating anode operating at 9 kW and equipped with a double Ge(220) monochromator to select the CuKα1 radiation (λ = 1.5406 Å). The x-ray experimental reflectivity curves were then simulated using the RCRefSimW software [35]. The fitting parameters were layer thickness, roughness and density associated with each layer. The absorption spectrum of the films was recorded using UV–vis spectrophotometer (Lambda900, Perkin Elmer). The emission spectra were achieved using a tunable pulsed laser EKSPLA NT340 as excitation source. The fluorescence measurements were performed at room temperature with a CCD sensor associated with a monochromator Triax 190 from Jobin-Yvon (1.4 nm resolution).
2. Experimental Aluminum thin films were grown at room temperature by ablating a rotating Al target (99.999% purity, Neyco) by means of an ArF excimer laser (Coherent CompexPro, λ = 193 nm, τ = 17 ns, f = 10 Hz) in a high vacuum deposition chamber (base pressure of 10−7 mbar). The laser beam was focused on the Al target at an incident angle of 45° with respect to the target normal. The laser fluence and substratetarget distance were kept constant at about 2.6 J cm−2 and 4 cm, respectively. Depending on the analysis to be performed, Al films were deposited on Suprasil or silicon (covered with its native oxide) substrates or on 300 nm thick Y2O3 buffer layers previously deposited by PLD. The Al thickness
3. Results and discussions 3.1. Thin films growth
XRR curves as a function of the nominal thickness of Al (tAl) are shown in figures 1(a) and (b) for deposition onto SiO2 and 2
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Nanotechnology 26 (2015) 115604
Table 1. Results of XRR curve fitting for Al films deposited on SiO2 (a) and Y2O3 (b) using a two-layer model (Al2O3 and Al). The thickness
( t ), density ( ρ ) and roughness ( R ) of both layers are listed for various nominal thicknesses of Al ( t XRR
XRR RAl −SiO2
XRR
and
XRR RAl −Y2 O3
XRR
Al ). The interfacial roughness are also given. The roughness of SiO2 and Y2O3 previously estimated by XRR were 0.57 and 1.12 nm, respectively.
(a) Al2O3
tAl (nm) 5 10 20 40
Al
SiO2
XRR tAl (nm) 2O3
XRR ρAl (g cm−3) 2O3
XRR RAl (nm) 2O3
XRR (nm) tAl
XRR (g cm−3) ρAl
XRR RAl (nm)
XRR RAl −SiO2 (nm)
1.49 1.84 1.91 2.14
1.98 1.98 1.98 1.98
0.69 0.60 0.41 0.59
4.46 11.89 21.56 39.76
2.83 2.70 2.73 2.70
0.69 0.40 0.40 0.59
0.26 0.62 0.84 0.30
(b) Al2O3
tAl (nm) 5 10 20 40
Al
XRR tAl (nm) 2O3
XRR ρAl (g cm−3) 2O3
XRR RAl (nm) 2O3
XRR (nm) tAl
XRR (g cm−3) ρAl
XRR RAl (nm)
XRR RAl −Y2 O3 (nm)
2.32 1.71 2.30 2.43
2.29 2.14 2.02 2.23
1.85 1.71 2.12 2.80
5.04 13.25 21.71 39.60
2.83 2.59 2.70 2.70
1.87 1.32 2.86 3.61
1.20 1.50 1.23 1.00
formation of dense films with low porosity, due to the high mobility of Al at the substrate surface. For Al2O3, the density (2.08 ± 0.13 g cm−3) is found to be significantly lower than the theoretical value of bulk Al2O3 (3.96 g cm−3) [38]. Although the origin of that discrepancy between our values and the theoretical one is not clearly understood, similar density values were found for Al2O3–Al–Ni–Co multilayers deposited by high e-beam evaporation [36]. It can be hypothesized that the Al oxidation leads to the formation of a porous outer oxide layer in different oxidized form. Last, the roughness values listed in table 1 show that a rougher surface is obtained when Al is deposited on Y2O3. These results clearly indicate that the roughness and then the morphology of the Al films are related to the properties of the underlying layer. The AFM images depicting the morphology of Al thin films deposited simultaneously onto SiO2 and onto Y2O3 are shown in figure 2. They reveal remarkably different Al growth characteristics depending on the properties of the underlying layer. On SiO2 (figure 2(a)), continuous Al films are observed whatever the film thickness. No aluminum nanostructure is evidenced, indicating a perfect wetting of aluminum on SiO2. This suggests that already in the initial stage of deposition (i.e. within the deposition of the first atomic layers) a complete coverage of the surface is obtained, followed by a growth in height by adsorption of further aluminum atoms arriving on the surface. We can note that the average roughness of Al films are found to range from 0.09 to 0.28 nm and are comparable to that of the substrate (rms = 0.10 nm). The apparent discrepancy with the roughness values obtained by XRR could be due to the size of the AFM tip used (∼10 nm radius), which combined with the roughness present in the sample, give low roughness values. Several studies have thus reported on the fact that different roughness can be obtained by XRR or AFM [39, 40]. In contrast, the
Y2O3, respectively. Note that Al was simultaneously deposited onto SiO2 and Y2O3 through a mask (diameter of 1.5 cm), and that therefore the resulting thickness of Al is expected to be similar on both substrates. The oscillation period is inversely proportional to the layer thickness, while the amplitude contains information on roughness and density. Also, XRR curves exhibit amplified oscillations when Al is deposited on Y2O3 (figure 1(b)). This important feature suggests a modification of the morphology of the Al films with the nature of the substrate. To clarify this point and then extract quantitative informations, the measured reflectivity data were fitted by modeling the deposition. For both substrates, a two-layer model consisting of aluminum with its native oxide (Al2O3) was considered. Such a model is in agreement with the literature, since Al is well-known to be rapidly oxidized upon exposure to air, leading to the formation of an outer oxide layer (up to 3–4 nm thick) [20, 22, 36]. The results of the reflectivity curves fitting (i.e., thickness, roughness and density) for all the samples are listed in XRR is limited table 1. As expected, the thickness of Al2O3 tAl 2 O3 to less than 2.50 nm. The total thickness, corresponding to Al XRR XRR and Al2O3 tAl + tAl , is higher than the nominal 2 O3 thickness ( tAl ). The latter being estimated from profilometric measurements performed on a thick calibrating sample, the relative error increases as the thickness decreases. However, it is important to note that the total thickness is almost similar on both substrates. The density of Al extracted from the fit is 2.72 ± 0.08 g cm−3. This value, which is almost identical whatever the substrate, is close to the density of bulk Al (2.70 g cm−3) [37]. The fact that the density of Al films is close to the theoretical value of bulk Al is due to the high kinetic energy deposition conditions that have been used in this work. Indeed, deposition under vacuum leads to the
(
(
Y2O3
)
)
3
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Nanotechnology 26 (2015) 115604
Figure 2. AFM images (5 × 5 μm) of Al thin films deposited by PLD onto SiO2 (a) and Y2O3 (b), with the nominal film thicknesses ranging from 5 to 40 nm, as indicated. The surface roughness (rms) evaluated from AFM images is given. The roughness of both substrates (for tAl = 0 nm) is less than 0.1 nm.
Y2O3 substrate (figure 2(b)) yields aluminum films with a macroporous-like structure, suggesting that at the first step of the growth due to its non-wetting behavior Al islands are formed on Y2O3. As the Al amount of matter increases a thin film of coalesced Al islands is grown with a roughness that is reminiscent of the initial aspect ratio of the Al islands. To quantify the changes in surface morphology, we evaluate the surface roughness from AFM images. The roughness slightly increases from 0.84 to 0.94 nm with the thickness, which is about 10 times higher than that of the Y2O3 underlying layer (rms = 0.09 nm). This radically different behavior for the two substrates must reflect a change in the aluminum growth mechanism. The two different growth mechanisms of aluminum, which depend on the surface properties, can be assessed by considering the spreading coefficient (S) expressed as S = γs−(γi + γAl), where γi, γs and γAl, are the surface energies of the metal/surface interface, the substrate surface and the aluminum surface, respectively [41, 42]. If S ⩾ 0, then we can expect a spreading of Al on the substrate (complete wetting). If, on the other hand, S ⩽ 0, the surface prefers to remain ‘dry’, i.e. the metal partially wets the substrate. The surface energies of SiO2 (γSiO2 ≈ 207–605 mJ m−2) [41], Y2O3
To outline the growing process of Al on Y2O3, we carefully analyzed by means of AFM the morphology of the percolated films as a function of the nominal film thickness tAl. Figures 3(a) and (b) show AFM images of Al films obtained for tAl = 5 and 20 nm, respectively. The corresponding AFM profiles are given in figure 3(c). For the lowest tAl, the percolated Al layer can be described as a disordered structure with apparent pore size of about 200 ± 25 nm full width at half maximum (FWHM). For thicker Al depositions, more orderly Al structures with relatively well-formed circular-pores (typical diameter of about 82 ± 17 nm) are observed. These observations indicate that the structure of Al percolated films, i.e. the shape, the density and the diameter of the holes, is controlled by the Al deposition thickness during PLD. More surprising is the evolution of the hole height (h) as a function of tAl, because a maximum of about 6 nm is obtained when tAl ⩾ 20 nm (figure 3(d)). To further understand this evolution, we should consider the aspect ratio of the Al percolated films defined as the ratio of the width to the height (i.e. FWHM/h). This parameter gives an insight into the ability of the incoming species to enter within the structure and to reach the bottom. From the AFM images and their cross section analyses, the aspect ratio is ranging from about 60 to 15 when tAl is increasing from 5 to 40 nm. These values clearly indicate that the species can easily reach the bottom of the Al pores, yielding finally to the formation of a continuous Al layer on the bottom. From this step, the growth of Al at the top and at the bottom becomes comparable and explains why there is no substantial change in the height for tAl ⩾ 20 nm.
( γY2 O3 ⩽ 83 mJ m−2) [43] and Al (γAl ≈ 40–70 mJ m−2) [44] are known with some confidence. In contrast, data on the interfacial energies are scarce in the literature, and we were unable to find values for γAl − SiO2 and γAl − Y2 O3. However, calculating γs − γAl for the two surfaces we consider here and using the values cited above, we find values between 137 and 565 mJ m−2 on SiO2 and ⩽43 mJ m−2 on Y2O3. Assuming positive values for γi, we can see that a negative value of S can be easily obtained on Y2O3, indicating that the dewetting of Al on Y2O3 occurs. In the case of SiO2, we can assume that S ⩾ 0 is more favorable suggesting a perfect wetting of SiO2 by Al. The formation of Al ultra-smooth nanolayers is therefore expected.
3.2. Optical properties
These characterized Al films are used for luminescence enhancement of europium. Eu3+ ions incorporated as doping ions in Y2O3 matrix present a wide UV absorption band corresponding to the Eu–O charge transfer (CT) band and produce intense red emission which can be enhanced by 4
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Nanotechnology 26 (2015) 115604
Figure 3. AFM images (500 × 500 nm) of Al thin films deposited by PLD onto Y2O3 for tAl = 5 nm (a) and tAl = 20 nm (b), and the corresponding surface profiles (c). Height of the Al structures as a function of tAl is shown in (d); the inset is a schematic illustration of the Al growth process on Y2O3.
to those of a reference film, i.e. a single Eu:Y2O3 layer (10 nm thick) without Al. Thus, the emission intensities were normalized in respect to the emission of the reference sample. A value higher than one indicates a fluorescence enhancement, while a value lower than one corresponds to a quenching (i.e. inhibition) of the luminescence (figure 5(c)). One can see that for the Al monolayer architecture a strong quenching (x 0.1) takes place when no spacer is introduced in the structure. A slight decrease of the quenching is then observed when d increases up to 20 nm. The quenching effect involves non radiative desexcitaton of the fluorescent emitter through transfer of energy to the metallic surface [45, 46]. Such mechanism is relevant for distances up to d = 80 nm for coupling with metallic layers [46], whereas it is limited to short distances (few nanometers) in the case of metal nanoparticles [46, 47]. For the Al bilayer architecture, the opposite tendency was evidenced, i.e. the maximum enhancement appears when no spacer is introduced in the structure (∼12 times higher in comparison with the previous architecture). Here, two possible mechanisms can be considered. First, the percolated Al layer could be described as a subwavelength metal nanostructure, which can scatter incident light into various angles leading to an increase of the path length of light in the Eu:Y2O3 layer. This mechanism leads to an increase of the absorption and therefore to the enhancement of fluorescence (quantum yield close to 1). However, such mechanism is only relevant for thicker metal nanostructures, i.e. typically few hundreds of nanometers [48]. The second mechanism is related to the coupling between localized plasmon resonance of the percolated upper Al layer and the europium absorption band in UV. The Al percolated films on Y2O3 promote a local field enhancement close to subwavelength Al nanostructure. As the optical absorption is proportional to the field intensity, a high local field leads to increased absorption in the emitting layer. The increase of d over 10 nm led to the decrease of luminescence down to
Figure 4. Absorption of Al on Y2O3 compare to the UV absorption band corresponding to the Eu–O charge transfer band.
introducing metallic structures close to Eu3+ ions. In that respect, Ray and al., have reported that rough aluminum nanofilms are interesting substrates for MEF in the UV range [7, 8]. MEF enhancement depends on the Al thicknesses and their optimal one was found for 15 nm thick films. Figure 4 shows the absorption spectrum of Al deposited on Y2O3 as well as the CT band of Eu3+ incorporated in Y2O3. The Al nanostructure supports large localized plasmon resonance typical for discontinuous films [11]. As far as we can tell from these measurements, the Al plasmon resonance overlaps the absorption band of Eu–O allowing us to expect a plasmonic coupling effect between Al and Eu3+. To highlight the effect of this overlapping on the fluorescence properties of the emitting layer, multilayers structures consisting of Eu:Y2O3/passive Y2O3/Al (figure 5(a)) or Al/ passive Y2O3/Eu:Y2O3/passive Y2O3/Al (figure 5(b)) were synthetized by PLD. For these both architectures, the thickness of the passive layer was varied from d = 0 to 20 nm. The effect of both architectures on the luminescence of Eu3+ ions at 612 nm was then evaluated, and the results were compared 5
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Nanotechnology 26 (2015) 115604
Figure 5. Effect of Al nanostructure on the fluorescence properties of Y2O3:Eu3+. Investigated architectures consist of (a) Y2O3:Eu3+
phosphor layer (10 nm) deposited on a continuous Al layer (tAl = 15 nm), and (b) the same as (a) with a percolated Al layer deposited on the top (tAl = 15 nm). A Y2O3 passive layer is inserted between the phosphor and the Al layers. (c) Enhancement and quenching of Y2O3:Eu3+ fluorescence as a function of the thickness d between Al and Y2O3:Eu3+; typical emission spectra under excitation at λexc = 240 nm corresponding to 5D0 → 7F0,1,2 transitions of Y2O3:Eu3+ are shown in inset for D = 0.
around 0.4, as shown in figure 5(c). Here, we can suppose that (i) there is a relative contribution of the percolated upper Al layer because we are out of range of the localized plasmon effect, and (ii) the quenching due to the Al bottom layer is then predominant. The mechanisms responsible of such enhancement as a function of d are not yet completely elucidated, but this is an experimental fact. In order to understand the impact of each structure as a function of the thickness, other multilayer architectures are investigated in the moment, which is however out of scope of this paper.
experimentally the potentiality of Al percolated layers for the fluorescence enhancement of emitting layers, such as Y2O3: Eu3+. Other multilayer architectures are under investigations in order to explore the ability of Al plasmonic nanostructures for the luminescence enhancement of emitting layers.
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4. Conclusion Ultra-smooth and percolated nanosized Al films were obtained by PLD on SiO2 and Y2O3, respectively. The Al film growth modes have been analyzed by XRR and AFM analysis. Based on values for surface energies reported in the literature, we concluded that the growth mode of Al depends on the wettability of the substrate surface. For ultra-smooth Al film on SiO2, the Al thickness increases with the number of laser pulses, and the roughness is estimated by AFM to range from 0.09 to 0.28 nm comparable to that of SiO2 substrate. For percolated Al film on Y2O3, the pore size decreases from 200 to 82 nm as the Al thickness increases from 5 to 40 nm, respectively. Beyond an Al thickness of 20 nm, a progressive fill of the pores is observed, however the depth remains stable. According to absorption measurement, the plasmon resonance of Al percolated layers is localized in UV range. A first approach to the understanding of the importance of Al films in plasmonic systems is proposed. We show 6
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Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 115604 (http://iopscience.iop.org/0957-4484/26/11/115604) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.93.16.3 This content was downloaded on 13/04/2015 at 08:31
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 115604 (7pp)
doi:10.1088/0957-4484/26/11/115604
Growth process of nanosized aluminum thin films by pulsed laser deposition for fluorescence enhancement N Abdellaoui1, A Pillonnet1, J Berndt2, C Boulmer-Leborgne2, E Kovacevic2, B Moine1, J Penuelas3 and A Pereira1 1
ILM- Université de Lyon, Université Lyon 1, CNRS UMR5306, Villeurbanne F-69622, France GREMI, UMR7344 CNRS and Université d’Orléans, 14 rue d’Issoudun BP6744-45067, Orléans Cedex 2, France 3 Institut des Nanotechnologies de Lyon—Université de Lyon, UMR 5270—CNRS, Ecole Centrale de Lyon, 36 avenue Guy de Collongue, F-69134 Ecully cedex, France 2
E-mail: [email protected] Received 24 November 2014, revised 13 January 2015 Accepted for publication 30 January 2015 Published 25 February 2015 Abstract
Pulsed laser deposition was used to deposit aluminum thin films of various thicknesses (tAl) ranging from 5 to 40 nm and to investigate their growth process when they are deposited onto SiO2 and Y2O3. Atomic force microscopy and x-ray reflectivity measurements show that the structure of the Al films are related to the wettability properties of the underlaying layer. Onto SiO2, ultra-smooth layers of aluminum are obtained, due to a perfect wetting of SiO2 by Al. In contrast when deposited onto Y2O3, percolated Al layers are observed with apparent pore size decreasing from 200 to 82 nm as tAl is increased from 5 to 40 nm, respectively. This particular morphology is related to partial dewetting of Al on Y2O3. These two different growth mechanisms of aluminum depend therefore on the surface properties of SiO2 and Y2O3. The plasmon resonance of such Al nanostructures in the UV region was then analyzed by studying the coupling between Eu3+ rare earth emitters and Al. Keywords: pulsed laser deposition, aluminum nanostructures, fluorescence enhancement, rare earth (Some figures may appear in colour only in the online journal) 1. Introduction
methods have been explored to synthetize nanostructured aluminum and control it in a desired fashion. For example, evaporation [7, 8, 11, 14], sputtering [15, 16] and molecular beam epitaxy [17] are certainly the most conventional techniques used to obtain Al thin films. Such techniques have also been used in connection with functionalized or patterned substrates to obtain a large variety of Al nanostructures [18– 20]. Other approaches, based on lithography techniques, have been investigated and developed as high-resolution techniques to pattern Al films [6, 9, 21, 22]. This work presents investigations concerning the use of pulsed laser deposition (PLD) for the growth of nanosized aluminum thin films. Indeed, PLD is a physical method that has emerged as an effective method for the growth of highpurity thin films and nanostructured materials [23]. By
The development of aluminum nanostructures has received considerable attention because of their plasmonic properties in the UV range [1, 2] making them interesting for many potential applications, e.g. photovoltaics [3–6], biosensing and biological imaging [7–9], and tip or surface-enhanced Raman scattering [10–12]. Different types of Al nanostructures have been actively investigated over the past years. For example, Al nanoparticles can be used to enhance the radiative rate of a fluorophore due to plasmonic-enhanced near fields [13]. Another prominent example relates to the use of rough Al thin films as substrates for metal enhanced fluorescence (MEF) [7, 8] or surface enhanced Raman spectroscopy [11] in the UV/blue range. In this context, several 0957-4484/15/115604+07$33.00
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Figure 1. XRR curves of Al thin films deposited by PLD onto SiO2 (a) and Y2O3 (b) for different values of nominal thickness of Al ( tAl ). The 4π scattering vector q is expressed as q = λ sin θ. The red solid line is the result of the fit.
adjusting the deposition parameters, this technique offers the possibility to deposit and control the morphology of a large variety of materials from small metallic nanoparticles, to semi-continuous or continuous ultra-thin layered or multilayered films [24–31]. Most studies dealing with the deposition of metals by PLD concern noble metals for catalytic and optical applications. There are only a few reports in the literature describing the deposition of Al by PLD [19, 32–34]. One reason is the high reflectivity of Al in the first steps of the ablation process (∼92% in the UV range), leading to a low ablation rate. In addition, plume deflection occurring as a result of Al target degradation under UV irradiation can also affect the deposition rate and the quality of the deposited layer [34]. For these reasons, it can be challenging to obtain thick aluminum films (>100 nm) by PLD. However, in the case of ultra-thin films these drawbacks should be limited. We report here on the growth of aluminum thin films onto SiO2 and Y2O3 with thicknesses ranging from 5 to 40 nm. To understand the Al growth process, the synthesized films are characterized by atomic force microscopy (AFM) and x-ray reflectivity (XRR). We comment the effect of the surface wettability on the change of the Al thin structure. At the end, we also outline the use of these nanostructured Al thin films which support UV plasmon for enhancing the luminescence of Eu3+ ions.
was varied by increasing the number of laser pulses taking into account the deposition rate (0.01 nm s−1) previously determined by profilometric measurements (Tencor Alpha Step 100). For luminescence investigations, composite films were prepared by alternating the ablation of three targets of Al, passive Y2O3, and emitting Y2O3:Eu, as described elsewhere [31]. For each material, the deposition rate was first determined by means of a profilometer for Al and m-lines spectroscopy measurements for Y2O3. The number of laser pulses was then adjusted to achieve the desired thickness. Al thin films were characterized by AFM and XRR. AFM (Asylum Research) in tapping mode with a scan rate of 2 Hz was used to investigate the morphology of the Al films. Silicon AFM tips (Tap190Al-G, Budgetsensors) were used to scan the Al films. Three different areas on the sample (5 × 5 μm) were scanned to obtain an average value of the root mean square roughness (rms). XRR analysis was used to investigate the thickness, density, surface and interface roughness of Al and Al/Y2O3 films. XRR spectra were recorded using a Rigaku SmartLab diffractometer. The x-ray source is a rotating anode operating at 9 kW and equipped with a double Ge(220) monochromator to select the CuKα1 radiation (λ = 1.5406 Å). The x-ray experimental reflectivity curves were then simulated using the RCRefSimW software [35]. The fitting parameters were layer thickness, roughness and density associated with each layer. The absorption spectrum of the films was recorded using UV–vis spectrophotometer (Lambda900, Perkin Elmer). The emission spectra were achieved using a tunable pulsed laser EKSPLA NT340 as excitation source. The fluorescence measurements were performed at room temperature with a CCD sensor associated with a monochromator Triax 190 from Jobin-Yvon (1.4 nm resolution).
2. Experimental Aluminum thin films were grown at room temperature by ablating a rotating Al target (99.999% purity, Neyco) by means of an ArF excimer laser (Coherent CompexPro, λ = 193 nm, τ = 17 ns, f = 10 Hz) in a high vacuum deposition chamber (base pressure of 10−7 mbar). The laser beam was focused on the Al target at an incident angle of 45° with respect to the target normal. The laser fluence and substratetarget distance were kept constant at about 2.6 J cm−2 and 4 cm, respectively. Depending on the analysis to be performed, Al films were deposited on Suprasil or silicon (covered with its native oxide) substrates or on 300 nm thick Y2O3 buffer layers previously deposited by PLD. The Al thickness
3. Results and discussions 3.1. Thin films growth
XRR curves as a function of the nominal thickness of Al (tAl) are shown in figures 1(a) and (b) for deposition onto SiO2 and 2
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Table 1. Results of XRR curve fitting for Al films deposited on SiO2 (a) and Y2O3 (b) using a two-layer model (Al2O3 and Al). The thickness
( t ), density ( ρ ) and roughness ( R ) of both layers are listed for various nominal thicknesses of Al ( t XRR
XRR RAl −SiO2
XRR
and
XRR RAl −Y2 O3
XRR
Al ). The interfacial roughness are also given. The roughness of SiO2 and Y2O3 previously estimated by XRR were 0.57 and 1.12 nm, respectively.
(a) Al2O3
tAl (nm) 5 10 20 40
Al
SiO2
XRR tAl (nm) 2O3
XRR ρAl (g cm−3) 2O3
XRR RAl (nm) 2O3
XRR (nm) tAl
XRR (g cm−3) ρAl
XRR RAl (nm)
XRR RAl −SiO2 (nm)
1.49 1.84 1.91 2.14
1.98 1.98 1.98 1.98
0.69 0.60 0.41 0.59
4.46 11.89 21.56 39.76
2.83 2.70 2.73 2.70
0.69 0.40 0.40 0.59
0.26 0.62 0.84 0.30
(b) Al2O3
tAl (nm) 5 10 20 40
Al
XRR tAl (nm) 2O3
XRR ρAl (g cm−3) 2O3
XRR RAl (nm) 2O3
XRR (nm) tAl
XRR (g cm−3) ρAl
XRR RAl (nm)
XRR RAl −Y2 O3 (nm)
2.32 1.71 2.30 2.43
2.29 2.14 2.02 2.23
1.85 1.71 2.12 2.80
5.04 13.25 21.71 39.60
2.83 2.59 2.70 2.70
1.87 1.32 2.86 3.61
1.20 1.50 1.23 1.00
formation of dense films with low porosity, due to the high mobility of Al at the substrate surface. For Al2O3, the density (2.08 ± 0.13 g cm−3) is found to be significantly lower than the theoretical value of bulk Al2O3 (3.96 g cm−3) [38]. Although the origin of that discrepancy between our values and the theoretical one is not clearly understood, similar density values were found for Al2O3–Al–Ni–Co multilayers deposited by high e-beam evaporation [36]. It can be hypothesized that the Al oxidation leads to the formation of a porous outer oxide layer in different oxidized form. Last, the roughness values listed in table 1 show that a rougher surface is obtained when Al is deposited on Y2O3. These results clearly indicate that the roughness and then the morphology of the Al films are related to the properties of the underlying layer. The AFM images depicting the morphology of Al thin films deposited simultaneously onto SiO2 and onto Y2O3 are shown in figure 2. They reveal remarkably different Al growth characteristics depending on the properties of the underlying layer. On SiO2 (figure 2(a)), continuous Al films are observed whatever the film thickness. No aluminum nanostructure is evidenced, indicating a perfect wetting of aluminum on SiO2. This suggests that already in the initial stage of deposition (i.e. within the deposition of the first atomic layers) a complete coverage of the surface is obtained, followed by a growth in height by adsorption of further aluminum atoms arriving on the surface. We can note that the average roughness of Al films are found to range from 0.09 to 0.28 nm and are comparable to that of the substrate (rms = 0.10 nm). The apparent discrepancy with the roughness values obtained by XRR could be due to the size of the AFM tip used (∼10 nm radius), which combined with the roughness present in the sample, give low roughness values. Several studies have thus reported on the fact that different roughness can be obtained by XRR or AFM [39, 40]. In contrast, the
Y2O3, respectively. Note that Al was simultaneously deposited onto SiO2 and Y2O3 through a mask (diameter of 1.5 cm), and that therefore the resulting thickness of Al is expected to be similar on both substrates. The oscillation period is inversely proportional to the layer thickness, while the amplitude contains information on roughness and density. Also, XRR curves exhibit amplified oscillations when Al is deposited on Y2O3 (figure 1(b)). This important feature suggests a modification of the morphology of the Al films with the nature of the substrate. To clarify this point and then extract quantitative informations, the measured reflectivity data were fitted by modeling the deposition. For both substrates, a two-layer model consisting of aluminum with its native oxide (Al2O3) was considered. Such a model is in agreement with the literature, since Al is well-known to be rapidly oxidized upon exposure to air, leading to the formation of an outer oxide layer (up to 3–4 nm thick) [20, 22, 36]. The results of the reflectivity curves fitting (i.e., thickness, roughness and density) for all the samples are listed in XRR is limited table 1. As expected, the thickness of Al2O3 tAl 2 O3 to less than 2.50 nm. The total thickness, corresponding to Al XRR XRR and Al2O3 tAl + tAl , is higher than the nominal 2 O3 thickness ( tAl ). The latter being estimated from profilometric measurements performed on a thick calibrating sample, the relative error increases as the thickness decreases. However, it is important to note that the total thickness is almost similar on both substrates. The density of Al extracted from the fit is 2.72 ± 0.08 g cm−3. This value, which is almost identical whatever the substrate, is close to the density of bulk Al (2.70 g cm−3) [37]. The fact that the density of Al films is close to the theoretical value of bulk Al is due to the high kinetic energy deposition conditions that have been used in this work. Indeed, deposition under vacuum leads to the
(
(
Y2O3
)
)
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Figure 2. AFM images (5 × 5 μm) of Al thin films deposited by PLD onto SiO2 (a) and Y2O3 (b), with the nominal film thicknesses ranging from 5 to 40 nm, as indicated. The surface roughness (rms) evaluated from AFM images is given. The roughness of both substrates (for tAl = 0 nm) is less than 0.1 nm.
Y2O3 substrate (figure 2(b)) yields aluminum films with a macroporous-like structure, suggesting that at the first step of the growth due to its non-wetting behavior Al islands are formed on Y2O3. As the Al amount of matter increases a thin film of coalesced Al islands is grown with a roughness that is reminiscent of the initial aspect ratio of the Al islands. To quantify the changes in surface morphology, we evaluate the surface roughness from AFM images. The roughness slightly increases from 0.84 to 0.94 nm with the thickness, which is about 10 times higher than that of the Y2O3 underlying layer (rms = 0.09 nm). This radically different behavior for the two substrates must reflect a change in the aluminum growth mechanism. The two different growth mechanisms of aluminum, which depend on the surface properties, can be assessed by considering the spreading coefficient (S) expressed as S = γs−(γi + γAl), where γi, γs and γAl, are the surface energies of the metal/surface interface, the substrate surface and the aluminum surface, respectively [41, 42]. If S ⩾ 0, then we can expect a spreading of Al on the substrate (complete wetting). If, on the other hand, S ⩽ 0, the surface prefers to remain ‘dry’, i.e. the metal partially wets the substrate. The surface energies of SiO2 (γSiO2 ≈ 207–605 mJ m−2) [41], Y2O3
To outline the growing process of Al on Y2O3, we carefully analyzed by means of AFM the morphology of the percolated films as a function of the nominal film thickness tAl. Figures 3(a) and (b) show AFM images of Al films obtained for tAl = 5 and 20 nm, respectively. The corresponding AFM profiles are given in figure 3(c). For the lowest tAl, the percolated Al layer can be described as a disordered structure with apparent pore size of about 200 ± 25 nm full width at half maximum (FWHM). For thicker Al depositions, more orderly Al structures with relatively well-formed circular-pores (typical diameter of about 82 ± 17 nm) are observed. These observations indicate that the structure of Al percolated films, i.e. the shape, the density and the diameter of the holes, is controlled by the Al deposition thickness during PLD. More surprising is the evolution of the hole height (h) as a function of tAl, because a maximum of about 6 nm is obtained when tAl ⩾ 20 nm (figure 3(d)). To further understand this evolution, we should consider the aspect ratio of the Al percolated films defined as the ratio of the width to the height (i.e. FWHM/h). This parameter gives an insight into the ability of the incoming species to enter within the structure and to reach the bottom. From the AFM images and their cross section analyses, the aspect ratio is ranging from about 60 to 15 when tAl is increasing from 5 to 40 nm. These values clearly indicate that the species can easily reach the bottom of the Al pores, yielding finally to the formation of a continuous Al layer on the bottom. From this step, the growth of Al at the top and at the bottom becomes comparable and explains why there is no substantial change in the height for tAl ⩾ 20 nm.
( γY2 O3 ⩽ 83 mJ m−2) [43] and Al (γAl ≈ 40–70 mJ m−2) [44] are known with some confidence. In contrast, data on the interfacial energies are scarce in the literature, and we were unable to find values for γAl − SiO2 and γAl − Y2 O3. However, calculating γs − γAl for the two surfaces we consider here and using the values cited above, we find values between 137 and 565 mJ m−2 on SiO2 and ⩽43 mJ m−2 on Y2O3. Assuming positive values for γi, we can see that a negative value of S can be easily obtained on Y2O3, indicating that the dewetting of Al on Y2O3 occurs. In the case of SiO2, we can assume that S ⩾ 0 is more favorable suggesting a perfect wetting of SiO2 by Al. The formation of Al ultra-smooth nanolayers is therefore expected.
3.2. Optical properties
These characterized Al films are used for luminescence enhancement of europium. Eu3+ ions incorporated as doping ions in Y2O3 matrix present a wide UV absorption band corresponding to the Eu–O charge transfer (CT) band and produce intense red emission which can be enhanced by 4
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Figure 3. AFM images (500 × 500 nm) of Al thin films deposited by PLD onto Y2O3 for tAl = 5 nm (a) and tAl = 20 nm (b), and the corresponding surface profiles (c). Height of the Al structures as a function of tAl is shown in (d); the inset is a schematic illustration of the Al growth process on Y2O3.
to those of a reference film, i.e. a single Eu:Y2O3 layer (10 nm thick) without Al. Thus, the emission intensities were normalized in respect to the emission of the reference sample. A value higher than one indicates a fluorescence enhancement, while a value lower than one corresponds to a quenching (i.e. inhibition) of the luminescence (figure 5(c)). One can see that for the Al monolayer architecture a strong quenching (x 0.1) takes place when no spacer is introduced in the structure. A slight decrease of the quenching is then observed when d increases up to 20 nm. The quenching effect involves non radiative desexcitaton of the fluorescent emitter through transfer of energy to the metallic surface [45, 46]. Such mechanism is relevant for distances up to d = 80 nm for coupling with metallic layers [46], whereas it is limited to short distances (few nanometers) in the case of metal nanoparticles [46, 47]. For the Al bilayer architecture, the opposite tendency was evidenced, i.e. the maximum enhancement appears when no spacer is introduced in the structure (∼12 times higher in comparison with the previous architecture). Here, two possible mechanisms can be considered. First, the percolated Al layer could be described as a subwavelength metal nanostructure, which can scatter incident light into various angles leading to an increase of the path length of light in the Eu:Y2O3 layer. This mechanism leads to an increase of the absorption and therefore to the enhancement of fluorescence (quantum yield close to 1). However, such mechanism is only relevant for thicker metal nanostructures, i.e. typically few hundreds of nanometers [48]. The second mechanism is related to the coupling between localized plasmon resonance of the percolated upper Al layer and the europium absorption band in UV. The Al percolated films on Y2O3 promote a local field enhancement close to subwavelength Al nanostructure. As the optical absorption is proportional to the field intensity, a high local field leads to increased absorption in the emitting layer. The increase of d over 10 nm led to the decrease of luminescence down to
Figure 4. Absorption of Al on Y2O3 compare to the UV absorption band corresponding to the Eu–O charge transfer band.
introducing metallic structures close to Eu3+ ions. In that respect, Ray and al., have reported that rough aluminum nanofilms are interesting substrates for MEF in the UV range [7, 8]. MEF enhancement depends on the Al thicknesses and their optimal one was found for 15 nm thick films. Figure 4 shows the absorption spectrum of Al deposited on Y2O3 as well as the CT band of Eu3+ incorporated in Y2O3. The Al nanostructure supports large localized plasmon resonance typical for discontinuous films [11]. As far as we can tell from these measurements, the Al plasmon resonance overlaps the absorption band of Eu–O allowing us to expect a plasmonic coupling effect between Al and Eu3+. To highlight the effect of this overlapping on the fluorescence properties of the emitting layer, multilayers structures consisting of Eu:Y2O3/passive Y2O3/Al (figure 5(a)) or Al/ passive Y2O3/Eu:Y2O3/passive Y2O3/Al (figure 5(b)) were synthetized by PLD. For these both architectures, the thickness of the passive layer was varied from d = 0 to 20 nm. The effect of both architectures on the luminescence of Eu3+ ions at 612 nm was then evaluated, and the results were compared 5
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Figure 5. Effect of Al nanostructure on the fluorescence properties of Y2O3:Eu3+. Investigated architectures consist of (a) Y2O3:Eu3+
phosphor layer (10 nm) deposited on a continuous Al layer (tAl = 15 nm), and (b) the same as (a) with a percolated Al layer deposited on the top (tAl = 15 nm). A Y2O3 passive layer is inserted between the phosphor and the Al layers. (c) Enhancement and quenching of Y2O3:Eu3+ fluorescence as a function of the thickness d between Al and Y2O3:Eu3+; typical emission spectra under excitation at λexc = 240 nm corresponding to 5D0 → 7F0,1,2 transitions of Y2O3:Eu3+ are shown in inset for D = 0.
around 0.4, as shown in figure 5(c). Here, we can suppose that (i) there is a relative contribution of the percolated upper Al layer because we are out of range of the localized plasmon effect, and (ii) the quenching due to the Al bottom layer is then predominant. The mechanisms responsible of such enhancement as a function of d are not yet completely elucidated, but this is an experimental fact. In order to understand the impact of each structure as a function of the thickness, other multilayer architectures are investigated in the moment, which is however out of scope of this paper.
experimentally the potentiality of Al percolated layers for the fluorescence enhancement of emitting layers, such as Y2O3: Eu3+. Other multilayer architectures are under investigations in order to explore the ability of Al plasmonic nanostructures for the luminescence enhancement of emitting layers.
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4. Conclusion Ultra-smooth and percolated nanosized Al films were obtained by PLD on SiO2 and Y2O3, respectively. The Al film growth modes have been analyzed by XRR and AFM analysis. Based on values for surface energies reported in the literature, we concluded that the growth mode of Al depends on the wettability of the substrate surface. For ultra-smooth Al film on SiO2, the Al thickness increases with the number of laser pulses, and the roughness is estimated by AFM to range from 0.09 to 0.28 nm comparable to that of SiO2 substrate. For percolated Al film on Y2O3, the pore size decreases from 200 to 82 nm as the Al thickness increases from 5 to 40 nm, respectively. Beyond an Al thickness of 20 nm, a progressive fill of the pores is observed, however the depth remains stable. According to absorption measurement, the plasmon resonance of Al percolated layers is localized in UV range. A first approach to the understanding of the importance of Al films in plasmonic systems is proposed. We show 6
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