Reduction of Plasmon Damping in Aluminum Nanoparticles with

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Reduction of Plasmon Damping in Aluminum Nanoparticles wth Rapid Thermal Annealing Feifei Zhang, Julien Proust, Davy Gerard, Jerome Plain, and Jérôme Martin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00909 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 3, 2017

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The Journal of Physical Chemistry

Reduction of Plasmon Damping in Aluminum Nanoparticles with Rapid Thermal Annealing Feifei Zhang, Julien Proust, Davy Gérard, Jérôme Plain, and Jérôme Martin∗ Laboratoire de Nanotechnologie et Instrumentation Optique, Institut Charles Delaunay, UMR CNRS 6281, Université de Technologie de Troyes, France E-mail: [email protected]

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Abstract Aluminum is now widely regarded as a promising plasmonic material, especially in the ultraviolet spectrum. In this paper, we propose rapid thermal annealing (RTA) as a simple method to significantly decrease the amount of intrinsic losses in aluminum nanoparticles. We study the structural and optical properties of aluminum nanoparticles before and after RTA at different temperatures. Our results unveil how RTA affects the plasmonic properties of Al nanoparticles through the competition between the reduction of the number of grain boundaries and oxidation. If RTA is performed below a threshold temperature of 400o C, oxidation is extremely weak and the plasmonic resonances sustained by Al nanoparticles are blue shifted with a decrease of their full width at half maximum. This improvement is due to a diminution of the number of grain boundaries inside the metal core. Hence, RTA appears as a simple, cost-effective and up-scalable technique to improve the plasmonic properties of aluminum. In contrast, above the threshold temperature, oxidation becomes predominant, resulting in a detrimental effect on the plasmon resonance. This effect should be taken into account in any industrial process involving heated Al nanoparticles.

Introduction The development of plasmonics outside the now well-explored visible and near-infrared ranges requires new materials capable of sustaining plasmon resonances. Aluminum (Al) appears as one of the most appealing candidates and is currently being rediscovered as a relevant material for plasmonics. 1–8 Aluminum is known for a long time to exhibit plasmonic properties, as it is actually the metal on which surface plasmons were first evidenced back in the 1950s. 9 However, it is only in recent years that a significant number of papers started to develop what is now called "aluminum plasmonics". 1 The main reason behind this regain of interest stems from aluminum excellent optical properties in the ultraviolet (UV) spectrum - a region where the classical plasmonic metals (i.e. gold and silver) cannot be used. Gold does not 2


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exhibit plasmonic resonances at wavelengths shorter than 520 nm due to its interband transitions. Silver nanostructures exhibit plasmonic resonances down to the very near UV (350 nm) but suffer from strong oxidation and lose their plasmonic properties over time. Another major interest of aluminum as a plasmonic material lies in its broadband nature: apart from a small region around λ=800 nm 10 (corresponding to the interband transition), aluminum sustains plasmonic resonances from the infrared to the deep-UV. Additionally, this metal is cheap and abundant, non-toxic, and compatible with metal-oxide-semiconductor (CMOS) technology. Moreover, Al does not oxidize in depth as it naturally forms a few nm thick selfprotecting oxide layer 11 making it chemically stable in time. Al nanoparticles have already found numerous applications including non-linear optics, 12 enhanced fluorescence, 13,14 UVSERS, 15 optoelectronics, 16 plasmonic assisted lasing, (by coupling Al with wide bandgpap semiconductors such as GaN), 17 photocatalysis, 18,19 structural colors 20 and data storage. 21 However, even if a self-limited oxide layer protects Al nanoparticles, their stability when exposed to various thermal processes is a key issue hindering their systematic use in industry. In this paper, we investigate the effect of Rapid Thermal Annealing (RTA) on arrays of Al nanoparticles made by Electron Beam Lithography (EBL). Particularly, we study the optical response of Al nanoparticles when annealed in air inside a RTA furnace at different temperatures and under atmospheric pressure. When the RTA temperature is lower than or equal to a threshold value of 400◦ C, the localized plasmonic resonances (LSPR) sustained by Al nanoparticles experience a blueshift with a simultaneous decrease of their full width at half maximum (FWHM). This effect is explained by the diminution of the number of grain boundaries inside the metal while oxidation remains very weak, improving the quality of the resonances. Above the threshold temperature, RTA affects differently the nanoparticles as oxidation becomes predominant, and a more complex variation of the resonances’ peak wavelength as well as a decrease of their intensity is observed. In brief, RTA affects the plasmonic properties of Al nanoparticles due to a competition between the reduction of grain boundaries and oxidation.

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Experimental methods Fabrication of the nanoparticles Quartz coverslips (AGAR reference L4465, thickness 250µm) were cleaned in a 50:50 acetone/isopropanol solution at 40◦ C in ultrasonic bath during 10 minutes. The same process was then applied in pure isopropanol followed by drying under N2 . Then coverslips were spincoated with layers of polymethyl methacrylate (160 nm of PMMA) and conductive polymer respectively followed by a 180◦ C and a 60◦ C baking on hot plate during 10 minutes. After exposition by an electron beam (with typical doses of 200 µC/cm−2 ) with EBL control unit (SEM FEG E-Line, Raith), the insolated patterns in PMMA layer have been revealed in a 1:3 MIBK:isopropanol solution during 60 s resulting in square arrays (50 x 50 µm2 ) of circular nanoapertures with diameters set to 80, 100 and 120 nm. The pitch between the circular apertures is set to 250 nm. Subsequently, a 40 nm thick aluminum film has been deposited on the sample by electron beam evaporation (Plassys MEB 400) at a constant deposition rate of 1.5 Å/s. The residual pressure in the deposition chamber was 5 × 10−6 Torr. Finally the resist has been lifted-off with acetone resulting in a quartz coverslips with well-defined Al nanoparticles with above mentioned diameters. Theses values have been chosen in order to get plasmonic resonances in the blue to near UV region, as this spectral range is of great interest in Al plasmonics. For TEM measurements, Al nanoparticles have been lithographed on TEM-compatible substrates. The latter consists in 20 nm thick Si3 N4 square membranes engraved in silicon circular wafers (NEYCO reference 4159SN-BA).

UV-VIS extinction spectroscopy Extinction measurements have been performed using a homemade extinction microscope. The sample is illuminated with an unpolarized lamp (Oriel, Series Q Deuterium Light Sources). The transmitted signal is collected by an objective lens and then spatially filtered thanks to an optical fiber (core diameter 200 µm) acting as a confocal pinhole. This 4


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defines a collection area of ∼ 40 µm2 on the sample surface. The collection spot has been carefully placed in the center of the lithographed areas for every extinction measurement.

Rapid Thermal Annealing All rapid thermal annealing processes were done in a JetFirst100/150 lamp-heated RTA furnace during 5 minutes in ambient atmosphere (i.e. the external atmosphere was simply let into the furnace).

SEM and TEM measurements Both SEM and TEM (Transmission Electron Microscopy) measurements have been done in a FEG system (Hitachi SU 8030). In TEM mode, the acceleration voltage was set to the maximum value of 30 kV allowing for the characterization of the grain boundaries inside the metal.

FDTD calculations Finite difference time domain method (FDTD) has been used to compute the optical response of Al nanoparticles. We used OptiFDTDTM software from the Optiwave Company. 3D modeling has been used, with PML as boundary conditions on both the top and bottom sides of the simulation box, while periodic boundary conditions were used on the remaining four sides. Refractive indices were taken from Ref. 22 Incident light is a plane wave, impinging from the substrate side.

Results and discussion The general principle of the experiment is depicted in Fig. 1, showing two typical extinction spectra measured on arrays of Al nanoparticles before (Fig. 1a) and after (Fig. 1b) annealing at 400◦ C. A peak centered around 420 nm corresponding to the plasmonic dipolar resonances 5



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sustained by the Al nanoparticles is evidenced on the pristine sample. After annealing, the peak wavelength is shifted to a value slightly below 400 nm while its FWHM is reduced by a factor ∼ 1.5. As evidenced in the following, we attribute this effect to a reduction of grain boundaries inside the metal during the annealing. Other studies 23 have shown that at this temperature, no additional or very few oxidation occurs inside the metal core of the nanoparticles. The native oxide layer remains amorphous under 400◦ C, with a constant thickness. It is worth noticing that the SEM images (Fig. 1, insets) evidence that the RTA process at 400◦ C does not affect the shape of the nanoparticles, whose cross-section remains circular. We can therefore argue that the effect of RTA on the plasmonic properties is not due to a deformation or reshaping of the nanoparticles. This stability is actually due to the high adhesion coefficient between Al and the substrate hindering any efficient dewetting process even at high temperature. 2

Evolution of the plasmonic properties with the temperature during RTA We now present a more detailed investigation of the plasmonic properties dependence on the annealing temperature. Extinction measurements on the Al nanoparticles after successive RTA treatments with increasing temperature have been carried out. Results are summarized in Fig. 2. Extinction spectra measured on arrays of Al nanoparticles after each step of the RTA process with temperature ranging from 300 to 800◦ C are plotted in Fig. 2a-c for three different diameters. Whatever the diameter of the structure, a clear trend is discernible: the dipolar plasmonic resonances experience an increasing blueshift accompanied with a reduction of their FWHM for annealing temperatures of 300 and 400◦ C. Above this temperature, the resonances behave differently. With RTA at 500◦ C resonances experience then a more or less pronounced redshift and a decrease in intensity; this effect is more pronounced for the smaller diameters. For temperatures equal to or above 600◦ C, the intensity of the resonances further decreases while they experiences a slight blueshift. This effect is particularly 6


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and thus of the peak intensity. Simultaneously, the increasing oxide shell thickness tends to redshift the spectral position of the LSPR, due to the higher refractive index of the surroundings. 11,26 Increasing the oxidation layer therefore leads to a more complex behavior where a redshift followed by a blueshift is experienced by the resonances as shown further by calculations. Such a behavior has been numerically predicted in the case of aluminum hemispheres. 27

Number of grain boundaries and plasmonic resonances improvement Depending on the temperature range, two distinct regimes are observed, where one or the other of the two suggested mechanisms dominates. This is illustrated in Fig. 2.d-f, where the evolution of both the resonances’ peak wavelength and FWHM have been plotted as a function of RTA temperature, for each diameter. In the first temperature range, (between 300 and 400◦ C), oxidation is extremely weak and the number of grain boundaries decreases (see below). Hence, the dipolar resonances experience a blueshift with a significant reduction of FWHM. At 400◦ C, FWHM is reduced by a factor ∼ 1.5 for every diameters and the blueshift is maximum. Above this threshold value (RTA at T = 500◦ C), the resonances experience a redshift while their intensity decreases, indicating that oxidation becomes predominant. Simultaneously, a slight increase of the FWHM is evidenced. The FWHM and the peak wavelength value are not plotted for RTA at highest temperatures (from 600 to 800◦ C) because the spectra present more complex behavior, due to the simultaneous shrinkage of the metal core and increase of the oxide layer as explained above. In addition, this behavior at highest temperatures will be explained further thanks to FDTD calculations. The previous assumption concerning the evolution of grain boundaries under thermal annealing has been ascertained by characterizing the structural properties of Al nanoparticles by TEM (Fig. 3). Two pristine Al nanoparticles (before annealing) and the same nanoparticles after 400◦ C RTA annealing were characterized. The polycrystallinity of the pristine structures is evidenced in the TEM image by the grains, highlighted by the dashed yellow lines on Fig. 3. The same 9



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nanoparticles after annealing exhibit fewer grain boundaries, likely due to grain-boundary migration at high temperatures as already shown for gold nanoparticles. 24 However, in the aforementioned work the gold nanoparticles needed to be encapsulated prior to annealing. In our case, the native oxide layer self-encapsulates the aluminum nanoparticles, removing the need for this step. The diameter and morphology of the nanoparticles after 400◦ C RTA remain the same as the pristine one, as expected. Consequently, the improvement of the dipolar plasmonic resonances quality factor Q, defined as:

Q=

λLSPR FWHM

can be attributed to a decrease in the number of grains boundaries inside the Al core. This leads to a lower number of scattering centers for electrons, increasing their mean free path and subsequently the lifetime of the plasmonic resonances. This results in the lowering of the imaginary part of the metal core’s dielectric function. This was recently observed in epitaxially-grown aluminum thin films compared to conventional thermally evaporated films. 28 We summarize the improvement of the plasmonic dipolar resonances in Fig. 4 by plotting the quality factor Q for several nanoparticles as a function of the RTA temperature. For every nanoparticle, a clear improvement of Q at 400◦ C is evidenced, with a relative increase by a factor of ∼ 1.4. The Q-factor is even higher than the ones measured for the dipolar modes of Al nanorods. 29 For smaller nanoparticles, even if the relative improvement is the same, the absolute values of Q are globally higher. We attribute this to the intrinsic losses in Al, which are lower for shorter wavelengths (for instance, ε2 (λ = 450nm) ≈ 2 × ε2 (λ = 350nm)). Whatever the diameter of the nanoparticles, the Q-factor decreases for RTA at temperatures above 500◦ C due to the increased oxidation and deformation as shown below.

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Figure 3: TEM images on the same single Al nanoparticles with diameter 150 nm (upper boxes) and 120 nm (lower boxes) pristine (blue boxes) and after RTA treatment at 400◦ C (red boxes). Scale bars 50 nm.

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nm, oxide thickness 0 nm) and the lower one to strongly oxidized nanoparticles (Al diameter 40 nm, oxide thickness 30 nm). The dipolar resonance experiences first a redshift due to the oxidation (actually due to the high refractive index of alumina) until the metallic core reaches a diameter of 80 nm, corresponding to an oxide layer of 10 nm. At this point, the redshift value is ∼ 8 nm. If the diameter further decreases, the blueshift due to the reduction of the metal core size overcomes the redshift due to the refractive index of alumina, resulting in a net blueshift of the resonance position as well as a decreasing intensity. Finally, when the metal core reaches a diameter of 40 nm, a maximum blueshift of the dipolar resonance is obtained as well as a dramatically decreased intensity, which is lowered by a factor ∼ 5. For lower diameters of the metal core, the optical signature of the dipolar resonance further decreases and eventually vanishes away (not shown here). As mentioned above, a similar behavior has been numerically observed for Al/Al2 O3 hemispheres. 27 Halas’s group 31 has also experimentally studied the effect of oxidation on the plasmonic properties of Al nanoparticles. They showed similar trend that observed here, redshift and decrease of resonances’ amplitude. These calculations explain the behavior of experimental data in Fig. 2 for RTA from 400 to 800◦ C. Once Al nanoparticles have reached their maximum structural improvement due to the reduction of grain boundaries, oxidation starts to dominate during RTA at higher temperatures and the oxide layer starts increasing. Consequently, dipolar resonances experience first a redshift and then a blueshift, while their intensity decreases. Eventually, oxidation is strong enough during the 5 minutes of RTA to almost transform the entire metal core resulting in very weak optical signature as experimentally shown for the smallest diameters.

Conclusion To conclude, a study on the evolution of plasmonic resonances sustained by aluminum nanoparticles under rapid thermal annealing in ambient air has been reported. The results evidenced the existence of two regimes during annealing. The first regime occurs with

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Acknowledgement Financial support of Nano’Mat (www.nanomat.eu) by the Ministère de l’enseignement supérieur et de la recherche, the Conseil régional Champagne-Ardenne, the FEDER fund and the Conseil général de l’Aube is acknowledged. Feifei Zhang thanks the Chinese Scholarship Council for funding his PhD scholarship in France.

Supporting Information Available The following files are available free of charge. • S1: AFM measurements on Al thin films (thickness 40 nm) before and after RTA at different temperatures. • S2: AFM measurements on large diameter Al nanoparticles (diameter 400 nm) before and after RTA at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org/.

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The Journal of Physical Chemistry Al 2O3

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30

Optical density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53



360

380


460

480

500

pristine 1 2 3 4 5 6 7 8 9 10 11

T = 400°C



26 of 27 T =Page 600°C

Page 27 The of Journal 27 of Physical Chemistry

1 2 3 4


Reduction of Plasmon Damping in Aluminum Nanoparticles with

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