Self-assembled aluminum nanoparticles for UV plasmonic based nano-optics Contact: Jerome.plain@utt.fr or Jerome.martin@utt.fr For several decades the optical properties of metallic NanoParticles (NPs) known to support Localized Surface Plasmon Resonances (LSPRs) have been almost exclusively studied in the visible region by using NPs made of gold or silver. Indeed, these noble metals support LSPRs in the visible and near-IR range. LSPRs are coherent, collective oscillation of the surface conduction electrons of the NPs [1]. When optically excited at the resonance frequency, LSPRs result in a strong and locally confined electromagnetic field in the vicinity of the NP, which intensively absorbs and scatters light. The resonance properties, frequency and linewidth, depend on the NP’s material, shape, and the refractive index of the surrounding medium. Consequently, NPs exhibit extremely attractive nano-optical properties allowing various technological applications such as ultra-sensitive sensing [2], surface enhanced Raman spectroscopy (SERS) [3], and metal-enhanced fluorescence [4].

Emerging applications will require the extension of nanoplasmonics toward higher energies, in the UV-range. However, the development of nanoscale devices with optical functionalities in the ultraviolet (UV) spectral range remains poorly addressed. For instance, plasmonics coupled with wide bandgap semiconductors, such as ZnO or GaN, could lead to more efficient light emission or harvesting devices [5]. Other applications include Surface Enhanced Raman Spectroscopy [6] (the intensity of Raman emission increases with the fourth power of the frequency of the source) or fluorescence enhancement [7, 8]. However, only a few studies of LSPRs in the UV region have been reported, implying that UV plasmonics is still in its infancy. The reported works related to UV LSPR use metals such as Ga, In, Pd, Pt or Al. The real part of the dielectric function of aluminum is negative in the visible and UV regime, down to a wavelength of 100 nm, with low losses. The excellent optical properties of aluminum have already been exploited to create UV optical antennas [9, 10] and to enhance the fluorescence of wide bandgap semiconductors [11].

Figure 1. (a) and (b): Normalized histogram of

diameters dispersion of Aluminum nanoparticles ((a) made on 1200 MNPs and (b) on 250 MNPs) by the both process: (a) chemical method: reduction of AlCl3 by Na @ 25 °C and (b) 175 °C.

Figure 2. AFM images of Au NPs self-

assembled into nanoring from reference [12] allowing from the tuning of plasmonic properties in the VIS-IR range.

This project is to develop Al NPs based nanostructures with tunable UV plasmonics properties by combining promising fabrication techniques with powerful characterization tools. Particularly, the project aims the fabrication and the optical characterizations of self-assembled aluminum NPs. The fabrication method of aluminum NPs is still in development in our laboratory and is based on the reduction of aluminum ions (present in aluminum chloride AlCl3) by metallic sodium. Much more subtle chemical mechanisms are involved in this reaction but are not described here and a patent is under progress. The resulting Al NPs are depicted in the SEM images in figure 1. The Al NPs support well defined UV plasmonic resonances as it as been shown by UV-VIS extinction spectroscopy. Those resonances can be tuned typically between 200 nm and 400 nm. The goal of this project is to control the location of such Al NPs on the substrate using self-assembly methods. We already developed such method for Au NPs allowing for the creation of self-assembled Au nano-rings allowing the possibility of tuning the plasmonic properties on the IR and visible12. The self-assembly is evaporation-induced and is driven using a template of a monolayer of dielectric microspheres. The resulting Au nano-rings made of Au NPs (playing the role of the building blocks) are depicted in the AFM micrograph on figure 2. By combining this method with the synthesis of Al NPs finalized in our laboratory, the possibility of controlling the plasmonics properties of such self-assembled structures in the UV range will be studied. The optical characterizations will be analyzed using a homemade confocal microscope operating in the UV-VIS range. Subsequent applications, for instance fluorescence or UV absorption enhancement will be also experimented.  References  [1]  A. V. Zayats, I. I. Smolyaninov, and A. A. Maradudin, “Nano-optics of surface plasmon polaritons,” Phys. Rep. 408, 131–314 (2005).  [2]  A. V. Kabashin, P. Evans, S. Pastkovsky, W. Hendren, G. A. Wurtz, R. Atkinson, R. J. Pollard, V. A. Podolskiy, and A. V. Zayats, “Plasmonic nanorod metamaterials for biosensing,” Nat. Mater. 8, 867–871 (2009). [3]  F. De Angelis, F. Gentile, F. Mecarini, G. Das, M. Moretti, P. Candeloro, M. L. Coluccio, G. Cojoc, A. Accardo, C. Liberale, R. P. Zaccaria, G. Perozziello, L. Tirinato, A. Toma, G. Cuda, R. Cingolani, and E. Di Fabrizio, “Breaking the diffusion limit with super-hydrophobic delivery of molecules to plasmonic nanofocusing SERS structures,” Nat. Photonics 5, 682–687 (2011). [4]  D. Gerard, J. Wenger, N. Bonod, E. Popov, H. Rigneault, F. Mahdavi, S. Blair, J. Dintinger, and T. W. Ebbesen, “Nanoaperture-enhanced fluorescence: towards higher detection rates with plasmonic metals,” Phys. Rev. B 77, 045413 (2008). [5]  R. Mupparapu, K. Vynck, I. Malfanti, S. Vignolini, M. Burresi, P. Scudo, R. Fusco, and D. Wiersma, “Enhanced downconversion of UV light by resonant scattering of aluminum nanoparticles,” Opt. Lett. 37, 368–370 (2012). [6]   S. K. Jha, Z. Ahmed, M. Agio, Y. Ekinci, and J. F. Lo ̈ffler, “Deep-UV surface-enhanced resonance Raman scattering of adenine on aluminum nanoparticle arrays,” J. Am. Chem. Soc. 134, 1966–1969 (2012). [7]  K. Ray, M. H. Chowdhury, and J. R. Lakowicz, “Aluminum nanostructured films as substrates for enhanced fluorescence in the ultraviolet-blue spectral region,” Anal. Chem. 79, 6480–6487 (2007). [8]  M. H Chowdhury, K. Ray, S. K Gray, J. Pond, and J. R. Lakowicz, “Aluminum nanoparticles as substrates for metal-enhanced fluorescence in the ultraviolet for the label-free detection of biomolecules,” Anal. Chem. 81, 1397–1403 (2009). [9]  A. Taguchi, Y. Saito, K. Watanabe, S. Yijian, and S. Kawata, “Tailoring plasmon resonances in the deep-ultraviolet by size-tunable fabrication of aluminum nanostructures,” Appl. Phys. Lett. 101, 081110 (2012).  [10]M.Knight,L.Liu,Y.Wang,L.Brown,S.Mukherjee,N.S.King,H.O.Everitt,P.Nordlander,and N.J.Halas, “Aluminum plasmonic nanoantennas,” Nano Lett. 12, 6000–6004 (2012). [11]  K. Wu, Y. Lu, H. He, J. Huang, B. Zhao, and Z. Ye, “Enhanced near band edge emission of ZnO via surface plasmon resonance of aluminum nanoparticles,” J. Appl. Phys. 110, 023510 (2011). [12] Thomas Lerond, Julien Proust, Hélène Yockell-Lelièvre, Davy Gérard, and Jérôme Plain, " Self-assembly of metallic nanoparticles into plasmonic rings ", Appl. Phys. Lett. 99, 123110 (2011)