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Electronic Structure of Core/shell Metal/oxide Aluminium Nanoparticles Giulia Maidecchi, Chinh Duc Vu, Renato Buzio, Andrea Gerbi, Gianluca Gemme, Maurizio Canepa, and Francesco Bisio J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07678 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 13, 2015
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The Journal of Physical Chemistry
Electronic Structure of Core/Shell Metal/Oxide Aluminium Nanoparticles Giulia Maidecchi,†,⊥ Chinh Vu Duc,‡,† Renato Buzio,¶ Andrea Gerbi,¶ Gianluca Gemme,§ Maurizio Canepa,k,§ and Francesco Bisio∗,¶ †Dipartimento di Fisica, Universit`a di Genova, via Dodecaneso 33, I-16146 Genova, Italy ‡Institute of Materials Science - Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay District, Ha Noi, Viet Nam ¶CNR-SPIN, C.so Perrone 24, I-16152 Genova, Italy §INFN, Sezione di Genova, via Dodecaneso 33, I-16146 Genova, Italy kDipartimento di Fisica, Universit`a di Genova and CNISM, Sede Consorziata di Genova, via Dodecaneso 33, I-16146 Genova, Italy ⊥Present address: Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy E-mail:
[email protected] 1 ACS Paragon Plus Environment
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Abstract We report a combined X-ray photoelectron spectroscopy and atomic-force microscopy study of ultrafine (< 30 nm) Al/Al-oxide core/shell nanoparticles. A complex fine structure is observed within O1s and Al2p core-level electronic spectra of the particles, indicative of a corresponding complex system morphology. The photoemission spectra of the Al metallic core exhibited a low-binding-energy component, ascribed to strongly undercoordinated metallic Al atoms in the particle core. Such a fraction of undercoordinated metallic atoms grows larger in relative weight as the particles get smaller, finally becoming completely dominant for nanoparticle size below 20 nm. This feature is interpreted as the fingerprint of vacancy-cluster formation within the metallic core as an effect of oxidation. For the smallest particle size investigated (< 10 nm), vacancy clusters coalesce leading to the core disruption, with a heavy impact on the corresponding electronic and plasmonic response.
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Introduction
Aluminum nanoparticles (NPs) and nanostructures are extremely appealing due to their potential application as low-cost plasmonic systems endowed with broadband plasmonic functionality extending from the visible to the deep-ultraviolet (DUV) photon-energy range. 1–26 The plasmonic response of aluminium NPs is extremely sensitive to the NP size and shape, 2,3,6,8,17 much more than the conventional plasmonic metals Au and Ag. This implies that the control over the NP morphology and composition is of paramount importance, the more so when DUV plasmonics, calling for NP size around or below the 20-nm mark, is involved. Aluminum undergoes a fast surface oxidation in atmosphere, leading to the quick formation of a 2-3 nm thick oxide layer. 27 Such a native oxide has a strong influence on the plasmonic response, leading to a significant redshift of the plasmon resonances, that can be also positively exploited to extend the application range of Al NPs towards the visible-light regime. 1–3,6–9,11 The impact of oxidation clearly becomes increasingly relevant for smaller NP 2 ACS Paragon Plus Environment
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size, where the mass transport associated with the anion/cation migration and their different migration velocity may strongly affect the morphology of the metallic core. 27,28 Understanding the morphology and composition of ultrafine metal/oxide Al core-shell NPs is therefore relevant for harnessing the fabrication procedures of Al nanostructures, and understanding their chemical and physical response for all the applications in atmosphere. Despite the relevance of the subject and the growing interest in Al NPs, however, the objective difficulty of fabricating very fine Al particles and of addressing this system have strongly limited the studies of Al NPs morphology and composition in the
