Nanopatterning and plasmonic properties of plasma sputtered gold on diatom frustules
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Nanopatterning and plasmonic properties of plasma sputtered gold on diatom frustules Julien Romann and Mari-Ann Einarsrud* Department of Material Science and Engineering Norwegian University of Science and Technology, NO-7491 Trondheim - NORWAY ABSTRACT Bio-silica nanostructures from diatoms (called frustules) featuring plasmonic gold nanoparticles (NPs) are elaborated using two methods based on plasma sputtering of gold. The first investigated method uses a thermal treatment to induce the thermal dewetting of a plasma sputtered gold layer on the diatom frustules. The second method first consists of coating the frustules with polyethylene glycol before sputtering gold on these frustules. For both methods, the amount of gold appears to be a key parameter regarding the final obtained layer, which can either be nanostructured by cavities or consist in individual gold NPs. For an amount of sputtered gold equivalent to form a 5 nm thick layer, both methods allow obtaining diatom frustules covered by gold NPs with a size around 20 nm and a narrow size distribution. The UV-visible characterization of the diatom frustules featuring gold NPs highlights a plasmon extinction band in agreement with individual gold NPs with a size below 25 nm. INTRODUCTION The plasmonic properties of noble metal nanoparticles (NPs) have been subject to widespread interest in recent years [1-9]. Many promising applications involving local surface plasmon resonance (LSPR) still drive a strong research activity in this area. More recently, the elaboration of plasmonic systems combining noble metal NPs and bio-synthesized photonic crystals opened a new way towards innovative optically active nanostructures [10]. In parallel, the use of diatoms for their particular optical properties is a field of increasing interest [11-17]. These unicellular algae build complex bio-synthesized silica nanostructures called frustules. The special optical behavior of the frustules, including light focusing effects [13], has been shown to result from photonic crystal properties [15], which are suspected to help the diatoms enhancing their photosynthetic activity [11]. This work aims at developing a simple method to combine the complex nanostructure of diatom frustules with the plasmonic properties of gold NPs. Several methods can be used to generate gold NPs on a substrate. Chemical routes often involve surfactants limiting the coalescence of the gold NPs, but also providing some control over their shape [6,9]. However, surfactant molecules often interact with the surface of the gold NPs influencing their resulting LSPR properties. On the other hand, physical methods do not involve any stabilization at the surface of the gold NPs, hence giving less control over their shape. The present study investigates two different methods using plasma sputtering to elaborate gold NPs at the surface of diatom frustules. The first method, based on the thermal dewetting of a gold layer, has already shown promising results on patterned substrates [18] and on one diatom species [10]. Th
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