Near‐Field Optical Properties of Fully Alloyed Noble Metal Nanoparticles

Abstract

1 of 6) 1600568 dielectric function. Alloying of these noble metals has been applied to tune the material dielectric function, where the LSPR can be modulated progressively from the UV (pure Ag) to the NIR (pure Au). [12][13][14][15] Thus, metallic nanostructures composed of Ag-Au can enable the rational design of building blocks for different applications, such as metamaterials, [16,17] hot carrier devices, [18] light absorption improvement in photovoltaics, [19,20] colored glasses, [21] displays, [22,23] and catalysis. [24] To date, different fabrication techniques have been successfully utilized to realize Ag x Au 1−x alloyed NPs. They can be formed by colloidal synthesis via the reduction of precursors containing metals in solution, [25,26] and by the sequential pulsed laser deposition of Ag and Au targets, [27,28] which can yield large amounts of NPs with narrow size distribution. However, the overall size of the NPs cannot be varied beyond 150 nm. [29] Alternatively, nanolithographic methods enable full control of NPs size, shape, and distribution. [13] Nevertheless, this technique is constrained to specific applications due to its high cost and very limited scalability. The dewetting of metallic thin films has also been used to fabricate pure [21,[30][31][32] and alloyed [19] metal NPs. In this simple and effective fabrication route, a very thin layer of metal (<50 nm) is initially deposited onto a substrate. Then, when the thin-film sample is annealed under a controlled environment (oxygen free), surface diffusion takes place and results in the formation of nanostructures to minimize the energy of the system. [33][34][35][36] This method has been particularly useful for optoelectronic devices, where these metallic NPs act as light scattering centers that ultimately increase light absorption within the semiconductor. [4,37] In this work, we fabricate fully alloyed Ag x Au 1−x NPs with controlled chemical composition by dewetting thin films and characterize their optical response at the macro-and nano-scale. Surprisingly, we find that the NPs' distribution heavily depends on the thin-film chemical composition, irrespective of the original film thickness. Simultaneously, we measure a shift of the LSPR due to the NPs' composition variation, which defines their optical response. We map the elemental distribution of Ag and Au and confirm that the NPs are fully alloyed, forming a solid solution at the nanoscale. To further illustrate how the chemical composition affects the material optical response, we perform a detailed analysis of the optical characteristics of fully alloyed Ag 0.5 Au 0.5 nanostructures in the visible range of the spectrum. For that, we combine spectrally dependent NSOM measurements and finite-difference time-domain (FDTD) simulations to locally resolve the optical response of individual NPs. Our results of the near-field light-matter interactions for Ag 0.5 Au 0.5 nanostructures reveal an electric field enhancement of 30 times in the visible range of the spectrum under the NPs Combining meta...

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