Substrate Dependence in the Formation of Au Nanoislands for Plasmonic Platform Application

Plasmonics https://doi.org/10.1007/s11468-019-01021-9 Substrate Dependence in the Formation of Au Nanoislands for Plasmonic Platform Application Marcin Łapiński 1 1 2 1 & Robert Kozioł & Anita Cymann & Wojciech Sadowski & Barbara Kościelska 1 Received: 17 June 2019 / Accepted: 12 August 2019 # The Author(s) 2019 Abstract In this work, the influence of the various substrates on Au nanoisland formation has been studied. Nanostructures were obtained via annealing of thin Au films. In order to determine nanoisland formation mechanisms, correlation between an initial film thickness and temperature of formation, shapes, and dimensions of nanostructures was examined. For the surface morphology studies, nanograin structure, and chemical composition analysis, SEM, HR TEM, and EDS measurements were performed, respectively. Morphology studies showed that the temperature at which nanostructures form varies for different substrates, which indicates high impact of the substrate material on the nanostructure formation. In the case of silicon substrate, besides the phenomenon of spinodal dewetting, the effect of eutectics on the nanostructures was additionally taken into consideration. Keywords Directional solidification . High-resolution electron microscopy (HREM) . Nanostructure . Scanning electron microscopy (SEM) . Thin-film annealing Introduction In recent years, much attention has been paid to the formation of noble metal nanostructures. This is mainly due to the possibility of using them as plasmonic platforms and nanosensors [1, 2]. One of the most commonly used metals in plasmonic platforms is gold. In addition to the high concentration of free electrons, leading to a high plasma frequency and a negative real permittivity over a wide range of frequencies, gold is characterized by high chemical resistance, which greatly facilitates the process of producing platforms [3]. The frequency and intensity of the plasmon resonance are highly dependent not only on the material of the nanostructures but also on its size, shape, and morphology. There is also a strong correlation * Marcin Łapiński 1 Faculty of Applied Physics and Mathematics, Department of Solid State Physics, Gdansk University of Technology, Gabriela Narutowicza 11/12, 80-233 Gdansk, Poland 2 Faculty of Chemistry, Department of Chemical Apparatus and Theory of Machines, Gdansk University of Technology, Gabriela Narutowicza 11/12, 80-233 Gdansk, Poland with dielectric properties of surrounding medium [4–6]. One of the easiest methods for the production of metal nanostructures is the way that is based on the thermal annealing of thin metal films [7, 8]. It is well known that several processes could be responsible for the formation of the nanostructures within this technique. One of them is directional solidification of eutectics that is widely cited in the literature. It may occur in the case of the gold film that is deposited on a silicon substrate. The melting temperature of Au–Si eutectic system drops to about 363 °C [9–12], so nanostructures can be formed in the temperatures much below the melting temperature of Au. However, it can lead to phase non-homogeneity of the gold nanostructures [10, 12–15]. On the other hand, in the cases of thin and ultra-thin layers, the leading process of nanostructure growths seems to be solid-state dewetting. It is widely discussed in the literature that the formation of nanostructures is based on nucleation of holes and their later growth [2, 16–19]. Nucleation can take place in two ways: as homogeneous nucleation, when holes appear as a consequence of small thermal density fluctuations. The second type is a heterogeneous nucleation, caused mainly by defects present or in the metal film or on the interface between the film and the substrate. Holes can be also created during the spinodal dewetting process, which occurs by the amplification of Plasmonics Fig. 1 SEM images of gold nanostructures on silicon substrate, with initial gold film of 10 nm, annealed at a 300 °C, b 325 °C, c 350 °C, d 375 °C, e 400 °C, and f 425 °C periodical film thickness fluctuation. In any case, dewetting occurs at temperatures well below the melting temperature of the film, so during this process the material remains in the solid state. It is difficult to determine experimentally which of the types of dewetting takes place in a specific case in the process of formation of metal nanostructures. Undoubtedly, the substrate plays a huge role here. It is also difficult to say whether the existence of eutectics has a significant impact on the formation of nanostructures at temperatures above the eutectic temperature, especially that the size of nanostructures or thin films greatly affects their melting point [20, 21]. In our previous works [22, 23], we described the method of synthesis, structure, and electromagnetic field distribution of Au nanostructures that form plasmonic platforms. However, at that time, we could not decide which process was mainly responsible for the growth of nanostructures. Present research are focused on processes leading to the formation of nanostructures as a result of heating thin layers deposited on various substrates when the annealing temperature far exceeds the temperature at which dewetting starts. Experimental Au nanostructures were prepared on quartz glass, Si(111), and well-polished Mo and Ta substrates. The substrates were cleaned with warm acetylacetone and then rinsed in ethanol. Thin Au films (with thickness in the range of 1–200 nm) were deposited using a tabletop dc magnetron sputtering coater (EM SCD 500, Leica) in pure Ar plasma condition (argon, Plasmonics Fig. 2 SEM images of gold nanostructures on tantalum substrate, with initial gold film of 10 nm, annealed at a 300 °C, b 325 °C, c 350 °C, d 375 °C, e 400 °C, and f 425 °C Air Products, 99.999%). The Au target had 99.99% purity, the rate of Au layer deposition was about 0.4 nm/s, and incident power was in a range of 30–40 W. The sputtering system was equipped with a quartz crystal microbalance for the film thickness in situ measurements. As prepared films of varying thicknesses were subsequently put to the hot furnace for formation of nanostructures. Samples were annealed at various temperatures in air atmosphere. To analyze the surface morphology of the samples, FEI Quanta FEG 250 scanning electron microscope (SEM) and Zeiss CrossBeam 540 SEM operated at 10 kV and 2 kV, respectively, were used. For nanograin structure and chemical composition analyses, a TALOS F200X high-resolution transmission electron microscope (HR TEM) equipped with an EDS detector was used. Results and Discussion In Figs. 1 and 2, selected SEM images of nanostructures formed during annealing of 10-nm thin Au films at different temperatures are presented. The films were deposited on silicon (Fig. 1) and tantalum (Fig. 2) substrates. The heating temperature’s as well as the substrate’s influence on the formation of nanostru (...truncated)