PERIODIC ARRAYS OF METAL NANOBOWLS AS SERS-ACTIVE SUBSTRATES

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1 PERIODIC ARRAYS OF METAL NANOBOWLS AS SERS-ACTIVE SUBSTRATES Lucie ŠTOLCOVÁ a, Jan PROŠKA a, Filip NOVOTNÝ a, Marek PROCHÁZKA b, Ivan RICHTER a a Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Department of Physical Electronics, Břehová 7, Prague 1, Czech Republic, lucie.stolcova@centrum.cz b Charles University, Faculty of Mathematics and Physics, Institute of Physics, Ke Karlovu 5, Prague 2, Czech Republic Abstract Surface-enhanced Raman spectroscopy (SERS) is a powerful technique which could compete in sensitivity with fluorescence spectroscopy while retaining the specificity of Raman spectroscopy. Enhancement of Raman signal by several orders of magnitude can be achieved when molecules are adsorbed on (or in proximity to) metal nanoparticles or nanostructured metal surfaces, referred to as SERS-active substrates. We have obtained SERS-active substrates with high reproducibility using self-assembly of polystyrene microspheres. These surfaces consist of highly ordered metal nanobowls with sharp edges which significantly contribute to high enhancement of the local electromagnetic field. Periodic arrays of closepacked nanobowls of various sizes were fabricated and their functionality was confirmed by measuring SERS spectra of methylene blue. Keywords: SERS, nanobowls, reproducible SERS substrates, self-assembly, methylene blue 1. INTRODUCTION Major disadvantage of Raman spectroscopy, very small cross section of Raman scattering, can be overcome by considerable enhancement of the local electromagnetic field due to excitation of localized surface plasmon resonances in metal colloidal particles or nanostructured metal surfaces [1]. This effect, surfaceenhanced Raman scattering (SERS), has great potential for applications in biology, medicine and diagnostics but commercial fabrication of the so-called SERS-active substrates remains a challenge. Large enhancement factors and reproducibility are two main requirements for high-quality SERS substrates. However, these features seldom coincide. While in the first two decades after the discovery of SERS in the 70 s, maximum enhancement was the most desired feature, now it is generally believed that reproducibility is the key factor for future commercialization of SERS-active surfaces because it allows quantitative analysis. Periodicity of these systems is an elegant way to obtain high reproducibility and homogeneity of enhancement over the whole surface. In the past years, a great advance has been made both in fabricating novel types of reproducible surfaces and designing their optimal shape and size using computer simulations of their interactions with incident light. Fabrication methods and obtained SERS-active surfaces have been reviewed in detail [2, 3]. A bottom-up approach based on self-assembly is particularly advantageous thanks to its low cost and the possibility to create large surfaces (i.e. compared to lithographic methods). Moreover, optical properties of these periodic structures determined by their characteristic dimensions can be easily tuned by the size of individual particles. Using self-assembled silica or polystyrene templates, surprisingly versatile periodic SERS-active surfaces can be obtained. Metal triangles fabricated by nanosphere lithography [4] and (metal) film over nanosphere (FON) substrates introduced by R. Van Duyne [5] appear to be the most widely known. Higher enhancement factors are expected on surfaces containing sharp features. Open shell nanostructures, such as nanobowls, have been shown to display large enhancement at the edges of the open apertures [6]. By depositing a 300 nm thick silver film over a self-assembled monolayer of spheres via thermal evaporation,

2 peeling the structure off with a tape and subsequent removal of the template, ordered array of metal nanobowls has been created and its functionality as a SERS substrate has been verified [7]. In this paper we report on the fabrication of periodic array of gold nanobowls obtained by magnetron sputtering using hexagonally ordered polystyrene (PS) spheres as a template and confirm SERS activity of this substrate. 2. EXPERIMENTAL 2.1 Materials and methods Aqueous dispersions (10 wt %) of PS microspheres with diameters (535 ± 16) nm and (920 ± 23) nm, respectively, were purchased from microparticles GmbH (Berlin, Germany). They were mixed with ethanol (1:1 v/v) and carefully deposited on water surface using a glass pipette. The close-packed microspheres formed compact monolayer with hexagonal order at the water-air interface. The obtained monolayer was then transferred onto cleaned glass substrates (approx. 1 x 1 cm) and left to dry at room temperature. Thin gold layers (20 or 40 nm in thickness) were prepared by magnetron sputtering deposition on the PS templates in Cressington 208HR high-resolution sputter coater. Poly(dimethylsiloxane) (PDMS) was purchased as Sylgard Elastomer Kit 184 from Dow Corning. Curing agent was mixed with polymer base at a ratio 10 (polymer):1.5 (curing agent), stirred and degassed by centrifugation at g. The mixture was then poured over glass substrates (approx. 1 x 1 cm) and left to cross-link for 1 day. The PS monolayer was then pressed onto PDMS by metalized side and peeled off with it (stamping). Polystyrene spheres were dissolved in toluene. Obtained substrates consisting of PDMS covered with gold nanobowls were, after drying, dipped in solution of methylene blue (ph 3,6, concentration M) for 11 hours and then dried in nitrogen flow. 2.2 Characterization Obtained samples were characterized by scanning electron microscope (SEM) JEOL JSM SERS spectra of methylene blue (MB) were measured on Raman microspectrometer LabRam HR800 (Horiba Jobin Yvon) using 633 nm excitation (He-Ne laser) of power 0,2 mw and accumulation time 1 x 60 s. 3. RESULTS AND DISCUSSION PS spheres deposited on the water surface formed hexagonal close-packed monolayer of good crystalline quality for both sphere sizes used (535 nm and 920 nm). After drying on a glass substrate, the spheres were sputtered with a thin layer of gold. Sputtering, unlike thermal evaporation, enables the metal particles to penetrate into regions shadowed by the PS spheres and to coat a region exceeding the upper half of the sphere. Fig. 1 shows SEM micrograph of a self-assembled monolayer of PS spheres with diameter of 920 nm sputtered with 20 nm of gold. Fabrication steps leading to gold nanobowl array are schematically illustrated in Fig. 2. Firstly, the goldcoated monolayer was turned upside-down by pressing into PDMS layer and removing the glass. Peeling the spheres off the glass substrate did not significantly disturb the hexagonal ordering (see Fig. 3). Secondly, the PS spheres were dissolved in toluene leaving ordered array of gold nanobowls in PDMS (see Fig. 4).

3 , Brno, Czech Republic, EU Fig 1: SEM micrographs of a self-assembled monolayer of PS spheres 920 nm in diameter sputtered with 20 nm of gold (tilting angles: (a) 0, (b) 45 ). Fig 2: Schematic illustration of the fabrication of gold nanobowls from gold-sputtered PS spheres Fig 3: SEM micrograph of gold-coated PS spheres (40 nm of gold) peeled off with PDMS. To evaluate the potential of this structure as a SERS-active substrate, we dipped the samples containing ordered arrays of nanobowls (diameter 920 nm, 20 nm of gold) into MB solutions of different concentrations. High-quality SERS spectra of MB were obtained for concentrations -7 down to 10 M (Fig. 5). Peak positions in the measured spectra correspond to those previously reported on gold substrates obtained by physical sputtering [8], but the detection limit in our case is at least one order of magnitude lower. Fig 4: SEM micrographs of gold nanobowls (40 nm of gold).

4 Fig 5: Measured SERS spectra of MB for very low concentrations: top M, bottom M. 4. CONCLUSION In this paper, we described a facile technique for the fabrication of periodic arrays of metal nanobowls using self-assembled polystyrene microspheres as templates. These open shell structures display very strong enhancement of the local electromagnetic field around the sharp edges which gives rise to the SERS effect. Moreover, periodicity of this structure ensures that it is highly reproducible. It is the combination of these two features what makes periodic arrays of nanobowls such a promising SERS-active substrate. Our method of fabrication is advantageous because of its low cost, high throughput and the possibility to easily tune the optical response of the metal array by choosing a template with appropriate dimensions. High quality SERS spectra of methylene blue were measured on the obtained surface down to concentrations 10-7 M. ACKNOWLEDGEMENTS This work was supported partially by grants of GAAV KAN , Ministry of Education, Youth and Sports, project COST OC09038, and MŠMT MSM

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