Optimization and characterization of Au cuboid nanostructures as a SERS device for sensing applications M. Chirumamilla a,b,⇑ , G. Das a , A. Toma a , A. Gopalakrishnan a,b , R. Proietti Zaccaria a , C. Liberale a , F. De Angelis a , E. Di Fabrizio a a Nanostructures, Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy b Università degli Studi di Genova, Genova 16145, Italy article info Article history: Available online 17 May 2012 Keywords: SERS Plasmonics Cuboid nanostructures Electron beam lithography Biosensor abstract Gold cuboid nanostructures with edge size in the range of 40–65 nm and inter-particle separation of 20 nm were fabricated by electron beam lithography. The aim of the present work is to investigate the effect of cuboid size on surface enhanced Raman scattering intensity and, thereby, to optimize the size of nanostructures in order to maximize signal enhancement. The electric field distribution of 4  4 array of Au cuboid nanostructures was numerically simulated by commercial software. A monolayer of Rhoda- mine-6G is deposited on the device using chemisorption technique, finding an enhancement factor 10 4 , which candidates the cuboid nanostructures as a promising enhanced Raman substrate. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Surface Enhanced Raman Scattering (SERS) has received much attention over years owing to its potential applications in detection of molecules at very low concentrations down to femto-molar (10 15 M) or even single-molecule detection [1–3]. The explana- tion of huge enhancement in SERS can be based on two mecha- nisms, the chemical [4] and the electromagnetic (EM) effects [5]. The former arises mainly when the electronic states of the mole- cules adsorbed on the surface interact with the electronic states of the metal. Usually it provides a contribution to the enhancement factor in the range of 10–100 [6]. The EM mechanism is based on the near field intensity as a result of the excitation of surface plas- mons [5,7]. The EM contribution to the Raman enhancement ranges between 10 4 and 10 12 [5–7]. Metallic nanostructures pos- sess topologically dependent Localized Surface Plasmon Reso- nances (LSPRs) which can promote a giant enhancement of local electromagnetic field on the surface. Therefore, metal nanostruc- tures comprising closely spaced gaps are of great interest, since the local electromagnetic field enhancement is strongly confined in the gap, leading to the formation of the ‘‘so-called’’ hot spots [8]. Control over the topology, size, inter-particle gap and dielectric environment [9–15] gives promising results for numerous applica- tions, such as bio-chemical sensing (SERS devices), CMOS, plas- monic solar cells, gas sensing [16–19], etc. Till to date, many studies were conducted, mainly on synthesis by colloidal tech- niques (e.g. nanorice, nanodisk, nanorods, nanocubes, etc. [20– 27]). A different approach is based on top-down fabrication technology [28,29] which allows to engineer devices with metal nanostructures. Lithographic techniques provide control over cou- pling between metal structures guaranteeing uniform enhance- ment over larger area. Owing to the high chemical stability, biological compatibility and optical properties (in the visible and near infrared spectral regions) fabrication of gold nanostructures offer a potential application in the field of nanoscale bio-sensors [30,31]. The figure of merit (FOM) was introduced by Sherry et al. [32] which represents the overall performance of single nano- particle as a chemical sensor. There have been many reports in the literature on FOM studies for different topologies, e.g. nanorice (1), nanoshell (1.7), triangular prism (1.7), crescent moon (3) and nanocube (5.4) [32–34]. According to the reported FOM values, nanocuboids appear to be the most suitable candi- dates for bio-chemical sensing applications. In this paper, we have fabricated Au nanocuboid structures to investigate the effect of cu- boid size on SERS intensity by varying the edge size from 40 to 65 nm. Rhodamine-6G (R6G) was used to examine the SERS sub- strate efficiency. To understand the electric field enhancement of 4  4 array of Au cuboid nanostructures, numerical simulations were also performed. 2. Experimental procedure 2.1. Device fabrication Au nanocuboid structures of 4  4 array with varying edge size from 40 to 65 nm were fabricated by using Electron Beam Lithog- 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2012.05.004 ⇑ Corresponding author at: Nanostructures, Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy. Tel.: +393203536299; fax: +3901071781236. E-mail address: manohar.chirumamilla@iit.it (M. Chirumamilla). Microelectronic Engineering 97 (2012) 189–192 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee