STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
OPTICAL AND STRUCTURAL INVESTIGATIONS OF ORDERED
METALLIC NANOSTRUCTURES FOR SERS EXPERIMENTS
M. Bolboaca 1 , L. Baia 1 , I. Chicinas 2 , D. Maniu 1 , T. Iliescu 1 , S. Astilean 1
1
Physics Faculty, Babes-Bolyai University, Cluj-Napoca,
Romania
2
Department of Materials Science and Technology,
Technical University, Cluj-Napoca, Romania
Abstract
We employ scanning electron microscopy in combination with transmission and reflectivity spectroscopy to characterize the structural, morphological and optical properties of thin gold film deposited on polystyrene colloidal crystal. We make interesting correlations between the optical response, nanometer-scale morphology and surface-enhanced
Raman scattering (SERS) activity of nanostructured film.
Introduction
Nanostructured metallic substrates offer the possibility of enhancing the electromagnetic field close to surfaces by factors up to 10 3 due to surface plasmon excitation [1]. Thereby, Raman signals from molecules adsorbed at these substrates are enhanced by up to six orders of magnitude. This surface-enhanced Raman scattering (SERS) offers high sensitivity and the possibility of detecting monolayers of molecules [2]. For this reason and the additional typical high selectivity of Raman scattering due to the fingerprint nature of the molecular spectra, SERS has a large potential for new developments in chemical and biochemical sensor applications [3].
Optimization of the Raman amplification is of major interest for applying the SERS technique in ultrasensitive molecular detection. A wide variety of substrates has been found to exhibit SERS [4]. In contrast to irregular metallic substrates (roughened metal electrodes, metal island films and colloidal metal films), regularly arranged metallic nanoparticles and metallic gratings can offer the possibility for a more systematic and reproducible study of the plasmon resonance contribution to SERS.
The rapid development of the nanofabrication technique has made possible to produce well-controlled and highly ordered nanostructured metallic surfaces.
Two types of nanofabrication techniques, i.e., lithography and template synthesis, play a dominant role for this purpose. Electron, ion, or light beam lithography can make highly ordered periodic arrays with optimized particle size, shape, and interparticle spacing. However, these point-by-point approaches make the fabrication process very slow and cost-consuming. In contrast, nanostructured surfaces with a large area can be prepared more easily and quickly by the template

M. BOLBOACA, L. BAIA, I. CHICINAS, D. MANIU, T. ILIESCU, S. ASTILEAN synthesis technique, especially when combined with the self-assembly method and nanosphere lithography.
In this paper, we applied the second route of nanofabrication to produce metallic nanostructures with SERS activity. Nanostructures exhibit valuable SERS properties due to highly ordered metallic features created by covering regular arrays of polystyrene nanospheres with a gold film. We were able to make correlations between optical response, nanometer-scale morphology and the amplification of surface-enhanced Raman scattering (SERS) spectra of rhodamine
6G molecules adsorbed on the substrate.
Experimental
a) Nanostructures preparation. The nanostructures were prepared in two steps, according to the drop-coat method of nanosphere ordering as described in literature [5]. Firstly, a suspension of polystyrene nanospheres of 220 nm diameter was drop-coated onto the a glass slide where they, after water evaporation, selfassembled into hexagonally close-packed two-dimensional (2D) colloidal crystals.
Then, the substrate was mounted into the vacuum chamber of a vapour deposition system and gold films of 64 nm thickness were thermally evaporated onto the substrate under a pressure of 5×10 -6 Torr.
For recording the SERS spectra, a drop of rhodamine 6G in methanol solution (10 -4 M concentration) was evaporated on the substrate. Rhodamine 6G and all materials involved in substrate preparation were purchased from commercial sources as analytical pure reagents. b) Experimental measurements.
Scanning electron microscopy investigations were performed with an JEOL JSM 5600-LV scanning electronic microscope.
Transmission and reflectivity spectra were measured at normal incidence using a
Perkin Elmer Lambda 9 uv-vis-nir spectrometer. The SERS measurements were performed on a Dilor Labram system by using an Olympus LMPlan Fl 50 microscope objective and two laser lines for excitation (514 nm and 633 nm respectively). The FT-Raman spectrum of Rhodamine 6G was recorded with a
Bruker IFS 120HR spectrometer equipped with a FRA 106 Raman module and a
Nd:YAG laser for excitation with the 1064 nm line.
Results and discussion
Figure 1 shows the scanning electron microscopy (SEM) picture the nanospheres assembly covered with a gold film. Depositing a thin film of metal over the nanospheres assembly produces regular arrays of quasi-interconnected metallic triangular nanoparticles. The ordered domains range from a few to tens of
 m. Figure 2 shows the transmission and reflection spectra of nanostructured gold film recorded at normal incidence. The optical spectra reflect the interplay between two distinct optical responses of nanostructure: (a) localized surface plasmon
(LSPR) modes associated with individual nanoparticles and (b) delocalized modes associated with the plasmon propagation on quasi-continuous corrugated film. Both characteristics enhance the versatility of SERS substrate.

OPTICAL AND STRUCTURAL INVESTIGATIONS OF ORDERED METALLIC NANOSTRUCTURES
Fig 1. SEM images of typical nanostructured gold film produced by metal coating crystalline assemblies of 220 nm polystyrene nanospheres.
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Fig 2. reflectivity
Transmission spectra nanostructured gold film and of
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Fig 3. FT-Raman, SERRS and SERS spectra of Rh6G adsorbed on nanostructured gold film
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M. BOLBOACA, L. BAIA, I. CHICINAS, D. MANIU, T. ILIESCU, S. ASTILEAN
The SERRS spectrum of Rh6G recorded with the 514 nm laser line (where molecules exhibit an electronic absorption maximum) together with SERS spectrum recorded with the 633 nm laser line, demonstrates the great capability of this substrate to provide Raman enhancements. We mention that these spectra were collected from the same region of substrate and two different laser lines. The differences between the SERRS and SERS spectra can be explained by considering the resonant contribution to the overall SERS enhancement. Previous theoretical studies [6,7] on aggregate structures suggest that the electromagnetic field effects responsible for SERS on solid substrates are localized in the interstitial regions of the arrays, and are accompanied by field depletion outside of these areas. This implies that the majority of the SERS signal measured from our sample is due to the excitation of very small percentage of adsorbate situated at these interstitial sites, and the individual enhancements are likely greater the surface-averaged measurements [8].
Conclusions
The absorption and reflectivity spectroscopy together with scanning electron microscopy have been performed in order to characterize the structural and optical properties of nanostructured gold film. The film deposited on colloidal crystal of polystyrene spheres exhibits structural periodicity, strong optical resonances and reproducible SERS properties. The approach of nanofabrication we employed here, provides great opportunity for not only optimizing the SERS activity but also searching for a new class of SERS active materials.
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