Raman enhancement of rhodamine adsorbed on Ag nanoparticles self-assembled into nanowire-like arrays
© Panagopoulou et al; licensee Springer. 2011
Received: 24 June 2011
Accepted: 14 December 2011
Published: 14 December 2011
This work reports on Raman scattering of rhodamine (R6G) molecules absorbed on either randomly distributed or grating-like arrays of approximately 8-nm Ag nanoparticles developed by inert gas aggregation. Optimal growth and surface-enhanced Raman scattering (SERS) parameters have been obtained for the randomly distributed nanoparticles, while effects related to the aging of the silver nanoparticles were studied. Grating-like arrays of nanoparticles have been fabricated using line arrays templates formed either by fracture-induced structuring or by standard lithographic techniques. Grating structures fabricated by both methods exhibit an enhancement of the SERS signal, in comparison to the corresponding signal from randomly distributed Ag nanoparticles, as well as a preferential enhancement in the areas of the sharp features, and a dependence on the polarization direction of the incident exciting laser beam, with respect to the orientation of the gratings structuring. The observed spectroscopic features are consistent with a line-arrangement of hot-spots due to the self- alignment of metallic nanoparticles, induced by the grating-like templates.
KeywordsSERS self-aligned silver nanoparticles R6G Raman spectra nanotechnology (design-applications)
The effect of the surface-enhanced Raman scattering (SERS) has been observed, since 1974, in (organic) molecules being in close contact with properly structured metallic surfaces, especially nanosized metallic particles. According to this effect, an enhancement, by many orders of magnitude, of the Raman signal from organic molecules was observed, when these molecules were attached to metallic (usually Ag, Au, but also Ni, Co, etc) nanoparticles. An intensive research activity has been carried out, in the following decade, concerning the mechanisms which are responsible for the strong enhancement of the scattering intensity, with respect to the signal of the conventional Raman scattering, with two models, namely, the electromagnetic enhancement and the electrochemical enhancement, proposed as the dominant ones (for a review of the related activities in the decade 1974 to 1984, see [1, 2]; see also .
Taking into account that in the SERS effect, in addition to the active molecule, which is the actual scattering center (e.g., pyridine, rhodamine, etc.), the most important role is played by the characteristics of the metallic surface, i.e. the metal itself (with prevailing, Ag and Au), and the nanostructure of the metallic surface (several geometries, with typical sizes from several nanometers up to a few tens of nanometers); the corresponding interest has been revitalized the last decade (2000 to 2010), due to the fast development of the nanotechnology and the related structures and devices. The shape and the dimensions of the metallic nanoparticles are strongly affecting the intensity enhancement, since those parameters have an important influence in the spectrum of the surface plasmons , while especially interesting are the two-dimensional arrays of nanopillars or nanowires, where SERS effect could be observed even for metals, such as Ni and Co, which were not considered as effective SERS-active  materials. According to theoretical studies, the characteristics of the plasmon resonances are correlated with the cross-section of the nanowires , with obvious selective scattering applications in the nanosystems . The arrangement of the nanowires in periodic arrays (as a result of either a self- or an induced organization) exhibits related polarization characteristics in the SERS effect , quite similar to polarization effects obtained through random parallel nanostructures in gold thin films .
The research in this field has been revived during the last years, from both aspects of view, the basic physics (plasmonic properties of random and periodic metallic nanostructures) and chemistry, as well as the potential of new technological tools in the area of chemical sensors. This generation of sensors, which is based on plasmonic excitation effects, is very promising in the fields of chemistry, biochemistry, and biomedical research.
Two sets of SERS-active samples are studied by microscopic and spectroscopic techniques. The first set, consisting of randomly distributed Ag nanoparticles, is used to obtain the optimal growth and SERS parameters. It is also used to check the influence, to the SERS efficiency, of the different substrates and of the time elapsed since the dying of the nanoparticles. The second set consists of silver nanoparticles evaporated over grating-like structured surfaces. Self-alignment effects of the nanoparticles, due to the structure of the surfaces, are monitored by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques, while its influence on the Raman enhancement is studied by micro-Raman scanning and polarization-dependent SERS measurements.
The study of the silver nanoparticle systems, by SERS spectroscopy, was carried out in macro- and micro-Raman configurations. The samples to be characterized by SERS spectroscopy were immersed, for 12 h, in methanol (or water) solution of rhodamine (R6G), with molarities of the order of magnitude of 10-4 M, and then dried by free evaporation of the solvent, for a few minutes. The Raman measurements were taken in ambient conditions. We have tried different wavelengths and have obtained optimum SERS spectra with the 514.5-nm Ar+ laser line. For the macro-Raman measurements, a SPEX 1403 double monochromator with standard photon-counting system was used. For this series of measurements, we have used 20-mW excitation beam power, focused by 75-mm focal-length lens, either cylindrical or spherical, depending on the specific measurements. In this series of measurements, a fluctuation of the SERS intensity was observed, immediately after illumination with the laser beam. All the measurements were taken after a stabilization interval of half an hour after illumination, in order to overcome this effect, known as photobleaching. For the micro-Raman measurements, a JY T64000 triple monochromator, with optical microscope of magnification up to ×100, and a liquid nitrogen-cooled CCD detector was used, equipped with motorized stepping drive motors for the scanning of the grating-like structures. For this series of measurements, an excitation beam power of 0.01 to 0.05 mW, was used, in order to minimize heating and photobleaching effects.
Results and discussion
Randomly distributed Ag nanoparticles
In conclusion, we have presented SERS-based investigation on randomly distributed and periodically self-arranged metallic nanoparticles. Optimization parameters obtained through the randomly distributed NPs, were taken into account in the study of the periodic structures. These studies confirm, through polarization measurements and micro-Raman scanning, the signal enhancement from the nanoparticles which are aligned in a form of nanowires, and prove that the synergetic combination of the two spontaneous organization processes (FIS patterning and nanoparticle self-alignment) can lead to further SERS enhancement with physically interesting aspects and potentially promising technological consequences.
Kind help by Dr. A.G. Kontos and M.-C. Skoulikidou for the AFM-SEM imaging is acknowledged with appreciation. We acknowledge financial support from Nanosource Marie-Curie project funded by EU.
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