Sep 24, 2008
Metallic nanocrystals near ultrasmooth metallic films for surface-enhanced Raman scattering application
Surface enhanced Raman scattering (SERS) is a phenomenon in which the Raman scattering cross sections of molecules residing at or near noble-metal nanostructures are dramatically enhanced. SERS enhancements as high as 1014 have been reported for random “hot-spots” on surfaces. However, the reproducible and reliable fabrication of SERS substrates with uniform enhancements over large areas will be essential for the wider application of SERS.
The size, shape, assembly and dielectric properties of the surroundings of nanostructures all play important roles in the electromagnetic contribution towards the SERS enhancement. In a recent paper published in Nanotechnology, uniform arrays of silver nanocrystals deposited on ultrasmooth gold and silicon surfaces were used as model systems to study the fundamental plasmonic coupling between the silver nanocrystals and the two underlying surfaces with very different electronic properties. Comparing the SERS intensities from these surfaces can shed light on the rational engineering of optimal SERS substrates.
Highly monodispersed 5 and 7 nm silver nanoparticles were obtained by colloidal synthesis, and ultrasmooth gold surfaces (RMS roughness <0.3 nm) by template-stripping. Closely packed arrays of silver nanoparticles were formed on both the ultrasmooth gold and silicon surfaces by drop casting, producing uniform SERS signals (standard deviation within 10%) over large areas of the substrates with 4-mercaptophenol and rhodamine 6G as probe molecules. The SERS intensity was about one order of magnitude higher on the ultrasmooth gold surface than on the silicon surface. This difference is attributed to the stronger plasmonic coupling between the nanoparticles and their charge images in the underlying metallic film, as confirmed by finite difference time domain calculations on the model systems.
These results provide significant insights in to the rational engineering of SERS substrates, which is the subject of future research efforts. The application of SERS for highly sensitive detection and identification of trace molecular species, including chemical and biological warfare agents, environmental toxins and disease indicators, will be of tremendous commercial potential.
About the author
All of the authors are affiliated with Hewlett-Packard (HP) Laboratories. Dr Jing Tang obtained her PhD in chemistry from Columbia University and is currently a research associate at HP Labs. Her research has been focused on the synthesis and applications of nanocrystals. Dr Ekaterina Ponizovskaya is a visiting scholar at HP Labs. Dr Alexander Bratkovsky, Dr Duncan Stewart and Dr Zhiyong Li are all senior research scientists at HP Labs. Dr Stanley Williams is a HP senior fellow and the director of the Information and Quantum Systems Laboratory of HP Labs.