Extending Raman Spectroscopy To The Nanoscale PDF Download

"The primary criteria for an analytical tool to meet to be effective in nanoscale science are high speed, ease of measurement, and simultaneous acquisition of multiple types of information. Even though the size of features that can be created in silicon using a commercialized nano-patterning process is down to 65 nm, techniques for analyzing structures with nanometer resolution and meeting the above criteria still need to be developed. Raman spectroscopy is an effective tool for the study of nano-structures due to its ability to observe the distribution of stress in samples while capturing the fingerprint spectra of chemical functionalities. The main drawbacks hindering the use of Raman spectroscopy for nano-analysis are the low signal intensity and the diffraction-limited resolution of current instruments. The strategy for overcoming the low intensity is to induce an enhancement in the electric field in the sample by placing it next to a roughened noble metal surface. With the aid of a thin noble metal layer, surface enhanced Raman scattering (SERS) spectra of films of polystyrene blends including different branched molecular architecture were acquired. These probe the composition within a depth of ~2 nm from the surface. The surface compositions gained using SERS were comparable to those gained from neutron reflectivity. To optimize the lateral resolution of SERS and to simultaneously access additional information such as surface topography, a Raman spectrometer and accompanying optics have been combined with a scanning probe microscope (SPM). The enhancement of the Raman signal on a very local, nanoscale level was realized by using the very small contact area between a noble metal layer on a SPM probe (tip) and a sample surface. Using a method based on such a combination, called scanning nano-Raman spectroscopy (SNRS), we have demonstrated Raman spectral imaging capability with ~20 nm lateral resolution with simultaneous topography measurement. For the successful application of SNRS to the analysis of silicon nano-structures, the contrast between the enhanced, localized signal and the far field signal has been maximized by optimizing the beam polarization. This optimized polarization was used to demonstrate an improvement in contrast by a factor of ~20 and a much better scanning capability than available with conventional polarization."--Abstract.