Product Code: ICAL08_N102

Surface- and Tip-enhanced Raman Spectroscopy of Silicon
Authors:
K.J. Yi, University of Nebraska-Lincoln; Lincoln NE USA
W.Q. Yang, University of Nebraska-Lincoln; Lincoln NE USA
X.N. He, University of Nebraska-Lincoln; Lincoln NE USA
Y.S. Zhou, University of Nebraska-Lincoln; Lincoln NE USA
W. Xiong, University of Nebraska-Lincoln; Lincoln NE USA
Y.F. Lu, University of Nebraska-Lincoln; Lincoln NE USA
Presented at ICALEO 2008

Strongly enhanced optical fields generated by nanoparticles due to Local Surface Plasmonic Resonance (LSPR) enable Surface-Enhanced Raman Spectroscopy (SERS) to perform chemical analysis with extremely high sensitivity. This technique, however, suffers from the disadvantage of low resolution because of the optical diffraction limit caused by conventional optics. On the other hand, a metallic tip irradiated by a laser beam can induce a highly localized and enhanced optical field with a resolution of 10 nm. In this study, both enhancement schemes (tip- and surface-enhancement) were integrated to achieve both high resolution and sensitivity at the same time. A SERS-active silicon substrate with metallic nanostructures was fabricated by Nanosphere Lithography (NSL) and Focused Ion Beam (FIB) techniques. A Au tip was prepared by the electrochemical etching method. In order to take advantage of the evanescent optical field underneath the tip apex, the tip was controlled to position above the substrate surface with a gap distance of 1 nm by a scanning tunneling microscope (STM). The tip was precisely aligned in the region where two adjacent nanostructures (dimer) were located by a nanopositioner. A resolution of 20 nm was achieved by mapping the metallic nanostructures and silicon dioxide patterns on the silicon substrate. The apparent enhancement of silicon Raman signals under the condition, in which the nanostructures and the Au tip were both present, was much higher than the case where only the Au tip was used. The extraordinary enhancement is attributed to the interplay between the enhanced optical fields from the dimer and the tip. Numerical simulation was performed to verify the suggestion by the FDTD algorithm based upon the Lorentz-Drude model. This technique has a great potential for single molecule detection, disease diagnosis and therapy control, and nanodevice analysis.