Optimum Length of Silver Nanorods for Fabrication of Hot Spots

The optimum length of silver nanorods for surface-enhanced Raman scattering (SERS) was studied. Uniform silver nanorods in diameter and length were fabricated on porous anodic aluminum oxide templates prepared in sulfuric acid, and their lengths were varied by controlling the electrodeposition time. The diameter of the nanorods was approximately 37 nm, and the distance between the nanorods was very uniform at 65 nm. The nanorods were highly ordered with a hexagonal pattern and perfectly vertical with respect to the plane of the templates. A good SERS spectrum was observed for benzenethiol adsorbed at the tip of the isolated silver nanorods. The SERS intensity sensitively varied with the length of the nanorods and attained the maximum when the length was approximately 62 nm for excitation with a 632.8-nm laser line. The enhancement factor calculated by comparing the SERS intensity with the normal Raman intensity and by excluding the resonance effect of molecules was 3.9 × 107, while that calculated from th...