Generation and optical properties of silver surface-enhanced Raman scattering substrates

Noble metal nanoparticles are capable of increasing the Raman scattering efficiency of nearby molecules with observed enhancements as large as 1014. Although the enhancement mechanism remains poorly understood, such immense enhancements have made single molecule detection possible using surface enhanced Raman scattering (SERS). The primary focus of this work was the design of novel SERS substrates and investigation of the enhancement mechanism to support the development of surfaces with predictable enhancements.; A novel type of SERS substrate was also explored based on nanoaperture arrays in silver films. Nanoaperture arrays fabricated using electron beam lithography showed well-controlled and reproducible aperture spacing. Control over the surface plasmon resonance and the SERS activity was evident by varying the array spacing.; Several reports of single molecule SERS have noted dramatic fluctuations, or blinking, in the intensity of the Raman spectra that have become widely accepted as a signature of single molecule detection. Silver colloids were demonstrated to exhibit blinking in the absence of a SERS probe and the various adsorbates associated with the typical sample preparation for single molecule studies. Blinking was also observed from colloids coated with a strongly bound alkanethiol monolayer as well as from thin silver films and bulk silver powder substrates. In addition, a continuum was observed from noble metal substrates. SERS and the SERS continuum exhibited correlated “on” and “off” blinking, fluctuations in intensity, and irreversible bleaching, all of which strongly suggest that the optical processes are intrinsically linked.; Calculations suggest that the continuum may be due to radiative recombination of electron-hole pairs in the metal and indicate that the generation of a significant electron-hole pair population is possible in a SERS active nanoparticle. A possible enhancement mechanism for both SERS and the continuum was proposed based on the population of electron-hole pairs in the metal providing a gain medium for stimulated radiative recombination. The proposed mechanism agrees with many experimental observations, including the similar behavior observed for SERS and the continuum.