Efficient SERS substrates made by electroless silver deposition into patterned silicon structures

A nano fabrication process using caesium chloride in an island lithography scheme has been used to make wells in a silicon dioxide layer on silicon. Silver structures have been made in these wells using an electroless process that involves the reduction of silver ions and oxidation of silicon in the presence of fluoride ions. The silver deposition process is found to be diffusion controlled. The "electroless" solution composition determines the structure of the deposited silver. Thus structures can be generated that resemble a torus or a pillar or a small set of spheres. The torus structure was found to give SERS signals for pyridine about 100 times higher than other reported structures. The stability of the torus structure in pure water is limited, and it is therefore necessary to store these structures in alkaline media or with an adsorbate layer as a protector, so making longterm active substrate storage possible. The free energy difference that drives the conversion of tori to pillars has its origin in the excess surface energy, determined by the surface curvature. Optimized substrates have a SERS sensitivity of about 10 for pyridine and some ssDNA molecules. The laser spot to spot variation in SERS signal intensity over the substrate is small, ca. ¡5%. For a molecule like R6G, where resonant absorption occurs, large enhancement is obtained such that a single molecule, in the field of view (3 micron spot size) of the Raman microscope, can be detected. The importance of the torus structure would seem to be empirically established. From a practical point of view, such as using SERS as an analytical tool or examining some resonant structure, these substrates offer a sensitive, uniform and reproducible adsorbent.