Sol-Gel SiO₂ Films Containing Colloidal Copper Particles for Surface-enhanced Raman Scattering of Graphite

Coatings on glass substrates with a weight composition of 60 CuO: 40 SiO₂ were prepared using the sol-gel method. To accomplish this, copper was incorporated in the starting solution prior to the film formation. Then thermal treatments in air on the freshly made coatings produced samples with the above-mentioned composition. Thermal annealings of the oxidized coatings under a CO-CO₂ atmosphere produce, first, the reduction of the copper oxide particles to form metallic copper particles and, second, the deposition of ultrathin graphite layers on the copper particles located near the surface of the coating. The strong graphite Raman signal observed in these samples is interpreted as being due to the surface enhancement of the graphite lines produced by the metallic copper particles. A simple approach is used to calculate the enhancement in the Raman lines and this agrees well with the enhancement measured experimentally.