Fullerene-enhanced Raman scattering: ZnO-covered C₆₀ as ultrasensitive CO gas sensor

We present extensive density functional theory (DFT) calculations dedicated to analyze the stability and Raman response of ZnO-covered C₆₀ fullerenes. The zinc oxide coating does not wet the carbon surface but instead prefers to assemble into small (ZnO)_<i>n</i> clusters of different sizes, leading to the formation of a highly nonuniform ZnO overlayer. There is a notable charge transfer within our hybrid C₆₀/metal-oxide material, occurring in both C₆₀ → ZnO and ZnO → C₆₀ directions, depending on the structure of the ZnO coating. This phenomenon is expected to modify the electronic properties of the fullerene compounds. The presence of the metal oxide overlayer considerably enhances the Raman signals associated to the C₆₀ molecule. Most interestingly, CO adsorption on ZnO-covered C₆₀ leads to enhancement factors (EF) for the intensity of the Raman active C-O stretching vibration as large as 5500. This value is considerably greater than that found in free-standing ZnO clusters, clearly defining our considered carbon nanostructures as ultrasensitive sensors for this toxic molecular species. The CO molecule interacts strongly with the Zn adatoms and, by analyzing different local atomic environments, hot spots on the ZnO surface have been identified. The here-reported CO recognition by our C₆₀-supported ZnO material, based only in the chemical contribution to the Raman intensity variations reveals, for the first time, the existence of the fullerene-enhanced Raman scattering (FERS) effect. We believe that CO sensing based in Raman spectroscopy measurements are complementary to previous studies where, upon CO adsorption, changes in the electrical resistance of zinc oxide/nanocarbon samples are mostly analyzed. Our theoretical predictions define additional applications for hybrid C₆₀/ZnO nanostructures through the appearance of the FERS effect.