Grouping of independent single molecules on silicon surfaces

We evaluated the possibilities of preparing single analyte molecules on silicon surfaces for chemical analysis based on fluorescence measurement. A single-molecule imaging technique involving far-field optical microscopy was used to visualize rhodamine B (rhB) molecules in submonolayers prepared on Si wafers. This technique revealed that fluorescence of solvent-free, adsorbed rhB molecules in submonolayers appeared in two different forms: isolated fluorescent spots in which a countable number of independent single rhB molecules were grouped together and diffused fluorescence in the region with no fluorescent spots. On the basis of the fluorescence imaging and time-resolved fluorometry of individual fluorescent spots, together with examination of light-scattering sites on Si wafers, we discussed the possibility that the isolated fluorescent spots occurred on crystal-originated sites dotted on Si wafers. Our findings should provide guiding principles for developing vials for single analyte molecules on Si surfaces.