Single-molecule studies of sol-gel-derived silicate films. Microenvironments and film-drying conditions

Single-molecule spectroscopy is used to characterize the microenvironments found in silicate thin films dried under different conditions. Local film p roperties are assigned on the basis of the fluorescence emission characteri stics of individual dopant (rhodamine B) molecules. The samples studied inc lude those characterized immediately after being spin cast onto a glass sub strate (fresh samples) and after drying at approximate to 80 degreesC in a vacuum oven for at least 12 h (dried samples). The single-molecule fluoresc ence spectra shift to the red for films dried under more rigorous condition s, reflecting increased average film polarity. The distribution of fluoresc ence emission maxima also broadens slightly with drying, reflecting an incr ease in film heterogeneity. Bimodal distributions in the widths of the emis sion maxima are observed. These distributions exhibit a narrowing of the si ngle-molecule emission with drying, pointing to greater microenvironmental rigidity. Studies of the time-dependent emission characteristics of the sin gle molecules show the total number of photons emitted (prior to bleaching) by the molecules in the dried films is four (3.6 +/- 0.6) rimes greater th an in the fresh films. A 4-fold (4.3 +/- 0.7) increase in the average survi val time of the molecules is also observed, proving that increased dye emis sion from the dried films results primarily from an increase in dye stabili ty, rather than an increase in fluorescence quantum yield. It is also shown that the single-molecule emission fluctuates more rapidly in the dried fil ms, possibly due to an increase in the rate of tripler formation and/or an increase in the triplet lifetime. Increased dopant stability is attributed to reduced oxygen and dye mobility within the more dense, highly cross-link ed silicate network of the dried films. FTIR studies of the thin films prov ide additional support for these conclusions.