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 properties are assigned on the basis of the fluorescence emission characteristics of individual dopant (rhodamine B) molecules. The samples studied include those characterized immediately after being spin cast onto a glass substrate (fresh samples) and after drying at approximate to 80 degreesC in a vacuum oven for at least 12 h (dried samples). The single-molecule fluorescence spectra shift to the red for films dried under more rigorous conditions, reflecting increased average film polarity. The distribution of fluorescence emission maxima also broadens slightly with drying, reflecting an increase in film heterogeneity. Bimodal distributions in the widths of the emission maxima are observed. These distributions exhibit a narrowing of the single-molecule emission with drying, pointing to greater microenvironmental rigidity. Studies of the time-dependent emission characteristics of the single 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 than in the fresh films. A 4-fold (4.3 +/- 0.7) increase in the average survival time of the molecules is also observed, proving that increased dye emission from the dried films results primarily from an increase in dye stability, rather than an increase in fluorescence quantum yield. It is also shown that the single-molecule emission fluctuates more rapidly in the dried films, 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-linked silicate network of the dried films. FTIR studies of the thin films provide additional support for these conclusions.