Single-Molecule Spectroscopic Studies of Nanoscale Heterogeneity in Organically Modified Silicate Thin Films

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Abstract

Single-molecule fluorescence spectroscopy is used to compare the nanoscale properties of organically modified sol−gel-derived silicate thin films prepared from different silicate precursors. Sols containing different mole fractions of isobutyltrimethoxysilane (BTMOS) and/or 3-(triethoxysilyl)propionitrile (CNS) and tetraethoxysilane (TEOS) are used for film preparation. The solvent-sensitive dye Nile red is doped into these films at nanomolar concentrations and is used to probe their nanoscale environments. The single-molecule fluorescence spectra obtained are analyzed using a form of Marcus theory for charge-transfer transitions. Important information on local film polarity and rigidity is obtained. The results show that films derived from CNS-containing sols are more polar than those prepared using BTMOS, as expected. Data obtained from a series of films as a function of film organic content (i.e., CNS or BTMOS, the remainder being TEOS) show that the local film environments become distinctly less polar and less rigid as the film organic content increases. However, the CNS and BTMOS sample series exhibit markedly different behaviors as a function of film organic content. CNS-containing materials exhibit gradual changes in their nanoscale polarity and rigidity, whereas BTMOS-containing materials exhibit a "discrete" change in these properties for films of greater than approximately 50% organic content. The latter result is attributed to phase separation and/or the formation of micelle-like domains in BTMOS-derived films. Comparisons between cohydrolyzed and separately hydrolyzed sols prepared from similar binary and ternary silane mixtures also show evidence for phase separation. Importantly, the single-molecule data indicate that the average and most common film environments are distinctly different in virtually all films studied.

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