Molecular friction as a tool to identify functionalized alkanethiols

By using the nanografting method, well-defined nanoscale patches of alkanethiols were constructed in a self-assembled monolayer (SAM) matrix on an atomically flat gold (Au(111)) surface. A series of nanografted patches, composed of alkanethiols with different end groups (-CH(3), -CF(3), -OH, -SH, -COOH, and -NH(2)), were analyzed in detail by a combination of atomic force microscopy (AFM) height and quantitative lateral friction measurements. By constructing a series of nanografted patches of methyl-terminated thiols with various chain lengths, it was shown that the absolute friction of the nanografted patches was always smaller than that of the surrounding SAM matrix, demonstrating that, because of the spatially confined self-assembly during nanografting, SAMs show less defects. In addition, the friction gradually increased for decreasing alkane chain length as expected, although a subtle odd-even effect was observed. The study of thiols with functionalized end groups (-CF(3), -OH, -SH, -COOH, and -NH(2)) gave specific insights in orientation, packing, and structure of the molecules in the SAMs. Depending on the thiol end groups, these nanografted patches exhibited large and specific differences in lateral friction force, which offers the unique possibility to use the friction as a molecular recognition tool for thiol-based self-assembled monolayers.