Differentiation of molecules in a mixed self-assembled monolayer of H-and Cl-terminated Mcyclo[2.2.2]octane derivatives

摘要

H- and Cl-terminal groups of bicyclo[2.2.2]octane (BCO) derivatives in a mixed self-assembled monolayer (SAM) on Au(111) were imaged using a modified Si tip with a CaF_2 nanocluster to differentiate the two terminals, which have different electronegativities. In order to achieve this we fabricated a new sample holder, on which a CaF_2 single crystal and the mixed SAM on Au(111) could be mounted side by side. We transferred the holder with the two samples into a ultrahigh vacuum (UHV) atomic force microscopy (AFM) chamber. Upon cleaving the CaF_2 single crystal under UHV, a fresh and clean CaF_2(111) surface parallel with the SAM surface appeared within 2 mm of the separation. The modified Si tip was prepared by repeatedly making contact between a Si tip and the CaF_2(111) surface. The resulting modified tip could image the atomic periodicity of a Ca~2+ and an F~- sublattice on the CaF_2(111) surface depending on the sign of the tip-terminating ion, i.e. an F~- and a Ca~2+ ion, respectively, as reported previously (Foster et al 2002 Phys. Rev. B 66 235417). Using the modified Si tip with the known tip-terminating ion, we observed the Cl-terminal in the surrounding H-terminals in the mixed SAM by noncontact (NC) AFM. Here, the Cl-terminal is negatively charged due to its electronegativity and thus the BCO moiety with the Cl-terminal is terminated by a C~delta+ - C1~delta- permanent dipole, while the H-terminal is almost neutral. The Cl-terminal appeared brighter (more attractive) and darker (more repulsive) than the surrounding H-terminals in NC-AFM images depending on the sign of the tip-terminating ion, i.e. a Ca~2+ and an F~- on the modified tip, respectively, although the relationship between the image contrast and the sign of the tip-terminating ion was not always perfect because of the instability of the tip-terminating ion on the nanocluster. The present method can be used to distinguish terminal groups with different electronegativities.