Effects of molecular geometry on the STM image contrast of methyl- and bromo-substituted alkanes and alkanols on graphite

Scanning tunneling microscopy (STM) images have been collected for a series of substituted alkanes and alkanols that form ordered overlayers at room t emperature on highly ordered pyrolytic graphite surfaces. Molecules that ha ve been imaged possess an internal bromide, with or without terminal alcoho l groups (HO(CH2)(9)CHBr(CH2)(10)OH and H3C(CH2)(16)CHBr(CH2)(16)CH3), an i nternal -OH group (H3C(CH2)(16)CHOH(CH2)(16)CH3), and an internal methyl gr oup (H3C(CH2)(16)CHCH3(CH2)(16)CH3). These data allow comparison to the STM image contrast reported previously for molecules in which -OH, -Br, and -C H3 groups were located in terminal positions of alkane chains adsorbed onto graphite surfaces. When the functional groups were in gauche positions rel ative to the alkyl chain, and thus produced molecular features that protrud ed toward the tip, the functional groups were observed to produce bright re gions in a constant current STM image, regardless of the STM contrast behav ior observed for these same functional groups when they were in terminal po sitions of adsorbed alkyl chains. These observations are in excellent agree ment with theoretical predictions of the STM behavior of such systems. Addi tionally, several interesting packing structures have been observed that ha ve yielded insight into the intermolecular forces that control the packing displayed by these overlayers.