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Monolayers of 1-octadecanol, 1-chlorooctadecane, and 1-octadecanoic acid were observed with scanning tunneling microscopy (STM). Chemical modification of STM tips allowed recognition of functional groups in monolayers of 1-octadecanol and 1-octadecanoic acid. Enhanced contrasts for hydroxyl and carboxyl residues were observed with gold tips modified with several aromatic mercaptans that can form hydrogen bonds (4-mercaptopyridine, 4-aminothiophenol, 4-hydroxythiophenol). On the other hand, no such enhancements were observed in monolayers of 1-chlorooctadecane. The enhanced contrasts for the OH and COOH groups seem to be due to electron tunneling through hydrogen bonds formed between the functional groups on tip and sample. These results suggest that tailored modification of STM tips may become a general method for the recognition of specific chemical species and functional groups in STM images.
Ito et al. (1998) studied this question.