Scannning Tunneling Microscopy Based on Chemically Modified Tips

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Monolayers of-mono-and diethers were studied with scanning tunneling microscopy (STM) as a new example for the chemical modification of STM tips for functional groups recognition. Bright lines revealing the positions of the ether oxygens of the monolayers were observed in STM images with tips modified with 4-mercaptobenzoic acid (4MBA) which can form hydrogen bond with ether oxygens. Such bright lines, however, were absent with unmodified tips or tips modified with thiophenol, which has no functional groups available for hydrogen bond formation. In the STM images of diethers, the brightness of the ether oxygens observed with 4MBA-modified tips was modulated depending upon the orientation of the ether oxygens. These results support that selective contrast enhancements are associated with hydrogen bond formation between sample and tip. It is demonstrated that chemical modification of STM tips allows not only selective recognition but also orientation-sensitive observation of functional groups.Chemical modification of scanning tunneling microscopy (STM) gold tips with 4-mercaptopyridine (4MP) was used for STM measurements of 5,15-bis(4-octadecyloxyphenyl) porphyrin (Por-H2) and its zinc(II) (Por-Zn) and nickel(II) complexes (Por-Ni). Monolayers of these compounds spontaneously form at the solution-HOPG (highly oriented pyrolytic graphite) interface. STM images of porphyrin centers measured with 4MP-modified tips exhibited bright spots while those measured with unmodified tips exhibited the porphyrin centers as dark depressions. The brightness of porphyrin centers seems to result from facilitated electron tunneling due to hydrogen bond interactions for Por-H2 and due to coordination interaction for Por-Zn and Por-Ni. Interestingly, the centers of Por-Zn were brighter than those of Por-H2 and Por-Ni. In addition, STM measurements with 4MP-modified tips allowed to distinguish Por-Zn from Por-H2 or Por-Ni in mixed monolayers.