Scanning tunneling microscopy with chemically modified tips:: orientation-sensitive observation of ether oxygens

Diethers, CH3(CH2)(15)O(CH2)(n)O(CH2)(15)CH3 (n = 10,11; C16OCnOC16), adsorbed onto highly ordered pyrolytic graphite were observed with scanning tunneling microscopy (STM) using unmodified tips and tips chemically modified with 4-mercaptobenzoic acid (4MBA). Pairs of bright fines corresponding to the ether oxygens were observed with the modified but not the unmodified tips. Using the modified tips, one of the oxygens Of C16OC10OC16 appears brighter than the other one, whereas no difference in the brightness of the two oxygens Of C16OC11OC16 is observed in the STM images. The relative difference in the brightness for the ether oxygens appear to result from the facilitation of electron tunneling by hydrogen bond interactions between the ether oxygens of the samples and the carboxyl group of 4MBA on the tips. These results demonstrate that the STM observation using chemically modified tips can reveal the orientation of the ether oxygens and furthermore the molecular orientation of the sample molecules. (C) 2001 Elsevier Science BY. All rights reserved.