Using non-contact atomic force microscopy with metallic tips enabled us to detect the electrostatic dipole moment of single carbon monoxide (CO) molecules adsorbed on three very different substrates. The observed distance dependent contrast can be explained by an interplay between the attractive van der Waals interaction and the repulsive electrostatic interaction, respectively, with the latter stemming from antiparallel aligned dipoles in tip and molecule. Our results suggest that metallic as well as CO-functionalized tips are able to probe electrostatic properties of polar molecules and that repulsive dipole-dipole interactions have to be considered when interpreting complex contrast patterns.
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Since the CO triple bond contains one dative bond with both electrons stemming from the oxygen, the corresponding shift of electron density overcompensates the larger electronegativity of oxygen.
Any additionally induced dipole moment due to polarization effects should be negligible, because ideally the electric field between tip and sample is zero for U = UCPD.
