Breaking the Intermolecular Hydrogen Bonds in Organic Dimers with Atomic Force Microscopy

Microcrystals of 1,1‘-bicyclohexyliden-4-one oxime (oxime) were prepared with the spin-coating technique. In these crystals, the oxime molecules form dimers through intermolecular hydrogen bonding. In this orientation, the crystals have a hydrophobic surface. The crystals were imaged with atomic force microscopy (AFM) at three different temperatures, i.e., 16, 20, and 30 °C. We show that it is possible to break the dimeric hydrogen bond with the AFM tip. The force necessary to accomplish this is temperature dependent and increases with decreasing temperature; its magnitude is on the order of the strength of the dimer bond. Breaking of the dimeric hydrogen bonds only occurs near the step edges. In these regions, a clear contrast in friction imaging was observed with lateral force microscopy together with a reduction of the step height to half the original value. We interpreted this as the removal of one layer of oxime molecules resulting in an (oxime-terminated) hydrophilic surface with a higher friction contrast as compared to the normal hydrophobic surface. In this way, the surface properties of the crystals can be tuned. This concept may be applicable to other systems.