AFM-induced melt growth on n-paraffin crystals

The {001} surfaces of n-C23H48 paraffin crystals have been investigated by ex situ alternative contact mode atomic force microscopy (AC-AFM). Although the crystals were scanned in air, without any solution present and below the melting temperature of n-C23H48, crystal growth or etching was revealed in all experiments, independently of the AC-AFM settings. Formation of 2D islands, growing spirals and pinning of advancing steps have been observed. It is shown that these crystal growth phenomena, which involve the nucleation and the propagation of monomolecular steps, are induced by the presence of the AFM tip. From temperature- and melting point-dependent measurements it is concluded that the observed crystal growth on n-C23H48 crystals is caused by a combination of local heating by the AFM laser beam and the action of capillary forces at the AFM tip, which generate a liquid paraffin bridge between tip and specimen surface. Control over the occurrence of growth versus etching was limited and is associated with the amount of paraffin stored in the liquid bridge. ype of ex situ investigations gives a good opportunity to study melt growth on a molecular scale, which would have been impossible with AFM in situ experiments, because of the extremely high requirements for temperature control needed for that case. Despite the different mechanism from that in ‘real’ melt growth experiments, the current system behaves remarkably similarly. Furthermore, the method may be suitable for surface patterning applications.