Alkanes

Chain molecules consisting of -CH2- groups with slightly bulky end groups -CH3 are chemically simple objects, which have some attractive features for AFM studies. First of all, alkane layers on graphite can be used as standard and calibration samples for imaging in the sub-100nm scale. In addition, these objects can be also considered as primitive heterogeneous system or primitive block copolymer in which -CH3 groups and -CH2- represent "blocks" dissimilar in their size, mobility and mechanical properties. This is a reason that these groups exhibit different contrast in the AFM images.

Fig.1 Height image of C390H782 normal alkane layer on graphite. Hi'Res-C18 AFM probe. Scan size 1µm. Height 3nm. Image courtesy of D. Ivanov.

Another interesting feature is the alkane arrangement on different substrates, which is a result of competition between the alkane molecules interaction with a substrate and their crystallization behavior. Therefore, alkane molecules either align parallel to the surface or form crystals with chains oriented perpendicularly to the substrate. AFM is one of the few techniques than can be used for characterization of these objects and their various transformations. The alkanes and, especially, those with ultra long chains with a number of carbon atoms over 100 are excellent models of polyethylene - one of the mass-produced polymers. AFM studies of chain folding, crystallization and melting in ultra long alkanes offer unique information about similar processes in this and other polymers.

Using the sharpest available AFM probes is always preferable for high-resolution studies of alkane layers. The sharp AFM tips can be less subjective to damage when softer AFM cantilevers are used. Therefore, AFM probes with sharp AFM tips should have AFM cantilever stiffness less than 5N/m. Actually, because sharp AFM tips experience less adhesive interactions, AFM probes with such AFM tips can have very soft AFM cantilevers and still be applied for tapping measurements. Please refer to the High Resolution page for details.