应用文章
High-resolution imaging has been the primary feature that attracted the researchers’ attention to scanning probe microscopy yet there are still a number of outstanding questions regarding this function of scanning tunneling microscopes and atomic force microscopes. Here I would like to address a few related issues starting with AFM imaging of alkane layers on graphite. Normal alkanes (chemical formula CnH2n+2) are linear molecules with a preferential zigzag conformation of the -CH2- groups. The terminal -CH3 groups are slightly larger than -CH2- groups but more mobile. At ambient conditions the alkanes with n=18 and higher are solid crystals (melting temperature of C18H38 – 28°C) with the chains oriented practically vertical to the larger faces of the crystals. Such surface of the C36H74 crystal, which is formed of -CH3 groups, was examined in contact mode, and the AFM images revealed the periodical arrangement of these groups 1 . It has been known for a long time that on the surface of graphite the alkane molecules are assembled in lat-laying lamellar structures, in which the fully extended molecules are oriented along three main graphite directions, Fig. 1. This molecular order is characterized by a number of periodicities: the 0.13nm spacing between the neighboring carbon atoms, the 0.25nm spacing between the -CH2- groups along the chain in the zigzag conformation, the 0.5nm interchain distance inside the lamellae and the lamellae width—the length of the extended CnH2n+2 molecule. The latter varies from 2.3nm for C18H38 to 49.5nm for C390H782 (the longest alkane synthesized).
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