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 ? at-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).