My two cents worth, but a point which is frequently poorly reported
in the literature. Neither AFM or STM produce true "images" in many
cases. While the images look like atoms, in AFM one is seeing those
components of the structure which produce a force (depending upon
details of exactly how the measurement is done); in STM the local
density of filled/unoccupied states at the Fermi level. Certainly for
STM there are numerous cases in the Surface Science literature where
erroneous interpretations were made. One of the early classic examples
was for gold on Si (111) where STM showed one "atom" -- it later turned
out that this was in fact 3 atoms in a trimer that could not be
resolved.
To the best of my knowledge one has to know the structure in order to
work
out what the images show.
I would not worry that AFM & STM images show different (smaller) sizes;
sometimes they are bigger (e.g. for small clusters due to tip
convolution
effects).
> Clarity on this point is very important to me because I notice that the
> widely reported diameter of each sugar-phosphate helix in the true
> side-by-side model of Lee et al. found in AFM & STM, and in other studies,
> at values around 1.2 - 1.3 nm, is just about root 3 smaller than the double
> helix diameter at about 2 - 2.2 nm.
>
--
L. D. Marks
Department of Materials Science and Engineering
Northwestern University, Evanston, IL. USA
mailto:l-marks at nwu.eduhttp://www.numis.nwu.edu
