  The encapsulation of hydrogen-terminated nanosized diamond fragments (the
so-called diamondoids) into armchair single walled carbon nanotubes with
diameters in the range of 1.0 up to 2.2 nm has been investigated using
classical molecular dynamics simulations. Diameter dependent molecular ordered
phases were found for the encapsulation of adamantane (C10H16), diamantane
(C14H20), and dihydroxy diamantane (C14H20O2). The same types of chiral ordered
phases (double, triple, 4- and 5-stranded helices) observed for the
encapsulation of C60 molecules were also observed for diamondoids. On the other
hand, some achiral phases comprising layered structures were not observed. Our
results also indicate that the modification of diamantane through
functionalization with hydroxyl groups can lead to an enhancement of the
packing of molecules inside the nanotubes compared to nonfunctionalized
compounds. Comparisons to hard-sphere models are also presented revealing
differences, specially when more asymmetrical diamondoids are considered. For
larger structures (adamantane tetramers) we have not observed long-range
ordering for nanotubes with diameters in the range of 1.49 to 2.17 nm but only
a tendency to form incomplete helical structures.
