  Recently we investigated from first principles screening properties in
systems where small molecules, characterized by a finite electronic dipole
moment, are encapsulated into different nanocages. The most relevant result was
the observation of an antiscreening effect in alkali-halide nanocages
characterized by ionic bonds. Here we extend the study to another class of
nanostructures, the nanotubes. Using first-principles techniques based on the
Density Functional Theory, we studied the properties of endohedral nanotubes
obtained by encapsulation of a water molecule or a linear HF molecule. A
detailed analysis of the effective dipole moment of the complexes and of the
electronic charge distribution suggests that screening effects crucially depend
not only on the nature of the intramolecular bonds but also on the size and the
shape of the nanotubes, and on the specific encapsulated molecule. As observed
in endohedral nanocages, screening is maximum in covalent-bond carbon
nanotubes, while it is reduced in partially-ionic nanotubes and an
antiscreening effect is observed in some ionic nanotubes. However in this case
the scenario is more complex than in corresponding ionic nanocages. In fact the
specific geometric structure of alkali-halide nanotubes turns out to be crucial
for determining the screening/antiscreening behavior: while nanotubes with
octagonal transversal section can exhibit an antiscreening effect, which
quantitatively depends on the number of layers in the longitudinal direction,
instead nanotubes with dodecagonal section are always characterized by a
reduction of the total dipole moment, so that a screening behavior is observed.
Our results therefore show that, even in nanotube structures, in principle one
can tune the dipole moment and generate electrostatic fields at the nanoscale
without the aid of external potentials.
