Screening and antiscreening in fullerene-like cages: Dipole-field amplification with ionic nanocages

The successful synthesis of endohedral complexes consisting of nanoscale carbon cages that can encapsulate small molecules has been a remarkable accomplishment since these systems are ideal models to investigate how confinement effects can induce changes in structural and electronic properties of encapsulated molecular species. We here investigate from first principles screening effects observed when small molecules, characterized by a finite electronic dipole moment, such as HF, LiF, NaCl, and H2O, are encapsulated into different nanoscale cages: C 60 , C 72 , B36N36, Be36O36, Li36F36, Li36Cl36, Na36F36, Na36Cl36, and K36Br36. Binding energies and electronic properties, of these complexes have been computed. In particular, detailed analysis of the effective dipole moment of the complexes and of the electronic charge distribution suggests that screening effects crucially depend on the nature of the intramolecular bonds of the cage: screening is maximum in covalent-bond carbon nanocages, while it is reduced in partially-ionic nanocages B36N36 and Be36O36, being very small in the latter cage which turns out to be almost "electrically transparent ". Interestingly, in the case of the ionic-bond nanocages, an antiscreening effect is observed: in fact, due to the relative displacement of positive and negative ions, induced by the dipole moment of the encapsulated molecule, these cages act as dipole-field amplifiers. Our results open the way to the possibility of tuning the dipole moment of nanocages and of generating electrostatic fields at the nanoscale without the aid of external potentials. Moreover, we can expect some transferability of the observed screening effects also to nanotubes and 2D materials.