Formation energies per carbon atom (a) and HOMO-LUMO gaps (b) of different carbon fullerenes, . The formation energy is defined as the total energy required to form a structure with respect to that of a graphene.
The optimized complexes displayed in the left panels of (a) and their difference charge densities are shown in the right panels. The difference electron charge density are defined as . The transferred electrons from a La atom to a carbon cage are highly localized inside a cage and very small density of charges are dispersed outside of the cage. The LDOS of pristine fullerenes (b) and metallofullerenes (c) are compared. For large fullerenes three extra electrons are occupied at the carbon-related states and the charging states become . For smaller fullerenes, less than three electrons are transferred from a La atom to a cage due to strong chemical coupling between a fullerene and a La atom. The charge densities are in the unit of and the dotted-yellow lines stand for Fermi level.
Binding energies of a hydrogen molecule to either pristine fullerenes (empty points) or metallofullerenes (solid points). The calculations performed by LDA (a) and GGA (b) are compared. We considered different hydrogen adsorption sites and found no strong dependence on the adsorption sites. The displayed results are taken from the highest values from different hexagon sites of each fullerene.
(a) Electric fields produced by metallofullerenes (red-dashed lines) and pristine fullerenes (black lines) of , 50, 60, and 82. The black arrows indicate the location of carbon cages and the red arrow the La location. (b) The charge differences of a hydrogen molecule are displayed. Here, the difference charge densities are shown in the right panels. The difference electron charge density are defined as or , where the charge densities are in the unit of .
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