Biomedical Engineering Reference
In-Depth Information
been studied by
calculations showing that these molecules
are only physisorbed on SWCNTs.
Most of the molecules studied are charge donors with small
charge transfer (between 0.01
ab initio
e
and 0.023
e
per molecule) and exhibit
a weak binding energy (
≤ 0.2 eV) with no substantial electron
density overlap between the adsorbate and the nanotube.
On the contrary, acceptors such as O
E
a
exhibit a significant
charge transfer, often accompanied by large adsorption energy, thus
indicating that chemical and physical adsorption characters coexist.
Aromatic compounds interacting with carbon nanostructures
show a similar uncertain nature of the bonding and their weak
intermolecular forces, including van der Waals interactions, are
often referred to as
and NO
2
2
-stacking interactions as they originate from the
states of the interacting systems [101]. Considering the adsorption of
C
p
on a CNT, the most favorable configuration is with the benzene
ring above the middle of a C-C bond (bridge position), when the
diameter of the nanotube is not too large. On the contrary, the most
stable adsorption configuration on graphene is with the benzene
molecule above a carbon atom (top position). Therefore the most
favorable adsorption geometry should evolve from bridge to top as
the nanotube diameter increases. The density of states (DOS) of the
entire system is practically the superposition of the benzene and the
CNT DOS, consistently with the fact that the
H
6
6
-stacking interaction
results in a very small binding energy. Consequently, it seems that
the adsorption of benzene on a CNT is more appropriately classified
as physisorption, although van der Waals interactions are not
involved. Woods and co-workers [112] have reconsidered critically
the adsorption of three different benzene-derived molecules on
semiconducting (8,0) SWCNT, namely, aniline (C
p
H
-NH
), toluene
6
5
2
(C
); more precisely they have
compared the properties of these complexes with the adsorption
features of benzene and of simple closed-shell molecules with the
same functional group: NH
H
-CH
), and nitrobenzene (C
H
-NO
6
5
3
6
5
2
.
The general trend found is that compounds with closed shells
are always physisorbed with minor changes in the nanotube
electronic structure. Different configurations, namely, parallel
and perpendicular to the nanotube axis, have been studied for
alanine, toluene, and nitrobenzene: in the case of the perpendicular
configuration, the interaction is promoted by the functional groups
and the benzene ring affects the adsorption only indirectly; for the
parallel configuration, instead, the interaction occurs through the
, CH
, and CH
NO
3
4
3
2
