Biomedical Engineering Reference
In-Depth Information
and RT by using several interaction potentials, sometimes quantum
corrected [29, 84, 99]. Nitrogen physical adsorption in CNT arrays
has been studied at subcritical (77 K and 100 K) and supercritical
(300 K) temperatures showing that type II isotherms at subcritical
temperatures can be explained by taking into account the outer
surface adsorption sites of the CNT bundles [50].
The most notable results concerning physical adsorption of
various gaseous species is reviewed in the following with the
important caution that some of them have been omitted due to the
complexity of the chemical scenario found that basically evidences
both chemisorption and physisorption features.
Graphene
Graphene is often considered for sensoring purposes due to the
enhanced effect of surface dopants that can vary the conductivity
even in the limit of zero carrier density. Charge carrier concentration
can be varied by adsorption of various gases even though controlling
and distinguishing between different molecular adsorbates is not
trivial.
Molecular doping with H
, and NO has been
investigated by first principles calculations [64] showing that the
charge transfer between the adsorbate and graphene depends
strongly on the adsorbate orientation and is nearly independent
on the adsorption site. For instance it has been demonstrated that,
contrarily to previous hypotheses [110], paramagnetic molecules
may not behave as strong dopants: indeed, NO functionalization
of graphene induces a negligible charge transfer (<0.02
O, NH
, CO, NO
2
3
2
) with the
equilibrium distance of 0.376 nm and thus no doping occurs. The
different behavior of NO and NO
e
on graphene is due to different
2
phenomena occurring: in the NO
case, a strong charge transfer
from the graphene to the molecule occurs causing strong doping;
on the contrary, in the NO case the charge transfer from/to the
graphene sheet is weak and the mixing between NO HOMO/LUMO
and the graphene orbitals below and above the Fermi energy leads
to physical adsorption.
Carbon Nanotubes
The detection of molecules that do not bind to SWCNTs and remain
physisorbed is an open problem of great technological interest. In
the pioneering work of Zhao and co-workers [121], the adsorption of
several gaseous species, such as H
2
O, NH
, CO
, CH
, NO
, and O
, has
2
3
2
4
2
2
