Biomedical Engineering Reference
In-Depth Information
When two impurities are adsorbed on the CNT side wall it is
shown that the transport behavior depends strongly on the position
of the second impurity so that conductivity is either unchanged with
respect to the single impurity case or it is completely suppressed.
The situation is different for semiconducting SWCNTs where
some molecular species may not be chemisorbed; it has been
shown, for instance, that carbon monoxide cannot be chemisorbed
onto a zig-zag CNT, except in the case it is deformed by applying a
uniaxial stress orthogonal to the tube axis; in this case, indeed, the
local chemical activity is altered and CO can be bonded onto the CNT
[90]. However it is still not clear how to exploit such deformation
properties in a sensor device.
Impurities in CNT may help to design possible chemical sensors;
in particular, metal impurities, indeed, are able to bind to different
molecular species that do not form chemical bonds onto pure CNTs.
Ab initio
O detection have
evidenced that metal impurities enhance the CNT chemical reactivity
so that the molecule binding energy is substantially increased [80]
being also accompanied by a large charge transfer from the nanotube
to the molecule.
Systematic investigations on small molecules adsorbed on a
Pt doped armchair SWCNT [115] have shown that most of the
examined species are chemisorbed with a significant charge transfer
from the nanotube to the adsorbate resulting in a change of the CNT
conductance. NH
calculations of B-doped CNTs for CO and H
2
behave differently (the charge transfer is opposite)
due to the high LUMO energy level of the adsorbed molecule that
inhibits the charge transfer from the nanotube.
TM-doped CNTs, perhaps, are the most promising candidates
for detection of small molecules under standard conditions. To this
aim, in recent experiments CNTs have been irradiated with Ar ion
beams to introduce (in a controlled way) vacancies that may host
TM impurities, strongly bonded to the CNTs, thanks to the partially
occupied TM
3
orbitals.
DFT total energy calculations have shown that substitutional
atoms of most of the 3
d
TMs exhibit a high binding energy in
different sites of an armchair CNT (see Fig. 8.11), with the exception
of Cu and Zn that are rather unstable because of their fully occupied
d
d
bands [35]. The general trend emerging is that light TMs can bind
several adsorbate molecular species (N
S)
with larger binding energy with respect to the one occurring with
, O
, H
O, CO, NH
, and H
2
2
2
3
2
