4
all N impurities. At this point it sufﬁces to see that the con-
servative estimates obtained from Eq. (7) predict measurable
signals in response to small changes in concentration of the
target molecules.
To our knowledge, controlled doping of CNTs with transi-
tion metal atoms has so far not been achieved. It has, how-
ever, been found that metal atoms incorporated into the CNT
lattice during catalytic growth are afterwards very difﬁcult to
remove [30]. Furthermore, it has been shown that CNT vacan-
cies, which are needed for the metallic doping, may be formed
in a controlled way by irradiation by Ar ions [31]. This sug-
gests that metallic doping of CNTs should be possible.
In summary, we have presented a general model of nanos-
tructured chemical sensors which takes the adsorption en-
ergies of the relevant chemical species and their individual
scattering resistances as the only input. On the basis of this
model we have performed a computational screening of tran-
sition metal doped CNTs, and found that Ni-doped CNTs are
promising candidates for detecting CO in a background of air.
The model may be applied straightforwardly to other nanos-
tructures than CNTs, other functionalizations than metal dop-
ing and other gas compositions than air.
The authors acknowledge ﬁnancial support from Span-
ish MEC (FIS2007-65702-C02-01), “Grupos Consolidados
UPV/EHU del Gobierno Vasco” (IT-319-07), e-I3 ETSF
project (Contract Number 211956), “Red Espa˜nola de Super-
computaci´on”, NABIIT and the Danish Center for Scientiﬁc
Computing. The Center for Atomic-scale Materials Design
(CAMD) is sponsored by the Lundbeck Foundation. JMG-L
acknowledges funding from Spanish MICINN through Juan
de la Cierva and Jos´e Castillejo programs.
∗ Electronic address: juanmaria.garcia@ehu.es
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