Nanosensores

PHYSICAL REVIEW B 81, 245429 2010

Modeling nanoscale gas sensors under realistic conditions: Computational screening
of metal-doped carbon nanotubes
J. M. García-Lastra,1,2 D. J. Mowbray,1,2 K. S. Thygesen,2 A. Rubio,1,3 and K. W. Jacobsen2
1

Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Dpto. Física de Materiales, Universidad del País Vasco,
Avenue Tolosa 72,E-20018 San Sebastián, Spain
2Center for Atomic-Scale Materials Design, Department of Physics, Technical University of Denmark,
DK-2800 Kongens Lyngby, Denmark
3
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany
Received 31 May 2010; published 22 June 2010
We use computational screening to systematically investigate the use of transition-metal-doped carbon
nanotubes forchemical-gas sensing. For a set of relevant target molecules CO, NH3, and H2S and the main
components of air N2, O2, and H2O , we calculate the binding energy and change in conductance upon
adsorption on a metal atom occupying a vacancy of a 6,6 carbon nanotube. Based on these descriptors, we
identify the most promising dopant candidates for detection of a given target molecule. From the fractionalcoverage of the metal sites in thermal equilibrium with air, we estimate the change in the nanotube resistance
per doping site as a function of the target molecule concentration assuming charge transport in the diffusive
regime. Our analysis points to Ni-doped nanotubes as candidates for CO sensors working under typical
atmospheric conditions.
DOI: 10.1103/PhysRevB.81.245429

PACS number s :73.63. b, 68.43. h, 73.50.Lw

I. INTRODUCTION

The ability to detect small concentrations of specific
chemical species is fundamental for a variety of industrial
and scientific processes as well as for medical applications
and environmental monitoring.1 In general, nanostructured
materials should be well suited for sensor applications because of their large surface to volume ratio which makesthem sensitive to molecular adsorption. Specifically, carbon
nanotubes CNTs Ref. 2 have been shown to work remarkably well as detectors of small gas molecules. This has been
demonstrated both for individual CNTs Refs. 3–8 as well
as for CNT networks.9,10
Pristine CNTs are known to be chemically inert—a property closely related to their high stability. As a consequence,
only radicals bindstrong enough to the CNT to notably affect its electrical properties.2,5,11–13 To make CNTs attractive
for sensor applications thus requires some kind of functionalization, e.g., through doping or decoration of the CNT
sidewall.13–21 Ideally, this type of functionalization could be
used to control not only the reactivity of the CNT but also
the selectivity toward specific chemical species.
In thiswork we consider the possibility of using CNTs
doped by 3d transition-metal TM atoms for chemical-gas
sensing. We use computational screening to systematically
identify the most promising dopant candidates for detection
of three different target molecules CO, NH3, and H2S under
typical atmospheric conditions. The screening procedure is
based on the calculation of two microscopicdescriptors: the
binding energy and scattering resistance of the molecules
when adsorbed on a doped CNT. These two quantities give a
good indication of the gas coverage and impact on the resistance. For the most promising candidates we then employ a
simple thermodynamic model of the CNT sensor. In this
model, the binding energies are used to obtain the fractional
coverage of the metallic sites as afunction of the target mol1098-0121/2010/81 24 /245429 10

ecule concentration under ambient conditions. Under the assumption of transport in the diffusive rather than localization
regime, the change in CNT resistivity may then be obtained
from the calculated coverages and single impurity conductances.
Over the last few years a large number of theoretical studies on CNTs and graphene doped...