Nanotechnology 24, 375702 (2013)
Bromophenyl functionalization of carbon nanotubes: an ab initio study
Jonathan Laflamme Janssen1, Jason Beaudin1, Nicholas D. M. Hine2,3, Peter D. Haynes2,3 and Michel Côté1
1Département de physique et Regroupement Québécois sur les Matériaux de Pointe (RQMP), Université de Montréal, C. P. 6128 Succursale Centre-ville, Montréal, QC, H3C 3J7, Canada
2Department of Physics, Imperial College London,
Exhibition Road, London SW7 2AZ, UK
3Department of Materials, Imperial College London,
Exhibition Road, London SW7 2AZ, UK
We study the thermodynamics of bromophenyl functionalization of carbon
nanotubes with respect to diameter and metallic/insulating character using
density-functional theory (DFT). On the one hand, we show that the
functionalization of metallic nanotubes is thermodynamically favoured over
that of semiconducting ones, in agreement with what binding energy
calculations previously suggested. On the other hand, we show that the
activation energy for the grafting of a bromophenyl molecule onto a
semiconducting zigzag nanotube ranges from 0.72 to 0.75 eV without any
clear diameter dependence within numerical accuracy. This implies that this
functionalization is not selective with respect to diameter at room
temperature, which explains the contradictory experimental selectivities
reported in the literature. This contrasts with what is suggested by the
clear diameter dependence of the binding energy of a single bromophenyl
molecule, which ranges from 1.52 eV for an (8,0) zigzag nanotube to
0.83 eV for a (20,0) zigzag nanotube. Also, attaching a single
bromophenyl to a nanotube creates states in the gap close to the
functionalization site. It therefore becomes energetically favourable for
a second bromophenyl to attach close to the first one on semiconducting
nanotubes. The para configuration is found to be favoured for
resulting bromophenyl pairs and their binding energy is found to decrease
with increasing diameter, ranging from 4.35 eV for a (7,0) nanotube to
2.26 eV for a (29,0) nanotube. An analytic form for this radius
dependence is derived using a tight binding Hamiltonian and first order
perturbation theory. The 1/R2 dependence obtained (where
R is the nanotube radius) is verified by our DFT results within
numerical accuracy. Finally, bromophenyl pairs are shown to be favoured by
only 50 meV with respect to separate moieties on (9,0) metallic
nanotubes, which suggests that pair formation is not significantly favoured
on some metallic nanotubes. This result explains the observation of stable isolated moieties at room
temperature in nanotube samples containing random nanotube chiralities.
Last updated: 27 August 2013