Effect of localized oxygen functionalization on the conductance of metallic carbon nanotubes

PHYSICAL REVIEW B 79, 115428 2009 Effect of localized oxygen functionalization on the conductance of metallic carbon nanotubes M. K. Ashraf, 1, * Nicolas A. Bruque, 1,† Rajeev R. Pandey, 2 Philip G. Collins, 3 and Roger K. Lake 1 1 Department of Electrical Engineering, University of California–Riverside, Riverside, California 92521, USA of Chemistry, University of the Pacific, Stockton, California 95211, USA 3 Department of Physics and Astronomy, University of California–Irvine, Irvine, California 92697, USA Received 9 September 2008; revised manuscript received 28 January 2009; published 20 March 2009 2 Department A comprehensive study of the effect of covalent oxygen attachment on the transmission and conductance of armchair and metallic zigzag carbon nanotubes CNTs is presented. In both armchair and zigzag CNTs covalent oxygen attachment favors an ether-type bond in which the C-C bond breaks. Oxygen atoms attached on the CNT surface within the same carbon ring on parallel bonds are energetically more stable than well- separated attachments. In an armchair CNT, oxygen attachment favors the C-C bonds orthogonal to the CNT axis. Cooperative addition propagates axially along parallel orthogonal bonds. In a zigzag CNT, oxygen attachment prefers the slanted bond, and cooperative addition propagates spirally along parallel slanted bonds. Closely spaced oxygen attachment on the armchair and zigzag CNT surfaces causes a dip in transmission symmetrically away from the Fermi level at the turn-on of the first excited modes. For both armchair and zigzag CNTs, as more oxygen atoms are placed in close proximity, their levels interact and split and move closer to the Fermi level which results in broader dips in transmission closer to the Fermi level. The transmis- sion of armchair CNTs near the charge-neutral Fermi level is relatively insensitive to a group of localized oxygen atoms compared to that of metallic zigzag CNTs. A clustered group of oxygen atoms covalently attached to a single-walled metallic zigzag CNT can result in a 1 order of magnitude drop in transmission that is asymmetric with respect to the Fermi energy resulting in a qualitative resemblance to conductance versus gate voltage curves observed experimentally. The covalent attachment of a single oxygen atom in any con- figuration, on either, an armchair, or zigzag metallic CNT does not give rise to a large change in conductance. Calculations use density-functional theory combined with nonequilibrium Green’s functions. DOI: 10.1103/PhysRevB.79.115428 PACS numbers: 73.63.Fg, 31.15.ae, 73.22.Ϫf I. INTRODUCTION Carbon nanotubes CNTs are a candidate for future na- noelectronics applications due to their exotic electronic prop- erties. Having their size comparable to the chemically active molecules, they can provide direct access to the chemical environment and provide information about single molecule events in chemical reactions and biological processes. 1 They are a candidate for elements of molecular level sensor devices. 2,3 The electrical detection of a single chemical at- tachment is regarded as a promising tool for future metrol- ogy applications. 4 The recent report describing the controlled addition of one or several localized molecular attachments on a CNT surface 5,6 provides the focus for this paper. One or several localized molecular attachments on a CNT were shown to significantly alter the conductance of the CNTs and even change the conductance versus gate voltage response of me- tallic CNTs to resemble that of p-type semiconducting CNTs. 5 We pose the question, “Can one or several localized covalently attached oxygen atoms significantly alter the con- ductance of a metallic CNT as observed experimentally?” We describe the results of a comprehensive set of simula- tions investigating the effect of covalently attached oxygen atoms on the CNT transmission and conductance. Interest in CNT functionalization began when Chen et al. 7 reported the first covalent functionalization of CNTs by nitric acid treatment. They demonstrated that covalent addition of dichlorocarbene to the sidewalls of CNTs can transform a metallic CNT to a semiconducting one when the Cl/C con- centration ratio reaches around 25%. 8,9 Theoretical studies followed the experiments and revealed many surprising phenomena. 10,11 The effect of functionalization on the energy bands and transport properties has been studied using various density-functional theory DFT methods. 12–17 One result that was established through these studies is that monovalent atoms when attached on the sidewall of CNTs break the sp 2 network at the site, and the ␲ bond at the C attachment site transforms to a sigma bond. 16 This bonding pattern affects the transmission spectra close to the Fermi level, and about 25% adatom to CNT C ratio can completely destroy the con- ducting properties of CNTs. 11 Covalent attachment of diva- lent atoms, on the other hand, retains the local sp 2 network of the pristine CNT and moderately perturb the conductance. Even at 25% concentration, the conductance only drops by a factor of 2, and the CNT remains conductive. These theoret- ical studies concentrated on adatoms distributed randomly over the CNT surface. Only a truly random process such as irradiation will in- troduce sparse, randomly distributed functional sites. When an atom attaches to a CNT, it perturbs nearby bonds and enhances the reactivity of adjacent sites. It then becomes energetically favorable for the next addition to attach in close proximity to the initial adatom. This type of cooperative be- havior in which consecutive additions occur in close proxim- ity in a cooperative pattern is referred to as cooperative ad- dition. Yumura et al. 18 reported an example of cooperative behavior in divalent group attachment on CNTs. They found cooperative behavior among functionalization groups only when the first group was attached endohedrally, on the inside of the CNT surface. They did not find any cooperative effect ©2009 The American Physical Society