Effects of strain and defects on the electron conductance of metallic carbon nanotubes Yao He, 1 Chun Zhang, 2 Chao Cao, 1 and Hai-Ping Cheng 1 1 Department of Physics and Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA 2 School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA Received 7 February 2007; revised manuscript received 30 April 2007; published 20 June 2007 Strain dependence of electronic structure and transport properties of 6,0 carbon nanotubes has been thor- oughly studied using first-principles calculations in conjunction with Green’s function techniques. We have found that the quantum conductance is very sensitive to structural deformation and relaxation. The conductance decreases monotonically with increasing strain, for both compression and elongation. In an elongated tube, strain-induced electron localization is the dominating mechanism that controls the contribution of molecular orbitals to conductance. Transport properties are also drastically affected by the presence of defects. Our results have demonstrated that the electronic transport properties of a nanoscale device are closely related to the nature of the band structure of the metallic lead, the details of chemical bonding in the scattering region, and the interaction between Bloch states and the molecular orbitals. DOI: 10.1103/PhysRevB.75.235429 PACS numbers: 85.65.+h, 73.63.-b I. INTRODUCTION Carbon nanotubes CNTs have attracted much attention since their discovery 1 in 1991. They are great candidates for future nanoscale electronic and mechanic device application because of their extraordinarily small diameter, robust stabil- ity, and versatile electronic properties. 2–8 These desirable properties hold the key to the quest of miniaturization of next era computer technology and also to the quest of new mate- rials that are light, strong, and controllable. It is therefore crucial to have a complete understanding of electronic and mechanical properties and the interplay between the two. In- spired by the foreseeable future potential, extensive effort has been made in the past 15 years to study variety proper- ties of CNT ranging from growth, electronic structures, chemical modification, to optical response. 1–8 Early investigations showed that the electronic properties of the CNTs are very sensitive to their geometric structure. The electronic properties of carbon nanotubes can be con- trolled by the structure of the nanotubes and by various de- formations of their geometries. 2–5 In a pioneering experi- ment, Tombler et al. showed that the conductance of a metallic CNT could decrease by orders of magnitude when strained by an atomic force microscope AFM tip. 2 The au- thors suggest that the formation of new C–C bonds within the CNT leads to fourfold coordination of some atoms, and a local distortion of the sp 2 bonding causes the drop in con- ductance. Maiti et al. explained the origin of such a large conductance drop using the opening of a band gap in a me- tallic tube under the tensile strain resulting from AFM deformation. 9 In another AFM experiment, Minot et al. have demonstrated that the conductance changes observed in CNT under AFM deformation are due to strain-induced changes of the band gap, 3 and they explain it by turning to previous theoretical works on strained CNTs which predicted that the rate of change of the band gap as a function of strain depends on the CNT chiral angle. 10–12 Rochefort et al. focus on the effect of twisting and bending on nanotube electronic struc- ture and find that the bending of the nanotubes decreases the transmission function in certain energy ranges and leads to increased electrical resistance. 13,14 Other theoretical calcula- tions find that defects such as Stone-Wales defect, vacancies, or dopant species modify the electronic properties of carbon nanotubes drastically, and the impurity and Stone-Wales de- fect could induce quasibound states and reduce the conductance. 15,16 As a practical issue in any device and/or material application, strain and defects in the carbon nano- tubes are always critically important for tailoring intrinsic properties of CNT and thus for designing new nanosize elec- tronic devices and nanostructured materials. This also sug- gests the possibility of designing nanoelectromechanical sen- sors in which carbon nanotubes are subjected to a uniform external strain. The transport properties of carbon nanotubes with a scat- tering center can be simply described in the Landauer formalism. 17–19 The conductance of a perfect CNT i.e., scat- tering potential is zero equals the number of conducting channels timing the conducting quantum, G 0 =2e 2 / h. This quantized conductance will change in the presence of strain or defects. In this paper, we report our studies of 6,0 CNTs focusing on the interplay between structure and conductance. The rest of the paper is organized as follows: Sec. II is about theoretical model and method, Sec. III presents detailed re- sults, and Sec. IV includes conclusion and discussion. II. MODELS AND METHODS The conductance of a one-dimensional system is calcu- lated using the Landauer-Butticker formula 17–19 in the frame- work of the Caroli model. 20–22 In our approach, we use density-functional method 23 to obtain the effective Hamil- tonian, which is evaluated in momentum space in the plane wave basis representation in this paper, the energy cutoff is taken to be 30.84 hartree in conjunction with Martin- Troullier pseudopotential. The effective potential of the sys- tem is written as V  eff = V  ps + V  H + V  xc , 1 where V  ps is the pseudopotential which represents interac- tions between ions and electrons, V  H is the Hartree potential, PHYSICAL REVIEW B 75, 235429 2007 1098-0121/2007/7523/2354296 ©2007 The American Physical Society 235429-1