Electronic and Structural Properties of Carbon Nanotubes Molecular Junction
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Electronic and Structural Properties of Carbon Nanotubes Molecular Junction M. Machado1, P. Piquini1, R. Mota1 and A. Fazzio1, 2 1 Departamento de Física, Universidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil. 2 Instituto de Física, Universidade de São Paulo, CxP 66318, 05315-970, São Paulo, SP, Brazil. ABSTRACT The electronic and structural properties of finite junctions produced by connecting two C nanotubes, saturated with hydrogens in both edges, are investigated using first-principles calculations, through self-consistent field Hartree-Fock-Roothaan method. The main target of our study is a molecular junction, which connects (10,0) and (6,6) tubes by the introduction of pentagon-heptagon pair defects diametrically opposed in the structure. The charge distributions, the character of the highest occupied molecular orbitals and the densities of states are determined for the finite (10,0) and (6,6) nanotubes and for the formed junction. An energetic analysis is also performed using ab-initio approach and empirical Tersoff potential. INTRODUCTION Molecular electronics has had enormous development during the last decade [1]. In the case of carbon nanotubes, in the recent literature, there are few works considering the electronic structure of tubes changing helicities within a single tube [2-5]. Such junctions could be the invaluable building blocks, in the future miniaturized electronics, of nanoscale electronic devices made entirely of carbon. These and related nanotube structures have been treated theoretically and they yield a rich spectrum of nanodevice characteristics, including highly nonlinear transport currents, asymmetries, and diode-like rectification [6-7]. The use of topological defects, like introduction of pentagon-heptagon pair defects, bends the structure at an angle that depends on the distance between both defects. With the two defects aligned along a side, straight connections can be realized. When the two defects are positioned at the two diametrically opposed sides of the structure, the axis of the two connected tubes makes an angle around 360. Also toroidal and helicoidal structures can be generated [8-9]. We have assumed finite tubes with hydrogens in both edges, which, due to their lower electronegativity in comparison with carbons, can be seen as simulating possible metallic contacts. We focus on tubes with a finite length since this could be one relevant architecture for molecular electronic applications. This scheme does not allow the use of the Bloch’s theorem and, consequently, the calculation is performed on the joint without any periodic boundary condition along the direction parallel to the axis of the nanotube. The (10,0)/(6,6) junction is usually called a semiconductor-metal hybrid, since the original infinite (10,0) zigzag nanotube is a semiconductor, whereas the infinite armchair (6,6) is a true metallic system. Of course, the concepts of metal and semiconductor in our case (length around 25 Å and diameter around 8 Å) become a little meanless, due to the truncated
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