Atomic Transformations and Quantum Transport in Carbon Nanotubes
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and Q. Zhao Department of Physics, North Carolina State University Raleigh, NC 27695, bernholcncsu.edu ABSTRACT High strain conditions can lead to a variety of atomic transformations in nanotubes, which usually occur via successive bond rotations. The energetic barrier for the rotation is dramatically lowered by strain, and ab initio results for its strain dependence are presented. While very high strain rates must lead to tube breakage, (n,m) nanotubes with n, m < 14 can display plastic flow under suitable conditions. This occurs through the formation of a 5-7-7-5 defect, which then splits into two 5-7 pairs. The index of the tube changes between the 5-7 pairs, potentially leading to metal-semiconductor junctions. The high strain conditions can be imposed on the tube via, e.g., AFM tip manipulations, and we show that such procedures can lead to intratube device formation. The defects and the index changes occurring during the mechanical transformations also affect the electrical properties of nanotubes. We have computed the quantum conductances of strained defective and deformed tubes using the tight binding method. The results show that the defect density and the contacts play key roles in reducing the conductance at the Fermi energy. We also explored the role of bending in changing the electrical properties and found that mechanical deformations affect differently the transport properties of achiral and chiral nanotubes. Our results are in good agreement with recent experimental data. INTRODUCTION The field of carbon nanotubes has seen an explosive growth in the recent years due to the substantial promise of these molecular structures for the use as high-strength, light-weight materials, superstrong fibers, novel nanometer scale electronic and mechanical devices, catalysts, and as energy storage media. Despite the potential impact that the materials based on carbon nanotubes would have in many areas of science and industry, a complete characterization of their mechanical and electrical properties is still lacking. Carbon nanotubes have already demonstrated exceptional mechanical properties: the excellent resistance to damage during bending has already been observed experimentally and studied theoretically [1, 2]. Their high stiffness combines with resilience and the ability to reversibly buckle and collapse: even largely distorted configurations (axial compression, twisting) can be due to elastic deformations with no atomic defects involved [1, 2, 3. 4]. We have focused on the theoretical analysis of the mechanism of strain release in carbon nanotubes under uniaxial tension, in an effort to address the question of the ultimate strength of these nanostructures. This issue requires the modeling of inherently mesoscopic phenomena, such as plasticity and fracture, on a microscopic, atomistic level, and constitutes a substantial challenge for both theory and experiment. Understanding the response of carbon nanotubes to large deformations is also important for device design. Usually, the positioning of an individual n
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