Qingzhong Zhao

Mechanical properties, defects and electronic behavior of carbon nanotubes

Abstract Using state-of-the-art classical and quantum simulations, we have studied the mechanical and electronic response of carbon nanotubes to external deformations, such as strain and bending. In strained nanotubes the spontaneous formation of double pentagon–heptagon defect pairs is observed. Tubes containing these defects are energetically preferred to uniformly stretched tubes at strains greater than 5%. These defects act as nucleation centers for the formation of dislocations in the originally ideal graphitic network and constitute the onset of further deformations of the carbon nanotube. In particular, plastic or brittle behaviors can occur depending upon the external conditions and tube symmetry. We have also investigated the effects that the presence of addimers has on strained carbon nanotubes. The main result is the formation of a new class of defects that wrap themselves about the circumference of the nanotube. These defects are shown to modify the geometrical structure and to induce the formation of nanotube-based quantum dots. Finally, we computed transport properties for various ideal and mechanically deformed carbon nanotubes. High defect densities are shown to greatly affect transport in individual nanotubes, while small diameter bent armchair nanotubes mantam thier basic electrical properties even in presence of large deformations with no defects involved.

Large-Scale Applications of Real-Space Multigrid Methods to Surfaces, Nanotubes, and Quantum Transport

The development and applications of real-space multigrid methods are discussed. Multigrid techniques provide preconditioning and convergence acceleration at all length scales, and therefore lead to particularly efficient algorithms. When using localization regions and optimized, non-orthogonal orbitals, calculations involving over 1000 atoms become practical on massively parallel computers. The applications discussed in this chapter include: (i) dopant incorporation and ordering effects during surface incorporation of boron, which lead to the formation of ordered domains at half-monolayer coverage; (ii) incorporation of Mg into GaN during growth, and in particular the conditions that would lead to maximum p-type doping; (iii) optical fingerprints of surface structures for use in real-time feedback control of growth: and (iv) mechanisms of stress release and quantum transport properties of carbon nanotubes.

Large-scale simulations of advanced materials and nanoscale devices

Recent advances in theoretical methods and parallel supercomputing allow for reliable ab initio simulations of the properties of complex materials. We describe two current applications: pyro- and piezoelectric properties of BN nanotubes and optical signatures of organic molecules on Si(001) surface. BN nanotubes turn out to be excellent piezoelectrics, with response values significantly greater than those of piezoelectric polymers. However, their symmetry leads to exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes. Breaking of this symmetry induces spontaneous polarization comparable to those wurtzite semiconductors. Turning to organics on Si(100), we calculated the atomic structure and the optical signatures of a cyclopentene overlayer on Si(001). Cyclopentene can be used to attach a variety of organic molecules to Si devices, including DNA, and can therefore form a basis of a sensor structure. The spectra turn out to be highly structure-dependent and can therefore be used to monitor interface formation.

Mechanical Properties and Electronic Transport in Carbon Nanotubes

The field of carbon nanotubes is undergoing an explosive growth due to both the intrinsic interest in these molecular structures and their technological promise in, e.g., high strength, light weight materials, superstrong fibers, novel nanometer scale electronic and mechanical devices, catalysts, and energy storage. Despite the potential impact that nanotubes could have in many areas of science and industry, the characterization of their mechanical and electrical properties is still incomplete. We show that nanotubes under high strain conditions can undergo a variety of atomic transformations, often occurring via successive bond rotations. The barrier for the rotation is dramatically lowered by strain. While very high strain rates must lead to breakage, (n,m) nanotubes with 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. We have also computed quantum conductances of strained tubes, defects, and nanotube junctions, since these deformations are likely to occur when nanotubes are used to form nanoscale electronic devices. The results show that the defect density and the contacts play key roles in reducing the conductance at the Fermi energy, while bending and mechanical deformations affect differently the conductance of achiral and chiral nanotubes. Our results are in good agreement with recent experimental data.

ATOMIC TRANSFORMATIONS, STRENGTH, PLASTICITY, AND ELECTRON TRANSPORT IN STRAINED CARBON NANOTUBES

Nanotubes are hollow cylinders consisting of ‘rolled-up’ graphitic sheets. They form spontaneously in the same apparatus as the famed C60 molecule, and have been predicted and/or observed to have even more spectacular properties than C60, including extremely high strength and flexibility, ability to form nanoscale electronic devices consisting entirely of carbon, strong capillary effects, cold cathode field emission, etc. Carbon nanotubes have also been theoretically predicted to be among the strongest materials known. Their strength, which has already been verified experimentally, may enable unique applications in many critical areas of technology. While very high strain rates must lead to tube breakage, nanotubes with (n,m) indices, where n,m < 14, can display plastic flow under suitable conditions. This occurs through the conversion of four hexagons to 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. Furthermore, carbon adatoms-induced transformations in strained nanotubes can lead to the formation of quantum dots. 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. The computed quantum conductances of strained defective and deformed tubes show that the defect density and the contacts play key roles in reducing the conductance at the Fermi energy. The role of bending in changing the electrical properties was also explored. It was found that mechanical deformations do not significantly affect the conductance of bent armchair nanotubes up to substantial bending angles, while a conductivity gap is induced by the bending of chiral nanotubes. These results are in good agreement with recent experimental data.

Atomic Transformations and Quantum Transport in Carbon Nanotubes

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 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.