Fermi-Energy-Dependent Structural Deformation of Chiral Single-Wall Carbon Nanotubes

Abstract

In this work, we use an extended tight-binding approach for calculating the Fermi-energy dependence of the structural deformation of chiral single-wall carbon nanotubes (SWNTs). We show that, in general, nanotube strains occur in such a way as to avoid a net charge from being accumulated on the nanotube. We also investigate the effect of the Fermi-energy-induced strains on the electronic structure of SWNTs, showing that the optical transition energies change by up to 0.5 eV due to the induced strains and that this change is nearly independent of how the nanotube is deformed. Finally, we also consider the contribution of the electron-electron Coulomb repulsion to the total energy by using an effective regularized potential energy model. We show that the inclusion of the Coulomb repulsion leads to larger strains and smaller net charges transferred to the nanotube. DOI: 10.1103/PhysRevApplied.2.014006 The nanomechanical actuation of carbon nanotubes can be controlled by using different approaches, each relying on a different mechanism for the coupling between the nanotube mechanical properties and some other controllable parameter. Modeling the nanotube nanomechanical responses to external stimuli is usually a difficult task which depends strongly on the processes involved and the type of stimulus used. For example, a nanoelectromechanical actuator works by tuning nanotube properties through an applied external electric field. However, an electric field can affect the nanotube in several different ways, which usually cannot be captured within a simple unique model. For this reason, the most common approach is to select the stimulus process which seems to be most relevant for the particular application and to use it to model the nanotube response. For example, Witkamp, Poot, and van der Zant [1] consider a direct electrostatic (capacitive) force between a gate and a suspended multiwall carbon nanotube to explain the actuation properties observed in their devices. Indeed, there are plenty of devices based on this electrostatic approach, such as the nanotweezers produced by Kim and Lieber [2], the nanobalance developed by Poncharal et al. by no means a general model and could not be applied in geometries for which the nanotube is not suspended above the gate.Another way that a nanoelectromechanical actuator may function is by a quantum-mechanical mechanism based on the change in the lattice parameters in the presence of a charge. This process has been extensively investigated by different authors using theoretical [7][8][9][10][11] and experimental [11][12][13][14][15] techniques for both graphene and carbon nanotubes. In such cases, the usual way to model the nanomechanical actuator mechanism is to include an extra charge (positive or negative) transferred to the nanotube and to optimize the carbon nanotube structure in order to balance the increased electronic energy [9,10]. Although this procedure can give interesting insights into the nanomechanical actuation of the carbon nanotubes, it is not the most appropri...