Electromechanical Instability in Suspended Nanowire-Based NEMS

Abstract: "Nanoelectromechanical systems'' (NEMS) are a class of nanometer-sized mechanical structures coupled to electronic device of comparable dimensions. Their peculiar features make them interesting both from the technological and fundamental research viewpoints. The dynamics of the movable part of these devices is affected on the conditions at which electric charge is transferred across them and vice versa, therefore NEMS offer the ideal playground to investigate the physical effects which originate from coupling mechanical and electronic degrees of freedom at the nanoscale. In this thesis I describe the properties of one of such effects, i.e. the electromechanical instability that can take place in a DC-biased suspended doubly clamped carbon nanotube in which extra charge is locally injected through the tip of a scanning tunneling microscope (STM). The sequence of tunneling events makes the charge state of the nanotube vary in time and accordingly the force exerted on it by the electrostatic field (which is created by the bias voltage) changes in time and can induce mechanical vibrations in the nanotube. Furthermore, the probability of electron tunneling between the tip of the STM and the nanotube depends on the distance between them, hence a coupling between the mechanical and electronic degrees of freedom of the system is established. The analysis described in this thesis indicates that if the energy pumped into the nanotube by the electrostatic field is sufficient to overcome the energy lost because of dissipative processes, the static equilibrium configuration of the nanotube becomes unstable. The onset of the electromechanical instability implies that initially quasi-periodic oscillations (to which all the unstable modes contribute) self-organize into periodic oscillations with a frequency corresponding to the eigenfrequency of one of the unstable modes. Furthermore, the results presented here suggests the possibility to selectively excite the transverse vibrational modes of the nanotube by optimizing their coupling to the electrostatic field through the positioning of the STM tip. This effect could provide the basic physical mechanism for a feasible procedure to control the dynamics of NEMS.