Nanomechanics with electron beam: detection and control of motion

Abstract

The study of nanomechanics using an electron beam has developed into an area of research, with recent works reporting on Brownian and ballistic motion detection, dynamic backaction,visualisation of sub-nm motion and mass sensing. It is important because the electron beam offers a platform for real-time observations of dynamics exhibited by nano- and microscale objects, with sub-nm scale displacement sensitivity and MHz bandwidth, as well as for controlling and characterising mechanical properties. In this study, I report the following yet unexplored aspects:• I have introduced a new technique for detecting and mapping the periodic motion of nano/microscale objects via cathodoluminescence, with nanometric displacement sensitivity and spatial resolution, and MHz bandwidth implemented in a modified scanning electron microscope. Its capability is demonstrated by detecting and mapping driven motion of nanomechanical cantilevers. The technique offers a noise equivalent displacement amplitude spectral density of 1 nm/√Hz.• I have observed the phenomenon of dependence of the frequency of oscillation of a cantilever on the presence of the electron beam. The repulsion between an electron beam and charge accumulated on a nanomechanical cantilever yields a stiffening that increases its resonance frequency, providing a mechanism for controlling resonators and sensing charge. For a cantilever of microscale length and nanoscale cross-section interacting with an electron beam, I observe a resonance shift on the order of 5% per nanocoulomb. The resonance frequency was expressed as a function of induced charge and electron beam parameters such as position, beam current and acceleration voltage. The model was tested experimentally by varying the current of an electron beam and its distance from the edge of grounded and isolated cantilevers.• Driving oscillations of a nanomechanical beam can lead to a bistable response related to the nonlinearity of the mechanical restoring force. I have observed for the first time that the nonlinear response of a nanowire and the regime of bistability can be controlled by the electron beam impinging on the oscillator. A nanowire that is fixed at both ends and driven to the nonlinear regime of bistable resonant oscillation was switched between its bistable states by changing the distance between a 10 kV, 1.3 nA electron beam and the nanowire. The control mechanism has been explained as a consequence of electronbeam-induced heating, leading to thermal expansion that affects stress in the nanowire, which controls its resonance frequency. Therefore, the electron beam can shift thenanowire's bistable resonance relative to a fixed frequency of driven oscillation, enabling it to switch between the bistable states.In summary this thesis reports on new ways for characterizing motion and controlling dynamics of nano- and microscale systems with electron beams