Nanomechanical resonators can detect electronic degrees of freedom through their sensitivity to small electrical forces. Here, we use few-layer graphene membranes as nanomechanical probes to investigate the slow dynamics of injected charge carriers in a trap-rich, leaky dielectric substrate. The dielectric consists of thermally grown SiO2 containing charge trapping centers introduced by the milling of cavities with a Ga+ focused ion beam. When suspended over these cavities, the resonators exhibit anomalous electromechanical behavior: their vibrational resonant frequencies are hysteretic when sweeping a dc gate voltage, and voltage steps result in an initial sharp increase in both frequency and static displacement, followed by a gradual relaxation toward steady values. We attribute this behavior to trap-assisted transport, in which injected charges undergo trapping and detrapping and gradually leak through the oxide, leading to a time-dependent evolution of the effective electrostatic force. Finally, we show that long-lived trapped charges give rise to a built-in potential that enables detection of the driven mechanical response even in the absence of an applied gate voltage, indicating opportunities for nanomechanical sensing of environmental charge dynamics.
Chen et al. (Mon,) studied this question.