Invisible Surface Charge Pattern on Inorganic Electrets

We propose an easy method to pattern the surface charge of ${\rm SiO}_{2}$ electrets without patterning the dielectric layer. By eliminating the use of metal guard electrodes, both the charge efficiency and the surface charge stability in humid environments improve. We apply the concept to a vibrational energy harvesting device, where a power output of 3 $\mu{\rm W}$ is successfully harvested at a low acceleration of 0.05 g and a resonant frequency of 414 Hz. No decay of the power output can be observed in a RH95% humidity test, which shows that the surface charge is stable and this is promising for application in harsh environments.