Converse Flexoelectric Two-Dimensional MoS₂ Actuator

Converse flexoelectricity generates a mechanical response under non-zero electric field gradient. Flexoelectric actuators operating based on this effect are promising candidates for many applications owing to their fast response, capability to control the displacement precisely without hysteresis, suitability for vacuum/extreme temperature conditions, and a wide range of material choices. In spite of these advantages, there has been limited studies on flexoelectric actuators, largely because flexoelectric response is negligible in bulk materials. For nanoscale actuators, however, converse-flexoelectric effect is a promising mechanism, as electric field gradient is increased quadratically as the material’s thickness decreases. For this reason, atomically thin two-dimensional (2D) materials, such as molybdenum disulfide (MoS₂), are promising candidates for flexoelectric actuators. Here, we report a flexoelectric actuator based on 2D MoS₂. A monolayer MoS₂ was used as an active layer, while a parylene-C layer with a thickness of 500 nm was used as a supporting material. We also employed a comb-shaped electrode to generate a strong electric field gradient. In response to 40 V AC excitation at ~20 kHz, the actuator shows resonant displacements up to 50 nm. A finite element analysis (FEA) simulation with COMSOL Multiphysics shows that this response is mainly induced by flexoelectric effect. Our 2D MoS₂ actuator outperforms other existing flexoelectric actuators by more than two orders in terms of relative displacement to active layer thickness. Moreover, displacement amplitude varies linearly with applied voltage, allowing precise actuation control. Most importantly, flexoelectric responses are preserved in vacuum/cryogenic environments with an amplitude of actuation (~35 nm) reduced only 30 % at 10 K. This greatly outperforms commercial piezoelectric actuators which experienced a 60 % reduction in actuation at 10 K. We also observed that our flexoelectric actuator can survive 10¹⁰ actuation cycles with less than 12 % of performance fluctuation, regardless of operating temperature. These results demonstrate the superiority of 2D materials based flexoelectric actuators in low temperature/vacuum conditions which will be useful in various extreme conditions such as actuators for space applications.