Two-Dimensional Mechanics of Atomically Thin Solids on Water

Movement of a three-dimensional solid at an air-water interface is strongly influenced by the extrinsic interactions between the solid and the water. The finite thickness and volume of a moving solid causes capillary interactions and water-induced drag. In this Letter, we report the fabrication and dynamical imaging of freely floating MoS₂ solids on water, which minimizes such extrinsic effects. For this, we delaminate a synthesized wafer-scale monolayer MoS₂ onto a water surface, which shows negligible height difference across water and MoS_2. Subsequently patterning by a laser generates arbitrarily shaped MoS₂ with negligible in-plane strain. We introduce photoswitchable surfactants to exert a lateral force to floating MoS₂ with a spatiotemporal control. Using this platform, we demonstrate a variety of two-dimensional mechanical systems that show reversible shape changes. Our experiment provides a versatile approach for designing and controlling a large array of atomically thin solids on water for intrinsically two-dimensional dynamics and mechanics.