Water Wetting Of Atomic Level Layered Materials

The interaction between two-dimensional?2D?and van der Waals?vdW?layered materials and their environment has been receiving tremendous research interest.One key challenge is that a well-defined and completely clean surface at the atomic level is required for studying wetting,which is difficult to obtain in macroscopic experiments.Based on pioneer reports,wetting properties of vdW-layered materials,such as graphene,h-BN,and MoS2,all are in great debating.It has been found the wetting behaviors of vdW-layered materials are largely dependent on thickness,defects,substrates,pollution from air,transfer methods,and so on.However,a clear physical picture of wetting for vdW-layered materials is still lacking.On the other hand,how to quantitatively characterize the surface morphology of vdW-layered materials is extremely important.The conventional way is to use atomic force microscopy to scan the surface roughness,which is slow,inaccurate,and impractical in industry fabrication.However,if we can clearly understand the wetting principles,it is possible to use wettability as a quick and simple method to characterize the surface morphology of vdW-layered materials.In the present work,we take clean and well-defined Bi₂Se₃ thin films,synthesized using molecular beam epitaxy?MBE?,as a particular example to study the wetting principles of vd W-layered materials.A linear relation between water wetting and surface roughness suggests a generalized Wenzel model is applicable for a wide range of vdW-layered materials.Such generalized Wenzel model provides a clear and accurate physical picture for wetting of vdW 2D materials for the first time.Moreover,the extrapolated CA?⁹8.4°?shows unexpected high hydrophobicity of the ideal Bi₂Se₃ surface,but the edge of terraces is extremely hydrophilic.Such an unusual hydrophobic/hydrophilic transition can be explained by its specific atomic and electronic structure,according to first principles calculations.The hydrophobic nature of high quality Bi₂Se₃ thin films enables robust,low-cost,and self-cleaning nanoelectronics and spintronics without sealing.