Nondestructive Picosecond Ultrasonic Probing of Intralayer and van der Waals Interlayer Bonding in alpha- and beta-In2Se3

The interplay between the strong intralayer covalent-ionic bonds and the weak interlayer van der Waals (vdW) forces between the neighboring layers of vdW crystals gives rise to unique physical and chemical properties. Here, the intralayer and interlayer bondings in alpha and beta polytypes of In2Se3 are studied, a vdW material with potential applications in advanced electronic and optical devices. Picosecond ultrasonic experiments are conducted to probe the sound velocity in the direction perpendicular to the vdW layers. The measured sound velocities are different in alpha- and beta-In2Se3, suggesting a significant difference in their elastic properties. Density functional theory and an effective spring model are used to calculate the elastic stiffness of the layer and vdW gap in alpha- and beta-In2Se3. The calculated elastic moduli show good agreement with experimental values and reveal the dominant contribution of interlayer atomic bonding to the different elastic properties of the two polytypes. The findings show the power of picosecond ultrasonics for probing the fundamental elastic properties of vdW materials. The data and analysis also provide a reliable description of the intra- and interlayer forces in complex crystal structures, such as the polytype phases of In2Se3.