Nondestructive Picosecond Ultrasonic Probing of Intralayer and van der Waals Interlayer Bonding in α‐ and β‐In₂Se₃

Abstract

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 α and β polytypes of In 2 Se 3 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 αand β-In 2 Se 3 , 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 αand β-In 2 Se 3 . 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 In 2 Se 3 .

Coherent Phonons in van der Waals MoSe2/WSe2 Heterobilayers

“…The measured value of f B allows us to calculate the stiffness k vdW of the vdW elastic bond between MoSe 2 and WSe 2 monolayers using the equation f normalB = 1 2 π k vdW ( 1 μ 1 + 1 μ 2 ) where μ 1,2 = 4.4 × 10 –6 (5.9 × 10 –6 ) kg m –2 is the sheet mass density for the MoSe 2 (WSe 2 ) monolayer. Substituting f B = 0.8 ± 0.03 THz in (eq ) we obtain k vdW = (6.3 ± 0.4)×10 19 N m –3 , which agrees with a typical value for vdW nanolayers. ,, …”

3D Hypersound Microscopy of van der Waals Heterostructures

“…Thin layered materials and VdW heterostructures remain insufficiently explored. In refs − , several picoacoustical approaches were first applied to thin flakes and VdW heterostructures. In particular, the bubbles were identified using the acoustical resonance technique .…”

Two‐dimensional In2Se3: A rising advanced material for ferroelectric data storage

“…52,127 According to previous studies, β In 2 Se 3 with a thickness of 7-760 nm has been successfully exfoliated. 52,127 Besides, CVD and MBE methods are also used to fabricate β In 2 Se 3 with thicknesses ranging from a single layer to tens of layers. 66,[73][74][75][76]128,129 In 2021, Rashid et al successfully fabricated γ In 2 Se 3 nanoflakes with thickness ranging from 13.6 to 90.5 nm by CVD method.…”

“…In 2018, through physical vapor transport, Balakrishnan et al grew a large‐area (>10 3 μm 2 ) β In 2 Se 3 film with the thinnest thickness of 4 nm on the surface of ε‐GaSe crystals 126 . Due to the natural van der Waals layered structure of β In 2 Se 3 , the mechanical exfoliation method can also be used to prepare ultrathin films 52,127 . According to previous studies, β In 2 Se 3 with a thickness of 7–760 nm has been successfully exfoliated 52,127 .…”

“…Due to the natural van der Waals layered structure of β In 2 Se 3 , the mechanical exfoliation method can also be used to prepare ultrathin films 52,127 . According to previous studies, β In 2 Se 3 with a thickness of 7–760 nm has been successfully exfoliated 52,127 . Besides, CVD and MBE methods are also used to fabricate β In 2 Se 3 with thicknesses ranging from a single layer to tens of layers 66,73–76,128,129 .…”