3D Hypersound Microscopy of van der Waals Heterostructures

Abstract

The mechanical properties of the layered crystals in the few layer limit are largely unexplored. We employ a picosecond ultrasonic technique to access the corresponding mechanical parameters. Temporal variation of the reflection coefficient of the Al film that covers hBN/WSe2/hBN (where hBN is hexagonal boron nitride) heterostructures on a sapphire substrate after the femtosecond laser pulse excitation is carefully measured using an interferometric technique with spatial resolution. The laser pulse generates a broadband sound wave packet propagating perpendicularly to the Al plane and partially reflecting from the heterostructural interfaces. The demonstrated technique allows one to resolve a WSe2 monolayer embedded in hBN. We apply a multilayered model of the optoacoustical response to evaluate the mechanical parameters, in particular, the rigidity of the interfaces. Mapping of the Fourier spectra of the response visualizes different composition regions and may serve as an acoustic tomography tool. Almost zero phonon dissipation below 150 GHz demonstrates the van der Waals heterostructures’ potential for nanoacoustical applications.

Revealing the interlayer van der Waals coupling of bi-layer and tri-layer MoS2 using terahertz coherent phonon spectroscopy

“…We can see that under this model, the substrate coupling effect and the next nearest neighbour effect can both be quantified and we also observed an increase of the force constant as the layer increase from 2 layers to 3 layers, agreeing with the ref [44] for bi-layer and tri-layer suspended 2H-MoSe 2 with ∼3 % increase. The study of the acoustic parameters for vdWs interfaces of exfoliated hBN and Al film on sapphire substrate from ref [41] gave a range of K S lower than 10 19 N/m 3 , and our deduced value 1.48 × 10 19 N/m 3 is slightly higher, agreeing with the expected higher binding energy between the sapphire and the epitaxial growth of MoS 2 .…”

“…Secondly, the bi-layer and tri-layer MoS 2 were epitaxially grown on sapphire substrate and thus the mechanical coupling at the interface would be stronger than the exfoliation samples. There have been studies [40] , [41] which report the quantification of the substrate coupling by introducing a force constant in the interface. Moreover, a next nearest neighbour effect has been applied in the study of twisted multilayer graphene to explain the LB modes’ difference between theory and experiment [42] .…”

Coherent Phononics of van der Waals Layers on Nanogratings

“…In this geometry, the vdW layer is excited only by coherent phonons transmitted through the nanograting to the layer. From the analysis of these experiments, we estimate η ∼ 10 17 –10 18 N m –3 , which is consistent with earlier studies of vdW layers on planar substrates. , …”

“…From the analysis of these experiments, we estimate η ∼ 10 17 −10 18 N m −3 , which is consistent with earlier studies of vdW layers on planar substrates. 24,25 For a qualitative analysis of phonon modes in our hybrid nanostructure, we have developed an analytical model (Supporting Information, Section 3). We assume q x a ≤ 1 and treat our periodic nanostructure as a 2D phononic crystal for flexural waves propagating laterally above the infinitively rigid grating with the same width of grooves and ridges as in the experiment.…”

Coherent Phonons in van der Waals MoSe2/WSe2 Heterobilayers

“…THz and sub-THz coherent phonons can be generated and detected by optical pulses. , This technique has been used widely in experiments with relatively thick vdW layers and heterostructures. − There are a few unique works where THz coherent phonons are studied in homobilayers fabricated from WSe 2 , MoSe 2 , and graphene, but to the best of our knowledge no coherent phonon experiments have been performed on heterobilayers. A specific physical feature for coherent phonon generation related to the Coulomb interaction between charged 2D monolayers is expected that distinguishes TMD heterobilayers from homobilayers and thicker vdW heterostructures.…”