Coherent acoustic phonons in van der Waals nanolayers and heterostructures

Abstract

Terahertz (THz) and sub-THz coherent acoustic phonons have been successfully used as probes of various quantum systems. Since their wavelength is in the nanometer range, they can probe nanostructures buried below a surface with nanometer resolution and enable control of electrical and optical properties on a picosecond time scale. However, coherent acoustic phonons have not yet been widely used to study van der Waals (vdW) two-dimensional (2D) materials and heterostructures. This class of 2D systems features strong covalent bonding of atoms in the layer planes and weak van der Waals attraction between the layers. The dynamical properties of the interface between the layers or between a layer and its supporting substrate are often omitted as they are difficult to probe. On the other hand, these play a crucial role in interpreting experiments and/or designing new device structures. Here, we use picosecond ultrasonic techniques to investigate phonon transport in vdW InSe nanolayers and InSe/hBN heterostructures. Coherent acoustic phonons are generated and detected in these 2D systems and allow us to probe elastic parameters of different layers and their interfaces. In particular, our study of the elastic properties of the interface between vdW layers reveals a strong coupling over a wide range of frequencies up to 0.1 THz, offering prospects for high-frequency electronics and technologies that require control over the charge and phonon transport across an interface. In contrast, we reveal a weak coupling between the InSe nanolayers and sapphire substrates, relevant in thermoelectrics and sensing applications, which can require quasi-suspended layers.

Time-Domain Investigations of Coherent Phonons in van der Waals Thin Films

“…They are characterized by an extracted frequency in the GHz range, which no longer depends on the probe wavelength, but increases when the material gets thinner, as shown in Figure 7 c,d. This has been observed in exfoliated few layer crystals of WSe 2 [ 133 ], MoS 2 [ 143 ], MoSe 2 [ 132 ], InSe [ 135 , 136 , 137 ], PtSe 2 [ 134 ], BP [ 126 ], Bi 2 Se 3 [ 243 ], and Bi 2 Te 3 [ 139 ]. In all these examples, the penetration depth of the pump beam gets larger than the thickness of the sample.…”

“…Noteworthily, even a good mechanical contact can yield an anti-node if the substrate has a low rigidity, for instance polymers [ 126 ]. This issue can be quantitatively discussed by introducing the acoustic impedance in the rigid-layer model [ 137 , 184 , 244 ] …”

Coherent acoustic phonon dynamics in chiral copolymers

“…In a system of interest, e.g., a metal, semiconductor, or polymer thin film, acoustic phonons are generated either directly by an ultrashort laser pulse 11–13 or by exciting an attached transducer layer, 14–17 which launches an acoustic pulse in the adjacent film structure. Such all-optical nanoultrasonics 8 has found widespread applications, and recent examples include buried nanointerfaces, multiple quantum wells, or organic-inorganic heterostructures 10,16,18,19 . Previous optical investigations largely employed detection at a single or a few probe wavelengths in order to extract information regarding longitudinal and transversal (shear) acoustic waves in various thin film materials, and many of these measurements have been performed in reflective geometry 4,6,11,16,17,20 …”

Employing graphene acoustoelectric switch by dual surface acoustic wave transducers

“…In addition, by properly utilizing four leads and IDTs, we can directly measure I ae to make GAET act as an acoustoelectric branch switch 28 . We note that recent studies reveal several intriguing interface elastic properties of van der Waals materials 42–44 and may offer a mean to speed up the switch rate of GAET by engineering the interfacial acoustoelectric properties. Maybe the switch rate is slow (audio frequencies) and the design is a little complex, we think that the GAET opens a route for developing graphene-based logic switch.…”