Abstract
Coherent phonons can be launched in materials upon localized pulsed optical excitation, and be subsequently followed in time-domain, with a sub-picosecond resolution, using a time-delayed pulsed probe. This technique yields characterization of mechanical, optical, and electronic properties at the nanoscale, and is taken advantage of for investigations in material science, physics, chemistry, and biology. Here we review the use of this experimental method applied to the emerging field of homo- and heterostructures of van der Waals materials. Their unique structure corresponding to non-covalently stacked atomically thin layers allows for the study of original structural configurations, down to one-atom-thin films free of interface defect. The generation and relaxation of coherent optical phonons, as well as propagative and resonant breathing acoustic phonons, are comprehensively discussed. This approach opens new avenues for the in situ characterization of these novel materials, the observation and modulation of exotic phenomena, and advances in the field of acoustics microscopy.
Highlights
Van der Waals materials, referred to as layered two-dimensional (2D) materials, are strongly anisotropic materials formed by layers of covalently bound atoms, stacked on top of each other and linked through vdW forces
Many lattice structures are encountered for the 2D layers, as depicted in Figure 1, such as hexagonal one-atom-thick layers in graphite and boron nitride, two-atom-thick layers in black phosphorous (BP), three-atom-thick layers with octahedral (PbI2, PtSe2 . . . ) or trigonal prismatic coordination (MoS2, WSe2 . . . ) usually referred to as transition metal dichalcogenides (TMDs), four-atom-thick layers (GaS, InSe . . . ), five-atom-thick layers (Sb2Se3, Bi2Te3 . . . ) usually referred to as V2VI3 chalcogenides or as quintuple layers (QLs), and many more complex structures [1,2]
We provide a contextualized panorama of the rising field of coherent phonon investigation in vdW systems
Summary
Van der Waals (vdW) materials, referred to as layered two-dimensional (2D) materials, are strongly anisotropic materials formed by layers of covalently bound atoms, stacked on top of each other and linked through vdW forces. Changing growth stoichiometry of QL materials yields natural superlattices with for instance intercalation of Bi2 vdW layers in Bi2Se3 stacks, or alternation of Bi2Te3 and Sb2Te3 layers with controllable periodicity [37,38,39]. Overall, this brings interesting potentials for scalable manufacturing and integration with the current CMOS technology [40]. State of the art nanofabrication yields heterostructures, i.e., composite vdW materials with atomic precision on the out-of-plane dimension, which can be processed as scalable and in situ tunable devices. Future perspectives are discussed building on the possible overcoming of current experimental limitations and on still-unexplored routes
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