3D Hypersound Microscopy of van der Waals Heterostructures.

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.

Structural and Optical Properties of Silicon Carbide Powders Synthesized from Organosilane Using High-Temperature High-Pressure Method.

The development of new strategies for the mass synthesis of SiC nanocrystals with high structure perfection and narrow particle size distribution remains in demand for high-tech applications. In this work, the size-controllable synthesis of the SiC 3C polytype, free of sp2 carbon, with high structure quality nanocrystals, was realized for the first time by the pyrolysis of organosilane C12H36Si6 at 8 GPa and temperatures up to 2000 °C. It is shown that the average particle size can be monotonically changed from ~2 nm to ~500 nm by increasing the synthesis temperature from 800 °C to 1400 °C. At higher temperatures, further enlargement of the crystals is impeded, which is consistent with the recrystallization mechanism driven by a decrease in the surface energy of the particles. The optical properties investigated by IR transmission spectroscopy, Raman scattering, and low-temperature photoluminescence provided information about the concentration and distribution of carriers in nanoparticles, as well as the dominant type of internal point defects. It is shown that changing the growth modes in combination with heat treatment enables control over not only the average crystal size, but also the LO phonon-plasmon coupled modes in the crystals, which is of interest for applications related to IR photonics.