Raman spectroscopy of layered metal chalcogenide 2D crystals

Two-dimensional (2D) atomic crystals represent a new classs of materials, whose unusual physical and chemical properties arise due to their atomic thicknesses and low dimensionality. The van der Waals layered metal chalcogenide compounds, especially in their 2D forms, have drawn great attention during the past few years for their novel optical and electronic properties. In this report, we focus on the investigations on the physical properties related to lattice vibrations, i.e., phonons, of two typical families of metal chalcogenides, the transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2) and bismuth chalcogenides (Bi2Se3, Bi2Te3), which are best known as the intriguing valleytronic materials and topological insulators, respectively. A comprehensive investigation on the phonon properties of these metal chalcogenide 2D crystals is conducted through a combination of Raman spectroscopy, group theory analysis, as well as first principles calculations. High-quality 2D crystals have been successfully prepared for both transition metal and bismuth chalcogenides, through a mechanical exfoliation method and a vapor transport method, respectively. The 2D crystals exhibit excellent optical contrast on top of SiO2/Si substrates, which enables the fast location and thickness determination of them simply through the optical microscopy. Moreover, a Te-seeded epitaxial growth mechanism has been uncovered for the vapor transport growth of Bi2Te3 nanoplates, which can be potentially extended to the growth of other metal chalcogenides through similar methods. Low-temperature electron transport measurements on the as-grown Bi2Te3 2D crystals reveals the weak anti-localization effect, as a result of the strong spin-orbit coupling in this topological insulator material. The intrinsic phonon properties of the layered metal chalcogenides are largely affected by the crystal symmetry and dimensionality. Group theory analysis indicate that more phonon modes exist in 2D than 3D, and the phonon symmetries and optical activities are distinctively dependent on the layer number of the crystal. In general, while a Raman-active phonon mode in 3D should also be Raman-active in 2D, the reverse is not necessarily true. This crystal symmetry and dimensionality effect is best reflected in the characteristic interlayer lattice vibrations of the layered materials. In both transition metal and bismuth chalcogenide 2D crystals, multiple interlayer breathing and shear modes have been uncovered through ultralow frequency (