Raman spectroscopy of layered metal chalcogenide 2D crystals
Zhao, Yan Yuan
Date of Issue2014
School of Physical and Mathematical Sciences
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 (<50 cm-1) Raman spectroscopy, with perfect agreement with the first principles calculations. Some of these interlayer vibrational modes are absent in the 3D bulk limit, due to the higher crystal symmetry. The Raman observed interlayer shear modes show redshift and blueshift, respectively, in the transition metal dichalcogenide and bismuth chalcogenide 2D crystals with decreasing crystal thickness; while the interlayer breathing modes in both families exhibit blueshift. These frequency evolution trends versus crystal thicknesses can be quantitatively interpreted through a simple linear chain model. The symmetry and dimensionality effect can also be reflected in the high frequency intralayer phonon property change from 3D to 2D. In the transition metal dichalcogenides, the intrinsic bulk phonon modes and experience a Raman-forbidden to Raman-allowed symmetry transition from the 3D bulk to 2D crystals, and turn into Raman-forbidden symmetries again in the limit of monolayer. Moreover, the anomalous lattice vibrations, commonly known as the blueshift of the mode and redshift of the mode with decreasing crystal thickness, have been identified through Raman spectroscopy in all the four transition metal dichalcogenide materials studied in this thesis. First principles calculations indicate the governing mechanism behind this phenomenon to be the competition between the surface effect and the thickness effect. Similarly, in bismuth chalcogenides, the Raman observed high frequency intralayer modes also exhibit frequency variations from 3D to 2D, as well captured by the calculated results. Besides the frequency changes, the Raman peaks generally experience broadening from 3D to 2D, which indicates a decrease of phonon lifetime and can be interpreted through the phonon confinement effect. Overall, our results in this thesis reveal that crystal symmetry and dimensionality play the key role in determining the intrinsic phonon properties of the layered metal chalcogenides. This study sheds light on a general understanding of the phonon behaviors in all van der Waals layered materials and their evolutions from 3D to 2D.