Low-Frequency Raman Study in Precisely Controlled Interlayer Twist Angle Using Etchant-Free Thin-Film Transfer

Two-dimensional transition metal dichalcogenides (TMDs) have attracted great attention because of their scalability and tunability of optical bandgaps depending on the number of stacked layers. Twisted TMDs have gained even more interest because of the additional degree of freedom arising from the twist angle, which provides a controllable playground for studying phonon-phonon couplings, twist angle-dependent interlayer excitons, and charge transfer dynamics. A monolayer of TMDs has a stoichiometry of MX₂where M is a transition metal (M = W or Mo) sandwiched between two layers of chalcogen atoms (X = S, Se). For twisted bilayer graphene, the magic angle is 1.1°, and the flat band appears only for a narrow range of twist angles (1.1°± 0.1°). For TMDs, however, there exists a wider range of twist angles, spanning from 4° to 5°. Moreover, long-wavelength Moiré superlattices arise either from a difference in the lattice constant or a twist angle between two stacked layers. These Moiré patterns in twisted TMD bilayers have emerged as a highly tunable platform for studying strongly correlated electron physics.<br/>Our research focuses on the precise control of the interlayer twist angle with a clean and large interfacing area. For making stacked bilayer homo and heterostructures, the individual monolayer used in our research is epitaxially grown coalesced films using the growth method of Metalorganic Chemical Vapor Deposition (MOCVD) on a 2’’ sapphire substrate. The highlight of our work firstly lies in making controlled twisted bilayer high-quality large-area samples (9 by 9 mm²). The degree of twist angle is controlled using a special transfer stage to stack WS₂ bilayers with a range of twist angles from 0°,1°, 2 °, 3°, 5°, 7°, 9°, 10°, 15°, 20° to 30°. The significance of our work includes creativity in stacking these large area bilayers using an etchant-free transfer method. As the monolayer is grown on a sapphire substrate and only the second layer is transferred, it makes the interface between the two layers cleanest which is very important to study the interlayer excitons and phonons. AFM images support the claim of high quality of transferred films as there are no pores or cracks in the films which usually arises in other transfer techniques like wet transfer methods. Our work makes a significant contribution to understanding phonon interactions using Raman spectroscopy for twisted bilayer structures. Low-frequency Raman modes are studied using 488 nm (ultra-frequency laser) excitation to map the dependence of inter-layer breathing and shear modes showing up in the range of -50 cm^-1 to 50 cm^-1 wavenumbers. For small twist angles (2°,3°, and °5) moiré phonons start appearing and then disappears in case of large twist angle. Our works report the first-time mapping of low-frequency Raman modes (-100 cm^-1to 100 cm^-1) and high-frequency Raman modes using resonant laser 532 nm and non-resonant laser 488 nm excitation source for WS₂twisted bilayers. We will also discuss the results of temperature-dependent photoluminescence and optical absorption showing the evolution of interlayer coupling in twisted TMDs.