Moire Structures of Transition Metal Dichalcogenides (TMDCs) and Oxide Perovskite Barium Titanate (BaTiO3)

Two-dimensional van Der Waal structures, such as Transition Metal Dichalcogenides (TMDCs) and recent fabrication of BTO twisted nanometer thick layers structures have brought about exciting research opportunities. These materials possess unique physical, electrical, and optical attributes that can be fine-tuned through twisting, rotations and mechanical deformations.<br/>In this talk, I will discuss the following three topics:<br/>1. Wrinkle-Ridge formation in 2-D MoSe₂under uniaxial and biaxial strain [1]<br/>2. Ferroelectric domains in MoS₂Moire structures under a variety of twist conditions<br/>3. Oxide Moire structures – Twisted 15-20nm thick layers of BaTiO3 (BTO) - that show vortex and antivortex structures [2]<br/>We have investigated the effects of lateral compression on out-of-plane deformation of 2-D MoSe₂ layers. A MoSe₂ monolayer develops periodic wrinkles under uniaxial compression and Miura-Ori patterns under biaxial compression. When a flat MoSe₂ monolayer is placed on top of a wrinkled MoSe₂ layer, the van der Waals (vdW) interaction transforms wrinkles into ridges and generates mixed 2H and 1T phases and chain-like defects. Biaxial strain induces regions of Miura-Ori patterns in bilayers. Strained systems analyzed using a convolutional neural network show that the compressed system consists of semiconducting 2H and metallic 1T phases. The energetics, mechanical response, defect structure, and dynamics are analyzed as bilayers undergo wrinkle–ridge transformations under uniaxial compression and moiré transformations under biaxial compression.<br/>Moire supercells, formed by the stacking of 2D TMDC materials with small twists, have been shown to exhibit novel electronic and optical properties. The stacking order of these 2D materials plays a crucial role in determining their electronic and optical characteristics. These Moire supercells have also been observed to give rise to a superlattice of out-of-plane ferroelectric domains. Specifically, we examine how the initial twist angle of the stacked 2D materials affects the formation of polarized domains. Additionally, we explore how the initial twist angle can be utilized to control the size of the ferroelectric domains.<br/>Exciting experiments involved stacking freestanding perovskite layers of BTO of thickness of 15- 20nm, at different twists of 3°, 6°, 10.4°, and 50° degrees. The system shows the emergence of electric polarization vortices and antivortices. The couplings across the interface between the twisted layers drive large strain gradients in the ferroelectric layers, resulting in vortex-like modulations of the homogeneous polarization state due to the flexoelectric effect. It is noted that the periodicity of the 2D vortex pattern can be largely tuned by controlling the twisting angle.<br/><b>References</b><br/>1.A. Aditya, A. Mishra, N. Baradwaj, K. Nomura, A. Nakano, P. Vashishta, and R. K. Kalia: Wrinkles, Ridges, Miura-Ori, and Moiré Patterns in MoSe 2 Using Neural Networks. The Journal of Physical Chemistry Letters 14(7), 1732 (2023).<br/>2.G. Sánchez-Santolino, V. Rouco, S. Puebla, H. Aramberri, V. Zamora, M. Cabero, F. A. Cuellar, C. Munuera, F. Mompean, M. Garcia-Hernandez, A. Castellanos-Gomez, J. Íñiguez, C. Leon, and J. Santamaria: A 2D ferroelectric vortex pattern in twisted BaTiO3 freestanding layers. Nature 626(7999), 529 (2024).<br/><br/>This Research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, Neutron Scattering and Instrumentation Sciences program under Award DE SC0023146. The simulations were performed at the Centre for Advanced Research and Computing of the University of Southern California.