Atomic Reconstruction and Interfacial Ferroelectricity in Twisted TMD Interfaces

Twisted heterostructures of two-dimensional crystals offer a broad scope for the design of novel metamaterials. In this talk I will review our latest work [1] on twisted transition metal fichalcogenide bilayers and discuss atomic reconstruction that occurs when the twist angles are small. For small twist near the 2H stacking, stable 2H domains dominate, with nuclei of a second MM metastable phase. This appears as a kagome-like pattern at ~1 degree twist, transitioning below 0.3 degree to a hexagonal array of large 2H domains. Our tunnelling measurements show that such reconstruction creates piezoelectric textures, opening a new avenue for engineering of 2D material properties.<br/>For twisted bilayer near 3R stacking (parrallel unit cells), a pattern of mirror reflected triangular 3R domains merges, featuring layer-polarized conduction band states caused by lack of both inversion and mirror symmetries. Surprisingly, the lack of inversion symmetry in 3R polytype leads to emergence of out-of-plane ferroelectricity due to layer-asymmetric interband hybridisation [2]. The resulting alternating domains can be moved by applying out-of-plane electrical fields, as visualized in situ using channelling contrast electron microscopy. The interfacial charge transfer for the observed ferroelectric domains is quantified using Kelvin probe force microscopy and agrees well with theoretical calculations. The movement of domain walls and their bending rigidity also agrees well with our modelling results. Furthermore, we demonstrate proof-of-principle field-effect transistors, where the channel resistance exhibits a pronounced hysteresis governed by pinning of ferroelectric domain walls. Our results show a potential venue towards room temperature electronic and optoelectronic semiconductor devices with built-in ferroelectric memory functions.<br/>[1]<i> Nat. Nanotechnol.</i> <b>2020</b>, <i>15</i> (7), 592–597.<br/>[2] arXiv:2108.06489