Van der Waals proximity induced structural polymorphism in atomically thin MoS₂.

Selective reversible structural switching of a crystal among its various polymorphs, is a fascinating problem in atomics scale crystal engineering. Atomically thin molybdenum disulphide (MoS₂), which can exist in two main coordination, namely octahedral (1T) and trigonal prismatic (1H) structures, is being endorsed as building blocks of future technologies. While both 1H and 1T polymorphs exhibit a remarkable range of physical properties, stabilizing octahedral polymorphs has been challenging. We show that thermodynamically stable octahedral phase of monolayer MoS₂ can be achieved at temperatures below ∼ 500 K using van der Waal epitaxy. At room temperature, ~ 10-15% of 2 × 1 1T’ octahedral phase, reversibly switchable to 1H by heating, is revealed by our temperature dependent Raman spectroscopy. Conducting-mode atomic force microscopy suggests that the octahedral phase forms a network of elongated (∼ 100 nm) patches/stripes within the 1H matrix. We attribute this, with support from density functional theory calculations, to local lattice relaxation due to incommensurability-driven stress fields. Our experiment establishes the van der Waals epitaxy as new tool for crystal structure engineering in atomic membranes.