Domain morphology and mechanics of the ${\mathrm{H}/\mathrm{T}}^{\ensuremath{'}}$ transition metal dichalcogenide monolayers

Domain morphology and mechanics of the ${H / T}^{'}$ transition metal dichalcogenide monolayers

Joel Berry, Songsong Zhou, Jian Han, David J. Srolovitz, and Mikko P. Haataja

Phys. Rev. Materials 2, 114002 – Published 5 November 2018

Abstract

The properties of two-dimensional (2D) transition metal dichalcogenide (TMD) monolayers can be dynamically controlled via strain-induced displacive structural transformations between semiconducting (H) and metallic or semimetallic ( $T^{'}$ ) crystal structures. The shapes, symmetries, and kinetics of crystalline domains generated during these transformations and the mechanical response of transforming monolayers are of fundamental and applied interest in, e.g., phase change memory devices and the study of topologically protected edge states in quantum spin Hall insulating $T^{'}$ crystals. We quantitatively characterize $T^{'}$ domain morphologies during $H \to T^{'}$ transformations in both flat and bendable TMD monolayers using a combination of first principles and continuum calculations. Wulff constructions for ${MoTe}_{2}$ and ${MoS}_{2}$ show that $T^{'}$ domains within much larger $T^{'}$ domains are either rhombi of fixed proportions (if nonmisfitting) or rectangles whose aspect ratio $A_{R}$ increases with domain size $L_{0}$ (if misfitting). Isolated $T^{'}$ domains within much larger H domains undergo a morphological crossover from compact to elongated shapes at $L_{0} \approx 100 – 200$ nm if the sheet is constrained to be flat or $L_{0} ≳ 2 μ m$ if the sheet is free to bend. This crossover is driven by a competition between anisotropic interfacial energy and elastic misfit energy, and its position can be tuned via the monolayer-substrate interaction strength. It is shown that the aspect ratio $A_{R}$ obeys a scaling law $A_{R} \sim L_{0}^{2 / 3}$ . Stress-strain response characterized as a function of strain orientation reveals extreme anisotropy in the effective elastic modulus through $H / T^{'}$ coexistence. Ferroelastic multidomain $T^{'} − {WTe}_{2}$ monolayers are found to exhibit two to three regimes of reversible mechanical response, and localized buckling in freely suspended $T^{'}$ monolayers is shown to qualitatively alter $T^{'}$ domain symmetries.

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Received 4 July 2018
Revised 14 September 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.114002

Physics Subject Headings (PhySH)

Bending Domain walls Mechanical deformation Microstructure Structural phase transition

2-dimensional systems Transition metal dichalcogenides

Density functional theory Phase-field modeling

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Joel Berry^1,2, Songsong Zhou¹, Jian Han¹, David J. Srolovitz^1,3,4, and Mikko P. Haataja^2,5,6

¹Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
²Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544, USA
³Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
⁴Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR, People's Republic of China
⁵Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey 08544, USA
⁶Program in Applied and Computational Mathematics (PACM), Princeton University, Princeton, New Jersey 08544, USA