Tuning the electronic properties of monolayer and bilayer transition metal dichalcogenide compounds under direct out-of-plane compression

Abstract

The band-gap modulation of atomically thin semiconductor transition metal dichalcogenides (MX₂; M = Mo or W, X = S or Se) under direct out-of-plane compression is systematically studied by means of the density functional theory (DFT) formalism including spin–orbit coupling (SOC) and dispersion correction (D3). The out-of-plane compared with other regimes stress regime significantly reduces the pressure threshold at which the semimetal state is achieved (2.7–3.1 and 1.9–3.2 GPa for mono- and bilayer systems, respectively). Structural, electronic and bonding properties are investigated for a better understanding of the electronic transitions achieved with compression. A notable relationship with the formal ionic radius (M⁴⁺ and X²⁻) is obtained. On one hand, the monolayer systems with the smallest transition metal radius (Mo⁴⁺ < W⁴⁺) reach the semimetal state at lower stress, on the other hand, for bilayer specimens the transition to semimetal is observed earlier for compounds with the smallest chalcogenide radius (S²⁻ < Se²⁻). Moreover, the appearance of non-covalent interaction (NCI) domains in the semimetal state confirms that the out-of-plane compression promotes the interaction between sulfur atoms in the single layered systems and reduces the interlayer space in bilayer configurations. Our predictions, supported by experimental evidences in the case of monolayered MoS₂, demonstrate new alternative methods for tuning the electronic properties of transition metal dichalcogenides under direct out-of-plane compression.