Negative piezoelectricity and enhanced electrical conductivity at the interfaces of two-dimensional dialkali oxide and chalcogenide monolayers

Abstract

The negative piezoelectric coefficient in ferroelectric polymers has been widely studied. However, the same in the two-dimensional (2D) materials is scarcely reported, which stresses the need for its thorough investigation. Here, the 2D van der Waals (vdW) heterobilayers (vdWHs) of the dialkali metal monochalcogenide ${M}_{2}X$ ($M=\mathrm{Na}$, K, Rb, or Cs; and $X=\mathrm{O}$, S, Se, or Te) monolayers show negative out-of-plane piezoelectricity. Out of all the explored heterobilayers of the family, the maximum piezoelectric strain coefficient in the out-of-plane direction ${d}_{33}=\ensuremath{-}39\phantom{\rule{0.28em}{0ex}}\mathrm{pm}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}$ is found in the ${\mathrm{Na}}_{2}\mathrm{Te}$/${\mathrm{Cs}}_{2}\mathrm{S}$ system. This strong piezoelectric coefficient arises from the notable electrostatic potential energy difference and band offset between the constituent monolayers. The small out-of-plane Young's modulus ($Y\ensuremath{\sim}11\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$), originating from the weak vdW interlayer bonding relative to the stronger intralayer bonding, accounts for the anomalous negative piezoelectricity in the heterostructure. The combination of minute out-of-plane ($Y=\ensuremath{\sim}11\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$) and in-plane ($Y=\ensuremath{\sim}23\phantom{\rule{0.28em}{0ex}}\mathrm{N}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}\mathbf{1}}$) Young's moduli with small bending modulus (8.83 eV) and negative piezoelectric coefficient underlines the suitability of the proposed vdWH for stretchable and flexible piezotronic devices. Moreover, external perturbations are found to modify the properties of the system. The vertical compressive strain widens the band gap by elevating the conduction band minimum in the heterostructure. An externally applied electric field of low strength ($\ensuremath{-}0.33$ and $+0.31\phantom{\rule{0.28em}{0ex}}\mathrm{V}/\AA{}$) is found to change the semiconducting heterostructure to metallic upon the closure of the band gap, which is attributed to the dropping of nearly free electron gas (NFEG) states to the Fermi level. A significant enhancement in electrical conductivity has been brought up by the NFEG states upon reaching the Fermi level, which inspires the use of the heterostructure in a nonresistive charge carrier transport application in electronics.