Coexisting 1T/2H polymorphs, reentrant resistivity behavior, and charge distribution in MoS2−hBN 2D/2D composite thin films

Abstract

In view of their immensely intriguing properties, two-dimensional (2D) materials are being intensely researched in search of novel phenomena and diverse application interests; however, studies on the realization of 2D/2D nanocomposites in the application-worthy thin-film platform are rare. Here we have grown $\mathrm{Mo}{\mathrm{S}}_{2}$-hexagonal boron nitride (hBN) 2D/2D composite thin films on different substrates by the pulsed laser deposition technique and made comparative studies with the pristine $\mathrm{Mo}{\mathrm{S}}_{2}$ and hBN films. The Raman and x-ray photoelectron spectroscopy techniques as well as high-resolution transmission electron microscopy confirm the concomitant presence of both the 1T (conducting) and 2H (semiconducting) polymorphs of $\mathrm{Mo}{\mathrm{S}}_{2}$ in the composite film. Interestingly, a peculiar reentrant semiconductor-metal-insulator transition is seen in the $\mathrm{Mo}{\mathrm{S}}_{2}\ensuremath{-}\mathrm{hBN}$ 2D/2D composite film which is absent in the $\mathrm{Mo}{\mathrm{S}}_{2}$ film, and it correlates well with the signatures of phonon softening seen in temperature-dependent Raman spectroscopy. Furthermore, electrostatic force microscopy reveals the presence of three distinct regions (metallic, semiconducting, and insulating) in the $\mathrm{Mo}{\mathrm{S}}_{2}\ensuremath{-}\mathrm{hBN}$ composite film with differing contact potentials and enhanced propensity for charge transfer with respect to pristine $\mathrm{Mo}{\mathrm{S}}_{2}$. A triboelectric nanogenerator device containing biphasic $\mathrm{Mo}{\mathrm{S}}_{2}\ensuremath{-}\mathrm{hBN}$ composite film as an electron acceptor exhibits more than twofold (sixfold) enhancement in peak-to-peak output voltage as compared to the pristine $\mathrm{Mo}{\mathrm{S}}_{2}$ (hBN) film. These observations bring out the potential of 2D/2D nanocomposite thin films for unfolding emergent phenomena and technological applications.