Tiled Monolayer Films of 2D Molybdenum Disulfide Nanoflakes Assembled at Liquid/Liquid Interfaces

Abstract

Thin films of MoS₂ bilayer nanoflakes, which are predominantly a single flake thick and with flakes in edge-to-edge contact, have been produced via self-assembled tiling at the planar interface between two immiscible liquids. Films of several square centimeters extent can be produced with a total covered area approaching 90% and over 70% of the film covered by single flakes without overlap. Films produced through liquid/liquid assembly are shown to produce a lower uncovered area fraction and more uniform thickness when compared with films of similar areal coverage produced by the "top-down" techniques of spin coating and spray coating. Statistical analysis of flake coverage data, measured by atomic force microscopy (AFM), shows that liquid/liquid assembly produces a distinctly different variation in film thickness than conventional top-down deposition. This supports the hypothesis that the two-dimensional (2D) confinement of liquid/liquid assembly produces more uniform films. Demonstrator field-effect transistors (FETs) manufactured from the films exhibit mobility and on/off current ratios of 0.73 cm² V^-1 s^-1 and 10⁵, respectively, comparable to FETs of similar layout and chemical vapor deposition (CVD)-grown or mechanically cleaved single-crystal MoS₂ channel material. This work demonstrates the use of liquid/liquid interfaces as a useful tool for the self-assembly of high-performance thin-film devices made from dispersions of 2D materials.