Growth of MoS2 films with vertical flakes via oxidative post-treatment of Mo precursor layers

Abstract

The sulfurization of pre-deposited Mo seed layers is a widely adopted method for synthesizing large-area MoS2 thin films with uniform thickness. However, upon exposure to air, nanometric native oxide films are formed on the Mo surface. Despite the extensive work using thin Mo layers, the oxidation state of the precursor remains poorly characterized. In this study, we investigate the role of Mo precursor oxidation state on MoS2 morphology using 10 nm Mo films on SiO2/Si substrates sulfurized at 900 ◦ C. Pretreatments of the Mo precursor layer included: freshly deposited Mo (having substoichiometric oxides), oxidative aging in air (yielding amorphous MoO3), and reductive annealing in Ar/2% H2 at 450 ◦C (producing Mo suboxides and MoO2). Scanning electron microscopy revealed the distinct topographies of the resulting MoS2 films: compact granular films from fresh precursors; vertical flakes with voids from aged precursors; and worm-like structures from reduced precursors.Spectroscopic ellipsometry modeling revealed the structure and thickness of the MoS2 films: those prepared from fresh precursors exhibited a compact single-film structure with a thickness of 29.1 nm; films from aged pre­cursors showed a single-film structure interspersed with void regions and a greater thickness of 41.6 nm; in contrast, films from the reduced precursor displayed a double-film structure comprising a compact inner film (12.4 nm thick) and a non-compact outer film (23.1 nm thick). We propose that the sublimation of amorphous MoO3 and its subsequent redeposition as oxythiomolybdate complexes are key to the formation of vertically aligned platelets separated by voids. In experiments where mass transport to the gas phase was suppressed, theresulting MoS2 films exhibited a smooth, compact morphology. MoS2 structures with vertically oriented platelets showed significantly higher electrocatalytic activity for the hydrogen evolution reaction than compact MoS2 films. This study demonstrates that the surface morphology of MoS2 can be precisely controlled by tuning the oxidation state and crystallinity of the Mo precursor.