Programmable Hydrogen-Assisted Chemical Vapor Deposition Growth and Bipolar Transport in Two-Dimensional MoO₂ Nanoflakes.

Abstract

Two-dimensional (2D) MoO₂ nanoflakes offer metallic conductivity and multiband structure, but their controlled growth remains limited by coupled precursor transport and reduction chemistry. Here we establish a programmable chemical vapor deposition approach with precisely timed H₂ introduction, decoupling precursor transport from surface reduction. This temporal gating yields thickness-controlled (5-30 nm), highly crystalline single-crystal MoO₂ nanoflakes. Time-resolved optical microscopy, X-ray diffraction, and Raman spectroscopy reveal a stepwise MoO₃ to MoO₂ pathway involving Mo₄O₁₁-like intermediates. Adjusting the H₂/Ar ratio controls nucleation density, lateral size, and thickness. The same timing principle also guides the 2D growth of WO₂ and Cr₂O₃. Temperature-dependent Hall measurements show nonlinear Hall behavior and, in thinner flakes, sign reversal of the Hall coefficient, providing direct evidence for bipolar transport with thickness-dependent electron-hole balance. This temporal gating approach provides a general strategy for nonlayered oxide growth and advances understanding of multicarrier transport in MoO₂.