Programmable Hydrogen-Assisted Chemical Vapor Deposition Growth and Bipolar Transport in Two-Dimensional MoO ₂ Nanoflakes

Two-dimensional (2D) MoO 2 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 H 2 introduction, decoupling precursor transport from surface reduction. This temporal gating yields thickness-controlled (5–30 nm), highly crystalline single-crystal MoO 2 nanoflakes. Time-resolved optical microscopy, X-ray diffraction, and Raman spectroscopy reveal a stepwise MoO 3 to MoO 2 pathway involving Mo 4 O 11 -like intermediates. Adjusting the H 2 /Ar ratio controls nucleation density, lateral size, and thickness. The same timing principle also guides the 2D growth of WO 2 and Cr 2 O 3 . 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 MoO 2 .