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[Display omitted] • WS 2 monolayers were synthesized by CVD based on liquid solution precursors containing organic surfactants for uniformity. • Carbons from the incompletely decomposed organic surfactant suppress crystal growth due to carbon amorphization during the thermal process. • These residual carbons were removed by introducing an O 2 pre-annealing process for residual carbon oxidation. • The growth of WS 2 monolayer crystals was systematically manipulated by the amount of O 2 gas for pre-annealing to obtain of triangular and hexagonal shapes. Chemical vapor deposition (CVD) using liquid solution precursors is a promising method for synthesizing uniform and large-area monolayers of transition metal dichalcogenides. Organic surfactants, such as iodixanol, are crucial additives for the uniform coating of substrates with liquid solution precursors. However, during the heating process of CVD, carbons from the incompletely decomposed organic surfactant suppress crystal growth unfavorably. Here, we investigate the role of such residual carbons in the crystal growth behavior of WS 2 monolayers. The crystal growth modes were precisely manipulated by residual carbon oxidation and the precursor etching rate, which was implemented by introducing O 2 pre-annealing into the CVD process. Without O 2 pre-annealing, flakes of limited size and multilayers were synthesized owing to the carbonization of surfactant, whereas sufficiently oxidized residual carbons via O 2 pre-annealing allow for crystal growth with a large monolayer size. Moreover, the growth modes of the flakes in terms of triangles and hexagons can be systematically controlled by the O 2 amount for pre-annealing because the remnant O 2 compensates for the role of H 2 , which manipulates the etching rate of the residual W precursors on the substrate. Our results pave the way for controlling the growth modes of CVD-grown WS 2 by introducing an optimized O 2 pre-annealing process. [ABSTRACT FROM AUTHOR]