CVD of MoS₂ single layer flakes using Na₂MoO₄ - impact of oxygen and temperature-time-profile

Two-dimensional (2D) materials are of great interest in many fields due to their astonishing properties at an atomic level thickness. Many fundamentally different methods to synthesize 2D materials, such as exfoliation or chemical vapor deposition (CVD), have been reported. Despite great efforts and progress to investigate and improve each synthesis method, mainly to increase the yield and quality of the synthesized 2D materials, most approaches still involve some compromise. Herein, we systematically investigate a chemical vapor deposition (CVD) process to synthesize molybdenum disulfide (MoS₂) single layer flakes using sodium molybdate (Na₂MoO₄), deposited on a silica (SiO₂/Si) substrate by spin-coating its aqueous solution, as the molybdenum source and sulfur powder as sulfur source, respectively. The focus lies on the impact of oxygen (O₂) in the gas flow and temperature-time-profile on reaction process and product quality. Atomic force microscopy (AFM), Raman and photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to investigate MoS₂ flakes synthesized under different exposure times of O₂ and with various temperature-time-profiles. This detailed study shows that the MoS₂ flakes are formed within the first few minutes of synthesis and elaborates on the necessity of O₂ in the gas flow, as well as drawbacks of its presence. In addition, the applied temperature-time-profile highly affects the ability to detach MoS₂ flakes from the growth substrate when immersed in water, but it has no impact on the flake.