This study reports the controlled vapor phase synthesis of molybdenum disulfide (MoS₂) atomic layers and investigates the nucleation, growth, and grain boundary formation in their crystalline monolayers. The authors developed a nucleation-controlled strategy to promote the formation of large-area single- and few-layered films. They examined the atomic structure and morphology of the grains and their boundaries using first-principles calculations to investigate the energy landscape. The electrical properties of the atomic layers were also evaluated, and the role of grain boundaries was assessed. The results show that the uniform thickness, large grain sizes, and excellent electrical performance of these materials indicate high-quality and scalable synthesis of MoS₂ atomic layers. The study highlights the importance of efficient control on nucleation for large-area growth and provides insights into the growth mechanisms and grain boundary structures, which are crucial for the development of high-performance MoS₂-based electronic devices.This study reports the controlled vapor phase synthesis of molybdenum disulfide (MoS₂) atomic layers and investigates the nucleation, growth, and grain boundary formation in their crystalline monolayers. The authors developed a nucleation-controlled strategy to promote the formation of large-area single- and few-layered films. They examined the atomic structure and morphology of the grains and their boundaries using first-principles calculations to investigate the energy landscape. The electrical properties of the atomic layers were also evaluated, and the role of grain boundaries was assessed. The results show that the uniform thickness, large grain sizes, and excellent electrical performance of these materials indicate high-quality and scalable synthesis of MoS₂ atomic layers. The study highlights the importance of efficient control on nucleation for large-area growth and provides insights into the growth mechanisms and grain boundary structures, which are crucial for the development of high-performance MoS₂-based electronic devices.