This study reports the controlled vapor-phase synthesis of molybdenum disulfide (MoS₂) atomic layers and elucidates the mechanisms of nucleation, growth, and grain boundary formation in its crystalline monolayers. The research establishes a nucleation-controlled strategy to systematically promote the formation of large-area single- and few-layered films. The atomic structure and morphology of the grains and their boundaries in the polycrystalline MoS₂ atomic layers are examined, and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are evaluated, and the role of grain boundaries is assessed. The uniformity in thickness, large grain sizes, and excellent electrical performance of these materials indicate high-quality and scalable synthesis of MoS₂ atomic layers. MoS₂ is a layered semiconductor with a bandgap in the range of 1.2-1.8 eV, whose physical properties are significantly thickness-dependent. The lack of inversion symmetry in single-layer MoS₂ and developments in controlling the available valley quantum states in this material have brought valleytronic applications closer to reality. However, the large-scale synthesis of high-quality MoS₂ atomic layers remains a challenge. Recent top-down approaches to obtain large areas of MoS₂ thin films have attracted considerable attention. However, the lack of uniformity in thickness and size undermines the viability of such approaches. Other techniques for the scalable synthesis of MoS₂ have focused on the direct sulfurization of molybdenum-containing films. Recent work on the solid phase sulfurization of molybdenum thin films revealed a straightforward method for large-area synthesis of MoS₂. However, the suboptimal quality of samples prepared using this synthetic route remains a problem. Traditionally, the sulfurization of molybdenum trioxide (MoO₃) has been the primary approach in MoS₂ nanomaterial synthesis. Lee et al. demonstrated that MoO₃ is a suitable precursor for chemical vapor deposition (CVD) of MoS₂ thin films. This work studied the nucleation and growth process in CVD grown MoS₂ atomic layers facilitated by seeding the substrate with graphene-like species, but lacked a comprehensive characterization of grains and grain boundary structures. The study develops a CVD-based procedure for the large-area synthesis of highly-crystalline MoS₂ atomic layers by vapor-phase MoO₃ sulfurization. The growth process, grain morphology, and grain boundary structure of the polycrystalline MoS₂ atomic layers are evaluated, and their corresponding electrical performances are characterized. The synthesis of MoS₂ atomic layers in a vapor-phase deposition process uses MoO₃ and pure sulfur as precursor and reactant materials. The experiments show that the synthesis of MoS₂ is limited by the diffusion of vapor-phase MoO₃₋x. Several stages were observed during the MoS₂ atomic layer growth. InitiallyThis study reports the controlled vapor-phase synthesis of molybdenum disulfide (MoS₂) atomic layers and elucidates the mechanisms of nucleation, growth, and grain boundary formation in its crystalline monolayers. The research establishes a nucleation-controlled strategy to systematically promote the formation of large-area single- and few-layered films. The atomic structure and morphology of the grains and their boundaries in the polycrystalline MoS₂ atomic layers are examined, and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are evaluated, and the role of grain boundaries is assessed. The uniformity in thickness, large grain sizes, and excellent electrical performance of these materials indicate high-quality and scalable synthesis of MoS₂ atomic layers. MoS₂ is a layered semiconductor with a bandgap in the range of 1.2-1.8 eV, whose physical properties are significantly thickness-dependent. The lack of inversion symmetry in single-layer MoS₂ and developments in controlling the available valley quantum states in this material have brought valleytronic applications closer to reality. However, the large-scale synthesis of high-quality MoS₂ atomic layers remains a challenge. Recent top-down approaches to obtain large areas of MoS₂ thin films have attracted considerable attention. However, the lack of uniformity in thickness and size undermines the viability of such approaches. Other techniques for the scalable synthesis of MoS₂ have focused on the direct sulfurization of molybdenum-containing films. Recent work on the solid phase sulfurization of molybdenum thin films revealed a straightforward method for large-area synthesis of MoS₂. However, the suboptimal quality of samples prepared using this synthetic route remains a problem. Traditionally, the sulfurization of molybdenum trioxide (MoO₃) has been the primary approach in MoS₂ nanomaterial synthesis. Lee et al. demonstrated that MoO₃ is a suitable precursor for chemical vapor deposition (CVD) of MoS₂ thin films. This work studied the nucleation and growth process in CVD grown MoS₂ atomic layers facilitated by seeding the substrate with graphene-like species, but lacked a comprehensive characterization of grains and grain boundary structures. The study develops a CVD-based procedure for the large-area synthesis of highly-crystalline MoS₂ atomic layers by vapor-phase MoO₃ sulfurization. The growth process, grain morphology, and grain boundary structure of the polycrystalline MoS₂ atomic layers are evaluated, and their corresponding electrical performances are characterized. The synthesis of MoS₂ atomic layers in a vapor-phase deposition process uses MoO₃ and pure sulfur as precursor and reactant materials. The experiments show that the synthesis of MoS₂ is limited by the diffusion of vapor-phase MoO₃₋x. Several stages were observed during the MoS₂ atomic layer growth. Initially