A novel synthesis approach for high-quality, large-scale transition-metal dichalcogenide monolayers is developed by SeungNam Cha, Jung Inn Sohn, and co-workers, as described in article number 1702206. The low supersaturation level, triggered by the evaporation of an extremely thin precursor layer, reduces the nucleation density dramatically under a thermodynamically stable environment, yielding uniform and clean large-area monolayer films. The vertically stacked heterostructured device shows a fast response time and a high photoresponsivity.