Atomically thin two-dimensional (2D) layered materials have potential applications in nanoelectronics,nanophotonics,and integrated optoelectronics.Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices,such as broad-band photodetectors,multi-color light emitting diodes (LEDs),and high-efficiency photovoltaic devices.In this review,we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range,as well as their induced applications in broadly tunable optoelectronic components.The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.