Spatially Graded Millimeter Sized Mo_1-WS₂ Monolayer Alloys: Synthesis and Memory Effect

The band gap engineering of two-dimensional (2D) transition metal dichacogenides (TMDs) could significantly broaden their applications, especially in electronics and optoelectronics. Alloying is a more effective approach to synthesize 2D ternary TMD materials with tunable bandgaps by regulating the compositions. Whether the alloying could induce memory effects is of interest as a scientific problem and worthy to be studied. A thermal evaporation-assisted chemical vapor deposition (CVD) method was proposed to grow millimeter size gradient alloyed monolayer Mo_1-WS₂. This method reveals a promising and universal methodology for the development of gradient alloyed TMDs because of the precise controlling of each precursor. The synthesized Mo_1-WS₂ monolayer crystal has a gradient composition with x ranging from 0.1 to 1. The W and Mo atoms homogeneously alloyed with random distribution in the Mo_1-WS₂ monolayer. As reported, the deep energy levels induced by sulfur vacancies can be effectively suppressed to shallow energy levels by alloying TMDs. The series distribution of the shallow energy levels in the band of the graded alloy semiconductor can act as multiple charge trapping states, which leads to obvious memory effects in the device. These results present a new opportunity for memory devices and related applications.