Layer-controlled precise fabrication of ultrathin MoS₂ films by atomic layer deposition

Monolayer and/or atomically thin transition metal dichalcogenides cover a wide range of two-dimensional (2D) materials, whose fascinating semiconducting and optical properties have made them promising candidate materials for optoelectronic devices. Controllable growth of these materials is critical for their device applications. By using MoCl₅ and H₂S as precursors, monolayer and ultrathin molybdenum disulfide (MoS₂) films with controlled lamellar structure have been directly built layer by layer on SiO₂ substrates without being followed by high-temperature annealing. Furthermore, the thickness of MoS₂ films can be precisely regulated by applying different atomic layer deposition (ALD) cycles. Once an ALD cycle is applied, one molecular layer of MoS₂ material will be 'added' on the substrate or original existing MoS₂ films. At the initial stage (one to three ALD cycles), the density of MoS₂ materials increases with an increase in ALD cycles, while a large area of continuous MoS₂ film on the substrate can be obtained when four or more ALD cycles are applied. In this way, excellent triangular crystals of MoS₂ with controlled atomic size in thickness and a highly oriented hexagonal crystal structures can be obtained by applying definite ALD cycles.