Chemical vapour deposition and characterization of uniform bilayer and trilayer MoS2 crystals

Abstract

Molybdenum disulfide (MoS₂) is a promising two-dimensional semiconductor for applications in electronics, optoelectronics and catalysis. Chemical vapor deposition (CVD) is a popular approach for the large-scale growth of thin MoS₂ crystals. As the properties of MoS₂ strongly depend on the number of layers, it is important to reliably grow MoS₂ crystals with different thicknesses. In this paper, we present a CVD procedure for MoS₂ growth from MoO₃ and S, which yields predominantly bilayer and trilayer MoS₂ triangular islands as opposed to monolayer MoS₂ triangles typically observed in similar CVD experiments. The growth of bilayer and trilayer MoS₂ crystals is achieved by increasing the flow rate of sulfur after the original nucleation of MoS₂ triangles. Most bilayer MoS₂ crystals are uniform in height, such that in a typical crystal the top layer fully extends to the edges of the bottom layer. While trilayer MoS₂ crystals grown by this procedure are in general less uniform than bilayers and often form terraced structures, it is still common to observe uniform trilayer MoS₂ triangles as well. In addition to standard characterization methods for MoS₂, such as Raman spectroscopy, atomic force microscopy and photoluminescence microscopy we demonstrate that scanning electron microscopy can be used to distinguish between monolayer and few-layered MoS₂ flakes at low accelerating voltages. The field-effect transistors based on CVD-grown MoS₂ triangles have electron mobilities reaching ∼10 cm² V⁻¹ s⁻¹ and ON/OFF ratios reaching ∼10⁵. The reported CVD procedure can be used for growing large quantities of uniform bilayer and trilayer MoS₂ crystals for materials studies.