The Growth Mechanism of Transition Metal Dichalcogenides by using Sulfurization of Pre-deposited Transition Metals and the 2D Crystal Hetero-structure Establishment.

A growth model is proposed for the large-area and uniform MoS2 film grown by using sulfurization of pre-deposited Mo films on sapphire substrates. During the sulfurization procedure, the competition between the two mechanisms of the Mo oxide segregation to form small clusters and the sulfurization reaction to form planar MoS2 film is determined by the amount of background sulfur. Small Mo oxide clusters are observed under the sulfur deficient condition, while large-area and complete MoS2 films are obtained under the sulfur sufficient condition. Precise layer number controllability is also achieved by controlling the pre-deposited Mo film thicknesses. The drain currents in positive dependence on the layer numbers of the MoS2 transistors with 1-, 3- and 5- layer MoS2 have demonstrated small variation in material characteristics between each MoS2 layer prepared by using this growth technique. By sequential transition metal deposition and sulfurization procedures, a WS2/MoS2/WS2 double hetero-structure is demonstrated. Large-area growth, layer number controllability and the possibility of hetero-structure establishment by using sequential metal deposition and following sulfurization procedures have revealed the potential of this growth technique for practical applications.

Establishment of 2D Crystal Heterostructures by Sulfurization of Sequential Transition Metal Depositions: Preparation, Characterization, and Selective Growth.

A nine-layer WS2/MoS2 heterostructure is established on a sapphire substrate after sequential growth of large-area and uniform five- and four-layer MoS2 and WS2 films by using sulfurization of predeposited 1.0 nm molybdenum (Mo) and tungsten (W), respectively. By using the results obtained from the ultraviolet photoelectron spectroscopy and the absorption spectrum measurements of the standalone MoS2 and WS2 samples, a type-II band alignment is predicated for the WS2/MoS2 heterostructure. Increasing drain currents and enhanced field-effect mobility value of the transistor fabricated on the heterostructure suggested that a channel with higher electron concentration compared with the standalone MoS2 transistor channel is obtained with electron injection from WS2 to MoS2 under thermal equilibrium. Selective 2D crystal growth with (I) blank sapphire substrate, (II) standalone MoS2, (III) WS2/MoS2 heterostructure, and (IV) standalone WS2 was demonstrated on a single sapphire substrate. The results have revealed the potential of this growth technique for practical applications.