Letter pubs.acs.org/NanoLett
Establishment of 2D Crystal Heterostructures by Sulfurization of Sequential Transition Metal Depositions: Preparation, Characterization, and Selective Growth Chong-Rong Wu,†,‡ Xiang-Rui Chang,‡ Tung-Wei Chu,†,§ Hsuan-An Chen,†,‡ Chao-Hsin Wu,† and Shih-Yen Lin*,†,‡ †
Graduate Institute of Electronics Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan Research Center for Applied Sciences, Academia Sinica, No. 128, Sec. 2, Academia Road, Taipei 11529, Taiwan
‡
ABSTRACT: 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. KEYWORDS: 2D crystal heterostructures, transition metal dichalcogenides
T
2D materials prepared by either mechanical exfoliation or chemical vapor deposition (CVD).11,12 However, this approach may bring additional contamination on the interfaces of the heterostructures during the transferring procedure. In one previous publication, we have demonstrated that by using sequential CVD growth of graphene and MoS2, graphene/ MoS2 heterostructures can be established without the transferring procedure.13 In the other publication, we have demonstrated that by repeating the same CVD growth cycle, multilayer MoS2 films can be grown on the sapphire substrate with good layer number controllability.14 The results have revealed that different 2D crystal heterostructures may be established by sequential CVD growth. Although the CVD technique is very promising for large-area 2D crystal growth, there are still several limitations for TMD growth by using this technique. Since the transition metal precursor and the sulfur (S) powder are uniformly distributed onto the substrates at high temperature, selective growth is difficult to achieve by using the traditional CVD technique. On the other hand, for the preparation of different TMDs to establish 2D crystal heterostructures, different precursors have to be located.
he major advantage of two-dimensional (2D) crystals for practical applications is that their unique material characteristics can be observed within one to few atomic layers.1,2 Compared with devices fabricated on bulk materials, this advantage has provided the possibility to achieve a reasonable aspect ratio when the device dimensions shrink down to nanometer scale. This is an important issue for the transistor fabrication in the
