The peculiar attributes of 2D transition metal dichalcogenides (TMDs) are increasingly being exploited to produce microelectronic devices. To meet the needs of specific applications, the properties of these materials – such as the band gap, photoresponse, or charge carrier mobility – can be tuned by using different numbers of layers, applying mechanical strain, or by introducing dopants.

Using MoS2, one of the more commonly studied 2D TMDs, Juan Xia and colleagues at Nanyang Technological University, Singapore, and collaborators at Oak Ridge National Laboratory in the US, and CINTRA CNRS in Singapore, have found an alternative method.

In a monolayer of MoS2, the constituent atoms form a hexagonal lattice, not unlike the familiar chicken-wire structure of graphene. When one monolayer is stacked on top of another, the degree of rotation between the layers determines whether certain sites within the structure are vacant or occupied by an atom. With two possible orientations for each layer, the stacking arrangement in a bilayer of MoS2 can be described as either AA or AB.

In research reported in 2D Materials, Xia and her team, led by Ze Xiang Shen of Nanyang Technological University, used chemical vapour deposition (CVD) to create junctions formed by the interfaces between differently stacked MoS2 bilayers (AA-AB). The group also used the same technique to combine a monolayer (1L) with each type of bilayer arrangement, resulting in either AA-1L or AB-1L trilayers.

Although the MoS2 monolayer initially deposited has a single crystal structure throughout, the layers laid down on top contain distinct phase domains. By carefully manipulating the deposition process, Xia and colleagues were able to produce flower-shaped structures in which the AA-1L, AB-1L and AA-AB junctions were all present.

The group then used electron-beam lithography to fabricate field-effect transistors, which were used to test the electronic and optoelectronic properties of the different junction types. All three junctions showed current rectification behaviour, though this was weaker in the AA-AB configuration. Shen's team also found an asymmetric photoresponse in every type of junction, with negligible photocurrent at zero bias, and a greater photocurrent at forward bias.

Stacks of applications

As Xia explained to, "these MoS2-based homojunctions are intrinsically formed by one-step chemical CVD growth without any heavy doping or mechanical transfer strategies, which guarantees a chemically homogeneous character and strong coupling among the layers. Our finding demonstrates the promise of using stacking-modulated 2D materials for future electronics and optoelectronics."

Next, says Xia, the team intends to study "the spin-valley polarization in bilayer and trilayer TMD systems with different stackings, through gate tuning. We will also try to grow other types of stacking-induced homo- and heterojunctions to realize diverse type-I, type-II and type-III electronic junctions based on their different band structures."

Full details of the research are reported in 2D Materials DOI: 10.1088/2053-1583/aa79db.