ABSTRACT: Two-dimensional (2D) layered materials such as MoS2 have been recognized as high on-off ratio semiconductors which are promising for electronic and optoelectronic devices. In addition to use individual 2D materials, the accelerated field of 2D heterostructures enables even more functionalities. Different device design is strongly controlled by the electronic band alignment. For examples, photovoltaic cells require type II HJs for light harvesting, and light-emitting diodes benefit from multiple quantum wells with the type I band alignment for high emission efficiency. The vertical tunneling field effect transistor for next-generation electronics counts on nearly broken-gap band alignment for boosting up its performance. To tailor them toward future applications, the understanding of 2D heterostructure band alignment become critically important.
In the first part of thesis, we discuss the band alignment of 2D heterostructures. To accomplish this, we firstly study the interlayer coupling between two dissimilar 2D materials. We conclude that a post-anneal process could enhance the coupling of as-transferred 2D heterostructures and the heterostructural stacking impose similar symmetry change as homostructural stacking. Later, we precisely determine the quasi-particle gap and band alignment of MoS2/WSe2 heterostructure by using scan tunneling microscopy/spectroscopy (STM/S) and micron-beam X-ray photoelectron spectroscopy (μ-XPS) techniques. Last, we prove the band alignment of 2D heterojunctions can be well predicted by Anderson's model, which is normally failed in predicting conventional bulk heterostructure.
In the second part of thesis, we develop a new CVD method capable of controlling the growth area of p- and n-type transition metal dichalcogenides (TMDCs) precisely and further form lateral/vertical 2D heterostructures. This method also allows to seperately grow p- and n-type TMDCs in selective area in one step. In addition, we demonstrate a first bottom-up 2D CMOS inverter based on hetero-TMDCs.