Characterization of 2D heterostructures

Abstract

Up to now, A great effort has been used to study the field of two dimensional (2D) materials. Reduced crystal symmetry existing in 2D materials when they are scaled down from bulk to monolayer leads to the change of their band structures. Graphene was the most widely 2D material in last decade because of its distinctive physical properties. A lot of devices based on graphene have been reported. However, the absence of a bandgap for graphene has limited its applications in electronics and optoelectronics. Taking advantages of reduced crystal symmetry, transition metal dichalcogenides (TMDs) have various advantages such as high electron mobilities, high ON/OFF ratio current ratio and excellent bendability, which are suitable for next generation low-power consumption and flexible electronic devices. Laterally, TMDs have strong covalent bonds which provide a great in-plane stability. Vertically, the van der Waals force allow TMDs to stack on other materials to form a 2D hybrid van der Waals (vdW) heterostructures without the need for considering the lattice mismatch of the two different materials. As a result, it opens the opportunities for developing novel device applications in the future. Recently, p-n junctions based on organic and two-dimensional (2D) materials have been recognized as the easiest way to fabricate hybrid 2D van der Waals heterojunction devices for electronic and optoelectronic applications. General speaking, organic materials on 2D materials is usually fabricated by thermal evaporation with the presence of high voltage and vacuum systems. In this thesis, we introduce a simple way to fabricate p-organic/n-2D heterostructure, where Pedot:PSS was chosen to be the p-organic material due to its high conductivity, excellent film forming ability and good stability, while MoS2 and WS2 were chosen as the n-2D material due to their well-known properties. We demonstrate that simple technique introduced in our fabrication of organic/2D van der Waals heterojunction could extend to include other organics and 2D materials.