Synthesis and Applications of Heterostructures of Two-Dimensional Materials

The potential of atomically thin layered materials, such as zero band gap semiconductor graphene, semiconducting transition metal dichalcogenides (TMDCs), semiconducting metal chalcogenides (MCs), and insulating hexagonal boron nitride (h-BN) are promising for future building blocks of electronic devices.The heterostructures of these ultimate thin materials offer new solutions for the problem in Si-based device miniaturization.For example, graphene with its high conductivity could be utilized as the future interconnection or electrical contact for semiconducting 2D materials.WS2 is a prospective choice for a semiconducting channel owing to predicted high electron mobility that is comparable with carrier mobility of Si.However, the poor electrical contact between single-layer WS2 and conventional metal electrodes retards the carrier mobility of WS2.In this thesis, the improvement of single-layer WS2 carrier mobility (50 cm 2 /Vs) with chemical vapor deposition (CVD)-grown multi-layer graphene (MLG) electrodes is demonstrated.The improved contact is explained by the tunable work function of MLG electrodes that can overcome the high Schottky barrier (SB) height usually formed with conventional metal electrodes.The high mechanical robustness of MLG-WS2 heterostructures allowed the demonstration of flexible fast (response time of 2 ms), and sensitive (responsivity of 4500 A/W) photodetector.Next, due to its unipolarity of n-type WS2, it is difficult to form a p-n junction or complementary field-effect transistors (FETs) with solely WS2.The heterostructures of two different materials with different polarities could solve this problem.Therefore, this thesis also presents the novel SnS-WS2 p-n junctions realized by using two-step CVD method.The SnS (a member of MCs) was found to have a p-type polarity with the mobility of 15 cm 2 /Vs.Then, the formation of SnS-WS2 p-n junction was confirmed based on its rectifying behavior.The p-n junction also exhibited a tunable rectification ratio and good ideality factor because of clean interface resulting from two-step CVD method.Moreover, the presence of SnS could enhance the photodetector performance due to high optical absorption of SnS.This thesis demonstrates the advantages of two-dimensional-based heterostructures which can be applied to cutting-edge electronic and optoelectronic applications in the near future