Van der Waals Heterojunctions for Emerging Device Applications

Abstract

Low-dimensional systems are an exciting platform for exploring new physics and realizing novel devices. The intriguing features, such as the existence of strongly bound multiparticle complexes and thickness-dependent band structures, enable us to utilize them to overcome many challenges faced by bulk materials and conceive new technologies. Since the isolation of graphene, the class of two-dimensional materials has grown tremendously. The array of materials one can choose from for implementing an idea is vast. Nevertheless, understanding the underlying physics is essential for utilizing these properties for real-life applications. Here, we explore the optical, electrical, and optoelectrical characteristics of heterostructures based on 2D layered systems. The strongly bound excitonic complexes hosted by monolayer transition metal dichalcogenide semiconductors (TMDC) are an excellent platform for probing many-body physics. The strong luminescence and a plethora of exciting properties make them a good candidate for applications such as single photon emitters and light-emitting diodes. In the first work, we explore new ways to tune the emission from these particles without compromising their luminescence. Using a high-quality graphene/hBN/WS2/hBN/Au vertical heterojunction, we demonstrate for the first time an out-of-plane electric field-driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. We also find that the encapsulating environment of the monolayer TMDC plays a vital role in wave function spreading and hence in determining the magnitude of the blue Stark shift. We also provide a theoretical framework to understand the underlying physics better. The findings have important implications in probing many-body interaction in the two dimensions and developing layered semiconductor-based tunable optoelectronic devices. A significant advantage of the 2D material...