Fabrication, Characterization of locally strained 2D Heterostructure

Abstract

Two-dimensional materials are atomic thick when exfoliated down to monolayers. 2D semiconductors are of special interests because they have a bandgap that graphene does not，hence they provide a good platform for studying physics and future devices. 2D semiconductors， especially monolayer of WSe2 and MoSe2, have a series of advantages, including light-matter interaction, excitons, and valleys. The abundant spectra information also enables the potentially applications in solar energy capture. With current in-lab fabrication techniques, researchers are able to fabricate heterostructures by precisely restacking different sheets into van der Waals force bound stack. The combination of self-designed heterostructures enables further study on emerging phenomenon and novel physics. Researchers are familiar with manipulating these heterostructures using electrical, magnetic, pressure and mechanical methods to study the abundant exciton physics. However, one extra degree of freedom, strain, remains poorly understood. The strain in bulk transition metal dichalcogenides (TMDS) is usually introduced during crystal growth. Once exfoliated, there is less confinement from both the upper and lower layers. Therefore, strain inherited from interfacial contact gives the heterostructure an complicated out-of-plane degree of freedom. Because the 2D material has a tensile feature and endures considerable stress and compression, the interface between the heterostructure and substrate becomes a dominant factor that affects the strain in heterostructure. In my thesis, substrate with different surface topographies is applied. Firstly, I placed the heterostructure of WSe2/MoSe2 on nano-diamond to introduce strain in the materials. To make the strain more controllable, one or two layers WSe2 was fixed on the top of pillars. With optical characterization combined, both energy shifts and fine features in photoluminescence are observed in different materials. These sample preparation experiments mark the first step towards more extensive related research.