Probing and manipulating the interlayer coupling in 2D structures

Abstract

In 2004, researchers reported the synthesis of planar graphene in the free state, overturning a decades-old prediction that 2D crystals were too thermodynamically unstable to exist in ambient. Since then, dozens of new monolayer crystals have been demonstrated, each with their own properties and capabilities. Very interestingly, as these monolayer materials are vertically stacked to create 2D structures, the properties of the structure are not simply a sum of the parent crystals, but a product of the interlayer coupling, which can redistribute charge, modify the band structures, and induce new properties unique to either parent crystal. The term interlayer coupling refers to the communication and interaction that exists between the parent materials, where the absence of a coupling suggests the parent materials are electronically independent. Understanding and controllably manipulating the interlayer coupling holds promise to not only engineer new capabilities, but to also retain desired properties that only exist in monolayer materials. This dissertation explores the interlayer coupling in a variety 2D structures, with an emphasis on a particular class of 2D structures: monolayer bismuth selenide (Bi2Se3) grown on monolayer transition metal dichalcogenides (TMDs), using vapor-phase chalcogenization. The physical charge redistribution induced by the interlayer coupling, and its effect on the band structure, was studied in as-grown samples. The interlayer coupling was then probed and manipulated using both electron beams in a vacuum, and the controlled absorption and desorption - possible intercalation and deintercalation - of oxygen. The results suggest that the properties could be drastically and controllably altered by varying the interlayer coupling strength. Chapter 1 is an introduction and background to monolayer materials and 2D structures, as well as bismuth selenide (Bi2Se3) and transition metal dichalcogenides (TMDs), the parent materials for the primary class of 2D structures studied in this dissertation. Chapter 2 is instrumentation and synthesis, where all the important methods and equipment are discussed, so as to enable reproduction of the work. Chapter 3 demonstrates that the interlayer coupling between dissimilar 2D heterostructures can be probed in-situ by manipulating the twist angle and structure using the focused electron beam of a transmission electron microscope (TEM). The electron beam imparts energy that induces the Bi2Se3 to break-up into grains, and for those grains to twist relative to the underlying monolayer TMD. Chapter 4 demonstrates oxygen-induced in-situ manipulation of the interlayer coupling and exciton recombination in Bi2Se3/MoS2 2D heterostructures. This chapter studies the possible intercalation and deintercalation of oxygen using both experimental and theoretical methods, providing justification for the claims, as well as other possible explanations for the manipulation of the interlayer coupling. Chapter 5 demonstrates the tunable photoluminescence in Bi2Se3/TMD 2D heterostructures for write-read-erase-reuse applications. Mono- and few-layer Bi2Se3 was grown on four different TMDs - MoS2, MoSe2, WS2, and MoSe2-2xS2x - and the interlayer coupling was manipulated possibly using the controlled intercalation and deintercalation of oxygen. Potential technologies include ultra high-density information storage, and tunable photoluminescing pixels (PLPs). Chapter 6 demonstrates evidence of a purely electronic two-dimensional lattice at room temperature that resides between the parent layers of a Bi2Se3/TMD 2D heterostructure. The results suggest that the interlayer coupling is inducing significant charge redistribution, a surprising result considering that that interlayer bonding in their bulk counterparts is "weak" van der Waals. Chapter 9.1 studies and briefly discusses work related to the interlayer coupling in Non-Bi2Se3 2D structures that include, graphene/MoS2, graphene/graphene, Sb2Se3/MoS2, and Bi2Te3/MoS2. The results suggest that while interlayer coupling is important in other 2D structures, the Bi2Se3/TMD 2D structures have a relatively strong interlayer coupling.