Title: Optical Characterization of Charge Transfer Excitons in Transition Metal Dichalcogenide Heterostructures
Description: Two-dimensional materials such as graphene, boron nitride and transition metal dichalcogenides have attracted significant research interest due to their unique optoelectronic properties. Transition metal dichalcogenides (TMDCs) are a family of two-dimensional semiconductors which exhibit strong light-matter interaction and show great promise for applications ranging from more efficient LEDs to quantum computing. One of the most intriguing qualities of TMDCs is their ability to be stacked on top of one another to tailor devices with specific properties and exploit interlayer phenomena to develop new characteristics. One such interlayer interaction is the generation of charge transfer excitons which span the interface between two different TMDC monolayers. This work aims to study the intrinsic optical properties of charge transfer excitons in TMDC heterostructures. We must first start by investigating methods to protect and isolate our sample of interest from its chemical and electrostatic environment.

We demonstrate that near intrinsic photoluminescence (PL) linewidth and exciton emission homogeneity from monolayer TMDCs can be achieved using a combination of BN encapsulation and passivation of substrate hydroxyl groups. Next, we develop clean stacking techniques and incorporate low defect density source crystals to maintain intrinsic properties and ensure a sufficiently high quality heterostructure interface to study characteristics of charge transfer excitons in 2D TDMCs. Strong photoluminescence emission from charge transfer excitons is realized and is shown to persist to room temperature. Charge transfer exciton lifetime is measured to be two orders of magnitude longer than previously reported. Using these high quality heterostructures, we study the behavior of charge transfer excitons under high excitation density. We observe the dissociation of charge transfer excitons into spatially separated electron-hole plasmas under optical excitation. We then probe properties of charge transfer exciton emission enhancement due resonant coupling to surface plasmon modes of gold nanorods.