MANIPULATION OF EXCITON DYNAMICS BY INTERFACIAL ENERGY/CHARGE TRANSFER IN TWO-DIMENSIONAL SEMICONDUCTORS
Creators: name: Sun, Dewei
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: classificationCode: 340606
subject: Photochemistry
classificationCode: 340699
subject: Physical chemistry not elsewhere classified

description: In the realm of two-dimensional (2D) materials, monolayer (ML) transition metal dichalcogenides (TMDCs) have gained significant interest due to their direct bandgap transition, high carrier mobility, strong light-matter interaction, and robust spin and valley degrees of freedom, starkly contrasting their bulk counterparts. Owing to their large surface-to-volume ratio, the integration of ML TMDCs with other various 2D semiconductors and microcavities offers opportunities to study fundamental photo-physics processes at the heterointerfaces, paving the way for implementation of next-generation devices.Chapter 1 provides a concise introduction to 2D materials, particularly TMDCs, and their fascinating optical and electronic properties. It examines the role of excitons in 2D materials, and the impact of energy transfer (ET) and charge transfer (CT) on exciton’s properties in TMDC through the construction of 2D van der Waals (vdW) heterostructures and coupling with optical microcavities. This chapter also delves into the potential enhancement of TMDCs’ optical properties by integrating 2D hybrid lead halide perovskites and ultra-thin three-dimensional (3D) halide perovskites with TMDCs. Furthermore, it sets the general context for light-matter interaction, another form of ET, considering both weak and strong coupling regimes.Chapter 2 outlines the optical techniques employed to gather data for this work. A focus is placed on ultrafast optical techniques like transient absorption spectroscopy, which allow for direct probing and analysis of ET and CT dynamics at the heterointerface.Photoinduced interfacial CT plays a critical role in the field of energy conversion involving vdW heterostructures constructed by inorganic nanostructures and organic materials. However, the control of atomic-scale stacking configurations to modulate charge separation at interfaces remains challenging. Chapter 3 aims to illustrate tunability of interfacial charge separation in a Type-II heterojunction between ML-WS<sub>2</sub> and an organic semiconducting molecule by rational design of relative stacking configurations using 2D perovskites as scaffoldings. This chapter investigates how different molecular stacking, face-to-face versus face-to-edge, affects CT at the heterointerface. Our findings reveal that the CT process heavily depends on the relative stacking configurations at the organic-TMDCs heterointerface, with charge separation being notably slowed down for face-to-edge configuration compared to face-to-face configuration. These investigations open new opportunities for designing efficient charge separation processes in energy conversion applications by judiciously engineering interfaces between organic and inorganic semiconductors, using 2D perovskites as scaffolds.Though TMDCs’ large surface-to-volume ratios make them excellent platforms for studying interfacial properties, the presence of bulky ligands on the surface of 2D perovskite poses a challenge, impeding direct interfacial coupling in their heterostructures. Chapter 4 details the fabrication of ML-WS<sub>2</sub> and ultra-thin CH<sub>3</sub>NH<sub>3</sub>PbX<sub>3</sub> (MAPbX<sub>3</sub>, X=Br, I) heterostructures with tunable energy levels, to study the dynamics of CT and ET at these hybrid interfaces. Notably, heterojunctions of WS<sub>2</sub> with pure MAPbBr<sub>3</sub> and MAPbI<sub>3</sub> were elucidated as Type-I and Type-II respectively, using photoluminescence (PL) and time-resolved photoluminescence (TR-PL) measurements. Transit absorption (TA) spectroscopy investigations unambiguously revealed a rapid ET facilitated by CT in the WS<sub>2</sub>/MAPbBr<sub>3</sub> heterostructure, with a time constant of ~20 ps, and a predominantly CT in the WS<sub>2</sub>/MAPbI<sub>3</sub> heterostructure with a time constant of ~50 femtosecond (fs). The successful interfacing of low-dimensional perovskites with an extensive array of traditional 2D materials such as TMDCs opens up possibilities for novel optoelectronic properties and applications within the field of 2D material systems. Furthermore, the ultrafast and efficient ET and CT processes hold promise for the creation of advanced energy conversion devices.In the last chapter, we successfully fabricated a ML-WS<sub>2</sub> in conjunction with a silver (Ag) nanoparticle (NP) array. Our findings affirmed a weak light-matter coupling between ML-WS<sub>2</sub> and the Ag NP array, as evidenced by angle-resolved photoluminescence spectroscopy. Furthermore, an enhancement in the bright exciton emission from ML-WS<sub>2</sub> was observed at reduced temperatures. The analysis of PL enhancement factor at varying temperatures suggested that an upper bound of the enhancement factor for the bright exciton could reach ~51 or even higher at 7 K, given the imperfect uniformity of the electric filed generated around the NPs. This discovery carries significant implications for the manipulation of excitons in TMDCs and expands their potential applications in the field of optoelectronics.