Optoelectronic and Electrochemical Properties of Hybrid Transition Metal Dichalcogenide Heterostructures
Transition metal dichalcogenides (TMDs) have attracted significant attention in recent years with their immense potential to revolutionize optoelectronics and electrochemical energy applications. However, several challenges have prevented their practical use, including fabrication difficulties, incompatibility with conventional doping techniques, and unwanted environmental effects. This thesis aims to address the issues by introducing novel strategies for transforming TMDs into organic-integrated hybrid structures. Furthermore, this study focuses on gaining a fundamental understanding and a tunability of the unique physical properties of TMDs. Finally, to unlock their full potential, this thesis explores synergetic effects among the hybrid components for the development of advanced optoelectronics and energy devices. By combining atomically thin TMDs with uniform organic layers, we have developed various two-dimensional (2D) hybrid junctions, including TMD/organic, TMD/TMD/organic, and TMD/organic/TMD. The TMD/organic hybrids are designed for type-II energy band alignments at the heterointerface and exhibit significantly improved (photo)conductivity and uniform photoresponse compared to pristine TMDs. The optoelectronic characteristics vary as a function of the layer number of TMDs, one of the unique features of ultrathin materials. We also find that integrating organic layers can tailor the charge density and polarity of TMD flakes, thus enabling controllable doping without damaging the crystallinity. The hybrid approach not only modulates the properties of individual TMD layers but also offers an opportunity to study unique phenomena of 2D heterostructures such as interlayer excitons (XIs). XIs are spatially separated bound states of an electron and a hole in TMD/TMD heterolayers. We prepared various TMD/TMD/organic hybrid heterostructures with distinct energy band alignments and demonstrated a selective modulation of XI emission. The photoluminescence from the radiative recombination of XIs can be preserved, quenched, or modulated based on the band alignments. Furthermore, we fabricated organic-layer-inserted heterolayers (TMD/organic/TMD) and investigated the environmental effects on XIs. The organic layers tailor the dielectric screening within XIs and the dipolar interaction among XIs, thus regulating the energy states of XIs. In addition to the rich potential in optoelectronics, the hybrid strategies are advantageous to improve electrochemical energy storage. We constructed hybrid composites from core carbon nanotubes, intermediate metal-organic frameworks (MOFs), and outer TMD layers for supercapacitor electrodes. The 3D hierarchical composites aim to achieve synergetic effects from the components and offer high energy density while maintaining excellent power density and durability. Percolated nanotube networks are highly conductive, MOFs ensure a fast ion diffusivity, and TMD offers a large ion capacity. We engineered the TMD morphologies via topochemical synthesis and determined the optimal structure maximizing faradaic-reactive surface areas for improved ion accumulation and redox energy storage. We found that the hybrid composite of a flower-like TMD structure interwoven with carbon networks exhibits an unprecedentedly high energy density of over 80 Wh/kg, superior to conventional supercapacitors. In summary, this thesis presents powerful strategies for engineering atomically thin TMDs and critical insights on relevant physics which may not be accessible otherwise. Given the extensive library of organic molecules, the hybrid approach may provide a versatile platform to study 2D materials and open new opportunities. The findings could serve as the foundation for the development of novel optoelectronic and energy storage applications.
Choi, Purdue University.
Materials science|Mechanical engineering|Condensed matter physics
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