Two Dimensional Materials for Renewable Energy Harvesting and Energy Efficient Devices

[摘要] Two-dimensional (2D) materials show promising electronical, optical,magnetic and mechanical properties. In this thesis, we investigate the preparation oftwo dimensional materials through chemical vapor deposition, and performelectronic, optical and magnetic measurements on these materials and theirheterstructures, including Molybdenum Disulfide (MoS2), Vanadium Sulfides (VS2,V5S8) and Iron oxide (Fe2O3). We argue that these materials can be potentialcandidates for harvesting solar energy to generate electricity and hydrogen, as wellas making energy efficient spintronic devices.Moisture content is one of the key parameters in controlling the synthesis of2D MoS2. We found that moisture present in the MoO3 powder will have significanteffect on the synthesis of monolayer MoS2. Having understood the role of moisturein the process, we achieved highly-reproducible, high-yield, and high-qualitymonolayer MoS2 growth by adopting an unglazed surface crucible to absorb thewater during the process.The photoluminescence (PL) performance of individual 2D material can bevery different when they are combined together. We study the inter-layer couplingbehaviors of hetero-bilayers of MoS2 and WS2 and MoS2-black phosphorous (BP),WS2-BP hetero-stacks. The interactions between the MoS2 and WS2 layers retain adirect band gap for MoS2 while form an indirect gap for the WS2. The MoS2-BPhetero-stack shows a type-II band alignment and WS2-BP hetero-stack has a type-Iband alignment. The build-in electric fields in these hetero-stacks are strong enoughto split the excitons, leading to a significant reduction in their recombination andsubsequently a strongly-quenched PL peak of the WS2 and MoS2.Singe crystalline VS2 nanosheets can be prepared by CVD. We determine thehexagonal 1T crystalline structures of VS2 using XRD and TEM. We investigate theelectrocatalytic hydrogen evolution reaction (HER) activities of the 1T-VS2, whichshows extremely low overpotential of -68 mV at 10 mA cm-2, and small Tafel slopesof ∼34 mV/decade as well as high stability, demonstrating its potential as acandidate non-noble metal catalyst for HER.Finally, two magnetic 2D materials: V5S8 and Fe2O3 were synthesized andstudied. V5S8 maintains its antiferromagnetic properties down to ~ 11 nmthickness, and possible quantum phase transition at accessible fields (~18 T). Dueto its minimum surface energy, the unique epsilon phase Fe2O3 was found to be amore stable phase among all other phases in 2D forms. Magnetic Kerr rotationmeasurements indicated the room-temperature ferromagnetism of epsilon phaseFe2O3. Combing its air-stable nature, ultra-thin 2D epsilon Fe2O3 can be an excellentplatform for compact and energy efficient spintronic devices.

[发布机构] Rice University

[效力级别] dimensional