| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系所 === 105 === Two-dimensional layered transition metal dichalcogenides (TMDs) materials such as Molybdenum disufide (MoS2) have been recognized as one of the low-cost and highly efficient electrocatalysts for hydrogen evolution reaction (HER). On the other hand, TMDs have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. The crystal edges, rather than the basal planes, of MoS2 have been identified as the active sites for HER performance, but they only account for a small percentage of the surface area, of MoS2 monolayer. Here, we report a simple and efficient approach that involves using a remote hydrogen-plasma process to creating S-vacancies on the basal plane of a monolayer crystalline MoS2; this process not only can generate high density of S-vacancies but also can maintain the morphology and structure of MoS2 monolayer, as confirmed with Atomic force microscopy (AFM) characterizations. The density of S-vacancies (defects) on the basal plane of MoS2 monolayers resulting from the remote hydrogen-plasma process can be tuned and play a critical role in HER, as evidenced by the results of electrical measurements. A lowered overpotential, from 727mV to 183mV, and a decreased Tafel slope, from 164mV/dec to 77mV/dec, as compared to those of a pristine MoS2 monolayer are observed. We found several times enhancement in the capacitance of the hydrogen- plasma-treated MoS2 monolayer from the electrical double layer capacitance (EDLC) measurement, Moreover, the stability test shows these materials have high durability in acid environment. The H2-plasma-treated MoS2 also provides an excellent platform for systematic and fundamental study of defect-property relationships in TMDs, which provides insights for future applications including electrical, optical and magnetic devices.
Second, we report a two-step epitaxial growth of lateral heterojunction WSe2-MoS2 monolayer with an atomically sharp interface, instead of preferred TMD alloy, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface which can be evidenced by high resolution TEM. From the electrical transport curves, we found the lateral heterostructure WSe2-MoS2 monolayer display apparent p-n junction and thus photovoltaic effect. Our spatially connected TMD lateral heterojunctions are potential candidates for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors.
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