Spectroscopic Study On Growth Mechanism And Physical Properties Of Two-Dimensional Transition Metal Dichalcogenides

The miniaturization,functionalization,and integration of optoelectronic devices is one of the most main factors for driving the development of information technologies,which makes that the optoelectronic materials for the fabrication of devices can keep the outstanding electronic and optical properties in the nanoscale size.The two-dimensional?2D?transition metal chalcogenides?TMDs?becomes the important candidates for the next-generation electronic and optoelectronic devices due to their two-dimensional characteristics,high carrier mobility,high stability,and fast light response.The realization of TMD materials in device applications firstly requires the fabrication of large-area,high quality atomically thin TMDs.The chemical vapor deposition?CVD?is the most promising method to reach this goal.However,these are few studies on the CVD growth of two-dimensional TMD materials,the microscopic nucleation and growth mechanisms remain unclear,and thus it strongly requires the further studies to guide the fabrication of TMD materials.In this thesis,we take the CVD-grown two-dimensional TMDs represented by MoS₂and MoSe₂ as the research objects and study the morphology,structure,growth mechanism and optical properties of MoS₂ and MoSe₂ by using the spectroscopic characterizations and theoretical calculations,and the main research contents and conclusions are as follows:1.Two-dimensional molybdenum disulfides?MoS2?were synthesized by the chemical vapor deposition?CVD?method,and their morphology,structure,growth mechanism and optical properties were deeply studied.It is found that the CVD growth of MoS₂ includes two growth nucleation modes:two-dimensional planar nucleation and self-seeding nucleation.The two nucleation modes can be regulated by the growth temperature and gas flow,and then the high quality,large area,single layer MoS₂ have been obtained.The morphological and structural characterization results prove that the fundamental reason of two growth nucleation modes is the change of substrate surface precursor?MoO3-x,sulfur?concentration.The moderate reactant concentration and the sufficient reaction lead to the uniform MoS₂ film on substrate surface,and the high reactant concentration and insufficient reaction will cause the formed MoS₂ film with a central nanoparticle?nucleus?.The spectroscopic analysis show that the two-dimensional planar nucleation easily produces single or few-layer structures,and self-seeding nucleation results in few-layer and multilayer MoS₂ structures.Therefore,the growth nucleation modes of MoS₂ can be controlled effectively by tuning the growth temperature and gas flow rate,which contributes to the realization of the controllable growth of the high quality,large area,monolayer MoS₂.In addition,these finding can help the understanding of growth mechanisms of other two-dimensional transition metal sulfides.2.Based on the spectroscopic analysis and theoretical calculations,the rapid nucleation/growth mechanisms and physical properties of salt-assisted CVD-grown molybdenum diselenide?MoSe₂?were studied.The research results indicate that salt can greatly promote the nucleation and growth of MoSe₂.Moreover,the prepared MoSe₂ exhibits has few grain boundaries and defects by using the small amount of salt?1 mg and 2 mg?,and the prepared MoSe₂ sample include a large number of grain boundaries at a large amount of salt?4 mg and 6 mg?due to the polycrystalline growth mode.The morphological and structural characterizations show that the use of salt greatly increase the nucleation and growth rates of MoSe₂,and the grain size reaches⁶0-80?m in a very short time?20 s?.Therefore,using a suitable salt concentration,it can obtain the high quality and large size atomically thin MoSe₂ with the low defect density.The calculated results suggest that the salt plays an important role in the nucleation and growth of MoSe₂,which reduces the nucleation barrier and the formation entropies make to increases the nucleation and growth rates by 5-8 and 2-3 orders of magnitude,respectively.The spectroscopic characterizations show that the prepared MoSe₂ samples with high salt quantity include a large number of defects and exhibit the multi-layered structures with the poor optical properties.To facilitate the rapid nucleation and growth of MoSe₂ with low defect density,the control of the amount of salt is crucial.In this thesis,the growth mechanism and physical properties of 2D transition metal chalcogenides were systematically studied,and it provides a solid foundation for the realization of high-quality and large-area growth of 2D MoS₂ and MoSe₂monolayers,matching with the current industrial production and commercial applications.In addition,it provides the important guidance and basis for the study of other 2D transition metal chalcogenides and their heterojunctions.