Study On The Regulations Of Photoluminescence Properties Of Few-layer Molybdenum Disulfide Via Advanced Plasma

Due to their unique layered-structure features,two-dimensional transition metal chalcogenide?TMDC?semiconductor materials such as molybdenum disulfide?MoS₂?and molybdenum diselenide?MoSe₂?have great potential applications in the fields of novel optoelectronic devices and micro-nanoelectronic devices,and thus have attracted extensive attention and research from both the academic and industry communities.For example,MoS₂has a limited bandgap in the visible light range,and its electrical devices exhibit large carrier mobility values and high on/off ratios.In addition,when MoS₂ is reduced from bulk to monolayer,its band structure transforms from the indirect bandgap to a direct one,displaying excellent optical properties.However,the inherent defects in MoS₂ may significantly affect its performance.Therefore,it is of great importance to study how to tune both the band structure and defects of MoS₂.In this thesis,we have succeeded in tuning the band structure of few-layer MoS₂ and MoSe₂ and thus achieved the photoluminescence?PL?regulation by using a self-designed advanced plasma technology.At the same time,the plasma treatment can induce both chemical and physical adsorptions and consequently influence the electrical properties of the treated few-layer materials.Then,the physical mechanisms of PL regulation and doping are systematically investigated by using optical microscopy,Raman/PL spectroscopy,atomic force microscopy and x-ray photoelectron spectroscopy.The major research content can be summarized as follows.1.Firstly,the preparation and characterization methods of few-layer Mo S₂ samples are introduced,as well as the capacitively coupled electrodeless plasma technology.2.Secondly,the few-layer MoS₂ samples are treated by soft oxygen plasma.Their PL intensity is greatly enhanced by a factor of 100 after optimizing the plasma processing parameters.For few-layer MoS₂ samples with different layers,the effect of plasma treatment time on the PL characteristics is systematically studied,and the optimal processing time for samples with different layers is determined for obtaining the largest increase of PL intensity.The microstructure and optical properties of few-layer MoS₂ samples before and after plasma treatment are systematically characterized by optical microscopy,atomic force microscopy,Raman spectroscopy and PL spectroscopy.In combination with the first-principle calculations,it is demonstrated that the soft oxygen plasma treatment can widen the interlayer distance of few-layer MoS₂,and the band structure of treated samples can transform from the indirect bandgap to a direct one,resulting in substantially enhanced PL intensity.Furthermore,this method is also proved applicable to tune the PL intensity of few-layer MoSe₂,indicating its universality for similar two-dimensional TMDC semiconductor materials.3.Finally,the physical mechanism of PL enhancement for the soft plasma treated few-layer MoS₂ is investigated in terms of doping.The chemical composition and electrical properties of few-layer MoS₂ samples before and after plasma treatment are studied systematically by x-ray photoelectron spectroscopy and electrical characteristics of field effect transistor?FET?.It is demonstrated that oxygen and molybdenum can form Mo-O bonds during the soft oxygen plasma treatment,thereby providing additional holes to reduce the electron concentration.As a result,the threshold voltage increases and the output circuit decreases.The PL characteristics measured at low temperature reveal that oxygen atoms can partly fill up the sulfur vacancies in the treated few-layer MoS₂,so that the defects are repaired and the PL intensity is enhanced.The comparison of PL characteristics measured under vacuum and atmosphere indicates that there exist not only chemical adsorption but also physical adsorption of oxygen in the few-layer MoS₂ after plasma treatment.