Preparation And Photoelectronical Properties Of Two-dimensional Molybdenum Disulfide Via Chemical Vapor Deposition

Transition metal dichalcogenides?TMDCs? materials have received significant attention owing to their unique fundamental physical and chemical properties. Among various semiconducting TMDCs, molybdenum disulfide?MoS₂? has attracted enormous research interest because two-dimensional?2D? MoS₂ is covalently bonded in the 2D plane similar to the structure of graphene. Monolayer MoS₂ is a direct band gap semiconductor with a band gap of 1.8-2.0 eV, while bulk MoS₂ is indirect bandgap?1.2 eV?. Compared with the zero energy gap of graphene, 2D MoS₂ has a great potential for applications in electronics and optoelectronics devices owing to its tuning band gap. Recently, chemical vapor deposition?CVD? was introduced as a more efficient preparation method to generate large-area MoS₂ layers, and has allowed for the successful preparation of functional devices combined with semiconductor manufacturing technology. However, based on the controllable synthesis methods of low cost, high efficiency still need further research. Further investigation of monolayer MoS₂ should ideally focus on photoelectric performance. In this thesis, we report an easy and fast CVD method for the growth of uniform MoS₂ thin film, and its growth mechanism was discussed. We systematically investigate the devices performance such as field effect transistor?FET? and photodetector of MoS₂ thin film, the electrochemical hydrogen evolution reaction?HER? of few layers MoS₂. The main results are concluded as follows:We report systematical studies on ambient pressure CVD growth of MoS₂ monolayer directly on molybdenum?Mo? foils. The Mo foils were annealed in high temperature under high-purity H2 atmosphere to smooth the surface of the foils and enlarge the grain boundaries. In oxidation method can be simple and fast determine the uniformity and integrity of the sample. The MoS₂ layers are transferred onto other insulated substrates such as quartz and silicon using PMMA coating and substrate etching methods. Scanning electron microscopy?SEM? and Atomic force microscopy?AFM? was used to characterize the structure of the obtained MoS₂ monolayer. The result showed that the large area of atomically smooth MoS₂ monolayer was formed on the surface of the Mo foil. Based on the theoretical calculation, we conducted an in-depth investigation of the atomistic mechanism for MoS₂ growth on Mo substrates.Furthermore, we attempts to produce large-area and continuous multilayer MoS₂ via a simple sulfurization of molybdenum dioxide formed on Mo foils at high temperature. In case of oxide morphology, the unique vertically aligned MoS₂ film with maximally exposing the edges could be formed under appropriate conditions. Such edge-terminated film with high-density edge sites represents a novel metastable structure of MoS₂, which greatly promoted applications in HER. We have confirmed their catalytic activity in a HER, in which the exchange current density correlates directly with the density of the exposed edge sites.We also studied the photoelectric properties of MoS₂-based FETs and photodetectors under various thicknesses MoS₂ films. The high vacuum annealing can effectively remove absorbates and ensure that the gold contacts are ohmic. We measure the transfer curves of MoS₂ devices in the air, and the result showed that our transistor exhibits high level room-temperature current on/off ratio. Environmental conditions, such as temperature, moisture, and physisorbed gases, critically affect the carrier mobility of transistor. In addition, we fabricated a vertical heterojunction photodetector composed of n-type MoS₂ and p-type silicon, and an obvious rectifying behavior was observed under varying illumination powers. We demonstrate that the ring Au thin film electrode as a negative electrode could be efficiently harvest the photocurrent, and exhibits an excellent photoresponse performance.