Synthesis And Properties Of Two-dimensional Layered Semiconductors

With unique crystal and band structures,excellent optical,electrical and catalytic properties,two-dimensional layered semiconductor?2DLS?has become an international research frontier and hot topic in materials,physics and other disciplines,due to its great potential applications in the fields of optoelectronics,microelectronics,electro-and photo-catalysis.At present,the study on controllable synthesis,novel effects and functional devices of new 2DLS is still in very early stage.In this dissertation,we focus on several 2DLSs such as graphene,molybdenum trioxide and metal dichalcogenides,and firstly investigate their synthesis mechanisms and controllable synthesis process,then study their electrical,optical,optoelectronic,photocatalytic and electrocatalytic properties.The main research contents and results are as follows.1.An ultrafast microwave-assisted method was presented for large-scale synthesis of nitrogen-doped graphene quantum dots?NGQDs?,whose optical properties were investigated.The results show that NGQDs can be synthesized within 1 min at room temperature and atmospheric pressure using glucose as a carbon source and ammonia as both catalyst and nitrogen doping source in a commercial microwave oven.The NGQDs with a nitrogen?only pyridine and pyrrolic N atoms?doping content of 4.1%is composed of 1³ graphene monolayers with an average size of 5 nm.The monodispersed NGQDs have excellent photoluminescence property and glow bright fluorescent blue under a UV light with a fluorescence quantum yield of 6.42%.2.A physical vapor deposition method was presented to synthesize centimeter-sized?-MoO₃ single crystals,whose Raman anisotropy and optoelectronic properties were investigated.The results show that centimeter-sized?-MoO₃ single crystals with a bandgap of 3.14 eV can be controllably prepared by adjusting the inner diameter of quartz tube,the mass and evaporation temperature of Mo source.The well-aligned out-of-plane orientation confirms the high crystallinity of as-prepared?-MoO₃ single crystals.The?-MoO₃-based photodetectors have a photoresponsivity of 2.62×10^-4 A/W and a photodetectivity of 1.15×10⁷ Jones under 405-nm incident laser.The performance of photodetectors can be greatly enhanced through introduction of oxygen vacancies by vacuum annealing:the responsivities and detectivities of devices can be significantly improved to 1.41×10^-2?65.6?A/W and 1.65×10⁷?4.43×10⁸?Jones by vacuum annealing at 200°C for 1?2?hours?3.With the aid of ultrasonication,liquid exfoliation of?-MoO₃ single crystals was carried out to prepare defect-rich?-MoO₃ nanosheets?dr-MoO₃?,whose photocatalytic properties were investigated.The results show that thin?-MoO₃ flakes were exfoliated and abundant surface defects are generated on?-MoO₃ flakes by the the ultrasonic waves generated cavitation bubbles and pitting effects.The dr-MoO₃ samples have abundant highly reactive sites and thus enhancing the photocatalytic performance.Within 50 min of irradiation,95%of Rhodamine B?RhB?in the solution have been degraded by dr-MoO_3.The degradation apparent rate constant k of dr-MoO₃ is 5.9×10^-2 min^-1,which is⁶.6 times higher than commercial?-MoO₃.The experiments with virous scavengers indicate that·OH is the main active species in the photodegradation process.4.The monolayer MoSe₂ domains and films have been controllably grown by chemical vapor deposition?CVD?in a furnace with three-temperature zones,and the electrocatalytic properties of MoSe₂ domains and films have been investigated.The results show that the thickness,morphology and grain size of MoSe₂ domains and films can be efficiently controlled by the growth pressure,temperature and time.The monolayer MoSe₂ domains have much better electrocatalytic performance for hydrogen evolution reaction?HER?than monolayer MoSe₂ films:the Tafel slope of domains is 63.3 mV/dec which is much better than films?119.1 mV/dec?;the electrochemical double-layer capacitance(C_dl)of domains is 75.3?F/cm² which is twice than films?38?F/cm²?.It confirms that MoSe₂ domains have much more catalytic active sites than films.5.The perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt?PTAS?was introduced as a seeding promoter in the CVD process to synthesize three-dimensional hierarchical MoSe₂ nanoarchitecture?3D-MoSe₂?,whose electrocatalytic properties were investigated.The results show that without PTAS seeding promoter,only horizontal MoSe₂ films?Ho-MoSe₂?were grown;in contrast,with PTAS,3D-MoSe₂nanoarchitecture constructed with curly few-layered vertical nanosheets onto the horizontal layer can be synthesized,and 3D-MoSe₂ can also be grown on carbon nanotube films?3D-MoSe₂@CNT?.The unique hierarchical structure and high conductivity of CNT increase the catalytic active sites and enhance the charge transfer,leading to better electrocatalytic performance of 3D-MoSe₂ and 3D-MoSe₂@CNT:the Tafel slope of3D-MoSe₂ and 3D-MoSe₂@CNT are 47.3 mV/dec and 32.5 mV/dec respectively,which are much better than Ho-MoSe₂?123.8 mV/dec?;the current densities of 3D-MoSe₂ and3D-MoSe₂@CNT are 1 and 2 orders of magnitude higher than Ho-MoSe₂ when overpotential is at 300 mV.6.A CVD strategy was proposed to synthesize the vertically oriented few-layered HfS₂ nanosheets?V-HfS₂?,whose growth mechanism and optoelectronic properties were investigated.The result show that large-area,highly crystalline and homogeneous V-HfS₂can be grown by using HfCl₄ as Hf source and optimizing process parameters.The as-grown V-HfS₂ are composed of vertically oriented nanosheets with uniform lateral width of 1?m,vertical height of 400 nm,thickness of 6 layers.A novel dangling-bond-assisted self-seeding growth mechanism is proposed to describe the growth ofV-HfS2 nanosheets.The photodetectors based on large-area V-HfS₂ have excellent optoelectronic performance with a low off state current of 1.5 pA and a high on state current of 1.2 nA,exhibiting high photosensitivity with a large on/off ratio of 10³.Especially,the photoresponse of V-HfS₂-based devices is 24 ms,which is the smallest resolution time of the instrument.Due to the high in-plane mobility of the HfS₂ layer,the vertically standing layered structure of V-HfS₂ offers high-speed paths for the transport of photogenerated carriers,resulting in a high response speed and photosensitivity.