Growth,Performance And Strain Studies Of Two-dimensional Transition Metal Sulfides

Since 2004,the discovery of the graphene indicates that the two-dimensional material with atomic scaled thickness has become a novel and promising subject.Two-dimensional(2D)transition metal sulfides(TMDs)is a class of the most typical 2D materials.It is easy for TMDs to dissociate into single layer structure because of the van der Waals interaction among the interlayers.There are various 2D TMDs,which exhibit abundant optical and electrical properties.MoS2 and WS2 are two kinds of most common 2D semiconductor TMDs materials.Bulk MoS2 and WS2 materials are indirect band gap semiconductors.However,monolayer MoS2 and WS2 materials are direct band gap semiconductors.The transition from bulk TMDs to monolayer TMDs not only greatly improves the light emission efficiency but also exhibits a high charge mobility.The aforementioned characteristics enable the 2D TMDs an ideal platform for electronics and optoelectronics.With the development of the research,the growth technology for 2D TMDs materials is also developing rapidly.Presently,2D TMDs materials can be prepared via mechanical exfoliation,physical deposition and chemical deposition method.However,these technologies have both advantages and disadvantages.Additionally,the size and quality of the product are uncontrollable.During the synthesis process,the change of photoelectric properties due to the deformation of the materials has attracted much attentions.In order to solve the above hot issues,this paper mainly focuses on improving the technology of chemical vapor deposition(CVD)and studying the properties of the synthezied materials.By means of our method,2D TMDs materials with large size,high quality,and various structures are obtained.In addition,the controllable synthesis,the electrical properties and the strains of the materials are thoroughly investigated.The main results are obtained as follows:(1)A new 2D material preparation technology is proposed,and various large-size 2D TMDs materials have been prepared through this method.In order to achieve the controllable growth of 2D TMDs materials,physical vapor deposition(PVD)assisted chemical vapor deposition(CVD)technology is designed.In the first step,PVD technology is used to deposit transition metal oxide onto the surface of the substrate,and precursor films with nanometer-level thickness are genereated.Subsequently,2D TMDs materials can be grown by CVD technology.Large-size MoS2 monolayer films and single crystals,WS2 single crystals and MoS2/WS2 lateral heterostructures can be controllably prepared by adjusting the reaction temperature and the carrier gas flow rate.(2)Spatially graded millimeter-sized 2D monolayer Mo1-xWxS2 alloy is synthesized by PVD-assisted CVD technology,and the memory effect is discovered in the 2D alloy material.In the PVD-assisted CVD synthesis technology,NaCl micron particles are used as the catalyst.It is found that the MoO3/WO3 composite precursor can nucleate and grow in the same crystal domain under the catalysis of salt.Finally,the spatially graded monolayer Mo1-xWxS2 alloy with orderly changes in composition is obtained.The 2D Mo1-xWxS2 alloy exhibits strong charge trapping ability,which leads to the memory effect in the device.By characterizing the structure and composition of the material,this effect can be attributed to the deep energy levels induced by sulfur vacancies can be effectively suppressed to shallow energy levels by alloying TMDs..(3)One kind of 2D poly crystalline MoS2-Mo1-xWxS2 heterostructure is prepared by PVD-assisted CVD technology,and the strain caused by lattice distortion is studied.In the PVD-assisted CVD synthesis process,the growth conditions are regulated in stages to induce the growth of polycrystalline heterostructures,which leads to the emergence of transition regions in the polycrystalline MoS2-Mo1-xWxS2 heterostructures.Through the characterization and analysis of different regions in the material,it is found that a slight lattice distortion existed in the transition region,which induces strain leading to the transformation from isotropy to anisotropy in this region.