Synthesis And Properties Of Two-dimensional Layered Transition Metal Silicon Nitrides

The successful exfoliation of graphene has opened the door to the field of two-dimensional(2D)layered materials.Its unique physical and chemical properties and huge application prospects in electrical,optical,thermal and mechanical aspects have attracted extensive attention from researchers.For more than ten years,many other 2D layered materials,such as hexagonal boron nitride(h-BN),transition metal chalcogenides(TMDCs),black phosphorene(BP),etc.,have also been developed.These 2D materials all have known bulk parent materials,which can be easily obtained by exfoliating their bulk parent materials.However,the number of bulk layered materials currently known in nature is limited.Therefore,exploring 2D layered materials without known parent materials can greatly expand the physical properties and applications of 2D materials,which has important scientific significance and practical value.Transition metal carbides and nitrides(TMCs and TMNs)are a class of non-layered materials that combine ceramic and metal properties.Early studies have shown that chemical vapor deposition(CVD)method can be applied to prepare high-quality 2D non-layered TMCs and TMNs crystals with different structures,such as orthogonal Mo2C,hexagonal WC,cubic TaC,etc.However,due to the limitation of surface energy,these non-layered materials tend to grow in island mode rather than layer-by-layer mode.Therefore,only 2D non-layered crystals with thickness of few nanometers and lateral size of about 100 ?m can be obtained,which still makes the synthesis of large-area crystals with uniform thickness challenging.Therefore,promoting the layered growth of TMCs or TMNs crystals is the key to successfully achieving their uniform growth to the limit of single layer thickness.This work creatively introduced silicon in the CVD growth process of non-layered molybdenum nitride to passivate its high-energy sites on the surface,thereby changing its island growth mode.Furthermore,a new type of 2D ternary layered MoSi2N4 single crystal and thin film were prepared,and then their basic physical properties were systematically studied.Using similar idea,2D WSi2N4 crystal with the same structure was also prepared.The main results achieved are as follows:The atmospheric pressure CVD method was adopted with Cu/Mo bimetallic laminate as the growth substrate.While the copper foil is melted and re-solidified at a high temperature,the Si source is introduced and ammonia is used as nitrogen source to prepare monolayer MoSi2N4 single crystals and centimeter-scale monolayer MoSi2N4 polycrystalline films,and then high-quality transfer is achieved by chemically etching the copper layer.In addition,multilayer MoSi2N4 crystals can be obtained by increasing the ammonia flow.Structural characterization showed that the monolayer MoSi2N4 crystal has a seven-atom stacking sequence,that is N-Si-N-Mo-N-Si-N,which can be regarded as a molybdenum nitride layer(N-Mo-N)sandwiched by two Si-N bilayers.And the Si layers and their two adjacent N layers form Si-N tetrahedral layers(the basic structural unit of Si3N4).The unique structure of MoSi2N4 gives it a series of unique physical and chemical properties.Studies have shown that the monolayer MoSi2N4 sample is a p-type semiconductor with an indirect bandgap of?1.94 eV;it has a Raman resonance effect under the 532 nm laser;its film has high transmittance(about 97.5%)in the visible range;at the same time,its single crystal has excellent mechanical properties,where the Young's modulus and breaking strength are as high as 491.4.GPa and 65.8 GPa,respectively;and it has good stability in a variety of chemical environments.The growth of MoiSi2N4 opened up the direction of preparing new 2D layered materials without three-dimensional parent materials,and opened up a new family of 2D layered materials.On the basis of the preparation of MoSi2N4,2D WSi2N4 crystal was successfully prepared with Cu/W bimetallic laminate as the growth substrate and the growth was carried out directly on the solid Cu/W substrate at high temperature.The stoichiometric ratio and crystal structure of WSi2N4 was verified to be same as MoSi2N4.With further cooperation,theoretical calculations have been used to predict that more than ten new 2D ternary layered materials can stably exist and have the same stoichiometric ratio and crystal structure as MoSi2N4,forming a new 2D ternary layered material family.