Growth Of Two-dimensional Single Crystals By Liquid Metal Chemical Vapor Deposition

The charge carriers,heat and photon transport of two-dimensional(2D)single crystals are all confined in a 2D plane,which makes them exhibit distinct properties from their bulk counterparts.These properties endow 2D single crystals great potentials in optoelectronic devices such as field effect transistors,light-emitting diodes and spintronics.However,reliable application is based on the premise of efficient synthesis of high quality 2D single crystals.Chemical vapor deposition(CVD)is recognized as one of the most promising methods for the efficient synthesis of high quality 2D single crystals.In the process of growing 2D single crystals by CVD,the metal substrates affect the elemental steps of adsorption,diffusion,nucleation and growth,and thus it will have a significant influence on the structure and the properties of 2D single crystals.Therefore,the design of metal substrate is important for the growth of 2D single crystals.Liquid metal shows a short-range order and long-range disorder in the atomic configuration.Due to the combined action of surface tension and atomic thermal motion,liquid metal tends to form an atomic isotropic smooth surface.At the same time,new pathways for mass transfer in liquid metal may be generated because the large number of clusters and vacancies in liquid Cu provide a possibility for the embedding of heterogeneous atoms.In addition,liquid metal has a high diffusion rate,which can accelerate the mass transfer of the precursors and promote the growth of the 2D crystal.Therefore,we choose the liquid metal as the growth substrate in this paper.We solved the challenge of fast growing graphene single crystals in the industrial production of graphene by liquid metal chemical vapor deposition(LMCVD).We achieved the low-density nucleation and the rapid growth of graphene single crystals on liquid Cu,and the maximum growth rate can be as fast as 79 ?m/s.Meanwhile,we also successfully breaked the three-dimensional isotropic growth balance of gallium phosphide(GaP)which has a three-dimensional covalent structure and prepared 2D GaP single crystals on liquid Ga by LMCVD.The main research contents are as follows:(1)The rapid growth of graphene single crystals was realized on the liquid Cu on account of the merits of liquid metal.The abundant free electrons in liquid Cu could effectively saturate the edge of carbon clusters,lower the nucleation barrier and accelerate the nucleation rate of graphene single crystals.The liquid Cu is featured by its atomic isotropic smooth surface,and it helps to avoid the inherent grain boundaries and defects of the solid polycrystalline metal substrate and significantly inhibits the nucleation density of graphene single crystals.At the same time,a large number of vacancies which is generated by the intense thermal motion of liquid Cu provide a possibility for the embedding of heterogeneous carbon atoms,resulting in a binary mass transfer pathway consisting of surface adsorption and bulk segregation,thus greatly accelerated the growth of graphene single crystals.In addition,the atomic distance and vacancy distribution in the liquid Cu were in dynamically movement,which greatly reduces the migration barrier of the precursor on the liquid Cu and accelerates the mass transfer rate.All of the above reasons contributed to the growth rate of graphene single crystals up to 79 ?m/s.(2)Based on the fact that liquid metal owns an isotropic atomic smooth surface and a high diffusion rate,we explored the possibility of the growth of 2D GaP single crystals on liquid Ga by LMCVD.It was found that liquid metal substrate could promote the nucleation and the growth of GaP crystals and then the h-BN encapsulation and salt-assisted confinement methods were used to inhibit the growth of GaP in vertical direction for breaking the isotropic growth balance of GaP in three dimensions.Therefore,the 2D GaP single crystals successfully grew on the liquid Ga.