CVD Growth Mechanism Of Hexagonal Boron Nitride On Fe₂B Surface

Two-dimensional（2D）hexagonal boron nitride（h-BN）has been regarded as excellent materials for protecting the intrinsic properties of other 2D materials due to its large bandgap,high temperature stability,excellent mechanical properties,low dielectric constant and high chemical stability.Currently,the large-scale application of h-BN in 2D devices faces the problem of controllable preparation of large-area and high-quality h-BN films.Chemical vapor deposition（CVD）method is the most promising method to synthesize large-area and high-quality h-BN film,in which the catalytic substrate is critical for the controllable synthesis of h-BN.Untill now,the preparation of large-area and single-crystal h-BN monolayers has been achieved experimentally on catalyst substrates such as Cu,Cu-Ni alloys and Au,while the controllable synthesis of large-area and high-quality multilayer h-BN films is still lacking effective catalytic substrate.In this paper,by combining first-principles calculations with experimental synthesis and characterization,we revealed the growth mechanism of single and multilayer h-BN by using Fe₂B as a catalytic substrate and boron source,and using nitrogen（N₂）as a nitrogen source.The vacancy-assisted mechanism was proposed for the synthesis of multilayer h-BN.Based on the growth mechanism predicted by theoretical calculations,we synthesized monolayer and multilayer h-BN by controlling the experimental temperature.The research coclusions of this thesis are as following:I.Growth mechanism and synthesis of monolayer h-BN on Fe₂B surfaceThis part studied the adsorption and dissociation of N₂ molecules,the migration of N atom on the surface of Fe₂B.The results show that N₂ can easily adsorb and dissociate on the Fe₂B surface,and the dissociated N atoms migrate to the hollow site of Fe₄ which is above the B atoms in the sublayer.The B atoms in the subsurface of Fe₂B migrate to the surface with the assistance of adsorbed N atoms on the surface,and eventually form a BN dimer on the surface of Fe₂B.The BN dimers diffuse and aggregate into a h-BN nucleus on the Fe₂B surface and such a nucleus grows into a h-BN film with the incorporation of more BN dimers.Our theoretical calculations show that the nucleation size of the single-layer h-BN thin film on the Fe₂B surface is B₅N₅,and the nucleation energy barrier is～2.0 e V.Based on this barrier,we predict that monolayer h-BN can grow on the Fe₂B surface at low temperature（～700 K）,and such a theoretical prediction is confirmed by the subsequent experiments.II.Growth mechanism of multilayer h-BN on Fe₂B surfaceIn this part,we studied the diffusion of B and N atoms in the bulk of Fe₂B,as well as the nucleation barrier of multilayer h-BN.The growth of multilayer h-BN requires a continuous supply of B and N atoms.Our research shows that the formation of BN dimer on Fe₂B subsurface consumes a large amount of B atoms near the surface,and the B vacancies produced near the surface promotes the diffusion of B atoms from the bulk to the surface of Fe₂B.The N atoms absorbed on Fe₂B surface can move down into the bulk phase through the B vacancy chain and precipitate out during the subsequent h-BN growth process.By combining theoretical calculations and experimental data,we found that the growth barrier of the multilayer h-BN is higher than that of the monolayer h-BN,which is about 2.65 e V.It shows that multilayer h-BN can be grown at an experimental temperature of about 900 K,which was confirmed by our subsequent experimental synthesis and characterization of multilayer h-BN films be obtained at a temperature of 960K.In addition,the growth of h-BN at high temperature is a process of isothermal segregation.The characterization of h-BN prepared at 1523 K shows that it is better to obtain uniform multilayer h-BN with large area and high quality than that of precipitation by solution during the cooling.