Controlled Synthesis And Physical Property Of Light-element BN And BCN 2D Materials

The discovery of graphene in 2004 has opened up the world of two-dimensional（2D）nanostructures.Known as"white graphene",hexagonal boron nitride（h-BN）holds similar crystal structure to that of graphene.However,their electronic structures are highly complementary:graphene is a zero band gap semi-metallic material,while h-BN is a wide band gap semiconductor（～6 e V）.In recent years,h-BN has received enormous attention due to its novel properties and great potential in the fields of deep ultraviolet optoelectronic devices,nonlinear optics,solid-state single-photon emission,phonon polaritons,and insulating thermal conductive composites.Furthermore,ternary boron carbon nitride（h-BCN）nanostructure,which belongs to the big family of light element materials along with graphene and h-BN,has been under attention of theoretical researchers since the early 1980s.In particular,since the huge difference in electronic structure between graphene and h-BN,ternary BCN with a composition between the two is considered as a new type of semiconductor material with continuously tunable band gap.This provides rich space for the discovery of new physical phenomenon and regulation of physical properties,which makes it worthwhile for basic researches and potential applications.This dissertation focused on the controlled synthesis of 2D h-BN and ternary BCN and investigations on related physical properties.The main findings are as follows:1.We developed the method of the gas/liquid interface self-limiting reaction for the growth of h-BN.First,we synthesized large-area,continuous,ultra-thin h-BN films with high crystalline quality via self-limiting reaction in the interface of liquid boron trioxide（B₂O₃）and NH₃.We elucidated that the oriented attachment mechanism is playing a role in mediating the growth of continuous large-area h-BN films.Then,we synthesized h-BN films with a high density of exposed edges through the introduction of Turing instability mechanism.Compared to the smooth h-BN ultra-thin film prepared under conventional conditions,the second harmonic generation（SHG）intensity of the edge-enriched h-BN film is three orders of the magnitude than the smooth one due to the symmetry-breaking nature of the edge sites.This work provides a unique platform for reliable implementation of h-BN layered materials in a range of nonlinear optical applications.2.We synthesized h-BN multilayer structures with two different stacking orders by CVD method,i.e.AA’stacking and AB stacking.The SHG measurements with 820nm femtosecond pulsed laser show that unlike the AA’stacking that shows SHG interlayer interference cancellation,the AB stacking"pyramid"type h-BN with thick layers exhibits constructive SHG and the SHG intensity shows a good proportional relationship with the square of the layer number.On this basis,considering the great resonance enhancement of SHG when the photon energy of SHG is close to the band gap of h-BN,we have conducted SHG experiments with 532 nm continuous-wave laser and 486 nm femtosecond pulse laser,and successfully observed the ultraviolet/deep ultraviolet SHG of"pyramid"h-BN.3.We prepared sub-monolayer BCN composed of single crystalline grains by CVD.It has been a long pursuit but an extreme challenge to synthesize homogeneous BCN nanostructures,since the bonding of B-N and C-C shows a strong thermodynamic tendency to form and therefore the homogeneous BCN structure is thermodynamically unfavorable.By using a precursor that contains B-C bonding,we successfully prepared homogeneous sub-monolayer BCN on molten Cu substrates,which solved the phase separation problem to a large extent.The near-edge X-ray absorption fine structure（NEXAFS）characterization showed that C is sp² hybridized to form C-B bonds and C-N bonds in the crystal lattice.The C-C bonds as in the graphene phase are rarely observed,indicating that the degree of phase separation is in our BCN sub-monolayer single crystalline grains are well controlled.We further studied the single crystalline BCN sub-monolayer by using transmission electron microscope（TEM）and electron energy loss spectroscopy（EELS）,and obtain direct experimental evidence of the uniform distributions of B,C and N atoms.4.Based on the molten salt reaction method,a reactive-etching synthesis method is designed and used to synthesize h-BN nanosheets.The difficulty in the controlled synthesis of h-BN nanosheets with bulk quantity is that two or more raw materials are required,and the synthesis temperature is relatively high.The molten salt reaction method transforms the reaction into a simple liquid phase reaction,which not only makes the raw materials evenly dispersed and fully reacted,but also lowers the reaction temperature.In the molten salt environment,the"etchant"Na₂CO₃ could etch away the generated defective h-BN core,and recrystallize to form a new h-BN structure with high crystal quality.Thus,h-BN nanosheets with high quality and bulk quantity were obtained.