Surface Modification And Property Investigation Of Hexagonal Boron Nitride Nanosheets:First-principles Calculations

Two-dimensional(2D)materials,including graphene,silene,black phosphorus(BP),hexagonal boron nitride(h-BN),graphitized carbonitride(g-C3N4),graphitized zinc oxide(g-ZnO),molybdenum disulfide(MoS2),etc.,have attracted wide attention due to their excellent properties and atomic-layered structure,as well as their wide application in electronic and photoelectric devices.Among many 2D materials,2D-h-BN is an isomer of graphene,which with very similar layered structure and unique photoelectric characteristics as well as mechanical robustness,thermal stability,chemical inertia and corrosion resistance.Therefore,it is widely used in field effect transistors,tunneling devices,deep ultraviolet transmitters and detectors,photoelectric devices,and micro and nano electromechanical systems(M/NEMS)as well as coating materials,lubricating materials and dielectric layers.The 2D-h-BN is considered one of the most promising materials,and it can also be integrated with other 2D materials,such as graphene and transition metal bihalogenated hydrocarbons(TMDCs),for the next generation of NEMS and other technologies.However,the application of h-BN nanosheets(h-BNNSs)in semiconductors or conductive M/NEMS is greatly restricted by its excellent electrical insulation.Secondly,compared with graphene,although 2D-h-BN nano materials have excellent lubrication performance and are widely used in various lubricants,h-BNNSs still have higher interlayer friction.This is because in addition to the weak van der Waals(vdW)force between h-BNNSs layers,h-BNNSs also have interlayer electrostatic interactions which due to the different electronegativity of B and N atoms.This phenomenon will weaken the interlayer slippage and reduce the lubrication properties of h-BNNSs as antifriction and antiwear protection layers.Moreover,the undesired energy dissipation caused by friction and wear almost exist in the every moving part of mechanical system,and because of the high surface area volume ratio of the micro-and nano-mechanical contact,this kind of friction and wear becomes more prominent for the M/NEMS,which reduces the accuracy and durability of the equipment,and even cannot be applied in practice.Hence,it is usually desirable to control or reduce interlayer friction in many applications of M/NEMS.Basing on the above problems,the new pursuit of material researchers is to find new h-BN based nano materials with excellent electrical properties and low friction properties.As we all know,surface functionalization is an important way to obtain new properties and applications of materials,that is,the properties and functionalization of 2D-h-BN can be adjusted by doping,substitution,surface functionalization and hybridization,so that 2D-h-BN can become a kind of multi-functional materials with a wide range of applications,especially to expand the application of h-BN based nano materials in M/NEMS.In this thesis,h-BNNSs are taken as the research object.Based on the first principle density functional theory(DFT),the effect of the single atom(e.g.,F,O and P atoms)doping and diatomic(e.g.F and H atoms)co-doping of the h-BNNSs on the surface modification,thermodynamic stability,electrical properties,interlayer tribological behavior and mechanism as well as the light absorption characteristics are studied.The tribological mechanism and the conclusions obtained in this study provide theoretical basis for the experimental research.At the same time,according to the mechanism and law obtained,OH radical and F atoms are also used to co-dope the h-BNNSs induces the generation of 2D-c-BNNSs with excellent electrical,magnetic and optical properties,which expands the application of h-BNNSs in the field of M/NEMS and the fabrication of new material.The main contents and conclusions of this thesis are as follows:(1)In this part of work,based on the above problems existing in practical application of h-BNNSs:insulation and higher interlayer friction than that of graphene,the influence and mechanism of F,O and P dopant atoms on the surface modification,electrical,magnetic,optical and tribological properties of h-BNNSs are studied by using the surface functionalization method via the first principle calculations within the framework of DFT.Firstly,the surface modification and properties investigation of h-BNNSs by F atom with different doping ratio are carried out.It is found that all F atoms have different effects on the electrical,magnetic,optical and interlayer friction behaviors of F-h-BNNSs.In terms of electrical properties,the introduction of different proportion of F atoms realizes the transformation of h-BNNSs from insulator to semiconductor and conductor.Some of the doping system has semi-metallicity and magnetism.This transformation is attributed to the introduction of F atoms,which changes the electronic energy distribution of Fermi level,the top of valence band and the bottom of conduction band.At the same time,through the comparative analysis of energy band structure and water redox potential,it is found that F-h-BNNSs with specific doping ratio can be used as visible light photocatalyst for water redox reaction.This discovery expands the application of h-BN based nano materials in the field of photocatalysis,and provides a new path for the generation of environment-friendly energy H2-In addition,the introduction of F atom will reduce the interlayer friction,and the magnetic production is more conducive to the reduction of the interlayer friction.Based on the analysis of the distribution of the deformation electron density of F-h-BNNSs at different sliding positions,we propose the mechanism of the interlayer friction behavior-electron redistribution mechanism,that is.the introduction of F atom changes the original electronic distributions between and within the layers,thus changes the interaction between and within the layers,then regulates the interlayer friction behavior.Secondly,by changing the doping mode and type of atoms,the mechanism of the influence of doping atoms on the interlayer friction behavior is further discussed.Through the analysis of the electrical properties and the interlayer friction behavior,it is found that the effect of O atom on the interlayer friction behavior is different because of the different doping mode and the different distribution of valence electrons.With the increase of the dopant numbers of O atom,the interlayer friction decreases first and then increases which is larger than that of the undoped system.Through the analysis of deformation electron density of the systems,there is an anchoring effect between layers at the O-doping position,which improves the interlayer friction;while the former study of F-doping system suggests that the magnetic production is conducive to the reduction of the interlayer friction.According to the analysis of electrical and magnetic properties,the competition mechanism of magnetostrictive effect and anchoring effect is proposed.Finally,in the case of the surface structure with zero bending deformation and zero magnetism of h-ENNSs,the influence mechanism of doped atom P on its electrical and optical properties is discussed.Through the calculation of absorption spectrum,it is found that the doping of P atom effectively improves the optical absorption performance of h-BNNSs which realizes the absorption of visible light,and even covers the whole visible light area.Through the analysis of energy band structure and electron density of state projected on orbit,it is found that the change is due to the decrease of the band gap due to the electronegativity of P is lower than that of N atom.In a word,the doping of single atom can effectively improve the electrical,magnetic,optical and interlayer friction behavior of h-BNNSs,and expand the application of h-BNNSs materials in the field of visible light photocatalysis,semiconductor,conductor and magnetic NEMS.(2)In this part of work,based on the friction mechanism and band gap engineering proposed in the first part,low friction h-BN based nanomaterials with excellent electrical properties are designed.Because the interlayer friction is not only affected by the slip layer but also by the substrate layer,a simple and effective method is proposed under the DFT framework,that is to adjust the conductivity and the interlayer friction behavior of the h-BNNSs bilayers by introducing F and H atoms.The surface structure and electronic density of state of F and H co-doped h-BNNSs bilayers were studied.The calculated cohesive energies show that F and H atoms can strongly combine with h-BNNSs.At the same time,the analysis of energy band structure shows that h-BNNSs have realized the transformation from insulator to semiconductor or conductor in the electrical properties.More importantly,due to the electron redistribution caused by the introduction of F and H atoms,the doped h-BNNS bilayers show excellent interlayer friction behavior,even under a certain compression interval it still shows very low interlayer friction,and the ultra-low interlayer friction can be also achieved by changing the doping mode.We call this phenomenon as pressure induced friction collapse,which is due to the energy crossover caused by quantum mechanical effect.The results show that these F-and H-doped h-BNNS bilayers have the advantages of good conductivity,high structural stability,small interlayer friction,high load-carrying capacity and so on,which will bring a broad application prospect for h-BNNSs in the field of semiconductor,conductor material and M/NEMS.(3)Development of novel structure,fabrication methods,formation mechanisms,and versatile applicability of boron nitride(BN)nanomaterials is still one of the research hotspots.In addition to h-BN,c-BN as the isoelectronic homomorphic body of diamond has attracted great interest of scientists because it has many extreme properties comparable to diamond,and even superior to diamond in chemical properties and thermal stability.In addition,it has been proved that c-BN has a strong wear resistance to the high temperature wetting of most molten metals.This unique performance of c-BN is better than that of h-BN,and it can provide many special applications,such as high-power and high-temperature nano devices operating in harsh environments.However,like h-BN,the realization of semi-conducting or conducting devices based on c-BN is still an arduous task which because of the 6.4 eV wide bandgap.Moreover,it is very difficult to synthesize c-BN with uniform thickness and controllable properties directly.Therefore,it is very important to develop a novel method to prepare c-BN-based nanomaterials with good electrical,magnetic or optical properties.In this part,we develop novel 2D c-BNNSs via first-principles calculation.This structure is converted from h-BN bilayers induced through OH radical and F atom codoping.The geometrical,electronic,and optical properties of this novel 2D OH radical and F codoped c-BNNSs(OH-F-c-BNNSs)have been systematically investigated.OH-F-c-BNNSs have excellent electrical,magnetic and optical properties.More importantly,when the doping positions of OH radicals and F atoms are exchanged it will only have electrical conductivity,which will make us to regulate the intrinsic properties of c-BNNSs for different application only by adjusting the element doping positions.They also have certain commonness.Through the analysis of energy band structure,it is found that both structures can be used as visible light photocatalysts for water redox reaction.This work can provide theoretical and experimental bases for designing and fabricating new types of 2D c-BNNS based materials for different applications in M/NEMS.