Doping Induced Giant Piezoelectricity In Square Graphynes

Piezoelectric materials can be widely used in the fields of heat,light,sound and electricity due to their unique properties,which effectively allow to transduce signals from the mechanical domain to the electrical domain and vice versa.In the recent years,many 2D piezoelectric materials such as h-BN,Mo S₂ and Ga N have been extensively researched.Compared with these materials,carbon materials exhibit more virtues including its low cost,light weight,large surface area and high chemical stability.However,there are few attentions in the piezoelectricity of carbon materials due to its centrosymmetric structures and special chemical and physical characters.To improve the piezoelectricity of carbon materials,the heteroatoms were introduced in this work.Significantly,the giant piezoelectricities were observed in the square graphynes after substitutions.Graphyne is new kind of 2D carbon material containing sp and sp² hybrid carbon atoms.It can be constructed by inserting acetylenic linkages between two bonded carbons in pristine graphene.In the recent years,various graphyne structures have been proposed,including?-??-??-??-?6,6,12-?R-?cp-?4,12,2-?4,12,4-graphyne.The diverse chemical bonds endow graphyne many fascinating properties,such as large surface?uniformly located pores and good chemical stability.Moreover,many theoretical studies based on density function theory(DFT)have revealed that the Dirac cones are not only in the hexagonal symmetries(?-??-graphyne)but also exist in the rectangle symmetry(6,6,12-graphyne)and square symmetries(cp-?4,12,2-?4,12,4-).While the?-??-graphyne are semiconductors with obvious band gaps.These special properties make them good candidates in lithium batteries,energy storage,catalysts and fuel cells.In our work,the elastic?electronic and optical and piezoelectric properties of the square symmetric graphyne(4,12,4-and 4,12,2-graphyne)containing B and N dopans were systematically studied based on density functional theory calculations(DFT).Afterwards,we will elaborate on these research contents in six chapters.The first chapter is the introduction.We have introduced the research background from the developing history and the allotropes of carbon.The carbon materials especially for 2D graphyne and graphene exhibit outstanding physical and chemical properties.Notably,numerous literatures have reported that the properties of the carbon allotropes can be controllable by regular doping,which can raise their application potentials in some fields.The second chapter is the theoretical basis and the computation details.The quantum mechanics is a science to investigate the phenomena in micro world.To reveal the motion of microscopic particles,schrodinger equation was established.For many-body systems,the corresponding schrodinger equation is very complicated.According to some important theorems and approximations,the schrodinger equation for many-body systems can be solved.In this work,we also consider the band theory to explore the electronic properties of graphyne.In the third chapter,we have systematically investigated the effects of boron(B)and nitrogen(N)substitutions in 4,12,2-graphyne on its geometric structure and mechanical as well as electronic properties with the aid of density functional theory(DFT).The trend in the elastic properties of the substituted systems is determined by the doping positions and the type of the dopants.The Bader charge analysis reveals that the N dopant at the sp-site destroys the acetylenic linkage in 4,12,2-graphyne,but instead tends to form a polar bond,or even possibly a charge-shift bond.In particular,an obvious in-plane piezoelectricity is induced by foreign atom substitutions owing to the deformation of the pristine square symmetry.In the fourth chapter,we have discussed the effects of boron(B)and nitrogen(N)dopants on 4,12,4-graphyne based on density functional theory(DFT)calculations.The B-and N-doped 4,12,4-graphyne systems exhibit P-and N-semiconductor characters,respectively.Some obvious spin splitting polarizations can be observed in their band structures and DOS.The directional electron and electronic thermal conductivities are also estimated by performing the Boltzmann's semiclassical transport calculations.In addition,both B-and N-doped systems are highly sensitive to light from the infrared to the ultra-violet regions under parallel polarization.However,the photoresponse is very weak under perpendicular polarization.In the fifth chapter,B-N pairs as dipole source were introduced into 4,12,4-graphyne.According to the density functional theory(DFT)simulations,the electronic configurations of the doped 4,12,4-graphyne systems were significantly modified owing to the built-in electric fields caused by the B-N dipoles.Different B-N concentrations and arrangements can alter the electronic structure of4,12,4-graphyne.The direct band gap can be delicately modulated from 150 me V to660 me V at PBE level.The B-N dipoles can also greatly enhance the light absorption instead of shifting the absorption region.These results indicate that B-N dipoles doped 4,12,4-graphyne can be used in the fields of solar cells and optoelectronics.Moreover,an obvious in-plane piezoelectricity can also be induced.In the sixth chapter,we have summarized the research contents in our work.According to our study,the manipulation of the heteroatoms and dipoles in 2D carbon materials is an effective way to acquire the functional materials with some desired physical properties.Therefore,the 2D carbon materials can be excellent candidates in some fields after properties controllable.