Theoretical Study On The Structural, Optical And Piezoelectric Properties Of Two-dimensional Nanomaterials

Science and technology are changing rapidly in this era,the discovery of new materials will greatly promote the rapid development of economy and the development of many advanced manufacturing fields are closely related to each breakthrough of materials.The successful preparation of graphene has caused a upsurge in exploring two-dimensional materials.The main research directions in low-dimensional nanomaterials is exploring and regulating the physicochemical properties of two-dimensional materials.Designing new materials by conventional experimental methods not only is certain limitations,but also will be time-consuming and poor efficiency.With the growth of computer technology and quantum mechanics,the way of solving the Schrodinger equation of complex system by numerical simulation has been realized,and the physicochemical properties of the system can also be obtained.According to the theoretical results,we can preliminarily screen some prepared or modified material systems to provide guidance for experimental preparation.In this paper,the existence of two dimensional?2D?pentagonal ZnO₂?penta-ZnO₂?sheet is predicted by first-principle method based on density functional theory.The crystal structure of the new two-dimensional material is different from that of like hexagonal graphene two-dimensional material and it is similar to the Cairo pentagonal tile pattern.Firstly,the structural stability of penta-Zn O₂ has been confirmed by energy,dynamics and mechanics,respectively.The binding energy of pentagonal zinc oxide is-4.203 eV/atom,lower than-4.123 eV/atom of g-ZnO but higher than-4.473 eV/atom of wurtzite ZnO,which proves that penta-Zn O₂ can exist stably in energy.And there is no imaginary frequency in phonon spectrum of penta-ZnO₂,which confirms that penta-ZnO₂ is dynamically stable.By fitting the energy curves of uniaxial,biaxial and shear strain,the elastic constants of pentagonal zinc oxide were calculated.The calculated results indicate that C₁₁C₂₂-C²₁₂>0 and C₆₆>0,satisfying Born-Huang criteria,thus the existence of penta-ZnO₂ is proved by mechanical mechanics.Penta-ZnO₂ is easy to bend and deform,and it also has a negative Poisson's ratio effect.The energy band structure and density of states of Penta-ZnO₂ are calculated by using HSE06 functional.It is found that the penta-ZnO₂is a quasi-direct band gap semiconductor material with band gap of 4.53 eV.In addition,the charge density difference diagram of penta-ZnO₂ illustrates its chemical bonding.By calculating the dielectric function of penta-ZnO₂,we get some parameters including that the absorption coefficient,reflectance and refractive index.and then deduce that the visible light transmittance of penta-ZnO₂ is almost 100%.Due to its unique electronic,elastic and optical properties,penta-ZnO₂ has a wide range of applications in optoelectronics field.Particularly,high flexibility and high light transmittance make pentagonal zinc oxide a promising flexible transparent electrode.In addition,we investigate the magnetic and piezoelectric properties of hexagonal boron nitride?h-BN?monolayer riddled with regularly spaced triangular holes by using first principle calculations.It is found that the triangular holes can indeed induce spontaneous magnetizations in the V_B-BN and V_N-BN systems.In V_B-BN,the doubly coordinated N atoms contribute most of the magnetism,followed by the the trebly coordinated N atoms.The atomic magnetic moments are 0.100 and0.059?_B,respectively.The doubly coordinated B atoms play a major role in magnetism of V_N-BN,and the trebly coordinated B atoms play a second role in the magnetic properties of V_N-BN.The atomic magnetic moments are 0.132 and 0.072?_B.More interesting,V_B-BN presents metallic property,V_N-BN displays a half-metallic behavior.In the spin-polarized band structure,we can see that the spin down electronic state of V_N-BN is an indirect band-gap insulator,and the band gap value is about 4.2 eV,and the spin-up electronic state is metallic.The metallic materials has no piezoelectric properties.Only the piezoelectric properties of the structure V_N-BN are calculated by DFPT method.Remarkably,its piezoelectric stress constant is 1.5times the amount of pristine h-BN and its piezoelectric strain constant is 20 times larger than that of pristine sheet.The defect regulation enables the h-BN lamellar to have both magnetic and piezoelectric properties,which makes it possible to be used in nanodevices with both magnetic and piezoelectric properties.