Piezoelectricity Of Group ?-?-?-? Nano Sheets

The piezoelectric materials are the materials with piezoelectric effect.When we apply a stress to the piezoelectric materials and make the materials strain,the piezoelectric materials will exhibit polarization phenomenon and form surface charge,converting the strained mechanical signals into the electrical signals.When we apply an electric field to the piezoelectric materials,the piezoelectric materials will be strained,converting the electrical signals of electric field into mechanical signals.In addition to the mutual conversion between mechanical signals and electrical signals,it can also realize the mutual conversion between mechanical energy and electrical energy.Because the piezoelectric materials have many advantages,such as easy to fabrication,low cost,high efficiency,it can be widely applied in electronic technology,laser technology,navigation technology,biotechnology and other engineering technologies.However,with the rapid development of science and technology,it is currently the most important and urgent issue to find the materials with higher efficiency piezoelectric effect,and it is also a hot issue for many scientists.In order to accurately find the materials with higher efficiency piezoelectric effect,we summarize the research background,native and foreign research status,application before our research.According to the native and foreign research,by analyzing the influence of the electronegativity of atoms and the symmetry of unit-cell structure on the internal dipolar polarization of the materials,we conclude that the efficiency of the piezoelectric materials is affected by the electronegativity of the contained atoms and degree of structural asymmetry.In 2017,Jia-He Lin et al proved that the group V-IV-III-VI compound nano sheets,including SiN-AlO,GeN-GaO,SnN-InO,SiP-AlS,GeP-GaS,SnP-InS,are stable,and are the semiconductors with indirect bandgap.We found that the structure of these six compound nano sheets belongs to the 3m point group,which does not have the symmetry center and mirror symmetry.It indicates that these six compound nano sheets have in-plane piezoelectric effect and out-of-plane piezoelectric effect.Since the electronegativity difference between the nitrogen atom and the oxygen atom is larger than that between the phosphorus atom and the sulfur atom,we expect that SiN-AlO,GeN-GaO,SnN-InO will exhibit high efficiency piezoelectric effect.In this paper,we thoroughly research the structural parameters,energy band,density of states,deformation charge density,Bader charge,elastic and piezoelectric properties of these three nano sheets?XN-MO sheets?by first-principles.The following is the contents of our research and the results obtained:First,we calculate the structural parameters,energy band,density of states,deformation charge density,and Bader charge of the XN-MO sheets.From the plots of deformation charge density,we found that there are obvious electron deletions around the M atom and X atom,forming a positive electric center in the sheets,and there are obvious electron aggregation around the O atom and N atom,forming a negative electric center in the sheets.The positive electric center and the negative electric center do not coincide in the x-direction,which indicates that the electric dipole moment?P₁?along the x-direction of the rectangular unit cell will change when the sheets are strained,and the sheets will exhibit an in-plane piezoelectric effect.From the Bader charge,we found that the MO layers of these three sheets exhibit positive charges,and the XN layers of these three sheets exhibit negative charges.It indicates that the electric dipole moment?P₃?in the z-direction of the unit cell will change the sheets is along the rectangle when the sheets are strained,and the sheets will exhibit a piezoelectric effect perpendicular to the sheets.Next,we calculated the elastic and piezoelectric properties of the XN-MO sheets.We found that the Poisson's ratio of SnN-InO?0.50?is the largest,which is larger than that of WTe₂?0.39?and InSe?0.40?;the Young's modulus of SnN-InO?109.80 N/m?is the smallest,and the Young's modulus of SiN-AlO?214.71 N/m?is the largest,which is much smaller than the graphene?341 N/m?.For piezoelectric properties,the in-plane piezoelectric constant d₁₁?12.69 pm/V?of SnN-InO is the largest,which is 3 times larger than that of MoS₂?3.73 pm/V?,9 times larger than that of BN?1.38 pm/V?,and 4 times larger than that of AlN?2.75 pm/V?.At the same time,SnN-InO also has the largest piezoelectric constant d₃₁?8.66 pm/V?perpendicular to the sheets,which is 18 times larger than that of the Janus group-III chalcogenide?0.07-0.46 pm/V?.Because the flexible SnN-InO exhibit large piezoelectric effect perpendicular to the sheets,it can be applied to nanoscale energy harvesters and nano-sensors.