Novel Two-dimensional Ferroelectric Materials:A First Principle Study

Material Science is a subject oriented towards the modification,or even creation of the building blocks of human civilization:Materials.Its multi-disciplinary properties render it able to combine physics,chemistry,biology altogether to study matters cross dimensions(from nanoscale to cosmos).From stone age to iron age and bronze age,the development of materials is closely related to the advancement of civilization.As can be seen,Material Science plays a deterministic role in the development of human society.With the recent development of ultra-thin 2D nano materials,the possibility of having devices with novel properties rises.In this information era,these exotic materials are unique to the industries because they have thickness of only few atomic layers while having a surface area of micrometers.Not only the dimension of these 2D structure trigger exotic phenomena,it also serves well with the trending of miniaturization of devices.At the same time,Ferroic materials,as a traditional research direction,also received its fair amount of interest in the last two decades since they are the most prominent candidates for non-volatile memories.Combining ferroelectricity and 2D properties,the 2D ferroelectrics have gradually gaining attention since they can have more interesting properties than its 3D conuterpart.Most recently,the discovery of low dimensional ferroelectric materials like SnSe has starting to surface.Based on First-principles calculaitons,this thesis studied two elemental 2D ferroelectric materials and origin of their electric polarization.The means of manipulating their properties via external field are also shown in details here.First,we propose the idea of elemental ferroelectricity,a real system constituting Group-V elements(As,Sb and Bi)is used to confirm this proposal.As a matter of fact,to our best knowledge,it's the first time these properties are found in real materials.The value of inplane spontaneous polarization for them are:0.046 ?C/m 0.075?C/m and 0.151 ?C/m for As,Sb and Bi respectively.Taking effective thickness into consideration,the polarization values are comparable to those prototypical perovskite ferroelectric materials and should be large enough for STM measurement.In the mean-time,we found the polarization relies heavily on the buckling of the structures:without the buckling,the system reverts back to centro-symmetry form and the buckling leads to charge transfer from the lower atom to the upper one,lowing the system's total energy while causing the dipole moment to arise.Further analysis shows that in monolayer Bi structure,there exists a centro-symmetric structure with neighboring cell buckling to the opposite direction.We found this structure to be antiferroelectric(AFE).This is the first time AFE was found in a 2D structures.To illustrate how it forms,we proposed a model based on Landau theory.By analyzing the model Landau polynomials,wo show the critical parameters for the AFE and FE to coexist.Our work may be the key to unlock designing of future 2D ferro/anti-ferroelectrics.It is also found that the ferro and anti-ferroelectricity can exist in Silicon(001)surface.The Silicon(001)surface constitute of two isolated Silicon atoms,when they are at the same hight,the system shows metallic properties.However,when they buckle to either left or right,they broke the mirror symmetry and induces a charge transfer effect,the whole system becomes semiconducting.This electronic induced structure distortion is called Pierels phase transition.The DFT results show that,the p(2×1)structure correspond to FE structre while the most stable structure c(4x2)is antiferroelectric.Also,by adding uniaxial compressive strain,the polarization can be greatly enhanced.By employing Monte-Carlo calculation,the alternating external field induces clear hysteresis loops in dielectric response,Based on the importance of silicon,our findings could have huge impact on future industrial applications.