Theoretical Investigation Of Piezoelectric Properties Of Several Low-dimensional Materials

Piezoelectric material refers to a material that electric charge is accumulated on its surface in response to applied mechanical stress.Nowadays,it has become one of the most important functional materials.In recent years,nanoscale piezoelectric materials has attracted much attention due to the demand for miniaturization and portability of devices.Since the pioneering work of Wang and Song[1]on the first prototype nano-generators based on ZnO nanowires,various piezoelectric nanodevices,such as nano piezoelectric sensors,piezoelectric electronic devices,piezoelectric optoelectronic de-vices,have been reported.However,at the nanoscale,the relationship between material structure and piezoelectric properties is not well established,and more experimental and theoretical investigations are highly requried to explore the microscopic mechanism of piezoelectricity.In this dissertation for Ph.D degree,we focus on piezoelectricity of several nanomaterials by performing extensive first-principles calculations.This dis-sertation contains the following six chapters.In chapter 1,the basic concept of piezoelectric effect,the definition of piezoelec-tric coefficient,and the relationship between piezoelectricity and crystal symmetry are briefly introduced,and then,we shortly summarize the research progress of piezoelec-tric nanomaterials from the aspects of experimental investigations,theoretical and sim-ulation activities.At the end of this chapter,we outline the research motivation and works in this dissertation.In chapter 2,we briefly introduces the basic assumptions and theoretical frame-work of density functional theory and the related computational methods for piezoelec-tric coefficient.Finally,the adoped DFT-based computational packages are list.In chapter 3,we focus on the effect of piezoelectric enhancement in nanowires.Several experimental and computational investigations have shown that the piezoelec-tric effect is enhanced at the nanoseale,which stimulates us to examine the piezoelectrie enhancement of several wurtzite nanowires.Theoretical results clearly reveal that the surface effect originating from the radial movement of ions and electrons near the free lateral surface is the key reason for the piezoelectric enhancement in ZnO nanowire.As for the case,fixing the axial lattice constant,the piezoelectric coefficients of the nanowire can be obtained by adding surface correction to the corresponding bulk piezo-electric coefficients.Then,the piezoelectric coefficients of ZnO nanowires with hexag-onal and triangular cross sections with different sizes(within the size range in experi-ments)are predicted,which may be used to understand the related experimental obser-vations.In chapter 4,we tune the piezoelectricity of nanowires via surface modification.Here,we examine the stability of dissociative adsorptions of HZ,C12,HCI and HF on ZnO nanowires,and calculate their piezoelectric coefficients.We find that the chem-ical adsorption can effectively stabilize the nanowires,and observe the strong dipole moments on the surface due to dissociative adsorption of these heteronuclear diatomic molecules.This leads to the obtained results that the piezoelectric properties of chemical modified ZnO nanowires with heteronuclear diatomic molecules are comparable with that of bare nanowires.At the end of this chapter,we also examine the piezoelectric properties of wurtzite core-shell ZnO nanowires.In chapter 5,we propose a theoretical method for calculating the local piezoelectric coefficients and applies it to examine piezoelectric films.Using the maximally local-ized Wannier function,one can obtain the spatial distribution of electrons in real space,and divide it into different regions for ions and electrons using the charge-neutral con?dition,then the local piezoelectric coefficients can be predicted,which do not depend on the selected reference position.Here,we take the rock salt MgO film with nei-ther piezoelectricity nor polarization,zinc blende ZnO film with piezoelectricity and non-polarization,and several wurtzite films with piezoelectricity and polarization are examples,and calculate their piezoelectric coefficients.We find that the piezoelectric coefficients for films are strongly related to the symmetry of exposed surface.If the exposed surface does not break the symmetry of the bulk,for example,the rock salt MgO films with(001)and(110)surfaces,this kind of dependence is not macroscopi-cally visible.As for the zinc blende ZnO film with(110)surface,we obtain non-zero piezoelectric coefficient components that does not exist in the bulk phase due to the mis-match of symmetry of the exposed surface with the bulk.Interestingly,in the wurtzite film with(100)surface,we find that the local piezoelectric coefficients depend on the valence state of these compounds,which may come from the effect of the number of lone pairs of electrons on the Poisson’s ratio.In chapter 6,we try to explore the coupling of piezoelectric properties and other physical properties.We firstly examine the effect of magnetism on the piezoelectric properties of BiFe03 and BaMn03.It is found that the relation between the magnetic structure and the equilibrium lattice constant contributes to the coupling of the magnetic and piezoelectric properties.Then,we investigates the ferroelectric properties of the newly confirmed polarized metal LiOsO3 film.It is clear that when the film thickness of LiOsO3 is comparable to the screening depth,the film is polarized due to the observed asymmetry erroelectric states.Moreover,this polarization can be reversed by applying an external electric field.As for the LiOsO3 film with larger thickness,they will be antiferroelectric or ferrielectric.