First-principles Study Of The Effects Of Environment On The Properties Of Ferroelectric Materials And Graphene

The properties of electronic materials will be affected by environmental factors. The ferroelectric material is a kind of electronic materials that is widely used in electronic devices. Water molecules (WMs) are easily adsorbed on ferroelectric material surfaces and change the electrical resistivity and dielectric constant. Graphene is also a kind of electronic materials, which conducts heat and electricity efficiently. The edge of graphene can interact with environment. The effects of environment on the properties of ferroelectric materials and graphene were studied in this paper. The conclusions are listed below:(1) The water-adsorption behavior on in-plane-polarized BaTiO3 surfaces was investigated using both experimental and theoretical methods. The FT-IR spectrum demonstrates three types of energy-nonequivalent active centers for water adsorption on the in-plane polarized BaTiO3 (100) surface. The XPS results suggest that hydroxyl groups exist on the surface and the interaction of WMs-Ba atoms differs from the interaction of WMs-Ti atoms. Based on First-principle calculations, the water-adsorption behavior was investigated on the in-plane polarized BaTiO3 (100) BaO and TiO2 terminated surfaces with WM 1/4 ML, half ML and 1 ML coverage. On the BaO terminated surface, the first adsorbed WM prefers to dissociate, while the second WM likes to interact with the first WM. For the TiO2 terminated surface, however, the chance for the adsorbed first WM to dissociate or not is only half, meaning that both types of adsorption compete on the TiO2 terminated surface. No matter of the first adsorbed WM is dissociated or not, the second WM favors energetically to interact with the first adsorbed WM. The calculation results are in good agreement of the experimental observations.(2) Based on First-principle calculations, the effect of the adsorption of water molecules on the atomic and electronic structures of the BaTiO3 surfaces was investigated. The results demonstrate that water molecules can be molecularly and dissociatively adsorbed on the BaTiO3 surface, which induced electron transfer from the adsorbed water molecules to the BaTiO3 surface and lead to the electron redistribution in the surface layers of BaTiO3. By analyzing the change in the atomic structure and the electron redistribution of the BaTiO3 surface layers before and after the adsorption of water molecules, we determined that the water molecule adsorption induces the turn and shift of Ti atoms in the TiO2 plane, indicating an in-plane domain switching. The in-plane domain switching occurs in several surface layers from the surface. However, the out-plane domain switching was not observed. In addition, the water molecule adsorption may result in a reduction in surface rumpling.(3) The water-adsorption behavior on LiNbO3 surfaces was investigated by first-principle calculation. WM can adsorbed on LiNbO3 surface and reduced the surface energy. The adsorption energy of c+ domain is higher then c" domain on the Nb surface. However, The adsorption energy of c+ domain is lower then c" domain on the Li or O surface. The different of adsorption energy is because of the structure of the surface and depolarization field.(4) The size-dependent elastic behavior of graphene nanoribbons were calculated by molecular dynamics. The eigenstress model for two-dimensional material was constructed to study the size-dependent of graphene nanoribbons. First principles calculations were conducted on armchair graphene nanoribbons (AGNRs) to simulate the elastic behavior of AGNRs with hydrogen-terminated and bare edges. The relaxation-induced initial in-plane strain and the edge energy density illustrate periodically modulated width-dependent behaviors; i.e., smaller widths correspond to higher initial strain and to smaller edge energy densities of the relaxed AGNRs. The nominal Young’s modulus and Poisson’s ratio of AGNRs and HAGNRs also exhibit periodically modulated width-dependent behaviors. As the width increases, the nominal Young’s modulus increases, whereas the nominal Poisson’s ratio decreases, and both values approach their respective core values. All of the width-dependent properties are periodically modulated by three families. Three sets of values for the Young’s modulus and Poisson’s ratio of the edges can describe the width-dependent nominal Young’s modulus and Poisson’s ratio.