Theoretical Study On The Structure And Properties Of Several Materials

Recent years have witness the tremendous developments in the electronic structure method and computer hardware, which made the quantum chemistry a powerful tool to calculate almost all kinds of molecule systems and play a very important role in almost all the branches of materials and chemistry. The chemistry and physics properties of materials were simulated by density functional theory in modern high speed computer to deeply understand our observe phenomenon and characteristics of materials on the scales from macrostructure to microstructure and to theoretically forecast configuration and characteristics of the materials, which provided theoretical direction and evidence for materials design and preparation. In this thesis, the structures and properties of piezoelectric crystals BTGS and CTGS were investigated by using density functional theory (DFT). Furthermore, we investigated the properties of semiconductor materials TiO₂ by doping other elements. And the effects of the structure of the branches on the two-photon absorption properties for the multi-branched molecules with nitrogen (N) as coupling center.In chapter 1, the importance of materials design and computer simulation were introduced and recently research on molecule simulation at home and abroad is summarized. General method of computational materials design is introduced. The density functional theory is an important method in quantum chemistry computational and materials simulation fields. At last, several kinds of familiar computational software were summarized in brief.In chapter 2, we briefly introduced the basic concept of DFT and reviewed its recent progress. Finding good approximation for exchange-correlation functional is one of the main targets in DFT research. With the development of functional research, DFT leads to more and more accurate results from the initial LDA, GGA to hybridization functional. Besides the improvement of exchange-correlation, extension of DFT to the time dependent region and combination of DFT with dynamic mean field theory (DMFT) are also active topics recently. All these progress lead DFT application to a broad range of problems.In chapter 3, we firstly introduced the development history of piezoelectric crystal and the theoretical research in recent years. We perform the systematic studies on the structural, electronic and optical properties of piezoelectric crystals BTGS and CNGS using DFT method for the first time, respectively. The imaginary part of the dielectric function has been obtained directly from the electronic structure calculation. With band gap correction, the real part of the dielectric function can be derived from the imaginary part by the Kramers-Kronig relationship. Based on atom coordinates from our experiments, the crystal structure is designed and optimized. Our optimized results are in good agreement with the experimental values from literatures. From the band structure of BTGS and CNGS crystals, we can analyze that the two kinds of crystals are wide band gap insulators. A lot of properties of BTGS and CNGS can't be obtained from the experiments, so we calculated the optical properties of two kinds of crystals. For example, the static dielectric constants and refractive index in the different directions were calculated. The shape of BTGS and CNGS was deduced from BFDH theory. At last, we calculated the elastic contants the two crystals. The investigation of these fundamental properties characteristics plays an important role in the experiment research on piezoelectric crystal.In chapter 4, we performed electronic structures and optical calculations for B-doped in anatase TiO₂ by using first-principles ultrasoft pseudpotential method in the generalized gradient approximation. The electronic structures demonstrate that substituting B for O may be an explanation to the redshift phenomenon as was found in the experiment. Whereas, TDOS and PDOS analyses suggest that the redshift of luminescence spectrum in B-doped TiO₂ result from the band to localized states transition.In chapter 5, the TPA properties of the multi-branched molecules with stilbene, dithienothiophene and flourene as branch and nitrogen as coupling center were investigated by using the ZINDO/SOS single and double configuration interaction (SDCI) method. Calculated results reveal that the multi-branched molecules based on stilbene, dithienothiophene and flourene show that the coupling effects between the branches can facilitate the enhancement of TPA cross section. The coupling effects are strongly affected by the structure of the branches, so the molecules that are more delocalized are selected as the branches can enhance the TPA cross section for multi-branched molecules. The results show that the molecules with stilbene, dithienothiophene as branches possess large TPA cross sections.In chapter 6, we summarized the conclusions of this dissertation and previewed the further studies.