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Defects, Band Structures and Optical Properties of Pr and Zn/Al Doped Titanates Luminescent Material

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Tutor: DuYong
School: Central South University
Course: Materials Science
Keywords: luminescent material,first-principles calculation,defect,band structure
CLC: TB34
Type: PhD thesis
Year:  2012
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Abstract:
Luminescent materials doped with rare earth, which are widely applied in the fields of color display, lighting and anti-counterfeiting and so on, are one of hot research topics among materials and physics. Pr doped CaTiO3and SrTiO3luminescent materials can be excited in the ultra violet and show a unique red emission as well as weak afterglow and high chemical stability at room temperature. Better red luminescent materials have been intensively investigated by broadening excitation area and improving the long decay properties during the recent years. It takes inevitably researchers long periods to obtain a better luminescent material through iterative experiments in the laboratory. Generally speaking, macroscopical properties of materials are the statistical results of all microcosmic exhibitions. Sometimes the macroscopical property of one sample is different from that of the other which is obtained by the same treatment. It is difficult to investigate all the relative microcosmic aspects for the macroscopical properties by using only experimental methods. Meanwhile, first-principles calculations are specialized in illustrating microcosmic exhibitions and inner mechanisms. Therefore, first-principles calculations are used to obtain defects, electronic structures of Pr doped CaTiO3, Pr and Al doped SrTiO3. Based on the two experimental findings that Zn2+ions are beneficial to the afterglow and red luminescent intensities of Pr doped CaTiO3and Al3+ions are helpful to red luminescent intensities of Pr doped SrTiO3, the optical properties of (Zn,Pr) doped CaTiO3and (Al,Pr) doped SrTiO3are investigated by first-principles calculations. Main conclusions are as follows:(I) Defects, band structures and optical propertis Pr and (Zn,Pr) doped CaTiO3¡¤ Formation energies and electronic structures of native defects in orthorhombic CaTiO3are explored using the first-principles calculations under A condition in which CaTiO3is in equilibrium with CaO and O2and under condition B (TiO2, CaTiO3and O2are in equilibrium). The Ca vacancy (VCa2), Ti vacancy (VTi4-) and Ca antisite (CaTi2-) are the acceptors energetically preferable. There is no defective gap state in electronic structures of decfective CaTiO3containing either of native defects. The bandgaps of VTi4-, CaTi2-and VCa2defective systems are reduced by0.21eV,0.12eV and0.06eV in comparasion with perfect system. Those of Vo2+and TiCa2+keep unchangeable. The shinking band gaps will low the absorption energy of the transition of excited phonon from the valence band to the conduction band. Therefore, it is possible to broaden the excitation spectra by modulating the compoition of native defects.¡¤ Formation energies and electronic structures of Pr impurities in orthorhombic CaTiO3are explored. In Pr-doped CaTiO3, Pr substituting for Ca (Prca) is likely to form under A condition. Under condition B, Pr substituting for Ti (PrTi) defect is energetically preferable depending on the Fermi levels. No gap state appears within the band gap of Prca0and PrCa1+defective system. For PrCa2+, the gap states within the band gap are suitable for luminescent centers, which agree with the proposed luminescent models of CaTiO3:Pr.¡¤ Band structures, electronic and optical properties are calculated for (Zn,Pr) codoped CaTiO3. There are gap states consisting of Pr4f, O2p and Ti3d hybrid orbitals above the top of valence bands of¡«1.30and2.06eV in Pr-doped systems, which are retained in (Zn,Pr)-codoped systems. There are new states above the top of valence bands of¡«0.18eV in (Zn,Pr) codoped systems. The calculated characteristic absorption peaks of (Zn,Pr) codoped systems are300nm,372nm and457nm, which are close to the experimental results of the corresponding material at330nm,375nm and458-495nm,respectively.(¢ò) Defcts, band structures and optical propertis (Al,Pr) doped SrTiO3¡¤ Formation energies and electronic structures of native defects in cubic SrTiO3are calculated under the condition that TiO2, SrTiO3and O2are in equilibrium. The Sr vacancies (VSr2-) and O vacancies (Vo2+) are found to be energetically preferable as acceptors and donors for the pure sysytem. There is no defective state in electronic structures of decfective SrTiO3. In comparasion with the perfect system, the band gap of TiSr1+defective system is enlarged by0.05eV while those of Vo2+, SrTi2,VTi4-and VSr2are kept unchangeable. The inflexible band gaps of native defects are not beneficial to the transition of excited phonon from the valence band to the conduction band.¡¤ Formation energies and electronic structures of Pr and Al impurities in cubic SrTiO3are calculated under the condition that TiO2, SrTiO3and O2are in equilibrium. Pr prefers to occupy Sr site while Al to Ti lattice site. The gap states of PrSr defects depend on charged states, which are similar to those of PrCa defects. The calculated Fermi-level pinning positions at1573K for Al-free and Al doped SrTiO3:Pr decline from0.645eV to¡«0.601eV. AlTi-defects substituting with VSr2-are major acceptors in the overall charge neutrality of (Al,Pr)-codoped SrTiO3.¡¤ Band structures, electronic and optical properties are calculated for (Al,Pr) doped SrTiO3. The multiple gap states of Pr orbitals within then band gap are deduced by the two Al atoms which are nearest to Pr atom. For (Al,Pr) doped SrTiO3, the calculated absorption peaks at287nm,308nm and350nm are similar to the experimental excitation peaks at270nm,300nm and360nm, respectively.In this paper, formation energies, band structures, electronic and optical properties are calculated for Pr and Zn/Al doped CaTiO3and SrTiO3systems. The calculated results agree reasonably with the experimental findings. The methodology of the dissertation is beneficial to obtain the logical orientation and improve the investigation efficiency for a better luminescent material and a higher efficiency photocatalyst, such as doping method, species and concentration and so on.
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