Dissertation > Excellent graduate degree dissertation topics show

First-principles Study of the Electronic Structure and Optical Properties of Ce-doped ZnO, Ce/N Codoped TiO₂, and Transparent Conducting Oxides In₄Sn₃O₁₂ and In₄Ge₃O₁₂

Author: ZhangYunGeng
Tutor: WangYuanXu
School: Henan University
Course: Condensed Matter Physics
Keywords: First-principles calculations Wide band gap semiconductors Electronic structure Optical Properties
CLC: TN304
Type: Master's thesis
Year: 2011
Downloads: 121
Quote: 0
Read: Download Dissertation

Abstract

Wide bandgap semiconductor materials much attention because of its special physical properties and a wide range of uses. ZnO is a direct band-gap semiconductor material of Ⅱ - Ⅵ room temperature band gap of 3.4 eV, the emission wavelength with respect to the near-UV 368 nm. Compared to other optoelectronic materials such as GaN, ZnO has a series of advantages, such as lower static dielectric constant, the larger the optical coupling coefficient, a high chemical stability as well as excellent photoelectric, piezoelectric characteristics. Its exciton binding energy of up to 60 meV, and the corresponding value is much higher than the GaN (about 21 meV ^{25 MeV), therefore, the device made with ZnO in a high temperature under the condition of having a better efficiency. ZnO ultraviolet light-emitting diodes, solar cells, liquid crystal displays, gas sensors, ultraviolet semiconductor laser, and a transparent conductive film has broad application prospects. In recent years, it was discovered that the doping can change the nature of ZnO, its performance is better play. Transparent conductive oxide material has a band gap in the visible spectrum (390 nm-760 nm, about 1.63 eV-3.18 eV) absorption rate and low reflectivity and low resistivity common optical and electrical properties, they have been widely used in solar cells, The flat-panel liquid crystal display, an electroluminescent display, electroluminescent color display, special features window coating and other areas of the photovoltaic device. Adjust the composition of the material and composition ratio regulation the materials electrical, optical, chemical and physical properties to meet the needs of special occasions. TiO ₂ having excellent physical and chemical properties, such as a strong catalytic activity, good chemical stability, non-toxic, low-cost and high-photogenerated carrier lifetime, and other characteristics make it in the photocatalytic water decomposition and the solar cell has a wide range of applications. However, on the one hand, the anatase TiO ₂ is a wide band gap semiconductor material, the intrinsic band gap of 3.2 eV can only absorb the ultraviolet part of the solar spectrum, while the UV light only accounted for about 5% of the total energy of the sunlight, and thus the optical conversion efficiency is very low; other hand, quickly compound resulting in the excitation of electrons and holes between the quantum efficiency is low. Therefore, various methods to improve TiO ₂ the electronic properties of its absorption edge extends to the visible range, to improve its catalytic efficiency. This paper first introduces the first-principles calculation based on density functional theory. Applications based on density functional theory GGA and GGA U method considering the strong correlation effects, we have calculated the electronic structure and optical properties of Ce-doped ZnO. Calculated found after incorporation of Ce in a very localized energy band, which corresponds to the spin-up of the Ce 4f states between the valence band and the conduction band of the ZnO. This localized energy band has set up a \ZnO: Ce is a degenerate semiconductor. Ce 4f 5d state spin-up and spin-down does not match and Ce incorporation caused by the O 2p states a spin - polarization hole of ZnO: Ce with the cause of the magnetic. We also calculated the absence of an electron and missing two electrons of ZnO: the Ce electronic structure and optical properties. With the increase in the missing number of electrons, and the Fermi level is gradually moved downward. Missing two electrons and the magnetic properties of the system is lost. The optical properties of the ZnO: Ce is a potential of the dielectric material in the field of photoelectric material has good application prospect. By first-principles calculations, we explore the In ₄ Sn ₃ O in Ge 12 and In _{4 3} O ₁₂ 18f position distribution of metal atoms. We found that the top of the valence band of these two materials are determined by hybridization between the d orbital of the metal element and the O 2p orbital; mixing of the bottom of the conduction band by the the O 2p track, In 5S, Sn, 5S or Ge 4s between tracks occupied. The analysis showed that the two materials are direct band gap semiconductor. The absorption coefficient of the two materials, and the reflectance and energy loss spectrum in the infrared and visible range are very low, indicating that the two materials are relatively good transparent conductive oxide. Using the the GGA method of density functional theory, and consider the strong correlation effects, we calculated a pure anatase anatase TiO ₂ N, Ce single doped and Ce / N co-doped TiO ₂ of the electronic structure and optical properties. Ce / N co-doped TiO ₂ the top of the valence band moved significantly to the high-energy end, we found occupied by the spin down N 2p states; almost no change in the location of the end of the conduction band by Ce 4f mixing between the states and Ti 3d states occupy. N single-doped and Ce / N co-doped TiO ₂ magnetic origin O 2p states, N 2p states and Ce 4f state spin down the difference between the number of electrons and the spin-up. Adulterated and TiO ₂ ions in the strong interaction with the main reason for the gap narrowing and the shift of the Fermi surface. N, Ce single doped and Ce / N codoped lead to the bandgap reduce the value of the order of 0.47 eV, 0.21 eV and 1.05 eV. Ce / N codoped lead to the red shift is most obvious, the size of the band gap to meet the demand for photocatalytic water. We predict that the Ce / N codoped raise TiO ₂ catalytic activity of a very good method.}

Related Dissertations

CLC: > Industrial Technology > Radio electronics, telecommunications technology > Semiconductor technology > General issues > Material
© 2012 www.DissertationTopic.Net Mobile

First-principles Study of the Electronic Structure and Optical Properties of Ce-doped ZnO, Ce/N Codoped TiO2, and Transparent Conducting Oxides In4Sn3O12 and In4Ge3O12

Abstract

Related Dissertations

First-principles Study of the Electronic Structure and Optical Properties of Ce-doped ZnO, Ce/N Codoped TiO₂, and Transparent Conducting Oxides In₄Sn₃O₁₂ and In₄Ge₃O₁₂