| With the development of our society,the demand of fossil fuels such as coal,petroleum and natural gas is steadily increasing.However,the fossil fuels as limited and non-renewable energies,will be exhausted in the near future.In addition,the fossil fuels will produce enormous harmful substances in the combustion process,leading to the water and air pollution,which is a serious threat to human survival and economic development.Therefore,to protect the environment and maintain the economic development,the development and utilization of new and clean energy has become one of the most important goals for researchers.As a clean and renewable energy,solar energy is unlimited and inexhaustible.The utilization of solar energy withthe large scale and high efficiency becomes the hot research topics in this field.In 1972,Fujishiman et al.firstly found that TiO2 photoelectrode can split water to produce hydrogen under the irradiation of light,which achieves the direct conversion of solar energy to chemical energy and explores a new way to use of solar energy.Over the past decaedes,photocatalytic technology has made great progress by the effort of researchers,and a large number of new photocatalytic materials have been prepared and characterized.However,few photocatalytic materials with wide light response rangeand high quantum efficiency were reported in the past.In addition,high stability,low costand environment-friendly are also the important standards for the practical application of this technology.Due to the low cost,high physical and chemical stability,no environmental pollution and visible light absorption,two-dimensional graphitic carbon nitride(g-C3N4)have attracted great research interest,which includes triazine and tri-s-triazine structure based on its primary building blocks.The experimental results show that the tri-s-triazine structure has the higher thermal stability compared with thetriazine structure.In addition,due to the suitable band edges,g-C3N4 with the tri-s-triazine structure can split water to produce hydrogen under irradiation of visible light.However,the quantum efficiency is very low due to the fast recombination of photo-generated carriers,which can not meet the requirement of practical application.Therefore,various methods have been employed to enhance the photocatalytic activity of g-C3N4,such as doping,deposition of co-catalyst and the construction of composites with other semiconductors.Recent theoretical studies suggested that another structure of the graphitic carbon nitride g-C6N6 has some excellent physical and chemical properties,possessing the potential applications in photocatalysis or electronic devices.The suitable band edge positions of g-C6N6 mean that it can split water under the light irridiation.In addition,it is found that g-C6N6 has a topological non-trivial electronic state,which may be one of the ideal topological insulators.On the other hand,solar cells can convert solar energy directly into electricity,which is also an effective strategy for direct and large-scale utilization of solar energy.In recent years,the solar cell technology is developing rapidly with the wide applications in lighting,automotive industry,geology and marine exploration,aerospace as well as solar power stations.However,it is difficult to store electrical energy,and the high efficiency solar cells require the using of the expensive monocrystalline silicon.Compared with Si-based materials,GaAs-based solar cell has the advantages of high solar energy utilization efficiency and stable operation.The single junction GaAs battery has 27%theoretical efficiency.Nevertheless,the band gap of GaAs is larger than the optimum value of solar cells,which limits the absorption and utilization of solar energy.Doping with elements of the same main group,such as B and In,is expected to further improve the utilization efficiency of solar energy for GaAs.The great effort has been maded by researchers,however,the variation tendency of the band gap with the concentration of impurity atoms is still unclear and there are some differences between the theoretical and experimental results.In this dissertation,we systematically studied the band structure and optical absorption spectra of graphitic carbon nitrides by using the Green' s function theory and the density functional theory.In addition,the band structure and excited state properties of B and/or In doped GaAs were also investigated.These studies can not only resolve the issues in the experiment,but also provide some newviewpoints and insights that can serve as guidance for the future research.In the first chapter,we briefly present the background and current research progress in photocatalytic properties of graphitic carbon nitrides.In addition,the current research progress of GaAs in solar cells is also presented.In the second chapter,the basic concepts and theoretical fundamentals of the first principles calculation are introduced,including density functional theory and the many-body Green's function theory.In the third chapter,we investigate the quasi-particle band structures and optical absorption properties of two kinds of carbon nitride materials with triazine and tri-s-triazine structures,especially focusing on the effects of optical polarization direction and material thickness on the optical absorption.In the fourth chapter,the electronic structures and excited state properties of graphitic g-C6N6 are investigated in detail.In the fifth chapter,we study the electronic structures and photocatalytic properties of g-C6N6/g-C3N4 heterostructure by density functional theory.In the sixth chapter,the effects of B and/In doping on the electronic structures and optical absorption properties of GaAs are investigated.The main contents and conclusions of this dissertation are listed as follows.(1)In recent years,the two-dimensional graphitic carbon nitride g-C3N4 as an important photocatalytic material has attracted wide attention.It has two kinds of structure,including triazine(named g-CNl in our work)and tri-s-triazine(named g-CN2 in our work)structures.The g-CN2 structure has higher thermodynamic stablility than g-CN1 structure and attracts much attention for researchers.It is found that g-CN2 has high photocatalytic activity in splitting water to produce hydrogen.However,the optical band gap of 2.7 eV was generally treated as electrical band gap,despite that they are two different conceptions.The difference is that the optical band gap includes the exciton effect.Compared with the bulk materials,the exciton effect of two-dimensional materials is obviously larger,and can not be ignored.Therefore,we systematically investigate the electronic structures,exciton effect and absorption spectra of g-CN2 by using the many-body Green's function theory.For comparison,the electronic structures and optical absorption properties of g-CN1 are also studied.To get the accurate band gap of the two materials,the partial self-consistent GW calculation is adopted.A scissor shift is used to reconcile the difference between the quasiparticle and the DFT-LDA band gap.It is found that the direct and indirect band gaps of g-C3N4 are 5.67 and 4.56 eV,respectively,while 2.7 eV obtained by experimental measurement is the position of the optical absorption edge,rather than the electrical band gap.Two kinds of polarization directions,including vertical and parallel to surfaces,are considered in the study of optical absorption spectra.By solving the Bethe-Salpeter equation and including the interaction between the electrons and holes,the positions of the calculated absorption peaks are in agreement with the experimental results.Further investigation demonstrates that the valence bands of the two structures are composed of ? and ? orbitals,while the top of the valence band is mainly composed of ? orbitals.The absorption peak is determined by the transition from low-energy ? orbitals to the conduction band.When the electric field direction is parallel to the surface,an obvious redshift of absorption peak can be realized owing to the electronic transitions from the a orbitals to the conduction band.This can extend the range of light absorption and improve the photocatalytic performance.Increasing the thickness of graphitic carbon nitrides can not only lead to the optical absorption redshift,but the exciton binding energy is decreased due to the increase of electronic screening.This is favorable for carrier separation and enhancement of photocatalytic activity.(2)A new structure of graphitic nitride g-C6N6 has potential application in photocatalysis and electronic devices.The electronic structures and optical absorption properties of this material have been studied by experimental and theoretical investigation.As a single particle theory,the DFT method has the well-known issue of band gap underestimation for semiconductor and is incapable to describe the interaction between the photo-generated electrons and holes.In this paper,we study the electronic structures and optical properties of g-C6N6 by using the many-body Green's function theory.The calculated absorption peaks are in agreement with the experimental results.Furthermore,it is implied that the exciton binding energy and absorption spectra can be significantly affected by the quasiparticle energies.To obtain accurate optical absorption spectra,we calculate the quasiparticle energies of dozens of k points in the first Brillouine zone by GW,which are required by BSE.When the influence of quasiparticle corrections to different k points and different electronic levels is not considered,the calculated absorption peak position and exciton binding energy have 0.7 and 0.14 eV errors,respectively.(3)Due to the potential application in photocatalytic water splitting,two-dimensional graphitic carbon nitride g-C3N4,which has many remarkable advantages such as the low cost,high stability,non-polluting and the visible light absorption,has attracted much attention during the past years.However,the rapid recombination of charge carriers for g-C3N4 leads to the low quantum efficiency.Thus,the overall photocatalytic activity of this material is not high.To improve its quantum efficiency,the construction of composites by coupling it with other semiconductors was generally adopted to improve the separation of photogenerated carriers.The g-C6N6 is a new two-dimensional nitride carbide,which has the matched lattice with g-C3N4.To increase the quantum efficiency of g-C3N4 and improve its photocatalytic performance,we study the electronic structures and photocatalytic properties of g-C6N6/g-C3N4 heterostructure by using the hybrid functional method.The thermodynamic stability of g-C6N6/g-C3N4 heterojunction is investigated and the most stable g-C6N6/g-C3N4 heterostructure is obtained.Compared with g-C3N4,the valence band of g-C6N6/g-C3N4 is more delocalized,indicating that the photo-generated hole has higher mobility.Moreover,the type-? heterojunction can be realized between g-C6N6 and g-C3N4 due to the suitable band edges,which is favorable for the separation and transfer of charge carriers at the interface,thus leading to the improvement of photocatalytic activity.In addition,g-C6N6/g-C3N4 heterojunction has a smaller band gap compared with the g-C3N4,indicating the enhanced absorption of visible light.Furthermore,the biaxial straincan be used to modulate the band gap of g-C6N6/g-C3N4 heterojunction,which is expected to improve the photocatalytic activity.(4)The III-V group compounds have many excellent electronic properties,attracting the great research interest.The binary arsenic compounds such as BAs,InAs and GaAs have attracted much attention for researchers.Among these materials,due to the high theoretical efficiency,GaAs has potential application in solar cells.However,its wide band gap limits the absorption of solar energy.The doping of cations,such as B and In,is expected to reduce the band gap,thus further improving the utilization efficiency of solar energy.Based on the many-body Green's function theory,the quasiparticle band structures and optical absorption properties of pure BAs,GaAs and InAs are investigated.The calculated band gap and absorption peak positions of GaAs are consistent with the experimental results,and the band gap of InAs is also consistent with the experimental reports.It is also found that BAs is an indirect gap semiconductor with direct and indirect band gap values of 4.24 and 1.91 eV,respectively.We also studied the effect of B and/or In doping on the electronic structures and optical absorption properties of GaAs under two lattice parameter conditions.By GW and BSE,it is found that the variation trend of the band gap and the optical absorption peak is related to the lattice parameter condition.Under the lattice constant condition based on the Vegard's Law,the reduced band gap and the redshift of the first excited state E0 can be observed for In doped GaAs,while increased band gap and the blueshift of E0 can be found for B doped GaAs.When the lattice constant is fixed to the experimental value,an opposite trend can be found.For B and In co-doped GaAs,the values of the band gap variation and the shifts of the state E0 are smaller compared with the case of single doping,which can be attributed to the cancellation effect of two kinds of impurity atoms.These conclusions can be used to resolve the controversies in experimental studies. |
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