Study On The Properties Of Excited States In Carbon Nanomaterials

Carbon nanotubes are made of graphene,which has a very bright future.In recent years,experiments and theoretical studies have been carried out on carbon nanotubes.The researchers found that they have excellent thermal conductivity,electrical conductivity,mechanical and optical properties.Therefore,further research of carbon nanotubes is necessary.As the experiment progressed,many phenomena that the experimental results could not be explained by single electron approximation were obtained.The main reason is due to ignore the interaction between electrons,so the so-called exciton effect is introduced to explain.The exciton effect can not only explain the problem that single electrons cannot explain,but also deeply study the electronic structure of carbon nanotubes.Therefore,it is urgent to study the exciton effect of carbon nanomaterials.In this paper we will discuss the exciton effect in carbon nanomaterials with details.After introducing the main classification of carbon nanomaterials and representative materials,such as fullerenes,graphenes,carbon nanotubes,etc.Taking carbon nanotubes as an example,the electronic structure of carbon nanotubes is introduced.Moreover,the exciton effect can well explain the process variation of electron transition and the interaction of electrons and holes in carbon nanotubes.For carbon nanotubes,the exciton effect has a large influence on optical absorption.Therefore,the linear absorption spectrum of exciton generation is studied.In the study of the exciton effect,the calculation methods mainly used are the tight-binding approximation calculation method,the first-principles calculation method,the variational calculations,and the effective mass method.In this paper,the exciton effect of carbon nanotubes under the combined action of magnetic field and deformation is studied by tight-binding approximation and Coulomb interaction.The results obtained in this paper are as follows:1.The exciton states of semiconducting carbon nanotubes are calculated by a tight-binding supplemented Coulomb interactions,under the combined effect of uniaxial strain and magnetic field.It is found that the excitation energy and absorption spectra of zigzag tubes?11,0?and?10,0?will show two different trends with the strain under the action of magnetic field.For?11,0?tube,the excitation energy decreases with increasing the uniaxial strain,for?10,0?tube,the variation trend firstly increases and then decreases,the excitation energy can be obtained by linear absorption spectroscopy,and both splitting occurs.2.The metallic zigzag tubes will open a band gap by applying the magnetic field or uniaxial strain.The exciton states of the nearest band to Fermi level(M₀₀)in the metallic carbon nanotubes are calculated by a tight-binding model,under the combined effect of the magnetic field and uniaxial strain.It is found that the excitation energy and binding energy of the metallic zigzag carbon nanotube?24,0?have the similar variation tendency,which are split intoM₀₀^?10?andM^-₀₀excitons by the applied magnetic field and strain.When the uniaxial strain is applied,forM^-₀₀,it increases with increasing magnetic field,while forM^-₀₀,it decreases.However,it is interesting to find that as the magnetic field increases,theM^-₀₀exciton will decrease to be zero and a metal-semiconductor-metal change.3.The exciton states of metallic carbon nanotubes were calculated by the tight-binding model,under the combined effect of magnetic field and uniaxial strain.Based on the exciton states,the linear absorption spectrum of the metallic carbon nanotubes can be obtained by using the standard formula.Due to the triangular warping effect,the absorption spectra of theM₁₁ andM₂₂transition energies of the metal carbon nanotubes are divided into four separate absorption spectra by the applied magnetic field and uniaxial strain,which is the largest for metallic zigzag tubes.Under the same magnetic field,the corresponding deformations of the equal energy of the four splitting absorption peaks ofM₁₁ andM₂₂ have the similar tendency,which increases with the increasing magnetic field.What is more interesting is that for the metallic carbon nanotubes with different diameters,as the magnetic field increases,the deformation corresponding to the two absorption spectra with equal energy also shows the same trend.In addition,this conclusion has certain reference value for optical applications,and the desired excitation energy can be obtained by adjusting the parameters of deformation and magnetic field.Finally,the fourth chapter gives the summaries and prospects of this thesis.