Dissertation
Dissertation > Industrial Technology > General industrial technology > Materials science and engineering > Special structural materials

Density-functional Thoery Calculations of the Optical Properties of Several Complex Solids and the Calculations and Experiments on the Field-emission of Carbon Nanotubes Model

Author ShangXueFu
Tutor TanMingQiu;WangZuo
School Zhejiang University
Course Condensed Matter Physics
Keywords Density-functional theory anisotropic optical properties MgB2 LaFeAsO magneto-optical Kerr effect AuMnSb AuMnSn carbon-nanotube field-emission screen-print
CLC TB383.1
Type PhD thesis
Year 2009
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This dissertation is represented into two parts. The first part is about the anisotropic optical properties of crystal of compounds and the magneto-optical Kerr effect (MOKE) of Heusler alloys, AuMnSb and AuMnSn, which were calculated by the density functional theory. The second part is on the field emission theory about carbon nanotube and carbon nanotube arrays. In this part we use the electrostatic field to enhance the screen-printed method for single-wall carbon-nanotubes film in experiment.Firstly, the density functional theory and full-potential linearized augmented plane wave (FLAPW) method are used to study the anisotropic optical properties of superconductor MgB2. The optical conductivity spectra, the reflectivity spectra and electron energy loss spectra have been calculated. The calculated optical conductivity spectra along xx-axis display quite different features in comparison with that of zz-axis, while the reflectivity spectra coincide well with the corresponding electron energy loss spectra especially about the characteristic peaks. In the optical conductivity spectra, the first intra-band absorption peak along xx-axis appears at 20000 cm-1 while along zz-axis it lies to a high frequency near 40000 cm-1. In order to simulate the temperature effects in these spectra, the Lorentz expanding (δ= 0.10 eV) is added into optical matrix elements during the calculation at 0 K. In comparison with existing experimental data, a minor mismatch of 1000 cm-1 (~0.124 eV) has been found in the calculated optical spectra.Then we calculated the anisotropic optical properties of another superconductor LaFeAsO using the same method. The optical conductivity spectra, the reflectivity spectra and electron energy loss spectra have been calculated. LaFeAsO is a anisotropic single crystal. The optical conductivity spectra along xx-axis display quite different features in comparison with that of zz-axis entirely, and the reflectivity spectra coincide well with the corresponding electron energy loss spectra especially about the characteristic peaks. In the optic conductivity spectra, the first intraband absorption on xx-axis appears at 1.3 eV while it on zz-axis is at 1.5 eV. In order to simulate the temperature effects in these spectra, the Lorentz expanding (δ= 0.10 eV) is introduced into optical matrix elements during the calculation at O K.What is more, we also have presented FLAPW calculation on the magneto-optical Kerr effect (MOKE) for both Heusler type alloys AuMnSb and AuMnSn. The differences with previous calculations on the Kerr spectra have been found explicitly. At proper Lorentzian such asδ= 0.4 eV, the calculated Kerr angle of AuMnSn reaches its maxima +0.3°near 0.6 eV and -0.5°at 5.2 eV, respectively while the MOKE spectra of AuMnSb exhibit less prominent peaks (+0.5°at 0.3 eV,-1.9°at 0.9 eV,-1.0°at 2.4 eV and -2.0°at 5.3 eV). The results on the spectra in this work showed quite a lot differences with all previous all-electron calculations.At first, the tip field distribution of the single carbon nanotube is calculated by the floating top segment model. It can simplify the calculation that taking the top segment replaces the whole single carbon nanotube. In the calculation, the linear charge equation about the field enhancement factor is obtained using the fictitious charge and image method. It conclude that the tip field distribution is related with the height-to-radius ratio weak, and the field emission intensity of the cylinder face can be ignored than that of the hemisphere face.And then the model of single carbon nanotube is extended to the CNT array. A set of modified linear charge equations have been derived for the CNT array using an approximation of multipole potential expansion. Fictitious charges inside a specific segment can be solved from these equations to calculate the field around the tips. Some numerical calculations of the field enhancement factor have been carried out, quantitatively accounting for the reduction of the factor with decreasing intertube distance. The relative field strength distribution on the top hemisphere of a CNT array is calculated too, which is observed to be close to that of a single CNT. Using Fowler-Nordheim’s formula, the field emission intensity of a CNT array is maximized with varying intertube-distance. The resulting optimum intertube-distance to height ratio decreases gradually with increasing height-to-radius ratio, and is close to 2 only for height-to-radius ratio around 3×102.Finally, we report a new method to improve the field emission performance of single-walled carbon nanotubes (SWCNTs) film by simple electrostatic field pretreatment. The screen-print process operated under electrostatic field~6 KV was used to prepare SWCNTs cathodes. For the FED treated by electrostatic, it has low electron emission turnon field (Eto= 1.22 V/um), low electron emission threshold field (Eth= 2.32 V/μm) and high brightness. By applying an electrostatic field, the tips of SWCNTs will emerge from the screen-printed film because of the repulsive force among SWCNTs. The figure below clearly shows that the pretreatment of electrostatic field can dramatically improve the field emission performance of SWCNTs.

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