Effect Of Doping On Electronic And Thermal Properties Of Inorganic Nanotubes

In order to provide considerable insight into physical properties of inorganic nanotubes, such as carbon and boron nitride nanotubes, we studied electronic and thermal properties of inorganic nanotubes. A number of interesting results are obtained. In this dissertation, we first comment current situation of investigations for inorganic nanotubes, including their structures, discovery and synthesis, properties and application perspective. By virtue of suitable theoretical methods, such as density functional theory and lattice dynamics theory, we investigated phonon, electronic structures, thermal properties and the effect of doping on electronic structures. The single-walled, multi-walled carbon nanotubes and double-walled nanotube with different species were also studied. Especially, we discussed the structural dependence of physical properties and potential mechanism. My dissertation is organized as follows:In Chapter 1, The history, background, structures, properties and application perspective were introduced.Chapter 2 is devoted to the well-known tight-binding and force constant methods, which have been widely used in previous calculations. We calculated the electronic and phonon structures of single-walled carbon nanotubes. Moreover, we developed the five-step-rotation method, which can be used to derive the force constant matrix between two atoms in carbon nanotubes.Chapter 3, Due to the curvature effect, the tight-binding and force constant methods mentioned above can not be directly used to calculate electronic and phonon structure of small diameter carbon nanotubes. Based on density functional theory, we perform systematic calculations of electronic structures of single-walled carbon nanotubes and discuss the effect of strong curvature on electronic structures. The results show that the variation of electronic structure due to curvature effect is small in large diameter nanotubes. However, strong curvature can invalidate the prediction of tight-binding theory. Because of the difference of structural curvature between inner and outer tubes, the shift of electronic states near the Fermi level occurs, leading to the pronounced decrease of band gap of semiconducting@semiconducting double-walled carbon nanotubes. Based on the curvature energy, we derived an analytical expression, which relates the radial force constant of nanotube to that of graphene and structural curvature. According to this expression, the nanotube force constants would be changed when the graphite sheet is rolled into carbon nanotube. This is due to the variation of total energy and bond length. Using the revised force constants, we calculated the radial breathing mode of 4? diameter carbon nanotubes. Our results are in good agreement with experimental observation and other theoretical calculations.Chapter 4, Based on the force constant model, the phonon structures of boron nitride nanotubes and diameter dependence of Raman mode frequency are obtained. We calculate the specific heat of boron nitride nanotubes and present the fitting formulas for diameter and chirality dependence of specific heat. Using Landauer theory, we calculate thermal conductance of single-walled boron nitride nanotube. The results show that thermal conductance at low temperature is quantized and increases with the increasing diameter. We calculate specific heat of multi-walled carbon nanotubes and single-crystal GaN nanotube and find that the specific heat is dominated by quantized size effect and surface effect. From the perturbation theory of three-phonon scattering process, we calculate thermal conductivity of single-walled and multi-walled carbon nanotubes. Thermal conductivity of chiral nanotubes is smaller than that of achiral nanotubes when both have the same diameter. Thermal transport of carbon nanotube can be suppressed by strong intertube coupling, which may be arisen from the structural defects of samples.Chapter 5, Using suitable interatomic interaction potential, we calculate the phonon structures and specific heat of single-walled carbon nanotube bundles doped with potassium. The frequency of high-frequency modes decreases linearly with the doping concentration. The low temperature specific heat deviates from the usual variation since the phonon modes of potassium dominate the low frequency phonon structures.Chapter 6, We calculate the electronic structures and radial charge distribution of potassium doped double-walled carbon nanotubes and carbon-boron nitride nanotubes. For large diameter double-walled carbon nanotubes, most of transferred charges reside on the outer tube. Due to curvature effect, a part of transferred charges are distributed on inner tube of semiconducting-semiconducting and metal-semiconducting nanotubes. If the outer tube is boron nitride nanotube, the transferred charges prefer residing on the inner tube owing to the predominance of electronic bands of inner tubes. Moreover, we discuss the effect of co-doping on electronic properties in potassium and bromine doped double-walled carbon nanotubes.Chapter 7, The summary and prospect are given in this chapter.