Study On Low-energy Light Absorption Of SWNTs

As a new kind of functional material, Carbon nanotubes composed of carbon atoms can be considered approximately as one-dimensional systems with nanostructures, which have singular and abundant physical capabilities. Carbon nanotubes can be classified as single-wall carbon nanotubes and multi-wall carbon nanotubes. Among them, electronic properties of single-wall carbon nanotubes are much popular on the subjects of material, physics, chemistry, micro-electronics and energy at the present time. Up to now, several kinds of nanotubular material have been synthesized, such as carbon nanotubes, silicon nanotubes, boron nitride nanotubes, sulfide nanotubes, etc. Many of their unique properties are closely related with their structures. The research on the structure and the properties of nanotubular materials is essential for better understanding the forming mechanism of the materials and exploring their potential applications.Computational simulation, which is the third extremely powerful tool to study physical world following experiments and theories, plays a very important role in science and technology. It's a bridge between theory and experiment and helps us not only to understand and interpret the experiments at the microscopic level, but also to study regions which are not accessible experimentally, or which would imply very expensive experiments, such as under extremely high pressure and temperature. There are two kinds of computational method to study nanoscale materials. One is empirical or semi-empirical method; the other is ab initio calculations or first-principle calculations. Empirical method calculates some properties of materials through analytic potential functions, while ab initio calculation starts with first-principlequantum chemistry and solves Scrodinger equation self-consistently by iteration.In the present work, by using semi-empirical method, we set up an approximately one-dimensional model on the basis of tight-binding theory. We have studied the electronic state, low-energy photon absorption, exciton binding energy of single-wall carbon nanotubes. This dissertation includes two parts. The first part introduces the theoretical fundamentals that we used in our research work. The second part introduces the author's main work done during three-year mater degree studies.The following gives a brief outline of the main contents of this dissertation.1. The theoretical fundamentals of our research workThe well-known Su-Schrieffer-Heeger(SSH) model simulation based on empirical potentials is an effective tool in the systematic design and the studies of basic characteristics of different materials. It's also the main method of the research work on carbon nanotubes in the present dissertation. In chapter 2, the principle and realization method of the one-dimensional Su-Schrieffer-Heeger model for carbon nanotubes are introduced. In addition, we introduce the structures of some sorts of carbon nanotubes and their unique physical,chemical properties and potential application.2. Low-energy light absorption in SWNTsCarbon nanotube is one kind of quasi-one-dimensional organic photo—electricalmaterials. Contrast to the traditional semiconductor, organic material has its uniqueproperties. Especially, the additional electrons or holes in organic materials will notspread in the whole chain. Instead, the electron-lattice interaction will induceself—trapped excitations, such as the exciton formed in Carbon nanotubes. When anelectron absorbs a photon, the electron—hole formed subsequently will interplay withsurrounding lattices and cause the distortion of lattices. Then a self—trapped excitonis formed. Ever since the discovery of single-wall carbon nanotubes by Iijima andIchihashi, there has been considerable interest in the optical properties of single-wallcarbon nanotubes. Carbon nanotube bundles containing all sorts of carbon nanotubeswith different diameters and chiralities have been synthesized. The variation ofdiameters of these tubes is 1～2nm. Through the analysis of the light absorptionspectrum on carbon nanotube bundles, three absorption peaks has been found in thelow energy level, 0.5～0.7ev, 1.1～1.3ev, 1.7～2.0ev respectively. By calculating statedensity of different sorts of carbon nanotubes and also the transition probabilitybetween different energy levels in singularities of DOS, we prove that electrontransition in semiconductor carbon nanotubes leads to the first absorption peak while the electron transition in metal carbon nanotubes produces the third peak. The reasonfor the second peak is unclear.3. Exciton binding energy in semiconductor SWNTsIn nanotube science, diameter and chirality are very essential and also very important concepts. In general, once the relationship between one property of carbon nanotubes and diameter and chirality is set, many phenomenons can be predicted. Therefore, a reliable determination of diameter and chirality trends of a given nanotube property, even when this is accomplished by simplified models, is often as important as determining accurately that property for a limited number of tubes. Many photo-electronic effects have relationships with features of exciton in carbon nanotubes. However, a full description of diameter and chirality dependences of exciton properties in SWNTs has not been provided. Nowadays many research works focus on the influence of electron—electron interplay on exciton binding energy, but at the same time, electron—lattice interplay is also an important characteristic of organic materials such as carbon nanotubes. In a special circumstance, an exciton is disassociated, which induces lattice distortion. Subsequently a positive polaron and a negative polaron are produced. We define the difference between the former energy sum and the latter energy sum is the amount of exciton binding energy. From this definition, we can examine the influence of electron—lattice interplay on exciton binding energy. We get the conclusion that in semiconductor carbon nanotubes of same chirality, with the increase of the diameter, the localization of exciton becomes weaker, exciton binding energy decreases. In the circumferential direction, the distribution of the excited electron is symmetrical.