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First-principles Density-functional Study Of The Field Emission Properties Of Carbon Nanotubes

Posted on:2008-07-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:J QiaoFull Text:PDF
GTID:1101360212997999Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Since their discovery in 1991, carbon nanotubes (CNTs) have been considered as a subject of research. Due to their unique geometrical structures and novel mechanical, electrical and chemical properties, various potential applications such as field emitters, microelectronic devices, structural reinforcement fibres in composite materials, hydrogen storage devices, and chemical and electrochemical sensors, have been suggested. Among these applications, the CNT-based field emission flat-panel display is the most promising. CNTs as the field emission electron source have many advantages: unusually high aspect ratio (long tube length and small curvature radius on the tip), high chemical stability, low extracting field, high current density, and long operating time as well. These excellent advantages make CNTs good field emission materials. Up to date, however, the most possible commercial applications of CNTs to flat-panel displays have not been realized. The main problems are the density and the uniformity of the electron emission sites. Hence, it is very important and necessary to modify CNTs to improve their field emission performance, which is also an important step to realize the industrialization of CNTs.Along with the rapid development of computational methods and computer technology, computational materials science has become more and more important in modern materials science. Due to its moderate computational consume and high precision, density functional theory (DFT) has become one of the most important methods in computational materials science. In order to further understand the field emission mechanism and enhance the field emission performance of CNTs, it seems very necessary to do theoretical simulations about the field emission process of CNTs. Taking advantage of rapid progress in computational physics, now it is possible to study the geometrical structures, electronic structures and many other properties of clusters from first-principles calculations, and get more and accurate information. This dissertation is to study the possible methods of enhancing the field emission properties of CNTs by using first-principles DFT code DMol3, which is a module from registered software package MATERIALS STUDIO, and to discuss the effects of adsorption and doping of extrinsic atoms or molecules on the field emission properties of CNTs. This will be helpful in the synthesis and design of field emission cathodes, which are made by CNTs.In Chapter 1, firstly, we give a brief introduction to the discovery, structures, properties, applications and synthesis of CNTs. Secondly, we give a review of the commonly used field emission cathode materials. Thirdly, experimental and theoretical studies on the field emission properties of CNTs are reviewed and previewed. Then we summarize the progress of the effects of adsorption and doping on the field emission properties of CNTs, which are two common methods to modify the field emission performance. At last, we simply describe the purpose and the results of our work.In Chapter 2, firstly, we introduce the basic concept and progress used in our work in detail, such as ab initio calculation, first-principles calculations and DFT. Secondly, we focus on the usage and computational flow of simulation package DMol3, which is used in our work. Then we give a brief introduction of the basic concept of field emission, such as tunnelling theory, work function, Fowler-Nordheim (F-N) formula and its modification. At the end of this chapter, we brief introduce several factors, which can modify the field emission of CNTs.In Chapter 3, the field emission properties of CNTs with water adsorbed on the tip are studied using first-principles DFT method. Since the working conditions of CNT cold cathodes can not be all under ultrahigh vacuum environment, it is very necessary to study the effect of working environment on the field emission properties of CNTs. Experimental investigations clearly denotes that at room temperature the field emission properties of CNTs can be significantly enhanced, which is due to the presence of water on CNT tips. And the saturation of field emission current at high applied electric field is also attributed to water adsorption. The enhancements of the field emission properties are attributed to the adjustment of the local electronic structures of the emitting regions by the adsorption of water molecules. However, no clear relationships between the water adsorbates and the electronic structures of CNTs under the applied electric field have been built, and the mechanism of the enhancement of the field emission by the water adsorbates is not very clearly revealed.In this chapter, we have calculated the geometrical structures and the field emission properties of capped (5, 5) single-walled carbon nanotubes (SWCNTs) with water adsorbed on the tip with and without the applied electric field. The adsorption of water molecules on the CNT tip is exothermic except for the CNT with one water molecule in the absence of the applied electric field. This denotes that these adsorbed structures of CNTs are remarkably stable under field emission conditions. The adsorption of water molecules can induce a dipole moment on the surface of the CNT tip and drive the electrons on the tip to emit to the vacuum and enhance the electron field emission. The energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) decreases on increasing the applied electric field, and further decreases with the adsorption of water molecules. The CNTs are polarized, and the Mulliken charge population shows that the charges are redistributed and accumulated on the tip, which is due to the electrostatic interaction under the applied electric field. Both the number of Mulliken charges that transfer from the CNT to water molecules and the number of Mulliken charges that accumulate on the CNT tip increase with the increase of the number of water molecules. When the applied electric field is applied, the Fermi level of the CNT with water molecules shifts towards the conduction band, and the local density of states (LDOS) at the Fermi level increases. On the other hand, at the Fermi level the LDOS for the CNT with water molecules increases to twice that without water molecules. This leads to the observation that electrons can emit more easily due to the effect of water adsorption. Our results reveal the role of water molecules in enhancing the field emission properties of CNTs, and provide the possible field emission mechanism, which can guarantee the application of CNTs to field emission devices.The doping of CNTs with other chemical elements is another practical and feasible way to modify the field emission properties of CNTs. In Chapter 4, we perform first-principles DFT calculations to investigate the field emission properties of N-doped CNTs. Nitrogen atom (contains one electron more than carbon atom) is the most common candidate of n-type dopant for carbon materials. With the development of synthesis techniques of CNTs, N-doped CNTs have been synthesized. The field emission experimental investigations indicate that N-doped CNTs show enhanced electron field emission and special emission properties. By incorporating nitrogen atom within the hexagonal lattice, it can introduce unsaturated dangling bond states at the tip of CNT and increase the emission current, which is owe to the shift in the energy level of localized states to the Fermi level. However, the actual emission current depends strongly on the doping configuration of CNT, which is related to the doping position of the extrinsic atom and the chirality of CNTs.In this chapter, we directly substitute one carbon atom by one nitrogen atom in the hexagonal lattice of capped (5, 5) SWCNT, which can lead to different doping positions of the doped CNTs. We calculate the geometrical structures and the field emission properties of pristine and N-doped CNTs. The doped capped CNT exhibits semiconducting property with a finite value of energy gap between LUMO and HOMO. The energy gap of the doped CNT decreases significantly when compared with pristine CNT. We can find "couple states" in N-doped CNT, which have a contribution to field emission. Upon nitrogen atom substitution, the work function decreases dramatically, indicating the enhancement of the field emission properties. Similarly, the IP of N-doped CNT reduces significantly due to the doping of nitrogen atom. The reduction of IP is correlated to the increase in the HOMO level. The doping of nitrogen atom into CNT will donate its extra electron to CNT, which results in the Mulliken charge redistribution. Due to the doping of nitrogen atom, the value of LDOS at the Fermi level increases dramatically and the Fermi level shifts towards the conduction band. Donor states can be observed above the Fermi level, and the additional states at the Fermi level are able to act as tunneling states. Furthermore, we can see that the most preferable doping position appears at the fourth layer, which is the link between the cap and the body of CNT. Our results provide the possible field emission mechanism and suggest that the doping of nitrogen atom can enhance the field emission properties of CNTs, which will open new avenues in the field emission devices.In Chapter 5, we perform first-principles DFT calculations to investigate the field emission properties of B-doped CNTs. Boron atom (contains one electron less than carbon atom) is another most common candidate of p-type dopant for carbon materials, and B-doped CNTs have been synthesized using different techniques. Compared with N-doped CNTs, the field emission properties of B-doped CNTs are more complicated, and up to now there exists two opposing experimental results. Some investigations demonstrate that the doping of boron atom can indeed enhance the electron field emission of CNTs. While others indicate that the incorporation of boron atom into the carbon network apparently increases the concentration of electron holes that become electron traps and eventually impedes the electron field emission properties. Due to the existence of the opposing experimental results, it is therefore interesting and necessary to study how the boron atom will affect the field emission properties of CNTs and the intrinsic field emission mechanism. Some investigations reveal that the field emission current is sensitive to both the doping position of the boron atom and the applied electric field, which helps us to gain insight into the effects of boron atom on the field emission properties of CNTs.In this chapter, we calculate the geometrical structures and the field emission properties of pristine and B-doped capped (5, 5) CNTs. The different doping positions of boron atom in carbon network are chosen, and from the calculated results of the heat of formation we can see that the boron atom is preferentially located at the CNT tip. Upon boron atom substitution, the work function increases dramatically, indicating that the field emission properties of B-doped CNT are impeded due to the substitution. We analyze the changes of the electronic structures of the CNT after substitution, and find that the HOMOs do not change too much, while the LUMOs decrease significantly. This leads to the decrease of the Fermi level, and the shift of the Fermi level to the valence band, which will increase the potential barrier on the surface of the CNT tip and therefore impede the field emission of B-doped CNTs.In conclusion, we perform first-principles DFT calculations of the field emission properties of the CNTs with water adsorbed on the tips, N-doped CNTs and B-doped CNTs. The results indicate that the field emission performance of CNTs can be enhanced by the adsorption of water molecules and the doping of nitrogen atom, and can be impeded by the doping of boron atom. In this dissertation, we give basic work on the study of the field emission properties of CNTs, and make progress in the study of the modification of the field emission properties of CNTs. These findings will be helpful in the synthesis and the design the CNT field emission cathode materials.
Keywords/Search Tags:Carbon Nanotubes, Field Emission, First-principles, Density-functional Theory, Adsorption, Doping
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