Electronic Structures of S-Doped Capped C-SWNT from First Principles Study
The semiconducting single-walled carbon nanotube (C-SWNT) has been synthesized by S-doping, and they have extensive potential application in electronic devices. We investigated the electronic structures of S-doped capped (5, 5) C-SWNT with different doping position using first principles calculations. It is found that the electronic structures influence strongly on the workfunction without and with external electric field. It is considered that the extended wave functions at the sidewall of the tube favor for the emission properties. With the S-doping into the C-SWNT, the HOMO and LUMO charges distribution is mainly more localized at the sidewall of the tube and the presence of the unsaturated dangling bond, which are believed to enhance workfunction. When external electric field is applied, the coupled states with mixture of localized and extended states are presented at the cap, which provide the lower workfunction. In addition, the wave functions close to the cap have flowed to the cap as coupled states and to the sidewall of the tube mainly as extended states, which results in the larger workfunction. It is concluded that the S-doped C-SWNT is not incentive to be applied in field emitter fabrication. The results are also helpful to understand and interpret the application in other electronic devices.
KeywordsSingle-walled carbon nanotube (C-SWNT) Electronic properties Workfunctions
Carbon nanotubes have attracted considerable attention due to their unique geometry and prominent electronic properties, which are promising materials for potential applications in field emitters, nanoheterojunction, scanning tunneling microscopy tip, and other vacuum microelectronic devices [1–3]. Recently, an approach for the synthesis of semiconducting single-walled carbon nanotube (C-SWNT) has been reported by S-doping with the method of graphite arc discharge. Such S-doped C-SWNTs are validated by experiments and theoretical calculations and have been preliminarily applied in field effect transistors (FET) fabrication . It is well known that the chemical and physical properties of C-SWNT can be modified by doping with other chemical elements. And it is believed that electronic structures of the carbon nanotubes should play a key role in determining their physical properties. In addition, the detailed electronic structure and the corresponding localized states for capped carbon nanotubes have been investigated . For the proposed applications, the detailed investigation into the electronic structures of semiconducting S-doped C-SWNTs is indispensable. In the same time, the workfunction is another critical quantity in understanding the field emission properties of carbon nanotubes. The workfunction of a metal surface is usually defined as Φ = φ − μ, where φ is the vacuum and φ is Fermi level, which describe the energy needed to take an electron from Fermi level to vacuum level.
In this work, we performed the first principles calculations to study the electronic properties of S-doped C-SWNT. We develop structural models for S-doping in capped (5, 5) C-SWNTs. The different doping positions of S atom are provided. We present the accurate values of workfunction of S-doped C-SWNT and analyze the change of the electronic structures without external electric field and under external electric field. It can be found that the electronic structures of S-doped C-SWNT depend strongly on the geometrical configuration of S atom in the C-SWNT. Under the external electric field, the electronic extended states of wave function are enhanced in the body wall of tubes. The electron distribution of S-doped C-SWNT is more localized than that of the pristine, which make the emission ability of S-doped C-SWNT lower. In the meantime, the coupled states with mixed properties of the localized and extended states occur in the tip of the S-doped C-SWNT. The coupled states increase the number of states with a large emission capability, which lowers the value of workfunction under external electric field than without external electric field. However, electrons obviously have two flow directions in the process of the redistribution of wave function close to the cap. One is as coupled states to the tip of C-SWNT and another is as extended states to the body wall farer away from the cap. The number of the former is less than that of the latter, which results in the lower value of workfunction compared with the pristine under equivalent external electric field. It is concluded that the S-doped C-SWNT is not incentive to be applied in field emitter fabrication.
Results and Discussion
The optimized geometry of the capped (5, 5) C-SWNT shows that the atoms at the top pentagon have an average bond length of 1.44 Å compared to that of 1.42 Å at the sidewall. However, the average C–S bond length was up to 1.80 Å, and the average C–S–C bond angles changed from 120° to 112°, which mean the implant of S atom into C-SWNT made the sp2 bonding in the perfect hexagonal lattices transmit to sp3-like bonding as tetrahedral-like lattices. The S-substitutional position has obvious dramatic local deformation, which should be believed to play an important role in the electronic properties. The structural changes are very small under applied electric field.
In summary, we investigated the electronic structures of S-doped capped (5, 5) C-SWNT with different doping position. We emphasized on analysis on how electronic structures have influence on the workfunction without and with external electric field. Due to the S-doping into the C-SWNT, the HOMO and LUMO charges distribution is mainly more localized at the sidewall of the tube than the pristine. The bonding charges accumulate on the S atom where the unsaturated dangling bond formed, which is believed to enhance the surface dipole with the increase in workfunction. When external electric field is applied, the coupled states with mixture of localized and extended states are presented at the cap, which provide the lower workfunction than without external electric field. In addition, the wave functions that distribute close to the cap have flowed to the cap as coupled states and to the sidewall of the tube mainly as extended states. The number of the former seems larger than that of the latter, which results in the larger workfunction than the pristine under the equivalent external electric field. The wave functions have redistributed at the sidewall of the tube due to the S-doping under external electric field. It is concluded that the S-doped C-SWNT is not incentive to be applied in field emitter fabrication. The results in this work are also helpful to understand and interpret the application in other electronics devices.
This work is supported by National Natural Science Foundation of China No. 50730008, Shanghai Science and Technology Committee Grant No. 09JC1407400 and 1052nm02000.
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