全国优秀博士学位论文中英文摘要精选(17)

　　Manipulation and electrical property measurements on carbon nanotubes inside the transmission electron microscope

　　Wang Ming-sheng

　　ABSTRACT

　　With the rapid development of nano science and technology, carbon nanotubes (CNTs) have been the focus in may science fields due to their remarkable physical and chemical properties. In electronics, CNTs can act as new type of electron sources, and in nanocircuits, they can also be used to construct the electric nanodevices or as interconnects instead of metal wires, etc. In this thesis, we study the field-emission properties of individual CNTs systematically inside transmission electron microscope using the sample holder integrated with a scanning tunneling microscope (TEM-STM). Using the technique of electron-beam-induced-deposition (EBID), we connected and shaped the CNTs in controllable manners, and the electrical and mechanical properties were measured in situ. As we know, the properties and applications of nanomaterials are closely related to their morphologies and structures. As a powerful tool, TEM-STM technique can be utilized to achieve the correlation between the electrical and mechanical properties of one-dimensional nanomaterials and their structural details in atomic scale. We also have developed some in-situ techniques that allow morphology control of CNTs with high precision.

　　(1) Individual multiwalled carbon nanotubes were obtained in situ by precise manipulation inside a TEM. The tip structure of CNTs could be further modified via some controllable processes, such as electrically-driven vaporization, current-cutting, and thermal assistant field evaporation, and the field emission properties were correlated with the morphology of CNTs, especially the tip structures. A large amount of measurements show that the field-emission characteristics of a CNT depend sensitively on its tip structure and onset voltage or field conversion factor may change for more than 100%. Open-ended CNTs show lower onset voltages than close-ended CNTs, and the irregular shaped graphitic sheets at the tip of the open-ended CNT may enhance the field emission of the CNT remarkably. Due to these atomic protrusions on the graphitic sheets, the standard F-N equation based on planner surface may no longer be valid for some open-ended CNTs and will need some modifications.

　　(2) Based on the new theory for the derivation of emission area from Fowler–Nordheim plot developed by Forbes, we use different approximation methods to substitute v(y), i.e. Nordheim function, and give the corresponding calculation method, so that we can extract the exact emission area conveniently. We find HK approximation is sufficiently accurate in the whole possible current density range, but may cause inconvenience in calculation. While some simple approximation (such as CM or Spindt approximation) may cause only very small error, which is accurate enough for estimation. Substitution of v(y) using v(y)=1 will not cause much error in calculation of field conversion factor, but may lead to much larger error exceeding two order of magnitude in emission area estimation. In addition, we give the new form of emission area extraction functionΓ(φ) with HK approximation. Due to the speciality of F-N formula, the error in the calculated emission area is not greater than 10% (an accuracy of 10% is usually sufficient in practice), even if the exact value of work function is unknown. We usually assume the work function to be 5eV for CNTs, and the corresponding emission area is actually a safe “medium value”, which will not cause much error in estimation of emission area.

　　(3) The damage of CNT tip structures caused by thermally assisted field evaporation has been investigated. We find the emission current heating and field evaporation may cause the multiple ranges in I-V and F-N curves, that is linear region at low temperature, deviation region at high temperature, thermally assisted field evaporation region and remaining emitting region. We also observed thermally assisted field evaporation processs at constant bias, and find that shortening of CNT proceeds via the removal of the irregular shaped graphitic sheets from the tip. In addition, field emission characteristics of a CNT depend far more sensitively on the tip structure than on the geometric length of the CNT. We believe thermally assisted field evaporation should be the main reason that restrict individual CNTs get higher FE current, and how to reduce resistive-heating should be crucial for obtaining higher current.

　　(4) We also demonstrated that field evaporation technique can be used constructively to modify the tip structures in a controllable manner. For example, self-cleaning by current-heating provides clean cap surface and stable FE characteristics; a capped CNT can be opened controllably. We concluded that the emission current-induced heating played an important role by comparing the structural change of a single CNT at positive and negative bias. Moreover, we have obtained individual CNTs with a conical tip via current-induced shell-by-shell breakdown technique, and the radius of curvature of the cap can be further modified by field evaporation technique. Our cone-shaped CNTs have the advantages of both the thin and thick CNTs, such as small and controllable cap radius, closed termination, thicker CNT shank, etc. All these structural features of the cone-shaped CNTs lead to a low onset emission voltage, good emission stability, relatively higher sustainable emission current, and convenience for mounting. More importantly, they can be tuned over a large range for the desired onset voltage or maximum stable emission current. Therefore, the conical CNT tips may be regarded being the “optimal tip structure” for CNT emitter.

　　(5) Furthermore, we investigated two types of abrupt failures behavior of CNTs field emitters. One is originated from the CNT/substrate contact, that is, tensile-loading under the applied electric field and local resistive-heating at the contact may lead to removal of CNTs from the substrate. The other is originated from the very tip of CNT and caused by field-evaporation-induced arc, which lead to removal of CNTs from tungsten substrate and even the damage of the tungsten tip itself. To avoid the above failures, we propose and show that the CNT/substrate contact can be stabilized by electron-beam-induced deposition (EBID) of amorphous carbon (a-C), which can not only enhances the mechanical stability, but also improves the conductance of the contact. In addition, by reducing the CNT length and using EBID technique, we acquired a field emission current more than 200μA from a single CNT.

　　(6) Using EBID technique, controllable growth of a-C nanowires has been conducted manually in situ inside TEM. By moving electron beam, we guide the deposition of a-C into the center of electron beam, so as to get the desired nanostrucures, such as nanowire, nanosopts, etc. The electron beam can not only be used as excitation for the deposition process, but also for monitoring and observation. We find that the size and shape of the deposit are directly determined by the spotsize and shape of the focused electron beam, and increasing deposition time may cause the growth of the deposit until saturation occurs. We study the electrical property of a-C nanowires by two-terminal measurement. The a-C is found to have poor conductance, while in nanometric-scale thickness, it exhibits small resistance and ohmic conduction. a-C nanowires also show considerable field emission capability, and can sustain a high current density of 106A/cm2. Furthermore, we discuss the possibility that controllable fabrication of CNTs in a large scale could be achieved by the combination of EBID technique and graphitization process of a-C.

　　(7) With the combination of STM manipulation and EBID technique, we connected the CNTs in a highly controllable manner for the first time, and all CNT structures have been successfully constructed. The measurements of their electrical and mechanical properties show that the deposited a-C can effectively increase the conductance of the contact and greatly improve the mechanical strength of the junction. We could construct bent and zigzag nanotube structures by controlling the contact angle, as well as a more-complex CNT network consisting of crossed and T-junctions. We also demonstrated for the first time that the current-induced graphitization of a-C, and have shown that this process may greatly reduce the junction resistance. The connection with disordered graphitized carbon or a-C can sustain a very high current, which may play an irreplaceable role in many practical applications, especially at extremely high current densities. CNTs and their connection with a-C can therefore take the place of metal and be used as interconnects in future nanocircuits.

　　(8) MWCNTs with diameters in the range of 20–50 nm have been shaped into morphologies with multiple bends or continuous curves by using an accurately controlled STM probe, followed by EBID of a-C, either at the buckling point or on the uniformly curved regions of the nanotubes. a-C provides excellent adhesion to the CNT and fixes the CNT into the desired shape with excellent mechanical strength, and can maintain the bent structures even under a large tensile-loading. In addition, the electrical conduction of the nanotube shows hardly any dependence on the bending deformation or on the deposition of amorphous carbon. Our findings suggest that in nanoelectronic and nanoelectromechanical systems, CNTs might be manipulated and processed as complex interconnections between electronic devices without much degradation in their electrical conductance. As nanometer-scale transport carriers of charge, mass, and energy, non-straight CNTs provide an important supplement to the straight ones and greatly extend potential applications of CNTs.

　　Key words: individual carbon nanotube, field emission, tip structure, amorphous carbon, electron-beam-induced deposition, carbon nanotubes interconnection

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