Study of Field Emission Characteristics and Applications on Carbon Nanomaterials
|關鍵字:||奈米碳管;微波電漿化學氣相沈積;場發射;奈米碳尖錐;Carbon nanotubes;Microwave plasma CVD;Field emission;Carbon nanotips|
|摘要:||在此篇論文中，將對奈米碳材的合成以及其在場發射的應用端做一番探討與研究。為了得到最佳效能，應用在場發射的材料必須具有高傳導性、高深寬比、好的均勻性以及耐久性。雖然奈米碳管(Carbon nanotubes, CNTs)的特性已經被研究多年，在場發射(Field emission)應用的效能以及技術上仍有許多值得探討的空間。事實上為了符合發射極的需求，各式各樣的奈米材料都可供選擇，然而考慮到將製程簡單化以及化合物的多樣性，使用碳材作為發射極不但具有多種的合成方式，也隨之有不同的表現。因此在本研究中，重點將討論奈米碳管以及奈米碳尖的合成技術以及改質之後的特性。
在合成奈米碳材的製程中，主要是以偏壓輔助微波電漿化學氣相沈積的方式(MPCVD)，在裝置中通入氫氣（H2）、甲烷(CH4)的氣體反應而成。藉由使用催化劑如鐵(Fe)、鉻(Cr)及鎳(Ni)，不同型態與特性的奈米碳管可以被合成。除了材料本身的特性，為了更進一步增加場發射的效能，如何控制奈米碳管密度的製程以及其特性的測量更顯重要。除此之外，本論文也成功合成新奈米碳材，如奈米碳尖錐以及自組裝碳化鉻奈米碳尖錐(chromium carbide capped carbon nanotips)之合成技術。值得一提的是，這些奈米碳材表現出全然不同的表面型態及特性。對於奈米碳尖來說，其超尖端(0.1nm)的形狀促使其可在較低的電場中(1.4V/μm)啟動，而自組裝碳化鉻奈米碳尖錐來說，則能得到更穩定的場發射電流。
In this thesis, synthesis of carbon nanomaterials were discovered and studied. The major applicable characters are focus on the field emission applications. To achieve optimizing performance, nanomaterials have to be with good conductivity, high aspect ratio, good uniformity and good durability. Carbon nanotubes, know for their novel properties, have been studied for more than a decade. For field emission applications, there are still works to be done to improve the performance and fabrication techniques. In fact, to meet the claims for an emitter, various kinds of nanomaterials are also good candidates. Due to the simplicity of the synthesis and diversity of compounds, carbon materials could be synthesized into various forms and characteristics. In this study, nanomaterials including CNTs and carbon nanotips were synthesized and major modifications to their morphologies were carried out to understand the characteristics. The nanomaterials were synthesized utilizing bias-assisted microwave plasma chemical vapor deposition (MPCVD) using H2/CH4 as reaction gases. Carbon nanotubes were synthesized using catalysts including Fe, Cr and Ni; each one shows unique surface morphology and field emission behavior. Except the intrinsic properties of the CNTs, to further more improve the field emission efficiency, density reduced CNTs were synthesized and measured. Besides, new kids of nanomaterials including bare carbon nanotips and chromium carbide capped carbon nanotips were synthesized. These nanomaterials show entirely different surface morphologies and characteristics. For bare carbon nanotips the ultra-sharp tip (0.1nm) makes it to turn-on at a low electric field (1.4V/μm). For chromium carbide capped carbon nanotips, the chromium carbide contributes to a more stable emission current. To achieve practical applications, fabrication of gated structure was also carried out. The suitable materials for the devices are used for fabrication and the advantages and disadvantages are studied. Due to the superior properties, selective growth of chromium carbide capped carbon nanotips were synthesized in the gated structure and low driving voltage (2.63V) of the field emission could be obtained.
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