利用鐵鈦共鍍催化金屬合成柱狀結構之奈米碳管之場發射特性的研究

Abstract

奈米碳管和基板附著性差會導致奈米碳管場發射特性的退化，而降低場發射的可靠度。在本論文，利用鐵和鈦的共鍍催化金屬來合成奈米碳管，可以改善場發射的可靠度，其中，鐵在催化金屬中的重量百分比為64 %。使用這種鐵鈦共鍍催化金屬，成長出來的奈米碳管，其根部會部分鑲嵌在基板中，如此，增加了奈米碳管和基板的附著性，進而提升場發射可靠度。在700 ℃下，利用這種催化金屬在熱化學氣相沈積中合成的奈米碳管，可以在7.7 V/μm的電場中維持一小時，保持約30 mA/cm2的穩定電流密度。除此之外，在前處理之後，發現使用鐵鈦共鍍的催化金屬顆粒，較單獨使用鐵作催化金屬的顆粒來得小且均勻，因此，在550 ℃的低溫下，成長出來的奈米碳管具有較大的成長速率，及較小的長度差異性。 使用這種鐵鈦共鍍作催化金屬的改良方式，前處理之後的催化金屬顆粒相當均勻，並且相較於使用純鐵作催化金屬的方式，更能合成出高準直性的奈米碳管。這是因為均勻的催化金屬顆粒可使奈米碳管以相同的速率成長，而得到持續的凡得瓦爾力(van der Waals force)。因此，我們使用鐵鈦共鍍催化金屬方式來合成柱狀結構的奈米碳管，並利用微影方式控制柱體密度，來降低電場遮蔽效應。可利用調變柱體間的間距和高度，折衷電場遮蔽效應和場發射面積效應，來獲得最佳的場發射特性。根據本實驗，最佳的場發射特性存在於R/H為2.5，開電場和起始電場均相當低，分別為1.01和2.67 V/μm，電流密度也高達256 mA/cm2，並且在施加固定電場5.33 V/μm一小時中，場發射的可靠度亦相當良好，電流變異大約只有20.59 %，平均電流密度約18.94 mA/cm2。因此，這種鐵鈦共鍍催化金屬合成的柱狀結構奈米碳管，具有相當潛力來應用在薄膜電晶體液晶顯示器上的背光源，以有效地降低製造材料成本。

The weak adhesion between the carbon nanotubes (CNTs) and the substrate resulted in the field emission degradation of CNTs. It reduced the reliability of CNTs for field emission application. In this thesis, a proposed method was achieved to improve the reliability of CNTs by using an Fe-Ti codeposited thin film whose weight percentages of Fe was 64 % as catalyst layer. With the Fe-Ti codeposited catalyst, the roots of the CNTs exhibited a little inserted in the substrate to enhance the adhesion between the CNTs and the substrate. The CNTs synthesized at 700 ℃ in thermal CVD exhibited a stable emission current density with 30 mA/cm2 at 7.7 V/μm for 3,600 sec. In addition, the catalyst nanoparticles after pretreatment for the Fe-Ti codeposited thin film were smaller and more uniform as compared with those for the pure Fe catalyst thin film. The growth rate and length variation of CNTs synthesized at a low temperature of 550 ℃ were thus improved by using an Fe-Ti codeposited catalyst. With this proposed method, the catalyst nanoparticles after pretreatment were very uniform and could be utilized to grow the highly aligned CNTs with respect to the results from the pure Fe thin film. It was attributed to the lasting van der Waals force since these uniform catalyst nanoparticles could lead to the grown CNTs with the equal growth rate. Therefore, we utilized the proposed method to synthesize CNT pillars to reduce the screening effect via the pillar density control. By adjusting the interpillar spacing (R) and height (H) of CNT pillars, the optimization of the field emission characteristics was obtained from the compromise of the screening effect and emission sites. According to the works, the maximum of the field emission was obtained by an optimal R/H of 2.5. The effective turn-on field and effective threshold field were as low as 1.01 and 2.67 V/μm, respectively. In the mean while, the maximum current density was as high as 256 mA/cm2. The reliability of the pillar arrays was determined by a stress test at 5.33 V/μm for 1 hour. It also showed an excellent reliability for the CNT pillars with the current variation coefficient of 20.59 %, and the mean current density of 18.94 mA/cm2. As a result, the CNT pillars could be potentially applied in the back light unit for TFT-LCD to reduce the material cost.