電漿處理成長奈米碳管與其表面特性研究

Abstract

奈米碳管具極佳的高寬比、小的曲率半徑、良好的化學穩定性、以及很高的機械強度，同時奈米碳管表面的幾何形狀具耐高電流密度、高導電度等，被廣為應用在元件上。本論文研究主題如下所述: (i) 運用微波電漿化學氣相沉積系統，探討氫氣電漿前處理對於奈米碳管合成之準直性影響。實驗以鎳金屬為觸媒，混和氫氣與甲烷氣體比(9:1)，其結構以電子顯微鏡分析，並研究微結構之變化。結果顯示，在不同氫氣流量製程中改變鎳金屬觸媒顆粒，使得奈米碳管合成密度與準直性可以有效控制。經由拉曼結果分析，G值和D值強度的比例因氫氣流量的增加而減少。再經由TEM分析結果顯示，氫氣電漿前處理可有效去除非晶碳與碳化合粒子。證實氫氣在合成奈米碳管的界面效應扮演重要角色。 (ii) 運用微波電漿化學氣相沉積法，合成奈米碳管。奈米碳管經由氟化碳與氧氣混合電漿PECVD後處理後，其微結構、結晶結構、與場發特性透過掃瞄式電子顯微鏡、穿透式電子顯微鏡、拉曼光譜、與場發系統進行分析。結果顯示，氟化碳與氧氣混合電漿對奈米碳管處理3分鐘後產生損壞，並於處理9分鐘後，產生些許非晶的碳微粒。而場發電流特性在氟化碳與氧氣電漿處理處理3分鐘後由1 mA/cm2提升到3.12 mA/cm2。由此可知，奈米碳管局部幾何結構的改變，具有良好的場發特性。 (iii) 運用熱化學氣相沉積法合成奈米碳管，經由高密度電偶蝕刻法通入氬氣電漿處理在300 瓦特時，以電子顯微鏡觀察微結構改變，研究其對應場發特性對應之變化，場發電流特性由0.65提升到 48 mA/cm2，電場驅動電壓從3.6 到 2.2 V/μm，兩者皆獲得明顯改善。結果顯示，氬氣電漿處理的主要效應可能將在奈米碳管修改局部場發射區域的幾何結構。無論如何，氬氣電漿處理顯示為電子發射提升場密度的一種有效的方法，因此顯著改進奈米碳管的電特性。 論文中探討氫氣、氟化碳-氧氣、與氬氣電漿運用在奈米碳管材料之表面處理，可有效去除非晶碳與碳化合粒子，與對奈米碳管表面改質等潛力，系列研究有效證明以奈米材料的運用潛力。

Keywords：奈米碳管、氫氣電漿、微波電漿化學氣相沉積、掃瞄式電子顯微鏡、穿透式電子顯微鏡、拉曼光譜、熱化學氣相沉積、高密度電偶蝕刻。

Carbon nanotubes (CNTs) have been applied for the process of devices, because of high aspect ratio, small radius curvature, high chemical stability, and high mechanical strength. CNTs arrays depend strongly on the work function and geometry of the surface of CNTs arrays, which have excellent properties for application. In this experiment, several parts of issues has been concluded as below: (i) The effects of H2 plasma pretreatment on the growth of vertically aligned CNTs by varying the flow rate of the precursor gas mixture during microwave plasma chemical vapor deposition (MPCVD) were investigated in this study. Gas mixture of H2 and CH4 with a ratio of 9:1 was used as the precursor for synthesizing CNTs on Ni-coated TiN/Si(1 0 0) substrates. The structure and composition of Ni catalyst nanoparticles were investigated by using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (XTEM). Results indicated that, by manipulating the morphology and density of the Ni catalyst nanoparticles via changing the flow rate of the precursor gas mixture, the vertically aligned CNTs could be effectively controlled. The Raman results also indicated that the intensity ratio of the G and D bands is decreased with increasing gas flow rate. TEM results suggest H2 plasma pretreatment can effectively reduce the amorphous carbon and carbonaceous particles and, thus, is playing a crucial role in modifying the obtained CNTs structures. (ii) CNT is synthesized using MPECVD. The effects of fluorocarbon and oxygen (CF4/O2) plasma by PECVD post-treatment on the micro-structural, crystal, and field emission characteristics of CNT by SEM, TEM, Raman, and field emission (FE) system were observed. Results showed that the presence of the damage CNT occurs at 3 min CF4/O2 plasma treated and, some amorphous carbon particles after 9 min CF4/O2 plasma treated. One can also observe that the emission current density of the CNTs is enhanced from 1 to 3.12 mA/cm2 as compared to that of the as-grown ones at 3 min CF4/O2 plasma treated. It is indicating a remarkable FE enhancement with the changed of the local geometrical structure in CNTs. (iii) The Field Emission(FE) characteristics of CNTs grown by thermal CVD and subsequently surface treated by high-density Ar plasma in an inductively coupled plasma reactive ion etching (ICP-RIE) with the various plasma powers were measured. Results indicate that, after treated by Ar plasma with power 300 W, the emission current density of the CNTs is enhanced from 0.65 to 48 mA/cm2 as compared to that of the as-grown ones. SEM and TEM were employed to investigate the structural features relevant to the modified FE properties of CNTs. Nevertheless, the turn-on fields decreased from 3.6 to 2.2 V/μm, indicating a remarkable FE enhancement. The results further suggest that the primary effect of Ar plasma treatment might be to modify the geometrical structures of the local emission region in CNTs. In any case, the Ar plasma treatment appears to be an efficient method to enhance the site density for electron emission and, hence markedly improving the electric characteristics of the CNTs. According to H2, CF4/O2, and Ar plasma on the surface treatment of CNTs, it can effectively changed the amorphous carbon and carbonaceous particles of CNTs and, therefore, are playing crucial role in modifying the obtained CNTs structures. This thesis report the present work to investigate important object of nanomaterials related to the application of CNTs.

Keywords: Carbon nanotubes; H2 pretreatment; MPECVD; PECVD; Thermal CVD; Field emission(FE); Raman spectroscopy; Scanning electron microscopy; Transmission electron microscopy; ICP-RIE.