微波電漿化學氣象沉積法合成準質性奈米碳管及其場發射性質研究

Abstract

在本實驗中，透過微波增強氣相沉積法，已可在矽基板上製作出準直性良好的奈米碳管薄膜。以此基礎，我們更試著在碳管成長過程中調變了許多參數，已達到調整碳管結晶性的目的。這其中包含了基板的偏壓及加入輔助性氣體(N2 和O2)，並試著找出結晶性與場發射行為上的關聯性，不過所得到的結果卻是不一致的!我們將此結果歸因為樣品表面型態的改變同時也影響了場發射的整體性質， 此假設並由SEM照片得到支持． 接著，我們選擇準分子雷射進行後處理的實驗，我們不只希望透過這個方式來研究碳管結晶性與場發間的關係，也希望驗證雷射處理是個改善碳管場發射效能的有效手段! 我們調變一系列參數來進行實驗，包含改變雷射功率，雷射脈衝數及後處理時的大氣環境，由實驗結果我們得到兩個結論: 首先，準分子雷射已在本實驗中被驗證能有效改善碳管的場發射行為，在本實驗最佳化的條件中，起始電場能有效降至原始電場值的一半，而電流密度亦能同時變為原始電流密度的2~3倍!對此結果，我們推論其原因，認為是入射光子對管璧造成了管璧結構上的破壞，而這破壞便形成了新的電子發射點而大大的改變了電子場發射的行為，這樣的推測也由高倍率TEM照片中獲的支持!

另外，我們從分析所得到的拉曼結果中，我們發現結晶性的好壞有趨勢地反映在場發射的表現上，簡而言之，便是隨著雷射處理所造成的結晶性變差，碳管的電流密度變大且起始電壓下降了!但在場致增強因子的關係上卻沒有得到類似的結果！我們相信影響場致因子的機制是非常複雜且受到許多因素的影響，這也是我們日後要做進一步研究的地方。

In this study, the optimal processing conditions for synthesizing the well-aligned CNTs on Si substrate (P type (100)) by microwave plasma chemical vapor deposition (MPCVD) were investigated. The carbon nanotubes and their properties are then characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and I-V measurements. The main results are summarized as follows. Firstly, the externally applied bias showed significant effects on CNTs growth and well-aligned CNTs were obtained when biased above 250 V. The Raman measurements also indicate that the greater applied bias the better the graphite crystallinity. The ID/IG ratio of the CNTs decreases with increasing negative applied bias which, in turn, improves their emission characteristics. We also studied the influences of N2 and O2 on the size distribution and morphology of Ni catalyst particles, and their effects on field emission characteristics. It is observed that clusters attacked on the top of carbon nanotubes can removed by adding N2 and O2 as assistant gases to get a appropriate flow rate of H2/CH4/N2 or H2/CH4/O2.The addition of N2 results in better crystallinity quality but degraded field emission characteristics. On the other hand, the addition of O2 results in poorer crystalline quality but better field emission properties. The underlying mechanisms for these intereling observations are not clear at present. Furthermore, post-treatment by KrF excimer laser irradiation to the CNTs was carried out to improve the emission characteristics. The maximum emission current density after laser irradiation reached 2.46mA/cm2 with applied electrical field of 3.55 V/μm. The turn–on field for CNTs emitters decreased from ~5 V/μm to 3 V/μm, corresponding to a decrease of turn-on voltage from~900V to ~550V with the laser irradiation treatment. The adding of oxygen during laser irradiation, however, degrades the performance of field emission properties, including current density, turn-on voltage, and field enhancement factor. Finally, the correlation between ID/IG and field emission performance was roughly conformed. The ratio does not have a strong relation with field emission performance by modifying various growth parameters, including bias voltage, N2 and O2 assistant gases. By contrast, modifying ID/IG with low power laser post-treatment shows that there is a strong relationship between the ratio and field emission performance. We suggest that morphology (distribution and spacing) and graphite structure (crystallite quality) may both have influences on field emission properties. The SEM and TEM images provide a evidence for our previous suggestion.