以陽極氧化鋁奈米孔洞為模板輔助合成垂直準直奈米碳管陣列

Abstract

自從1991年發現奈米碳管以來，專家學者經過十餘年的研究，雖然對其比以往了解得多，但在應用方面仍有許多問題尚待克服。其中，場發射顯示器一直是眾人認為最有潛力實現的碳管商品，其所需要的基板便是擁有高度準直的奈米碳管陣列。時至今日，場發射顯示器的商品依舊是只聞其聲，不聞其人，其中極重要的一環即是無法有效的控制奈米碳管的生長，包括準直性、長度以及密度。本實驗已成功的製造出規則排列的陽極氧化鋁奈米孔洞，並利用電鍍法沉積觸媒於孔洞內，再以此為模板，進行以氫氣與甲烷為反應氣體的電子迴旋共振化學氣相沉積法來合成奈米碳管陣列。由於模版的作用再加上合成的參數中允許加入偏壓，如此合成的奈米碳管具有極佳的準直性。值得注意的是，由於電子迴旋共振化學氣相沉積法擁有較低的合成速率，碳管的長度可以經由合成的時間來做細微的控制；相較於文獻上類似的做法，本實驗可以合成均勻長度之碳管，且擁有良好的石墨化結構。在密度的控制上，利用改變反應氣體的流量比例，造成特定量的非晶質碳沉積在孔洞上，某些尚未長出洞口的碳管因得不到成長所需碳源而停止生長，經過如此非晶質碳與成長碳管之競爭反應，即可達成密度控制的目的。另一方面，碳管陣列的場發射特性亦為吾人所關注的焦點，此一性質為碳管做為場發射源的首要考量。針對不同長度及密度的碳管，我們皆加以量測其場發射效應；在長度方面，我們發現在相同的密度下，最長的碳管擁有最佳的場發射效應。而在不同密度方面，由於屏蔽效應，最密的碳管反而擁有較差的場發射表現，但額外必須考慮的是，因為利用反應氣體濃度的改變為控制的條件，當碳源氣體變多，使得非晶質碳不但容易在奈米孔洞內沉積，長出孔洞表面的碳管上也容易被其覆蓋，間接的驅動觸媒的毒化現象，密度低的碳管同時也具有較短的長度，由此，我們也得到了一個在長度及密度恰好時的最佳場發射特性，場發射增強因子，也在這些量測中被一一的計算出來。其它，我們也做了許多定性與定量的分析，包括各種的電子顯微鏡影像、拉曼光譜、歐傑電子光譜，也將在以下的文章中詳述。

In order to utilize the excellent field emission properties as specific applications, ordered carbon nanotubes (CNTs) arrays with controllable length and density has become an important issue. Highly CNTs have been successfully grown in vertical channels of the anodic aluminum oxide (AAO) template by microwave plasma electron cyclotron resonance chemical vapor deposition (ECR-CVD). Nanoporous AAO templates with hexagonal pore pattern were prepared by the two-step anodization of Al films. Following the electroplating of Co catalyst into the pore bottom, multiwalled CNTs were synthesized in the ECR-CVD system using a gas mixture of CH4 and H2. The microstructure of the CNTs was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The CNTs with a very high packing density and a uniform size distribution are well-graphitized, and Co particles embedded at their tips implies the tip growth mechanism. The segments of CNTs stretching out of the AAO nanopores still maintain relatively good alignment, and have a very slow growth rate, which allows us to obtain reproducible tube length by tuning the growth time. Field emission measurements of the CNTs showed derivable electron emission properties, attributed to their uniformity in size, good alignment, and good graphitization properties. Moreover, a simple method was proposed to control the tube number density of the Co-catalyzed CNTs on AAO template, which was realized by in situ regulating the flow rate ratio of CH4/H2 precursor gases during the CNT growth. The amorphous carbon byproduct of CNT growth was employed to confine the CNT outgrowth from AAO nanopores. There was a competition reaction between the CNT growth and the amorphous carbon deposition. It was found that the number of CNTs escaped from the AAO nanopores decreases linearly following the increase of the CH4 concentration. The field emission of the AAO assisted CNTs can be optimized by tuning the tube number density.