利用電泳法將單壁碳奈米管擇區沉積在各種基材及其性質探討

Abstract

本研究中，利用電泳沉積法及後續的退火處理將單壁碳奈米管均勻的選擇性沉積在各種材料之圖案基板上之製程已成功被開發。實驗的製程參數包括：電極材料種類、電泳懸浮液成份、陽極與陰極間之距離、外加電壓、沉積時間及退火之溫度與時間。本研究所採用的陽極與陰極基板材料有：鍍鋁、鈦及二氧化矽(SiO2)之矽晶片、純矽晶片、導電玻璃(ITO)及鋁箔。電泳懸浮液之成份有：電弧放電法製備之單壁碳奈米管、去離子水、作為電解質之硝酸銅(Cu(NO3)2)及多種界面活性劑(作為分散劑)包括SDS (Sodium dodecyl sulfate)、CTAB (Hexadecyl trimethyl ammonium bromide)及TOPO (Trioctylphosphine oxide)。藉由紫外-可見光吸收光譜技術(UV-visible absorbance spectroscopy)進行懸浮液之穩定性分析，單壁碳奈米管及沉積物之結構與性質將利用場發射掃描式電子顯微技術(FESEM)、高解析度穿透電子顯微技術(HRTEM)、拉曼光譜技術(Raman spectroscopy)、能量散佈光譜技術(EDS)及場發射J-E測量法加以分析。從實驗結果可歸納出以下結論： 為了得到組成及厚度均勻性佳的電泳沉積物，結果顯示所使用之電泳懸浮液必須具備分散性佳及穩定性好的優點。根據紫外-可見光吸收光譜儀的結果指出，其穩定性依序為：SDS > TOPO > CTAB，歸因於碳奈米管與分散劑SDS之間的凡得瓦爾力大於碳管之間的凡得瓦爾力。就在懸浮液中添加電解質增加懸浮液的導電性對電泳沉積之影響而言，結果指出添加硝酸銅確實可以降低電泳沉積所需要之最小電壓及電泳沉積的時間。 在擇區沉積碳奈米管圖案(pattern)的製備方面，其可行性可以藉由均勻沉積對電極材料導電性之高相依性而獲得。實驗結果顯示，在研究的電極中，鍍鋁之矽晶基材由於具最佳導電性所以功能亦最佳，這項結論可從電泳沉積時有最小外加電壓而得到。換句話說，利用具有導電及不導電材料組成之圖案基板可以使碳奈米管薄膜選擇性沉積在導電部位，例如鍍鋁相對於鍍二氧化矽之基材在本研究中已成功地被證實。另外，實驗結果亦指出陽極與陰極間之距離、外加電壓及沉積時間等製程參數，除了在薄膜厚度或沉積速率方面外，對薄膜之形貌並沒有顯著的影響。 就後續退火處理製程對薄膜與基材間之附著性的影響而言，實驗結果指出附著性、薄膜裂縫、薄膜碎片捲曲之情形與加熱的溫度及時間有關。隨退火時間的增加，薄膜首先形成網狀裂縫，接著裂縫會擴展，最後在裂開薄膜邊緣位置向上捲曲，隨著溫度增加，這樣的情形會越容易在短時間內發生。另一方面，薄膜附著性可以經由提高退火溫度至本研究之最高溫度(~300oC)而得，但是最佳的附著性會出現在中間的退火時間。換言之，根據膠帶黏貼及刮痕測試的結果，本研究在300oC、5分鐘時可以得到最佳的附著性。值得一提的是，在退火處理過程中所產生的裂縫內有很多橫向排列的單壁碳奈米管，這提供了一個量測碳管各別性質的可能性。 在場發射方面，實驗結果顯示電泳參數對薄膜的場發射特質並沒有太大影響，倒是退火溫度及時間對薄膜之場發射特性有較顯著之影響，起始電場大約在2.4 V/□m ~3.5 V/□m之間，優於目前文獻中利用電泳法所沉積之碳奈米管薄膜的起始電場( > 4.0 V/□m)，當外加電場為5.0 V/□m時其電流密度大約在0.7 mA/cm2∼16 mA/cm2之間，最好的場發射性質是屬於薄膜具有最佳附著性以降低場發射阻抗。此外，鋁基材上電泳沉積之含單壁碳奈米管薄膜與表面塗佈螢光粉之導電玻璃(ITO)螢光板作成場發射元件的發光測試結果，發光區域均勻且穩定。 將電泳沉積製程應用於其他具有不同形狀之基材，例如導電玻璃或鋁箔紙，結果顯示即使在起皺之鋁箔上電泳沉積之薄膜仍然具有良好的場發射能力(起始電場約為2.7 V/□m、外加電場5.0 V/□m下的電流密度約為4.2 mA/cm2)及附著性。總括來說，電泳沉積的製程具有：室溫即可進行之簡單製程、製程費用便宜、量產及大面積沉積之可行性以及幾乎不受限於基材的形狀等優點。

In this work, processes of using the same materials at anode and cathode electrodes to selectively deposit the single walled carbon nanotubes (SWNTs) on various substrate patterns at the anode side with good uniformity were successfully developed by electrophoresis deposition (EPD) method and post air annealing treatment. The process parameters include electrode materials, suspensions compositions, applied voltage, anode-cathode distance, deposition time, annealing temperature and time. The investigated anode and cathode electrode substrates were Al-, Ti-, SiO2-coated or pure Si wafers, ITO glass and pure Al-foil. The electrophoresis suspensions were made of the arc-discharge-prepared SWNTs, D.I. water, Cu(NO3)2 electrolyte and various surfactants (acting as dispersants), including SDS (sodium dodecyl sulfate), CTAB (hexadecyl trimethyl ammonium bromide) and TOPO (trioctylphosphine oxide). The stability of suspensions was characterized by UV-visible absorbance spectroscopy. Structures and properties of the SWNTs or the deposited films at each processing step were characterized by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, energy dispersive spectroscopy (EDS) and field emission J-E measurements. From the experimental results, the following conclusions can be drawn. In order to form the deposit with good uniformity in thickness and compositions, the results show that the suspension with good stability and well dispersion is necessary. In terms of the stability of the suspensions as indicated by UV-visible absorbance spectroscopy, it is in the order of suspensions with SDS > TOPO > CTAB, due to a greater Van der Waals force between CNTs and SDS dispersant than between tubes. Regarding effect of suspension compositions on its conductivity by adding additional electrolyte ( in this work, ~ 0.08 wt.% of Cu(NO3)2), the results indicate that the additional electrolyte could give a better function to reduce the minimum applied voltage and electrophoresis time. On selectivity to fabricate the CNTs pattern, the feasibility can be obtained by finding a strong dependence of uniform deposition on the conductivity of electrode material. The Al-coated Si wafer demonstrates the best function due to its best conductivity among the investigated electrode materials, which can be concluded from its minimum applied voltage of electrophoresis deposition. In other words, the selective area deposition to fabricate pattern made of conductive and nonconductive substrates, such as Al versus SiO2 substrates in this work, has been successfully demonstrated. The results also appeal that the anode-cathode distance, applied voltage and deposition time have no significant effects on morphologies of the deposits, except on its thickness or deposition rate. Regarding effect of the post annealing treatment on the film-substrate adhesion, the results indicate that the adhesion, film cracking and curling are a function of annealing temperature and time. With progress of annealing time, the films start to form more net-like cracks, crack enlarging and then curling at the cracked areas, which can occur in shorter time at higher temperature. On the other hand, the film adhesion can be increased by increasing the annealing temperature up to the maximum investigated temperature in this work (300oC), but it shows a maximum value at some intermediate annealing time. In other words, the best adhesion in this work is found at 300oC for 5 min, basing on results of the tape-pulling and scratching tests. It is interesting to note that lots of the horizontally oriented SWNTs across the crack straits, which may provide the opportunity to measure the individual properties of the tubes. Considering the field emission, it is found that the EPD parameters have no significant effects on its properties. However, the annealing temperature and time have a notable influence on their field emission properties, where the turn-on field strength vary from 2.4 V/□m to 3.5 V/□m, which are much better than the reported values in the literature by EPD process ( > 4.0 V/□m) and the current density vary from 0.7 mA/cm2 to 16 mA/cm2 at an applied field of 5.0 V/□m. The best field emission properties are the film with the best adhesion to minimize the emission resistance, which is annealed at 300oC for 5 min. The device luminance experiment of phosphorus-ITO screen versus the EPD-deposited SWNTs films on the Al-coated substrate did show steady emission brightness. On feasibility to apply the EPD process on other substrates with different shapes, such as ITO or the wrinkled Al-foil, it demonstrates a good performance in emission properties (turn-on field strength ~ 2.7 V/□m and current density approximately 4.2 mA/cm2 at an applied field of 5.0 V/□m ) and adhesion even on the wrinkled surface. In summary, the novel EPD process possesses the following advantages: room temperature simple low cost process, high possibilities for mass production and large area deposition, almost no limitations on the shapes of the substrates.