奈米高分子薄膜接枝於奈米碳管在奈米複材內之導電性質研究

Electric Conductivity of Polymeric Nanolayers Grafted on Multi-walled Carbon Nanotubes in Nanocomposites

黃才銘(Tsai-ming Huang)

指導教授 ： 楊長謀

國立清華大學/工學院/材料科學工程學系/碩士(2010年)

摘要

本研究藉由接枝共軛高分子P3HT (poly(3-hexylthiophene))的方式，對奈米碳管進行表面改質，增加碳管的分散性，並利用反應條件的控制，合成出不同包覆厚度的奈米碳管接枝P3HT。改質過後的奈米碳管與PS (polystyrene)混合製成奈米複合薄膜，以上下平板電極與四點探針量測其複合材料的電阻率。結果發現，垂直膜面方向的電阻率較平行膜面的電阻率高出約三個數量級，此外垂直方向的臨界展透值 (percolation threshold)也較水平方向高出許多。這是由於碳管在複合薄膜內以平行膜面的方向排列，因此當電子在垂直膜面的方向傳遞時，其所需克服的高分子障礙比水平方向多，電阻也較高。同時也由於奈米碳管在薄膜中的排列，導致碳管在垂直方向比水平方向更難形成展透團簇 (percolation cluster)，臨界展透值也相對較高。
我們更進一步利用波茲曼分佈(Boltzmann distribution)以及單位投影長度來描述奈米碳管的排列狀況以及計算碳管接觸點的數目，並藉由垂直與水平方向接觸電阻(contact resistance)的逼近，將不同P3HT包覆厚度碳管的接觸電阻及接枝的P3HT電阻率估算出來。從計算的結果發現，隨著包覆厚度的減少，接枝的P3HT電阻率成指數下降，且其值皆小於P3HT塊材的電阻率。同時我們也利用旋轉塗佈的方式製備不同厚度的P3HT薄膜，並藉由上下平板電極量測其電阻率隨厚度的變化。量測的結果發現，當厚度小於約100nm時，薄膜電阻率也隨著厚度的降低而成指數下降，同時其電阻率也較塊材低。
這是由於高分子在薄膜狀態時，Miller-Abraham的電子躍遷機制(electron hopping mechanism)主導電子在薄膜中的導電行為：隨著電子傳遞距離的縮短，形成導通路徑的機率成指數上升。因此當厚度降低時，薄膜的電阻率會成指數型下降，且較塊材的電阻率低。此外，奈米碳管與P3HT包覆薄膜之間的交互作用力，可能影響P3HT在碳管上的構形及排列，導致接枝P3HT的侷限狀態(localization condition)受到改變，因此造成其電阻率與薄膜厚度的相依性和旋轉塗佈製成的P3HT薄膜不同。
藉由接枝厚度對於電阻率影響的研究，我們了解在包覆碳管的薄膜中，主導電性行為的傳導機制。這有助於未來奈米碳管應用複材 (例如：太陽能電池、發光二極體等) 的光電性質相關研究，使奈米元件的效率得以改善。

關鍵字

奈米碳管 ； 接觸電阻 ； 高分子複合材料 ； 薄膜電阻率

並列摘要

In this study, multi-walled carbon nanotube (MWCNT) was modified by surface grafted conjugated polymer, poly(3-hexylthiophene) (P3HT) to improve the dispersion of MWNCT. By controlling the reaction condition, different coating thickness of P3HT on MWCNT was synthesized. The functionalized MWCNT was blended with polystyrene (PS) to make nano-composites for electric resistivity measurement with 4-point probe and parallel electrodes. The resistivity of composites for the vertical direction to film surface was three orders higher than it for horizontal direction, and the percolation threshold of vertical direction was also higher than it of horizontal direction. Due to the horizontal orientation of MWCNTs in composite film, electron needs to overcome more polymer barriers than it transports along horizontal direction. Also because of the orientation of MWCNTs, the percolation cluster is more easily formed in the horizontal direction than vertical direction. Therefore, the percolation threshold of vertical direction is higher than it of horizontal direction.
Boltzmann distribution and unit projected length were used to describe the orientation status of MWCNTs and estimate the number of contact points. By approximation from perpendicular and parallel direction of composite with percolation model, the contact resistance and the resistivity of grafted P3HT for different coating thickness can be estimated. From the calculation result, the resistivity of P3HT coating exponentially decreased with reducing coating thickness and the resistivity of grafted P3HT was lower than it of bulk. Different thickness of P3HT thin film was also prepared by spin coating and measured the resistivity by parallel plane electrodes. For the spin-coated P3HT thin film, the resistivity also exponentially decreased with reducing thickness from 100nm to 5nm and the resistivity was also lower than it of bulk.
The low resistivity of P3HT thin film is because that the electric conductivity will be mainly dominated by the microscopic electron transport mechanism, the Miller-Abraham hopping theory, in the thin polymer film. The possibility of forming a percolation route for electron transport exponentially increases with reducing transport distance, thus the resistivity will exponentially decrease with reducing film thickness and the resistivity is lower than it of bulk. Since there are an interaction force between MWCNT and grafted P3HT film, the conformation or arrangement of P3HT may be affected and thus the localization situation of grafted P3HT will be different from spin-coated P3HT film.
By studied the thickness effect of resistivity, the mechanism which dominates the conduction behavior can be figured out. This will aid the photoelectric research of applied CNT composites such as solar cell and light-emitting diode, and improve the efficiency of composite device.

並列關鍵字

carbon nanotubes ； contact resistance ； polymer composites ； resistivity of thin films

參考文獻

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