本篇論文利用第一原理的計算模擬分子接面的電子傳輸性質,以 及計算出系統的熱電效應與電流感應力,分別為兩種系統:碳鏈系統 與苯環分子直接與鉑電極連接的奈米接面系統。
在碳鏈系統中, Seebeck 係數會隨著在電極中間的碳原子數目不 同其正負號會有奇偶效應。絕對值會隨著碳鏈中碳原子數的而有上下 起伏的現象,碳原子數為奇數時的絕對值較高、偶數時較低,以及正 負號會隨著碳原子的增加而改變。這種現象在一般塊材中並不存在,
一般塊材在同個溫度下只會有一種 Seebeck 係數,而在碳鏈系統,卻 會因碳數不同而改變 Seebeck 的值,算是一種特別的物理性質變化。
而奇偶效應的原因為:在碳鏈系統中,每增加一個碳原子會提供 四個自由電子,其中兩個電子增加到整個系統中的 core state,另外兩 個則填到了系統費米能階附近的軌域上,進而造成態密度在費米能 階附近的軌域將會呈現填滿與半填滿相間隔,因此造成穿隧方程式 在費米能階附近的斜率會變號。
在苯環分子直接與鉑電極連接的奈米接面系統中,我們發現此不 對稱系統的在苯環上方的電流感應力會略大於下方感應力,使其產生 如水驅動水車旋轉般的奈米分子轉子,而電流感應力的產生是由
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Direct force 跟 Wind force 所組成,Direct force 是由電壓差所產生的 靜電力,而 Wind force 則是由電流所攜帶的電子風感應來驅動苯環分 子產生旋轉。
苯環在以 z 方向為軸旋轉會更易於以 n 為軸,最主要的原因可由 鍵結原理得知,我們知道共價鍵是具有方向性的,而金屬鍵上則是不 具有方向性,所以從理論上來推斷:此分子接面系統旋轉 z 方向的金 屬鍵會相較於旋轉法線 n 方向的鉑(Pt)與苯環中的碳(C)所鍵結的共價 鍵來的容易。
最後我們觀察此系統中,在苯環以 z 方向旋轉所需的力矩雖還是 略為大於感應所得的力矩,但我們認為當電壓接近 0 時,則不對稱苯 環旋轉所需的力矩將會小於感應所得的力矩,使整個分子接面系統能 夠完整的旋轉 360 度,而成為一個由電流所驅動的分子轉子。
分子轉子的產生,對於製作奈米機器人的夢想更往前了一步,其 廣泛的用途,例如可用於組裝其他機械,或是在人體內移動、投送藥 物或進行微型手術。因此分子轉子的前景是非常誘人的。我們也希望 透過這些模擬研究,可以協助實驗未來在製作奈米尺度元件上會有幫 助。
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