探討Ni-DNA記憶電容與熱電效應

Abstract

DNA為一種長度可調的分子奈米線，由於其導電性不佳，因此加入二價金屬離子形成可導電之金屬離子DNA，譬如Zn-DNA、Co-DNA和Ni-DNA等。本實驗使用Ni2+離子螯合而成的Ni-DNA奈米線，利用此奈米線製作電子元件，來探討記憶電容特性與熱電效應。 由於Ni-DNA中鎳離子的氧化還原反應，在循環電壓掃描 (cyclic-voltage scan)中，正電壓可將Ni2+氧化成Ni3+，反之負電壓將Ni3+還原成Ni2+，因此可藉由不同極性的正、負偏壓來控制Ni-DNA中Ni2+與Ni3+離子數目。Ni-DNA元件可用一模擬電路來描述其氧化還原狀態，而模擬電路中的可變電容可藉由外加偏壓改變狀態並儲存電荷，所儲存的電荷可穩定持續一段時間。即使改變外加偏壓寫入的時間，電荷依舊穩定且長時間存在，可確認Ni-DNA具有記憶電容的特性。 我們共同合作研究Ni-DNA的團隊中，理論計算預測Ni-DNA有高的熱電轉換效率，即較高的席貝克係數 (Seebeck coefficient)。本實驗中以二倍頻技術與金屬電阻溫度測量方式來量測Ni-DNA的熱電效應，結果發現Ni-DNA所產生的熱電轉換效率值約為310 mV K-1，此數值略高於理論計算所預測的數值，推測是實驗中使用的元件所包含之Ni-DNA數目較理論預測多的緣故。此外，我們藉由不同極性電壓控制Ni-DNA中Ni2+與Ni3+的離子濃度，發現Ni-DNA的熱電轉換效率因2價3價Ni離子濃度改變而下降。

DNA is one kind of molecular nanowires whereas its length is adjustable. Due to its poor conductivity, metallic DNA nanowires was made by chelating divalent metal ions, such as Zn2+, Co2+, Ni2+, etc. In this work, we’ll use the DNA chelated by nickel ions to explore memory capacitance and thermoelectric effect in the Ni-DNA nanowires. The measurement of current response in a cyclic-voltage scan exhibits the redox reaction between Ni2+ and Ni3+ within Ni-DNA base pairs. As a result, the redox of Ni ions can be controlled by applying different polarity of voltages. The Ni-DNA nanowires could be simulated by analog circuits. When a high bias voltage from 2 to 4 V is applied at Ni-DNA for a certain period of time, the Ni-DNA show variable capacitance and charge storage features. The stored charges are maintained for a long time and they can be read by a small bias voltage of 0.1 V. The Ni-DNA nanowire device demonstrates characteristics of a memory capacitor. Our theoretical cooperators calculate thermoelectric properties of Ni-DNA nanowires and predict a high thermoelectric conversion efficiency (Seebeck coefficient) Here we try experimentally to confirm their calculations. We design a pattern of electrodes for making Ni-DNA thermoelectric devices to measure thermoelectric power. The Seebeck coefficient is estimated to be about 310 mV K-1 which is much higher than theoretical predictions. It is argued that the high Seebeck coefficient is owing to a large bundle of Ni-DNA nanowires in our experiments whereas the theoretical calculations only predict the coefficient of small bundle of Ni-DNA nanowires. In addition, we apply a high voltage to change the ratio of concentration between Ni2+ and Ni3+ ions and we observe a decrease of the Seebeck coefficient.