奈米接面系統的熱電效應

Abstract

我們利用第一原理計算的方式去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。我們分別模擬了碳鏈以及Br-Al-Al-Br兩種系統。我們發現在碳鏈系統中，Seebeck係數會隨著在電極中間的碳原子的數目不同其正負號會有奇偶效應。其中當碳鏈中碳數為奇數時Seebeck係數為負的，在碳數為偶數時Seebeck係數為正的。而奇偶效應的原因為：當碳鏈中的碳數增加時，穿隧方程式在費米能階附近的斜率會變號。而在Br-Al-Al-Br的系統中，我們計算了此系統的熱電效應以及能量轉換功率。因為這系統在費米能階附近擁有一個較尖銳的峰值，所以這系統擁有較大的Seebeck係數，利用這較大Seebeck係數以及溫差可將此系統視為一個電源供應器。我們另外將系統模擬成電晶體，發現改變閘極電壓可調控熱電效應所產生的電位差、電流以及熱電轉換效率。擁有這樣的I-V特性的奈米系統在電路設計上可當作電晶體或是開關。

Using first-principles approaches, we have investigated the thermoelectric properties of two different systems of nano-junction. The first system is Carbon-atoms chains connecting bimetal junction, and the other one is paired metal-Br-Al junction. In Carbon-atoms chains system, we observe odd-even effects in the Seebeck coefficient of carbon-atom chains as the number of carbon atoms increases. The Seebeck coefficients show minus signs for the odd-numbered atomic chains and plus signs for the even-numbered atomic chains in low temperatures regime. The reason for the odd-even effects is that the tangent of transmission functions near the chemical potential (Fermi level) change signs as the number of carbon atoms in the junction increases. In the paired metal-Br-Al junction, we have investigated the thermoelectric properties and energy conversion efficiency. Owing to the narrow states in the vicinity of the chemical potential, the nanojunction has large Seebeck coefficients such that it can be considered an efficient thermoelectric power generator.We also consider the nanojunction in a three-terminal geometry, where the current, voltage, power, and efficiency can be efficiently modulated by the gate voltages. Such current_voltage characteristics could be useful in the design of nanoscale electronic devices such as a transistor or switch.