奈米元件的熱電性質

Abstract

應用第一原理計算的方法，我們研究了分子熱電元件在趨近零外加偏壓和有外加偏壓情兩種不同況下的熱電性質。在利用butanethiol為實驗對象下，我們發現有用氨基取代的butanethiol和沒有取代的butanethiol，它們的Seebeck 係數會呈現非常大的不同。而且若把閘極電壓加下去我們還能調控Seebeck 係數使它達到最佳化。在有用氨基取代的butanethiol下我們發現調控外加偏壓和閘極偏壓可以改變Seebeck 係數的正負號，這意味著我們可以改變分子熱電元件的導電狀態，從N型到P型。除了Seebeck 係數以外，整個熱電元件的轉換效率也是我們所關心的。熱電轉化效率重要的指標為ZT，我們在趨近零外加偏壓和兩邊電極溫度差趨近於零的情況下對碳原子線和鋁原子線的系統做了一些研究。我們發現當電極溫度遠小於特徵溫度T_0=√(β/(γ(l)))時ZT∝T^2；相對的，當電極溫度遠大於特徵溫度時ZT會趨於飽和。而且ZT和原子線長度是有相依性的並不會和長度(原子數目)無關。鋁原子線越長ZT就越大；相反的，碳原子線越長ZT就越小。這應該是和一個是導體一個是非導體有很大的關係。同時我們在研究Seebeck 係數和ZT也注意到鋁原子線有很強的元件性質(跟元件形狀有關)而碳原子線則有部分的元件性質(Seebeck 係數和碳原子線長度無關)。這是個有趣的現象，我們希望能藉著更多的研究數據來找出其背後更多新的物理。

A field-theoretic theory combined with first-principles approaches is presented for the thermoelectricity in molecular junctions formed by a single molecule contact. The study compares the Seebeck coefficient in the amino-substituted and unsubstituted butanethiol junction and observes interesting thermoelectric properties in the amino-substituted junction. Due to the novel states around the Fermi levels introduced by the amino-substitution, the Seebeck coefficient could easily be controlled using gate voltages and biases. When the temperature in one of the electrodes is fixed, the Seebeck coefficient varies pronouncedly with the temperature in the other electrode, and such dependence could be enhanced by varying gate voltages. At large biases, richer features in the Seebeck coefficient are observed, which are closely related to the density of states in the vicinity of the left and right Fermi levels. In addition, we investigated the efficiency of energy conversion in nanojunctions, described by the thermoelectric figure of merit ZT. We obtained the qualitative and quantitative descriptions for the dependence of ZT on temperatures and lengths. A characteristic temperature: T_0=√(β/(γ(l))) was observed. When T≪T_0, ZT∝T^2. When T≫T_0, ZT tends to a saturation value. The dependence of ZT on the wire length for the metallic atomic chains is opposite to that for the insulating molecules: for aluminum atomic (conducting) wires, the saturation value of ZT increases as the length increases; while for alkanethiol (insulating) chains, the saturation value of ZT decreases as the length increases. We show that the thermoelectric quantities in nanojunctions unnecessarily display material properties. The Seebeck coefficient and the thermoelectric figure of merit are shows that metallic atomic chains reveal strong junction properties while the insulating molecular wires partially possess the material properties.