原子級分子線與金屬線奈米接面的熱導、電導與熱電性質比較

Abstract

我們利用密度泛函理論(Density Function Theory)結合非平衡格林函數(Non-Equilibrium Green's Function)的理論，並透過第一原理的計算方法來研究不同長度與結構的苯環(BDT)分子線接面與不同長度與粗細的金奈米線接面系統的電子的電導、電子與聲子熱導、與Seebeck係數，也透過古典非平衡態分子動力學模擬計算的方法來研究奈米系統的聲子熱導性質，同時也考慮了其在低溫時，基於Bose-Einstein distributions的量子修正，並綜合比較分線與分子線與金屬線接面的熱電轉換效率[熱電優值(ZT)]。首先，我們計算雙苯環的最佳化結構，並發現雙苯環單分子接面中苯環間夾角增加會導致電導上升。單苯環至五苯環不同長度苯環單分子線接面的電導會隨著苯環分子數增加而下降，而在金屬線的電導則不會因為長度增加而下降。而在分子線與金屬的接面熱導研究中發現溫度在低於得拜溫度下，原子線與金屬線的聲子聲子熱導與溫度呈現正比關係，而在高於得拜溫度時，因為聲子的振頻已經完全被激發，所以聲子熱導與溫度無明顯關係，而在分子線與金屬線的熱導比較中可以發現在分子線熱導是由聲子所主導，而在金屬線的熱導是由電子所主導，而在分子線與金屬線的熱電優值研究發現，奈米系統的熱電優值(ZT)的趨勢會與Power Factor一致。

Density Function Theory coupled to Non-Equilibrium Green's Function (NEGF-DFT) has been applied to study the electrical conductance, electron thermal conductance, and the Seebeck coefficient for various lengths and different structures of benzene ring (BDT) single-molecule junctions and gold nanowires systems. Classical non-equilibrium molecular dynamics simulation (NEMD) has been applied to investigate the phonon thermal conductivity of these two nanometer scale system, whereas quantum correction based on Bose-Einstein distributions at low temperature has also been considered to correct the phonon thermal conductance in low-temperature regime. We further investigate and compare their thermoelectric figure of merit (ZT).Firstly, we optimize the structure of the di-benzenedithiol(DBDT), and investigate the dependence of the conductance on the angle between two benzene rings. It was found that the conductance of the DBDT increase with the rising of the angle. We find that the thermal conductance of DBDT single-molecule junctions and gold nanowires junction systems are proportional to temperature in the low-temperature regime. The phonon has been completely excited at temperatures larger than the Debye temperature, so the phonon thermal conductivity and temperature no significant relationship. In contrast to the fact that the phonon’s thermal conductance increases as the temperature increases in low-temperature regime. We observe that the phonon’s thermal current dominates the thermal current in the DBDT single-molecule junctions, while the electron’s thermal current dominates the thermal current in the gold nanowires. The trends of thermoelectric figure of merit (ZT) and the Power Factor with respective to temperature are similar in both the DBDT single-molecule junctions and gold nanowires.