- 學位論文
Stretching effect on the spin transport properties of single molecular junctions: A first-principle study
Stretching effect on the spin transport properties of single molecular junctions: A first-principle study
摘要
近年來隨著自旋電子學的蓬勃發展，對於「分子通道的傳導特性」的量測也有新的構想，像是將原本接在分子通道兩側的金屬電極換成具有磁性的金屬 (磁性電極/單分子/磁性電極)，以移動STM探針來達到拉扯分子的效果，使分子通道隨著拉扯過程逐漸斷裂成「單分子通道」。 在理論模擬的部分，我們選用的單分子通道為BDT(硫-C_6 H_6-硫)與ADT(硫-C_3 H_6-硫)兩種，並將其接在鈷電極(111)中間。為了模擬真實的實驗狀況，我們緩緩的增加右側電極與分子的距離，使其具有類似STM探針拉扯的效果，然後計算系統的能量直到收斂，重複上述循環直至分子通道斷裂。我們發現以π-鍵為主的C_6 H_6 與 以σ-鍵為主的C_3 H_6，在分子被拉伸的過程中，分子會影響其附近的原子並產生截然不同的結果。我們也藉由改變左側電極的磁矩方向，使系統成為平行(↑↑/單分子/↑↑)與反平行(↓↓/單分子/↑↑)的狀況，我們發現由於中間通道的差異，會影響自旋的注入，更值得注意的是C_6 H_6在平行與反平行的兩種情況下，電導會有巨大的差異，並產生顯著的磁阻。
並列摘要
The manipulation of spin transport properties in FM/single molecule/FM junctions has attracted intensive attentions due to their potential applications in molecular spin electronics, where FM denotes the ferromagnetic materials. In this study, we employ the first-principles calculation with the Keldysh Green’s function method [8] to calculate the spin transport properties of the σ-saturated Co/Alkanedithiols(ADT)/Co and the π-saturated Co/Benzenedithiol(BDT)/Co single molecular junctions. In order to simulate the single molecular magnetic junction in realistic experimental system [5], we stretch the junction by increasing the distance between two Co electrodes in small steps, optimize again, and continued to do so, until the junction is broken down. The calculated total energy, bond lengths, and bond angles conclude that the central molecule indeed plays an important role on the stretching process and the breakdown situation. Once we obtain the relaxed junction geometry under stretching process, we employ the DFT+NEGF+LDA calculation to calculate the spin-polarized transmission spectra and PDOS’s for both PC and APC situations. For both junctions, the dramatic variation from the highly spin-polarized transmission in PC to the non-spin-polarized transmission in APC indicates the possibility for the high MR value under bias. However, the more conductive BDT molecule enhances the spin transport probabilities near EF may be a promising candidate for the molecular spintronics application.