三核鈷金屬串錯合物[Co3(dpa)4(NCS)2]的自旋電子傳輸機制

Abstract

本研究探討三核鈷金屬串錯合物[Co3(dpa)4(NCS)2]的自旋電子傳輸行為，討論磁場對「電極-單分子-電極」量測平台測得的單分子導電值的影響。量測方法是掃描式穿隧顯微術斷裂接合法(scanning tunneling microscopy-based break junction, STM BJ)，檢視的量測平台的電極材料有金/金和金/鎳兩種組合，前者的導電值不受磁場的影響，後者會因強力磁鐵的磁力方向而有兩個數量級的差異。非平衡態格林函數-密度泛函理論(non-equilibrium Green's function-density functional theory, NEGF-DFT)計算電子穿透能譜(transmission spectrum)的結果顯示零偏壓的β-spin電子穿透峰(transmission peak)與電極的費米能階相距0.03 eV，小於α-spin的0.35 eV，可見主導傳輸的是β-spin電子。論文中以能態密度(density of states, DOS)的簡化模型解釋導電值量測的自旋選擇效應：磁力方向影響鎳電極的DOS與β-spin電子穿透峰能量；此外，金元素之自旋-軌域耦合(spin-orbit coupling, SOC)與金屬串分子磁矩非定域化的兩者性質共同造成金電極能態密度自旋極化、DOS分裂。上述以DOS圖像描繪出的幾個機制，其加乘的結果定性地描述了自旋選擇效應。本研究呈現金屬串錯合物的磁性所致的電性表現，顯示金屬串錯合物作為自旋電子元件的可能性。

The conductance of single-molecule junctions for a tricobalt metal-string complex [Co3(dpa)4(NCS)2] was investigated under magnetic influence and measured by scanning tunneling microscopy-based break junction (STM BJ) technique. The metal-molecule-metal (MMM) junctions were configured by Au- or Ni-tips of the STM and a piece of Au substrate. For the Ni-Au pair of electrodes, the junction conductance was associated with whether the magnet’s north or south pole faced the Ni-tip. For the control experiments using Au-Au electrodes, the magnetic field had little effect on conductance. Spin-polarized transmission spectra were calculated by non-equilibrium Green's function-density functional theory (NEGF-DFT) and showed a transmission peak of β-spin closer to the electrode Fermi energy than that of the α-spin one, indicating that the magnetic and tranpsport properties of the molecule were dominated by β-spin electron. Proposed herein is a DOS-based model (density of states) in which the magnetized nickel electrode influenced the energy of β-spin transmission peaks. Additionally, the spin-orbit coupling (SOC) effect from gold and the spin delocalization of the molecule led to spin polarization and the consequent DOS splitting in the gold electrode. In conclusion, this study shows the electron-transporting behavior across Au-[Co3(dpa)4(NCS)2]-Ni junctions under the influence of a 0.22-Tesla magnet.