Wu Kunlin, Bai Meilin, Sanvito Stefano, Hou Shimin
Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China.
School of Physics, AMBER and CRANN Institute, Trinity College, Dublin 2, Ireland.
J Chem Phys. 2014 Jul 7;141(1):014707. doi: 10.1063/1.4886378.
The transition voltage of vacuum-spaced and molecular junctions constructed with Ag and Pt electrodes is investigated by non-equilibrium Green's function formalism combined with density functional theory. Our calculations show that, similarly to the case of Au-vacuum-Au previously studied, the transition voltages of Ag and Pt metal-vacuum-metal junctions with atomic protrusions on the electrode surface are determined by the local density of states of the p-type atomic orbitals of the protrusion. Since the energy position of the Pt 6p atomic orbitals is higher than that of the 5p/6p of Ag and Au, the transition voltage of Pt-vacuum-Pt junctions is larger than that of both Ag-vacuum-Ag and Au-vacuum-Au junctions. When one moves to analyzing asymmetric molecular junctions constructed with biphenyl thiol as central molecule, then the transition voltage is found to depend on the specific bonding site for the sulfur atom in the thiol group. In particular agreement with experiments, where the largest transition voltage is found for Ag and the smallest for Pt, is obtained when one assumes S binding at the hollow-bridge site on the Ag/Au(111) surface and at the adatom site on the Pt(111) one. This demonstrates the critical role played by the linker-electrode binding geometry in determining the transition voltage of devices made of conjugated thiol molecules.
采用非平衡格林函数形式理论结合密度泛函理论，研究了用银和铂电极构建的真空间隔和分子结的跃迁电压。我们的计算表明，与之前研究的金 - 真空 - 金情况类似，电极表面带有原子突出的银和铂金属 - 真空 - 金属结的跃迁电压由突出部分p型原子轨道的局域态密度决定。由于铂6p原子轨道的能量位置高于银和金的5p/6p，铂 - 真空 - 铂结的跃迁电压大于银 - 真空 - 银和金 - 真空 - 金结的跃迁电压。当转而分析以联苯硫醇为中心分子构建的不对称分子结时，发现跃迁电压取决于硫醇基团中硫原子的特定键合位点。特别与实验结果相符的是，当假设硫在银/金(111)表面的中空桥位以及铂(111)表面的吸附原子位键合时，银的跃迁电压最大，铂的最小。这证明了连接体 - 电极键合几何结构在确定由共轭硫醇分子制成的器件的跃迁电压中所起的关键作用。
