金属弦单分子器件中构象可控的非弹性电荷输运和散粒噪声行为
Conformation Controllable Inelastic Charge Transport and Shot Noise Behavior in Metal-String Single Molecular Devices
摘要：
它往往是有趣的实验，以评估如何在设备配置的构象变化可能会影响单分子结的整体电荷传输行为。基于密度泛函理论和非平衡态格林函数理论，我们研究了结的异质性对各种金属串单分子器件中非弹性电荷输运的影响.组成的活性元素是敏感的共振能级，过渡金属中心被发现影响拉伸，弯曲和扭转的激发模式，而摇摆和剪切模式主要由轴向配体控制。对于特定的分子构象和电极取向，声子辅助的量子干涉效应可能会突然出现，导致高波数振动模式的抑制。由此产生的非弹性光谱很可能采取占主导地位的Fano共振或反共振的形状，这取决于声子是发射还是吸收。这种纳米级量子干涉效应尤其表现在那些金属弦分子结中，对于这些金属弦分子结，能隙（在定域虚态和离域虚态之间）完全位于光学声子能量（$Δ{E}_{|HOMO-LUMO|} <$ 40 meV）内。如果通道间通过前线轨道的隧穿过程伴随着声子的吸收或发射，那么单分子散粒噪声可以表现出近似泊松行为。因此，我们的研究结果表明，跨金属串复合物的电荷传输特性可以潜在地通过金属中心的选择性结构来调节，并且还可以通过纳米级电极的优选取向来构建具有理想可控性的分子器件。
英文摘要：
It is often intriguing experimentally to take stock of how conformational changes in the device configuration may impact the overall charge transport behavior of single-molecule junctions. Based on the allied approach of density functional theory and non-equilibrium Green's function formalism, we explore here the effect of junction heterogeneity on inelastic charge transport in various metal-string based single-molecule devices. The constituent active elements being sensitive to the resonant levels, transition metal centers are found to influence stretching, bending, and torsional excitation modes, while rocking and scissoring modes are controlled largely by the axial ligands. For certain molecular conformations and electrode orientations, phonon-assisted quantum interference effect may crop up, leading to the suppression of higher wavenumber vibrational modes. The resulting inelastic spectra are likely to take the shape of dominant Fano resonance or anti-resonance, depending on whether phonons are emitted or absorbed. Such nanoscale quantum interference effect is manifested especially in those metal-string molecular junctions for which the energy gap (between localized and delocalized virtual states) lies well within the optical phonon energies ($Δ{E}_{|HOMO-LUMO|} <$ 40 meV). It also turns out that single molecular shot noise can exhibit nearly Poissonian behavior if the inter-channel tunneling through frontier orbitals is accompanied by phonon absorption or emission following a slow relaxation process. Our results thus suggest that charge transport properties across metal-string complexes can be potentially tuned by selective architecture of the metal centers and also, by preferred orientation of nanoscale electrodes in a bid to build up molecular devices with desirable controllability.
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