Theoretical studies of light emission from molecular junctions

电子与集体电磁模式的相互作用以及这些模式与光子的耦合为涉及局部等离子体模式的非弹性电子隧穿激发的尖端表面相互作用提供了独特的探针，并已成为桥接纳米电子学和等离子体学的重要课题。由于表面等离子激元极化子的阻尼，stm诱导的光发射显示出光发射强度与光频率电流波动之间有很强的相关性。然而，分子的作用以及它如何影响stm诱导的光发射仍然存在争议。我的主要目标是描述与应用偏压相比，在较低光子能量下分子结中的等离子体光发射，以及在光子能量明显超过应用偏压时发射的光。前者将通过探索不同量子模型的频率相关非对称电流-电流相关函数的特征来研究。虽然我将为一般的多层系统制定我的理论，但我将在概念研究中集中于单层和双层模型。两层模型特别有趣。它们构成了分子连接的最小模型，它们既可以处理范诺共振，也可以描述通过显性HOMO和LUMO水平的传输。此外，我将基于Keldysh非平衡格林函数（NEGF）形式主义，发展分子结等离子体光发射的微观理论，考虑到能量依赖的传输，并考虑单能级和双能级输运，特别是解释实验观察到的阈值以上发射。特别是，三电子和四电子散射过程的可能性以及由于这些高阶修正而产生的相关自能图将被确定。最后，利用多尺度量子力学/电磁学（QM/EM）方法，定量计算了这些结的散粒噪声特性和发光率。为此，使用DFT方法结合NEGF对QM区域进行模拟，而EM区域则通过实现麦克斯韦方程进行模拟。QM和EM计算通过两个区域之间的边界条件通过接口相互通信，并迭代求解，直到达到自一致性。

The interaction of electrons with collective electromagnetic modes and the coupling of these modes to photons provide a unique probe for tip-surface interactions involving inelastic electron tunneling excitations of local plasmon modes and has become an important topic bridging nanoelectronics and plasmonics. STM-induced light emission due to the damping of the surface plasmon polaritons revealed a strong correlation between the light emission intensity and the current fluctuations at optical frequencies. However, the role of the molecule and how it effects the STM-induced light emission remains debated.My main objective is to describe the emission of plasmonic light in molecular junctions at lower photon energies compare to the applied bias as well as the emitted light at photonenergies which exceed the applied bias significantly . The former will be investigatedby exploring the characteristics of frequency-dependent non-symmetrized current-current correlation function for different quantum models. While I will formulate my theory for a general multilevel system, I will in conceptual studies concentrate on single and two-level models. The two-level model are of particular interest. They constitute a minimal model for a molecular junction, they can treat both Fano resonance and describe transport through dominant HOMO and LUMO levels.Furthermore, I will develop a microscopic theory for plasmonic light emission in molecular junctions based on the Keldysh nonequilibrium Green's function (NEGF) formalism taking into account an energy-dependent transmission and by considering single level and two-level transport, especially to explain the experimentally observed above-threshold emission. Particularly, the possibility of three- and four-electron scattering processes and the relevant self-energy diagrams due to these higher order corrections will be identified.Finally, using a multiscale quantum mechanics/electromagnetics (QM/EM) method, the shot noise characteristics and the light emission rate of such junctions will be calculated quantitatively. To this aim, the QM region is simulated using the DFT method in combination with NEGF, while the EM region is simulated by implementation of Maxwell's equations. QM and EM calculations communicate with each other through the interface by means of boundary conditions between the two regions and are solved iteratively until self-consistency achieved.