Theoretical Investigations on Transition Voltage Spektroscopy (TVS)

本项目主要从事跃迁电压谱（TVS）的理论研究。TVS是Frisbie等人最近提出的一种分子电子学实验工具，旨在从Fowler-Nordheim （FN）图的最小值（跃迁）电压确定相对于金属费米能级的分子轨道的相对能量排列。FN图直接由测得的I-V曲线得到。轨道能量偏移是分子输运的关键参数，它控制着分子的电荷转移效率。在与两个实验小组的密切合作下，本项目计划开展的工作将试图为TVS提供坚实的理论基础。为此，需要一种基于具有明确物理意义的分子轨道（与Kohn-Sham轨道相反）的分子输运方法。本项目的主要方法论目标是开发和实施一种从头开始（不基于密度泛函理论（DFT））的方法，该方法依赖于外价格林函数（OVGF），这是一种成熟的量子化学方法。它得到了量子化学40年工作的支持，证明了外层价区可以在单粒子图像中精确描述。这种基于包含后hf （Hartree-Fock）电子-电子相互作用效应的穿衣准粒子的从头算输运方法不仅对TVS而且对一般的分子输运理论都有意义。在实现基于ovgf的传输方法时，将利用Cholesky分解提供的计算优势，它能够以高质量的基集和合理的计算成本计算大分子。TVS的基本想法是为实验家提供一个简单的工具来直接确定轨道能量偏移，而不需要求助于复杂的传输计算。只有利用足够简单的模型才能实现这一实际目标，这些模型的参数可以从实验数据中提取出来，并通过微观计算进行验证。现实的模型将被提出，以扩展纽恩斯-安德森模型，该模型已被用于最近的研究，并被证明成功地解释了一系列的电视实验结果。本项目设想的扩展将包括锚定群和电极功功能的影响，它们对过渡电压的重大影响已在最近的实验研究中得到证明。随机波动对二维过渡-电压-电导直方图的影响（由于Tao等人最近的实验成就而变得可用）以及具有软自由度的分子的TVS的内部重组也将构成重要的研究主题。

The project is devoted to the theoretical investigation of the Transition Voltage Spectroscopy (TVS). TVS is an experimental tool of molecular electronics proposed recently by Frisbie et al, which aims at determining the relative energetic alignment of the molecular orbitals with respect to metal's Fermi level from the (transition) voltage at the minimum of the Fowler-Nordheim (FN) diagram. The FN diagrams are directly obtained from the measured I-V curves. The orbital energy offset is a key parameter for molecular transport, because it controls the charge transfer efficiency. In close collaboration with two experimental groups, the work planned in this project will attempt to provide a firm theoretical foundation of TVS. To this aim, an approach of molecular transport based on molecular orbitals with a clear physical meaning (as opposed to Kohn-Sham orbitals) is needed. The main methodological goal of this project is to develop and implement an ab initio (not based on Density functional theory (DFT)) approach to molecular transport relying upon the outer valence Green's functions (OVGF), a well-established method of quantum chemistry. It is backed by four decades of work in quantum chemistry, demonstrating that the outer valence region can be accurately described within a single-particle picture. Such an ab initio transport approach based on dressed quasi-particles that incorporate post-HF (Hartree-Fock) electron-electron interaction effects is not only of interest for TVS but also for the molecular transport theory in general. In implementing the OVGF-based transport approach, the computational advantage offered by the Cholesky decomposition will be exploited, which enables calculations for large molecules with high-quality basis sets and reasonable computing cost. The basic TVS idea was to provide experimentalists with a simple tool to directly determine the orbital energy offset without the need to resort to sophisticated transport calculations. This practical goal, at which the project also aims, can be achieved only if sufficiently simple models can be utilized, whose parameters can be extracted from experimental data and validated by microscopic calculations. Realistic models will be proposed that will extend the Newns-Anderson model, which has been employed in recent studies and turned out to successfully explain a series of experimental TVS findings. Extensions envisaged in this project will incorporate, e. g., the effect of anchoring groups and electrodes' work function, whose significant impact on the transition voltage has been demonstrated in recent experimental studies. The impact of stochastic fluctuations on two-dimensional transition-voltage-conductance histograms (that became available due to recent experimental achievements of Tao et al) and of the inner reorganization on TVS for molecules with a floppy degree of freedom will also constitute important topics of investigation.