Single-Electron Alternate-Charging Scanning Tunneling Microscopy on Insulating Surfaces

在原子尺度上理解物质的电子结构是纳米科学的中心目标之一。特别是，许多化学过程是由原子和分子之间的电子交换控制的，这种机制被称为电子转移。探索这种现象需要在原子和分子自身的长度尺度上研究它们的能力，同时能够控制它们的电荷状态。虽然扫描隧道显微镜（STM）允许以埃分辨率成像电子状态，但导电衬底的要求阻碍了对给定电荷状态下完全电子隔离的吸附质结构的研究。最近，我们结合STM和AFM的元素，成功地开发了一种新的成像和光谱方法，称为单电子交替充电扫描隧道显微镜（AC-STM）。在这里，在我们的更新建议中，我们的目标是将这种新方法应用于各种物理问题，没有AC-STM技术，这些问题一直难以捉摸。事实上，通过在绝缘体上实现电子状态成像，用这种方法也可以研究更高的激发态，特别是多重电离分子。我们在五种不同的语境中使用这些可能性。首先，我们的目标是研究单个低聚分子内的电子-电子排斥，作为导电聚合物的模型系统。其次，我们想用AC-STM来鉴定由电荷注入形成的反应产物。在这些单分子反应中需要访问多个电荷态，这抑制了传统的STM表征，突出了AC-STM带来的可能性。第三，我们希望将该技术扩展到其全光谱能力，这样它不仅可以提供不同电荷态跃迁的能量，还可以提供能级展宽、极化位移甚至振动激发。第四，我们想要探索在每个电荷态中获得非平衡态的可能性。例如，通过适当的顺序注入不同的载流子，应该有可能制备具有HOMO-LUMO激发（激子）的分子，从而提供通过电子转移反应获得电荷守恒光学跃迁的能量学。最后，我们提出将AC-STM与厚绝缘子上的发光实验结合起来。这个项目将为单分子实验开辟一个新的领域，使我们能够接触到所有涉及非平衡电荷态的现象。

Understanding the electronic structure of matter on the atomic scale is one of the central goals of nanoscience. In particular, many chemical processes are governed by the exchange of electrons between atoms and molecules, a mechanism known as electron transfer. Probing such phenomena requires the capability to study atom and molecules at their own length scales, while at the same time being able to control their charge states.While Scanning Tunneling Microscopy (STM) allows imaging electronic states with Angstrom resolution, the requirement of a conductive substrate hinders the investigation of completely electronically isolated adsorbate structures in given charge states. Recently, combining elements of STM and AFM, we successfully developed a novel imaging and spectroscopy method, referred to as single-electron Alternate-Charging Scanning Tunneling Microscopy (AC-STM), in the framework of a running project. Here, in our renewal proposal, we aim at applying this novel method to various physical questions that, without the AC-STM technique, have been elusive. In fact, by enabling electronic state imaging on insulators, with this method one can also study higher lying excited states, in particular multiply-ionized molecules. We make use of these possibilities in five different contexts. First, we aim to study the electron-electron repulsion within single oligomeric molecules, as model systems for conducting polymers. Second, we would like to use AC-STM to identify reaction products, formed by charge injection. The need to access multiple charge states in these single-molecule reactions inhibits conventional STM characterization, highlighting the possibilities brought about by AC-STM. Third, we wish to expand the technique to its full spectroscopic capabilities, such that it can provide not only the energy of different charge state transitions, but also level broadening, polaronic shifts and maybe even vibrational excitations. Fourth, we would like to explore the possibilities to also access non-equilibrium states within each charge state. For example, by a suitable sequential injection of different charge carriers, it should be possible to prepare a molecule with a HOMO-LUMO excitation (exciton), providing access to the energetics of charge conserving optical transitions with electron-transfer reactions. Finally, we propose to combine AC-STM with luminescence experiments on thick insulators. This project will open up a new arena of single-molecule experiments with access to the entire class of phenomena involving out-of-equilibrium charge states.