Direct Imaging of Monovacancy-Hydrogen Complexes in a Single Graphitic Layer

Understanding how foreign chemical species bond to atomic vacancies in graphene layers can advance our ability to tailor the electronic and magnetic properties of defective graphenic materials. Here we use ultra-high vacuum scanning tunneling microscopy (UHV-STM) and density functional theory to identify the precise structure of hydrogenated single atomic vacancies in a topmost graphene layer of graphite and establish a connection between the details of hydrogen passivation and the electronic properties of a single atomic vacancy. Monovacancy-hydrogen complexes are prepared by sputtering of the graphite surface layer with low energy ions and then exposing it briefly to an atomic hydrogen environment. High-resolution experimental UHV-STM imaging allows us to determine unambiguously the positions of single missing atoms in the defective graphene lattice and, in combination with the ab initio calculations, provides detailed information about the distribution of low-energy electronic states on the periphery of the monovacancy-hydrogen complexes. We found that a single atomic vacancy where each sigma-dangling bond is passivated with one hydrogen atom shows a well-defined signal from the non-bonding pi-state which penetrates into the bulk with a (\sqrt 3 \times \sqrt 3)R30^ \circ periodicity. However, a single atomic vacancy with full hydrogen termination of sigma-dangling bonds and additional hydrogen passivation of the extended pi-state at one of the vacancy's monohydrogenated carbon atoms is characterized by complete quenching of low-energy localized states. In addition, we discuss the migration of hydrogen atoms at the periphery of the monovacancy-hydrogen complexes which dramatically change the vacancy's low-energy electronic properties, as observed in our low-bias high-resolution STM imaging.

理解外来化学物质如何与石墨烯层中的原子空位结合，能够提高我们定制有缺陷的石墨烯材料的电子和磁性性能的能力。在此，我们利用超高真空扫描隧道显微镜（UHV - STM）和密度泛函理论来确定石墨最顶层石墨烯层中氢化单原子空位的精确结构，并在氢钝化的细节与单原子空位的电子性能之间建立联系。单空位 - 氢复合物是通过用低能离子溅射石墨表面层，然后将其短暂暴露于原子氢环境中制备的。高分辨率的实验性UHV - STM成像使我们能够明确确定缺陷石墨烯晶格中单个缺失原子的位置，并且结合从头算计算，提供了关于单空位 - 氢复合物周边低能电子态分布的详细信息。我们发现，每个σ - 悬空键都被一个氢原子钝化的单原子空位显示出来自非键合π态的明确信号，该信号以（\(\sqrt{3}×\sqrt{3}\)）\(R30^{\circ}\)周期性渗透到体相中。然而，在空位的一个单氢化碳原子上，σ - 悬空键完全被氢终止且扩展的π态被额外氢钝化的单原子空位，其特征是低能局域态完全猝灭。此外，我们讨论了单空位 - 氢复合物周边氢原子的迁移，正如我们在低偏压高分辨率STM成像中所观察到的，这种迁移极大地改变了空位的低能电子性能。