Sakthivel Naga Arjun, Shabaninezhad Masoud, Sementa Luca, Yoon Bokwon, Stener Mauro, Whetten Robert L, Ramakrishna Guda, Fortunelli Alessandro, Landman Uzi, Dass Amala
Department of Chemistry and Biochemistry, University of Mississippi, Oxford, Mississippi 38677, United States.
Department of Physics, Western Michigan University, Kalamazoo, Michigan 49008, United States.
J Am Chem Soc. 2020 Sep 16;142(37):15799-15814. doi: 10.1021/jacs.0c05685. Epub 2020 Sep 3.
Understanding the evolution of the structure and properties in metals from molecule-like to bulk-like has been a long sought fundamental question in science, since Faraday's 1857 work. We report the discovery of a Janus nanomolecule, Au(SPh-Bu) having both molecular and metallic characteristics, explored crystallographically and optically and modeled theoretically. Au has an anisotropic, singly twinned structure with an Au core protected by a ligand shell made of 24 monomeric [-S-Au-S-] and 6 dimeric [-S-Au-S-Au-S-] staples. The Au core is composed of an 89-atom inner core and 66 surface atoms, arranged as [Au@Au@Au]@Au concentric shells of atoms. The inner core has a monotwinned/stacking-faulted face-centered-cubic (fcc) structure. Structural evolution in metal nanoparticles has been known to progress from multiply twinned, icosahedral, structures in smaller molecular sizes to untwinned bulk-like fcc monocrystalline nanostructures in larger nanoparticles. The monotwinned inner core structure of the ligand capped Au nanomolecule provides the critical missing link, and bridges the size-evolution gap between the molecular multiple-twinning regime and the bulk-metal-like particles with untwinned fcc structure. The Janus nature of the nanoparticle is demonstrated by its optical and electronic properties, with metal-like electron-phonon relaxation and molecule-like long-lived excited states. First-principles theoretical explorations of the electronic structure uncovered electronic stabilization through the opening of a shell-closing gap at the top of the occupied manifold of the delocalized electronic superatom spectrum of the inner core. The electronic stabilization together with the inner core geometric stability and the optimally stapled ligand-capping anchor and secure the stability of the entire nanomolecule.
自1857年法拉第的研究工作以来，理解金属中结构和性质从分子状到块状的演变一直是科学界长期以来寻求的基本问题。我们报告了一种兼具分子和金属特性的Janus纳米分子Au(SPh-Bu)的发现，对其进行了晶体学和光学研究，并进行了理论建模。Au具有各向异性的单孪晶结构，其Au核由由24个单体[-S-Au-S-]和6个二聚体[-S-Au-S-Au-S-]钉组成的配体壳保护。Au核由一个89原子的内核和66个表面原子组成，排列成[Au@Au@Au]@Au同心原子壳。内核具有单孪晶/堆垛层错面心立方(fcc)结构。已知金属纳米颗粒的结构演变从较小分子尺寸的多重孪晶二十面体结构发展到较大纳米颗粒中无孪晶的块状fcc单晶纳米结构。配体封端的Au纳米分子的单孪晶内核结构提供了关键的缺失环节，并弥合了分子多重孪晶状态与具有无孪晶fcc结构的块状金属样颗粒之间的尺寸演变差距。纳米颗粒的Janus性质通过其光学和电子性质得以证明，具有类似金属的电子-声子弛豫和类似分子的长寿命激发态。对电子结构的第一性原理理论探索揭示了通过在内核离域电子超原子光谱的占据流形顶部打开一个壳闭合间隙实现的电子稳定。电子稳定与内核几何稳定性以及最佳钉扎的配体封端相结合，确保了整个纳米分子的稳定性。
