Steering Electron-Induced Surface Reaction via a Molecular Assembly Approach.

Electrons not only serve as a "reactant" in redox reactions but also play a role in "catalyzing" some chemical processes. Despite the significance and ubiquitousness of electron-induced chemistry, many related scientific issues still await further exploration, among which is the impact of molecular assembly. In this work, microscopic insights into the vital role of molecular assembly in tweaking the electron-induced surface chemistry are unfolded by combined scanning tunneling microscopy and density functional theory studies.

Assembling Surface Molecular Sierpiński Triangle Fractals via K-Invoked Electrostatic Interaction.

Molecular Sierpiński triangles (STs), a family of elegant and well-known fractals, can be prepared on surfaces with atomic precision. Up to date, several kinds of intermolecular interactions such as hydrogen bond, halogen bond, coordination, and even covalent bond have been employed to construct molecular STs on metal surfaces. Herein a series of defect-free molecular STs have been fabricated via electrostatic attraction between potassium cations and electronically polarized chlorine atoms in 4,4″-dichloro-1,1':3',1″-terphenyl (DCTP) molecules on Cu(111) and Ag(111).

Single-Cation Catalyst: Ni Cation in Monolayered CuO for CO Oxidation.

It is vital to differentiate catalytic properties between cationic and metallic single atoms at the atomic level. To achieve this, we fabricated well-defined cationic Ni atoms snugged in and metallic Ni atoms supported on monolayered CuO. The Ni cations are chemically inert for CO adsorption even at 70 K but highly active toward O dissociation at room temperature.

Visualization of on-surface ethylene polymerization through ethylene insertion.

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal-carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface.

Steering Effect of Bromine on Intermolecular Dehydrogenation Coupling of Poly(-phenylene) on Cu(111).

Among the multitudinous methodologies to steer on-surface reactions, less attention has been paid to the effect of externally introduced halogen atoms. Herein, highly selective -dehydrogenation coupling at the specific -C-H site of two poly(-phenylene) molecules, -quaterphenyl (Ph) and -quinquephenyl (Ph), is achieved on Cu(111) by externally introduced bromine atoms. Scanning tunneling microscopy/spectroscopy experiments reveal that the formed molecular assembly structure at a stoichiometric ratio of 4:1 for Br to Ph or 5:1 for Br to Ph can efficiently promote the reactive collision probability to trigger the -coupling reaction at the -C-H site between two neighboring Ph or Ph molecules, leading to an increase in the coupling selectivity.

Selective Intramolecular Dehydrocyclization of Co-Porphyrin on Au(111).

The on-surface C-H bond activation and coupling reaction is a powerful approach to constructing fine-tuned surface nanostructures. It is quite challenging to control its regioselectivity due to the inertness of the C-H bond involved. With scanning tunneling microscopy/spectroscopy and theoretical calculations, the C-H activation and sequential intramolecular dehydrocyclization of -tetra(p-methoxyphenyl)porphyrinatocobalt(II) was explored on Au(111), showing that the methoxy groups in the molecule could kinetically mediate the selectivity of the intramolecular reaction over its intermolecular coupling counterpart.

Reversibly Switching the Charge State and Adsorption Location of A Single Potassium Atom on Ultrathin CuO Films.

Potassium (K) cations are spontaneously formed upon thermal deposition of low-coverage K onto an ultrathin CuO monolayer grown on Cu(110) and they were explored by low-temperature scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy. The formed K cations are highly immobile and thermally stable. The local work function around an individual K cation decreases by 1.

Atomic structures and local electronic properties of K- and Rh-modified ceria/Pt(111) inverse model catalysts.

Ceria has been widely applied as a support in heterogeneous catalysis due to its unique capability to store and release oxygen. As a typical inverse model catalyst, a ceria/Pt(111) system has attracted much attention due to its strong metal-oxide interaction. The structural and electronic properties of the ceria/Pt(111) system can be effectively modified by the introduction of alien K and Rh atoms.

Long-Range Ordered Structures of Corannulene Governed by Electrostatic Repulsion and Surface-State Mediation.

The adsorption and assembly of sub-monolayered bowl-shaped corannulene (COR) on Cu(111) and Ag(111) are investigated by scanning tunneling microscopy (STM). Three COR configurations, namely, up, down, and tilted ones, are formed on Cu(111), as unraveled by high-resolution STM images. It is also experimentally revealed that monodispersed, hexagonal, and evenly spaced stripe patterns develop on both Cu(111) and Ag(111).

Chiral features of metal phthalocyanines sitting atop the pre-assembled TiOPc monolayer on Ag(111).

The chiral features of the top-layer TiOPc molecules on monolayered TiOPc assembly on Ag(111) were carefully investigated by scanning tunnelling microscopy and local work function measurements. Combined with the density functional theory calculations, systematic experimental explorations of the TiOPc/TiOPc, CuPc/TiOPc and TiOPc/CuPc systems on Ag(111) revealed that the chirality originated from asymmetric electronic interactions rather than conformational change, which might be related to the high performance of the photoelectronic devices based on the MPc complexes.

Steering the Achiral into Chiral with a Self-Assembly Strategy.

Chirality transfer from self-assembly of achiral titanyl phthalocyanine (TiOPc) to its top-sitting TiOPc molecule has been successfully achieved. The TiOPc molecules first assemble into a porous network on Au(111) that contains periodic chiral voids, each being fenced by four axially rotating TiOPc molecules in upward adsorption geometry where their ending O atoms exclusively point away from the substrate. The additional top-sitting TiOPc molecule turns out to be chiral upon adsorption on a chiral void with its ending O atom toward the substrate.

On Controllability and Applicability of Surface Molecular Self-Assemblies.

Molecular self-assembly (MSA) refers to spontaneous arrangement of molecular building blocks into ordered structures governed by weak interactions. Due to its high versatility and reversibility, MSA has been widely employed as a robust bottom-up approach to fabricating low-dimensional functional nanostructures, which are used in various applications in nanoscience and technology. To date, tremendous effort has been devoted to constructing various MSAs at surfaces, ranging from self-assembled monolayers and two-dimensional (2D) nanoporous networks to complex 2D quasicrystals and Sierpiński triangle fractals.

On the shuttling mechanism of a chlorine atom in a chloroaluminum phthalocyanine based molecular switch.

An intermediate shuttling structure of a chloroaluminum phthalocyanine(ClAlPc)-based molecular switch is transiently created and analyzed by combined scanning tunneling microcopy/spectroscopy and density-functional theory calculations, which suggests that the Cl atom is squeezed into the space between the central Al atom and the inner N-containing ring in ClAlPc.