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

Molecular Sierpinski 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). The electrostatic interaction is confirmed both experimentally by scanning tunneling microscopy and theoretically by density functional theory calculations. These findings illustrate that electrostatic interaction can serve as an efficient driving force to construct molecular fractals, which enriches our toolbox for the bottom-up fabrication of complex functional supramolecular nanostructures.

Molecular insight into on-surface chemistry of an organometallic polymer.

A molecular investigation of Cu-elimination and subsequent C-C coupling of DCTP (4,4''-dichloro-1,1':3',1''-terphenyl)-Cu organometallic (OM) polymers on Cu(111) is conducted by scanning tunneling microscopy and spectroscopy, revealing that the Cu adatoms embedded in the DCTP-Cu chains are located at the hollow and bridge sites on the Cu(111) surface. The difference in the catalytic activities of these surface sites leads to stepwise elimination of Cu adatoms in the OM chains. Moreover, the interchain interaction plays an important role in the Cu-elimination process of the DCTP-Cu chains as well. The interchain steric hindrance, on the one hand, induces the formation of Cu-eliminated intermediates that are scarcely observed in other Ullmann coupling systems, and on the other hand, promotes the cooperative Cu-elimination and C-C coupling of the OM segments in neighboring chains. These findings demonstrate the key role of the molecule-substrate and intermolecular interactions in mediating the reaction processes of the extended molecular systems on the surface.

Adaptive List Flip Decoder for Polar Codes with High-Order Error Correction Capability and a Simplified Flip Metric.

Designing an efficient decoder is an effective way to improve the performance of polar codes with limited code length. List flip decoders have received attention due to their good performance trade-off between list decoders and flip decoders. In particular, the newly proposed dynamic successive cancellation list flip (D-SCLF) decoder employs a new flip metric to effectively correct high-order errors and thus enhances the performance potential of present list flip decoders. However, this flip metric introduces extra exponential and logarithmic operations, and the number of these operations rises exponentially with the increase in the order of error correction and the number of information bits, which then limits its application value. Therefore, we designed an adaptive list flip (ALF) decoder with a new heuristic simplified flip metric, which replaces these extra nonlinear operations in the original flip metric with linear operations. Simulation results show that the simplified flip metric does not reduce the performance of the D-SCLF decoder. Moreover, based on the in-depth theoretical analyses of the combination of the adaptive list and the list flip decoders, the ALF decoder adopts the adaptive list to further reduce the average complexity.

On-Surface Synthesis of Highly Ordered Covalent Sierpinski Triangle Fractals.

The Sierpinski triangle (ST) is a well-known fractal structure. Synthesis of stable molecular STs with robust covalent linkages is attractive but challenging. Here, we demonstrate the formation of a series of high-quality covalent STs via the on-surface dehydration reaction of 1,3-benzenediboronic acid with the presence of water as an equilibrium regulator at ambient atmosphere. Extended molecular fractals up to third generation are obtained, as disclosed by scanning tunnel microscope. The covalent STs show intriguing bright and dark contrasts irrespective of the fractal generations, which is related with epitaxial relationship of fractal structure to the underlying graphite lattice, as supported by theoretical simulations.

Dechlorinated Ullmann Coupling Reaction of Aryl Chlorides on Ag(111): A Combined STM and XPS Study.

Surface-assisted Ullmann coupling was widely applied to construct various molecular nanostructures on surfaces due to its reliability and controllability. By using 4,4''-dichloro-1,1':3',1''-terphenyl (DCTP) as the precursor, covalently bonded zig-zag oligophenylene chains and hexagonal hyperbenzene rings, e. g., [18]-honeycombenes, were successfully fabricated on Ag(111) via dechlorinated Ullmann coupling reaction. Stepwise annealing was applied to investigate the reaction process in detail. Scanning tunneling microscopy and synchrotron X-ray photoemission spectroscopy were utilized to explore the thermal evolution of the DCTP molecules on Ag(111) under ultrahigh vacuum conditions, evidencing the existence of intact DCTP molecules, chemisorbed Cl atoms, covalently bonded DCTP dimers as well as organometallic C-Ag-C-containing intermediates. These results may help understand dechlorinated Ullmann coupling reaction of aryl chlorides on metal surfaces.

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.

Estimation of Activity Interaction Parameters in Fe-S-j Systems.

It is important to know the activity interaction parameters between components in melts in the process of metallurgy. However, it's considerably difficult to measure them experimentally, relying still to a large extent on theoretical calculations. In this paper, the first-order activity interaction parameter ( e s j ) of j on sulphur in Fe-based melts at 1873 K is investigated by a calculation model established by combining the Miedema model and Toop-Hillert geometric model as well as considering excess entropy and mixing enthalpy. We consider two strategies, with or without using excess entropy in the calculations. Our results show that: (1) the predicted values are in good agreement with those recommended by Japan Society for Promotion of Science (JSPS); and (2) the agreement is even better when excess entropy is considered in the calculations. In addition, the deviations of our theoretical results from experimental values | e S ( exp ) j - e S ( cal ) j | depend on the element j's locations in the periodic table.

Steering on-surface reactions with self-assembly strategy.

The control of assembly structures that subsequently help achieve viable functionalities has been one of the key motivations for the exploration of surface molecular assembly. In terms of its functionality and applicability, the assembly is explored as a strategy to steer on-surface reactions primarily by two methods: assembly-assisted and assembly-involved reactions. The functions of the self-assembly strategy are threefold: tweaking reaction selectivities, steering reaction pathways, and directing reaction sites. The governing principle herein is that the assembly strategy can apply a surface confinement effect that affects the energy barrier and pre-exponential factor of the Arrhenius equation for the dynamics of the target reaction. Development of such a strategy may reveal new routes to steer on-surface reactions and even single molecule properties in surface chemistry.

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.

Insulating and Robust Ceramic Nanorod Aerogels with High-Temperature Resistance over 1400 °C.

Ceramic aerogels, which present a unique combination of low thermal conductivity and excellent high-temperature stability, are attractive for thermal insulation under extreme conditions. However, most ceramic aerogels are constructed by oxide ceramic nanoparticles and thus are usually plagued by their brittleness and structural collapse at elevated temperatures (less than 1000 °C). Despite great progress achieved in this regard recently, it still remains a big challenge to design and fabricate intriguing ceramic aerogels with enhanced mechanical strength and remarkable thermal stability at ultrahigh temperature up to 1400 °C. To this end, we herein report a facile and scalable strategy to manufacture ceramic nanorod aerogels (CNRAs) with hierarchically macroporous and mesoporous structures by the controllable assembly of Al2O3 nanorods and SiO2 nanoparticles. Subsequently, the high-temperature annealing treatment of CNRAs significantly maximizes mechanical strength and promotes thermal tolerance. The obtained CNRAs demonstrate the integrated properties of super-strong heat resistance (up to 1400 °C), low thermal conductivity (0.026 W/m·K at 25 °C and 0.089 W/m·K at 1200 °C), high mechanical robustness (compressive strength 1.5 MPa), and low density (0.146 g/cm3). We envision that this novel nanorod-assembled ceramic aerogels offer considerable advantages than most of the state-of-the-art ceramic aerogels for thermal superinsulation upon exposure to extremely harsh environments.

Steering Effect of Bromine on Intermolecular Dehydrogenation Coupling of Poly(p-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 trans-dehydrogenation coupling at the specific meta-C-H site of two poly(p-phenylene) molecules, p-quaterphenyl (Ph4) and p-quinquephenyl (Ph5), 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 Ph4 or 5:1 for Br to Ph5 can efficiently promote the reactive collision probability to trigger the trans-coupling reaction at the meta-C-H site between two neighboring Ph4 or Ph5 molecules, leading to an increase in the coupling selectivity. Such Br atoms can also affect the electronic structure and adsorption stability of the reacting molecules. It is conceptually demonstrated that externally introduced halogen atoms, which can provide an adjustable halogen-to-precursor stoichiometry, can be employed to efficiently steer on-surface reactions.

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). A quantitative evaluation of the long-range intermolecular interaction on Cu(111) mediated by electrostatic repulsion and surface-state mediation is presented. At 0.05 monolayer (ML), the long-range monodispersed pattern is mainly induced by electrostatic interaction. At 0.24 and 0.47 ML, however, surface-state mediation plays a dominant role, and the electrostatic interaction is leveled due to the identical static environment for each molecule.

Assembling molecular Sierpinski triangle fractals.

Fractals, being "exactly the same at every scale or nearly the same at different scales" as defined by Benoit B. Mandelbrot, are complicated yet fascinating patterns that are important in aesthetics, mathematics, science and engineering. Extended molecular fractals formed by the self-assembly of small-molecule components have long been pursued but, to the best of our knowledge, not achieved. To tackle this challenge we designed and made two aromatic bromo compounds (4,4″-dibromo-1,1':3',1″-terphenyl and 4,4‴-dibromo-1,1':3',1″:4″,1‴-quaterphenyl) to serve as building blocks. The formation of synergistic halogen and hydrogen bonds between these molecules is the driving force to assemble successfully a whole series of defect-free molecular fractals, specifically Sierpinski triangles, on a Ag(111) surface below 80 K. Several critical points that govern the preparation of the molecular Sierpinski triangles were scrutinized experimentally and revealed explicitly. This new strategy may be applied to prepare and explore various planar molecular fractals at surfaces.