Gold-Organic Hybrids: On-Surface Synthesis and Perspectives

Scanning Tunneling Spectroscopy Investigation of Au-bis-acetylide Networks on Au(111): The Influence of Metal-Organic Hybridization

Graphdiyne (GDY) is an appealing two-dimensional carbon material, but the on-surface synthesis of a single layer remains challenging. Demetalation of well-crystalline metal acetylide networks, though in its infancy, provides a new avenue to on-surface synthesized GDY substructures. In spite of the synthetic efforts and theoretical concerns, there are few reports steeped in elaborate characterization of the electronic influence of metalation. In this context, we focused on the surface supported Au-bis-acetylide network, which underwent demetalation after further annealing to form hydrogen-substituted GDY. We made a comprehensive study on the geometric structure and electronic structure and the corresponding demetalized structure on Au(111) through STM, noncontact atomic force microscopy (nc-AFM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) simulations. The bandgap of the Au-bis-acetylide network on Au(111) is measured to be 2.7 eV, while the bandgap of a fully demetalized Au-bis-acetylide network is estimated to be about 4.1 eV. Our findings reveal that the intercalated Au adatoms are positioned closer to the metal surface compared with the organic skeletons, facilitating electronic hybridization between the surface state and unoccupied frontier molecular orbitals of organic components. This leads to an extended conjugation through Au-bis-acetylene bonds, resulting in a reduced bandgap.

Topology selectivity of a conformationally flexible precursor through selenium doping

Conformational arrangements within nanostructures play a crucial role in shaping the overall configuration and determining the properties, for example in covalent/metal organic frameworks. In on-surface synthesis, conformational diversity often leads to uncontrollable or disordered structures. Therefore, the exploration of controlling and directing the conformational arrangements is significant in achieving desired nanoarchitectures. Herein, a conformationally flexible precursor 2,4,6-tris(3-bromophenyl)-1,3,5-triazine is employed, and a random phase consisting of C3h and Cs conformers is firstly obtained after deposition of the precursor on Cu(111) at room temperature to 365 K. At low coverage (0.01 ML) selenium doping, we achieve the selectivity of the C3h conformer and improve the nanopore structural homogeneity. The ordered two-dimensional metal organic nanostructure can be fulfilled by selenium doping from room temperature to 365 K. The formation of the conformationally flexible precursor on Cu(111) is explored through the combination of high-resolution scanning tunneling microscopy and non-contact atomic force microscopy. The regulation of energy diagrams in the absence or presence of the Se atom is revealed by density functional theory calculations. These results can enrich the on-surface synthesis toolbox of conformationally flexible precursors, for the design of complex nanoarchitectures, and for future development of engineered nanomaterials.

Highly Selective On-Surface [2 + 2] Cycloaddition Induced by Hierarchical Metal-Organic Hybrids

On-surface synthesis of phenylenes is a promising strategy to form extended π-conjugated frameworks but normally lacks selectivity in achieving uniform products. Herein we demonstrate that the debromination reaction of 2,3-dibromophenazine (DBPZ) on Au(111) and Ag(111) surfaces can vary significantly considering the involvement of metal-organic hybrids (MOHs). On Au(111), [2 + 2] and [2 + 2 + 2] cycloadditions facilitate instantaneously upon the debromination occurring, while on Ag(111), several MOHs have been observed under sequential thermal annealing, leading to finally the uniform [2 + 2] cycloaddition product exclusively. By means of scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (BR-AFM), we have unambiguously depicted the chemical structure of related reaction intermediates and unraveled the undocumented role of hierarchical evolution of MOHs in steering the chemical selectivity.

Tandem Desulfurization/C-C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer

Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C-C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C-C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.

Substrate-Modulated Synthesis of Metal-Organic Hybrids by Tunable Multiple Aryl-Metal Bonds

Assembly of semiconducting organic molecules with multiple aryl-metal covalent bonds into stable one- and two-dimensional (1D and 2D) metal-organic frameworks represents a promising route to the integration of single-molecule electronics in terms of structural robustness and charge transport efficiency. Although various metastable organometallic frameworks have been constructed by the extensive use of single aryl-metal bonds, it remains a great challenge to embed multiple aryl-metal bonds into these structures due to inadequate knowledge of harnessing such complex bonding motifs. Here, we demonstrate the substrate-modulated synthesis of 1D and 2D metal-organic hybrids (MOHs) with the organic building blocks (perylene) interlinked solely with multiple aryl-metal bonds via the stepwise thermal dehalogenation of 3,4,9,10-tetrabromo-1,6,7,12-tetrachloroperylene and subsequent metal-organic connection on metal surfaces. More importantly, the conversion from 1D to 2D MOHs is completely impeded on Au(111) but dominant on Ag(111). We comprehensively study the distinct reaction pathways on the two surfaces by visually tracking the structural evolution of the MOHs with high-resolution scanning tunneling and noncontact atomic force microscopy, supported by first-principles density functional theory calculations. The substrate-dependent structural control of the MOHs is attributed to the variation of the M-X (M = Au, Ag; X = C, Cl) bond strength regulated by the nature of the metal species.

On-purpose design of dual active sites in single V atom anchored C2N nanosheet for propane dehydrogenation catalysis

As an emerging propylene production technology, propane dehydrogenation (PDH) has attracted much attentions from researchers. In this work, we designed the single transition metal atom anchored C2N nanosheets (TM1/C2N) with...

Surfactant-mediated morphology evolution and self-assembly of cerium oxide nanocrystals for catalytic and supercapacitor applications

Surfactant plays a remarkable role in determining the growth process (facet exposition) of colloidal nanocrystals (NCs) and the formation of self-assembled NC superstructures, the underlying mechanism of which, however, still requires elucidation. In this work, the mechanism of surfactant-mediated morphology evolution and self-assembly of CeO2 nanocrystals is elucidated by exploring the effect that surfactant modification has on the shape, size, exposed facets, and arrangement of the CeO2 NCs. It is directly proved that surfactant molecules determine the morphologies of the CeO2 NCs by preferentially bonding onto Ce-terminated {100} facets, changing from large truncated octahedra (mostly {111} and {100} exposed), to cubes (mostly {100} exposed) and small cuboctahedra (mostly {100} and {111} exposed) by increasing the amount of surfactant. The exposure degree of the {100} facets largely affects the concentration of Ce3+ in the CeO2 NCs, thus the cubic CeO2 NCs exhibit superior oxygen storage capacity and excellent supercapacitor performance due to a high fraction of exposed active {100} facets with great superstructure stability.

On-surface preparation of coordinated lanthanide-transition-metal clusters

The study of lanthanide (Ln)-transition-metal (TM) heterometallic clusters which play key roles in various high-tech applications is a rapid growing field of research. Despite the achievement of numerous Ln-TM cluster compounds comprising one Ln atom, the synthesis of Ln-TM clusters containing multiple Ln atoms remains challenging. Here, we present the preparation and self-assembly of a series of Au-bridged heterometallic clusters containing multiple cerium (Ce) atoms via on-surface coordination. By employing different pyridine and nitrile ligands, the ordered coordination assemblies of clusters containing 2, 3 and 4 Ce atoms bridged by Au adatoms are achieved on Au(111) and Au(100), as revealed by scanning tunneling microscopy. Density functional theory calculations uncover the indispensable role of the bridging Au adatoms in constructing the multi-Ce-containing clusters by connecting the Ce atoms via unsupported Ce-Au bonds. These findings demonstrate on-surface coordination as an efficient strategy for preparation and organization of the multi-Ln-containing heterometallic clusters.

A Fundamental Role of the Molecular Length in Forming Metal-Organic Hybrids of Phenol Derivatives on Silver Surfaces

In on-surface chemistry, the efficient preparation of metal-organic hybrids is regarded as a primary path to mediate controlled synthesis of well-ordered low-dimensional organic nanostructures. The fundamental mechanisms in forming these hybrid structures, however, are so far insufficiently explored. Here, with scanning tunneling microscopy, we studied the bonding behavior of the adsorbed phenol derivatives with different molecular lengths. We reveal that shorter molecules favor bonding with extracted metal adatoms and result in metal-organic hybrids, whereas longer molecules prefer to bond with lattice metal atoms. The conclusions are further confirmed by density functional theory calculations.

Oxygen-Induced 1D to 2D Transformation of On-Surface Organometallic Structures

While surface-confined Ullmann-type coupling has been widely investigated for its potential to produce π-conjugated polymers with unique properties, the pathway of this reaction in the presence of adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″-dibromo-p-terphenyl or 1,4-dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host-guest chemistry.

Single Vanadium Atoms Anchored on Graphitic Carbon Nitride as a High-Performance Catalyst for Non-oxidative Propane Dehydrogenation

In comparison with oil-based cracking technologies, the on-purpose dehydrogenation of propane (PDH) is a more eco-friendly and profitable approach to produce propylene. By means of density functional theory calculations, the present work reveals that the single vanadium (V) atom anchored on graphitic carbon nitride (V₁/g-C₃N₄) may serve as a promising single-atom catalyst for non-oxidative PDH with industrially practical activity, selectivity, and thermal stability. The high activity of V₁/g-C₃N₄ for PDH is attributed to the low-coordinated 3d orbitals of single V atoms, while the propylene selectivity is originated from the inhibition of the di-σ binding mode of propylene on the single V atoms. This work provides a guideline to design and screen out promising single-atom catalysts for selective dehydrogenation of alkanes.

On-surface synthesis of graphyne nanowires through stepwise reactions

We have achieved on-surface synthesis of graphyne nanowires through stepwise reactions involving two different types of dehalogenative homocoupling reactions (i.e., C(sp³)–Br and C(sp²)–Br).

Gold-linked strings of donor–acceptor dyads: on-surface formation and mutual orientation

Strings of fused donor–acceptors with their dipoles following a complicated correlation driven partially by next-nearest-neighbor effects on Au(111).

On-surface synthesis of gold–coronene molecular wires

Perchlorocoronene undergoes selective dehalogenation under the one-dimensional constraint by intermolecular interactions, resulting in the formation of gold–coronene wires on Au(111) surfaces.

On-Surface Synthesis of Graphyne-Based Nanostructures

The successful synthesis of stacking graphdiynes has stimulated numerous fascinating applications. However, it still remains challenging to prepare atomically precise 2D graphdiyne and other graphyne-based structures. The development of on-surface synthesis has contributed to many secondary graphyne-based structures that are directive in fabricating extended graphyne networks. Herein, the recent progress concerning on-surface synthesis of graphyne-based nanostructures, especially atomically precise graphdiyne nanowires, is summarized.

Mechanocombinatorially Screening Sensitivity of Stretchable Strain Sensors

Stretchable strain sensors have aroused great interest for their application in human activity recognition, health monitoring, and soft robotics. For various scenarios involving the application of different strain ranges, specific sensitivities need to be developed, due to a trade-off between sensor sensitivity and stretchability. Traditional stretchable strain sensors are developed based on conductive sensing materials and still lack the function of customizable sensitivity. A novel strategy of mechanocombinatorics is proposed to screen the sensor sensitivity based on mechanically heterogeneous substrates. Strain redistribution over substrates is optimized by mechanics and structure parameters, which gives rise to customizable sensitivity. As a proof of concept, a local illumination method is used to fabricate heterogeneous substrates with customizable mechanics and structure parameters. A library of mechanocombinatorial strain sensors is created for extracting the specific sensitivity. Thus, not only is an effective strategy for screening of sensor sensitivity demonstrated, but a contribution to the mechanocombinatorial strategy for personalized stretchable electronics is also made.

On-Surface Synthesis of One-Dimensional Carbon-Based Nanostructures via C-X and C-H Activation Reactions

The past decades have witnessed the emergence of low-dimensional carbon-based nanostructures owing to their unique properties and various subsequent applications. It is of fundamental importance to explore ways to achieve atomically precise fabrication of these interesting structures. The newly developed on-surface synthesis approach provides an efficient strategy for this challenging issue, demonstrating the potential of atomically precise preparation of low-dimensional nanostructures. Up to now, the formation of various surface nanostructures, especially carbon-based ones, such as graphene nanoribbons (GNRs), kinds of organic (organometallic) chains and films, have been achieved via on-surface synthesis strategy, in which in-depth understanding of the reaction mechanism has also been explored. This review article will provide a general overview on the formation of one-dimensional carbon-based nanostructures via on-surface synthesis method. In this review, only a part of the on-surface chemical reactions (specifically, C-X (X=Cl, Br, I) and C-H activation reactions) under ultra-high vacuum conditions will be covered.

Selective on-surface covalent coupling based on metal-organic coordination template

Control over on-surface reaction pathways is crucial but challenging for the precise construction of conjugated nanostructures at the atomic level. Herein we demonstrate a selective on-surface covalent coupling reaction that is templated by metal-organic coordinative bonding, and achieve a porous nitrogen-doped carbon nanoribbon structure. In contrast to the inhomogeneous polymorphic structures resulting from the debrominated aryl-aryl coupling reaction on Au(111), the incorporation of an Fe-terpyridine (tpy) coordination motif into the on-surface reaction controls the molecular conformation, guides the reaction pathway, and finally yields pure organic sexipyridine-p-phenylene nanoribbons. Emergent molecular conformers and reaction products in the reaction pathways are revealed by scanning tunneling microscopy, density functional theory calculations and X-ray photoelectron spectroscopy, demonstrating the template effect of Fe-tpy coordination on the on-surface covalent coupling. Our approach opens an avenue for the rational design and synthesis of functional conjugated nanomaterials with atomic precision.

Synthesizing precise conjugated nanostructures on a surface requires fine control over the covalent reaction pathways. Here, the authors show that reversible coordinative bonds can be used to template on-surface C-C coupling reactions, guiding the formation of porous organic nanoribbons.

Two-dimensional delocalized states in organometallic bis-acetylide networks on Ag(111)

The electronic structure of surface-supported organometallic networks with Ag-bis-acetylide bonds that are intermediate products in the bottom-up synthesis of graphdiyne and graphdiyne-like networks were studied. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal a frontier, unoccupied electronic state that is delocalized along the entire organometallic network and proves the covalent nature of the Ag-bis-acetylide bonds. Density-functional theory (DFT) calculations corroborate the spatial distribution of the observed delocalized state and attribute it to band mixing of carbon and silver atoms combined with n-doping of the metal surface. The metal-bis-acetylide bonds are typical metal-organic bonds with mixed character containing covalent and strong ionic contributions. Moreover, the organometallic networks exhibit a characteristic graphene-like band structure with linear band dispersion at each K point.

Complex supramolecular interfacial tessellation through convergent multi-step reaction of a dissymmetric simple organic precursor

Interfacial supramolecular self-assembly represents a powerful tool for constructing regular and quasicrystalline materials. In particular, complex two-dimensional molecular tessellations, such as semi-regular Archimedean tilings with regular polygons, promise unique properties related to their nontrivial structures. However, their formation is challenging, because current methods are largely limited to the direct assembly of precursors, that is, where structure formation relies on molecular interactions without using chemical transformations. Here, we have chosen ethynyl-iodophenanthrene (which features dissymmetry in both geometry and reactivity) as a single starting precursor to generate the rare semi-regular (3.4.6.4) Archimedean tiling with long-range order on an atomically flat substrate through a multi-step reaction. Intriguingly, the individual chemical transformations converge to form a symmetric alkynyl-Ag-alkynyl complex as the new tecton in high yields. Using a combination of microscopy and X-ray spectroscopy tools, as well as computational modelling, we show that in situ generated catalytic Ag complexes mediate the tecton conversion.

Polymorphism and metal-induced structural transformation in 5,5′-bis(4-pyridyl)(2,2′-bispyrimidine) adlayers on Au(111)

Addition of iron to a self-assembled molecular network can lift polymorphism and leads to the expression of one single metal–organic structure on a surface.

A new on-surface synthetic pathway to 5-armchair graphene nanoribbons on Cu(111) surfaces

We report a new pathway to fabricate armchair graphene nanoribbons with five carbon atoms in the cross section (5-AGNRs) on Cu(111) surfaces. Instead of using haloaromatics as precursors, the 5-AGNRs are synthesized via a surface assisted decarboxylation reaction of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). The on-surface decarboxylation of PTCDA can produce extended copper–perylene chains on Cu(111) that are able to transform into graphene nanoribbons after annealing at higher temperatures (ca. 630 K). Due to the low yield (ca. 20%) of GNRs upon copper extrusion, various gases are introduced to assist the transformation of the copper–perylene chains into the GNRs. Typical reducing gases (H₂ and CO) and oxidizing gas (O₂) are evaluated for their performance in breaking aryl–Cu bonds. This method enriches on-surface protocols for the synthesis of AGNRs using non-halogen containing precursors.