On-Surface Synthesis of Porous Carbon Nanoribbons from Polymer Chains

Bottom-up Synthesis and Characterization of Porous 12-Atom-Wide Armchair Graphene Nanoribbons

Although several porous carbon/graphene nanoribbons (GNRs) have been prepared, a direct comparison of the electronic properties between a nonporous GNR and its periodically perforated counterpart is still missing. Here, we report the synthesis of porous 12-atom-wide armchair-edged GNRs from a bromoarene precursor on a Au(111) surface via hierarchical Ullmann and dehydrogenative coupling. The selective formation of porous 12-GNRs was achieved through thermodynamic and kinetic reaction control combined with tailored precursor design. The structure and electronic properties of the porous 12-GNR were elucidated by scanning tunneling microscopy/spectroscopy and density functional theory calculations, revealing that the pores induce a 2.17 eV band gap increase compared to the nonporous 12-AGNR on the same surface.

Construction of Two-Dimensional Organometallic Coordination Networks with Both Organic Kagome and Semiregular Metal Lattices on Au(111)

Two-dimensional metal-organic networks (2D MONs) having heterogeneous coordination nodes (HCNs) could exhibit excellent performance in catalysis and optoelectronics because of the unbalanced electron distribution of the coordinating metals. Therefore, the design and construction of 2D MONs with HCNs are highly desirable but remain challenging. Here, we report the construction of 2D organometallic coordination networks with an organic Kagome lattice and a semiregular metal lattice on Au(111) via the in situ formation of HCNs. Using a bifunctional precursor 1,4-dibromo-2,5-diisocyanobenzene, the coordination of isocyano with Au adatom on a room-temperature Au(111) yielded metal-organic coordination chains with isocyano-Au-isocyano nodes. In contrast, on a high-temperature Au(111), a selective debromination/coordination cascade reaction occurred, affording 2D organometallic coordination networks with phenyl-Au-isocyano nodes. By combining scanning tunneling microscopy and density functional theory calculations, we determined the structures of coordination products and the nature of coordination nodes, demonstrating a thermodynamically favorable pathway for forming the phenyl-Au-isocyano nodes.

Porous Nanographenes, Graphene Nanoribbons, and Nanoporous Graphene Selectively Synthesized from the Same Molecular Precursor

We demonstrate a family of molecular precursors based on 7,10-dibromo-triphenylenes that can selectively produce different varieties of atomically precise porous graphene nanomaterials through the use of different synthetic environments. Upon Yamamoto polymerization of these molecules in solution, the free rotations of the triphenylene units around the C-C bonds result in the formation of cyclotrimers in high yields. In contrast, in on-surface polymerization of the same molecules on Au(111) these rotations are impeded, and the coupling proceeds toward the formation of long polymer chains. These chains can then be converted to porous graphene nanoribbons (pGNRs) by annealing. Correspondingly, the solution-synthesized cyclotrimers can also be deposited onto Au(111) and converted into porous nanographenes (pNGs) via thermal treatment. Thus, both processes start with the same molecular precursor and end with a porous graphene nanomaterial on Au(111), but the type of product, pNG or pGNR, depends on the specific coupling approach. We also produced extended nanoporous graphenes (NPGs) through the lateral fusion of highly aligned pGNRs on Au(111) that were grown at high coverage. The pNGs can also be synthesized directly in solution by Scholl oxidative cyclodehydrogenation of cyclotrimers. We demonstrate the generality of this approach by synthesizing two varieties of 7,10-dibromo-triphenylenes that selectively produced six nanoporous products with different dimensionalities. The basic 7,10-dibromo-triphenylene monomer is amenable to structural modifications, potentially providing access to many new porous graphene nanomaterials. We show that by constructing different porous structures from the same building blocks, it is possible to tune the energy band gap in a wide range.

Unzipping Carbon Nanotubes to Sub-5-nm Graphene Nanoribbons on Cu(111) by Surface Catalysis

Graphene nanoribbons (GNRs) are promising in nanoelectronics for their quasi-1D structures with tunable bandgaps. The methods for controllable fabrication of high-quality GNRs are still limited. Here a way to generate sub-5-nm GNRs by annealing single-walled carbon nanotubes (SWCNTs) on Cu(111) is demonstrated. The structural evolution process is characterized by low-temperature scanning tunneling microscopy. Substrate-dependent measurements on Au(111) and Ru(0001) reveal that the intermediate strong SWCNT-surface interaction plays a pivotal role in the formation of GNRs.

On-surface polymerization reactions of dibrominated hexaphenylbenzene influenced by densely packed self-assembly

Controlled bottom-up fabrication of molecular nanostructures through on-surface reactions of tailor-made precursors is of scientific and technological interest. Recently, on-surface polymerization reactions influenced by precursor self-assembly have been reported. Thus, a fundamental understanding of the reaction process is a prerequisite for controlled formation. Herein, we report on the influence of molecular self-assembly of dibrominated hexaphenylbenzene (Br2-HPB) on the on-surface polymerization reactions on a Au(111) substrate. By using low-temperature scanning tunnelling microscopy (STM), we find that the polymerization of Br2-HPB proceeds while maintaining the long-range ordered self-assembly, despite a decrease in HPB-HPB distance due to debromination and successive covalent bonding of Br2-HPB. From the STM investigations of the polymerization process, we conclude that the polymerization of Br2-HPB is accompanied by molecular rotations to maintain the periodic array of the self-assembled structure, contrary to the conventional understanding of the polymerization of the self-assembled precursor layer.

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.

Silver Surface-Assisted Dehydrobrominative Cross-Coupling between Identical Aryl Bromides

Cross-coupling reactions represent an indispensable tool in chemical synthesis. An intriguing challenge in this field is to achieve selective cross-coupling between two precursors with similar reactivity or, to the limit, the identical molecules. Here we report an unexpected dehydrobrominative cross-coupling between 1,3,5-tris(2-bromophenyl)benzene molecules on silver surfaces. Using scanning tunneling microscopy, we examine the reaction process at the single-molecular level, quantify the selectivity of the dehydrobrominative cross-coupling, and reveal the modulation of selectivity by substrate lattice-related catalytic activity or molecular assembly effect. Theoretical calculations indicate that the dehydrobrominative cross-coupling proceeds via regioselective C-H bond activation of debrominated TBPB and subsequent highly selective C-C coupling of the radical-based intermediates. The reaction kinetics plays an important role in the selectivity for the cross-coupling. This work not only expands the toolbox for chemical synthesis but also provides important mechanistic insights into the selectivity of coupling reactions on the surface.

Predicting Organometallic Intermediates in the Surface-Assisted Ullmann Coupling of Chrysene Isomers

On-surface polymerization of functional organic molecules has been recently recognized as a promising route to persistent low-dimensional structures with tailorable properties. In this contribution, using the coarse-grained Monte Carlo simulation method, we study the initial stage of the Ullmann coupling of doubly halogenated chrysene isomers adsorbed on a catalytically active (111) crystalline surface. To that end, we focus on the formation of labile metal-organic precursor structures preceding the covalent bonding of chrysene monomers. Four monomeric chrysene units with differently distributed halogen substituents were probed in the simulations, and the resulting precursor structures were compared and quantified. Moreover, the effect of (pro)chirality of chrysene tectons on the structure formation was elucidated by running separate simulations in enantiopure and racemic systems. The calculations showed that suitable manipulation of the halogen substitution pattern allows for the creation of diverse precursor architectures, ranging from straight and winded chains to cyclic oligomers with enantiopure, racemic, and nonracemic composition. The obtained findings can be helpful in developing synthetic strategies for covalent polymers with predefined architecture and functionality.

Diels-Alder cycloaddition polymerization for porous poly-phenylenes with exceptional gas uptake properties

We report the synthesis of two-dimensional and three-dimensional porous polyphenylenes (2D/3D-pPPs) via the Diels-Alder cycloaddition polymerization reaction. The resulting 2D and 3D-pPPs showed surface areas up to 1553 m2 g-1, pore volumes of 1.45 cm3 g-1 and very high H2 uptake capacities of 7.4 and 7.1 wt% at 77 K, respectively, along with a competitive high-pressure CO2 and CH4 uptake performance.

Stepwise on-surface synthesis of nitrogen-doped porous carbon nanoribbons

Precise synthesis of carbon-based nanostructures with well-defined structural and chemical properties is of significance towards organic nanomaterials, but remains challenging. Herein, we report on a synthesis of nitrogen-doped porous carbon nanoribbons through a stepwise on-surface polymerization. Scanning tunneling microscopy revealed that the selectivity in molecular conformation, intermolecular debrominative aryl-aryl coupling and inter-chain dehydrogenative cross-coupling determined the well-defined topology and chemistry of the final products. Density functional theory calculations predict that the ribbons are semiconductors, and the band gap can be tuned by the width of the ribbons.

Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

On-surface synthesis of Au-C4 and Au-O4 alternately arranged organometallic coordination networks via selective aromatic C-H bond activation

Selective activation of the C-H bond of aromatic hydrocarbons is significant in synthetic chemistry. However, achieving oriented C-H activation remains challenging due to the poor selectivity of aromatic C-H bonds. Herein, we successfully constructed alternately arranged Au-C4 and Au-O4 organometallic coordination networks through selective aromatic C-H bond activation on Au(111) substrate. The stepwise reaction process of the 5, 12-dibromopyrene 3,4,9, 10-tetracarboxylic dianhydride precursor is monitored by high-resolution scanning tunneling microscopy. Our results show that the gold atoms in C-Au-C organometallic chains play a crucial role in promoting the selective ortho C-H bonds activation and forming Au-C4 coordination structure, which is further demonstrated by a comparative experiment of PTCDA precursor on Au(111). Furthermore, our experiment of 2Br-PTCDA precursor on Cu(111) substrate confirms that copper atoms in C-Cu-C organometallic chains can also assist the formation of Cu-C4 coordination structure. Our results reveal the vital effect of organometallic coordination on selective C-H bond activation of reactants, which holds promising implications for controllable on-surface synthesis.

Synthesis of a Porous [14]Annulene Graphene Nanoribbon and a Porous [30]Annulene Graphene Nanosheet on Metal Surfaces

The electrical and mechanical properties of graphene-based materials can be tuned by the introduction of nanopores, which are sensitively related to the size, morphology, density, and location of nanopores. The synthesis of low-dimensional graphene nanostructures containing well-defined nonplanar nanopores has been challenging due to the intrinsic steric hindrance. Herein, we report the selective synthesis of one-dimensional (1D) graphene nanoribbons (GNRs) containing periodic nonplanar [14]annulene pores on Ag(111) and two-dimensional (2D) porous graphene nanosheet containing periodic nonplanar [30]annulene pores on Au(111), starting from a same precursor. The formation of distinct products on the two substrates originates from the different thermodynamics and kinetics of coupling reactions. The reaction mechanisms were confirmed by a series of control experiments, and the appropriate thermodynamic and kinetic parameters for optimizing the reaction pathways were proposed. In addition, the combined scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations revealed the electronic structures of porous graphene structures, demonstrating the impact of nonplanar pores on the π-conjugation of molecules.

On-surface synthesis of nitrogen-doped nanographene with an [18]annulene pore on Ag(111)

On-surface synthesis is of importance to fabricate low dimensional carbon-based nanomaterials with atomic precision. Here, we synthesize nitrogen-doped nanographene with an [18]annulene pore and its dimer through sequential reactions of debromination, aryl-aryl coupling, cyclodehydrogenation and C-N coupling on Ag(111) from 3,12-dibromo-7,8-diaza[5]helicene. The inner structures of the products were characterized with scanning tunneling microscopy with a CO terminated tip at low temperature. Furthermore, the first four unoccupied electronic states of the nanographene were investigated with a combination of scanning tunneling spectroscopy and theoretical calculations. Except for the LUMO + 2 state observed at +1.3 V, the electronic states at 500 mV, 750 mV and 1.9 V were attributed to the superatom molecular orbitals at the [18]annulene pore, which were significantly shifted towards the Fermi level due to the hybridization with the confined surface state.

On-Surface Synthesis of Graphene Nanoribbons with Atomically Precise Structural Heterogeneities and On-Site Characterizations

Graphene nanoribbons (GNRs) are strips of graphene, with widths of a few nanometers, that are promising candidates for future applications in nanodevices and quantum information processing due to their highly tunable structure-dependent electronic, spintronic, topological, and optical properties. Implantation of periodic structural heterogeneities, such as heteroatoms, nanopores, and non-hexagonal rings, has become a powerful manner for tailoring the designer properties of GNRs. The bottom-up synthesis approach, by combining on-surface chemical reactions based on rationally designed molecular precursors and in situ tip-based microscopic and spectroscopic techniques, promotes the construction of atomically precise GNRs with periodic structural modulations. However, there are still obstacles and challenges lying on the way toward the understanding of the intrinsic structure-property relations, such as the strong screening and Fermi level pinning effect of the normally used transition metal substrates and the lack of collective tip-based techniques that can cover multi-internal degrees of freedom of the GNRs. In this Perspective, we briefly review the recent progress in the on-surface synthesis of GNRs with diverse structural heterogeneities and highlight the structure-property relations as characterized by the noncontact atomic force microscopy and scanning tunneling microscopy/spectroscopy. We furthermore motivate to deliver the need for developing strategies to achieve quasi-freestanding GNRs and for exploiting multifunctional tip-based techniques to collectively probe the intrinsic properties.

Chirality variation from self-assembly on Ullmann coupling for the DBCh adsorbate on Au(111) and Ag(111)

On-surface Ullmann coupling has been considered an appealing approach for the precise fabrication of carbon-based covalent nanostructures under solution-free conditions. However, chirality has seldom been discussed in Ullmann reactions. In this report, self-assembled two-dimensional chiral networks are initially constructed in a large area on Au(111) and Ag(111) after adsorption of the prochiral precursor, 6,12-dibromochrysene (DBCh). Self-assembled phases are then transformed into organometallic (OM) oligomers after debromination, preserving the chirality; in particular, the formation of scarcely reported OM species on Au(111) is discovered herein. With the aryl-aryl bonding induced after intensive annealing, covalent chains are fabricated via the cyclodehydrogenation between chrysene blocks, resulting in the formation of 8-armchair graphene nanoribbons with staggered valleys on both sides. Before chiral polymer chains are constructed by chrysene blocks, the high structural flexibility of OM intermediates on Ag(111) is also revealed during reactions, which is derived from the twofold coordination of Ag atoms and conformationally flexible metal-carbon bonding. Our report not only provides solid evidence of atomically precise fabrication of covalent nanostructures with a feasible bottom-up approach but also sheds insights into the comprehensive investigation of chirality variation from monomers to artificial architectures via surface coupling reactions.

Bonding character of intermediates in on-surface Ullmann reactions revealed with energy decomposition analysis

On-surface synthesis has become a thriving topic in surface science. The Ullmann coupling reaction is the most applied synthetic route today, but the nature of the organometallic intermediate is still under discussion. We investigate the bonding nature of prototypical intermediate species (phenyl, naphthyl, anthracenyl, phenanthryl, and triphenylenyl) on the Cu(111) surface with a combination of plane wave and atomic orbital basis set methods using density functional theory calculations with periodic boundary conditions. The surface bonding is shown to be of covalent nature with a polarized shared-electron bond supported by π-back donation effects using energy decomposition analysis for extended systems (pEDA). The bond angle of the intermediates is determined by balancing dispersion attraction and Pauli repulsion between adsorbate and surface. The latter can be significantly reduced by adatoms on the surface. We furthermore investigate how to choose computational parameters for pEDA of organic adsorbates on metal surfaces efficiently and show that bonding interpretation requires consistent choice of the density functional.

On-Surface Synthesis of Self-Assembled Covalently Linked Wavy Chains with Site-Selective Conformational Switching

Conformational arrangements in polymers on surfaces determine the overall shape as well as the potential properties. It is generally believed that conformational diversity leads to uncontrollable or disordered structures in on-surface synthesis. However, in this study, we obtain two well-ordered self-assembled covalently linked wavy chains with site-selective conformational switching via the Ullmann reaction of 1,2-bis(3-bromophenyl)ethane with multiple conformations on Ag(111). Two kinds of wavy chains exhibit distinct conformational arrangements, where chain I contains one repeating unit conformation of -cis-trans1-cis-trans1-cis-cis-trans1-, while the adjacent parallel parts in wavy chain II have two different conformational arrangements of -cis-cis-trans1- and -cis-cis-trans2-. Wavy chains coassemble with dissociated bromine atoms, suggesting that the Br···H-C interactions between Br atoms and molecular chains are crucial for the construction of ordered wavy chains. High-resolution scanning tunneling microscopy is employed to reveal the surface reaction process at the molecular scale. In depth growth mechanism analysis combined with density functional theory calculations unveils that the substrate also plays an important role in the fabrication of well-ordered wavy chains. The present work extends the surface reaction of conformational flexible precursors.

Tuning Dehalogenative Coupling of Br2Py on Bimetallic Templates

Considerable attention has been paid to on-surface Ullmann coupling during the past decade owing to the feasible synthesis of artificial nanostructures. While previous reports mainly concentrated on coupling reactions on single-metal-atom surfaces, herein we present the Ullmann coupling of 2,7-dibromopyrene (Br₂Py) on bimetallic surfaces, Bi-Ag(111) and Bi-Au(111), respectively, with scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). On the Bi-decorated Ag(111), self-assembly of intact Br₂Py is realized due to the reduced activity at the interface. Subsequent annealing promotes the dehalogenation of Br₂Py on Bi-Ag(111), while Bi adatoms do not bring any visible influence on coupling reactions. Furthermore, post-deposition of Bi onto preassembled nanostructures on Ag(111) immediately initiates the Ullmann coupling by inducing more Ag adatoms available on the surface, while stepwise annealing afterward leads to complete polymerization and formation of covalent chains with lateral displacement compared to that on the bare Ag(111), probably due to the space hindrance and confinement. For Bi-Au(111) with the modified reconstruction, higher-temperature annealing is required to trigger Ullmann coupling compared to that on Au(111). The exception is that the C-C coupling reaction remains impervious to Bi adatoms, and recovery of the Bi-Au reconstruction is realized after intensive annealing. In principle, bimetallic surfaces herein present intriguing behavior toward the controllable Ullmann coupling, and this report might provide different insights into the comprehensive atomistic elucidation of reaction mechanisms as well as the design of a new platform to effectively regulate Ullmann coupling.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies

The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface. From a single molecular precursor correspondingly following a trans- and cis-coupling, the DV8-aGNR and another porous nanographene are respectively formed at step edges and on terraces at 720 and 570 K. Combining scanning tunneling microscopy/spectroscopy, atomic force microscopy, and first-principles calculations, we determine the out-of-plane conformation, wide bandgap (∼3.36 eV), and wiggly shaped frontier orbitals of the DV8-aGNR. Nudged elastic band calculations further quantitatively reveal that the additional steric hindrance effect in the cyclodehydrogenative reactions has a higher barrier of 1.3 eV than that in the planar porous nanographene, which also unveils the important role played by the monatomic Au step and adatoms in reducing the energy barriers and enhancing the thermodynamic preference of the oxidative cyclodehydrogenation. Our results provide the first case of GNRs containing periodic pores as small as divacancies with a nonplanar configuration and demonstrate the strategy by utilizing the chemical heterogeneity of a substrate to promote the formation of novel carbon nanomaterials.

Low-Dimensional Porous Carbon Networks Using Single-/Triple-Coupling Polycyclic Hydrocarbon Precursors

Polycyclic hydrocarbons (PHs) share the same hexagonal structure of sp² carbons as graphene but possess an energy gap due to quantum confinement effect. PHs can be synthesized by a bottom-up strategy starting from small building blocks covalently bonded into large 2D organic sheets. Further investigation of the role of the covalent bonding/coupling ways on their electronic properties is needed. Here, we demonstrate a surface-mediated synthesis of hexa-peri-hexabenzocoronene (HBC) and its extended HBC oligomers (dimers, trimers, and tetramers) via single- and triple-coupling ways and reveal the implication of different covalent bonding on their electronic properties. High-resolution low-temperature scanning tunneling microscopy and noncontact atomic force microscopy are employed to in situ determine the atomic structures of as-synthesized HBC oligomers. Scanning tunneling spectroscopy measurements show that the length extension of HBC oligomers narrows the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, the energy gaps of triple-coupling HBC oligomers are smaller and decrease more significantly than that of the single-coupling ones. We hypothesize that the triple coupling promotes a more effective delocalization of π-electrons than the single coupling, according to density functional theory calculations. We also demonstrate that the HBC oligomers can further extend across the substrate steps to achieve conjugated polymers and large-area porous carbon networks.

Monte Carlo Simulations of the Metal-Directed Self-Assembly of Y-Shaped Positional Isomers

The rational fabrication of low-dimensional materials with a well-defined topology and functions is an incredibly important aspect of nanotechnology. In particular, the on-surface synthesis (OSS) methods based on the bottom-up approach enable a facile construction of sophisticated molecular architectures unattainable by traditional methods of wet chemistry. Among such supramolecular constructs, especially interesting are the surface-supported metal–organic networks (SMONs), composed of low-coordinated metal atoms and π-aromatic bridging linkers. In this work, the lattice Monte Carlo (MC) simulation technique was used to extract the chemical information encoded in a family of Y-shaped positional isomers co-adsorbed with trivalent metal atoms on a flat metallic surface with (111) geometry. Depending on the intramolecular distribution of active centers (within the simulated molecular bricks, we observed a metal-directed self-assembly of two-dimensional (2D) openwork patterns, aperiodic mosaics, and metal–organic ladders. The obtained theoretical findings could be especially relevant for the scanning tunneling microscopy (STM) experimentalists interested in a surface-assisted construction of complex nanomaterials stabilized by directional coordination bonds.

Chemically imaging nanostructures formed by the covalent assembly of molecular building blocks on a surface with ultrahigh vacuum tip-enhanced Raman spectroscopy

Surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. However, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogeneous two-dimensional (2D) system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution. Here, low temperature ultrahigh vacuum scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the formation of 2D covalent networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br₄TPP) molecules on a Ag(100) substrate. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirm the covalent nature of the 2D networks and suggest that the apparent disorder arises from molecular flexibility.

Supramolecular Tiling of a Conformationally Flexible Precursor

Supramolecular self-assembly offers a possible pathway for nanopatterning and functionality. In particular, molecular tiling such as trihexagonal tiling (also known as the Kagome lattice) has promising chemical and physical properties. Distorted Kagome lattices are not well understood due to their complexity, and studies of their controllable fabrication are few. Here, by employing a conformationally flexible precursor, 2,4,6-tris(3-bromophenyl)-1,3,5-triazine (mTBPT), we demonstrate two-dimensional distorted Kagome lattice p3, (333) by supramolecular self-assembly and achieve tuning of the metastable phases, including the homochiral porous network and distorted Kagome lattice p3, (333) by steering deposition rates on a cold Ag(111) substrate. By a combination of scanning tunneling microscopy and density functional theory calculations, the distorted Kagome lattice is energetically unfavorable but can be trapped at a high deposition rate, and the process mainly depends on surface kinetics. This work using conformationally flexible mTBPT molecules provides a pathway for the controllable growth of different phases, including metastable Kagome lattices.

Theoretical modeling of the metal-organic precursors of anthracene-based covalent networks on surfaces

Surface-assisted fabrication of molecular network architectures has been a promising route to low-dimensional materials with unique physicochemical properties and functionalities. One versatile way in this field is the Ullmann coupling reaction of halogenated organic monomers on catalytically active metallic surfaces. In this work, using the coarse grained Monte Carlo simulations, we studied the on-surface self-assembly of metal-organic precursors preceding the covalent Ullman-type linkage of tetrahalogenated anthracene building blocks. To that end a series of positional isomers was examined and classified with respect to their ability of creation of extended network structures. Our simulations focused on the identification of basic types of self-assembly scenarios distinguishing enantiopure and racemic systems and producing periodic and aperiodic networks. The calculations carried out for selected tectons demonstrated wide possibilities of controlling porosity (e.g. pore size, shape, periodicity, chirality, heterogeneity) of the networks by suitable functionalization of the monomeric unit. The findings reported here can be helpful in rational designing of 2D polymeric networks with predefined structures and properties.

Borazatruxenes as precursors for hybrid C-BN 2D molecular networks

Synthesizing atomically thin, crystalline two-dimensional (2D) molecular materials which combine carbon with other elements is an emerging field requiring both custom-designed molecular precursors and their ability to organize into networks (hydrogen-bonded or covalent). Hybrid carbon-boron nitride (C-BN) networks face the additional challenge of needing hydrolytically-stable BN-containing molecular precursors. Here, we show that borazatruxenes (truxene-like molecules with a borazine core) and their halogenated derivatives are highly stable precursors suitable for on-surface assembly. Using scanning tunneling microscopy (STM) and density functional theory (DFT) simulations we demonstrate hierarchical H-bonded assembly based on chiral homodimers of tribromo-borazatruxenes (3Br-borazatruxenes) as building blocks for both 1D chains and 2D networks. A low-symmetry, H-bonded chiral 2D lattice forms on Au(111) from the C₃-symmetric 3Br-borazatruxenes, leading to large enantiomorphic domains that are molecularly homochiral. Such homochiral segregation is a necessary condition if chiral C-BN covalent networks are to be obtained via subsequent on-surface reactions. We show via DFT that up to two Na atoms can be trapped within the small pores of this dense lattice, while further Na atoms can adsorb on preferred network sites; this leads to hybrid Na-molecular network electronic bands with anisotropic dispersion and significant (up to hundreds of meV) bandwidths, as well as significant doping, that can engender anisotropic transport through the network. Finally, electronic structure comparisons (combining both experiment and computation) between borazatruxene, its tri-brominated derivative and truxene show that the borazine core controls the band gap increase, while also inducing C-B p_z-p_z electron delocalization that facilitates a continuous electron path across the molecule. Furthermore, as shown by DFT, the borazine core drives inter-layer B-N polar interactions that promote adsorption of BN containing molecules in a staggered configuration, a mechanism to be exploited in layer-by-layer supra-molecular assembly of novel hybrid C-BN materials.

On-Surface Synthesis of Variable Bandgap Nanoporous Graphene

Tuning the bandgap of nanoporous graphene is desirable for applications such as the charge transport layer in organic-hybrid devices. The holy grail in the field is the ability to synthesize 2D nanoporous graphene with variable pore sizes, and hence tunable band gaps. Herein, the on-surface synthesis of nanoporous graphene with variable bandgaps is demonstrated. Two types of nanoporous graphene are synthesized via hierarchical CC coupling, and are verified by low-temperature scanning tunneling microscopy and non-contact atomic force microscopy. Nanoporous graphene-1 is non-planar, and nanoporous graphene-2 is a single-atom thick planar sheet. Scanning tunneling spectroscopy measurements reveal that nanoporous graphene-2 has a bandgap of 3.8 eV, while nanoporous graphene-1 has a larger bandgap of 5.0 eV. Corroborated by first-principles calculations, it is proposed that the large bandgap opening is governed by the confinement of π-electrons induced by pore generation and the non-planar structure. The finding shows that by introducing nanopores or a twisted structure, semi metallic graphene is converted into semiconducting nanoporous graphene-2 or insulating wide-bandgap nanoporous graphene-1.

On-surface synthesis of π-conjugated ladder-type polymers comprising nonbenzenoid moieties

On-surface synthesis provides a powerful approach toward the atomically precise fabrication of π-conjugated ladder polymers (CLPs). We report herein the surface-assisted synthesis of nonbenzenoid CLPs from cyclopenta-annulated anthracene monomers on Au(111) under ultrahigh vacuum conditions. Successive thermal annealing steps reveal the dehalogenative homocoupling to yield an intermediate 1D polymer and the subsequent cyclodehydrogenation to form the fully conjugated ladder polymer. Notably, neighbouring monomers may fuse in two different ways, resulting in six- and five-membered rings, respectively. The structure and electronic properties of the reaction products have been investigated via low-temperature scanning tunneling microscopy and spectroscopy, complemented by density-functional theory calculations. Our results provide perspectives for the on-surface synthesis of nonbenzenoid CLPs with the potential to be used for organic electronic devices.

Surface-assisted fabrication of low-dimensional carbon-based nanoarchitectures

On-surface synthesis, as an alternative to traditional in-solution synthesis, has become an emerging research field and attracted extensive attention over the past decade due to its ability to fabricate nanoarchitectures with exotic properties. Compared to wet chemistry, the on-surface synthesis conducted on atomically flat solid surfaces under ultrahigh vacuum exhibits unprecedented characteristics and advantages, opening novel reaction pathways for chemical synthesis. Various low-dimensional nanostructures have been fabricated on solid surfaces (mostly metal surfaces) based on this newly developed approach. This paper reviews the classic and latest works regarding carbon-based low-dimensional nanostructures since the arrival of on-surface synthesis era. These nanostructures are categorized into zero-, one- and two-dimensional classes and each class is composed of numerous sub-nanostructures. For certain specific nanostructures, comprehensive reports are given, including precursor design, substrate choice, synthetic strategies and so forth. We hope that our review will shed light on the fabrication of some significant nanostructures in this young and promising scientific area.

An intermolecular hydrogen bond plays a determining role in product selection of a surface confined Schiff-base reaction

Herein, we illustrate how the cooperation of intermolecular hydrogen bonds and conformation flexibility leads to the formation of diverse complex covalent nanostructures on the surface, while the relative abundance of the final products can be further tuned by adjusting the molar ratio and concentration of monomers.

Topology Selectivity in On-Surface Dehydrogenative Coupling Reaction: Dendritic Structure versus Porous Graphene Nanoribbon

Selective control on the topology of low-dimensional covalent organic nanostructures in on-surface synthesis has been challenging. Herein, with combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), we report a successful topology-selective coupling reaction on the Cu(111) surface by tuning the thermal annealing procedure. The precursor employed is 1,3,5-tris(2-bromophenyl)benzene (TBPB), for which Ullmann coupling is impeded due to the intermolecular steric hindrance. Instead, its chemisorption on the Cu(111) substrate has triggered the ortho C-H bond activation and the following dehydrogenative coupling at room temperature (RT). In the slow annealing experimental procedure, the monomers have been preorganized by their self-assembly at RT, which enhances the formation of dendritic structures upon further annealing. However, the chaotic chirality of dimeric products (obtained at RT) and hindrance from dense molecular island make the fabrication of high-quality porous two-dimensional nanostructures difficult. In sharp contrast, direct deposition of TBPB molecules on a hot surface led to the formation of ordered porous graphene nanoribbons and nanoflakes, which is confirmed to be the energetically favorable reaction pathway through density functional theory-based thermodynamic calculations and control experiments. This work demonstrates that different thermal treatments could have a significant influence on the topology of covalent products in on-surface synthesis and presents an example of the negative effect of molecular self-assembly to the ordered covalent nanostructures.

Searching for kagome multi-bands and edge states in a predicted organic topological insulator

Recently, mixed honeycomb-kagome lattices featuring metal-organic networks have been theoretically proposed as topological insulator materials capable of hosting nontrivial edge states. This new family of so-called "organic topological insulators" are purely two-dimensional and combine polyaromatic-flat molecules with metal adatoms. However, their experimental validation is still pending given the generalized absence of edge states. Here, we generate one such proposed network on a Cu(111) substrate and study its morphology and electronic structure with the purpose of confirming its topological properties. The structural techniques reveal a practically flawless network that results in a kagome network multi-band observed by angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. However, at the network island borders we notice the absence of edge states. Bond-resolved imaging of the network exhibits an unexpected structural symmetry alteration that explains such disappearance. This collective lifting of the network symmetry could be more general than initially expected and provide a simple explanation for the recurrent experimental absence of edge states in predicted organic topological insulators.

Designing 2D covalent networks with lattice Monte Carlo simulations: precursor self-assembly

Organic synthesis reactions in the adsorbed phase have been recently an intensively studied topic in heterogeneous catalysis and material engineering. One of such processes is the Ullmann coupling in which halogenated organic monomers are transformed into covalently bonded polymeric structures. In this work, we use the lattice Monte Carlo simulation method to study the on-surface self-assembly of organometallic precursor architectures comprising tetrasubstituted naphthalene building blocks with differently distributed halogen atoms. In the coarse grained approach adopted herein the molecules and metal atoms were modeled by discrete segments, two connected and one, respectively, placed on a triangular lattice representing a (111) metallic surface. Our simulations focused on the influence of the intramolecular distribution of the substituents on the morphology of the resulting superstructures. Special attention was paid to the molecules that create porous networks characterized by long-range order. Moreover, the structural analysis of the assemblies comprising prochiral building blocks was made by running simulations for the corresponding enantiopure and racemic adsorbed systems. The obtained results demonstrated the possibility of directing the on-surface self-assembly towards networks with controllable pore shape and size. These findings can be helpful in designing covalently bonded 2D superstructures with predefined architecture and functions.

Initiating Ullmann-like coupling of Br2Py by a semimetal surface

Intensive efforts have been devoted to surface Ullmann-like coupling in recent years, due to its appealing success towards on-surface synthesis of tailor-made nanostructures. While attentions were mostly drawn on metallic substrates, however, Ullmann dehalogenation and coupling reaction on semimetal surfaces has been seldom addressed. Herein, we demonstrate the self-assembly of 2, 7-dibromopyrene (Br₂Py) and the well controllable dehalogenation reaction of Br₂Py on the Bi(111)–Ag substrate with a combination of scanning tunnelling microscopy (STM) and density functional theory calculations (DFT). By elaborately investigating the reaction path and formed organic nanostructures, it is revealed that the pristinely inert bismuth layer supported on the silver substrate can initiate Ullmann-like coupling in a desired manner by getting alloyed with Ag atoms underneath, while side products have not been discovered. By clarifying the pristine nature of Bi–Ag(111) and Ullmann-like reaction mechanisms, our report proposes an ideal template for thoroughly exploring dehalogenative coupling reaction mechanisms with atomic insights and on-surface synthesis of carbon-based architectures.

Halogen bonding controlled 2D self-assembled polymorphism of regioisomeric thienophenanthrene derivatives by coadsorption

5,10-BTD-C14 molecules display self-assembled polymorphism in different bromo-hydrocarbons with different solution concentrations at the liquid/solid interface.

Metalated Graphyne-Based Networks as Two-Dimensional Materials: Crystallization, Topological Defects, Delocalized Electronic States, and Site-Specific Doping

Graphyne-based two-dimensional (2D) carbon allotropes feature extraordinary physical properties; however, their synthesis as crystalline single-layered materials has remained challenging. We report on the fabrication of large-area organometallic Ag-bis-acetylide networks and their structural and electronic properties on Ag(111) using low-temperature scanning tunneling microscopy combined with density functional theory (DFT) calculations. The metalated graphyne-based networks are robust at room temperature and assembled in a bottom-up approach via surface-assisted dehalogenative homocoupling of terminal alkynyl bromides. Large-area networks of several hundred nanometers with topological defects at domain boundaries are obtained due to the Ag-acetylide bonds' reversible nature. The thermodynamically controlled growth mechanism is explained through the direct observation of intermediates, which differ on Ag(111) and Au(111). Scanning tunneling spectroscopy resolved unoccupied states delocalized across the network. The energy of these states can be shifted locally by the attachment of a different number of Br atoms within the network. DFT revealed that free-standing metal-bis-acetylide networks are semimetals with a linear band dispersion around several high-symmetry points, which suggest the presence of Weyl points. These results demonstrate that the organometallic Ag-bis-acetylide networks feature the typical 2D material properties, which make them of great interest for fundamental studies and electronic materials in devices.

On-Surface Assembly of Hydrogen- and Halogen-Bonded Supramolecular Graphyne-Like Networks

Demonstrated here is a supramolecular approach to fabricate highly ordered monolayered hydrogen‐ and halogen‐bonded graphyne‐like two‐dimensional (2D) materials from triethynyltriazine derivatives on Au(111) and Ag(111). The 2D networks are stabilized by N⋅⋅⋅H−C(sp) bonds and N⋅⋅⋅Br−C(sp) bonds to the triazine core. The structural properties and the binding energies of the supramolecular graphynes have been investigated by scanning tunneling microscopy in combination with density‐functional theory calculations. It is revealed that the N⋅⋅⋅Br−C(sp) bonds lead to significantly stronger bonded networks compared to the hydrogen‐bonded networks. A systematic analysis of the binding energies of triethynyltriazine and triethynylbenzene derivatives further demonstrates that the X₃‐synthon, which is commonly observed for bromobenzene derivatives, is weaker than the X₆‐synthon for our bromotriethynyl derivatives.

Simulations of the 2D self-assembly of tripod-shaped building blocks

We introduce a molecular dynamics (MD) coarse-grained model for the description of tripod building blocks. This model has been used by us already for linear, V-shape, and tetratopic molecules. We wanted to further extend its possibilities to trifunctional molecules to prove its versatility. For the chosen systems we have also compared the MD results with Monte Carlo results on a triangular lattice. We have shown that the constraints present in the latter method can enforce the formation of completely different structures, not reproducible with off-lattice simulations. In addition to that, we have characterized the obtained structures regarding various parameters such as theoretical diffraction pattern and average association number.

On-surface synthesis of planar acenes via regioselective aryl-aryl coupling

The reaction of 2,2'-dibromo-biphenyl on a Ag(111) surface leads to the formation of planar acenes with a high-regioselectivity rather than nonplanar saddle-shaped tetraphenylene as the typical product in solution chemistry. The regioselective aryl-aryl coupling reaction is attributed to the hydrogen repulsion between the reactants on the confined two-dimensional surface.

Direct observation of the geometric isomer selectivity of a reaction controlled via adsorbed bromine

Methods to improve the specificity of stereoselective reactions are paramount to the viability of reaction-based processes. Surface-bound methods are a powerful means to carry out reactions with selectivity in the pursuit of specific products or nanoarchitectures through bottom-up assembly. The Ullmann-like coupling reaction has come to represent one of the most useful methods to form two-dimensional structures through covalent couplings of aromatic molecules following the dissociation of an aryl carbon-halide bond. The leaving halogen atoms are proven to remain adsorbed on the surface and can be deleterious to the fabrication of larger conjugated superstructures. However, on Au(100) we have found the leaving halogen atoms generate a new adsorbate surface that leads to geometric isomer selectivity compared to the unmodified metal surface. The covalent coupling of 3,6-dibromo-phenanthrenequinone (DBPQ) was studied and leaving bromine atoms were found to form self-assembled islands and modify the reconstruction of Au(100). Subsequently, the coupling reaction yielded total selectivity towards a radical trans dimer when surrounded by bromine atoms, while only cis dimers were observed on the undecorated Au surface. This selectivity induced by bromine networks on the surface ultimately results in another potent way to control the stereoselectivity of surface-bound coupling reactions.

Synthesis of nanoporous graphenes via decarboxylation reaction

Two new crystalline structures of nanoporous graphenes were synthesized via decarboxylation reaction and provided an ideal platform for oxygen evolution reaction.