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.

Exploring Approaches for the Synthesis of Few-Layered Graphdiyne

Graphdiyne (GDY) is an emerging carbon allotrope in the graphyne (GY) family, demonstrating extensive potential applications in the fields of electronic devices, catalysis, electrochemical energy storage, and nonlinear optics. Synthesis of few-layered GDY is especially important for both electronic applications and structural characterization. This work critically summarizes the state-of-art of GDY and focuses on exploring approaches for few-layered GDY synthesis. The obstacles and challenges of GDY synthesis are also analyzed in detail. Recently developed synthetic methods are discussed such as i) the copper substrate-based method, ii) the chemical vapor deposition (CVD) method, iii) the interfacial construction method, and iv) the graphene-templated method. Throughout the discussion, the superiorities and limitations of different methods are analyzed comprehensively. These synthetic methods have provided considerable inspiration approaching synthesis of few-layered or single-layered GDY film. The work concludes with a perspective on promising research directions and remaining barriers for layer-controlled and morphology-controlled synthesis of GDY with higher crystalline quality.

On-Surface Activation of Trimethylsilyl-Terminated Alkynes on Coinage Metal Surfaces

The controlled attachment of protecting groups combined with the ability to selectively abstract them is central to organic synthesis. The trimethylsilyl (TMS) functional group is a popular protecting group in solution. However, insights on its activation behavior under ultra-high vacuum (UHV) and surface-confined conditions are scarce. Here we investigate a series of TMS-protected alkyne precursors via scanning tunneling microscopy (STM) regarding their compatibility with organic molecular beam epitaxy (OMBE) and their potential deprotection on various coinage metal surfaces. After in-situ evaporation on the substrates held in UHV at room temperature, we find that all molecules arrived and adsorbed as intact units forming ordered supramolecular aggregates stabilized by non-covalent interactions. Thus, TMS-functionalized alkyne precursors with weights up to 1100 atomic mass units are stable against OMBE evaporation in UHV. Furthermore, the TMS activation through thermal annealing is investigated with STM and X-ray photoelectron spectroscopy (XPS). We observe that deprotection starts to occur between 400 K and 500 K on the copper and gold surfaces, respectively. In contrast, on silver surfaces, the TMS-alkyne bond remains stable up to temperatures where molecular desorption sets in (≈600 K). Hence, TMS functional groups can be utilized as leaving groups on copper and gold surfaces while they serve as protecting groups on silver surfaces.

Functionalized Graphdiyne Nanowires: On-Surface Synthesis and Assessment of Band Structure, Flexibility, and Information Storage Potential

Carbon nanomaterials exhibit extraordinary mechanical and electronic properties desirable for future technologies. Beyond the popular sp² -scaffolds, there is growing interest in their graphdiyne-related counterparts incorporating both sp² and sp bonding in a regular scheme. Herein, we introduce carbonitrile-functionalized graphdiyne nanowires, as a novel conjugated, one-dimensional (1D) carbon nanomaterial systematically combining the virtues of covalent coupling and supramolecular concepts that are fabricated by on-surface synthesis. Specifically, a terphenylene backbone is extended with reactive terminal alkyne and polar carbonitrile (CN) moieties providing the required functionalities. It is demonstrated that the CN functionalization enables highly selective alkyne homocoupling forming polymer strands and gives rise to mutual lateral attraction entailing room-temperature stable double-stranded assemblies. By exploiting the templating effect of the vicinal Ag(455) surface, 40 nm long semiconducting nanowires are obtained and the first experimental assessment of their electronic band structure is achieved by angle-resolved photoemission spectroscopy indicating an effective mass below 0.1m₀ for the top of the highest occupied band. Via molecular manipulation it is showcased that the novel oligomer exhibits extreme mechanical flexibility and opens unexplored ways of information encoding in clearly distinguishable CN-phenyl trans-cis species. Thus, conformational data storage with density of 0.36 bit nm^-2 and temperature stability beyond 150 K comes in reach.

Chemical Vapor Deposition Growth of Linked Carbon Monolayers with Acetylenic Scaffoldings on Silver Foil

Graphdiyne analogs, linked carbon monolayers with acetylenic scaffoldings, are fabricated by adopting low-temperature chemical vapor deposition which provides a route for the synthesis of two-dimensional carbon materials via molecular building blocks. The electrical conductivity of the as-grown films can reach up to 6.72 S cm^-1 . Moreover, the films show potential as promising substrates for fluorescence suppressing and Raman advancement.

Template-controlled Sonogashira cross-coupling reactions on a Au(111) surface

We employed three metal-coordination structures as templates to control the Sonogashira cross-coupling reactions on a Au(111) surface.

Reaction mechanisms for on-surface synthesis of covalent nanostructures

In recent years, on-surface synthesis has become an increasingly popular strategy to form covalent nanostructures. The approach has great prospects for facilitating the manufacture of a range of fascinating materials with atomic precision. However, the on-surface reactions are enigmatic to control, currently restricting its bright perspectives and there is a great need to explore how the reactions are governed. The objective of this topical review is to summarize theoretical work that has focused on comprehending on-surface synthesis protocols through studies of reaction mechanisms.

Surface-Activated Coupling Reactions Confined on a Surface

Chemical reactions may take place in a pure phase of gas or liquid or at the interface of two phases (gas-solid or liquid-solid). Recently, the emerging field of "surface-confined coupling reactions" has attracted intensive attention. In this process, reactants, intermediates, and products of a coupling reaction are adsorbed on a solid-vacuum or a solid-liquid interface. The solid surface restricts all reaction steps on the interface, in other words, the reaction takes place within a lower-dimensional, for example, two-dimensional, space. Surface atoms that are fixed in the surface and adatoms that move on the surface often activate the surface-confined coupling reactions. The synergy of surface morphology and activity allow some reactions that are inefficient or prohibited in the gas or liquid phase to proceed efficiently when the reactions are confined on a surface. Over the past decade, dozens of well-known "textbook" coupling reactions have been shown to proceed as surface-confined coupling reactions. In most cases, the surface-confined coupling reactions were discovered by trial and error, and the reaction pathways are largely unknown. It is thus highly desirable to unravel the mechanisms, mechanisms of surface activation in particular, of the surface-confined coupling reactions. Because the reactions take place on surfaces, advanced surface science techniques can be applied to study the surface-confined coupling reactions. Among them, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) are the two most extensively used experimental tools. The former resolves submolecular structures of individual reactants, intermediates, and products in real space, while the latter monitors the chemical states during the reactions in real time. Combination of the two methods provides unprecedented spatial and temporal information on the reaction pathways. The experimental findings are complemented by theoretical modeling. In particular, density-functional theory (DFT) transition-state calculations have been used to shed light on reaction mechanisms and to unravel the trends of different surface materials. In this Account, we discuss recent progress made in two widely studied surface-confined coupling reactions, aryl-aryl (Ullmann-type) coupling and alkyne-alkyne (Glaser-type) coupling, and focus on surface activation effects. Combined experimental and theoretical studies on the same reactions taking place on different metal surfaces have clearly demonstrated that different surfaces not only reduce the reaction barrier differently and render different reaction pathways but also control the morphology of the reaction products and, to some degree, select the reaction products. We end the Account with a list of questions to be addressed in the future. Satisfactorily answering these questions may lead to using the surface-confined coupling reactions to synthesize predefined products with high yield.

On-surface synthesis of carbon-based scaffolds and nanomaterials using terminal alkynes

The covalent linking of acetylene compounds is an important synthetic tool to control carbon-carbon bond formation and has been extensively studied for more than a century. Notably, Glaser coupling and subsequently developed refined procedures present an important route for the fabrication of distinct carbon-based scaffolds incorporating units with both sp(2)- and sp-hybridizations, such as carbyne chains, or two-dimensional (2D) graphyne or graphdiyne networks. However, the realization of the envisioned regular low-dimensional compounds and nanoarchitectures poses formidable challenges when following conventional synthesis protocols in solution, which we briefly overview. Now, recent developments in on-surface synthesis establish novel means for the construction of tailored covalent nanostructures under ultrahigh vacuum conditions. Here we focus on the exploration of pathways utilizing interfacial synthesis with terminal alkynes toward the atomically precise fabrication of low-dimensional carbon-rich scaffolds and nanomaterials. We review direct, molecular-level investigations, mainly relying on scanning probe microscopy, providing atomistic insights into thermally activated reaction schemes, their special pathways and products. Using custom-made molecular units, the employed homocoupling, cyclotrimerization, cycloaddition, and radical cyclization processes indeed yield distinct compounds, extended oligomers or 2D networks. Detailed insights into surface interactions such as bonding sites or conformational adaptation, and specific reaction mechanisms, including hierarchic pathways, were gained by sophisticated density functional theory calculations, complemented by X-ray spectroscopy measurements. For the fabrication of regular nanostructures and architectures, it is moreover imperative to cope with spurious side reactions, frequently resulting in chemical diversity. Accordingly, we highlight measures for increasing chemo- and regioselectivity by smart precursor design, substrate templating, and external stimuli. The ensuing preorganization of functional groups and control of side reactions increases product yields markedly. Finally, the electronic band structures of selected cases of novel low-dimensional hydrocarbon materials accessible with the monomers employed to date are discussed with a specific focus on their differences to theoretically established graphyne- and graphdiyne-related scaffolds. The presented methodology and gained insights herald further advancements in the field, heading toward novel molecular compounds, low-dimensional nanostructures, and coherently reticulated polymeric layers, eventually presenting well-defined arrangements with specific carbon-carbon bond sequencing and electronic characteristics. The functional properties of these or other foreseeable scaffolds and architectures bear significant prospects for a wide range of applications, for example, in nanoelectronics, photonics, or carbon-based technologies.

Quantum confinement in self-assembled two-dimensional nanoporous honeycomb networks at close-packed metal surfaces

Quantum confinement of a two-dimensional electron gas by supramolecular nanoporous networks is investigated using the boundary elements method based on Green's functions for finite geometries and electron plane wave expansion for periodic systems. The "particle in a box" picture was analyzed for cases with selected symmetries that model previously reported architectures constructed from organic and metal-organic scattering centers confining surface state electrons of Ag(111) and Cu(111). First, by analyzing a series of cases with systematically defined parameters (scattering geometry, potentials, and effective broadening), we demonstrate how the scattering processes affect the properties of the confined electrons. For the features of the local density of states reported by scanning tunneling spectroscopy (STS), we disentangle the contributions of lifetime broadening and splitting of quantum well states due to coupling of neighboring quantum dots. For each system, we analyze the local electron density distribution and relate it to the corresponding band structure as calculated within the plane-wave expansion framework. Then, we address two experimental investigations, where in one case only STS data and in the other case mainly angle-resolved photoemission spectroscopy (ARPES) data were reported. In both cases, the experimental findings can be successfully simulated. Furthermore, the missing information can be complemented because our approach allows to correlate the information obtained by STS with that of ARPES. The combined analysis of several observations suggests that the scattering potentials created by the network originate primarily from the adsorbate-induced changes of the local surface dipole barrier.

Synthesis, characterization, monolayer assembly and 2D lanthanide coordination of a linear terphenyl-di(propiolonitrile) linker on Ag(111)

As a continuation of our work employing polyphenylene-dicarbonitrile molecules and in particular the terphenyl derivative 1 (TDCN), we have synthesized a novel ditopic terphenyl-4,4"-di(propiolonitrile) (2) linker for the self-assembly of organic monolayers and metal coordination at interfaces. The structure of the organic linker 2 was confirmed by single crystal X-ray diffraction analysis (XRD). On the densely packed Ag(111) surface, the terphenyl-4,4"-di(propiolonitrile) linkers self-assemble in a regular, molecular chevron arrangement exhibiting a Moiré pattern. After the exposure of the molecular monolayer to a beam of Gd atoms, the propiolonitrile groups get readily involved in metal-ligand coordination interactions. Distinct coordination motifs evolve with coordination numbers varying between three and six for the laterally-bound Gd centers. The linker molecules retain an overall flat adsorption geometry. However, only networks with restricted local order were obtained, in marked contrast to previously employed, simpler polyphenylene-dicarbonitrile 1 linkers.

Synthesis of extended graphdiyne wires by vicinal surface templating

Surface-assisted covalent synthesis currently evolves into an important approach for the fabrication of functional nanostructures at interfaces. Here, we employ scanning tunneling microscopy to investigate the homocoupling reaction of linear, terminal alkyne-functionalized polyphenylene building-blocks on noble metal surfaces under ultrahigh vacuum. On the flat Ag(111) surface, thermal activation triggers a variety of side-reactions resulting in irregularly branched polymeric networks. Upon alignment along the step-edges of the Ag(877) vicinal surface drastically improves the chemoselectivity of the linking process permitting the controlled synthesis of extended-graphdiyne wires with lengths reaching 30 nm. The ideal hydrocarbon scaffold is characterized by density functional theory as a 1D, direct band gap semiconductor material with both HOMO and LUMO-derived bands promisingly isolated within the electronic structure. The templating approach should be applicable to related organic precursors and different reaction schemes thus bears general promise for the engineering of novel low-dimensional carbon-based materials.