Carbon-Carbon Coupling on Inert Surfaces by Deposition of En Route Generated Aryl Radicals

Growth of Single Crystalline 2D Materials beyond Graphene on Non-metallic Substrates

The advent of 2D materials has ushered in the exploration of their synthesis, characterization and application. While plenty of 2D materials have been synthesized on various metallic substrates, interfacial interaction significantly affects their intrinsic electronic properties. Additionally, the complex transfer process presents further challenges. In this context, experimental efforts are devoted to the direct growth on technologically important semiconductor/insulator substrates. This review aims to uncover the effects of substrate on the growth of 2D materials. The focus is on non-metallic substrate used for epitaxial growth and how this highlights the necessity for phase engineering and advanced characterization at atomic scale. Special attention is paid to monoelemental 2D structures with topological properties. The conclusion is drawn through a discussion of the requirements for integrating 2D materials with current semiconductor-based technology and the unique properties of heterostructures based on 2D materials. Overall, this review describes how 2D materials can be fabricated directly on non-metallic substrates and the exploration of growth mechanism at atomic scale.

Surface-confined alternating copolymerization with molecular precision by stoichiometric control

Keen desires for artificial mimicry of biological polymers and property improvement of synthesized ones have triggered intensive explorations for sequence-controlled copolymerization. However, conventional synthesis faces great challenges to achieve this goal due to the strict requirements on reaction kinetics of comonomer pairs and tedious synthetic processes. Here, sequence-controlled alternating copolymerization with molecular precision is realized on surface. The stoichiometric control serves as a thermodynamic strategy to steer the polymerization selectivity, which enables the selective alternating organometallic copolymerization via intermolecular metalation of 4,4"-dibromo-p-terphenyl (P-Br) and 2,5-diethynyl-1,4-bis(phenylethynyl)benzene (A-H) with Ag adatoms on Ag(111) at P-Br: A-H = 2, as verified by scanning tunneling microscopy and density functional theory studies. In contrast, homopolymerization yield increases as the stoichiometric ratio deviates from 2. The microscopic characterizations rationalize the mechanism, providing a delicate explanation of the stoichiometry-dependent polymerization. These findings pave a way to actualizing an efficient sequence control of copolymerization by surface chemistry.

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.

On-Surface Two-Dimensional Polymerization: Advances, Challenges, and Prospects

Two-dimensional polymers (2DPs) are molecularly thin networks consisting of monomers covalently linked in at least two directions in the molecular plane. Because of the unique structural features and emergent physicochemical properties, 2DPs promise application potentials in catalysis, chemical sensing, and organic electronic devices. On-surface synthesis is of great interest to fabricate 2DPs with atomic precision, and the properties of the 2DPs can be characterized in situ through scanning probe techniques. In this Perspective, we first introduce the recent developments of on-surface 2D polymerization, including the design principle, the synthetic reactions, and the factors affecting the synthesis of 2DPs on surface. Then, we summarize some major challenges in this field, including the fabrication of high-quality 2DPs and the study of the intrinsic electronic properties of 2DPs, and we discuss some of the available solutions to address these issues.

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.

On-Surface Chemistry on Low-Reactive Surfaces

Zero-dimensional (0D), mono-dimensional (1D), or two-dimensional (2D) nanostructures with well-defined properties fabricated directly on surfaces are of growing interest. The fabrication of covalently bound nanostructures on non-metallic surfaces is very promising in terms of applications, but the lack of surface assistance during their synthesis is still a challenge to achieving the fabrication of large-scale and defect-free nanostructures. We discuss the state-of-the-art approaches recently developed in order to provide covalently bounded nanoarchitectures on passivated metallic surfaces, semiconductors, and insulators.

Initial Coupling and Reaction Progression of Directly Deposited Biradical Graphene Nanoribbon Monomers on Iodine-Passivated Versus Pristine Ag(111)

The development of widely applicable methods for the synthesis of C-C-bonded nanostructures on inert and insulating surfaces is a challenging yet rewarding milestone in the field of on-surface synthesis. This would enable studies of nearly unperturbed covalent nanostructures with unique electronic properties as graphene nanoribbons (GNR) and π-conjugated 2D polymers. The prevalent Ullmann-type couplings are almost exclusively carried out on metal surfaces to lower the temperature required for initial dehalogenation well below the desorption threshold. To overcome the necessity for the activation of monomers on the target surface, we employ a recently developed Radical Deposition Source (RaDeS) for the direct deposition of radicals onto inert surfaces for subsequent coupling by addition reactions. The radicals are generated en route by indirect deposition of halogenated precursors through a heated reactive tube, where the dehalogenation reaction proceeds. Here, we use the ditopic 6,11-diiodo-1,2,3,4-tetraphenyltriphenylene (DITTP) precursor that afforded chevron-like GNR on Au(111) via the usual two-staged reaction comprised of monomer-coupling into covalent polymers and subsequent formation of an extended GNR by intramolecular cyclodehydrogenation (CDH). As a model system for inert surfaces, we use Ag(111) passivated with a closed monolayer of chemisorbed iodine that behaves in an inert manner with respect to dehalogenation reactions and facilitates the progressive coupling of radicals into extended covalent structures. We deposit the DITTP-derived biradicals onto both iodine-passivated and pristine Ag(111) surfaces. While on the passivated surface, we directly observe the formation of covalent polymers, on pristine Ag(111) organometallic intermediates emerge instead. This has decisive consequences for the further progression of the reaction: heating the organometallic chain directly on Ag(111) results in complete desorption, whereas the covalent polymer on iodine-passivated Ag(111) can be transformed into the GNR. Yet, the respective CDH proceeds directly on Ag(111) after thermal desorption of the iodine passivation. Accordingly, future work is aimed at the further development of approaches for the complete synthesis of GNR on inert surfaces.

Evolution of adsorption heights in the on-surface synthesis and decoupling of covalent organic networks on Ag(111) by normal-incidence X-ray standing wave

Structural characterization in on-surface synthesis is primarily carried out by Scanning Probe Microscopy (SPM) which provides high lateral resolution. Yet, important fresh perspectives on surface interactions and molecular conformations are gained from adsorption heights that remain largely inaccessible to SPM, but can be precisely measured with both elemental and chemical sensitivity by Normal-Incidence X-ray Standing Wave (NIXSW) analysis. Here, we study the evolution of adsorption heights in the on-surface synthesis and post-synthetic decoupling of porous covalent triazine-phenylene networks obtained from 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) precursors on Ag(111). Room temperature deposition of TBPT and mild annealing to ∼150 °C result in full debromination and formation of organometallic intermediates, where the monomers are linked into reticulated networks by C-Ag-C bonds. Topologically identical covalent networks comprised of triazine vertices that are interconnected by biphenyl units are obtained by a thermally activated chemical transformation of the organometallic intermediates. Exposure to iodine vapor facilitates decoupling by intercalation of an iodine monolayer between the covalent networks and the Ag(111) surface. Accordingly, Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and NIXSW experiments are carried out for three successive sample stages: organometallic intermediates, covalent networks directly on Ag(111) and after decoupling. NIXSW analysis facilitates the determination of adsorption heights of chemically distinct carbon species, i.e. in the phenyl and triazine rings, and also for the organometallic carbon atoms. Thereby, molecular conformations are assessed for each sample stage. The interpretation of experimental results is informed by Density Functional Theory (DFT) calculations, providing a consistent picture of adsorption heights and molecular deformations in the networks that result from the interplay between steric hindrance and surface interactions. Quantitative adsorption heights, i.e. vertical distances between adsorbates and surface, provide detailed insight into surface interactions, but are underexplored in on-surface synthesis. In particular, the direct comparison with an in situ prepared decoupled state unveils the surface influence on the network structure, and shows that iodine intercalation is a powerful decoupling strategy.

Synthesis on inert surfaces

Extraordinarily robust extended covalent organic nanostructures with unprecedented structures and intriguing chemical and electronic properties are currently synthesized on metal surfaces. Envisaged electronic applications, for instance in field effect transistors or sensors, however, demand insulating supports. To obviate the need for a cumbersome post-synthetic transfer from the metal growth surface to the target substrate, synthesis directly on inert surfaces is highly desirable. Albeit reversible polycondensations are broadly established on inert graphite surfaces, carbon-carbon (C-C) coupling remains mostly elusive. Thermally activated coupling on weakly interacting supports suffers from the "desorption problem", that is the premature desorption of reactants due to increased reaction barriers, which becomes even worse on inert surfaces due to diminished desorption barriers. Consequently, C-C coupling on inert surfaces requires new paradigms. We propose either photochemical coupling or activation of monomers prior to deposition as possible alternatives, discuss the current state-of-the-art and identify future challenges.

Atomically precise graphene nanoribbons: interplay of structural and electronic properties

Graphene nanoribbons hold great promise for future applications in nanoelectronic devices, as they may combine the excellent electronic properties of graphene with the opening of an electronic band gap - not present in graphene but required for transistor applications. With a two-step on-surface synthesis process, graphene nanoribbons can be fabricated with atomic precision, allowing precise control over width and edge structure. Meanwhile, a decade of research has resulted in a plethora of graphene nanoribbons having various structural and electronic properties. This article reviews not only the on-surface synthesis of atomically precise graphene nanoribbons but also how their electronic properties are ultimately linked to their structure. Current knowledge and considerations with respect to precursor design, which eventually determines the final (electronic) structure, are summarized. Special attention is dedicated to the electronic properties of graphene nanoribbons, also in dependence on their width and edge structure. It is exactly this possibility of precisely changing their properties by fine-tuning the precursor design - offering tunability over a wide range - which has generated this vast research interest, also in view of future applications. Thus, selected device prototypes are presented as well.

Collective radical oligomerisation induced by an STM tip on a silicon surface

Over the past decade, on-surface fabrication of organic nanostructures has been widely investigated for the development of molecular electronic components, catalysts, and new materials. Here, we introduce a new strategy to obtain alkyl oligomers in a controlled manner using on-surface radical oligomerisations that are triggered by electrons between the tip of a scanning tunnelling microscope and the Si(111)√3 ×√3 R30°-B surface. This electron transfer event only occurs when the bias voltage is below -4.5 V and allows access to reactive radical species under exceptionally mild conditions. This transfer can effectively 'switch on' a sequence leading to the formation of oligomers of defined size distribution thanks to the on-surface confinement of the reactive species. Our approach enables new ways to initiate and control radical oligomerisations with tunnelling electrons, leading to molecularly precise nanofabrication.

Carbon-Carbon Coupling on Inert Surfaces by Deposition of En Route Generated Aryl Radicals

To facilitate C-C coupling in on-surface synthesis on inert surfaces, we devised a radical deposition source (RDS) for the direct deposition of aryl radicals onto arbitrary substrates. Its core piece is a heated reactive drift tube through which halogenated precursors are deposited and en route converted into radicals. For the proof of concept we study 4,4''-diiodo-p-terphenyl (DITP) precursors on iodine-passivated metal surfaces. Deposition with the RDS at room temperature results in highly regular structures comprised of mostly monomeric (terphenyl) or dimeric (sexiphenyl) biradicals. Mild heating activates progressive C-C coupling into more extended molecular wires. These structures are distinctly different from the self-assemblies observed upon conventional deposition of intact DITP. Direct deposition of radicals renders substrate reactivity unnecessary, thereby paving the road for synthesis on application-relevant inert surfaces.