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.

Iodine passivation facilitates on-surface synthesis of robust regular conjugated two-dimensional organogold networks on Au(111)

Two-dimensional conjugated organogold networks with anthra-tetrathiophene repeat units are synthesized by thermally activated debrominative coupling of 2,5,9,12-tetrabromoanthra[1,2-b:4,3-b':5,6-b'':8,7-b''']tetrathiophene (TBATT) precursor molecules on Au(111) surfaces under ultra-high vacuum (UHV) conditions. Performing the reaction on iodine-passivated Au(111) surfaces promotes formation of highly regular structures, as revealed by scanning tunneling microscopy (STM). In contrast, coupling on bare Au(111) surfaces results in less regular networks due to the simultaneous expression of competing intermolecular binding motifs in the absence of error correction. The carbon-Au-carbon bonds confer remarkable robustness to the organogold networks, as evidenced by their high thermal stability. In addition, as suggested by density functional theory (DFT) calculations and underscored by scanning tunneling spectroscopy (STS), the organogold networks exhibit a small electronic band gap in the order of 1.0 eV due to their high π-conjugation.

Barth J, Lobo-Checa J, Corso M, Rogero C. Enhancing Haloarene Coupling Reaction Efficiency on an Oxide Surface by Metal Atom Addition

The bottom-up synthesis of carbon-based nanomaterials directly on semiconductor surfaces allows for the decoupling of their electronic and magnetic properties from the substrates. However, the typically reduced reactivity of such nonmetallic surfaces adversely affects the course of these reactions. Here, we achieve a high polymerization yield of halogenated polyphenyl molecular building blocks on the semiconducting TiO2(110) surface via concomitant surface decoration with cobalt atoms, which catalyze the Ullmann coupling reaction. Specifically, cobalt atoms trigger the debromination of 4,4″-dibromo-p-terphenyl molecules on TiO2(110) and mediate the formation of an intermediate organometallic phase already at room temperature (RT). As the debromination temperature is drastically reduced, homocoupling and polymerization readily proceed, preventing presursor desorption from the substrate and entailing a drastic increase of the poly-para-phenylene polymerization yield. The general efficacy of this mechanism is shown with an iodinated terphenyl derivative, which exhibits similar dehalogenation and reaction yield.

Long and oriented graphene nanoribbon synthesis from well-ordered 10,10'-dibromo-9,9'-bianthracene monolayer on crystalline Au surfaces

Bottom-up synthesis on metal surfaces has attracted attention for the fabrication of graphene nanoribbons (GNRs) with atomically-precise chemical structures to realize novel electronic devices. However, control of length and orientation on surfaces during GNR synthesis is difficult, thus, achieving longer and aligned GNR growth is a significant challenge. Herein, we report GNR synthesis from a well-ordered dense monolayer on Au crystalline surfaces for long and oriented GNR growth. Scanning tunneling microscopy showed that 10,10'-dibromo-9,9'-bianthracene (DBBA) precursors deposited on Au(111) at room temperature self-assembled into a well-ordered dense monolayer, and the straight molecular wire structure was formed where Br atoms in each precursor were adjacent along the wire axis. The DBBAs in the monolayer were found to be hardly desorbed from the surface under subsequent heating and efficiently polymerize along with the molecular arrangement, resulting in more long and oriented GNR growth compared to the conventional growth method. The result is attributed to be suppression of random diffusion and desorption of the DBBAs on the Au surface during polymerization due to the densely-packed DBBA structure. Additionally, an investigation of the effect of the Au crystalline plane on the GNR growth revealed further anisotropic GNR growth on Au(100) compared to Au(111) due to the stronger interactions of DBBA with Au(100). These findings provide fundamental knowledge for controlling GNR growth from a well-ordered precursor monolayer to achieve more long and oriented GNRs.

Ordering of monomers, dimers and polymers of deposited Br2I2Py molecules: a modeling study

We propose a lattice model describing the ordering of 1,6-dibromo-3,8-diiodopyrene (Br₂I₂Py) molecules on the Au(111) surface into two-dimensional structures and correlated one dimensional rows. Our model employs three (intact, singly and doubly deiodinated) types of Br₂I₂Py molecules and mimics the situation which occurs with increasing temperature, where the majority of intact molecules form ordered two-dimensional networks, while most of the doubly deiodinated molecules assemble into long organometallic polymeric rows. We use DFT calculations to determine the values of intermolecular interactions for intact molecules and propose a strategy for estimating the interactions for deiodinated molecules, where the organometallic interaction with Au atoms plays the dominant role. Our model is solved using Monte Carlo calculations and allows us to obtain the monomeric structure of intact molecules, the dimeric structure of singly deiodinated molecules and the polymeric row structure of (mostly) doubly deiodinated molecules. We obtain the coexistence of ordered intact Br₂I₂Py molecules and organometallic dimers, as well as their separation at large values of intermolecular interaction with Au. Similar results are obtained by studying mixtures of singly and doubly deiodinated molecules: dimer rows can be either incorporated into the two dimensional pattern of correlated polymeric chains or separated into their own dimeric structures.

Actinide Coordination Chemistry on Surfaces: Synthesis, Manipulation, and Properties of Thorium Bis(porphyrinato) Complexes

Actinide-based metal-organic complexes and coordination architectures encompass intriguing properties and functionalities but are still largely unexplored on surfaces. We introduce the in situ synthesis of actinide tetrapyrrole complexes under ultrahigh-vacuum conditions, on both a metallic support and a 2D material. Specifically, exposure of a tetraphenylporphyrin (TPP) multilayer to an elemental beam of thorium followed by a temperature-programmed reaction and desorption of surplus molecules yields bis(porphyrinato)thorium (Th(TPP)₂) assemblies on Ag(111) and hexagonal boron nitride/Cu(111). A multimethod characterization including X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption, and complementary density functional theory modeling provides insights into conformational and electronic properties. Supramolecular assemblies of Th(TPP)₂ as well as individual double-deckers are addressed with submolecular precision, e.g., demonstrating the reversible rotation of the top porphyrin in Th(TPP)₂ by molecular manipulation. Our findings thus demonstrate prospects for actinide-based functional nanoarchitectures.

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.

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.

Polycyclic aromatic chains on metals and insulating layers by repetitive [3+2] cycloadditions

The vast potential of organic materials for electronic, optoelectronic and spintronic devices entails substantial interest in the fabrication of π-conjugated systems with tailored functionality directly at insulating interfaces. On-surface fabrication of such materials on non-metal surfaces remains to be demonstrated with high yield and selectivity. Here we present the synthesis of polyaromatic chains on metallic substrates, insulating layers, and in the solid state. Scanning probe microscopy shows the formation of azaullazine repeating units on Au(111), Ag(111), and h-BN/Cu(111), stemming from intermolecular homo-coupling via cycloaddition reactions of CN-substituted polycyclic aromatic azomethine ylide (PAMY) intermediates followed by subsequent dehydrogenation. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry demonstrates that the reaction also takes place in the solid state in the absence of any catalyst. Such intermolecular cycloaddition reactions are promising methods for direct synthesis of regioregular polyaromatic polymers on arbitrary insulating surfaces.

Hollow Zn-Co Based Zeolitic Imidazole Framework as a Robust Heterogeneous Catalyst for Enhanced CO2 Chemical Fixation

The efficient chemical conversion of carbon dioxide (CO₂ ) into value-added fine chemicals is an intriguing but challenging route in sustainable chemistry. Herein, a hollow-structured bimetallic zeolitic imidazole framework composed of Zn and Co as metal centers (H-ZnCo-ZIF) has been successfully prepared via a post-synthetic strategy based on controllable chemical-etching of the preformed solid ZnCo-ZIF in tannic acid. The creation of hollow cavities inside each monocrystalline ZIFs could be achieved without destroying the intrinsic frameworks, as characterized by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction technologies. The as-synthesized H-ZnCo-ZIF exhibited remarkable catalytic activity in the cycloaddition of CO₂ with epoxides to the corresponding cyclic carbonates, outperforming the solid ZnCo-ZIF analogue due to the improved mass transfer originating from the hollow structure. More importantly, due to stabilization of metal centers in the ZIF framework by the tannic acid shell, H-ZnCo-ZIF exhibited good recyclability, and no activity loss could be observed in six runs. The present study provides a simple and effective strategy to enhance the catalytic performance of ZIFs by creating a hollow structure via chemical etching.

Two-Sidedness of Surface Reaction Mediation

A heterogeneous catalytic process involves many surface elementary steps that affect the overall catalytic performance in one way or another. In general, a high-performance heterogeneous catalyst should meet the main criteria: excellent catalytic activity and high selectivity toward target products. Using surface science techniques, the two-sidedness of the surface reaction mediations can be explored, from the perspectives of the surface and the molecule manipulations. The surface manipulation refers to a reaction that is mediated by composition and structure of the substrate as well as surface species, while the molecular manipulation relates to a reaction that is mediated by the reacting molecule via the precursor selection, environmental control, or external excitation. The best catalytic system should consist of the most efficient catalyst and the best suitable reacting molecule, in addition to its economic benefit and environmental amity. Recent research progress in surface reaction mediation is outlined, and its two-sidedness is governed by the Arrhenius equation. This should shed new light on the connection between basic theory and surface reaction mediation strategies. To conclude, challenges and possible opportunities are elaborated for efficient surface reaction mediations.

PdAu alloy nanoparticles supported on nitrogen-doped carbon black as highly active catalysts for Ullmann coupling and nitrophenol hydrogenation reactions

Noble metal-based catalysts have been proven to be active for catalytic organic reactions. The selectivity and conversion can be improved by integration with proper carrier materials, and further modulated by tuning the composition as well as the electronic structure of the active noble metals. Compared with unsupported monometallic catalysts, the synergistic interactions between neighboring metals and the combined effects between the carrier materials and the active components often give rise to positive influences on the enhancement of the catalytic efficiency and selectivity. In this work, we report a facile process for the fabrication of nitrogen-doped carbon black (NCB) supported PdAu bimetallic nanoparticles (NPs) with a uniform dispersion and narrow size distribution. The PdAu/NCB catalyst with a Pd/Au mole ratio of 1/1 shows the highest activity towards both Ullmann coupling reactions of aryl halides and the hydrogenation reaction of nitrophenols. Moreover, this bimetallic catalyst also exhibits a superior recycling durability to that of monometallic Pd/NCB and Au/NCB catalysts. The enhanced catalytic performance of the bimetallic catalyst is mainly due to the large BET specific surface area (125.45 m2 g-1) and the synergy between the individual components of the catalyst.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Ho-Mediated Alkyne Reactions at Low Temperatures on Ag(111)

Low-temperature approaches to catalytic conversions promise efficiency, selectivity, and sustainable processes. Control over certain coupling reactions can be obtained via the pre-positioning of reactive moieties by self-assembly. However, in the striving field of on-surface synthesis atomistic precision and control remains largely elusive, because the employed coupling reactions proceed at temperatures beyond the thermal stability of the supramolecular templates. Here, utilizing scanning tunneling microscopy, we demonstrate terminal alkyne on-surface reactions mediated by Ho atoms at a weakly reactive Ag(111) substrate at low-temperatures. Density functional theory calculations confirm the catalytic activity of the involved adatoms. Pre-deposited Ho induces alkyne dehydrogenation starting at substrate temperatures as low as 100 K. Ho arriving at molecularly pre-covered surfaces held at 130 and 200 K produces covalent enyne-linked dimers and initiates cyclotrimerization, respectively. Statistical product analysis indicates a two-step pathway for the latter, whereby the enyne intermediates influence the distribution of the products. High chemoselectivity results from the absence of cyclotetramerization and diyne-forming homocoupling. Our analysis indicates that mainly the arriving Ho adatoms enable the coupling. These findings support the concept of dynamic heterogeneity by single-atom catalysts and pave the way for alternative means to control on-surface reactions.

Incorporation of CuO NPs into modified UiO-66-NH2 metal–organic frameworks (MOFs) with melamine for catalytic C–O coupling in the Ullmann condensation

The UiO-66-NH₂ is initially modified with melamine via a post-synthetic approach. CuO NPs are then anchored via the available functional groups on the surface of the modified MOF.

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.

Surface-assisted Ullmann coupling

Surface-assisted Ullmann coupling is both drosophila and workhorse of on-surface synthesis. The fabrication of novel covalent low-dimensional organic nanostructures is accompanied by fundamental studies of surface chemistry.