Two-dimensional metal carbides and nitrides–MXenesrepresent a group of materials which have attained growing attention over the last decade due to their chemical versatility, making them highly ...promising in areas such as energy storage, superconductivity, and heterogenous catalysis. Surface terminations are a natural consequence of the MXene synthesis, conventionally consisting of O, OH, and F. However, recent studies have extended the chemical domain of the surface terminations to other elements, and they should be considered as an additional parameter governing the MXene properties. There is a shortfall in the understanding of how various chemical species could act as terminations on different MXenes. In particular, there is limited comprehension in which chemical environments different terminations are stable. Here, we present an extensive theoretical study of the surface terminations of MXenes in different atmospheres by considering in total six experimentally achieved MXenes (Ti2C, Nb2C, V2C, Mo2C, Ti3C2, and Nb4C3) and twelve surface terminations (O, OH, N, NH, NH2, S, SH, H, F, Cl, Br, and I). We consider fully terminated (single termination) MXenes and also the impact of substituting individual terminations. Our study provides insights into what terminations are stable on which MXenes in different chemical environments, with predictions of how to obtain single-termination MXenes and which MXenes are resilient under ambient conditions. In addition, we propose synthesis protocols of MXenes which have not yet been realized in experiments. It is anticipated that alongside the development of new synthesis routes, our study will provide design rules for how to tailor the surface terminations of MXenes.
We computationally study the reaction mechanisms of halogen-based covalent self-assembly, a major route for synthesizing molecular nanostructures and nanographenes on surfaces. Focusing on biphenyl ...as a small model system, we describe the dehalogenation, recombination, and diffusion processes. The kinetics of the different processes are also investigated, in particular how diffusion and coupling barriers affect recombination rates. Trends across the periodic table are derived from three commonly used close-packed (111) surfaces (Cu, Ag, and Au) and two halogens (Br and I). We show that the halogen atoms can poison the surface, thus hindering long-range ordering of the self-assembled structures. Finally, we present core-level shifts of the relevant carbon and halogen atoms, to provide reference data for reliably detecting self-assembly without the need for atomic-resolution scanning tunneling microscopy.
The fabrication of graphene nanoribbons (GNRs) requires a high degree of precision due to the sensitivity of the electronic structure on the edge shape. Using Br-substituted molecular precursors, ...this atomic precision can be achieved in a thermally induced two-step reaction following Br dissociation on a Au(111) surface. Using DFT, we find evidence that the Br atoms are bound to the intermediate polyanthrylene chains. We employ temperature-programmed desorption to demonstrate the associative desorption of HBr and molecular hydrogen during the final cyclodehydrogenation step of the reaction. Both processes are found to have similar activation barriers. Furthermore, we are able to remove Br atoms from the polyanthrylene chains by providing molecular hydrogen. The subsequent formation of GNR via a cyclodehydrogenation demonstrates that Br does not influence this part of the overall reaction.
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 sp2- 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.
More than a decade after the discovery of MXene, there has been a remarkable increase in research on synthesis, characterization, and applications of this growing family of two-dimensional (2D) ...carbides and nitrides. Today, these materials include one, two, or more transition metals arranged in chemically ordered or disordered structures of three, five, seven, or nine atomic layers, with a surface chemistry characterized by surface terminations. By combining M, X, and various surface terminations, it appears that a virtually endless number of MXenes is possible. However, for the design and discovery of structures and compositions beyond current MXenes, one needs suitable (stable) precursors, an assessment of viable pathways for 3D to 2D conversion, and utilization or development of corresponding synthesis techniques. Here, we present a critical and forward-looking review of the field of atomic scale design and synthesis of MXenes and their parent materials. We discuss theoretical methods for predicting MXene precursors and for assessing whether they are chemically exfoliable. We also summarize current experimental methods for realizing the predicted materials, listing all verified MXenes to date, and outline research directions that will improve the fundamental understanding of MXene processing, enabling atomic scale design of future 2D materials, for emerging technologies.
Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. To shed light on individual ...reaction steps and their progression with temperature, temperature-programmed X-ray photoelectron spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5-tris(4-bromophenyl)benzene precursor is comparatively studied on Ag(111) versus Au(111), whereby intermolecular bonds and network topologies are additionally characterized by scanning tunneling microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and density functional theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long lifetime of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces.
We have computationally studied the effects of temperature on the reaction pathway of an electrocyclic ring-closure reaction on the Au(111) surface, particularly focusing on thermodynamic aspects of ...the reaction. The electrocyclic ring closure is accompanied by a series of dehydrogenation steps, and while it is found that temperature, in terms of vibrational entropy and enthalpy, has a reducing effect on most energy barriers, it does not alter the qualitative appreciation of the reaction kinetics. However, it is found that the way the abstracted hydrogen atoms are treated is crucial for the thermodynamics of the reaction. The overall reaction is highly endothermic but becomes thermodynamically favorable due to the entropy gain of the hydrogen byproducts, which desorb associatively from the surface as H2. The study provides new outlooks for the theoretical treatment of reactions related to on-surface synthesis, anticipated to be instructive for future studies.
We investigate the cooperative effects controlling the synthesis of a graphene nanoribbon on the Au(111) surface starting from an anthracene polymer using density functional calculations including ...van der Waals interactions. We focus on the high-temperature cyclodehydrogenation step of the reaction and find that the reaction proceeds by simultaneously transferring two H-atoms from the anthracene units to the Au surface, leaving behind a C–C bond in the process. This step is significantly more favorable than the three other potential reaction paths. Moreover, we find that successive dehydrogenations proceed from one end of the polyanthracene and propagate step-by-step through the polymer in a domino-like fashion.
The synthesis of ammonia (NH3) from nitrogen (N2) under ambient conditions is of great significance but hindered by the lack of highly efficient catalysts. By performing first-principles ...calculations, we have investigated the feasibility for employing a transition metal (TM) atom, supported on Ti3C2T2 MXene with O/OH terminations, as a single-atom catalyst (SAC) for electrochemical nitrogen reduction. The potential catalytic performance of TM single atoms is evaluated by their adsorption behavior on the MXene, together with their ability to bind N2 and to desorb NH3 molecules. Of importance, the OH terminations on Ti3C2T2 MXene can effectively enhance the N2 adsorption and decrease the NH3 adsorption for single atoms. Based on proposed criteria for promising SACs, our calculations further demonstrate that the Ni/Ti3C2O0.19(OH)1.81 exhibits reasonable thermodynamics and kinetics toward electrochemical nitrogen reduction.
Assembly of semiconducting organic molecules with multiple aryl–metal covalent bonds into stable one- and two-dimensional (1D and 2D) metal–organic frameworks represents a promising route to the ...integration of single-molecule electronics in terms of structural robustness and charge transport efficiency. Although various metastable organometallic frameworks have been constructed by the extensive use of single aryl–metal bonds, it remains a great challenge to embed multiple aryl–metal bonds into these structures due to inadequate knowledge of harnessing such complex bonding motifs. Here, we demonstrate the substrate-modulated synthesis of 1D and 2D metal–organic hybrids (MOHs) with the organic building blocks (perylene) interlinked solely with multiple aryl–metal bonds via the stepwise thermal dehalogenation of 3,4,9,10-tetrabromo-1,6,7,12-tetrachloroperylene and subsequent metal–organic connection on metal surfaces. More importantly, the conversion from 1D to 2D MOHs is completely impeded on Au(111) but dominant on Ag(111). We comprehensively study the distinct reaction pathways on the two surfaces by visually tracking the structural evolution of the MOHs with high-resolution scanning tunneling and noncontact atomic force microscopy, supported by first-principles density functional theory calculations. The substrate-dependent structural control of the MOHs is attributed to the variation of the M–X (M = Au, Ag; X = C, Cl) bond strength regulated by the nature of the metal species.
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