Polycyclic conjugated hydrocarbons have acquired increased interests recently because of their potential applications in electronic devices. On metal surfaces, the selective synthesis of four- and five-membered carbon rings remains challenging due to the presence of diverse reaction pathways. Here, utilizing the same precursor molecule, we successfully achieved substrate-controlled highly selective cycloaddition reactions towards four- and five-membered carbon rings. A 97 % yield for four-membered carbon rings on Au(111), while a 96 % yield towards five-membered carbon rings is achieved on Ag(111). The detailed topological structures of the reaction products are carefully examined by bond-resolving scanning tunneling microscopy (BR-STM) imaging with a CO functionalized tip. The underlying mechanism of the novel surface-directed reaction selectivity is elucidated by extensive density functional theory (DFT) calculations. Our study paves the way for high selective synthesis of polycyclic conjugated hydrocarbons with non-benzenoid rings.

Selective etching has emerged as a key method for synthesizing 2D materials, with the conversion of MAX phases to MXenes being by far the most widely studied and reported example. While traditional methods rely on etching in primarily acidic aqueous media, molten salts offer an intriguing alternative. However, the current understanding of MAX phase reactivity in molten salts is limited, restricting our ability to predict reaction outcomes. In this study, we present a computational framework that uses process-specific phase diagrams to model A-element substitution and MXene formation, as well as competing side reactions. Applying this approach to Ti3AlC2, V2AlC, and Ti2AlN in ZnCl2 molten salt, we reveal distinct reaction behaviors despite identical redox potentials-defined here by the Al-to-Zn exchange-of key processes. Our findings underscore the limitations of predicting reactions based solely on redox potentials and show that our model can capture key trends in MXene synthesis. Beyond MXenes, our methodology lays the groundwork for identifying new 2D materials accessible through molten salt etching.

Sulfur-terminated single sheet (ss-)MXene was recently achieved by delamination of multilayered van der Waals bonded (vdW)-MXenes Nb2CS2 and Ta2CS2 synthesized through solid-state synthesis, rather than via the traditional way of selectively etching A-layers from the corresponding MAX phase. Inspired by this, we perform a computational screening study of vdW-MXenes M(2)CCh(2) isotypical to Nb2CS2 and Ta2CS2, with M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, or W and Ch = S, Se, or Te. The thermodynamic stability of each vdW-MXene M(2)CCh(2) is assessed, and the dynamical stability of both vdW- and ss-MXene is considered through phonon dispersions. We predict seven stable vdW-MXenes, out of which four have not been reported previously, and one, V2CSe2, incorporates a new transition metal element into this family of materials. Electronic properties are presented for the vdW- and ss-forms of the stable vdW-MXenes, suggesting that the materials are either metallic, semimetallic, or semiconducting. In previous experimental reports the vdW-MXene Nb2CS2 is synthesized by manipulation of the corresponding M(2)AX phase Nb2SC. Therefore, we also evaluate the thermodynamic stability of the corresponding M(2)AX phases, identifying 15 potentially stable phases. Six of these are experimentally reported, leaving nine new M(2)AX phases for future experimental investigation.

On-surface synthesis often proceeds under kinetic control due to the irreversibility of key reaction steps, rendering kinetic studies pivotal. The accurate quantification of reaction rates also bears potential for unveiling reaction mechanisms. Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) has emerged as an analytical tool for kinetic studies with splendid chemical and sufficient temporal resolution. Here, we demonstrate that the common linear temperature ramps lead to fitting ambiguities. Moreover, pinpointing the reaction order remains intricate, although this key parameter entails information on atomistic mechanisms. Yet, TP-XPS experiments with a stepped temperature profile comprised of isothermal segments facilitate the direct quantification of rate constants from fitting time courses. Thereby, rate constants are obtained for a series of temperatures, which allows independent extraction of both activation energies and pre-exponentials from Arrhenius plots. By using two analogous doubly versus triply brominated aromatic model compounds, we found that their debromination on Ag(111) is best modeled by second-order kinetics and thus proceeds via the involvement of a second, non-obvious reactant. Accordingly, we propose that debromination is activated by surface supplied Ag adatoms. This hypothesis is supported by Density Functional Theory (DFT) calculations. We foresee auspicious prospects for this TP-XPS variant for further exploring the kinetics and mechanisms of on-surface reactions. The temporal evolution of the reactant concentrations as measured by XPS for different temperature profiles reveals that the debromination of organic molecules on Ag(111) is activated by Ag adatoms.

Aluminum (Al)-ion batteries have emerged as a potential alternative to conventional ion batteries that rely on less abundant and costly materials like lithium. Nonetheless, given the nascent stage of advancement in Al-ion batteries (AIBs), attaining electrode materials that can leverage both intercalation capacity and structural stability remains challenging. Herein, we demonstrate a C3N4-derived layered N,S heteroatom-doped carbon, obtained at different pyrolysis temperatures, as a cathode material for AIBs, encompassing the diffusion-controlled intercalation and surface-induced capacity with ultrahigh reversibility. The developed layered N,S-doped corbon (N,S-C) cathode, synthesized at 900 degrees C, delivers a specific capacity of 330 mAh g(-1) with a relatively high coulombic efficiency of similar to 85% after 500 cycles under a current density of 0.5 A g(-1). Owing to its reinforced adsorption capability and enlarged interlayer spacing by doping N and S heteroatoms, the N,S-C900 cathode demonstrates outstanding energy storage capacity with excellent rate performance (61 mAh g(-1) at 20 A g(-1)) and ultrahigh reversibility (90 mAh g(-1) at 5 A g(-1) after 10 000 cycles).

On-surface reactions of terminal alkynes in ultrahigh vacuum have attracted widespread attention due to their high technological promise. However, employing different precursors and substrate materials often intricate reaction schemes appear far from being well-understood. Thus, recent investigations of alkyne coupling on noble metal surfaces suggest non-dehydrogenative scenarios, contradicting earlier reports. Herein, the study employs noncontact atomic force microscopy (nc-AFM) with high spatial resolution to conclusively characterize exemplary alkyne coupling products. Contrary to initial interpretations proposing dehydrogenative homocoupling on Ag(111), bond-resolved AFM imaging reveals the expression of enyne motifs. Based on complementary, extensive density functional theory calculations, the pertaining reaction mechanisms are explored. It is proposed that enyne formation initiates with a direct carbon-carbon coupling between two alkyne groups, followed by surface-assisted dehydrogenation-hydrogenation processes. Thereby consecutive steps of atomic hydrogen cleavage, surface migration and recombination to a different carbon atom enable bridging via carbon-carbon double bonding. The new results shed light on subtle, but crucial surface-mediated hydrogen transfer processes involved in the chemical bond formation, which are suggested to be of general relevance in on-surface synthesis. Terminal alkyne coupling on Ag(111) in ultrahigh vacuum is conclusively examined by bond-resolved atomic force microscopy and density functional theory modeling. The prevailing bonding motif is the enyne moiety, originating from a distinct surface-mediated dehydrogenation-hydrogenation reaction pathway. The findings highlight the important role of hydrogen transfer in the course of on-surface synthesis procedures. image

Nanocars are carbon-based single-molecules with a precise design that facilitates their atomic-scale control on a surface. The rational design of these molecules is important in atomic and molecular-scale manipulation to advance the development of molecular machines, as well as for a better understanding of self-assembly, diffusion and desorption processes. Here, we introduce the molecular design and construction of a collection of minimalistic nanocars. They feature an anthracene chassis and four benzene derivatives as wheels. After sublimation and adsorption on an Au(111) surface, we show controlled and fast manipulation of the nanocars along the surface using the tip of a scanning tunneling microscope (STM). The mechanism behind the successful displacement is the induced dipole created over the nanocar by the STM tip. We utilized carbon monoxide functionalized tips both to avoid decomposition and accidentally picking the nanocars up during the manipulation. This strategy allowed thousands of maneuvers to successfully win the Nanocar Race II championship.

A highly distorted chiral nanographene structure composed of triple corannulene-fused [5]helicenes is prepared with the help of the Heck reaction and oxidative photocyclization with an overall isolated yield of 28%. The complex three-dimensional (3D) structure of the bowl-helix hybrid nanostructure is studied by a combination of non contact atomic force microscopy (AFM) and scanning tunneling microscopy (STM) on the Cu(111) surface, density functional theory calculations, AFM/STM simulations, and high-performance liquid chromatography-electronic circular dichroism analysis. This examination reveals a molecular structure in which the three bowl-shaped corannulene bladesd position themselves in a C3-symmetric fashion around a highly twisted triphenylene core. The molecule appears to be shaped like a propeller in which the concave side of the bowls face away from the connected [5]helicene motif. The chirality of the nanostructure is confirmed by the direct visualization of both MMM and PPP enantiomers at the single-molecule level by scanning probe microscopies. These results underline that submolecular resolution imaging by AFM/STM is a powerful real-space tool for the stereochemical characterization of 3D curved chiral nanographene structures.

On-surface acetylenic homocoupling has been proposed to construct carbon nanostructures featuring sp hybrid-ization. However, the efficiency of linear acetylenic coupling is far from satisfactory, often resulting in undesired enyne products or cyclotrimerization products due to the lack of strategies to enhance chemical selectivity. Herein, we inspect the acetylenic homocou-pling reaction of polarized terminal alkynes (TAs) on Au(111) with bond-resolved scanning probe microscopy. The replacement of benzene with pyridine moieties significantly prohibits the cyclotrimerization pathway and facilitates the linear coupling to produce well-aligned N-doped graphdiyne nanowires. Combined with density functional theory calculations, we reveal that the pyridinic nitrogen modification substantially differentiates the coupling motifs at the initial C-C coupling stage (head-to-head vs head-to-tail), which is decisive for the preference of linear coupling over cyclotrimerization.

Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, its role remains largely unexplored in on-surface chemistry, where the interaction with the substrate can alter the electronic and geometric structure of the adsorbates. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity in the reactions by scanning tunnelling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, which show a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights are helpful to understand, and in turn design, reactions with aromatic species in on-surface chemistry and heterogeneous catalysis. While aromaticity is a useful concept for assessing the reactivity of organic compounds, the connection between aromaticity and on-surface chemistry remains largely unexplored. Now, scanning probe experiments on cyclization reactions of porphyrins on Au(111) show that the peripheral carbon atoms outside of the aromatic 18-& pi; electron pathway exhibit a higher reactivity.