Chemical transformations at the interface between solid/liquid or solid/gaseous phases of matter lie at the heart of key industrial-scale manufacturing processes. A comprehensive study of the molecular energetics and conformational dynamics that underlie these transformations is often limited to ensemble-averaging analytical techniques. Here we report the detailed investigation of a surface-catalysed cross-coupling and sequential cyclization cascade of 1,2-bis(2-ethynyl phenyl)ethyne on Ag(100). Using non-contact atomic force microscopy, we imaged the single-bond-resolved chemical structure of transient metastable intermediates. Theoretical simulations indicate that the kinetic stabilization of experimentally observable intermediates is determined not only by the potential-energy landscape, but also by selective energy dissipation to the substrate and entropic changes associated with key transformations along the reaction pathway. The microscopic insights gained here pave the way for the rational design and control of complex organic reactions at the surface of heterogeneous catalysts.

With first principles calculations, we predict a novel stable 2D layered structure for group VI elements Se and Te that we call square selenene and square tellurene, respectively. They have chair-like buckled structures similar to other layered materials such as silicene and germanene but with a square unit cell rather than hexagonal. This special structure gives rise to anisotropic band dispersions near the Fermi level that can be described by a generalized semi-Dirac Hamiltonian. We show that the considerably large band gap (∼0.1 eV) opened by spin-orbit coupling makes square selenene and tellurene topological insulators, hosting non-trivial edge states. Therefore, square selenene and tellurene are promising materials for novel electronic and spintronic applications. Finally, we show that this new type of 2D elemental material can potentially be grown on proper substrates, such as a Au(100) surface.The isolation of graphene in 2004 [1] opened up a new avenue in condensed matter physics: two-dimensional (2D) materials research.Following the success of graphene [2], intensive efforts have been devoted to exploring other 2D materials [3]. Among them, elemental 2D materials (composed of only one element) have attracted much attention because of their simple composition and intriguing properties [4]. A number of elemental 2D materials beyond graphene have been predicted and synthesized, such as silicene [5-8], germanene [9], stanene [10,11], phosphorene [12], and borophene [13,14], with elements ranging from group III to group V. However, no studies on group VI elemental 2D materials have ever been reported. Unlike the aforementioned elements, at ambient conditions, most of the group VI elements have 3D bulk structures composed of 1D atomic chains or 0D atomic rings with only two-fold coordination bonding. The question remains whether these elements can form 2D atomic layers as elements from groups III-V do.During the exploration of 2D materials, some are predicted to be topological insulators (TIs), a new quantum state of matter recently discovered [15][16][17][18][19]. TIs have different band topology than normal insulators, giving rise to non-trivial gapless surface states at the interface between TIs and normal insulators or vacuum. These nontrivial surface states are protected by time-reversal symmetry and the spin of these states is locked with their momentum, significantly reducing back-scattering. For a 2D TI, these non-trivial symmetry-protected edge states even give rise to spin-polarized conduction channels without dissipation as back-scattering is strictly prohibited [16,17]. This special property makes 2D TIs extremely appealing in novel electronic and spintronic applications.In this study, we report for the first time the theoretical prediction of a new type of 2D layered structures for group VI elements Se and Te, which we call square selenene and tellurene, following the convention of graphene and the symmetry of the unit cell. We confirm their thermal stability with ab initio phonon calculations and...

Owing to their higher intrinsic electrical conductivity and chemical stability with respect to their oxide counterparts, nanostructured metal sulfides are expected to revive materials for resistive chemical sensor applications. Herein, we explore the gas sensing behavior of WS 2 nanowire-nanoflake hybrid materials and demonstrate their excellent sensitivity (0.043 ppm-1) as well as high selectivity towards H 2 S relative to CO, NH 3 , H 2 , and NO (with corresponding sensitivities of 0.002, 0.0074, 0.0002, and 0.0046 ppm-1 , respectively). Gas response measurements, complemented with the results of X-ray photoelectron spectroscopy analysis and first-principles calculations based on density functional theory, suggest that the intrinsic electronic properties of pristine WS 2 alone are not sufficient to explain the observed high sensitivity towards H 2 S. A major role in this behavior is also played by O doping in the S sites of the WS 2 lattice. The results of the present study open up new avenues for the use of transition metal disulfide nanomaterials as effective alternatives to metal oxides in future applications for industrial process control, security, and health and environmental safety.

The tetrapyrrole macrocycle of porphine is the common core of all porphyrin molecules, an interesting class of π-conjugated molecules with relevance in natural and artificial systems. The functionality of porphines on a solid surface can be tailored by the central metal atom and its interaction with the substrate. In this study, we present a local adsorption geometry determination for cobalt porphine on Cu(111) by means of complementary scanning tunneling microscopy, high-resolution X-ray photoelectron spectroscopy, X-ray standing wave measurements, and density functional theory calculations. Specifically, the Co center was determined to be at an adsorption height of 2.25 ± 0.04 Å occupying a bridge site. The macrocycle adopts a moderate asymmetric saddle-shape conformation, with the two pyrrole groups that are aligned perpendicular to the densely packed direction of the Cu(111) surface tilted away from the surface plane.

Surface diffusion, molecular conformation, and on-surface coupling reactions are key processes for building tailored molecular nanostructures such as graphene nanoribbons, polycyclic aromatic hydrocarbons, and one-dimensional/twodimensional (2D) polymers. Here, we study the surface diffusion and coupling in situ of a chlorinated porphyrin, namely 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (Cl 4 TPP), using a combined scanning tunneling microscopy (STM), density functional theory (DFT), and X-ray photoelectron spectroscopy approach. Using STM, we obtain surface migration and rotation barriers ΔE of 0.77 ± 0.09 and 0.93 ± 0.28 eV, respectively, indicative of covalent binding to the surface. In fact, we find that the precursors as well as all the reaction species exclusively (≈100%) adopt a peculiar "inverted" conformation covalently bonded to Cu(111). Using DFT, we have mapped two coupling reaction pathways: direct dechlorination and Cu adatom-mediated Ullmann coupling. We find that the latter is essentially barrierless, whereas the former faces a barrier of about 0.9 eV for inverted Cl 4 TPP on Cu(111). Our STM measurements show that C−Cu−C organometallic species are the main final products in the presence of Cu adatoms, which is explained by our DFT reaction profile when heat dissipation to the substrate is taken into account. This work not only highlights the relevance of surface adatoms in selecting the reaction pathway but also opens the possibility of precisely tailoring 2D molecular assemblies by controlling the supply of Cu adatoms.

Carbon nanotubes are a natural choice as gas sensor components given their high surface to volume ratio, electronic properties, and capability to mediate chemical reactions. However, a realistic assessment of the interaction of the tube wall and the adsorption processes during gas phase reactions has always been elusive. Making use of ultraclean single-walled carbon nanotubes, we have followed the adsorption kinetics of NO2 and found a physisorption mechanism. Additionally, the adsorption reaction directly depends on the metallic character of the samples. Franck–Condon satellites, hitherto undetected in nanotube–NOx systems, were resolved in the N 1s X-ray absorption signal, revealing a weak chemisorption, which is intrinsically related to NO dimer molecules. This has allowed us to identify that an additional signal observed in the higher binding energy region of the core level C 1s photoemission signal is due to the C=O species of ketene groups formed as reaction byproducts . This has been supported by density functional theory calculations. These results pave the way toward the optimization of nanotube-based sensors with tailored sensitivity and selectivity to different species at room temperature.

Confining carbyne to a space that allows for stability and controlled reactivity is a very appealing approach to have access to materials with tunable optical and electronic properties without rival. Here, we show how controlling the diameter of single-walled carbon nanotubes opens the possibility to grow a confined carbyne with a defined and tunable band gap. The metallicity of the tubes has a minimal influence on the formation of the carbyne, whereas the diameter plays a major role in the growth. It has been found that the properties of confined carbyne can be tailored independently from its length and how these are mostly determined by its interaction with the carbon nanotube. Molecular dynamics simulations have been performed to interpret these findings. Furthermore, the choice of a single-walled carbon nanotube host has been proven crucial even to synthesize an enriched carbyne with the smallest energy gap currently reported and with remarkable homogeneity.

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 nonmetal 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.