Tuning On-Surface Synthesis of Graphene Nanoribbons by Noncovalent Intermolecular Interactions

On-surface synthesis has been widely used for the precise fabrication of surface-supported covalently bonded nanostructures. Here, we report on tuning the on-surface synthesis of graphene nanoribbons by noncovalent intermolecular interactions on Au(111) surfaces. By introducing noncovalent intermolecular interactions with the companion molecules (dianhydride derivative), intramolecular cyclodehydrogenation of nonplanar precursor molecules (bianthryl derivative) are promoted at 200 °C, with the monomers interlinked by gold atoms instead of the formation of polyanthrylene. By adjusting the deposition sequence of precursor and companion molecules, conjugated graphene nanoribbons can be finally obtained at a temperature of 240 °C, much lower than the synthesis procedures without companion molecules. Density functional theory calculations indicate that intermolecular interactions result in a dramatic shrinkage of the torsional angle between the adjacent anthryl groups of the precursor molecule, aiding the cyclodehydrogenation process. Our work demonstrates an intermolecular strategy for controllable fabrication of covalently bonded nanostructures by on-surface synthesis

Linear Alkane Polymerization on Au-Covered Ag(110) Surfaces

Efficiently utilizing alkanes as the precursors to realize various chemical reaction processes is challenging due to the inertness of alkane C–H bonds. We report here the selective C–H activation and C–C coupling reaction of linear alkanes on Au-covered Ag(110) surfaces. Based on density functional theory calculations, thin gold films with a thickness of several atomic layers deposited on Ag(110) resemble the bulk Au(110) surface for alkane C–H bond activation. By using scanning tunneling microscopy (STM) we have observed that, instead of linear C–C coupling on unalloyed Au(110), alkane molecules desorb from Ag(110) surfaces at elevated temperatures. The featured missing-row (1 × 2) reconstruction of Au(110) surfaces has been obtained by deposition of ∼20 ML (<3 nm) gold atoms on Ag(110). On such reconstructed surface, linear alkane polymerization through selective C–H activation and C–C coupling has been achieved at mild temperatures. Our work demonstrates the possibility to utilize thin gold films replacing bulk Au(110) substrates for selective alkyl C–H bond activation

Thermally Induced Transformation of Nonhexagonal Carbon Rings in Graphene-like Nanoribbons

Exploring the structural transformation of nonhexagonal rings is of fundamental importance for understanding the thermal stability of nonhexagonal rings and revealing the structure–property relationships. Here, we report on the thermally induced transformation from the fused tetragon-hexagon (4–6) carbon rings to a pair of pentagon (5–5) rings in the graphene-like nanoribbons periodically embedded with tetragon and octagon (4–8–4) carbon rings. A distinct contrast among tetragon, pentagon, hexagon, and octagon carbon rings is provided by noncontact atomic force microscopy with atomic resolution. The thermally activated bond rotation with the dissociation of the shared carbon dimer between the 4–6 carbon rings is the key step for the 4–6 to 5–5 transformation. The energy barrier of the bond rotation, which results in the formation of an irregular octagon ring in the transition state, is calculated to be 1.13 eV. The 5–5 defects markedly change the electronic local density of states of the graphene-like nanoribbon, as observed by scanning tunneling microscopy. Our density functional theory calculations indicate that the introduction of periodically embedded 5–5 rings will significantly narrow the electronic band gap of the graphene-like nanoribbons

Surface-Assisted Alkane Polymerization: Investigation on Structure–Reactivity Relationship

Surface-assisted polymerization of alkanes is a remarkable reaction for which the surface reconstruction of Au(110) is crucial. The surface of (1×2)-Au(110) precovered with molecules can be completely transformed into (1×3)-Au(110) by introducing branched methylidene groups on both sides of the aliphatic chain (18, 19-dimethylidene­hexatriacontane) or locally shifted into (1×3)-Au(110) under exposure to low-energy electrons (beam energy from 3.5 to 33.6 eV, for alkane dotriacontane). Scanning tunneling microscopy investigations demonstrate that alkane chains adsorbed on (1×3)-Au(110) are more reactive than on (1×2)-Au(110), presenting a solid experimental proof for structure–reactivity relationships. This difference can be ascribed to the existence of an extra row of gold atoms in the groove of (1×3)-Au(110), providing active sites of Au atoms with lower coordination number. The experimental results are further confirmed by density functional theory simulations

Diverse Structures and Magnetic Properties in Nonlayered Monolayer Chromium Selenide

Thickness-dependent magnetic behavior has previously been observed in chemical vapor deposition-grown chromium selenide. However, the low-dimensional structure in nonlayered chromium selenide, which plays a crucial role in determining the low-dimensional magnetic order, needs further study. Here, we report the structure-dependent magnetic properties in monolayer CrSe2 and Cr2Se3 grown by molecular beam epitaxy. In the monolayer CrSe2, 1T-CrSe2 with a lattice constant of 3.3 Å has a metallic character, coexisting with the 1T″ phase with 2 × 2 surface periodicity. Monolayer CrSe2 can be transformed into Cr2Se3 with a lattice constant of 3.6 Å by annealing at 300 °C. X-ray magnetic circular dichroism (XMCD) measurements combined with DFT calculations reveal that while the MBE-grown monolayer CrSe2 is antiferromagnetic, monolayer Cr2Se3 is ferromagnetic with a Curie temperature of ∼200 K. This work demonstrates the structural diversity in nonlayered chromium selenide and the critical effect of different structures on its electronic and magnetic properties

Atomically Thin 1T-FeCl₂ Grown by Molecular-Beam Epitaxy

Two-dimensional (2D) magnetic materials have attracted much attention due to their unique magnetic properties and promising applications in spintronics. Here, we report on the growth of ferrous chloride (FeCl2) films on Au(111) and graphite with atomic thickness by molecular-beam epitaxy (MBE) and the layer-dependent magnetic properties by density functional theory (DFT) calculations. The growth follows a layer-by-layer mode with adjustable thickness from sub-monolayer to a few layers. Four types of moiré superstructures of a single-layer FeCl2 on graphite and two types of atomic vacancies on Au(111) have been identified based on high-resolution scanning tunneling microscopy (STM). It turned out that the single- and few-layer FeCl2 films grown on Au(111) exhibit a 1T structure. The DFT calculations reveal that a single-layer 1T-FeCl2 has a ferromagnetic ground state. The minimum-energy configuration of a bilayer FeCl2 is satisfied for the 1T–1T structure with ferromagnetic layers coupled antiferromagnetically. These results make FeCl2 a promising candidate as ideal electrodes for spintronic devices providing large magnetoresistance

Epitaxial Growth of Single-Layer Kagome Nanoflakes with Topological Band Inversion

The kagome lattice has attracted intense interest with the promise of realizing topological phases built from strongly interacting electrons. However, fabricating two-dimensional (2D) kagome materials with nontrivial topology is still a key challenge. Here, we report the growth of single-layer iron germanide kagome nanoflakes by molecular beam epitaxy. Using scanning tunneling microscopy/spectroscopy, we unravel the real-space electronic localization of the kagome flat bands. First-principles calculations demonstrate the topological band inversion, suggesting the topological nature of the experimentally observed edge mode. Apart from the intrinsic topological states that potentially host chiral edge modes, the realization of kagome materials in the 2D limit also holds promise for future studies of geometric frustration