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.

Linear alkane C-C bond chemistry mediated by metal surfaces.

Linear alkanes undergo different C-C bond chemistry (coupling or dissociation) thermally activated on anisotropic metal surfaces depending on the choice of the substrate material. Owing to the one-dimensional geometrical constraint, selective dehydrogenation and C-C coupling (polymerization) of linear alkanes take place on Au(110) surfaces with missing-row reconstruction. However, the case is dramatically different on Pt(110) surfaces, which exhibit similar reconstruction as Au(110). Instead of dehydrogenative polymerization, alkanes tend to dehydrogenative pyrolysis, resulting in hydrocarbon fragments. Density functional theory calculations reveal that dehydrogenation of alkanes on Au(110) surfaces is an endothermic process, but further C-C coupling between alkyl intermediates is exothermic. On the contrary, due to the much stronger C-Pt bonds, dehydrogenation on Pt(110) surfaces is energetically favorable, resulting in multiple hydrogen loss followed by C-C bond dissociation.

Unexpected versatile electrical transport behaviors of ferromagnetic nickel films.

Perpendicular magnetic anisotropy (PMA) of magnets is paramount for electrically controlled spintronics due to their intrinsic potentials for higher memory density, scalability, thermal stability and endurance, surpassing an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental element ferromagnet, yet its electrical transport behavior associated with magnetism has not been comprehensively studied, hindering corresponding spintronic applications exploiting nickel-based compounds. Here, we systematically investigate the highly versatile magnetism and corresponding transport behavior of nickel films. As the thickness reduces within the general thickness regime of a magnet layer for a memory device, the hardness of nickel films' ferromagnetic loop of anomalous Hall effect increases and then decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA. Additionally, the square ferromagnetic loop changes from a hard to a soft one at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we observe a butterfly magnetoresistance resulting from the anisotropic magnetoresistance effect, which evolves in conjunction with the thickness and temperature-dependent magnetic transformations as a complementary support. Our findings unveil the rich magnetic dynamics and most importantly settle down the most useful guiding information for current-driven spintronic applications based on nickel film: The hysteresis loop is squarest for the ∼8 nm-thick nickel film, of highest hardness withRxyr/Rxys∼ 1 and minimumHs-Hc, up to 125 K; otherwise, extra care should be taken for a different thickness or at a higher temperature.

Phase-selective in-plane heteroepitaxial growth of H-phase CrSe2.

Phase engineering of two-dimensional transition metal dichalcogenides (2D-TMDs) offers opportunities for exploring unique phase-specific properties and achieving new desired functionalities. Here, we report a phase-selective in-plane heteroepitaxial method to grow semiconducting H-phase CrSe2. The lattice-matched MoSe2 nanoribbons are utilized as the in-plane heteroepitaxial template to seed the growth of H-phase CrSe2 with the formation of MoSe2-CrSe2 heterostructures. Scanning tunneling microscopy and non-contact atomic force microscopy studies reveal the atomically sharp heterostructure interfaces and the characteristic defects of mirror twin boundaries emerging in the H-phase CrSe2 monolayers. The type-I straddling band alignments with band bending at the heterostructure interfaces are directly visualized with atomic precision. The mirror twin boundaries in the H-phase CrSe2 exhibit the Tomonaga-Luttinger liquid behavior in the confined one-dimensional electronic system. Our work provides a promising strategy for phase engineering of 2D TMDs, thereby promoting the property research and device applications of specific phases.

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.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities.

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.

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.

Room Temperature Ferromagnetism of Monolayer Chromium Telluride with Perpendicular Magnetic Anisotropy.

The realization of long-range magnetic ordering in 2D systems can potentially revolutionize next-generation information technology. Here, the successful fabrication of crystalline Cr3 Te4 monolayers with room temperature (RT) ferromagnetism is reported. Using molecular beam epitaxy, the growth of 2D Cr3 Te4 films with monolayer thickness is demonstrated at low substrate temperatures (≈100 °C), compatible with Si complementary metal oxide semiconductor technology. X-ray magnetic circular dichroism measurements reveal a Curie temperature (Tc ) of v344 K for the Cr3 Te4 monolayer with an out-of-plane magnetic easy axis, which decreases to v240 K for the thicker film (≈7 nm) with an in-plane easy axis. The enhancement of ferromagnetic coupling and the magnetic anisotropy transition is ascribed to interfacial effects, in particular the orbital overlap at the monolayer Cr3 Te4 /graphite interface, supported by density-functional theory calculations. This work sheds light on the low-temperature scalable growth of 2D nonlayered materials with RT ferromagnetism for new magnetic and spintronic devices.

On-surface synthesis of gold-coronene molecular wires.

Perchlorocoronene undergoes random dehalogenation at elevated temperatures, resulting in the formation of a disordered gold-coronene complex on Au(111) surfaces. The dehalogenation can be guided by introducing intermolecular constraint from accompanying molecules coexisting on the surface. Owing to the intermolecular electrostatic interactions, gold-coronene wires are intercalated between the polyphenylene chains, which hinders the transverse dehalogenation and results in an improved tendency toward linear gold-coronene molecular wires.

Graphene-like nanoribbons periodically embedded with four- and eight-membered rings.

Embedding non-hexagonal rings into sp2-hybridized carbon networks is considered a promising strategy to enrich the family of low-dimensional graphenic structures. However, non-hexagonal rings are energetically unstable compared to the hexagonal counterparts, making it challenging to embed non-hexagonal rings into carbon-based nanostructures in a controllable manner. Here, we report an on-surface synthesis of graphene-like nanoribbons with periodically embedded four- and eight-membered rings. The scanning tunnelling microscopy and atomic force microscopy study revealed that four- and eight-membered rings are formed between adjacent perylene backbones with a planar configuration. The non-hexagonal rings as a topological modification markedly change the electronic properties of the nanoribbons. The highest occupied and lowest unoccupied ribbon states are mainly distributed around the eight- and four-membered rings, respectively. The realization of graphene-like nanoribbons comprising non-hexagonal rings demonstrates a controllable route to fabricate non-hexagonal rings in nanoribbons and makes it possible to unveil their unique properties induced by non-hexagonal rings.

Atomic Structures of CH3NH3PbI3 (001) Surfaces.

We report on the atomic structures of methylammonium (MA) lead iodide (CH3NH3PbI3) perovskite surfaces, based on a combined scanning tunneling microscopy and density functional theory calculation study. A reconstructed surface phase with iodine dimers, coexisting with the pristine zigzag phase, was found at the MA-iodine-terminated (001) surfaces of the orthorhombic perovskite films grown on Au(111) surfaces. The reorientation of surface MA dipoles, which strengthens the interactions with surface iodine anions, resulting in a slight energy reduction of 34 meV per unit cell, is responsible for the surface iodine dimerization. According to our calculation, the surface MA dipoles weaken the surface polarity and are therefore considered to be stabilizing the surface structures.