In-Situ Manipulation of the Magnetic Anisotropy of Single Mn Atom via Molecular Ligands.

Magnetic anisotropy is essential for permanent magnets to maintain their magnetization along specific directions. Understanding and controlling the magnetic anisotropy on a single-molecule scale are challenging but of fundamental importance for the future's spintronic technology. Here, by using scanning tunneling microscopy (STM), we demonstrated the ability to control the magnetic anisotropy by tuning the ligand field at the single-molecule level. We constructed a molecular magnetic complex with a single Mn atom and an organic molecule (4,4'-biphenyldicarbonitrile) as a ligand via atomic manipulation. Inelastic tunneling spectra (IETS) show that the Mn complex has much larger axial magnetic anisotropy than individual Mn atoms, and the anisotropy energy can be tuned by the coupling strength of the ligand. With density functional theory calculations, we revealed that the enhanced magnetic anisotropy of Mn arising from the carbonitrile ligand provides a prototype for the engineering of the magnetism of quantum devices.

Realization of Regular-Mixed Quasi-1D Borophene Chains with Long-Range Order.

The polymorphism of borophene makes it a promising system to realize tunable physical or chemical properties. Various pure borophene phases consisting of quasi-1D boron chains with different widths have been commonly obtained in experimental studies. Here, it is shown that, due to a substrate mediation effect, artificial long-range ordered phases of borophene consisting of different combinations of boron chains seamlessly joined together can be achieved on Ag(100). Scanning tunneling microscopy measurements and theoretical calculations reveal that mixed-chain phases are more stable than the pure phase, and interact only weakly with the substrate. The mixed-chain phases with various proportions of different chains can be well separated based on the crystal direction of the substrate. The successful growth of mixed-chain phases is expected to deepen the impact of substrate tailored synthesis of borophene.

The effect of moiré superstructures on topological edge states in twisted bismuthene homojunctions.

Creating and controlling the topological properties of two-dimensional topological insulators is essential for spintronic device applications. Here, we report the successful growth of bismuth homostructure consisting of monolayer bismuthene and single-layer black phosphorus-like Bi (BP-Bi) on the HOPG surface. Combining scanning tunneling microscopy/spectroscopy with noncontact atomic force microscopy, moiré superstructures with twist angles in the bismuth homostructure and the modulation of topological edge states of bismuthene were observed and studied. First-principles calculations reproduced the moiré superlattice and indicated that the structure fluctuation is ascribed to the stacking modes between bismuthene and BP-Bi, which induce spatially distributed interface interactions in the bismuth homostructure. The modulation of topological edge states is directly related to the variation of interlayer interactions. Our results suggest a promising pathway to tailor the topological states through interfacial interactions.

One-dimensional nearly free electron states in borophene.

Two-dimensional boron (borophene) features structural polymorphs and distinct in-plane anisotropy, opening opportunities to achieve tailored electronic properties by intermixing different phases. Here, using scanning tunneling spectroscopy combined with first-principles calculations, delocalized one-dimensional nearly free electron states (NFE) in the (2,3) or ß12 borophene sheet on the Ag(111) surface were observed. The NFE states emerge from a line defect in borophene, manifested as a structural unit of the (2,2) or χ3 sheet, which creates an in-plane potential well that shifts the states toward the Fermi level. The NFE states are held near the 2D plane of borophene, rather than in the vacuum region as observed in other nanostructures. Furthermore, borophene can provide a rare prototype to further study novel NFE behaviors, which may have potential applications in transport or field emission nanodevices based on boron.

Dynamics of Single-Molecule Dissociation by Selective Excitation of Molecular Phonons.

Breaking bonds selectively in molecules is vital in many chemistry reactions and custom nanoscale device fabrications. The scanning tunneling microscope (STM) has proved to be an ideal tool to initiate and view bond-selective chemistry at the single-molecule level, offering opportunities for the further study of the dynamics in single molecules on metal surfaces. We demonstrate H─HS and H─S bond breaking on Au(111) induced by tunneling electrons using low-temperature STM. An experimental study combined with theoretical calculations shows that the dissociation pathway is facilitated by vibrational excitations. Furthermore, the dissociation probabilities of the two different dissociation processes are bias dependent due to different inelastic-tunneling probabilities, and they are also closely linked to the lifetime of inelastic-tunneling electrons. Combined with time-dependent ab initio nonadiabatic molecular dynamics simulations, the dynamics of the injected electron and the phonon-excitation-induced molecule dissociation can be understood at the atomic scale, demonstrating the potential application of STM for the investigation of excited-state dynamics of single molecules on surfaces.

Binary Two-Dimensional Honeycomb Lattice with Strong Spin-Orbit Coupling and Electron-Hole Asymmetry.

Two-dimensional (2D) materials consisting of heavy atoms with particular arrangements may host exotic quantum properties. Here, we report a unique 2D semiconducting binary compound, a Sn_{2}Bi atomic layer on Si(111), in which hexagons are formed by bonding Bi with a triangular network of Sn. Because of the unique honeycomb configuration, the heavy elements, and the energy-dependent hybridization between Sn and Bi, 2D Sn_{2}Bi not only shows strong spin-orbit coupling effects but also exhibits high electron-hole asymmetry: Nearly free hole bands and dispersionless flat electron bands coexist in the same system. By tuning the Fermi level, it is possible to preserve both nearly free and strongly localized charge carriers in the same 2D material, which provides an ideal platform for the studies of strongly correlated phenomena and possible applications in nanodevices.

Scanning tunneling microscopy investigations of unoccupied surface states in two-dimensional semiconducting ß-√3 × âˆš3-Bi/Si(111) surface.

Two-dimensional surface structures often host a surface state in the bulk gap, which plays a crucial role in the surface electron transport. The diversity of in-gap surface states extends the category of two-dimensional systems and gives us more choices in material applications. In this article, we investigated the surface states of ß-√3 × âˆš3-Bi/Si(111) surface by scanning tunneling microscopy. Two nearly free electron states in the bulk gap of silicon were found in the unoccupied states. Combined with first-principles calculations, these two states were verified to be the Bi-contributed surface states and electron-accumulation-induced quantum well states. Due to the spin-orbit coupling of Bi atoms, Bi-contributed surface states exhibit free-electron Rashba splitting. The in-gap surface states with spin splitting can possibly be used for spin polarized electronics applications.

The Pentagonal Nature of Self-Assembled Silicon Chains and Magic Clusters on Ag(110).

The atomic structures of self-assembled silicon nanoribbons and magic clusters on Ag(110) substrate have been studied by high-resolution noncontact atomic force microscopy (nc-AFM) and tip-enhanced Raman spectroscopy (TERS). Pentagon-ring structures in Si nanoribbons and clusters have been directly visualized. Moreover, the vibrational fingerprints of individual Si nanoribbon and cluster retrieved by subnanometer resolution TERS confirm the pentagonal nature of both Si nanoribbons and clusters. This work demonstrates that Si pentagon can be an important element in building silicon nanostructures, which may find important applications for future nanoelectronic devices based on silicon.

Abnormal phase transition between two-dimensional high-density liquid crystal and low-density crystalline solid phases.

Some two-dimensional liquid systems are theoretically predicted to have an anomalous phase transition due to unique intermolecular interactions, for example the first-order transition between two-dimensional high-density water and low-density amorphous ice. However, it has never been experimentally observed, to the best of our knowledge. Here we report an entropy-driven phase transition between a high-density liquid crystal and low-density crystalline solid, directly observed by scanning tunneling microscope in carbon monoxide adsorbed on Cu(111). Combined with first principle calculations, we find that repulsive dipole-dipole interactions between carbon monoxide molecules lead to unconventional thermodynamics. This finding of unconventional thermodynamics in two-dimensional carbon monoxide not only provides a platform to study the fundamental principles of anomalous phase transitions in two-dimensional liquids at the atomic scale, but may also help to design and develop more efficient copper-based catalysis.

Experimental realization of honeycomb borophene.

We report the successful preparation of a purely honeycomb, graphene-like borophene, by using an Al(1 1 1) surface as the substrate and molecular beam epitaxy (MBE) growth in ultrahigh vacuum. Scanning tunneling microscopy (STM) images reveal perfect monolayer borophene with planar, non-buckled honeycomb lattice similar as graphene. Theoretical calculations show that the honeycomb borophene on Al(1 1 1) is energetically stable. Remarkably, nearly one electron charge is transferred to each boron atom from the Al(1 1 1) substrate and stabilizes the honeycomb borophene structure, in contrast to the negligible charge transfer in case of borophene/Ag(1 1 1). The existence of honeycomb 2D allotrope is important to the basic understanding of boron chemistry, and it also provides an ideal platform for fabricating boron-based materials with intriguing electronic properties such as Dirac states.