Giant Magnetic Anisotropy of Transition-Metal Dimers on Defected Graphene

Continuous miniaturization of magnetic units in spintronics and quantum computing devices inspires efforts to search for magnetic nanostructures with giant magnetic anisotropy energy (MAE) and high structural stability. Through density functional theory calculations, we found that either Pt–Ir or Os–Ru dimer forms a stable vertical structure on the defected graphene and possess an MAE larger than 60 meV, sufficient for room-temperature applications. Interestingly, their MAEs can be conveniently manipulated by using an external electric field, which makes them excellent magnetic units in spintronics and quantum computing devices

Engineering Topological Surface States of Cr-Doped Bi₂Se₃ Films by Spin Reorientation and Electric Field

The tailoring of topological surface states in topological insulators is essential for device applications and for exploring new topological phase. Here, we propose a practical way to induce the quantum anomalous Hall phase and unusual metal–insulator transitions in Cr-doped Bi₂Se₃ films based on the model Hamiltonian and first-principles calculations. Using the combination of in-plane and plane-normal components of the spin along with external electric fields, we demonstrate that the topological state and band structures of topological insulating films exhibit rich features such as the shift of Dirac cones and the opening of nontrivial band gaps. We also show that the in-plane magnetization leads to significant suppression of inter-TSS scattering in Cr-doped Bi₂Se₃. Our work provides new strategies to obtain the desired electronic structures for the device, complementary to the efforts of an extensive material search

Increasing the Band Gap of Iron Pyrite by Alloying with Oxygen

Systematic density functional theory studies and model analyses have been used to show that the band gap of iron pyrite (FeS₂) can be increased from ∼1.0 to 1.2–1.3 eV by replacing ∼10% of the sulfur atoms with oxygen atoms (i.e., ∼10% O_S impurities). O_S formation is exothermic, and the oxygen atoms tend to avoid O–O dimerization, which favors the structural stability of homogeneous FeS_2–<i>x</i>O_<i>x</i> alloys and frustrates phase separation into FeS₂ and iron oxides. With an ideal band gap, absence of O_S-induced gap states, high optical absorptivity, and low electron effective mass, FeS_2–<i>x</i>O_<i>x</i> alloys are promising for the development of pyrite-based heterojunction solar cells that feature large photovoltages and high device efficiencies

Formation of Pd Monomers and Dimers on a Single-Crystal Pd₃Fe(111) Surface

Surface reconstruction of binary alloys is important in heterogeneous catalysis because it modifies both surface composition and structure and thus affects the catalytic activity and selectivity. We report here on segregation and surface morphology at a Pd₃Fe(111) single-crystal model catalyst investigated by low-energy ion scattering (LEIS) and scanning tunneling microscopy (STM). Annealing in vacuum causes Pd segregation, and STM reveals a complex surface structure with 0.17 monolayers of Pd monomer and dimer adatoms on top of the outermost alloy layer. This result is explained by density functional theory (DFT) calculations, which reveal that the contribution from vibrational free energy causes Pd atoms to detach from step edges at high temperature (>1200 K) and then become trapped at room temperature at Fe defect sites due to a large diffusion barrier. This adlayer structure differs from surface structures observed for other binary alloy systems and is likely to offer new opportunities for manipulating catalytic properties of bimetallic alloys

Visualization of Nanoplasmonic Coupling to Molecular Orbital in Light Emission Induced by Tunneling Electrons

The coupling between localized plasmon and molecular orbital in the light emission from a metallic nanocavity has been directly detected and imaged with sub-0.1 nm resolution. The light emission intensity was enhanced when the energy difference between the tunneling electrons and the lowest unoccupied molecular orbital (LUMO) of an azulene molecule matches the energy of a plasmon mode of the nanocavity defined by the Ag-tip and Ag (110) substrate of a scanning tunneling microscope (STM). The spatially resolved image of the light emission intensity matches the spatial distribution of the LUMO obtained by scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. Our results highlight the near-field coupling of a molecular orbital to the radiative decay of a plasmonic excitation in a confined nanoscale junction

Quantitative Understanding of van der Waals Interactions by Analyzing the Adsorption Structure and Low-Frequency Vibrational Modes of Single Benzene Molecules on Silver

The combination of a sub-Kelvin scanning tunneling microscope and density functional calculations incorporating van der Waals (vdW) corrections has been used successfully to probe the adsorption structure and low-frequency vibrational modes of single benzene molecules on Ag(110). The inclusion of optimized vdW functionals and improved <i>C</i>₆-based vdW dispersion schemes in density functional theory is crucial for obtaining the correct adsorption structure and low-energy vibrational modes. These results demonstrate the emerging capability to quantitatively probe the van der Waals interactions between a physisorbed molecule and an inert substrate

A Chemically-Responsive Nanojunction within a Silver Nanowire

The formation of a nanometer-scale chemically responsive junction (CRJ) within a silver nanowire is described. A silver nanowire was first prepared on glass using the lithographically patterned nanowire electrodeposition method. A 1–5 nm gap was formed in this wire by electromigration. Finally, this gap was reconnected by applying a voltage ramp to the nanowire resulting in the formation of a resistive, ohmic CRJ. Exposure of this CRJ-containing nanowire to ammonia (NH₃) induced a rapid (<30 s) and reversible resistance change that was as large as Δ<i>R</i>/<i>R</i>₀ = (+)­138% in 7% NH₃ and observable down to 500 ppm NH₃. Exposure to water vapor produced a weaker resistance increase of Δ<i>R</i>/R_0,H₂O = (+)­10–15% (for 2.3% water) while nitrogen dioxide (NO₂) exposure induced a stronger concentration-normalized resistance decrease of Δ<i>R</i>/<i>R</i>_0,NO₂ = (−)­10–15% (for 500 ppm NO₂). The proposed mechanism of the resistance response for a CRJ, supported by temperature-dependent measurements of the conductivity for CRJs and density functional theory calculations, is that semiconducting p-type Ag_<i>x</i>O is formed within the CRJ and the binding of molecules to this Ag_<i>x</i>O modulates its electrical resistance

Intrinsically Conductive Organo–Silver Linear Chain Polymers [−S–Ag–S–Biphenyl−]_<i>n</i> Assembled on Roughened Elemental Silver

A combined experimental and theoretical study of the facile polymerization of biphenyl-4,4′-dithiol (BPDT) to form intrinsically conductive linear chain [−Ag–S–BP–S−]_<i>n</i> polymers is described. BPDT readily polymerizes and extrudes on roughened surfaces of elemental silver under ambient conditions. The self-assembled polymers can be sharply imaged through scanning electron microscopy because of their silver content and conductivity. Cyclic current versus voltage measurements (<i>I</i>/<i>V</i> curves) using a scanning tunneling microscope establish that the conductivity is intrinsic, consistent with the metallic conductivity of the linear polymer predicted through density functional theory. Systematic calculations identify that the roughness-catalyzed polymerization is driven by mobile Ag adatoms and adatom-mobilized monomers

Revealing Surface Elemental Composition and Dynamic Processes Involved in Facet-Dependent Oxidation of Pt₃Co Nanoparticles via <i>in Situ</i> Transmission Electron Microscopy

Since catalytic performance of platinum–metal (Pt–M) nanoparticles is primarily determined by the chemical and structural configurations of the outermost atomic layers, detailed knowledge of the distribution of Pt and M surface atoms is crucial for the design of Pt–M electrocatalysts with optimum activity. Further, an understanding of how the surface composition and structure of electrocatalysts may be controlled by external means is useful for their efficient production. Here, we report our study of surface composition and the dynamics involved in facet-dependent oxidation of equilibrium-shaped Pt₃Co nanoparticles in an initially disordered state via <i>in situ</i> transmission electron microscopy and density functional calculations. In brief, using our advanced <i>in situ</i> gas cell technique, evolution of the surface of the Pt₃Co nanoparticles was monitored at the atomic scale during their exposure to an oxygen atmosphere at elevated temperature, and it was found that Co segregation and oxidation take place on {111} surfaces but not on {100} surfaces

Correlating Electronic Transport to Atomic Structures in Self-Assembled Quantum Wires

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi₂ are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal–insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale