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We develop an atomic-scale model for an ordered incommensurate gold sulfide (AuS) adlayer which has previously been demonstrated to exist on the Au(111) surface, following sulfur deposition and annealing to 450 K. Our model reproduces experimental scanning tunneling microscopy images. Using state-of-the-art Wannier-function-based techniques, we analyze the nature of bonding in this structure and provide an interpretation of the unusual stoichiometry of the gold sulfide layer. The proposed structure and its chemistry have implications for related S–Au interfaces, as in those involved in self-assembled monolayers of thiols on Au substrates. [ABSTRACT FROM AUTHOR]

Organic spintronics is a promising emerging field, but the sign of the tunneling magnetoresistance (TMR) is highly sensitive to interface effects, a crucial hindrance to applications. A key breakthrough in molecular electronics was the discovery of amine–Au link groups that give a reproducible conductance. Using first-principles calculations, we predict that amine–Au links give improved reproducibility in organic spintronics junctions with Au-covered Fe leads. The Au layers allow only states with sp character to tunnel into the molecule, and the flexibility of amine–Au links results in a narrow range of TMR for a fixed number of Au layers. Even as the Au thickness changes, the TMR remains positive as long as the number of Au layers is the same on both sides of the junction. Since the number of Au layers on Fe surfaces or Fe nanoparticles can now be experimentally controlled, amine–Au links provide a route towards robust TMR in organic spintronics. [ABSTRACT FROM AUTHOR]

Abstract: MoO3 is an interesting oxide prototype because its catalytic activity is sensitive to the presence and nature of defects. In this work, we demonstrate that we can control the number of defects in single-layer MoO3 nanostructures grown on Au(111) by a thermal reduction treatment. X-ray photoelectron spectroscopy demonstrates the formation of Mo5+ species and oxygen vacancies during annealing at 650K. The percentage of Mo5+ increases with the duration of annealing, until a stable composition containing 50% Mo6+ and 50% Mo5+ is obtained. Surprisingly, the formation of lower oxidation states such as Mo4+ was not observed. The reduced MoO x islands remain one layer high, based on scanning tunneling microscope (STM) images. The two-dimensional nature of the reduced oxide nanocrystals may be due to a large barrier for structural reorganization and, thus, may account for the absence of Mo oxidation states lower than +5. Based on scanning tunneling microscopy images and density functional calculations, we propose that the formation of Mo5+ ions during annealing is not associated with formation of oxygen point defects, but can be attributed to the formation of extended one-dimensional shear defects. These reduced structures are useful for studying the dependence of reactivity on defect type, and present exciting possibilities for chemical sensors and other applications. [Copyright &y& Elsevier]

Abstract: Ultrathin oxide layers can exhibit special behavior by enabling the coupling of structural distortions and charge transfer beyond that allowed in the bulk. In this work, we show from first-principles calculations that ultrathin layers of titania, a prototypical oxide, are active in stabilizing adsorption of O2 on Au overlayers. The adsorbed O2 molecules induce charge redistribution in Au that penetrates to the Au–titania interface, which responds through structural distortions that lower the total energy of the system. We suggest that this effect may be of more general nature and useful in catalysis. [Copyright &y& Elsevier]

Two-dimensional (2D) materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, Eext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, Ec. Increasing Eext beyond Ec can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semimetal transition. [ABSTRACT FROM AUTHOR]

Abstract: We report experimental and theoretical investigations of monolayer MoO3 nanocrystals grown on Au(111), in contrast to the bilayered structure of the bulk oxide crystal. The Au surface acts as the other half of the bilayer by satisfying local bonding requirements through charge redistribution at the interface. Epitaxy with the Au lattice is achieved through the ability of the Mo–O bonds to rotate about one another. The oxide layer becomes semimetallic as it strains to enhance bonding with the Au substrate. This flexibility of the oxide lattice suggests the possibility of tuning electronic properties of transition metal oxides via interface interactions. The effects of electronic structure on the surface chemistry of oxides are in turn illustrated by H adsorption energetics on MoO3. [Copyright &y& Elsevier]

Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single‐photon sources and valleytronic devices. For these applications, defects with well‐defined optical characteristics need to be generated in a precisely controlled manner. However, defect‐induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton‐beam irradiation. Defect‐induced emissions are found to occur ≈100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high‐excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe‐Salpeter‐equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state. [ABSTRACT FROM AUTHOR]

This work is a systematic experimental and theoretical study of the in‐plane dielectric functions of 2D gallium and indium films consisting of two or three atomic metal layers confined between silicon carbide and graphene with a corresponding bonding gradient from covalent to metallic to van der Waals type. k‐space resolved free electron and bound electron contributions to the optical response are identified, with the latter pointing towards the existence of thickness dependent quantum confinement phenomena. The resonance energies in the dielectric functions and the observed epsilon near‐zero behavior in the near infrared to visible spectral range, are dependent on the number of atomic metal layers and properties of the metal involved. A model‐based spectroscopic ellipsometry approach is used to estimate the number of atomic metal layers, providing a convenient route over expensive invasive characterization techniques. A strong thickness and metal choice dependence of the light–matter interaction makes these half van der Waals 2D polar metals attractive for quantum engineered metal films, tunable (quantum‐)plasmonics and nano‐photonics. [ABSTRACT FROM AUTHOR]

Since the first isolation of graphene, new classes of two-dimensional (2D) materials have offered fascinating platforms for fundamental science and technology explorations at the nanometer scale. In particular, 2D transition metal dichalcogenides (TMD) such as MoS2 and WSe2 have been intensely investigated due to their unique electronic and optical properties, including tunable optical bandgaps, direct–indirect bandgap crossover, strong spin–orbit coupling, etc., for next-generation flexible nanoelectronics and nanophotonics applications. On the other hand, organics have always been excellent materials for flexible electronics. A plethora of organic molecules, including donors, acceptors, and photosensitive molecules, can be synthesized using low cost and scalable procedures. Marrying the fields of organics and 2D TMDs will bring benefits that are not present in either material alone, enabling even better, multifunctional flexible devices. Central to the realization of such devices is a fundamental understanding of the organic–2D TMD interface. Here, we review the organic–2D TMD interface from both chemical and physical perspectives. We discuss the current understanding of the interfacial interactions between the organic layers and the TMDs, as well as the energy level alignment at the interface, focusing in particular on surface charge transfer and electronic screening effects. Applications from the literature are discussed, especially in optoelectronics and p–n hetero- and homo-junctions. We conclude with an outlook on future scientific and device developments based on organic–2D TMD heterointerfaces. [ABSTRACT FROM AUTHOR]

Distorted octahedral T′ phase of MoTe2 has recently attracted significant interest due to its predicted topological states and novel charge transport properties. Here, we report a nondestructive method for determining the crystal orientation of few-layer T′-MoTe2 flakes by polarized Raman spectroscopy. The experimentally observed Raman modes are assigned to eigenmodes of vibrations predicted by density functional theory calculations. Polarized Raman measurements reveal four distinct types of angle-dependent intensity variations. From group theory, it can be deduced that the intensity of the Bg mode reaches a maximum in the [ABSTRACT FROM AUTHOR]

Abstract: Molecular wires constitute the building blocks for nanoscale interconnects. However, the exponential decrease in conductance with wire length severely limits their applications. We predict, using first principles calculations, that armchair graphene nanoribbon (AGNR) wires, connected by transverse zigzag edges to wider AGNR electrodes, can exhibit anomalous resonant transmission peaks that are nearly independent of the wire length. We propose a new model to explain the unusual length independence of peak energies from the locally repeating property of the wavefunction in the middle-AGNR. We further uncover that this locally repeating pattern originates from states of a perfect AGNR with infinite length. The pattern can be well preserved when the AGNR is connected to wider AGNR leads because of the zigzag edges serving as electron sources and drains. The length independence of peak widths results from the zigzag edges absorbing most of the wavefunction renormalization as the length increases, so that the coupling strength to the electrodes does not change significantly. These anomalous properties arising from intrinsic wavefunction properties of the AGNRs are in sharp contrast to typical transmission properties of traditional molecular wire junctions, which suggests promising potential application as “molecular wire” interconnects in nano-electronics. [Copyright &y& Elsevier]

We measure conductance and thermopowerof single Au–4,4′-bipyridine–Aujunctions in distinct low and high conductance binding geometriesaccessed by modulating the electrode separation. We use these datato determine the electronic energy level alignment and coupling strengthfor these junctions, which are known to conduct through the lowestunoccupied molecular orbital (LUMO). Contrary to intuition, we findthat, in the high-conductance junction, the LUMO resonance energyis further away from the Au Fermi energy than in the low-conductancejunction. However, the LUMO of the high-conducting junction is bettercoupled to the electrode. These results are in good quantitative agreementwith self-energy corrected zero-bias density functional theory calculations.Our calculations show further that measurements of conductance andthermopower in amine-terminated oligophenyl–Au junctions, whereconduction occurs through the highest occupied molecular orbitals,cannot be used to extract electronic parameters as their transmissionfunctions do not follow a simple Lorentzian form. [ABSTRACT FROM AUTHOR]