Understanding the Electronic Transport Through Single Noble Gas Atoms

We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold electrodes. This explains the observations reported in metallic atomic-size contacts with adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the conductance due to the additional current path provided by their valence p states.Comment: 3 figure

C6H6/Au(111): Interface dipoles, band alignment, charging energy, and van der Waals interaction

The following article appeared in Journal of Chemical Physics 134.4 (2011): 044701 and may be found at http://scitation.aip.org/content/aip/journal/jcp/134/4/10.1063/1.3521271.We analyze the benzene/Au(111) interface taking into account chargingenergy effects to properly describe the electronic structure of the interface and van der Waals interactions to obtain the adsorption energy and geometry. We also analyze the interface dipoles and discuss the barrier formation as a function of the metal work-function. We interpret our DFT calculations within the induced density of interface states (IDIS) model. Our results compare well with experimental and other theoretical results, showing that the dipole formation of these interfaces is due to the charge transfer between the metal and benzene, as described in the IDIS model.This work is supported by Spanish MICIIN under Contracts No. MAT2007-60966 and No. FIS2010-16046, the CAM under Contract No. S2009/MAT-1467, and the European Project MINOTOR (Grant No. FP7-NMP-228424). E.A. gratefully acknowledges financial support by the Consejería de Educación of the CAM and the FSE. J.I.M. acknowledges funding from Spanish MICINN through Juan de la Cierva Program

Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2 - a natural one and one prepared at high pressure and high temperature - and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials

Molecular detection on a defective MoS2_2 monolayer by simultaneous conductance and force simulations

International audienceBased on simultaneous force and conductance simulations, a proof of concept for a potential method of molecular detection is presented. Using density functional theory calculations, a metallic tip has been approached to different small inorganic molecules such as CO, CO$_2$ , H$_2$O, NO, N$_2$ , or O$_2$. The molecules have been previously chemisorbed on a defect formed by two Mo atoms occupying a S divacancy on a MoS 2 monolayer where they are strongly bonded to the topmost substitutional molybdenum. At that site, the fixed molecules can be imaged by a conductive atomic-force-microscopy tip. Due to the differences in atomic composition and electronic configurations, each molecule yields specific conductance/force curves during the tip approach. A molecule-tip contact is established at the force minimum, followed by the formation of a characteristic plateau in the conductance in most of the cases. Focusing our attention on the position and values of such force minimum and conductance maximum, we can conclude that both characteristic properties can give a clear signature of each molecule, proposing a different method of detecting molecules adsorbed on highly reactive sites

Carbon tips for all-carbon single-molecule electronics

International audienceWe present here an exhaustive ab initio study of the use of carbon-based tips as electrodes in single-molecule junctions. Motivated by recent experiments, we show that carbon tips can be combined with other carbon nanostructures, such as graphene, to form all-carbon molecular junctions with molecules like benzene or C 60. Our results show that the use of carbon tips can lead to relatively conductive molecular junctions. However, contrary to junctions formed with standard metals, the conductance traces recorded during the formation of the all-carbon single-molecule junctions do not exhibit clear conductance plateaus, which can be attributed to the inability of the hydrogenated carbon tips to form chemical bonds with the organic molecules. Additionally, we explore here the use of carbon tips for scanning tunneling microscopy and show that they are well suited for obtaining sample images with atomic resolution

Van der Waals forces in the local-orbital Density Functional Theory

Van der Waals forces are analyzed in a Density Functional Theory, using a “local orbital occupancy” formulation and second-order perturbation theory. In this approach, the exchange-correlation energy as well as the van der Waals forces are written as a function of the orbital occupation numbers. We present a detailed discussion of the \chem{He}-\chem{He} case and calculate the Density Functional-van der Waals energy of the system. Our analysis also suggests an alternative approach for including van der Waals forces in the local-orbital DFT formulation, namely, to introduce an effective hopping interaction between the orbitals of both atoms. Our results for the \chem{He} and \chem{Ne} dimers show the validity and the accuracy of our proposed DFT-van der Waals approach

Dynamical screening of van der Waals interaction between graphene layers

The interaction between graphene layers is analyzed combining local orbital DFT and second order perturbation theory. For this purpose we use the Linear Combination of Atomic Orbitals -Orbital Occupancy (LCAO-OO) formalism, that allows us to separate the interaction energy as a sum of a weak chemical interaction between graphene layers plus the van der Waals interaction [1]. In this work, the weak chemical interaction is calculated by means of corrected-LDA calculations using an atomic-like sp3d5 basis set. The van der Waals interaction is calculated by means of second order perturbation theory using an atom-atom interaction approximation and the atomiclike orbital occupancies. We also analyze the effect of dynamical screening in the van der Waals interaction using a simple model. We find that this dynamical screening reduces the van der Waals energy between graphene layers by 22 meV/atom , which represents a 40% reduction in the van der Waals interaction. Taking into account this dynamical screening, we obtain a graphene-graphene interaction of 64 meV/atom, in good agreement with the experimental evidence. 2Fil: Dappe, Y. J.. Service de Physique de l´etat Conndensé. DSM/IRAMIS/SPEC; FranciaFil: Bolcatto, Pablo Guillermo. Universidad Nacional del Litoral. Facultad de Humanidades y Ciencias; Argentina. Universidad Nacional del Litoral. Facultad de Ingenieria Quimica. Departamento de Fisica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Flores, F.. Universidad Autónoma de Madrid; EspañaFil: Ortega, J.. Universidad Autónoma de Madrid; Españ

Reactivity Enhancement and Fingerprints of Point Defects on a MoS₂ Monolayer Assessed by <i>ab Initio</i> Atomic Force Microscopy

The effect of topological point defects, vacancies, and substitutional antisites, in a monolayer MoS₂, has been analyzed by <i>ab initio</i> atomic force microscopy (AFM) simulations. Our calculations based on density functional theory (DFT) show how a careful combination of measurements at different distances enables the characterization of each defect on the monolayer in future noncontact AFM experiments. Taking into account the minimum in the forces, atomic displacements, and charge transfer, a great enhancement has been found on the reactivity of MoS₂ when some defects are included in the monolayer. We demonstrate the strong influence of the chemical composition of the tip and the environment of the chosen site on the calculated force. Furthermore, we show that the results can be mostly understood considering a standard metal–semiconductor junction model. Finally, our study exhibits the possibility of local atomic doping using the AFM tip