21 research outputs found
Evaluation of conduction eigenchannels of an adatom probed by an STM tip
Ballistic conductance through a single atom adsorbed on a metallic surface
and probed by a scanning tunneling microscope (STM) tip can be decomposed into
eigenchannel contributions, which can be potentially obtained from shot noise
measurements. Our density functional theory calculations provide evidence that
transmission probabilities of these eigenchannels encode information on the
modifications of the adatom's local density of states caused by its interaction
with the STM tip. In the case of open shell atoms, this can be revealed in
nonmonotonic behavior of the eigenchannel's transmissions as a function of the
tip-adatom separation.Comment: 4.5 pages, 5 figures, REVTe
Charge Transport in Single Au|Alkanedithiol|Au Junctions: Coordination Geometries and Conformational Degrees of Freedom
Recent STM molecular break-junction experiments have revealed multiple series
of peaks in the conductance histograms of alkanedithiols. To resolve a current
controversy, we present here an in-depth study of charge transport properties
of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM
measurements unambiguously confirm features showing more than one set of
junction configurations. Based on quantum chemistry calculations, we propose
that certain combinations of different sulfur-gold couplings and trans/gauche
conformations act as the driving agents. The present study may have
implications for experimental methodology: whenever conductances of different
junction conformations are not statistically independent, the conductance
histogram technique can exhibit a single series only, even though a much larger
abundance of microscopic realizations exists.Comment: 19 pages, 9 figures, 1 table; published versio
Transport properties of single atoms
We present a systematic study of the ballistic electron conductance through
sp and 3d transition metal atoms attached to copper and palladium crystalline
electrodes. We employ the 'ab initio' screened Korringa-Kohn-Rostoker Green's
function method to calculate the electronic structure of nanocontacts while the
ballistic transmission and conductance eigenchannels were obtained by means of
the Kubo approach as formulated by Baranger and Stone. We demonstrate that the
conductance of the systems is mainly determined by the electronic properties of
the atom bridging the macroscopic leads. We classify the conducting
eigenchannels according to the atomic orbitals of the contact atom and the
irreducible representations of the symmetry point group of the system that
leads to the microscopic understanding of the conductance. We show that if
impurity resonances in the density of states of the contact atom appear at the
Fermi energy, additional channels of appropriate symmetry could open. On the
other hand the transmission of the existing channels could be blocked by
impurity scattering.Comment: RevTEX4, 9 pages, 9 figure
Atomically wired molecular junctions: Connecting a single organic molecule by chains of metal atoms
Using a break junction technique, we find a clear signature for the formation
of conducting hybrid junctions composed of a single organic molecule (benzene,
naphthalene or anthracene) connected to chains of platinum atoms. The hybrid
junctions exhibit metallic-like conductance (~0.1-1G0), which is rather
insensitive to further elongation by additional atoms. At low bias voltage the
hybrid junctions can be elongated significantly beyond the length of the bare
atomic chains. Ab initio calculations reveal that benzene based hybrid
junctions have a significant binding energy and high structural flexibility
that may contribute to the survival of the hybrid junction during the
elongation process. The fabrication of hybrid junctions opens the way for
combining the different properties of atomic chains and organic molecules to
realize a new class of atomic scale interfaces
Magnetoresistance through a single molecule
The use of single molecules to design electronic devices is an extremely
challenging and fundamentally different approach to further downsizing
electronic circuits. Two-terminal molecular devices such as diodes were first
predicted [1] and, more recently, measured experimentally [2]. The addition of
a gate then enabled the study of molecular transistors [3-5]. In general terms,
in order to increase data processing capabilities, one may not only consider
the electron's charge but also its spin [6,7]. This concept has been pioneered
in giant magnetoresistance (GMR) junctions that consist of thin metallic films
[8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains,
however, a challenging endeavor. As an important first step in this field, we
have performed an experimental and theoretical study on spin transport across a
molecular GMR junction consisting of two ferromagnetic electrodes bridged by a
single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though
H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can
enhance the magnetoresistance by one order of magnitude to 52%.Comment: To appear in Nature Nanotechnology. Present version is the first
submission to Nature Nanotechnology, from May 18th, 201
Conduction Properties of Bipyridinium-Functionalized Molecular Wires
We present a detailed analysis of the coherent electron transport through a redox-active, 4,4′-bipyridinium (viologen)-functionalized molecular wire, which was studied in several recent experiments. Our calculations for the bare viologen predict conductances differing by 2 orders of magnitude depending on the contact geometry. For the alkyl-wired viologen unit, we obtain an exponential decay of the conductance with the wire length. Because this exponent also governs the conductance in the incoherent transport regime, comparison with experiments is legitimate and we find a good agreement. Furthermore, our calculations indicate that the experimentally observed conductance switching behavior is not amenable to an explanation inside a coherent transport picture. A possible incoherent mechanism is being discussed
Local Current Density Calculations for Molecular Films from Ab Initio
We present a formalism relying on density functional theory for the calculation of the spatially continuous electron current density j(r) and induced magnetic fields B(r) in molecular films in dc transport. The proposed method treats electron transport in graphene ribbons containing on the of order 103 atoms. The employed computational techniques scale efficiently when using several thousand CPUs. An application to transport through hydrogenated graphene will be presented. As we will show, the adatoms have an impact on the transmission function not only because they introduce additional states but also because their presence modifies the geometry of the carbon host lattice (lattice relaxation)
Ab initio simulations of scanning-tunneling-microscope images with embedding techniques and application to C58-dimers on Au(111)
We present a modification of the standard electron transport methodology based on the (non-equilibrium) Green's function formalism to efficiently simulate STM-images. The novel feature of this method is that it employs an effective embedding technique that allows us to extrapolate properties of metal substrates with adsorbed molecules from quantum-chemical cluster calculations. To illustrate the potential of this approach, we present an application to STM-images of C58-dimers immobilized on Au(111)-surfaces that is motivated by recent experiments
Ab Initio Transport Calculations for Single-Atom Copper Junctions in the Presence of Hydrogen Chloride
We study the transport properties of single-atom-thick Cu wires submerged in an electrochemical solvent containing HCl. As a first step, we investigate the stability of hydrogen coadsorbing with chlorine on the Cu(111) surface in an implicit electrochemical environment. We find that adding hydrogen to a Cl-covered Cu surface is energetically unfavorable. The result serves as an estimate for the number of Cl atoms that adsorb near the single-atom wire. We use it to construct model junctions (Cu wire plus adsorbates), the electron transport properties of which we investigate with density functional theory. We find that the Cl and H adsorbates tend to deplete the density of states of the Cu wire near the Fermi energy. As a consequence, the transmission is reduced. Interestingly, we observe that in the case of H-adsorption, the amount of depletion is quite sensitive to the wire geometry (relaxed vs unrelaxed), but this is not the case with Cl