In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Green’s functions to show that precisely the opposite is true in the extended metal atom chain Cr3(dpa)4(NCS)2 where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr3 core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actuall...

Influence of low-symmetry distortions on electron transport through metal atom chains: when is a molecular wire really "broken"?

Understanding the effects of each component of a molecular device is at the heart of designing a useful device. Molecular cores and linkers are well studied, but relatively few studies have been devoted to investigating the electrode effect on a molecular electronic device. Here, we study unique characteristics of Au, Ru, and carbon nanotube electrodes using the nonequilibrium Green function method combined with a density functional theory. By systematic modification of the device region, we extract the effect of the electrode materials on the electron transport. We show that the band structure and surface density of states of an electrode material, independent of the choice of other device components, have unique influences on the transmission curve. We note that carbon nanotube electrodes can offer unusual nonlinear current-voltage characteristics.

Effect of electrodes on electronic transport of molecular electronic devices.

Density functional theory is used to reconcile the structural, magnetic and electron transport properties of a triruthenium extended metal atom chain, Ru3(dpa)4(NCS)2. The distinct bending of the Ru–Ru–Ru core in this species is traced to strong second-order mixing between levels of σ and π symmetry that are near degenerate in the linear geometry. The dominant electron transport channel is formed by the LUMO, an orbital of π* symmetry that lies just above the Fermi level of the gold electrode. The bending has a substantial impact on electron transport in that it induces a spin crossover from a quintet to a singlet which in turn brings the LUMO much closer to the Fermi level. The presence of significant net π bonding in the metal chains also broadens the π/πnb/π* manifold, such that the channel is not strongly perturbed by the electric field, even at a bias of 1.0 V. The presence of a robust π symmetry conduction channel marks the triruthenium systems out as quite distinct from its first-row counterparts, Cr3(dpa)4(NCS)2 and Co3(dpa)4(NCS)2, where current flows primarily through the σ framework.

Periodic trends in electron transport through extended metal atom chains: comparison of Ru3(dpa)4(NCS)2 with its first-row analogues

We present a theoretical study of spin transport in a series of organometallic iron-cyclopentadienyl, FenCpn+1, multidecker clusters sandwiched between either gold or platinum electrodes. Ab initio modeling is performed by combining the non-equilibrium Green's function formalism with spin density functional theory. Due to the intrinsic bonding nature, the low-bias conductance of the FenCpn+1 clusters contacted to gold electrodes is relatively small even for strong cluster–electrode coupling. However, a nearly 100% spin polarization of the transmitted electrons can be achieved for the FenCpn+1 (n>2) clusters. In contrast, the FenCpn+1 (n>2) clusters attached to platinum electrodes through Pt adatoms not only can act as nearly perfect spin filters but also show a much larger transmission around the Fermi level, demonstrating their promising applications in future molecular spintronics.

The spin filter effect of iron-cyclopentadienyl multidecker clusters: the role of the electrode band structure and the coupling strength

High resolution electron energy loss spectroscopy (HREELS) data for C6H6 and C6D6 on Pt(111) were measured as a function of coverage. The spectra show five additional loss peaks with respect to earlier reports that were used in density functional theory (DFT) calculations of the properties of benzene on Pt(111). The spectra display a strong coverage dependence in the CH stretching region. HREELS and STM data for the adsorption of 2,2,2-trifluoroacetophenone (TFAP) on Pt(111) are used in the analysis of the benzene/Pt(111) system. The TFAP spectra display the same coverage dependence as benzene in the CH stretching region. An analysis of STM images of TFAP dimers shows that their azimuthal orientation is not consistent with the occupation of hcp0 sites. The data for benzene adsorption cannot be explained on the basis of existing density functional theory (DFT) calculations. In particular, the spectra are not consistent with either a C2v symmetry bridge adsorption or a combination of adsorption at C2v symme...

Surface Vibrational Spectroscopy Study of Benzene and 2,2,2-Trifluoroacetophenone on Pt(111)

The conductance of a benzene molecule connected to two ruthenium (Ru) electrodes through two C(H) anchoring groups is investigated using a self-consistent ab initio approach that combines the non-equilibrium Green’s function formalism with density functional theory. Our calculations demonstrate that a nearly perfect conductance with magnitude exceeding 84% of the conductance quantum G 0 can be obtained when the two C(H) anchoring groups are bonded to a Ru adatom on the Ru(0 0 0 1) surface, independently from whether this is a Ru C double bond or a Ru C triple bond. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the benzene backbone interact with the Ru–C π bonds in the contact region to form efficient charge transport channels, illustrating the high conducting nature of benzene.

High transmission in ruthenium-benzene-ruthenium molecular junctions

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High transmission in ruthenium-benzene-ruthenium molecular junctions