Ziyong Shen

First-principles calculation of the conductance of a single 4,4 bipyridine molecule.

The conductance of a single 4,4 bipyridine (44BPD) molecule connected to two gold electrodes is calculated using a density functional theory based Green function method. The atomic geometry of such a molecular junction is constructed from the optimized structure of a gold trimer-44BPD-gold trimer complex. Resonant conduction is the main feature of its transport properties. The magnitude of the transmission coefficient at the Fermi level is determined to be T = 1.01 × 10(-2), which is in excellent agreement with the experimental value. The dependence of the transmission on the Au-N bond length and the torsion angle is also discussed.

Analysis on the contribution of molecular orbitals to the conductance of molecular electronic devices

We present a theoretical approach which allows one to extract the orbital contribution to the conductance of molecular electronic devices. This is achieved by calculating the scattering wave functions after the Hamiltonian matrix of the extended molecule is obtained from a self-consistent calculation that combines the nonequilibrium Greens function formalism with density functional theory employing a finite basis of local atomic orbitals. As an example, the contribution of molecular orbitals to the conductance of a model system consisting of a 4,4-bipyridine molecule connected to two semi-infinite gold monatomic chains is explored, illustrating the capability of our approach.

An accurate and efficient self-consistent approach for calculating electron transport through molecular electronic devices: including the corrections of electrodes

A self-consistent ab initio approach for calculating electron transport through molecular electronic devices is developed. It is based on density functional theory (DFT) calculations and the Greens function technique employing a finite basis of local orbitals. The device is rigorously separated into the extended molecule region and the electrode region. In the DFT part calculating the Hamiltonian matrix of the extended molecule from its density matrix, the electrostatic correction induced by electrodes and the exchange?correlation correction due to the spatial diffuseness of localized basis functions are included. Our approach is efficient and accurate, with a controllable error to deal with such open systems. A one-dimensional infinite gold monatomic chain, whose electronic structure can be known from conventional DFT calculations with periodic boundary conditions (PBCs), is employed to validate the accuracy of our approach. With both corrections, our result for the gold chain at equilibrium is in excellent agreement with the PBC DFT result. We find that, for the gold chain, the exchange?correlation correction is more significant than the electrostatic correction.

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

We present a theoretical study of spin transport in a series of organometallic iron-cyclopentadienyl, Fe(n)Cp(n+1), multidecker clusters sandwiched between either gold or platinum electrodes. Ab initio modeling is performed by combining the non-equilibrium Greens function formalism with spin density functional theory. Due to the intrinsic bonding nature, the low-bias conductance of the Fe(n)Cp(n+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 Fe(n)Cp(n+1) (n>2) clusters. In contrast, the Fe(n)Cp(n+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.

Tuning the Magneto-Transport Properties of Nickel−Cyclopentadienyl Multidecker Clusters by Molecule−Electrode Coupling Manipulation

Spin transport in a series of organometallic multidecker clusters made of alternating nickel atoms and cyclopentadienyl (Cp) rings is investigated by using first-principles quantum transport simulations. The magnetic moment of finite NinCp(n+1) clusters in the gas phase is a periodic function of the number of NiCp monomers, n, regardless of the cluster termination and despite the fact that the band structure of the infinite [NiCp]infinity chain is nonmagnetic. In contrast, when the clusters are sandwiched between gold electrodes, their spin polarization is found to strongly depend on the molecule-electrode coupling. On the one hand, a substantial magnetic moment and a large spin polarization can be detected for NiCp2 and Ni4Cp5 with both weak and modest molecule-electrode coupling. On the other hand, when the coupling of the clusters is strong and mediated by Ni adatoms, the spin polarization of all NinCp(n+1) (n = 1-4) clusters is destroyed, although their low-bias conductance is large. This demonstrates that the magnetism and the spin-transport properties of fragile molecular magnets, such as NinCp(n+1), can be tuned in a controllable way by changing the contact geometry.

Quantum chemistry study on the open end of single-walled carbon nanotubes

Abstract Geometrical and electronic structures of open-ended single-walled carbon nanotubes (SWCNTs) are calculated using density functional theory (DFT) with hybrid functional (B3LYP) approximation. Due to different distances between carbon atoms along the edge, reconstruction occurs at the open end of the (4,4) armchair SWCNT, i.e., triple bonds are formed in the carbon atom pairs at the mouth; however, for the (6,0) zigzag SWCNT, electrons in dangling bonds still remain at ‘no-bonding’ states. The ionization potential (IP) of both (4,4) and (6,0) SWCNTs is increased by their negative intrinsic dipole moments, and localized electronic states existed at both of their open ends.

First-principles calculation on the conductance of a single 1,4-diisocyanatobenzene molecule with single-walled carbon nanotubes as the electrodes

The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between two single-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistent ab initio approach which combines nonequilibrium Greens function formalism with density functional theory calculations. Several metallic zigzag and armchair SWCNTs with different diameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogen atoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couple strongly only with the frontier molecular orbitals that are related to nonlocal pi bonds. Although the chirality of SWCNT electrodes has significant influences on this coupling and thus the molecular conductance, the diameter of electrodes, the distance, and the torsion angle between electrodes have only minor influences on the conductance, showing the advantage of using SWCNTs as the electrodes for molecular electronic devices.

Spin transport properties of single metallocene molecules attached to single-walled carbon nanotubes via nickel adatoms

The spin-dependent transport properties of single ferrocene, cobaltocene, and nickelocene molecules attached to the sidewall of a (4,4) armchair single-walled carbon nanotube via a Ni adatom are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Greens function formalism with the spin density functional theory. Our calculations show that the Ni adatom not only binds strongly to the sidewall of the nanotube, but also maintains the spin degeneracy and affects little the transmission around the Fermi level. When the Ni adatom further binds to a metallocene molecule, its density of states is modulated by that of the molecule and electron scattering takes place in the nanotube. In particular, we find that for both cobaltocene and nickelocene the transport across the nanotube becomes spin-polarized. This demonstrates that metallocene molecules and carbon nanotubes can become a promising materials platform for applications in molecular spintronics.

Local oxidation of titanium thin films using an atomic force microscope under static and pulsed voltages

Scanning probe microscope tip-induced local oxidation is a promising tool for the fabrication of nanometre-scale structures and devices. In this study, oxide line patterns were fabricated on the surface of a titanium thin film using a conductive atomic force microscope (AFM). Geometrical characters of the oxide line patterns and their dependence on the exposure parameters in fabricating, i.e. the applied voltage amplitude and duration, ambient humidity, AFM set point value, and the mode of applied voltage, are investigated. The dependence of the oxide width on the applied voltage duration was found to have two distinct growth rates and a two-stage growth model was proposed to account for it. Application of pulsed voltages was proved to be an efficient method for suppressing the growth of oxide width by repeatedly breaking the directional transport of OH− ions in the process of oxidation. A line-width of 8 nm was achieved with an optimized pulsed voltage. Based on the experimental results, optimal controlling of exposure parameters to improve the fabricating resolution and reliability are discussed.

Electronic transport calculations for the conductance of Pt–1,4-phenylene diisocyanide–Pt molecular junctions

The low-bias transport properties of a single 1,4-phenylene diisocyanide (PDI) molecule connected to two platinum (Pt) electrodes are investigated using a self-consistent ab initio approach that combines the non-equilibrium Greens function formalism with density functional theory. Our calculations demonstrate that the zero-bias conductance of an asymmetric Pt-PDI-Pt junction, where the PDI molecule is attached to the atop site at one Pt(111) electrode and to a Pt adatom at the other, is 2.6 x 10( - 2)G(0), in good agreement with the experimental value (3 x 10( - 2)G(0)) measured with break junctions. Although the highest occupied and the lowest unoccupied molecule orbitals in PDI are both pi-type, delocalized along the entire molecule, their electronic coupling with the highly conducting states of the Pt electrode is blocked at the atop site, leading to the small transmission. This indicates that more efficient electronic contacts are needed to fabricate molecular devices with a high conductance using Pt electrodes and aromatic isocyanides such as PDI.