Xingyu Zhao

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.

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.

Atomically resolved field emission patterns of single-walled carbon nanotubes

The electron emission and structural properties of single-walled carbon nanotubes (SWCNTs) were investigated by using field emission microscopy (FEM). The transmission electron microscopy micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions, an FEM image with an elliptic ring-like structure composed of separated bright dots was obtained, a reasonable interpretation of it is that it was produced from the open end for a zigzag (16,0) SWCNT protruding from the SWCNT bundle, each bright dot corresponding to a single atom at the open end. And, if true, this means that the FEM demonstrated 0.2nm resolution, which was theoretically possible for the assumed geometry. The calculated value of the magnification of the pattern was also consistent with the measured value if the value of the compression factor beta was set at 1.76.

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.

Field emission of individual carbon nanotubes on tungsten tips

Individual multiwalled carbon nanotubes (MWCNTs) were assembled onto tungsten tips in a transmission-electron microscope. Then they were transferred into a field-emission microscope for the measurement of field-emission properties. Stable field emission was established after repeated heat treatment and extraction of field-emission current, which are believed to have cleaned and blunted the MWCNT ends. Even under high voltages and large currents, most of the emitted electrons that hit the screen were found to be still restricted within an ∼10−2 solid angle, indicating the possible availability of a high brightness.

Study on a new organic-complex thin film with electrical bistable properties using a scanning tunneling microscope

Abstract A new organic complex thin film of tetrathiofulvalene/ m- nitrobenzylidene propanedinitrile (TTF/ m- NBP) with electrical bistable properties has been fabricated by using a hot-wall deposition method. The films were analyzed by Fourier transform infrared (FTIR) spectroscopy and transmission electron microscope (TEM). Data recording experiments on the film were realized by applying voltage pulses between the scanning tunneling microscope (STM) tip and the substrate. The small recording dot was 1.2 nm in diameter. The width of the pulse voltage has influence upon the diameter of the recording marks. The current–voltage ( I–V ) curves of the film showed a conductive behavior for the recorded region, while a high resistance behavior for the unrecorded region. The possible conductive mechanism was also discussed.