We report on a self-consistent ab initio technique for modeling quantum transport properties of atomic and molecular scale nanoelectronic devices under external bias potentials. The technique is based on density functional theory using norm conserving nonlocal pseudopotentials to define the atomic core and nonequilibrium Green's functions (NEGF's) to calculate the charge distribution. The modeling of an open device system is reduced to a calculation defined on a finite region of space using a screening approximation. The interaction between the device scattering region and the electrodes is accounted for by self-energies within the NEGF formalism. Our technique overcomes several difficulties of doing first principles modeling of open molecular quantum coherent conductors. We apply this technique to investigate single wall carbon nanotubes in contact with an Al metallic electrode. We have studied the current-voltage characteristics of the nanotube-metal interface from first principles. Our results suggest that there are two transmission eigenvectors contributing to the ballistic conductance of the interface, with a total conductance $G\ensuremath{\approx}{G}_{0}$ where ${G}_{0}{=2e}^{2}/h$ is the conductance quanta. This is about half of the expected value for infinite perfect metallic nanotubes.

/pdf/ab-initio-modeling-of-quantum-transport-properties-of-2siyvgwd35.pdf

Ab initio modeling of quantum transport properties of molecular electronic devices

Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.

Electron transport in molecular junctions.

First-principles based matrix Green's function approach to molecular electronic devices: general formalism

We present an ab initio analysis of electron conduction through a ${\mathrm{C}}_{60}$ molecular device. Charge transfer from the device electrodes to the molecular region is found to play a crucial role in aligning the lowest unoccupied molecular orbital of the ${\mathrm{C}}_{60}$ to the Fermi level of the electrodes. This alignment induces a substantial device conductance of $\ensuremath{\sim}2.2\ifmmode\times\else\texttimes\fi{}{(2e}^{2}/h).$ A gate potential can inhibit charge transfer, and introduce a conductance gap near ${E}_{F},$ changing the current-voltage characteristics from metallic to semiconducting, thereby producing a field-effect molecular current switch.

/pdf/ab-initio-modeling-of-open-systems-charge-transfer-electron-4ntwe4desl.pdf

Ab initio modeling of open systems: Charge transfer, electron conduction, and molecular switching of a C 60 device

A review with 228 references on experimental investigation of conductor/molecule/metal molecular electronic junctions is presented. Devices based on covalent and Langmuir−Blodgett bonding of single molecules or molecular monolayers to conducting substrates are reviewed, as characterized by scanning probe microscopy and microelectronic techniques. Phenomena observed to date include molecular rectification, conductance switching, and nonlinear current/voltage behavior. Prospects and problems for the application of molecular junctions to the more general area of molecular electronics are discussed.

Molecular Electronic Junctions

In the interpretation of scanning-tunneling spectroscopy data on molecular nanostructures the tunneling conductance is often assumed to be proportional to the local density of states of the molecule. This precludes the possibility of observing negative differential resistance (NDR). We report here the observation of NDR in the current-voltage $(I\ensuremath{-}V)$ characteristics of a self-assembled monolayer of $4\ensuremath{-}p\ensuremath{-}\mathrm{terphenylthiol}$ molecules on the Au(111) surface measured using a platinum tip. We argue that the NDR arises from narrow features in the local density of states of the tip apex atom and show that depending on the electrostatic potential profile across the system, NDR could be observed in one or both bias directions.

Negative differential resistance in the scanning-tunneling spectroscopy of organic molecules

The dihalide capped trinuclear nickel clusters Ni 3(I3-X)2(I2-dppm)3 (X ) I( 1a), Br (1b); dppm ) Ph2- PCH2PPh2) were synthesized and converted to their respective monocations (Ni3(I3-X)2(I2-dppm)3) + (X ) I( 1a + ), Br (1b + )) Via a single electron oxidation. Clusters 1a and 1a + were characterized by X-ray crystallography. Displacement of a triply-bridging iodide ligand in Ni3(I3-I)2(I2-dppm)3 (1a )b y ﷿-acceptor ligands produces a class of 48-electron trinuclear nickel clusters of the general formula (Ni 3(I3-L)(I3-I)(I2-dppm)3) + (L ) CO (2a); CNR, R ) CH3 (3a), 2,6-(CH3)2C6H3 (4a), i-C3H7 (5), C6H11 (6), t-C4H9 (7), CH2C6H5 (8), C6H5 (9), p-C6H4 I( 10), p-C6H4- Br (11), p-C6H4Cl (12), p-C6H4 F( 13), p-C6H4CH3 (14), p-C6H4CF3 (15), p-C6H4OCH3 (16), and p-C6H4CN (17)). A similar but less extensive study was conducted with the dibromide capped cluster 1b. The X-ray crystal structure of cluster 2a, as the PF6 - salt, was also obtained. Clusters 2-17 possess strikingly similar spectroscopic and electrochemical properties. This is ascribed to the lack of interaction between the a2 LUMO of clusters 2-17 and the molecular orbitals of the capping ﷿-acceptor ligands. The unexpected appearance of two O(CtN) bands in the FT-IR spectra of clusters 3-17 was demonstrated to be the result of a Fermi resonance involving the O(CtN) fundamental and the first overtone of the O(N-C(alkyl)) fundamental of the capping isocyanide. In addition, molecular orbital calculations on 1-17 provide insights into the differences in the physical properties and reactivities of clusters of this class capped by ﷿-donor (1 )o r ﷿-acceptor ligands (2-17).

I - ,B r - ; n )0, 1; dppm ) Ph2PCH2PPh2): Novel Physical Properties and the Fermi Resonance of Symmetric Ì3- 1 Bound Isocyanide Ligands

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Negative differential resistance in the scanning-tunneling spectroscopy of organic molecules

I - ,B r - ; n )0, 1; dppm ) Ph2PCH2PPh2): Novel Physical Properties and the Fermi Resonance of Symmetric Ì3- 1 Bound Isocyanide Ligands