Rapid progress of nanotechnology requires developing novel theoretical methods to explain complicated experimental results and predict new functions of nanodevices. Thus, for the last decade, one of the challenging works of quantum chemistry is to understand the electron and spin transport phenomena in molecular devices. This critical review provides an extensive survey of on-going research and its current status in molecular electronics with the focus on theoretical applications to diverse types of devices along with a brief introduction of theoretical methods and its practical implementation scheme. The topics cover diverse molecular devices such as molecular wires, rectifiers, field effect transistors, electrical and optical switching devices, nanosensors, spin-valve devices, negative differential resistance devices and inelastic electron tunnelling spectroscopy. The limitations of the presented method and the possible approaches to overcome the limitations are addressed (183 references).

Graphene is a promising material for spintronics due to its outstanding spin transport property. Its maximally exposed 2p z orbitals allow tuning of electronic structure toward better functionality in device applications. Because the positions of carbon atoms are commensurate with those of Ni atoms on the substrate, we design a graphene spin-valve device based on the epitaxial graphene grown on the Ni (111) surface. We explored its transport properties with non equilibrium Green function theory combined with density functional theory. We show that the device has magnetoresistance (∼110%) due to the strong spin-dependent interaction between the Ni surface and the epitaxial graphene sheet.

Ionic liquids (ILs) are promising materials for application in a new generation of Li-batteries. They can be used as electrolyte, interlayer, or incorporated into other materials. ILs have ability to form a stable Solid Electrochemical Interface (SEI) which plays an important role, preventing Li-based electrode from oxidation and electrolyte from extensive decomposition. Experimentally, it is hardly possible to elicit fine details of the SEI structure. To remedy this situation, we have performed a comprehensive computational study (DFT-MD) to determine the composition and structure of the SEI compact layer formed between Li anode and [Pyr14][TFSI] IL. We found that the [TFSI] anions quickly reacted with Li and decomposed, unlike the [Pyr14] cations which remained stable. The obtained SEI compact layer structure is non-homogeneous and consists of the atomized S, N, O, F, and C anions oxidized by Li atoms.

The structures, energetics, and transition states of water clusters (trimer to pentamer, n = 3-5) are investigated as a function of electric field by using ab initio calculations. With an increasing strength of the field, the most stable cyclic structures of trimer, tetramer, and pentamer open up to align their dipole moments along the direction of the field. For the lower strength (below 0.3 V/angstroms) of the electric field, the dipole moment of each water monomer is along the same direction with the field, while it retains the cyclic structure. For the higher strength of the field, to have a higher dipole moment for the cluster along the field direction, each cyclic structure opens up to form a linear chain or "water wire." We have investigated the transition state structures between the cyclic and linear forms for the field strengths of 0.3-0.4 V/angstroms where both cyclic and linear forms are energetically comparable.

We have systematically analyzed the structure of imogolite and their energetics, to understand the physics governing control over imogolite nanotube diameter and strain energy. In this work, we have presented evidence that the arrangements of inner and outer hydroxyl (OH) groups, that is hydrogen bond (HB) networks, play an important role in the formation of imogolite nanotubes and their structural stability. The outer HB significantly affects the Al−O distances and generates two different structures. Even though the relaxation of inner and outer Al−O and Si−O bond distances is the driving force for tubular imogolite formation, we show that the origin of the strain energy minimum, that is high monodispersity of diameter in imogolite nanotubes, is the inner HB networks. The preference for zigzag chirality and the formation mechanism of tubular imogolite also can be understood based on HB networks. Therefore, we present that a comprehensive understanding of the origin of high monodispersity of diameter and the preference for zigzag chirality in imogolite nanotubes provide useful insight into the researches of technical applications of imogolite nanotubes.

On the basis of recently synthesized calix[4]hydroquinone (CHQ) nanotubes which were self-assembled with infinitely long one-dimensional (1-D) short hydrogen bonds (SHB), we have investigated the nature of 1-D SHB using first-principles calculations for all the systems including the solvent water. The H-bonds relay (i.e., contiguous H-bonds) effect in CHQs shortens the H...O bond distances significantly (by more than 0.2 A) and increases the bond dissociation energy to a large extent (by more than approximately 4 kcal/mol) due to the highly enhanced polarization effect along the H-bond relay chain. The H-bonds relay effect shows a large increase in the chemical shift associated with the SHB. The average binding energies for the infinite 1-D H-bond arrays of dioles and dions increase by approximately 4 and approximately 9 kcal/mol per H-bond, respectively. The solvent effect (due to nonbridging water molecules) has been studied by explicitly adding water molecules in the CHQ tube crystals. This effect is found to be small with slight weakening of the SHB strength; the H...O bond distance increases only by 0.02 A, and the average binding energy decreases by approximately 1 kcal/mol per H-bond. All these results based on the first-principles calculations are the first detailed analysis of energy gain by SHB and energy loss by solvent effect, based on a partitioning scheme of the interaction energy components. These reliable results elucidate not only the self-assembly phenomena based on the H-bond relay but also the solvent effect on the SHB strength.

A recent landmark achievement (Jadzinsky et al. Science 2007, 318, 430) reported the successful crystallization and X-ray measurements of para-mercaptobenzoic acid (p-MBA)-protected gold nanoparticles containing 102 gold atoms and 44 p-MBAs. To gain more insight into the Au 102 (SR) 44 nanoclusters, we have performed density functional calculations for Au 102 (SMe) 44 and its neighboring systems. The exceptional stability of Au 102 (SR) 44 can be attributed to three coexisting factors: effective staple-motif formation, high stability against dissociation, and a large HOMO-LUMO gap.

In an effort to examine the intricacies of electronic nanodevices, we present an atomistic description of the electronic transport properties of an isolated benzene molecule. We have carried out ab initio calculations to understand the modulation of the molecular orbitals (MOs) and their energy spectra under the external electric field, and conducting behavior of the benzene molecule. Our study shows that with an increase in the applied electric field, the energy of the third lowest unoccupied molecular orbital (LUMO) of benzene decreases, while the first and second LUMO energies are not affected. Above a certain threshold of the external electric field, the third LUMO is lowered below the original LUMO and becomes the real LUMO. Since the transport through a molecule is to a large extent mediated by the molecular orbitals, the change in MOs can lead to a dramatic increase in the current passing through the benzene molecule. Thus, in the course of this study, we show that the modulation of the molecular orbitals in the presence of a tuning parameter(s) such as the external electric field can play important roles in the operation of molecular devices. We believe that this understanding would be helpful in the design of electronic nanodevices.