Shigeru Tsukamoto

Molecular Scale Control of Unbound and Bound C60 for Topochemical Ultradense Data Storage in an Ultrathin C60 Film

2010 WILEY-VCH Verlag Gmb Rapidly advancing information technologies require innovative data storage that combines greatly larger data density, rewritability, and nonvolatility. One solution is to use individual molecules to overcome the limitation of data storage. So far, various recording schemes such as charge storage inmolecules, controlling molecular conformation, and the nanomechanical deformation of molecules have been investigated. To date, reversible charge transfer between organic molecules via intermolecular p p and donor-acceptor interactions has been extensively studied and examples of rewritable data storage with bit sizes of 1.8 2.5 nm are reported. In this Communication, we demonstrate that a thin film of C60 molecules [9] and the use of a scanning tunneling microscope (STM) provides excellent controllability for reversible switching between the unbound and bound states of two or three adjacent C60 molecules in the film at room temperature (RT) by changing the polarity of an electric field that is locally applied to any designated position. This chemical reactionmethod enables topochemical data storage with a bit size of a single C60 molecule (about 1 nm) and with a data density of 190 Tbit/in, which is 10–10 times higher than those achieved by today’s data storage methods. An ultrathin film of C60 molecules was prepared on a Si(111)H3 H3R308-Ag substrate in ultrahigh vacuum (UHV) and then transferred to an STM operated in UHV at RT (see Fig. 1a). Figure 1b shows an STM image of the C60 film with a thickness of three layers (all C60 films in this study had the same thickness). In this STM image, the observed C60 molecules exhibit no internal structure due to their thermal rotation at RT. The STM tip was then scanned along the broken line in Figure 1b applying a relatively large negative sample bias (Vs) of 3.5 V. As a result, C60 molecules near the scan line became dark in the STM image contrast, as shown in Figure 1c. In the magnified image in Figure 1c, the dark C60 molecules exhibit an internal structure, indicating that their thermal rotation is suppressed due to chemical bonding with neighboring C60 molecules. In general, the STM image contrast of a molecule is altered by its vertical displacement and change in electronic state. In the present case, however, the latter effect is negligibly small because the STM is probing a part of the bound C60 molecule away from the newly created intermolecular bonds, where the original molecular orbital is almost preserved as the molecular orbital of the unbound C60 molecule. Furthermore, the STM images used for the following analysis were taken at a Vs of 1 V. This condition corresponds to the observation of electronic density of states

Electron-transport properties of Na nanowires under applied bias voltages

We present first-principles calculations on electron transport through Na nanowires at finite bias voltages. A nanowire exhibits a nonlinear current-voltage characteristic and negative differential conductance (NDC). NDC is induced by the drastic suppression of the electron-transmission peaks which is attributed to the reduction of the electron conductance through the negatively biased side of the system. The mechanism of the occurrence of NDC is also demonstrated based on the finding that a voltage drop locally occurs on the negatively biased end of the nanowire.

Real-space electronic structure calculations with full-potential all-electron precision for transition metals

We have developed an efficient computational scheme utilizing the real-space finite-difference formalism and the projector augmented-wave (PAW) method to perform precise first-principles electronic-structure simulations based on the density functional theory for systems containing transition metals with a modest computational effort. By combining the advantages of the time-saving double-grid technique and the Fourier filtering procedure for the projectors of pseudopotentials, we can overcome the egg box effect in the computations even for first-row elements and transition metals, which is a problem of the real-space finite-difference formalism. In order to demonstrate the potential power in terms of precision and applicability of the present scheme, we have carried out simulations to examine several bulk properties and structural energy differences between different bulk phases of transition metals, and have obtained excellent agreement with the results of other precise first-principles methods such as a plane wave based PAW method and an all-electron full-potential linearized augmented plane wave (FLAPW) method.

Mechanisms of electron transport through bellows-shaped fullerene tubes

A bellows-shaped fullerene tube is featured by diameter modulation along the tube, where C60 molecules polymerize with tubular linkages between the molecules. The electronic structures of two types of bellows-shaped fullerene tube are theoretically found to exhibit band gaps indicating semiconducting characteristics. Although the effective masses of the conduction states of the two tubes are similar, these states have different spatial distributions. One of the two tubes is expected to exhibit thermally assisted electron transport.

First-principles electronic structure calculations for peanut-shaped C120 molecules

Abstract Using the first-principles real-space finite-difference method, we have theoretically examined optimized structures and electronic energy levels of three peanut-shaped C120 molecules (C60 dimers), namely, P55-, P56-, and P66-C120 molecules. Our calculations show that as the number of eight-membered rings included in each C120 molecule increases, the total energy becomes large and the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gap becomes small. For the P56-C120 molecule, the LUMO is found to be localized at one C60 component, while for the other molecules, the LUMOs are extended over the entire molecule. This fact is understood from the symmetry/asymmetry in the atomic configuration of the three C120 molecules.

Magnetic orderings in Al nanowires suspended between electrodes

A theoretical analysis of a relation between atomic and spin-electronic structures for the ground state of single-row aluminum nanowires suspended between Al(001) electrodes is demonstrated using first-principles structural optimizations. We obtain an unusual result that a three-aluminum-atom nanowire sandwiched between the electrodes does not manifest magnetic ordering, although an isolated aluminum trimer molecule in a straight line is spin-polarized. On the other hand, a five-atom nanowire exhibits ferromagnetic ordering, where three central atoms form a spin-polarized trimer. Moreover, in the case of an eight-atom nanowire, the middle atoms in the nanowire form two spin-polarized trimers with antiferromagnetic ordering.

Real-space method for first-principles electron transport calculations: Self-energy terms of electrodes for large systems

We present a fast and stable numerical technique to obtain the self-energy terms of electrodes for first-principles electron-transport calculations. Although first-principles calculations based on the real-space finite-difference method are advantageous for execution on massively parallel computers, large-scale transport calculations are hampered by the computational cost and numerical instability of the computation of the self-energy terms. Using the orthogonal complement vectors of the space spanned by the generalized Bloch waves that actually contribute to transport phenomena, the computational accuracy of transport properties is significantly improved with a moderate computational cost. To demonstrate the efficiency of the present technique, the electron-transport properties of a Stone-Wales (SW) defect in graphene and silicene are examined. The resonance scattering of the SW defect is observed in the conductance spectrum of silicene since the

Self-energy matrices for electron transport calculations within the real-space finite-difference formalism

\sigma^\ast

Real-space calculations for electron transport properties of nanostructures

state of silicene lies near the Fermi energy. In addition, we found that one conduction channel is sensitive to a defect near the Fermi energy, while the other channel is hardly affected. This characteristic behavior of the conduction channels is interpreted in terms of the bonding network between the bilattices of the honeycomb structure in the formation of the SW defect. The present technique enables us to distinguish the different behaviors of the two conduction channels in graphene and silicene owing to its excellent accuracy.

First-principles calculation method for electron transport based on the grid Lippmann-Schwinger equation

The self-energy term used in transport calculations, which describes the coupling between electrode and transition regions, is able to be evaluated only from a limited number of the propagating and evanescent waves of a bulk electrode. This obviously contributes toward the reduction of the computational expenses in transport calculations. In this paper, we present a mathematical formula for reducing the computational expenses further without using any approximation and without losing accuracy. So far, the self-energy term has been handled as a matrix with the same dimension as the Hamiltonian submatrix representing the interaction between an electrode and a transition region. In this work, through the singular-value decomposition of the submatrix, the self-energy matrix is handled as a smaller matrix, whose dimension is the rank number of the Hamiltonian submatrix. This procedure is practical in the case of using the pseudopotentials in a separable form, and the computational expenses for determining the self-energy matrix are reduced by 90% when employing a code based on the real-space finite-difference formalism and projector-augmented wave method. In addition, this technique is applicable to the transport calculations using atomic or localized basis sets. Adopting the self-energy matrices obtained from this procedure, we present the calculation of the electron transport properties of C_{20} molecular junctions. The application demonstrates that the electron transmissions are sensitive to the orientation of the molecule with respect to the electrode surface. In addition, channel decomposition of the scattering wave functions reveals that some unoccupied C_{20} molecular orbitals mainly contribute to the electron conduction through the molecular junction.