Tomofumi Tada

Design principle for increasing charge mobility of π-conjugated polymers using regularly localized molecular orbitals

The feasibility of using π-conjugated polymers as next-generation electronic materials is extensively studied; however, their charge mobilities are lower than those of inorganic materials. Here we demonstrate a new design principle for increasing the intramolecular charge mobility of π-conjugated polymers by covering the π-conjugated chain with macrocycles and regularly localizing π-molecular orbitals to realize an ideal orbital alignment for charge hopping. Based on theoretical predictions, insulated wires containing meta-junctioned poly(phenylene–ethynylene) as the backbone units were designed and synthesized. The zigzag wires exhibited higher intramolecular charge mobility than the corresponding linear wires. When the length of the linear region of the zigzag wires was increased to 10 phenylene–ethynylene units, the intramolecular charge mobility increased to 8.5 cm2 V−1 s−1. Theoretical analysis confirmed that this design principle is suitable for obtaining ideal charge mobilities in π-conjugated polymer chains and that it provides the most effective pathways for inter-site hopping processes.

Rectifying Electron-Transport Properties through Stacks of Aromatic Molecules Inserted into a Self-Assembled Cage.

Aromatic stacks formed through self-assembly are promising building blocks for the construction of molecular electronic devices with adjustable electronic functions, in which noncovalently bound π-stacks act as replaceable modular components. Here we describe the electron-transport properties of single-molecule aromatic stacks aligned in a self-assembled cage, using scanning probe microscopic and break junction methods. Same and different modular aromatic pairs are noncovalently bound and stacked within the molecular cage holder, which leads to diverse electronic functions. The insertion of same pairs induces high electronic conductivity (10(-3)-10(-2) G0, G0 = 2e(2)/h), while different pairs develop additional electronic rectification properties. The rectification ratio was, respectively, estimated to be 1.4-2 and >10 in current-voltage characteristics and molecular orientation-dependent conductance measurements at a fixed bias voltage. Theoretical calculations demonstrate that this rectification behavior originates from the distinct stacking order of the internal aromatic components against the electron-transport direction and the corresponding lowest unoccupied molecular orbital conduction channels localized on one side of the molecular junctions.

Single-Molecule Conductance of π-Conjugated Rotaxane: New Method for Measuring Stipulated Electric Conductance of π-Conjugated Molecular Wire Using STM Break Junction

An electronic conductance with small fluctuations, which is stipulated in single-molecule junctions, is necessary for the precise control of single-molecule devices. However, the suppression of conductance fluctuations in conventional molecular junctions is intrinsically difficult because the fluctuations are related to the contact fluctuations and molecular motion. In the present study involving experimental and theoretical investigations, it is found that covering a single π-conjugated wire with an α-cyclodextrin molecule is a promising technique for suppressing conductance fluctuations. The conductance histogram of the covered molecular junction measured with the scanning tunneling microscope break-junction technique shows that the conductance peak for the covered junction is sharper than that of the uncovered junction. The covering technique thus has two prominent effects: the suppression of intramolecular motion, and the elimination of intermolecular interactions. Theoretical calculations of electronic conductance clearly support these experimental observations.

Water Durable Electride Y5Si3: Electronic Structure and Catalytic Activity for Ammonia Synthesis

We report an air and water stable electride Y5Si3 and its catalytic activity for direct ammonia synthesis. It crystallizes in the Mn5Si3-type structure and confines 0.79/f.u. anionic electrons in the quasi-one-dimensional holes. These anionic electrons strongly hybridize with yttrium 4d electrons, giving rise to improved chemical stability. The ammonia synthesis rate using Ru(7.8 wt %)-loaded Y5Si3 was as high as 1.9 mmol/g/h under 0.1 MPa and at 400 °C with activation energy of ∼50 kJ/mol. Its strong electron-donating ability to Ru metal of Y5Si3 is considered to enhance nitrogen dissociation and reduce the activation energy of ammonia synthesis reaction. Catalytic activity was not suppressed even after Y5Si3, once dipped into water, was used as the catalyst promoter. These findings provide novel insights into the design of simple catalysts for ammonia synthesis.

First-principles simulations on bulk Ta2O5 and Cu/Ta2O5/Pt heterojunction: Electronic structures and transport properties

Electronic structures and transport properties of bulk Ta2O5 and Cu/Ta2O5/Pt heterojunction have been studied from first principles. Of the two room-temperature phases of bulk Ta2O5, β-, and δ-Ta2O5, our calculated results showed that the β phase has much narrower band gap than the δ-Ta2O5. For Cu/δ-Ta2O5/Pt heterojunction, the p-type Schottky barriers between the Cu (Pt) and Ta2O5 were estimated as 0.9–1.2 eV. Both the standard density-functional calculation and the nonequilibrium Green’s function showed that no conducting channels were formed from Cu to Pt through δ-Ta2O5.

Conductive path formation in the Ta₂O₅ atomic switch: first-principles analyses.

The conductive path formed by the interstitial Cu or oxygen vacancies in the Ta(2)O(5) atomic switch were investigated in detail by first-principles methods. The calculated results indicated that the defect state induced by the interstitial Cu is located just at the Fermi level of the Cu and Pt electrodes in the Cu/Ta(2)O(5)/Pt heterostructure and that a conduction channel is formed in the Ta(2)O(5) film via the interstitial Cu. On the other hand, oxygen vacancies in Ta(2)O(5) do not form such a conduction channel because of the lower energy positions of their defect states. The above results suggest that the conductive path could be formed by interstitial Cu in the Ta(2)O(5) atomic switch, whereas the oxygen vacancies do not contribute to the formation of the conductive path.

Excess-silver-induced bridge formation in a silver sulfide atomic switch

Structural properties and electron transport of a Ag2S atomic switch composed of Ag–Ag2S–Ag heterostructure are investigated by nonequilibrium Green’s function calculations considering the effect of excess Ag in the Ag2S layer. In addition to confirming experimentally the formation of the Ag bridge inside Ag2S, the bridge is found to consist of units having a structure similar to that of the Ag (111) face in the bulk Ag. The analyses of Mulliken population, transmission spectra, and current-voltage characteristics reveal that the bridge has a conductive and metallic nature.

High-throughput ab initio screening for two-dimensional electride materials.

High-throughput ab initio screening of approximately 34000 materials in the Materials Project was conducted to identify two-dimensional (2D) electride materials, which are composed of cationic layers and anionic electrons confined in a 2D empty space. The screening was based on three indicators: (1) a positive total formal charge per formula unit; (2) layered structures for two-dimensionality; (3) empty spaces between the layer units. Three nitrides, Ca2N, Sr2N, and Ba2N, and the carbide Y2C were identified as 2D electrides, where Ca2N is the only experimentally confirmed 2D electride (Lee, K.; et al. Nature 2013, 494, 336-341). Electron density analysis using ionic radii revealed a smaller number of anionic electrons in Y2C than those in the three nitrides as a result of the partial occupation of the anionic electrons in the d orbitals of Y. In addition, no candidates were identified from the p-block elements, and thus the ab initio screening indicates that the s-block elements (i.e., alkali or alkaline-earth metals) are highly preferable as cation elements. To go beyond the database screening, a tailored modeling was conducted to determine unexplored compounds including the s-block elements that are suitable for 2D electrides. The tailored modeling found that (1) K2Cl, K2Br, Rb2Cl, and Rb2Br dialkali halides are highly plausible candidates, (2) Li2F and Na2Cl dialkali halides are highly challenging candidates, and (3) the Cs2O(1-x)F(x) halogen-doped dialkali oxide is a promising candidate.

Triphosphasumanene Trisulfide: High Out-of-Plane Anisotropy and Janus-Type π-Surfaces

A triphosphasumanene trisulfide was designed and synthesized as an out-of-plane anisotropic π-conjugated molecule. Incorporating three anisotropic phosphine sulfide moieties into a sumanene skeleton induced a cumulative anisotropy with a large dipole moment (12.0 D), which is aligned in perpendicular direction with respect to the π-framework and more than twice as large as those of conventional out-of-plane anisotropic molecules. In the crystal, the molecules align to form columnar structures, in which electron-rich and electron-deficient sides of the π-framework face each other. The interactions between the electron-rich surfaces, which contain three sulfur atoms, and Au(111) were examined by X-ray photoelectron spectroscopy.

Resolving metal-molecule interfaces at single-molecule junctions.

Electronic and structural detail at the electrode-molecule interface have a significant influence on charge transport across molecular junctions. Despite the decisive role of the metal-molecule interface, a complete electronic and structural characterization of the interface remains a challenge. This is in no small part due to current experimental limitations. Here, we present a comprehensive approach to obtain a detailed description of the metal-molecule interface in single-molecule junctions, based on current-voltage (I-V) measurements. Contrary to conventional conductance studies, this I-V approach provides a correlated statistical description of both, the degree of electronic coupling across the metal-molecule interface, and the energy alignment between the conduction orbital and the Fermi level of the electrode. This exhaustive statistical approach was employed to study single-molecule junctions of 1,4-benzenediamine (BDA), 1,4-butanediamine (C4DA), and 1,4-benzenedithiol (BDT). A single interfacial configuration was observed for both BDA and C4DA junctions, while three different interfacial arrangements were resolved for BDT. This multiplicity is due to different molecular adsorption sites on the Au surface namely on-top, hollow, and bridge. Furthermore, C4DA junctions present a fluctuating I-V curve arising from the greater conformational freedom of the saturated alkyl chain, in sharp contrast with the rigid aromatic backbone of both BDA and BDT.