Susumu Shiraki

Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity

We produce precise chiral-edge graphene nanoribbons on Cu{111} using self-assembly and surface-directed chemical reactions. We show that, using specific properties of the substrate, we can change the edge conformation of the nanoribbons, segregate their adsorption chiralities, and restrict their growth directions at low surface coverage. By elucidating the molecular-assembly mechanism, we demonstrate that our method constitutes an alternative bottom-up strategy toward synthesizing defect-free zigzag-edge graphene nanoribbons.

Self-Assembly Strategy for Fabricating Connected Graphene Nanoribbons

We use self-assembly to fabricate and to connect precise graphene nanoribbons end to end. Combining scanning tunneling microscopy, Raman spectroscopy, and density functional theory, we characterize the chemical and electronic aspects of the interconnections between ribbons. We demonstrate how the substrate effects of our self-assembly can be exploited to fabricate graphene structures connected to desired electrodes.

Negligible “Negative Space-Charge Layer Effects” at Oxide-Electrolyte/Electrode Interfaces of Thin-Film Batteries

In this paper, we report the surprisingly low electrolyte/electrode interface resistance of 8.6 Ω cm(2) observed in thin-film batteries. This value is an order of magnitude smaller than that presented in previous reports on all-solid-state lithium batteries. The value is also smaller than that found in a liquid electrolyte-based batteries. The low interface resistance indicates that the negative space-charge layer effects at the Li3PO(4-x)N(x)/LiCoO2 interface are negligible and demonstrates that it is possible to fabricate all-solid state batteries with faster charging/discharging properties.

Growth processes of lithium titanate thin films deposited by using pulsed laser deposition

We have investigated the pulsed laser deposition(PLD)growth processes of spinel lithiumtitanates based on the preparation of Li4Ti5O12 and LiTi2O4 from a Li4Ti5O12 target. The Li/Ti atomic ratio of the species arriving at substrate during the deposition was only ∼0.5. The LiTi2O4epitaxialthin films fabricated on MgAl2O4 (111) substrate exhibited high conductivity at room temperature (∼3.0 × 103 Ω−1 cm−1) and a superconducting transition temperature of ∼12 K. These values are the highest reported for epitaxialthin films. Our results demonstrate the importance of the target composition, providing further insights into the Li-containing metal oxide deposition processes using PLD.

Atomic-Scale Visualization of Initial Growth of Homoepitaxial SrTiO3 Thin Film on an Atomically Ordered Substrate

The initial homoepitaxial growth of SrTiO(3) on a (√13 × √13)-R33.7° SrTiO(3)(001) substrate surface, which can be prepared under oxide growth conditions, is atomically resolved by scanning tunneling microscopy. The identical (√13 × √13) atomic structure is clearly visualized on the deposited SrTiO(3) film surface as well as on the substrate. This result indicates the transfer of the topmost Ti-rich (√13 × √13) structure to the film surface and atomic-scale coherent epitaxy at the film/substrate interface. Such atomically ordered SrTiO(3) substrates can be applied to the fabrication of atom-by-atom controlled oxide epitaxial films and heterostructures.

Effect of oxygen deficiency on SrTiO3(001) surface reconstructions

The contribution of oxygen deficiencies to SrTiO3(001) surface reconstructions is studied using low-energy electron diffraction and scanning tunneling microscopy. We have prepared a SrTiO3 sample with spatially graded oxygen deficiencies, in which R33.7°-(13×13), (2×1), and R26.6°-(5×5) surface reconstructions are observed while increasing the amount of oxygen deficiencies. This indicates that oxygen nonstoichiometry has an influence on the formation of various surface reconstructions as one of the important factors. This concept is also applicable to other transition metal oxides to prepare atomically ordered surfaces in a reproducible manner.

One-dimensional Fe Nanostructures Formed on Vicinal Au(111) Surfaces

In this study of fabricated one-dimensional (1D) nanostructures of Fe adatoms on vicinal Au(111) surfaces, the growth mechanism and electronic structures are investigated by scanning tunneling microscopy (STM) and by angle-resolved photoemission spectroscopy (ARPES). STM observations reveal that dosed Fe atoms are trapped at the lower corners of the steps. They create nucleation centers near the intersections between steps and discommensuration lines, and grow into evenly spaced Fe fragments located at face-centered-cubic (fcc) stacking regions of the substrate. The connection of these fragments aligned along the steps results in the formation of Fe monatomic rows. As the Fe coverage increases, the Fe growth proceeds predominantly at the fcc stacking regions, and forms quasi-1D nanostructures with undulating edges. At an Fe coverage of ∼0.6 ML, the fast-growing parts connect with the adjacent Fe structures and a two-dimensional network structure is built up. ARPES measurements reveal that the decoration o...

A Single-Atom-Thick TiO2 Nanomesh on an Insulating Oxide

The electronic structures and macroscopic functionalities of two-dimensional (2D) materials are often controlled according to their size, atomic structures, and associated defects. This controllability is particularly important in ultrathin 2D nanosheets of transition-metal oxides because these materials exhibit extraordinary multifunctionalities that cannot be realized in their bulk constituents. To expand the variety of materials with exotic properties that can be used in 2D transition-metal-oxide nanosheets, it is essential to investigate fabrication processes for 2D materials. However, it remains challenging to fabricate such 2D nanosheets, as they are often forbidden because of the crystal structure and nature of their host oxides. In this study, we demonstrate the synthesis of a single-atom-thick TiO2 2D nanosheet with a periodic array of holes, that is, a TiO2 nanomesh, by depositing a LaAlO3 thin film on a SrTiO3(001)-(√13×√13)-R33.7° reconstructed substrate. In-depth investigations of the detailed structures, local density of states, and Ti valency of the TiO2 nanomesh using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, and density functional theory calculations reveal an unexpected upward migration of the Ti atoms of the substrate surface onto the LaAlO3 surface. These results indicate that the truncated TiO5 octahedra on the surface of perovskite oxides are very stable, leading to semiconducting TiO2 nanomesh formation. This nanomesh material can be potentially used to control the physical and chemical properties of the surfaces of perovskite oxides. Furthermore, this study provides an avenue for building functional atomic-scale oxide 2D structures and reveals the thin-film growth processes of complex oxides.

Nanoscale structural variation observed on the vicinal SrTiO3(001) surface

The vicinal (001) surface of a Nb-doped SrTiO3 single crystal has been investigated by scanning tunneling microscopy and low energy electron diffraction. The stepped surface prepared by annealing in ultrahigh vacuum at 250 °C exhibits a complex atomic structure composed of four types of reconstructions, which shows short-range variation within nanoscale regions. SrO layers show a c(6×2) structure being stable up to 1000 °C, while √13×√13-R33.7°, c(√13×√13)-R33.7°, and c(√2×√18)-R45° structures are formed on TiO2 layers, which disappear at 450–750 °C followed by the formation of 2×2 and √5×√5-R26.6° structures. These results indicate instability of the reconstructions on the TiO2 terminated surface due to the variation in Sr adatom density caused by multikinetic processes, in contrast to the thermodynamically stable SrO terminated surface.

Quasiparticle Interference on Cubic Perovskite Oxide Surfaces

We report the observation of coherent surface states on cubic perovskite oxide SrVO_{3}(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a d_{xy}-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO_{2} plane play a critical role in controlling the coherent surface state via modulating orbital state.