Katsumi Nagaoka

Electronic Structure of Ultrathin Bismuth Films with A7 and Black-Phosphorus-like Structures

Using scanning tunneling spectroscopy and first-principles calculations, we have studied the electronic structure of two different ultrathin bismuth films on a Si(111)-7×7 substrate; a hexagonal film (HEX film) having a bulklike (A7-like) structure and a film having a black-phosphorus-like structure (BP film). The HEX film is metallic because of spin–orbit (SO)-split surface-state bands lying inside the projected bulk band gap near the Fermi level. Another SO-split surface state is also observed inside the SO gap. The BP film exhibits a significant reduction in metallicity in contrast to the HEX film. This is related to the formation of a very stable paired-layer structure, the mechanism of which is similar to that of the stabilization of semiconducting bulk black P. However, unlike bulk black P, a certain extent of metallicity still remains. This slight metallicity can be associated with buckling and strain in the BP film, which is analogous to the fact that shear angle distortion in bulk Bi is responsib...

Encapsulated inorganic nanostructures: a route to sizable modulated, noncovalent, on-tube potentials in carbon nanotubes.

The large variety of hybrid carbon nanotube systems synthesized to date (e.g., by encapsulation, wrapping, or stacking) has provided a body of interactions with which to modify the host nanotubes to produce new functionalities and control their behavior. Each, however, has limitations: hybridization can strongly degrade desirable nanotube properties; noncovalent interactions with molecular systems are generally weak; and interlayer interactions in layered nanotubes are strongly dependent upon the precise stacking sequence. Here we show that the electrostatic/polarization interaction provides a generic route to designing unprecedented, sizable and highly modulated (1 eV range), noncovalent on-tube potentials via encapsulation of inorganic partially ionic phases where charge anisotropy is maximized. Focusing on silver iodide (AgI) nanowires inside single-walled carbon nanotubes, we exploit the polymorphism of AgI, which creates a variety of different charge distributions and, consequently, interactions of varying strength and symmetry. Combined ab initio calculations, high-resolution transmission electron microscopy, and scanning tunneling microscopy and spectroscopy are used to demonstrate symmetry breaking of the nanotube wave functions and novel electronic superstructure formation, which we then correlate with the modulated, noncovalent electrostatic/polarization potentials from the AgI filling. These on-tube potentials are markedly stronger than those due to other noncovalent interactions known in carbon nanotube systems and lead to significant redistribution of the wave function around the nanotube, with implications for conceptually new single-nanotube electronic devices and molecular assembly. Principles derived can translate more broadly to relating graphene systems, for designing/controlling potentials and superstructures.

Phase-operation for conduction electron by atomic-scale scattering via single point-defect

In order to propose a phase-operation technique for conduction electrons in solid, we have investigated, using scanning tunneling microscopy, an atomic-scale electron-scattering phenomenon on a 2D subband state formed in Si. Particularly, we have noticed a single surface point-defect around which a standing-wave pattern created, and a dispersion of scattering phase-shifts by the defect-potential against electron-energy has been measured. The behavior is well-explained with appropriate scattering parameters: the potential height and radius. This result experimentally proves that the atomic-scale potential scattering via the point defect enables phase-operation for conduction electrons.

Scanning Tunneling Microscope Study of a Local Electronic State Surrounding Mn Nanoclusters on Graphite

We have investigated the influence of Mn nanoclusters on the electronic structure of graphite using the scanning tunneling microscope. The dI/dV spectra and the dI/dV images indicate that a number of Mn nanoclusters of sizes less than one nanometer induce an unexpected local electronic state that surrounds the nanocluster and spreads out by 6–8 nm diameter. This localized state is not observed around a Mn monomer (or trimer) or Mn nanoparticles larger than 1 nm. The localized state is presumably caused by the electronic coupling between the Mn nanocluster and graphite surface, or by a local magnetic field generated by the Mn nanocluster.

Controlling molecular condensation/diffusion of copper phthalocyanine by local electric field induced with scanning tunneling microscope tip

We have discovered the condensation/diffusion phenomena of copper phthalocyanine (CuPc) molecules controlled with a pulsed electric field induced by the scanning tunneling microscope tip. This behavior is not explained by the conventional induced dipole model. In order to understand the mechanism, we have measured the electronic structure of the molecule by tunneling spectroscopy and also performed theoretical calculations on molecular orbitals. These data clearly indicate that the molecule is positively charged owing to charge transfer to the substrate, and that hydrogen bonding exists between CuPc molecules, which makes the molecular island stable.