Using low-energy electron microscopy we show that Au deposition leads to the arrangement of three-dimensional islands at surface atomic steps on Si(111). Because the islands nucleate within a narrow coverage window, they have a small size distribution and their size can be precisely controlled by the deposition time. During the Au deposition on the surface with a controlled island density, the islands move into upper terraces with trenches left behind. This indicates that the islands are Au-Si alloy droplets. The Au-Si alloy islands act as catalysts for the nanowire formation based on the vapor-liquid-solid (VLS) mechanism, which enables us to fabricate the one-dimensional alignment of vertical GaP nanowires. Controlling the size and position of the nanostructures by using surface atomic steps as templates means “top-down” lithographic techniques are not needed. Our method is a pure “bottom-up” self-assembly one and is expected to greatly contribute to establishing self-assembled nano-device fabrication methods.
An electrostatic potential profile was obtained on the Si(111)√3×√3-Ag surface at a subnanometer spatial resolution. The potential was measured from an energy-level shift of electronic states on the surface. The obtained potential images reveal that the potential drops around steps and Ag adsorbates, upon which positive charges are presumably accumulated. The profiles of the reduced potentials are explained with the screening of potential due to the charges in two-dimensional electron gas (2DEG) existing on the surface. The Friedel oscillation, which results from the screening and has a period of the half Fermi wavelength of the 2DEG, was also observed in the potential images. The relative variation of the measured potential is in sub-10 meV range, demonstrating high energy resolution of the potential measurement method.
Using angle-resolved photoemission spectroscopy, we have investigated changes in the band dispersion of a free-electron-like surface state of Si(111)-√3×√3-Ag, induced by adsorption of submonolayer Au adatoms. At room temperature, where the adatoms are in a two-dimensional adatom-gas phase, electrons are transferred from the Au adatoms to the substrate, shifting the surface band downwards and increasing the band occupation (carrier doping), and also causing it to deviate from a parabolic dispersion. At 135 K where the Au adatoms are frozen at specific sites of the substrate, the surface-state band splits into two. This is the first experimental verification of band splitting that can be explained in terms of hybridization between the unperturbed surface-state band and the localized virtual bound states induced by the adatoms. This kind of interaction between localized states around individual impurity atoms and extended carriers are very important and common in various physical phenomena such as Kondo effect and diluted magnetic semiconductors.
Oxide materials with strong ionicity are associated with a large number of lattice defects because the defect formation enthalpy is very low. The control of growth mode and lattice defects in film growth of hexagonal ZnO plays an important role in producing new functional applications utilizing optical, magnetic and electrical properties. Thus far, we have investigated growth mode, optical and electrical characterizations in ZnO films in relation to polarity and non-polarity. In this report, we present a correlation between surface morphology and growth direction, and then show that optical and electrical properties were closely related to lattice defects generated in the host. Furthermore, we introduce self-organization of surface nanostructures and a local phase separation induced by interface defects in heteroepitaxial growth with lattice mismatch.
Adsorption on a doped semiconductor surface often induces a gradual formation of a carrier-depletion or carrier-accumulation layer at the surface, which has a great influence on the surface electronic excitations often coupled with surface optical phonons in compound semiconductors. Firstly, we give a brief survey of formation of depletion layers at n-type GaAs and InSb surfaces and of accumulation layers at n-type InAs surfaces. Secondly, we describe how the subband structure varies in an accumulation-layer formation process at an n-type InAs(110) surface, with special emphasis on the growing influence of nonparabolicity of the conduction-band dispersion. Finally, we elucidate the evolution of surface elementary excitations, namely, two coupled plasmon-phonon modes at the surface and a surface optical-phonon mode involving screening charges, in a depletion-layer formation process at an n-type GaAs(110) surface. Our systematic analysis provides a clear explanation of the change in the electron energy-loss spectrum.
We demonstrated a fabrication method for high-density and high-uniformity InAs quantum dots (QDs) on Sb-terminated GaAs(001) buffer layers using molecular beam epitaxy (MBE) of Stranski-Krastanov (SK) mode. The high-density formation of QDs is caused by the initial formation of 2-dimensional wire structures and small islands, which are induced by a surface exchange reaction of Sb atoms and As atoms. The surface segregation of Sb atoms also suppresses coalescence between neighboring QDs. In addition, the non-radiative recombination process is effectively suppressed by developed Sb-termination conditions. Furthermore, we found a new in-plane self-arrangement of high-density InAs QDs on the GaAsSb/GaAs(001) buffer layers.
Cataplasm containing anti-inflammatory and analgestic drug, ketoprofen, was developed. Cataplasm containing ketoprofen features the cross-linking network between aluminum and sodium polyacrylate and is a swollen hydrogel. Cataplasm can contain large amount of drug, water and other additives. Ketoprofen dispersed as microemulsions in the cataplasm is easily released from cataplasm and absorbed into the skin. Adhesion properties between cataplasms and between cataplasm and hairless mice skin were evaluated using phosphor-bronze thin plate technique. Skin-cataplasm adhesion force and cataplasm-cataplasm separation force are in good agreement with human sensory test. The diseased part was also cooled and moistured by cataplasm. It was found that cataplasm containing ketoprofen is a friendly formulation for patients.