Tomoya Konishi

Investigation of glass formation and color properties in the P2O5–TeO2–ZnO system

Abstract The compositional dependence of glass-forming tendency and thermal properties was investigated for the P2O5–TeO2–ZnO system with the aid of an automatic sample preparation system including a batch-preparing apparatus and a multi-sample casting machine. The samples exhibited a variety of colors, such as transparent, yellow, wine red and brown depending on composition. Analyses with scanning electron microscopy and X-ray photoelectron spectroscopy revealed that the reddish coloration results from a colloidal suspension of nanometer-sized elemental Te particles.

Green Chemical Catalyst Supported on S-Terminated GaN(0001)

A novel function of nitride-based semiconductor is successfully developed for organic synthesis, in which palladium supported on the surface of sulfur-terminated GaN(0001) serves as a unique green chemical catalyst. It efficiently catalyzes Heck reaction with simple manipulations and its catalytic activity is retained for several repeat reactions. Moreover, it is easily reused without any special treatment. A plausible mechanism for Pd adsorption is provided for the first time; the –SH groups on the surface of the substrate attract Pd2+, and reduce to Pd0. The presence of Pd0 on the surface was confirmed by X-ray photoelectron spectroscopy measurements.

Hard x-ray photoemission spectroscopic investigation of palladium catalysts immobilized on a GaAs(001) surface

We present studies on the structure and chemical states of a catalyst developed by immobilizing palladium on S-terminated GaAs(001). Hard x-ray photoelectron spectroscopy (HX-PES) of core-level and valence band photoemission consistently indicates that the organopalladium molecules are reduced on the surface yielding Pd nanoparticles with a metallic nature. This finding is supported by high-resolution observations using scanning electron microscopy and backscattered electron image. HX-PES results also reveal that a portion of S atoms forming the S-termination is oxidized during the formation of Pd nanoparticles.

Statistical Analysis of Surface Reconstruction Domains on InAs Wetting Layer Preceding Quantum Dot Formation

Surface of an InAs wetting layer on GaAs(001) preceding InAs quantum dot (QD) formation was observed at 300°C with in situ scanning tunneling microscopy (STM). Domains of (1 × 3)/(2 × 3) and (2 × 4) surface reconstructions were located in the STM image. The density of each surface reconstruction domain was comparable to that of subsequently nucleated QD precursors. The distribution of the domains was statistically investigated in terms of spatial point patterns. It was found that the domains were distributed in an ordered pattern rather than a random pattern. It implied the possibility that QD nucleation sites are related to the surface reconstruction domains.

Spatial point analysis of quantum dot nucleation sites on InAs wetting layer

We perform spatial point analysis of InAs quantum dot (QD) nucleation sites and surface reconstruction domain patterns on an InAs wetting layer to investigate QD nucleation mechanisms in Stranski-Krastanow growth mode. An InAs wetting layer on a GaAs(001) substrate has been observed at 300 °C by using in situ scanning tunneling microscopy (STM) preceding QD formation. A nearest-neighbor analysis of the STM images finds that the point pattern of QD precursors is similar to that of (1 × 3)/(2 × 3) surface reconstruction domains which are specific to Ga-rich fluctuation. This provides the evidence that InAs QD nucleation is induced by Ga-rich fluctuation within an InAs wetting layer, as a technical implication for site-controlled QD growth for various QD devices.

Hopkins-Skellam index and origin of spatial regularity in InAs quantum dot formation on GaAs(001)

We investigate the origin of the spatial regularity of arrays of InAs quantum dots (QDs) grown on GaAs(001). The Hopkins-Skellam index (HSI) is used with a newly developed calculation algorithm to quantify the spatial regularity both of QDs and of nm-sized surface reconstruction territories (SRTs) present in the InxGa1−xAs wetting layer prior to QD nucleation. The SRT is the minimum extent of a surface reconstruction region needed for one QD to nucleate. By computing the evolving HSI of SRTs from sequences of in situ scanning tunnelling microscopy images during growth, we find that the spatial regularity of QDs is traced back to that of the (n × 3) SRTs as early as 0.6 monolayers of InAs coverage. This regularity is disturbed by the (n × 4) SRTs which appear at higher coverage. The SRT approach is discussed in comparison to conventional capture zone theories of surface growth.

Organopalladium catalyst on S-terminated GaAs(001)-(2×6) surface

Organopalladium molecules immobilized on the S-terminated GaAs(001), termed GaAs-S-{Pd}, have high catalytic activity and stability in the Heck reaction. It is thought that the presence of Ga-S bonds in the single atomic layer S-termination is essential for these catalytic properties despite the much higher thickness (~100 nm) of the organopalladium films. In this study, we demonstrate the retention of Ga-S bonds in ultra-thin GaAs-S-{Pd} by using reflection high-energy electron diffraction (RHEED) and scanning tunnelling microscopy (STM) to show that the organopalladium molecules are immobilized on an intact S-terminated GaAs(001)-(2times6) surface.

Sulfur-mediated palladium catalyst immobilized on a GaAs surface

We present a hard x-ray photoelectron spectroscopy study on the preparation process of palladium catalyst immobilized on an S-terminated GaAs(100) surface. It is revealed that Pd(II) species are reduced on the GaAs surface and yield Pd nanoparticles during the process of Pd immobilization and the subsequent heat treatment. A comparison with the results on GaAs without S-termination suggests that the reduction of Pd is promoted by hydroxy groups during the Pd immobilization and by S during the heat treatment.

Optimization of host glass composition to make soda borosilicate glasses doped with reduced rare earth ions

Soda borosilicate glasses with various compositions containing 1 mol% of Sm 3+ were prepared in a reductive atmosphere to reduce Sm 3+ ions. For the glass preparation, a multi-sample glass-formation tester was adopted. This apparatus is capable of handling 24 samples at once. The existence of thermally reduced Sm 2+ was confirmed by their fluorescent emission spectrum. By using compositional combinatorial approach, the glass-forming region and the optimal region for reducing Sm 3+ ions were determined in a ternary compositional diagram of soda borosilicate system. It was found that both regions overlapped each other and located near B 2 O 3 -rich side of the diagram.

Spatial regularity of InAs-GaAs quantum dots : quantifying the dependence of lateral ordering on growth rate

The lateral ordering of arrays of self-assembled InAs-GaAs quantum dots (QDs) has been quantified as a function of growth rate, using the Hopkins-Skellam index (HSI). Coherent QD arrays have a spatial distribution which is neither random nor ordered, but intermediate. The lateral ordering improves as the growth rate is increased and can be explained by more spatially regular nucleation as the QD density increases. By contrast, large and irregular 3D islands are distributed randomly on the surface. This is consistent with a random selection of the mature QDs relaxing by dislocation nucleation at a later stage in the growth, independently of each QD’s surroundings. In addition we explore the statistical variability of the HSI as a function of the number N of spatial points analysed, and we recommend N > 103 to reliably distinguish random from ordered arrays.