Unraveling structures of protection ligands on gold nanoparticle Au68(SH)32

New low-energy atomic structures of the thiolate-protected gold nanoparticle Au68(SH)32 are uncovered, where the atomic positions of the Au atoms are taken from the recent single-particle transmission electron microscopy measurement by Kornberg and co-workers, whereas the pattern of thiolate ligands on the gold core is attained on the basis of the generic formulation (or rule) of the “divide and protect” concept. Four distinct low-energy isomers, Iso1 to Iso4, whose structures all satisfy the generic formulation, are predicted. Density-functional theory optimization indicates that the four isomers are all lower in energy by 3 to 4 eV than the state-of-the-art low-energy isomer reported. Further analysis of the optimized structures of Au68(SH)32 shows that the structure of gold core in Iso1 to Iso4 is consistent with the experiment, whereas the positions of a few Au atoms at the surface of gold core are different. The computed optical absorption spectra of the four isomers are consistent with the measured spectrum. Computation of catalytic properties of Au68(SH)32 toward CO oxidation suggests that the magic number cluster can be a stand-alone nanoscale catalyst for future catalytic applications.

Characterization of Surface Defects of Highly N-Doped 4H-SiC Substrates that Produce Dislocations in the Epitaxial Layer

The structures of defects that form different types of etch pits on highly N-doped 4H-SiC substrates, that were produced by a sublimation method, after molten KOH etching were characterized. It was found that most of the dislocations in the epitaxial layer originated from defects at the surface of substrate whose etch pit structures were clearly different from the conventional structures. The etch pits were classified into drop, oval, round and caterpillar pits. The drop and oval pits were concluded to be formed by the deformation of conventional etch pits. Round pits were concluded to originate from half loop dislocations and were transformed to complex dislocations by epitaxial growth. Analysis by transmission electron microscopy measurement indicates that slipped edge dislocations (or screw dislocations) on the basal plane form caterpillar pits.

Optical Properties of Quantum-Wires Grown Using Lateral Composition Modulation Induced by (InP)1/(GaP)1 Short-Period Superlattices

ABSTRACTThe optical properties of quantum wires (QWRs) grown using lateral composition modulation (LCM) were studied by photoluminescence (PL) measurement as a function cryostat temperature (Tcr). 3 stacked arrays of QWRs were formed by sequential growth of ∼ 180 Å-thick LCM layers (lateral period: ∼ 90 Å) induced by (InP)1/(GaP)1 short-period superlattices, and 200 Å-thick InGaP spacers at the growth temperature of 490 °C. The formation of QWRs was confirmed by a transmission electron microscopy measurement. By the analysis of the dependence of PL intensity and peak energy of the QWRs on Tcr, the origin of higher energy peak (H) and lower energy peak (L) were investigated. While behavior of the H peak is similar to that of an ordered InGaP, the L peak shows the insensitivity of PL peak energy to Tcr. This is attributed to compensation of the bandgap by competition of strain in the QWR region and indicates the L peak is related to the QWRs. Strong dependence of the L peak on the position of polarizer also supports this. Additionally, the PL peak intensity of the L peak has the maximum value not at the lowest Tcr (10 K) but at 50 K, while the H peak decrease continuously as T increases. We introduced the idea of compensation of the thermal expansion coefficient to explain this phenomenon.

Fabrication of Nanocrystalline Si by SiH4 Plasma Cell

ABSTRACTNanocrystalline silicon (nc-Si) has been fabricated by a very-high-frequency plasma cell attached to an ultra-high-vacuum chamber using SiH4 gas. Nanocrystalline Si is formed in the gas phase of the plasma cell and is extracted out of plasma cell through the orifice to the ultra-high-vacuum chamber. The shape of nc-Si is spherical or octahedral with the diameter of 3–30nm. Giant Si particles about 100nm in diameter are also formed at the lower cell pressure condition. A 1000keV transmission electron microscopy measurement has revealed that the core region of giant Si particle with the diameter about 30nm was crystalline and the shell region is amorphous. We have demonstrated that the spread of particle size can be decreased using pulsed gas supply of H2 into SiH4 plasma.