Chulho Song

We have studied the lattice-plane modulation of Mg-doped GaN homoepitaxial layers by X-ray diffraction topography. X-ray rocking curve images and full-width at half maxima images of a 2 inch GaN wafer were obtained by an image stacking method. Low frequency components evaluated from Fourier transformations of the X-ray rocking curves tended to increase as the Mg-doping concentration increased, which indicates an increase of film lattice modulation. For Mg-dopant concentrations greater than 10¹⁹ cm⁻³, the film lattice-plane changed from a convex to concave shape and became wrinkled. We attribute this lattice-plane modulation to the relaxation of strain in the in-plane direction after the generation of dislocations by Mg doping.

Pd_xRu_1−x nanoparticles (NPs) were observed to display enhanced CO oxidation activity with the maximum performance obtained at the composition x = 0.5. To unveil the origin of this superior CO oxidation activity, we investigated the local structure, valence state, and electronic properties of Pd_xRu_1−x NPs using synchrotron-based X-ray techniques. Site specific information obtained from X-ray absorption fine structure (XAFS) spectroscopy revealed that the local disorder around Pd and Ru atoms and their valence state can be systematically tuned by varying the Pd composition. Furthermore, the XAFS results indicated a strong correlation among the structural and valence state and the observed CO oxidation catalytic properties of Pd_xRu_1−x NPs. Hard X-ray photoelectron spectroscopy (HAXPES) analysis suggested that the capability of CO oxidation requires an optimum balance between the adsorption and desorption energy for CO adsorption and eventually conversion to CO₂. A comparison between the experimental valence band (VB) HAXPES spectra of Pd_xRu_1−x NPs and the linear combination of VB HAXPES spectra of Pd and Ru NPs revealed that the charge transfer from Pd to Ru occurs in the Pd_xRu_1−x alloy at intermediate compositions, causing electron enrichment of the Ru surface. In addition, the maximum red-shift in the edge-position relative to that of bulk Pd/Ru and high structural disorder were observed for the PdRu alloy at the intermediate composition. This coupled behavior of structure and electronic properties followed the experimental trend of CO oxidation activity in this system.

We have investigated the crystal quality of a 4-inch GaN wafer by X-ray diffraction topography. GaN (114) diffraction images at various incident angles were obtained to determine the image of maximum intensity and full-width at half-maximum (FWHM). The images reconstructed from the maximum intensity at each detector pixel position indicated that the inhomogeneous crystal quality of the wafer originated from seed crystals during wafer manufacturing. The evaluated FWHM distribution tended to increase and become broader from the center to the edge of the wafer. The q-vector components evaluated from the two rocking-curve images at different azimuthal angles combined with the rotation matrix revealed that the overall lattice planes bowed towards the diagonal direction. A histogram on the tilting angle of the wafer showed that the most frequently observed angle was about 0.03°. We expect that our findings could be applied to wafer quality estimation.

The previously published version of this Article contained an error in Fig. 1. In panel c, the thickness of the HKUST-1 layer (3 nm) was incorrectly labelled. This error has been corrected in both the PDF and HTML versions of the Article.

Hybrid materials composed of metal nanoparticles and metal-organic frameworks have attracted attention for various applications because of the synergistic functionality between their constituent materials. Interfacial interaction is expected however the mechanism remains ambiguous. Here we report the valence bands of palladium nanocubes covered by copper(II) 1, 3, 5-benzenetricarboxylate (HKUST-1), denoted as Pd@HKUST-1, and the charge transfer from the palladium nanocubes to HKUST-1 at the Pd/HKUST-1 interface is investigated quantitatively. Interfacial density of states are different from those of internal constituents and imply that the Cu–O group in HKUST-1 acts as a charge accepter. The role of Cu–O group in charge transfer behaviour is also observed experimentally. Finally, we reveal the charge transfer mechanism from the Pd 4dbands to the Cu 3d(4sp) – O 2phybridization bands of HKUST-1 at the Pd/HKUST-1 interface, which explains the enhanced hydrogen storage capacity in Pd@HKUST-1.

The Li–O ₂ electrochemical reaction involving the formation and decomposition of crystalline Li ₂ O ₂ was clearly demonstrated by using an operando synchrotron-based XRD in a transmission mode and a special airtight LAB cell.

The smallest Rh NPs have the largest structural disorder and volume fraction of cavities and so exhibit excellent hydrogen-storage capacity.