S.-Y. Seo

Surface roughness and strain effects on ZnO nanorod growth

Vertically aligned ZnO nanorods were fabricated on Al2O3(001) substrates with various GaN interlayers by a catalyst-free metal-organic chemical vapor deposition. We observed that the shape and quality of ZnO nanorods grown on the GaN interlayers were considerably sensitive to the surface roughness of the interlayers. We also investigated orientation-dependent residual strain in the ZnO nanorods grown on Al2O3 substrates using polarized x-ray absorption fine structure (XAFS) measurements at Zn K edge. The XAFS revealed that the residual strain relaxation of Zn–O pairs in ab plane played a key role in the ZnO nanorod growth.

In-situ X-ray Absorption Fine Structure Study of TiO2 Nanoparticles under Ultraviolet Light

The local structural properties of TiO2 nanoparticles (NPs) were investigated under ultraviolet light (UV) using in-situ X-ray absorption fine structure measurements at the Ti K edge. XAFS analysis showed that the bond lengths of the atomic pairs were distorted by 0.1 A and that the Debye–Waller factors of the atomic pairs were increased substantially under UV light, compared with those of no UV light. These results suggested that the local structural changes were caused by the photocatalyst reactivity of oxygen atoms in the NPs. Unlike other techniques, in-situ XAFS is sensitive to detect local structural changes during the photocatalytic reactivity.

Structural and electrical properties of ZnO nanorods and Ti buffer layers

Vertically-well-aligned ZnO nanorods were synthesized on Ti buffer layers by a metal-organic chemical-vapor deposition process. Structural analyses demonstrated that the ZnO nanorods were well-aligned in the c-axis and ab-plane. Transmission electron microscopy (TEM) showed that the Ti buffer layer was amorphous and interdiffused into the ZnO nanorods. Energy-dispersive spectroscopy (EDS) analysis revealed the Ti buffer layers to be slightly oxide. Extended x-ray absorption fine structure confirmed the TEM and EDS results. The I-V characteristic measurements showed a 20-fold increase in current density with the Ti buffer layer, suggesting excellent electrical contact between the Ti buffer layer and ZnO nanorods.

Orientation‐Dependent Structural Properties and Growth Mechanism of ZnO Nanorods

We present the local structural properties of ZnO nanorods studied by using extended x‐ray absorption fine structure (EXAFS). Vertically aligned ZnO nanorods were fabricated on Al2O3 substrates by a catalyst free metal organic chemical vapor deposition (MOCVD). The polarized EXAFS measurements on the ZnO nanorods were performed at Zn K‐edge. The polarized EXAFS study revealed that the nanorods had a wurtzite structure, and that there were substantial amount of structural disorders in Zn‐O pairs in the beginning of the nanorod growth. The EXAFS measurements revealed that the orientation‐dependent disorders of the Zn‐O pairs were directly related to the growth mechanism and crystal quality of the ZnO nanorods.

Local structural properties and growth mechanism of ZnO nanorods grown on various substrates

We present the systematic study of the growth mechanism of ZnO nanorods grown on Al2O3 substrates with ZnO homo-buffer, n-GaN and p-GaN interlayers. Vertically aligned ZnO nanorods with diameter of 50 nm and lengths of range of 0.1 - 2 mum were synthesized at the substrate temperature of 350 - 500degC by catalyst-free metal-organic chemical vapor deposition. A thin ZnO film was observed underneath the ZnO nanorods grown on Al2O3, ZnO homo-buffer layers and p-GaN interlayers and the film thickness varied from 0.1 mum to 0.55 mum depending on the substrates while the film was negligible on an n- GaN interlayer. The residual strain of the ZnO nanorods grown on Al2O3 substrates was reduced to less than one fifth of the original value by employing a n-GaN interlayer. The n-GaN interlayer also enhanced the orientations among the nanorods in the ab-plane. The extended x-ray absorption fine structure measurements revealed that there were a substantial amount of disorders in the bonding lengths of the Zn-O pairs in the beginning of the ZnO nanorod growth on the Al2O3 substrates with and without the n-GaN interlayer. The disorders of the ZnO pairs located near the ab-plane were decreased as the nanorod length was increased above 0.1 mum implying that the strain relaxation of the ZnO crystals in the ab-plane was critical in the formation of ZnO nanorods.