Kim H.,Texas A&M University |
Kim Y.,Advanced Materials Analysis Laboratory
Nanotechnology | Year: 2013
The influence of the high surface-to-volume ratio of ZnO nanomaterials, whose sizes are large enough to exclude the quantum effect, on electronic properties was investigated by spatially resolved valence electron energy loss spectroscopy. ZnO nanowires, nanoplates, and nanotubes with different sizes were fabricated and characterized. Both the reduced volume and the increased surface area of the large ZnO nanomaterials were found to be able to modify electronic properties significantly. Hence, a nanoplate and a nanotube with very small volumes show unique energy loss functions and dielectric functions different from those of bulk ZnO at all the probe points. On the other hand, a nanowire with a relatively large diameter (70 nm) has electronic properties similar to those of bulk ZnO at the center. However, they are dissimilar at the edge of the nanowire due to the component of surface parallel to the electron path and the reduced interaction volume. Moreover, some interband transitions shift positions and bulk plasmons change oscillator strength depending upon the size of the volume and the geometry of the surface. These empirical results demonstrate that semiconducting nanomaterials larger than the exciton Bohr radius can still behave differently from bulk materials due to the high ratio between surface area and volume. © 2013 IOP Publishing Ltd.
Kim Y.,Advanced Materials Analysis Laboratory |
Kim H.,Texas A&M University
Journal of Nanoparticle Research | Year: 2014
Multiwalled carbon nanotubes (MWCNTs) with outermost diameters of ∼5 to more than 100 nm are modified and investigated for structural and optical properties. MWCNTs are sonicated for a long time (100 min) to intentionally introduce defects and functional groups as well as annealed at a high temperature (2,000 C) for further graphitization. They are examined for absorption and emission behavior and compared to the initial material. The MWCNTs are found to show plenty of highly-resolvable emission bands. Since they are observed from all the samples, the bands are considered to be intrinsic to MWCNTs. The mechanisms previously suggested for the emission of MWCNTs are compared using the structural features and the optical data. It was deduced that defects or functional groups are not involved in the luminescence. Rather, the emission bands imply interband transitions between van Hove singularities in the electronic band structure. These results not only help to understand inherent properties of MWCNTs but may also give insight into the luminescence phenomena of other carbon nanotube family such as double-walled carbon nanotubes and functionalized MWCNTs. © 2013 Springer Science+Business Media Dordrecht.