In Situ Confocal Raman Mapping Study of a Single Ti-Assisted ZnO Nanowire
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In this work, we succeeded in preparing in-plane zinc oxide nanowires using a Ti-grid assisted by the chemical vapor deposition method. Optical spatial mapping of the Confocal Raman spectra was used to investigate the phonon and geometric properties of a single ZnO nanowire. The local optical results reveal a red shift in the non-polar E2 high frequency mode and width broadening along the growth direction, reflecting quantum-confinement in the radial direction.
KeywordsNanocrystalline materials Confocal Raman Spectroscopy Nanowire
One-dimensional nanostructures, such as nanowires (NWs), nanorods, nanobelts and nanoparticles have become the focus of intensive research in the twentyfirst century due to their unique applications in microscopic physics and for the fabrication of nanoscale devices [1–6]. Recently, a variety of novel devices including nanoscale lasers, electrochemical gated quantum dot transistors have been investigated. The highly efficient exciton UV lasing action under optical pumping from the nano-clusters and thin films make for an exceptionally important semiconductor. The wide band gap (~3.37 eV) and large excitation binding energy (~60 meV) at room temperature of zinc oxide (ZnO) make it a very promising material for nanoscale optoelectronic applications [7–11]. The availability of large quantities of well-aligned ZnO nanowires (ZnONWs) in a single crystalline form is extremely important for the development of high-efficiency, short-wavelength optoelectronic nano-devices. However, very little is known about the optical vibrational spectra of ZnO nanorods, except that the vibrational frequencies are expected to be different from the bulk material, since a large quantity of the bonding partners are missing at the surface . Raman scattering is a very useful nondestructive technique for examination of phononic behavior, providing information about the crystal structure, lattice dynamics and defects. Recently, Raman scattering has been used to investigate individual carbon nanotubes , SiC nanowires  and ZnO belts . In the present study, we report on the quantum confined effects in a single ZnONW using in situ confocal Raman mapping technique. Comprehensive structural analysis shows that these ZnONWs are single crystal in nature with excellent crystal quality.
Results and Discussion
Morphology and SEM Images of ZnONWs
Confocal Raman Scattering
Two-Dimensional Raman Mapping and Analysis
The chemical vapor deposition (CVD) technique was used to grow single crystalline ZnONWs on a Ti-grid with a hexagonal cross-section without using any catalyst at 650°C. The experimental results indicate that, without choosing any specific direction for the orientation of the Ti substrate, there is still growth of ZnO in a c-axis direction that results in uniform and crystalline ZnONWs. Confocal Raman scattering was performed to study the phonon behavior and geometric anisotropy of a single ZnONW. As the diameter along the growth direction is reduced, the peak position and linewidth of the non-polar E2 (high) mode resulted in red shift, broadening, and asymmetry of the peak profile. The results reveal the local optical behavior and the uniformity of crystal growth, confirming that the quantum-confinement in the radial direction is in agreement with the theoretical calculations .
We would like to thank the National Science Council of the Republic of China for their financial support through project number NSC 97-2112-M-259-004-MY3.
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