Electron Microscopy Characterization of Al2O3 and ZnO Coated Carbon Nanotubes

JM Green, T Gutu, L Dong, J Jiao, JF Conley, Y Ono
2006 Microscopy and Microanalysis  
Because of their many unique structural and electronic properties, carbon nanotubes (CNTs) have been investigated for use in devices ranging from flat-panel displays to electron microscopes. ZnO has also demonstrated its usefulness in applications ranging from UV photonics to nanowire-based electronics. Al 2 O 3 is often used in conjunction with ZnO films to increase their conductivity. In this study, we combined the properties of these disparate materials in an effort to creat a new form of
more » ... at a new form of materials with enhance properties. This approach resulted in exotic morphologies of coated carbon nanotubes. Multi-walled carbon nanotubes (as shown in Fig. 1 ) were grown using a chemical vapor deposition (CVD) process, with Fe as a catalyst. They were then coated, using atomic layer deposition (ALD), with a thin layer of Al 2 O 3 followed by a layer of ZnO. The samples were characterized both at as-made stage and also following a post-deposition anneal. Using a scanning electron microscope (SEM), the morphologies of the coated CNTs were studied. After the ALD process but before annealing, the deposited films appeared as a thick uniform layer around the carbon nanotubes, increasing the diameters of the structures. Small particles were also present on the surface of the coated nanotubes as shown in Fig. 2 . After annealing, small dendritic protrusions appeared on the surface of the coated nanotubes, as demonstrated in Fig. 3 . These dendrite-shaped growths appear to have diameters approximately similar to those of the original uncoated CNTs. A transmission electron microscope (TEM) equipped with an energy-dispersive x-ray (EDX) spectrometer was used to determine the internal structure and chemical composition of the coated nanotubes. Fig. 4 is a low magnification TEM image revealing the dendrite-shaped ZnO/Al 2 O 3 /CNT morphologies. These structures were found to be single-crystalline, indicated by the HRTEM image in the inset To further characterize the chemical composition distribution, an EDX line scan was performed. The line scan, Fig. 5 (b) , revealed the Zn and O to be concentrated in the dendritic protrusions while Al and C were evenly distributed throughout the structure. All of this evidence suggests that ZnO forms the dendrite-shaped growths during the annealing process. The initial nucleation of ZnO is affected by the surface conditions of the substrate. The coating layer of Al 2 O 3 on the CNTs creates a unique surface morphology; thus, the deposited ZnO forms novel structures under these conditions. Further tests are needed to discover the exact growth mechanism of these structures.
doi:10.1017/s1431927606067018 fatcat:ltu4zyeclrd7hbirtfxggghysa