Preparation Of ZnO Nano-arrays And Optical Properties Of Rare Earth Dopant Process

On the basis of preparing high-ordered porous alumina template, the nanoneedle, nanotube and nanowire arrays are prepared by the electric field-assisted deposition method in this paper. Furthermore, by means of X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, and thermo-gravimetric analyzer, the as-prepared samples are explicitly analyzed and illuminated, respectively. The photoluminescence properties of samples are explored through the near-field scanning optical microscopy and fluorescence spectrophotometer, and the light-emitting mechanisms are proposed. The steps, contents, and conclusions of this paper are presented as follows:1. Through the two-step anodic oxidation in 0.3 mol/L solution, the high-ordered alumina template is prepared. With the large density, the form of the holes on the surface of alumina membrane is very uniform. The aperture size of the alumina pores is between 60 nm and 80 nm, and most pores shapes are hexagonal arrangement, with small parts of individual holes round and ellipse though. By the two-step anodic oxidation method, the half-wall alumina nanotube arrays are synthesized in oxalic acid solution. The 302 nm emission band of the half-wall alumina nanotube array is contributed to the 1B→1A electronic transition of alumina, which belongs to the emission of F⁺ centers. Combined with the process of high-temperature annealing and the ultrasound in anhydrous ethanol, the alumina micron-trees are prepared, which involve the boles and branches. The average width and length of the boles are about 1μm and 20-30μm, respectively. The average width and length of the branches are 1-2μm and 3-5μm, respectively. Moreover, the results show that the formation of micron tree boles is not along a single direction of growth, but along different directions at the same time, which exhibits a clear X crossover-type growth direction.2. At room temperature, the ZnC₂O₄ nanoneedle arrays were prepared in the side of anodic alumina membrane (AAM) by the electric field-assisted deposition method. Subsequently, ZnC₂O₄ precipitate has been transformeded into ZnO nanoneedle arrays in the annealing process at 800 oC for 2 h. The pores of AAM provide the space for the growth of ZnC2O4 nanoneedle arrays, which result from the accumulation of ZnC₂O₄ particles and deposit at the bottom of cathode in electrolytic cell. The results of scanning electron spectroscopy (SEM) show that the tip size of ZnO nanoneedle is about 80 nm, which corresponds to the size of pores of AAM. Based on this result, the formation mechanism of ZnO nanoneedle arrays is proposed. The XRD pattern and the HRTEM image indicate that the grown ZnO nanoneedles have the preferential growth toward [101] direction and poly-crystal structure. The study on the PL spectrum of those ZnO nanoneedles at room temperature shows that the nanoneedles have excellent optical properties. Especially, with the increasing annealing temperature and time, the green emission band (510 nm) decreases and the near band-edge emission band (379 nm) increases. ZnO nanoneedle arrays have potential applications in field emission, which deserve further research.3. ZnO:Tb³⁺ nanotube arrays are synthesized by using anodic alumina membranes (AAMs) via electrodeposited method. The prepared ZnO:Tb³⁺ nanotubes were characterized by XRD, SEM, EDS and PL. The XRD patterns show that the prepared samples unannealed involve the phases of ZnC₂O₄, Zn(OH)₂ and Tb(OH)₃. The ZnO:Tb³⁺ nanotubes are transformed by the mixtures of oxalates and hydroxides, which can be testified by the XRD patterns and IR spectrum. SEM images show the morphology of the ZnO:Tb³⁺ nanotubes and the diameter and thickness of those grown ZnO:Tb³⁺ nanotubes are about 80 nm and 15 nm, respectively. The PL emission spectrum shows that a 344 nm emission band emerges and the PL emission of ZnO decreases with the increase of the concentration doped with Tb³⁺. It can be concluded that the 344 nm emission band may result from the AAMs itself. Namely, as the ZnO:Tb³⁺ nanotubes are excited by the 235 nm line, Al2O3 also can be excited simultaneously.4. ZnO:Eu³⁺ nanowire arrays (NWAs) are synthesized using anodic alumina membranes (AAMs) via electrodeposited method. XRD patterns suggest that Eu³⁺ ions occupy Zn sites or interstitial in the ZnO crystal lattice. SEM images show that the uniform diameters of those grown NWAs are about 80 nm. The HRTEM and Fourier Transform pattern show that the NWAs are single crystals with preferential growth in the [0001] direction, which is consistent with the results of XRD patterns. The PL results all show that there is a possibility of energy transfer between ZnO and Eu³⁺ due to the absence of 510 nm emission band in the emission spectrum. The 306 nm emission peak results from the excitation of Al₂O₃ nanotubes, as the NWAs are excited by 395 nm.