Browsing by Author "Jia, Shijun"

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    Polyelectrolyte assisted preparation and characterization of nanostructured ZnO thin films
    (2005) Jia, Shijun; Aldinger, Fritz (Prof. Dr.)
    The present work focuses on the synthesis and characterization of nanostructured ZnO thin films onto silicon wafers modified by self-assembled-monolayers (SAMs) via chemical bath deposition (CBD). Two precursor solutions were designed and used for the film deposition, in which two different polymers were introduced respectively to control the growth of the ZnO colloidal particles in solution. ZnO films were deposited from an aqueous solution containing zinc salt and hexamethylenetetramine (HMTA) in the presence of a graft-copolymer (P (MAA0.50-co(MAA-EO20)0.50)70). A film-formation-diagram was established based on the results obtained by scanning electron microscopy (SEM) and atomic force microscopy (AFM), which describes the influence of the concentration of HMTA and copolymer on the ZnO film formation. According to the film morphology, film formation can be classified into three categories: (a) island-like films, (b) uniform films and (c) canyon-like films. The ZnO films annealed at temperatures of 450°C, 500°C, 600°C and 700°C were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). After annealing, the films are polycrystalline ZnO with wurtzite structure. XRD measurements indicate that with increasing annealing temperature, the average grain size increases accordingly and the crystallinity of the films is improved. Upon heating to 600°C, the ZnO films exhibit preferred orientation with c-axis normal to substrate, whereas the films annealed at 700°C even show a more explicit texture. By annealing at temperatures above 600°C the ZnO film reacts with the substrate to form an interfacial layer of Zn2SiO4, which grows thicker at elevated annealing temperatures. The ZnO films annealed at 600°C and 700°C show strong UV emission. Another non-aqueous solution system for ZnO thin film deposition was established, in which 2- propanol was used as a solvent and Zn (CH3COO)2·2H2O as well as NaOH as reactants. Polyvinylpyrrolidone (PVP) was used as an additive. The influence of PVP on film formation was investigated by varying the [PVP]/[Zn] ratio. XRD investigations reveal that the films are of ZnO wurtzite structure and consist of nanometer-sized crystals. TEM results indicate that the film growth proceeds by oriented attachment of nanocrystallites from the reaction solution. The room temperature photoluminescence spectra for the as-deposited ZnO thin films shows a strong UV emission and a broad green emission peak. The growth of the nanostructured films in both reaction systems can be interpreted according to the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. In the initial stage of the film formation electrostatic interactions between the pristine surface and the first particles to deposit are dominant. Thus a significant electrostatic difference of potential exists between the charged ZnO/polymer particles and the surface. The subsequent attachment of further ZnO/polymer particles must then proceed mainly by Van der Waals interactions as the electrostatic difference of potential between similar particles is small. As can be seen in Zeta potential measurements the polymer molecules which are coordinated to the ceramic particles affect the electrostatic potential of ZnO. Polymer molecules in the solution might also contribute to the film growth by depletion flocculation. The present study has demonstrated a novel route to prepare nanostructured ZnO thin films via CBD. Two deposition systems with different polymers as additives were investigated. In the first deposition system, which contains a graft-copolymer, the as-deposited film is amorphous. In contrast, the second deposition system, which contains PVP, yields crystalline ZnO films at low deposition temperature, which can offer a wide use for future applications.