Synthesis of well arrayed structures with assistance of statistical experimental design

• Main Author: Cheng, Yajuan
• Format: Doctoral Thesis
• Language: English
• Published: KTH, Materialvetenskap 2015
• Subjects: optimization; experimental design; statistical analysis; monolayer arrays of silica spheres; vertically aligned ZnO rod arrays
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-172938; http://nbn-resolving.de/urn:isbn:978-91-7595-676-3

During the synthesis of well arrayed nano/micro structures through wet chemical methods, plenty of parameters are usually involved. Consequently, it is extremely time- and cost-consuming to find out the optimized synthesis conditions by using the conventional "changing one separate factor at a time" (COST) strategy. Instead, the "statistical experimental design" method has been proven in a few works to be an efficient method for experiments involving many parameters.  With this method, the responses could be optimized efficiently by using only a few experiments. Besides, several responses can be optimized simultaneously. Also, models could be built up and the changing tendency can be plotted to predict the required experimental settings for specific tasks. Two types of well arrayed structures including monolayer arrays of silica spheres and vertically aligned ZnO rod arrays were investigated in this work. Monolayer arrays of silica spheres were synthesized by using a dual-speed spin coating method. With assistance of statistical experimental design, the accelerating rate, the second rotation speed and time of the dual-speed spin coating system were found as non-significant parameters to the ordering degree of the obtained monolayer, and thus they can be fixed. This finding could remarkably increase the feasibility of optimizing the practical process. On the other hand, the relative humidity, the first rotation speed and the suspension concentration are identified as the significant parameters to the structures of the monolayer. Moreover, the optimal values for these three parameters were identified: 23% for the relative humidity, 1000 rpm for the first rotation speed and 30 wt.% for the suspension concentration. With these optimized parameters, the area of the obtained silica sphere monolayers reached over 1 cm2 and the defect-free domain size reached over 4000 μm2. These values are considerably higher compared to the previously reported values. Vertically aligned ZnO rod arrays were fabricated by chemical bath deposition. Parameters including precursor concentration, pH value, reaction temperature, reaction time and addition of capping agent were optimized by using statistical experimental design to improve and optimize the growth quality of ZnO rod arrays. Through several stages of optimization, the growth quality of the obtained structures was remarkably enhanced from sparse or clustered ZnO rods to upright and dense ZnO rods. The boundary conditions to achieve vertically aligned ZnO rods, such as a neutral solution and a precursor concentration over 0.02M, were determined. The changing tendency of the texture coefficient and aspect ratio with the factors was also plotted to predict the required experimental settings for specific requests. The points or regions to achieve the optimal properties were identified as well. For instance, the concentration should be as close as to 0.1 M, while the reaction temperature should be limited to 80-90 ◦C, to achieve the ideal preferential growth. With the optimized parameters, the texture coefficient reached almost the perfect value 1, and the aspect ratio was elevated to 21. Moreover, to obtain a dense ZnO thin film, tri-sodium citrate was added to the reaction system. The diameter was systematically controlled through varying the parameters. When both the diameter and the texture coefficient reached the optimal values, the rods were merged together to form a dense ZnO thin film. Furthermore, comments on the statistical experimental method are proposed, and both the advantages and disadvantages are presented according to the present thesis work. This might help the researchers to avoid the disadvantages and thus to employ this method more efficiently in the future.  === <p>QC 20150903</p>