Study On Characteristic Stimuli Responsive Polymer Thin Films

Author : Md Zahabul Islam
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (119 download)

DOWNLOAD NOW!

Download or read book Multi-stimuli Effects on Thin Films and Devices written by Md Zahabul Islam and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Physical properties of materials are known to depend on microstructure and defects across multiple length-scales. The structure-property scaling becomes enhanced at the micro and nano scales, an example of which is the 'smaller is stronger' phenomenon. Size effects also render nanoscale materials more sensitive to external stimuli such as stress, temperature, electrical current, light compared to their bulk counterparts. Even more interesting is the observation of the breakdown of classical physical laws at length scales (grain size, thickness) at or below characteristic length scales for physical domains. For example, scaling of yield stress (known as the Hall-Petch law) breaks down below ~25 nm, where a grain cannot accommodate statistically significant number of dislocations to induce plasticity. Similar breakdown phenomena have been observed for other (electrical, thermal) domains. Fundamentals of the mechanics and physics of nanomaterials is a prerequisite for the development of nanotechnology, which makes the length scale and external stimuli effect on materials behavior as an attractive field of research. While extensive efforts are ongoing to explore nanoscale structure-properties relationship in single domains, this dissertation is rooted in processing-structure aspects of materials by exploiting pronounced coupling exists among physical domains. Since the core of materials processing relates to the response of the material to external stimuli (such as temperature), our approach is to explore size or confinement effects that could make materials more sensitive to external stimuli compared to conventional bulk materials. This lays down our hypothesis, 'size-induced coupling of multiple domains is manifested in form of unprecedented synergy of multiple stimuli, which can be exploited to tailor microstructure or defect density to achieve control over physical properties'. This hypothesis is aligned to the ulterior goal of this dissertation, which is to develop novel materials processing techniques that are faster, more effective and energy efficient compared to the conventional (high temperature) thermal annealing. Accordingly, the objective of this dissertation is to validate the hypothesis and demonstrate it on two classes of materials spanning nano to micro scales namely, (i) ~2 nm thick two-dimensional (2D) and (ii) 100 nm to 100 micron thick metals and additive manufactured alloys. For each of these cases, we have explored the multi-stimuli synergy to achieve control over crystallinity, grain size and defect density. Since nanoscale characterization is challenging even in single domains, a key hurdle for this research is to develop an experimental setup, which can simultaneously apply multiple stimuli, or conversely, characterize the materials in multiple domains. We achieved this with a Micro Electro-Mechanical System (MEMS) based framework, where strain, temperature and electrical current are simultaneously applied on the specimen inside a high-resolution microscope that visualizes the microstructural changes in real time. The setup is small enough to fit inside a transmission electron microscope (TEM), which can provide atomic resolution. We have compared the magnitude of these stimuli for both cases (i) when they are applied simultaneously and (ii) separately to quantify the synergistic effect of the stimuli. In addition, we have also quantified the time rate or the dynamics of microstructural evolution. We have also analyzed the stimuli magnitude and microstructural dynamics to demonstrate the efficiency of our proposed multi-stimuli materials processing technique. Our findings shows the enhanced atomic and defect mobility due to the electrical current. It also reveals the microstructural transformation of near-amorphous material to nanocrystalline materials. This type of transformation is difficult or energy extensive for conventional thermal annealing. We have also investigated the pronounced effect of multi-stimuli (instead of single stimuli) to observe the microstructural changes. In this research, at first we have chosen e-beam deposited thin films and additive manufactured (AM) alloys as platform materials. Additive manufacturing is a highly non-equilibrium manufacturing process where laser sintering/melting results in defects spanning micro to nanoscales. While the pores and voids can be eliminated by conventional thermal annealing, more challenging tasks are the nanoscale defects, such as sub-grain structures. Thus, we have explored the effectiveness of the multi-stimuli processing on microstructure control of additive manufactured alloys as well as thin films (zirconium and palladium, gold). We also used ion irradiation to generate controlled defects in polycrystalline gold films and then investigated the effectiveness of multi-stimuli on defects annihilation. Another platform material is 2D materials for their extremely small length-scale (mono to few atomic layers configuration). Our study on chemical vapor deposited (CVD) MoS2, with few atomic layers, inside a TEM shows the effectiveness of the stimulus effects on defects annihilation and microstructural changes at low temperatures. Later on, we extend our multi-stimuli synergy on 2D material based back-gated field effect transistor (FET). External stimulus such as electrical current generates both resistive heating i.e., Joule heating and atomic scale force also known as electron wind force (EWF) in a material. Study shows that this external stimulus can induce significant amount of momentum on the defective sites even at low temperature due to the EWF. Study also reveals that this unique EWF accompanied at low temperature can enhance device performance in a short period of time span, which indicates this proposed technique will potentially lead to time and cost-effective post-processing of two-dimensional materials and their devices. The scientific contribution of this research will be experimental validation of the hypothesis that simultaneously applied stimuli are more effective, energy efficient and faster in achieving control over defects and microstructure compared to conventional thermal annealing process. The potential impact of successful validation is a novel material processing technique, whose unprecedented atomic and defect mobility at lower temperatures will open a new horizon in defect engineering to modulate physical properties to find applications from nanotechnology to advanced manufacturing.