Designing nanostructures for application in electronic devices

Abstract

A worldwide great investment, in terms of scientists’ time and money, in the field of nanoscience and nanotechnology has delivered its promises upto a great extent especially in the field of optoelectronics, materials science and energy. In current scenario, new nanomaterials need to be designed by adopting a dual approach in making electronic devices power efficient and can be used for multiple potential application which can minimize electronic garbage, which is very hard to dispose. An electrochromic device is one of the important members of electronics and lot of efforts are going on in developing novel materials to be used for such kind of power efficient device applications. Devices based on electrochemical activity show interesting behavior by controlling nanoscale architecture of the active material when used as the working electrode. Among these, electrochromism, of new nanomaterials fabricated as electrodes, is one such promising technology which has various potential application in the current era of advancement from automobile to smart buildings to display system and many more. Hence, primary focus has been to develop novel and multifunctional materials on one hand, and sustainable technologies on the other several approaches have been explored todevelop conducting polymer nanostructures. Keeping this in mind the work presented in this thesis having primary objectives of develop an understanding on electrochemical approach towards device fabrication and understanding device operational mechanism. Along with the operation, explore new techniques to track molecular changes within the electrochromic device like: in-situ spectroscopies to probe the device behavior under its operational condition. Explore the possibility of nanostructures of organic and inorganic material which can be incorporated in the device for better performance. As far as structure is concern understanding the growth mechanism of various nanostructures involving hydrothermal and electrodeposition methods along with high temperature and pressure treatment so that the electrochromic device parameters can be optimized. Apart from electrochromic device incorporation of these nanostructures in other areas of electrochemical based application such as energy storage and sensing. Other than this field emission is also one of the application we have gone through it.In an attempt to understand the color switching mechanism of organic electrochromic devices, live spectroscopy has been done to probe the internal mechanism of the device. Role of redox reactions taking place at the electrode/electrolyte interface has been identified using Raman and UV-Vis spectroscopies which have been carried out during the device operation. The origin of color change has been attributed to the bias induced redox switching between its dication and free radical forms which have different optical properties from each other. Raman spectra collected from negative and positive electrodes of the device reveal that blue color species (free radical) are present at the negative electrode which is created due to reduction of the dicationic form. In-situ UV-Vis spectra reveals that the navy blue color of the device under biased condition. Absorption modulation has been reported from the device with good ON/OFF contrast of the device. After understanding the fundamental mechanism of color switching within the electrochromic device we have prepared a new electrochromic gel (EC-Gel) combine of ethyl viologen (EV) - graphene nanoflake (GNFs) - tetrathiafulvalene (TTF) show faster and efficient electrochromism. A prototype flexible electrochromic device has been fabricated using the above-mentioned EC-Gel as active layer which shows overall improved coloring efficiency. Aftersuccessfully understanding pure organic electrochromic devices, we have deigned inorganic core-shell nanorods made of TiO2/Co3O4 exhibits improved electrochromic properties. The core shell hetrostructure shows better performances as compared to the individual nanostructures of either of the metal oxides. The structures grown on FTO coated glass substrate using hydrothermal electrodeposition technique. The core-shell electrode exhibit high stable and power efficient bias induced color change between transparent (sky blue) and opaque (dark brown) state with coloration efficiency of ~90cm2/C. Improvement in electrochromic performance is likely due to increased surface area and modified charge dynamics within the core-shell heterojunction with solid foundation of single crystalline nanorods. Additionally, these core-shells also exhibit porous morphology and strong adhesion to the surface of transparentconducting glass electrode gives rise to superior cyclic stability in both, energy storage and electrochromic application. After understanding pure organic andpure inorganic electrochromic devices, we have demonstrated hybrid core-shell nanostructures based on transition metal oxide as core and conducting polymer as shell to improve electrochromic properties. Nickel oxide nanopetals (NiO-NPs) transition metal used as backbone templates for polyaniline (PANI) conducting polymer as shell shows efficient and stable electrochromic performance. Apart from electrochromic applications, we have also explored various other applications such as energy storage to field emission to glucose sensing of the same multifunctional nanostructures used in electrochromic application. The core-shell nanostructures have been grown on an FTO coated glass substrate by preparing TiO2 nanorods through hydrothermal reaction followed by growing Co3O4 shell layer by electrodeposition shows high specific and areal capacitance. A power efficient and stable field emission (FE) has been observed from ultrathin nanothorns covered nickel oxide (NiO) nanopetals (NPs) where three orders of magnitude improved electron FE, in terms of threshold and turn-on fields, has been observed. Glucose sensing properties of mesoporous well-aligned, dense nickel oxide (NiO) nanostructures (NSs) in nanopetals (NPs) shape grown hydrothermally on the FTO coated glass substrate has been demonstrated.