Dielectric studies of Liquid Crystal Nanocomposites and Nanomaterial systems.

Liquid crystals (LCs) have revolutionized the display and communication technologies. Doping of LCs with inorganic nanoparticles such as carbon nanotubes, gold nanoparticles and ferroelectric nanoparticles have garnered the interest of research community as they aid in improving the electro-optic performance. In this thesis, we examine a hybrid nanocomposite comprising of 5CB liquid crystal and block copolymer functionalized barium titanate ferroelectric nanoparticles. This hybrid system exhibits a giant soft-memory effect. Here, spontaneous polarization of ferroelectric nanoparticles couples synergistically with the radially aligned BCP chains to create nanoscopic domains that can be rotated electromechanically and locked in space even after the removal of the applied electric field. The resulting non-volatile memory is several times larger than the non-functionalized sample and provides an insight into the role of non-covalent polymer functionalization. We also present the latest results from the dielectric and spectroscopic study of field assisted alignment of gold nanorods

Electromechanical Coupling of Graphene With Cells

Nanomaterials have been studied extensively in the last decade in the context of many applications such as polymer composites, energy harvesting systems, sensors, ‘transparent’-like materials, field-effect transistors (FETs), spintronic devices, gas sensors and biomedical applications. Graphene, a recently discovered two-dimensional form of carbon has captured the interest of material scientists, and physicists alike due to its excellent electrical, mechanical and thermal properties. Graphene has also kindled a tremendous interest among chemists and cell biologists to create cellular-electronic interface in the context of bio-electronic devices as it can enable fabricating devices with enhanced potential as compared to conventional bio-electronics. Graphene’s unique electronic properties and sizes comparable with biological structures involved in cellular communication makes it a promising nanostructure for establishing active interfaces with biological systems. In the recent past Field effect transistors (FETs) have been successfully fabricated using carbon nanotubes (CNTs) and nanowires (NWs) and electrical characterization of these FETs were done by interfacing them with various cell cultures, tissues and muscle cells. In these cases, exceptionally high surface area to thickness ratio of FETs provides high percentage of collectible signals and the cells that are used for the study are typically placed on the FET. In this thesis, we examine a different approach towards forming bio-electronic interfaces by covering the graphene oxide (reduced) sheets on the yeast cells. Graphene oxide and reduced graphene oxide sheets as two-dimensional electronic materials have very high charge carrier mobility, extremely high surface area to thickness ratio, mechanical modulus and elasticity. We report the synthesis of graphene oxide using wet chemistry method, reduction of graphene oxide using different reducing agents and electrical characterization of graphene oxide’s conductivity. Micro-meter sized graphene sheets are used to encapsulate the yeast cells with the aid of calcium and gold nanoparticle chains. We also demonstrate that graphene sheets form electrically conductive layers on the yeast cells and developing an electromechanical coupling with the cell. The mechanical and electrical characteristics of graphene sheets are highly dependent on the cell volume and structure which are in turn related to the environment around the cell. Furthermore, using the same principle of electromechanical coupling we study the dynamics of cell surface stresses and cell volume modification, which are of importance in processes such as cell growth, division, and response to physiological factors such as osmotic stresses

Synergistic pH effect for reversible shuttling aptamer-based biosensors between graphene oxide and target molecules

DNA aptamers are known to desorb from graphene oxide (GO) surface in the presence of target molecules. We demonstrate herein that the binding equilibrium can be shifted by simply tuning the solution pH. At lower pH, the aptamer/GO binding is enhanced while aptamer/target binding is weakened, making this system a regenerable biosensor without covalent conjugation.University of Waterloo || Natural Sciences and Engineering Research Council |

Adsorption and Desorption of DNA on Graphene Oxide Studied by Fluorescently Labeled Oligonucleotides

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © American Chemical Society after peer review and technical editing by publisher. To access the final edited and published work see http://dx.doi.org/10.1021/la1037926Being the newest member of the carbon materials family, graphene possesses many unique physical properties resulting is a wide range of applications. Recently, it was discovered that graphene oxide can effectively adsorb DNA, and at the same time, it can completely quench adsorbed fluorophores. These properties make it possible to prepare DNA-based optical sensors using graphene oxide. While practical analytical applications are being demonstrated, the fundamental understanding of binding between graphene oxide and DNA in solution received relatively less attention. In this work, we report that the adsorption of 12-, 18-, 24-, and 36-mer single-stranded DNA on graphene oxide is affected by several factors. For example, shorter DNAs are adsorbed more rapidly and bind more tightly to the surface of graphene. The adsorption is favored by a lower pH and a higher ionic strength. The presence of organic solvents such as ethanol can either increase or decrease adsorption depending on the ionic strength of the solution. By adding the cDNA, close to 100% desorption of the absorbed DNA on graphene can be achieved. On the other hand, if temperature is increased, only a small percentage of DNA is desorbed. Further, the adsorbed DNA can also be exchanged by free DNA in solution. These findings are important for further understanding of the interactions between DNA and graphene and for the optimization of DNA and graphene-based devices and sensors.University of Waterloo || Natural Sciences and Engineering Research Council |

First-principles Investigation of Lithium Intercalation and Transport in Manganese-oxides

The transportation and energy storage industries are currently undergoing a technology revolution involving electrification of every market segment. Top automakers such as GM, Ford, and Tesla are investing upwards of $30 billion each, building the material supply chain and battery manufacturing capabilities to accommodate the expected growth. As a result of this intense effort, the battery prices have reduced from$1160/kWh in 2010 to $150/kWh in 2020. Lithium (Li) intercalation chemistry has remained the gold standard since its discovery in 1972. Layered oxides such as lithium cobalt oxide, derivatives of lithium nickel aluminum oxides have increased the energy density to ~260 Wh/kg and offer up to 1000 full depth-of-discharge cycles. However, there are several remaining challenges: (i) the safety concerns associated with energy-dense chemistries such as nickel-manganese-cobalt oxide (e.g., LiNi0.8Mn0.1Co0.1O2) have proven to be challenging to solve. This issue necessitates complex battery cooling requirements in an EV application, (ii) batteries based on intercalation chemistry utilize less than 30% of their theoretical capacity: for example, lithium cobalt oxide/graphite in 18650 format has a theoretical volumetric energy density of 2950 WhL-1, but only 20% of it is utilized, resulting in an energy density of 570 WhL-1. The specific capacities of cathodes have been a prime limiting factor in improving the performance characteristics of modern Li-ion batteries. In addition, excessive reliance on Nickel and Cobalt has caused a supply chain bottleneck and instability in commodity pricing. So, there has been a coordinated effort from academia and industry to eliminate the expensive and limited-supply transition metals, which further require toxic chemicals for processing, from cathodes. In this context, there is a renaissance of cathode chemistries such as Mn-oxides that are inexpensive and non-toxic. Although positive electrode materials made of Mn (spinel, LiMn2O4) have been in usage since the 1980s, other polymorphs such as tunnel structured Mn-oxides have not found broader application in Li-ion intercalation chemistry. Understanding the key structure-property evolution in the context of intercalation processes and Li-ion transport in Mn-oxides with [2×2] tunnel structures form the basis of this dissertation work. [2×2] α-MnO2 material was chosen because of its varied electrochemical properties and its application as a 3V cathode with a potential specific capacity of ~ 504 Wh kg-1. These materials offer a larger annular space within their channels for ionic transport (in comparison to their [1x1] counterparts], and the surface facets are conducive for catalytic processes. At the same time, the α-MnO2 material system suffers from capacity fade due to structural degradation and loss of tunnel geometry. The underpinning physics behind such material degradation is not well understood. This dissertation has resulted in three different research manuscripts addressing several unanswered questions in regards to the electrochemical performance of α-MnO2. The research community is still investigating the exact role of cationic stabilization of tunnels and its role in structural degradation. To address this knowledge gap and offer theoretical insights, we studied the effect of stabilizing cations (K+) on the lithiation-induced evolution in structure and electronic as well as ion transport properties of the underlying material system. This study considered different K+-ion concentrations in α-KxMn8O16 and found that higher cationic content (2 K+/tunnel) impedes intercalation and transforms the structure into the layered configuration, leading to poor capacity retention. The discharge voltage predicted by Density Functional Theory (DFT) calculations is in agreement with the experimentally reported values in literature. Surface reconstruction and Mn-dissolution are a few of the key degradation mechanisms in Mn-rich electrodes. When this polymorph is synthesized and used in nanoscopic sizes, the high surface-to-volume ratio alters the interfacial diffusion dynamics, and the factors that lead to the dissolution of Mn at the surface under such size regimes and conditions are not well understood. So, this thesis directed substantive efforts towards understanding the nature of surface facets of these nanoscopic materials and offered important new insights. We found that 3D radial diffusion is a major limiting factor in realizing higher diffusivity and capacity retention. Finally, the role of cationic size and valence on Li-ion intercalation and capacity retention has been elucidated, and the findings from our work offer guidelines for the synthesis of [2×2] tunnel structures with different stabilizing cations. Overall, this body of work offers several theoretical insights into the structure-property relationship of α-MnO2 in the context of rechargeable batteries