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School of Materials Science and Engineering

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College of Engineering

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Polymer-Ligated Nanocrystals with Tunable Dimensions, Compositions, and Architectures

(Georgia Institute of Technology, 2022-11-01) Zhang, Mingyue

The ability to produce monodisperse nanocrystals with stable and tunable surface chemistry is of key importance to render investigation into their size- and shape-dependent physical properties and thus an array of applications including electronics, photonics, catalysis, sensors, energy storage, information technology, bionanotechnology, etc. In this context, nonlinear block copolymer nanoreactor has emerged as a general and robust route to synthesis of a gallery of nanocrystals with precisely controlled sizes, shapes, compositions, and surface chemistry. In this thesis, I capitalized on a set of rationally designed star-like and bottlebrush-like block copolymer to template the growth of a host of functional 0D and 1D nanocrystals with controlled dimensions, compositions, and architectures, and scrutinize the dependence of physical properties and energy-related applications on their size, shape, and surface chemistry. First, a series of star-like copolymers were synthesized via sequential atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) and styrene from star-like macroinitiators, brominated β-cyclodextrin (β-CD). Due to the living nature of ATRP, the molecular weight of each polymer block can be precisely controlled by simply tuning polymerization time and a low polydispersity index (PDI) can be achieved. The inner hydrophobic poly(tert-butyl acrylate) (PtBA) blocks were then converted into hydrophilic poly(acrylic acid) (PAA), which strongly coordinates with the metal moieties of precursors of targeted nanocrystals, leading to the nucleation and growth of nanocrystals confined within the space occupied by the PAA blocks. As a result, the size and shape of nanocrystals can be readily controlled by the molecular weight of PAA blocks (i.e., diameter of nanoparticles). Moreover, the outer PS blocks, originally covalently linked to the inner PAA blocks, form a layer of permanently anchored ligands on the nanocrystal surface to enable stable surface chemistry. This synthetic strategy were successfully applied for preparing a diversity of functional nanoparticles for the investigation into their physical properties and applications. Specifically, this judiciously designed nanoreactor was utilized to craft monodispersed magnetic spinel CoFe2O4 nanoparticles, which was studied for their magnetic and surface chemistry related electrocatalytic activity. It was the first systematic scrutiny of the influence of spin-pinning effect in spinel nanoparticles realized via surface reconstruction on the oxygen evolution reaction (OER). Second, using the same chemistry, 1D bottlebrush-like PAA-b-PS templates can be realized by employing brominated cellulose (Cell-Br) as macroinitiators. Due to the larger number of side chains on one Cell-Br macroinitiator (ranging from about 40 chains to more than 150 chains), high quality bottlebrush-like block copolymers are more challenging to synthesize than star-like block copolymers, which only have 21 arms. Systematic scrutiny was made to investigate the reaction conditions (e.g., catalyst ratio, ligand ratio, reaction concentration, degassing method, etc.) that affect the uniform growth of the highly dense block copolymer side chains, which has a determining effect on the quality of the bottlebrush-like templates and their application as nanoreactors for the synthesis of 1D nanocrystals. In addition to focusing on the precise synthesis of 0D and 1D nanocrystals via nanoreactor strategy, this thesis also covers the practical application of multi-functional nanocomposites. In this work, a ternary nanocomposite consisting of antibacterial silver (Ag) NPs, photocatalytic titania oxide (TiO2) NPs, and upconverting NPs are prepared, manifesting a greatly enhanced biocidal performance under ambient environment. It was found that the visible light (blue) and ultraviolet (UV) light which were converted from near infrared (NIR) radiation by the NaYF4@Yb:Tm upconverting NPs can be effectively absorbed by Ag and TiO¬2 NPs to generate electrons and electron-hole pairs, respectively. Reactive oxygen species (ROS) could then be produced from the reactions between environment and the electrons and holes to terminate bacteria. The outstanding antibacterial performance of this nanocomposite system renders it the potential to be used in food packaging industry. Moreover, reversible photo responsive Ruddlesden-Popper 2D perovskite nanoplatelets were explored in this thesis. Colloidal two-dimensional RP perovskite nanoplatelets with a general formula L2(ABX3)n-1BX4 are a rapidly emerging type of semiconductor materials with excellent optical and electronic properties. Research on the functional organic spacers (L) has become a popular direction in the past several years. Inspired by our previous research about reversible photo-crosslinkable nanoparticles realized by capitalizing on star-like nanoreactors with photo responsive coumarin containing repeat units in the outer block, the preparation of RP lead halide perovskite nanoplatelets with coumarin containing ammonium as organic spacers was attempted. Molecular modification and mixed organic spacer strategies were adopted to overcome the solubility limitation of coumarin containing molecules in non-polar solvents. Such coumarin containing 2D perovskite nanoplatelets will undergo controllable and reversible layer-by-layer crosslinking and de-crosslinking under radiation of certain wavelength, leading to many intriguing and tunable optical and electronic properties.
Nanoscale Characterization of Hierarchical, Natural and Synthetic Low-Dimensional Composites

(Georgia Institute of Technology, 2022-08-01) Adstedt, Katarina M.

Low-dimensional nanocomposites are promising material composites for use as high-performance materials due to their unique strength-to-weight ratios and multifunctional properties. Their application fields are diverse, acting as structural materials in lightweight electronics, defense systems, or health care devices, or as biosensors and energy storage devices. However, these low-dimensional nanocomposites have yet to reach their ultimate performance due to a gap in fundamental understanding of nanofiller intercalation mechanisms and the ultimate role of chemistry-structure-property relationships. This work will focus on the chemical and structural changes in nanocomposite systems with the introduction of nanofillers, revealing the intercalation mechanisms and role of chemistry and structure in influencing the final properties. The first task aims to understand how nanofiller content intercalates with one-dimensional cellulose nanocrystals to create an entirely natural, bio-based nanocomposite, and reveal the critical limit to nanofiller integration and correlating the resulting structural and chemistry changes to the optical and mechanical performance. The second task aims to uncover what unique structural formations exist on carbon fiber surfaces, revealing how nanostructures increase mechanical performance in synthetic composites. Finally, the third task unveils how two-dimensional architecture and chemistry, for graphene oxide and MXene nanoflakes, alters the mechanical, electrostatic, chemical, and morphological properties of synthetic nanocomposites. This work will inspire the optimal design of low-dimensional nanocomposites, demonstrating how to achieve enhanced mechanics while sustaining or enhancing the optical or conducting properties. These studies provide a framework for understanding the intercalation and absorbing mechanisms of nanocomposites and which aspects can be tailored for ultimate property control.
BRANCHED POLYMER ELECTROLYTES: RESPONSIVE NANOMATERIALS FOR CONTROLLED ION MOBILITY

(Georgia Institute of Technology, 2021-05-01) Erwin, Andrew J.

Polymers containing ionic groups such as polyelectrolytes and polymerized ionic liquids are promising candidates for the design of organized ionically conductive media due to their controlled morphology, robust chemical and thermal stability, and single-ion conductivity. However, while polymerization of ionic groups affords electrolytes a greater degree of dimensional control, the effect of nonlinear chain architecture remains mostly an unexplored consideration, despite the unique functional group densities, chain conformations, counterion condensation, and dynamics of branched polymers. First, the stimuli-responsive interfacial assembly and tunable morphologies of star-shaped polyelectrolyte block-copolymers and polymerized ionic liquids in monolayers and multicomponent systems are examined. In the former case, a dual-responsive star-graft block-quarterpolymer with variable arm number, arm length, and grafting density are integrated into hydrogen-bonded multilayer films and their morphologies were evaluated in different environments using surface probe microscopy and neutron reflectivity. The results point toward the amphiphilicity endowed by the star-graft architecture as the chief factor controlling the temperature and pH-induced conformational changes which lead to the diverse star-like clustering at the molecular scale. Likewise, the surface organization of linear and star-shaped polymerized ionic liquids in monolayers and multilayers is compared under variable adsorption conditions for polymers with the different branching architectures. Both studies demonstrate how polyelectrolytes and polymerized ionic liquids with branched architecture assemble into multilayer films with variable porosity, thickness, and textured morphologies featuring compartmentalized internal morphologies that are remarkably distinct from traditional multilayer systems. The second part of this work focuses on the ion transport in polyelectrolytes comprised of star and hyperbranched polymerized ionic liquids. Long-chain arms were found to exhibit more sluggish and elastic dynamics at longer timescales while the glass transition temperature, rates of segmental relaxation, ion disassociation, and dc conductivity were similar regardless of the polymer architecture and arm length. But when polymerized ionic liquids are branched on a smaller scale, such as in the ionic liquid tethered macromolecules consisting of both POSS and hyperbranched polyester cores, considerable shifts in the glass transition temperatures and conductivities were observed. This ability to control the ion mobility in polymerized ionic liquids near the Tg is critical for the development of solid-state electrolytes in which it is desirable to have high conductivities in the near glassy state. Overall, this dissertation provides an initial view of branched polymer electrolytes as uniquely versatile nanomaterials in the assembly of multifunctional polymer electrolytes with tunable morphologies and controlled ion transport properties.
Interfacial Assembly of Natural and Synthetic Components for Functional Bionanocomposites

(Georgia Institute of Technology, 2021-05-01) Krecker, Michelle C.

Organized bionanocomposites are promising new materials since they are biocompatible, biodegradable, and can be used in a variety of applications such as flexible electronics, wearable sensors, and molecular sieving membranes. However, their mechanical and functional performance is not up to theoretical predictions due to a gap in understanding fundamental interactions between the biopolymeric and synthetic components of multilayered composite films. This work will focus on chemical and morphological changes of structural proteins in intimate contact with two-dimensional nanofillers when exposed to differing chemical environments and the resulting mechanical and conductive properties of multilayers composites. The first task aims to unveil the mechanisms behind silk fibroin’s natural morphological reorganization in direct contact with the surface of Ti3C2Tx MXene over time in aqueous solution. In the second task, the mechanical properties of silk-MXene multilayered composite films were investigated. Finally, suckerin-12 protein encapsulated MXene flakes are fabricated and their morphological reorganization in response to salt annealing was studied. These studies showed that MXene can be uniformly encapsulated by proteins with secondary structures that can be manipulated with non-covalent methods. Organized layered nanocomposites formed from these hybrid materials display enhanced mechanical properties dependent upon protein concentration and secondary structure. This work will inspire the fabrication of functional bionanocomposites with tailorable properties facilitated through interfacial interaction manipulation via non-covalent methods. These studies provide a framework for understanding interfacial interactions between structural proteins and two-dimensional synthetic materials and outlines techniques with which functional organized protein composites can be designed.
Modulated optical behavior of electrochromic conjugated polymer hybrids

(Georgia Institute of Technology, 2019-07-30) Zhou, Jing

Noble metal nanoparticles and semiconducting quantum dots are promising building blocks for tailoring light-matter interactions at the nanoscale, which find applications in miniaturization of photonic devices, high-throughput optical sensing, and super-resolution imaging. While the optical properties of these inorganic nanoparticles alone have been largely understood, active control of their optical behavior with external fields remains challenging. This dissertation aims to understand and develop novel nanostructured electrochromic conjugated polymer (ECP)/optical inorganic nanoparticle hybrids with electrochemically modulated extinction of noble metal nanoparticles and photoluminescence of semiconducting quantum dots. Important focuses are placed on developing synthetic strategies to fabricate ECP-inorganic nanoparticle hybrids and understanding their optical response as affected by the interfacial assembly, spectral overlap, refractive properties and redox states of ECP during the in-situ electrochemical reaction. As a result of this study, the core-shell hybrid plasmonic nanostructures assembled from a gold nanoparticle core with a various of ECP shells were synthesized by in-situ chemical oxidative polymerization. Different electrochemical plasmon tuning systems have been realized with these hybrids including a dual responsive system with reversible plasmonic shift reaching 150 nm and a system possessing an easily identifiable narrow visible-near infrared absorption band. Next, a manyfold reversible increase in dark-field scattering intensity was revealed for the single hybrid nanoparticle when the conjugated polymer shell was electrochemically switched, which enables the real-time visualization of the redox reaction of conjugated polymer down to single nanoparticle level. Last, by maximizing the spectral change in an ECP with emission of quantum dot in a nanostructured assembly, we demonstrate the electrochemical modulation of quantum dot photoluminescence with a remarkable optical contrast. Overall, we suggest that unique modulated optical behavior that is unachievable in individual components can be accomplished through careful design of spectrally matched hybrid materials. The knowledge obtained in this study can be used to improve the design of high-throughput optical sensors with enhanced sensitivity and scalable, flexible, high-performance displays.
Biopolymer and synthetic polymer nanocomposite reinforcement via interfacial assembly

(Georgia Institute of Technology, 2019-07-19) Grant, Anise

Protein biopolymer composites bring together the tunability and flexibility of protein matrices and functionality of filler components. Graphene-based biocomposites are particularly popular for design of aqueously processible and strong flexible electronics for sensing, nanowires, and semiconductors. However, a lot of trial and error is required to determine biopolymer and co-constituent chemistry as well as the assembly process needed to capitalize on their synergistic properties. This dissertation identifies non-covalent methods to control interfacial interactions that drive and stabilize assembly of silk fibroin from Bombyx mori silkworm cocoons in order to induce mechanical reinforcement. This work, then shows the cross-applicability of assembly triggers for silk with other semi-crystalline, amphiphilic biopolymers using Humbolt squid sucker ring teeth protein suckerin. And, lastly, synthetic copolymers are used to clarify the role of biopolymer and surface properties on interfacial assembly without post-processing treatments. The main drivers of assembly and interfacial binding studied here include temperature, shear force, hydropathy, and pH. Surface topography and polymer chemistry/conformation were studied concurrently via atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR). This data was supported by simulation to better define assembly mechanisms at the interface of biopolymers and inorganic 2D fillers and their timescales. Then, mechanical characterization via bulging tests and scanning probe microscopy methods (SPM), force distance spectroscopy (FDS) and quantitative nanomechanical mapping (QNM). Mechanical performance is evaluated at the macro and nanoscales using quantitative nanomechanical mapping, FDS, and buckling tests. Overall, this dissertation shows how interfacial assembly driven by the hydrophobic effect can be manipulated using non-covalent means to study to tune mechanical performance. Thus, this work a roadmap for further optimization of biopolymer-based nanocomposites through interface-minded design.
Manipulating photoluminescent properties via spatial control of optically active media within polymer matrices and templates

(Georgia Institute of Technology, 2019-05-21) Smith, Marcus J.

Fundamentally, light management plays a central role in industries such as life science, health, communication, energy, and agriculture. Research involving further understanding and control of light to the benefit of humankind continues to be a driving force for advancing society. Of particular interest is furthering our understanding of light-matter interactions. By controlling these interactions, intriguing optical phenomenon such as directional light modulation and low threshold lasing can be achieved, particularly with photonic assemblies of gain medium. This study focuses on understanding the fundamental aspect of light-matter interactions and propagation in gain medium such as organic dyes, quantum dots (QD), and QD-polymer nanocomposites. Specific emphasis is placed on understanding the QD-polymer interface to realize guided assembly in nanocomposites and on finding the parameters governing optical coupling between nanocomposite structures with particular focus on photonic cavity size, shape, position, and obtaining dynamic tunability. This work provides a scientific framework which demonstrates useful methodologies for designing photonic systems that require control of light-matter interactions including emission, mode activity, and resonator coupling. Specifically, Cd based core and core/shell QDs with different interfacial architectures, including core/shell with sharp interface, and core/graded shell, are investigated to understand dynamic photoluminescence behavior. Core/shell QDs with a CdSe/ZnS composition showed the most dynamic PL behavior with an emission signature that showed semi-reversible recovery behavior based on exposure conditions. Next, an ultrafast crosslinking technique incorporating thiol-ene chemistry was used to realize QD-polymer nanocomposites with high loading, minimal aggregation and optical scattering, and tunable mechanical properties. Finally, lithographic techniques were used to fabricate high resolution templates for whispering gallery mode microsphere resonator assembly, and individual and coupled microdisk resonators with strong coupling. Taking advantage of 3D lithography, with ~200nm resolution, and unique system design we obtain microsphere resonators with sub-100nm gap spacing, equating to strong evanescently coupled resonators. The knowledge obtained herein can be used to aid in the design of robust optical materials with added functionality and tunability for sensing and enhanced light modulation leading to unique optical phenomenon such as directional lasing and unidirectional light propagation.
Plasmonic hybrid nanoconstructs for subwavelength manipulation of optical properties

(Georgia Institute of Technology, 2019-04-24) Zhang, Shuaidi

Plasmonic nanostructures give rise to intriguing optical phenomena such as deep-subwavelength light focusing, precisely tunable scattering/absorption, and super-linearity enhancement. These properties could be harnessed by combining plasmonic components with functional organics into plasmonic hybrid nanoconstructs for practical applications, such as molecular sensing, advanced displays, and photonic logic processing. However, to fully realize the potential of these plasmonic hybrid nanoconstructs, critical questions on fundamental aspects of organic-inorganic assembly and their coupling behavior need to be addressed. This work seeks to understand some of these aspects and fill knowledge gaps regarding the efficient microfabrication and utilization of hybrid plasmonic nanoconstructs. Specifically, an advanced nanoscale characterization method based on scanning probe microscopy and secondary ion mass-spectrometry is developed to monitor the morphological and compositional changes on the surface of plasmonic nanocrystal as ligand exchange reactions proceeds, leading to new discoveries on reaction dynamics. Detailed characterization of nanoparticle distribution in organic matrices and corresponding electrodynamic modeling have also been combined to aid the rational design of a cellulose nanofiber-gold nanorod hybrid surface enhanced Raman spectroscopy based molecular sensing platform that outperforms traditional design by two orders of magnitude. In addition, new discoveries have been made regarding the optical response of electrochromic polymer infused plasmonic nanohole arrays upon complex permittivity modulation: the forward and backward scattering shows drastically different response whose origin is explained by advanced electrodynamic simulation. Finally, using high resolution hyperspectral mapping and high-fidelity sub-nm resolution electrodynamic simulation, we furthered the understanding of coupling between plasmonic nanocrystals and photonic microcavities and proposed a new method that uses the near-field coupling between plasmonic nanoparticle antennas to regulate the optical output of a plasmonic-photonic hybrid cavity, which could lead to extremely compact designs for nanolasers.
Responsive nanostructures for controlled alteration of interfacial properties

(Georgia Institute of Technology, 2017-11-15) Geryak, Ren

Responsive materials are a class of materials that are capable of “intelligently” changing properties upon exposure to a stimulus. Silk ionomers are introduced as a promising candidate of biopolymers that combine the robust, biocompatible properties of silk fibroin with the responsive properties of poly-l-lysine (PL) and poly-l-glutamic acid (PG). These polypeptides can be assembled using the well-known technique of layer-by-layer processing, allowing for the creation of finely tuned nanoscale multilayers coatings, but their properties remain largely unexplored in the literature. Thus, this research explores the properties of silk ionomer multilayers assembled in different geometries, ranging from planar films to three-dimensional microcapsules with the goal of created responsive systems. These silk ionomers are composed of a silk fibroin backbone with a variable degree of grafting with PG (for anionic species) or PL or PL-block- polyethylene glycol (PEG) (for cationic species). Initially, this research is focused on fundamental properties of the silk ionomer multilayer assemblies, such as stiffness, adhesion, and shearing properties. Elastic modulus of the materials is considered to be one of the most important mechanical parameters, but measurements of stiffness for nanoscale films can be challenging. Thus, we studied the applicability of various contact mechanics models to describe the relationship between force distance curves obtained by atomic force microscopy and the stiffness of various polymeric materials. Beyond considerations of tip size, we also examine the critical regions at which various commonly used indenter geometries are valid. Following this, we employed standard AFM probes and colloidal probes coated with covalently bonded silk ionomers to examine the friction and adhesion between silk ionomers layers. This technique allowed us to compare the interactions between silk ionomers of different chemical composition by using multilayer films containing standard silk ionomers or silk ionomers grafted with polyethylene glycol PEG. This led to the unexpected result that the PEG grafted silk ionomers experienced a higher degree of adhesion and a larger friction coefficient compared to the standard silk ionomers. Next, we move to microscale responsive systems based on silk ionomer multilayers. The first of these studies looks at the effect of assembly pH and chemical composition on the ultimate properties of hollow, spherical microcapsules. This study shows that all compositions and processing conditions yield microcapsules that show a substantial change in elastic modulus, swelling, and permeability, with maximum changes in property values (from acidic pH to basic pH) of around a factor of 6, 1.5, and 5, respectively. In addition, it was discovered that the use of acidic pH assembly inverts the permeability response (i.e. causes a drastic reduction in permeability at higher pH), whilst the use of PEG largely damps any observable trend in permeability, without adversely affecting the swelling or elastic modulus responses. In the second part of these studies, we constructed tri-component photopatterned arrays for the purpose of creating self-rolling films. This study demonstrated that the ultimate geometry of the final rolled shape can be tuned by controlling the thickness of various components, due to the creation of a stress mismatch at high pH conditions. Additionally, it was revealed that pH-driven, semi-reversible delamination of silk ionomers from polystyrene exhibited a change in both magnitude and wavelength with the addition of methanol treated silk fibroin as a top layer. Finally, we showcase examples of biologically compatible systems that incorporate non-polymeric materials in order to generate tunable optical behavior. In one study, we fabricated composite nanocellulose-silk fibroin meshes that contained genetically engineered bacteria that acted as chemically sensitive elements with a fluorescent response. The addition of silk fibroin was found to drastically improve the mechanical properties of the cellulose composite structures, safely contain the bacteria to prevent efflux into the medium, and protect the cells from moderate ultraviolet radiation exposure. The final study concludes with the creation of a self-assembled segmented gold-nickel nanorod array used as a responsive element when anchored into a hydrogen-bonded polymer multilayer. Because of the mild tethering conditions and the magnetic nickel component, the nanorods were able to tilt in response to an external magnetic field. This, in turn, allowed for the creation of a never before reported magnetic-plasmonic system capable of continuously-shifting multiple surface polariton scattering peaks (up to 100 nm shifts) with nearly complete reversibility and rapid (<1 s) response times. Overall, this research develops the understanding of the fundamental properties of several different species of silk ionomers and related polymeric materials. This understanding is then utilized to fabricate pH-responsive systems with drastic changes in modulus, permeability, and geometry. In the end, the research prototypes two types of systems with optical responses and chemical/magnetic stimuli, using materials that are chemically (i.e. silk fibroin-based) or structurally (i.e. multilayers) translatable to future work on silk ionomers. These projects all serve the purpose of advancing the understanding of materials and assembly strategies that will allow for the next generation of bioinspired responsive materials.
Design of multicomponent nanostructured surfaces with tailored optical properties

(Georgia Institute of Technology, 2017-07-05) Geldmeier, Jeffrey A.

Two promising ways of manipulating light-matter interactions at the nanoscale are through the use of noble metal plasmonic nanostructures and quantum dots. However, the majority of previous studies focus on single particle properties in solution instead of in mesoscale, organized, substrate-bound arrays and films. Understanding and guiding the assembly behavior of nanostructures in a large-scale, bottom-up, and controllable manner has important ramifications for controlling resultant unique properties for emerging optical applications. The primary goal of this research is therefore understanding, both experimentally and computationally, the principles that govern plasmonic and emissive properties of nanostructure assemblies that possess novel emergent optical properties. This work was focused into three concrete tasks for understanding, controlling, and tuning nanoscale optical properties through the use of nanoparticle coupling interactions, polymeric components, and large-scale assemblies: • Understanding the nanostructure assembly fundamentals that can result in broadband absorbing plasmonic nanostructure assemblies through controlled coupling and assembly behavior; • Gaining insight into the various morphologies of conjugated polymer and plasmonic nanostructure composites and how their combination can be utilized for reversible and stimuli-responsive plasmonic resonances; • Examining the morphology of quantum dot/polymer composite films and how their interfacial properties can be altered for the enhancement of quantum dot fluorescence using dewetting-induced far-field scattering. Overall, the integration of multiple components in nanoscale assemblies and the subsequent characterization processes presented in this work can be used to address several existing challenges in present photonic and sensor applications. The controlled combination and assembly of noble metal and semiconductor nanostructures realized during the course of this work can serve as future guides and frameworks for further control of light-matter interactions at the nanoscale.