acase@tulane.edu / 1 / hong zhang

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Mary E. Galvin-Donoghue, Dept. of Materials Science & Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Dionisios G. Vlachos, Dept. of Chemical Engineering. Includes bibliographical references.

This thesis is focused on mixed systems of various copolymers (double hydrophilic block and gradient polyelectrolytes, hydrophobic graft copolymers) and low-molar-mass compounds (sodium dodecyl sulfate as a representative of a ionic surfactant or superparamagnetic iron oxides in the form of nanocrystals). The electrostatic and hydrophobic interactions in the studied systems in aqueous solutions leading to aggregation behavior and to the formation of co-assembled nanoparticles were investigated by combination of scattering and microscopy techniques, including light, X-ray and neutron scattering, electron microscopy and atomic force microscopy. Powered by TCPDF (www.tcpdf.org)

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 246-276).

Supracrystals are highly symmetrical ordered superstructures built up from nanoparticles via self-assembly. While the NP assembly has been intensively investigated, the formation mechanism is still not understood. To shed some light onto the formation mechanism, we are using one of the most common supracrystal morphologies, the trigonal structures, as a model system to investigate the formation process in solution. To explain the formation of the trigonal structures and determining the size of the supracrystal seeds formed in solution, we introduce the concept of substrate-affected growth. Furthermore, we show the influence of the NP concentration on the seed size and extend our investigations from Ag towards Au. 1.

Magnetické nanočástice a nanostrukturované materiály jsou velkým příslibem v mnoha oblastech, včetně biomedicíny, sanace životního prostředí nebo získávání energie. Proto neustále roste zájem o výzkum jejich jedinečných vlastností a vývoj uskutečnitelných výrobních postupů. Tato práce se zabývá metodou samouspořádávání, spočívající v povrchové precipitaci tenkých vrstev, za účelem výroby epitaxních polí nanoostrůvků ze slitiny FeRh na různých monokrystalických substrátech. Při tomto výrobním postupu zůstává zachována metamagnetická fázová přeměna nanoostrůvků. Morfologie a magnetické vlastnosti samouspořádaných nanomagnetů z FeRh jsou charakterizovány kombinací experimentálních technik a modelování, přičemž bylo zjištěno, že jejich rovnovážné tvary a magnetické uspořádání jsou navzájem propojeny. Kromě toho je navržena cesta pro získání volných nanočástic, která by mohla potenciálně umožnit využití metamagnetických nanostruktur v buněčných kulturách a biomedicíně obecně. Za tímto účelem jsou nanoostrůvky FeRh uvolňovány ze substrátu chemickým leptáním. Chování nanočástic a jejich reakce na teplotu a magnetické pole jsou studovány v kapalném prostředí. Metamagnetické vlastnosti separovaných nanočástic jsou charakterizovány pomocí vibrační magnetometrie.

The emergence of novel electronic, optical and magnetic properties in ordered two-dimensional (2D) nanoparticle ensembles, due to collective dipolar interactions of surface plasmons or excitons or magnetic moments have motivated intense research efforts into fabricating functional nanostructure assemblies. Such functional assemblies (i.e., highly-integrated and addressable) have great potential in terms of device performance and cost benefits. Presently, there is a paradigm shift from lithography based top-down approaches to bottom-up approaches that use self-assembly to engineer addressable architectures from nanoscale building blocks. The objective of this dissertation was to develop appropriate processing tools that can overcome the common challenges faced in fabricating floating gate memory devices using self-assembled 2D metal nanoparticle arrays as charge storage nodes. The salient challenges being to synthesize monodisperse nanoparticles, develop large scale guided self-assembly processes and to integrate with Complementary Metal Oxide Semiconductor (CMOS) memory device fabrication processes, thereby, meeting the targets of International Technology Roadmap for Semiconductors (ITRS) – 2017, for non-volatile memory devices. In the first part of the thesis, a simple and robust process for the formation of wafer-scale, ordered arrays using dodecanethiol capped gold nanoparticles is reported. Next, the results of ellipsometric measurements to analyze the effect of excess ligand on the self-assembly of dodecanethiol coated gold nanoparticles at the air-water interface are discussed. In a similar vein, the technique of drop-casting colloidal solution is extended for tuning the interparticle spacing in the sub-20 nm regime, by altering the ligand length, through thiol-functionalized polystyrene molecules of different molecular weights. The results of characterization, using the complementary techniques of Atomic Force Microscopy (AFM) and Field-Emission Scanning Electron Microscopy (FESEM), of nanoparticle arrays formed by polystyrene thiol (average molecular weight 20,000 g/mol) grafted gold nanoparticles (7 nm diameter) on three different substrates and also using different solvents is then reported. The substrate interactions were found to affect the interparticle spacing in arrays, changing from 20 nm on silicon to 10 nm on a water surface; whereas, the height of the resultant thin film was found to be independent of substrate used and to correlate only with the hydrodynamic diameter of the polymer grafted nanoparticle in solution. Also, the mechanical properties of the nanoparticle thin films were found to be significantly altered by such compression of the polymer ligands. Based on the experimental data, the interparticle spacing and packing structure in these 2D arrays, were found to be controlled by the substrate, through modulation of the disjoining pressure in the evaporating thin film (van der Waals interaction); and by the solvent used for drop casting, through modulation of the hydrodynamic diameter. This is the first report on the ability to vary interparticle spacing of metal nanoparticle arrays by tuning substrate interactions alone, while maintaining the same ligand structure. A process to fabricate arrays with square packing based on convective shearing at a liquid surface induced by miscibility of colloidal solution with the substrate is proposed. This obviates the need for complex ligands with spatially directed molecular binding properties. Fabrication of 3D aggregates of polymer-nanoparticle composite by manipulating solvent-ligand interactions is also presented. In flash memory devices, charges are stored in a floating gate separated by a tunneling oxide layer from the channel, and the tunneling oxide thickness is scaled down to minimize power consumption. However, reduction in tunneling oxide thickness has reached a stage where data loss can occur due to random defects in the oxide. Using metal nanoparticles as charge-trapping nodes will minimize the data loss and enhance reliability by compartmentalizing the charge storage. In the second part of the thesis, a scalable and CMOS compatible process for fabricating next-generation, non-volatile, flash memory devices using the self-assembled 2D arrays of gold nanoparticles as charge storage nodes were developed. The salient features of the fabricated devices include: (a) reproducible threshold voltage shifts measured from devices spread over cm2 area, (b) excellent retention (>10 years) and endurance characteristics (>10000 Program/Erase cycles). The removal of ligands coating the metal nanoparticles using mild RF plasma etching was found, based on FESEM characterization as well as electrical measurements, to be critical in maintaining both the ordering of the nanoparticles and charge storage capacity. Results of Electrostatic Force Microscope (EFM) measurements are presented, corroborating the need for ligand removal in obtaining reproducible memory characteristics and reducing vertical charge leakage. The effect of interparticle spacing on the memory characteristics of the devices was also studied. Interestingly, the arrays with interparticle spacing of the order of nanoparticle diameter (7 nm) gave rise to the largest memory window, in comparison with arrays with smaller (2 nm) or larger interparticle spacing (20 nm). The effect of interparticle spacing and ligand removal on memory characteristics was found to be independent of different top-oxide deposition processes employed in device fabrication, namely, Radio-frequency magnetron sputtering (RF sputtering), Atomic Layer Deposition (ALD) and electron-beam evaporation. In the final part of the thesis, a facile method for transforming polydisperse citrate capped gold nanoparticles into monodisperse gold nanoparticles through the addition of excess polyethylene glycol (PEG) molecules is presented. A systematic study was conducted in order to understand the role of excess ligand (PEG) in enabling size focusing. The size focusing behavior due to PEG coating of nanoparticles was found to be different for different metals. Unlike the digestive ripening process, the presence of PEG was found to be critical, while the thiol functionalization was not needed. Remarkably, the amount of adsorbed carboxylate-PEG mixture was found to play a key role in this process. The stability of the ordered nanoparticle films under vacuum was also reported. The experimental results of particle ripening draw an analogy with the well-established Pechini process for synthesizing metal oxide nanostructures. The ability to directly self-assemble nanoparticles from the aqueous phase in conjunction with the ability to transfer these arrays to any desired substrate using microcontact printing can foster the development of applications ranging from flexible electronics to sensors. Also, this approach in conjunction with roll-to-roll processing approaches such as doctor-blade casting or convective assembly can aid in realizing the goal of large scale nanostructure fabrication without the utilization of organic solvents.

The understanding of self-assembly processes is important for fabrication of well-defined structures with new functionalities for applications in the area of biomedical sciences, material sciences and electronics. In this thesis, two types of self-assembly processes are described: (1) self-assembly of fullerene derivatives in water and (2) self-assembly on surfaces using layer-by-layer (LbL) approach. The various interactions and parameters involved in the self-assembly are detailed in the introductory chapter 1. The various internal parameters like molecular geometry, intramolecular and intermolecular forces that guides the self-assembly process of amphiphiles in water are discussed. The experimental procedures used in the present thesis for the fabrication of nanostructures via self-assembly approach are also described. In the later part of the chapter, the LbL technique for fabrication of thin films and microcapsules is reviewed where various interactions involved in the growth of LbL assembly are discussed. The effect of ionic strength and pH on the growth and property of LbL assemblies is elaborated. A brief discussion of the materials used in the thesis ‒ fullerene, bovine serum albumin (BSA) and nanocrystalline cellulose (NCC) is also provided The self-assembly behaviour of amphiphilic fullerene derivatives are described in chapter 2. Fullerene is anisotropically substituted with five polar hydroxyl groups using organo-copper reagent. The derivative can interact in water via the van der Waals and hydrophobic interactions of the fullerene moiety as well as the intermolecular hydrogen bonding among the hydroxyl groups and also with water. The penta-hydroxy fullerene derivative self-assembles in water as vesicular structures. The size of these vesicles can be varied by modifying the kinetics of self-assembly which was done by changing the rate of addition of non-solvent (water) to the solution of the fullerene derivative. In the second derivative, the hydroxyl groups are substituted with less polar methoxy groups. The penta-methoxy fullerene derivative cannot participate in inter-molecular hydrogen bonding formation unlike the penta-hydroxy derivative but there is possibility of hydrogen bond formation with water where oxygens on methoxy group can act as hydrogen bond acceptor. The penta-methoxy fullerene does not show any vesicle formation in water. The computational simulation studies were carried out on the two fullerene derivatives to understand the self-assembly behaviour of these two derivatives. Furthermore, the vesicle structures formed by the penta-hydroxy fullerene derivative are used for entrapment of hydrophobic polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and also hydrophilic dye, Rhodamine B. In both the cases, fluorescence quenching is observed due to electron transfer reaction with fullerene and hence these fullerene vesicles can be used to study the effect of confinement on electron transfer reactions and other chemical dynamics. The layer-by-layer self-assembly approach for the fabrication of biopolymeric thin films and microcapsules is discussed in the chapters 3 to 6. The biocompatible nanoparticles and nanofibers were used as the components of the assembly. In chapter 3, we have described fabrication of thin film of bovine serum albumin (BSA) nanoparticles via LbL approach using biopolymer chitosan as the complementary polymer. The driving force for the assembly growth of the assembly was the electrostatic and complementary hydrogen bond formation between the two components. The idea of incorporating nanoparticles in the thin film was that the nanoparticles can act as reservoirs for functional materials. The films were loaded with anticancer drug doxorubicin and show pH dependent release of the drug. The various interactions involved in the LbL assembly of BSA nanoparticles and polymers were investigated towards understanding the growth mechanism of the assembly in chapter 4. The understanding of the interactions involved in the assembly formation is important in order to modify the conditions of the assembly for enhancing the growth. It is inferred from the study reported in this chapter that not only the interaction of nanoparticles with polymers but also the inter-particle interactions are important factors in determining the growth of LbL assembly of nanoparticles/polymers. The growth of the assembly is enhanced on minimizing the inter-particle repulsions, which was achieved in case of BSA nanoparticles by modifying the pH of the assembly. We also utilized the LbL self-assembly approach for the delivery of lipophilic drugs. The lipophilic drugs are difficult to administer in the body due to their poor water solubility and hence show poor pharmacokinetic profile. The methods for incorporating hydrophobic drugs in LbL assembled thin films and microcapsules are described in chapters 5 and 6. In chapter 5, hydrophobic molecules binding property of albumin has been exploited for solubilisation of a water-insoluble molecule, pyrene (model drug) and hydrophobic drug, curcumin, by preparation of non-covalent conjugates with BSA. The interaction with BSA provided negative zeta potential to the previously uncharged molecules and hence they can be incorporated in the LbL assembled thin films and microcapsules using electrostatic as well as hydrogen bonding interaction with biopolymer, chitosan. The fabrication of protein encapsulated stable microcapsules with hydrophobic molecules incorporated in the shell of the microcapsules has also been demonstrated. The microcapsules were further capable of loading hydrophilic molecules like Rhodamine B. Thus, this approach can be employed for fabrication of multi-agent carrier for hydrophobic and hydrophilic drugs as well as therapeutic macromolecules. In chapter 6, we have incorporated nanocrystalline cellulose (NCC) LbL assembled thin films and microcapsules. The assembly formed was porous in nature due to the nano-fibrous morphology of NCC. The nanoassemblies can act as potential drug delivery carrier, which has been demonstrated by loading anticancer drug doxorubicin, and a lipophilic drug, curcumin. Doxorubicin hydrochloride, the salt form of the drug, doxorubicin, has good water solubility and hence can be postloaded in the assembly by diffusion from its aqueous solution. In the case of curcumin, which has limited solubility in water, a stable aqueous dispersion of the drug was prepared via noncovalent interaction with NCC prior to incorporation in the LbL assembly. The interaction of various other lipophilic drugs with NCC was analysed computationally.