One of the current challenges in materials science is establishing a simple way to generate an ultradense arrays of addressable nanoscopic elements on macroscopic scales. The addressability of nanomaterials is essential for many applications, ranging from high-density magnetic storage to high-density, ultrahigh resolution displays to photovoltaics. Among the strategies available, "photocombing" has been proposed as a promising route to create long-range ordered nanostructures in self-assembled block copolymers (BCPs) over macroscopic distances through a photocontrollable ordering transition. In this process, bands of light act as a "comb" to sweep across BCP thin films unidirectionally, reversibly bringing the BCPs through an ordering transition, like the disorder-to-order transition (DOT) and the order-to-order transition (OOT). Thus, defects are "combed" out, forming arrays of highly ordered BCP microdomains on a macroscopic length scale. It is similar in principle to the classic zone refining method, which is used to produce large single crystals of metals and semiconductors. In this dissertation, I will focus on three systems to investigate photocombing. System I is the supramolecular assembly of poly(2-vinylpyridine)-block-poly(n-butyl methacrylate) and polystyrene-block-poly(2-vinylpyridine) di-BCPs with azobenzene-containing 2-(4-hydroxyphenylazo)benzoic acid chromophores. In these systems, an ordering transition from lamellae to hexagonally packed cylinders was observed after one hour of UV radiation at 150 °C. System II is the deuterated polystyrene-block-poly(n-butyl methacrylate) BCPs with photoisomerizable azobenzene functionalities. They exhibit an entropy-driven lower DOT, the characteristic of "compressibility", similar to their parent BCPs. System III is anthracene-functionalized tri-BCPs containing deuterated polystyrene (d8-PS) and poly(methyl methacrylate) (PMMA) blocks, as well as a small middle block of poly(2-hydroxyethyl methacrylates) that is randomly functionalized by anthracene. Under UV exposure, the junction between d8-PS and PMMA blocks in the tri-BCPs is joined together through anthracene photodimers, thereby resulting in a significantly increase in the total molecular weight of the tri-BCPs. As a consequence, the tri-BCPs undergo an ordering transition from a disordered state to an ordered state, when it is phase-mixed but close to the boundary of the ordering transition.

Interfacing inorganic nanoparticles with biological systems to develop a variety of novel imaging, sensing and diagnostic tools has generated great interest and much activity over the past two decades. However, the effectiveness of this approach hinges on the ability to prepare water dispersible nanoparticles, with compact size and long term colloidal stability in biological environments, and the development of controlled conjugation to various biomolecules. The primary focus of this dissertation is the design and synthesis, characterization and use of a series of new multidentate and multifunctional coordinating polymers as ligands that render various inorganic nanocrystals water soluble, with emphasis on: i) how to improve the colloidal stability, ii) how to design compact coating, iii) how to engineer the nanoparticle surface with tunable functionalities to achieve bioconjugation, and iv) how to develop the resulting nanoprobes into biological sensing and imaging. This dissertation is organized as follows: In Chapter 1 we introduce the basic physical properties of quantum dots (QDs), gold nanocrystals and magnetic nanocrystals along with brief description of their syntheses. We then provide an overview of surface functionalization strategies and recent progress in the ligand chemistry, followed by highlights of a few conjugation approaches applied to nanoparticles in biology. We then discuss modulation of the optical and spectroscopic properties of QDs via energy and charge transfer interactions. We conclude by presenting a few related examples on the incorporation of QD-conjugates into sensor design and intracellular imaging. In Chapter 2, we report the design of a series of multifunctional polymers as ligands for surface engineering of QDs and facilitating their use in bioconjugation. This Chapter includes three sections: • First, we introduce a novel PEGylated polymer that combines the synergies of metal-chelation promoted by lipoic acid and imidazole groups, as effective coating for the surface functionalization of QDs; one of the goals was to address the problems associated with thiol oxidation and weak imidazole affinity. We detail the ligand synthesis via the highly efficient, one-step nucleophilic addition reaction between a central poly(isobutylene-alt-maleic anhydride) chain (as a scaffold) and several distinct but complementary amine-presenting functionalities. We then demonstrate the use of in situ phase transfer of hydrophobic QDs into water mediated by a mild photoligation strategy under borohydride-free conditions. Ligation with these polymers provides QDs with excellent colloidal stability over a wide range of conditions; this improves on what has been reached using coating with ligands presenting lower coordination of thiol only or imidazole only. • Second, to minimize the hydrodynamic radius of the QDs without sacrificing aqueous solubility, a set of polymer ligands appended with zwitterion and imidazole motifs have been synthesized applied for the surface engineering of QDs. By using zwitterion as the hydrophilic block, this design provides QDs with a thin coating and very compact overall dimension; this has, for example, allowed the self-assembly of QDs with polyhistidine-tagged proteins via metal−histidine coordination. We further show that the assembled QD-dopamine conjugates can be used to detect iron ions and amino acid cysteine through charge-transfer interactions. Finally, we demonstrate that QDs ligand exchanged with folic acid-functionalized ligand are capable of targeting cancerous cells. • Third, modulation of the nanoparticle’s interaction with biological systems requires access to an effective conjugation of these materials with bioactive targets in a controlled manner. To fulfill this goal, we have developed several zwitterion-based multifunctional ligands presenting tunable functional groups, including carboxyl, amine, azide and biotin. This has allowed conjugation of the QDs to biomolecules via bio-orthogonal coupling chemistries, including (1) amine-isothiocyanate reaction; (2) biotin-streptavidin self-assembly; (3) copper-free click chemistry. The resulted QD-bioconjugates have been tested in sensor design and for cell imaging. We also find that the efficiency of polyhistidine-mediated metal coordination is not only determined by the ligand lateral extension but also greatly influenced by the nature of metal coordination on the QDs. In Chapter 3, we have applied the various multi-coordinating and multi-reactive polymers, in particular, those presenting lipoic acid and PEG for the functionalization of gold nanoparticles and nanorods. Gold nanocrystals coated with this polymer exhibit excellent long-term colloidal stability over a broad range of conditions, and furthermore prevent the formation of protein corona. This was verified using dynamic light scattering measurements combined with agarose gel electrophoresis. The diffusion properties of polymer-coated nanocrystals were further characterized using dynamic light scattering; this has yielded valuable information on the nature of the interparticle interactions in biological media. In Chapter 4, an additional set of modular ligands were synthesized and applied for the surface modification of iron oxide nanoparticles. These ligands feature several dopamines for tight binding on iron oxide nanoparticle surface, a short PEG for water solubility and reactive groups (amine, carboxyl, azide and thiol) for bioconjugation. Nanoparticles functionalized with these polymers show extended stability in biologically relevant conditions and little to no cytotoxicity. We demonstrate that covalent attachment of dye enables producing luminescent probe for cell imaging. A summary of the major contributions assembled in this dissertation along with a discussion of the future outlook is provided in Chapter 5. / A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the Doctor of Philosophy. / Fall Semester 2016. / September 21, 2016. / Bioconjugation, Inorganic Nanocrytsals, Ligand Chemistries, Quantum Dots, Sensing and Imaging, Surface Functionalization / Includes bibliographical references. / Hedi Mattoussi, Professor Directing Dissertation; Hengli Tang, University Representative; Joseph B. Schlenoff, Committee Member; Kenneth L. Knappenberger, Committee Member.

Electro-optic phase modulators play a vital role in various large-scale photonic systems including Light Detection and Ranging (LIDAR), quantum circuits, optical neural networks and optical communication links. The key requirement of these modulators include strong phase change with low modulation induced optical loss, low electrical power consumption, small device footprint and low fabrication complexity. Conventional silicon phase modulators have either high power consumption (thermo-optic effect) or high optical loss (plasma-dispersion effect). On the other hand, low-loss phase modulation can be achieved using electro-optic 𝑋² effect such as LiNbO₃ which has a large device footprint (in mm's) and requires complex fabrication. The need of the hour is a material or a device that is strongly tunable with low optical loss and capable of picosecond switching speed. Transition metal dichalcogenides (TMDs) have been widely studied for optoelectronic applications due to their strong and tunable excitonic response. In fact, TMDs have been shown to experience massive changes of upto 20 % in their refractive index with doping, but this modulation is accompanied with large absorption change (60 %), which greatly limits their utility in photonic applications. In contrast, very litte is known about the effect of doping on the electro-optic response of TMDs at energies far below the exciton resonances, where the material is transparent and therefore could be used for photonic circuits. In this work, we first probe the electro-optic properties of TMDs in the near-infrared using a dielectric SiN microring resonator platform. We measure a strong doping induced change in the refractive index (Δn) of 0.52 in WS2 with minimal induced absorption (Δk) of 0.004. The |Δn/Δk| of 125, is an order of magnitude higher than the measured |Δn/Δk| for 2D materials including graphene and TMD monolayer at excitonic resonances, and for bulk electro-refractive materials commonly employed in silicon photonics. We next utilize this strong electro-refractive response to demonstrate low power, lossless optical phase modulation based on a composite SiN-TMD platform. The WS₂ based photonic modulator achieves a modulation efficiency (V_π⋅L) of 0.8 V ⋅ cm with a RC limited bandwidth of 0.3 GHz and DC electrical power consumption of 0.64 nW. The measured index change in monolayer TMDs (∼ 15%) in TMDs is unprecedented, considering the change in index of bulk (LiNbO₃) - the 'gold standard' for photonics - is typically 0.04 %. Despite the observed strong electro-refractive effect in TMDs and the enhanced light-matter interaction, the change in effective index of the propagating mode is 6.5 × 10⁻⁴ RIU, thereby requiring WS₂ phase modulators that are 1.3 mm long. This is due to the low optical mode overlap of 0.03 % with the monolayer that necessitates long phase shifter length. There is an urgent need for a compact, low-loss and high-speed optical phase shifter. Conventional phase modulators with low optical loss require long lengths to achieve strong phase change. On the contrary, traditional intensity modulators leverage compact high-finesse ring resonators to modulate output intensity. However, such cavities with conventional electro-refractive materials such as silicon where Δn/Δk = -20 cannot be used for phase modulation, owing to the high insertion loss associated with the phase change. Here, we show that we can leverage high-finesse ring resonators to achieve strong phase change with low optical loss. We achieve this by simultaneously modulating both the real and imaginary part of the effective index in the cavity to the same extent i.e. Δn/Δk ≈1. We design a hybrid SiN-2D platform that modulates the complex effective index of the propagating mode, by tuning the loss and index in monolayer graphene (Gr) and WSe₂ embedded on a SiN waveguide, respectively. We engineer the Gr-WSe₂ capacitor design to achieve a linear phase change of (0.50 ± 0.05)π radians with a low transmission modulation of 1.73 ± 0.20 dB and insertion loss of 2.96 ± 0.34 dB. We measure a 3 dB electro-optic bandwidth of 14.9 ± 0.1 GHz in the SiN-2D hybrid platform. We measure a phase modulation efficiency (V_(π/2)⋅L) of 0.045 V ⋅ cm with an insertion loss of 4.7 dB for a phase change of π/2 radians in the 25 μm SiN-2D platform. We show that the V_(π/2)⋅L for our SiN-2D hybrid platform is significantly lower than V_(π/2)⋅L of electro-refractive phase modulators based on silicon PN, PIN and MOS capacitors with comparable insertion loss. The TMD or TMD-graphene capacitor is incorporated as a post-fabrication process, transforming any passive substrate into an active photonic platform. The demonstrated enhanced light-matter interaction in monolayer TMDs could open up routes to a range of novel applications with these 2D materials and enable highly reconfigurable photonic circuits with low optical loss and power dissipation. We estimate that the efficiency of our TMD platform can be improved by optimizing the optical mode overlap with the monolayer through photonic mode optimization or reducing the dielectric thickness. For large-scale photonic systems, wafer-scale integration of TMD materials with silicon photonics can be done either as a direct TMD growth process on silicon wafers or a post-processing step where large wafer-scale TMD films are transferred onto a silicon photonics platform fabricated in a standard foundry.

The discovery of strongly correlated electronic phases in twisted bilayer graphene has led to an enormous interest in twisted van der Waals (vdW) heterostructures. While twisting vdW layers provides a new control knob and never before seen functionalities, it also leads to large spatial variations in the electronic properties. Scanning probe experiments are therefore necessary to fully understand the properties of twisted vdW heterostructures. In this thesis, we studied twisted bilayer graphene (TBG) with two scanning probe techniques at two twist angle regimes. At small twist angles, our nano-infrared images resolved the spatial variations of the electronic structure occurring within a Moiré unit cell and uncovered a quantum photonic crystal. Meanwhile, with nano-photocurrent experiments, we resolved DC Seebeck coefficient changes occurring in domain walls on nanometer length scales. At larger twist angles, we mapped the twist angle variations naturally occurring in our device with a combination of nano-photocurrent and nano-infrared imaging. Finally, we also investigated different materials for use as nano-optics compatible top gates in future experiments on TBG. Our results demonstrate the power of nano-optics techniques in uncovering the rich, spatially inhomogeneous physics of twisted vdW heterostructures.

Since the single-molecule diode was first envisioned by Aviram and Ratner in 1974, researchers have investigated how the electronic properties of molecules might be designed to achieve a variety of device functionality. However, although electron-phonon and electron-photon interactions have been studied in systems where the molecule is poorly electronically coupled to the environment, only a few experimental modalities exist for studying inelastic transport in two-terminal single-molecule junctions. Furthermore, each phenomena typically has a few possible mechanisms which must be distinguished. The objective of this dissertation is to expand the experimental tools available for probing inelastic transport in single-molecule junctions, with special attention to electron-photon interactions. Throughout the dissertation, we utilize the scanning tunneling microscope break-junction technique to form either tunnel junctions or single-molecule junctions. By repeatedly pushing and pulling a Au STM tip into a Au-coated mica substrate, a variety of junction geometries are sampled to give a distribution of device performances. Transport and optical measurements are made while controlling the electrode displacement and junction bias independently, which permits flexible experimental design. The body of the dissertation is divided into three chapters, each chapter exploring a different phenomenon. In the first chapter, I study light emission from tunnel junctions driven at high bias. It was shown previously that electroluminescence from tunnel junctions can have photon energies exceeding the classical limit, so-called overbias emission. Multi-electron processes and blackbody radiation have been proposed as possible explanations for this extraordinary result. We demonstrate that the intensity of the overbias emission depends superlinearly on the junction conductance even at room temperature, which strongly supports the theory from multi-electron processes. Additionally, we show that blackbody radiation-like effects can be produced by multi-electron processes. In the second chapter, I demonstrate experimentally the enhanced conductance of single-molecule junctions under illumination. Again, we consider two mechanisms for enhancement, namely photon-assisted tunneling and hot-electron distributions. By carefully comparing the two theories, we find that their steady-state signatures are nearly identical, but that the contribution from hot-electron distributions is larger in our system. This is confirmed by measuring a conductance enhancement at a polarization where photon-assisted tunneling is negligible. In the third chapter, I explore both junction rupture and nonlinear transport phenomena in single-molecule junctions around the resonant tunneling regime. Importantly, we develop nonlinear regression curve-fitting to allow straightforward extraction of key transport parameters on individual single-molecule junctions. We observe a strong correlation between the bias at which the junction ruptures and the level alignment of the dominant transport orbital, which suggests that, in the resonant tunneling regime, the tunneling electrons interact strongly with the nuclear degrees of freedom. However, we also find that not all junctions rupture and those that sustain display negative differential resistance and hysteresis. We hypothesize that this nonlinear behavior is due to a change in the charge state of the molecule. We study the stability of this charge state and find that the dynamics of charging and discharging occur on millsecond timescales. Although the blocking-state and polaron models each predict parts of our data, neither are fully consistent with the experiments in their entirety. This reveals opportunities for further experimental and theoretical investigations into transport in the resonant tunneling regime.

No description available.

Graphene materials, exhibiting outstanding mechanical properties, are excellent candidates as reinforcement in high-performance polymer nanocomposites. In this dissertation, advanced atomic force microscopy (AFM) techniques are applied to study the nanomechanics of graphene/polymer nanocomposites, specifically the graphene/polymer interfacial strength and the stress transfer at the interface.;Two novel methods to directly characterize the interfacial strength between individual graphene sheets and polymers using AFM are presented and applied to a series of polymers and graphene sheets. The interfacial strength of graphene/polymer varies greatly for different combinations. The strongest interaction is found between graphene oxide (GO) and polyvinyl alcohol (PVA), a strongly polar, water-based polymer. On the other hand, polystyrene, a non polar polymer, has the weakest interaction with GO. The interfacial bond strength is attributed to hydrogen bonding and physical adsorption.;Further, the stress transfer in GO/PVA nanocomposites is studied quantitatively by monitoring the strain in individual GO sheet inside the polymer via AFM and Raman spectroscopy. For the first time, the strains of individual GO sheets in nanocomposites are imaged and quantified as a function of the applied external strains. The matrix strain is directly transferred to GO sheets for strains up to 8%. at higher strain levels, the onset of the nanocomposite failure and a stick-slip behavior is observed. This study reveals that GO is superior to pure graphene as reinforcement in nanocomposites. These results also imply the potential to make a new generation of nanocomposites with exceptional high strength and toughness.;In the second part of this dissertation, AFM is used to study the structure of silk proteins and the morphology of spider silks. For the first time, shear-induced self-assembly of native silk fibroin is observed. The morphology of the Brown Recluse spider silk is investigated and a novel silk/GO nanocomposite is proposed.;Finally, the growth, capacitance and frequency response of vertically oriented graphene sheets prepared by radio frequency plasma-enhanced chemical vapor deposition and used in electric double layer capacitors (EDLC) are presented. These capacitors exhibit the highest frequency response observed, to date, for carbon based materials, providing EDLC suitable for AC filtering. The results also suggest mechanisms other than surface area are operative in the double layer charge storage, such as a stronger polarization from graphene edges and vacancies.

No description available.

In modern agriculture, nanotechnology has been at the forefront of agrochemical product innovation. For crop protection, researchers have turned to nano-zinc oxide (nano-ZnO) products that could potentially serve as an alternative to copper-based pesticides while mitigating micronutrient Zn deficiency. In this thesis, Zinkicide®, a nano-ZnO (4.5% Zn) based agriculture-grade product formulation has been investigated for their potential use as a broad-spectrum bactericide with systemic activity. Initial studies showed that Zinkicide® exhibits phytotoxicity to susceptible plants and experiences limited rainfastness. It is hypothesized that a suitable spray adjuvant will improve rainfastness and zinc absorption without compromising the antimicrobial efficacy. To test these hypotheses, the effect of three commercially available spray adjuvants – FitoFix®, Photon®, and AgriOil® – on Zinkicide®'s physico-chemical properties including wettability, zinc mobility and rainfastness were evaluated using citrus plants. Effects of adjuvant on Zinkicide® antimicrobial properties were also examined. Characterization results indicated that the composition of spray adjuvants has minimal effect on the deposition pattern (coffee ring effect) of Zinkicide® on glass and almost no effect on citrus leaf substrates. The wettability of Zinkicide® was slightly altered by the addition of adjuvants when tested on both substrates. FTIR data indicates that the adjuvants do not chemically interact with Zinkicide®. The effect of spray adjuvants on Zinkicide® antimicrobial properties were investigated using two model pathogens, Xanthamonas alfalfae and Pseudomonas syringae. The results suggest that the addition of adjuvants had no noticeable effect on the antimicrobial properties of Zinkicide®. The zinc in-planta mobility and rainfastness studies showed that the spray adjuvants have no observable effect on these properties. The above research findings could help advance Zinkicide® research in finding other potential adjuvant candidates in tank-mix settings.

<p> Polymer blending facilitates the combination of the attractive attributes of two or more polymers while compensating for the unfavorable ones. Most polymers are thermodynamically incompatible with one another, and their blending yields a two-phase microstructure. This morphology generally determines the mechanical and rheological properties of the blend system which then determine its applications. Morphology development typically involves deformation of the dispersed phase followed by drop breakup. However, drop coalescence competes with this process, and ultimately a balance must be reached between these two competing processes. Extensional flow fields are known to promote drop breakup and are especially important for blends with high viscosity ratios, that is for blends where the viscosity of the dispersed phase is at least about 3.8 times greater than that of the matrix phase. Coalescence may be attenuated by compatibilizers that modify the interface between the polymer phases. Nanoparticles with tuned surface chemistry may also be used as compatibilizers. A combination of extensional flow and nanoparticle stabilization should, therefore, result in a fine, stable morphology. </p><p> To begin the investigation toward the effects of extensional flow blending with and without the incorporation of nanoparticles, preliminary results were obtained using two different polymer blend systems: polycarbonate (PC)/styrene acrylonitrile (SAN) and polystyrene (PS)/linear low-density polyethylene (LLDPE). However, the majority of the presented results involve blends of high-density polyethylene (HDPE) dispersed in PS. With this blend system, with the material grades selected, the viscosity ratio exceeded 3.8 over the entire domain of deformation rates anticipated in the processing used. Coarse blends of various compositions were formulated using shear flow in an internal mixer or in a twin-screw extruder. These blends were subjected to extensional flow in converging dies of different geometries and where more than one stretching episode was possible; the temperature, total strain, and flow rate were varied, among other factors, in a systematic manner. Experiments were repeated in the presence of various grades of fumed nanosilica of different sizes and surface treatments, which imparted different surface tension and relative surface polarity (hydrophilic versus hydrophobic) for the nanoparticles. The mixing sequence was varied including premixing the nanosilica into the thermodynamically non-preferred polymer phase. </p><p> Scanning electron microscopy (SEM) was used to determine the size and size distribution of the dispersed polymer phase. The material was typically sectioned in the flow direction, but sectioning in the direction perpendicular to flow and etching, or selectively dissolving, one phase or the other was also investigated. The primary effect of extensional flow blending was to reduce the volume-average diameter of the dispersed polymer phase, especially with increasing strains and flow rates, or strain rates, which is directly dependent on both. Finding suitable conditions for the nanoparticles to selectively localize at the HDPE/PS interface was challenging, but relatively small amounts of nanoparticles dispersed in the PS matrix decreased the volume-average diameter of HDPE drops. When the nanosilica was preloaded into the HDPE dispersed phase, very coarse initial blends were produced which then exhibited dramatic decreases in phase size with extensional flow. These and other results are properly organized and presented.</p>