Chemistry & Biochemistry Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2752

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UNVEILING THE SELF-ASSEMBLY OF POLYMER-GRAFTED NANOPARTICLES IN SELECTIVE SOLVENTS
(2023) Lamar, Chelsey; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The self-assembly of inorganic nanoparticles (NPs) has garnered considerable attention due to the potential for fabricating functional structures with unique collective properties. In recent years, polymers have emerged as valuable candidates in assisting the organization of NPs into complex architectures with multiple capabilities. Researchers have shown that polymer-grafted nanoparticles (PGNPs) facilitate the use of advanced nanostructures with tailored properties in biomedical applications. Although, continued exploration of the rational design and tailoring of PGNP assemblies is needed to expand our understanding before we can fully realize the potential of these structures in desired applications. My dissertation aims to investigate the fundamental aspects and elucidate the underlying mechanisms in the self-assembly of PGNPs for modern biomedical applications. A facile and versatile solution-based strategy was utilized to explore the individual self-assembly of PGNPs with anisotropic NPs and the co-assembly of binary PGNPs with distinct sizes. We focused on designing, characterizing, and exploring the optical properties of hierarchical assembly structures produced from inorganic NPs tethered with amphiphilic block copolymers (BCPs). Individual PNGPs with anisotropic NPs and binary mixtures of small and large PGNPs produce vesicle structures with well-defined packing arrangements. My work shows how key parameters, including polymer chain length, nanoparticle size, and concentration, influence the self-assembly behavior and the formation of vesicles in each system. Through a combination of experimental observations and theoretical considerations, I highlight the significance of polymer shell shape in dictating the self-assembly behavior of individual anisotropic PGNPs. Moreover, I demonstrate that elevated temperatures impacted the stability and optical responses of the vesicle structures. In co-assembly studies, my work describes the macroscopic segregation of PGNPs with different sizes in the vesicular membrane, which is attributed to the conformation entropy gain of the grafted copolymer ligands. This research will provide valuable insights into the self-assembly behavior and fundamental design of PGNP structures relevant to biomedical applications.
Solution-Processed Clean SWCNTs and Their Use as Templates for One-Dimensional van der Waals Heterostructures
(2022) Zhang, Chiyu; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Single-walled carbon nanotubes (SWCNTs) have shown exceptional electrical, optical, mechanical, and thermal properties. Solution processing is a critical first step to harness these nanomaterials for applications in electronics, biomedicine, and energy technologies. However, dispersion of SWCNTs in solutions requires assistance by surfactants or polymers, which cannot be cleanly removed easily and become unwanted contaminants, resulting in degraded performance of SWCNTs.In this dissertation, I developed strategies to attain clean, solution-processed SWCNTs and further demonstrated their applications as templates for the synthesis of van der Waals heterostructures. We investigated the role of surfactants in dispersing SWCNTs and found that the highest efficiency in dispersing SWCNTs occurs at the critical micelle concentration of surfactants, which is well below the typically required surfactant concentrations. Furthermore, we synthesized a thermally removable surfactant, ammonium deoxycholate (ADC) which can be removed cleanly at a relatively low temperature without damaging the SWCNT structure. Compared to a commonly used surfactant, sodium deoxycholate (DOC), ADC features the same anion, but contains an ammonium (NH4+) cation in place of the metal ion (Na+). ADC exhibits the same high dispersion efficiency for SWCNTs as DOC, but the peak thermal decomposition temperature of ADC is nearly 70 oC lower than that of DOC. A two-step annealing process can remove this new nanotube surfactant while keeping the SWCNTs intact, even with a small diameter of just 0.76 nm. This work also reveals the chemical origin of residues from thermal annealing of surfactant-processed carbon nanomaterials. The clean SWCNTs enable the synthesis of van der Waals heterostructure consisting of pure chiral single-wall carbon nanotubes nested in boron nitride (SWCNT@BN). Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the successful synthesis of SWCNT@BN from the solution-purified nanotubes. The photoluminescence peak of (7,5)-SWCNT@BN heterostructure is found to redshift by 10 nm relative to that of (7,5)-SWCNT and the Raman G peak of (7,5)-SWCNTs downshift by 10 cm-1 after BN coating.
Fluorescent Carbon Nanotubes as Molecular Sensors and Color-Center Hosts
(2022) Qu, Haoran; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This thesis demonstrates the application of single-walled carbon nanotubes (SWCNTs) as single-digit nanopores for molecular sieving and addresses a fundamental challenge pertaining to controlled synthesis of organic color-centers (OCCs) on the sp2 carbon lattice of SWCNTs. First, I describe a hyperspectral single-defect photoluminescence imager system that provides both hyperspectral imaging and super-resolution capabilities in the shortwave infrared. Second, I aim to understand the relationship between nanotube photoluminescence and encapsulated molecules. Using carbon nanotubes with sub-1 nm pores, I demonstrate molecular sieving of n-hexane from cyclohexane, which are nearly identical in size. Furthermore, I discovered a light irradiation method to drive structural transformation of OCCs which allow us to narrow the spectral distribution of defect emissions by 26%. Finally, I show that [2+2] cycloaddition can efficiently create OCCs. Remarkably, this novel defect chemistry reduces the number of OCC bonding configurations from six, which are commonly observed with monovalent defect chemistries, to just three. This work may have broad implications to the potential applications of SWCNTs and OCCs in chemical sensing, bioimaging, and quantum information science.
Hybrid plasmonic tubular nanostructures: synthesis, optical and photoelectrocatalytic property
(2022) Zhang, Qian; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Utilizing sustainable and clean solar energy in the visible-NIR range to drive chemical transformation has been a key resolution to solve the energy crisis. Recently, rational design of photocatalysts that conjugate plasmonic nanostructures with catalytic metal nanostructures has attracted particular research interests. This dissertation describes the design principles of plasmonic-catalytic hybrid tubular nanostructures and investigates their application in photoelectrocatalytic reactions.First, we introduced the mechanism of plasmon-mediated catalysis and current research of plasmonic photocatalysts. Specifically, managing the energy flow from the plasmonic entity to the catalytic entity is the crucial part for customizable design of photoelectrocatalysts. We also summarized the design principles of general electrocatalysts and synthesis of hollow nanostructures. Second, we reported a facile but highly reproducible synthetic strategy to fabricate catalytic Pt hollow nanotubes (NTs). Scalable fabrication of ultrathin Pt NTs can be achieved through simple mediation of the concentration of the surfactant employed in the reaction. Third, we reported a template-directed synthesis of PtAu NTs with tunable localized surface plasmon resonance (LSPR) bands in the visible-near infrared region (NIR). The geometric dependent LSPR band shift was systematically studied based off the spectra from both experiment and finite-difference time-domain (FDTD) simulations. It was found that the PtAu NTs exhibited the LSPR characters of both rodlike and hollow nanostructures. Finally, we investigated the photoelectrocatalytic performance of the PtAu NTs under visible-NIR light irradiation. The optimized photocatalytic activity of the PtAu NTs towards the electrooxidation of methanol was achieved by maximizing their LSPR absorption cross sections at longer wavelengths in the visible-NIR region. Meanwhile, combining the superior intrinsic electrocatalytic performance of PtAu NTs towards methanol oxidation, we expected the bimetallic PtAu tubular nanostructure could effectively convert visible light energy to drive the electrochemical transformation.
SYNTHESIS AND POLYMER-MEDIATED REGIOSELECTIVE SELF-ASSEMBLY OF SHAPED PLASMONIC NANOPARTICLES
(2021) Lin, Xiaoying; Fourkas, John T; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Plasmonic nanoparticles with collective excitations of the conduction-band electrons have many potential applications in surfaceenhanced spectroscopies, photocatalysis, photovoltaics, and biomedicine. The plasmonic properties are highly tunable via the size, shape, chemical composition, and surrounding media of individual nanoparticles, as well as by the interactions with other nanoparticles in close proximity. Fabricating plasmonic nanostructures with precisely controlled size, shape, and interparticle distance is critical for harnessing desirable properties. Although top-down lithographic techniques are widely used for fabricating shaped plasmonic nanoparticles and ensembles, the products are limited to 2D planar structures and the cost can be prohibitive. As low-cost and versatile alternatives, colloidal synthesis and self-assembly have been explored to fabricate complex plasmonic nanostructures.In this dissertation, wet-chemistry synthetic and self-assembly approaches are explored for fabricating well-defined plasmonic nanostructures with high structural complexity. Chapter 2 describes a facile synthetic method for circular and triangular gold nanorings with tunable diameters, ring thicknesses, surface roughness, and hence the plasmonic response. The gold nanorings with rough surface show 100-fold higher enhancement factor than solid gold nanoparticles as substrate for surface-enhanced Raman spectroscopy. In chapter 3, we demonstrate regioselective bonding between nanospheres and nanoplates originating from the steric hindrance of polymeric ligand brushes and the anisotropy of nanoparticles. The regioselectivity enables a self-assembly system with precise control over the relative orientation of Au nanospheres on Ag nanoplates and the stoichiometry of reactive groups of copolymeric ligands dictates the number of nanoparticles in one nanocluster. The yield of each assembly was ~70% without further purification. Optical study reveals that different bonding modes affect the plasmonic coupling of assembled structures. In chapter 4, the regioselective bonding is applied to fabricate complex plasmonic nanocluster, nanoflowers and nanobuds, with distinct bonding modes. Compared with nanobuds, nanoflowers with the same number of petals show stronger electric field enhancement and further localized surface plasmon resonance peak shifts. The synthetic and self-assembly methods demonstrated in this dissertation have great potentials and versatility in designing not only plasmonic nanoclusters, but also other inorganic nanoparticle-based functional structures with high complexity.
Functionalized 3D DNA Crystals through Core-Shell and Layer-by-Layer Assembly
(2019) McNeil, Ronald; Paukstelis, Paul; Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
A fundamental goal of DNA nanotechnology has been assembly of DNA crystals for use as molecular scaffolds to organize arrays of guest molecules. We use previously described 3D DNA crystals to demonstrate core-shell and layer-by-layer assembly of DNA crystals capable of accommodating tethered guest molecules within the crystals’ pervasive solvent channel network. We describe the first example of epitaxial biomacromolecular core-shell crystallization through assembly of the crystals in two or more discrete layers. The solvent channels also allow post-crystallization guest conjugation with layer-specific addressability. We present microfluidics techniques for core-shell crystal growth which unlock greater potential for finely tunable layer properties and assembling complex multifunctional crystals. We demonstrate assembly of these DNA crystals as nanoscale objects much smaller than previously observed. These techniques present new avenues for using DNA to create multifunctional micro- and nanoscale periodic biomaterials with tunable chemical and physical properties.
Immobilized Seed-mediated Growth of Two Dimensional Arrays of Shaped Metallic Nanocrystals
(2017) Perez Cardenas, Maria Teresa; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Acknowledging that the optical properties of noble metal nanocrystals (NMNCs) are largely determined by their size, composition, and shape, the demand for NMNCs with controlled shapes is expected to increase. To expand the property discovery and application development of polyhedral NMNCs, it is pivotal to understand the key factors involve in the nucleation and growth processes of NMNCs for better control over the crystal facets. Furthermore, to implement polyhedral NMNCs into functional devices for applications in such as chemical sensors, photovoltaics, and catalysis, it is essential to design cost-effective methods to assemble NMNCs into two-dimensional arrays with controlled orientation and particle distance. This dissertation describes the stability and interaction of molecular species formed during the reduction of gold metal precursor, as well as factors that influence the formation of nanocrystals with different shapes. Our study suggests that during the Au reduction step, an intermediate complex is formed. Over time the complex degrades decreasing the concentration of gold ions and subsequently slowing down or inhibiting the nucleation; thereby, affecting the reproducibility of synthetic methods. My findings will provide guidance for the development of more simple, reliable methods to control the shapes of the nanocrystals. Additionally, I developed an immobilized seed-mediated growth strategy for the fabrication of two-dimensional arrays of mono- and bi-metallic polyhedral nanocrystals with well-defined shapes and orientations on a substrate. This method relies on the controlled solution-phase deposition of gold and palladium metals on a selectively exposed surface of self-assembled seed nanoparticles that are immobilized on a substrate through collapsed polymer brushes. The synthetic approach I developed presents an important addition to current tools for the fabrication of substrate-supported functional nanocrystals as new materials and devices.
RESOLUTION IMPROVEMENT OF PHOTOLITHOGRAPHIC TECHNIQUES BASED ON VISIBLE LIGHT
(2017) Tomova, Zuleykhan; Fourkas, John T; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The semiconductor industry is planning to use Extreme Ultraviolet lithography as its next-generation patterning technique. However, this technique has run into many roadblocks due to its cost and complexity. An alternative approach employs light in the near-UV. A 2-color photolithographic technique based on combination of two colors on the near-UV or visible light has shown promising results in creating structures with sizes at a fraction of the excitation light wavelength. One color of light excites photoinitiator molecules to a chemically active state that leads initiation of polymerization. A second color of light deactivates photoinitiator molecules before they form radicals, inhibiting polymerization. In this thesis we show how extending 2-color lithography to include a third color (3CL) can achieve super-resolution for applications requiring fabrication of closely packed structures. The advantage of the 3CL process is in its separation of polymerization initiation and deactivation steps by involving different chemical states that allow for more efficient deactivation and for increased resolution. Some of the crucial elements needed to achieve an optimized scheme for 3CL are the determination of the intramolecular transitions that participate in the process, the lifetimes of the photoinitiators, and the exposure parameters. Several photoinitiators were studied to determine the optimal exposure conditions. Polymerization action spectra and deactivation action spectra were used to determine the combinations of excitation and deactivation parameters resulting in the most efficient deactivation. The 2-beam initiation threshold (2-BIT) method was introduced for in situ measurement of the order of eective nonlinearity of photoresists. The order of the effective nonlinearity was determined for a series of photoinitiators under various excitation wavelengths and fabrication velocities. Additionally, a photoinitiator with a proportional velocity (PROVE) dependence, in which feature size increases with the velocity, was found to undergo efficient self-deactivation at increased temperatures. This dependence was demonstrated by gradually heating the sample and analyzing the fabricated feature sizes. Spot heating with a laser beam was also used to locally prevent polymerization. The correlation between polymerization rate and temperature opens opportunities for high speed fabrication that uses temperature gradients to create finer structures.
In Situ Characterization of Optical Absorption by Carbonaceous Aerosols: Calibration and Measurement
(2016) You, Rian; Zachariah, Michael R.; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Light absorption by aerosols has a great impact on climate change. A Photoacoustic spectrometer (PA) coupled with aerosol-based classification techniques represents an in situ method that can quantify the light absorption by aerosols in a real time, yet significant differences have been reported using this method versus filter based methods or the so-called difference method based upon light extinction and light scattering measurements. This dissertation focuses on developing calibration techniques for instruments used in measuring the light absorption cross section, including both particle diameter measurements by the differential mobility analyzer (DMA) and light absorption measurements by PA. Appropriate reference materials were explored for the calibration/validation of both measurements. The light absorption of carbonaceous aerosols was also investigated to provide fundamental understanding to the absorption mechanism. The first topic of interest in this dissertation is the development of calibration nanoparticles. In this study, bionanoparticles were confirmed to be a promising reference material for particle diameter as well as ion-mobility. Experimentally, bionanoparticles demonstrated outstanding homogeneity in mobility compared to currently used calibration particles. A numerical method was developed to calculate the true distribution and to explain the broadening of measured distribution. The high stability of bionanoparticles was also confirmed. For PA measurement, three aerosol with spherical or near spherical shapes were investigated as possible candidates for a reference standard: C60, copper and silver. Comparisons were made between experimental photoacoustic absorption data with Mie theory calculations. This resulted in the identification of C60 particles with a mobility diameter of 150 nm to 400 nm as an absorbing standard at wavelengths of 405 nm and 660 nm. Copper particles with a mobility diameter of 80 nm to 300 nm are also shown to be a promising reference candidate at wavelength of 405 nm. The second topic of this dissertation focuses on the investigation of light absorption by carbonaceous particles using PA. Optical absorption spectra of size and mass selected laboratory generated aerosols consisting of black carbon (BC), BC with non-absorbing coating (ammonium sulfate and sodium chloride) and BC with a weakly absorbing coating (brown carbon derived from humic acid) were measured across the visible to near-IR (500 nm to 840 nm). The manner in which BC mixed with each coating material was investigated. The absorption enhancement of BC was determined to be wavelength dependent. Optical absorption spectra were also taken for size and mass selected smoldering smoke produced from six types of commonly seen wood in a laboratory scale apparatus.
Ionics and Electrochemical Reactions in 1D and 3D Crosslinked Porous Electrodes
(2015) Gillette, Eleanor I.; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Lithium ion batteries are a critical component enabling many modern technologies, including portable electronics, hybrid electric vehicles and more. While interest in nanomaterials for lithium ion batteries has been growing in recent years, very few systematic studies have been carried out on controlled architectures to explore of the impact of nanoscale and mesoscale structure on the reaction mechanisms, kinetics and resulting rate performance in these electrodes. Here we utilize a combination of anodized aluminum oxide templates and atomic layer deposition to fabricate a variety of systematically variable electrode architectures. The structural control and electrode design are described in detail. Then, analysis of the rate performance, with a focus on distinguishing between diffusion and charge transfer limited reaction mechanisms, is carried out for two distinct electrode systems, focusing on different issues which face advanced electrode architectures. First, we analyze the impact of nanotube length in 1D structures to establish a quantitative understanding of the balance between the loss of capacity due to resistance increases and improvements due to surface area increases. Second, we analyze the impact of transitioning from arrays of 1D nanostructures to crosslinked electrode networks. While 1D alignment is often considered favorable for reducing defects that may lead to capacity loss and degradation, our results indicate that the 3D structures gain more from increased surface area and mass loading than they lose from the introduction of defects. This observation opens up opportunities for rationally designed advanced electrode architectures to optimize the performance of electrochemical energy storage devices in novel ways that are unavailable to conventional, particle based electrode configurations.

Chemistry & Biochemistry Theses and Dissertations

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