Because of the high sensitivity, molecular specificity, and non-invasive sampling technique, surface enhanced Raman scattering (SERS) is one of the most powerful analytical techniques. Additionally, its low interference from water makes SERS uniquely effective for biological sensing. In the last decade, the emergence of life-threatening pathogens and debilitating diseases has outpaced the development of treatments. Detection and monitoring have become the first guard approach to containing the spread of deadly infections such as Ebola, H1N1, and SARS-CoV-2. As a result, recent SERS research has heavily focused on studying biological processes and point of care diagnostics.The choice of the nanostructure of the SERS substrate determines its localized surface plasmon resonance which directly contributes to the overall SERS enhancement. Hollow nanostructures are advantages as SERS substrates due to their highly tunable optical properties, which arise from varying the outer diameter to shell thickness. However, hollow nanoparticles have been underexplored due to a more complex synthesis needed to control their physical dimensions. The work in this dissertation explores using hollow gold nanospheres (HGNs) as a template to synthesize rugose hollow nanostructures. HGNs enabled fine control over the hollow (void space) core. However, isotropic geometry yields a single plasmon mode limiting the number of hot spots for SERS enhancement. Rugose nanostructures have improved electric field strength as compared to smooth nanoparticles. More specifically, spiny nanostructures have better electron confinement at their tips. A multibranched structure has an increased number of hot spots. Multibranched hollow gold nanostructures were synthesized using hollow gold nanospheres as a seed particle. Chapter 1 introduces general concepts of plasmonics applied to Raman spectroscopy, specifically how the nanostructure of noble metals influences the localized surface plasmon resonance. The latter part of chapter 1 discusses the characterization of metal nanoparticles through absorption spectroscopy and electron microscopy. Chapter 2 highlights the use of hollow gold nanostars and PEG-based conjugation to overcome the difficulty of detecting amyloid beta 1-14. As a result, we increased our understanding of the parameters needed to detect low Raman scatterers in aqueous media. The key findings were that hollow gold nanostars enable improved SERS enhancement compared to hollow gold nanospheres and solid gold nanostars. The analytical enhancement factor of the hollow gold nanostars was 1.29 x 109 for the 1367 cm-1 peak in R6G was shown to be 11.5 times increased compared to hollow gold nanospheres. Chapter 3 focuses on developing a new method to detect the SARS-CoV-2 S1 spike protein and the anti-SARS-CoV-2 antibodies by anchoring the his-tagged spike protein to a multibranched gold nanoshell for detection via SERS. Multibranched gold nanoshells were prepared using HAuCl4, AgNO3, and ascorbic acid to improve the optical and morphological properties of smooth ~150 nm gold nanoshells for detecting proteins. The multibranched gold nanoshells were gently mixed with a his-tagged SARS-CoV-2 S1 spike protein, purified, and mixed with Anti-SARS-CoV-2 monoclonal antibodies. Raman and SERS measurements of the spike protein and monoclonal antibody with a histidine tag showed a significant increase in peak intensity compared to samples without histidine. Enhanced SERS peaks from the spike protein at 1079, 1178, and 1592 cm-1 are attributed to δ =N-H, νC-N, νC=C, respectively. Enhanced peaks from the Anti-SARS-CoV-2 monoclonal antibody at 861, 1205, 1454, and 1630 cm-1 can be assigned to νN-H, νC-C, C-H def, and νC=O associated with the phenylalanine and tyrosine rich region closest to the binding sites on mAb. Chapter 4 discusses the work to apply fiber-enhanced SERS to monitor the repair process of a self-healing wastewater purification membrane. The use of a SERS substrate with hollow core photonic crystal fiber (HCPCF) as a waveguide increased the signal intensity for major distinguishing hydrocarbon tail peaks at 1436cm-1 (CH2 bend) and 1296cm-1 (CH2 twist) by 1.5 and 2x as compared to SERS alone. The increase in signal for lower intensity C-C peaks such as 1157cm-1 was 14x compared to SERS alone. The results show that SERS coupled with a waveguide can enable highly sensitive in situ measurements.

Two-photon photoluminescence (2PPL) together with hollow gold nanosphere’s (HGN's) photothermal properties are utilized as a single theranostic agent for cancer. HGNs bioconjugated with folic acid-PEG-thiol (HGN-FA) selectively bind to the overexpressed folate receptor of cervical cancer HeLa cells and the 2PPL from HGN-FA captures high-resolution cancer cells images using multiphoton microscopy. Subsequent power increase and laser scanning dwell time results in highly efficient photothermal destruction of cancer cells. Using femtosecond laser pulses, microseconds of laser exposure generates well-localized superheating of HGNs, yielding subcellular thermal damage and cell death.

The following chapters transition to the synthesis and characterization of colloidal and solid film perovskite quantum dots (PQDs). Important hindrances for commercialization of PQD solid film in optoelectronic devices are insufficient charge transport and long-term stability. This is mostly contributed to their lack of charge transfer in PQD solid films and degradation in the presence of moisture, oxygen, heat, and light. These hindrances can be overcome by exploring different surface ligand passivation strategies and understanding their functionality associated with PQD solid film's optical, electronic, and electrical properties. Enhancing the charge transport properties of PQDs is dependent on mitigating the tunneling barrier which is imposed by the distance charges must travel between PQDs. Using long alkyl chain passivating ligands instills greater stability, however, a diminishment in charge transport. Moreover, short alkyl chain or counter ion passivation leads to instability but improved charge transport. To overcome these inversely related properties, an effective strategy to mitigate the charge tunneling barrier and simultaneously provide stability is passivating with exciton-delocalization ligands (EDLs). EDLs consist of π delocalizing orbitals that hybridize with the electronic wave function of PQDs. This enables charges to delocalize to the PQD’s surface and eliminates the tunneling barrier imposed by surface ligands. The following chapters discuss the dependence of charge transport and stability of PQD solid film on the capping ligand chain length, and then, the investigations of using EDLs for enhancing the optical and charge transport properties of PQD solid film.

The final chapter discusses the synthesis of perovskite magic sized clusters (PMSCs). Specifically, the ligand dependence of their growth and optical properties. Here, PMSCs are synthesized using an excess amount of capping ligands. The two different ligand combinations used to synthesized PMSCs are butylamine-valeric acid (BTYA-VA) and 3,3-diphenylpropylamine-valeric acid (DPPA-VA). These PMSCs were observed to have the bluest shifted absorption and PL peaks that correspond to their smallest stable size. For BTYA-VA PMSCs, the excitonic absorption peak is at 424 nm and PL peak at 434 nm, while the DPPA-VA PMSCs exhibited an absorption peak at 428 nm and PL peak at 451 nm. Due to the shift in absorption, the size of PMSCs is determined to be ligand dependent. In addition, it was discovered that different size PQDs can be synthesized from DPPA-VA PMSCs that correspond to a specific dilution. This method to precisely tune size, and hence, optical properties simply by dilution may provide a more exact and facile synthesis of PQDs.

Doped semiconductor quantum dots (QDs) comprise an important subclass of nanomaterials in which a small quantity of impurity is added intentionally, adding another degree of freedom to alter their size-dependent physical and electronic properties. Intense, tunable, long lived and stable photoluminescence make them quintessential candidates for many opto-electronic applications including solid-state lighting, display devices and biomedical imaging. ZnSe QDs, which are blue-emitting fluorophores, were doped with Cu⁺¹ to redshift their photoluminescence (PL) to green region of the visible spectrum. These Cu-doped ZnSe QDs were then codoped with Al³⁺, Ga³⁺ and In³⁺ to improve the PL quantum yield (QY) by eliminating the defect states originating from charge imbalance created by aliovalent doping. Codoping also resulted in further redshifting of the PL, covering most of the visible spectrum, making them potential candidates for use in solid-state lighting and as optical down converters in next generation light emitting diodes (LEDs). To better understand the optical properties of these materials, local structure around the luminescent centers was investigated by extended X-ray absorption fine structure (EXAFS). Cu was found to occupy a distorted tetrahedral site with the codopant residing in a substitutional Zn site. Based on the structural information obtained by EXAFS, density functional theory calculations (DFT) were performed to get a clear picture of the energy levels associated with the electronic transitions. Furthermore, the dynamics studies of the exciton and charge carriers were carried out to get deeper insight of the various photophysical processes involved. The fluorescence lifetime was increased approximately 10 times after doping.

The multielectron catalytic reactions often involve multimetallic clusters, where the reaction is controlled by the electronic and spin coupling between metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. A method was developed to detect X-ray emission signal from multiple elements simultaneously to probe the electronic structure and sequential chemistry that occurs between the elements. A wavelength dispersive spectrometer based on the von-Hamos geometry was used, that disperses K&beta emission signals of multiple elements onto an area detector, and enables an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the Rowland circle spectrometers, and the data is free from temporal and normalization errors, and therefore ideal to follow sequential chemistry at multiple sites. This method was applied to MnOx based electrocatalysts, and the effect of Ni addition was investigated. Electro-deposited Mn oxide catalyses oxygen-evolution reaction (OER) and oxygen-reduction reaction (ORR) at different electrochemical potentials under alkaline condition. Incorporation of Ni reduced the low valent Mn component resulting in higher average oxidation state of Mn in MnNiOx under ORR and OER conditions, when compared to MnOx under similar conditions. The reversibility of the electrocatalyst was also found to improve by the inclusion of Ni.

Abstract

Staci A. Adams

Magnetic and Plasmonic Properties of Metal and Metal-Oxide Nanoparticles and Their Applications

Nanomaterials are designed and synthesized for a wide range of applications including clinical diagnostics, therapeutics, the targeting of bioterrorism agents, wastewater treatment, energy, environmental remediation and sensing. In order to enhance performance in these fields researchers often work with materials that are either magnetic, plasmonic or a combination of both. Nanomaterials can be customized through synthetic variation in size, shape, aspect ratio, the dielectric constant of the surrounding media, surface morphology and whether particles are aggregated. Chapter 1 serves as an introduction to hollow gold nanospheres (HGNs) including their unique plasmonic properties and how these properties can be refined and harnessed for emerging applications. HGNs have hollow solvent filled dielectric cores and polycrystalline gold shells that, due to the two surfaces or interfaces, can generate an enhanced electromagnetic (EM) field. They possess a unique combination of properties that include small size (20-125 nm), large surface to volume (S/V) ratios, spherical shape, narrow and tunable SPR (~520-1000 nm) and biocompatibility. Their surfaces can also be easily functionalized to target and deliver biomolecules and are resistant to photobleaching. Additionally, their scattering and absorption cross-sections can be tailored, making them excellent candidates for a variety of applications including surface enhanced Raman scattering (SERS), sensing, imaging, drug delivery, site specific silencing and photothermal therapies (PTTs). Chapter 2 describes the detailed synthetic mechanism for creating highly reproducible near infrared (NIR) absorbing HGNs with an emphasis on the cobalt seed particle growth step of the synthesis. Several studies describing HGNs and their applications are found in chapter 3. The first study investigates the interaction of HGNs with Cu2+, commonly found in vivo, and the role that these ions play in aggregation, since aggregation can strongly influence the optical and photothermal properties of HGNs. The second study utilizes HGNs to visualize the Primo Vascular System (PVS) in a rat model. The use of hollow gold−silica double-shell (HGSDS) and hollow gold-silica composite (HGSC) nanostructures as surface enhanced Raman scattering (SERS) substrates for the detection of glucose is detailed in the third study found in chapter 3. Chapter 4 describes the synthesis of large Fe3O4@SiO2 nanoparticles (~200 nm) functionalized with gold and poly(vinylpyrrolidone) synthesized for bio-separation and SERS sensing applications. These particles have a unique surface morphology comprised of roughened gold nodules. The surface coatings prevent oxidation and render the particles easy to functionalize in order to target a wide range of moieties. The gold coverage is not only uniform across the entire particle surface but also ultra-thin so as to maintain a high percentage of the cores magnetic saturation (~68%) when compared to bare magnetite. The gold nodules facilitate the generation of hot spots that enhance the EM associated with the particle surface and are useful in sensing applications like SERS spectroscopy whereas the strong magnetic core allows for rapid separation (~30 s) of target molecules from solution once they are bound to the particles. Finally, in Chapter 5 the effect of polymer and gold functionalization on the magnetic properties of Fe3O4 nanoparticles is examined by superconducting quantum interference device (SQUID) and electron paramagnetic resonance (EPR) along with thoughts on future experimentation that can help to create a more complete story with respect to this unique nanocomposite material.

Nanomaterials including metals, metal oxide, and semiconductors have been studied as powerful tools for various applications including photovoltaic, photocatalyst, biomedical therapeutics, sensing, energy storage, wastewater treatment and so many more. Based on the specific application, these nanomaterials can be tuned and optimized to achieve their best performance. These modifications can be in the form of altering size, shape, crystal structure, surface passivation, surface morphology, and doping. In this dissertation, the effect of size and surface modification on optical, photophysical and photocatalytic properties of perovskite nanocrystals, CdS based nanocomposites, and hollow gold nanoparticles are investigated.

In chapter 1, a comprehensive literature review was done on photophysical properties and improved stability of organic-inorganic perovskite nanomaterials by surface passivation. In this chapter, it is discussed how by changing the capping ligand and surface passivation strategies, the size, shape and crystal structure of these nanomaterials were controlled to produce high quality nanostructured and bulk film perovskites. The degradation mechanism and surface passivation approach to address the instability issue toward environmental factors were also highlighted. This information emphasizes the importance of defect passivation and surface modification in achieving high performance in photovoltaic applications.

In chapter 2, a new surface passivation strategy was developed using peptide molecules with amine and carboxylic functional groups to synthesize methylammonium lead bromide (CH3NH3PbBr3) perovskite nanocrystals (PNCs) with excellent optical properties. It was demonstrated in this work that well-passivated PNCs can only be obtained when both amino and carboxylic groups were involved, and this is attributed to the protonation reaction between –NH2 and –COOH. In addition to their improved optical properties, using peptide as capping ligand resulted in increasing the product yield up to ~44%. This is due to the polar nature of peptides, which cause aggregation and precipitation of peptide-passivated PNCs from nonpolar toluene solvent. PNCs size was also controlled by adjusting the concentration of the peptide, resulting in tunable optical properties due to the quantum confinement effect. Furthermore, generality and versatility of this strategy were shown by passivating different types of PNCs such as CsPbBr3.

In chapter 3, three differently sized (3.1, 5.7, and 9.3 nm) methylammonium lead bromide (CH3NH3PbBr3) perovskite nanocrystals (PNCs) were synthesized using (3-Aminopropyl) triethoxysilane (APTES) and oleic acid (OA) as capping ligands. The size dependence of charge carrier dynamics was studied to decipher the radiative and non-radiative components. These PNCs showed size-dependent absorption and photoluminescence (PL), with the middle-sized PNCs exhibiting the highest PL quantum yield (~91%). The effect of size on the exciton/charge carrier dynamic of PNCs was studied using transient absorption spectroscopy (TA) and time-resolved photoluminescence (TRPL). The middle-sized PNCs (PNC35_APTES) showed slower early time recombination compared to that of the larger and smaller PNCs, suggesting optimized passivation of surface trap states. However, the radiative lifetime was found to decrease with decreasing PNC size, which seems to be primarily determined by the PNC core, while the non-radiative lifetime is longest for the middle-sized PNCs, which is strongly influenced by the presence of bandgap states that depend on surface passivation. A kinetic model is proposed to explain the observed dynamics results, including size dependence. This study demonstrates the competing effect between size and surface properties in determining the dynamics and optical properties of PNCs.

In chapter 4, the same three differently sized methylammonium lead bromide (CH3NH3PbBr3) perovskite nanocrystals (PNCs) were studied using UV-Vis, photoluminescence (PL), temperature dependent PL, temperature dependent time-resolved PL (TRPL), cryo-XRD, and cryo-EM to investigate crystal phase stability of the PNCs as a function of size at different temperatures ranging from 20 K to 300 K. The preliminary results showed a spectral blue shift of the PL peak by decreasing the temperature, which is the opposite of what was observed for large and middle-sized particles. The lifetime of the smaller PNCs also decreased by increasing the temperature. These results can be possibly due to crystal phase transition in the small particles. Future work is suggested to further investigate the phase transition possibility using cryo-XRD at slow scanning rate and temperature dependent Raman spectroscopy.

In chapter 5, different compositions of CdS nanowires with MoS2, NiS, NiS2, Ni3S2, and NiCo2S4 nanoparticles composites were synthesized and the photocatalytic activities of them in hydrogen evolution reaction (HER) was investigated. These hierarchical structures provide high activation potentials for HER and suppress the photo-corrosion of CdS. The results indicated there is an optimal amount of nanoparticles decorated on the CdS surface for optimized performance. This optimum nanoparticle ratio will provide uniform coverage of the CdS surface resulting in efficient charge transfer. While the higher ratios form aggregate structures, which act as carrier trap states. To gain deeper insight into the mechanism behind the enhanced performance, ultrafast transient absorption (TA) techniques were used to probe the charge carrier dynamics of these CdS-based heterostructures. At the for each nanocomposite, mechanism of charge transfer between components was proposed.

In chapter 6, the effect of targeting on photothermal therapy (PTT) efficiency of hollow gold nanoparticles (HGNs) was investigated. HGNs are class of plasmonic nanomaterials which showed great potential for biomedical applications and specifically PTT. These HGNs can be easily modified on the surface and be conjugated to various targeting ligands including antibody, peptide and other small molecules such as folate. In this chapter, HGNs with an average outer diameter of 40 nm were synthesized and conjugated to anti_EGFR antibody and GE-11 peptide, which both target EGF receptors on cancer cells. The conjugated HGNs were characterized using UV-Vis, photoluminescence (PL), Inductively coupled plasma-optical emission spectrometry (ICP_OES), bicinchoninic acid assay (BCA), fluorescence microscopy, and confocal microscopy. The preliminary results showed significantly higher cell death with peptide conjugated HGNs (91%) compared to antibody-conjugated HGNs (54%). Different hypotheses were discussed as a possible explanation for this enhanced PTT efficiency including possible particle internalization, better binding to receptors due to small ligand size, and most importantly closer proximity of HGNs to the cell surface resulting in more efficient heat transfer.

On the nanoscale, size and structure are powerful dictators of optical and electronic response. As such, they may be rationally manipulated in order to obtain desired properties for specific applications. The dissertation projects herein are aimed toward understanding how structure affects the photophysical properties of three nanoparticle systems: hollow gold nanospheres (HGNs), doped α-Fe2O3 nanostructures, and PbS/CdS core/shell quantum dots (QDs). The dissertation is divided into two parts: Part I: The Highly Tunable Hollow Gold Nanosphere: Synthesis, Size, and Surface Morphology and Part II: Ultrafast Charge Carrier Dynamics of Hematite Nanostructures and PbS/CdS Quantum Dots

Part I focuses on synthetic control and characterization of HGNs, solvent-filled plasmonic shells of gold ranging from 20-200 nm in diameter. HGNs have shown promising performance in drug loading, targeted delivery, surface-enhanced Raman scattering, and photothermal therapy. Their optical properties are very sensitive to their aspect ratio, the ratio of diameter to shell thickness. As such, a well-controlled synthesis is highly desired. In Chapter 1, the HGN synthesis was updated to enable simultaneous control of both diameter and SPR while maintaining monodispersity and uniformity of the resultant shells. This was possible through a detailed and systematic investigation of the synthesis of the sacrificial cobalt-based scaffolds onto which HGNs are formed through galvanic exchange.

In Chapter 2, additional synthetic adjustments were introduced to systematically control the HGN surface morphology from smooth to very bumpy. Rugose structures are perhaps the most versatile nanoparticles, with an increased density of active sites for catalysis as well as local electric field enhancement around the surface features for sensing and detection. As hollow particles have displayed enhanced plasmonic performance in comparison with their solid counterparts for a number of applications, the combination of hollow cores and rugose surfaces is highly attractive. One of the most attractive applications of HGNs is plasmonic photothermal therapy (PTT). In this application, plasmonic nanoparticles are targeted to cancer cells where they convert incident light to heat, raising the temperature of their environment above the point of cell viability. A systematic comparison of HGNs with different surface morphologies revealed that bumpy HGNs retain the excellent photothermal conversion efficiency (PCE) of their smooth counterparts. Next, in Chapter 3, PCE was investigated for HGNs of different diameters, theoretically and experimentally. The findings revealed that 50 nm HGNs generate ~2 times the heat per µg gold as their 70 nm counterparts and ~1.5 times the heat per µg gold as their 30 nm counterparts. In vitro HGN-mediated PTT of oral squamous cell carcinoma was also carried out. Ongoing efforts are needed to assess the PCE and potential size dependence of HGNs in vitro and in vivo.

In Part II, transient absorption spectroscopy (TAS) is used to probe the charge carrier dynamics in α-Fe2O3 (hematite) and PbS/CdS nanostructures. In both systems, structural modification has been crucial for obtaining enhanced photophysical performance. Hematite is considered to be an especially promising material for solar water splitting due to its chemical stability, abundance, and non-toxicity. Additionally, its band gap of approximately 2.0–2.2 eV facilitates the absorption of about 40% of incident solar light. However, it suffers from a number of limitations which hinder its practical implementation as a photoanode material. Chapter 4 discusses these limitations and structural approaches to overcome them, including nanostructuring and doping to facilitate charge transfer and improve PEC performance. Charge carrier recombination dynamics are investigated before and after doping to aid understanding of the mechanism of performance enhancement in Zr and Ti-doped hematite films. Although nanostructuring and doping are beneficial, performance gains have been modest and a review of the current approaches for the rational design of hematite heterostructures is provided in Chapter 5. In these reports, TAS has been employed as a useful tool to gain deeper insight into the mechanisms of photogenerated electron-hole recombination and their relation to PEC performance.

Finally, in Chapter 6, the passivation of PbS QDs with a CdS shell, and the dependence of the passivation on particle size, is investigated. Semiconductor QDs like PbS are highly attractive components in solar cells, sensing, and detection due to their size-dependent optical and electronic properties. Broad absorption, narrow photoluminescence (PL), and high PL quantum yield arise from quantum confinement. The formation of a thin shell atop the QD core has become a common method for passivating the surface bonds and thereby enhancing and stabilizing resultant PL. In this work, it was found that surface passivation of PbS with a CdS shell is core-size-dependent; ultrasmall PbS QDs did not benefit from CdS passivation like their larger counterparts, but instead experienced a decrease in PL. Coupling TAS with steady state PL and QY measurements enabled a comprehensive understanding of the radiative and nonradiative relaxation pathways of the PbS/CdS nanosystem. Insights into the size-dependent variation in trap state density and relaxation pathways may serve as a guide for future structural modification.

Nanomaterials composed of metals and semiconductors have been studied as powerful tools for various applications in the fields of diagnostics, therapy, sensing, and photovoltaics. Based on the specific application, these nanomaterials can be tuned and optimized to achieve their best performance. Modifications can be made to alter the size, shape, surface morphology, and surface passivation of these nanoparticles. In this dissertation, the effect of these changes on the optical characteristics necessary for effective performance of hollow plasmonic nanoparticles and perovskite nanocrystals are investigated. In chapter 1, a comprehensive literature review was completed on hollow plasmonic nanoparticles, their synthetic procedures, and their applicability to the biomedical field. This chapter, describes the physical properties of hollow plasmonic structures, such as hollow gold nanospheres (HGNs), by discussing SPR and Mie theory. Numerous synthetic methods leading to hollow structures, including galvanic exchange, and ways to effectively tune size, shell thickness, and surface properties were described. Finally, hollow plasmonic particles’ ability to sense, image, diagnose, and treat diseases were assessed based off the changes in their physical properties. In chapter 2, HGNs, hollow gold nanostars (HNSs), and silver coated hollow gold nanostars (AgHNSs) were synthesized and compared for their SERS applicability. It was demonstrated in this work that both HNSs and AgHNSs exhibit tunable structural and optical properties. Using rhodamine 6G as a probe molecule, it was determined that HNSs reported an order of magnitude higher relative enhancement over previously reported HNSs and AgHNSs have a four-fold increase in SERS signal compared to HNSs reported herein. Theoretical analyses were compared to the experimental findings, which suggested that the increase in enhancement was not completely due to the addition of silver, but also potentially the branching of individual spikes or increased binding efficacy of the analyte to the nanoparticle. To further explore the applicability of these nanoparticles, capping ligand exchange was completed from citrate to pentanethiol to increase their interaction with negative analytes like bovine serum albumin. In chapter 3, methylammonium lead bromide magic sized clusters (MSCs) and quantum dots (QDs) were synthesized using a new heated ligand assisted reprecipitation method. A mixture of QDs and MSCs were tuned to monodispersed MSCs by increasing the temperature of the precursor solution. Additionally, the size of the MSCs themselves was tuned by increasing the temperature of the precursor solution and increasing the concentration of capping ligand. The MSCs were analyzed with Raman spectroscopy and determined to be similar to bulk perovskite. Using IR, mass spectrometry, and control experiments it was determined that oleylamine was the primary capping ligand on the surface of the magic-sized cluster and therefore that the surface must be cationic in nature. In chapter 4, a comparison of the stability of previous ligand assisted reprecipitation method and newer temperature dependent synthesis of MSCs was made. The MSCs were placed under ambient conditions and exposed to water and observed until they no longer emitted light. Then, they were stabilized with noncoordinating polymer matrix, paraffin, and the solution was then similarly exposed to water and observed until both particles degraded. The paraffin MSCs mixtures were then dried as a film. The LARP MSCs degraded after 3 hours, but the HLARP MSCs remained luminous for 8 days when stabilized with paraffin. Both solids were analyzed with Raman spectroscopy. HLARP MSCs were then characterized using X-ray diffraction and TEM. It was determined that HLARP MSCs are more ordered that LARP MSCs and have an orthorhombic quasi-crystalline structure.

II-VI semiconducting nanostructures have been intensively researched as promising materials in applications including sensing, light emitting diodes (LEDs), lasers, photoelectrochemical (PEC) materials for water splitting and photo-catalysis, and dye or quantum dot sensitized solar cells. On the nanoscale, structural morphology plays a significant role in determining optical, electronic, and physical properties, and thus consequently affects the ultimate device properties.

In zero-dimensional quantum dots (QDs), control of the crystal size can allow for tunability of properties such as the absorption and emission spectrum. As a result, these nanostructures have proven attractive for QD-LEDs, biological labeling, and sensing. Because of the high surface to volume ratio of QDs, the surface quality and structure of these nanocrystals play a significant role in carrier trapping and non-radiative decay processes. QD surfaces can be effectively passivized by increasing the coordination number of surface atoms through coordinating capping ligands. Ligands improve photoluminescence (PL) quantum yield by healing dangling bond bandgap states, but they can also insert electron acceptor states below the conduction band (CB), which can trap photoelectrons and quench QD PL. This latter approach has been investigated for sensing applications such as in explosives detection. Characterization using density function theory DFT and time dependent DFT has been applied to study the acceptor levels in six explosives to help target different II-VI materials as potential fluorescent probes.

In one-dimensional quantum rods or wires, the exciton can be confined in two dimensions, leaving the third available for transport. These structures are promising in light harvesting applications such as PEC water splitting and solar cells. Nanowire (NW) photoanodes allow for efficient collection of photogenerated electrons by providing a direct route to the back contact while minimizing the distance the photohole must travel to reach the solution interface. At the same time, the added surface area increases the surface/electrolyte contact area, as well as improves sensitizer loading, the latter of which has been studied to improve performance in the visible portion of the spectrum. Common sensitizers for metal oxide (MO) anodes, which have large bandgaps and therefore do not absorb visible photons, include organic dyes, QDs, and in some studies the use of metal nanoparticles (NPs) has been proposed. Metal NPs, with their size and shape tunable surface plasmon resonance, can be controlled through the visible-NIR portion of the spectrum, and have been investigated as potential sensitizers on ZnO photoanodes using ultrafast transient absorption spectroscopy.

Enhanced performance of MO photoanodes can also be achieved through various annealing treatments meant to improve crystallinity, decrease intrinsic trapping defects, increase donor concentration by encouraging advantageous defects in n-type semiconductors, or passivate undesirable defects. The effect of air and hydrogen annealing of ZnO NWs on PEC performance was investigated with in-situ TA spectroscopy which showed that the combination of these methods improves PEC water splitting efficiency.

Finally, the application of II-VI materials in three-dimensional thin films has shown great potential in next generation photovoltaics which include the CdS/CdTe solar cell. The n-type CdS window layer, which forms the p-n junction with p-type CdTe, can be deposited by several different techniques but commonly include: DC pulse sputtering (DCPS) and chemical bath deposition (CBD). The dynamics of photoexcited charge carriers in CdS films prepared these methods was investigated with ultrafast transient absorption spectroscopy and the results was analyzed using singular value decomposition global fitting. The CBD sample had significant donor, acceptor, and donor acceptor pair recombination attributed to high oxygen content in the films while the DCPS had intrinsic donor level defects which are expected for n-type CdS. The results suggest that DCPS made thin films should offer improved performance in solar cell applications.

Photoelectrochemical data is reported for P25 TiO_{2/ films deposited as a pristine film on FTO. The pristine P25 films show a photocurrent of 0.06 mA/cm² and an onset potential of -0.5 V vs. Ag/AgCl. After TiCl4 Treatment to form a compact TiO2 layer on the bottom and the surface layer, the TiCl4 sandwich films have a photocurrent of 0.19 mA/cm² and an onset potential of -0.8 V. We have also investigated the effects of hydrogen treatment, ammonia treatment, and the combination of hydrogen treatment followed by ammonia treatment (co-treatment) on the photocurrent, the incident photon-to-current efficiency (IPCE), and the electron donor density of the P25 TiCl4 sandwich films. Treating the P25 films had almost no effect on the size and d-spacing of the particles, but EPR evidence, as well as color change, indicated the formation of N 2p nitrogen sites and oxygen vacancies (VO) for each film treatment. I-V data for the treated TiCl4 sandwich films show an increase in photocurrent from 0.19 mA/cm² for the pristine P25 TiO2 film to 0.4 mA/cm² for the co-treated TiO2 film compared to 0.23 mA/cm² for hydrogen treatment and 0.25 mA/cm² for ammonia treatment. For the P25 treated films, there is negligible increase of UV absorption in the visible for the singly treated films as well as the co-treated films. However, the improved photocurrent for the ammonia treated, hydrogen treated, and co-treated films may be explained by increased donor density. Mott-Schottky plots are used to characterize donor density, showing that the co-treated P25 TiCl4 sandwich films have an increased donor density over the nitrogen-treated P25 TiO2 and pristine P25 TiO2 films. The improved donor density of the treated P25 films over the untreated films may prove to be useful when completing future dye or semiconducting quantum dot sensitization experiments.}

Semiconductor nanostructures, also known as quantum dots (QDs), have shown great promise as optical materials in solar cells, light emitting devices, and as fluorescent probes. When combined with other materials, the properties of QDs can be modified to suit a particular application. QD heterostructures were synthesized and investigated using time resolved spectroscopy in order to understand how to control the fate of excitons. A one pot approach to the synthesis of CdSe/ZnSe/ZnS core/shell/shell QDs was developed, and the shell growth was shown to be an effective means of extending the exciton lifetime and increasing fluorescence quantum yield. Both the extended lifetime and increased fluorescence were attributed to passivation of surface states by the double shell. Furthermore, this approach was adapted to alloyed CdxZn(1-x)Se QDs to produce CdxZn(1-x)Se/ZnSe/ZnS alloyed core/shell/shell QDs with high fluorescence quantum yield. Transient absorption spectroscopy showed that the shell coating slowed exciton recombination on the ps timescale by passivating surface states. Time resolved fluorescence revealed identical ns dynamics before and after shell growth, this was attributed to very fast surface state recombination that can is complete within a nanosecond. In addition, homogenous CdSe/TiO2 composite nanorods and pure CdSe nanorods were grown using physical vapor deposition. SEM images showed that the CdSe and CdSe/TiO2 nanorod arrays are vertically aligned, with very similar morphology. Ultrafast transient absorption spectroscopy showed that the composite nanorods exhibit very fast electron injection from CdSe into TiO2. The fast electron injection is due to the large interfacial area between the two materials in the composite nanorods.