UC Santa Cruz

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.