Recent discoveries of highly efficient solar cells based on methylammonium lead iodide (MAPbI3) perovskites (three dimensional, 3D, structure) attract a surge in research activity on the photo-generated carriers and how carrier and/or exciton interact in these materials. Understanding the photo-carrier dynamics and interactions as well as the nature of the trap states are crucial for elucidating the working mechanisms of perovskite solar cells. Lead iodide perovskites can also be prepared in two-dimensional (2D) structures, which are essentially self-assembled quantum wells. Questions remain on whether the photo-excited species are free carriers or excitons and how they interact and recombine. The nature of trap states and how to minimized them in these materials are also unclear. In this thesis, the carrier/ exciton interactions and the trap states in 3D and 2D lead iodide perovskites and the Auger recombination in 3D perovskites are studied with ultrafast Time-Resolved Transient Absorption (TA) Spectroscopy. The first part of this thesis is the carrier generation and carrier/carrier interaction study in 3D MAPbI3 along with a comparative study on the exciton/carrier interaction in 2-dimentional (2D) lead iodide perovskites (Chapter 4 and 5). The major photo-generated species are charge carriers in 3D perovskites and excitons in 2D perovskites. Upon high photon energy excitation, the hot electrons and holes are created instantaneously which induce a red-shift on the band-edge optical transition in 3D perovskites while a broadening effect on the 1S exciton in 2D perovskites. The red-shift is the result of the Stark effect from the hot carriers and the broadening comes from the scattering by the carriers. The band-edge carriers in 3D perovskite recombine following two-molecular recombination at low density and Auger recombination at higher density. In 2D perovskite, we observed a blue-shift in 1S exciton transition due to the localized exciton-exciton interaction. The 6th chapter is the discussion on the below-gap trap states, depending on the dimensionality and the organic/inorganic interfaces. We observed trap states in both 3D and 2D perovskites below the optical band-gap, and in 2D perovskites the trap states increase with the decrease of the quantum well thickness. With the help of surface sensitive UPS and temperature dependent PL measurements, we concluded the trap states localize at the “soft” organic/inorganic interfaces, which in 3D are the grain boundaries and surfaces and in 2D are the barrier/well interfaces. Aside from the TA studies on perovskites, Time-Resolved Second Harmonic Generation (TR-SHG) study on the transient electric field in neat C70 film and CuPc/C70 bilayer film are reported at the end of the thesis. TR-SHG has been applied to study the interfacial electric field generation at donor/acceptor interface but the total SHG signal may have contributions from the donor, the acceptor and the interface. All of these contributions need to be considered in order to fully understand the TR-SHG signal. With ultrafast laser excitation with ~100 fs time scale, we observed an internal E-field generated in C70 film due to charge drift and diffusion, with ~ 10 ps rise time. For CuPc/C70 bilayer film, an additional interfacial E-field appears with a time constant of ~0.1 ps due to charge separation at the donor/acceptor interface. The E-Field induced SHG signal from these two E-fields interfere with each other giving rise to the overall SHG, which is dependent on both the probe polarization and the film thickness.

Nonlinear optical microscopy (NOM) has become increasingly popular in biomedical research in recent years with the developments of laser sources, contrast mechanisms, novel probes and etc. One of the advantages of NOM over the linear counterpart is the ability to image deep into scattering tissues or even on the whole animals. This is due to the adoption of near-infrared excitation that is of less scattering than visible excitation, and the intrinsic optical sectioning capability minimizing the excitation background beyond focal volume. Such an advantage is particularly prominent in two-photon fluorescence microscopy compared to one-photon fluorescence microscopy. In addition, NOM may provide extra molecular information (e.g. second harmonic generation and third harmonic generation) or stronger signal (e.g. stimulated Raman scattering and coherent anti-Stokes Raman scattering compared to spontaneous Raman scattering), because of the nonlinear interaction between strong optical fields and molecules. However, the merits of NOM are not yet fully exploited to tackle important questions in biomedical research. This thesis contributes to the developments of NOM in two aspects that correspond to two fundamental problems in biomedical imaging: first, how to non invasively image small functional biomolecules in live biological systems (Chapters 1-4); second, how to extend the optical imaging depth inside scattering tissues (Chapters 5-6). The ability to non-perturbatively image vital small biomolecules is crucial for understanding the complex functions of biological systems. However, it has proven to be highly challenging with the prevailing method of fluorescence microscopy. Because it requires the utilization of large-size fluorophore tagging (e.g., the Green Fluorescent Protein tagging) that would severely perturb the natural functions of small bio-molecules. Hence, we devise and construct a nonlinear Raman imaging platform, with the coupling of the emerging stimulated Raman scattering (SRS) microscopy and tiny vibrational tags, which provides superb sensitivity, specificity and biocompatibility for imaging small biomolecules (Chapters 1-4). Chapter 1 outlines the theoretical background for Raman scattering. Chapter 2 describes the instrumentation for SRS microscopy, followed with an overview of recent technical developments. Chapter 3 depicts the coupling of SRS microscopy with small alkyne tags (C≡C) to sensitively and specifically image a broad spectrum of small and functionally vital biomolecules (i.e. nucleic acids, amino acids, choline, fatty acids and small molecule drugs) in live cells, tissues and animals. Chapter 4 reports the combination of SRS microscopy with small carbon-deuterium (C-D) bonds to probe the complex and dynamic protein metabolism, including protein synthesis, degradation and trafficking, with subcellular resolution through metabolic labeling. It is to my belief that the coupling of SRS microscopy with alkyne or C-D tags will be readily applied in answering key biological questions in the near future. The remaining chapters of this thesis (Chapters 5-6) present the super-nonlinear fluorescence microscopy (SNFM) techniques for extending the optical imaging depth into scattering tissues. Unlike SRS microscopy that is an emerging technique, multiphoton microscopy (mainly referred as two-photon fluorescence microscopy), has matured over 20 years with its setup scheme and biological applications. Although it offers the deepest penetration in the optical microscopy, it still poses a fundamental depth limit set by the signal-to-background ratio when imaging into scattering tissues. Three SNFM techniques are proposed to extend such a depth limit: unlike the conventional multiphoton microscopy whose nonlinearity stems from virtual-states mediated simultaneous interactions between the incident photons and the molecules, the high-order nonlinearity of the SNFM techniques that we have conceived is generated through real-state mediated population-transfer kinetics. In particular, Chapter 5 demonstrates the multiphoton activation and imaging (MPAI) microscopy, which adopts a new class of fluorophores, the photoactivatable fluorophores, to significantly extend the fundamental imaging depth limit. Chapter 6 theoretically and analytically depicts two additional SNFM techniques of stimulated emission reduced fluorescence (SERF) microscopy and focal saturation microscopy. Both MPAI and focal saturation microscopies exhibit a fourth order power dependence, which is effectively a four-photon process. SERF presents a third order power dependence for a three-photon process.

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The focus of this thesis is the design of low-dimensional coordination polymers (CPs) using semi-rigid N-donor ligands based on heterocyclic molecules, especially benzotriazole, and the investigation of their potential magnetic and catalytic properties. Chapter 1 serves as a general introduction to the chemistry discussed in the thesis. The first part emphasizes on the synthetic aspects and applications of CPs. The second part presents the unique chemical characteristics of benzotriazole and includes a thorough literature review on its use as a ligand in coordination chemistry, culminating to the development of a ligand system for the design of the targeted materials. Chapter 2 introduces the main family of benzotriazole-based ligands (L1-L3) employed in this thesis, focusing on their coordination chemistry with cobalt salts. The synthesis and characterisation of a series of novel 0D, 1D and 2D compounds with a large structural variety is reported. Synthetic aspects and magnetic properties of selected compounds are discussed. Chapters 3, 4 and 5 report a series of copper coordination compounds employing L1-L3 as well as analogous N-donor ligands (L4-L8). A system of 1D CPs is established and investigated for its catalytic properties in a range of organic transformations that includes the synthesis of 1,4-dihydropyridines through a previously unreported route, the A3 coupling and the ‘click' azide-alkyne cycloaddition reaction. Investigations into optimising the catalytic behaviour and mechanistic aspects of this system are presented. In Chapter 6 the coordination capabilities of L1-L3 are combined with the rich chemistry of silver salts to generate a structurally diverse family of 0D, 1D and 2D compounds. Investigations of their potential catalytic properties in the A3 coupling and alkyne hydration reactions are additionally presented. Chapter 7 provides an overall conclusion to the work presented in the thesis, including its contributions to the reported literature as well as potential future directions. Finally, experimental and synthetic details as well as crystallographic data are presented in Chapter 8 and Chapter 9 respectively.

This dissertation explores the electronic characteristics of silicon at the single molecule level. This idea is born as we enter the post-Moore’s law era when the exponential shrinking of conventional silicon microelectronics has begun to stall and an investigation of molecular materials is timely. Single-molecule electronic components have shown promising functionalities such as conductors, switches, and diodes, and single molecule junctions have become a widely used test-bed for probing electron transport properties at the molecular level. In this thesis, we use scanning tunneling microscope break junction method to create single molecule junctions with a variety of silicon molecular wires. Our results demonstrate electronic properties of silicon beyond it being a semiconductor in its bulk form. We begin this work in pursuit of an expanded understanding of low-k dielectric components with an experimental goal on determining the cause of its breakdown. Low-k dielectrics are beneficial as they enable faster switching speeds and lower heat dissipation, however, they tend to breakdown after prolonged usage under an applied voltage. At the atomic level, low-k dielectric breakdown involves bond rupture. To determine which bond breaks easily, we conduct experimental studies on the robustness of individual chemical bonds that are commonly found in low-k dielectrics. We subject the single molecule junctions to a high bias and investigate the breakdown phenomenon of individual Si-Si, Ge-Ge, Si-O, and Si-C bonds. Among these, Si-C proved to be significantly more durable than the others. To further prove our hypothesis that the Si-Si bond ruptures under the applied high bias, we design a two-path molecular structure consisting of a Si-Si bond in parallel with a naphthyl group. The broken junction shows conduction through the naphthyl pathway, strongly indicating that the Si-Si bond is breaking. This demonstrates a method for probing the bond cleavage under an electric field and provides insights to the weak links in low-k dielectrics. Next, we study the fundamental charge transport characteristics of single molecule junctions comprised of Si and Ge-based molecular wires, starting with the simplest form - linear atomic chains. We observe a slower decay of conductance with increasing length in the silanes and germanes than in alkanes, indicating that the electronic delocalization in the Si-Si and Ge-Ge -bonds is stronger than that of the well-studied C-C bonds. Furthermore, we demonstrate that this electronic delocalization in the Si-Si and Ge-Ge bonded backbones enables single-molecule conductance switching. This conductance switch, induced by a mechanical modulation, relies on the nature of the terminal groups and constitutes the first example of a stereoelectronic switch. We also study the molecular conductance of these silanes with metal contacts other than Au, which can potentially open up interesting avenues as metal varies in its electronic states and catalytic activities. We find that Ag electrodes enable higher conductance for thiol-terminated silanes than Au or Pt electrodes. The electrical properties of more complex silicon structures - silicon rings - were probed. We choose a five-membered silicon ring as a target system to investigate the effect of isomerism on single molecule conductance. We find that due to the flexibility of the ring, multiple conformations contribute to the spread in the measured conductance for each isomer. This provides us with a starting point to further compare the conductance of a variety of silicon rings. We find that most of the silicon rings are less conductive than their linear counterparts due to the suboptimal backbone conformation for electronic coupling. In particular, destructive quantum interference appears in one of the bicyclic structures and leads to an exceptionally low conductance. This is the first example of a destructive quantum interference feature ever observed experimentally in a π-bonded rather than a σ-bonded system. Finally, we investigate the impact of strain on molecular conductance of silanes. In one case, we introduce the strain using a silacyclobutane ring in the backbone. Unexpectedly, we find that ring strain enables a new Au-silacycle binding mode, resulting in a much higher conductance state. In another molecular design, we choose disilaacenaphthene in the backbone. This strained disilane is found to constitute an example of a direct Si-to-Au contact in single molecule circuits, thereby demonstrating a new binding motif that is valuable for designing high conducting molecular components. Taken together, this body of work provides important knowledge about the transport properties of silicon at the nano-scale, as well as insights on the design of silicon components for nanoelectronics. This work represents one step forward to create functional silicon molecular components.

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis (M. Sc.)--University of Hong Kong, 1955. / Type-written copy. Includes bibliographical references (p. 54-55).

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