In this perspective, we take a critical look at the research progress within the nanowire community for the past decade. We discuss issues on the discovery of fundamentally new phenomena versus performance benchmarking for many of the nanowire applications. We also notice that both the bottom-up and top-down approaches have played important roles in advancing our fundamental understanding of this new class of nanostructures. Finally we attempt to look into the future and offer our personal opinions on what the future trends will be in nanowire research.

We present an all-oxide solar cell fabricated from vertically oriented zinc oxide nanowires and cuprous oxide nanoparticles. Our solar cell consists of vertically oriented n-type zinc oxide nanowires, surrounded by a film constructed from p-type cuprous oxide nanoparticles. Our solution-based synthesis of inexpensive and environmentally benign oxide materials in a solar cell would allow for the facile production of large-scale photovoltaic devices. We found that the solar cell performance is enhanced with the addition of an intermediate oxide insulating layer between the nanowires and the nanoparticles. This observation of the important dependence of the shunt resistance on the photovoltaic performance is widely applicable to any nanowire solar cell constructed with the nanowire array in direct contact with one electrode.

The ability to manipulate light in subwavelength photonic and plasmonic structures has shown great potentials in revolutionizing how information is generated, transformed and processed. Chemically synthesized nanowires, in particular, offers a unique toolbox not only for highly compact and integrated photonic modules and devices, including coherent and incoherent light sources, waveguides, photodetectors and photovoltaics, but also for new types of nanoscopic bio-probes for spot cargo delivery and in-situ single cell endoscopy and sensing. Such nanowire probes would enable us to carry out intracellular imaging and probing with high spatial resolution, monitor in-vivo biological processes within single living cells and greatly improve our fundamental understanding of cell functions, intracellular physiological processes, and cellular signal pathways. My work is aimed at developing a material and instrumental platform for such single nanowire probe.

Successful optical integration of Ag nanowire plasmonic waveguides, which offers deep subwavelength mode confinement, and conventional photonic waveguides was demonstrated on a single nanowire level. The highest plasmonic-photonic coupling efficiency coupling was found at small coupling angles and low input frequencies. The frequency dependent propagation loss was observed in Ag nanowire and was confirmed by quantitative measurement and in agreement with theoretical expectations. Rational integration of dielectric and Ag nanowire waveguide components into hybrid optical-plasmonic routing devices has been demonstrated. This capability is essential for incorporating sub-100nm Ag nanowire waveguides into optical fiber based nanoprobes for single cell endoscopy.

The nanoprobe system based on single nanowire waveguides was demonstrated by optically coupling semiconductor or metal nanowire with an optical fiber with tapered tip. This nanoprobe design requires minimal instrumentation which makes it cost efficient and readily adaptable to average bio-lab environment. These probes are mechanically robust and flexible and can withstand repeated bending and deformation without significant deterioration in optical performance, which offers an ideal instrumental platform for out subsequent effort of using these nanoprobes in chemical sensing as well as single cell endoscopy and spot delivery. Parameters affecting the coupling efficiency and output power of the nanoprobe were studied and chemical etched of single mode fiber with small cone angle was established to be optimized for highly effective optical nanoprobes.

The versatility of the nanoprobe design was first tested by transforming the nanowire probe into a pH sensor with near-field photopolymerization of a copolymer containing pH sensitive dye on the tip of the nanowire. The pH-sensitive nanoprobe was able to report the pH difference in micro-droplets containing buffer solution with the excitation of light waveguided on the nanoprobe with internal calibration, fast response time and good photostability and reversibility. Such nanoprobe sensors are ideal for high definition spatial and temporal sensing of concentration profile, especially for the kinetic processes in single cell studies for which chemical probes of minute sizes and fast response are desired.

The nanoprobe was then applied into spot cargo delivery and in-situ single cell endoscopy. It was demonstrated that nanowire-based optical probe can deliver payloads into the cell with a high spatiotemporal precision, guide and confine visible light into intracellular compartments selectively and detect optical signals from the subcellular regions with high spatial resolution. The nanoprobe was proven to be biocompatible and non-invasive. The effective optical coupling between the fiber optics and the nanowire enables highly localized excitation and detection, limiting the probe volume to the close proximity of the nanowire. None the less, this versatile technique does not rely on any expensive or bulky instrumentation, and relies only on micromanipulator and optical microscope that are readily available in most biological labs. The different functions can be further integrated to make the whole nanoprobe system more compact and even portable.

In addition, my research also includes the first demonstration of the synthesis of the longitudinal heterostructured SiO₂/Al₂O₃ nanotubes and the nanofluidic diode device based on the discontinuity of their internal surface charge. Comprehensive characterization shows that the nanotubes has heterostructured inner tube walls, as well as a discontinuity of surface charge. The ionic transport through these nanotube heterojunctions exhibits clear current rectification, a signature of ionic diode behavior. The development of such nanofluidic devices would enable the modulation of ionic and molecular transport at a more sophisticated level, and lead to large-scale integrated nanofluidic networks and logic circuits.

Metal halide perovskites, a class of semiconducting materials with remarkable optoelectronic properties, have attracted considerable attention in recent years. This dissertation delves into innovative design and synthesis approaches, as well as structural transformations and applications of halide perovskites, with a focus on the fundamental properties of the metal halide ion octahedron [MX6]n– (M = metal cation, X = halide anion) as the primary building block and functional unit. By examining the assembly, connection, and interaction of these octahedra, this research aims to establish a solid foundation for the future development and application of halide perovskites.Chapter 1 provides a comprehensive overview of halide perovskites, covering their fundamentals, structural chemistry, stimulus response, and applications, with particular emphasis on the metal halide octahedron as the key building block and functional unit. In Chapter 2, we propose a new design principle for halide perovskite structures based on ionic octahedron networks (IONs) and report the first experimental synthesis of a novel halide perovskite, Cs8Au3.5In1.5Cl23, which adopts an ABO3-type ION. Chapter 3 examines the stimulus-responsive behavior of metal halide perovskites, using Cs3Bi2Br9 as a model compound. By employing in situ characterization techniques, we identify two distinct distortion classes of [BiBr6]3– octahedra and analyze the changes in exciton emissions in relation to the octahedral distortion. In Chapter 4, we utilize our understanding of [MX6]n– octahedra to establish the electronic band structure and photoexcitation model of molecule-like halide perovskite Cs2TeBr6. We demonstrate that different LED wavelengths can generate holes in various valence bands and differentially activate benzyl alcohol molecules. Lastly, Chapter 5 summarizes the research findings and provides insights into future directions for halide perovskite studies, emphasizing the significance of understanding the metal halide ion octahedron.

Halide perovskites are emerging new semiconductors that have excellent optoelectronic performance. The great optical efficiency has triggered research in all fields of physical sciences, including electronic property studies, chemical synthesis, electrical engineering, and structural characterization. Different from traditional covalent semiconductors such as silicon or II-VI semiconductors, halide perovskites resemble ionic crystals to have lower mechanical strength, solution processability, and dynamical lattices. The uniqueness of halide perovskites evades conventional categorization so that renewed conceptual frameworks are necessary. Generally speaking, in the field of physical chemistry of halide perovskites, both the structural and electronic properties of this class of material are of great interest. The coupling between structural dynamics and electronic behaviors contributes to the intriguing carrier dynamical phenomena, such as the formation of polaron, delayed photoluminescence, dynamical screening of hot carriers, etc. The major obstacle to unravel the unprecedented properties of halide perovskites has been the lack of control over the atomic configuration of perovskite microstructures, which requires both synthetic and characterization advancement. Therefore, this Dissertation centers around the structural and carrier Dynamics of low-dimensional halide perovskites by making use of atomically defined nanostructures and advanced spectroscopic/microscopic tools.

After a brief background introduction to halide perovskites, I discuss the scientific questions to be addressed in this Dissertation. In Chapter 2 and 3, I focus on a material platform composed of one-dimensional halide perovskite nanowires. With this material platform, an optical scaling law is observed and explained with Monte Carlo simulation. In these two chapters, I demonstrate the power of well-defined perovskite nanostructures and how to use these nanostructures to resolve intriguing carrier dynamics of halide perovskites in general. In Chapter 4, I introduce synthesis of atomically thin halide perovskite nanostructures. Both one-dimensional atomically thin nanowires and two-dimensional atomically thin nanosheets are used as case studies to demonstrate the advanced synthetic control over the atomic structure of halide perovskites. Electron microscopy is used to resolve the structure of atomic resolution. In Chapter 5, I select the atomically thin nanowires as a material platform to study the transient structural dynamics of halide perovskite nanostructures. Ultrafast electron camera is used to resolve the structural dynamics with millisecond resolution while atomic spatial resolution is maintained. I thoroughly investigate the structural behaviors of one-dimensional halide perovskites under electron beams and demonstrate the resilient while mobile lattice dynamics as a unique character of halide perovskites. Finally, in Chapter 6, I summarize the Dissertation by providing an outlook and new perspectives on the study of halide perovskites. Overall, this Dissertation is aimed to address the structural and electronic properties of halide perovskite with an emphasis on the synthetic control. The research logic of this Dissertation can be used as a new paradigmatic framework for the future exploration and investigation of this emerging new material.

Increasing fossil fuel consumption and CO2 emission have raised great concerns about our energy and environment future. In order to solve these problems, on the one hand, it is critical to develop alternative, renewable energy sources, such as fuel cell powered by H2, on the other hand, it is also important to reduce the existing CO2 into value-added products, such as fuels and fine chemicals. Electrocatalysis using nanomaterials plays an essential role in completing these two tasks by transforming the reactants into the target products efficiently along with the energy conversion process. The activity, stability, and selectivity of nanocatalysts highly depend on their structures, which require careful and rational design to achieve desired properties. Therefore, this dissertation focuses on the development of multiple effective strategies to improve the performance of different nanocatalysts for either fuel cell or CO2 reduction reaction (CO2RR) electrocatalysis.

First, post-synthesis treatment has been recognized as a key step to tailor the catalytic behavior of Pt-based alloys. In the second chapter, we present the effects of catalyst processing on the electrocatalytic property of Pt-Ni nanoframes for cathodic oxygen reduction reaction (ORR) in fuel cell. The Pt-Ni nanoframes are made by corroding the Ni-rich phase from solid rhombic dodecahedral particles. Among the three different corrosion procedures, electrochemical corrosion leads to the highest initial specific activity by retaining more Ni in the nanoframes. However, the high activity gradually goes down in a subsequent stability test due to continuous Ni loss and concomitant surface reconstruction. In contrast, the best stability is achieved by a more-aggressive corrosion using oxidative nitric acid. Although the initial activity is compromised, this procedure imparts a less-defective surface, and thus, the specific activity drops by only 7% over 30,000 potential cycles. These results indicate a delicate trade-off between the activity and stability of Pt-Ni nanoframe electrocatalysts.

Second, controlled synthesis of nanoparticles with optimal morphology and composition is crucial to promoting their catalytic performance. In the third chapter, we demonstrate the integration of highly open nanoframe morphology and catalytically active Pt-Co composition to develop Pt-Co nanoframes. Their ORR mass activity in acidic media is as high as 0.40 A mgPt-1 initially and 0.34 A mgPt-1 after 10,000 potential cycles at 0.95 V versus reversible hydrogen electrode (VRHE). Moreover, their mass activity for anodic methanol oxidation reaction (MOR) in alkaline fuel cell is up to 4.28 A mgPt-1 and is 4-fold higher than that of commercial Pt/C catalyst. Experimental studies indicate that the weakened binding of intermediate carbonaceous poisons contributes to the enhanced MOR behavior. More impressively, the Pt-Co nanoframes also show excellent stability under long-term testing, which could be attributed to the negligible electrochemical Co dissolution.

Third, introducing a second material, such as the metal-organic framework (MOF), is a promising strategy to add catalytic functions beyond metal nanoparticles. In the fourth chapter, we demonstrate the combination of Pt-Ni nanoframe and zeolitic imidazolate framework-8 (ZIF-8), which is a special MOF, into an individually encapsulated frame-in-frame structure. Via surface functionalization, the Pt-Ni nanoframe is first embedded in ZIF-8 to achieve a single core-shell structure, as evidenced by the three-dimensional tomography. The growth trajectory of such frame-in-frame nanocomposite is tracked, revealing that ZIF-8 first nucleates in the solution, then attaches to the surface of the nanoframe, and finally grows to capture the entire nanoframe, enabling the one-in-one encapsulation. Next, by utilizing ZIF-67 as the sacrificial layer, the Pt-Ni nanoframe is further solely encased in ZIF-8 to form a single yolk-shell structure, which has a cavity between the core and the shell. The obtained frame-in-frame structures have potential applications in size-selective or tandem catalysis to produce fine chemicals.

Fourth, long-range atomic ordering in nanocrystals holds the promise of unique catalytic properties for many reactions. In the fifth chapter, we report the preparation of Cu3Au intermetallic nanowires by using Cu@Au core-shell nanowires as the precursors. With appropriate Cu/Au stoichiometry, the Cu@Au core-shell nanowires are transformed into fully ordered Cu3Au nanowires under thermal annealing. Thermally-driven atomic diffusion, which is facilitated by the abundant twin boundaries, accounts for the ordering process. The resulting Cu3Au intermetallic nanowires have uniform and accurate atomic positioning in the crystal lattice, which enhances the nobility of Cu. No obvious copper oxides are observed in fully ordered Cu3Au nanowires after annealing in air at 200 oC, a temperature that is much higher than those observed in Cu@Au core-shell and pure Cu nanowires. The acquired Cu3Au intermetallic nanowires are promising candidates for either ORR or CO2RR electrocatalysis.

Fifth, covalently bonded surface ligands often block the active metal sites and limit the reactivity of nanocluster catalysts. In the sixth chapter, we investigate the ligand removal process for Au25 nanoclusters using both thermal and electrochemical treatments, as well as its impact on the CO2 electroreduction to CO. The Au25 nanoclusters are synthesized with 2-phenylethanethiol as the capping agent and anchored on sulfur-doped graphene. The thiolate ligands can be readily removed under either thermal annealing at >180 oC or electrochemical biasing at <-0.5 VRHE. However, these ligand-removing conditions also trigger the structural evolution of Au25 nanoclusters concomitantly. The thermally and electrochemically treated Au25 nanoclusters show enhanced activity and selectivity for the electrochemical CO2-to-CO conversion than their pristine counterpart, which is attributed to the increased exposure of undercoordinated Au sites on the surface after ligand removal.

Anthropogenic CO2 emissions from human dependence on fossil fuel are causing rising concentrations of CO2 in the atmosphere and oceans, increasing the average temperature of the planet and acidifying the oceans, among other negative effects. Reliance on carbon-based fuels is not sustainable and must be replaced by alternative, renewable energy sources, including solar, wind, hydroelectric, and geothermal. The water cycle, splitting water into hydrogen and oxygen and electrochemically oxidizing the hydrogen to regenerate water, can be an efficient and sustainable energy storage cycle. An electrolyser powered by renewable electricity splits water, and a proton exchange membrane fuel cell uses hydrogen fuel to produce electricity on demand. The performance of proton exchange membrane fuel cells is significantly limited by the platinum-based oxygen reduction catalyst, which must be improved in activity and durability in order to commercialize applications such as fuel cell vehicles.

Bimetallic nanoframes have great potential for achieving new levels of catalytic activity in various heterogeneous reactions due to their high surface area dispersion of expensive noble metals on the exterior and interior surfaces of the structure. Pt-based nanoframes such as Pt-Ni and Pt-Co can be specifically tuned for excellent oxygen reduction activity and application in the cathode of proton exchange membrane fuel cells. Nanoframes were made by first synthesizing polyhedral nanoparticles with composition such as PtCo3 or PtNi3 by a hot-injection method. Scanning transmission electron microscopy combined with energy dispersive x-ray spectroscopy showed that these nanoparticles demonstrate elemental segregation of platinum to the edges of the polyhedron, forming the basis for a framework nanostructure. The process of preferential oxidative leaching was used to remove the non-noble metal from the interior of the framework, leaving the platinum-rich edges intact to form Pt3Co or Pt3Ni nanoframes.

Understanding the atomic structure of the nanoframe is crucial to exposing the source of its performance characteristics. It is highly unlikely that a catalyst remains the same under reaction conditions when compared to as-synthesized. Hence, the ideal experiment to study the catalyst structure should be performed in situ. X-ray absorption spectroscopy is an ideal in situ technique to study Pt3Ni nanoframes, which are an excellent electrocatalyst for the oxygen reduction reaction. First, the surface characteristics of the nanoframes were probed through electrochemical hydrogen underpotential deposition and carbon monoxide electrooxidation, which proved that nanoframe surfaces with different structure exhibit varying levels of binding strength to adsorbate molecules. It is well-known that Pt-skin formation on Pt–Ni catalysts will enhance oxygen reduction activity by weakening the binding energy between the surface and adsorbates. Ex situ and in situ X-ray absorption spectroscopy revealed that nanoframes which bind adsorbates more strongly have a rougher Pt surface caused by insufficient segregation of Pt to the surface and consequent Ni dissolution. In contrast, nanoframes which exhibit extremely high oxygen reduction activity demonstrated more significant segregation of Pt over Ni-rich subsurface layers, allowing better formation of the critical Pt-skin.

The nanoframe structure is made possible by the compositional heterogeneity in polyhedral Pt-Ni or Pt-Co nanocrystals, which offers additional variables to maneuver the functionality of the nanocrystal. However, understanding how to manipulate anisotropic elemental distributions in a nanocrystal is a great challenge in reaching higher tiers of nanocatalyst design. The evolutionary trajectory of phase segregation in Pt–Ni rhombic dodecahedra was studied in order to elucidate the mechanisms that enable synthesis of the Pt3Ni nanoframe. The anisotropic growth of a Pt-rich phase along the <111> and <200> directions at the initial growth stage resulted in Pt segregation to the 14 axes of a rhombic dodecahedron, forming a highly branched, Pt-rich tetradecapod structure embedded in a Ni-rich shell. With longer growth time, the Pt-rich phase selectively migrated outwards through the 14 axes to the 24 edges such that the rhombic dodecahedron became a Pt-rich frame enclosing a Ni-rich interior phase. The Ni-rich interior was then selectively oxidized and corroded to form the hollow Pt3Ni nanoframe that demonstrated exceptional catalytic activity for the oxygen reduction reaction.

The anisotropic phase segregation and migration mechanism offers a radically different approach to synthesis of nanocatalysts with desired compositional distributions and performance. The newly acquired understanding of these mechanisms was applied to manipulate the morphology of Pt-Ni rhombic dodecahedral nanoframes into excavated nanoframes that also exhibit high catalytic activity for the oxygen reduction reaction without need for Pt-skin formation. Controlling the ratio of platinum and nickel precursors within the nanoframe synthetic system adjusted the three-dimensional platinum anisotropy in the rhombic dodecahedron, resulting in a transition from the previous hollow nanoframe to a unique excavated structure. Excavated nanoframes showed ~10 times higher specific and ~6 times higher mass activity than commercial Pt/C, and twice the mass activity of the hollow nanoframe. The high specific activity of the excavated nanoframe is attributed to enhanced Ni content in the near-surface region and the extended two-dimensional sheet structure within the nanoframe that minimizes the number of low-coordinated Pt sites.