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.

While Raman is a powerful tool that can be used to identify chemicals in a mixed environment, two of the major issues with the conventional Raman are weak signal intensity and low spatial resolution. To resolve these issues, surface and tip-enhanced Raman spectroscopy have been developed that surface-enhanced Raman spectroscopy (SERS) mainly focuses on solving the problem of weak Raman signal, while tip-enhanced Raman spectroscopy (TERS) can improve the spatial resolution overcoming the diffraction limit while enhancing Raman signal. In this work, we report a one-pot synthesis method for monodispersed silver nanocubes with tunable sizes ranging from 60 to 180 nm and investigate the size effect of silver nanocubes when monolayer close-packed silver nanocubes are used as SERS substrate. With the tunable size of silver nanocubes, the size-dependent of SERS substrate was demonstrated. TERS combines traditional Raman spectroscopy and near-field scanning optical microscopy that overcomes the diffraction limit, enabling to obtain of Raman spectroscopy and topological image simultaneously at the nanoscale. Chemically synthesized silver nanowires (AgNWs) have been reported to be used as scanning probes. Specifically, AgNW with sharp tips can significantly improve the Raman signals and spatial resolution. Here a two-step polyol synthetic method was developed to promote oxidative etching selectively to the ends of AgNWs, which eventually produced sharp-tip AgNWs with over 96% selectivity and an average tip radius of 7.87 nm. Among atomic force microscope (AFM) probes fabricated by mounting silver nanowires with different tip sharpness onto commercial AFM cantilevers, the probe based on ultra-sharp AgNW showed significant improvement in spatial resolution in topographic scanning and enhancement factor of TERS tests over the same carbon nanotube sample compared to the probes based on AgNWs with pentagonal pyramid or rounded ends. To broaden the application of TERS in the catalysis field, a second noble metal (Pd, Pt) was grown onto ultra-sharp AgNWs creating a uniform core-shell structure with tunable shell thickness. The resulting nanowires show minimal light scattering and strong tip emission when mounted onto optical fiber under excitation light of 532 nm and 650 nm wavelength suggesting feasibility as TERS probes.

Transparent conductive electrodes are essential components in many optoelectronic devices such as solar cells, touch panels, and organic light-emitting diodes (OLEDs), all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO, which alone accounts for 93% of the entire market of the transparent conductor. However, there are several attributes of ITO that are undesirable. (1), ITO is a ceramic material that is brittle and prone to cracking. (2), The abundance of indium in the earth’s crust is low (0.05 ppm), and its cost is correspondingly high, approximately $700 kg-1. (3), ITO is produced by a slow vapor phase sputtering process, leading to high fabrication cost that dominates the cost of ITO (Indium costs only ~2% for 100 nm thick ITO). In addition, the rate of film throughput decreases with increasing film thickness, making thicker, high-conductivity ITO (~$26/m2 for 10 Ω sq-1) more expensive than thinner, low-conductivity ITO (~$5.5/m2 for 150 Ω sq-1), which is especially problematic for OLEDs and solar cells due to their need to carry higher currents, and thus use relatively expensive ITO with a low sheet resistance. In these devices, ITO can account for over 50% of the material cost.

This proposal will focus on developing scalable and cost-efficient methods to fabricate air-stable metal nanowire networks for transparent flexible electronics, targeting at a sheet resistance (Rs) of <10Ωsq-1 at 90% transmittance (T) and low cost at <$5/m2. This overall objective will be realized through two thrusts: (1), developing air-stable, highly-oriented epitaxial Ag@Au core-shell nanowire networks, aiming for large-scale roll-to-roll solution-phase fabrication of high-performance transparent conducting films for flexible optoelectronics; (2), developing air-stable super-stretchable Cu@rGO core-shell nanowire networks, aiming at cost-efficient foldable and wearable electronics.

In the past century, electric current dominated the field of information generation, transportation, and process. In the last decades, the photon has been replacing the electric current due to its extraordinarily large bandwidth and faster transmission speed. 1D nanostructures possess an exceptional advantage over the other nanostructures for the guidance of light because it has a smaller small footprint and high mechanical stability and flexibility and requires low materials. Metal nanowires that are synthesized with a polyol methods exhibit atomically smooth surface, high crystallinity, and low impurities, has been intensively studied recently due to its ability of deep-subwavelength confinement. Recently, localized surface plasmon resonance (LSPS) using a plasmonic hotspot formed in the gap (<5nm) remotely excited by metallic nanowire waveguide have stirred-up great research interests in chemical imaging and sensing. In this work, I seek to improve the performance and reliability of chemical imaging and analysis at the nanoscale by developing an effective method for the confinement and guidance of the light. The enabling technology is a novel adiabatic nanofocusing to concentrate light to a nano-confined Raman sensing volume with high efficiency and energy throughput. Here, the adiabatic means no energy loss in the process of focusing light at the nanoscale. The critical component of the adiabatic nanofocusing is selective excitation of fundamental surface plasmon polariton (SPP) modes in AgNW using a tapered optical fiber with a minimal scattering and propagation losses. The SPP propagates along the NW and eventually creates a localized light at the sharp NW tip, with a hotspot size (<5nm). This optical path can be used not only for concentrating of light to but also extracting the vibrational spectra from the hot spot. Compared to the objective lens based conventional light focusing method, this NW waveguide works as a tunnel for light to propagates without a significant propagation loss due to a high absorption or diffraction in or reflection from the liquid sample. Therefore, imaging or sensing in the liquid phase is also available such as living cell analysis in culture media. The versatility and performance of our adiabatic nanofocusing were successfully demonstrated by measuring Raman spectra of specific molecules on the solid surface and in a single living cell. The transformative advances in light focusing and extracting are instrumental in increasing the prevalence of chemical imaging and analysis at the nanoscale as a practical, reliable and powerful tool for researchers in materials science, catalysis, energy conversion, electronics, and biology. This novel light focusing technique will be the pillar of the chemical imaging and analysis at the nanoscale and contribute to innovative findings in diverse scientific fields in the future.

Flexible electronics have been proposed as the next generation of electronic devices. They have advantages over traditional electronics in that they use less material, are more durable and have greater versatility in their proposed applications. However, there are a variety of types of devices being developed that have specific engineering challenges. This dissertation addresses two of those challenges. The first challenge involves lowering contact resistance in MoS2 based flexible thin film transistor devices using a photochemical phase change method while the second addresses using silver nanowire networks as a replacement flexible electrode for indium tin oxide in flexible electronics.

In this dissertation, a scalable method was developed for making monolayer MoS2 using ambient pressure chemical vapor deposition. These films were then characterized using spectroscopic techniques and atomic force microscopy. A photochemical phase change mechanism was then proposed to improve contact resistance in MoS2 based devices. The central hypothesis is that the controllable partial transition from a semiconducting 2H to metallic 1T phase can be realized in monolayer TMDs through photo-reduction in the presence of hole scavenging chemicals. Phase-engineering in monolayer TMDs would enable the fabrication of high-quality heterophase structures with the potential to improve carrier mobility and contact. Phase change as a result of the proposed photochemical method was confirmed using Raman spectroscopy, photoluminescence measurements, X-Ray photoelectron spectroscopy and other supporting data.

Gold coated silver nanowires were then created to serve as flexible nanowire based electrodes by overcoming galvanic replacement in solution. This was confirmed using various forms of electron microscopy. The central hypothesis is that a thin gold coating will enable silver nanowire meshes to remain electrically stable in atmosphere and retain necessary low resistance values and transparencies over time. It was shown that gold coated silver nanowire meshes could be created with sheet resistances comparable to indium tin oxide and outlast their bare silver nanowire counterparts in environments at 80 deg C.

Biosensors have begun to attract and find widespread use in the medical field due to their potential for ultrasensitive detection, and as a reliable, high-throughput, and nondestructive diagnostic immunoassay, specifically aimed towards the early screening of cancer. Specifically, there is need to screen the early stages of tumor growth, as 90% of all cancer related deaths are due to metastasis. Matrix metalloproteinase-14 (MMP-14) is an important target for cancer research due to its ability to induce tissue remodeling, extracellular matrix degradation, tumor invasion and angiogenesis. Surface Enhanced Raman Spectroscopy (SERS) has emerged as a popular method of detection due to the high optical enhancement offered by the surface plasmon resonances (SPRs) of metallic nanostructures while preserving the structural specificity, rapid screening, and high flexibility of Raman spectroscopy. The direct SERS analysis of biomolecules often suffer from poor selectivity and sensitivity due to their weak Raman activities, lack of characteristic functional groups/polarizable moieties, and poor surface selection rules with the nanoparticle surface. This study hopes to take advantage of a novel indirect approach termed “nano-stress” sensing which provides an excellent alternative to conventional direct sensing. In short, a Raman reporter with a highly resolved, well characterized spectrum is chemically functionalized with a biorecognition element (e.g. antibodies) and undergoes a change in peak intensity or position in response to a molecular recognition event (e.g. antibody-antigen binding). Chapter 2 describes the synthesis of silver octahedral nanoparticles using a modified polyol process and their assembly into a large-scale, close-packed, two-dimensional, thin film SERS platform, using a homemade Langmuir-Blodgett (LB) trough. SEM images reveal many well-ordered, close-packed polycrystalline domains scattered throughout. Chapter 3 presents the fabrication of the SERS immunosensor using a self-assembled monolayer (SAM) of 4-MBA, its chemical modification to bind to Fab3A2, and the detection of cdMMP-14 through extrinsic nano-stress sensing. Optimization experiments using TRIS Buffer indicate that a 0.1M/0.1M EDC/NHS solution at pH 6.0 with stirring gives the highest 4-MBA conjugation (~50%). About 38.58% Fab3A2 conjugation was observed along with the successful sensing of the biorecognition event between Fab3A2 and cdMMP-14 at various concentrations. A maximum frequency shift of 0.95±0.37cm-1 was observed for 5x10-7M cdMMP-14. A linear semilog relationship (R2=97.79%) was also observed giving the equation y = 0.31136x + 2.9109 for the concentration range 5x10-7M to 5x10-10M. Control experiments shows slight to relatively no binding with cdMMP-9 as well as minimal non-specific protein adsorption therefore confirming the specificity of the immunosensor.

Carbon nanomaterials have received a great deal of attention and investment in recent years, not only because of their excellent properties, but also driven by the fact that conventional Si-based electronics is pushed to its limits, and when the size of a Si based transistor is miniaturized to several nanometers, the cost increases dramatically. Chapter 1 reviews the synthesis, properties and application of carbon nanotubes and graphene. Chemical functionalization is an efficient way to manipulate and tune the properties of carbon nanomaterials and to broaden their applications in different fields.

Transfer of CVD transfer without degrading its quality is essential to the wide applications of this carbon material in electronics. In Chapter 2, two novel graphene transfer methods are reported. The availability of clean graphene is critical for the chemical modification of graphene and its application in flexible electronics.

The interaction between carbon surfaces and transition metals, discussed in Chapter 3, has long been an interesting topic. Graphene half sandwich complexes and graphene sandwich complexes were successfully synthesized and the electrical conductivity was used to characterize the synthetic process. Besides, the fascinating electrical properties of individual carbon nanotubes were extended toward macroscale by aligning individual tubes on a large scale and applying organometallic chemistry to constructively interconnect the sidewalls of parallel tubes and two ends of nanotubes in series with transition metals, which greatly improved the electrical performance of aligned carbon nanotube devices and paves the way for the fabrication of carbon nanotube-based high performance microprocessors.

The absence of a bandgap in graphene inhibits its application to transistors, meanwhile from a chemical standpoint the unique band structure of graphene dictates the chemical reactivity of graphene as exemplified in the Diels Alder (DA) reaction discussed in Chapter 4. The effect of a facile room temperature DA reaction on the magnetoresistance of epitaxial graphene (Epigraphene) was studied in a temperature range from 300 K to 2 K. The transition from the positive magnetoresistance in pristine Epigraphene to a large negative magnetoresistance in naphthazarin-Epigrahene adduct provides an alternative way to develop graphene based magneto-electronic devices.