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In this work a Finite Difference Time Domain (FDTD) simulation is employed to explore local field enhancement, plasmonic coupling, and charge distribution patterns. This 3D simulation calculates the magnetic and electric field components in a large matrix of Yee cells using Maxwell’s equations. An absorbing boundary condition is included to eliminate reflection back into the simulation chamber, and a sample system of cells is checked for convergence. In the specific simulations considered here, a laser pulse of single wavelength is incident on a silicon substrate, travels through an embedded ZnO nanowire (NW) waveguide only (due to an Ag filter), then incites plasmonic coupling at the gap between an Au nanoparticle tip and an Au substrate, an Au nanoparticle (NP), or a trio of Au nanoparticles incident on an angled Si substrate. The angle between the axis of the NW and the normal of the substrate is varied from 0-60°. The NP perpendicular deflection with respect to the NW axis is also varied from -115 - 75 nm. The enhancement patterns reveal superior signal to noise ratio compared to Near Field Scanning Optical Microscopy (NSOM), three times smaller than the NP diameter 100 nm, as well as resolution and spot size of less than 50 nm. This method of Apertureless NSOM (ANSOM) using a NW waveguide grown on a transparent microcantilever therefore shows promise for imaging of single molecules incident on a substrate and NP-labeled cell membrane. / text

Interest in nanomaterials is motivated partly by their potential for sensor arrays to detect different gases. Nanowires in particular are of interest because their high surface-to-volume ratio promises the possibility of high sensitivity. However, because of their discrete quasi-one-dimensional geometry, electrical integration of nanowires into photolithographically defined devices and circuits is challenging and remains one of the obstacles to their widespread use. In this thesis, a novel method for fabricating electrically integrated zinc oxide nanobridge devices using carbonized photoresist is investigated. The conductivity of carbonized photoresist is known and nanowire growth on carbonized photoresist has recently been reported, suggesting the possibility of simultaneous use as a nucleation layer and electrode. However, these reports did not characterize the contact between the ZnO nanowires and carbonized photoresist. In this work, ZnO nanobridges are fabricated between opposing carbonized photoresist electrodes and characterized both electrically and with electron microscopy. Operation of nanobridge devices as bottom gate transistors, UV sensors, and gas sensors is demonstrated. / Graduation date: 2010

Clean and affordable energy, especially solar energy, is becoming more and more important as our annual total energy consumption keeps rising. However, to make solar energy more affordable and accessible, the cost for fabrication, transportation and assembly of all components need to be reduced. As a crucial component for solar cells, transparent conductive electrode (TCE) can determine the cost and performance. A light weight, easy-to-fabricate and cost-effective new generation TCE is thus needed. While indium-doped tin oxide (ITO) has been the most widely used material for commercial applications as TCEs, its cost has gone up due to the limited global supply of indium. This is not only due to the scarcity of the element itself, but also the massive production of various opto-electronic devices such as TVs, smartphones and tablets. In order to reduce the cost for fabricating large area solar cells, substitute materials for ITO should be developed. These materials should have similar optical transmittance in the visible wavelength range, as well as similar electrical conductivity (sheet resistance) to ITO. This work starts with synthesizing ITO-replacing nano-materials, such as copper nanowires (CuNWs), derivative zinc oxide (ZnO) thin films, reduced graphene oxide (rGO) and so on. Further, we applied various deposition techniques, including spin-coating, spray-coating, Mayer-rod coating, filtration and transferring, to coat transparent substrates with these materials in order to fabricate TCEs. We characterize these materials and analyze their electrical/optical properties as TCEs. Additionally, these fabricated single-material-based TCEs were tested in various lab conditions, and their shortcomings (instability, rigidity, etc.) were highlighted. In order to address these issues, we hybridized the different materials to combine their strengths and compared the properties to single-material based TCEs. The multiple hybridized TCEs have comparable optical/electrical metrics to ITO. The doped-ZnO TCEs exhibit high optical transmittance over 90% in the visible range and low sheet resistance under 200Ω/sq. For CuNW-based composite electrodes, ~ 85% optical transmittance and ~ 25Ω/sq were observed. Meanwhile, the hybridization of materials adds additional features such as flexibility or resistance to corrosion. Finally, as a proof of concept, the CuNW-based composite TCEs were tested in dye-sensitized solar cells (DSSCs), showing similar performance to ITO based samples.

In recent years, quasi-one-dimensional materials have attracted a lot of research attention due to their remarkable properties, and their potential as building blocks for nanoscale electronic and optoelectronic devices. A modified chemical vapor deposition (CVD) method has been used to synthesize ZnO nanowires. Electron microscopy and other characterization techniques show that nanowires having distinct morphologies when grown under different conditions. The effects of reaction parameters including reaction time, temperature, carrier gas flow rate, substrates and catalyst material upon the size, shape, and density of ZnO nanowire arrays have been investigated. Excitonic solar cells —including Gratzel-type cells, organic and hybrid organic/inorganic solar cells—are promising devices for inexpensive, large-scale solar energy conversion. Hybrid organic/inorganic solar cells are made from composites of conjugated polymers with nanostructure metal oxides, in which the polymer component serves the function of both light absorber and hole conductor, and the ZnO nanowire arrays act as the electron conductors. Organic solar cells have been fabricated from environmentally friendly water-soluble polymers and ZnO nanowire arrays.

Doctor of Philosophy / Department of Physics / Bret N. Flanders / Directed electrochemical nanowire assembly (DENA) is a method for fabricating nano-structured materials via electrochemical dendritic solidification. This thesis presents two new applications of nano-structured materials that are fabricated via the DENA methodology: cellular force sensors to probe adhesive sites on living cells and single-crystalline metallic dendrites as surface coating materials. Fast migrating cells like D. discoideum, leukocytes, and breast cancer cells migrate by attachment and detachment of discrete adhesive contacts, known as actin foci, to the substrate where the cell transmits traction forces. Despite their importance in migration, the physics by which actin foci bind and release substrates is poorly understood. This gap is largely due to the compositional complexity of actin foci in living cells and to a lack of technique for directly probing these sub-cellular structures. Recent theoretical work predicts these adhesive structures to depend on the density of adhesion receptors in the contact sites, the receptor-substrate potential, and cell-medium surface tension. This thesis describes the fabrication of sub-microscopic force sensors composed of poly(3,4-ethylene dioxythiophene) fibers that can interface directly with sub-cellular targets such as actin foci. The spring constants of these fibers are in the range of 0.07-430 nN m-1. These fibers were used to characterize the strength and lifetime of adhesion between the single adhesive contacts of D. discoideum cells and the fibers, finding an average force of 3.1 ± 2.7 nN and lifetime of 23.4 ± 18.5 s. This capability is significant because direct measurement of these properties will be necessary to measure the cell-medium surface tension and to characterize the receptor-substrate potential in the next (future) stage of this project. The fabrication of smart materials that are capable of the high dynamic range structural reconfiguration would lead to their use to confer hydrophobic, lipophobic, and anti-corrosive character to substrates in a regenerative manner. As a step towards this goal, we have extended the DENA method to enable repetitive growth and dissolution of metallic dendrites to substrates. The experimental parameters that control this process are the frequency and duty cycle of the alternating voltage signal that initiates the dendritic growth.

In this thesis, we demonstrate in-situ scanning electron microscopy techniques for both electron beam lithography (EBL) and in-situ one dimensional nano materials electrical characterization. A precise voltage contrast image positioning for in-situ EBL to integrate nanowires into suspended structures for nanoswitch fabrication has been developed. The in-situ EBL eliminates the stage movement error and field stitching error by preventing any movements of the stage during the nanolithography process; hence, a high precision laser stage and alignment marks on the substrate are not needed, which simplifies the traditional EBL process. The ZnO piezoelectronics is also studied using nano-manipulators in scanning electron microscope. Methods to improve the contact have been demonstrated and the contacts between probe tips and the nanowires are found to have significant impact on the measurement results.

Nanowire solar cells have great potential as candidates for high efficiency, next-generation solar cell devices. To realize their potential, accurate and efficient modeling techniques en- compassing both optical and electrical phenomena must be developed. In this work, a coupled optical and electronic model of GaAs nanowire solar cells was developed, with the goal of building a platform for automated, algorithmic device optimization. Significant work was done on the optical portion of model, with the goal of reducing run- times and improving the level of automation. Enhancements were made to an open-source implementation of the Rigorous Coupled Wave Analysis method for solving Maxwell’s equations, to make it more accurate for modeling nanowire solar cells. Its accuracy and efficiency were thoroughly investigated, and with the enhancements presented here it was shown to be an effective technique for rapid optical modeling of nanowire devices. Purely optical optimizations of a sample AlInP-passivated GaAs nanowire on a GaAs substrate were performed to demonstrate the efficacy of the technique using a Nelder-Mead simplex optimization of device geometry. The optical model was then coupled into a finite volume method based electrical model implemented in TCAD Sentaurus, to compute device efficiencies and ultimately optimize electrical device performance. As a first step, an algorithmic optimization of a p-i-n nanowire solar cell consisting of an AlInP-passivated GaAs nanowire on a Si substrate was performed using the generation rates computed by the enhanced RCWA implementation. The overall geometry was fixed to the result of the optical optimization, and only internal electrical parameters were optimized. The results showed that significant performance improvements can be obtained with the right choice of doping levels and doping region configurations, even without optimizing the global device geometry.

Conjugated polymers have been found useful in a wide range of applications such as solar cells, sensor elements and printed electronics, due to their optical and electronic properties. Functionalization with charged side chains has enabled water solubility, resulting in an enhanced interaction with biomolecules. This thesis focus on the emerging research fields, where these conjugated polyelectrolytes (CPEs) are combined with biomolecules for biological sensing and DNA nanowire assembling. CPEs have shown large potential in biomolecular detection where the optical read out is due to the geometrical alternation in the backbone and aggregation state. This thesis focused on transferring the biomolecular detection to a surface of CPEs. The characterization of the CPE layer show that a hydrogel can be formed, and how the layer can undergo geometrical changes upon external stimulus such as pH change. A selective sensor surface can be created by imprinting ssDNA or an antibody in the CPE layer. The discrimination for complementary DNA hybridization and specific antibody interaction can be monitored by surface plasmon resonance or quartz crystal microbalance. We have also taken the step out from the controlled test tube experiments to the complex environment of the cell showing the potential for staining of compartments and structures in live and fixed cell. Depending on the conditions and CPE used, cell nuclei, acidic vesicles and cytoskeleton structure can be visualized. Furthermore, the live staining shows no sign of toxic effect on cultured fibroblasts. CPEs can also be a valuable element when assembling electronics in the true nano regime. I have used DNA as building template due to its attractive size features, with a width of around 2 nm and a length scale in the µm regime, and the inbuilt base-paring recognition elements. This thesis shows how DNA can be decorated with CPEs and stretched on surfaces into a model for aligned semiconducting nanowire geometries. Not only making the template structures is of importance, but also how to place them on the correct surface position, i.e. on electrodes. Strategies for positioning DNA nanowires using transfer printing and surface energy patterning methods have therefore been developed in the thesis. The stretched DNA decorated with CPE also offers a way to further study the molecular binding interaction between the two molecules. Single molecular spectroscopy in combination with polarization has given information of the variation of the CPE binding along a DNA chain.

Due to a lack of fundamental knowledge about the role of molecular structures in molecular electronic devices, this research is focused on the development of instruments to understand the relation between device design and the electronic properties of electroactive components. The overall goal is to apply this insight to obtain a more efficient and reliable scheme and greater functional control over each component. This work developed a fabrication method for porphyrinoids on graphene-based field effect transistors (FETs), and a chemical sensing platform under an ambient environment by integrating a tip-enhanced Raman spectroscope (TERS), atomic force microscope (AFM), and electronic testing circuit. The study is divided into three aspects. The first is aimed at demonstrating fabrication processes of nanoscale FETs of graphene and porphyrinoid composites based entirely on scanning probe lithography (SPL). A nanoshaving mechanism was used to define patterns on octadecanethiol self-assembled monolayers on gold film evaporated on graphene flakes, followed by metal wet etching and/or oxygen plasma etching to develop patterns on Au films and graphene, respectively. The integrity and optoelectronic properties were examined to validate the processes. The second area of study focused on the development of the chemical sensing platform, enabling chemical changes to be monitored during charge transports under an ambient environment. The localized Raman enhancement was induced by exciting surface plasmon resonance in nanoscale silver enhancing probes made by thermal silver evaporation on sharp AFM tips. As the system was designed along an off-axis illumination/collection scheme, it was demonstrated that it was capable of observing molecular decomposition on opaque and conductive substrates induced by an electric bias. The third line of work proposed a novel TERS system and a probe preparation method. Silver nanowires mounted on AFM tips were used to locally enhance the Raman scattering. The observed Raman enhancement allows quick chemical analysis from a nanoscale region, and thus enables chemical mapping beyond the diffraction limit. Compared with other TERS geometries, the new optical design not only allows analysis on large or opaque samples, but also simplifies the design of the optical components and the alignment processes of the setup.