This dissertation describes experimental studies of the superconducting proximity effect in single-crystal Pb, Sn, and Zn nanowires of lengths up to 60 um, with both ends of the nanowires in contact with macroscopic electrodes that are either superconducting (Sn or Pb) or non-superconducting (Au). The Pb, Sn, and Zn nanowires are fabricated using a template-based electrochemical deposition method. Electric contacts to the nanowires are formed in situ during electrochemical growth. This method produces high transparency contacts between a pair of macroscopic electrodes and a single nanowire, circumventing the formation of oxide or other poorly conducting interface layers. Extensive analyses of the structure and the composition of the nanowire samples are presented to demonstrate that (1) the nanowires are single crystalline and (2) the nanowires are clean without any observable mixing of the materials from the electrodes. The nanowires being investigated are significantly longer than the nanowires with which electrode-induced superconductivity was previously investigated by other groups. We have observed that in relatively short (~6 um) Sn and Zn nanowires, robust superconductivity is induced at the superconducting transition temperatures of the electrodes. When Sn and Pb nanowires are in contact with a pair of Au electrodes, superconductivity is suppressed completely. For nanowires of 60 um in length, although the suppression of superconductivity by Au electrodes is only partial, the induced superconductivity at the higher transition temperatures of the electrodes remains full and robust. Therefore, an anomalous superconducting proximity effect has been observed on a length scale which far exceeds the expected length based on the existing theories of the proximity effect. The measured current-voltage characteristic of the nanowires reveals more details such as hysteresis, multiple Andreev reflection, and phase-slip centers. An interesting relation between the proximity effect and the residual-resistance-ratio of the nanowires has also been observed. Possible mechanisms for this proximity effect are discussed based on these experimental observations.

Silicon as a mono-crystalline bulk semiconductor is today the predominant material in many integrated electronic and photovoltaic applications. This has not been the case in lighting technology, since due to its indirect bandgap nature bulk silicon is an inherently poor light emitter.With the discovery of efficient light emission from silicon nanostructures, great new interest arose and research in this area increased dramatically.However, despite more than two decades of research on silicon nanocrystals and nanowires, not all aspects of their light emission mechanisms and optical properties are well understood, yet.There is great potential for a range of applications, such as light conversion (phosphor substitute), emission (LEDs) and harvesting (solar cells), but for efficient implementation the underlying mechanisms have to be unveiled and understood.Investigation of single quantum emitters enable proper understanding and modeling of the nature and correlation of different optical, electrical and geometric properties.In large numbers, such sets of experiments ensure statistical significance. These two objectives can best be met when a large number of luminescing nanostructures are placed in a pattern that can easily be navigated with different measurement methods.This thesis presents a method for the (optional) simultaneous fabrication of luminescent zero- and one-dimensional silicon nanostructuresand deals with their structural and optical characterization.Nanometer-sized silicon walls are defined by electron beam lithography and plasma etching. Subsequent oxidation in the self-limiting regime reduces the size of the silicon core unevenly and passivates it with a thermal oxide layer.Depending on the oxidation time, nanowires, quantum dots or a mixture of both types of structures can be created.While electron microscopy yields structural information, different photoluminescence measurements, such as time-integrated and time-resolved imaging, spectral imaging, lifetime measurements and absorption and emission polarization measurements, are used to gain knowledge about optical properties and light emission mechanisms in single silicon nanocrystals.The fabrication method used in this thesis yields a large number of spatially separated luminescing quantum dots randomly distributed along a line, or a slightly smaller number that can be placed at well-defined coordinates. Single dot measurements can be performed even with an optical microscope and the pattern, in which the nanostructures are arranged, enables the experimenter to easily find the same individual dot in different measurements.Spectral measurements on the single dot level reveal information about processes that are involved in the photoluminescence of silicon nanoparticles and yield proof for the atomic-like quantized nature of energy levels in the conduction and valence band, as evidenced by narrow luminescence lines (~500 µeV) at low temperature. Analysis of the blinking sheds light on the charging mechanisms of oxide-capped Si-QDs and, by exposing exponential on- and off-time distributions instead of the frequently observed power law distributions, argues in favor of the absence of statistical aging. Experiments probing the emission intensity as a function of excitation power suggest that saturation is not achieved. Both absorption and emission of silicon nanocrystals contained in a one-dimensional silicon dioxide matrix are polarized to a high degree. Many of the results obtained in this work seem to strengthen the arguments that oxide-capped silicon quantum dots have universal properties, independently of the fabrication method, and that the greatest differences between individual nanocrystals are indeed caused by individual factors like local environment, shape and size (among others). / <p>QC 20120920</p>

Point-of-care (POC) diagnostics presents a giant market opportunity with profound societal impact. In particular, specific detection of DNA and protein markers can be essential for early diagnosis of e.g. cancer, cardiovascular disease, infections or allergies. Today, identification of these markers often requires extensive laboratory work and hence is expensive and time consuming. Current methods for recognition and detection of specific biomolecules are mostly optics based and thus impose severe limitations as to convenience, specificity, sensitivity, parallel processing and cost reduction. Electronic sensors based on silicon nanowire field-effect transistors have been reported to be able to detect biomolecules with concentrations down to femtomolar (fM) level with high specificity. Although the reported capability needs further confirmation, the CMOS-compatible fabrication process of such sensors allows for low cost production and high density integration, which are favorable for POC applications. This thesis mainly focuses on the development of a multiplex detection platform based on silicon nanowire field-effect sensors integrated with a microfluidic system for liquid sample delivery. Extensive work was dedicated to developing a top-down fabrication process of the sensors as well as an effective passivation scheme. The operation mechanism and coupling efficiencies of different gate configurations were studied experimentally with the assistance of numerical simulation and equivalent circuits. Using pH sensing as a model system, large effort was devoted to identifying sources for false responses resulting from the instability of the inert-metal gate electrode. In addition, the drift mechanism of the sensor operating in electrolyte was addressed and a calibration model was proposed. Furthermore, protein detection experiments were performed using small-sized Affibody molecules as receptors on the gate insulator to tackle the Debye screening issue. Preliminary results showed that the directionality of the current changes in the sensors was in good agreement with the charge polarities of the proteins. Finally, a graphene-based capacitor was examined as an alternative to the nanowire device for field-effect ion sensing. Our initial attempts showed some attractive features of the capacitor sensor.

Elongated nanostructures have attracted a great deal of interest due to unique optical, electrical and physical properties. In particular, silver nanowires and nanobeams have proven to be top contenders for a variety of applications. Due to their nano-sized dimensions, however, electrical access to individual nanowires is difficult and expensive. Here, a simple and economical procedure was designed to electrically contact small elongated structures using common facilities available at most universities. A common lithographic procedure is used to pattern gold pads and electrodes on top of nanowires already dispersed on a substrate. This process is tested by first characterizing, using a 4-point-probe measurement, a novel nanobeam created by fusing silver nanodisks. The resistivity of the nanobeams was found to be as low as 2.7x10^−8 Ω·m, which is only slightly above that of bulk silver. These measurements corroborate modeling done by another group that the nanodisks align to create a nearly continuous crystal rather than disjointed grains. In the second application, Joule-heating was used to actualize a reliable weld between silver nanowires synthesized using the polyol method. The nanowires were situated in series between two metal pads, and a procedure was designed to use electrical current to break down intermediate layers without destroying the nanowires themselves. In the last enterprise, individual silver nanowires were isolated between two gold pads and then using the same electrical recipe used for welding nanowires, the contact resistance was reduced to a negligible portion of its original value. It was found that due to the reduction in contact resistance, the 2-point-probe resistivity of the nanowire was similar to those conducted using 4 probes. The invented procedure can thus allow accurate resistivity measurements of individual metal nanowires to be done with only 2 contacts rather than 4, thereby simplifying contact fabrication and allowing appropriate contacts to be deposited on nanowires as short as 4 μm using standard photolithography.

Silver nanowire transparent electrodes have recently received much attention as a replacement for indium tin oxide (ITO) for use in various electronic devices such as touch panels, organic solar cells, and displays. The fabrication of silver nanowire electrodes on glass substrates with a sheet resistance as low as 9 Ω/□ and 90% optical transparency at 550 nm is demonstrated. These resistance and transparency values match that of commercially available indium tin oxide and are superior to other alternatives such as carbon nanotube electrodes. The nanowire electrodes are low cost and easy to fabricate. Moreover, by depositing nanowire films on plastic substrates, mechanically flexible electrodes are obtained. The silver nanowire electrodes are integrated into several electronic devices: transparent heaters, organic solar cells, and switchable privacy glass. The concerns about the suitability of silver nanowire electrodes for use in commercial electronic devices are discussed. High surface roughness, one of the major concerns, is addressed by introducing a new method of embedding silver nanowires in a soft polymer. The instability of silver nanowire electrodes under current flow is also demonstrated for the first time. It is shown that silver nanowire electrodes fail under current flow after ass little as 2 days. This failure is caused by Joule heating which causes the nanowires to break up and thus create an electrical discontinuity in the nanowire film. Suggestions for improving the longevity of the electrodes are given.

Photodetectors are semiconductor devices that can convert optical signals into electrical signals. There is a wide range of photodetector applications such as fiber optics communication, infrared heat camera sensors, as well as in equipment used for medical and military purposes. Nanowires are thin needle-shaped structures made of semiconductor materials, e.g. gallium arsenide (GaAs), indium phosphide (InP) or silicon (Si). Their small size, well-controlled crystal structure and composition as well as the possibility to fabricate them monolithically on silicon make them ideally suited for sensitive photodetectors with low noise. In this project, Fourier Transform Infrared (FTIR) Spectroscopy is used to investigate the optical characteristics of InP nanowire-based PIN photodetectors. The corresponding electrical characteristics are also measured using very sensitive instrumentation. A total of 4 samples consisting of processed nanowires with 80 nm diameter but different density and length have been examined. The experiments were conducted from 78K (-196oC) to room temperature 300K (27oC). The spectrally resolved photocurrent and current-voltage (IV) curves (in darkness &amp; under illumination) for different temperatures have been studied and analyzed. The samples show excellent IV performance with very low leakage currents. The photocurrent scales with the number of nanowires, from which we conclude that most photocurrent is generated in the substrate. Spectrally resolved photocurrent data, recorded at different temperatures, display strong absorption in the near-infrared region with interesting peaks that reveal the underlying optical processes in the substrate and nanowires, respectively. The nature of the absorption peaks is discussed in detail. This study is an important step towards integration of optically efficient III-V nanoscale devices on cheap silicon substrates for applications e.g. on-chip optical communication and solar cells for energy harvesting.

To allow electronic devices to operate within allowable temperatures, heat sinks and fans are employed to cool down computer chips. However, cooling performance is limited by air gaps between the computer chip and the heat sink, due to the fact that air is a poor heat conductor. To alleviate this problem, thermal interface material (TIM) is often applied between mating substrates to fill air gaps. Carbon nanotube (CNT) based TIM has been reported to have excellent thermal impedance; however, because it is non biodegradable, its potential impact on the environment is a concern. In this thesis research, two types of TIMs were designed, synthesized, and characterized. The first type, Designed TIM 1, consisted of anodic aluminum oxide (AAO) templates with nanochannels (pore size=80nm) embedded with copper nanowires by electrodeposition. This type of nanostructure was expected to have low thermal impedance because the forest-like structure of copper nanowires can bridge two mating surfaces and efficiently transport heat one dimensionally from one substrate to the other. The second type, Designed TIM 2, was fabricated by sandwiching Designed TIM 1 with commercially available thermal grease to further reduce thermal impedance. It was expected that the copper nanowire structures would secure the thermal grease in place, thus preventing grease pump-out under contact pressure, which is a common problem associated with the usage of thermal grease. The morphologies of the two designed TIMs were studied using scanning electron microscopy (SEM), and their thermal properties were determined using ASTM D5470-06, the standard method for testing thermal transmission properties of thermally conductive materials. Experiments were conducted to evaluate the proposed TIMs, as well as commercially available TIMs, under different temperature and pressure settings. Experimental results suggest that the thermal impedance of TIMs can be reduced by increasing contact pressure or reducing thickness. Designed TIM 2 yielded 0.255℃-cm2/W, which is lower than thermal grease and other available TIMs at the operating temperature of 50 to 60℃. Considering the application limitations and safety issues of thermal grease, phase change material, and CNT-based TIMs, our designed TIMs are safe and promising for future applications.

This dissertation describes experimental studies of the superconducting proximity effect in single-crystal Pb, Sn, and Zn nanowires of lengths up to 60 um, with both ends of the nanowires in contact with macroscopic electrodes that are either superconducting (Sn or Pb) or non-superconducting (Au). The Pb, Sn, and Zn nanowires are fabricated using a template-based electrochemical deposition method. Electric contacts to the nanowires are formed in situ during electrochemical growth. This method produces high transparency contacts between a pair of macroscopic electrodes and a single nanowire, circumventing the formation of oxide or other poorly conducting interface layers. Extensive analyses of the structure and the composition of the nanowire samples are presented to demonstrate that (1) the nanowires are single crystalline and (2) the nanowires are clean without any observable mixing of the materials from the electrodes. The nanowires being investigated are significantly longer than the nanowires with which electrode-induced superconductivity was previously investigated by other groups. We have observed that in relatively short (~6 um) Sn and Zn nanowires, robust superconductivity is induced at the superconducting transition temperatures of the electrodes. When Sn and Pb nanowires are in contact with a pair of Au electrodes, superconductivity is suppressed completely. For nanowires of 60 um in length, although the suppression of superconductivity by Au electrodes is only partial, the induced superconductivity at the higher transition temperatures of the electrodes remains full and robust. Therefore, an anomalous superconducting proximity effect has been observed on a length scale which far exceeds the expected length based on the existing theories of the proximity effect. The measured current-voltage characteristic of the nanowires reveals more details such as hysteresis, multiple Andreev reflection, and phase-slip centers. An interesting relation between the proximity effect and the residual-resistance-ratio of the nanowires has also been observed. Possible mechanisms for this proximity effect are discussed based on these experimental observations.

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Integrated photonic circuits require small photonic elements. Recent progress in nanowire synthesis and nanofabrication enables us to investigate the potential of nanowires in novel integrated photonic devices. This thesis explores light manipulation on two material platforms – metallic nanostructures that support surface plasmon polaritons (SPPs), and periodic dielectric arrays for mode engineering. In Chapters 2 and 3, I will show that chemically-synthesized metallic nanowires are attractive candidates to support SPPs and enhance light- matter interactions. The first model device consists of a single quantum emitter in close proximity to a highly crystalline Ag nanowire. When the quantum emitter is optically excited, its emission rate is enhanced by a factor of 2.5, and 60% of the emission couples into the Ag nanowire, generating single SPPs. In addition to optically exciting SPPs, we demonstrate an optoelectronic device that generates and detects SPPs electrically, paving the way for seamless integration between electronic and plasmonic elements in a single circuit. In Chapter 4, I present a general strategy to create stretchable and flexible photonic devices. Flexible photonics has garnered a lot of interest because mechanical properties can be exploited to generate highly conformal devices with novel optical characteristics. We fabricated Si nanowire photonic crystal cavities and transferred them into polydimethylsiloxane (PDMS). The composite photonic crystal cavity supports high quality factor (Q) modes in the telecommunication range. We achieve mechanical reconfiguration of the cavity by stretching it, and observe tuning of the resonance wavelength over 67 nm, 134 times the resonance linewidth. The above demonstrations, when taken together, underscore the promise and potential of nanowires in integrated photonic circuits. / Chemistry and Chemical Biology