Understanding the Mechanisms that Affect the Quality of Electrochemically Grown Semiconducting Nanowires

Template-assisted synthesis of nanowires is a simple electrochemical technique commonly used in the fabrication of semiconducting nanowires. It is an easy and cost-effective approach compared to conventional lithography, which requires expensive equipment. The focus of this chapter is on the various mechanisms involving mass transport of ions during successive stages of the template-assisted electrochemical growth of indium antimonide (InSb) nanowires. The nanowires were grown in two different templates such as commercially available anodic aluminum oxide (AAO) templates and polycarbonate membranes. The chapter also presents the results of characterizing the InSb nanowires connected in a field effect transistor (FET) configuration. The Sb-rich InSb nanowires that were fabricated by DC electrodeposition in nanoporous AAO exhibited hole-dominated transport (p-type conduction). Temperature-dependent transport measurement shows the semiconducting nature of these nanowires

Light-Driven Nanoscale Vectorial Currents

Controlled charge flows are fundamental to many areas of science and technology, serving as carriers of energy and information, as probes of material properties and dynamics, and as a means of revealing or even inducing broken symmetries. Emerging methods for light-based current control offer promising routes beyond the speed and adaptability limitations of conventional voltage-driven systems. However, optical manipulation of currents at nanometer spatial scales remains a basic challenge and a key step toward scalable optoelectronic systems and local probes. Here, we introduce vectorial optoelectronic metasurfaces as a new class of metamaterial in which ultrafast charge flows are driven by light pulses, with actively-tunable directionality and arbitrary patterning down to sub-diffractive nanometer scales. In the prototypical metasurfaces studied herein, asymmetric plasmonic nanoantennas locally induce directional, linear current responses within underlying graphene. Nanoscale unit cell symmetries are read out via polarization- and wavelength-sensitive currents and emitted terahertz (THz) radiation. Global vectorial current distributions are revealed by spatial mapping of the THz field polarization, also demonstrating the direct generation of elusive broadband THz vector beams. We show that a detailed interplay between electrodynamic, thermodynamic, and hydrodynamic degrees of freedom gives rise to these currents through rapidly-evolving nanoscale forces and charge flows under extreme spatial and temporal localization. These results set the stage for versatile patterning and optical control over nanoscale currents in materials diagnostics, nano-magnetism, microelectronics, and ultrafast information science

A mechanically-guided approach to three-dimensional functional mesostructures towards unconventional applications

Controlled formation of three-dimensional functional mesostructures (3DFMs) has broad engineering implications in biomedical devices, microelectromechanical systems (MEMS), optics, and energy storage. Most existing 3D techniques, however, not only lack compatibility with essential electronic materials (silicon, metals, ceramics) that exist in solid-state or crystalline forms, but also produce in a slow and inefficient manner. This is in stark contrast to the planar technologies widely adopted by the modern semiconductor industry. I propose to solve these challenges by a novel 3D assembly strategy based on the planar technologies, which involves precisely controlled 2D-to-3D transformations via the substrate-induced mechanical buckling. This lithography-based, mechanically-guided 3D approach is compatible with virtually any engineering thin films including semiconductors, metals, and polymers, applies to a wide range of length scales and geometries and produces in a high throughput. In this dissertation, I present strategies that combine fabrications and mechanics to achieve a set of complex 3D geometries. I also study the potentials of the 3DFMs in micro-robotics. I further demonstrate the unique applications in energy harvesting, bio-integrated systems, and nanoscale sensing. The results may enlighten the development of advanced, multi-functional 3D electronic micro-systems inaccessible to other 3D techniques

ELECTROSTATIC AND ELECTRICAL TRANSPORT ANALYSIS OF NANOMATERIALS AND NUMERICAL METHODS DEVELOPMENT

The nanotechnology today is continuously boosting the application of nanostructured materials in the development and innovation of electronic devices, such as Nano &ndash Electromechanical Systems (NEMS), electrical transistors, thermoelectric devices, and solar cells. Due to the size miniaturization, quantum mechanical effects play important roles in the performance of such devices. To correctly capture the quantum mechanical effects and understand how these effects influence the electrostatic and electrical transport properties of nanomaterials, efficient and accurate computational models are highly desirable. Currently, the commonly used model for electrostatic analysis of nanoscale devices is based on self &ndash consistent solution of the effective &ndash mass Schroedinger equation coupled with the Poisson equation. However, a major drawback of this model is its inefficiency to simulate systems with large Degrees of Freedom (DOFs). To reduce the computational cost, in this thesis, two Component Mode Synthesis (CMS) approaches, namely the fixed &ndash interface CMS and the free &ndash interface CMS, are incorporated into the Schroedinger &ndash Poisson model to speed up the electrostatic analysis in nanostructures. The new model is employed to analyze the quantum electrostatics in both nanowires and FinFETs. Numerical results demonstrate the superior computational performance in terms of efficiency and accuracy. In addition to the electrostatic analysis, carrier transport in nanostructures with perturbation from quantum effects also merits careful consideration. Among the computational models developed for quantum mechanical carrier transport analysis, the Non &ndash Equilibrium Green &rsquo s Function (NEGF) coupled with Poisson equation has gained vast application in both ballistic and diffusive transport analysis of nanodevices. In this thesis, the NEGF model is expanded to include mechanical strain and carrier scattering effects. Two important multiphysics systems are investigated in this work. We first study the effect of mechanical strain on the electrical conductivity of Si/Si 1 &minus x Ge x nanocomposite thin films. The strain effect on the bandstructures of nano &ndash thin films is modeled by a degenerate two &ndash band k · p theory. The strain induced bandstructure variation is then incorporated in the NEGF &ndash Poisson model. The results introduce new perspectives on electrical transport in strained nano &ndash thin films, which provides useful guidance in the design of flexible electronics. Secondly, nanoporous Si as an efficient thermoelectric material is studied. The Seebeck coefficient and electrical conductivity of nanoporous Si are computed by using the NEGF &ndash Poisson model with scatterings modeled by Buttiker probes. The phonon thermal conductivity is obtained by using a Boltzmann Transport Equation (BTE) model while the electron thermal conductivity is captured by the Wiedemann &ndash Franz law. The thermoelectric figure of merit of nanoporous Si is computed for different doping density, porosities, temperature and pore size. An optimal combination of the material design parameters is explored and the result proves that nanoporous Si has better thermoelectric properties than its bulk counterpart. In the electrical transport analysis of nanomaterials, we found that the standard NEGF &ndash Poisson model using the Finite Difference (FD) method has a high computational cost, and is inapplicable to devices with irregular geometries. To overcome these difficulties, an accelerated Finite Element Contact Block Reduction (FECBR) method is developed in this thesis. The performance of the accelerated FECBR is evaluated through the simulation of two types of electronic devices: taper &ndash shaped DG &ndash MOSFETs and DG &ndash MOSFETs with Si/SiO 2 interface roughness. Numerical results show that the accelerated FECBR can be applied to model ballistic carrier transport in devices with multiple leads, arbitrary geometry and complex potential profile. The accelerated FECBR significantly improves the flexibility and efficiency of electrical transport analysis of nanomaterials and nanodevices

Hybrid Nanomaterials

Two of the hottest research topics today are hybrid nanomaterials and flexible electronics. As such, this book covers both topics with chapters written by experts from across the globe. Chapters address hybrid nanomaterials, electronic transport in black phosphorus, three-dimensional nanocarbon hybrids, hybrid ion exchangers, pressure-sensitive adhesives for flexible electronics, simulation and modeling of transistors, smart manufacturing technologies, and inorganic semiconductors

Novel Multiphysics Phenomena in a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures’ internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors