Electrical and Electronic Engineering - Theses
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ItemEnhancing thermoelectric performance of graphene through nanostructuring( 2017)The field of thermoelectrics has the potential to solve many problems that are predominant in the electronics industry. However, due to low-efficiency, high material cost, and toxicity, this field is yet to meet its expectations. On another, graphene, a two-dimensional allotrope of carbon has attracted a great deal of attention due to its unique electronic, thermal and mechanical properties. In this work, we explore the suitability of planar materials like graphene for thermoelectric application and propose techniques that have the potential to address the issues that are holding back thermoelectrics from large scale applications. Due to the high thermal conductivity and lack of electrical band gap graphene has not been thoroughly investigated for thermoelectric application. In this work, the inherent properties of graphene are modified through nanostructuring in order to make it suitable for the thermoelectric application. First, the graphene sheet is nanostructured into graphene nanoribbon (GNR). The focus is given to the electronic properties that affect the thermoelectric performance. Graphene nano-ribbon with different width and array combinations are analyzed. To explain the experimental results, a model that considers the effect of scattering mechanism and random charge carrier fluctuations is proposed. Based on the model, a route to further enhance the thermoelectric properties of graphene is presented. In the next part of this thesis, further nanostructuring of graphene nano-ribbon is investigated. Simulation is carried out for GNR with pores. Pores impede the phonon transmission while electron transmission continues to take place at the edges. There have been several reports on utilizing GNRs with pores for thermoelectric application, but the design methodology of such structures has not been thoroughly investigated. In this work, the effect of pore dimensions on thermoelectric parameters is studied through quantum mechanical simulations. The results report a surprising relation between pore width and TE parameters which is later explained using physical insights. Similarly, another approach of nano-structuring GNRs through the introduction of break junctions is investigated. The motivation behind this work is to utilize the tunneling mechanism that is observed in GNR with break junction, in order to achieve delta like transmission spectra. Moreover, the nano-break impedes phonon transmission. As a result, the overall thermoelectric performance of the device is enhanced significantly. Finally, a novel approach of bio-sensing based on the Seebeck coefficient measurement of graphene is proposed and validated using quantum mechanical simulation. This proof of concept study indicates the wide range of applications that are enabled through exploiting thermoelectric properties of planar devices. Overall, the techniques and insights presented throughout the thesis are based on graphene but can be applied and investigated on other two-dimensional materials as well.
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ItemGraphene nanoelectronics( 2016)Graphene is a two-dimensional material consists of tightly packed carbon atoms in a hexagonal lattice with surprising electrical properties. Patterning graphene into nanostructures to achieve desirable electronic properties is an active research area. The aim of the thesis is to explore diverse graphene nanostructures for electronic and biological applications. Novel graphene structures have been proposed including graphene nanoribbons, break junctions and nanopores and investigated for circuit interconnects, negative differential resistance devices and biosensors. Interconnects and nanoscale transmission lines are critical components in the design of nanoelectronic systems. In this thesis, high frequency characteristics of chemical vapour deposition graphene nanoribbon (GNR) interconnects and radio frequency propagation in GNRs embedded in a coplanar waveguide structure up to 20 GHz have been studied. An equivalent transmission line model is proposed to model the GNRs at high frequencies. The solid agreement between the model and the measured data suggests that it can be used in the design of nanoscale circuits in which GNRs are utilized as the interconnect elements. The study provides insight into microwave behavior of GNRs for developing high speed graphene devices. Graphene-based negative differential resistance (NDR) devices hold great potential for enabling the implementation of several elements required in electronic circuits and systems. However, previously proposed devices manifest several drawbacks due to their complex structures and the multiple steps required in the fabrication processes. This research presents a novel structure based on GNR junctions for NDR devices, which can be fabricated using standard lithography techniques. Theoretical simulation shows that GNR junctions with a nano gap in the transport direction of the ribbons can manifest a pronounced NDR phenomenon. The predicted NDR effect is then confirmed through the experimental investigation on the current-voltage characteristics of the fabricated devices. A natural extension of GNR systems is to incorporate graphene nanopores (GNPs). GNPs are promising building blocks for electronic circuitry and bio applications. A large numbers of studies on the fabrication of GNPs have been reported, in which GNPs were realized from GNRs by drilling a tiny hole in the middle of GNRs. However, methods to design GNPs that achive desirable conduction performance and sensing characteristics have not been well understood. Therefore, this dissertation investigated the quantum transport properties of GNPs created by drilling pores in armchair and zigzag GNRs. The study reveals that the quantum transmission spectra of GNPs are highly tunable and GNPs with specific transport properties able to be produced by properly designing pore shapes. This thesis shows that the biological sensing capabilities of GNPs are transmission dependent and can vary dramatically with pore geometry. Finally, it is shown that GNPs with suitable edge passivation can potentially be used as NDR devices. The insight presented in this thesis serves as a guideline for developing several graphenebased devices to obtain required performance characteristics for various applications.
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ItemPlanar nanoelectronic devices and biosensors using two-dimensional nanomaterials( 2015)Graphene, a monolayer of carbon atoms and the first two-dimensional (2D) material to be isolated, has sparked great excitement and vast opportunities in the global research community. Its isolation led to the discovery of a new family of materials that are completely 2D, each of which exhibits unique properties in its own right. Such a wide range of new nanomaterials in a completely unexplored 2D platform offers a potential treasure for the electronics industry, which is yet to be explored. However, after more than a decade of research, nanoelectronic devices based on 2D nanomaterials have not yet met the high expectations set for them by the electronics industry. This thesis hopes to drive these efforts forward by proposing a different approach for the conceptualization of nanoelectronic devices, in light of the new opportunities offered by 2D nanomaterials. The proposed approach is centred on exploiting the truly unique property of two-dimensionality, which defines and distinguishes this exciting family of 2D nanomaterials, for the realization of completely 2D planar nanoelectronic devices. Less reliance is made on individual properties that are unique to individual 2D nanomaterials, however, wherever possible; such properties are exploited in enhancing the performance of the proposed devices. The proposed approach is applied to the conceptualization of a number of planar nanoelectronic devices that have a potential in a range of direct as well as long term envisioned applications, complementing conventional electronics on the short term but also having the potential to revolutionize electronics on the long term. All of the proposed devices are planar, completely 2D and realizable within a single 2D monolayer, reducing the required number of processing steps and enabling extreme miniaturization and CMOS compatibility. For the first time, a 2D Graphene Self-Switching Diode (G-SSD) is proposed and investigated, showing promising potential as a nanoscale rectifier. By exploiting some of graphene’s unique properties, the G-SSD is transformed into different types of planar devices that can achieve rectification, Negative Differential Resistance (NDR) operation and tunable biosensing. The extension of the proposed approach to other types of 2D nanomaterials is also investigated, by exploring the implementation of SSDs using MoS2 and Silicene. Finally, new classes of graphene resonant tunneling diodes (RTDs), with completely 2D planar architectures, are proposed, showing unique transport properties and with promising performance, while requiring minimal process steps during fabrication.
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ItemSilicon nanowire and graphene nanoribbon biological sensors( 2014)One-dimensional (1D) nanoscale devices such as silicon nanowires or graphene nanoribbons have recently received massive research interest thanks to their extraordinary properties. While silicon nanowires (SiNWs) exhibit remarkable surface sensitivity due to their large surface-to-volume ratio, graphene nanoribbons (GNRs) with their unique structure and electrical characteristics promise to deliver rapid and highly sensitive sensors that are flexible and biocompatible. In this thesis, a reliable top-down fabrication technique using CMOS compatible processes is presented, from which state-of-the-art SiNW and GNR device structures are fabricated. The core of the technique is the electron beam lithography (EBL) technology which allows for the formation of nanometre scaled patterns. SiNWs and GNRs fabricated using this method are uniform, well-aligned and have excellent control over device dimensions. The technique can be applied to fabricate other nanoscale structures with a variety of materials. For the first time, a simulation study of a three-dimensional (3D) SiNW biological sensor model demonstrates that the SiNW exhibits an AC-transfer function that resembles that of a high pass filter. It is illustrated that as molecules with a higher net charge attach to the nanowire and displace more charge carriers within the nanowire channel, the filter's corner frequency decreases. This property of SiNW leads to a novel frequency-based detection mechanism that enables one to estimate the concentration of target analytes. To experimentally verify above simulation findings, a boron-doped SiNW with specially designed test pads is fabricated and experimentally characterized at frequencies up to 30 GHz. A transmission line model with a combination of passive circuit elements is proposed. Fitting is then performed based on the SiNW's de-embedded scattering parameters to extract its equivalent circuit components. The study makes significant contribution to the broadband response of SiNWs. It also serves as a guideline towards designing SiNW for biological sensing applications. GNRs are graphene that is cut into ultra-thin strips. Broadband measurement and characterization of GNR, a potential candidate for biological sensor devices, is also demonstrated. Accurate high frequency lumped circuit equivalent models are proposed from which equivalent circuit GNR sensing device components are extracted. It is determined that for narrow devices, inductance dominates. This finding provides important insights into the utilization of GNR as a flexible and biological compatible sensing device. Finally, SiNW devices are then constructed to produce highly sensitive sensors. Here, a functionalized SiNW sensor is demonstrated to provide label-free and sensitive detection of single-stranded deoxyribonucleic acid (DNA). Electrical measurements exhibit changes in device conductance upon specific binding of complementary target DNA. An application is demonstrated where SiNW-based sensors are produced using the technique and new theory described in this thesis. The work in this thesis serves as a development platform for a point-of-care genetic testing device that promises significant contribution to the field of personalized drug treatment and clinical analysis.