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As electronics reach nanometer size scales, new avenues of integrating biology and electronics become available. For example, nanoscale field-effect transistors have been integrated with single neurons to detect neural activity. Researchers have also used nanoscale materials to build electronic ears and noses. Another exciting development is the use of nanoscale biosensors for the point-of-care detection of disease biomarkers. This thesis addresses many issues that are relevant for electrical sensing applications in biological environments. As an experimental platform we have used carbon nanotube field-effect transistors for the detection of biological proteins. Using this experimental platform we have probed many of properties that control sensor function, such as surface potentials, the response of field effect transistors to absorbed material, and the mass transport of proteins. Field effect transistor biosensors are a topic of active research, and were first demonstrated in 1962. Despite decades of research, the mass transport of proteins onto a sensor surface has not been quantified experimentally, and theoretical modeling has not been reconciled with some notable experiments. Protein transport is an important issue because signals from low analyte concentrations can take hours to develop. Guided by mass transport modeling we modified our sensors to demonstrate a 2.5 fold improvement in sensor response time. It is easy to imagine a 25 fold improvement in sensor response time using more advanced existing fabrication techniques. This improvement would allow for the detection of low concentrations of analyte on the order of minutes instead of hours, and will open the door point-of-care biosensors. / Graduation date: 2011

acase@tulane.edu / Ever since the successful isolation of graphene, plenty of researches have been pursued to study fundamental physics in low-dimensional van der Waals materials, referred to as materials with the existence of out of plane vdW force. Not only graphene but also many other novel vdW materials start to emerge and play important roles in quantum physics. Due to the highly preserved crystal quality of the nanostructures achieved by micromechanical exfoliation, a variety of new phenomenon have been discovered in these novel materials. This dissertation focuses on the discovery and electronic properties study of new vdW materials both in 2D and 1D systems. Semiconducting transition metal dichalcogenides with layered structure have been viewed as the promising channel materials for field-effect transistors (FETs) in modern electronics. To characterize the performance, we have fabricated FETs based on multilayer WS2 thin crystals. By using gold as the contact metal and varying the thickness of the crystal, high-performance FETs with on/off ratio of 108 and mobility up to 234 cm2V-1s-1 at room temperature have been realized. The high performance is associated with the minimized Schottky barrier and a shallow impurity level below the conduction band. Elementary substance and binary compound crystals have limited members belong to 2D or 1D family. Thus, expanding the research to ternary compound materials is necessary. In this regard, we focused on a novel ternary compound 2D material Nb3SiTe6 and studied its magneto-transport. We have discovered that by using such a high crystalline 2D metal, we could study the inelastic electron-phonon (e-ph) interactions involved with reducing dimensions. From 3D bulk to 2D films with a rigid substrate, the weak antilocalization (WAL) signature is gradually enhanced according to our magnetoresistance (MR) measurements. Systematic studies of the temperature dependence of the dephasing rate in the crystal with various thicknesses suggest the suppression of electron-phonon interaction due to quantum confinement of the phonon spectrum. Our work shows great consistency with the long-standing predicted theory. We have successfully expanded the mechanical exfoliation method to 1D material group. As demonstrated by semiconducting quasi-1D materials, Ta2Pd3Se8 (TPdS) and Ta2Pt3Se8 (TPtS), the external force can efficiently break the weak vdW interactions between ribbons. In our work, we have produced ultrathin 1D TPdS and TPtS nanowires, and fabricated 1D FETs showing p-type and n-type transistor behavior respectively. Moreover, we have successfully built the functional logic NOT gate using these two different 1D FETs. / 1 / Xue Liu

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.

Despite the widespread interest in graphene electronics over the last decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this work, we investigate the realization of high-performance graphene field effect transistors implemented on flexible plastic substrates. The optimum device structure for high-mobility and high-bendability is suggested with experimental comparison among diverse structures including top-gate GFETs (TG-GFETs), single/multi-finger embedded-gate GFETs with high-k dielectrics (EG-highk/GFETs), and embedded-gate GFETs with hexagonal boron nitride (h-BN) dielectrics. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobility of 8,000 cm²/V·s, and importantly 32 GHz cut-off frequency. Mechanical studies reveal robust transistor performance under repeated bending down to 0.7 mm bending radius whose tensile strain corresponds to 8.6%. Passivation techniques, with robust mechanical and chemical protection in order to operate under harsh environments, for embedded-gate structures are also covered. The integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene. / text

abstract: Nanofluidic devices in which one single-walled carbon nanotube (SWCNT) spans a barrier between two fluid reservoirs were constructed, enabling direct electrical measurement of the transport of ions and molecules. Ion current through these devices is about 2 orders of magnitude larger than that predicted from the bulk resistivity of the electrolyte. Electroosmosis drives excess current, carried by cations, and is found to be the origin of giant ionic current through SWCNT as shown by building an ionic field-effect transistor with a gate electrode embedded in the fluid barrier. Wetting of inside of the semi-conducting SWCNT by water showed the change of its electronic property, turning the electronic SWCNT field-effect transistor to "on" state. These findings provide a new method to investigate and control the ion and molecule behavior at nanoscale. / Dissertation/Thesis / Ph.D. Physics 2011

The emergence of organic semiconductors is exciting since it promises to open up for straightforward and low-cost fabrication of a wide range of efficient and novel electronic devices. However, in order for this promise to become reality it is critical that new and functional fabrication techniques are developed. This thesis demonstrates the conceptualization, development, realization and implementation of a particularly straightforward and scalable fabrication process: the photo-induced and resist-free imprint patterning technique.Initial experiments revealed that some members of a group of carbon-cage molecular semiconductors – termed fullerenes – can be photochemically modified into dimeric or polymeric structures during exposure to laser light, and, importantly, that the exposed fullerene material retains its good electron-transport property while its solubility in common organic solvents is drastically lowered. With this information at hand, it was possible to design and create well-defined patterns in a solution-deposited fullerene film by exposing selected film areas to laser light and then developing the entire film in a tuned developer solution. An electronically active fullerene pattern emerges at the locations defined by the incident laser beam, and the patterning technique was successfully utilized for the fabrication of arrays of efficient field-effect transistors.In a later stage, the capacity of the photo-induced and resist-free imprint technique was demonstrated to encompass the fabrication of ubiquitous and useful CMOS circuits. These are based on a combination of p-type and n-type transistors, and a blend between a p-type organic semiconductor and an n-type fullerene compound was designed so that the latter dominated. By solution-depositing the blend film on an array of transistor structures, exposing selected transistors to laser light, and then developing the entire transistor array in a developer solution, it was possible to establish a desired combination of (non-exposed) p-type transistors and (exposed) n-type transistors. We finally utilized this combination of transistors for the fabrication of a CMOS circuit in the form of well a-functional organic inverter stage.

We have grown the high quality AlGaN/GaN heterostructure by plasma-assisted molecular beam epitaxy. We obtained the mobility of two-dimensional electron gas of the AlGaN/GaN is 9300 cm2/Vs and carrier concentration is 7.9¡Ñ1012 cm-2 by conventional van der Pauw Hall measurement at 77K. The samples made of the AlGaN/GaN heterostructure were patterned to Hall bar geometry with a width of 20£gm by conventional photolithography. After the photolithography, the nanowire was fabricated by the process of focus ion beam (FIB), and the widths of nanowire were reduced to 900 nm, 500 nm, 300 nm, 200nm, 100 nm, 80 nm and 50 nm respectively. The SiO2 layer and Al electrode were deposed on the samples to form nanowired MOSFETs. We have studied the leakage current measurement on the AlGaN/GaN nanowired MOSFETs at 300K. On the 100 nm and 200 nm width of nanowires, we did not observe the leakage current for the gate voltage work range from -2.5 to 3.0 V and from -0.5 to 0.5 V respectively.

The fabrication processes development for on III-nitride (III-N) heterojunction bipolar transistors (HBTs), heterojunction field-effect transistors (HFETs) and metal-insulator-semiconductor field-effect transistors (MISFETs) were performed. D.c, microwave and quasi-static I-V and C-V measurements were carried out to characterize the fabricated III-N transistors and diodes. The GaN/InGaN direct-growth HBTs (DG-HBTs) grown on free-standing GaN (FS-GaN) substrates demonstrated a high current gain (hfe) > 110, high current density (JC) > 141 kA/cm2, and high power density (Pdc) > 3 MW/cm2. The first III-N DG-HBT showing fT > 8 GHz and fmax > 1.3 GHz were also demonstrated on sapphire substrates. Recessed-gate AlGaN/AlN/GaN HFETs demonstrated Vth = 0 V with 0.17 V deviation across the sample. Baliga's figure of merit is 240 MW/cm2 was achieved. Current collapse was eliminated and the dynamic on-resistance was reduced by 67% after using a remote-oxygen-plasma treatment. Normally-off recessed-gate AlGaN/AlN/GaN MISFETs with Vth = 0.9 V were also fabricated with the remote-oxygen-plasma treatment. Low leakage current (< 1 pA/mm), high on-off ratio (> 2.2E11) are achieved. These achievements suggest that high-performance III-N transistors are very promising for high-power switching and microwave amplification. Findings concerning remaining process issues and implications for future research are also discussed.

La unión de las propiedades de los CNT con los principios de reconocimiento molecular se presenta como una base adecuada para el desarrollo de sensores altamente específicos. El objetivo de la presente tesis ha sido desarrollar sensores químicos, del tipo transistores de efecto campo (CNTFET), basados en interacciones receptor-analito, mediante el empleo de los nanotubos de pared sencilla (SWCNT), que actúan como transductores de la señal analítica.Las principales etapas de la parte experimental han sido: Crecimiento de SWCNT con la técnica de deposición química en fase vapor. Integración de los SWCNTs en sistemas CNTFET. Empleo del CNTFET como base del sensor en distintos campos utilizando modelos de reconocimiento molecular. Dependiendo del tipo de funcionalización de los SWCNTs se pueden obtener sensores para proteínas, iones, etc. Como resultado, se han desarrollado y estudiado sensores basados en CNTFETs para la detección distintos analitos de interés, como son la Inmunoglobulina G Humana, los iones potasio y el dióxido de azufre. / The general objective of this thesis is to develop chemical sensors whose sensing capacities are based on the principle of molecular recognition and where the transduction is carried out by single-walled carbon nanotubes (SWCNT).The sensing device used is the carbon nanotube field-effect transistor (CNTFET). The new structure of the CNTFET allows nanotubes to be integrated at the surface of the devices, thus exploiting SWCNTs' sensitivity to changes in their environment. The functionalization of SWCNTs with several types of molecular receptors such as antibodies, ion selective membranes, and synthetic receptors, achieve a high selectivity towards the analyte of interest. This thesis shows that CNTFETs can be used for the successful selective detection of different types of target analytes. These can be biomolecules such as antigens, small compounds such as cations or gas-phase compounds such as SO2.