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This thesis presents fabrication, characterization and initial results of vertically aligned carbon nanofibers (VACNF)-based electrodes for use as electrochemical sensors. VACNFs are nanostructures that can be fabricated to the desired specifications using a plasma-enhanced chemical-vapor deposition process and are ideal candidates for electrode material because of their excellent electrical and structural properties. The first step of the fabrication of VACNFs on silicon substrates involved photolithography to pattern the interconnects and the catalysts (nickel dots). VACNFs were then grown on silicon substrates from the nickel catalysts, whose size determines the growth of a single nanofiber or a forest of nanofibers. This work presents a method for growth of nanofiber forest for redundancy and uniform vertical growth. A reservoir was built around the nanofibers to keep the liquid samples in contact with the nanofibers during testing. Nanofiber electrodes were characterized electrochemically using ruthenium hexamine trichloride to ensure proper functionality. The biosensor is customizable to selectively detect various elements or compounds depending on the binding materials used on the nanofibers. One example of a sample that can be detected with VACNF electrodes is glucose. The enzymes, horseradish peroxidase and glucose oxidase, were applied to the nanofibers and were immobilized for the testing of glucose. The reference electrode of the electrochemical analyzer was inserted into the reservoir containing the glucose and multiple analyses were performed. The nanofiber electrodes were able to collect the electrons from the electrochemical reactions of glucose and the enzymes. Amperometric data was gathered for the oxidation and reduction potentials and the current was measured as a function of the glucose concentration. Vertically aligned carbon nanofibers fabricated on silicon substrates are ideal electrodes for integration with silicon compatible structures such as complementary metal-oxide-semiconductor (CMOS) microelectronics based transmitting and signal processing integrated circuits.
Exciting new developments are enabling sensors to go beyond the realm of simple sensing of movement or capture of images to deliver information such as location in a built environment, the sense of touch, and the presence of chemicals. These sensors unlock the potential for smarter systems, allowing machines to interact with the world around them in more intelligent and sophisticated ways. Featuring contributions from authors working at the leading edge of sensor technology, Technologies for Smart Sensors and Sensor Fusion showcases the latest advancements in sensors with biotechnology, medical science, chemical detection, environmental monitoring, automotive, and industrial applications. This valuable reference describes the increasingly varied number of sensors that can be integrated into arrays, and examines the growing availability and computational power of communication devices that support the algorithms needed to reduce the raw sensor data from multiple sensors and convert it into the information needed by the sensor array to enable rapid transmission of the results to the required point. Using both SI and US units, the text: Provides a fundamental and analytical understanding of the underlying technology for smart sensors Discusses groundbreaking software and sensor systems as well as key issues surrounding sensor fusion Exemplifies the richness and diversity of development work in the world of smart sensors and sensor fusion Offering fresh insight into the sensors of the future, Technologies for Smart Sensors and Sensor Fusion not only exposes readers to trends but also inspires innovation in smart sensor and sensor system development.
Vertically aligned carbon nanofibers (VACNFs) have found a variety of electronic applications. To further realize these applications, a good understanding of the charge transport properties is essential. In this work, charge transport properties have been systematically measured for three types of VACNF forests with Ni as catalyst, namely VACNFs grown by direct current PECVD, and inductively coupled PECVD at both normal pressure (3.6 Torr) and low pressure (50 mTorr). The structure and composition of these nanofibers have also been investigated in detail prior to the charge transport measurements. It has been found that the dc VACNF body consists of three parts: a 10-15 nm thick graphitic outer layer, cross-struts, and a layer with darker contrast in between. Carbon, nitrogen, silicon, nickel and oxygen are all present in the dc VACNF body. Ni is distributed along the entire dc VACNF body, as first reported in this work. Auger electron spectroscopy results indicate that Ni is primarily located in fiber walls, not in the center catalytic part. Four-probe I-V measurements on individual nanofibers have been enabled by the fabrication of multiple metal ohmic contacts on individual fibers that exhibited resistance of only a few k[Omega]. An O2 plasma reactive ion etch method has been used to achieve ohmic contacts between the nanofibers and Ti/Au, Ag/Au, Cd/Au, and Cr/Au electrodes. Dc VACNFs exhibit linear I-V behavior at room temperature, with a resistivity of approximately 4.2x10−3 [Omega]·cm. Gate effect is not observed when the heavily doped Si substrate is used as a back gate. Our measurements are consistent with a dominant transport mechanism of electrons traveling through intergraphitic planes in the dc VACNFs. The resistivity of these fibers is almost independent of temperature, and the contact resistance decreases as temperature increases. Further studies reveal that the 10-15 nm thick graphitic outer layer dominates the charge transport properties of dc VACNFs. This is demonstrated by comparison of charge transport properties of as-grown VACNFs and VACNFs with the outer layer partially removed by oxygen plasma reactive ion etch. The linear I-V behavior of the fibers does not vary as this outer layer becomes thinner, but displays a drastic shift to a rectifying behavior when this layer is completely stripped away from some regions of the nanofiber. This shift may be related with the compositional differences in the outer layer and the inner core of the nanofibers. Our results imply that by varying the extent of graphitization and structure of the outer layer, it may be possible to achieve controllable charge transport properties for dc VACNFs. VACNFs grown by inductively coupled PECVD at normal and low pressure both have a defective outer layer and a more crystalline inner core. The composition of these fibers is predominately carbon, and Ni is not observed along the fiber body. Nitrogen is present possibly as a result of sample storage in air. Two-probe charge transport measurements indicate linear I-V behavior, and the resistivity of both types of inductively coupled PECVD grown VACNFs is on the order of 10−3 to 10−4 [Omega]·cm.
A combination of Carbon Nanotubes (CNTs) and Ion Selective Field Effect Transistor (ISFET) is designed and experimentally verified in order to develop the next generation ion concentration sensing system. Micro Electro-Mechanical System (MEMS) fabrication techniques, such as photolithography, diffusion, evaporation, lift-off, packaging, etc., are required in the fabrication of the CNT-ISFET structure on p-type silicon wafers. In addition, Atomic Force Microscopy (AFM) based surface nanomachining is investigated and used for creating nanochannels on silicon surfaces. Since AFM based nanomanipulation and nanomachining is highly controllable, nanochannels are precisely scratched in the area between the source and drain of the FET where the inversion layer is after the ISFET is activated. Thus, a bundle of CNTs are able to be aligned inside a single nanochannel by Dielectrophoresis (DEP) and the drain current is improved greatly due to CNTs' remarkable and unique electrical properties, for example, high current carrying capacity. ISFET structures with or without CNTs are fabricated and tested with different pH solutions. Besides the CNT-ISFET pH sensing system, this dissertation also presents novel AFM-based nanotechnology for learning the properties of chemical or biomedical samples in micro or nano level. Dimensional and mechanical property behaviors of Vertically Aligned Carbon Nanofibers (VACNFs) are studied after temperature and humidity treatment using AFM. Furthermore, mechanical property testing of biomedical samples, such as microbubbles and engineered soft tissues, using AFM based nanoindentation is introduced, and the methodology is of great directional value in the area.
Abstract: Recently, nanomaterials have been vigorously studied for the development of biosensors. Among them, carbon nanotubes (CNTs) have stimulated enormous interest for constructing biosensors due to their unique physical and chemical properties such as high surface-to-volume ratio, high conductivity, high strength and chemical inertness. Our study is to develop a general design of biosensors based on vertically aligned CNT arrays. Glucose biosensor is selected as the model system to verify the design of biosensors. In the preliminary design, glucose oxidase (GOx) is attached to the walls of the porous alumina membrane by adsorption. Porous highly ordered anodized aluminum oxide (AAO) prepared by two-step anodization are used as templates. Deposited gold on both sides of template surfaces serve as a contact and prevent non-specific adhesion of GOx on the surface. In order to find out optimized thickness of gold coating, the oxidation and reduction (redox) reaction in [Fe(CN)6]3\168C /[Fe(CN)6]4\168C system is monitored by Cyclic Voltammetry (CV). Subsequently, enzymatic redox reaction in glucose solutions is also attempted by CV. We expect protein layers with GOx form a conductive network. However, no obvious enzymatic redox reaction is detected in the voltammogram. To take advantage of the attractive properties of CNTs, the design of enzyme electrodes is modified by attaching CNT onto the sidewalls of AAO template nanopores and then immobilizing GOx to the sidewalls and tips of CNTs. AAO templates provided vertical, parallel, well separated and evenly spacing nanochannels for CNT growth. Cobalt is used as a catalyst to fabricate CNTs. As a result, multi-walled carbon nanotubes (MWCNTs) are fabricated inside the AAO templates by catalytic chemical vapor deposition (CCVD). Characterization of AAO templates and cobalt electrochemical deposition are employed by scanning electron microscope (SEM), and energy dispersive X-ray spectrometry (EDS). Detailed structure and texture of CNTs are examined by transmission electron microscope (TEM).
The main objective of this research is to design, fabricate, characterize, and test the silicon based vertical electrode Nanogap biosensor which will be used to detect and identify target proteins in aqueous solution.