Nanomaterials characterization and bio-chemical sensing using microfabricated devices
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A variety of nanostructured materials have been synthesized in recent years. These nanomaterials have potential applications in areas spanning computing, energy conversion, sensing, and biomedicine. Because of size confinement effects, furthermore, these nanomaterials are expected to show very different physical properties from those of their bulk counterparts. The measurement of their properties, however, has been very challenging due to their small dimensions. Similarly, it remains a challenge to detect chemical and biomolecular species due to their small dimensions. This dissertation presents the development of microelectromechanical systems (MEMS) devices for the characterization of thermophysical properties of nanomaterials and for the detection of chemical species and biological cells. The thermophysical property of one-dimensional (1D) nanomaterials was measured using a batch-fabricated microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams. Three methods were developed to assemble nanomaterials with the measurement devices. Those three methods include a wet deposition process, an in-situ chemical vapor deposition technique, and an electric-field-assisted assembly method. During the measurement, one membrane is Joule-heated to cause heat conduction through the nanomaterials to the other membrane, allowing for the measurement of thermal conductance and Seebeck coefficient. The electrical conductance can also be measured using the microdevice. The temperaturedependent properties of an individual single-wall carbon nanotubes (SWCNs) and SWCN bundles were measured. Measurement sensitivity, errors, and uncertainty were examined. The obtained thermal conductivity of an individual SWCN is found to be much higher than bundles of SWCNs in the range of 2000-11000 W/m-K at room temperature, in agreement with theoretical predictions. Furthermore, the thermal conductivity of bundles of SWCNs are found to be suppressed by contact resistance between interconnected SWCNs in the bundle. The microdevice has also been integrated with metal oxide nanobelts for chemical sensing. The sensing mechanism is based on surface oxidation-reduction (redox) processes that change the electrical conductance of the nanobelt. The sensor was found to be highly sensitive to inflammable and toxic gas species including nitrogen dioxide (NO2), ethanol, and dimethyl methylphosphonate (DMMP). Furthermore, it eliminated the sensor poisoning effects that have limited the wide use of polycrystalline metal-oxide based sensors. The experiment is a step towards the large scale integration of nanomaterials with microsystems, and such integration via an electric-field-directed assembly approach can potentially enable the fabrication of low-power, ultra-sensitive, and selective integrated nanosensor systems. The electric field manipulation technique has not only been used to assemble nanomaterials with MEMS, but also been used to focus biological cells in a microfluidic channel for cytometry applications. Flow cytometry is a powerful and versatile method of rapidly analyzing large populations of cells and other particulate or molecular analytes that have been captured on the surface of carrier particles. However, the key components of the system, hydrodynamic focusing and optical systems, make conventional cytometers complex, large, and expensive. To eliminate these drawbacks, a dielectrophoretic particle focusing technique combined with MEMS is explored to replace the hydrodynamic focusing mechanism. To focus particles, microelectrodes are patterned on the circumference of the channel to generate AC fringing fields that result in negative dielectrophoretic forces directing cells from all directions to the center of the channel. An ellipticlike microfluidic channel has been fabricated by isotropic etching of soda lime glass wafers and a subsequent wafer-bonding process. Experiments with microbeads and human leukemia HL60 cells and an analysis using a thin shell model indicate that biological cells can be focused using an AC voltage of an amplitude up to 15 Vp-p and a frequency below 100 kHz, respectively. This design eliminates the sheath flow and the fluid control system that makes conventional cytometers bulky, complicated, and difficult to operate, and offers the advantages of a portable standalone instrument as well as a module that could potentially be integrated with on-chip impedance or optical sensors into a micro total analysis system.