Year: 2004
Type: dissertation
Abstract: A variety of nanostructured materials have been synthesized in recent \nyears. These nanomaterials have potential applications in areas spanning \ncomputing, energy conversion, sensing, and biomedicine. Because of size \nconfinement effects, furthermore, these nanomaterials are expected to show very \ndifferent physical properties from those of their bulk counterparts. The \nmeasurement of their properties, however, has been very challenging due to their \nsmall dimensions. Similarly, it remains a challenge to detect chemical and \nbiomolecular species due to their small dimensions. \nThis dissertation presents the development of microelectromechanical \nsystems (MEMS) devices for the characterization of thermophysical properties of \nnanomaterials and for the detection of chemical species and biological cells. \nThe thermophysical property of one-dimensional (1D) nanomaterials was \nmeasured using a batch-fabricated microdevice consisting of two adjacent \nsymmetric silicon nitride membranes suspended by long silicon nitride beams. \nThree methods were developed to assemble nanomaterials with the measurement \ndevices. Those three methods include a wet deposition process, an in-situ \nchemical vapor deposition technique, and an electric-field-assisted assembly \nmethod. During the measurement, one membrane is Joule-heated to cause heat \nconduction through the nanomaterials to the other membrane, allowing for the \nmeasurement of thermal conductance and Seebeck coefficient. The electrical \nconductance can also be measured using the microdevice. The temperaturedependent \nproperties of an individual single-wall carbon nanotubes (SWCNs) and \nSWCN bundles were measured. Measurement sensitivity, errors, and uncertainty \nwere examined. The obtained thermal conductivity of an individual SWCN is \nfound to be much higher than bundles of SWCNs in the range of 2000-11000 \nW/m-K at room temperature, in agreement with theoretical predictions. \nFurthermore, the thermal conductivity of bundles of SWCNs are found to be \nsuppressed by contact resistance between interconnected SWCNs in the bundle. \nThe microdevice has also been integrated with metal oxide nanobelts for \nchemical sensing. The sensing mechanism is based on surface oxidation-reduction \n(redox) processes that change the electrical conductance of the nanobelt. The \nsensor was found to be highly sensitive to inflammable and toxic gas species \nincluding nitrogen dioxide (NO2), ethanol, and dimethyl methylphosphonate \n(DMMP). Furthermore, it eliminated the sensor poisoning effects that have \nlimited the wide use of polycrystalline metal-oxide based sensors. The experiment \nis a step towards the large scale integration of nanomaterials with microsystems, \nand such integration via an electric-field-directed assembly approach can \npotentially enable the fabrication of low-power, ultra-sensitive, and selective \nintegrated nanosensor systems. \nThe electric field manipulation technique has not only been used to \nassemble nanomaterials with MEMS, but also been used to focus biological cells \nin a microfluidic channel for cytometry applications. Flow cytometry is a \npowerful and versatile method of rapidly analyzing large populations of cells and \nother particulate or molecular analytes that have been captured on the surface of \ncarrier particles. However, the key components of the system, hydrodynamic \nfocusing and optical systems, make conventional cytometers complex, large, and \nexpensive. To eliminate these drawbacks, a dielectrophoretic particle focusing \ntechnique combined with MEMS is explored to replace the hydrodynamic \nfocusing mechanism. \nTo focus particles, microelectrodes are patterned on the circumference of \nthe channel to generate AC fringing fields that result in negative dielectrophoretic \nforces directing cells from all directions to the center of the channel. An ellipticlike \nmicrofluidic channel has been fabricated by isotropic etching of soda lime \nglass wafers and a subsequent wafer-bonding process. Experiments with \nmicrobeads and human leukemia HL60 cells and an analysis using a thin shell \nmodel indicate that biological cells can be focused using an AC voltage of an \namplitude up to 15 Vp-p and a frequency below 100 kHz, respectively. This \ndesign eliminates the sheath flow and the fluid control system that makes \nconventional cytometers bulky, complicated, and difficult to operate, and offers \nthe advantages of a portable standalone instrument as well as a module that could \npotentially be integrated with on-chip impedance or optical sensors into a micro \ntotal analysis system.
Author: Choongho Yu
Language: English
Subfield: Electrical and Electronic Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2279242484