Semiconducting metal oxide nanostructures for scalable sensing applications

Abstract

Semiconducting metal oxides (SMO) are widely known to respond strongly to changes in their environment through their surface chemistry. Sensors built from field?effect transistors (FET) that use nanostructured SMOs as the active channel have the combined advantages of having an ultra sensitive surface with large surface?to?volume ratio and being equipped with an electronic read?out that can be label?free, fast?responding, portable, and accessible. However, the obstacle of applying metal oxide nanostructures to FET sensing technology in a scalable and controllable process that?s necessary for practical applications must first be tackled. In this thesis two approaches are demonstrated to overcome this problem. ? In chapter one, background information for semiconducting metal oxides, the effect of nanoscale materials on sensitivity, and the mechanism of chemical and biological sensing are presented. ? In chapter two, the growth of epitaxially aligned SnO? nanowires are demonstrated as a ?bottom?up? approach that can be used to fabricate FETs, photo?detectors, polarizers, and chemical sensors in a salable process. Nanowire growth study shows good alignment guided by substrate lattice, and the electronic quality of the aligned SnO? nanowires is demonstrated through its ultra sensitive 0.2ppb detection level of NO?. ? In chapter three, a ?top?down? process is developed to fabricate In?O? nanoribbon biosensors that are highly uniform and sensitive. The fabrication process requires only 2 conventional photolithography steps that are scalable for different wafer sizes and can use a versatile range of substrates. The sensors are demonstrated to have strong and fast response to pH, and a stable surface chemistry is used to apply the nanoribbon sensors to specific and selective biomarker detection. ? In chapter four, polysilicon is investigated as the optimal silicon-based material for developing ?top?down? nanoribbon biosensors. By comparing its fabrication process, pH and biomarker sensitivity with the In?O? nanoribbon, it is concluded that although poly?silicon nanoribbon fabrication is a scalable way to achieve silicon?based ?top?down? nanobiosensors, In?O? nanoribbons are more advantageous in terms of sensitivity, uniformity, and versatility. ? In chapter five, In?O? nanoribbon biosensors are used to perform an electronic enzyme?linked immuno assay (ELISA) for the detection of multiple chest pain biomarkers. Quantitative detection of the cardiac biomarkers troponin, creatine kinase?MB, and B-type natriuretic peptide (BNP) were achieved within clinically relevant concentrations. Detection of BNP in whole blood was also achieved with concentration response within 3% of prediction. ? Finally, chapter 6 presents a summary of the thesis and future directions.