Physics and Applications of Semiconducting Binary Oxide Nanobelts Field-Effect Transistors

Nanotechnology is a frontier area of scientific research that involves materials and devices with at least one dimension at or below 100 nm scale. The ability to rationally synthesize and characterize functional materials at the nanometer scale has opened up numerous opportunities for unprecedented progress in many fields of research including materials science, solid state physics, chemistry, biology and engineering, and particularly at the interface of these disciplines. Nanoscale solid-state devices have found important applications in electronics and sensing, offering the potential for high-density, low-power, efficient devices with novel functionalities. Nanoscale materials also offer unique opportunities for interfacing with chemical and biological molecular systems for effective transduction of chemical reactions and biological interactions into measurable electrical, mechanical, optical, or magnetic signals. In this thesis, we perform a set of systematic studies on a group of binary oxide nanobelts, primarily to evaluate their performance in electronic device and chemical/biological sensor applications. These nanobelts, synthesized through a simple physical vapor deposition growth method under controlled conditions, have emerged as a class of powerful quasi-one-dimentional nanomaterials with high purity, uniformity and crystallinity. The nanoscale rectangular cross-section, long lengths up to hundreds of micrometers, and large width-to-thickness ratio make them a unique class of nanomaterials with "belt"-like morphology. To study the electrical properties of individual nanobelts, and most importantly, to provide an effective platform for building nanobelt sensors, through photolithography and metallization we successfully define multiple electrical contacts onto a single nanobelt. The electrical transport properties of a single nanobelt are studied through measurement of the temperature dependence of the resistivity. The results reveal that the nanobelts act as doped semiconductors possibly due to atomic defects dopants from high levels of surface oxygen deficiency. To explore potential electronic device applications of the nanobelts, we construct field-effect transistors (FETs) based on individual semiconducting oxide (SnO2 and ZnO) nanobelts , using a back-gated structure with a heavily-doped silicon wafer as the gate and a thin layer of silicon dioxide on top as the delectric. Simultaneous two-terminal and four-terminal measurements enable direct correlation of the FET characteristics with the nature of the contacts. Devices with high-resistance non-Ohmic contacts exhibit Schottky barrier FET behavior. In contrast, low-resistance Ohmic contacts on the nanobelt lead to high-performance n-channel depletion mode FETs with well-defined linear and saturation regimes, large "on" current, and an on/off ratio as high as 107. The excellent intrinsic characteristics of these nanobelt FETs make them ideal candidates as nanoscale biological and chemical sensors based on field-effect modulation of the channel conductance. SnO2 nanobelt FETs with low-resistance Ohmic contacts are characterized as room temperature hydrogen gas sensors. The mechanism for the hydrogen sensing is determined to be the change of the oxygen stoichiometric state of the SnO2 nanobelt surface in the presence of H2. The advantages of these sensors include high sensitivity, short response time, low operating temperature, and low power consumption. Using the same setup and measurements, the important physical parameters such as resistivity, effective carrier concentration, and mobility of the nanobelts are determined from measurements of the transistors' I-V characteristics in varying concentrations of ambient oxygen. Intrinsic channel-limited SnO2 nanobelt FETs are shown to operate as excellent in-solution pH sensors as well, owing to the conductance responses to the surface hydroxyl groups undergoing protonation and deprotonation. Integration of microfluidics and passivation of the electrodes are incorporated for the in-solution sensing. Detailed sensor responses to pH and ion concentration, and the effect of surface molecular modification are characterized. The nanosensor conductance response to the surface electrical field caused by positively charged hydrogen ions and the electrostatic screening effect are discussed in detail. The in-solution pH sensing with SnO2 nanobelt FETs have laid much of the groundwork towards the realization of biosensing. By working with the oxide surface chemistry and controlled organic molecular assembly, we successfully achieve selective nanoscale manipulation on the tin oxide surface with amino silane. This leads to the covalent surface linkage of a B-complex vitamin of interest, biotin. Biotin-functionalized tin oxide nanobelt FET device is characterized as a protein sensor. The specific protein detected is streptavidin, which has high specific binding affinity to biotin. As the streptavidin molecules bind to the biotin on the nanobelt surface, their charges induce local electrostatic gating of the FET channel and a change in its conductance. The electrical detection is corroborated by fluorescence from the quantum dot tags on the streptavidin. Moreover, since the protein charging level is pH dependent, the effect is expected to vary with the solution pH; this effect is demonstrated and the results provide useful guidelines in optimizing the protein detection sensitivity. The detection of a biomedically significant biomolecule using this platform is demonstrated through the sensing of cTnI, one of the protein markers as reliable early indicators of cardiac damage from a cardiac arrest. The successful experiment is realized by functionalizing the nanobelt surface with the cTnI antibody through the biotin-stretavidin linkage. The binding of the cTnI antigen results in a change in the FET channel conductance, which exhibits the expected pH dependence. Control experiments on an unmodified tin oxide nanobelt are performed and no significant conductance response is observed. The results provide a powerful proof of concept for the nano FETs as viable devices for ultrasensitive, highly specific, label-free, and most importantly, portable real-time biomedical sensors. They also showcase the great promise of the nanobelt FET platform in particular, and the combination of surface molecular assembly with nanoscale solid state electronics in general, in basic studies of biomolecular interactions and in novel biomedical applications.