Self-Assembled Liquid-Gated Zinc Oxide Nanowire Transistors: Fabrication and Sensing Operation

This thesis aims at the site-specific realization of self-assembled field-effect transistors (FETs) based on semiconducting Zinc oxide NWs and their application towards chemical and bio-sensing in liquid medium. At first, a solution based growth method for hierarchical ZnO nanostructures was devised in order to achieve synthesis of high quality ZnO NWs. This solution based growth method was then deployed for the growth of NWs from preferred sites on a substrate. In order to make a transistor, microelectrode pairs prewritten on a Si/SiO2 chip (4mm◊4mm) using standard photolithography procedure served as growth sites and also formed source and drain terminals. The NWs bridging these "source" and "drain" electrodes formed the transistor channel. The procedure here yields ZnO NWs in up to 100 percent of the positions available for growth. The site-specific self-assembled fabrication of ZnO NW FETs was evaluated in terms of scalability and applications. The procedure was extended from 4mm◊4mm silicon chips to large area silicon wafers (80mm◊80mm) and flexible substrates of Kapton polyimide of the same size. On the large area substrates this procedure yields ZnO NWs in up to 80 percent of the positions available for growth, which can be bettered. Furthermore, the method was also employed to fabricate ZnO NW FETs in situ in a microfluidic channel. For this purpose the precursor solution was let to flow on to the microelectrode pairs with the help of microfluidic channels assembled on top of the substrate. The substrate was heated locally under the microelectrode pairs to stimulate the NW growth. Thus a solution-based self-assembled fabrication of NW FETs was achieved locally inside a microfluidic channel. Microfluidic channels were made of silicon nitride (Si3N4) on top of Si/SiO2 chips and facilitated the characterization of FETs in liquid medium. Alternatively, microfluidic channels made of polydimethylsiloxane (PDMS) were used for characterization of transistor devices in liquids. In order to deploy FETs in liquids, the back-gate is replaced by a reference electrode (Ag/AgCl) and the potential is applied through a liquid surrounding the transistor channel (ZnO NW). The liquid surrounding the ZnO NW creates an electrochemical double layer (EDL) on the NW surface which in turn gives rise to the gate capacitance. The resulting gating effect can be used to modulate the NW conductance depending on the voltage applied through the liquid. The ZnO NW transistors showed a current modulation of up to 6 orders of magnitude, high field-effect mobilities (around 1.85 cm2/Vs) and sub-threshold slopes as low as 105 mV/decade. This is the first demonstration of liquid-gated FETs using ZnO NWs. The liquid-gated FETs are used as a basic device for further sensing trials in liquids. For this purpose, the FETs were functionalized with receptor molecules. These so called ion-selective FETs (ISFETs) were demonstrated as functional pH sensors for liquids with pH values from 6 to 10. The NWs functionalized with analyte-sensitive molecules on their surface are influenced electrically by the presence of analytes in the surrounding liquid. Any change in the amount of analyte present in the surrounding liquid is thus reflected in the electrical transport characteristics of the FET. 3-aminopropyltriethoxysilane (3-APTES) molecules were used to functionalize the ZnO NWs in order to construct the pH sensors. Subsequently, the realization of a biosensor based on ZnO NW transistors is demonstrated. Label-free direct detection of urea molecules was carried out from a solution of urea in buffer. ZnO NWs were electrochemically functionalized with the enzyme urease incorporated into an electro-polymerized polypyrrole matrix. Urease molecules act as specific receptors for urea molecules and catalyze an enzymatic reaction. This reaction causes a pH change in the vicinity of NWs, which is reflected in the field-effect characteristics of transistor. The performance of liquid-gated ZnO NW FETs used as chemical and biological sensor can be further improved by employing a metal gate in liquid medium. This was established by fabricating liquid gated metal-semiconductor FET (MESFET) based on ZnO NWs. ZnO NWs were decorated with metal nanoparticles (NPs) by using an electrochemical deposition method. The NPs were then used as metal gate and devices were characterized by applying a gate voltage on the NPs through the liquid. The realization of metal-semiconductor gate in liquids considerably improved the field-effect characteristics of liquid-gated FETs based on ZnO NWs. The realization of high performance liquid-gated transistors based on ZnO NWs thus constitutes a suitable platform for label-free detection of biomolecules and shows promise for future applications in chemical analysis and medical diagnostics.


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