Un-doped and metal-doped ZnO nanowires for energy harvesting applications based on piezoelectric and triboelectric effects

Abstract

This dissertation describes an experimental study on synthesis and characteristics of metal doped Zinc oxide (ZnO) nanowires (NWs). In addition, the fabrication and analysis of piezoelectric energy harvesting application with ZnO NWs and triboelectric energy harvesting application are demonstrated in the thesis.

Part I : Characteristics of metal doped ZnO nanowires

Among various wide-band-gap semiconductors, zinc oxide (ZnO) is an increasingly promising candidate as a building block material for optoelectronic devices, such as light-emitting diodes, photo detectors/sensors, and laser diodes, due to its wide band gap of 3.37 eV and large exciton binding energy of 60 meV. In particular, one-dimensional (1D) nanostructures such as nanowires (NWs) have attracted great interest over the past decade because of their specific physical properties and potential as basic elements for next-generation nano-optoelectronic devices. First, structural properties of ZnO NWs synthesized by hydro thermal process were investigated. The ZnO NWs were synthesized on ZnO thin film seed layers via an aqeous solution method with zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and hexamethylenetetramine (HMT). The growth speed and the shape of the ZnO NWs were determined for various mole concentrations. The structural analysis of the ZnO NWs was performed using X-ray diffraction, SEM, and TEM measurements. The correlation of structural results wit growth conditions, such as the mole concentration and the growth temperature of chemical precursors based on Gibbs free energy. Second, because un-doped ZnO NWs are naturally n-type, their use in practical devices has been hindered; thus, significant effort has been dedicated toward the development of p-type ZnO NWs. To date, many groups have successfully reported p-type ZnO NWs using group I and group V elements as the initial p-type dopants. Among these reports, the p-type ZnO NWs tested with group V elements are highly susceptible to the large lattice mismatch in the radii between group V dopants and oxygen ions. Due to the challenges associated with group V element doping, efforts have focused on the synthesis of p-type ZnO NWs using group I elements, In case of Li dopants, Li has the smallest ionic radius (0.76 Å) that is very close to that of Zn (0.74 Å). TEM meausrements of the c-axis oriented and highly vertically aligned NWs demonstrated that Li defect in as-grown Li-doped ZnO NWs can occupy the empty cages of the wurtzite structure at octahedral sites, and that Li substitution of Zn occurred because of thermally-induced migration due to post-annealing in the presence of oxygen. The low-temperature and temperature-dependent photoluminescence spectra clearly exhibited emission peaks that confirmed the presence of a lithium impurity as an acceptor dopant. Particularly, the acceptor energy level of the Li dopant was estimated to be 121 meV from the PL spectra. This value was also indicated from an Arrhenius plot of the integrated PL intensity of the AoX emission as a function of temperature. The stable formation of p-type Li-doped ZnO NWs using a NWFET and a simple n-type thin film / p-type annealed Li -doped ZnO NWs homojunction diode. In case of Ag dopants, zinc nitrate hexahydrate (Zn(NO3)2•6H2O) and hexamethylenetetramine (HMTA) solution was prepared in deionized water at 25 mmol and silver nitrate AgNO3 solution was also prepared in deionized water at 1 mmol without using any high temperature post-annealing processes. The optical properties taken at 10 K indicated significant insight into the effects of Ag-doping. The low-temperature and temperature-dependent photoluminescence feature revealed emission peaks that indirectly confirmed the feasibility of a silver impurity as an acceptor dopant. In particular, the acceptor energy level of the Ag dopant was estimated to be 119 meV from the PL spectra. Finally, we demonstrate the rationally modulate surface electrostatic energies for crystallographic-selective growth of ZnO wires with an electronic configuration scheme. The facets and orientations of ZnO wires are transformed between hexagonal and rectangular/diamond cross-sections with polar and non-polar growth directions, exhibiting different optical and piezoelectrical properties. Our novel synthetic route for ZnO wire fabrication provides new opportunities for future opto-electronics, piezoelectronics, and electronics, with new topological properties.

Part II : Characteristics of energy harvesting applications.

The anticipated increasing use of key natural energy resources, which become extremely scarce, in the coming decades have sparked significant world-wide efforts toward the search for cost-effective, renewable and green energy sources to meet the global energy demands of the future. In this regard, advances in self-powered nanotechnology, that allow for the design of efficient energy harvesting, offer an enormous potential for the creation of sustainable systems utilizing unlimited natural ambient energy sources. Recent developments in nanogenerators harvesting energy steadily from ambient mechanical vibrations without regard to time, place, or any external conditions, present innovative and emerging research topics in the area of a green energy technology. First, we describe that the device performance of a sound-driven piezoelectric energy nanogenerator (SPENG) is remarkably improved by controlling both the carrier density and the interfacial energy in a semiconducting ZnO NWs, thereby achieving its intrinsic efficiency limits. A SPENG with carrier-controlled ZnO NWs exhibits excellent energy harvesting characteristics with an average power density of 0.9 mW cm-3, as well as a near 50 fold increase in both output voltage and current compared to those of a conventional ZnO NW. In addition, we demonstrate for the first time that an optimized SPENG is large enough and very suitable to drive electrophoretic ink displays based on voltage-drive systems. Second, we report solution-based synthesis of Ag-doped ZnO nanowires on flexible polyester substrates without using any high temperature annealing processes. Along with the demonstration of the efficient features of Ag-doped nanogenerators through the measurement of a sound-driven piezoelectric energy device with an output power of 0.5 mW, which is nearly 2.9 times that of a nanogenerator with un-doped ZnO NWs. This finding could provide the possibility of high output nanogenerators for practical applications in future portable/wearable personal display and motion sensors. Finally, we present the electrical responses of a textile substrate-based triboelectric nanogenerator (T-TENG), including nanostructured surface configurations provided by Al nanoparticles and PDMS, in which no intricate fabrication process was performed or required. Along with analysis of the working principle and finite element simulation, the textile-based output power density of 33.6 mW/cm2 was obtained from the periodic mechanical stress of an adjustable bending machine, and this was used to activate commercially available light emitting diode (LED) bulbs. Additionally, we demonstrated the generation of sufficient energy from clothes attached to a commercial arm sleeve to power the LED devices from the activity of a human arm. Our practical cloth approach may contribute to a general framework for developing functional and self-powered wearable electronic devices.