| Abstract |
Zinc oxide (ZnO) is a well-known and studied II-IV wide band gap compound semiconductor that, in recent years, has been in the center of intensive research efforts by scientists around the world, because of enhanced technological interest as regards the development of novel electronic, optoelectronic and spintronic materials and devices. Important driving force for this research have been the plethora of ZnO-based nanostructures, that have been grown in the form of particles, rods, wires, sheets and other nano-architectures, by use of a number of varied methodologies. These novel structures often exhibit properties that are critically modified with respect to those of the bulk semiconductor, and enable scientists to explore their potential in demanding applications.
An interesting application of ZnO nanostructures have been in the field of chemical sensor technology. Sensing relies on a number of redox processes that take place as the analyte gas is adsorbed on the surface of the nanostructure, which lead to changes of its electrical conductance. Monitoring variations in electrical conductance enables quantitative detection of a number of molecular gases and volatile compounds, such as hydrocarbons, alcohols or small gaseous oxides, with appreciable detection limits, often in the low ppm (part per million) range. However a considerable drawback of this approach relates to the fact that sensors based on ZnO nanostructures achieve efficient performance at relatively high temperature, typically in the range of 200 - 400οC, and this represents a technological barrier to a number of applications.
In this context, monitoring the optical properties of semiconductor materials, for example, transmittance, reflectance or photoluminescence emission that have also been found to undergo changes in the presence of external chemical stimuli, is a promising alternative for gas sensing, and such a technology is, to date, much less investigated. Importantly, changes in optical properties are observable at room temperature and their recording requires straightforward experimental equipment, while the sensing material does not necessitate any special preparation or conditioning for conducting a measurement. These features make optical sensing an attractive approach for achieving fast and reliable gas detection and monitoring.
The subject of this thesis is the growth and study of key optical properties of ZnO nanostructures, with the aim to develop optical gas sensors, based on the characteristic photoluminescence (PL) of ZnO in the ultraviolet, which would operate at room temperature. To accomplish this objective, the PL emission and its dependence on the presence of ethanol vapors was studied for a number of different systems with the sensing material being a) in the form of organized structures, ZnO nanorods (ZnO-NRs), grown on hard surfaces (glass, silica, Si or optical fibers) and b) in the form of ZnO nanopatricles dispersed in a soft, flexible polymeric matrix (polydimethylsiloxane-PDMS).
Regarding the synthesis of the ZnO nanorods, the suggested methodology is based on a two-step procedure: this involves the selective growth of a metallic zinc (Zn) precursor layer, followed by an aqueous chemical growth of ZnO nanocrystalline rods, out of an aqueous solution of zinc nitrate hexahydrate (Zn(NO3)2) in the presence of ammonia. The proposed procedure is a simple and flexible scheme, carried out at relatively low temperature (θ < 100 οC) and is consistent with different type of substrates. Additionally, the laser-based techniques employed for the deposition of the Zn layer, allow the deposition of ZnO NRs in the form of micro-structured patterns (patterned growth) or as a continuous layer, on substrates with flat or cylindrical geometry (optical fibers). More specifically, Laser-Induced Forward Transfer (LIFT) or Pulsed Laser Deposition (PLD) was employed for the fabrication of the micro-patterned or the continuous film precursor layer, respectively.
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Ανάπτυξη, ιδιότητες και εφαρμογές νανοδομών ZnO
