Abstract |
Chemical sensor technology is one of the most studied field in nano science. Sensing relies on the modification
of the properties (electrical, optical, mechanical, etc.) of the sensing material of, when chemical compounds
(analyte) are absorbed on its surface.
To date, sensing has been achieved almost entirely by monitoring variations in the electrical conductance of
different nanostructured semiconducting materials exposed in varying environments. This approach has
enabled the detection of hydrocarbons, alcohols, oxides, etc. often in ppm range. A serious drawback of this
type of sensors is that efficient performance is achieved at high operation temperatures, in the range 200-400
οC, condition that inhibits their application in the presence of flammable- gases or biological materials
Monitoring the optical properties of semiconducting materials (transmittance, reflectance,
photoluminescence), which also change in the presence of external stimuli, is an alternative approach for
chemical compounds detection, however it has been much less investigated to date. Changes in optical
properties are observable at room temperature, minimal experimental equipment is required for their
recording, while the sensing material is not subjected to kind of processing. These parameters as well as the
capability of remote control make optical sensors very attractive devices for gas detection.
In terms of sensing material, numerous research reports are available, investigating application of
nanostructured (nanoparticles, nanorods or nanowires) semiconducting metal oxides, mainly zinc oxide
(ZnO), as the active sensor element while very limited information is available. for other materials or systems.
The subject of the present work is the investigation of zinc oxide nano hybrids (ZnO / polymer) as sensors that
exploit the Photoluminescence (PL) of the semiconductor, aiming to explore the influence of polymer on
sensor characteristics. To achieve this goal the UV PL emission of ZnO nanoparticles (ZnO-NPs) and ZnO
nanoparticles dispersed in different organic polymeric matrices {ZnO/PEGMA (poly (ethylene glycol
methacrylate), ZnO/PEGMA (ZnO-NPs) ZnO/PVA (poly (vinyl alcohol) and ZnO/PMMA (poly (methyl
methacrylate)}, was investigated, in the presence of oxygen and ozone.
Regarding the fabrication of the specimens, films of ZnO nanoparticles were prepared by dispersing
commercially available ZnO nanoparticles (in powder form) in ethanol solvent and then deposited on glass by
drop casting. A series of ZnO / polymer hybrids (containing 30wt% ZnO) were prepared by mixing appropriate
amounts of ZnO nanoparticles and polymer (PEGMA, PDMS, PVA, PMMA) using the same method.
The optical excitation of the semiconductor was performed with a pulsed ultraviolet laser source (KrF excimer
laser, λex = 248 nm, τ = 15ns), with photon energy (Eph = 5eV) higher than the band gap of ZnO (3.37 eV), inside
a stainless-steel optical chamber, which enabled the controlled introduction and removal of gases. The PL
emission was collected at 45ο with respect to the sample normal by a fused silica optical fiber that was coupled
into the entrance slit of a spectrograph. Emission spectra were recorded on an intensified charge- coupled
device detector (ICCD). In order to quantify the changes in the photoluminescence emitted by the specimens
as a function of changes in the ambient atmosphere, spectra were acquired at 10sec intervals and the
integrated emission intensity I = ∫I(λ)dλ was calculated, in the spectral range 350 - 420 nm.
The ability of the studied materials (ZnO-NPs and ZnO hybrids) to exhibit sensing behavior was first
investigated in a set involving exposure of the samples to low vacuum and at atmospheric air environment
and measuring the variation in their PL emission intensity at room temperature. The ZnO nanoparticles as well
as ZnO/PEGMA and ZnO/PDMS hybrids were found to have sensing capability, while ZnO/PVA and ZnO/PMMA
hybrids were found to have very low response. The influence of the excitation conditions and particularly the
pumping energy density on the sensor performance was also investigated. In accordance with previous
reports, it was confirmed that low pump energy density is crucial for optimizing the sensor response.
The sensing properties of ZnO nanoparticles, ZnO/PEGMA and ZnO/PDMS hybrids towards oxygen sensing was
investigated. Upon exposure to oxygen the materials exhibit a clear decrease in their room temperature
photoluminescence emission, allowing the detection of oxygen in a wide range of pressures from 200 to 0.4
mbar. The materials preserve their sensing properties for an extended exposure of several cycles. The relative
change of the emission intensity (response, % DI) exhibits a linear dependence on oxygen pressure at the lowpressure regime followed by saturation at higher pressures, implying a Langmuir-type adsorption.
Comparing the response of the materials examined, ZnO/PEGMA and ZnO/PDMS hybrids show a lower
response compared with the response of pure ZnO nanoparticles.
In the last stage of the present thesis, ZnO nanoparticles and their hybrids are investigated, for the first time,
as PL based ozone sensors. The performance of these sensing systems was investigated with respect to
response, reversibility, and dynamic characteristics. All experimental measurements revealed that their PL
emission is sensitive to the ozone presence, at room temperature. They exhibit dynamic response for an
extended exposure of several cycles, towards different ranges of ozone concentrations. The PL variations show
a near Langmuir dependence on the ozone concentrations (1600 to 50 ppb), while the limit of detection is less
than 50ppb, value that corresponds to the maximum acceptance level of exposure (World Health
Organization-WHO). Comparing the response of the materials examined, ZnO/PDMS hybrids, show the
optimum response.
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