When size is
reduced in one or more dimensions, the electromagnetic, thermal, optical,
mechanical and chemical properties of materials change dramatically. Furthermore, in
materials with one or more nanoscale dimensions, these properties can be
tailored to enhance the performances of sensors (in particular the sensing
efficiency). This causes an alternation of the number and density of states
that are fundamental to sensors’ operation [1].
The development of
metal oxide semiconductor sensors goes back to 1952, when Bardeen and Brattain
[2] discovered that the electrical resistance of oxide semiconductors could be
modified under the action of surrounding gases. In
1962, Seyama et al. suggested that semiconducting oxides can be used as gas sensors [3]. Present day studies
are mainly focused to build up new devices and faster technologies. Modern electronic devices require thin
semiconductor films and nanometric structures with accurately tailored properties for specific applications. Oxides of many metals are at present actively studied for their
semiconducting, electrochromic and photochromic properties [4, 5], which are a
function of their band gap values, which in turn depend on the oxide stoichiometry.
Likewise, the
adsorbed gases on surface of semiconductor oxides exert significant electronic
influence on the individual crystalline particles. Such gas-solid state
interaction results in a change in electron or hole density at the surface (a
space charge forms), which in turn causes a change in general conductivity of
the semiconductor oxide. The transition from polycrystalline structures to
amorphous ones results in increasing of surface interaction of detected gas
with the semiconductor oxide. So, one
can postulate that decreasing the size of the sensing material to less than 100
nm results in increasing the sensitivity. Furthermore, the reduction to 10 nm size results in
increasing a ratio of sensor surface to their volume which determines
the increasing of gas sensitivity.
Fe2O3 is a material of
high interest because it can act as a chemical sensor for various gases and
vapors, when doped with metallic ions or in combination with other oxides. Its
efficiency was proven for the detection of H2S [6], CO and CH4 [7],
methanol [8], water vapors [9] among others. In form of micro- and
nano-particles, it can act as a chemical or biological sensor, able to detect
acetone [10], E. Coli, DNA [11], H2S [12].
TiO2 is
an efficient sensor with confirmed specificity for CO, NO2 [13],
acetone, ethanol [14], hydrogen [15], CO [16] or NH3 [17]. The
increase of its efficiency was achieved when in films, nanotubes or nanoparticles.
The refractive index of a TiO2 thin film changes with temperature.
Small variations of reflectivity can be detected by ellipsometry and a
correlation with temperature can be established. Based on this mechanism, a
thermo-optical TiO2 sensor for optical fibers was proposed in [18].
WO3 is an n-type metal oxide semiconductor with oxygen vacancies. It was
studied as gas sensing material and found to be able to detect traces of NH3
[19], ozone [20] H2S, SO2, CH4, NO or NO2
[21] Generally, WO3 sensing structures are synthesized in form of
thin films and doped with ions of noble metals to increase the detection
sensitivity. As nanowires, WO3 can be used as photo chemical sensor [22].Via
laser irradiation, oxygen and water evaporate from the surface, thus producing a
change in the electrical conductivity that can be quantified.
ZnO is a wide-bandgap semiconductor vastly studied for its sensing capabilities. To ensure an increased detection range, ZnO is usually combined with other oxides or doped with metal ions that act as catalysts. A newer trend is to increase the ZnO sensor’s active surface, by growing the structures as nanoparticles or nanowires. Ozone [23], NO2 [24], CO, CH4, butane [25], thanol [26], lucose [27] are among the materials detectable via modified ZnO nanostructures. Nanowires of ZnO can be used as biological sensor for DNA, by/using photo induced currents detection [28].
Significant efforts have been recently focused upon metal oxide doping or surface clustering with nanoscaled metal additives (Pt, Pd, Sb, Au, Ru). In case of optical sensors, an enhanced sesnitivity was obtained when the active oxide surfaces were sputtered with noble metal nanoparticles. It was suggested that the sharply increased sensitivity of the optical metal oxide sensors partially covered with nanoclusters is due to the surface plasmon resonance [29].
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