JSSSJournal of Sensors and Sensor SystemsJSSSJ. Sens. Sens. Syst.2194-878XCopernicus PublicationsGöttingen, Germany10.5194/jsss-5-179-2016High-temperature stable indium oxide photonic crystals: transducer material
for optical and resistive gas sensingAmrehnSabrinaWuXiaWagnerThorstenthorsten.wagner@upb.deDepartment of Chemistry, University of Paderborn, Paderborn,
GermanyThorsten Wagner (thorsten.wagner@upb.de)25May2016511791859November20157March201622April2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://jsss.copernicus.org/articles/5/179/2016/jsss-5-179-2016.htmlThe full text article is available as a PDF file from https://jsss.copernicus.org/articles/5/179/2016/jsss-5-179-2016.pdf
Indium oxide (In2O3) inverse opal is a promising new transducer
material for resistive and optical gas sensors. The periodically ordered and
highly accessible pores of the inverse opal allow the design of resistive
sensors with characteristics independent of structure limitations, such as
diffusion effects or limited conductivity due to constricted crosslinking.
Additionally the photonic properties caused by the inverse opal structure can
be utilized to read out the sensors' electronical state by optical methods.
Typically semiconducting sensors are operated at high temperatures
(> 300 ∘C). To maintain a good thermal stability of the
transducer material during operation is a minimum requirement. We present
results on the synthesis and investigation of the structural stability of the
In2O3 inverse opal structure up to a temperature of 550 ∘C
(limit of substrate material). As will be shown, their optical properties are
maintained with only slight shifts of the photonic band gaps which can be
explained by the results from the structural characterization using X-ray
diffraction and electron microscopy combined with optical simulations.
Introduction
Indium oxide is not only known as a sensing material for resistive
semiconducting gas sensors for the detection of NO2, O3, ethanol,
hydrogen or CO (Ivanovskaya et al., 2001; Wagner et al., 2011, 2013; Takada
et al., 1993; Zheng et al., 2009; Martin et al., 2004; Yamaura et al., 1996).
Because of its optical properties it is also utilized for transparent
conductors (typically doped with tin, ITO) for electronic components such as
flat screen displays, solar cells and LEDs (Lewis and Paine, 2000; Kim et
al., 1998).
The combination of these two properties, namely the gas sensitivity and the
transparency in the visible regime combined with its high refractive index,
makes indium oxide an interesting candidate for building optical transducers
for gas sensors. Periodically ordered nanostructures of dielectric materials
with a periodicity in the order of the wavelength of interested
electromagnetic waves show interesting new, structure related optical
properties. Therefore these structures are commonly referred to as photonic
crystals (Joannopoulos et al., 1997). The most prominent feature of a
photonic crystal is its photonic band gap. Light of certain wavelengths
cannot propagate along one (stop band) or all (complete band gap) directions
of the photonic crystal. Photons with the energy within the photonic band gap
region are reflected. Therefore, the photonic crystals operating in visible
range could show intense color impressions. The position and size of the band
gap are determined by structure parameters (e.g., periodicity, symmetry,
geometry, filling fraction) and the refractive index contrast between
the wall material and the fluid in the pore (Joannopoulos et al., 1997).
Variation of one of these properties can be observed by a change in the
reflectance spectra.
Photonic crystals can be synthesized with different methods, e.g.,
electron beam lithography (Cheng and Scherer, 1995), direct laser writing
(Deubel et al., 2004) or laser holography (Miklyaev et al., 2003).
In the following, we focus on the inverse opal structure, a specific type of
three-dimensional photonic crystals. By utilizing self-assembly of spherical
particles (artificial opals) as a template and a consecutive casting step the
inverse opal structure offers a relatively simple method of production
(Stein et al., 2008) with the drawback of a high amount of lattice defects and
macroscopic cracks due to shrinkage. These structural imperfections are
crucial for some highly demanding applications such as informatics and
telecommunications; but for most sensing applications the reflectance at the
photonic band gap of these chemically prepared photonic crystals is high
enough to be used as an optical signal for sensing. So the advantage of a
fast and cheap synthesis combined with the possibility of scaling up for mass
production allows this method to find its own niches of application where
extremely high structural quality is not required.
As for sensing applications, a typical sensing mechanism of photonic crystals
is to optically read out the change of their reflection spectra resulting
from the change of their refractive index contrast between the solid phase
and the introduced fluids (Nair and Vijaya, 2010). This mechanism can be
used, for example, to detect liquids with different refractive indices (Amrehn et
al., 2015). Another typical mechanism is based on the reaction of the
detected species with the solid phase itself, inducing a variation of
electronic and optical properties of the photonic crystals (Xie et al.,
2012).
Besides sensors utilizing the inverse opal structure for optical readout
also resistive type sensors take some advantage of the highly accessible
pores of the inverse opal. The structure allows the design of sensing layers
with characteristics independent of structure limitations such as diffusion
effects or limited conductivity due to constricted crosslinking (Scott et
al., 2001).
As for some special sensing applications, such as sensors operating at high
temperature (above 500 ∘C), remote sensing using optical readout
of photonic crystals is of special interest. Because of remote sensing there
is no need for the wire connection, which may break down at high
temperature, between the transducer and the optical devices for measuring
the signal change. Since the optical signal of a photonic crystal is caused
by its structure, any degradation of structure at high temperature can be
detected by the optical devices, which is difficult to be realized for
sensors based on other porous materials.
Despite of all these advantages for photonic crystals used as sensors at
high temperature, the thermal stability of the photonic crystals needs to be
tested before they are applied to this new type of sensor concept. In this
paper, indium oxide is synthesized in the inverse opal structure and heat
treated at different temperatures. Afterwards structural changes and the
corresponding optical properties changes are investigated, to evaluate the
potential for indium oxide with the inverse opal structure for high
temperature sensing applications.
Experimental
The indium oxide inverse opal structures in this study are prepared utilizing
a modified three-step version (Fig. 1) of a literature-known, template-based
synthesis protocol (Stein et al., 2008). In the first step, monodisperse
polymethylmethacrylat (PMMA) spheres are synthesized by surfactant-free
emulsion polymerization. Controlled deposition of these spheres (second step)
leads to artificial opals which are, in the third step, used as rigid structure
matrices for the casting of the indium oxide inverse opal, which results from
thermal conversion of an infiltrated indium precursor species and removal of
the matrix.
Scheme of the inverse opal synthesis by casting a PMMA opal: the
PMMA spheres are deposited onto a glass slide at 60 ∘C, and then the
opal pores are filled with an indium nitrate solution. This composite is
dried, the indium nitrate is thermally converted to indium oxide and the PMMA
template is removed by combustion.
To investigate the thermal stability of indium oxide in the inverse opal
structure, the photonic band gap position (optical spectroscopy), the
crystallite size (powder X-ray diffraction) and the structure of the indium
oxide films (electron microscopy) are compared before and after heat
treatment. The temperature limit of this investigation is given by the glass
transition of the glass slides as substrates (550 ∘C).
Synthesis
The polymethylmethacrylat (PMMA) spheres were synthesized by surfactant-free
emulsion polymerization (Egen and Zentel, 2004), this leads to monodisperse
spheres. A total of 400 mL water was heated to reflux in nitrogen atmosphere. After
stopping the nitrogen flow, methylmethacrylat (21.3 mL, 0.5 mol L-1,
Merck, 99 %) and ethylene glycol dimethacrylat, (0.57 mL, Merck,
97.5 %) as a crosslinking agent, were destabilized by filtrating with
Al2O3 and added to the flask. The mixture was stirred for 10 min
(300 rpm). K2S2O8 (27 mg, Bayer) was dissolved in 1 mL
water and added. The mixture was stirred for 2 h at 100 ∘C, then
the flask was opened and the mixture cooled to room temperature. By filtering
the mixture through a paper filter, large aggregates were removed.
For the opal template structure preparation, microscopy slides were cleaned
with ethanol and acetone, then heated up to 60 ∘C on a heating
plate. A total of 40 µL of the PMMA dispersion was placed on the slide with a
microliter pipette and the solvent was removed by evaporation at
60 ∘C.
The In2O3 inverse opal films were synthesized by casting the PMMA
opal. Therefore, the opal pores were filled with an indium nitrate solution
(0.016 g In(NO3)3⋅xH2O (Sigma Aldrich, 99.99 %) in
0.043 mL ethanol) by placing 2 µL of this solution on top of the
opal film. The resulting composite was dried at room temperature for 24 h
and at 60 ∘C for 72 h. Finally, the indium nitrate was converted to
indium oxide in an incineration furnace at 300 ∘C (2 h; heating
rate 0.5 ∘C min-1). A scheme of the synthesis procedure is
shown in Fig. 1.
Thermal stability tests
The optical and structural change of the indium oxide inverse opal films
before and after each heat treatment step were characterized with a modified
Fourier transform infrared spectrometer with wavelength extension to the
visible regime (FTIR-vis) and with the X-ray diffraction method,
respectively. The heat treatment was performed in an incineration furnace
under air. The heating rate was 5 ∘C min-1, the dwell time was
5 h and the tested temperatures were 350, 400, 450, 500 and 550 ∘C.
For each temperature, a fresh sample was prepared to avoid the superposition
of time-dependent aging effects. The sample annealed at 550 ∘C was
again tested after characterization at 550 ∘C for 72 h to
investigate the influence of prolonged heating on the stability. Before this
additional heating step the sample was cooled down slowly over 24 h to
reduce unwanted effects by thermal stress and maintain comparability with the
other samples. To gain some information on reproducibility, an additional
series of samples was synthesized and annealed at 550 ∘C for 72 h.
Characterization
The reflectance spectra were recorded with a Vertex 70 FTIR spectrometer
under the Hyperion 1000 light microscope (Bruker) modified with aluminum
mirrors and a xenon light source to extend the wavelength range to visible
regime (referred to as FTIR-vis). A silicon wafer with known reflectance was
used as a reference for normalization to obtain the absolute reflectance of
samples. For every sample, four measurement spots were randomly chosen and
their reflectance was averaged. Gauss fit was used for determination of the
reflectance maximum. The scanning electron microscope (SEM) images were taken
with a Zeiss Neon 40. The powder X-ray diffraction (PXRD) patterns were
recorded with a D8 Advance (Bruker). The X-rays have a wavelength of
0.154 nm, generated by a CuKα-tube. The measurement range was
from 20 to 70∘ 2θ with a step size of 0.02∘ and an
integration time of 3 s. The crystallite size is calculated with the
Scherrer equation. Therefore the full width at half maximum and the peak
maximum of the (222) peak is evaluated with Lorentzian fit; the shape factor
is 0.94.
Simulation
The photonic band structures of both the inverse opal structure and rod
structure were calculated using a free software package “MIT
Photonic-Bands” (MPB 1.4.2). MPB calculates fully vectorial eigenmodes of
Maxwell's equations with periodic boundary conditions by preconditioned
conjugate-gradient minimization of the block Rayleigh quotient in a planewave
basis (Johnson and Joannopoulos, 2001). The structural model of the inverse
opal structure is constructed by placing air spheres in the lattice points of
fcc structure and filling the complementary solid phase with In2O3
with the refractive index of 1.85 taken from literature (Senthilkumar and
Vickraman, 2010). The dispersion and absorption of the light in visible range
is assumed to be negligible. The radius of the air sphere is varied to model
the structure with different volume fractions of the solid phase varying from
1.8 to 62.0 %. The rod structure is still an fcc structure. This structure
is constructed by connecting the octahedral and tetrahedral interstitial
sites in the conventional unit cell of fcc structure by rods to form eight
tetrahedral bonded structures in the unit cell, which corresponds to two
tetrahedral boned structures in the fcc primitive cell. The rod is an ideal
structural motif to model the materials connecting the interstitial sites,
where the detailed surface roughness and topology as observed in SEM is
simplified.
Optical microscope images from an as-synthesized indium oxide film
(left) and after annealing at 550 ∘C for 77 h (double heated for
5+72 h, see text) (right) at the same location of the same sample.
These rods are filled with In2O3 with the same
refractive index as in the inverse opal structure. The radius of the rod is
varied to vary the volume fraction of the solid phase from 4.7 to 47.7 %.
For the simulation of both structural models, the grid resolution is 32
pixels per basis vector in a given direction. The mesh size used to average
the refractive index at each grid point is set to be 7. At each k point of high symmetry, 10 bands are
calculated. The stop gaps along
Γ-L direction, which corresponds to the 〈111〉 surface
normal of the real photonic crystal, are used to evaluation. The resulting
photonic band structure diagram in frequency is calculated into a diagram in
wavelength for the specific photonic crystals synthesized here with a lattice
constant of 339 nm.
Results and discussion
A great advantage for sample evaluation of the here presented inverse opals
compared to many other types of nanostructured materials is the structural
color in the visible regime due to the photonic stop bands. This allows fast
evaluation of the quality of the synthesized product even by naked eye and
might be later used as a feature for self-testing a high-temperature sensor
device by colorimetry. As can be seen in the optical microscope (Fig. 2),
the indium oxide inverse opals do show an intense blue color after synthesis.
This color as well as the crack pattern is preserved after the heat treatment
which is a first indication for thermal stability of the photonic crystals'
framework. A more detailed optical characterization of the indium oxide
inverse opal films before and after heat treatment (Fig. 3) reveals
reflection bands with maxima between 446 and 466 nm (corresponding to blue).
Since there is no such reflection observed in nonstructured indium oxide
films and the wavelength range corresponds to the results of theoretical
simulations (see later) this color is taken as a strong evidence for (i) the
successful casting of the inverse opal structure and (ii) the conservation of
the framework after heat treatment. Figure 3 also shows the evolution of the
maximum reflectance. As can be seen, it is conserved (ca. 3.5 %)
throughout the heat treatment. This implies that (i) the size of the ordered
regions is conserved, (ii) the position and size of the macroscopic cracks
remains constant and (iii) there is no large change of refractive index of
the In2O3. However, especially after the prolonged 550 ∘C
treatment there is a strong increase in the background reflectance. In
principle, the background reflectance is caused by the opal–substrate
interface. Therefore we assume that this strong increase is due to changes of
the utilized glass substrates which, at 550 ∘C, is close to the glass
transition temperature.
Reflectance spectra of the indium oxide inverse opals: average of
the spectra before heat treatment, and spectra after heat treatments at
different temperatures. The grey horizontal line marks the maximum
reflectance value of 3.5 % after 77 h treatment at 550 ∘C. The
vertical red line at 450 nm serves as a guide for the eye to recognize the
shift of the maximum intensity.
A more detailed analysis of the position of the reflectance maximum
(Fig. 4) reveals a shift to shorter wavelengths. Before the heat treatment
of the indium oxide the mean value of the band gap maximum for the different
samples is 454 nm. After the heat treatment the reflectance maximum shifts
e.g., to 449 nm after the 550 ∘C treatment for the 5 h treated
samples. However, as discussed before, the absolute value of the reflectance
of the indium oxide photonic crystal does not decrease. For the samples
treated for 72 h, a stronger change in the reflectance peak position is
observed. As will be discussed below, this is most likely due to the thinning
of the inverse opal framework by diffusion of an indium species into gas
phase. For later applications this has to be evaluated more carefully in the
context of the targeted working conditions. Especially the oxygen
concentration in the surrounding atmosphere will have a strong impact on the
diffusion behavior.
Reflectance maximum after heat treatments at different temperatures.
Error bars: standard deviation of the mean value of the measurement of five
different spots on each sample. Values were measured from 6 samples for the cases
of both as-synthesized and 72 h treatment, and the rest of the values were measured from 2 samples for each case.
Summarizing the results from the optical characterization it can be concluded
that indium oxide inverse opals are a thermally stable transducers for high-temperature
applications in the range of 550 ∘C for a limited timespan
(72 h) since their optical properties are preserved. For lower
temperature as e.g., 500 ∘C it is assumed that even hundreds of
hours can be achieved according to Arrhenius law, since the processes which lead to
the degradation of the material are thermally activated.
To further investigate the origin of the observed shift of the reflection
bands, a detailed structural analysis utilizing PXRD and SEM was carried out.
The XRD results (Fig. 5) show the typical reflection pattern for cubic
crystalline indium oxide phase (JCPDS 71-2194). As the heat treatment
temperatures increase from 300 to 550 ∘C, the crystallite size
calculated by the Scherrer equation increases from about 11.5 to 20 nm
(Fig. 6). Similar increase of crystallite size after heat treatment was
also reported for In2O3 film deposited by the electron beam evaporation
method (Senthilkumar and Vickraman, 2010).
PXRD data: average of the as-synthesized samples and after heat
treatments at different temperatures.
Crystallite sizes derived from the (222) peaks using the Scherrer
equation for samples before and after heat treatment. Error bars: standard
deviation of the mean value of the crystallite size of 12 different “as-synthesized” samples.
As the position of the stop band is strongly affected by the periodicity of
the inverse opal structure, SEM analysis was carried out. Figure 7 shows the
evolution of the structure after different temperature treatments. Contrary
to the expected change in periodicity, which was not observed, the
periodicity remains stable at about 240 nm (distance between the centers of
two neighboring pores), but there are strong variations in the pore shape to
be observed. The as-synthesized samples show a typical nanocast inverse opal
structure with spherical pores interconnected by circular windows (Fig. 7a,
spherical pore marked in green, windows in red). For the 450 ∘C
treated sample the window size is increased (Fig. 7b) and after
550 ∘C the windows are widened in such a way that the inverse opal
framework is formed by a rod-type structure (Fig. 7c).
SEM images of the inverse opal structures: as
synthesized (a), annealed at 450 ∘C (b) and at
550 ∘C for 5+72 h (c). Insets show zoomed out region
of (a) and (c) with spherical pore marked in green, windows
connecting pores in red and rods forming after 550 ∘C treatment in
blue.
Compared to the as-synthesized sample, which shows a smooth surface in the
SEM, the indium oxide annealed at 550 ∘C shows thickness variations
along the rods. The narrow necking on the rods' surface might be correlated
to the thermal grooves typically formed along grain boundaries on the surface
of thin polycrystalline materials after annealing (Gottstein, 2004). Mainly
driven by the energy reduction of the system via eliminating grain
boundaries, the crystallite size (grain size) increases as the heat treatment
temperature increases which is shown above in the XRD results. Meanwhile, the
margin of the window of the as-synthesized inverse opal structure changes its
shape to the rod-type structure driven by the reduction of surface energy.
The thermal grooves on the surface of rods form when the average grain size
reaches the size of the smallest sample dimension, which is the diameter of
these rods here in nanometer range. These grooves lead to a retarding force
on the grain boundary migration reducing the grain growth rate, which is
beneficial for the thermal stability we are aiming at. These grooves are, in
principle, even able to stop grain growth when the grain size is about twice
the sample thickness in the case of a thin sheet sample (Gottstein, 2004).
This effect may explain the cease of grain growth after prolonged heat
treatment (72 h) at high temperature (550 ∘C) (Fig. 6). However,
to further prove this, prolonged treatments have to be carried out.
Simulated mid-gap wavelengths for indium oxide in the inverse opal
structure and the after annealing rod-type structure (structural model as
inset) at different volume fractions of solid phase, the dashed lines show
the measured reflectance maxima.
We consider Ostwald ripening and surface diffusion to be the dominating
mechanisms which might lead to the formation of the new rod-type structure.
Ostwald ripening is driven by indium oxide vapor pressure difference at the
highly curved window margins (high pressure) and at the less curved surface
of rods (low pressure). The indium oxide evaporates from the margin and
condenses on the rods. Surface diffusion (mass transfer diffusion) can also
occur since the surfaces of as-synthesized inverse opal wall structures most
probably contain many surface defects, and therefore allow the formation
of mobile adparticles to transfer material.
To test if these structural changes due to heat treatment might be
responsible for the observed blueshift of the reflectance maxima, simulations
of the photonic band structures of the as-synthesized sphere-type as well as
of the rod-type structure have been carried out utilizing MPB (Johnson and
Joannopoulos, 2001). Results (Fig. 8) show a band gap of the
mid-gap wavelength of 460 nm (the same as the average wavelength of the
reflection maxima of the as-synthesized samples) would be opened for the
inverse opal structure when the volume fraction of indium oxide is assumed to
be 19 %. Given the shrinkage of materials during the casting process,
this smaller volume fraction is considered to be a reasonable and close-to-reality
volume fraction of the solid phase in the as-synthesized inverse opal
structure, instead of 26 % for the ideal case of face-centered cubic
close packing. For the very same volume fraction, the simulation results show
that the band gap blueshifts by 2 nm (from 460 to 458 nm) from sphere-type
to rod-type structure. This theoretical blueshift qualitatively agrees well
with the experimentally observed shift in the same direction, but the amount
of the measured shift is larger (21 nm). However, the simulation results
show that loss in indium oxide mass also will have a strong impact on the
position of the reflectance maximum. A reduction of the solid volume fraction
by about 7 %, probably due to evaporation of the indium oxide, can lead
to the observed shift to 433 nm.
Summarizing the results of the SEM, XRD characterizations and the photonic
band structure simulations it can be concluded that the structural change
from sphere-type to rod-type in combination with a 7 % mass loss can
explain the observed changes in optical properties. However, this theoretical
estimation is only accurate under the assumption that the refractive index of
indium oxide remains constant. At the current stage, it is difficult to
measure the refractive index change of the inverse opal films as a porous
material after the heat treatment. In the literature, it was reported that the
refractive index of In2O3 film deposited by electron beam
evaporation increases as the temperature increases, which may be attributed
to the improved crystallinity, and the changes of the packing density and
porosity of the film after annealing (Senthilkumar and Vickraman, 2010).
Since this synthesis method is completely different from the one used in the
present study, it is difficult to estimate whether the similar trend of
refractive index change occurs here. In case of increased refractive index
after annealing, the expected mass loss which can lead to the observed
blueshift should be larger than 7 %.
Conclusions
Indium oxide inverse opals show stable optical properties up to the
temperature of 550 ∘C in air for 72 h. The observed structural
changes due to heat treatment, such as grain growth and pore shape
modifications, occur in the length scale which is 1 order of magnitude smaller than
the periodicity of the photonic crystals. The visible light as a sensing
signal is not sensitive to these minor structural changes as long as the
periodicity and refractive index of the photonic crystals remain stable at
high temperature. This inherent optical “inertness” of photonic crystals
to structural changes at certain level makes them interesting candidates for
a new class of optical gas sensing transducers at high temperatures.
Furthermore, it is practically very convenient that the structural color of
photonic crystals can be utilized as an indicator for the structural
integrity since it can be readout by the naked eye. Not only is the possibility
of remote sensing using optical readout attractive for high-temperature
application but also for conventional resistive sensing the open inverse
opal structure might be of interest since it allows the design of sensors
with characteristics independent of structure limitations.
Acknowledgements
We thank the Federal Ministry of Education and Research (BMBF, 13N12969) for
financial support. Edited by:
J. Zosel Reviewed by: two anonymous referees
ReferencesAmrehn, S., Wu, X., Schumacher, C., and Wagner, T.: Photonic crystal-based
fluid sensors: Toward practical application, Phys. Status Solidi A, 212,
1266–1272, 10.1002/pssa.201431875, 2015.Cheng, C. C. and Scherer, A.: Fabrication of photonic band-gap crystals, J.
Vac. Sci. Technol. B, 13, 2696–2700, 10.1116/1.588051, 1995.Deubel, M., von Freymann, G., Wegener, M., Pereira, S., Busch, K., and
Soukoulis, C. M.: Direct laser writing of three-dimensional photonic-crystal
templates for telecommunications, Nat. Mater., 3, 444–447,
10.1038/nmat1155, 2004.Egen, M. and Zentel, R.: Surfactant-Free Emulsion Polymerization of Various
Methacrylates: Towards Monodisperse Colloids for Polymer Opals, Macromol.
Chem. Phys., 205, 1479–1488, 10.1002/macp.200400087, 2004.
Gottstein, G.: Physical Foundations of Materials Science, Springer Berlin
Heidelberg, Berlin, Heidelberg, ISBN 978-3-662-09291-0, 2004.Ivanovskaya, M., Gurlo, A., and Bogdanov, P.: Mechanism of O3 and NO2
detection and selectivity of In2O3 sensors, Proceedings of the 8th
International Meeting on Chemical Sensors IMCS-8 – Part 2, 77, 264–267,
10.1016/S0925-4005(01)00708-0, 2001.Joannopoulos, J. D., Villeneuve, P. R., and Fan, S.: Photonic crystals:
putting a new twist on light, Nature, 386, 143–149, 10.1038/386143a0,
1997.Johnson, S. and Joannopoulos, J.: Block-iterative frequency-domain methods
for Maxwell's equations in a planewave basis, Opt. Express, 8, 173–190,
10.1364/OE.8.000173, 2001.Kim, J. S., Granström, M., Friend, R. H., Johansson, N., Salaneck, W. R.,
Daik, R., Feast, W. J., and Cacialli, F.: Indium–tin oxide treatments for
single- and double-layer polymeric light-emitting diodes: The relation
between the anode physical, chemical, and morphological properties and the
device performance, J. Appl. Phys., 84, 6859–6870, 10.1063/1.368981,
1998.Lewis, B. G. and Paine, D. C.: Applications and Processing of Transparent
Conducting Oxides, MRS Bulletin, 25, 22–27, 10.1557/mrs2000.147, 2000.
Martin, L. P., Pham, A.-Q., and Glass, R. S.: Electrochemical hydrogen sensor
for safety monitoring, Solid State Ionics, 175, 527–530,
10.1016/j.ssi.2004.04.042, 2004.Miklyaev, Y. V., Meisel, D. C., Blanco, A., von Freymann, G., Busch, K.,
Koch, W., Enkrich, C., Deubel, M., and Wegener, M.: Three-dimensional
face-centered-cubic photonic crystal templates by laser holography:
fabrication, optical characterization, and band-structure calculations, Appl.
Phys. Lett., 82, 1284–1286, 10.1063/1.1557328, 2003.Nair, R. V. and Vijaya, R.: Photonic crystal sensors: An overview, Prog.
Quant. Electron., 34, 89–134, 10.1016/j.pquantelec.2010.01.001, 2010.Scott, R. W. J., Yang, S. M., Chabanis, G., Coombs, N., Williams, D. E., and
Ozin, G. A.: Tin Dioxide Opals and Inverted Opals: Near-Ideal Microstructures
for Gas Sensors, Adv. Mater., 13, 1468–1472,
10.1002/1521-4095(200110)13:19<1468::AID-ADMA1468>3.0.CO;2-O, 2001.Senthilkumar, V. and Vickraman, P.: Annealing temperature dependent on
structural, optical and electrical properties of indium oxide thin films
deposited by electron beam evaporation method, Curr. Appl. Phys., 10,
880–885, 10.1016/j.cap.2009.10.014, 2010.Stein, A., Li, F., and Denny, N. R.: Morphological Control in Colloidal
Crystal Templating of Inverse Opals, Hierarchical Structures, and Shaped
Particles, Chem. Mater., 20, 649–666, 10.1021/cm702107n, 2008.Takada, T., Suzuki, K., and Nakane, M.: Highly sensitive ozone sensor,
Sensor. Actuat. B-Chem., 13, 404–407, 10.1016/0925-4005(93)85412-4,
1993.Wagner, T., Hennemann, J., Kohl, C.-D., and Tiemann, M.: Photocatalytic ozone
sensor based on mesoporous indium oxide: Influence of the relative humidity
on the sensing performance, Proc. 7th International Workshop on Semiconductor
Gas Sensors, 520, 918–921, 10.1016/j.tsf.2011.04.181, 2011.Wagner, T., Kohl, C.-D., Malagù, C., Donato, N., Latino, M., Neri, G.,
and Tiemann, M.: UV light-enhanced NO2 sensing by mesoporous In2O3:
Interpretation of results by a new sensing model, Selected Papers from the
14th International Meeting on Chemical Sensors, 187, 488–494,
10.1016/j.snb.2013.02.025, 2013.Xie, Z., Xu, H., Rong, F., Sun, L., Zhang, S., and Gu, Z.-Z.: Hydrogen
activity tuning of Pt-doped WO3 photonic crystal, Thin Solid Films, 520,
4063–4067, 10.1016/j.tsf.2012.01.027, 2012.Yamaura, H., Jinkawa, T., Tamaki, J., Moriya, K., Miura, N., and Yamazoe, N.:
Indium oxide-based gas sensor for selective detection of CO, Sensor. Actuat.
B-Chem., 36, 325–332, 10.1016/S0925-4005(97)80090-1, 1996.Zheng, W., Lu, X., Wang, W., Li, Z., Zhang, H., Wang, Y., Wang, Z., and Wang,
C.: A highly sensitive and fast-responding sensor based on electrospun
In2O3 nanofibers, Sensor. Actuat. B-Chem., 142, 61–65,
10.1016/j.snb.2009.07.031, 2009.