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Highly transparent ITO thin films on photosensitive glass: Sol-gel synthesis, structure, morphology and optical properties

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Highly transparent ITO thin films on photosensitive glass: Sol-gel synthesis, structure, morphology and optical properties

Abstract

Conductive and highly transparent indium tin oxide (ITO) thin films were prepared on photosensitive glass substrates by the combination of sol–gel and spin-coating techniques. First, the substrates were coated with amorphous Sn-doped indium hydroxide, and these amorphous films were then calcined at 550∘C to produce crystalline and electrically conductive ITO layers. The resulting thin films were characterized by means of scanning electron microscopy, UV-Vis spectroscopy, X-ray photoelectron spectroscopy and spectroscopic ellipsometry. The measurements revealed that the ITO films were composed of spherical crystallites around 20 nm in size with mainly cubic crystal structure. The ITO films acted as antireflection coatings increasing the transparency of the coated substrates compared to that of the bare supports. The developed ITO films with a thickness of ∼170–330 nm were highly transparent in the visible spectrum with sheet resistances of 4.0–13.7 kΩ/sq. By coating photosensitive glass with ITO films, our results open up new perspectives in micro- and nano-technology, for example in fabricating conductive and highly transparent 3D microreactors.
1 23
Applied Physics A
Materials Science & Processing
ISSN 0947-8396
Appl. Phys. A
DOI 10.1007/s00339-012-6765-1
Highly transparent ITO thin films on
photosensitive glass: sol–gel synthesis,
structure, morphology and optical
properties
László Kőrösi, Szilvia Papp, Szabolcs
Beke, Béla Pécz, Róbert Horváth, Péter
Petrik, Emil Agócs & Imre Dékány
1 23
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Appl Phys A
DOI 10.1007/s00339-012-6765-1
Highly transparent ITO thin films on photosensitive glass: sol–gel
synthesis, structure, morphology and optical properties
László K˝
orösi ·Szilvia Papp ·Szabolcs Beke ·
Béla Pécz ·Róbert Horváth ·Péter Petrik ·Emil Agócs ·
Imre Dékány
Received: 24 August 2011 / Accepted: 22 December 2011
© Springer-Verlag 2012
Abstract Conductive and highly transparent indium tin ox-
ide (ITO) thin films were prepared on photosensitive glass
substrates by the combination of sol–gel and spin-coating
techniques. First, the substrates were coated with amor-
phous Sn-doped indium hydroxide, and these amorphous
films were then calcined at 550C to produce crystalline
and electrically conductive ITO layers. The resulting thin
films were characterized by means of scanning electron mi-
croscopy, UV-Vis spectroscopy, X-ray photoelectron spec-
troscopy and spectroscopic ellipsometry. The measurements
revealed that the ITO films were composed of spherical crys-
tallites around 20 nm in size with mainly cubic crystal struc-
ture. The ITO films acted as antireflection coatings increas-
ing the transparency of the coated substrates compared to
that of the bare supports. The developed ITO films with a
thickness of 170–330 nm were highly transparent in the
visible spectrum with sheet resistances of 4.0–13.7 k/sq.
By coating photosensitive glass with ITO films, our results
open up new perspectives in micro- and nano-technology,
Electronic supplementary material The online version of this article
(doi:10.1007/s00339-012-6765-1) contains supplementary material,
which is available to authorized users.
L. K˝
orösi ()·S. Papp ·I. Dékány
Supramolecular and Nanostructured Materials Research Group of
the Hungarian Academy of Sciences, University of Szeged, Aradi
vértanúk tere 1, 6720 Szeged, Hungary
e-mail: l.korosi@chem.u-szeged.hu
Fax: +36-62-544042
S. Beke
Department of Nanophysics, Italian Institute of Technology,
Via Morego 30, 16163 Genova, Italy
B. Pécz ·R. Horváth ·P. Petr i k ·E. Agócs
Research Institute for Technical Physics and Materials Science,
Konkoly-Thege út 29-33, 1121 Budapest, Hungary
for example in fabricating conductive and highly transpar-
ent 3D microreactors.
1 Introduction
By virtue of its excellent optical and electrical properties,
indium tin oxide (ITO) is one of the most extensively stud-
ied transparent conductive oxides [1]. ITO thin films with
high transmittance in the visible spectral range and with low
resistivity have previously been prepared by several meth-
ods, such as sputtering [2], spray pyrolysis [3], chemical
vapor deposition [4], electron-beam evaporation [5], screen
printing [6], pulsed-laser deposition [7], and the sol–gel pro-
cess [8]. The sol–gel method is a popular technique to pro-
duce high-quality thin films as it has a number of advan-
tages, such as relatively low cost, a need for only simple
equipment, and a high degree of control over the chemical
composition of the resulting metal oxides.
Sol–gel-based ITO films can be prepared either by the
deposition of coating solutions [9] or from sols containing
colloids [10]. In the coating solutions, the precursors are not
hydrolyzed before the film deposition step [11]. In this case,
after film deposition, the resulting, partially hydrolyzed pre-
cursors transform to ITO upon calcination. The coating so-
lutions are frequently used to prepare ITO films, but they
contain inorganic ions of the precursors, which could be
unfavorable during calcination. Sols (or colloidal disper-
sions) containing ITO [12] or other types of nanoparticles
(e.g. indium tin hydroxide, ITH) can also be used for thin-
film preparation [10]. ITH sols are generally synthesized by
the hydrolysis of InCl3or In(NO3)3and SnCl4in aqueous
medium [13]. Since the hydrolysis and condensation of the
precursors result in colloidal particles, the undesired ions
(Cland NO
3) can be removed from the media of the sol
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L. K˝
orösi et al.
by washing or use of dialysis. Another advantage of the sol-
based process is that the morphology or crystal phase of the
ITO particles can be controlled via the synthesis tempera-
ture and pH. Sol–gel-based films are usually prepared on a
glass or quartz substrate by dip- or spin-coating techniques.
During film deposition, the medium of the sol evaporates
and subsequently a metal oxide/hydroxide xerogel film is
formed [10]. The as-prepared layers are often amorphous,
and heat treatment at 500–550C is therefore necessary to
obtain crystalline and conductive ITO films. This heat treat-
ment is an important step because it causes particle sinter-
ing, which leads to a decrease in the electrical resistance of
the ITO layers. For sol preparations, the well-known hydro-
[14] and solvothermal [15] methods can also be employed.
These synthesis methods, with the application of elevated
temperature and pressure, can produce ITO nanoparticles di-
rectly instead of ITH [16].
In a recent study [10], we presented a short survey of the
preparation of ITO films with well-controlled layer thick-
ness (40–1160 nm) by applying a combination of sol–gel
and dip-coating methods. The adhesive ITH sols applied al-
lowed the rapid preparation of high-quality ITO films. It was
revealed that the sheet resistance of the ITO films was signif-
icantly lower when the ITH sols contained polyvinylpyrroli-
done (PVP). In another study [17], we demonstrated a novel
concept through which to achieve embedded 3D conductive
and completely transparent structures in glass microchips
by using a combination of femtosecond laser microfabrica-
tion and a sol–gel method. The application of ITO coatings
on photosensitive glass can be a breakthrough toward the
further exploitation of microchip technology by fabricating
microchips with novel features. The idea relies on the wide-
ranging applicability and sensitivity of ITO films of value
for sensitive biochemical analysis and biological, chemi-
cal, and medical inspections based on the development of
highly functional microdevices. Photosensitive glass (under
the trade name Foturan®) has all of the unique properties of
traditional glass (e.g. transparency, hardness, chemical, etc.)
and is an excellent material with which to embed hollow mi-
crochannels and other complicated 3D structures by using
femtosecond laser direct writing [1820]. Moreover, Fotu-
ran glass can withstand thermal treatment at 550C, which
is a requirement for the production of sintered, conductive
ITO films.
In the present study, we report on the preparation of sol–
gel-based ITO thin films on photosensitive glass from PVP-
containing amorphous ITH sol through use of spin-coating
method and subsequent calcination. The structural, optical
and electrical properties of the resulting ITO thin films are
reported and the effects of the aging time on the crystallinity
and crystal phase composition are discussed.
2 Experimental details
2.1 Synthesis of ITH sols
1.1727 g of InCl3·4H2O and 0.1402 g of SnCl4·5H2Owere
dissolved in 100 ml of deionized water, and the precur-
sors were hydrolyzed at room temperature by the addi-
tion of 2.0 ml of 25% NH3solution during intensive stir-
ring. The resulting dispersion was centrifuged and the sed-
iment obtained was washed with water and subsequently
with ethanol. After the washing procedure, a stable sol
was prepared by redispersion of the sediment in 25 ml of
ethanol with the addition of PVP in a final concentration of
0.1 w/v%. This sol was designated ITH-NA.
The ITH-A sol was prepared as previously reported [10].
In this synthesis the aqueous dispersion of the as-prepared
precipitate was dialyzed against deionized water until the
conductivity had decreased below 1 µScm1. The dialysis
was continued for a further 3 days, during which the precipi-
tate was aged. After dialysis, the dispersion was centrifuged
and the sediment obtained was washed with ethanol. Finally,
the resulting precipitate was dispersed in 50 ml of ethanol
with the addition of PVP as described above.
2.2 Preparation of ITH and ITO thin films
ITH thin films were prepared on Foturan glass substrates
(10 ×10 ×2 mm) by the spin-coating method. 50 µl of
ITH-NA or ITA-A sol was dropped onto the substrate ro-
tating with 3000 rpm and the as-prepared layer was subse-
quently dried for 30 s in the spin-coater. To prepare multi-
layer films the above deposition step was applied again with
the desired number of repetition. To obtain ITO thin films,
the deposited ITH layers were calcined for 30 min in air at
550C in a preheated furnace.
2.3 Characterization
Thermogravimetric (TG) investigations were carried out in
air with a TGA/SDTA 851e (Mettler Toledo) derivatograph
at a heating rate of 5Cmin
1. X-ray diffraction (XRD)
patterns were collected on a Bruker D8 Advance diffrac-
tometer equipped with a Göbel mirror. The measurements
were made in θ-θconfiguration using CuKαradiation. The
operating voltage and current were 40 kV and 40 mA, re-
spectively. Transmission electron microscopy (TEM) im-
ages were obtained with a Philips CM-10 electron micro-
scope at an accelerating voltage of 100 kV. Scanning elec-
tron microscopy was performed with a Hitachi S-4700 FE-
SEM cold-field emission electron microscope operated at
5 kV. UV-Vis spectrophotometry was performed with an
Ocean Optics Chem2000-UV-VIS diode array spectropho-
tometer at wavelengths in the range 250–800 nm.
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Highly transparent ITO thin films on photosensitive glass: sol–gel synthesis, structure, morphology
The X-ray photoelectron spectroscopy (XPS) was carried
out with VG ESCALAB 250 spectrometer (Thermo Fisher
Scientific K.K.) using monochromatic Al K X-ray radiation.
The X-ray gun was operated at 200 W (15 kV, 13.3 mA).
The C 1s binding energy of adventitious carbon was used as
energy reference; it was taken at 284.8 eV.
Ellipsometry measures the complex reflection ratio (ρ=
tan(Ψ )e, where Ψand Δare the ellipsometric angles),
the ratio of the reflection coefficients of light polarized par-
allel and perpendicular to the plane of incidence [21]. By
sensitively measuring phase changes of the light passing
through the deposited layers, ellipsometry has a sensitivity
of 0.1 nm and 0.0001 in the thickness and the in refrac-
tive index, respectively. However, as it is an indirect method,
the accuracy depends on the choice of an appropriate opti-
cal model. The ellipsometric measurements were performed
with a Woollam M-2000DI rotating compensator spectro-
scopic ellipsometer in the wavelength range 400–800 nm, at
angles of incidence ranging from 50to 70. A microspot
was used to avoid backside reflection from the transparent
substrate. The refractive index of the bare Foturan substrate
was determined by using the multiple-angle spectra with
backside roughening.
The sheet resistance of ITO films was determined by
four-point probe measurements with a Keithley 2400 source-
meter and a cylindrical four-point probe head (Jandel Engi-
neering Ltd). The tip array was linear with a probe spacing
of 1.0 mm. The 100-micron radius tips were made of tung-
sten carbide. The spring load was 60 g per tip. The sheet
resistance (Rs) was calculated via the following relation-
ship:
Rs=π/ln 2 ×(V /I )
where Vand Iare the voltage and the current, respectively.
3 Results and discussion
3.1 Structure, morphology and surface composition
ITO thin films were prepared by a combination of conven-
tional sol–gel and spin-coating techniques. In the first part of
the procedure, indium tin hydroxide (ITH) sol was synthe-
sized, while in the second step ITH thin films were prepared
by a spin-coating. Finally, the ITH films were calcined at
550C. To synthesize ITH, the precursors (InCl3and SnCl4)
were hydrolyzed in aqueous medium by the addition of NH3
solution until the pH reached 9. Following the hydrolysis
of In3+and Sn4+and the condensation of the resultant hy-
droxide species, the hydrous gel obtained was subsequently
washed with water and ethanol. Before the washing proce-
dure, the precipitate was not left to age which is a significant
difference as compared with our previously reported synthe-
sis [10]. As we presented earlier, the electric resistance of
Fig. 1 (a)TGand(b) DTG curves of ITO-NA xerogel dried at 50C
ITO films can be decreased by the addition of PVP to the
ITH sol [10]. After the washing procedure, therefore the gel
was dispersed ultrasonically in ethanol, and PVP was added
to the sol in final concentration of 0.1 w/v%. The solid con-
tent of the sol was 3.5 w/v%. To obtain ITH xerogel powder,
the ethanol was evaporated from the sol at 50C. The ther-
mogravimetric (TG) curve of the ITH-NA xerogel obtained
is presented in Fig. 1, curve a. The total mass loss in the
range 25–1000C was 25.2%. The differential thermogravi-
metric (DTG) curve (Fig. 1, curve b) exhibited minima at
50, 150, 280, 350 and 480C. Two main processes can be
distinguished; the first (25–215C) is due to dehydration,
while the mass loss above 215C is assigned to the dehy-
droxylation of ITH and the decomposition of PVP. In the in-
terval 550–1000C, the xerogel lost its residual OH groups,
which caused only a minor mass loss (0.7%). Since total
mass loss of the xerogel without PVP was 19.9%, the PVP
content of the ITO-NA xerogel was 5.3%. It should be noted
that the total mass loss of the previously synthesized aged
ITH xerogel [10] was slightly lower, which may be due to
the higher drying temperature (80C) applied prior to the
TG analysis.
The XRD patterns of the ITH-NA powder both before
and after calcination are displayed in Fig. 2. The XRD pat-
tern of the sample dried at 50C (Fig. 2, curve a) did not con-
tain any peaks, which indicates that the ITH-NA was amor-
phous. However, the ITH-A sample was nanocrystalline,
as reported previously [10]. In contrast with ITH-NA, the
ITH-A xerogels consisted of cubic In(OH)3and orthorhom-
bic InOOH crystal phases. It should be noted that the na-
ture of the medium during the aging process is of crucial
importance for the crystal phase evolution. When the as-
prepared precipitate (hydrous gel) was subsequently washed
with ethanol, the ITH did not become crystalline even after
several months of storage. We therefore conclude that the
aging of the gel in an aqueous medium favors the formation
of nanocrystalline ITH.
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L. K˝
orösi et al.
Fig. 2 XRD patterns of (a) ITH-NA and (b) ITO-NA powders. The
characteristic reflections of In2O3with cubic and rhombohedral struc-
tures are indexed
In the XRD pattern of the calcined ITH-NA at 550C
(hereafter designated ITO-NA), cubic (JCPDS No. 06-0416)
and rhombohedral (JCPDS No. 22-0336) In2O3can be iden-
tified (Fig. 2, curve b). Other crystal phases (e.g. SnO and
SnO2) could not be detected, indicating a homogeneous dis-
tribution of Sn4+in the In2O3host. The broadened XRD
lines revealed that the ITO-NA was nanocrystalline. Due to
this broadening, several reflections of the two crystal phases
overlapped. The predominant crystal phase was cubic; the
(104) reflection of the rhombohedral phase appeared only
as a shoulder on the wide angle side of the (222) peak. To
determine the average crystallite size, the broad peak in the
range 29–32was deconvoluted in accordance with the posi-
tions of (222) and (104) reflections. From the FWHM of the
(222) and (110) reflections, the average sizes (determined
via the Scherrer equation) were 19.5 and 11.6 nm for the cu-
bic and rhombohedral crystallites, respectively. The effects
of aging on the crystallinity and structure can be compared
with the aid of the XRD patterns presented in Fig. 3.From
a comparison of the line broadening it is clearly seen that
the calcined ITH-A at 550C (hereafter designated ITO-A)
consisted of smaller crystallites than those of ITO-NA. For
ITO-A, the sizes were 8.3 and 9.4 nm for the cubic and
rhombohedral crystallites, respectively. The XRD patterns
also revealed that the ratio of the cubic and rhombohedral
phases was influenced by the aging. For ITO-NA, the inten-
sities of the lines relating to the rhombohedral phase were
significantly lower. Consequently, the aging of ITH favors
the formation of rhombohedral ITO crystallites. However, it
should be noted that the phase evolution may also depend on
the pH during aging. Kim et al. [22] reported that lower pH
(8)promotes the formation of rhombohedral ITO.
TEM picture of the ITH-NA (Fig. 4a) indicated an amor-
phous structure confirming the XRD results. In contrast, in
the case of ITH-A (Fig. 4b), polymorphic particles could be
observed: round particles measuring a few nanometers were
Fig. 3 Comparison of XRD patterns of (a) ITO-NA and (b) ITO-A
powders
present together with 40–70-nm cubic and columnar parti-
cles. These findings revealed that these rectangular objects
were formed during the aging process. The ITO-NA sam-
ple (Fig. 4c) was composed of round particles with a diam-
eter of 8–26 nm. The estimated average particle size was
17 nm, although the precise particle size distribution could
not be given because of the sintering, which caused the parti-
cles to assemble into large aggregates. The electron diffrac-
tion pattern of ITO-NA (inset in Fig. 4c) confirmed the pres-
ence of both cubic and rhombohedral phases. As may be
seen from the TEM picture of ITO-A (Fig. 4d), the rect-
angular morphology remained after calcination; the cubes
and columns were still present and only a minor coarsen-
ing of the round particles could be observed. It is clear from
the TEM image of ITO-A that the large rectangular particles
were composed of primary particles 5–10 nm in size, in ac-
cordance with the value (8.3 nm) calculated via the Scherrer
equation.
ITH-NA thin films with different layer numbers were pre-
pared on photosensitive glass from ITH-NA sol by a spin-
coating technique. To obtain crystalline and conductive ITO
films, the as-deposited ITH layers were calcined at 550C.
No heat treatment was applied between the film deposition
steps. The surface morphology of the ITO-NA films ob-
tained is presented in Fig. 5a. The SEM picture revealed
that ITO-NA consisted of fine particles with uniform round-
shaped morphology, resulting in lower surface roughness as
compared with ITO-A (Fig. S1 in the Supplementary Ma-
terial). The SEM picture also revealed that the film was sig-
nificantly porous. The cross-sectional view (Fig. 5b) of ITO-
NA film made up of 3 layers showed a uniform thickness of
360 nm.
XPS measurements on the ITO-NA film yielded the high-
resolution spectra of In 3d,Sn3dand O 1sregions depicted
in Fig. 6. Both the In 3dand the Sn 3dspectra were sym-
metric, indicating a single chemical state and the O–In–O
Author's personal copy
Highly transparent ITO thin films on photosensitive glass: sol–gel synthesis, structure, morphology
Fig. 4 TEM images of (a) ITH-NA, (b) ITH-A, (c) ITO-NA and (d) ITO-A samples. The inset in (c) shows the corresponding electron diffraction
pattern
and O–Sn–O chemical environments. The In 3d5/2and Sn
3d5/2peaks were located at 444.3 and 486.6 eV, respec-
tively. These binding energies reveal In and Sn oxidation
states of 3+and 4+, respectively [16,23]. Table 1lists the
binding energies of In, Sn and O components in various ITO
samples prepared by different synthesis methods.
Our XPS results are in good agreement with those re-
ported for ITO films elsewhere [24,25]. Quantitative analy-
sis using the atomic sensitivity factors (ASF) of In (ASF =
4.51) and Sn (ASF =4.89) resulted in an In:Sn atomic ratio
of 8.73, a value slightly lower than the estimated theoret-
ical atomic ratio In:Sn =10. While the In 3dand Sn 3d
spectra are symmetrical, reflecting one chemical state, the
O1sspectrum has a shoulder on the high binding energy
side at 531.6 eV, due to surface OH groups. Similar asym-
metric O 1sspectra were observed for other OH-containing
metal oxides (SnO2and TiO2)[23,26]. The main compo-
nent of the O 1sspectrum is at 530 eV, which corresponds
to the lattice oxygen. The atomic ratio (In+Sn):O was found
to be 0.664.
3.2 Optical and electrical properties
The UV-Vis transmittance spectra of ITO-NA films are pre-
sented in Fig. 7a. It is clearly seen that in the visible wave-
length range the uncoated photosensitive glass has a lower
transmittance (T)than that of the ITO-NA-coated substrate
(Fig. 7a, inset). This phenomenon is due to the ITO layers
on the substrate acting as an antireflection coating. After de-
position of an ITO-NA monolayer, Tincreased from 91.6
to 95% throughout the whole visible wavelength range. In
the UV range, Tdecreased slightly with increasing number
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L. K˝
orösi et al.
Fig. 5 SEM images of (a) surface morphology and (b) cross-sectional view of ITO-NA thin film made up of 3 layers on Foturan glass
Fig. 6 High-resolution XP
spectra of In 3d,Sn3dand O 1s
regions for ITO-NA thin film
calcined at 550C
Tabl e 1 Binding energy values
of In, Sn and O in ITO samples
prepared by different synthesis
method
Method Binding energy (eV) Ref.
In 3d5/2Sn 3d5/2O1s
Sol–gel 444.3 486.9 530.6 532.0 [24]
444.3 486.6 529.9 531.6 In this work
Solvothermal 444.1 486.1 529.6 530.7 [16]
Thermal evap.-ox. 444.4 486.0 529.9 532.4 [25]
of layers. The Tspectra of the ITO-NA multilayers exhib-
ited interference fringes, and hence the Tvalues depended
on the wavelength of the incident light. It should be noted
that the ITH-NA layers also exhibited higher transparency
than that of the uncoated substrate (Fig. S2 in the Supple-
mentary Material). The Tvalues of all ITO-A-coated Fotu-
ran glasses also exceeded the Tof the uncoated substrate
(Fig. 7b, inset). For ITO-A films, the transparency increased
with increasing layer number in the whole UV-Vis range.
The highest Twas 96% after three deposited ITO-A lay-
ers.
For the ellipsometric measurements, the wavelength
range of high T(400–800 nm, see Fig. 7) was used because
this allows application of a simple polynomial dispersion
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Highly transparent ITO thin films on photosensitive glass: sol–gel synthesis, structure, morphology
Fig. 7 UV-Vis transmittance spectra of (a) ITO-NA and (b) ITO-A thin films composed of different numbers of layers on Foturan glass
function for the refractive index [27] (in a broader range,
complex oscillator models must be used [28]). The spec-
tra could be fitted by using a linearly graded depth profile
of the refractive index in the range 3–8% (graded models
have also been described for RF sputtered ITO films [27]).
Figure 8shows the better fit with the graded model relative
to non-graded one. We note that the application of a sur-
face nano-roughness model did not improve the fit, which
indicates that the thin films were of high surface quality,
in agreement with the above SEM results. For the ITO-NA
samples, the inset in Fig. 8demonstrates a linear increase
of total thickness (from 166 to 330 nm) as a function of the
number of layers. For the ITO-A samples, the thickness var-
ied from 26 to 72 nm. The lower film thickness of ITO-A is
due to the lower concentration of the ITH-A sol. The refrac-
tive indices at the He–Ne laser wavelength of 633 nm were
in the range 1.3–1.4 and 1.1–1.3 for the ITO-NA and ITO-A
samples, respectively. This is lower than the published val-
ues for evaporated and sputtered ITO films (in the range
1.7–2.0 [28,29]), which is in agreement with the porous
structure revealed by SEM (Fig. 5).
The measured sheet resistance (Rs)ofITO-NAfilmsde-
creased significantly with increasing layer number. For film
thickness of 166, 254 and 328 nm, Rswas 13.72 ±0.04,
5.96 ±0.05 and 4.02 ±0.02 k/sq, respectively, i.e. lower
than the values measured earlier [10] for ITO-A at similar
layer thickness. The differences may be due to the lower
surface roughness and better particle-to-particle contacts in
ITO-NA films.
4 Conclusions
ITO thin films were successfully prepared on photosensi-
tive glass substrates by the combination of sol–gel and spin-
coating techniques. For film deposition, an amorphous ITH
Fig. 8 Measured (symbols)andfitted(lines) ellipsometric spectra of
a bilayer of ITO-NA on Foturan glass. Solid and dashed lines indi-
cate graded and non-graded models, respectively. The inset shows total
thickness as a function of layer number
sol in ethanol medium was used. Whereas the as-deposited
films were amorphous, nanocrystalline ITO was formed af-
ter calcination at 550C. These ITO nanoparticles displayed
spherical morphology and high crystallinity. It was estab-
lished that the aging process in aqueous medium promotes
the crystallization of ITH and favors the formation of large
(40–70 nm) particles with rectangular morphology. With-
out the application of aging, a lower surface roughness is
achieved and the resulting ITO film on photosensitive glass
acts as an antireflection coating. After deposition of an ITO
monolayer, the transmittance in the visible wavelength range
increased from 91.6 to 95%. As the thickness increased from
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L. K˝
orösi et al.
166 to 328 nm, the sheet resistance decreased from 13.7 to
4.0 k/sq.
We believe that the developed ITO films, especially their
successful processing on photosensitive glass, can find ap-
plications in diverse areas of micro- and nano-technology,
such as the development of conductive and highly transpar-
ent 3D structures for microreactor applications.
Acknowledgement The authors thank Dr. Aiko Nakao (Cooperative
Support Team, Riken—Advanced Science Institute, Wako, Saitama,
Japan) for the assistance on XPS measurements. The authors also
thank the Hungarian National Scientific Fund (OTKA) K81842 and
PD 73084 for the financial support.
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Author's personal copy
... 39 It was also used to deposit a conductive anti-reflective coating. 40 Spin-coating method was also studied by Liu et al. for deposition of ATO porous and transparent films to establish the relationship between conductivity and doping level and it a c d b 12 was found that a resistivity of 5.2*10 -1 Ω cm could have been achieved while increasing the doping up to 20 % doping with 300 nm pore size. However, at the same time the transparency went down from 80 % (for undoped SbO 2 ) to around 61% (in 300 -800 nm range, Figure 1.3). ...
... The first group of such methods are vacuum methods like sputtering 88,138 , glancing angle deposition 16,139 , metalorganic chemical vapor deposition 100 or pulsed laser deposition. 140 Next group are wet 48 and evaporation methods like spin coating 38,40,54,64 , nanocrystal assembly 24,70 , doctor blade 23,141 , dip-coating 37,58 , sol-gel deposition 40 and nanocasting. 142 As mentioned, resulting mesoporous layers have some advantages like high surface area but at the same time diffusion ...
... The first group of such methods are vacuum methods like sputtering 88,138 , glancing angle deposition 16,139 , metalorganic chemical vapor deposition 100 or pulsed laser deposition. 140 Next group are wet 48 and evaporation methods like spin coating 38,40,54,64 , nanocrystal assembly 24,70 , doctor blade 23,141 , dip-coating 37,58 , sol-gel deposition 40 and nanocasting. 142 As mentioned, resulting mesoporous layers have some advantages like high surface area but at the same time diffusion ...
Thesis
There is a growing interest in finding new and commercially viable methods of performing a spectroelectrochemical analysis which combines electrochemical and spectral techniques. For this purpose, an electrode material that is transparent and conductive needs to be prepared. In this work, such electrode was prepared by electrospinning which is a technique capable of forming very thin fibers with high surface area. Those electrodes were also covered with additional layer of porous and functionalized silica to maximize the surface area and introduced additional sensing properties. This material was used in the detection of methylene blue which is an industrial dye and an environmental pollutant. It was found that using such electrode it was possible to detect concentrations that are smaller than the harmful environmental levels. Finally, the layers were also used with success to generate luminescence which is opening new prospects for the design of spectroelectrochemical sensors
... Gas sensors play an important role in detecting, monitoring and controlling the presence of dangerous and poisonous gases in the atmosphere at very low concentrations [3][4][5]. Nanostructured semiconductor gas sensors are highly sensitive and dependable, and have a performance/price ratio as good as to that of microelectronic components [6][7][8]. It is well known that the physicochemical properties which control gas adsorption on the surface of a semiconductor can significantly influence its electrical conductivity [9,10]. ...
Article
Full-text available
In this work In2O3 and Sn-doped ITO nanoparticles were prepared by sol-gel method and deposited on quartz substrate by dip coating technique at different doping concentration of (5, 10 and 15%). The samples were annealed at 550 oC at constant time (60 min). X-ray analysis confirmed the formation of polycrystalline cubic phase that decreases in crystalline size with increasing doping concentration. The optical properties of Sn-ITO nanostructure thin film were studied. The transmittance was measured in the wavelength range of (300nm to 1100 nm) for all thin films. The sensitivity towards NO2 gas was measured, when In2O3 was doped with Sn at different concentrations.
... ITO-based TCO films can exhibit different electro-optical, mechanical, electronic structure and composition properties depending on manufacturing technique. Many deposition techniques are used to prepare TCO coatings such as Dc and/or RF sputtering, chemical vapour deposition, reactive evaporation, molecular beam epitaxy, sol-gel method, electron beam evaporation and pulsed laser deposition [7][8][9][10][11][12][13][14]. However, the sol-gel technique has many advantages including cost effectiveness, short processing time, simplicity and the opportunity to fabricate coatings with high scalability along with the flexibility of finetuning coatings to have preferred shapes and surface areas [15][16][17][18]. ...
Article
Titanium-doped indium tin oxide thin films were synthesized via a sol-gel spin coating process. Surface chemical bonding states and mechanical properties have been investigated as a function of titanium content (2 and 4 at%) and annealing temperature ranging from 400 to 600 °C with increments of 100 °C. Raman analysis was performed to study the phonon vibrations for the prepared samples and the results revealed the existence of ITO vibrational modes. The elemental compositions, bonding states and binding energies of the film materials were investigated using X-ray photoelectron spectroscopy (XPS) technique. The XPS results indicated that the ratio of the metallic elements (In, Sn, Ti) to the oxygen on the surface of the thin film coatings decreased due to the increase of the oxide layer on the surface of the thin films. Also, by increasing the annealing temperature up to 600 °C, the Ti 2p and Cl 2p signals were no longer detected for both 2 and 4 at% Ti contents, due to the thicker surface oxidation layer. Mechanical properties of the synthesized films were also evaluated using a nanoindentation process. Variations in the hardness (H) and the elastic modulus (E) were observed with different Ti at% and annealing temperatures. The hardness is within the range of 6.3–6.6 GPa and 6.7–6.8 GPa for 2 and 4 at% Ti content samples, respectively, while the elastic modulus is within the ranges of 137–143 and 139–143 GPa for 2 at% and 4 at% Ti contents samples, respectively. A combination of the highest H and E were achieved in the sample of 4% Ti content annealed at 600 °C. Furthermore, the H/E ratio ranges from 4.5 × 10⁻² to 5.0 × 10⁻² which reflects a reasonable level of wear resistance.
Thesis
Al-rich AlGaN is required for light-emitting diodes (LEDs) and lasers operating in the deep-ultraviolet (UV) spectral range, solar-blind photodetectors, integrated UV photonics, and future high-power electronic devices. For many of these applications, it is essential that AlGaN with atomically smooth surface and with a minimal level of defects and dislocations can be epitaxially grown on foreign lattice-mismatched substrates, which are of lower cost, larger size and more widely available than bulk GaN or AlN substrates. In this work, with the use of molecular beam epitaxy (MBE), superior quality AlN and Al-rich AlGaN grown on sapphire are demonstrated. For AlN epilayers grown directly on sapphire, the X-ray diffraction (XRD) (002) rocking curve peak is significantly narrower than that previously reported for samples of comparable thicknesses. By employing a careful sequence of cycled in situ high-temperature annealing, many of the dislocations and stacking faults generated at the AlN/sapphire interface are reduced within the first 50 nm of growth. The photoluminescence (PL) emission is twice as strong as commercial AlN epitaxial templates that are over 10 times thicker, without the presence of defect-related emissions. With increasing thicknesses, the (002) and (102) rocking curve peak widths are among the best reported for AlN epitaxially grown on sapphire. Furthermore, a detailed study of Al-rich AlGaN epilayers was conducted. A method was developed to precisely control the alloy composition by tuning the Al flux and N flow rate. Under optimized conditions, Al0.6Ga0.4N epilayers exhibit a surface roughness < 0.4 nm, and strong PL emission at room temperature. Despite the lattice mismatch between AlGaN, AlN and sapphire, the formation and propagation of dislocations is significantly suppressed. This work presents important insights into obtaining superior-quality wide-bandgap Al(Ga)N epilayers on lattice-mismatched substrate without the limitations of thick buffer layers, in order to break the efficiency bottleneck of deep-UV optoelectronics. Hexagonal boron nitride (h-BN) has shown tremendous promise when used alongside other two-dimensional (2D) materials such as graphene, and as a wide-bandgap semiconductor for deep-ultraviolet optoelectronics and quantum photonics. Owing to its large bandgap energy comparable or higher than Al(Ga)N, h-BN can be used to form heterostructures to address some of the critical challenges of Al(Ga)N-based systems. In this context, dislocation-free AlN/BN nanowire heterostructures were grown directly on Si substrate. AlN/BN deep-UV LEDs, exhibiting a relatively low turn-on voltage (< 7 V) and strong electroluminescence (EL) emission at ~210 nm at room temperature were demonstrated for the first time. The epitaxy of h-BN was then studied. Using high-temperature MBE, domains of exceptional crystalline quality were obtained on Ni substrate, with strong excitonic PL emission. It was theoretically and experimentally demonstrated that, even though the energy gap of h-BN is indirect, it luminesces as strongly as direct-gap materials, because of unusually strong phonon coupling. The luminescence intensity (~220 nm) of such a h-BN sample was 10 to 100 times stronger than that of commercially grown direct-bandgap AlN, demonstrating the extraordinary potential of epitaxial h-BN for deep-UV optoelectronics and quantum photonics. By forming a p-i-n structure using this high-quality h-BN as the active region, the current-voltage (I-V) and EL characteristics of a h-BN deep-UV LED is reported for the first time.
Article
Artificial photosynthesis based on photoelectrochemical (PEC) strategy is another solar-to-chemical energy conversion method besides photocatalysis. Owing to the diversity of structure and the adjustability of synthesis, the metal-organic frameworks (MOFs) can enrich this important research field in the form of constructing a photoelectrode. In addition to solving the problems of difficult recovery for powder and the easy recombination for carriers in photocatalysis, MOFs and their derivatives-modified photoelectrodes can reasonably adjust the PEC activity at the molecular level and increase the contact area between the electrolyte and electrode, thereby facilitating the diffusion of electrolyte and reaction substrate in the electrode. In this review, we comprehensively reviewed representative studies in this area. Firstly, functions of MOFs in photoelectrodes are outlined, and the various synthesis strategies of MOFs-modified photoelectrodes are elaborated. Subsequently, special attention has been paid to the application and mechanism of MOFs-modified photoelectrodes (MIL, ZIF, UiO and PCN) in photo-electrochemistry. And we discuss the stability, reproducibility and reusability of MOFs-modified photoelectrodes. Finally, the challenges and improvements of MOFs-modified photoelectrodes in promoting practical application are proposed. Overall, MOFs and their derivatives-modified photoelectrodes achieved the integration of adsorption, photocatalysis and electrocatalysis. Notably, the research in this field is in infancy, many improvements are required before practical applications.
Article
A review on the preparation methods for porous and transparent metal-oxide electrodes is provided alongside a discussion of existing and possible electrochemical and electroanalytical applications. The elaboration routes include non-templated particle deposition, template approaches, physical deposition methods, etching and electrospinning. Applications of such materials are mainly found in energy conversion and storage (photovoltaics, water splitting) and electroanalysis.
Article
Single walled carbon nanotubes (SWCNTs) incorporated in indium tin oxide (ITO) was developed to fabricate transparent conductive thin films via a sol-gel spin coating technique. The fabricated thin films were annealed at 350°C. The effects of incorporating SWCNTs and varying film thickness on crystal structure, Raman shift, surface elemental compositions, surface topography, optoelectronic characteristics and mechanical properties of the SWCNTs/ITO thin films were systematically investigated. XRD results confirmed the body centered cubic structure of indium oxide polycrystalline phase, indicating that the structural properties of the ITO films were not significantly altered by incorporating CNTs. The presence of CNTs in ITO matrix was confirmed by Raman spectroscopy. FESEM images revealed the formation of SWCNTs/ITO nanoparticles, and the average crystallite size increased along with increasing film thickness. Electrical characteristics also improved as the film thickness increased. The lowest electrical resistivity (4.6×10-4 Ω.cm), as well as the highest carrier concentration (3.3×1020 cm-3) and carrier mobility (41 cm2/V s) were achieved from the 320 nm thick film. However, the optical transparency decreased from 91% to 87.5% as the film thickness increased from 150 to 320 nm. The hardness and Young’s modulus of the prepared samples improved, with the increase of SWCNTs doping level, and achieved the maximum values of 28 GPa and 306 GPa respectively.
Article
Anatase phase titanium dioxide (titania) is a potential anodic material for lithium-ion batteries (LIBs), while the serious drawback is its low electrical conductivity. As an attempt to address this issue, a new hierarchically structured nanotubular anatase-titania/indium-tin-oxide (titania/ITO) composite was fabricated. Nanotubular structured anatase titania with a tube wall thickness of ca. 10 nm was firstly synthesized by employing natural cellulose substance (ordinary laboratory filter paper) as the structural template, and a thin layer of ITO with a thickness of ca. 15 nm composed of fine ITO crystallites (particle sizes ca. 5 nm) was deposited by means of a solvothermal process. When this composite was utilized as an anodic material for LIBs, it delivered a high initial discharge capacity of 1773.4 mAh g⁻¹, and a stable discharge capacity of 298.6 mAh g⁻¹ after 120 charge/discharge processes cycled at 1 C. The unique hierarchical structure of the composite reduced the diffusion length for both electronic and ionic transport, improved the electrode-electrolyte contact area, and enhanced the accommodation of the volume expansion and constriction during the lithium ion insertion/extraction process. Hence improved electrochemical performances such as high reversible capacity, good rate performance and high cycling stability were achieved.
Article
Aluminum-rich AlGaN is the ideal material system for emerging solid-state deep-ultraviolet (DUV) light sources. Devices operating in the near-UV spectral range have been realized; to date, however, the achievement of high efficiency LEDs operating in the UV-C band (200-280 nm specifically) has been hindered by the extremely inefficient p-type conduction in AlGaN, and the lack of DUV-transparent conductive electrodes. Here we show that these critical challenges can be addressed by Mg dopant-free Al(Ga)N/h-BN nanowire heterostructures. By exploiting the acceptor-like boron vacancy formation, we have demonstrated that h-BN can function as a highly conductive, DUV-transparent electrode; the hole concentration is ~10(20) cm(-3) at room temperature, which is ten orders of magnitude higher than that previously measured for Mg-doped AlN epilayers. We have further demonstrated the first Al(Ga)N/h-BN LED, which exhibits strong emission at ~210 nm. This work also reports the first achievement of Mg-free III-nitride LEDs that can exhibit high electrical efficiency (80% at 20 A/cm(2)).
Article
We report on the experimental fabrication and functioning of an optical fiber sensing probe operating on the principle of a localized surface plasmon resonance (LSPR) at near infrared (NIR) wavelengths, around the 2- $\mu \text{m}$ window. As indium-tin oxide exhibits an absorption peak at NIR wavelengths, we have synthesized and characterized ITO nanoparticles with a doping concentration of 9:1 of In:Sn and attached these onto the unclad core of the fiber. Corresponding to the absorption peak of the bare nanoparticles, the fiber probe displays an LSPR peak at 1950 nm, which falls in the signature band of many bacterial contaminants. The probe is further used for the sensing of analytes of refractive indices varying from 1.335 to 1.375. The results reveal that the average peak absorbance of the probe changes by 3.46 absorbance units for a unit change in refractive index.
Article
Full-text available
Indium-tin-oxide (ITO) films deposited by sputtering and e-gun evaporation on both transparent (Corning glass) and opaque (c-Si, c-Si/SiO2) substrates and in c-Si/a-Si:H/ITO heterostructures have been analyzed by spectroscopic ellipsometry (SE) in the range 1.5-5.0 eV. Taking the SE advantage of being applicable to absorbent substrate, ellipsometry is used to determine the spectra of the refractive index and extinction coefficient of the ITO films. The effect of the substrate surface on the ITO optical properties is focused and discussed. To this aim, a parametrized equation combining the Drude model, which considers the free-carrier response at the infrared end, and a double Lorentzian oscillator, which takes into account the interband transition contribution at the UV end, is used to model the ITO optical properties in the useful UV-visible range, whatever the substrate and deposition technique. Ellipsometric analysis is corroborated by sheet resistance measurements.
Article
Full-text available
Two kinds of nanocrystalline indium tin oxide (ITO) powders with different crystal structures—rhombohedral and cubic—were prepared using a coprecipitation process through the control of pH of a mixing solution and aging time after coprecipitation. The two powders have the same particle size of 15nm in diameter but different morphologies (spherical for rhombohedral and rectangular for cubic). The gaseous ethanol sensing characteristics of the sensors prepared by the two ITO powders were quite different. The sensitivity of rhombohedral ITO sensor was high compared to that of the cubic ITO sensor across all temperatures. The reason for this is explained through the viewpoint of the binding energy as shown in XPS measurement and the surface structure relating to the crystal structure.
Article
Indium tin oxide (ITO) thin films were prepared on glass substrates by an electron beam evaporation technique from a mixture of In2O3 and SnO2. The films were annealed in air for 30 min at 350°C. The electrical and optical properties of these films were investigated as a function of the substrate temperature. The films were deposited at substrate temperatures ranging from 50 to 350°C at an oxygen partial pressure of 5×10−5 mbar. The dopant concentration, resistivity, electrical conductivity, activation energy, optical transmission and band gap energy were investigated. It was found that the activation energy decreased with an increase in the film thickness. A transmittance value of 92% in the visible region of the spectrum and a resistivity of 3×10−6 Ωm was obtained at a substrate temperature of 350°C. Structural studies showed that the films were polycrystalline.
Article
Indium tin oxide (ITO) thin films were prepared by pulsed laser deposition (PLD) on glass substrate at room temperature. Structural, optical, and electrical properties of these films were analyzed in order to investigate its dependence on oxygen pressure, and rapid thermal annealing (RTA) temperature. High quality ITO films with a low resistivity of 3.3 × 10−4 Ω cm and a transparency above 90% were able to be formed at an oxygen pressure of 2.0 Pa and an RTA temperature of 400 °C. A four-point probe method, X-ray diffraction (XRD), atomic force microscopy (AFM), and UV–NIR grating spectrometer are used to investigate the properties of ITO films.
Article
Indium tin oxide (ITO) thin-films were deposited on soda-lime-silicate glass using sols prepared from alcoholic solutions of indium chloride and stannic chloride with different In:Sn atomic ratios, namely 95/5, 90/10, 85/15 and 80/20. The electrical properties, structure and morphology of the thin-films were investigated. All the films studied, with a thickness range of 10–490 nm were polycrystalline with grain sizes in the range of 25–60 nm. Uniform and dense microstructure apparently devoid of cracks and voids were observed. Only cubic In2O3 phase was observed in the X-ray diffraction (XRD) and transmission electron microscopy (TEM). SnO and SnO2 phases were not detected. The sheet resistance values decreased with increase in coating thickness. A significant decrease in the resistance values was also noted after annealing in N2\H2 (96–4%) atmosphere. The minimum sheet resistance values were noted for Sn concentration of 10 at.%. The lowest value, 11 ohms per square, was obtained after annealing for a 490-nm film.
Article
Aiming at a facile and low-cost liquid phase synthesis of indium tin oxide (ITO), a microwave-assisted preparation in high-boiling, multidentate alcohols (so-called polyols) is evaluated. While heating for 2 h at 200 °C and ambient pressure, transparent ITO suspensions are obtained exhibiting a bright blue color. According to dynamic light scattering, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and Brunauer–Emmett–Teller analysis, as-prepared particles turn out to be single crystalline with an average diameter of 17 nm and a near monodisperse size distribution. Four-point probing of powder pellets reveals a low resistivity (1.1×10−2 Ω cm) of as-prepared In2O3:Sn (5 mol.%). Fourier-transform infrared spectra show a reflectivity in the infrared close to 100%. As a proof of concept, thin layers are deposited on glass plates using a simple solvent evaporation technique. After a certain post-treatment, these layers exhibit a visible transmittance similar to the uncoated glass substrate and a low resistivity (1.2×10−2 Ω cm).
Article
Indium tin oxide thin films were prepared by a chemical solution deposition route based on a one pot reaction of indium acetate, tin tetrachloride pentahydrate and propionic acid. The films were heat-treated in oxidising atmosphere and post-annealed in forming gas containing 10% hydrogen. It was found that the coating solution concentration exerted a major influence on the morphology of the films. Low-concentrated solutions yielded layers with columnar structure while higher molarities led to fine-grained granular films. Under optimised conditions, thin films of indium tin oxide exhibiting a resistivity of 1·10−3 Ω cm were obtained. The transparency was determined to a high level of above 90% in the entire visible range of the spectrum.
Article
Thin films of ITO (Sn-doped In2O3) were coated on Corning #1737 glass substrates (50×50 mm2) heated at 600 °C, after introducing an aerosol with air to a furnace heated at 620 °C. The aerosol was produced by ultrasonic vibration of 0.2 M In(III)-acetylacetonate together with 5 mol% Sn(IV)-bis-acetylacetonate-dibromide dissolved in acetylacetone. Film structure and electrical resistivity at room temperature depend on air flow rate: (1) at 10 l/min ITO films grew with (100)-preferred orientation because (400) and (800) peaks were clearly observed and the electrical resistivity was approximately (1–2)×10−4 Ωcm; (2) when the air flow rate was reduced to 7.5 and 5 l/min, the (400) and (800) peak intensities became small and their resistivity was (2–4)×10−4 Ωcm. The optical energy band gap (3.90 eV), which is estimated from the relationship between the square of the optical absorption coefficient times the frequency of the electromagnetic wave and the frequency near the absorption edge, is independent of film thickness and air flow rate. Some comments on the (100)-preferred orientation thin-film growth and on the resistivity change due to the flow rate are provided.
Article
Indium tin oxide (ITO) films have been prepared by the sol-gel method using both organic and inorganic precursors. A computer-controlled dip-coating unit is designed and fabricated in our laboratory for a precise control of the parameters during the dip-coating process. These films have been characterized by X-ray diffraction optical and electrical study and also by atomic absorption spectroscopy. The optimized coatings exhibit a sheet resistance of ara und 100 Ω/□ and an average visible solar transmission of around 85%.A five-layer electrochromic system using these ITO layers as transparent electrodes was fabricated and tested. The performance of the electrochromic system indicates the high potential of these films for such applications, especially for large area coaling.
Article
Gas sensors using metal oxides have several advantageous features such as simplicity in device structure and low cost fabrication. In this work, Tin-doped indium oxide (ITO) films were prepared by the screen printing technique onto glass substrates. The granular and porous structure of screen-printed ITO are suitable for its use in gas sensing devices. The resistance of the ITO films was found to be strongly dependent on working temperatures and the nature and concentration of the ambient gases. We show that screen-printed ITO films have good sensing properties toward NH3 vapours. The observed behaviors are explained basing on the oxidizing or the reducer nature of the gaseous species that react on the surface of the heated semi-conducting oxide.