Formation of zinc inverted opals on indium tin oxide and silicon substrates by electrochemical deposition

Article abstract of DOI:10.1021/jp047475n

Zinc inverted opals have been fabricated by means of electrochemical deposition templated on polystyrene artificial opals grown on indium tin oxide (ITO) and n-doped silicon substrates. Several differences in the infiltration were observed between samples

Full text of DOI:10.1021/jp047475n

16708
J. Phys. Chem. B 2004, 108, 16708-16712
Formation of Zinc Inverted Opals on Indium Tin Oxide and Silicon Substrates by
Electrochemical Deposition
Beatriz H. Ju a´ rez and Cefe L o´ pez*
Instituto de Ciencia de Materiales de Madrid C/ Sor Juana In e´ s de la Cruz 3, 28049 Madrid, Spain
Concepci o´ n Alonso
Departamento de Qu ´ı mica F ´ı sica Aplicada, UniVersidad Aut o´ noma de Madrid, Cantoblanco,
2
8049 Madrid, Spain
ReceiVed: June 11, 2004; In Final Form: August 19, 2004
Zinc inverted opals have been fabricated by means of electrochemical deposition templated on polystyrene
artificial opals grown on indium tin oxide (ITO) and n-doped silicon substrates. Several differences in the
infiltration were observed between samples infilled by linear cyclic voltammetry and square wave pulsating
potential approaches, leading to the fabrication of metallic photonic crystals with different topologies on
semiconducting substrates. The samples have been characterized by scanning electronic microscopy (SEM),
energy dispersive X-ray analysis (EDX), and X-ray powder diffraction. This is the first time that Zn inverse
opals have been fabricated and this work proves the possibility of growing macroporous structures by
electrochemical methods on different semiconductor substrates.
I. Introduction
Macroporous materials have gathered great attention in the
its own but also for the extraordinary optical properties of its
oxide. Although ZnO has been electrochemically grown in
colloidal templates, to the best of our knowledge, this is the
22
past decades due to the promising future in many areas such as
first time the growth of Zn inverted opals is reported. ZnO and
ZnO/Zn structures derived from Zn opals could be tackled by
proper and controlled oxidation of Zn opals in order to study
luminescent properties in metallic face-centered cubic (fcc)
structures. In this work Zn growth in opals on semiconductor
substrates, namely, indium tin oxide (ITO) and n-doped silicon
substrates is presented. There is little work concerning the
1
2
3
catalysis, chemical sensors, and photonic crystals. In the latter
case, large photonic band gaps are predicted for periodic metallic
structures opening up new alternatives in the development of
potential photonic applications.4 Measuring and interpreting
the optical response of metallic photonic crystals has become a
-7
8-11
challenging task.
Due to the large refractive index, complete
photonic band gaps (CPBG) can be opened in regions where
the metal absorption is negligible. In thin periodic metallic
structures, new interesting phenomena can be expected from
the interaction of light with plasmon resonances.1
23
electrodeposition of metallic films on silicon substrates, and
to the best of our knowledge, none of them about zinc. Two
different electrochemical methods with different growth mech-
anisms open up the possibility of controlling the fabrication of
metallic photonic crystals with different topologies on semi-
conducting substrates. The growth of periodic metallic zinc on
transparent substrates (ITO) and silicon surfaces can be of
relevant importance in optoelectronics and other technologies.
2
Owing to several reasonssthey are widely used, profusely
investigated, cheap, and easily fabricatedsartificial opals are
excellent systems to study the physics involved in the optical
response of metallic photonic crystals. Electrodeposition de-
serves special attention due to its low cost, low-temperature
working conditions, and precise control over the thickness and
composition of the electrogenerated material. It is a suitable
method to grow metals, semiconductors, alloys, and a large
number of different compounds. Braun and Wiltzius13 used this
approach to infill II-VI compounds in silica opals. Bartlett et
Cyclic voltammetry and square wave pulse potential meth-
ods24 were used to grow and improve the homogeneity and
density of zinc deposits on both substrates. At variance with
commercial plating baths that contain additives to improve the
final appearance of the deposits, no additives were used in this
work. The samples were used as working electrode without
masking so that the whole area was allowed for growth.
1
4
al. have reported several macroporous structures fabricated
by electrochemical methods. Many other authors have used the
1
5
electrochemical approaches to build up III-V compounds,
II. Experimental Section
16
17
18
metallic structures, alloys, semiconductor composites, and
19
conducting polymers based on artificial opals. Electroless
baths2 have been employed as well to fill the macroporous bare
structures with metals. Other approaches such as photolithog-
raphy and chemical vapor deposition (CVD) methods were
combined to grow one unit cell thick wood-pile arrangements
A. Fabrication of Thin Artificial Opals. Anionic polystyrene
0
spheres were synthesized by a nonsurfactant emulsion polym-
2
5
-1
erization method. ITO-coated glass (R ) 10 Ω‚cm ) and
n-Si(111) surfaces were used as conducting substrates. Si
substrates were cleaved from n-wafers highly doped (R < 20
2
1
-1
of tungsten. The metal here reported is of interest not only on
Ω cm ). The native oxide was removed with 3% vol HF
solution for 10 min and finally rinsed with double deionized
water. ITO and SiO2-free silicon substrates were hydrophilized
*
Corresponding author. E-mail: cefe@icmm.csic.es.
1
0.1021/jp047475n CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/01/2004

Electrochemical Deposition of Zinc Inverted Opals
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16709
in two different solutions. SiO2-free n-Si surfaces were immersed
in an acid mixture (H2SO4/H2O2 3:1) during 10 min. In the case
of ITO substrates a mixture of H2O/NH3/H2O2 (17:3:1) was used
to immerse the samples for 20 min.
The backside of the Si samples was properly preserved from
the electrolyte by painting with insulator paste.
Polystyrene (PS) artificial opals were grown by means of the
26
convective method to obtain highly ordered fcc structures with
controlled thickness. As a typical procedure, in this work 0.3%
vol colloids were used, obtaining 5-10 layers of opals after 35
h at 45 °C.
The samples at this stage are not mechanically robust enough
to withstand immersion in a liquid without being partially
destroyed. A sintering process at appropriate temperature can
27
improve the mechanical strength of artificial opals. However,
this treatment is not appropriate for opals attached to a substrate.
An alternative method recently described, based on the synthesis
of SiO2 in vapor phase at room temperature,28 shows important
advantages over temperature treatments. It produces a SiO2 film
covering the spheres that acts as a framework and strengthens
the structure without provoking cracks as sintering procedures
do and is compatible with the presence of the supporting
substrate.
By means of numerical calculations,29 precise estimations in
filling fractions can be done by comparing experimental
reflectance spectra from bare and silica-infiltrated opals with
the photonic bands (not shown). A mere 8% of the pore volume
Figure 1. Cyclic voltammetry for n-Si in 100 g/L zinc acetate aqueous
-
1
solution at pH ) 6.3. The scan rate was 10 mV s
.
III. Results and Discussion
A. Electrochemical Response. Current-potential curves
were recorded beforehand in order to choose the optimal
working conditions for opals grown on both ITO and n-Si
substrates. Regarding the reduction potential, no differences
were found between bare substrates and substrates with an opal.
Figure 1 shows the voltammogram for a bare n-Si substrate at
3
0°C in a 100 g/L zinc acetate aqueous solution. On the initial
cathodic scan, zinc deposition starts at -1.05 V, with the current
increasing for increasing (more negative) potentials. In these
conditions, once a zinc monolayer is formed, bulk zinc
deposition takes place. On the other hand, hydrogen evolution
reaction on n-Si in background electrolyte (potassium acetate)
starts at E ) -1.35 V. However, this reaction in the presence
of zinc is not observed since, once a monolayer has been
electrodeposited, the hydrogen development begins taking place
on zinc. Hydrogen overtension on zinc is known to be about
(2% of the total opal volume or a layer of SiO2 around 1% of
the sphere radius) revealed enough. Although opals grown on
both ITO and n-Si substrates were covered with silica, the latter
were immersed in diluted HF before acting as working
electrodes to remove the native SiO2. By doing so, the silica
shell previously deposited was also partially dissolved. After
this process the samples remained mechanically stable.
B. Infiltration of Metallic Zinc. A three-electrode setup was
used for zinc electrodeposition. A platinum plate served as
auxiliary electrode. All the potentials are quoted with respect
to Ag/AgCl reference electrode. The cell was properly deaerated
by 99.999% N2 during 20 min before each experiment. The zinc
solution (100 g/L) was prepared from zinc acetate (Aldrich,
0
.7 V (EdH /Zn ≈ -1.8 V). This is the reason hydrogen evolution
2
does not interfere with the zinc deposition process.
On the anodic scan, a stripping peak corresponding to the
removal of bulk zinc, with an onset at -1.05 V and a current
peak centered at -0.77 V, can be seen.
In the case of samples grown on n-Si by cyclic voltammetry,
the potential range was -1.05 and -1.35 V at scan rate of 10
mV s . Under these conditions, the redissolution of the deposit
is prevented. The growth proceeds under kinetic control,
avoiding diffusion-limited growth, which is characterized by
low-density and high-rugosity deposits.
9
9.95%) and ultrapure water (Millipore, 18.2 MΩ). The pH of
-1
the solution is 6.3. The experiments were performed in solutions
thermostated at 25 and 30 °C with the help of a thermostatic
bath.
Two different methods have been used to infiltrate thin
artificial opals with metallic zinc: cyclic voltammetry and
square wave pulsating potential electrodeposition.
Voltammograms for ITO substrates were recorded under the
same conditions of concentration and temperature (not shown).
In the absence of electroactive substance (zinc acetate), the
hydrogen evolution reaction starts at E ) -1.2 V and zinc
deposition begins at -1.0 V. The stripping current peak at
potential of -0.550 V is observed. However, in this case, the
degradation of electrode material occurs just below E ) -1.055
C. Inversion Procedure (Oxygen Plasma Etching). In this
study, polystyrene spheres were removed by means of oxygen
plasma etching at room temperature to avoid the remainders
left by organic solvents such as toluene, styrene, or tetrahydro-
furan (THF) after the polystyrene dissolution. Adequate oxygen
plasma etching yields cleaner structures. The removing was done
in several cycles at room temperature (RT) conditions to avoid
metal oxidation. Ten cycles, 2 min each with an oxygen pressure
of PO2 ) 400 mTorr revealed enough for removing the
polystyrene opal without damaging the metallic network.
V. Zn electrodeposition was carried out between -0.850 and
-1
-
1.055 V, at scan rate of 10 mV s . As in the former case,
the electrodeposition process does not involve redissolution. The
nuclei generated under these conditions grow as the number of
scans increases.
Cyclic voltammograms have been recorded as well at 25 °C,
showing no differences in the potential range as compared with
those at 30 °C.
The square wave pulsating potential method consists of a
periodic repetition of overpotential pulses. The pulse cycle is
characterized by the pulse potentials, the cathodic deposition
D. Characterizations. The morphology of the specimens was
observed by scanning electron microscopy (SEM). An energy-
dispersive X-ray analyzer (EDX) equipped on the SEM was
used to determine the compositions. Powder X-ray diffraction
was recorded from blank substrates.

16710 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Ju a´ rez et al.
Figure 2. Schematic description of the modes of infiltration of artificial
opals: (a) cyclic voltammetry and (b) square wave pulsating potential
methods.
time (ton), and the time interval between two pulses (toff), in
which the system relaxes. 24 According to theory, this method
yields smoother uniform surfaces with finer grains as a result
of higher instantaneous current densities and is an efficient way
to control the particle size distributions. During the on time,
the formation of zinc nuclei takes place, and during off time, a
recrystallization occurs, obtaining larger grains.
In our case, the pulsed potential waveform applied for zinc
deposition on n-Si substrates was Edeposition) -1.8 V during
ton)1 ms and Eoff ) -1.05 V during 10 ms. The pause-to-
pulse ratio (p ) 10 ms/1 ms) is 10 and the frequency is 90 Hz.
These values are inside a range, optimal to obtain compact
deposits, since the depletion in the diffusion layer at the end of
the pulse is small and, therefore, there is not influence of mass
transport. Besides, the system has time enough (10 ms) to reach
a state termodynamically more stable. For samples on ITO
substrates Edeposition ) -1.05 V and Eoff ) -0.85 V. The
deposition time was varied between 2 and 48 h.
B. SEM Characterization of Zinc Inverted Opals. The
cyclic voltammetry and pulsing potential electrodeposition
approaches are characterized by different ways of infiltration
of artificial opals. Feature filling during the Zn deposition under
cyclic conditions occurs by growth of an increasingly thicker
layer following the spherical geometry of spheres. This approach
yields metallic shells around the spheres that become thicker
as electrodeposition time evolves. On the other hand, by means
of pulsing potential electrodeposition, the electrodeposits nucle-
ate in preferential sites on the substrate around the beads and
grow to flood the space between spheres, giving rise to smooth
deposits in which opals finally become immersed. Figure 2
clarifies both ways of infiltration. These differences can be
related to the previously cited tendencies in the growth of
electrodeposits. Since the off time favors the recrystallization,
smoother and more uniform surfaces are expected for elec-
trodeposits obtained under pulsating conditions.
Figure 3. (a) First stages of Zn growth on n-Si under optimized pulsing
electrodeposition conditions in the presence/absence of opal template.
Scale bar is 2 µm. (b) zinc inverted monolayer on n-Si. Scale bar is 2
µm. (c) Larger area. Scale bar is 10 µm.
can be appreciated: the upper right zone shows the initial steps
of Zn growth on opal-free n-Si surfaces. Here, the deposit
comprises large needle crystals stacked and randomly oriented,
giving rise to an irregular film. On the lower left zone, these
wires’ growth has been directed by a monolayer of beads
(removed upon etching) acting as a template. In the absence of
a second layer of spheres, once the cavities between beads have
been filled, further growth proceeds again as Zn spikes oriented
at random, covering the spheres as in the upper left zone. Figure
3b shows a SEM image of a templated zinc film grown through
a single monolayer of 505 nm diameter polystyrene spheres
under pulsing conditions during 2 h. The image shows that the
spherical voids obtained after removal of the initial pattern are
well-ordered and close-packed. The differences in the pore sizes
result from film thickness variations in the flooding level.
Ordered regions can extend over several hundred square
micrometers as can be seen in Figure 3c.
Although not shown, similar SEM images are obtained for
samples grown on ITO surfaces. After removal of the initial
template, highly ordered and hexagonal structures are obtained.
From these pictures it can be clearly observed that the
morphology of the deposits is markedly different in the presence
of ordered spheres. High quality and smooth deposits are
obtained between close-packed beads. As expected, thickness
and density of the electrodeposits increase with the electrodepo-
sition time.
On the other hand, cyclic voltammetry yields nuclei that grow
as the electrodeposition time increase, giving rise to bigger
grains.
Figure 3a shows the first stages of zinc growth by pulsing
potential electrodeposition on n-Si inside and outside a single
monolayer of opal (after opal removal). Three different zones

Electrochemical Deposition of Zinc Inverted Opals
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16711
Figure 4. Several stages in the growth of zinc by pulsing electrodepo-
sition. The oblong appearance of some holes is due to the distortion of
the metallic lattice under low scanning SEM process. The scale bar is
1
µm.
For short periods of electrodeposition, more open structures
are observed with incompletely filled spaces in underneath layers
that become filled with time. It is worth noting that pulsing
electrodeposition yields very homogeneous surface appearance
because higher instantaneous current density provokes an
increase in the nucleation rate, leading to the formation of finer-
grained deposits. On the other hand, during the off time or
relaxing time, the nuclei increase their size due to the diffusion
of the adatoms. At variance with Figure 2, which is only a
schematic description to clarify the way infiltration proceeds,
it is worth mentioning that differences in the stages of growth
can be appreciated along the millimeter area of the opal. These
deviations allow us to easily understanding the infiltration
process. Figure 4 depicts an SEM image of a sample grown by
pulsing potential electrodeposition during 8 h on n-Si. Several
stages of growth can be distinguished.
Figure 5. (a) (111) surface in a zinc inverted opal obtained by means
of cyclic voltammetry on ITO surface. (b) (111) Zn inverted surface
on n-Si surface. Scale bars are 1.2 and 5 µm, respectively.
Figure 6. Zinc deposits on n-Si at (a) 25 °C and (b) 30 °C under
identical experimental conditions. Scale bar is 1 µm.
The picture suggests that, by means of pulsating potential
electrodeposition, the Zn nucleation takes place preferentially
around the contact points of spheres with the electrode surface,
growing forward to the center of the cavities between the beads.
In the upper right zone (circled for clarity), six triangles
separated by the contact points between the neighbor spheres
in a (111) plane can be observed. In this particular point, the
thickness of the deposit is in the equator level of the spheres.
In every triangle can be seen three nuclei of growth located
near the contact points of the spheres corresponding to
preferential sites of growth. This geometry leaves an empty dark
central spot which is the last space to be filled as the deposit
progresses, as can be seen in several points, as the one marked
with an arrow.
With increasing film thickness, due to the geometric arrange-
ment of the spheres, it is possible to observe three windows in
the second layer, seen as dark circles, in each spherical cavity.
These small holes are the internal channels between macropores,
resulting from points where the original spheres were in contact
in the initial opal.
Relevant differences are observed in opals infilled with zinc
by means of cyclic voltammetry. Figure 5 illustrates two stages
in the process of Zn growth in opals laying on ITO and n-Si
substrates under cyclic voltammetry conditions. Figure 5a shows
a (111) surface area from an opal infiltrated for 20 h by cyclic
voltammetry on an ITO surface (after oxygen plasma inversion).
It can be clearly seen that the electrodeposits grow in a
conformal way around the beads, replicating the spherical
morphology. Thus, in a particular layer, triangular voids between
adjacent spheres are usually empty and bounded by zinc shells.
However, further evolution of the electrodeposits with time
evidence the complete infilling of pores in underneath layers,
which is a sign that the thickness of the deposit is larger in
layers closer to the electrode (substrate). Figure 5b depicts a
(111) inverted opal surface on n-Si. Although the electrodeposits
are inhomogeneous in thickness across the whole area of the
electrode, which can be due to inhomogeneities in the opal
template, large and high-quality areas in the order of hundreds
of square micrometers can be obtained. Since the type of growth
is shared by opals grown on n-Si surfaces and those on ITO,
the mode of infiltration is cathode-independent.
C. SEM Characterization of Zinc on Bare Substrates.
Although no significant differences can be appreciated between
inverted opals grown at 25 or 30 °C, nor in the reduction
potentials, marked tendencies can be extracted from the ap-
pearance of electrodeposits on blank substrates. Figure 6 shows
a nontemplated Zn film on n-Si deposited under the same
conditions at (a) 25 and (b) 30 °C. The increase in temperature
favors diffusion of the adatoms, giving rise to smoother deposits.
D. EDX and X-ray Diffraction. EDX analyses optimized
for detection of light elements were performed on each sample,
confirming the principal presence of zinc in all the structures.
However, since commercial ITO films are grown on glass
containing oxygen and silicon is naturally oxidized in air, all
samples show the presence of oxygen, making it difficult to
demonstrate that metallic zinc was not oxidized after the plasma
etching. To minimize the detrimental effect derived from the

16712 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Ju a´ rez et al.
square wave pulsating potential approaches yield different ways
of infiltration of these structures. This work demonstrates the
possibility of fabricating high-quality metallic structures on
semiconductor substrates. Further improvement in the thickness
control of the electrodeposits will help us understand the optical
behavior of metallic opals.
Acknowledgment. This work is partially financed by the
Comunidad Aut o´ noma de Madrid through 07T/0048/2003 and
Spanish MCyT through MAT 2003-01237 projects. We thank
E. Castillo-Martinez, E. Palacios-Lid o´ n and Dr. D. Golmayo
for experimental help.
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