LETTER
pubs.acs.org/NanoLett
Probing the Lithium Ion Storage Properties of Positively and
Negatively Carved Silicon
Sang Hoon Nam,† Ki Seok Kim,† Hee-Sang Shim,‡ Sang Ho Lee,† Gun Young Jung,*,† and Won Bae Kim*,†,‡
†
School of Materials Science and Engineering and ‡Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of
Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, South Korea
bS Supporting Information
ABSTRACT: Here, we report Si pillar and well arrays as
tailored electrode materials for advanced Li ion storage devices.
The well-ordered and periodic morphologies were formed on a
Si electrode thin film via laser interference lithography followed
by a dry etch process. Two different patterns of negatively or
positively carved Si electrodes exhibited highly improved cycle
performance as a consequence of the enlarged surface area and
the nanoscale pattern effects. The Si well arrays showed the
highest energy density, rate capability, and cycling retention
among the prepared Si electrodes. This tailored electrode
platform demonstrates that these design principles could be applied to future developments in Si electrodes.
KEYWORDS: Silicon, laser interference lithography, nanopattern, anode, lithium ion battery
S
ilicon is attracting increasing attention as an alternative anode
electrode material for use in Li ion batteries (replacing
carbonaceous materials), due to its very high theoretical specific
capacity (up to 4200 mAh g1) and relatively low working
potential (∼0.5 V vs Li/Li+).1,2 Although deposited Si film
electrodes exhibit a high specific capacity at initial stages, the
cycle performance is likely to decay rapidly, with cracking and
crumbling occurring with increasing cycle number.35 This poor
cycle retention, which occurs because of the volume changes
(approximately 400%) in Si during Li alloying/dealloying,1 has
delayed the development of Si as a next-generation material for Li
ion batteries. The most important requirement for successful Si
electrode development is the accommodation of the volume
expansion/contraction during the cycles. Recently, several research groups have reported interesting Si nanostructures for
Li ion batteries, such as nanowires,68 nanotubes,9,10 and
nanoparticles,11,12 prepared by various bottom-up methods. The
nanostructured Si can directly affect the transport characteristics
of charges and show induced stress relaxation during lithium
insertion/extraction since the interval distance formed between
neighboring nanostructures can act as structural buffer space
preventing the agglomeration of Si,13 resulting in enhanced
capacity and cycle performance. These earlier works provided a
base for surface nanopatterning techniques capable of manipulating surface regions through control of both the periodicity
and diameter to enter into the picture in the development of Si
electrodes for Li ion batteries. Over the last few decades a
number of new nanopatterning techniques have been developed,
including photolithography,14 nanoimprinting,15 electron beam
lithography,16 dip-pen writing,17 and variants thereof; the use of
these techniques has allowed a scaling down of the feature
size to below 100 nm. The process starts with the design of a
r 2011 American Chemical Society
nanopattern in the form of a mask, which is subsequently used for
template etching or deposition to create two-dimensional nanopatterns on a substrate. This has also been accomplished using
laser interference lithography (LIL), which is a powerful method
for fabricating periodic structures over large areas at the nanoscale level. This technique provides an economical, rapid, and
reproducible system for manufacturing patterned arrays on flat
surfaces.1821 However, there are few reports on the Li ion
storage properties using the nanopattering techniques despite
many possible opportunities in Li ion batteries.
In this work, we produced carved Si electrodes using the LIL
technique under controlled exposure conditions, in combination
with a dry etch process. We demonstrated their electrochemical
properties and showed that, as a consequence of the structural
benefits of tailored Si nanostructures, they are good electrode
materials for Li ion storage. The direct fabrication of arrayed or
integrated architectures on the substrate provides fast charge
transport, thereby allowing every individual nanostructure to
participate in the electrochemical reaction.6,22 In the current
study, we probed the effects of nanopattern type on the electrochemical properties and Li ion storage performance, using
consistently carved Si electrodes. The pattern size, etch depth,
and periodicity of the Si electrodes used in this work were
controlled to allow a comparative study.
Results and Discussion. The LIL technique confers several
advantages, including easy processing, large-area patterning, and
simple modulation of the density or diameter in periodic dot or
hole patterns. In this work, we have utilized LIL with a positive or
Received: May 9, 2011
Revised:
July 27, 2011
Published: August 22, 2011
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Figure 1. Schematic diagrams illustrating the fabrication of positively or
negatively carved Si arrays. The scheme and images for the laser
interference setup and pitch size differentiation with increasing incident
angle (θ), and the developed polymer templates were additionally
described in Figure S1 in the Supporting Information.
Figure 3. X-ray spectra of the sputtered Si thin film: (a) X-ray
diffraction and (b) X-ray photoelectron spectroscopy of Si 2p obtained
from the bare Si film. The inset figure indicates the depth profile using Ar
ion etch.
Figure 2. Scanning electron microscopy plane- and cross-view images
of SPA (a, b), SWA (c, d), and STF (e, f). The inset figures present highly
magnified images of the plane-view of the prepared Si electrodes.
negative photoresist mask to prepare nanoscale patterned periodic Si pillar arrays (SPA) or Si well arrays (SWA), as illustrated
in Figure 1. The prepared Si thin film (STF) was spin coated with
a diluted positive- or negative-tone photoresist. The 4-fold
symmetry array pattern was modulated by irradiating a standing
wave onto the sample, which was rotated by α (α = 0° and 90°).
A double exposure was performed to fabricate a square lattice
array. Two beams, one directly from a 325 nm HeCd laser and
the other reflected from a Lloyd mirror, converged coherently
and caused constructive and destructive interference on the
photoresist surface. The periodic pattern array was given by
Λ = λ/(2 sin θ), where Λ, λ, and θ indicate the pitch, wavelength
of the UV laser, and incident angle, respectively. After the
development process, two-dimensional periodic patterns of
photoresist dots or holes were obtained (see Figure S1 in the
Supporting Information). The development process was then
performed with acetone. The positively or negatively carved Si
electrodes were then constructed by reactive ion etching (RIE).
Scanning electron microscopy (SEM) images of the SPA
(Figure 2a,b) and SWA (Figure 2c,d) revealed that the carved
Si nanostructures showed clear, well-defined periodic nanopatterns of SPA or SWA, consistent with the schematics in Figure 1.
The period, radius, and etch depth of the carved shapes were
fixed to ca. 400, 200, and 150 nm, respectively. The developed
pattern of 1.29 109 dots was generated uniformly over an area
of 1.77 cm2. For comparison, an STF without a pattern was also
prepared (Figure 2e,f).
The structural and chemical properties of the prepared Si
electrodes were investigated using X-ray diffraction (XRD) and
X-ray photoelectron spectroscopy (XPS). Figure 3a shows the
XRD patterns for the sputtered Si on the stainless steel substrate.
Except the characteristic peaks observed at 2θ = 43.8°, 44.7°, and
50.8° from the substrate itself, the sputtered Si indicated no
crystalline peaks due to its amorphous nature. Also, transmission
electron microscopy (TEM) image and selective area electron
diffraction (SAED) pattern indicated that the phase of deposited
Si in this work is amorphous (see Figure S2, Supporting Information). It is reported that many nanostructures and thin
films of Si remain amorphous with cyclings.612 It is reported
that the amorphous feature can show a higher electrochemical
performance than crystalline Si.2325 Figure 3b shows the XPS
spectra of the sputtered Si, which gave two main peaks, corresponding to elemental Si (at 99.3 eV) and SiOx (at 103.3 eV).
The SiOx is believed to be oxidized Si species present on the
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Figure 4. The galvanostatic process of the prepared Si electrodes: the voltage profiles for the first, second, and tenth cycle at a constant current density of
0.04C rate: (a) SPA, (b) SWA, and (c) STF, respectively. (d) Corresponding calculated dQ/dV plots for tenth cycle.
surface layer, which can contribute to an irreversible capacity
resulting from the decomposition of SiOx to Si and xLi2O.26
After Ar ion sputtering, the XPS spectrum of the fresh surface was
obtained (inset Figure 3b). The etched Si indicated a single
strong peak at 99.3 eV, without any SiOx peak.
The demonstrated Si (pillar arrays, well arrays, and thin film)
electrodes were inserted into a coin-type half-cell, and the LiSi
alloying/dealloying properties were investigated with discharge/
charge processes. As shown in the panels of Figure 4, a potential
window from 0.01 to 1.5 V (vs Li/Li+) was studied at a constant
C rate of 0.04C (ca. 168 mA g1). The discharge capacity mainly
occurred below 0.3 V, and the charge capacity appeared from 0.3
to 0.8 V. As shown in panels ac of Figures 4, the voltage profiles
of the prepared Si electrodes indicated a plateau in the first
discharge and a smoother and sloped shape after the second
discharge. The irreversible voltage profile at the initial cycles
seems to be associated with the formation of a solid electrolyte
interface (SEI) layer or native oxide reduction which is related to
phase transformation.27 It was also found that the voltage profiles
at the initial cycles could be affected by the surface morphology of
the electrodes. In the first discharge, the potentials of the SPA,
SWA, and STF rapidly dropped to the voltages of 0.10, 0.43, and
0.22 V, respectively. The potential plateau formed around 0.2 V
in cases of the SPA and STF and then gradually decreased to 0.01 V.
The STF showed the steeper decreases of potential from the
second discharge curves than those of the carved Si patterns in
the range 0.20.6 V. The physical constraint involved in the
silicon electrodes can influence the capacity and energy dissipation due to plastic deformation.28,29 These voltage fluctuations
may be related to the electrochemical resistance of Si against
electrochemical reactions. Consequently, the control of active Si
via the pattern type in this manner could have led to the different
potential profiles and cell performances.
The reversible 20th cycle showed a distinct difference between
the carved Si electrodes and the STF electrodes. Whereas the
STF electrode potential rapidly dropped to about 0.4 V, the
carved Si electrodes showed sluggish potential drops in the
discharge process, implying that the carved Si electrodes have
smaller internal resistance compared with the STF. From
the calculated differential capacity (dQ/dV) in Figure 4d, the plot
of dQ/dV vs V shows redox couples at the 10th cycle in terms of
the positions and relative intensity of the peaks. The obtained
peaks were attributed to the potential dependence of LiSi alloys
of different compositions. Cathodic peaks below 0.3 V, as well as
anodic peaks over 0.25 V, were observed; these were related to
the alloying/dealloying in the Si electrode. In the alloying
process, two broad cathodic peaks at around 0.23 and 0.10 V
appear in the case of SWA and they are associated with the
formation of amorphous LixSi phase via a single-phase transition.
The appearance of the two peaks might be caused from two
different sites with different energies in the structures.30 In the
dealloying processes, lithium is extracted from the amorphous
LixSi, and the amorphous silicon is then formed. The constructed
Si structures will be re-formed when the potential reaches 0.8 V,
which is consistent with the changes in the Si electrodes after
cycling, as will be discussed below. A decrease in the anodic-tocathodic peak-to-peak separation was observed in the carved Si
electrodes, as compared with the STF. The peak separations in
the SWA sample appeared to be ca. 0.15 V, whereas the SPA and
STF were found to have a peak separation of ca. 0.30 V. In
previous research,31,32 such peak separations occurred at low
overpotentials, and the peak positions were usually reported for
doped Si and metal-incorporated Si. We can therefore postulate
that the carved Si electrodes had better conductivity (see Table
S1, Supporting Information) along with the small potential
differences, which implies that the constructed Si electrode had
a small internal resistance with a lower overpotential needed for
the cycling of the carved Si electrode. The SWA sample showed
the largest peak magnitudes, due to the effective integration of
active Si in the alloying/dealloying process associated with the
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Figure 5. (a) Capacity vs cycle number of the prepared Si electrodes
and (b) Coulombic efficiency vs cycle number for a half-cell cycled
between 1.5 and 0.01 V. (c) The relative capacity at the different C rates
(0.02C to 0.2C), shown in percentage form.
phase transition. Thereby, the carved Si is a better anode material
with low discharge/charge voltage hysteresis then that of STF.
Panels a and b of Figure 5 compare the cycle performance of Si
electrodes in terms of their discharge/charge capacity and
Coulombic efficiency, which is summarized in panels ac of
Figure 4. In Figure 5a, the carved Si electrodes appear to show
substantially higher capacities, with improved capacity retention
compared with that of bare STF. Small capacity fading of below
0.4% per cycle (after the fifth cycle) was observed in the carved Si
electrode, whereas the STF showed 1.3% per cycle. This means
that the carved Si electrodes had greater facile stress relaxation
than the bare STF, since the capacity retention is related to the
film stress during cycling.33 Considering that the Si nanopatterns
were etched to 50% of the total Si film thickness, the change and
improvement of the electrochemical properties for the carved Si
nanostructures should be averaged by the remaining film layer
part because the unetched Si layer also contributes to the electrochemical performance. A plot of Coulombic efficiency as a
function of cycle number is provided in Figure 5b. The Coulombic efficiency is the ratio of the number of charges entering
the electrode to the number of charges extracted from the
electrode. For SWA, the Coulombic efficiency at the first cycle
was ca. 77.5%, which was similar to those reported for onedimensional nanostructured Si,610,13 implying an improved
reversibility in electrochemical reaction compared with that of
the bulk Si. SWA showed the largest capacity and reversibility
LETTER
among the Si electrodes, indicating a more effective integration of
active Si compared with SPA or STF. Within five cycles, the
carved Si electrode reached the highest Coulombic efficiency of
more than 99%, indicating that the carved Si was highly reversible than the STF. In addition, the improved capacity retention of the carved Si was also correlated to the enhanced
Coulombic efficiency, which could contribute to the long-term
cycling stability.34 Figure 5c shows results obtained from experiments for the performance of the carved Si electrode at higher C
rates, varying from 0.02C (ca. 84 mA g1) to 0.2C (ca. 840 mA g1)
rates between 1.5 and 0.01 V in the coin-type half-cells. (See also
Figure S3 in the Supporting Information for the discharge
capacity profiles with cycle numbers.) Comparing the discharge
capacity at 0.2C rate, the carved Si electrodes preserved 80% of
the capacity at 0.02C rate. In contrast, the capacity of the STF
sample rapidly decreased to 53% retention by the increase in the
input current density to 0.2C rate. We believe that the remarkable
rate capability of the carved Si electrode resulted from the wellconstructed architecture. A well-constructed architecture can
ensure enlarged active sites for electrochemical reactions with
open space among neighboring nanostructures for Li ion diffusion
of the electrolyte.35 This feature is particularly helpful for highpower applications when the battery is discharged or charged at
high current.
The Li diffusion coefficient in the carved Si electrode was
measured by cyclic voltammetry,36,37 with various scan rates
from 0.05 to 0.5 mV s1. As shown in panels ac of Figure 6
(which show current vs potential profiles with respect to various
scan rates, shown from low scan rates to high scan rates), the ratedetermining steps of the electrochemical reaction may have
changed from surface reactions to semi-infinite solid-state diffusion of Li ions. For semi-infinite diffusion, the peak current is
proportional to the square root of the scan rate (υ1/2). We found
that the peak current at different scan rates was in proportion to
square root of the scan rate (Figure 6d), indicating that in this
work the reaction kinetics were controlled by the semi-infinite
diffusion of Li. Peak currents for both the cathodic and anodic
peaks increased with increasing potential scan rate. The Li
diffusion coefficient can be derived by the following peak current
equation38
Ip ¼ ð2:69 105 Þn3=2 AD1=2 C0 υ1=2
where n is the number of electrons transferred (1 for Li+), Ip is the
peak current (A), A is the apparent surface area of the electrode
(cm2), D is the diffusion coefficient of the Li ions (cm2 s1), C0 is
bulk concentration of Li ions, and υ is the scan rate (V s1). The
surface areas of SPA and SWA were calculated to be ca. 3.48 cm2,
while that of STF was ca. 1.77 cm2. For the relation of Ip and υ1/2
shown in Figure 6d, SPA and SWA had steeper slopes than STF.
SWA appeared to have a diffusion coefficient of ca. 5.9 109 cm2 s1, which was approximately four times higher than
that for the STF sample (ca. 1.6 109 cm2 s1). The diffusion
coefficient for SPA (ca. 1.3 109 cm2 s1) was similar to that of
STF. The electrode performance of SPA showed insignificant
improvement over that of STF, despite the enlarged active sites.
Therefore, the improved SPA performance may have been
related to the reduced diffusion length of Li ions and the decrease
in resistance.39 From the nanopattern carving, the effective
thickness of the silicon layer could be changed as compared with
that of uncarved STF. Considering the results of Figures 4 and 6,
the SPA appeared to show an analogous behavior to the film
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Figure 6. Cyclic voltammetry of (a) SPA, (b) SWA, and (c) STF at various scan rates from 0.05 to 0.5 mV s1. (d) Relationship between the peak
current density and the square root of the scan rate.
structure that having a decreased film thickness,23,39,40 while the
SWA seemed to reveal the different features from the others with
the nanostructure effects,613,41 in light of electrochemical characteristics such as irreversible capacity, internal resistance, overpotential, and lithium ion diffusion coefficient.
To seek a correlation between the tailored Si electrode
configuration and the electrochemical results, we compared the
morphology of the samples before and after the cycles. SEM
images for SPA and SWA samples after 30 cycles are shown in
panels a and b of Figure 7. The carved Si electrodes seemed to
remain relatively stable, with a few cracks and delamination
appearing in the surface region, whereas the STF showed serious
pulverization from the substrate. In the STF case, repetitive
aggregation, pulverization, and large volume expansions/contractions may have led to the formation of a large cracked surface.
The results showed that the capacity retention of the STF was
unstable and subject to prolonged degradation. Accordingly, part
of the active Si became detached from the current collector,
resulting in a rapid fading of the capacity. In contrast, the carved
Si electrodes can play an effective role in facile stress relaxation,
thereby alleviating the cracking and crumbling of the Si electrode.
This capability may have been responsible for the better reversible capacity performance in the Li ion battery. The carved
Si electrodes showed remarkable electrochemical properties
compared with the STF. The carving and nanopatterning of
the Si electrodes reduced the internal resistance by increasing
the number of active sites for Li alloying/dealloying. Simultaneously, the extended surface area and generated interior nanocavities provided extra contact regions for the electrolyte,
resulting in a facilitated Li ion transport. Furthermore, the carved
Si electrodes exhibited improved capacity retention upon repeated cycling. The enlarged active sites in these Si electrodes
may have contributed to the facile stress relaxation, with smaller
volume changes during cycling. This approach with the precisely
patterned electrodes of SPA and SWA using the LIL method can
allow a comparative model study through the systematically
carved Si nanopatterns. It is also important to probe the electrochemical properties of nanostructured Si electrodes for the Li ion
battery with respect to the morphological evolution. The promise of the carved architectures for more advanced shapes in the
Si electrodes can provide more general design guidelines for the
advanced structures of the Li ion battery electrodes.
In summary, we have fabricated positively and negatively
carved Si nanopattern electrodes using laser interference lithography and have investigated them as anode materials for
rechargeable Li ion batteries. To ensure a systematic study, the
fabricated Si nanopattern arrays were controlled in terms of the
pattern size, etch depth, and periodicity. The electrochemical
results described in this work suggest that nanopatterning may
play a significant role in next-generation electrode materials. The
improved electrochemical features of the carved Si electrodes
could be attributed to the low internal resistance, facilitated
charge transport, and facile stress relaxation resulting from the
engraving of the Si into periodic nanopatterns. Consequently, the
carved Si electrodes prepared using LIL exhibited a superior Li
storage capacity, a high rate capability, and long cycling properties, indicating their potential as anode materials in high energydensity Li ion batteries.
Experimental Section. Preparation of Si Thin Film (STF). A Si
thin film with ca. 300 nm thickness was deposited on a substrate
with size of 1.77 cm2 using a radio frequency magnetron
sputtering system at a base pressure of less than 5 106 Torr
and a working pressure of 10 mTorr. An inert Ar (99.999%) gas
at 40 sccm was carried to the chamber at room temperature. The
thickness and mass of as-deposited Si thin film was carefully
measured using a surface profiler (KLA Tencor, Alpha-step IQ)
and microbalance (Sartorius, M3P). The measured mass and
thickness was ca. 0.175 mg and ca. 300 nm, respectively.
Fabrication of Si Carved Structures (SPA and SWA). The Si
thin film for pattern fabrication was first treated by coating a
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line transferred samples were rotated 90° and another exposure
was performed to generate a 4-fold symmetry array pattern. The
development process was performed using a developer (MIF500,
Clariant), for a time period of 35 s at room temperature. This
process removed the undeveloped regions, and the fabricated
resists were baked at 110 °C for 1 min on a hot plate. A dryetching process with a CHF3 gas mixture (100 W power at
10 mTorr) was performed for 65 s to transfer the positively or
negatively developed photoresist nanopatterns on the STF.
Finally, the masses of the SPA and SWA were characterized,
giving 0.09 and 0.15 mg, respectively, of active material.
Microstructural and Electrochemical Characterization. The
morphologies of the prepared electrodes were examined by SEM
(Hitachi S-4800). After cycling, delithiated electrodes were
removed from the configured cell in an Ar filled glovebox, since
the reacted electrodes were sensitive to air and moisture. The
electrodes were washed with acetonitrile to remove the remaining electrolyte and dried at room temperature before the SEM
characterization. The microstructures were characterized by
XRD (Rigaku Ru-200B), TEM (JEOL S7600), and XPS (VG
Multilab2000). The composition of the deposited Si was characterized by XPS with a monochromatic Al Kα X-ray source
(E = 1486.6 eV). Data processing was performed using Avantage
4.54 software. The background was corrected using the Shirley
method, and the binding energy of the C 1s peak from the
support at 284.5 eV was taken as an internal standard.
The electrochemical tests were performed, using a twoelectrode system fabricated with the prepared materials for the
working electrode and metallic lithium for the counter electrode
in an Ar-circulating glovebox. The mirror-like polished stainless
steel disk was used as the substrate for all the electrochemical
tests and characterizations in this work except the samples for the
cross-section SEM images that employed crystalline Si substrates. A 1 M LiPF6 solution in a 1:1 volume mixture of ethylene
carbonate and diethyl carbonate was used as the electrolyte. The
galvanostatic discharge/charge process at various C rates from
0.02C to 0.2C was conducted with a potential window of 1.5
0.01 V (vs Li/Li+) using a battery cycler (WonA tech, WBCS3000).
Cyclic voltammetry was performed over a potential range of
1.500.01 V at various scan rates of 0.05, 0.1, 0.2, and 0.5 mV s1,
using a Solartron 1470E multistat system.
’ ASSOCIATED CONTENT
Figure 7. The morphological changes of Si electrodes after the 30th
cycle in the galvanostatic process at 0.04C rate. SEM images of (a) SPA,
(b) SWA, and (c) STF after the finished discharge/charge process.
20 nm thick adhesion promoter layer of hexamethyldisilazane
(HMDS, Fluka), followed by annealing at 90 °C for 2 min (these
steps are not included in Figure 1). An HeCd laser (λUV =
325 nm, intensity = 0.75 mW cm2) was used for the LIL
process. The positive (AZ6612, Clariant) or negative (AZ6600
series, Clariant) tone photoresists were mixed with a thinner
(AZ1512, Clariant), using a 1:2 volume ratio. The photoresists
were spin-coated on the HMDS layer-coated Si thin film. In this
method, two coherent HeCd laser beams were used to produce
a periodic interference pattern on a photoresist-coated Si thin
film, as shown in Figure 1. In the first exposure, some parts of the
photoresist coated Si thin film were exposed, and nanoscale
parallel lines were formed. After the first exposure, the parallel
bS
Supporting Information. Figures showing schematic
diagram of laser interference setup and developed polymer
template, TEM image and SAED pattern for the Si thin film,
and discharge capacity vs cycle number for the prepared Si
electrodes and table of sheet resistance and the initial voltage
drop of the prepared Si electrodes. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (W. B. Kim); [email protected]
(G. Y. Jung).
’ ACKNOWLEDGMENT
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government
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(MEST) (No. 20110016600, Midcareer Researcher Program)
and by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (No. R15-2008-006-03002-0).
We also appreciate the financial support by the GIST Specialized
Research International Cooperation (GSR_IC/GIST) Project,
the Program for Integrated Molecular Systems (PIMS/GIST),
and the Core Technology Development Program from the
Research Institute of Solar and Sustainable Energies (RISE/GIST).
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