C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1
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Rheological properties of graphene oxide liquidcrystal
http://dx.doi.org/10.1016/j.carbon.2014.08.0850008-6223/� 2014 Elsevier Ltd. All rights reserved.
* Corresponding author.E-mail address: [email protected] (S.O. Kim).
Pradip Kumar, Uday Narayan Maiti, Kyung Eun Lee, Sang Ouk Kim *
Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Department of Materials Science & Engineering KAIST,
Daejeon 305-701, Republic of Korea
A R T I C L E I N F O
Article history:
Received 3 June 2014
Accepted 22 August 2014
Available online 29 August 2014
A B S T R A C T
We report the rheological properties of liquid crystalline graphene oxide (GO) aqueous dis-
persion. GO dispersions exhibit typical shear thinning behaviors of liquid crystals, which is
described by power law or simple Curreau model. Irrespective of the shear rate, shear vis-
cosity exhibits sudden decrease with the increase of GO composition around a critical vol-
ume fraction, /c = 0.33%, demonstrating typical colloidal isotropic–nematic phase
transition. Dynamic measurements reveal the liquid-like (isotropic phase, G 0 > G00) behavior
at a low GO composition (/ � 0.08%) and solid-like (liquid crystalline) behavior at higher
compositions (/ � 0.45%), where G 0 exceeds over G00. Nematic gel-like phase is confirmed
at a higher GO composition over / > 0.83%, where both G 0 and G00 moduli are nearly inde-
pendent of frequency (x). Simple power law scaling arguments are introduced to model
the dependence of yield stress and viscoelastic moduli on the GO composition. We also
observed the yield stress and rigidity percolation transition above phase transition compo-
sition /c > 0.33% with a percolation exponent of 1.3 ± 0.1. These rheological insights provide
valuable information for the liquid crystalline processing of GO based materials including
fibers, sheets and other complex structures for electronic/optoelectronic and energy stor-
age/conversion applications.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Mesoscopic ordering of anisotropic molecules or particles in
fluid states, known as liquid crystalline (LC) phase, has
attracted a great deal of research attention for the signifi-
cance in display devices and highly ordered materials pro-
cessing [1–4]. Among various liquid crystals, carbonaceous
mesophases are of particular research interest, as they offer
valuable routes to functional carbon based materials [5–10].
Recently, graphene oxide (GO), an oxygenated form of graph-
ene with spontaneous solution dispersibility, has also been
reported to form discotic colloidal liquid crystalline phase in
aqueous dispersion as well as in several organic solvents
[11–19]. Liquid crystalline GO dispersions have already been
utilized in a number of areas including carbon based fiber
spinning, alignment of one-dimensional nanomaterials, and
multifunctional composites, liquid crystal displays, etc [13–
19]. Manipulation of atomic thick platelet ordering by external
stimuli or dynamic perturbations may provide unprecedented
routes to highly ordered carbon based materials assembled
from genuine two-dimensional building blocks.
Processing of liquid crystalline materials frequently
encounters technological issues involved with complex mul-
tiscale structures and dynamics. For many applications,
454 C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1
liquid crystalline phase formation and corresponding visco-
elastic properties play significant roles to determine the
material structures and properties [20–22]. In the particular
case of GO liquid crystals, Gao and Xu reported the shear vis-
cosity reduction at isotropic to nematic phase transition as an
evidence for LC phase formation [23]. Yong and co-workers
reported typical shear thinning and relevant shear alignment
behavior of GO liquid crystals. [24]. Nonetheless, detailed rhe-
ological properties of GO liquid crystals have not been inves-
tigated thus far, despite their crucial significance for
optimized processing of high-performance graphene-based
materials [23–25]. The aim of this work is to investigate on
the various aspects of the viscoelastic properties of GO liquid
crystals. Thorough investigation over a broad GO composi-
tional range up to A = 1.8% reveals the typical rheological
behaviors involved with the isotropic-to-nematic phase tran-
sition and nematic gel formation.
2. Experimental
GO dispersion was prepared by the oxidative exfoliation of
graphite powder (Bay Carbon, SP1 Graphite powder; lot #
060706), following modified Hummers method, as described
elsewhere [11]. Briefly, 1 g of graphite powder was oxygenated
in 40 mL H2SO4, while stirring for 15 min, which was followed
by slow adding of 3.5 g potassium permanganate under an ice
bath. The mixture was then continuously stirred for 2 h at
35 �C. After completion oxygenation, the excess amount of
deionized water and 35% H2O2 were added to the mixture.
The obtained yellow mixture was thoroughly filter washed
with 1 M HCl solution and deionized water and re-dispersed
in 1 L of deionized water. The monolayer exfoliation was
achieved by the sonication of the washed GO dispersion in
water bath ultrasonicator. Subsequent purification was per-
formed by dialysis and centrifugation to remove ionic impuri-
ties and unexfoliated graphite oxides. A predetermined
amount of the concentrated dispersion was diluted in deion-
ized water to prepare the GO dispersions with a desired
composition.
To characterize the GO sheet size and thickness, extremely
diluted GO dispersion was dried at Si substrate surface.
Atomic force microscopy (SPA400) image was recorded in
the non-tapping mode under ambient conditions. Orienta-
tional arrangement of GO sheets was characterized by FE-
SEM (Hitachi S-4800 SEM). Optical birefringence of GO liquid
crystal dispersion was observed with light microscopy
between crossed polarizers. Rheological characterization of
GO dispersions was performed on a stress controlled
Advanced Rheometer (Bohlin Instrument) in both steady
and dynamic mode. All measurements were performed with
cone-plate configuration (radius: 44 mm, cone angle: 4�) with
truncated gap of 150 lm. In the steady-state measurements,
shear viscosity and shear stress were monitored as function
of shear rate (from 0.01 to 100 s�1). The shear rate was
increased continuously with the 30 s integration time for each
shear rate. Dynamic frequency sweep tests were performed
from 0.1 to 100 rad/s. All samples are compressed slowly to
the set gap and allowed to equilibrate for 10 min prior to each
measurement. All measurements were performed at room
temperature (20 �C).
3. Results and discussion
3.1. Characterization of GO liquid crystal dispersion
Fig. 1a shows the SEM image of as-synthesized GO platelets
with the mean lateral size of �700 nm. AFM image of GO
sheets with a height profile is shown in Fig. 1b, which con-
firms the monolayer thickness of �1 nm. The structure and
properties of GO dispersions are strongly dependent on the
interactions between the platelets. The steric hindrance from
overlapping of GO platelets arises at high composition, results
in configurational entropy driven excluded-volume effects
[12]. Well-known Onsager’s model accounts for the formation
of nematic liquid crystals based on purely entropy-driven
mechanism [26]. The thermodynamic equilibrium liquid crys-
tal phase originates from the competition between orienta-
tional and positional entropies. In dilute dispersion, the
orientational entropy is maximized due to random orienta-
tion of GO platelets. By contrast, above critical GO concentra-
tion, the GO platelets orientate parallel to each other in order
to minimize the excluded volume of the approaching plate-
lets. Thus, the parallel arrangement of anisotropic GO plate-
lets maximizes the packing or positional entropy that
governs the formation of nematic liquid crystalline GO disper-
sions. The liquid crystalline phase can be easily identified
from polarized optical microscopy. GO dispersions does not
show any optical birefringence at low volume fraction, /
� 0.08%. Biphasic dispersion is observed between 0.15% and
0.45%. Entire nematic phase is confirmed above a volume
fraction of 0.45%. Typical optical birefringent texture of single
nematic LC phase for / � 0.45% is shown in Fig. 1c. This bire-
fringence texture indicates the characteristic of anisotropic
phases originates from the alignment of anisotropic platelets.
The liquid crystalline GO dispersions are further confirmed
from freeze-dried sample. Aqueous dispersion of / = 0.45%
quenched in liquid N2 and subsequently freeze-dried. The
freeze-dried sample was subjected to SEM observation.
Fig. 1d clearly demonstrates the ordered GO platelets in the
nematic phase.
3.2. Rheological study
3.2.1. Steady shear flowMolecular ordering is particularly significant in the systems of
anisotropic colloidal particles, polymers or worm-like
micelles, where external shear can readily induce alignment
and orientational order. To understand the orientational
behavior of GO dispersions under shear, steady state shear
flow curves were measured. Fig. 2a shows the evolution of
shear viscosity against shear rate for various GO composi-
tions. GO dispersion exhibits Newtonian and non-Newtonian
(shear thinning) behavior strongly depend on the /. At low /
= 0.08%, shear viscosity shows Newtonian behavior at low
and high shear rate with a shear thinning region at interme-
diate range. At a high GO composition, GO dispersion exhibits
typical shear thinning behaviors. Moreover, the viscosity is
found to show non-monotonic behavior with composition,
which will be discussed later. Observed shear thinning is
associated to the orientation of GO platelets in nematic dis-
persion. The flow behavior can be approximated by power
(a) (b)
(c) (d)
Fig. 1 – (a) SEM image of synthesized GO flakes deposited on Si substrate. (b) AFM image of GO sheets with height profile
showing the typical monolayer thickness of �1 nm. (c) Polarized light microscopy of GO aqueous dispersion showing
optically birefringent texture. (d) Freeze dried SEM image of GO liquid crystalline phase. (A colour version of this figure can be
viewed online.)
C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1 455
law and Curreau shear thinning models. In power law shear
thinning behavior (g � cx�1) with g the viscosity, k a constant,
the shear rate and x the power law exponent [27]. The expo-
nent x = 1 is retrieved for Newtonian flow and x < 1 is for
shear thinning fluids. In Fig. 2a, we have fitted the power
law in the low shear rate regime (0.01–1 s�1). The exponents,
x decrease from 0.9 to 0.29 with volume fraction (see Table 1).
The exponent, x = 0.9 matches well with the one for isotropic
dispersion at / = 0.08% with Newtonian behavior, while
x = 0.29 for / = 1.8% corresponds the typical values of weak
gels or concentrated dispersions. The shear thinning over
the entire shear rate range can be also described by the sim-
ple Curreau equation [28], gðcÞ � gðc! 0Þ=½1þ ðsrcÞ2�m; where
m, and sr define the shear thinning exponent and orienta-
tional relaxation time in the system, respectively. We find that
shear thinning exponent of m = 0.38 for / = 0.08%, is close to
liquid-like (isotropic) behavior. By contrast, exponent,
m = 0.61 for / = 1.8 close to highly entangled polymer melts
and concentrated solutions [29]. Moreover, relaxation time,
sr increase from 4.5 to 45 s when / increase from 0.08% to
1.8%. This increase in relaxation time of GO platelets should
manifest the maximization of packing and enhanced orienta-
tion of GO platelets along the shear direction.
Viscosity behavior of liquid crystalline dispersions
depends not only on the GO composition but also on molecu-
lar arrangements in the dispersion. Fig. 2b shows the evolu-
tion of shear viscosity with volume fraction, / for shear
rates of 0.01, 8.5, and 92 s�1. Interestingly, viscosity exhibits
non-monotonic behavior with increasing /. At a low fraction
of A = 0.08, GO dispersion is in isotropic phase. Afterwards,
viscosity increased up to a critical fraction (/c � 0.33) and
get maxima. With the further increase in /c, viscosity goes
down and exhibits minima before increasing again. This vis-
cosity reduction above a critical volume fraction, /c � 0.33, is
caused by the formation of low-viscosity nematic liquid crys-
talline phase. The viscosity minima approximately corre-
spond to the GO composition for entire nematic phase
formation. The transition from isotropic to nematic phase
can be observed for the GO composition range between the
maxima and minima in shear viscosity [30]. In this work,
/c = 0.33% is considered as the critical GO composition for iso-
tropic–nematic phase transition, which is close to theoretical
predicated value (0.25%) for polydispersed infinitely thin
platelets [31]. We note that this GO liquid crystal phase tran-
sition behavior agrees well with our previous optical charac-
terization with polarized optical microscopy [11].
The shear effect on GO liquid crystal dispersion is further
described by shear stress-shear rate flow curve (Fig. 3). The
flow curves were measured for a wide range of GO composi-
tion from 0.08% to 1.8%. The representative shear stress-shear
rate plots for / = 0.08% and 0.83% are presented. At / = 0.08%,
shear stress varies almost linearly up to the shear rate of 1 s�1
with a constant viscosity (see Fig. 2a), which confirm the typ-
ical Newtonian behavior in isotropic phase. As shear rate
increases, horizontal stress plateau are observed for both
samples, where the constant shear stress values are
Fig. 2 – (a) Evolution of shear viscosity with shear rate at different GO composition. (b) Non-monotonic constitutive relation
between shear viscosity vs GO composition demonstrating isotropic to nematic phase transition.
Table 1 – Power law (g � cx�1), and Carreau equationðgðcÞ � gðc! 0Þ=½1þ ðsrcÞ2�
mÞ fitted parameters.
/ (vol%) x sr/s m
0.08 0.9 4.4 0.380.16 0.77 15 0.300.33 0.53 24 0.520.45 0.68 22 0.330.83 0.41 33 0.531.20 0.28 45 0.561.80 0.29 42 0.61
456 C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1
measured from 1 to 18 s�1 for / = 0.08% and from 0.1 to 5.9 s�1
for / = 0.83%, respectively. For / = 0.83% we performed mea-
surements decreasing the shear rate, as shown in inset of
Fig. 3. As arrows indicate sheer rate change direction, hyster-
esis is clearly observed. The observed stress plateaux and
hysteresis loops in steady state flow curves are often associ-
ated to the typical bistability of various soft materials, includ-
ing aqueous surfactant solutions, emulsions, and liquid
crystals to concentrated colloidal suspensions [32–36]. In
liquid crystalline materials, such bistability can be linked to
the phenomenon of shear banding; separation of the flow in
high and low shear regions. Under a shear flow, many com-
plex fluids have been reported to form macroscopic bands,
perpendicular to the flow direction at different shear rate lev-
els. In liquid crystalline systems, an ordered phase co-exits
with a disordered one in the shear-banding regime. Our obser-
vation of such a bistability can be associated to the different
orientational states of GO platelets in the direction of shear
flow as well as banded structure formation. In addition to
shear banding GO liquid crystal also exhibits yield stress (ry)
behavior. This steady shear yield stress can be estimated from
the observed stress plateau (Fig. 3), where shear stress starts
again increase as shear rate further increases. The observed
yield stress is plotted as a function of GO composition along
with dynamic yield stress in the inset of Fig. 4b.
Fig. 3 – Shear stress plotted against shear rate. The inset shows hysteresis observed when the shear stress was measured
with increasing and decreasing the shear rate for / = 0.83%.
C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1 457
3.2.2. Dynamic characteristicsTo establish the limits of the linear viscoelastic regime (LVR),
where dynamic moduli are independent on the applied stress,
oscillatory stress sweep measurements are performed. We
note that non-Newtonian systems such as polymer solution,
concentrated colloidal suspensions, and liquid crystalline dis-
persion do not show true LVR. For practical measurement, lin-
ear viscoelastic regime can be defined as the region in which
both G 0 and G00 are nearly constant with stress. Fig. 4a shows
the typical stress sweep measurements for liquid crystalline
GO dispersion of / = 1.2%, where both G 0 (solid) and G00 (open)
moduli are nearly constant up to a critical stress of 1.27 Pa.
Above the critical value, both moduli become stress depen-
dent, G 0 begin to decrease due to rupture of the GO network
and eventually cross G00. The crossover of G 0 and G00 can be
employed to estimate the yield stress (ry) of GO dispersions
[37]. Fig. 4b shows the strong dependency of yield stress on
GO composition. Along with the increase of /, ry first moves
to high values (/c = 0.33) and then decreases at single nematic
phase fraction (/N = 0.45) and finally increases again to higher
values. This intermittent decrease in yield stress at single
nematic phase composition is caused by the alignment or ori-
entation of GO platelets that reduces the entanglements
among platelets. The yield stress values are plotted against
GO composition up to / P 0.45 in the inset of Fig. 4b. Simple
scaling argument; ry � am is used to quantitatively link the /
dependence of the yield stress with m = 2.86 and 3.14 for
dynamic and steady yield stresses, respectively. Similar scal-
ing exponents have been reported for discotic laponite gels
and concentrated yield stress fluids [38].
Frequency-dependent measurements for the storage G 0(x)
and loss G00(x) moduli, exhibit typical liquid- or solid-like
behavior for volume fraction ranged from / = 0.08% to 1.8%
in the linear viscoelastic regime at constant stress of 0.1 Pa.
Fig. 5a shows the typical frequency sweep measurements
for the GO dispersion with / = 0.08, 0.45, and 1.8%. For /
= 0.08%, both moduli, G 0 (solid symbol) and G00 (open symbol)
have very low amplitudes and fluctuate continuously with
frequency. By contrast, for / > 0.08%, G 0 and G00 increase
monotonically up to a critical frequency and exhibits a sharp
minima before fluidization. This observation of minima
(dynamic instability) will be discussed later. We further
observed that G 0 and G00 become independent of frequency
as / P 0.83%. Low-frequency G 0 and G00 moduli can be
described by the power-law relationship, G(x) � kxn, where
the exponent n is the magnitude of slope in the double loga-
rithmic plot and k is the intercept which defines the degree of
network connectivity [39,40]. The linear viscoelasticity model
proposed by Winter [41] predicts that the stress relaxation fol-
lows the power-law frequency dependence G 0 � xn with
0 < n < 1. Stoichiometric balanced and imbalanced cross-
linked networks showed n < 0.5 in the presence of excess
cross-linker, and n > 0.5 in the short of cross-linker, respec-
tively. This description strictly applies to chemically cross-
linked gels. It has been known that n > 0 for a physical (weak)
gel, whereas n = 0 for a chemical (strong) gel [42,43]. The expo-
nent n can therefore be used as a quantitative measure of the
resemblance of an elastic network system. In viscoelastic
measurements, the liquid-like behavior is governed by G 0 / x2
and G00 / x. The data in Fig. 5a are fitted with the aforemen-
tioned power law and the obtained exponent values are plot-
ted as a function of / (Fig. 5b). At low /, both G 0 and G00 are of
the same order of magnitude and exhibits frequency depen-
dence, G 0 � x1.65 and G00 � x0.79, confirms the isotropic phase.
At / P 0.33, G 0 exceeds over G00 across the low frequency
range and indicates that GO sheets relaxation time is longer
than experimentally accessible value, exhibiting nematic
phase (solid-like) behavior, whereas n = 0.28 for / = 0.45%.
For even higher volume fraction of / = 1.8%, G 0 and G00 are
independent of frequency with very low exponent of
n = 0.05, confirming the gel-like phase formation. It is note-
worthy that our finding is qualitatively similar to previous
Fig. 4 – (a) Dynamic oscillatory stress sweep measurements for / = 0.16% (sphere), and / = 0.83% (triangle) GO dispersions.
Solid symbol for storage (G 0) and open symbol for loss (G00) moduli. (b) Double logarithmic plot of estimated yield stress as a
function of GO composition. Inset shows the double logarithmic plot of dynamic yield stress (solid sphere) and steady shear
yield stress (open sphere) for fully nematic GO dispersions.
458 C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1
observations of aqueous CNT dispersions [44–46]. Taken
together, we may conclude that the elasticity arises from
GO platelet network and nematic ordering of GO platelets.
Our results are consistent with the recent observations of
gel-like phase formation of aqueous GO dispersion at high
composition [47,48]. The formation of GO liquid crystalline
gel or three-dimensional network is attributed to the large
and flexible two-dimensional structure of GO platelets and
the force balance between their static repulsion and bonding
interaction. Increasing the bonding force or decreasing the
repulsion force between GO sheets in dispersion will reinforce
the gel network. In GO liquid crystalline dispersions, two sorts
of inter-sheets forces work together; repulsive forces due to
the negatively charged functional groups (oxygenated func-
tional group) and attractive forces arising from p–p interaction
among the graphitic patches and vander Walls interaction. At
a high GO composition, the strength of attractive forces may
dominant due to the small inter-sheets separation. This
resulted in strong sheets interlinked GO gel-like phase
formation.
We have above discussed that GO dispersion exhibits
liquid crystalline and thixotropic fluid like features. This
thixotropic behavior may arise from the breakup of intercon-
nected network and the enhanced orientational ordering of
Fig. 5 – (a) Frequency sweep measurements for / = 0.08% (circle), 0.33% (triangle) and 1.2% (square). Solid and open symbol
represents for storage and loss modulus, respectively. (b) Variation of scaling exponents (n) as a function of GO composition.
(c) Storage modulus (G 0) vs. GO composition. Inset shows the double logarithmic plot of G00 vs. reduced volume fraction (/ � /*)
obtained at angular frequency (x) of 0.01 rad/s. (d) GO composition dependent frequency minima where instability occur in
storage (G 0, open sphere), loss (G00, solid sphere) moduli.
C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1 459
GO platelets. In frequency sweep measurements elastic stor-
age modulus at low frequencies defined as plateau modulus
G00 is plotted as a function of / as shown in Fig. 5c. The notice-
able increase of G00 more than five orders of magnitude
between / = 0.16% and 0.33% is due to rigidity percolation.
Near the percolation threshold, G00 can be correlated to the
volume fraction / through a power law relationship as
G00 / u� u0ð Þp; where /* is the percolation volume fraction,
and p is the percolation exponent. We obtain percolation vol-
ume fraction of /* = 0.30 and exponent of p = 1.32 ± 0.08 by
varying /* to minimize the error in the slope of a double log-
arithmic plot of G00 vs (/ � /*) (see inset of Fig. 5c). This power
law increase in the elastic modulus is possible due to the
nematic ordering of GO platelets. The percolation exponent
of 1.32 obtained in current work is comparable to single-
walled, multi-walled nanotube dispersions and their polymer
nanocomposites [46,49–51]. A comparison of our measured
value of p = 1.32 ± 0.08, with previously speculated three-
dimensional elastic percolated systems, shows that the
inter-particle interaction for GO platelets is weak. This obser-
vation of rigidity percolation transition in GO liquid crystal-
line dispersions should be useful for the analysis of GO
fillers in a polymer matrix to estimates the percolation
threshold.
Lastly, we found the interesting dynamic instabilities in
GO liquid crystalline dispersion at a critical frequency, which
strongly depends on the GO composition. The dynamic insta-
bilities in a fluid mechanics are responsible for a variety of
important common phenomena, such as waves on the sea
surface or Taylor vortices in Couette flow [52]. Observation
of distinct minima in the frequency sweet test (G 0, G00) dis-
played in Fig. 5a signifies existence of dynamic instability.
As obvious from the Fig. 5a, the critical instability frequency
shifts to the higher values with the increase of GO composi-
tion (Fig 5d). The mechanism for such kind of instability has
not been clearly understood, however, there are few reports
on the oscillatory flow induced instabilities for other colloidal
systems. Torralba et al. [53] have reported that oscillatory flow
of the Maxwell fluid is unstable at high driving frequencies. In
other report, the binary mixture under horizontal vibrations
have revealed the existence of a new dynamical instability
due to size segregation under oscillatory shear and its con-
nections to pattern formation in granular flows [52]. Our find-
ing of dynamic instability should be significantly considered
in the processing of GO based materials.
4. Conclusions
We have reported that aqueous GO dispersion exhibit the
well-defined LC phases and gel-like behavior based on
rheological measurements. The GO dispersion exhibit shear
460 C A R B O N 8 0 ( 2 0 1 4 ) 4 5 3 – 4 6 1
thinning behavior, which is well described by power law and
Carreau models. Concentration dependent shear viscosity
confirms the typical colloidal isotropic–nematic phase transi-
tion at /c = 0.33%. Viscoelastic measurements revealed that
liquid crystals and gel-like phase exhibit well-defined elastic
shear modulus and yield stress, which increase with GO com-
position following a power law. Moreover, GO liquid crystal-
line dispersion shows the rigidity percolation transition
with an onset of solid-like elasticity at /* = 0.3% and the per-
colation exponent of 1.32 ± 0.08. Our observations of strongly
nonlinear rheology under flow (shear thinning, banding and
dynamic instabilities) demonstrate that precise rheological
characterization of GO liquid crystalline dispersion is
required for their processing into desired structures and
properties.
Acknowledgment
This work is financially supported by Institute for Basic Sci-
ence (IBS) [IBS-R004-G1-2014-a00], and Asian Office of Aero-
space Research and Development (AOARD) [AOARD-14-1403].
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