Detection of graphene chirality using achiral liquid crystalline platforms
Rajratan Basu,a) Daniel Kinnamon, and Alfred GarveySoft Matter and Nanomaterials Laboratory, Department of Physics, The United States Naval Academy,Annapolis, Maryland 21402, USA
(Received 21 July 2015; accepted 5 September 2015; published online 16 September 2015)
Monolayer graphene flakes were dispersed at low concentrations into two achiral liquid crystals
(LCs) alkoxyphenylbenzoate (9OO4) and 4-cyano-40-pentylbiphenyl (5CB), separately. The
presence of graphene resulted in two types of chiral signatures in the LCs: an electroclinic effect
(a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the
smectic-A phase of 9OO4, and a macroscopic helical twist of the LC director in the nematic phase
of 5CB. Graphene flakes generally possess strain chirality and edge chirality. The non-covalent
interactions between the LC molecules and chiral graphene flakes induce molecular conformational
deracemization in the LC, exhibiting a bulk electroclinic effect and a macroscopic helical twist.
[http://dx.doi.org/10.1063/1.4931147]
I. INTRODUCTION
Chirality is one of the most important and interesting
topics in science. In nanoscience, carbon nanotubes are a
prime example of chiral macromolecule. Apart from carbon
nanotubes, it has been recently demonstrated that graphene
also possesses chirality.1–6 Graphene is a crystalline allo-
trope of carbon with 2-dimensional properties. The carbon
atoms in a graphene sheet are densely packed in a regular
sp2-bonded atomic-scale hexagonal pattern. An infinitely
large graphene sheet is not expected to show any chirality
due to the symmetry of the hexagonal pattern. However,
when graphene is present in form of nano flakes, the symme-
try breaking of the graphene lattice due to a soft shear mode
in the graphene sheet shows circular dichroism,1,2 indicating
the presence of surface chirality. Apart from this strain chir-ality, graphene also exhibits edge chirality. Due to the pres-
ence of different chiralities, i.e., armchair or zigzag at the
edge,3–6 the flakes show different optical,7 superconductive,8
and magnetic properties.9 This edge chirality of graphene
can be detected from the intensity of the D band using
Raman spectroscopy.10,11 In soft matter, chirality is an
important aspect of the phases, as the component molecules
often can exist both in chiral and achiral form. In rod-like
liquid crystals (LC), the introduction of chirality in the mole-
cules breaks the mirror symmetry and shows unique features
in chiral LC phases, such as macroscopic director twist in
the chiral nematic, electroclinic effect in the chiral smectic-
A, and ferroelectricity in the chiral smectic-C. Thus, the pres-
ence of chirality in soft matter results in a wide range of
physical properties, from electro-optical to hierarchical and
ultimately macroscopic helical twist.
In a different direction, two-dimensional honeycomb
structure of graphene shows several interesting interactions
with LCs. The graphene nanostructure can be used to enhance
the tilted smectic-C order.12 Multilayer graphene flakes can
improve the electro-optic response in a nematic phase.13 The
vertical alignment of LC molecules can be achieved by
graphene-oxide without any surface treatment of the sub-
strate.14 Graphene with benzene can enhance the orientational
order in a nematic phase.15 Functionalized graphene can
mutually self-assemble discotic liquid crystals.16 Transparent
graphene-conducting-layers, instead of indium-tin-oxide, can
be used as electrodes to produce high-transmittance liquid
crystal displays.17,18 In an LC – nanocolloidal dispersion, the
effect of nonmesogenic guest-nanoparticles on the LC’s bulk
properties often rests on the molecular identification at the
nanoscale in order to share and disseminate the “information”
coded into the nanostructure of the nanoparticles.19–22
Because most of the graphene flakes can have strain chirality,1
we ask the question: Can this surface chirality be transmitted
into the LC? If so, the resulting LC-graphene mixture could
exhibit a spatially averaged bulk chirality. Here, we report ex-
perimental observations of chirality transfer from graphene
surface into two different achiral LCs, alkoxyphenylbenzoate
(9OO4) and 4-cyano-40-pentylbiphenyl (5CB). An electro-
clinic effect23 (a polar tilt of the LC director perpendicular to,
and linear in, an applied electric field) is observed in the
smectic-A phase of 9OO4 when the LC is doped with mono-
layer graphene flakes. On the other hand, a macroscopic
helical twist24 of the LC director is observed in the nematic
phase of 5CB in the presence of graphene flakes.
II. EXPERIMENTS, RESULTS, AND DISCUSSION
The LC 9OO4 (obtained from LC Vision) has the cooling
phase sequence as: Isotropic – 87� – nematic – 73.5� – smectic-
A – 62.5� – smectic-C – 50.2� – smectic-B – 35� – Crystal. The
LC 5CB has the cooling phase sequence as: Isotropic – 35� –
nematic – 22� – Crystal. The chemical structures of the two
achiral liquid crystals used in this experiment are shown in
Figure 1. The pristine graphene (GP) sample, obtained from
Graphene Supermarket, Inc., contained more than 97% of
monolayer flakes (and 3% of multilayer flakes) of an average
thickness of 0.35 nm and an average lateral size of 550 nm. The
GP sample was ultrapure with no oxidation, no surfactants, and
99.99% carbon content. The GP sample was available in etha-
nol solvent in 1 mg/l. The ethanol þ GP solution was first
remixed by a micro-homogenizer tip of 5 mm diameter at
a)Author to whom correspondence should be addressed. Electronic mail:
0021-8979/2015/118(11)/114302/5/$30.00 118, 114302-1
JOURNAL OF APPLIED PHYSICS 118, 114302 (2015)
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35 000 rpm for 3 h, followed by sonication for 10 h. The LC
then was added to the ethanol þ GP mixture and sonicated for
5 h, allowing the LC to dissolve completely into the solution.
The ethanol was evaporated slowly at an elevated temperature,
leaving a pure LC þ GP mixture. Finally, the LC þ GP mix-
ture was degassed under a vacuum for 1 h. The final concentra-
tion of 9OO4 þ GP mixture was 0.0035 wt. % and the same
for the 5CB þ GP mixture was 0.0025 wt. %. For consistency,
the pure LCs were also treated the same way, such as dissolv-
ing in ethanol followed by a slow evaporation and degassing.
An electroclinic effect is observed in a bulk chiral smec-
tic-A LC.23 In this effect, an electric field ~E applied parallel to
the smectic layer induces a polar tilt h [/ E] of the director in
a plane perpendicular to ~E. The tilt susceptibility, also known
as the electroclinic coefficient ec¼ dhdE, diverges on cooling to-
ward the chiral smectic-C* phase (the asterisk denotes the chi-
ral phase). The electroclinic effect involves the reduced C2
symmetry and is absent when the system is achiral or racemic.
Chiral-based effects at LC/substrate interfaces also have been
known for some time.25–28 In these systems, either the LC or
the interaction surface layer is chiral, and the interface’s
reduced symmetry gives rise to surface electroclinic behavior.
Commercially manufactured LC cells (LC2-5.0, planar
antiparallel rubbed, from Instec, Inc.) with a 1.5� pre-tilt
angle, 0.5� 0.5 cm2 semitransparent indium tin oxide (ITO)
coated area, and a d¼ 5 lm spacing were used for our elec-
troclinic experiment. Two cells were filled at temperature in
the isotropic phase by capillary action, one with 9OO4 only
and the other with the 9OO4þGP mixture; from then on
both cells always remained in the smectic-C phase (or higher
temperature). Before performing any electroclinic measure-
ments, the 9OO4þGP cell was examined using a polarizing
optical microscope, which revealed uniform nematic,
smectic-A, and smectic-C textures like that of the pure LC
cell, indicating a uniform director field.
The optical setup consisted of a beam from a 5-mW
He-Ne laser at wavelength 633 nm that passed through the
polarizer, the cell, a crossed analyzer, and into a photodetec-
tor (PDA100A, Thor Labs). The beam was polarized at an
angle of p/8 with respect to the cells’ rubbing direction; this
is the classical “electroclinic geometry.”29 The output of the
detector was fed into both a lock-in amplifier (Stanford
Research Systems SR830 DSP) that was referenced to the
driving frequency f of the applied electric field (Stanford
Research Systems SR345) and to a dc voltmeter (Keithley
2002), allowing us to measure the ac intensity Iac at fre-
quency f and the dc intensity Idc, respectively. The setup and
data acquisition were computer controlled using LabVIEWVR
software. From the measured intensities, the field-induced
spatially average tilt angle h was obtained using the formula
h ¼ Iac
4 Idc.29 This is a common technique to measure h vs. E (f)
for the electroclinic effect in chiral LCs.28,30–33 In mean field
theory, the field-induced tilt angle h ¼ kEðT�TAC� Þ,
23 where TAC*
is the smectic-A to smectic-C* transition temperature and kis a proportionality constant, which reflects the chiral cou-
pling to the electric field, and vanishes in the absence of
chirality.
Figure 2 shows the tilt angle h as a function of the rms
applied field E at f¼ 1 kHz for (a) pure 9OO4 and for (b) the
9OO4þGP mixture. As expected, 9OO4 in the absence of GP
does not exhibit any electroclinic effect in the smectic-A phase,
confirming the absence of molecular chirality associated with
FIG. 1. Chemical structures of 5CB and 9OO4. The schematic diagram
represents that two parallel graphene flakes in 5CB LC induce a helical twist
of the LC director due to p–p stacking. Note that the flakes show different
chiralities along the edges. The p–p electron stacking is illustrated by match-
ing 5CB LC’s benzene rings (red) on the graphene-honeycomb structure.
FIG. 2. Electroclinic effect in the smectic-A phase: (a) tilt angle h vs. E(f¼ 1 kHz) for bulk 9OO4 at two different values of T, listed in the legend;
(b) tilt angle h vs. E (f¼ 1 kHz) for 9OO4þGP at eight different values of Tlisted in the legend. Lines represent linear fits.
114302-2 Basu, Kinnamon, and Garvey J. Appl. Phys. 118, 114302 (2015)
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pure 9OO4. On the other hand, the GP-doped 9OO4 clearly
shows a bulk electroclinic effect that grows on cooling toward
TAC*, demonstrating a net chirality in the smectic-A phase. We
also examined the temperature behavior of the electroclinic
effect of the mixture above TAC* (¼ 62 �C), where ec ¼ dhdE
was found to increase rapidly on cooling, with an apparent
divergence at TAC*. In Figure 3, we plot e�1c vs. T, observing
an approximately linear variation with T close to TAC*. This is
consistent with mean-field behavior and with previous results
on fully chiral LCs.32,33 Importantly, ec diverges at TAC*, indi-
cating that the observed effect indeed corresponds to coupling
between ~E and the graphene-induced chiral LC domains,
rather than a direct electric field—graphene coupling. We
repeated the experiments at frequencies f¼ 500 Hz and
10 kHz, with no significant differences. It is also important to
point out that the magnitude of ec observed in this experiment
is a few orders of magnitude smaller that of the fully chiral
LCs,23,29 indicating weak chirality at graphene surface.
9OO4 and 5CB are configurationally achiral LCs. But a
chiral conformation in the ground state can be adopted by
rotating left or right22 about the Ph–O bond for 9OO4 and
Ph–Ph bond for 5CB. An equal probability distribution
between left and right twist makes them dynamically race-
mic. In an LCþGP system, the LC director aligns along the
graphene surface,34 employing the p–p electron stacking
between the honeycomb structure of graphene and benzene
rings of the LC to reduce the elastic distortion. When the gra-
phene surface possess strain chirality, the LC can adopt that
surface chirality by shifting its equilibrium between left- and
right-handed chiral conformers to produce a statistically
larger population of chiral conformers of one handedness,
breaking the symmetry between right- and left-handed mo-
lecular conformations: this is also called deracemization. We
observed the electroclinic effect in LC 9OO4 due to this
symmetry breaking in the LC through deracemization.
Parallel graphene sheets present in the LC medium can
induce a twist of the LC director due to the p–p electron
stacking as schematically shown in Figure 1. A twist of the
director can also induce conformational deracemization,28
i.e., chirality. This helical twist (due to chiral induction) can
be experimentally observed in a 90� twist cell.
Due to the chiral induction by graphene flakes, we quali-
tatively observed that the director in the nematic phase of
5CB undergoes a helical twist with a pitch length P, where Pis the distance over which the director rotates by an angle of
2p. We have used LC 5CB for this twist experiment because
of its availability with a room temperature nematic phase. In
this experiment, we used LC cells in which the two surfaces
were arranged to induce a 90� twist of the nematic director
from one substrate to the other. A uniform cell thickness was
maintained by randomly dispersed 6.8 lm spacer particles
during the fabrication of the cells. In the absence of an inher-
ent nematic twist (P!1), there are two possible domains in
the 90� twist cell: one with right-handed twist and the other
with left-handed twist. The spacer particles pin the LC discli-
nations (i.e., the walls between the right-handed and left-
handed domains) in the cell. Since both domains are equally
likely, the disclination lines that run between the cell’s spacer
particles are straight. However, for a chiral liquid crystal with
an intrinsic P, domains having the same sense of surface-
imposed twist grow at the expense of those having the oppo-
site sense of surface-imposed twist, and thus the disclination
lines that connect the spacer particles become bowed. The
equilibrium shape of the disclination line is determined by the
minimization of the total energy, which involves the energy of
the disclination and the twist elastic energy, where it is
assumed that the LC director is anchored strongly at the two
substrates. Thus, a measure of the disclination’s radius of cur-
vature yields the inherent chiral pitch P.24
Figures 4(a)–4(d) show typical polarized micrographs of
pure 5CB at four randomly chosen regions inside a 90� twist
cell. Each micrograph has an area of 1.173� 0.899 mm2. As
expected for a pure achiral LC, the disclination lines between
the equally likely left- and right-handed domains are linear
(P ! 1), minimizing the line tension associated with the
disclination. The dark spots observed in Figure 4 are 6.8 lm
spacer particles, which pin the disclinations. Under the same
conditions, a chiral LC would exhibit curved disclinations in
order to minimize the total energy, and the chiral pitch
length P¼ 2R, where R is the radius of curvature of the
bowed disclination line.24 This method is sensitive to weak
chirality, i.e., large P. Figures 4(e)–4(h) show polarized
micrographs of 5CBþGP at four randomly chosen regions
inside a 90� twist cell. In these four micrographs, clearly
visible curved disclination lines between spacer particles
confirm that the graphene flakes induce chiral twist in the ne-
matic phase. These micrographs also reveal a non-uniform
curvature-distribution throughout the cell. Because the radius
of curvature varies significantly from region to region in the
cell, this behavior suggests a non-uniform density distribu-
tion of graphene flakes in the nematic medium. This curva-
ture variation also suggests that the flakes possess different
chiral strengths due to their size distribution. For the
5CBþGP sample, the radius of curvature R was obtained
from the 1.173� 0.899 mm2 micrographs along several dif-
ferent segments by computer identification and digitization
of the disclination lines. Figure 5 shows the variation of the
chiral pitch length P (¼ 2R) at seven randomly chosenFIG. 3. Inverse of electroclinic coefficient e�1
c vs. T. A linear fit is shown
near TAC* (¼ 62 �C).
114302-3 Basu, Kinnamon, and Garvey J. Appl. Phys. 118, 114302 (2015)
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regions (i.e. seven 1.173� 0.899 mm2 micrographs) in the
5CBþGP cell. Regions 1 to 4 in Figure 5 correspond to
micrographs (e) to (h) in Figure 4, respectively. With a wide
variation, the average P is found to be 2.9 mm. This large Pindicates that the graphene-induced chirality is weak——
this is consistent with weak electroclinic signal for the 9OO4
þGP sample.
III. SUMMARY
Based on transmission electron microscopy, electron
energy loss spectroscopy, Raman spectroscopy, and electron
diffraction35 data from the manufacturer, there is no sugges-
tion of chiral impurities in the graphene sample (i.e., 99.99%
carbon content). The appearance of an electroclinic effect for
the graphene-doped 9OO4 LC and its absence for pure
9OO4 is, therefore, an unambiguous signature of induced
chirality in an achiral LC, arising from the graphene flakes.
On the other hand, the presence of bowed disclinations for
the GP-doped-5CB LC and linear disclinations for pure 5CB
is a distinct characteristic of induced chirality and propaga-
tion of twist through nematic correlation in an achiral ne-
matic LC. These two independent experiments coherently
suggest that the chirality of graphene can be detected using
FIG. 4. Micrographs (1.173
� 0.899 mm2) showing disclination
lines in two 90� twist cells under the
cross polarized microscope; (a)–(d)
pure 5CB showing straight disclination
lines at four different places in the
cell; (e)–(h) 5CBþGP showing bowed
disclination lines at four different pla-
ces in the cell. Black dots are 6.8 lm
spacer particles.
114302-4 Basu, Kinnamon, and Garvey J. Appl. Phys. 118, 114302 (2015)
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LC platforms. These results suggest that the graphene sample
contains a small quantity of an enantiomeric excess.
However, the origin of this enantiomeric excess of graphene
is still unknown. The chiral induction in the LC may occur in
two different ways in this case. The strain chirality on gra-
phene surface can break the symmetry between the right-
and left-handed molecular conformations in the LC. Another
way, the dispersed graphene flakes self-align in the LC with
their flat surface parallel to the LC director.36 Therefore,
when they are parallel to each other, they can also induce a
macroscopic twist on the LC director, inducing deracemiza-
tion in the LC. In addition to the existing chirality detection
techniques of graphene, such as circular dichroism1 and
Raman spectroscopy,10 this new technique with the LCs will
not only make graphene technologically useful but will also
lead directly to the central role played by LC-influenced
organizational themes in the development of nanoscale self-
assembled systems, such as the propagation of chirality from
the graphene surface to the LC.
ACKNOWLEDGMENTS
This work was supported by the Office of Naval Research
(Award No. N0001415WX01534) and the investment grant at
the U.S. Naval Academy.
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FIG. 5. The distribution of pitch length P for 5CBþGP at seven randomly
chosen regions in the 90� twist cell. The dotted line shows the average P.
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