Graphene-Based Liquid Crystal Device
Peter Blake1, Paul D. Brimicombe2, Rahul R. Nair2, Tim J. Booth3, Da Jiang4, Fred Schedin4,
Leonid A. Ponomarenko2, Sergey V. Morozov5, Helen F. Gleeson2, Ernie W. Hill1, Andre K.
Geim4, Kostya S. Novoselov2*
1 School of Computer Science, University of Manchester, Manchester, M13 9PL, UK
2 School of Physics & Astronomy, University of Manchester, Manchester, M13 9PL, UK
3 Graphene Industries, Manchester, UK
4 Centre for Mesoscience and Nanotechnology, University of Manchester, Manchester M13 9PL,
5 Institute for Microelectronics Technology, 142432 Chernogolovka, Russia
Tel.: +44-(0)161-275-41-19 Fax.: +44-(0)161-275-40-56 E-mail: firstname.lastname@example.org
Graphene is only one atom thick, optically transparent, chemically inert and an excellent
conductor. These properties seem to make this material an excellent candidate for applications in
various photonic devices that require conducting but transparent thin films. In this letter we
demonstrate liquid crystal devices with electrodes made of graphene which show excellent
performance with a high contrast ratio. We also discuss the advantages of graphene compared to
conventionally-used metal oxides in terms of low resistivity, high transparency and chemical
Graphene is the first example of truly two-dimensional materials1. Only one atom thick, it
demonstrates high crystallographic quality2 and ballistic electron transport on the micrometer
scale. Such unique properties make it a realistic candidate for a number of electronic
applications. In particular, graphene is an attractive material for optoelectronic devices, in which
its high optical transmittance, low resistivity, high chemical stability and mechanical strength
seems to make it an ideal optically-transparent conductor.
Transparent conductors are an essential part of many optical devices. Traditionally, thin
metallic or metal-oxide films are used for these purposes (for a review see3). At the same time
there is a constant search for new types of conductive thin films, as existing technologies are
often complicated (e.g. thin metallic films require anti-reflection coating) and expensive (often
using noble or rare metals). Furthermore, many of the widely used metal oxides exhibit
nonuniform absorption across the visible spectrum4 and are chemically unstable (for instance the
commonly used Indium Tin Oxide (ITO, In2O3:Sn) is known to inject oxygen5 and indium6 ions
into the active media of a device). Recently carbon nanotube films have been produced7 and
used as an alternative transparent conductor in various photonic devices including electric field-
activated optical modulators, organic solar cells8 and liquid crystal displays9. The experimental
discovery of graphene10 brought a new alternative to the ubiquitous ITO. The optical properties
of this material are now being widely tested11,,,, 12 13 14 15, and graphene films have recently been
used as transparent electrodes for solar cells16.
In this letter we report on the use of graphene as a transparent conductive coating for photonic
devices and show that its high transparency and low resistivity make this two-dimensional
crystal ideally suitable for electrodes in liquid crystal devices. We will also argue that graphene,
being mechanically strong, chemically stable and inert, should improve the durability and
simplify the technology of potential optoelectronic devices.
Figure 1. (a) Schematic diagram of our liquid crystal devices with typical layer thicknesses in
brackets: 1 – glass (1mm); 2 – graphene; 3 – Cr/Au contact surrounding graphene flake (5nm Cr
+ 50nm Au); 4 – alignment layer (polyvinyl alcohol) (40 nm); 5 – liquid crystal (20μm); 6 –
alignment layer (40 nm); 7 – ITO (150 nm); 8 – glass (1 mm). The graphene flake is surrounded
by a non-transparent Cr/Au contact. (b-e) Optical micrographs of one of our liquid crystal device
using green light (505 nm, FWHM 23 nm) with different voltages applied across the cell: (b)
V=8 Vrms; (c) V=13 Vrms; (d) V= 22Vrms; (e) V=100 Vrms. Overall image width is 30μm. The
central hexagonal window is covered by graphene, surrounded by the opaque Cr/Au electrode.
(f) An optical micrograph (in reflection, using white light) of a graphene flake on the surface of a
1 mm thickness glass slide. The contrast is of the order of 6%. Overall image width is 10μm. (g)
The same image but in transmission. The flake is practically invisible. (h) Control device with no
graphene in the opening of the Cr/Au contacts with V=100 Vrms applied across the cell. Since
the electrode on the ITO-coated surface is continuous, there is a significant stray field within the
window that distorts the liquid crystal structure, leading to the pattern shown.
Graphene flakes were prepared by micromechanical cleavage10,17 on a glass microscope slide.
They were first located using an optical microscope18 (Figure 1f,g) and then further identified as
monolayer graphene using Raman microscopy19. Thin (70 nm) chromium/gold contacts were
then deposited around the flakes, so the graphene crystal was effectively covering a window in
the metallization, Figure 1a,b (this geometry also eliminates stray electric fields from the edges
of the electrode). A planar-aligned liquid crystal devices were then fabricated using such
graphene-on-glass films as one of the transparent electrodes, Figure 1a. The other substrate was
of a glass slide coated with conventional ITO. Both substrates were coated with a polyvinyl
alcohol alignment layer which was subsequently baked and then unidirectionally rubbed (ITO-
coated substrate only) in order to promote uniform alignment of the liquid crystal director. The
device was then capillary-filled with nematic liquid crystal material E7 (Merck). Applying a
voltage across the liquid crystal layer distorts the crystal alignment, changing the effective
birefringence of the device and altering the transmitted light intensity20. A control sample, with
an opening in the metallization not covered by graphene, was also prepared (Figure 1h). Note
that although we will limit our consideration of graphene-based liquid crystal devices by those
with planar untwisted nematic liquid crystals, this technology could equally be applied to any of
the various nematic liquid crystal device types (e.g. twisted nematic21, supertwisted nematic22,
in-plane switching23 and vertically aligned nematic24 devices) and also to ferroelectric and other
liquid crystal devices that use smectic phases.
Figure 2. Light transmission through the liquid crystal device as a function of voltage applied
across the cell, normalized to the maximum transmission. Inset: the same at low voltages. Solid
blue curve: in green light, 505 nm, FWHM 23 nm; dashed red curve: in white light. The data
taken in white light practically coincide with those in green light for voltages above 10Vrms and
are omitted from the main panel for clarity. From the oscillatory behavior the thickness of the
liquid crystal layer is estimated to be ~20 μm, assuming that the birefringence of E7 is 0.225.
An AC (square-wave) voltage was applied across the cells in order to reorient the liquid crystal
director. The electro-optic properties were observed using an optical microscope with the device
placed between crossed polarizers and the rubbing direction oriented 45o with respect to the
polarizers. Above the expected threshold voltage of around 0.9 Vrms, a strong change in the
transmission is observed (Figure 1b-e, 2) both in white and monochromatic light. The fact that
the whole electrode area changes contrast uniformly suggests that the electric field is applied
uniformly through the area of graphene and that the graphene has no negative effect on the liquid
crystal alignment. The contrast ratio (between maximum transmission and the transmission when
100 Vrms is applied across the cell) is better than 100 under illumination using white light,
which is very good for this type of cell and demonstrates that graphene could indeed be used
effectively as a transparent electrode for liquid crystal displays. No significant changes in
transmission were observed for the control sample, with only edge effects appearing due to the
finite thickness of the cell, Figure 1h.
We will now assess the quality of our liquid crystal devices, concentrating on such important
issues as the transparency of graphene, its resistivity and chemical stability. Light absorption by
graphene is presented on Figure 3(right inset) as a function of the number of layers. Each layer of
graphene absorbs about 2%, which is significantly lower than that of conventionally used ITO
(15-18% ). Such high transmittance is explained by a low electronic density of states in
Figure 3. Sheet resistance of a graphene device as a function of gate voltage with (solid red
curve) and without (dashed blue curve) a layer of polyvinyl alcohol on top. Polyvinyl alcohol
provides n-type doping, shifting the curve to negative gate voltages. The sheet resistance at zero
gate voltage is ∼400 Ω. Left inset: capacitance of one of our liquid crystal devices as a function
of voltage applied. Right inset: light absorption of free hanging graphene of different
The sheet resistance of undoped graphene is of the order of 6kΩ (one conductivity quantum
per species of charge carriers). However, it can be reduced down to 50Ω by chemical doping10,25,
and even unintentional doping (due to molecules absorbed from the surrounding atmosphere, e.g.
water) can be of the order of 1012 cm-2). In liquid crystal devices an electrode is usually in direct
contact with an alignment layer (in our case polyvinyl alcohol). We have tested the doping of
graphene with polyvinyl alcohol, by preparing a standard graphene device on a 300nm SiO2/Si
wafer and measuring its gate response with and without a layer of polyvinyl alcohol on top of
graphene (Figure 3). The introduction of a layer of polyvinyl alcohol produces n-type doping of
about 3×1012 cm-2. For this particular sample it resulted in a drop in the sheet resistance down to
400Ω, which is an impressive result for a conductive coating with optical transmission of about
98%. It is difficult to compare this result to ITO, as the resistance of In2O3:Sn films diverges
strongly (in the order of tens of kΩ) when trying to obtain optical transmittance above 95%. ITO
films with 95% transmittance demonstrate comparable sheet resistances of a few hundred Ohms,
dropping to tens of Ohms at an optical transmittance of about 90%26. Similar or even lower
resistances can be achieved for graphene by a variety of means: increasing the number of
layers27, intentional doping, or by using samples with higher mobility28,29.
An important issue for most ITO-based liquid crystal devices and other photonic devices is the
chemical stability of the metal-oxide and the diffusion of ions into the active media. Such
processes deteriorate the active media (for example via oxidation if oxygen is injected) and can
lead to break-down at lower voltages. Furthermore, in liquid crystal displays the injected ions get
trapped at the alignment layer, thus screening the applied electric field. This leads to the so-
called image sticking problem30 which is usually avoided by driving the liquid crystal cells with
One can generally expect that such issues can be avoided when using graphene, where its
chemical stability should minimize the level of ion diffusion. To check this we have measured
the capacitance of one of our liquid crystal devices, which has one electrode made of graphene
and the other from ITO, when applying DC voltages of different polarities (Figure 3 left inset,
here positive voltage corresponds to higher potential on the ITO electrode). There is clearly a
highly hysteretic response when applying positive biases, but no hysteresis has been observed at
the opposite polarity. We attribute this observation to positive indium ions drifting into the liquid
crystal from the ITO electrode, whereas no ions are injected from the graphene electrode. Similar
liquid crystal devices constructed using ITO electrodes on both substrates produce the hysteretic
response for both polarities.
Although it is important to demonstrate the possibility and advantages of using graphene as a
transparent conductive coating, the feasibility of its mass production is essential when
considering realistic applications. No industrial technology can rely on the micromechanical
cleavage technique that allows only minute quantities of graphene and, although sufficient for
fundamental research and proof-of-concept devices, is unlikely to become commercially viable.
Recently, large-area conductive films have been demonstrated by using chemical exfoliation of
graphite oxide and then reducing it to graphene16,, 31 32. This could lead to a viable way of making
thin graphene-based films with properties similar to those discussed earlier and using them for
various photonic devices. However, so far this technique has not demonstrated the ability to fully
recover the excellent conductive properties of graphene to recover33. We propose an alternative
approach. It involves making a graphene suspension by direct chemical exfoliation of graphite
(rather than graphite oxide), which is subsequently used to obtain transparent conductive films
on top of glass by spin- or spray-coating.
Crystals of natural graphite (Branwell Graphite Ltd.) were exfoliated by sonication in
dimethylformamide (DMF) for over 3 hours. DMF “dissolves” graphite surprisingly well, and
the procedure resulted in a suspension of thin graphitic platelets with large proportion of
monolayer graphene flakes, DMF also wets the flakes preventing them from conglomerating34.
The suspension was then centrifuged at 13,000 rpm for 10 minutes to remove thick flakes. The
remaining suspension, consisting mostly of graphene and few-layer graphite flakes of sub-
micrometer size, was spray-deposited onto a preheated glass slide (Figure 4a,b) which yielded
thin (∼1.5nm) films over centimeter sized areas. These films were then annealed for 2 hours in
argon(90%)/hydrogen(10%) atmosphere at 250oC. The transparency of such graphitic layers was
approximately 90% (Figure 4b).
Figure 4. (a) Scanning electron micrograph of a thin graphitic film obtained by chemical
exfoliation and spray-coating. Inset shows the same area under higher magnification. (b) Light
transmission through an original glass slide (left) and the one covered with the graphitic film
(right). (c) Temperature dependence of the film’s sheet resistance (R∼exp(T0/T1/3) behavior is
observed at T>10K, where T0 is a constant). Inset: the same data but for the low temperature
interval (R∼exp(Δ/T) behavior is observed at T<10K, where Δ is a constant). The red lines are
guides for the eye.
In order to measure resistivity of our films, a mesa structure in the shape of the Hall bar with
typical dimensions of 1mm was prepared, and the four-probe resistance was measured as a
function of temperature (Figure 4c). The high temperature region (above 10 K) is well described
by exp(T0/T1/3) dependence, characteristic for variable range hopping in two dimensions35. The
room temperature sheet resistance is of the order of 5 kΩ, which is already acceptable for some
applications3,16, and can be decreased further by increasing the film thickness. Resistance at low
temperatures deviates from the variable-range-hopping dependence but can be described by the
simple activation dependence exp(-Δ/T) (see inset in Figure 4c). We attribute this low-
temperature behaviour to weak tunneling-like coupling between different flakes, possibly due to
contamination with organic (DMF) residues. This indicates some potential for improvements as
better cleaning and annealing procedures can potentially improve coupling between graphene
crystallites and decrease the film resistance further.
To conclude, high optical transparency, low resistivity and high chemical stability of graphene
makes it an excellent choice for transparent electrodes in various optoelectronic devices.
Furthermore, there are already several technologies that potentially allow mass production of
thin graphene-based transparent conductors (besides the chemical exfoliation of graphite
described in the present letter, one can also think of epitaxial growth of graphene on top of a
metal surface, followed by a transfer of such a layer onto a transparent substrate). These
techniques are capable of producing continuous graphene films of thickness below 5 monolayers,
which is required for realistic applications.
The authors are grateful to EPSRC for financial support. AKG and KSN also acknowledge
support from the Royal Society, UK.
(1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183.
(2) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S.
Nature 2007, 446, 60.
(3) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2007, 91, 1529.
(4) Phillips, J. M.; Kwo, J.; Thomas, G. A.; Carter, S. A.; Cava, R. J.; Hou, S. Y.; Krajewski,
J. J.; Marshall, J. H.; Peck, W. F.; Rapkine, D. H.; van Dover, R. B. Appl. Phys. Lett. 1994, 65,
(5) Scott, J. C.; Kaufman, J. H.; Brock, P. J.; DiPietro, R.; Salem, J.; Goitia, J. A. J. Appl. Phys.
1996, 79, 2745 .
(6) Schlatmann, A. R.; Wilms Floet, D.; Hilberer, A.; Garten, F.; Smulders, P. J. M.; Klapwijk,
T. M.; Hadziioannou, G. Appl. Phys. Lett. 1996, 69, 1764 .
(7) Wu, Z. C.; Chen, Z. H.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.;
Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273.
(8) van de Lagemaat, J.; Barnes, T. M.; Rumbles, G.; Shaheen, S. E.; Coutts, T. J.; Weeks, C.;
Levitsky, I.; Peltola, J.; Glatkowski, P. Appl. Phys. Lett. 2006, 88, 233503.
(9) Chan Yu King, R.; Roussel, F. Appl. Phys. A 2007, 86 159.
(10) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;
Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666.
(11) Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth,
T. J.; Geim, A. K. Appl. Phys. Lett. 2007, 91, 063124.
(12) Abergel, D. S. L.; Russell, A.; I. Fal'ko, V. Appl. Phys. Lett. 2007, 91, 063125.
(13) Casiraghi, C.; Hartschuh, A.; Lidorikis, E.; Qian, H.; Harutyunyan, H.; Gokus, T.;
Novoselov, K. S.; Ferrari, A. C. Nano Lett. 2007, 7, 2711.
(14) Kuzmenko, A. B.; van Heumen, E.; Carbone, F.; van der Marel D. arXiv:0712.0835.
(15) Nair, R. R. private communication.
(16) Wang, X.; Zhi, L.; Mullen, K. Nano Lett. 2008, 8, 323.
(17) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T.; Khotkevich, V. V.; Morozov S. V.;
Geim, A. K. PNAS 2005, 102, 10451.
(18) Graphene flakes on glass are feebly visible in reflection, giving rise to about 6% contrast
(confirmed by both experiment and theoretical modeling, similar to).
(19) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec,
S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97, 187401.
(20) Fréedericksz, V.; Zolina, V. Trans. Faraday Soc. 1933, 29, 919.
(21) Schadt, M.; Helfrich, W. Appl. Phys. Lett. 1971, 18, 127.
(22) Watters, C. M.; Brimmell, V.; Raynes, E. P. Proc. 3rd Int. Display Res. Conf., Kobe,
Japan, 1983, 396.
(23) Soref, R. A. J. Appl. Phys. 1974, 12, 5466.
(24) Schiekel, M. F.; Fahrenchon, K. Appl. Phys. Lett. 1971, 19, 391.
(25)Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.;
Novoselov, K. S. Nat. Mater. 2007, 6, 652.
(26) Sheet resistance of ITO films diverges strongly with decreasing its thickness. See for
example Wong, F. L.; Fung, M. K.; Tong, S. W.; Lee, C. S.; Lee, S. T. Thin Solid Films 2004,
(27) Morozov, S. V.; Novoselov, K. S.; Schedin, F.; Jiang, D.; Firsov, A. A.; Geim, A. K.
Phys. Rev. B 2005, 72, 201401(R).
(28) The sample shown in Figure 3 exhibits a mobility of about 0.5 m2/V·s. Graphene samples
with room-temperature mobility as high as 2 m2/V·s have obtained by micromechanical
(29) Morozov, S. V.; Novoselov, K. S.; Katsnelson, M. I.; Schedin, F.; Elias, D. C.; Jaszczak,
J. A.; Geim, A. K. Phys. Rev. Lett. 2008, 100, 016602.
(30) Bremer, M.; Naemura, S.; Tarumi, K. Jpn. J. Appl. Phys. 1998, 37, L88.
(31) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach,
E. A.; Piner, R. D.; Nguyen S. T.; Ruoff, R. S. Nature 2006, 442, 282.
(32) Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3,
(33) Gomez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.;
Kern, K. Nano Lett. 2007, 7, 3499.
(34) Other strong organic solvents can also be used (Coleman, J. N. private communication).
(35) Mott, N. F. Phil. Mag. 1969, 19, 835.