In liquid crystal devices (LCDs) the indium tin oxide (ITO) films are traditionally used as transparent and conductive electrodes. However, today, due to the development of multichannel optical communication, the need for flexible LCDs and multilayer structures has grown. For this application ITO films cannot be used in principle. For this problem, graphene (an ultrathin material with unique properties, e.g., high optical transparency, chemical inertness, excellent conductivity) is an excellent candidate. In this work, the electro-optical and dynamic characteristics of a liquid crystal (LC) cell with graphene and ITO transparent conducting layers are investigated. To insure uniform thickness of the LC layer, as well as the same orientation boundary conditions, a hybrid LC cell containing graphene and ITO conductive layers has been prepared. The characteristics of LC cells with both types of conducting layers were found to be similar, indicating that graphene can be successfully used as a transparent conductive layer in LC devices

Abrahamyan, VK

Aroutiounian, VM

Gharghani, ASD

Hakobyan, NH

Kostanyan, AB

Margaryan, HL

Tabirian, N

Wilkinson, TD

Beilstein Journal of Nanotechnology

English

CUED - Cambridge University Engineering Department

Electro-optical characteristics of a liquid crystal cell with graphene electrodes

Nune H. Hakobyan

Hakob L. Margaryan

Valeri K. Abrahamyan

Vladimir M. Aroutiounian

Arpi S. Dilanchian Gharghani

Amalya B. Kostanyan

Timothy D. Wilkinson

Nelson Tabirian

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In liquid crystal devices (LCDs) the indium tin oxide (ITO) films are traditionally used as transparent and conductive electrodes. However, today, due to the development of multichannel optical communication, the need for flexible LCDs and multilayer structures has grown. For this application ITO films cannot be used in principle. For this problem, graphene (an ultrathin material with unique properties, e.g., high optical transparency, chemical inertness, excellent conductivity) is an excellent candidate. In this work, the electro-optical and dynamic characteristics of a liquid crystal (LC) cell with graphene and ITO transparent conducting layers are investigated. To insure uniform thickness of the LC layer, as well as the same orientation boundary conditions, a hybrid LC cell containing graphene and ITO conductive layers has been prepared. The characteristics of LC cells with both types of conducting layers were found to be similar, indicating that graphene can be successfully used as a transparent conductive layer in LC devices.</jats:p

Nune H Hakobyan

Hakob L Margaryan

Valeri K Abrahamyan

Vladimir M Aroutiounian

Arpi S Dilanchian Gharghani

Amalya B Kostanyan

Timothy D Wilkinson

Crossref

Hakobyan, Nune H

Margaryan, Hakob L

Abrahamyan, Valeri K

Aroutiounian, Vladimir M

Dilanchian Gharghani, Arpi S

Kostanyan, Amalya B

Wilkinson, Timothy D

Tabirian, Nelson

Apollo (Cambridge)

2802Electro-optical characteristics of a liquid crystal cell withgraphene electrodesNune H. Hakobyan*1, Hakob L. Margaryan1, Valeri K. Abrahamyan1,Vladimir M. Aroutiounian1, Arpi S. Dilanchian Gharghani1, Amalya B. Kostanyan2,Timothy D. Wilkinson2 and Nelson Tabirian3Full Research Paper Open AccessAddress:1Center of Semiconductor Devices and Nanotechnologies, YerevanState University, 1 Alex Manoogian Str., Yerevan 0025, Armenia,2Centre of Molecular Materials for Photonics and Electronics,Department of Engineering, University of Cambridge, 9 J. J. ThomsonAvenue, Cambridge, CB3 0FA, UK and 3BEAM Co., 1300 Lee Road,Orlando, FL, 32810, USAEmail:Nune H. Hakobyan* - nune.hakobyan@ysu.am* Corresponding authorKeywords:conductive layer; graphene; ITO; liquid crystal cell; optical switchingtimeBeilstein J. Nanotechnol. 2017, 8, 2802–2806.doi:10.3762/bjnano.8.279Received: 27 July 2017Accepted: 29 November 2017Published: 28 December 2017This article is part of the Thematic Series "Nanostructured liquid crystalsystems and applications".Guest Editor: A. Emelyanenko© 2017 Hakobyan et al.; licensee Beilstein-Institut.License and terms: see end of document.AbstractIn liquid crystal devices (LCDs) the indium tin oxide (ITO) films are traditionally used as transparent and conductive electrodes.However, today, due to the development of multichannel optical communication, the need for flexible LCDs and multilayer struc-tures has grown. For this application ITO films cannot be used in principle. For this problem, graphene (an ultrathin material withunique properties, e.g., high optical transparency, chemical inertness, excellent conductivity) is an excellent candidate. In this work,the electro-optical and dynamic characteristics of a liquid crystal (LC) cell with graphene and ITO transparent conducting layers areinvestigated. To insure uniform thickness of the LC layer, as well as the same orientation boundary conditions, a hybrid LC cellcontaining graphene and ITO conductive layers has been prepared. The characteristics of LC cells with both types of conductinglayers were found to be similar, indicating that graphene can be successfully used as a transparent conductive layer in LC devices.IntroductionIn modern optical devices based on liquid crystals (LCs) theelectro-optical control is realized using a transparent conduc-tive layer, which is often a thin film of metal (e.g., Au, Ag) orindium tin oxide (ITO). However, the use of these coatingsoften complicates the technology because of the need for antire-flection coatings, or they disturb the optical homogeneity andchemical stability [1-3]. Hence a number of technologies havebeen developed to obtain alternative transparent conducting ma-terials, many of which are based on nanomaterials.Among the various competing technologies, graphene films areshowing great promise due to their high conductivity, opticalBeilstein J. Nanotechnol. 2017, 8, 2802–2806.2803transparency, chemical resistance and mechanical flexibility [4].Many studies have been conducted on the application ofgraphene electrodes for light-emitting devices [5-7], photo-voltaic devices [8,9], and touch screens [10], with muchpromise as a replacement for ITO. However, fundamentalstudies on the application of graphene as transparent electrodesfor LC devices [11,12] have yet to be properly explored.In this work we present the results of the investigation of theelectro-optical and dynamic characteristics of LC cells employ-ing graphene electrodes as compared with the characteristics ofLC cells containing conventional ITO electrodes.Results and DiscussionSynthesis of graphene filmsThe graphene was obtained by a chemical vapor deposition(CVD) process. Details of synthesis and extensive characteriza-tion of the CVD graphene can be found in prior works [13,14].The monolayer graphene film was then transferred from the Cufoil to a precleaned glass substrate by using a polymer supportlayer of polystyrene (PS, Mw 35k, 2% w/w in toluene) and anetchant acid (FeCl3 aq, 0.5 M) to remove the Cu, as describedelsewhere [15]. This was followed by a wash in a warm ethylacetate bath to dissolve the supporting polymer layer.Device fabricationFigure 1 presents the LC cell design on glass substrates. Toinsure the same processing conditions, the same LC layer thick-ness, and the same orientation boundary conditions, two struc-tures were made in one device, each using different transparentconductive layers. Commercially available 15–20 Ohm/sq ITOcoated glass substrates with 0.7 mm thickness were used as abase of the structure, which was patterned to develop two divi-sions in each device. The left side of the device was developedto create a graphene–graphene structure together with the rightside an ITO–ITO structure, as shown in Figure 1.Figure 1: A schematic of the graphene–ITO hybrid liquid crystal cell.Figure 3: The transmission spectra of graphene and indium tin oxidesections of the hybrid liquid crystal cell.To avoid inhomogeneity caused by the mechanical treatment ofthe substrates (rubbing), the orientation of LC molecules wasrealized optically (photo-orientation) [16] by irradiation of aphoto-orienting polymer with linearly polarized radiation froma He–Cd laser at a wavelength of 325 nm. 6CHBT nematic LC(Δn = 0.15) was filled by capillary action, and the thickness ofthe LC layer (20 μm) was set by spacers under planar orienta-tion. Images of the fabricated cell, obtained between crossedpolarizers when voltage is applied to different areas, are shownin Figure 2.Figure 2: Images of the graphene–ITO hybrid liquid crystal (LC) cellbetween crossed polarizers: voltage not applied (a), peak-to-peakvoltage Vpp = 40 V applied to LC cell with graphene electrodes (b), andwith ITO electrodes (c).MeasurementsAll measurements were carried out on both sides (i.e., thegraphene–graphene and ITO–ITO sides) of the hybrid LC cell.The transmission spectra of the both sections were measured inthe visible range (Figure 3). As can be seen from the figure, thecell with ITO electrodes is more transparent for the 400–500 nmwavelengths, and the cell with graphene electrodes is moretransparent for wavelengths exceeding 800 nm. The trans-parency is the same for the 550–800 nm range.Beilstein J. Nanotechnol. 2017, 8, 2802–2806.2804Figure 5: Light intensity vs peak-to-peak (pp) voltage applied, passingthrough the liquid crystal cell with ITO and graphene layers.The dynamic characteristics of both sections were investigated.A schematic of the experiment is presented in Figure 4. Mea-surements were carried out with a specially designed driverwhich allows selection of the form of applied voltage, includ-ing the transient nematic effect (TNE [17]). The driver allows asmooth change of the amplitude of the control voltage with aselected rate within the given range and also registers the signalfrom the output of a photodetector.Figure 4: Schematic of the liquid crystal characterization experiment.When the He–Ne laser beam (632 nm) propagates through theLC cell placed between crossed polarizers (so that the directionof the polarization of the incident beam makes a 45° angle withthe LC director), then the influence of an external electric fieldallows typical oscillations due to the reorientation of the LC tobe observed (Figure 5). Hence the LC cell can operate as anelectrically controlled light valve, where the intensity of thetransmitted light is changed from minimum to maximum andvice versa with an associated π-phase shift.The switching time of the light valve depends on the value ofthe applied voltage. In order to determine the voltage corre-sponding to the minimum switching time, an experiment wascarried out to obtain the dependence of the light intensity (trans-mitted through the cell located between crossed polarizers) onthe control voltage (see Figure 5). А square-wave voltage of1 kHz frequency with the amplitude varying at a rate of10 mV/s was applied to the cell. As presented (Figure 5), thelight intensity at the output is in the form of characteristic oscil-lations with increasing voltage. It is important to note that bothsides of the hybrid LC cell, with different conducting elec-trodes (i.e., graphene–graphene and ITO–ITO), show almostidentical characteristics.A similar pattern was observed during the study of the reorien-tation and relaxation processes under the influence of anexternal bipolar pulse voltage (Figure 6).Figure 6: Time dependence of the intensity of light passing throughthe LC cells with graphene and ITO electrodes.As shown, at the initial moment of the pulse action (sharp reori-entation oscillation) the system tends to keep its originalordered state. Here, the translation of the highly ordered sur-face layer into the LC bulk occurs under the influence of elasticforces. However, later, the ordering in the bulk is lost due to theunmatched response of the LC molecules to the sharp forcedimpact of the electric field and the appearance of nonviscousflow (the oscillations fade away due to the strong light scat-tering). By the end of the pulse action, the system tends towardsordered restoration (restoration of the reorientation oscillations).Then the relaxation process takes place only under the influ-ence of the molecular forces of elasticity. The system returns toits original oriented state, similar to relaxation after influence ofan electric field. The above-mentioned feature of the reorienta-Beilstein J. Nanotechnol. 2017, 8, 2802–2806.2805Figure 7: Time characteristic of liquid crystal cells with ITO (a) and graphene (b) transparent conductive layers operating as a light valve in the fastswitching mode.Figure 8: Time characteristic of liquid crystal cells with ITO (a) and graphene (b) transparent conductive layers operating as a light valve in the slowswitching mode.tion process under the influence of an electric pulse, namely, amore ordered reorientation of the LC at the initial moment of itsaction, can be used when forming the control signal shape toimprove the operating speed of LC devices.In general, a liquid crystal cell working as a light valve canoperate in two modes: fast switching (high voltage, V >> Vth)and slow switching (the voltage is close to the threshold,V < 5Vth). In the fast switching mode (Figure 7), under the in-fluence of a strong electric field, the bulk and boundary layersof the LC are involved in the reorientation process. This reducesthe relaxation time due to the binding energy of the LC mole-cules with the surface of the substrate. Despite the shortresponse time there are undesirable backflows, making a deviceoperating in such mode less attractive.In the slow switching mode, reorientation occurs in the bulk ofthe LC. In this case, the operation time is essentially muchlarger than in the previous mode due to its dependence on onlythe rotational viscosity and elasticity of the LC.However, it is possible to achieve higher performance byapplying a control voltage with a special shape, which is basedon the transition nematic effect (TNE), when a step change involtage leads to the switching between the quasi-steady states ofthe LC. Optical switching time characteristics are measured byusing the TNE control signal (Figure 8).As shown Figure 7, during the fast switching mode, the reorien-tation times for LC cells with ITO and graphene conductinglayers are 0.66 ms and 0.88 ms, respectively, and the relaxationBeilstein J. Nanotechnol. 2017, 8, 2802–2806.2806times are 2.2 ms (ITO) and 2.1 ms (graphene). In the slowswitching mode (Figure 8) the switching times for tje LC cellwith a graphene conducting layer (23.2 ms – reorientation,12.4 ms – relaxation) are slightly higher than for the LC cellwith ITO (18.6 ms – reorientation, 11.7 ms – relaxation).ConclusionHybrid graphene–ITO nematic LC devices have been investi-gated to characterize the electronic properties of graphene. Theoptical switching time characteristics of LC cells with grapheneare slightly worse than those of cells with ITO. 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Electro-optical characteristics of a liquid crystal cell with graphene electrodes.

Electro-optical characteristics of a liquid crystal cell with graphene electrodes.

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