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Surfaces, Interfaces, and Applications

Nanoid Canyons on-Demand: Electrically Switchable Surface Topography in Liquid Crystal Networks Eser Metin Akinoglu, Laurens T. de Haan, Songru Li, Zhike Xian, Lingling Shui, Jinwei Gao, Guofu Zhou, and Michael Giersig ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15203 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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ACS Applied Materials & Interfaces

Nanoid Canyons on-Demand: Electrically Switchable Surface Topography in Liquid Crystal Networks Eser Metin Akinoglu1,§,*, Laurens Theobald de Haan2,3,§, Songru Li4, Zhike Xian4, Lingling Shui3 Jinwei Gao4, Guofu Zhou1,2,3, *, Michael Giersig1,5, * Affiliations: 1

International Academy of Optoelectronics at Zhaoqing, South China Normal University, 526238

Guangdong, P. R. China 2

SCNU-TUE Joint Lab of Device Integrated Responsive Materials (DIRM), National Center for

International Research on Green Optoelectronics, South China Normal University, 510006 Guangdong, P.R. China 3 Guangdong

Provincial Key Laboratory of Optical Information Materials and Technology & Institute of

Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, 510006 Guangdong, P. R. China. 4 Institute

for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering

and Quantum Materials, Academy of Advanced Optoelectronics, South China Normal University, 510006 Guangdong, China 5 Department

of Physics, Freie Universität Berlin, 14195 Berlin, Germany

§ These authors contributed equally *Corresponding authors:

Eser M. Akinoglu International Academy of Optoelectronics at Zhaoqing South China Normal University Liyuan Street National High-Tech Zone, Zhaoqing, Guangdong, 526238, P. R. China Tel.: +8615820290944 E-mail: [email protected]

Guofu Zhou (Email: [email protected]) Michael Giersig (Email: [email protected])

Keywords: actuators, liquid crystal elastomer, switchable surface, topography

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Abstract Topography is a key factor that governs important properties of surfaces, such as adhesion and wettability, and materials with switchable surface topographies will have switchable surface properties. We demonstrate a principle to generate electrically switchable surface topographies on the surface of a thin nematic liquid crystal elastomer film which is sandwiched between a continuous electrode and a random metal network. Voltage-controlled displacement of the metal network towards the continuous electrode is achieved, resulting in unprecedented topographical modulations in the range of 0 – 2.5 micron. We show, that this depth variation is significantly larger than the expected deformation due to electrostatic attraction between the network and the continuous electrode. This effect is explained by deformation due to the rotation of the liquid crystal side groups along the electric field lines.


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1. Introduction Topography is a key characteristic of surfaces that crucially determines the properties of material interfaces. Whether a surface is flat or rough has great impact on how it looks, how it feels, how it interacts with an adjacent medium and in which respect and to which extend fundamental material properties differ at the surface versus in the bulk.1 Implications of surface topography are widely observed in nature.2 For instance, plant leaves,3 insect eyes4 and animal skins5 exhibit hydrophobic or hydrophilic wetting properties, and insect wings feature bactericidal properties6 or colorful patterns7 based on light diffraction, all of which is determined to large extend by surface topography. Commonly, artificially tailored surfaces are static, which in turn results in static material properties, however a dynamic surface topography would enable a broad range of innovative applications based on switchable material properties. To this end, electrostrictive systems which can be controlled with an electrical circuit are an ideal candidate to design microactuators. Electroactive polymers (EAP) that exhibit large deformations under an electrical stimulus are often employed in actuator research and applications.8 Ionic EAPs, such as polymer gels, have the advantage of a response at low electrical fields, but their instability, short lifetime and high response time are major drawbacks.9 Electronic EAPs are also responsive to applied electric fields and can exhibit longer lifetimes and, most importantly, fast response times.9 Dielectric actuators are devices where the electrostrictive response is a result of Maxwell stress originating from the electrostatic attraction between the electrodes.10 Liquid crystal (LC) polymer networks are commonly used in stimuliresponsive devices,11 as the a change in alignment of the liquid crystal groups due to a stimulus can cause significant changes in their shape.12–14 Whereas thermally and optically triggered liquid crystal elastomers have been well-known for some time,15 electrically driven liquid crystal elastomer technology is still only slowly emerging.16–20 A notable recent example are subtle protrusions (up to 8% of total thickness) created in thin LC polymer films by in-plane electric fields that induce a reduction in order parameter and an increase in free volume.21,22 3 ACS Paragon Plus Environment


In other work, a polydomain isotropic-genesis nematic liquid crystal elastomer (IG-LCE) with a large electromechanical effect, exhibiting strains up to 35% based on the increase of the order parameter, was reported.19 However, while this effect seems ideal to generate large shape deformations, it was only shown in bulk material or as a single surface protrusion.23 In the work presented here, we use this mechanism to generate electrically switchable surface topographies with protrusions exceeding 2.5 µm (28%) in height in an electroresponsive polydomain liquid crystal elastomer (LCE). We obtain significantly larger deformations in LCEs compared to both the commonly used dielectric elastomers with the same viscoelastic properties that solely exploit the Maxwell effect,24,25 and previously reported LC-based systems.21

2. Experimental Section 2.1. Electrode Fabrication The bottom electrode is a 200 nm thick Ag film that was deposited on a glass slide by magnetron sputtering system (ATC Orion8, AJA International INC.) at a pressure of 4 mTorr and 200 W DC Power. A 5 nm Cr adhesive layer is utilized to ensure film adhesion to the glass slide. For the top electrode, we exploit the crack formation of a thin film of nail polish (Dongguan Husheng Product Co., Ltd) as a lithographic template. The nail polish is coated on the glass slide by rod-coating. In a next step a 20 nm thin amorphous carbon layer is deposited via carbon wire flash evaporation in a Quorum Technologies Q150T ES turbomolecular-pumped coating system as a lubricant to facilitate transfer. It is followed by 200 nm Ag Magnetron sputtering. The nail polish based lithographic template is then removed in chloroform.

Liquid Crystal Mixture The monoacrylate mesogen 4-cyanophenyl 4-((6-(acryloyloxy)hexyl)oxy)benzoate and the diacrylate mesogen 2-methyl-1,4-phenylene bis(4-((6-(acryloyloxy)hexyl)oxy)benzoate) were purchased from HCCH. The photoinitiator 1-hydroxycyclohexyl phenyl ketone was purchased from HEOWNS. A mixture 4 ACS Paragon Plus Environment

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of 94-97 w% mesogen, 1-4 w% diacrylate crosslinker and 2 w% photoinitiator was prepared by dissolving the components in dichloromethane a vial, followed by removal of the solvent through heating and stirring.

Materials Characterization The perforated electrodes were characterized with a profiler (P-6, KLA Tencor) to obtain film thicknesses, a Keithley 2400 source meter (Keithley, USA) was used in a four-point-probe measurement setup to obtain a sheet resistance of 3.7 Ω□ for the perforated electrode, and an optical microscope (MA 2002, Chongqing Optical&Electrical Instrument Co., Ltd.) was used to characterize the dimensions of the perforated electrode. Its wire widths and wire-to-wire spacing were approximately 5 µm and 30-50 µm respectively. The viscoelastic properties of the LC polymer network were probed with a Mettler Toledo DMA 1 dynamic mechanical analyzer. The dielectric constant of the LC polymer network was calculated from parallel plate capacitance measurements.

Device Assembly The device fabrication is depicted in Figure S1. A gap between the two parallel glass slides supporting the electrodes, which were separated by spherical 10 µm silica spacers, was filled with the liquid crystal mixture through capillary forces. Polymerization of the reactive mesogens was performed at 90 °C in the high-temperature isotropic phase via the excitement of a photoinitiator with UV irradiation using an Omnicure 1500 series curing lamp with a 320-500 nm filter for 5 min at full intensity. As the glass slide supporting the perforated electrode was lifted off, the metal network remained embedded into the polymer. Contact pads on the support substrate were used to integrate the device into an electrical circuit.

Device Characterization A direct current (DC) electrical potential up to 250 V was obtained using a laboratory power supply with In soldering contacts to the contact pads. The device was then fixed on a Linkam hotstage using



double-sided tape, followed by stage heating to 90 °C. Surface topography changes were monitored with a 3D optical profiler using interferometry.

3. Results and Discussion Principle and Materials


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The architecture of the device with electrically switchable surface topography consists of a thin liquid crystal elastomer film sandwiched between a continuous and a perforated silver electrode as shown in Figure 1a,b. A glass substrate supports a bottom electrode. The top electrode is a thin perforated Ag film embedded into the surface of a ~10 µm thick polydomain IG-LCE film photopolymerized from an LC monomer mixture (Figure 1c). An emergent metallic network is employed as the perforated top electrode, with wire widths of about 5 µm and a large, random dispersion of the perforation area (Figure



1d), prepared as described in previous work.26 Immobilization in the isotropic phase by photopolymerization is important to obtain super soft multi-domain LCEs.27,28 The small size of the domains was revealed using polarized optical microscopy (Figure S2). The transition temperature of the polydomain state into the isotropic phase is approximately 130 °C, as concluded from differential scanning calorimetry (Figure S3). Because of the device architecture, when an electric potential is applied between the electrodes a capacitor is formed below the metal fraction of the perforated


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electrode that results in a contractive electrostrictive response of the LC film. From a different perspective, protrusions arise through the perforations in the metal network, which are determined in shape and area by the morphology of the perforations, and by height through the electric field strength (ON-state). Upon removal of the electric field the electromechanical effect is completely reversed and a planar surface (OFF-state) is obtained. This effect appears instantaneous to the human eye. In this context, Okamoto et al. found that the response speed of this material class can reach up to 10 Hz.19

Figure 1. Device architecture and operation. a) Schematic representation of the device, with a continuous bottom electrode, a perforated top electrode and an LCE as the three key components and b) its cross-section. c) Chemical composition of the liquid crystal mixture to form the LCE by

photopolymerization. d) Optical micrograph of the emergent perforated metal network employed as the top electrode. e) Schematic presentation of the device operating principle with electrically switchable and reversible ON- and OFF- states. OFF: Planar surface topography. ON: Textured surface topography. 9 ACS Paragon Plus Environment


These experiments were performed at 90 °C. A schematic illustration of the electrically switchable and reversible states is depicted in Figure 1e.

Actuation and Characterization


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A three dimensional (3D) optical profiler was used to characterize the surface topography of IG-LCE devices with 1 w%, 2 w% and 4 w% crosslinker. Figure 2 shows the OFF-state and ON-states of 2 w% a IG-LCE devices. The OFF-state with no applied potential is a flat LCE surface with the embedded metal network visible as small (~50 µm) protrusions, as shown in Figure 2a. The 3D profiles of two ON-states

Figure 2. Actuation and surface topography. a) 3D profiles of a IG-LCE (2 w% crosslinker) device in the OFF-state state at 0 V applied potential. Two ON-states are shown at b) 150 V and c) 250 V.

Compression indentation is observed beneath the metal network yielding protrusions of uniform height but largely varying size and shape.



with 150 V and 250 V DC-potential applied are shown to illustrate the effect of the electric potential. It is evident that protrusions arise within the perforations of the metal network, with a large range of sizes and shapes that is governed by the morphology of the perforations. The height of the protrusions increases with the strength of the applied electric field, and at the apex a walled plateau deformation occurs at the highest levels of electric field strength. The protrusions occur because of the metal network moving into the LCE, and the protrusion plateau remains at the same height compared to the OFF-state (Figure S4). From the height difference between metal network and protrusion plateau we obtain the electric field dependent thickness modulation (Figure 3). This modulation is dependent on the polymer crosslink density which we investigated for IG-LCE networks with 1 w% (red), 2 w% (green) and 4 w% (blue) crosslinker respectively (Figure 3). The recorded maximum modulation of 28%, equal to an absolute height variation of 2.57 µm in a 9.15 µm thick film, was observed in an IG-LCE network with 1w% crosslinker under 25 V/µm bias. We find, that the modulation is greatly increased for lower crosslink densities, i.e. one order of magnitude difference in modulation at 20 V/µm electric field

Figure 3. Modulation of LCE networks. The modulation vs. applied electric field strength for IG-LCE network films is shown for 1 w% (red), 2 w% (green) and 4 w% (blue) crosslinker respectively. The

dashed lines are guides for the eye. Smaller crosslink densities yield superior modulation due to simplified mesogenic realignment. 12 ACS Paragon Plus Environment

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strength between LCE networks with 1 w% and 4 w% crosslinker. This can be explained due to an increased mobility of the liquid crystal side chains at lower crosslink densities. Furthermore, we have observed that the modulation is consistent for film thicknesses between 8 – 13 µm. For films with thickness below or beyond this range the modulation can be expected in the same order of magnitude. However, thicker films require higher potentials to achieve the same electric fields, which may become a limiting factor.

Mechanism To explore the mechanism of the compression, we estimate the magnitude of the expected Maxwell effect of the IG-LCE networks with 2 w% crosslinker to approximate the expected thickness modulation because of electrostatic pressure, and put it in perspective to the observed modulation in experiment (Figure 4). Beneath the perforated electrode we can make a parallel plate capacitor approximation as schematically illustrated in. The electrostatic pressure is then given by ிಶ ஺

௎ ଶ

= ߝ଴ ߝ௥ ቀௗ ቁ

(1)

with A the area of the electrode, ε0 the vacuum permittivity, εr the relative permittivity, U the applied electrical DC-potential, d the separation of the electrodes, and FE the electrostatic force on the perforated electrode into the film. The latter is in equilibrium with a restoring force of the elastic medium following Hooke’s law given by ௒஺

‫ܨ‬ோ = ቀ ௗ ቁ ∆݀

(2)

with Y the elastic modulus, ∆d the thickness difference compared to the initial thickness, and FR the restoring force describing the strain-stress response of the elastomer. From equation (1) and (2) follows ∆ௗ ௗ

=

ఌబ ఌೝ ௎ ଶ ௒

ቀௗ ቁ

(3)

Dynamic mechanical analysis provides insights into the viscoelastic properties of the polymer (Figure 4a), which exhibits a glass transition temperature Tg ≈ 30 °C observed both through a decrease 13 ACS Paragon Plus Environment


in elastic modulus and a peak in energy dissipation measured through tan(δ).29 On the rubbery plateau T > Tg, a second transition Tα* is observed, which hints to a double glass transition as a result of inhomogeneous polymerization yielding locally densely and loosely crosslinked areas respectively. The temperature dependent, static dielectric constant εr of the LC polymer (Figure 4b) was probed with capacitance measurements in parallel plate configuration, which accurately represents operating conditions of our device.30 However, it should be noted that the dielectric constant of liquid crystals31 and their polymers32 is frequency dependent and exhibits strong anisotropy. We find, that the static dielectric constant of the used LC polymer is increasing with temperature, especially in the elastomeric regime above Tg. This can be attributed to facilized polarization processes in the polydomain LCE networks at elevated temperature leading to an increase of the dielectric constant.33 For the operating conditions of the device with 2 w% crosslinker then follows Y ≈ 1.6 MPa and εr ≈ 7.86, which yields a thickness modulation ∆d/d0 based on the Maxwell effect, shown as a black dashed line in Figure 4c. It can be seen that the actual modulation response (Figure 4c, red markers and dashed line) is well beyond the predictions of the Maxwell effect. It is important to note that the calculated Maxwell effect generally constitutes an upper limit to the modulation observed for dielectric elastomer, and the measured modulations usually lie well below the calculated values.34 The liquid crystal elastomer showing a response clearly surpassing the Maxwell effect (Figure 4d) is the result of the realignment of the LC side groups from a multidomain orientation towards uniaxial orientation along the electric field direction as is schematically shown in Figure 4e.19 Okamoto et al. showed that such realignment causes the material to become strongly stretched in the plane perpendicular to the realigned director (i.e., Efield axis) and compressed in the plane parallel to the realigned director.19 This, combined with the predicted Maxwell compression explains the dramatic deformation observed. The expansion of the LCE in the LCE film plane beneath the metal network also explains the formation of a walled plateau shape at the protrusions apexes.


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Figure 4. Modulation and mechanism. a) Viscoelastic properties of the LCE film. Next to the glass transition at approximately 30 °C a second Tα* transition is observed. The elastic modulus decreases significantly in the rubbery plateau of the polymer b) Temperature dependent dielectric constant of the LCE extracted from capacitance measurements in a static parallel plate configuration. An an increaseas in the dielectric constant is evident above Tg. c) The modulation of the LCE film sandwiched between the perforated metal network and the bottom electrode is given in red. The black dashed line represents the predicted Maxwell effect. d) Schematic illustration of the Maxwell effect. e) Schematic illustration of the correlation between themacroscopic deformation and mesogen realignment under E-fields.



4. Conclusion We have demonstrated a new method to generate dynamic surface topographies that are electrically switchable, through the application of an electric field to an electroactive liquid crystal elastomer sandwiched between a perforated metal network electrode and a continuous electrode. Protrusions arise through the perforations of the metal network, which allows indirect control of their size and shape through morphology of the perforations. The maximum protrusion height of more than 2.5 µm, corresponding to 28% thickness modulation, is far larger compared to any similar liquid crystal-based system previously reported. The effect also goes well beyond the expected Maxwell effect based on the force balance between the electrostatic pressure of the electrodes on the elastomer and the repulsive force of the elastomer under compression, in which is due to the deformation of the isotropic genesis polydomain nematic elastomer as a consequence of the realignment of randomly oriented liquid crystal domains along the electric field lines. Further engineering of the LCE material properties or utilizing alternative electroactive polymers could be used to exceed the achieved modulation of 28% in this work, as well as to decrease the operation temperature towards room temperature. A possible strategy could be to maximize the Maxwell effect through the engineering of the polymers dielectric constant and elastic modulus, as has been done for dielectric elastomer actuators.35,36 In fact, prestretching is a common method to decrease the elastic modulus of dielectric elastomer actuator materials to values orders of magnitude lower than in our material. The principle described here enables the generation of surfaces with electrically switchable surface topography, featuring large protrusions with well-defined size, height and shape as a promising approach to engineer devices with switchable topographyinduced surface properties.


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Acknowledgements Funding: The research leading to these results has been funded by the Guangdong Innovative and Entrepreneurial Team Program titled “Plasmonic Nanomaterials and Quantum Dots for Light Management in Optoelectronic Devices” (No.2016ZT06C517), the National Natural Science Foundation of China (No. 51561135014, U1501244) and the Science and technology project of Guangdong Province (No. 2017B020240002).

Authors Contributions: E. M. A. and L. T. d. H. conceived the investigation and executed the work. S. L. and Z. X. contributed to the sample fabrication. L. S., J. G., G. Z. and M. G. helped to understand the underlying science with discussions. All authors contributed to the manuscript.

Conflicts of interest: The authors declare no competing interests.

Data and materials availability: All data is available in the manuscript or the supplementary materials.

Supporting Information Supporting Information is available online. Fig. S1 is a schematic presentation of the device fabrication. Fig. S2 shows a polarized optical microscope image of IG-LCE. Fig. S3 shows the differential scanning calorimetry analysis of the IG-LCE. Fig. S4 is a 3D profile obtained with optical profilometry demonstrating that the perforated electrode sinks into the IG-LCE film when it is under electrical potential.



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