Conformational Transitions of Polymer Brushes for ReversiblySwitching Graphene TransistorsSong Liu,† Safa Jamali,† Qingfeng Liu,† Joao Maia,† Jong-Beom Baek,‡ Naisheng Jiang,† Ming Xu,†

and Liming Dai*,†

†Department of Macromolecular Science and Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio44106, United States‡School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks, Ulsan National Institute ofScience and Technology (UNIST), 50, UNIST, Ulsan 44919, South Korea

*S Supporting Information

ABSTRACT: We developed a facile, but efficient, approach tographene field-effect transistors (FET) functionalized withpolymer brushes, in which the conductance can be reversiblyswitched by solvent-induced polymer conformational changes.Our experimental and stimulation results demonstrated that thesolvent-induced conformational transition of the polymer brushcould affect the carrier concentration by changing the number ofscattering sites associated with the graphene−polymer contactareas, leading to reversible electrical switching for the grapheneFET device. Both end-adsorbed diblock and triblock copolymersshowed similar switching effect through the solvent-induced chain stretching−collapse and tail-to-loop conformational changes,respectively. This work provides new platform technologies for developing novel electronic devices with tunable electricalproperties and for studying macromolecular conformations and conformational transitions.

■ INTRODUCTION

Responsive polymers can reversibly change their conforma-tions/properties in response to external stimuli, such as solvent,pH, and temperature.1−4 The stimulus-induced conformation/property changes make the responsive polymers attractive asactive components in various smart devices for a wide range ofapplications,1−4 including sensors, drug carriers, actuators, andsmart textiles, to mention a few. Polymer brushes comprise amajor class of responsive polymers, in which polymer chains aretethered at one end to a solid substrate through either covalentattachment or physical adsorption.5 Polymer brushes have beenwidely demonstrated to change conformations in response toexternal stimuli, including changes in solvent, temperature, pH,ionic strength, light, and mechanical stress, leading to manysmart functional devices with controllable switching proper-ties.6−8

Graphene, on the other hand, possesses many excellentproperties, including an extremely large specific area (2630 m2

g−1), outstanding thermal conductivity (up to 5000 W m−1 K−1

for a single-layer graphene), high Young’s modulus (1.0 TPa),good electrical conductivity (106 S cm−1) and charge mobility(200 000 cm2 V−1 s−1), and excellent optical transparency,9

which makes it attractive for various potential applications,ranging from novel composites, through electronics, tobiosensors.10−12 Of particular interest, field-effect transistors(FETs) based on functional graphene sheets hold great promisefor electronic switching, sensing, and detecting due to theirultrahigh carrier mobility, along with the two-dimensional (2D)

structure ideal for the existing fabrication process compatiblewith the standard planar Si technology.13 Since the electronicproperties of graphene are susceptible to its surface character-istics, various molecules, macromolecules, and nanoparticles,such as DNA, peptide, and quantum dots, have been used tomodify the graphene surface to impart specific functions toFET devices based on graphene sheet(s).14−18

The 2D structure of graphene serves as an ideal substrate forthe preparation of polymer brushes, while controllablepolymerization methods have been devised to create newfunctional graphene (or graphene oxide) composites withpolymer brushes.19−22 This prompted us to integrateresponsive polymer brushes with graphene FETs, leading tohighly sensitive devices for reversible sensing. We havepreviously used highly asymmetric polystyrene−poly(ethyleneoxide) (PSm−PEOn, m and n are weight-averaged degree ofpolymerization for polystyrene and poly(ethylene oxide)blocks, respectively, and m ≫ n, Table 1) diblock copolymersto form polystyrene brushes through anchoring the non-adsorbing PS block, as a dangling chain, by the terminal PEOblock at a liquid−solid interface (e.g., mica in toluene).23,24 Bydirectly measuring the interactions of a single layer of the end-adsorbed PS−PEO polymer brush against a bare mica surface,we have also demonstrated that PS−PEO chains can undergo

Received: May 14, 2016Revised: September 19, 2016Published: September 30, 2016

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© 2016 American Chemical Society 7434 DOI: 10.1021/acs.macromol.6b01011Macromolecules 2016, 49, 7434−7441

fast and reversible stretching−collapse conformational tran-sition on mica surface by simply changing the solvent fromtoluene (good solvent) to cyclohexane (poor solvent, Tθ = 34°C), or vice versa (Figure 1).23,24

In this study, we develop a facile, but efficient, approach tographene FET transistors with terminally anchored PS−PEOpolymer brushes, in which the conductance can be reversiblyswitched by the solvent-induced conformational changes(Figure 1). As we shall see later, our experimental andstimulation results demonstrate that the solvent-inducedconformational transition of the polymer brush can affect thecarrier concentration by changing the number of scattering sitesassociated with the graphene−polymer contact areas (Figure 1)and hence conductivity of the graphene surface in the FETdevice. By changing the solvents to cause the reversiblecollapse−stretching conformation transition of the end-adsorbed diblock copolymer chains on the graphene surface(Figure 1), reversible electrical switching was demonstrated.Moreover, loop−tail conformational changes were alsoobserved when end-adsorbed PEO−PS−PEO triblock copoly-mer chains (Table 1) were used. These results indicate that thenewly developed graphene FET transistors functionalized withpolymer brushes can provide a novel pathway toward stimuli-responsive graphene electronics with tunable electrical proper-ties for specific applications.

■ EXPERIMENTAL SECTIONMaterials. The PS−PEO and PEO−PS−PEO copolymers were

synthesized by Polymer Laboratories (U.K.). The weight-averagemolecular weight and molecular weight distribution of the sampleswere determined by gel permeation chromatography (GPC) measure-ments, while the PEO content was measured by 1HNMR and checkedby infrared spectroscopy and elemental analysis. Analytical gradetoluene and cyclohexane used in this study were purchased fromAldrich Chemical Inc. PS with a weight-average molecular weight of350 000 was ordered from Sigma-Aldrich, and PEO with a weight-average molecular weight of 400 000 was purchased from ScientificPolymer Products, Inc. Both have a molecular weight polydispersity of

about 1.06. Other reagents and solvents were purchased from AldrichChemical Inc.

Device Fabrication. Briefly, graphene was synthesized through achemical vapor deposition (CVD) process on copper foil (Alfa Aesar).After graphene growth, poly(methyl methacrylate) (PMMA) with 300nm thickness was spin-coated on the graphene-deposited Cu films.The whole samples were then immersed into a freshly-prepared ironchloride saturated aqueous solution for about 1−2 h to remove the Cusubstrate. The resultant transparent sheet floating in the aqueoussolution was transferred to a heavily doped silicon substrate with a 400nm layer of thermally grown oxide. Then the PMMA was removed in aboiling acetone solution. To aid in the alignment for the subsequentphotolithographic process, gold marks were thermally evaporatedthrough a shadow mask. In order to form the graphene devices,selective water plasma was applied through a photolithographicallypatterned resist mask to etch away the unprotected graphene. Finally,by using another photolithographic process, high-density patternedmetallic electrodes (5 nm of Cr followed by 50 nm of Au) separatedby 5 μm in the center (Figure S1) were deposited onto the graphenesheets through thermal evaporation. The doped silicon wafer served asa global back-gate electrode for these devices.

Device Measurement. Polymer brush was attached on graphenesurface by incubation the device in the polymer solution withconcentration of 0.005 g/mL in toluene for about 20 h. Afterabsorption, fresh toluene was used to wash the transistor to removenonadsorbed polymer chain, if any. For the wet measurement, thesolvent was dropped on the surface of the device to keep the liquidenvironment. To change the solvent, the device was immersed in thepure solvent. To test the transistor characteristics of these transistors,the I−V measurements were carried out at room temperature in theambient atmosphere using a Keithley 2636A system SourceMeterconnected with a Signatone 1160 series probe station.

Characterization. X-ray photoelectron spectroscopic (XPS)measurements were carried out on a VG Microtech ESCA 2000using a monochromic Al X-ray source (97.9 W, 93.0 eV). The UV−visspectra were measured with a Jasco V-670 spectrometer. The Ramanspectra were collected using a Raman spectrometer (Renishaw) with a514 nm laser. Contact angles were measured with Future DigitalScientific Corp. After the treatment of toluene or cyclohexane, thesamples were dried with N2 gas flow immediately and then tested thecontact angles with water. Atomic force microscopic images wereobtained by an Agilent 5500 AFM. Adhesion force was tested with ahomemade force measurement system. The adhesion force wasobtained from the force curve of interactions between the tip and PS−PEO functionalized graphene surface.

■ RESULTS AND DISCUSSION

Transistors based on graphene sheets generated by chemicalvapor deposition (CVD) were fabricated using a nondestructivemethod involving polymer-mediated transfer, standard multi-step photolithography, and etching.25 Judicious application ofthis method allows for mass production of high qualitygraphene transistors with a high yield. Figure S1 shows anoptical image of a graphene transistor thus prepared, in whichgraphene is clearly evident in the central part of the device.With the metal pads as the source (S) and drain (D) contacts,the graphene device can be tested by applying gate bias voltagefrom the silicon substrate as the global back-gate. Prior to test,the graphene FET was subjected to self-assembling of apolymer brush in solution (toluene, 0.005 g/mL), usingprocedures similar to those reported previously.26,27 In thisstudy, three different copolymers (Table 1) were used tomodify the graphene surface for demonstrating the effects ofmolecular structure, weight, and composition on the grapheneFET performance.In a typical experiment, end-adsorbed PS−PEO (150K)

diblock chains were terminally anchored onto the graphene

Table 1. Molecular Characteristics of the CopolymerSamples Used in This Study

sample 10−3MW MW/MN

wt %PS xl y x2

PS−PEO (150K) 150 1.16 98.5 0 1420 51PEO−PS−PEO(128K)

128 1.02 99.7 5 1225 5

PEO−PS−PEO (49K) 49 1.09 92.4 42 435 42ax1, y, and x2 refer to the polymerization index of the blockcopolymer: (PEO)x1(PS)y(PEO)x2.

Figure 1. Schematic representation of the reversible conformationaltransition of polymer brush PEO−PS functionalized graphenetransistor induced by sequentially solvent change.

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surface to switch the graphene FET devices. To form thegraphene-supported polymer brush, a graphene FET wasimmersed into a solution of PEO−PS (150K) in toluene(∼0.005 g/mL) for about 20 h. After the PS dangling chainswere anchored onto the graphene surface by the terminal PEOto form the polymer brush,23,24,26,27 the FET device wasremoved from the polymer solution and washed with freshtoluene for subsequent surface characterization. A typicalRaman spectrum of graphene before and after being assembledwith the PS−PEO polymer brush is given in Figure 2a, whichshows a 2D peak (2690 cm−1), G band (1580 cm−1), and Dband (1350 cm−1) characteristic of graphene. The large G/2Dpeak ratio (IG/I2D > 1/4) indicates a multilayered graphenewith a high graphitization degree, as evidenced by the low D/Gpeak ratio (ID/IG = 0.09).28 The increased ID/IG ratio from 0.09to 0.24 after functionalization with the PS−PEO polymer brushis due to the increased sp3 carbon atom fraction on graphenesurface associated with the physically adsorbed PS−PEOchains. The presence of a PS−PEO adsorbed layer on thegraphene substrate was further confirmed by the decrease inoptical transmittance of graphene from about 94.5% to 90% at550 nm after adsorption of the PS−PEO chains (Figure2b).29,30

To investigate the surface chemistry, we further performedX-ray photoelectron spectroscopy (XPS) measurements.Figures 2c and 2d show the high-resolution XPS C 1s peakfor graphene before and after adsorption of PS−PEO chains,respectively. As expected, sp3-hybridized saturated carbons(285.2 eV) increased significantly upon the adsorption of PS−PEO chains (Figure 2d), along with the appearance of a newpeak C−O (286.1 eV), attributable to the absorption of PEO−

PS onto the graphene surface. These results from the driedsurface characterization suggest that the PS−PEO polymerchains have been successfully attached on the graphene surface.However, the possible formation of some PS−PEO micelles viathe PEO segment aggregation in toluene followed by directlyattaching the micelle to the graphene surface through π−πinteraction between graphene and PS cannot be ruled out.Having confirmed the presence of the end-adsorbed PS−

PEO chains on the graphene surface in the FET device, weperformed electrical measurements in solvents to demonstratethe effects of polymer brush conformation transition (Figure3a) on the FET performance. As discussed above, we havepreviously reported the reversible stretching−collapse con-formational transition for PS−PEO polymer brushes on micasurfaces by changing the solvent from toluene to cyclohexane,or vice versa.23,24 Figure 3b shows the source-drain current (ID)as a function of the gate voltage (VG) at a fixed source-drainbias voltage (VD) for a representative device under variousconditions. As can be seen, we obtained very stable I−V curvesat fixed experimental conditions, suggesting a reliable procedurehas been established. A decrease in conductance was observedupon the end-adsorption of PEO−PS diblock copolymer chainsfrom toluene onto the graphene surface (red curve, Figure 3b),due most probably to the scattering effect on charge carrierscaused by physical coverage of the graphene surface withPEO−PS chains.31−33 We noted that the polymer-adsorption-induced decrease in conductance of graphene was quite general,as also confirmed by a control experiment in whichhomopolymer chains of PEO were absorbed on graphenefrom either cyclohexane (or toluene) or PS from cyclohexane(Figures S2 and S3).

Figure 2. Characterization of PEO−PS functionalized graphene transistors. (a) Raman spectra of an individual graphene devices before (black) andafter (red) assembly of PEO−PS diblock copolymer molecules. (b) Transmittance change before (black) and after (red) PEO−PS wasfunctionalized to a graphene sheet on glass slide. (c, d) High-resolution C 1s XPS spectra of pristine graphene and PEO−PS assembled graphene.The black curves are experimental data with other colored curved for deconvoluted fitting.

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As seen in Figures S2 and S3, however, changes of thesolvent between toluene and cyclohexane did not cause anyconductance change for the graphene FETs adsorbed witheither PEO or PS homopolymers. In contrast, reversible andsignificant changes in ID were observed when the PS−PEOadsorbed device was subject to sequential solvent changesbetween toluene and cyclohexane (Figures 3b and 3c).Specifically, a significantly decreased ID was observed over theentire range of gate bias covered in this study when toluene wasreplaced by cyclohexane (blue curve in Figure 3b). When thesolvent was changed from cyclohexane back to toluene, it wasinteresting to note that ID of the same device returned toalmost its original value at any of the gate bias, as shown by thegreen curve in Figure 3b. The observed solvent-induced currentswitching is highly reversible and reproducible, as exemplifiedby the data from 35 tested devices shown in Figure 3c.By changing the solvent quality, the end-adsorbed PS−PEO

brush underwent reversible stretching−collapse conformationtransition.23,24 As mentioned earlier, the solvent-inducedreversible stretching−collapse conformation transition causeda concomitant reversible change in charge carrier concentrationby reversibly changing the number of scattering sites associatedwith the graphene−polymer contact areas31−33 and hence theobserved reversible current change of the graphene FET undera constant gate bias (Figures 3b and 3c). As shown in Figure3b, the slopes of the I−V curves were essentially constant,which reflected negligible change in transistor mobility. This isbecause conductivity is defined by G = neμ, where e is theelectron charge, μ is the mobility, and n stands for the carrier

concentration.34 Furthermore, Figure 3b shows no obviousDirac point shift, and thus charge-transfer-induced doping ofgraphene could be eliminated. So, the conductance switching ofour transistor was mainly from the carrier concentration changein graphene. This is because the absorbed polymer chains couldserve as scattering sites to quench the charge carriers (hole orelectron), leading to a reduced carrier concentration. Byreplacing toluene with cyclohexane, the PS brush collapsed toform a more compact polymer layer at the graphene surface toenhance the surface coverage on graphene. The increasedpolymer−graphene contact scattered more the charge carriersand hence the decreased conductance of the device. Oncecyclohexane was changed back to toluene, the collapsed PSchains returned to the stretched brush conformation through afast dynamic with a concomitant increase in the conductance,leading to the reversible switching of the graphene FETassociated with the solvent-induced reversible stretching−collapse conformation transition.Upon thermal annealing (300 °C in Ar and H2 for 3 h) to

decompose the physically adsorbed polymer chains, currentresponses of the graphene FET virtually returned to the originalstate characteristic of the pristine graphene device with almostno solvent response (Figure 3d and Figure S4). No solventresponse was observed for either the pristine graphene FETwithout adsorbed polymer (Figure S5) or the graphene FETswith adsorbed homopolymer chains (i.e., Figure S2 for PEOand Figure S3 for PS). Clearly, therefore, the observedreversible current switching effect can be exclusively attributedto the absorbed PS−PEO polymer brush.

Figure 3. (a) Schematic representation of the conformation transition of PEO−PS in toluene and cyclohexane (H: brush height; Rg: radius ofgyration). (b) Changes in drain current of a typical device functionalized by PEO−PS as a function of VG at different states: pristine graphene(black), after PEO−PS assembly on graphene surface (red), after the first treatment of cyclohexane (blue), and after the treatment of toluene again(green). (c) Switching cycles for the same device upon alternate treatment with cyclohexane and toluene. (d) Current changes of different states ofthe same device before and after annealing to remove the PEO−PS polymers in alternate treatment with toluene and cyclohexane at fixed conditions.All current values were taken at VG = 0 V in (c) and (d). All the measurements of solvent treatments were carried out at VD = 1 mV in a liquidenvironment.

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To gain a better understanding of the end-adsorbed polymerconformation transition as a response to solvent change and theassociated electrical switching for the graphene FETs function-alized with polymer brushes, we further performed computersimulations, along with relevant experiments. First, wemeasured the water contact angle to be ∼72.0° for the pristinegraphene sheet (Figure 4a, see Supporting Information fordetails). The contact angle increased to ∼84.0° after the end-adsorption of PEO−PS in toluene, followed by a furtherincrease up to 101.6° after replacement of toluene bycyclohexane. This distinct change of about 17° in the contactangle as a result of the solvent change can be attributed to thefact that the conformation transition altered the interaction ofPEO−PS surface with water. For PEO−PS chains in the brushconformation in toluene, the end-adsorbed PS chains swelledand stretched away from the surface, facilitating the contactwith water to become rather hydrophilic. Conversely, PS chainscollapsed on the graphene surface after the cyclohexanetreatment and became relatively hydrophobic.35,36 Figure 4ashows the reversible contact angle changes. Moreover, atomicforce microscopic (AFM) images collected in toluene andcyclohexane show an increase in the surface roughness from0.76 nm in toluene to 5.85 nm in cyclohexane (Figure S6),indicating, once again, the solvent-induced conformationaltransition37though the presence of some residual tail PSchains in the collapsed PS−PEO adsorbed layer cannot beruled out.23,24 Adhesion force between the PS−PEO function-alized graphene surface and the tip of force measurementsystem was also measured. As shown in Figure 4b, theexpanded structure after toluene treatment exhibited a muchstronger adhesion force compared to the collapsed PS−PEOsurface and the pristine graphene sheet as a control. This isbecause the polymer brush enhanced the contact area andhence the increased van der Waals forces for adhesion in the

same manner as the vertically aligned polymer fibers on geckotoes and vertically aligned carbon nanotubes.38−40

Our experimental results were complemented by dissipativeparticle dynamics (DPD) simulations.41 DPD is a particulatesimulation method that utilizes pairwise interactions betweencoarse-grained particles (each particle in DPD represents agroup of molecules) to write the equation of motion for eachcomponent. In order to mimic the main characteristics of thebrush, simulation parameters were set based on the solubilityand subsequently the Flory−Huggins interaction parameterbetween each component of the system.42 A detailedexplanation of simulation method and the parameters aregiven in the Supporting Information.Figure 4c provides the simulation results of distance from

surface changing with the polymer density, which clearlyrevealed the collapse−stretching conformation transition asshown in Figure 3a. Figure 4d exhibits the contact pointsbetween PEO−PS and graphene in the two solvents. Regardlessof the polymer density, more contact points can be observed incyclohexane (the poor solvent for PS) than toluene (goodsolvent). In cyclohexane, the distance from surface decreased toform a collapsed coil and increasing the contact area withsurface in accordance with the experimental results.According to the plasmonic scattering effect,43 larger-size

nanoparticles have a higher scattering efficiency; this explainedthe conductance change in FET devices caused by polymerconformational changes. Furthermore, it should be noted thatthe simulation results expected that the change of contactpoints increased in higher density. In order to demonstrate ourhypothesis that the change of scattering sites induced byconformational transition dominated the conductance changeof the transistor, we compared the relative current change indifferent polymer concentrations. The results shown in FigureS7 agree well with the simulation expectation. Whenconcentration increase from 0.002 to 0.02 g/mL, more polymer

Figure 4. (a) Cycles of water contact angle change in sequential solvent treatment on PEO−PS functionalized graphene sheet. Insets exhibit theimages of water droplets in the two states. (b) Adhesion force comparison between graphene and PEO−PS functionalized surface in sequentialsolvents treatments. (c) Relationship of distance from surface with density in PEO−PS system in toluene and cyclohexane. (d) Relationship ofcontact points and density of PEO−PS system from DPD simulation results for PEO−PS system functionalized on the graphene surface.

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molecules with high density absorbed on the graphene surface,which then caused more significant conductance change from15% to 27% between the two solvent changes.The solvent-induced reversible conductance switching

phenomenon discussed above is quite general, which can alsobe applied to the conformational transition of triblockcopolymer brushes. As demonstrated in our previouspublications, PEO−PS−PEO triblock copolymer chains withshort PEO sticking blocks could form polymer brushes with amixed population of loops and tails by end-anchoring the PSchains at the liquid/solid interface via either one or both PEOend-block(s) of a single polymer chain.23,24,44,45 For PEO−PS−PEO (128K, Table 1) with relatively long PEO sticking blocks,the “tail” is the predominant conformation in the polymerbrush formed in a good solvent (e.g., toluene), whereas PEO−PS−PEO (49K, Table 1) forms more loop conformationsunder the same condition due to the longer PEO end blocks,and hence the stronger interactions with the substrate surface,than those in PEO−PS−PEO (128K).23,24 Nevertheless, a tail-to-loop transition has been previously demonstrated for boththe PEO−PS−PEO (49K) and PEO−PS−PEO (128K) chainsupon the solvent change from toluene to cyclohexane.23,24 Inthis study, the solvent-induced reversible tail-to-loop conforma-tional transition (Figure 5a) was found to also lead to thereversible current switching (Figures 5b and 5c) similar to thatof the diblock PS−PEO brush (Figure 3c). However, thePEO−PS−PEO (49K) showed faster (the chain transitionkinetics is much faster than that experimentally measurable)and more pronounced conductance switching (Figure 5b) thanthat of the PEO−PS−PEO (128K) brush (Figure 5c) due tothe stronger affinity to the substrate surface associated with thelonger PEO segments in PEO−PS−PEO (49K) triblockcopolymer chains.46

■ CONCLUSIONS

In summary, we have developed a facile and reliable approachto reversibly switchable graphene transistors functionalized withpolymer brushes. Both experimental and stimulation resultsdemonstrated that the solvent-induced conformation transitionof the polymer brush could affect the carrier concentration bychanging the number of scattering sites associated with thegraphene−polymer contact areas, leading to a revisibleconductance switch for the graphene FET device. The newlydiscovered solvent-induced reversible conductance switchingphenomena is quite general, which was observed for both thestretching−collapse conformational transition associated withend-adsorbed diblock copolymer brush and tail-to-loopconformational transition of triblock copolymer brushes. Thefacile and versatile methodology developed can serve as apromising platform for developing novel electronic devices for alarge variety of potential applications, ranging from sensingthrough control release to electronic switches. Furthermore,this work also offers new technologies for the investigation ofmacromolecular conformations and conformational transitions.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.macro-mol.6b01011.

All the measurements, characterization, and controlexperiments; the method for dissipative particle dynam-ics simulation (PDF)

■ AUTHOR INFORMATION

Corresponding Author*(L.D.) E-mail liming.dai@case.edu.

Figure 5. (a) Schematic representation of the reversible conformational transition of the polymer brushes PEO−PS−PEO in good (toluene) andpoor (cyclohexane) solvent. (b, c) Electrical switching of the polymer in good (toluene) and poor (cyclohexane) solvent of the triblock polymerbrushes for PEO−PS−PEO (49K) and PEO−PS−PEO (128K). All current values were taken at VG = 0 V. All the measurements were carried out atVD = 1 mV in a liquid environment.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge the financial support from NSF(CMMI-1266295). This work was partially supported by theCreative Research Initiative (CRI, 2014R1A3A2069102)through National Research Foundation of Korea.

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