Article pubs.acs.org/Macromolecules
Conformational Transitions of Polymer Brushes for Reversibly Switching Graphene Transistors Song 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, Ohio 44106, United States ‡ School of Energy and Chemical Engineering/Center for Dimension-Controllable Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 50, UNIST, Ulsan 44919, South Korea S Supporting Information *
ABSTRACT: We developed a facile, but efficient, approach to graphene field-effect transistors (FET) functionalized with polymer brushes, in which the conductance can be reversibly switched by solvent-induced polymer conformational changes. Our experimental and stimulation results demonstrated that the solvent-induced conformational transition of the polymer brush could affect the carrier concentration by changing the number of scattering sites associated with the graphene−polymer contact areas, leading to reversible electrical switching for the graphene FET device. Both end-adsorbed diblock and triblock copolymers showed 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 electrical properties and for studying macromolecular conformations and conformational transitions.
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INTRODUCTION Responsive polymers can reversibly change their conformations/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 as active components in various smart devices for a wide range of applications,1−4 including sensors, drug carriers, actuators, and smart textiles, to mention a few. Polymer brushes comprise a major class of responsive polymers, in which polymer chains are tethered at one end to a solid substrate through either covalent attachment or physical adsorption.5 Polymer brushes have been widely demonstrated to change conformations in response to external stimuli, including changes in solvent, temperature, pH, ionic strength, light, and mechanical stress, leading to many smart functional devices with controllable switching properties.6−8 Graphene, on the other hand, possesses many excellent properties, 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, to biosensors.10−12 Of particular interest, field-effect transistors (FETs) based on functional graphene sheets hold great promise for electronic switching, sensing, and detecting due to their ultrahigh carrier mobility, along with the two-dimensional (2D) © XXXX American Chemical Society
structure ideal for the existing fabrication process compatible with the standard planar Si technology.13 Since the electronic properties of graphene are susceptible to its surface characteristics, various molecules, macromolecules, and nanoparticles, such as DNA, peptide, and quantum dots, have been used to modify the graphene surface to impart specific functions to FET devices based on graphene sheet(s).14−18 The 2D structure of graphene serves as an ideal substrate for the preparation of polymer brushes, while controllable polymerization methods have been devised to create new functional graphene (or graphene oxide) composites with polymer brushes.19−22 This prompted us to integrate responsive polymer brushes with graphene FETs, leading to highly sensitive devices for reversible sensing. We have previously used highly asymmetric polystyrene−poly(ethylene oxide) (PSm−PEOn, m and n are weight-averaged degree of polymerization for polystyrene and poly(ethylene oxide) blocks, respectively, and m ≫ n, Table 1) diblock copolymers to form polystyrene brushes through anchoring the nonadsorbing PS block, as a dangling chain, by the terminal PEO block at a liquid−solid interface (e.g., mica in toluene).23,24 By directly measuring the interactions of a single layer of the endadsorbed PS−PEO polymer brush against a bare mica surface, we have also demonstrated that PS−PEO chains can undergo Received: May 14, 2016 Revised: September 19, 2016
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DOI: 10.1021/acs.macromol.6b01011 Macromolecules XXXX, XXX, XXX−XXX
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about 1.06. Other reagents and solvents were purchased from Aldrich Chemical Inc. Device Fabrication. Briefly, graphene was synthesized through a chemical vapor deposition (CVD) process on copper foil (Alfa Aesar). After graphene growth, poly(methyl methacrylate) (PMMA) with 300 nm thickness was spin-coated on the graphene-deposited Cu films. The whole samples were then immersed into a freshly-prepared iron chloride saturated aqueous solution for about 1−2 h to remove the Cu substrate. The resultant transparent sheet floating in the aqueous solution was transferred to a heavily doped silicon substrate with a 400 nm layer of thermally grown oxide. Then the PMMA was removed in a boiling acetone solution. To aid in the alignment for the subsequent photolithographic process, gold marks were thermally evaporated through a shadow mask. In order to form the graphene devices, selective water plasma was applied through a photolithographically patterned resist mask to etch away the unprotected graphene. Finally, by using another photolithographic process, high-density patterned metallic electrodes (5 nm of Cr followed by 50 nm of Au) separated by 5 μm in the center (Figure S1) were deposited onto the graphene sheets through thermal evaporation. The doped silicon wafer served as a global back-gate electrode for these devices. Device Measurement. Polymer brush was attached on graphene surface by incubation the device in the polymer solution with concentration of 0.005 g/mL in toluene for about 20 h. After absorption, fresh toluene was used to wash the transistor to remove nonadsorbed polymer chain, if any. For the wet measurement, the solvent was dropped on the surface of the device to keep the liquid environment. To change the solvent, the device was immersed in the pure solvent. To test the transistor characteristics of these transistors, the I−V measurements were carried out at room temperature in the ambient atmosphere using a Keithley 2636A system SourceMeter connected with a Signatone 1160 series probe station. Characterization. X-ray photoelectron spectroscopic (XPS) measurements were carried out on a VG Microtech ESCA 2000 using a monochromic Al X-ray source (97.9 W, 93.0 eV). The UV−vis spectra were measured with a Jasco V-670 spectrometer. The Raman spectra were collected using a Raman spectrometer (Renishaw) with a 514 nm laser. Contact angles were measured with Future Digital Scientific Corp. After the treatment of toluene or cyclohexane, the samples were dried with N2 gas flow immediately and then tested the contact angles with water. Atomic force microscopic images were obtained by an Agilent 5500 AFM. Adhesion force was tested with a homemade force measurement system. The adhesion force was obtained from the force curve of interactions between the tip and PS− PEO functionalized graphene surface.
Table 1. Molecular Characteristics of the Copolymer Samples Used in This Study sample
10−3MW
MW/MN
wt % PS
xl
y
x2
PS−PEO (150K) PEO−PS−PEO (128K) PEO−PS−PEO (49K)
150 128
1.16 1.02
98.5 99.7
0 5
1420 1225
51 5
49
1.09
92.4
42
435
42
a
x1, y, and x2 refer to the polymerization index of the block copolymer: (PEO)x1(PS)y(PEO)x2.
fast and reversible stretching−collapse conformational transition on mica surface by simply changing the solvent from toluene (good solvent) to cyclohexane (poor solvent, Tθ = 34 °C), or vice versa (Figure 1).23,24
Figure 1. Schematic representation of the reversible conformational transition of polymer brush PEO−PS functionalized graphene transistor induced by sequentially solvent change.
In this study, we develop a facile, but efficient, approach to graphene FET transistors with terminally anchored PS−PEO polymer brushes, in which the conductance can be reversibly switched by the solvent-induced conformational changes (Figure 1). As we shall see later, our experimental and stimulation results demonstrate that the solvent-induced conformational transition of the polymer brush can affect the carrier concentration by changing the number of scattering sites associated with the graphene−polymer contact areas (Figure 1) and hence conductivity of the graphene surface in the FET device. By changing the solvents to cause the reversible collapse−stretching conformation transition of the endadsorbed diblock copolymer chains on the graphene surface (Figure 1), reversible electrical switching was demonstrated. Moreover, loop−tail conformational changes were also observed when end-adsorbed PEO−PS−PEO triblock copolymer chains (Table 1) were used. These results indicate that the newly developed graphene FET transistors functionalized with polymer brushes can provide a novel pathway toward stimuliresponsive graphene electronics with tunable electrical properties for specific applications.
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RESULTS AND DISCUSSION Transistors based on graphene sheets generated by chemical vapor deposition (CVD) were fabricated using a nondestructive method involving polymer-mediated transfer, standard multistep photolithography, and etching.25 Judicious application of this method allows for mass production of high quality graphene transistors with a high yield. Figure S1 shows an optical image of a graphene transistor thus prepared, in which graphene 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 voltage from the silicon substrate as the global back-gate. Prior to test, the graphene FET was subjected to self-assembling of a polymer brush in solution (toluene, 0.005 g/mL), using procedures similar to those reported previously.26,27 In this study, three different copolymers (Table 1) were used to modify the graphene surface for demonstrating the effects of molecular structure, weight, and composition on the graphene FET performance. In a typical experiment, end-adsorbed PS−PEO (150K) diblock chains were terminally anchored onto the graphene
EXPERIMENTAL SECTION
Materials. The PS−PEO and PEO−PS−PEO copolymers were synthesized by Polymer Laboratories (U.K.). The weight-average molecular weight and molecular weight distribution of the samples were determined by gel permeation chromatography (GPC) measurements, while the PEO content was measured by 1HNMR and checked by infrared spectroscopy and elemental analysis. Analytical grade toluene and cyclohexane used in this study were purchased from Aldrich Chemical Inc. PS with a weight-average molecular weight of 350 000 was ordered from Sigma-Aldrich, and PEO with a weightaverage molecular weight of 400 000 was purchased from Scientific Polymer Products, Inc. Both have a molecular weight polydispersity of B
DOI: 10.1021/acs.macromol.6b01011 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. Characterization of PEO−PS functionalized graphene transistors. (a) Raman spectra of an individual graphene devices before (black) and after (red) assembly of PEO−PS diblock copolymer molecules. (b) Transmittance change before (black) and after (red) PEO−PS was functionalized 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.
PS onto the graphene surface. These results from the dried surface characterization suggest that the PS−PEO polymer chains have been successfully attached on the graphene surface. However, the possible formation of some PS−PEO micelles via the PEO segment aggregation in toluene followed by directly attaching the micelle to the graphene surface through π−π interaction 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, we performed electrical measurements in solvents to demonstrate the effects of polymer brush conformation transition (Figure 3a) on the FET performance. As discussed above, we have previously reported the reversible stretching−collapse conformational transition for PS−PEO polymer brushes on mica surfaces 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-drain bias voltage (VD) for a representative device under various conditions. As can be seen, we obtained very stable I−V curves at fixed experimental conditions, suggesting a reliable procedure has been established. A decrease in conductance was observed upon the end-adsorption of PEO−PS diblock copolymer chains from toluene onto the graphene surface (red curve, Figure 3b), due most probably to the scattering effect on charge carriers caused by physical coverage of the graphene surface with PEO−PS chains.31−33 We noted that the polymer-adsorptioninduced decrease in conductance of graphene was quite general, as also confirmed by a control experiment in which homopolymer chains of PEO were absorbed on graphene from either cyclohexane (or toluene) or PS from cyclohexane (Figures S2 and S3).
surface to switch the graphene FET devices. To form the graphene-supported polymer brush, a graphene FET was immersed into a solution of PEO−PS (150K) in toluene (∼0.005 g/mL) for about 20 h. After the PS dangling chains were anchored onto the graphene surface by the terminal PEO to form the polymer brush,23,24,26,27 the FET device was removed from the polymer solution and washed with fresh toluene for subsequent surface characterization. A typical Raman spectrum of graphene before and after being assembled with the PS−PEO polymer brush is given in Figure 2a, which shows a 2D peak (2690 cm−1), G band (1580 cm−1), and D band (1350 cm−1) characteristic of graphene. The large G/2D peak ratio (IG/I2D > 1/4) indicates a multilayered graphene with a high graphitization degree, as evidenced by the low D/G peak ratio (ID/IG = 0.09).28 The increased ID/IG ratio from 0.09 to 0.24 after functionalization with the PS−PEO polymer brush is due to the increased sp3 carbon atom fraction on graphene surface associated with the physically adsorbed PS−PEO chains. The presence of a PS−PEO adsorbed layer on the graphene substrate was further confirmed by the decrease in optical transmittance of graphene from about 94.5% to 90% at 550 nm after adsorption of the PS−PEO chains (Figure 2b).29,30 To investigate the surface chemistry, we further performed X-ray photoelectron spectroscopy (XPS) measurements. Figures 2c and 2d show the high-resolution XPS C 1s peak for 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 new peak C−O (286.1 eV), attributable to the absorption of PEO− C
DOI: 10.1021/acs.macromol.6b01011 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. (a) Schematic representation of the conformation transition of PEO−PS in toluene and cyclohexane (H: brush height; Rg: radius of gyration). (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 of the 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 liquid environment.
concentration.34 Furthermore, Figure 3b shows no obvious Dirac point shift, and thus charge-transfer-induced doping of graphene could be eliminated. So, the conductance switching of our transistor was mainly from the carrier concentration change in graphene. This is because the absorbed polymer chains could serve as scattering sites to quench the charge carriers (hole or electron), leading to a reduced carrier concentration. By replacing toluene with cyclohexane, the PS brush collapsed to form a more compact polymer layer at the graphene surface to enhance the surface coverage on graphene. The increased polymer−graphene contact scattered more the charge carriers and hence the decreased conductance of the device. Once cyclohexane was changed back to toluene, the collapsed PS chains returned to the stretched brush conformation through a fast dynamic with a concomitant increase in the conductance, leading to the reversible switching of the graphene FET associated 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, current responses of the graphene FET virtually returned to the original state characteristic of the pristine graphene device with almost no solvent response (Figure 3d and Figure S4). No solvent response was observed for either the pristine graphene FET without adsorbed polymer (Figure S5) or the graphene FETs with adsorbed homopolymer chains (i.e., Figure S2 for PEO and Figure S3 for PS). Clearly, therefore, the observed reversible current switching effect can be exclusively attributed to the absorbed PS−PEO polymer brush.
As seen in Figures S2 and S3, however, changes of the solvent between toluene and cyclohexane did not cause any conductance change for the graphene FETs adsorbed with either PEO or PS homopolymers. In contrast, reversible and significant changes in ID were observed when the PS−PEO adsorbed device was subject to sequential solvent changes between toluene and cyclohexane (Figures 3b and 3c). Specifically, a significantly decreased ID was observed over the entire range of gate bias covered in this study when toluene was replaced by cyclohexane (blue curve in Figure 3b). When the solvent was changed from cyclohexane back to toluene, it was interesting to note that ID of the same device returned to almost its original value at any of the gate bias, as shown by the green curve in Figure 3b. The observed solvent-induced current switching is highly reversible and reproducible, as exemplified by 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 conformation transition.23,24 As mentioned earlier, the solvent-induced reversible stretching−collapse conformation transition caused a concomitant reversible change in charge carrier concentration by reversibly changing the number of scattering sites associated with the graphene−polymer contact areas31−33 and hence the observed reversible current change of the graphene FET under a constant gate bias (Figures 3b and 3c). As shown in Figure 3b, the slopes of the I−V curves were essentially constant, which reflected negligible change in transistor mobility. This is because conductivity is defined by G = neμ, where e is the electron charge, μ is the mobility, and n stands for the carrier D
DOI: 10.1021/acs.macromol.6b01011 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a) Cycles of water contact angle change in sequential solvent treatment on PEO−PS functionalized graphene sheet. Insets exhibit the images of water droplets in the two states. (b) Adhesion force comparison between graphene and PEO−PS functionalized surface in sequential solvents treatments. (c) Relationship of distance from surface with density in PEO−PS system in toluene and cyclohexane. (d) Relationship of contact points and density of PEO−PS system from DPD simulation results for PEO−PS system functionalized on the graphene surface.
same manner as the vertically aligned polymer fibers on gecko toes and vertically aligned carbon nanotubes.38−40 Our experimental results were complemented by dissipative particle dynamics (DPD) simulations.41 DPD is a particulate simulation method that utilizes pairwise interactions between coarse-grained particles (each particle in DPD represents a group of molecules) to write the equation of motion for each component. In order to mimic the main characteristics of the brush, simulation parameters were set based on the solubility and subsequently the Flory−Huggins interaction parameter between each component of the system.42 A detailed explanation of simulation method and the parameters are given in the Supporting Information. Figure 4c provides the simulation results of distance from surface changing with the polymer density, which clearly revealed the collapse−stretching conformation transition as shown in Figure 3a. Figure 4d exhibits the contact points between PEO−PS and graphene in the two solvents. Regardless of the polymer density, more contact points can be observed in cyclohexane (the poor solvent for PS) than toluene (good solvent). In cyclohexane, the distance from surface decreased to form a collapsed coil and increasing the contact area with surface in accordance with the experimental results. According to the plasmonic scattering effect,43 larger-size nanoparticles have a higher scattering efficiency; this explained the conductance change in FET devices caused by polymer conformational changes. Furthermore, it should be noted that the simulation results expected that the change of contact points increased in higher density. In order to demonstrate our hypothesis that the change of scattering sites induced by conformational transition dominated the conductance change of the transistor, we compared the relative current change in different polymer concentrations. The results shown in Figure S7 agree well with the simulation expectation. When concentration increase from 0.002 to 0.02 g/mL, more polymer
To gain a better understanding of the end-adsorbed polymer conformation transition as a response to solvent change and the associated electrical switching for the graphene FETs functionalized with polymer brushes, we further performed computer simulations, along with relevant experiments. First, we measured the water contact angle to be ∼72.0° for the pristine graphene sheet (Figure 4a, see Supporting Information for details). The contact angle increased to ∼84.0° after the endadsorption of PEO−PS in toluene, followed by a further increase up to 101.6° after replacement of toluene by cyclohexane. This distinct change of about 17° in the contact angle as a result of the solvent change can be attributed to the fact that the conformation transition altered the interaction of PEO−PS surface with water. For PEO−PS chains in the brush conformation in toluene, the end-adsorbed PS chains swelled and stretched away from the surface, facilitating the contact with water to become rather hydrophilic. Conversely, PS chains collapsed on the graphene surface after the cyclohexane treatment and became relatively hydrophobic.35,36 Figure 4a shows the reversible contact angle changes. Moreover, atomic force microscopic (AFM) images collected in toluene and cyclohexane show an increase in the surface roughness from 0.76 nm in toluene to 5.85 nm in cyclohexane (Figure S6), indicating, once again, the solvent-induced conformational transition37though the presence of some residual tail PS chains in the collapsed PS−PEO adsorbed layer cannot be ruled out.23,24 Adhesion force between the PS−PEO functionalized graphene surface and the tip of force measurement system was also measured. As shown in Figure 4b, the expanded structure after toluene treatment exhibited a much stronger adhesion force compared to the collapsed PS−PEO surface and the pristine graphene sheet as a control. This is because the polymer brush enhanced the contact area and hence the increased van der Waals forces for adhesion in the E
DOI: 10.1021/acs.macromol.6b01011 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. (a) Schematic representation of the reversible conformational transition of the polymer brushes PEO−PS−PEO in good (toluene) and poor (cyclohexane) solvent. (b, c) Electrical switching of the polymer in good (toluene) and poor (cyclohexane) solvent of the triblock polymer brushes 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 at VD = 1 mV in a liquid environment.
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CONCLUSIONS In summary, we have developed a facile and reliable approach to reversibly switchable graphene transistors functionalized with polymer brushes. Both experimental and stimulation results demonstrated that the solvent-induced conformation transition of the polymer brush could affect the carrier concentration by changing the number of scattering sites associated with the graphene−polymer contact areas, leading to a revisible conductance switch for the graphene FET device. The newly discovered solvent-induced reversible conductance switching phenomena is quite general, which was observed for both the stretching−collapse conformational transition associated with end-adsorbed diblock copolymer brush and tail-to-loop conformational transition of triblock copolymer brushes. The facile and versatile methodology developed can serve as a promising platform for developing novel electronic devices for a large variety of potential applications, ranging from sensing through control release to electronic switches. Furthermore, this work also offers new technologies for the investigation of macromolecular conformations and conformational transitions.
molecules with high density absorbed on the graphene surface, which then caused more significant conductance change from 15% to 27% between the two solvent changes. The solvent-induced reversible conductance switching phenomenon discussed above is quite general, which can also be applied to the conformational transition of triblock copolymer brushes. As demonstrated in our previous publications, PEO−PS−PEO triblock copolymer chains with short PEO sticking blocks could form polymer brushes with a mixed population of loops and tails by end-anchoring the PS chains at the liquid/solid interface via either one or both PEO end-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 polymer brush formed in a good solvent (e.g., toluene), whereas PEO− PS−PEO (49K, Table 1) forms more loop conformations under 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 tailto-loop transition has been previously demonstrated for both the PEO−PS−PEO (49K) and PEO−PS−PEO (128K) chains upon the solvent change from toluene to cyclohexane.23,24 In this study, the solvent-induced reversible tail-to-loop conformational transition (Figure 5a) was found to also lead to the reversible current switching (Figures 5b and 5c) similar to that of the diblock PS−PEO brush (Figure 3c). However, the PEO−PS−PEO (49K) showed faster (the chain transition kinetics is much faster than that experimentally measurable) and more pronounced conductance switching (Figure 5b) than that of the PEO−PS−PEO (128K) brush (Figure 5c) due to the stronger affinity to the substrate surface associated with the longer PEO segments in PEO−PS−PEO (49K) triblock copolymer chains.46
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01011.
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All the measurements, characterization, and control experiments; the method for dissipative particle dynamics simulation (PDF)
AUTHOR INFORMATION
Corresponding Author
*(L.D.) E-mail
[email protected]. F
DOI: 10.1021/acs.macromol.6b01011 Macromolecules XXXX, XXX, XXX−XXX
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support from NSF (CMMI-1266295). This work was partially supported by the Creative Research Initiative (CRI, 2014R1A3A2069102) through National Research Foundation of Korea.
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