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Reversible and Irreversible Responses of Defect-Engineered Graphene-Based Electrolyte-Gated pH Sensors Sun Sang Kwon,† Jaeseok Yi,† Won Woo Lee,† Jae Hyeok Shin,† Su Han Kim,† Seunghee H. Cho,† SungWoo Nam,‡ and Won Il Park*,† †
Division of Materials Science and Engineering, Hanyang University, Seoul 04763, Korea Department of Mechanical Science and Engineering, University of Illinois, Urbana−Champaign, Urbana, Illinois 61801, United States
‡
S Supporting Information *
ABSTRACT: We have studied the role of defects in electrolytegated graphene mesh (GM) field-effect transistors (FETs) by introducing engineered edge defects in graphene (Gr) channels. Compared with Gr-FETs, GM-FETs were characterized as having large increments of Dirac point shift (∼30−100 mV/pH) that even sometimes exceeded the Nernst limit (59 mV/pH) by means of electrostatic gating of H+ ions. This feature was attributed to the defect-mediated chemisorptions of H+ ions to the graphene edge, as supported by Raman measurements and observed cycling characteristics of the GM FETs. Although the H+ ion binding to the defects increased the device response to pH change, this binding was found to be irreversible. However, the irreversible component showed relatively fast decay, almost disappearing after 5 cycles of exposure to solutions of decreasing pH value from 8.25 to 6.55. Similar behavior could be found in the Gr-FET, but the irreversible component of the response was much smaller. Finally, after complete passivation of the defects, both Gr-FETs and GM-FETs exhibited only reversible response to pH change, with similar magnitude in the range of 6−8 mV/ pH. KEYWORDS: graphene, graphene mesh, electrolyte-gated field effect transistor, pH sensor, nanosensor, defect-mediated chemisorption, defect passivation
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INTRODUCTION Graphene (Gr), a single-layer material of sp2-hybridized C atoms in a honeycomb lattice, is attractive for use in sensors because its charge carrier transport is sensitively affected by the surrounding chemical and biological environment.1−11 In particular, Gr has excellent electrical conductivity and mobility12−15 and a low level of 1/f noise,1,3 which, combined with its two-dimensional (2D) nature, enables the electrical detection of chemical and biological species at the singlemolecule level of sensitivity. Because pristine Gr is a semimetal with a zero band gap,16 Gr field-effect transistors (Gr-FETs) exhibit unique ambipolar conduction characteristics. In source−drain current versus gate voltage (I−Vg) transfer characteristics of Gr-FETs, a conductance minimum is observed at the so-called Dirac point, and the corresponding voltage is termed the Dirac point voltage (VDir); at Vg lower than VDir, holes become the major carrier type and channel current decreases with increasing Vg, whereas at Vg higher than VDir, the major carriers are electrons, and accumulate with increasing Vg. As a result, the Gr-FET responds to external chemical and biological species in such a way that the adsorption and desorption of charged molecules on the Gr surface causes the Dirac point to shift. Generally, the © XXXX American Chemical Society
sensitivity of Gr-FET sensors is evaluated by the horizontal shift of the Dirac point, i.e., the change in VDir (ΔVDir) in response to chemical stimuli (changes in ion/molecular concentration, pH, etc.).17 However, previous experimental results on Dirac point shift in Gr-FET sensors have varied from case to case, and its mechanism is still controversial. For example, some research groups have reported relatively small shift of the Dirac point in response to changes in acidity, of less than ∼10−20 mV/ pH,18,19 whereas others have shown much larger shift, close to 100 mV/pH.1 Poorer sensitivity is often reported for sensors made with high-quality Gr, such as that prepared by mechanical exfoliation from highly ordered pyrolytic graphite.20,21 Such Gr sheets are considered to be perfect surfaces with fully saturated C atoms that are chemically inert to ionic molecules. Accordingly, it has been suggested that change in VDir is mediated by physisorbed species through the so-called “electrostatic gating” effect. Larger ΔVDir has generally been reported for Gr with higher defect concentration (e.g., chemical Received: October 24, 2015 Accepted: December 21, 2015
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DOI: 10.1021/acsami.5b10183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces vapor deposition (CVD) synthesized Gr tends to be more sensitive than mechanically exfoliated Gr and plasma-treated Gr tends to be more sensitive than untreated Gr).18,19,22,23 In some cases, the sensitivity of VDir to pH change even exceeds the Nernst value of 59 mV/pH, which is the maximum limit of the gating effect by H+ ions in aqueous solution.1 In addition to changes in the VDir, vertical shifting of the I− Vg curve (and thereby Dirac Point) is commonly found in GrFETs, which is not associated with the gating effect. To explain these discrepancies, a “defect doping” effect has been proposed that involves direct charge carrier transfer between the adsorbed ions and graphene.24−26 According to this proposal, ionic species interact with unsaturated C atoms at defects, especially by means of strong chemisorption. However, the influence of defects upon sensing characteristics and the sensing mechanism remains largely unclear. Moreover, reversibility of the interactive reaction between the ionic species and the defects is unresolved, despite its importance for sensing applications. To engineer defects and elucidate the related sensing characteristics, Gr nanoribbons have been fabricated by conventional lithography and plasma etching processes,27−30 and enhanced pH response of the sensors has been demonstrated. 19 However, the edges formed by these approaches are readily terminated with disordered C atoms and contaminated by residual chemicals and polymers, causing them to lose their intrinsic characteristics.31 Herein, to minimize the side effects associated with Gr patterning, we have introduced the direct synthesis of Gr mesh (GM) structure by using silica as a growth mask.32 This approach minimizes exposure to contamination and reactive plasma, enabling the edges to be more energetically reactive than those fabricated by top-down patterning processes. The resulting GMs have nearly ideal edges consisting of 2-fold coordinated C atoms, and thus can be exploited as a platform to study the intrinsic response of defects to ionic molecules. We found that strong Dirac point shifts occur in GM-FETs owing to the chemisorption of H+ ions to the edges, and this reaction is irreversible and thus undesirable for sensing. However, we further demonstrated that the irreversible component (i.e., Dirac point shift by the chemisorption of H+ ions to the edge defects) lessens gradually with repeated exposure to acidic solutions and finally becomes negligible, with only the reversible response remaining, making the resulting sensor suitable for multiple-cycle operations.
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Figure 1. (a, b) (top) Schematics and (bottom) SEM images of (a) GM and (b) Gr. (c, d) Cross-sectional and schematic 3D views of the electrolyte-gated Gr-based FET sensor, respectively. (e) Experimental setup. (f) Typical I−Vg curves of GM-FET against the back-gate and electrolyte-gate voltage. an acetone solution for 2 min to remove the PMMA layer. The overall dimensions of the Gr samples were ∼12 × 3 mm2 (length × width). Both ends of these samples were deposited with metal electrodes, followed by vacuum annealing at 300 °C for 30 min. To characterize the pH response of the sensors, a microfluidic delivery devices were fabricated from polydimethylsiloxane (PDMS) following a previously reported procedure,33 and were then integrated with the Gr-FETs and GM-FETs (Figure 1c−e). The outer dimension of the PDMS devices was ∼15 × 5 × 10 mm3 (length × width × height), and the inner dimension of the fluidic channels was 6 × 0.5 × 0.05 mm3 (length × width × height). The microfluidic delivery device was carefully placed and clamped such that the inner microfluidic channel overlapped the central region of the surface of the Gr or GM channel, and the channel and the fluidic channel crossed at right angles. A silver/silver chloride (Ag/AgCl) wire was also inserted into the microfluidic channel as a reference gate electrode, and was used to measure the sensors’ transfer characteristics. The sensor responses under flows of pH buffer solutions at the typical rate of 0.1 mL/min were monitored using either a semiconductor parameter analyzer (HP4145) or a custommade I−V measurement system consisting of a picoammeter (Keithley 6485) equipped with data acquisition hardware (PCI 6221 card and BNC-2090a). pH solutions were prepared using 10 mM phosphatebuffered saline (PBS) as a buffer solution and a small amount of hydrogen chloride (HCl)/potassium hydroxide (KOH) solution to adjust the pH within the range of ∼6.55−8.25.
EXPERIMENTAL SECTION
Preparation of Gr and GM Samples. Gr and GM were synthesized on a 25 μm-thick catalytic Cu foil (99.999%, Alfa Aesar) by CVD, following previously published procedures. Bare Cu foil was used to form mostly single-layer Gr, and Cu foil covered with a hexagonally close-packed monolayer of silica spheres was used to directly grow GM by dissociation of C atoms at the Cu−silica interface (Figure 1a,b). After GM growth, remaining silica spheres were selectively removed by etching with a 10% aqueous solution of hydrofluoric acid for 5 min. The resulting GM had a hexagonal array of circular holes, with average diameter and interspacing of ∼300 and ∼600 nm, respectively. Fabrication of Gr-FET and GM-FET Sensors. As-grown Gr and GM were each coated with a poly(methyl methacrylate) (PMMA) protecting layer and were then immersed in a Cu etchant (0.05 M ammonium persulfate solution) to remove the Cu foil. The floating PMMA/Gr samples were transferred to ∼150 × 150 mm2 pieces of thermally oxidized Si substrate, and these substrates were immersed in
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RESULTS AND DISCUSSION pH Response of Graphene-Based FETs Sensors. Figure 2a,b shows typical source−drain current versus gate voltage (I− Vwg) curves for the electrolyte-gated Gr-FETs and GM-FETs, measured at the source−drain voltage (Vsd) of 50 mV using a reference gate electrode (Ag/AgCl wire) in buffer solutions (10 mM) of various pH. Compared with typical back-gate FETs, the electrolyte-gated FETs show much more sensitive B
DOI: 10.1021/acsami.5b10183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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The large shifts of VDir and IDir in the GM-FETs, and, in particular, the fact that the magnitude of ΔVDir exceeded the theoretical Nernst limit, suggest that this case cannot be explained solely by the electrostatic gating effect. Compared with Gr, GM is characterized to have an increased number of unsaturated C atoms at edge defects. This suggests that H+ ions attached to the defect sites might be associated with the increased Dirac point shifts observed in the GM-FETs. Raman Spectra of Graphene Meshes. To study the interaction between H+ ions and GM edges, Raman analysis was carried out on three types of GM representing three sequential stages of fabrication: (i) as-prepared GM, (ii) GM treated in an acidic solution (pH 6.55) followed by rinsing with deionized (DI) water and then with methanol, and (iii) GM further subjected to vacuum annealing (Figure 3a). Two
Figure 2. Channel current versus gate voltage (I−Vwg) curves for electrolyte-gated FETs: (a, b) I−Vwg curves for FETs operated in buffer solutions of various pH at Vds = 50 mV: (a) Gr-FET, (b) GMFET. The buffers were applied in the order of the highest pH (8.25) to the lowest (6.55). (c) VDir position versus pH for Gr (black) and GM (red), including a linear fit to the data points (solid line). (d) IDir versus pH for Gr (black) and GM (red).
responses of Isd to Vwg (Figure 1f). This feature is associated with the extremely thin electric double layer developed at the Gr−electrolyte interface. The double layer thickness (or Debye length) is reciprocally proportional to the square root of ion concentration, and is typically a few nanometers or less in aqueous ionic solutions.34 Both Gr-FETs and GM-FETs exhibited nearly symmetrical ambipolar behaviors, with p-type characteristics at near zero gate bias, although VDir was more positive for the GM. The greater sensitivity in GMs is attributed to dangling bonds or adsorbed molecules such as oxygen species at edge sites. Negative Dirac point shifts with decreasing pH are observed in Gr-FETs and GM-FETs. When the pH decreased from 8.25 to 6.55, the VDir of the Gr-FET shifted from 0.22 to 0.19 V, corresponding to a sensitivity of ∼16.2 mV/pH. In contrast, the GM-FET sensitivities were significantly higher, at ∼89.7 mV/ pH. pH-dependent I−Vwg curves were measured for other sensors, showing similar behavior (see Figure S1). The GMFETs were on average ∼3 times more sensitive than the normal Gr-FETs. We note also that the pH sensitivity of the GM-FETs exceeded the maximum limit predicted by the electrostatic gating effect. According to the Nernst limit (59 mV/pH), device sensitivity cannot exceed this limit assuming that Dirac point shifts are caused by the physisorption of H+ ions on the Gr surface. In addition to larger ΔVDir, GM-FETs also undergo greater changes in the minimum current at the Dirac point (IDir); that is to say, the Dirac point shifts larger amount vertically with varying pH. The GM-FETs exhibited such an IDir shift more than 3 times than that of the Gr-FETs (Figure 2 and Figure S1). Moreover, in the GM-FETs, the IDir shifts were mostly positive with decreasing pH, whereas the shifts in Gr-FETs were scattered in shifting directions and were of nearly negligible magnitude, typically less than 2%. Similar phenomena have been found in Gr-FETs in aqueous solutions or under H2 atmosphere,35 and competition between residual carriers and impurities has been suggested to explain shifts in the Dirac point in either direction.36
Figure 3. Representative Raman spectra of three types of (a) GM, and (b) Gr: as-prepared (black), treated in an acidic solution followed by rinsing with DI water and methanol (red), and further subjected to vacuum annealing at 573 K (blue).
distinct Raman peaks, characterized as a G peak near 1590 cm−1 and a 2D peak near 2700 cm−1, were observed for all three samples. The intensity ratio of the 2D and G peaks (I2D/IG) of 1.4 and the single Gaussian-shaped 2D peak indicated that the GMs were mostly single- or double-layered films. In addition to the G and 2D peaks, a weak D peak was observed near 1350 cm−1; it was ascribed to the presence of disorders and defects (edges, dislocations, and/or vacancies) in the Gr lattices. Importantly, the D peak of the GM exposed to the acidic solution was 2 times as intense as that of the as-prepared GM. The fact that this change in the D peak was observed after rinsing with water and then with organic solutions indicates that the H+ ions attached strongly to the Gr surface. Moreover, the binding of the H+ ions to the GM also bathochromically shifted the G peak; this was ascribed to in-plane elongation of C−C bonds.37 Considered together, these observations suggest that the bonding between the H+ ions and Gr was much stronger than physisorption; it was most likely covalent bonding, namely, chemisorption to C atoms at the defect sites. This theory also explains the observed ΔVDir in excess of the Nernst limit. Kim et al. also studied the chemisorption of dissociated H2 molecules on Gr under a high-pressure H2 atmosphere at elevated temperature, and observed the anomalous evolution of the Raman spectra.35 However, in the present work chemisorption occurred at much lower pressure and high temperature. This difference arose from the lower activation barrier for the dissociation of H2O in aqueous solution than that for the dissociation of H2 in the vapor phase. After vacuum annealing at 300 °C, the D and G peaks in the Raman spectrum were restored to original states. This indicates that once ions adsorb to the edge-defects, they bind so strongly as only to be detached after high temperature annealing.38 In C
DOI: 10.1021/acsami.5b10183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 4. Cyclic I−Vwg characteristics of the GM-FET and Gr-FET: (a, b) I−Vwg characteristic curves of the GM-FET in different pH buffer solutions from 8.25 to 6.55 pH, recorded during the (a) first cycle and (b) fifth cycle; (c) corresponding plot of VDir versus pH during 5 cycles. (d, e) I−Vwg characteristic curves of the Gr-FET during the (d) first and (e) fifth cycle; (f) corresponding plot of VDir versus pH during 5 cycles. After the fifth cycle, no significant change was noticed.
mV). This reflects the fact that intrinsic defects affect the pH response much more weakly than the extrinsic edge defects do. After the stabilization of the Gr-FET, however, the remaining reversible component of the VDir shift was measured to be ∼10 mV (pH sensitivity of ∼7.0 mV/pH), quite close to that of the GM-FET. Overall, our results suggest that both intrinsic and extrinsic defects of the Gr provide strong interaction sites for H+ ions, presumably involving covalent bonding. Despite its considerable influence on the Dirac point shift, this defectmediated reaction is irreversible and thus not desirable for sensing applications (Figure S3a). However, our cyclic tests also demonstrated that the chemisorption reaction was relatively fast and could be easily inactivated by exposure to acidic aqueous solution. Accordingly, when the defects were completely passivated with chemisorbed ions, the Gr-based FET sensors dominantly exhibited the reversible response associated with the electrostatic gating effect on a pristine graphene surface (Figure S3b). As confirmed with Raman spectra, the strong H+ ion bonding to defect sites could be broken during thermal annealing. When the GM-FET after H+ ion passivation was thermally annealed at 300 °C, it reverted back to the original state and exhibited again the considerable irreversible response in cyclic test (Figure S4).
the case of graphene, very weak D peak was observed for the asgrown sample and also no significant change was noticed after acidic treatment and thermal annealing (Figure 3b). Cycling Behaviors of Graphene-Based FETs Sensors. Despite the large contribution of the chemisorbed H+ ions to the Dirac point shift, any reactions causing such a shift should be reversible for practical use of the Gr-based FET sensors. Accordingly, we conducted cyclic tests in which the FET sensors were repeatedly immersed in aqueous solutions of various pH. Specifically, solutions of pH 8.25, 7.85, 7.40, 6.95, and 6.55 were sequentially introduced for 3 min each, and the I−Vwg characteristics were measured during the middle of each of these segments (Figure 4a−c and Figure S2a). During the first cycle of the GM-FET, a large translation of the I−Vwg curve was observed, with a VDir shift from 0.36 to 0.27 V, similar to the case of Figure 2b. When the pH was restored to the initial value of 8.25, however, the I−Vwg curve did not return to the previous curve, and VDir remained relatively unchanged. In the second cycle, VDir started to shift from 0.27 V, but by only 30 mV, one-third the shift observed during the first cycle. In addition, the IDir shift became almost negligible from the second cycle. The sensitivity of VDir to pH changes decreased gradually with cycling, and after 5 cycles, the ΔVDir converged to 10 mV for each cycle (Figure 4c and Figure S2a). In addition, the change of the I−Vwg characteristic curves and the corresponding Dirac point shift with pH became reversible, with the resulting pH sensitivity of ∼7.0 mV/pH (Figure 4c and Figure S4d−f). The cycling characteristics of the Gr-FETs were consistent with those of the GM-FETs insofar as the irreversible response was concerned during initial cycles but gradually decreased with continued cycling, and only the reversible component remained after 5 cycles (Figure 4d−f and Figure S2b). The magnitude of the irreversible components of the Gr-FET was ∼30 mV, based on the change in VDir observed during the initial cycle (Figure 4d), considerably smaller than that of the GM-FETs (∼90
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CONCLUSIONS Our ability to engineer the defects in Gr enabled us to study the effect of the defects upon the response of Gr-based electrolytegated FET sensors. Our study illustrated that defects consisting of unsaturated C atoms provide strong adsorption sites for H+ ions, which significantly increases the Dirac point shift, but the irreversible nature of this adsorption would limit its application to multiple-cycle sensor operations. Fortunately, we also found that the defect sites are easily inactived after binding with H+ ions, as only the reversible component can be dominant in the sensitivity of the GM-FET sensors. Comparison between GrFETs and GM-FETs showed that the intrinsic and extrinsic D
DOI: 10.1021/acsami.5b10183 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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(8) Sudibya, H. G.; He, Q. Y.; Zhang, H.; Chen, P. Electrical Detection of Metal Ions Using Field-Effect Transistors Based on Micropatterned Reduced Graphene Oxide Films. ACS Nano 2011, 5 (3), 1990−1994. (9) An, J. H.; Park, S. J.; Kwon, O. S.; Bae, J.; Jang, J. HighPerformance Flexible Graphene Aptasensor for Mercury Detection in Mussels. ACS Nano 2013, 7 (12), 10563−10571. (10) Zhang, S.; Zhang, D.; Sysoev, V. I.; Sedelnikova, O. V.; Asanov, I. P.; Katkov, M. V.; Song, H.; Okotrub, A. V.; Bulusheva, L. G.; Chen, X. Wrinkled Reduced Graphene Oxide Nanosheets for Highly Sensitive and Easy Recoverable Nh3 Gas Detector. RSC Adv. 2014, 4 (87), 46930−46933. (11) Han, T. H.; Huang, Y. K.; Tan, A. T. L.; Dravid, V. P.; Huang, J. X. Steam Etched Porous Graphene Oxide Network for Chemical Sensing. J. Am. Chem. Soc. 2011, 133 (39), 15264−15267. (12) Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3 (8), 491−495. (13) Lee, D. H.; Yi, J.; Lee, J. M.; Lee, S. J.; Doh, Y. J.; Jeong, H. Y.; Lee, Z.; Paik, U.; Rogers, J. A.; Park, W. I. Engineering Electronic Properties of Graphene by Coupling with Si-Rich, Two-Dimensional Islands. ACS Nano 2013, 7 (1), 301−307. (14) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.; Stormer, H. L. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146 (9−10), 351− 355. (15) Rana, K.; Singh, J.; Ahn, J. H. A Graphene-Based Transparent Electrode for Use in Flexible Optoelectronic Devices. J. Mater. Chem. C 2014, 2 (15), 2646−2656. (16) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Electronic Confinement and Coherence in Patterned Epitaxial Graphene. Science 2006, 312 (5777), 1191−1196. (17) Shao, Y. Y.; Wang, J.; Wu, H.; Liu, J.; Aksay, I. A.; Lin, Y. H. Graphene Based Electrochemical Sensors and Biosensors: A Review. Electroanalysis 2010, 22 (10), 1027−1036. (18) Fu, W. Y.; Nef, C.; Knopfmacher, O.; Tarasov, A.; Weiss, M.; Calame, M.; Schonenberger, C. Graphene Transistors Are Insensitive to Ph Changes in Solution. Nano Lett. 2011, 11 (9), 3597−3600. (19) Tan, X. B.; Chuang, H. J.; Lin, M. W.; Zhou, Z. X.; Cheng, M. M. C. Edge Effects on the Ph Response of Graphene Nanoribbon Field Effect Transistors. J. Phys. Chem. C 2013, 117 (51), 27155− 27160. (20) Lim, C. X.; Hoh, H. Y.; Ang, P. K.; Loh, K. P. Direct Voltammetric Detection of DNA and Ph Sensing on Epitaxial Graphene: An Insight into the Role of Oxygenated Defects. Anal. Chem. 2010, 82 (17), 7387−7393. (21) Cheng, Z. G.; Li, Q.; Li, Z. J.; Zhou, Q. Y.; Fang, Y. Suspended Graphene Sensors with Improved Signal and Reduced Noise. Nano Lett. 2010, 10 (5), 1864−1868. (22) Robinson, J. A.; Snow, E. S.; Badescu, S. C.; Reinecke, T. L.; Perkins, F. K. Role of Defects in Single-Walled Carbon Nanotube Chemical Sensors. Nano Lett. 2006, 6 (8), 1747−1751. (23) Zhang, Y. H.; Chen, Y. B.; Zhou, K. G.; Liu, C. H.; Zeng, J.; Zhang, H. L.; Peng, Y. Improving Gas Sensing Properties of Graphene by Introducing Dopants and Defects: A First-Principles Study. Nanotechnology 2009, 20 (18), 185504. (24) Wei, D. C.; Liu, Y. Q.; Wang, Y.; Zhang, H. L.; Huang, L. P.; Yu, G. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9 (5), 1752−1758. (25) Dan, Y. P.; Lu, Y.; Kybert, N. J.; Luo, Z. T.; Johnson, A. T. C. Intrinsic Response of Graphene Vapor Sensors. Nano Lett. 2009, 9 (4), 1472−1475. (26) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6 (9), 652−655. (27) Brenner, K.; Yang, Y. X.; Murali, R. Edge Doping of Graphene Sheets. Carbon 2012, 50 (2), 637−645.
defects contributed to irreversible changes on the sensing characteristics. More importantly, when the defect sites were fully passivated, both sensors exhibited a similar reversible ΔVDir of 6−8 mV/pH, driven by the response from the Gr surface plane only. Although the defect-mediated irreversible contribution was much smaller in the Gr-FETs than in the GMFETs, it was still 2−4 times the magnitude of the reversible contribution (16−26 mV/pH versus 6−8 mV/pH). This result explains the large variations in reported pH response of Grbased FETs and illustrates the importance of controlling Gr defects to enable reliable multiple-cycle sensor operations.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10183. Channel current versus gate voltage (I−Vwg) curves for electrolyte-gated GM-FETs, Evolution of I−Vwg characteristic curves of GM- and Gr-FETs with increasing cycles, Source−drain current (Id) versus time for GMFETs compared with as-prepared and defect-passivated, Corresponding I−Vwg characteristic curves before and after annealing (PDF).
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AUTHOR INFORMATION
Corresponding Author
*W. I. Park. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program (2015R1A2A2A11001426) and the International Research & Development Program (2013K1A3A1A32035393) of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea. S.N. acknowledges support from the Air Force Office of Scientific Research/Asian Office of Aerospace Research Development (AFOSR/ AOARD) through the Nano Bio Info Technology (NBIT) Phase III Program (AOARD-13-4125).
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