www.advmat.de www.MaterialsViews.com
COMMUNICATION
Highly Transparent and Stretchable Field-Effect Transistor Sensors Using Graphene-Nanowire Hybrid Nanostructures Joohee Kim, Mi-Sun Lee, Sangbin Jeon, Minji Kim, Sungwon Kim, Kukjoo Kim, Franklin Bien, Sung You Hong,* and Jang-Ung Park*
Recently, with the rapid advances in flexible and wearable electronics, electrically conductive, transparent, and stretchable films have been studied extensively to replace conventional indium tin oxide (ITO) due to its brittleness and the scarcity of indium. There are several potential alternatives to ITO, such as conducting polymers,[1,2] carbon nanotubes,[1,3–5] graphene,[6–10] metal nanowires (mNWs),[4,11–16] metal grids,[16–18] and their hybrid forms.[19–24] Among them, graphene-mNW hybrid nanostructures have attracted considerable attention due to their low electrical resistance (90% in the visible range) and their outstanding mechanical flexibility and stretchability.[19–22] The formation of these hybrids can complement the disadvantages of the single material of graphene or mNWs, such as limitations of mNW percolating networks on decreasing their pattern sizes, weakness of mNWs against electrical breakdown, and chemical oxidation, and the relatively high sheet resistance (Rs) of chemical vapor deposition (CVD) graphene (≥200 Ω sq−1).[15,16,25,26] Although the graphene-mNW hybrid has been explored only for electrodes as the single component of devices,[27,28] approaches to fabricate the entire structures of integrated electronic devices using this hybrid structure have not been reported yet. Herein, we report an unconventional approach to form monolithically integrated devices based on the graphene– AgNW hybrid film for highly transparent and stretchable electronic devices. Spatial patterning of AgNWs in the hybrid permits the full integration of field-effect transistor (FET) sensors for RLC resonant circuits, with graphene as channels and the
J. Kim, Dr. M.-S. Lee, M. Kim, S. Kim, Dr. K. Kim, Prof. J.-U. Park School of Materials Science and Engineering Wearable Electronics Research Group Low-Dimensional Carbon Materials Research Center Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798, Republic of Korea E-mail: [email protected] S. Jeon, Prof. S. Y. Hong School of Energy and Chemical Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798, Republic of Korea E-mail: [email protected] Prof. F. Bien School of Electrical and Computer Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan Metropolitan City 689-798, Republic of Korea
DOI: 10.1002/adma.201500710
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
graphene–AgNW hybrid parts as electrodes such as source/ drain/interconnect and even antenna. The superior transconductance values of graphene are appropriate for the active channels of FETs. In contrast, the formation of the hybrid by patterning AgNW networks on graphene selectively reduces Rs and field-effect responses, which is advantageous for applications in electrodes. In this FET structure, the graphene layer is continuous and monolithically connected through all conductive components of the channels and electrodes, which can reduce the contact resistance (Rc) between the channel and electrodes and enhance FET mobility (≈3000 cm2 V−1 s−1). The RLC resonant circuits integrated with the FET arrays can be operated as real-time, wireless sensors for detection of carbohydrate binding proteins via glycosylation of the graphene channel. These fully integrated devices have remarkable transparency and stretchability. Furthermore, these ultrathin and light devices can be transferred onto a variety of substrates, and they also are flexible and work consistently, even when they are bent with radii of curvature as small as ≈27 µm. On the basis of these multiple functionalities, we demonstrate their real-time, wireless operations on human skin as a transparent electronic tattoo for applications in wearable electronics. Graphene–AgNW hybrid film can be produced by transferring the 2D layer of CVD-synthesized graphene onto the spun, random networks of 1D AgNWs, and vice versa (Figure S1, Supporting Information).[19] After producing the graphene–AgNW hybrid films with different spin rates of AgNWs, the optical and electrical properties of the hybrid films were characterized (Figure S2, Supporting Information). Negligible field-effect responses, such as low transconductances, as well as good electric conductivities with high transparencies, are necessary for transparent electrodes of electronic circuits in order to retard undesired changes in the resistance of these electrodes by bias during the operation of the devices. Field-effect responses of graphene, AgNWs, and their hybrid structures were characterized after patterning these three materials with identical dimensions on a Si wafer with a 300-nm-thick SiO2 top layer (Figures 1a and S3, Supporting Information). Figure 1b–d shows the transfer characteristics of the three materials. The current versus back-gate characteristics of AgNWs shows negligible change of current with the back-gate voltage (VG), as presented in Figure 1b. In contrast, Figure 1c shows stronger modulation of the current in graphene according to VG.[29] Similar to the AgNW case, the graphene–AgNW hybrid shows negligible transconductance behaviors (Figure 1d), but it provides significantly lower resistance than the AgNW sample, as compared from the current (ID) values of Figures 1b,d. These
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. a) Schematic image of the transistor layout with various channel materials. ID–VG characteristics of the back-gate FETs with the channel made of b) AgNW, c) graphene, and d) graphene–AgNW hybrid (VD = 0.01 V).
low resistance and field-effect responses of the hybrid are advantageous for applications in electrodes. In contrast, the superior transconductance level of graphene is appropriate for the active channels of FETs. These results enable the fabrication of FETs composed of graphene as the channel and the graphene–AgNW hybrid as the electrodes, which offers promising potential for transparent and stretchable electronic devices. For graphene FETs, Rc between the graphene channel and metal electrodes is an important factor in determining the performances of the devices, such as on-state currents and mobilities.[30,31] Different work functions and density of states of the 2D graphene and 3D metallic source (S)/drain (D) electrodes can result in Rc at the interface of the metals on graphene.[30,31] The contact properties and Rc values between graphene and various metals, including Au, Pd, Cr, Cu, and Ni, have been studied extensively.[30,32,33] In this study, we fabricated the graphene FETs with the graphene–AgNW hybrid as S/D electrodes rather than conventional metals, and we characterized their
contact properties. To fabricate these FETs, the AgNW suspension was spun on a highly-doped Si-wafer with a 300-nmthick SiO2 dielectric layer and then photolithographically patterned for the S/D geometry of FET arrays. A CVD graphene layer was transferred and then patterned to cover the entire top surfaces of the isolated transistors where the graphene connects the channel and S/D part monolithically. Therefore, graphene and the hybrid parts serve as the channel and the S/D, respectively, in this FET layout, and the back-gate responses of the FET arrays were measured using Si as the bottom gate (G). Figure 2a shows the as-fabricated graphene FETs with the hybrid S/D. Rc between the channel and S/D can be estimated by utilizing the transfer length method (TLM).[31,32,34] In this measurement, the width of the channel was fixed at 5 µm, and the length of the channel (L) was varied from 10 to 110 µm. The characterization of the S/D current (ID) versus back-gate bias (VG) of these FETs was performed at room temperature with a drain bias of 0.1 V, as shown in Figure S4 (Supporting
Figure 2. a) Schematic illustration of a FET composed of a graphene channel and hybrid electrodes. b) ID–VG characteristic of the graphene FETs with hybrid electrode or metal (Cr/Au) electrode (VD = 0.1 V). The width of the channel is 5 µm, and the length of the channel is 70 µm. c) Device mobility as a function of channel length. The width of the channel was 5 µm in all cases, and the lengths of the channel ranged from 10 to 110 µm. d) Dependence of total resistance on the length of the channel at the gate voltages from −40 to 60 V (channel width: 5 µm). An inset indicates the estimation of the contact resistance by the transfer length method. e) Contact resistance as a function of gate voltage. Each data point indicates the average for 50 samples, and error bars represent the standard deviation.
2
wileyonlinelibrary.com
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
COMMUNICATION
Information). These FETs presented ambipolar behavior consistent with the expected semimetallic characteristics of graphene,[9,35–37] with positive charge neutrality points of ≈20–30 V (red curves of Figure 2b). The hole (electron) mobility of the FET, calculated using a standard metal-oxide-semiconductor FET model, was 2925 ± 78 cm2 V−1 s−1 (892 ± 45 cm2 V−1 s−1) for L = 70 µm (Figure 2b). For the comparison, graphene FETs using the S/D electrode of Cr/Au (thickness: 3 nm/100 nm) instead of graphene–AgNW hybrid also were fabricated with the identical dimensions of the channel. As shown in the blue curves of Figure 2b, this device presented a similar charge neutrality point of ≈30 V, but slightly lower transconductance and mobility (i.e., µCr/Au = 2340 ± 56 cm2 V−1 s−1 in the p-type regime and 690 ± 28 cm2 V−1 s−1 in the n-type regime). These results indicated that the hybrid electrode can display similar or slightly better performances of graphene FETs as S/D than the Cr/Au case with potential added advantages of transparency and stretchability. To provide better understanding of the contact properties, we plotted the L dependence of the device mobility, as shown in Figure 2c. This graph shows a steady decrease of mobility as L decreases in the range of 10–50 µm, since Rc dominates the total resistance of the device when L is small enough.[38] And, then, Rtotal was obtained from the reciprocal of the slope of the output curves plotted as a function of L (Figure 2d). From the equation, Rtotal = Rchannel + 2Rc, Rc corresponds to the y-axis intercept of the extrapolated linear fitting of Rtotal versus L.[39] In addition, the transfer length (LT), which was defined as the “1/e” distance of the voltage curve and obtained from the x-axis intercept of the TLM plot,[39] was determined to be lower than 10 µm in our system (Figure 2d). As shown in Figure 2e, Rc of the FETs with S/D of the hybrid was dependent on VG in the range of ≈6–9 kΩ·µm. The max Rc was estimated as 9 kΩ·µm near the charge neutrality point. These values are commensurate with Rc between graphene and other metals, such as Pd, Au, Ni, Cu, and Cr/Au; the Rc values of the Pd, Au, Ni, Cu, and Cr/Au at the charge neutrality point of FETs were approximately 5, 14, 20, 28, and 100 kΩ·µm, respectively.[30,32,33] Among these metals, the hybrid electrode had the smallest Rc with the graphene channel except for the case of Pd. In our FETs, the graphene layer was continuous and monolithically connected through the channel and S/D without physical disconnections, which was advantageous in reducing Rc. Moreover, the use of the graphene–AgNW hybrid for S/D enabled a larger flow of ID when the high VG of 60 V was applied than was the case for the graphene FETs in which the single material of AgNWs was used as S/D, due to the robust stability of the hybrid against electrical breakdown.[19] Layer-by-layer stacking of the hybrid films can reduce Rs further and hence improve the conductance of the electrodes and interconnects (Figure S5, Supporting Information). This layerby-layer stacking of the hybrid can decrease the value of Rs to less than 4 Ω sq−1 and preserve the semi-transparent property, thereby widening the possible range of the resistance of the hybrid electrode for diversifying circuit designs. This stacking approach to enhancing conductance provides the feasibility on the use of this hybrid structure as interconnects of various integrated circuits. With regard to wireless sensors, for example, high conductance is preferred to reduce power consumptions
and therefore to enhance antenna signals. Also, to come close to achieving the required properties of wearable electronics, we demonstrated the mechanical flexibility of graphene FET arrays based on hybrid electrodes, as shown in Figures S6,S7 (Supporting Information). Figure S6d (Supporting Information) shows the resistance variations by stretching them up to 20% in uniaxial tensile strain, and the change was negligible. Since this stretchability was well above the general upper limit (≈15%) for the elastic mechanics of human skin,[40,41] these devices offer promising potential for skin-attachable electronics. Furthermore, we investigated the durability of the graphene–AgNW hybrid structure against repeated stretching and releasing in our previous work and there was no significant performance degradation even after a few thousands of stretching.[42] The mechanical stability of these devices against bending and stretching was superior to that of ITO and other inorganics, such as Si, which can be cracked by applying a tensile strain of ≈1%, so the devices offer advantages that could lead to the development of transparent, wearable electronic devices. Owing to the simple process, good electrical properties, high flexibility, and facile surface functionalization, graphene FETs have been utilized for biosensors.[43] Inherent high carrier mobility and 2D structural integrity with high surface area of graphene allows low noise levels and high detection sensitivity for biomaterials.[44–46] Together with proteins, lipids, and nucleic acids, carbohydrate-containing structures (glycans) belong to four major biological compounds that mediate cellcell communications, immune responses, cellular differentiations via glyco-conjugates, and the interactions of carbohydrate recognition protein (lectins).[47–50] A lectin biosensor can be constructed by utilizing graphene–pyrene conjugates. The noncovalent functionalization method has been utilized to integrate glyco-conjugates into FET while preserving the intrinsic properties of graphene. Although covalent functionalization methods to 2D, π-conjugated systems (e.g., oxidation followed by amidation, cycloaddition, and radical reaction) allow stable chemical conjugation between graphene and organic compounds, they alter the intrinsic chemical structures of graphene from the sp2 planar carbon allotrope to sp3 tetrahedral carbon and can degrade the electrical properties of graphene.[50] To prepare glyco-pyrene derivatives (Figure S8, Supporting Information), thio-glycosyl derivatives 1 and 4 were chosen due to their facile preparation and chemical stability during organic transformations.[51] The p-anilinyl type of rigid aglycones was introduced since they can further enhance the bindings with graphene and lectins. Anilinyl termini were coupled with 1-pyrenebutyric acid, and, then, acetyl groups of 2 and 5 were cleaved under the Zemplen condition to afford glyco-pyrene derivatives 3 and 6. To prepare glycosylated graphene FET, integrated graphene FET was incubated with glyco-pyrene derivatives for 12 h, followed by extensive washing. The chemical structures of compounds 3 and 6 were confirmed by 1H and 13C nuclear magnetic resonance, 2D correlation spectroscopy, and heteronuclear single quantum coherence experiments. In addition, noncovalent conjugation of mannosyl pyrene 3 to graphene was examined by UV–vis and Raman spectroscopic analyses. (see Figure S8–S14, Supporting Information.) The devices with the graphene channel and hybrid S/D electrodes can be used as FET biosensors with transparent and
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. a) Schematic illustration and principle of Con A detection with the glyco-pyrene incorporated graphene. (i) Functionalization of graphene with glyco-pyrene derivatives by using noncovalent π–π stacking. (ii) Con A sensing by using mannosyl–pyrene coated graphene via specific mannoselectin binding. (iii) Separating Con A from mannosyl–pyrene via competitive binding assay with excess mannose. b) Photograph of the flexible and transparent graphene field-effect sensor arrays based on graphene–AgNW hybrid electrodes. Scale bar, 1 cm. Transfer characteristics (ID–VG curves, VD = 0.1 V) of solution-gated graphene field-effect sensors with c) mannosyl–pyrene-linked graphene and d) galactosyl–pyrene-linked-graphene before and after flowing Con A solutions. e) Real-time, reversible Con A sensing using mannosyl–pyrene-coated graphene field-effect sensors (VSG = 0.1 V).
stretchable functions for wearable electronics. For this demonstration, AgNWs were coated and patterned as array forms of S/D electrodes, and, then, graphene covered the dumbbell-like device surfaces connecting the channel and S/D continuously. The S/D areas of the graphene–AgNW hybrid structure were covered sequentially with a 500-nm-thick SU-8 passivation layer with openings around the graphene channels. Each block of the sensor array contained nine of the graphene field-effect sensors with a single common source and independent drains of the hybrid. The sensor chip had four such blocks composed of a total of 36 sensor devices. Figure 3b shows a photograph of this sample. Then, a microfluidic channel based on a polydimethylsiloxane (PDMS, Sylgard 184) mold was integrated onto the fabricated FET sensors for real-time, solution-gated sensing. (Figure S15a, Supporting Information) To detect lectins selectively, graphene FET sensors were noncovalently functionalized with the synthesized pyrene-based glyco-conjugates, as described above. Figure 3a shows the binding processes between glycans and mannose-binding lectins (Concanavalin A). Mannosyl–pyrenes, as receptor molecules, were immobilized on the graphene surfaces due to π–π stacking between graphene and the pyrene-linker part during incubation of the prepared sensors in the mannosyl–pyrene solutions. After incorporation of mannosyl–pyrene solutions into the graphene surfaces, specific interactions between lectin and mannosyl–pyrene were transduced to induce the current change (ΔID) by flowing Concanavalin A (Con A) solutions into the microfluidic channel. As shown in Figure 3c, a decrease in transconductance with a negative shift of −0.2 V in VG was observed using solution gating due to this Con A binding. To investigate the reversibility of the sensors, excess mannose solutions were injected into the fluidic channel by a syringe pump. As a result, the current level of the device was recovered to the original ID curve before the lectin solutions were added (Figure S15b, Supporting Information). The pristine graphene channel functionalized with mannosyl–pyrene can be recovered by flowing excess mannose over
4
wileyonlinelibrary.com
the channel, which reacts with Con A and, in turn, leads to the separation from the channel. In addition, as a control experiment, sensing was conducted with galactosyl–pyrene, which does not react with Con A, and a negligible change in the current was observed, as shown in Figure 3d. Real-time sensing of the protein using the sensor device is shown in Figure 3e. In this experiment, Con A and excess mannose solutions flowed alternately with similar time intervals. For the sensor that was functionalized using mannosyl–pyrene, the current decreased with ΔID of ≈2 µA on switching solutions to Con A in the p-type regime (solution-gate potential: 0.1 V), and it responded in the opposite way on switching to the excess mannose solution. In contrast, the sensor with galactosyl–pyrene group showed no response during the flow. Conventional designs of electronic sensors contain the rigid and big power sources as well as direct physical connections between the sensing probes and data processing electronics.[52,53] In order to eliminate the need for an onboard battery and external connections in order to detect the biomaterials wirelessly, we incorporated a resonant coil into the device. This sensor system was composed of a passive RLC resonant circuit with an inductive (L) coil for wireless transmission and a capacitive (C) electrode contacting a resistive (R) sensor channel. For this circuit, graphene and a single-layered hybrid structure were used for the channel and all electrodes, respectively. And graphene monolithically connected all conductive components with continuous interfaces. The entire circuit system is transferrable to other substrates, and its ultrathin and light characteristics are advantageous for conformable adhesion on the planar or nonplanar surfaces of the substrates.[54] In our experiment, we selected parylene as the substrate because it provided the advantages of (1) good adhesion, (2) biocompatibility, (3) resistance to chemicals, such as acetone and acid, which are necessary for the transfer step, and (4) stretchability.[55,56] To fabricate the circuit, first, a parylene film (with a thickness of 500 nm) was coated onto a Cu foil. Then, AgNWs were spun and patterned for all electrode
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. a) Schematic image of the wireless sensor, which is composed of the antenna and electrodes based on hybrid structures, and a graphene channel. b) Frequency response of the reflection coefficient of the antenna on the plastic substrates after buffer and Con A treatment. c) Photographs of the floating devices on the water (top) and fabricated sensor onto the 1.4-µm-thick PET films (bottom). Scale bar: 1 cm (inset: whole image of the device floating on water. Scale bar: 2 cm). d) Schematic of the biosensor attached to the skin on the back of a human hand. e) Photograph of the transparent wireless sensor attached to the human skin. Scale bar: 1 cm (inset: close-up image of the device. Scale bar: 0.5 cm). f) Wireless sensing curves of a sensor on a hand before and after stretching.
areas, including the inductive coil antenna. Subsequently, graphene was transferred to cover all of the circuit patterns for the channel and all electrodes. Finally, an SU-8 passivation layer was coated with openings around the graphene channels. Figures 4a and S16 (Supporting Information) show the circuit layout and photographs of this sample, and Figure 4b shows the measured reflection of this wireless sensor circuit with a resonant frequency of 4.1 GHz. It also presents the RF responses of the wireless device when exposed to Con A. The measured reflection value (S11 parameter) at the resonance frequency, which is inversely proportional to the electric resistance of the graphene channel, was decreased by sensing Con A. Furthermore, the high elastic modulus of graphene and AgNWs with the hydrophobic nature of parylene enabled this entire circuit (in a free-standing form with the parylene substrate) to float sustainably on water, as shown in Figure 4c. Also, this ultrathin circuit can be conformably attached onto a variety of substrates. As an example, Figure 4d and 4e shows the transparent, wireless sensor circuit attached to the skin of a human hand. Its good stretchability enabled its consistent operation during the diverse deformations of the skin, such as bending, twisting, rolling, and stretching motions (Figure 4f and Movie S1, Supporting Information). In addition, the stability of graphene, AgNWs, and the hybrid films against thermal oxidation has been studied previously.[19] Because oxygen gas and moisture cannot permeate through the graphene layer, the graphene covering AgNWs in the hybrid structure of our devices can act as a passivation layer to retard Ag oxidation as well as providing conductive paths. These results suggested substantial potential for applications in transparent, wearable electronics, including invisible electronic tattoos.
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
In conclusion, we described the advantages of FET sensors with graphene channels and all electrodes of the graphene– AgNW hybrid nanostructures, as transparent and stretchable electronic devices. Graphene is continuous through all of the conducting components of channels and electrodes in this device, which is advantageous to enhance FET mobility with reducing contact resistance between the channel and S/D. Glycosylation of the graphene channel was studied for real-time detection of protein using these FET sensors. Furthermore, graphene and its hybrid structures with AgNWs enabled the fabrication of RLC resonant circuits integrated with these sensors for wireless monitoring without the need for a power source. This transparent and stretchable sensor circuit can be transferable onto diverse substrates, and the very thin and light characteristics of the device can improve its conformable adhesion on the substrates. Consistent operation of this invisible sensor circuit attached on the skin of a human hand suggests a promising strategy for developing transparent and wearable electronic devices, indicating substantial promise for next-generation electronics.
Experimental Section Experimental details are given in the Supporting Information.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Acknowledgements J.K and M.-S.L. contributed equally to this work. This work was supported by the Ministry of Science, ICT & Future Planning and the Ministry of Trade, Industry and Energy of Korea through Basic Science Research Program of National Research Foundation (Grant No. 2013R1A2A2A01068542), IT R&D program (10041416), Materials Original Technology Program (10041222), Technology Innovation Program (Grant No. 10044410), Convergence Technology Development Program for Bionic Arm (NRF-2014M3C1B2048198), and Pioneer Research Center Program (NRF-2014M3C1A3001208). Received: February 10, 2015 Revised: March 18, 2015 Published online: [1] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539. [2] M. Vosgueritchian, D. J. Lipomi, Z. Bao, Adv. Funct. Mater. 2012, 22, 421. [3] Z. Wu, Z. Chen, X. Du, J. M. Logan, J. Sippel, M. Nikolou, K. Kamaras, J. R. Reynolds, D. B. Tanner, A. F. Hebard, A. G. Rinzler, Science 2004, 305, 1273. [4] G. Yu, A. Cao, C. M. Lieber, Nat. Nanotechnol. 2007, 2, 372. [5] a) J.-U. Park, M. A. Meitl, S.-H. Hur, M. L. Usrey, M. S. Strano, P. J. A. Kenis, J. A. Rogers, Angew. Chem. 2006, 118, 595; b) J.-U. Park, M. A. Meitl, S.-H. Hur, M. L. Usrey, M. S. Strano, P. J. A. Kenis, J. A. Rogers, Angew. Chem. Int. Ed. Engl. 2006, 45, 581. [6] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim, Science 2008, 320, 1308. [7] J.-U Park, S. W. Nam, M.-S. Lee, C. M. Lieber, Nat. Mater. 2012, 11, 120. [8] S. Bas, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozyilmaz, J.-H. Ahn, B. H. Hong, S. Iijima, Nat. Nanotechnol. 2010, 5, 574. [9] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, R. S. Ruoff, Science 2009, 324, 1312. [10] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [11] C. M. Lieber, MRS Bull. 2011, 36, 1052. [12] Y. Wu, J. Xiang, C. Yang, W. Lu, C. M. Lieber, Nature 2004, 430, 61. [13] C. Wang, Y. Hu, C. M. Lieber, S. J. Sun, Am. Chem. Soc. 2008, 130, 8902. [14] D. Wang, C. M. Lieber, Nat. Mater. 2003, 2, 355. [15] S. De, T. M. Higgins, P. E. Lyons, E. M. Doherty, P. N. Nirmalraj, W. J. Blau, J. J. Boland, J. N. Coleman, ACS Nano 2009, 3, 1767. [16] L. Hu, H. Wu, Y. Cui, MRS Bull. 2011, 36, 760. [17] Y. Zhu, Z. Sun, Z. Yan, Z. Jin, J. M. Tour, ACS Nano 2011, 5, 6472. [18] P. B. Catrysse, S. Fan, Nano Lett. 2010, 10, 2944. [19] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. W. Nam, J.-U. Park, Nano Lett. 2013, 13, 2814. [20] I. N. Kholmanov, C. W. Magnuson, A. E. Aliev, H. Li, B. Zhang, J. W. Suk, L. L. Zhang, E. Peng, S. H. Mousavi, A. B. Khanikaev, R. Piner, G. Shvets, R. S. Ruoff, Nano Lett. 2012, 12, 5679. [21] C. Jeong, P. Nair, M. Khan, M. Lundstrom, M. A. Alam, Nano Lett. 2011, 11, 5020. [22] R. Chen, S. R. Das, C. Jeong, M. R. Khan, D. B. Janes, M. A. Alam, Adv. Funct. Mater. 2013, 23, 5150. [23] L. Li, J. Liang, S.-Y. Chou, X. Zhu, X. Niu, Z. Yu, Q. Pei, Sci. Rep. 2014, 4, 04307.
6
wileyonlinelibrary.com
[24] H. Koga, M. Nogi, N. Komoda, T. T. Nge, R. Sugahara, K. Suganuma, NPG Asia Mater. 2014, 6. [25] L. Yang, T. Zhang, H. Zhou, S. C. Price, B. J. Wiley, W. You, ACS Appl. Mater. Interfaces 2011, 3, 4075. [26] W. Hu, X. Niu, L. Li, S. Yun, Z. Yu, Q. Pei, Nanotechnology 2012, 23, 344002. [27] K. Lee, J. Park, M.-S. Lee, J. Kim, B. G. Hyun, D. J. Kang, K. Na, C. Y. Lee, F. Bien, J.-U. Park, Nano Lett. 2014, 14, 2647. [28] S.-K. Lee, H. Y. Jang, S. Jang, E. Choi, B. H. Hong, J. Lee, S. Park, J.-H. Ahn, Nano Lett. 2012, 12, 3472. [29] Y. Zhang, J. P. Small, W. V. Pontius, P. Kim, Appl. Phys. Lett. 2005, 86, 073104. [30] S. M. Song, J. K. Park, O. J. Sul, B. J. Cho, Nano Lett. 2012, 12, 3887. [31] A. Venugopal, L. Colombo, E. M. Vogel, Appl. Phys. Lett. 2010, 96, 013512. [32] S. K. Hong, S. M. Song, O. J. Sul, B. J. Cho, Carbon Lett. 2013, 14, 171. [33] F. Xia, V. Perebeinos, Y.-M. Lin, Y. Wu, P. Avouris, Nat. Nanotechnol. 2011, 6, 179. [34] K. Nagashio, T. Nishimura, K. Kita, A. Toriumi, Electron Devices Meeting (IEDM), IEEE Int., Baltimore 2009. [35] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, Nature 2005, 438, 197. [36] Y. Zhang, Y.-W. Tan, H. L. Stormer, P. Kim, Nature 2005, 438, 201. [37] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus, J. Kong, Nano Lett. 2009, 9, 30. [38] J. Zaumseil, K. W. Baldwin, J. A. Rogers, J. Appl. Phys. 2003, 93, 6117. [39] D. K. Schroder, Semiconductor Material and Device Characterization, 3rd ed., John Wiley & Sons, Canada 2006. [40] D.-H. Kim, N. Lu, R. Ma, Y.-S. Kim, R.-H. Kim, S. Wang, J. Wu, S. M. Won, H. Tao, A. Islam, K. J. Yu, T.-I. Kim, R. Chowdhury, M. Ying, L. Xu, M. Li, H.-J. Chung, H. Keum, M. McCormick, P. Liu, Y.-W. Zhang, F. G. Omenetto, Y. Huang, T. Coleman, J. A. Rogers, Science 2011, 333, 838. [41] J. A. Fan, W.-H. Yeo, Y. Su, Y. Hattori, W. Lee, S.-Y. Jung, Y. Zhang, Z. Liu, H. Cheng, L. Falgout, M. Bajema, T. Coleman, D. Gregoire, R. J. Larsen, Y. Huang, J. A. Rogers, Nat. Commun. 2014, 5, 3266. [42] M.-S. Lee, J. Kim, J. Park, J.-U. Park, Nanoscale Res. Lett. 2015, 10, 27. [43] a) W. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson, J. J. Gooding, F. Braet, Angew. Chem. 2010, 122, 2160; b) W. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson, J. J. Gooding, F. Braet, Angew. Chem. Int. Ed. 2010, 49, 2114. [44] Y. Huang, X. Dong, Y. Shi, C. M. Li, L.-J. Li, P. Chen, Nanoscale 2010, 2, 1485. [45] O. S. Kwon, S. H. Lee, S. J. Park, J. H. An, H. S. Song, T. Kim, J. H. Oh, J. Bae, H. Yoon, T. H. Park, J. Jang, Adv. Mater. 2013, 25, 4177. [46] O. S. Kwon, S. J. Park, J.-Y. Hong, A.-R. Han, J. S. Lee, J. S. Lee, J. H. Oh, J. Jang, ACS Nano 2012, 6, 1486. [47] R. A. Dwek, Chem. Rev. 1996, 96, 683. [48] A. Varki, Glycobiology 1993, 3, 97. [49] B. G. Davis, Chem. Rev. 2002, 102, 579. [50] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril, K. S. Kim, Chem. Rev. 2012, 112, 6156. [51] F. M. Ibatullin, S. I. Selivanov, A. G. Shavva, Synthesis 2001, 2001, 33. [52] C. M. Li, H. Dong, X. Cao, J. H. T Luong, X. Zhang, Curr. Med. Chem. 2007, 14, 937. [53] M. O. Schurr, S. Schostek, C. N. Ho, F. Rieber, A. Menciassi, Minim. Invasiv. Ther. 2007, 16, 76. [54] Y. J. Park, S.-K. Lee, M.-S. Kim, H. Kim, J.-H. Ahn, ACS Nano 2014, 8, 7655. [55] E. Meng, P.-Y. Li, Y.-C. Tai, Sens. Actuator A. 2008, 144, 18. [56] D. C. Rodger, J. D. Weiland, M. S. Humayun, Y.-C. Tai, Sens. Actuator B. 2006, 117, 107.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
COMMUNICATION
Highly Transparent and Stretchable Field-Effect Transistor Sensors Using Graphene-Nanowire Hybrid Nanostructures Joohee Kim, Mi-Sun Lee, Sangbin Jeon, Minji Kim, Sungwon Kim, Kukjoo Kim, Franklin Bien, Sung You Hong,* and Jang-Ung Park*
Recently, with the rapid advances in flexible and wearable electronics, electrically conductive, transparent, and stretchable films have been studied extensively to replace conventional indium tin oxide (ITO) due to its brittleness and the scarcity of indium. There are several potential alternatives to ITO, such as conducting polymers,[1,2] carbon nanotubes,[1,3–5] graphene,[6–10] metal nanowires (mNWs),[4,11–16] metal grids,[16–18] and their hybrid forms.[19–24] Among them, graphene-mNW hybrid nanostructures have attracted considerable attention due to their low electrical resistance (90% in the visible range) and their outstanding mechanical flexibility and stretchability.[19–22] The formation of these hybrids can complement the disadvantages of the single material of graphene or mNWs, such as limitations of mNW percolating networks on decreasing their pattern sizes, weakness of mNWs against electrical breakdown, and chemical oxidation, and the relatively high sheet resistance (Rs) of chemical vapor deposition (CVD) graphene (≥200 Ω sq−1).[15,16,25,26] Although the graphene-mNW hybrid has been explored only for electrodes as the single component of devices,[27,28] approaches to fabricate the entire structures of integrated electronic devices using this hybrid structure have not been reported yet. Herein, we report an unconventional approach to form monolithically integrated devices based on the graphene– AgNW hybrid film for highly transparent and stretchable electronic devices. Spatial patterning of AgNWs in the hybrid permits the full integration of field-effect transistor (FET) sensors for RLC resonant circuits, with graphene as channels and the
J. Kim, Dr. M.-S. Lee, M. Kim, S. Kim, Dr. K. Kim, Prof. J.-U. Park School of Materials Science and Engineering Wearable Electronics Research Group Low-Dimensional Carbon Materials Research Center Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798, Republic of Korea E-mail: [email protected] S. Jeon, Prof. S. Y. Hong School of Energy and Chemical Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798, Republic of Korea E-mail: [email protected] Prof. F. Bien School of Electrical and Computer Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan Metropolitan City 689-798, Republic of Korea
DOI: 10.1002/adma.201500710
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
graphene–AgNW hybrid parts as electrodes such as source/ drain/interconnect and even antenna. The superior transconductance values of graphene are appropriate for the active channels of FETs. In contrast, the formation of the hybrid by patterning AgNW networks on graphene selectively reduces Rs and field-effect responses, which is advantageous for applications in electrodes. In this FET structure, the graphene layer is continuous and monolithically connected through all conductive components of the channels and electrodes, which can reduce the contact resistance (Rc) between the channel and electrodes and enhance FET mobility (≈3000 cm2 V−1 s−1). The RLC resonant circuits integrated with the FET arrays can be operated as real-time, wireless sensors for detection of carbohydrate binding proteins via glycosylation of the graphene channel. These fully integrated devices have remarkable transparency and stretchability. Furthermore, these ultrathin and light devices can be transferred onto a variety of substrates, and they also are flexible and work consistently, even when they are bent with radii of curvature as small as ≈27 µm. On the basis of these multiple functionalities, we demonstrate their real-time, wireless operations on human skin as a transparent electronic tattoo for applications in wearable electronics. Graphene–AgNW hybrid film can be produced by transferring the 2D layer of CVD-synthesized graphene onto the spun, random networks of 1D AgNWs, and vice versa (Figure S1, Supporting Information).[19] After producing the graphene–AgNW hybrid films with different spin rates of AgNWs, the optical and electrical properties of the hybrid films were characterized (Figure S2, Supporting Information). Negligible field-effect responses, such as low transconductances, as well as good electric conductivities with high transparencies, are necessary for transparent electrodes of electronic circuits in order to retard undesired changes in the resistance of these electrodes by bias during the operation of the devices. Field-effect responses of graphene, AgNWs, and their hybrid structures were characterized after patterning these three materials with identical dimensions on a Si wafer with a 300-nm-thick SiO2 top layer (Figures 1a and S3, Supporting Information). Figure 1b–d shows the transfer characteristics of the three materials. The current versus back-gate characteristics of AgNWs shows negligible change of current with the back-gate voltage (VG), as presented in Figure 1b. In contrast, Figure 1c shows stronger modulation of the current in graphene according to VG.[29] Similar to the AgNW case, the graphene–AgNW hybrid shows negligible transconductance behaviors (Figure 1d), but it provides significantly lower resistance than the AgNW sample, as compared from the current (ID) values of Figures 1b,d. These
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. a) Schematic image of the transistor layout with various channel materials. ID–VG characteristics of the back-gate FETs with the channel made of b) AgNW, c) graphene, and d) graphene–AgNW hybrid (VD = 0.01 V).
low resistance and field-effect responses of the hybrid are advantageous for applications in electrodes. In contrast, the superior transconductance level of graphene is appropriate for the active channels of FETs. These results enable the fabrication of FETs composed of graphene as the channel and the graphene–AgNW hybrid as the electrodes, which offers promising potential for transparent and stretchable electronic devices. For graphene FETs, Rc between the graphene channel and metal electrodes is an important factor in determining the performances of the devices, such as on-state currents and mobilities.[30,31] Different work functions and density of states of the 2D graphene and 3D metallic source (S)/drain (D) electrodes can result in Rc at the interface of the metals on graphene.[30,31] The contact properties and Rc values between graphene and various metals, including Au, Pd, Cr, Cu, and Ni, have been studied extensively.[30,32,33] In this study, we fabricated the graphene FETs with the graphene–AgNW hybrid as S/D electrodes rather than conventional metals, and we characterized their
contact properties. To fabricate these FETs, the AgNW suspension was spun on a highly-doped Si-wafer with a 300-nmthick SiO2 dielectric layer and then photolithographically patterned for the S/D geometry of FET arrays. A CVD graphene layer was transferred and then patterned to cover the entire top surfaces of the isolated transistors where the graphene connects the channel and S/D part monolithically. Therefore, graphene and the hybrid parts serve as the channel and the S/D, respectively, in this FET layout, and the back-gate responses of the FET arrays were measured using Si as the bottom gate (G). Figure 2a shows the as-fabricated graphene FETs with the hybrid S/D. Rc between the channel and S/D can be estimated by utilizing the transfer length method (TLM).[31,32,34] In this measurement, the width of the channel was fixed at 5 µm, and the length of the channel (L) was varied from 10 to 110 µm. The characterization of the S/D current (ID) versus back-gate bias (VG) of these FETs was performed at room temperature with a drain bias of 0.1 V, as shown in Figure S4 (Supporting
Figure 2. a) Schematic illustration of a FET composed of a graphene channel and hybrid electrodes. b) ID–VG characteristic of the graphene FETs with hybrid electrode or metal (Cr/Au) electrode (VD = 0.1 V). The width of the channel is 5 µm, and the length of the channel is 70 µm. c) Device mobility as a function of channel length. The width of the channel was 5 µm in all cases, and the lengths of the channel ranged from 10 to 110 µm. d) Dependence of total resistance on the length of the channel at the gate voltages from −40 to 60 V (channel width: 5 µm). An inset indicates the estimation of the contact resistance by the transfer length method. e) Contact resistance as a function of gate voltage. Each data point indicates the average for 50 samples, and error bars represent the standard deviation.
2
wileyonlinelibrary.com
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
COMMUNICATION
Information). These FETs presented ambipolar behavior consistent with the expected semimetallic characteristics of graphene,[9,35–37] with positive charge neutrality points of ≈20–30 V (red curves of Figure 2b). The hole (electron) mobility of the FET, calculated using a standard metal-oxide-semiconductor FET model, was 2925 ± 78 cm2 V−1 s−1 (892 ± 45 cm2 V−1 s−1) for L = 70 µm (Figure 2b). For the comparison, graphene FETs using the S/D electrode of Cr/Au (thickness: 3 nm/100 nm) instead of graphene–AgNW hybrid also were fabricated with the identical dimensions of the channel. As shown in the blue curves of Figure 2b, this device presented a similar charge neutrality point of ≈30 V, but slightly lower transconductance and mobility (i.e., µCr/Au = 2340 ± 56 cm2 V−1 s−1 in the p-type regime and 690 ± 28 cm2 V−1 s−1 in the n-type regime). These results indicated that the hybrid electrode can display similar or slightly better performances of graphene FETs as S/D than the Cr/Au case with potential added advantages of transparency and stretchability. To provide better understanding of the contact properties, we plotted the L dependence of the device mobility, as shown in Figure 2c. This graph shows a steady decrease of mobility as L decreases in the range of 10–50 µm, since Rc dominates the total resistance of the device when L is small enough.[38] And, then, Rtotal was obtained from the reciprocal of the slope of the output curves plotted as a function of L (Figure 2d). From the equation, Rtotal = Rchannel + 2Rc, Rc corresponds to the y-axis intercept of the extrapolated linear fitting of Rtotal versus L.[39] In addition, the transfer length (LT), which was defined as the “1/e” distance of the voltage curve and obtained from the x-axis intercept of the TLM plot,[39] was determined to be lower than 10 µm in our system (Figure 2d). As shown in Figure 2e, Rc of the FETs with S/D of the hybrid was dependent on VG in the range of ≈6–9 kΩ·µm. The max Rc was estimated as 9 kΩ·µm near the charge neutrality point. These values are commensurate with Rc between graphene and other metals, such as Pd, Au, Ni, Cu, and Cr/Au; the Rc values of the Pd, Au, Ni, Cu, and Cr/Au at the charge neutrality point of FETs were approximately 5, 14, 20, 28, and 100 kΩ·µm, respectively.[30,32,33] Among these metals, the hybrid electrode had the smallest Rc with the graphene channel except for the case of Pd. In our FETs, the graphene layer was continuous and monolithically connected through the channel and S/D without physical disconnections, which was advantageous in reducing Rc. Moreover, the use of the graphene–AgNW hybrid for S/D enabled a larger flow of ID when the high VG of 60 V was applied than was the case for the graphene FETs in which the single material of AgNWs was used as S/D, due to the robust stability of the hybrid against electrical breakdown.[19] Layer-by-layer stacking of the hybrid films can reduce Rs further and hence improve the conductance of the electrodes and interconnects (Figure S5, Supporting Information). This layerby-layer stacking of the hybrid can decrease the value of Rs to less than 4 Ω sq−1 and preserve the semi-transparent property, thereby widening the possible range of the resistance of the hybrid electrode for diversifying circuit designs. This stacking approach to enhancing conductance provides the feasibility on the use of this hybrid structure as interconnects of various integrated circuits. With regard to wireless sensors, for example, high conductance is preferred to reduce power consumptions
and therefore to enhance antenna signals. Also, to come close to achieving the required properties of wearable electronics, we demonstrated the mechanical flexibility of graphene FET arrays based on hybrid electrodes, as shown in Figures S6,S7 (Supporting Information). Figure S6d (Supporting Information) shows the resistance variations by stretching them up to 20% in uniaxial tensile strain, and the change was negligible. Since this stretchability was well above the general upper limit (≈15%) for the elastic mechanics of human skin,[40,41] these devices offer promising potential for skin-attachable electronics. Furthermore, we investigated the durability of the graphene–AgNW hybrid structure against repeated stretching and releasing in our previous work and there was no significant performance degradation even after a few thousands of stretching.[42] The mechanical stability of these devices against bending and stretching was superior to that of ITO and other inorganics, such as Si, which can be cracked by applying a tensile strain of ≈1%, so the devices offer advantages that could lead to the development of transparent, wearable electronic devices. Owing to the simple process, good electrical properties, high flexibility, and facile surface functionalization, graphene FETs have been utilized for biosensors.[43] Inherent high carrier mobility and 2D structural integrity with high surface area of graphene allows low noise levels and high detection sensitivity for biomaterials.[44–46] Together with proteins, lipids, and nucleic acids, carbohydrate-containing structures (glycans) belong to four major biological compounds that mediate cellcell communications, immune responses, cellular differentiations via glyco-conjugates, and the interactions of carbohydrate recognition protein (lectins).[47–50] A lectin biosensor can be constructed by utilizing graphene–pyrene conjugates. The noncovalent functionalization method has been utilized to integrate glyco-conjugates into FET while preserving the intrinsic properties of graphene. Although covalent functionalization methods to 2D, π-conjugated systems (e.g., oxidation followed by amidation, cycloaddition, and radical reaction) allow stable chemical conjugation between graphene and organic compounds, they alter the intrinsic chemical structures of graphene from the sp2 planar carbon allotrope to sp3 tetrahedral carbon and can degrade the electrical properties of graphene.[50] To prepare glyco-pyrene derivatives (Figure S8, Supporting Information), thio-glycosyl derivatives 1 and 4 were chosen due to their facile preparation and chemical stability during organic transformations.[51] The p-anilinyl type of rigid aglycones was introduced since they can further enhance the bindings with graphene and lectins. Anilinyl termini were coupled with 1-pyrenebutyric acid, and, then, acetyl groups of 2 and 5 were cleaved under the Zemplen condition to afford glyco-pyrene derivatives 3 and 6. To prepare glycosylated graphene FET, integrated graphene FET was incubated with glyco-pyrene derivatives for 12 h, followed by extensive washing. The chemical structures of compounds 3 and 6 were confirmed by 1H and 13C nuclear magnetic resonance, 2D correlation spectroscopy, and heteronuclear single quantum coherence experiments. In addition, noncovalent conjugation of mannosyl pyrene 3 to graphene was examined by UV–vis and Raman spectroscopic analyses. (see Figure S8–S14, Supporting Information.) The devices with the graphene channel and hybrid S/D electrodes can be used as FET biosensors with transparent and
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. a) Schematic illustration and principle of Con A detection with the glyco-pyrene incorporated graphene. (i) Functionalization of graphene with glyco-pyrene derivatives by using noncovalent π–π stacking. (ii) Con A sensing by using mannosyl–pyrene coated graphene via specific mannoselectin binding. (iii) Separating Con A from mannosyl–pyrene via competitive binding assay with excess mannose. b) Photograph of the flexible and transparent graphene field-effect sensor arrays based on graphene–AgNW hybrid electrodes. Scale bar, 1 cm. Transfer characteristics (ID–VG curves, VD = 0.1 V) of solution-gated graphene field-effect sensors with c) mannosyl–pyrene-linked graphene and d) galactosyl–pyrene-linked-graphene before and after flowing Con A solutions. e) Real-time, reversible Con A sensing using mannosyl–pyrene-coated graphene field-effect sensors (VSG = 0.1 V).
stretchable functions for wearable electronics. For this demonstration, AgNWs were coated and patterned as array forms of S/D electrodes, and, then, graphene covered the dumbbell-like device surfaces connecting the channel and S/D continuously. The S/D areas of the graphene–AgNW hybrid structure were covered sequentially with a 500-nm-thick SU-8 passivation layer with openings around the graphene channels. Each block of the sensor array contained nine of the graphene field-effect sensors with a single common source and independent drains of the hybrid. The sensor chip had four such blocks composed of a total of 36 sensor devices. Figure 3b shows a photograph of this sample. Then, a microfluidic channel based on a polydimethylsiloxane (PDMS, Sylgard 184) mold was integrated onto the fabricated FET sensors for real-time, solution-gated sensing. (Figure S15a, Supporting Information) To detect lectins selectively, graphene FET sensors were noncovalently functionalized with the synthesized pyrene-based glyco-conjugates, as described above. Figure 3a shows the binding processes between glycans and mannose-binding lectins (Concanavalin A). Mannosyl–pyrenes, as receptor molecules, were immobilized on the graphene surfaces due to π–π stacking between graphene and the pyrene-linker part during incubation of the prepared sensors in the mannosyl–pyrene solutions. After incorporation of mannosyl–pyrene solutions into the graphene surfaces, specific interactions between lectin and mannosyl–pyrene were transduced to induce the current change (ΔID) by flowing Concanavalin A (Con A) solutions into the microfluidic channel. As shown in Figure 3c, a decrease in transconductance with a negative shift of −0.2 V in VG was observed using solution gating due to this Con A binding. To investigate the reversibility of the sensors, excess mannose solutions were injected into the fluidic channel by a syringe pump. As a result, the current level of the device was recovered to the original ID curve before the lectin solutions were added (Figure S15b, Supporting Information). The pristine graphene channel functionalized with mannosyl–pyrene can be recovered by flowing excess mannose over
4
wileyonlinelibrary.com
the channel, which reacts with Con A and, in turn, leads to the separation from the channel. In addition, as a control experiment, sensing was conducted with galactosyl–pyrene, which does not react with Con A, and a negligible change in the current was observed, as shown in Figure 3d. Real-time sensing of the protein using the sensor device is shown in Figure 3e. In this experiment, Con A and excess mannose solutions flowed alternately with similar time intervals. For the sensor that was functionalized using mannosyl–pyrene, the current decreased with ΔID of ≈2 µA on switching solutions to Con A in the p-type regime (solution-gate potential: 0.1 V), and it responded in the opposite way on switching to the excess mannose solution. In contrast, the sensor with galactosyl–pyrene group showed no response during the flow. Conventional designs of electronic sensors contain the rigid and big power sources as well as direct physical connections between the sensing probes and data processing electronics.[52,53] In order to eliminate the need for an onboard battery and external connections in order to detect the biomaterials wirelessly, we incorporated a resonant coil into the device. This sensor system was composed of a passive RLC resonant circuit with an inductive (L) coil for wireless transmission and a capacitive (C) electrode contacting a resistive (R) sensor channel. For this circuit, graphene and a single-layered hybrid structure were used for the channel and all electrodes, respectively. And graphene monolithically connected all conductive components with continuous interfaces. The entire circuit system is transferrable to other substrates, and its ultrathin and light characteristics are advantageous for conformable adhesion on the planar or nonplanar surfaces of the substrates.[54] In our experiment, we selected parylene as the substrate because it provided the advantages of (1) good adhesion, (2) biocompatibility, (3) resistance to chemicals, such as acetone and acid, which are necessary for the transfer step, and (4) stretchability.[55,56] To fabricate the circuit, first, a parylene film (with a thickness of 500 nm) was coated onto a Cu foil. Then, AgNWs were spun and patterned for all electrode
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. a) Schematic image of the wireless sensor, which is composed of the antenna and electrodes based on hybrid structures, and a graphene channel. b) Frequency response of the reflection coefficient of the antenna on the plastic substrates after buffer and Con A treatment. c) Photographs of the floating devices on the water (top) and fabricated sensor onto the 1.4-µm-thick PET films (bottom). Scale bar: 1 cm (inset: whole image of the device floating on water. Scale bar: 2 cm). d) Schematic of the biosensor attached to the skin on the back of a human hand. e) Photograph of the transparent wireless sensor attached to the human skin. Scale bar: 1 cm (inset: close-up image of the device. Scale bar: 0.5 cm). f) Wireless sensing curves of a sensor on a hand before and after stretching.
areas, including the inductive coil antenna. Subsequently, graphene was transferred to cover all of the circuit patterns for the channel and all electrodes. Finally, an SU-8 passivation layer was coated with openings around the graphene channels. Figures 4a and S16 (Supporting Information) show the circuit layout and photographs of this sample, and Figure 4b shows the measured reflection of this wireless sensor circuit with a resonant frequency of 4.1 GHz. It also presents the RF responses of the wireless device when exposed to Con A. The measured reflection value (S11 parameter) at the resonance frequency, which is inversely proportional to the electric resistance of the graphene channel, was decreased by sensing Con A. Furthermore, the high elastic modulus of graphene and AgNWs with the hydrophobic nature of parylene enabled this entire circuit (in a free-standing form with the parylene substrate) to float sustainably on water, as shown in Figure 4c. Also, this ultrathin circuit can be conformably attached onto a variety of substrates. As an example, Figure 4d and 4e shows the transparent, wireless sensor circuit attached to the skin of a human hand. Its good stretchability enabled its consistent operation during the diverse deformations of the skin, such as bending, twisting, rolling, and stretching motions (Figure 4f and Movie S1, Supporting Information). In addition, the stability of graphene, AgNWs, and the hybrid films against thermal oxidation has been studied previously.[19] Because oxygen gas and moisture cannot permeate through the graphene layer, the graphene covering AgNWs in the hybrid structure of our devices can act as a passivation layer to retard Ag oxidation as well as providing conductive paths. These results suggested substantial potential for applications in transparent, wearable electronics, including invisible electronic tattoos.
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
In conclusion, we described the advantages of FET sensors with graphene channels and all electrodes of the graphene– AgNW hybrid nanostructures, as transparent and stretchable electronic devices. Graphene is continuous through all of the conducting components of channels and electrodes in this device, which is advantageous to enhance FET mobility with reducing contact resistance between the channel and S/D. Glycosylation of the graphene channel was studied for real-time detection of protein using these FET sensors. Furthermore, graphene and its hybrid structures with AgNWs enabled the fabrication of RLC resonant circuits integrated with these sensors for wireless monitoring without the need for a power source. This transparent and stretchable sensor circuit can be transferable onto diverse substrates, and the very thin and light characteristics of the device can improve its conformable adhesion on the substrates. Consistent operation of this invisible sensor circuit attached on the skin of a human hand suggests a promising strategy for developing transparent and wearable electronic devices, indicating substantial promise for next-generation electronics.
Experimental Section Experimental details are given in the Supporting Information.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Acknowledgements J.K and M.-S.L. contributed equally to this work. This work was supported by the Ministry of Science, ICT & Future Planning and the Ministry of Trade, Industry and Energy of Korea through Basic Science Research Program of National Research Foundation (Grant No. 2013R1A2A2A01068542), IT R&D program (10041416), Materials Original Technology Program (10041222), Technology Innovation Program (Grant No. 10044410), Convergence Technology Development Program for Bionic Arm (NRF-2014M3C1B2048198), and Pioneer Research Center Program (NRF-2014M3C1A3001208). Received: February 10, 2015 Revised: March 18, 2015 Published online: [1] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539. [2] M. Vosgueritchian, D. J. Lipomi, Z. Bao, Adv. Funct. Mater. 2012, 22, 421. [3] Z. Wu, Z. Chen, X. Du, J. M. Logan, J. Sippel, M. Nikolou, K. Kamaras, J. R. Reynolds, D. B. Tanner, A. F. Hebard, A. G. Rinzler, Science 2004, 305, 1273. [4] G. Yu, A. Cao, C. M. Lieber, Nat. Nanotechnol. 2007, 2, 372. [5] a) J.-U. Park, M. A. Meitl, S.-H. Hur, M. L. Usrey, M. S. Strano, P. J. A. Kenis, J. A. Rogers, Angew. Chem. 2006, 118, 595; b) J.-U. Park, M. A. Meitl, S.-H. Hur, M. L. Usrey, M. S. Strano, P. J. A. Kenis, J. A. Rogers, Angew. Chem. Int. Ed. Engl. 2006, 45, 581. [6] R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, A. K. Geim, Science 2008, 320, 1308. [7] J.-U Park, S. W. Nam, M.-S. Lee, C. M. Lieber, Nat. Mater. 2012, 11, 120. [8] S. Bas, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. R. Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozyilmaz, J.-H. Ahn, B. H. Hong, S. Iijima, Nat. Nanotechnol. 2010, 5, 574. [9] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo, R. S. Ruoff, Science 2009, 324, 1312. [10] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [11] C. M. Lieber, MRS Bull. 2011, 36, 1052. [12] Y. Wu, J. Xiang, C. Yang, W. Lu, C. M. Lieber, Nature 2004, 430, 61. [13] C. Wang, Y. Hu, C. M. Lieber, S. J. Sun, Am. Chem. Soc. 2008, 130, 8902. [14] D. Wang, C. M. Lieber, Nat. Mater. 2003, 2, 355. [15] S. De, T. M. Higgins, P. E. Lyons, E. M. Doherty, P. N. Nirmalraj, W. J. Blau, J. J. Boland, J. N. Coleman, ACS Nano 2009, 3, 1767. [16] L. Hu, H. Wu, Y. Cui, MRS Bull. 2011, 36, 760. [17] Y. Zhu, Z. Sun, Z. Yan, Z. Jin, J. M. Tour, ACS Nano 2011, 5, 6472. [18] P. B. Catrysse, S. Fan, Nano Lett. 2010, 10, 2944. [19] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.-Y. Lee, S. W. Nam, J.-U. Park, Nano Lett. 2013, 13, 2814. [20] I. N. Kholmanov, C. W. Magnuson, A. E. Aliev, H. Li, B. Zhang, J. W. Suk, L. L. Zhang, E. Peng, S. H. Mousavi, A. B. Khanikaev, R. Piner, G. Shvets, R. S. Ruoff, Nano Lett. 2012, 12, 5679. [21] C. Jeong, P. Nair, M. Khan, M. Lundstrom, M. A. Alam, Nano Lett. 2011, 11, 5020. [22] R. Chen, S. R. Das, C. Jeong, M. R. Khan, D. B. Janes, M. A. Alam, Adv. Funct. Mater. 2013, 23, 5150. [23] L. Li, J. Liang, S.-Y. Chou, X. Zhu, X. Niu, Z. Yu, Q. Pei, Sci. Rep. 2014, 4, 04307.
6
wileyonlinelibrary.com
[24] H. Koga, M. Nogi, N. Komoda, T. T. Nge, R. Sugahara, K. Suganuma, NPG Asia Mater. 2014, 6. [25] L. Yang, T. Zhang, H. Zhou, S. C. Price, B. J. Wiley, W. You, ACS Appl. Mater. Interfaces 2011, 3, 4075. [26] W. Hu, X. Niu, L. Li, S. Yun, Z. Yu, Q. Pei, Nanotechnology 2012, 23, 344002. [27] K. Lee, J. Park, M.-S. Lee, J. Kim, B. G. Hyun, D. J. Kang, K. Na, C. Y. Lee, F. Bien, J.-U. Park, Nano Lett. 2014, 14, 2647. [28] S.-K. Lee, H. Y. Jang, S. Jang, E. Choi, B. H. Hong, J. Lee, S. Park, J.-H. Ahn, Nano Lett. 2012, 12, 3472. [29] Y. Zhang, J. P. Small, W. V. Pontius, P. Kim, Appl. Phys. Lett. 2005, 86, 073104. [30] S. M. Song, J. K. Park, O. J. Sul, B. J. Cho, Nano Lett. 2012, 12, 3887. [31] A. Venugopal, L. Colombo, E. M. Vogel, Appl. Phys. Lett. 2010, 96, 013512. [32] S. K. Hong, S. M. Song, O. J. Sul, B. J. Cho, Carbon Lett. 2013, 14, 171. [33] F. Xia, V. Perebeinos, Y.-M. Lin, Y. Wu, P. Avouris, Nat. Nanotechnol. 2011, 6, 179. [34] K. Nagashio, T. Nishimura, K. Kita, A. Toriumi, Electron Devices Meeting (IEDM), IEEE Int., Baltimore 2009. [35] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, Nature 2005, 438, 197. [36] Y. Zhang, Y.-W. Tan, H. L. Stormer, P. Kim, Nature 2005, 438, 201. [37] A. Reina, X. Jia, J. Ho, D. Nezich, H. Son, V. Bulovic, M. S. Dresselhaus, J. Kong, Nano Lett. 2009, 9, 30. [38] J. Zaumseil, K. W. Baldwin, J. A. Rogers, J. Appl. Phys. 2003, 93, 6117. [39] D. K. Schroder, Semiconductor Material and Device Characterization, 3rd ed., John Wiley & Sons, Canada 2006. [40] D.-H. Kim, N. Lu, R. Ma, Y.-S. Kim, R.-H. Kim, S. Wang, J. Wu, S. M. Won, H. Tao, A. Islam, K. J. Yu, T.-I. Kim, R. Chowdhury, M. Ying, L. Xu, M. Li, H.-J. Chung, H. Keum, M. McCormick, P. Liu, Y.-W. Zhang, F. G. Omenetto, Y. Huang, T. Coleman, J. A. Rogers, Science 2011, 333, 838. [41] J. A. Fan, W.-H. Yeo, Y. Su, Y. Hattori, W. Lee, S.-Y. Jung, Y. Zhang, Z. Liu, H. Cheng, L. Falgout, M. Bajema, T. Coleman, D. Gregoire, R. J. Larsen, Y. Huang, J. A. Rogers, Nat. Commun. 2014, 5, 3266. [42] M.-S. Lee, J. Kim, J. Park, J.-U. Park, Nanoscale Res. Lett. 2015, 10, 27. [43] a) W. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson, J. J. Gooding, F. Braet, Angew. Chem. 2010, 122, 2160; b) W. Yang, K. R. Ratinac, S. P. Ringer, P. Thordarson, J. J. Gooding, F. Braet, Angew. Chem. Int. Ed. 2010, 49, 2114. [44] Y. Huang, X. Dong, Y. Shi, C. M. Li, L.-J. Li, P. Chen, Nanoscale 2010, 2, 1485. [45] O. S. Kwon, S. H. Lee, S. J. Park, J. H. An, H. S. Song, T. Kim, J. H. Oh, J. Bae, H. Yoon, T. H. Park, J. Jang, Adv. Mater. 2013, 25, 4177. [46] O. S. Kwon, S. J. Park, J.-Y. Hong, A.-R. Han, J. S. Lee, J. S. Lee, J. H. Oh, J. Jang, ACS Nano 2012, 6, 1486. [47] R. A. Dwek, Chem. Rev. 1996, 96, 683. [48] A. Varki, Glycobiology 1993, 3, 97. [49] B. G. Davis, Chem. Rev. 2002, 102, 579. [50] V. Georgakilas, M. Otyepka, A. B. Bourlinos, V. Chandra, N. Kim, K. C. Kemp, P. Hobza, R. Zboril, K. S. Kim, Chem. Rev. 2012, 112, 6156. [51] F. M. Ibatullin, S. I. Selivanov, A. G. Shavva, Synthesis 2001, 2001, 33. [52] C. M. Li, H. Dong, X. Cao, J. H. T Luong, X. Zhang, Curr. Med. Chem. 2007, 14, 937. [53] M. O. Schurr, S. Schostek, C. N. Ho, F. Rieber, A. Menciassi, Minim. Invasiv. Ther. 2007, 16, 76. [54] Y. J. Park, S.-K. Lee, M.-S. Kim, H. Kim, J.-H. Ahn, ACS Nano 2014, 8, 7655. [55] E. Meng, P.-Y. Li, Y.-C. Tai, Sens. Actuator A. 2008, 144, 18. [56] D. C. Rodger, J. D. Weiland, M. S. Humayun, Y.-C. Tai, Sens. Actuator B. 2006, 117, 107.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500710
