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Water-Free Transfer Method for CVD-Grown Graphene and Its Application to Flexible Air-Stable Graphene Transistors Hyun Ho Kim, Yoonyoung Chung, Eunho Lee, Seong Kyu Lee, and Kilwon Cho* Since graphene was experimentally obtained by mechanical exfoliation from graphite,[1] researchers have developed a number of synthetic methods for making large-area graphene that improve the yield and increase the throughput of graphene fabrication.[2–5] Chemical vapor deposition (CVD) is one of the most promising approaches to fabricate large-area high-quality graphene.[2,3,5] In the CVD process, graphene is generally grown on a catalytic metal substrate such as copper[3,6] or nickel,[2,5] so an additional transfer process is generally required to place the graphene layer onto the target substrate. The wet transfer method, in which a polymeric supporting layer is coated onto the graphene/metal layers followed by wet etching of the catalytic metal and scooping of the polymer/graphene onto the target substrate, has become widely used for CVD-grown graphene because it is a simple process that results in a few crack formations.[3,7,8] However, the wet transfer process requires water to float and stretch the polymer/graphene layers without shrinking prior to the scooping process. Thus, the wet transfer cannot be used to place graphene onto water-sensitive substrates, which limits the applications of CVD-grown graphene. Graphene has several unique properties including high mobility (200 000 cm2/V·s),[9] optical transparency (97.7%),[10] elongation (∼20%),[2] and thermal conductivity (∼5000 W/m·K),[11] which makes this material a strong candidate as a principal building block in future flexible stretchable electronic applications.[2] In most of these applications, graphene field-effect transistors (GFETs) are essential because they perform signal conditioning such as modulation,[12] frequency multiplication,[13] and amplification.[14] Although previous studies have reported the fabrication of flexible GFETs,[15] several drawbacks, such as poor stability in air, poor reliability under mechanical stress, and high gate leakage, still need to be overcome for practical applications. We report herein a water-free method for the transfer of CVD-grown graphene onto a water-sensitive substrate and the application of this method to fabricate flexible, air-stable, low-voltage GFETs. The key feature of our transfer method is preventing the contact of water with the target substrate H. H. Kim,[+] E. Lee, S. K. Lee,[+] Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology Pohang 790–784, Republic of Korea E-mail: [email protected] Dr. Y. Chung, Prof. K. Cho Polymer Research Institute Pohang University of Science and Technology Pohang 790–784, Republic of Korea [+]H. H. Kim and Dr. Y. Chung contributed equally to this work.
DOI: 10.1002/adma.201305940
Adv. Mater. 2014, DOI: 10.1002/adma.201305940
through the lamination of dried graphene; thus, graphene can be transferred onto a variety of materials even those that are easily damaged or dissolved by water, such as water-sensitive inorganics, silk, and organic semiconductors. The trapping of water between the substrate and graphene is also effectively prevented. In contrast to a previous study,[16] in which CVDgrown graphene coated with poly(methylmethacrylate) (PMMA) was dry-transferred by using a polydimethylsiloxane stamp, our method adds a polybutadiene (PBU) layer to the graphene. This nonpolar PBU layer prevents undesirable Fermi level change[7] and reduces the charged-impurity scattering from a polar adjacent layer or residues such as PMMA.[17] A polymeric bilayer consisting of PBU (bottom) and PMMA (top) rather than PMMA alone can be used as a supporting layer for graphene transfer without affecting its promising intrinsic characteristics. Furthermore, the polymeric layers coated onto graphene protect it against detrimental ambient species such as water, and so enhance the air stability of the resulting graphene devices. The Dirac voltage (VDirac) and field-effect mobility (µFET) values of the GFETs with the polymeric layers did not change appreciably in air for more than 130 days, whereas the GFETs without the polymers underwent significant and rapid degradation. Air-stable graphene devices are vital for practical applications of this novel material. In order to achieve low-voltage operation, we utilized nanometer-thick aluminum oxide (AlOX) made by atomic layer deposition (ALD) as the gate dielectric. When conventional wet transfer is used, such thin AlOX is difficult to be used because it is easily damaged by water.[18] Our water-free transfer process prevents the trapping of water between the graphene and the substrate, and so also minimizes the effects of water molecules such as charged impurities[19] and graphene wrinkles.[20] The water-free transfer process is illustrated in Figure 1a. A graphene monolayer was grown on a catalytic copper foil by CVD,[3] followed by sequential spin coatings of polymeric supporting layers consisting of PBU and PMMA. After removing the copper foil with a wet etchant and rinsing the polymers/ graphene layers with deionized (DI) water, we scooped up the polymers/graphene layers, floating on DI water, by using a sample holder with a square hole (hole area = ∼4 cm2). The suspended polymers/graphene layers were kept in a vacuum oven at 60 °C for 1 hour to dry the sample. Then, we placed the sample holder onto a target substrate (plastic film or silicon) and blew nitrogen gas to press the polymers/graphene layers onto the substrate. During the nitrogen-gas pressing, the substrate was heated to 120 °C, which is higher than the glass transition temperature (Tg) of the polymers (Tg_PBU = −95 °C and Tg_PMMA = 105 °C). This substrate heating improves the adhesion and the uniformity of the polymers/graphene layers on the substrate. The gas flow from a regular nitrogen gun was sufficiently strong to break the edges of the suspended
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polymers/graphene and to laminate the layers onto the target substrate. The polymeric bilayer (PMMA/PBU) provides not only robust support during the transfer process, but also effective passivation that protects graphene from undesirable charged impurities. When a passivation layer is unnecessary, PMMA can be used alone as a rigid supporting layer during the graphene transfer and then removed with organic solvents (e.g., acetone or chloroform). When a PBU supporting layer was used without PMMA, the PBU/graphene layers immediately shrank in water during the etching of the catalytic copper foil because the rubbery PBU layer is not sufficiently rigid. The quality of the water-free-transferred graphene was examined with field-emission scanning electron microscopy (FESEM), optical microscopy (OM), atomic force microscopy (AFM), and Raman spectroscopy, as shown in Figures 1b–e. These results show that the graphene was uniformly transferred onto the substrate with only a few cracks. In the Raman spectroscopy results, no D band (1350 cm−1) was observed, and the G and 2D bands were located at 1585 and 2678 cm−1, respectively, with a 2D/G ratio of 2.23. The almost negligible intensity of the D band indicates that our transfer technique does not generate any defects (sp3) in the graphene. Moreover, the location of the G and 2D bands and the 2D/G ratio confirm that the Fermi level of the graphene does not change during the transfer process.[21] Comparing to the graphene obtained with the widely used wet transfer process,[3] our water-free-transferred graphene has almost equivalent high quality. Utilizing the water-free transfer method, we fabricated flexible GFETs on a polyimide substrate (125 µm). A nickel layer (30 nm) was deposited in an e-beam evaporator with a shadow mask for gate electrodes, and an AlOX layer was made by ALD 2
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on the nickel layer as the gate dielectric. The thickness of the AlOX was 12.6 (±0.2) nm, as measured with transmission electron microscopy (see Supporting Information Figure S1). A tetradecylphosphonic acid self-assembled monolayer was formed on the AlOX to passivate any charge traps on the surface.[22] The capacitance of the gate dielectric was 0.411 (±0.003) µF/cm2, which means that our GFETs can operate at only a few volts. This capacitance value is equivalent to that of 8.4-nm-thick silicon dioxide (SiO2). Gold source/drain electrodes (40 nm) were thermally evaporated on the gate dielectric by using a shadow mask. PMMA/PBU/graphene layers were then placed on the sample by using the water-free transfer process, followed by reactive ion etching (RIE) with O2 gas to pattern the graphene. During the RIE, the polymer layers reduce the detrimental effects on the graphene from O2 radicals, whereas the graphene etched in the absence of the polymers was found to be severely damaged, confirmed by Raman spectroscopy (see Supporting Information Figure S2). Figure 2a shows a schematic diagram and images of a flexible GFET. We measured the current-voltage (I–V) characteristics of the flexible GFETs in air, as shown in Figures 2b and 2c. Our GFETs have a high IMAX/IMIN ratio of more than 6 at a supply voltage of only 4 V, and no hysteresis is evident. The I–V data are in good agreement with a previous model,[23] which we used to extract µFET. The devices have hole and electron µFET values of 2575 (±278) and 3075 (±193) cm2/V·s, respectively, and an average VDirac of 0.38 (±0.22) V (see Supporting Information Figure S3 for the histograms). The gate leakage current of our GFETs ranges from only 1 to 20 pA. In contrast, the GFETs fabricated via the conventional wet transfer of graphene exhibit significant gate leakage, as shown in Figure 2d, which indicates that the
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Figure 2. Schematic diagram and electrical properties of a flexible GFET fabricated by the water-free transfer of graphene. All measurements were performed in air, and the drain-source voltage (VDS) was fixed at 1 mV, except for the results in Figure 2b. The width (W) and length (L) of the channel are 200 µm and 100 µm, respectively. (a) Schematic diagram and images of the flexible GFET. Red dashed lines indicate the patterned edges of the PMMA/PBU/graphene layers. (b, c) Output and transfer curves of the flexible GFETs. (d) Gate leakage current of the flexible GFETs made by using the water-free transfer of graphene and the wet transfer of graphene. The leakage current is significantly higher when the graphene is wet-transferred because water dissolves the gate dielectric made of aluminum oxide. (e) Normalized Dirac voltage (VDirac) and hole mobility (µhole) values of the flexible GFETs after consecutive bending tests, and photographs during the test. The GFET performance does not change appreciably up to 5000 bendings with a bending radius of 0.5 cm.
AlOX layer (gate dielectric) has been damaged by water.[18] One of the most important features of graphene is that it is highly flexible. When the flexible GFETs were bent with a bending radius of 0.5 cm and measured, the I–V characteristics remained the same. To further test the durability against mechanical strain, we bent our flexible GFETs up to 5,000 times with the same bending radius and measured their I–V characteristics. As shown in Figure 2e, the performance of the flexible GFETs does not change much with the consecutive bendings and releasings; the variations in µFET and VDirac are less than 10% of their initial values. The surface strain induced by the bending can be estimated with the formula t/2r, where t is the sample thickness and r is the radius of curvature.[24] The total thickness of the GFETs, including the substrate, is approximately 125 µm,
Adv. Mater. 2014, DOI: 10.1002/adma.201305940
so the surface strain is ∼1.25% when the GFETs are fully bent. We conclude that this amount of strain does not degrade the electrical characteristics of the metal layers (less than 40 nm), the AlOX gate dielectric (12.6 nm), and the PMMA/PBU layers (approximately 100 nm), so our GFETs are highly flexible (see Supporting Information Figure S4 for the surface image of PMMA/PBU before/after the bending tests). The long-term stability of our flexible GFETs was measured for 133 days, while the devices were kept in air (23–25 °C and a relative humidity of 47–52%) with no voltage applied. The results in Figure 3a show the outstanding air stabilities of both µFET and VDirac of the flexible GFETs. The average hole mobility was maintained for more than 130 days without degradation. During the first day, the average VDirac shifted positively from
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Figure 3. Long-term air stability of the GFETs. The drain-source voltage was fixed at 1 mV during the measurements, and no voltage was applied between each test. (a) Transfer characteristics of the flexible GFETs in Figure 2 while the devices were kept in ambient air, and their average hole mobility data over 133 days. The current-voltage characteristics were maintained owing to the PMMA/PBU passivating layers on graphene, except the Dirac voltage increase during the first day. The average hole mobility did not degrade over more than 130 days. (b) The air stability of the GFETs under high-humidity conditions (25 °C and a relative humidity of 80%). Silicon (Si) and thermally-grown silicon dioxide (SiO2) were used as the gate electrode and the gate dielectric respectively, with gold (Au) source/drain electrodes (W: 200 µm & L: 90 µm). The transfer curves of the GFETs were measured while the devices were exposed to the humid conditions for 5 hours. The current-voltage characteristics of the GFETs with PMMA/PBU did not change, whereas the GFETs without the polymers underwent a rapid degradation and change.
0.2 to 1.2 V, which we attribute to the adsorption of water molecules at the patterned edges of the PMMA/PBU/graphene (see the red dashed lines in Figure 2a).[19,25] In spite of the VDirac shift at the beginning, the average µFET did not change appreciably. Owing to the PMMA/PBU layers, the charged impurities in air only affect the patterned edges of the graphene, and charged-impurity scattering[17] in the channel of the GFETs is minimized. To further investigate the effects of the PMMA/ PBU layers on the air stability of the GFETs, we compared GFETs with and without the polymeric bilayer. As previous
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studies have shown that water changes the electronic structure of graphene,[19,25,26] we kept the samples under high-humidity conditions (25 °C and a relative humidity of 80%). For the GFETs without the PMMA/PBU layers, conventional wet graphene transfer[3] was used. Thermally grown 300-nm-thick SiO2 on highly doped silicon was used as the gate dielectric and the common gate electrode for both sets of GFETs since thin AlOX is damaged by water during the wet transfer of graphene.[18] No graphene patterning was performed in this experiment. As shown in Figure 3b, there is almost no change in either the µFET or the VDirac of one of the GFETs with PMMA/PBU. In contrast, the GFETs without PMMA/PBU underwent considerable degradation during the first 5 hours (see Supporting Information Figure S5 for the VDirac and µFET data). After 5 hours, the performance remained unchanged, which indicates the adsorption of water on the graphene is saturated within 5 hours under these experimental conditions. We have confirmed that water molecules severely degrade the performance of GFETs and that the PMMA/PBU layers can be used as good passivation for graphene devices. In conclusion, we have developed a water-free transfer method for CVD-grown large-area graphene. Utilizing this method, we successfully fabricated flexible GFETs that operate at a low supply voltage of 4 V. The polymer layers used in the transfer process were found to provide excellent passivation and to protect graphene against detrimental ambient species; thus, the stable operation of the GFETs was maintained over a long period of time. As all device components are tens of nanometers thick, our flexible GFETs showed almost negligible changes in I–V characteristics after 5,000 bending and releasing repetitions with a bending radius of 0.5 cm. We believe that our transfer method expands the usage of graphene to a variety of substrates and that the flexible and air-stable GFETs will be used in future flexible electronic applications.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank Professor H.-J. Lee and Professor W. H. Lee for discussions. This work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT, and Future Planning (2011–0031628) and by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2013R1A1A2012046). Received: December 3, 2013 Revised: December 31, 2013 Published online: [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666. [2] 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. [3] 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.
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[4] a) D. C. Wei, Y. Q. Liu, Adv. Mater. 2010, 22, 3225; b) S. Y. Bae, I. Y. Jeon, J. Yang, N. Park, H. S. Shin, S. Park, R. S. Ruoff, L. M. Dai, J. B. Baek, ACS Nano 2011, 5, 4974. [5] a) Z. P. Chen, W. C. Ren, L. B. Gao, B. L. Liu, S. F. Pei, H. M. Cheng, Nat. Mater. 2011, 10, 424; b) S. J. Chae, F. Gunes, K. K. Kim, E. S. Kim, G. H. Han, S. M. Kim, H. J. Shin, S. M. Yoon, J. Y. Choi, M. H. Park, C. W. Yang, D. Pribat, Y. H. Lee, Adv. Mater. 2009, 21, 2328. [6] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Ri Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozyilmaz, J.-H. Ahn, B. H. Hong, S. Iijima, Nat. Nano. 2010, 5, 574. [7] W. H. Lee, J. Park, S. H. Sim, S. Lim, K. S. Kim, B. H. Hong, K. Cho, J. Am. Chem. Soc. 2011, 133, 4447. [8] W. H. Lee, J. Park, S. H. Sim, S. B. Jo, K. S. Kim, B. H. Hong, K. Cho, Adv. Mater. 2011, 23, 1752. [9] X. Du, I. Skachko, A. Barker, E. Y. Andrei, Nat. Nano. 2008, 3, 491. [10] 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. [11] A. A. Balandin, S. Ghosh, W. Z. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C. N. Lau, Nano Lett. 2008, 8, 902. [12] S. Lee, K. Lee, C. H. Liu, G. S. Kulkarni, Z. H. Zhong, Nat. Commun. 2012, 3, 1018. [13] H. Wang, D. Nezich, J. Kong, T. Palacios, IEEE Electron Device Lett. 2009, 30, 547. [14] S.-J. Han, K. A. Jenkins, A. Valdes Garcia, A. D. Franklin, A. A. Bol, W. Haensch, Nano Lett. 2011, 11, 3690. [15] a) B. J. Kim, H. Jang, S.-K. Lee, B. H. Hong, J.-H. Ahn, J. H. Cho, Nano Lett. 2010, 10, 3464; b) J. Lee, T.-J. Ha, H. Li, K. N. Parrish,
M. Holt, A. Dodabalapur, R. S. Ruoff, D. Akinwande, ACS Nano 2013, 7, 7744. J. W. Suk, A. Kitt, C. W. Magnuson, Y. Hao, S. Ahmed, J. An, A. K. Swan, B. B. Goldberg, R. S. Ruoff, ACS Nano 2011, 5, 6916. J. H. Chen, C. Jang, S. Adam, M. S. Fuhrer, E. D. Williams, M. Ishigami, Nat. Phys. 2008, 4, 377. a) R. K. Nahar, Sensor Actuat B-Chem 2000, 63, 49; b) A. I. Abdulagatov, Y. Yan, J. R. Cooper, Y. Zhang, Z. M. Gibbs, A. S. Cavanagh, R. G. Yang, Y. C. Lee, S. M. George, ACS Appl. Mater. Interfaces 2011, 3, 4593. T. O. Wehling, A. I. Lichtenstein, M. I. Katsnelson, Appl. Phys. Lett. 2008, 93, 202110. W. Zhu, T. Low, V. Perebeinos, A. A. Bol, Y. Zhu, H. Yan, J. Tersoff, P. Avouris, Nano Lett. 2012, 12, 3431. A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari, A. K. Sood, Nat. Nanotechnol. 2008, 3, 210. a) W. H. Lee, J. Park, Y. Kim, K. S. Kim, B. H. Hong, K. Cho, Adv. Mater. 2011, 23, 3460; b) Y. Chung, O. Johnson, M. Deal, Y. Nishi, B. Murmann, Z. Bao, Appl. Phys. Lett. 2012, 101, 063304. S. Kim, J. Nah, I. Jo, D. Shahrjerdi, L. Colombo, Z. Yao, E. Tutuc, S. K. Banerjee, Appl. Phys. Lett. 2009, 94, 062107. V. D. d. Silva, Mechanics and strength of materials Springer, Berlin; New York 2006. F. Yavari, C. Kritzinger, C. Gaire, L. Song, H. Gullapalli, T. BorcaTasciuc, P. M. Ajayan, N. Koratkar, Small 2010, 6, 2535. J. Shim, C. H. Lui, T. Y. Ko, Y. J. Yu, P. Kim, T. F. Heinz, S. Ryu, Nano Lett. 2012, 12, 648.
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Water-Free Transfer Method for CVD-Grown Graphene and Its Application to Flexible Air-Stable Graphene Transistors Hyun Ho Kim, Yoonyoung Chung, Eunho Lee, Seong Kyu Lee, and Kilwon Cho* Since graphene was experimentally obtained by mechanical exfoliation from graphite,[1] researchers have developed a number of synthetic methods for making large-area graphene that improve the yield and increase the throughput of graphene fabrication.[2–5] Chemical vapor deposition (CVD) is one of the most promising approaches to fabricate large-area high-quality graphene.[2,3,5] In the CVD process, graphene is generally grown on a catalytic metal substrate such as copper[3,6] or nickel,[2,5] so an additional transfer process is generally required to place the graphene layer onto the target substrate. The wet transfer method, in which a polymeric supporting layer is coated onto the graphene/metal layers followed by wet etching of the catalytic metal and scooping of the polymer/graphene onto the target substrate, has become widely used for CVD-grown graphene because it is a simple process that results in a few crack formations.[3,7,8] However, the wet transfer process requires water to float and stretch the polymer/graphene layers without shrinking prior to the scooping process. Thus, the wet transfer cannot be used to place graphene onto water-sensitive substrates, which limits the applications of CVD-grown graphene. Graphene has several unique properties including high mobility (200 000 cm2/V·s),[9] optical transparency (97.7%),[10] elongation (∼20%),[2] and thermal conductivity (∼5000 W/m·K),[11] which makes this material a strong candidate as a principal building block in future flexible stretchable electronic applications.[2] In most of these applications, graphene field-effect transistors (GFETs) are essential because they perform signal conditioning such as modulation,[12] frequency multiplication,[13] and amplification.[14] Although previous studies have reported the fabrication of flexible GFETs,[15] several drawbacks, such as poor stability in air, poor reliability under mechanical stress, and high gate leakage, still need to be overcome for practical applications. We report herein a water-free method for the transfer of CVD-grown graphene onto a water-sensitive substrate and the application of this method to fabricate flexible, air-stable, low-voltage GFETs. The key feature of our transfer method is preventing the contact of water with the target substrate H. H. Kim,[+] E. Lee, S. K. Lee,[+] Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology Pohang 790–784, Republic of Korea E-mail: [email protected] Dr. Y. Chung, Prof. K. Cho Polymer Research Institute Pohang University of Science and Technology Pohang 790–784, Republic of Korea [+]H. H. Kim and Dr. Y. Chung contributed equally to this work.
DOI: 10.1002/adma.201305940
Adv. Mater. 2014, DOI: 10.1002/adma.201305940
through the lamination of dried graphene; thus, graphene can be transferred onto a variety of materials even those that are easily damaged or dissolved by water, such as water-sensitive inorganics, silk, and organic semiconductors. The trapping of water between the substrate and graphene is also effectively prevented. In contrast to a previous study,[16] in which CVDgrown graphene coated with poly(methylmethacrylate) (PMMA) was dry-transferred by using a polydimethylsiloxane stamp, our method adds a polybutadiene (PBU) layer to the graphene. This nonpolar PBU layer prevents undesirable Fermi level change[7] and reduces the charged-impurity scattering from a polar adjacent layer or residues such as PMMA.[17] A polymeric bilayer consisting of PBU (bottom) and PMMA (top) rather than PMMA alone can be used as a supporting layer for graphene transfer without affecting its promising intrinsic characteristics. Furthermore, the polymeric layers coated onto graphene protect it against detrimental ambient species such as water, and so enhance the air stability of the resulting graphene devices. The Dirac voltage (VDirac) and field-effect mobility (µFET) values of the GFETs with the polymeric layers did not change appreciably in air for more than 130 days, whereas the GFETs without the polymers underwent significant and rapid degradation. Air-stable graphene devices are vital for practical applications of this novel material. In order to achieve low-voltage operation, we utilized nanometer-thick aluminum oxide (AlOX) made by atomic layer deposition (ALD) as the gate dielectric. When conventional wet transfer is used, such thin AlOX is difficult to be used because it is easily damaged by water.[18] Our water-free transfer process prevents the trapping of water between the graphene and the substrate, and so also minimizes the effects of water molecules such as charged impurities[19] and graphene wrinkles.[20] The water-free transfer process is illustrated in Figure 1a. A graphene monolayer was grown on a catalytic copper foil by CVD,[3] followed by sequential spin coatings of polymeric supporting layers consisting of PBU and PMMA. After removing the copper foil with a wet etchant and rinsing the polymers/ graphene layers with deionized (DI) water, we scooped up the polymers/graphene layers, floating on DI water, by using a sample holder with a square hole (hole area = ∼4 cm2). The suspended polymers/graphene layers were kept in a vacuum oven at 60 °C for 1 hour to dry the sample. Then, we placed the sample holder onto a target substrate (plastic film or silicon) and blew nitrogen gas to press the polymers/graphene layers onto the substrate. During the nitrogen-gas pressing, the substrate was heated to 120 °C, which is higher than the glass transition temperature (Tg) of the polymers (Tg_PBU = −95 °C and Tg_PMMA = 105 °C). This substrate heating improves the adhesion and the uniformity of the polymers/graphene layers on the substrate. The gas flow from a regular nitrogen gun was sufficiently strong to break the edges of the suspended
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Sample holder with square hole
N2 gas pressing
Polymers(PMMA/PBU) /graphene
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Raman Shift (cm-1) Figure 1. Water-free transfer method for CVD-grown graphene and spectroscopy results. (a) Illustration of the water-free transfer method. (b, c, d) Microscopy results for water-free-transferred graphene on SiO2: (b) FESEM, (c) OM, and (d) AFM. The graphene is of high quality with only a few cracks. (e) Raman spectra of graphene on SiO2. Red and black curves are for graphene obtained with water-free transfer and conventional wet transfer, respectively. The water-free-transferred graphene has almost identical Raman characteristics to the wet-transferred graphene.
polymers/graphene and to laminate the layers onto the target substrate. The polymeric bilayer (PMMA/PBU) provides not only robust support during the transfer process, but also effective passivation that protects graphene from undesirable charged impurities. When a passivation layer is unnecessary, PMMA can be used alone as a rigid supporting layer during the graphene transfer and then removed with organic solvents (e.g., acetone or chloroform). When a PBU supporting layer was used without PMMA, the PBU/graphene layers immediately shrank in water during the etching of the catalytic copper foil because the rubbery PBU layer is not sufficiently rigid. The quality of the water-free-transferred graphene was examined with field-emission scanning electron microscopy (FESEM), optical microscopy (OM), atomic force microscopy (AFM), and Raman spectroscopy, as shown in Figures 1b–e. These results show that the graphene was uniformly transferred onto the substrate with only a few cracks. In the Raman spectroscopy results, no D band (1350 cm−1) was observed, and the G and 2D bands were located at 1585 and 2678 cm−1, respectively, with a 2D/G ratio of 2.23. The almost negligible intensity of the D band indicates that our transfer technique does not generate any defects (sp3) in the graphene. Moreover, the location of the G and 2D bands and the 2D/G ratio confirm that the Fermi level of the graphene does not change during the transfer process.[21] Comparing to the graphene obtained with the widely used wet transfer process,[3] our water-free-transferred graphene has almost equivalent high quality. Utilizing the water-free transfer method, we fabricated flexible GFETs on a polyimide substrate (125 µm). A nickel layer (30 nm) was deposited in an e-beam evaporator with a shadow mask for gate electrodes, and an AlOX layer was made by ALD 2
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on the nickel layer as the gate dielectric. The thickness of the AlOX was 12.6 (±0.2) nm, as measured with transmission electron microscopy (see Supporting Information Figure S1). A tetradecylphosphonic acid self-assembled monolayer was formed on the AlOX to passivate any charge traps on the surface.[22] The capacitance of the gate dielectric was 0.411 (±0.003) µF/cm2, which means that our GFETs can operate at only a few volts. This capacitance value is equivalent to that of 8.4-nm-thick silicon dioxide (SiO2). Gold source/drain electrodes (40 nm) were thermally evaporated on the gate dielectric by using a shadow mask. PMMA/PBU/graphene layers were then placed on the sample by using the water-free transfer process, followed by reactive ion etching (RIE) with O2 gas to pattern the graphene. During the RIE, the polymer layers reduce the detrimental effects on the graphene from O2 radicals, whereas the graphene etched in the absence of the polymers was found to be severely damaged, confirmed by Raman spectroscopy (see Supporting Information Figure S2). Figure 2a shows a schematic diagram and images of a flexible GFET. We measured the current-voltage (I–V) characteristics of the flexible GFETs in air, as shown in Figures 2b and 2c. Our GFETs have a high IMAX/IMIN ratio of more than 6 at a supply voltage of only 4 V, and no hysteresis is evident. The I–V data are in good agreement with a previous model,[23] which we used to extract µFET. The devices have hole and electron µFET values of 2575 (±278) and 3075 (±193) cm2/V·s, respectively, and an average VDirac of 0.38 (±0.22) V (see Supporting Information Figure S3 for the histograms). The gate leakage current of our GFETs ranges from only 1 to 20 pA. In contrast, the GFETs fabricated via the conventional wet transfer of graphene exhibit significant gate leakage, as shown in Figure 2d, which indicates that the
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Figure 2. Schematic diagram and electrical properties of a flexible GFET fabricated by the water-free transfer of graphene. All measurements were performed in air, and the drain-source voltage (VDS) was fixed at 1 mV, except for the results in Figure 2b. The width (W) and length (L) of the channel are 200 µm and 100 µm, respectively. (a) Schematic diagram and images of the flexible GFET. Red dashed lines indicate the patterned edges of the PMMA/PBU/graphene layers. (b, c) Output and transfer curves of the flexible GFETs. (d) Gate leakage current of the flexible GFETs made by using the water-free transfer of graphene and the wet transfer of graphene. The leakage current is significantly higher when the graphene is wet-transferred because water dissolves the gate dielectric made of aluminum oxide. (e) Normalized Dirac voltage (VDirac) and hole mobility (µhole) values of the flexible GFETs after consecutive bending tests, and photographs during the test. The GFET performance does not change appreciably up to 5000 bendings with a bending radius of 0.5 cm.
AlOX layer (gate dielectric) has been damaged by water.[18] One of the most important features of graphene is that it is highly flexible. When the flexible GFETs were bent with a bending radius of 0.5 cm and measured, the I–V characteristics remained the same. To further test the durability against mechanical strain, we bent our flexible GFETs up to 5,000 times with the same bending radius and measured their I–V characteristics. As shown in Figure 2e, the performance of the flexible GFETs does not change much with the consecutive bendings and releasings; the variations in µFET and VDirac are less than 10% of their initial values. The surface strain induced by the bending can be estimated with the formula t/2r, where t is the sample thickness and r is the radius of curvature.[24] The total thickness of the GFETs, including the substrate, is approximately 125 µm,
Adv. Mater. 2014, DOI: 10.1002/adma.201305940
so the surface strain is ∼1.25% when the GFETs are fully bent. We conclude that this amount of strain does not degrade the electrical characteristics of the metal layers (less than 40 nm), the AlOX gate dielectric (12.6 nm), and the PMMA/PBU layers (approximately 100 nm), so our GFETs are highly flexible (see Supporting Information Figure S4 for the surface image of PMMA/PBU before/after the bending tests). The long-term stability of our flexible GFETs was measured for 133 days, while the devices were kept in air (23–25 °C and a relative humidity of 47–52%) with no voltage applied. The results in Figure 3a show the outstanding air stabilities of both µFET and VDirac of the flexible GFETs. The average hole mobility was maintained for more than 130 days without degradation. During the first day, the average VDirac shifted positively from
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Figure 3. Long-term air stability of the GFETs. The drain-source voltage was fixed at 1 mV during the measurements, and no voltage was applied between each test. (a) Transfer characteristics of the flexible GFETs in Figure 2 while the devices were kept in ambient air, and their average hole mobility data over 133 days. The current-voltage characteristics were maintained owing to the PMMA/PBU passivating layers on graphene, except the Dirac voltage increase during the first day. The average hole mobility did not degrade over more than 130 days. (b) The air stability of the GFETs under high-humidity conditions (25 °C and a relative humidity of 80%). Silicon (Si) and thermally-grown silicon dioxide (SiO2) were used as the gate electrode and the gate dielectric respectively, with gold (Au) source/drain electrodes (W: 200 µm & L: 90 µm). The transfer curves of the GFETs were measured while the devices were exposed to the humid conditions for 5 hours. The current-voltage characteristics of the GFETs with PMMA/PBU did not change, whereas the GFETs without the polymers underwent a rapid degradation and change.
0.2 to 1.2 V, which we attribute to the adsorption of water molecules at the patterned edges of the PMMA/PBU/graphene (see the red dashed lines in Figure 2a).[19,25] In spite of the VDirac shift at the beginning, the average µFET did not change appreciably. Owing to the PMMA/PBU layers, the charged impurities in air only affect the patterned edges of the graphene, and charged-impurity scattering[17] in the channel of the GFETs is minimized. To further investigate the effects of the PMMA/ PBU layers on the air stability of the GFETs, we compared GFETs with and without the polymeric bilayer. As previous
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studies have shown that water changes the electronic structure of graphene,[19,25,26] we kept the samples under high-humidity conditions (25 °C and a relative humidity of 80%). For the GFETs without the PMMA/PBU layers, conventional wet graphene transfer[3] was used. Thermally grown 300-nm-thick SiO2 on highly doped silicon was used as the gate dielectric and the common gate electrode for both sets of GFETs since thin AlOX is damaged by water during the wet transfer of graphene.[18] No graphene patterning was performed in this experiment. As shown in Figure 3b, there is almost no change in either the µFET or the VDirac of one of the GFETs with PMMA/PBU. In contrast, the GFETs without PMMA/PBU underwent considerable degradation during the first 5 hours (see Supporting Information Figure S5 for the VDirac and µFET data). After 5 hours, the performance remained unchanged, which indicates the adsorption of water on the graphene is saturated within 5 hours under these experimental conditions. We have confirmed that water molecules severely degrade the performance of GFETs and that the PMMA/PBU layers can be used as good passivation for graphene devices. In conclusion, we have developed a water-free transfer method for CVD-grown large-area graphene. Utilizing this method, we successfully fabricated flexible GFETs that operate at a low supply voltage of 4 V. The polymer layers used in the transfer process were found to provide excellent passivation and to protect graphene against detrimental ambient species; thus, the stable operation of the GFETs was maintained over a long period of time. As all device components are tens of nanometers thick, our flexible GFETs showed almost negligible changes in I–V characteristics after 5,000 bending and releasing repetitions with a bending radius of 0.5 cm. We believe that our transfer method expands the usage of graphene to a variety of substrates and that the flexible and air-stable GFETs will be used in future flexible electronic applications.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors thank Professor H.-J. Lee and Professor W. H. Lee for discussions. This work was supported by the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT, and Future Planning (2011–0031628) and by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2013R1A1A2012046). Received: December 3, 2013 Revised: December 31, 2013 Published online: [1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science 2004, 306, 666. [2] 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. [3] 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.
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Adv. Mater. 2014, DOI: 10.1002/adma.201305940
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