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off current ratio based on large-area single-crystal graphene
Carbon 163 (2020) 417e424
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Carbon journal homepage: www.elsevier.com/locate/carbon
Flexible field-effect transistors with a high on/off current ratio based on large-area single-crystal graphene Jing Ning a, b, *, 1, Ying Wang a, b, 1, Xin Feng a, b, Boyu Wang a, b, Jianguo Dong a, b, Dong Wang a, b, Chaochao Yan a, b, Xue Shen a, b, Xinran Wang b, Jincheng Zhang a, b, **, Yue Hao a, b a b
The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, 710071, China Shaanxi Joint Key Laboratory of Graphene, Xidian University, Xi’an, 710071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2020 Received in revised form 5 March 2020 Accepted 20 March 2020 Available online 21 March 2020
Large-area single-crystal graphene (LSG) over half a millimeter were synthesized via chemical vapor deposition, and a flexible ion gel-gated graphene field-effect transistor (FET) was fabricated to explore the electrical properties of the graphene based on the mechanism of electronic double layers. A procedure of cyclical oxidation/hydrogen-annealing processes by chemical vapor deposition (CVD) was proposed and optimized as a new growth method for the large single-crystal graphene, to facilitate the reconstruction of copper foil surface. To form a flexible FET on the polyethylene terephthalate substrate, LSG was utilized as a channel layer, and a spin-coated ion gel film was used as a gate dielectric. The flexible device demonstrates excellent electrical properties and high degree of bendability. When the device was bent by 8.1%, the on/off current ratio exceeded 400 because the deformation of the graphene crystal lattice widened graphene’s bandgap. This work is a fundamental study on the growth mechanism of LSG and the two-dimensional-material-based ion gel-gated transistor for application in flexible electronic devices. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction Since being exfoliated from highly oriented pyrolytic graphite for the first time in 2004, graphene has attracted much attention owing to its unique properties, such as mechanical flexibility, ultrahigh carrier mobility, optical transparency, and thermal stability [1e3,29]. The synthesis of graphene by the chemical vapor deposition (CVD) method further made it possible to exploit graphene’s outstanding properties for practical device applications [3e6]. The growth of large-area single-crystal graphene (LSG) and even graphene single-crystal thin films has always been a focus in the field of graphene research. Large-area graphene films are generally merged from small single-crystal domains and therefore have
* Corresponding author. The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, 710071, China. ** Corresponding author. The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, 710071, China. E-mail addresses: [email protected] (J. Ning), [email protected] (J. Zhang). 1 Jing Ning and Ying Wang contributed equally to this work. https://doi.org/10.1016/j.carbon.2020.03.040 0008-6223/© 2020 Elsevier Ltd. All rights reserved.
lattice defects, which result in challenges in the consistency of graphene materials [7]. Based on this, the performance of graphene devices is uneven, and accordingly, it is necessary to study the growth and preparation of graphene [8]. However, several factors affect the growth of graphene during the preparation of graphene by the CVD method, such as the choice of metal substrate, surface treatment of the metal substrate, selection of the carbon source, annealing treatment, gas flow ratio, and growth temperature, which need to be researched and optimized for realization of highquality LSG growth [9]. A field-effect transistor (FET) fabricated using LSG can demonstrate good performance because of the properties of high-quality graphene [10]. However, back-gated FETs on conventional oxidized silicon substrates usually exhibit poor gate control owing to the lack of carriers induced by the oxidation layer dielectric [11]. To realize better carrier control in two-dimensional materials, some methods are often utilized, including chemical doping, ion intercalation, ionic gating, and ion gel gating [12e14]. Ionic liquids, also known as room-temperature molten organic salts, have been very attractive owing to their favorable properties such as large ionic capacitance, wide electrochemical window, high thermal stability,
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and negligible vapor pressure [15]. Ionic liquids are promising candidates for use as gate dielectrics with high dielectric constants in FETs [16]. Additionally, ion gels, which are made from ionic liquids along with copolymers at temperatures lower than 100 C, are especially suitable for flexible electronic devices because the processing temperature of ion gels is low enough to be in the tolerant temperature range of almost all kinds of flexible substrates and is compatible with the fabrication process of flexible electronics [17,18]. Due to the lack of an intrinsic bandgap in graphene, highperformance transistors based on this excellent two-dimensional material are difficult to produce. Therefore, bandgap opening in graphene is of great importance, and has become an attractive topic for researchers over the past decade. So far, many theoretical and experimental advances have been made in interfacial engineering to achieve bandgap opening. Among these developments, strain and corrugation are promising to realize tuning the band gap of graphene, especially in the field of flexible electronic devices [19]. Ni et al. reported that ab initio calculations showed a band-gap opening of ~300 meV for graphene under 1% uniaxial tensile strain, and the size of the band gap increases almost linearly with the increase of tensile strain [20]. However, Pereira et al. analyzed the effect of tensional strain in the electronic structure of graphene and demonstrated that the gapless Dirac spectrum is robust for small and moderate deformations, and that the gap appears as a consequence of the merging of the two inequivalent Dirac points, only under considerable deformations of the lattice. Thus, there are large threshold values for bandgap opening in graphene [21]. In this study, LSG over half a millimeter was synthesized by CVD with staged oxidization/annealing process, ion-gated graphene FETs on a flexible polyethylene terephthalate (PET) substrate were fabricated with an ionic liquid, and their electrical properties as well as mechanical flexibility were explored. 2. Experimental methods 2.1. Graphene growth A procedure of pre-oxidation before annealing with staged oxygen supply was proposed and optimized as a growth method for the large single-crystal graphene, because the oxidation and reduction of the copper foil surface, which occurred in oxidation and hydrogen annealing process respectively, facilitate the reconstruction of copper foil surface. Single-crystal graphene was grown on a polished copper foil by CVD in a quartz tube. Before the growth, the gas in the tube was pumped out, and the furnace was heated to the growth temperature, which in this case was in the range of 850 C-1050 C with 20 sccm H2 and 700 sccm Ar. When the required temperature was reached, the copper foil was oxidized for 30 s with a flow of 2 sccm O2 and then annealed with 100 sccm H2 and 700 sccm Ar for 15 min. The oxidization and annealing process was repeated once, twice and three times, respectively. A CH4 flow of 0.4 sccm was introduced into the tube as the carbon source along with 100e900 sccm H2 during the 30-min growth process. Afterwards, the temperature in the tube was cooled down rapidly while the same gas was still flowing in it.
structural integrity of the graphene crystals, and then, the PMMA/ graphene film was rinsed with deionized water and gently picked up from the solution using a rigid substrate. The PMMA was removed by soaking the samples in acetone for 24 h. 2.3. Fabrication of back-gated graphene FETs A heavily P-doped silicon substrate and the oxidized layer (thickness of 300 nm) on top were used as the back gate and gate dielectric, respectively. The transferred graphene on the substrate was used as the channel material, and the channel areas were defined with a channel width of 350 mm and different channel lengths ranging from 120 mm to 20 mm using photolithography and an O2 plasma etching process for 60 s. The Ti/Au (60 nm/120 nm) electrodes were patterned by electron beam evaporation through a hard mask. Rapid thermal annealing was conducted for 10 min at 400 C in a N2 atmosphere to improve the contact between the source/drain electrodes and the graphene channel. 2.4. Fabrication of ion-gated graphene FETs In the electrolyte gate configuration, a gate electrode was located by the side of the channel. An ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI), was used as a component in the solid electrolyte gate. To form a thin film of gelated ionic liquid, EMIM-TFSI was dissolved in acetone along with a copolymer, polyvinylidene fluoride-six fluoropropene (PVDF-HFP), at a weight ratio of 4:15:1. After spin coating the solution on the devices described above and evaporating the solvent in an ambient atmosphere, a uniform ion gel film covering the graphene channel and the gate electrode was formed. Fig. 1a illustrates the fabrication process of both back-gated and ion-gated graphene FETs. The optical microscopy images of the back-gated and ion-gated graphene FETs are shown in Fig. 1bec respectively. The ion gel contains mobile cations and anions, [EMIM]þ and [TFSI], which can accumulate on the surface of the graphene and gate electrode under a certain gate voltage and form an electric double layer [15,22]. A diagram of the ion gel-gated graphene FETs operating under a positive gate voltage is shown in Fig. 1d. Fig. 1eef shows the chemical structures of [TFSI] and [EMIM]þ, respectively. 2.5. Fabrication of flexible ion-gated graphene FET on PET substrates CVD grown single-crystal graphene was transferred on the substrate by the wetting transfer method, and the channel areas were defined using photolithography and an O2 plasma etching process. The Ti/Au (10 nm/50 nm) electrodes were patterned by electron beam evaporation through a hard mask. Rapid thermal annealing was not conducted because the annealing temperature was too high to be tolerable for the PET substrates. The acetone solution of EMIM-TFSI and PVDF-HFP was spin-coated on the graphene channel and electrodes with acetone vapors in the ambient atmosphere for several minutes. 3. Results and discussion
2.2. Wetting transfer of graphene The single-crystal graphene on a copper foil was transferred onto rigid substrates by the wetting transfer method. Polymethyl methacrylate (PMMA) was spin-coated on the graphene/Cu film and baked at 50 C for 30 min to form a supportive layer. Subsequently, the Cu foil was chemically etched in aqueous solution of ammonium persulfate, during which the PMMA protected the
Pre-oxidization was performed during the preparation of LSG before the annealing process. This step facilitates the growth of LSG because a layer of oxygen molecules is adsorbed on the surface of the copper foil at high temperature and reacts with the surface of the copper foil to form a thin layer of copper oxide, which is then reduced by the annealing process in a hydrogen and argon atmosphere. During this process, the surface structure of copper is
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Fig. 1. Device fabrication: (a) Diagram of the fabrication of back-gated graphene FETs and ion-gated graphene FETs, (b) optical microscopy image of back-gated graphene FETs, (c) optical microscopy image of ion gel-gated graphene FETs, (d) ion distribution in the ion gel under a positive gate voltage, and (eef) chemical structures of the anions and cations in the ion gel, respectively. (A colour version of this figure can be viewed online.)
restructured, which reduces the density of the active areas on the surface of the copper foil. Moreover, crystal domains on the surface of the copper foil also grow, and the surface quality of the copper foil is significantly improved [23]. Control experiments were performed by changing the cycle of the oxidation/hydrogen-annealing processes. Fig. 2 shows the optical microscopy images with different scales of graphene synthesized with pre-oxidation/annealing procedure repeated once, twice and three times, respectively. As the results show, the nucleation density of graphene decreases with the repeating cycle of the preoxidation/annealing procedure increasing, meanwhile, the size of its crystal domain becomes larger. During the process of graphene growth, the growth temperature is a critical factor for suppressing the nucleation density of graphene. The effect of temperature on the nucleation density of graphene was investigated. Considering the safety restrictions of the CVD growth furnace, the melting point of copper foil, and the slower pyrolysis process of methane on a copper substrate because of the excessive low temperature, the growth temperatures for the experiment was 800 C, 850 C, 900 C, 950 C, 1000 C, and 1050 C, sequentially, with a methane flow of 0.4 sccm, hydrogen flow rate of 500 sccm, and growth time of 60 min. Finally, the copper foil was rapidly cooled down. Cooling required 5 min. The optical microscope characterization of graphene samples prepared at different temperatures is shown in Fig. 3aed. The scale bar in the figure is 100 mm. The outline of the graphene singlecrystal domain shows regular polygons. As the statistical results shown in Fig. 3e, the upper limit of the nucleation density range of graphene was 10 mm2, and the average size distribution of single crystal domains ranged from 20 mm to over 1 mm; the distribution of the size of graphene domains at different temperatures was
concentrated, indicating that the distribution of time when graphene began to nucleate during the growth period was basically concentrated. Additionally, the size of graphene single-crystal domains was only 20 mm at 850 C, and no nucleation point of graphene was found when the growth temperature was 800 C when other conditions were kept consistent. This reveals that the temperature of graphene growth during CVD on copper foil substrates using methane as the carbon source cannot be below 800 C. This is because when the growth temperature is below 800 C, even with copper foil used as a catalyst, the decomposition of methane adsorbed on the surface of the copper foil is insufficient. As the growth temperature increases, the nucleation density of graphene appears to increase first and then decreases, whereas the average diameter of its crystal domain size becomes larger. This is due to the co-determination of multiple processes. First, the lower the temperature, the smaller the kinetic energy of the carbon precursor cracked from methane, the shorter the average free path on the surface of the copper foil, and the easier it is for the carbon atoms to gather and reach a threshold number and crystallize into nucleation. Second, because the decomposition of methane into the carbon precursor is an endothermic reaction, the methane decomposition rate decreases with the growth temperature, and the number of carbon precursors catalyzed by the copper foil becomes smaller, which leads to a slower growth rate of graphene single-crystal domains. Third, with a higher temperature, the activated carbon precursor adsorbed on the surface of the copper foil that participates in growth is easily detached from the surface of the copper foil, which effectively reduces the number of activated carbon precursors on the surface of the copper foil and helps reduce the nucleation density of graphene. The graphene was transferred to Si/SiO2 (the thickness of the
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Fig. 2. (aec) Optical microscopy images of graphene synthesized with pre-oxidation procedure performed only once. (def) Optical microscopy images of graphene synthesized with pre-oxidation procedure repeated twice. (gei) Optical microscopy images of graphene synthesized with pre-oxidation procedure repeated three times. (A colour version of this figure can be viewed online.)
silicon oxide layer was 300 nm), and its Raman spectrum was characterized, as shown in Fig. 3f. All the samples exhibited the characteristic G and 2D peaks of single-layer graphene, indicating that the prepared graphene was single-layer graphene [24]. The D peak, which indicates the existence of defects, did not appear, showing that the transferred graphene had few defects [25]. Additionally, Table S1 shows the centre position and FWHM of G and 2D peaks of graphene synthesized at different temperature. From the results we can see that as the growth temperature increases, G and 2D peaks both appear to have a tendency of blue shift and the FWHM of G and 2D peaks both increase first, reach a maximum at 950 C and then decrease. This may indicate that graphene prepared at different temperatures exhibits different degrees of doping after the transfer process. During the growth of graphene, hydrogen plays an important role in the preparation of graphene, affecting the number of layers, shape, nucleation density, size, and crystal quality of the graphene [26]. First, it promotes the reaction of methane adsorbed on the surface of copper foil into activated carbon precursors that participate in the growth of graphene. Second, hydrogen suppresses the forward reaction in the dynamic equilibrium reaction of methane and hydrogen. Therefore, it acts as an inhibitor of methane decomposition and as an etchant for graphene crystal domains [27]. In this experiment, the effect of hydrogen flow on the growth of graphene was studied.
Fig. 4aee shows the optical microscopy images of single-crystal graphene grown with CH4/H2 flow ratios of 0.4:100, 0.4:300, 0.4:500, 0.4:700, and 0.4:900, respectively, and transferred onto Si/ SiO2 substrates. The growth temperature was fixed at 1050 C, the growth time was 30 min, and the temperature was rapidly reduced to room temperature in 5 min. With an increase in the flow rate of hydrogen, the crystal domain size of graphene decreased first and then increased. Moreover, the graphene nucleation density showed the opposite trend. This is because as the flow of hydrogen increases, the carbon source adsorbed on the surface of the copper foil is divided into different regions and nucleates in a specific area. The nucleation density of the graphene crystal domains increases. Meanwhile, the competition was fierce between graphene crystal domains for activated carbon precursors; thus, the graphene crystal domain size showed a decreasing trend [28]. As the hydrogen flow rate continued to increase, a large amount of carbon precursors on the surface of the copper foil desorbed, and a small number of discretely activated carbon precursors formed nucleation points; then, the nucleation density of graphene began to decrease. Additionally, the graphene crystal domain size increased owing to the weakening of the competitive relationship between the graphene crystal domains for the activated carbon precursors. Fig. 4f shows the atomic force microscopy image of the single-crystal graphene on a Si/SiO2 substrate. The root mean square of the roughness was 1.34 nm.
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Fig. 3. (aed) Optical microscopy images of single-crystal graphene grown under 1050 C, 1000 C, 950 C and 900 C, respectively, transferred onto Si/SiO2 substrates. (e) Nucleation density and domain average diameter of single crystal graphene synthesized at different growth temperatures. (f) Raman spectra of the single-crystal graphene grown under different temperature. (A colour version of this figure can be viewed online.)
Fig. 4. (aee) Optical microscopy images of single-crystal graphene grown with CH4/H2 flow ratios of 0.4:100, 0.4:300, 0.4:500, 0.4:700, and 0.4:900, respectively, transferred onto Si/SiO2 substrates and (f) atomic force microscopy image of the single-crystal graphene on a Si/SiO2 substrate. (A colour version of this figure can be viewed online.)
Fig. 5aec shows the output characteristics of the back-gated graphene FETs and the ion-gated graphene FETs, respectively. The back-gated graphene FETs show completely linear behavior owing to graphene’s zero band gap. The heavily doped silicon substrate has poor gate control ability, and it is difficult to switch the transistors off even with a fairly large gate voltage (60 V). In contrast, ion-gated graphene FETs exhibit much more effective gate control so that the transistors can be easily switched on and off within the
gate voltage range of several volts. Additionally, the mobile ions that moved onto the surface of the graphene induced a large carrier density in the channel and therefore increased the conductivity and the current density. With a drainesource voltage of 1 V and a gate voltage of 4 V, the drainesource current can reach over 5 mA. Fig. 5bed shows the transfer characteristics of the back-gated graphene FETs and the ion-gated graphene FETs, respectively. Because of oxygen adsorption and charged impurities that originate
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Fig. 5. Electrical characterization: (aeb) Output and transfer curves of back-gated graphene FETs, (ced) output and transfer curves of ion-gated graphene FETs, and (e) drain voltage dependence on the minimum drain current and Dirac point voltage in the ion-gated graphene FETs. (A colour version of this figure can be viewed online.)
from the silicon dioxide substrate, the back-gated graphene FETs exhibit only p-type, and no Dirac point voltage was observed in the large range of gate voltage, although graphene is known to be ambipolar. In the ion-gated graphene FETs, ambipolar behavior was observed, and the Dirac point voltages were near 1 V. This phenomenon liked resulted from the neutralization of impurity charges realized by the ions in the ion gel. Additionally, the Dirac point voltage increased with the drainesource voltage. The dependence of the Dirac point voltage on the drainesource voltage is plotted in Fig. 5e. As the drainesource voltage increased from 0.2 to 0.8 V, the Dirac point voltage increased from 1.36 to 1.96 V. From this dependence, the actual Dirac point voltage of the graphene is inferred to be 1.12 V. The ratio of on current to off current was significantly increased up to ~6 compared to back-gated transistors. The hole and electron mobilities can be extracted from the transfer curves using the relationship between the drainesource current and gate voltage, which is as follows:
ID ¼ m
W CVD ðVG VDirac Þ; L
where m is the field-effect mobility, W and L are the channel width and length, and C is the specific capacitance of the gate dielectric
[17]. The hole and electron mobilities are 30.06 ± 2.03 cm2/(V$s) and 50.06 ± 3.12 cm2/(V$s) respectively. The ion-gated graphene FETs fabricated on the PET substrates are shown in Fig. 6a. Fig. 6b and c shows the output characteristics of the flexible ion-gated graphene FETs on PET substrates without and with 2.7% bending, respectively. When the FETs operate under bending conditions, the drainesource current decreases by approximately 60%. Fig. 6d shows the transfer characteristics of the flexible ion-gated graphene FETs operating under different bending conditions when the drainesource voltage was 0.3 V. As the bending degree increased from 0% to 8.1%, both the on and off currents decreased. From the transfer curves, the on/off current ratio and the Dirac point voltage were extracted. The dependence of the Dirac point voltage and on/off current ratio on the bending degree is plotted in Fig. 6e. As the bending degree increased, the on/ off current ratio first decreased slightly and then increased rapidly, reaching approximately 400 when 8.1% bending was conducted. This may have resulted from the opened bandgap owing to the deformation of the graphene crystal lattice. The Dirac point voltage increased from 1.99 to 2.17 V, while the bending degree of the device increased from 0% to 8.1%. Additionally, the hole and electron mobilities decreased with increasing bending degree.
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Fig. 6. Flexible devices: (a) Photo of flexible ion-gated graphene FETs (bec) output characterization of the flexible ion-gated graphene FETs without bending and with 2.7% bending, (d) transfer characterization of the flexible ion-gated graphene FETs under different bending percentages, and (e) bending percentage dependence on the Dirac point voltage and on/off current ratio in the flexible ion-gated graphene FETs. (A colour version of this figure can be viewed online.)
The experimental results in our work are basically consistent with Pereira’s theory [21]. When the bending degree was increased in a small range from zero, the on/off current ratio basically maintained, or even decreased a little. But as the bending percentage increased to a relatively high value, the on/off current ratio increased rapidly, reaching approximately 400 when 8.1% bending was conducted.
flexible electronic devices.
4. Conclusion
CRediT authorship contribution statement
In this study, LSG of over half a millimeter was synthesized by CVD. A growth method with a procedure of cyclical oxidation/ hydrogen-annealing processes for large graphene crystal domains was proposed, and effects of the cycle of the oxidation/hydrogenannealing processes on the nucleation density and domain size of graphene were studied. Flexible ion-gated graphene FETs were fabricated on PETs, and their electrical properties were studied while they were bent. The devices exhibit excellent flexibility and an on/off current ratio up to 400, due to the formation of graphene bandgap induced by the bending. In conclusion, this work demonstrates that combining LSG with an ion-gel dielectric provides an ion-gated graphene FET that has good application prospects in
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Jing Ning: Conceptualization, Methodology, Writing - review & editing. Ying Wang: Methodology, Writing - original draft. Xin Feng: Resources. Boyu Wang: Visualization. Jianguo Dong: Formal analysis. Dong Wang: Project administration. Chaochao Yan: Visualization. Xue Shen: Formal analysis. Xinran Wang: Project administration. Jincheng Zhang: Supervision. Yue Hao: Funding acquisition. Acknowledgements The work was supported by the Natural Science Basic Research Plan in Shaanxi Province of China (Program No.2017ZDCXL-GY-11-
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03, 2019ZDLGY16-08, 2019ZDLGY16-03, 2019ZDLGY16-02), Youth Science and Technology Nova Program of Shaanxi Province, Wuhu and Xidian University special fund for industry-university-research cooperation (Program No.HX01201909039), Young Talent fund of University Association for Science and Technology in Shaanxi, China (Program No.20170106), the Natural Science Basic Research Plan in Shaanxi Province of China (Program No.2019JQ-860). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2020.03.040. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. [2] Wonbong Choi, Indranil Lahiri, Raghunandan Seelaboyina, Yong Soo Kang, Synthesis of graphene and its applications: a review, Crit. Rev. Solid State Mater. Sci. 35 (1) (2010) 52e71. [3] J. Ning, D. Wang, J. Yan, D. Han, Z. Chai, W. Cai, et al., Combined effects of hydrogen annealing on morphological, electrical and structural properties of graphene/r-sapphire, Carbon 75 (2014) 262e270. ~ oz, Cristina Go mez-Aleixandre, Review of CVD synthesis of graphene, [4] R. Mun Chem. Vap. Depos. 19 (10e11-12) (2013) 297e322. [5] P.R. Somani, S.P. Somani, M. Umeno, Planer nano-graphenes from camphor by CVD, Chem. Phys. Lett. 430 (1e3) (2006) 56e59. [6] Xuesong Li, Weiwei Cai, Jinho An, Seyoung Kim, Junghyo Nah, Dongxing Yang, et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1312e1314. [7] O.V. Yazyev, S.G. Louie, Topological defects in graphene: dislocations and grain boundaries, Phys. Rev. B 81 (19) (2010). [8] J. Zhang, J. Zhao, J. Lu, Intrinsic strength and failure behaviors of graphene grain boundaries, ACS Nano 6 (3) (2012) 2704e2711. [9] E. Loginova, N.C. Bartelt, P.J. Feibelman, K.F. McCarty, Factors influencing graphene growth on metal surfaces, New J. Phys. 11 (6) (2009), 63046-0. [10] J. Chen, Y. Guo, L. Jiang, Z. Xu, L. Huang, Y. Xue, et al., Near-equilibrium chemical vapor deposition of high-quality single-crystal graphene directly on various dielectric substrates, Adv. Mater. 26 (9) (2014). [11] F. Chen, Q. Qing, J. Xia, J. Li, N. Tao, Electrochemical gate-controlled charge transport in graphene in ionic liquid and aqueous solution, J. Am. Chem. Soc. 131 (29) (2009) 9908e9909. [12] H. Liu, Y. Liu, D. Zhu, Chemical doping of graphene, J. Mater. Chem. 21 (10) (2011), 3335-0.
[13] W. Shi, J. Ye, Y. Zhang, R. Suzuki, M. Yoshida, J. Miyazaki, et al., Superconductivity series in transition metal dichalcogenides by ionic gating, Sci. Rep. 5 (2015), 12534. [14] M. Kettner, I. Vladimirov, A.J. Strudwick, M.G. Schwab, R.T. Weitz, Ionic gel as gate dielectric for the easy characterization of graphene and polymer fieldeffect transistors and electrochemical resistance modification of graphene, J. Appl. Phys. 118 (2) (2015), 025501. [15] V. Narasimhan, S.Y. Park, An ion gel as a low-cost, spin-coatable, highcapacitance dielectric for electrowetting-on-dielectric (EWOD), Langmuir 31 (30) (2015) 8512e8518. [16] K.H. Lee, M.S. Kang, S. Zhang, Y. Gu, T.P. Lodge, C.D. Frisbie, “Cut and stick” rubbery ion gels as high capacitance gate dielectrics, Adv. Mater. 24 (2012) 4457e4462. [17] B.J. Kim, H. Jang, S.K. Lee, B.H. Hong, J.H. Ahn, J.H. Cho, High-performance flexible graphene field effect transistors with ion gel gate dielectrics, Nano Lett. 10 (9) (2010) 3464e3466. [18] S.K. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, et al., Stretchable graphene transistors with printed dielectrics and gate electrodes, Nano Lett. 11 (11) (2011) 4642. [19] X. Xu, C. Liu, Z. Sun, T. Cao, Z. Zhang, E. Wang, et al., Interfacial engineering in graphene bandgap, Chem. Soc. Rev. 47 (9) (2018) 3059e3099. [20] Z.H. Ni, T. Yu, Y.H. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening, ACS Nano 2 (11) (2008) 2301e2305. [21] V.M. Pereira, A.C. Neto, N.M.R. Peres, Tight-binding approach to uniaxial strain in graphene, Phys. Rev. B 80 (4) (2009), 045401. [22] T. Fujimoto, K. Awaga, Electric-double-layer field-effect transistors with ionic liquids, Phys. Chem. Chem. Phys. 15 (23) (2013) 8983. [23] Y. Hao, M.S. Bharathi, L. Wang, Y. Liu, H. Chen, S. Nie, et al., The role of surface oxygen in the growth of large single-crystal graphene on copper, Science 342 (6159) (2013) 720e723. [24] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, et al., Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006), 18: 187401. [25] C. Wang, W. Chen, C. Han, G. Wang, B. Tang, C. Tang, et al., Growth of millimeter-size single crystal graphene on Cu foils by circumfluence chemical vapor deposition, Sci. Rep. 4 (2014) 4537. [26] X. Zhang, J. Ning, X. Li, B. Wang, L. Hao, M. Liang, et al., Hydrogen-induced effects on the CVD growth of high-quality graphene structures, Nanoscale 5 (18) (2013) 8363. [27] I. Vlassiouk, M. Regmi, P. Fulvio, S. Dai, P. Datskos, G. Eres, et al., Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene, ACS Nano 5 (7) (2011) 6069e6076. [28] B. Luo, E. Gao, D. Geng, H. Wang, Z. Xu, G. Yu, Etching-controlled growth of graphene by chemical vapor deposition, Chem. Mater. 29 (3) (2017) 1022e1027. [29] J Ning, D Wang, Y Chai, X Feng, M Mu, L Guo, et al., Review on mechanism of directly fabricating wafer-scale graphene on dielectric substrates by chemical vapor deposition, Nanotechnology 28 (2017).
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Carbon journal homepage: www.elsevier.com/locate/carbon
Flexible field-effect transistors with a high on/off current ratio based on large-area single-crystal graphene Jing Ning a, b, *, 1, Ying Wang a, b, 1, Xin Feng a, b, Boyu Wang a, b, Jianguo Dong a, b, Dong Wang a, b, Chaochao Yan a, b, Xue Shen a, b, Xinran Wang b, Jincheng Zhang a, b, **, Yue Hao a, b a b
The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, 710071, China Shaanxi Joint Key Laboratory of Graphene, Xidian University, Xi’an, 710071, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2020 Received in revised form 5 March 2020 Accepted 20 March 2020 Available online 21 March 2020
Large-area single-crystal graphene (LSG) over half a millimeter were synthesized via chemical vapor deposition, and a flexible ion gel-gated graphene field-effect transistor (FET) was fabricated to explore the electrical properties of the graphene based on the mechanism of electronic double layers. A procedure of cyclical oxidation/hydrogen-annealing processes by chemical vapor deposition (CVD) was proposed and optimized as a new growth method for the large single-crystal graphene, to facilitate the reconstruction of copper foil surface. To form a flexible FET on the polyethylene terephthalate substrate, LSG was utilized as a channel layer, and a spin-coated ion gel film was used as a gate dielectric. The flexible device demonstrates excellent electrical properties and high degree of bendability. When the device was bent by 8.1%, the on/off current ratio exceeded 400 because the deformation of the graphene crystal lattice widened graphene’s bandgap. This work is a fundamental study on the growth mechanism of LSG and the two-dimensional-material-based ion gel-gated transistor for application in flexible electronic devices. © 2020 Elsevier Ltd. All rights reserved.
1. Introduction Since being exfoliated from highly oriented pyrolytic graphite for the first time in 2004, graphene has attracted much attention owing to its unique properties, such as mechanical flexibility, ultrahigh carrier mobility, optical transparency, and thermal stability [1e3,29]. The synthesis of graphene by the chemical vapor deposition (CVD) method further made it possible to exploit graphene’s outstanding properties for practical device applications [3e6]. The growth of large-area single-crystal graphene (LSG) and even graphene single-crystal thin films has always been a focus in the field of graphene research. Large-area graphene films are generally merged from small single-crystal domains and therefore have
* Corresponding author. The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, 710071, China. ** Corresponding author. The State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, Xidian University, Xi’an, 710071, China. E-mail addresses: [email protected] (J. Ning), [email protected] (J. Zhang). 1 Jing Ning and Ying Wang contributed equally to this work. https://doi.org/10.1016/j.carbon.2020.03.040 0008-6223/© 2020 Elsevier Ltd. All rights reserved.
lattice defects, which result in challenges in the consistency of graphene materials [7]. Based on this, the performance of graphene devices is uneven, and accordingly, it is necessary to study the growth and preparation of graphene [8]. However, several factors affect the growth of graphene during the preparation of graphene by the CVD method, such as the choice of metal substrate, surface treatment of the metal substrate, selection of the carbon source, annealing treatment, gas flow ratio, and growth temperature, which need to be researched and optimized for realization of highquality LSG growth [9]. A field-effect transistor (FET) fabricated using LSG can demonstrate good performance because of the properties of high-quality graphene [10]. However, back-gated FETs on conventional oxidized silicon substrates usually exhibit poor gate control owing to the lack of carriers induced by the oxidation layer dielectric [11]. To realize better carrier control in two-dimensional materials, some methods are often utilized, including chemical doping, ion intercalation, ionic gating, and ion gel gating [12e14]. Ionic liquids, also known as room-temperature molten organic salts, have been very attractive owing to their favorable properties such as large ionic capacitance, wide electrochemical window, high thermal stability,
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and negligible vapor pressure [15]. Ionic liquids are promising candidates for use as gate dielectrics with high dielectric constants in FETs [16]. Additionally, ion gels, which are made from ionic liquids along with copolymers at temperatures lower than 100 C, are especially suitable for flexible electronic devices because the processing temperature of ion gels is low enough to be in the tolerant temperature range of almost all kinds of flexible substrates and is compatible with the fabrication process of flexible electronics [17,18]. Due to the lack of an intrinsic bandgap in graphene, highperformance transistors based on this excellent two-dimensional material are difficult to produce. Therefore, bandgap opening in graphene is of great importance, and has become an attractive topic for researchers over the past decade. So far, many theoretical and experimental advances have been made in interfacial engineering to achieve bandgap opening. Among these developments, strain and corrugation are promising to realize tuning the band gap of graphene, especially in the field of flexible electronic devices [19]. Ni et al. reported that ab initio calculations showed a band-gap opening of ~300 meV for graphene under 1% uniaxial tensile strain, and the size of the band gap increases almost linearly with the increase of tensile strain [20]. However, Pereira et al. analyzed the effect of tensional strain in the electronic structure of graphene and demonstrated that the gapless Dirac spectrum is robust for small and moderate deformations, and that the gap appears as a consequence of the merging of the two inequivalent Dirac points, only under considerable deformations of the lattice. Thus, there are large threshold values for bandgap opening in graphene [21]. In this study, LSG over half a millimeter was synthesized by CVD with staged oxidization/annealing process, ion-gated graphene FETs on a flexible polyethylene terephthalate (PET) substrate were fabricated with an ionic liquid, and their electrical properties as well as mechanical flexibility were explored. 2. Experimental methods 2.1. Graphene growth A procedure of pre-oxidation before annealing with staged oxygen supply was proposed and optimized as a growth method for the large single-crystal graphene, because the oxidation and reduction of the copper foil surface, which occurred in oxidation and hydrogen annealing process respectively, facilitate the reconstruction of copper foil surface. Single-crystal graphene was grown on a polished copper foil by CVD in a quartz tube. Before the growth, the gas in the tube was pumped out, and the furnace was heated to the growth temperature, which in this case was in the range of 850 C-1050 C with 20 sccm H2 and 700 sccm Ar. When the required temperature was reached, the copper foil was oxidized for 30 s with a flow of 2 sccm O2 and then annealed with 100 sccm H2 and 700 sccm Ar for 15 min. The oxidization and annealing process was repeated once, twice and three times, respectively. A CH4 flow of 0.4 sccm was introduced into the tube as the carbon source along with 100e900 sccm H2 during the 30-min growth process. Afterwards, the temperature in the tube was cooled down rapidly while the same gas was still flowing in it.
structural integrity of the graphene crystals, and then, the PMMA/ graphene film was rinsed with deionized water and gently picked up from the solution using a rigid substrate. The PMMA was removed by soaking the samples in acetone for 24 h. 2.3. Fabrication of back-gated graphene FETs A heavily P-doped silicon substrate and the oxidized layer (thickness of 300 nm) on top were used as the back gate and gate dielectric, respectively. The transferred graphene on the substrate was used as the channel material, and the channel areas were defined with a channel width of 350 mm and different channel lengths ranging from 120 mm to 20 mm using photolithography and an O2 plasma etching process for 60 s. The Ti/Au (60 nm/120 nm) electrodes were patterned by electron beam evaporation through a hard mask. Rapid thermal annealing was conducted for 10 min at 400 C in a N2 atmosphere to improve the contact between the source/drain electrodes and the graphene channel. 2.4. Fabrication of ion-gated graphene FETs In the electrolyte gate configuration, a gate electrode was located by the side of the channel. An ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI), was used as a component in the solid electrolyte gate. To form a thin film of gelated ionic liquid, EMIM-TFSI was dissolved in acetone along with a copolymer, polyvinylidene fluoride-six fluoropropene (PVDF-HFP), at a weight ratio of 4:15:1. After spin coating the solution on the devices described above and evaporating the solvent in an ambient atmosphere, a uniform ion gel film covering the graphene channel and the gate electrode was formed. Fig. 1a illustrates the fabrication process of both back-gated and ion-gated graphene FETs. The optical microscopy images of the back-gated and ion-gated graphene FETs are shown in Fig. 1bec respectively. The ion gel contains mobile cations and anions, [EMIM]þ and [TFSI], which can accumulate on the surface of the graphene and gate electrode under a certain gate voltage and form an electric double layer [15,22]. A diagram of the ion gel-gated graphene FETs operating under a positive gate voltage is shown in Fig. 1d. Fig. 1eef shows the chemical structures of [TFSI] and [EMIM]þ, respectively. 2.5. Fabrication of flexible ion-gated graphene FET on PET substrates CVD grown single-crystal graphene was transferred on the substrate by the wetting transfer method, and the channel areas were defined using photolithography and an O2 plasma etching process. The Ti/Au (10 nm/50 nm) electrodes were patterned by electron beam evaporation through a hard mask. Rapid thermal annealing was not conducted because the annealing temperature was too high to be tolerable for the PET substrates. The acetone solution of EMIM-TFSI and PVDF-HFP was spin-coated on the graphene channel and electrodes with acetone vapors in the ambient atmosphere for several minutes. 3. Results and discussion
2.2. Wetting transfer of graphene The single-crystal graphene on a copper foil was transferred onto rigid substrates by the wetting transfer method. Polymethyl methacrylate (PMMA) was spin-coated on the graphene/Cu film and baked at 50 C for 30 min to form a supportive layer. Subsequently, the Cu foil was chemically etched in aqueous solution of ammonium persulfate, during which the PMMA protected the
Pre-oxidization was performed during the preparation of LSG before the annealing process. This step facilitates the growth of LSG because a layer of oxygen molecules is adsorbed on the surface of the copper foil at high temperature and reacts with the surface of the copper foil to form a thin layer of copper oxide, which is then reduced by the annealing process in a hydrogen and argon atmosphere. During this process, the surface structure of copper is
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Fig. 1. Device fabrication: (a) Diagram of the fabrication of back-gated graphene FETs and ion-gated graphene FETs, (b) optical microscopy image of back-gated graphene FETs, (c) optical microscopy image of ion gel-gated graphene FETs, (d) ion distribution in the ion gel under a positive gate voltage, and (eef) chemical structures of the anions and cations in the ion gel, respectively. (A colour version of this figure can be viewed online.)
restructured, which reduces the density of the active areas on the surface of the copper foil. Moreover, crystal domains on the surface of the copper foil also grow, and the surface quality of the copper foil is significantly improved [23]. Control experiments were performed by changing the cycle of the oxidation/hydrogen-annealing processes. Fig. 2 shows the optical microscopy images with different scales of graphene synthesized with pre-oxidation/annealing procedure repeated once, twice and three times, respectively. As the results show, the nucleation density of graphene decreases with the repeating cycle of the preoxidation/annealing procedure increasing, meanwhile, the size of its crystal domain becomes larger. During the process of graphene growth, the growth temperature is a critical factor for suppressing the nucleation density of graphene. The effect of temperature on the nucleation density of graphene was investigated. Considering the safety restrictions of the CVD growth furnace, the melting point of copper foil, and the slower pyrolysis process of methane on a copper substrate because of the excessive low temperature, the growth temperatures for the experiment was 800 C, 850 C, 900 C, 950 C, 1000 C, and 1050 C, sequentially, with a methane flow of 0.4 sccm, hydrogen flow rate of 500 sccm, and growth time of 60 min. Finally, the copper foil was rapidly cooled down. Cooling required 5 min. The optical microscope characterization of graphene samples prepared at different temperatures is shown in Fig. 3aed. The scale bar in the figure is 100 mm. The outline of the graphene singlecrystal domain shows regular polygons. As the statistical results shown in Fig. 3e, the upper limit of the nucleation density range of graphene was 10 mm2, and the average size distribution of single crystal domains ranged from 20 mm to over 1 mm; the distribution of the size of graphene domains at different temperatures was
concentrated, indicating that the distribution of time when graphene began to nucleate during the growth period was basically concentrated. Additionally, the size of graphene single-crystal domains was only 20 mm at 850 C, and no nucleation point of graphene was found when the growth temperature was 800 C when other conditions were kept consistent. This reveals that the temperature of graphene growth during CVD on copper foil substrates using methane as the carbon source cannot be below 800 C. This is because when the growth temperature is below 800 C, even with copper foil used as a catalyst, the decomposition of methane adsorbed on the surface of the copper foil is insufficient. As the growth temperature increases, the nucleation density of graphene appears to increase first and then decreases, whereas the average diameter of its crystal domain size becomes larger. This is due to the co-determination of multiple processes. First, the lower the temperature, the smaller the kinetic energy of the carbon precursor cracked from methane, the shorter the average free path on the surface of the copper foil, and the easier it is for the carbon atoms to gather and reach a threshold number and crystallize into nucleation. Second, because the decomposition of methane into the carbon precursor is an endothermic reaction, the methane decomposition rate decreases with the growth temperature, and the number of carbon precursors catalyzed by the copper foil becomes smaller, which leads to a slower growth rate of graphene single-crystal domains. Third, with a higher temperature, the activated carbon precursor adsorbed on the surface of the copper foil that participates in growth is easily detached from the surface of the copper foil, which effectively reduces the number of activated carbon precursors on the surface of the copper foil and helps reduce the nucleation density of graphene. The graphene was transferred to Si/SiO2 (the thickness of the
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Fig. 2. (aec) Optical microscopy images of graphene synthesized with pre-oxidation procedure performed only once. (def) Optical microscopy images of graphene synthesized with pre-oxidation procedure repeated twice. (gei) Optical microscopy images of graphene synthesized with pre-oxidation procedure repeated three times. (A colour version of this figure can be viewed online.)
silicon oxide layer was 300 nm), and its Raman spectrum was characterized, as shown in Fig. 3f. All the samples exhibited the characteristic G and 2D peaks of single-layer graphene, indicating that the prepared graphene was single-layer graphene [24]. The D peak, which indicates the existence of defects, did not appear, showing that the transferred graphene had few defects [25]. Additionally, Table S1 shows the centre position and FWHM of G and 2D peaks of graphene synthesized at different temperature. From the results we can see that as the growth temperature increases, G and 2D peaks both appear to have a tendency of blue shift and the FWHM of G and 2D peaks both increase first, reach a maximum at 950 C and then decrease. This may indicate that graphene prepared at different temperatures exhibits different degrees of doping after the transfer process. During the growth of graphene, hydrogen plays an important role in the preparation of graphene, affecting the number of layers, shape, nucleation density, size, and crystal quality of the graphene [26]. First, it promotes the reaction of methane adsorbed on the surface of copper foil into activated carbon precursors that participate in the growth of graphene. Second, hydrogen suppresses the forward reaction in the dynamic equilibrium reaction of methane and hydrogen. Therefore, it acts as an inhibitor of methane decomposition and as an etchant for graphene crystal domains [27]. In this experiment, the effect of hydrogen flow on the growth of graphene was studied.
Fig. 4aee shows the optical microscopy images of single-crystal graphene grown with CH4/H2 flow ratios of 0.4:100, 0.4:300, 0.4:500, 0.4:700, and 0.4:900, respectively, and transferred onto Si/ SiO2 substrates. The growth temperature was fixed at 1050 C, the growth time was 30 min, and the temperature was rapidly reduced to room temperature in 5 min. With an increase in the flow rate of hydrogen, the crystal domain size of graphene decreased first and then increased. Moreover, the graphene nucleation density showed the opposite trend. This is because as the flow of hydrogen increases, the carbon source adsorbed on the surface of the copper foil is divided into different regions and nucleates in a specific area. The nucleation density of the graphene crystal domains increases. Meanwhile, the competition was fierce between graphene crystal domains for activated carbon precursors; thus, the graphene crystal domain size showed a decreasing trend [28]. As the hydrogen flow rate continued to increase, a large amount of carbon precursors on the surface of the copper foil desorbed, and a small number of discretely activated carbon precursors formed nucleation points; then, the nucleation density of graphene began to decrease. Additionally, the graphene crystal domain size increased owing to the weakening of the competitive relationship between the graphene crystal domains for the activated carbon precursors. Fig. 4f shows the atomic force microscopy image of the single-crystal graphene on a Si/SiO2 substrate. The root mean square of the roughness was 1.34 nm.
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Fig. 3. (aed) Optical microscopy images of single-crystal graphene grown under 1050 C, 1000 C, 950 C and 900 C, respectively, transferred onto Si/SiO2 substrates. (e) Nucleation density and domain average diameter of single crystal graphene synthesized at different growth temperatures. (f) Raman spectra of the single-crystal graphene grown under different temperature. (A colour version of this figure can be viewed online.)
Fig. 4. (aee) Optical microscopy images of single-crystal graphene grown with CH4/H2 flow ratios of 0.4:100, 0.4:300, 0.4:500, 0.4:700, and 0.4:900, respectively, transferred onto Si/SiO2 substrates and (f) atomic force microscopy image of the single-crystal graphene on a Si/SiO2 substrate. (A colour version of this figure can be viewed online.)
Fig. 5aec shows the output characteristics of the back-gated graphene FETs and the ion-gated graphene FETs, respectively. The back-gated graphene FETs show completely linear behavior owing to graphene’s zero band gap. The heavily doped silicon substrate has poor gate control ability, and it is difficult to switch the transistors off even with a fairly large gate voltage (60 V). In contrast, ion-gated graphene FETs exhibit much more effective gate control so that the transistors can be easily switched on and off within the
gate voltage range of several volts. Additionally, the mobile ions that moved onto the surface of the graphene induced a large carrier density in the channel and therefore increased the conductivity and the current density. With a drainesource voltage of 1 V and a gate voltage of 4 V, the drainesource current can reach over 5 mA. Fig. 5bed shows the transfer characteristics of the back-gated graphene FETs and the ion-gated graphene FETs, respectively. Because of oxygen adsorption and charged impurities that originate
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Fig. 5. Electrical characterization: (aeb) Output and transfer curves of back-gated graphene FETs, (ced) output and transfer curves of ion-gated graphene FETs, and (e) drain voltage dependence on the minimum drain current and Dirac point voltage in the ion-gated graphene FETs. (A colour version of this figure can be viewed online.)
from the silicon dioxide substrate, the back-gated graphene FETs exhibit only p-type, and no Dirac point voltage was observed in the large range of gate voltage, although graphene is known to be ambipolar. In the ion-gated graphene FETs, ambipolar behavior was observed, and the Dirac point voltages were near 1 V. This phenomenon liked resulted from the neutralization of impurity charges realized by the ions in the ion gel. Additionally, the Dirac point voltage increased with the drainesource voltage. The dependence of the Dirac point voltage on the drainesource voltage is plotted in Fig. 5e. As the drainesource voltage increased from 0.2 to 0.8 V, the Dirac point voltage increased from 1.36 to 1.96 V. From this dependence, the actual Dirac point voltage of the graphene is inferred to be 1.12 V. The ratio of on current to off current was significantly increased up to ~6 compared to back-gated transistors. The hole and electron mobilities can be extracted from the transfer curves using the relationship between the drainesource current and gate voltage, which is as follows:
ID ¼ m
W CVD ðVG VDirac Þ; L
where m is the field-effect mobility, W and L are the channel width and length, and C is the specific capacitance of the gate dielectric
[17]. The hole and electron mobilities are 30.06 ± 2.03 cm2/(V$s) and 50.06 ± 3.12 cm2/(V$s) respectively. The ion-gated graphene FETs fabricated on the PET substrates are shown in Fig. 6a. Fig. 6b and c shows the output characteristics of the flexible ion-gated graphene FETs on PET substrates without and with 2.7% bending, respectively. When the FETs operate under bending conditions, the drainesource current decreases by approximately 60%. Fig. 6d shows the transfer characteristics of the flexible ion-gated graphene FETs operating under different bending conditions when the drainesource voltage was 0.3 V. As the bending degree increased from 0% to 8.1%, both the on and off currents decreased. From the transfer curves, the on/off current ratio and the Dirac point voltage were extracted. The dependence of the Dirac point voltage and on/off current ratio on the bending degree is plotted in Fig. 6e. As the bending degree increased, the on/ off current ratio first decreased slightly and then increased rapidly, reaching approximately 400 when 8.1% bending was conducted. This may have resulted from the opened bandgap owing to the deformation of the graphene crystal lattice. The Dirac point voltage increased from 1.99 to 2.17 V, while the bending degree of the device increased from 0% to 8.1%. Additionally, the hole and electron mobilities decreased with increasing bending degree.
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Fig. 6. Flexible devices: (a) Photo of flexible ion-gated graphene FETs (bec) output characterization of the flexible ion-gated graphene FETs without bending and with 2.7% bending, (d) transfer characterization of the flexible ion-gated graphene FETs under different bending percentages, and (e) bending percentage dependence on the Dirac point voltage and on/off current ratio in the flexible ion-gated graphene FETs. (A colour version of this figure can be viewed online.)
The experimental results in our work are basically consistent with Pereira’s theory [21]. When the bending degree was increased in a small range from zero, the on/off current ratio basically maintained, or even decreased a little. But as the bending percentage increased to a relatively high value, the on/off current ratio increased rapidly, reaching approximately 400 when 8.1% bending was conducted.
flexible electronic devices.
4. Conclusion
CRediT authorship contribution statement
In this study, LSG of over half a millimeter was synthesized by CVD. A growth method with a procedure of cyclical oxidation/ hydrogen-annealing processes for large graphene crystal domains was proposed, and effects of the cycle of the oxidation/hydrogenannealing processes on the nucleation density and domain size of graphene were studied. Flexible ion-gated graphene FETs were fabricated on PETs, and their electrical properties were studied while they were bent. The devices exhibit excellent flexibility and an on/off current ratio up to 400, due to the formation of graphene bandgap induced by the bending. In conclusion, this work demonstrates that combining LSG with an ion-gel dielectric provides an ion-gated graphene FET that has good application prospects in
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Jing Ning: Conceptualization, Methodology, Writing - review & editing. Ying Wang: Methodology, Writing - original draft. Xin Feng: Resources. Boyu Wang: Visualization. Jianguo Dong: Formal analysis. Dong Wang: Project administration. Chaochao Yan: Visualization. Xue Shen: Formal analysis. Xinran Wang: Project administration. Jincheng Zhang: Supervision. Yue Hao: Funding acquisition. Acknowledgements The work was supported by the Natural Science Basic Research Plan in Shaanxi Province of China (Program No.2017ZDCXL-GY-11-
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03, 2019ZDLGY16-08, 2019ZDLGY16-03, 2019ZDLGY16-02), Youth Science and Technology Nova Program of Shaanxi Province, Wuhu and Xidian University special fund for industry-university-research cooperation (Program No.HX01201909039), Young Talent fund of University Association for Science and Technology in Shaanxi, China (Program No.20170106), the Natural Science Basic Research Plan in Shaanxi Province of China (Program No.2019JQ-860). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2020.03.040. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, et al., Electric field effect in atomically thin carbon films, Science 306 (5696) (2004) 666e669. [2] Wonbong Choi, Indranil Lahiri, Raghunandan Seelaboyina, Yong Soo Kang, Synthesis of graphene and its applications: a review, Crit. Rev. Solid State Mater. Sci. 35 (1) (2010) 52e71. [3] J. Ning, D. Wang, J. Yan, D. Han, Z. Chai, W. Cai, et al., Combined effects of hydrogen annealing on morphological, electrical and structural properties of graphene/r-sapphire, Carbon 75 (2014) 262e270. ~ oz, Cristina Go mez-Aleixandre, Review of CVD synthesis of graphene, [4] R. Mun Chem. Vap. Depos. 19 (10e11-12) (2013) 297e322. [5] P.R. Somani, S.P. Somani, M. Umeno, Planer nano-graphenes from camphor by CVD, Chem. Phys. Lett. 430 (1e3) (2006) 56e59. [6] Xuesong Li, Weiwei Cai, Jinho An, Seyoung Kim, Junghyo Nah, Dongxing Yang, et al., Large-area synthesis of high-quality and uniform graphene films on copper foils, Science 324 (5932) (2009) 1312e1314. [7] O.V. Yazyev, S.G. Louie, Topological defects in graphene: dislocations and grain boundaries, Phys. Rev. B 81 (19) (2010). [8] J. Zhang, J. Zhao, J. Lu, Intrinsic strength and failure behaviors of graphene grain boundaries, ACS Nano 6 (3) (2012) 2704e2711. [9] E. Loginova, N.C. Bartelt, P.J. Feibelman, K.F. McCarty, Factors influencing graphene growth on metal surfaces, New J. Phys. 11 (6) (2009), 63046-0. [10] J. Chen, Y. Guo, L. Jiang, Z. Xu, L. Huang, Y. Xue, et al., Near-equilibrium chemical vapor deposition of high-quality single-crystal graphene directly on various dielectric substrates, Adv. Mater. 26 (9) (2014). [11] F. Chen, Q. Qing, J. Xia, J. Li, N. Tao, Electrochemical gate-controlled charge transport in graphene in ionic liquid and aqueous solution, J. Am. Chem. Soc. 131 (29) (2009) 9908e9909. [12] H. Liu, Y. Liu, D. Zhu, Chemical doping of graphene, J. Mater. Chem. 21 (10) (2011), 3335-0.
[13] W. Shi, J. Ye, Y. Zhang, R. Suzuki, M. Yoshida, J. Miyazaki, et al., Superconductivity series in transition metal dichalcogenides by ionic gating, Sci. Rep. 5 (2015), 12534. [14] M. Kettner, I. Vladimirov, A.J. Strudwick, M.G. Schwab, R.T. Weitz, Ionic gel as gate dielectric for the easy characterization of graphene and polymer fieldeffect transistors and electrochemical resistance modification of graphene, J. Appl. Phys. 118 (2) (2015), 025501. [15] V. Narasimhan, S.Y. Park, An ion gel as a low-cost, spin-coatable, highcapacitance dielectric for electrowetting-on-dielectric (EWOD), Langmuir 31 (30) (2015) 8512e8518. [16] K.H. Lee, M.S. Kang, S. Zhang, Y. Gu, T.P. Lodge, C.D. Frisbie, “Cut and stick” rubbery ion gels as high capacitance gate dielectrics, Adv. Mater. 24 (2012) 4457e4462. [17] B.J. Kim, H. Jang, S.K. Lee, B.H. Hong, J.H. Ahn, J.H. Cho, High-performance flexible graphene field effect transistors with ion gel gate dielectrics, Nano Lett. 10 (9) (2010) 3464e3466. [18] S.K. Lee, B.J. Kim, H. Jang, S.C. Yoon, C. Lee, B.H. Hong, et al., Stretchable graphene transistors with printed dielectrics and gate electrodes, Nano Lett. 11 (11) (2011) 4642. [19] X. Xu, C. Liu, Z. Sun, T. Cao, Z. Zhang, E. Wang, et al., Interfacial engineering in graphene bandgap, Chem. Soc. Rev. 47 (9) (2018) 3059e3099. [20] Z.H. Ni, T. Yu, Y.H. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, Uniaxial strain on graphene: Raman spectroscopy study and band-gap opening, ACS Nano 2 (11) (2008) 2301e2305. [21] V.M. Pereira, A.C. Neto, N.M.R. Peres, Tight-binding approach to uniaxial strain in graphene, Phys. Rev. B 80 (4) (2009), 045401. [22] T. Fujimoto, K. Awaga, Electric-double-layer field-effect transistors with ionic liquids, Phys. Chem. Chem. Phys. 15 (23) (2013) 8983. [23] Y. Hao, M.S. Bharathi, L. Wang, Y. Liu, H. Chen, S. Nie, et al., The role of surface oxygen in the growth of large single-crystal graphene on copper, Science 342 (6159) (2013) 720e723. [24] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, et al., Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006), 18: 187401. [25] C. Wang, W. Chen, C. Han, G. Wang, B. Tang, C. Tang, et al., Growth of millimeter-size single crystal graphene on Cu foils by circumfluence chemical vapor deposition, Sci. Rep. 4 (2014) 4537. [26] X. Zhang, J. Ning, X. Li, B. Wang, L. Hao, M. Liang, et al., Hydrogen-induced effects on the CVD growth of high-quality graphene structures, Nanoscale 5 (18) (2013) 8363. [27] I. Vlassiouk, M. Regmi, P. Fulvio, S. Dai, P. Datskos, G. Eres, et al., Role of hydrogen in chemical vapor deposition growth of large single-crystal graphene, ACS Nano 5 (7) (2011) 6069e6076. [28] B. Luo, E. Gao, D. Geng, H. Wang, Z. Xu, G. Yu, Etching-controlled growth of graphene by chemical vapor deposition, Chem. Mater. 29 (3) (2017) 1022e1027. [29] J Ning, D Wang, Y Chai, X Feng, M Mu, L Guo, et al., Review on mechanism of directly fabricating wafer-scale graphene on dielectric substrates by chemical vapor deposition, Nanotechnology 28 (2017).