- Email: [email protected]
Effects of chemical and physical defects on the humidity sensitivity of graphene surface
Chemical Physics Letters 689 (2017) 206–211
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Effects of chemical and physical defects on the humidity sensitivity of graphene surface Young Jun Son a,1, Kyoung-Yong Chun b,1, Jun-Sik Kim c, Jong-Heun Lee c, Chang-Soo Han a,b,⇑ a
School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea Development Group for Creative Research Engineers of Convergence Mechanical System, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea c Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 22 August 2017 In final form 12 October 2017 Available online 14 October 2017 Keywords: Graphene Humidity sensing Defect Poly(methyl methacrylate)
a b s t r a c t We investigate the effect of the chemical and physical defects on the humidity sensitivity of graphene. For this we apply reactive ion etching for physical defects and the poly(methyl methacrylate) (PMMA) coating for chemical defects to the CVD graphene. The tendency of humidity sensing is hardly found by the physical defects while the distinct changes are observed with chemical defects by control of the thickness and the coverage area of the PMMA on the graphene surface. The graphenes covered with thinner or smaller area of PMMA show an enhanced humidity sensitivity, indicating the possibility of H2O sensing. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction As one of the crystalline carbon structures, graphene is well known for its various kinds of applications [1–4]. Among them, one of the representative applications is the gas sensors. Electrical properties graphene are effected by a few molecules adsorbed on the surface of graphene. [5]. Among them, H2O molecules can be readily doped on graphene surfaces [6,7], implying that graphene could be used as a promising material in humidity sensing applications, but, it is still challenging to achieve high sensitivity for water molecule. In some reports, graphene derivatives like graphene oxide (GO) and reduced graphene oxide (rGO) had been chosen for the sensing material for humidity change [8,9]. GO and rGO are hydrophilic due to oxygen functional groups [10,11], so it is possible to obtain high humidity sensitivity. However, the structure of the GO layer on the substrate, is layer by layer structure of GO flakes [8,9], so it is hard to apply photolithography process and GO itself can have degradation in humid air because of its high solubility in water. The graphene grown by chemical vapor deposition (CVD) is favored in most of the graphene devices, because it is easy to obtain the mono layer of graphene in a large scale [12,13], so it is useful to apply mass production. There are already some reports about the
⇑ Corresponding author at: School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea. E-mail address: [email protected] (C.-S. Han). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.cplett.2017.10.028 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
humidity sensor using CVD graphene [14,15]. Despite these advantages of the CVD graphene, there are some factors to carefully consider when we fabricate the humidity sensor using graphene. The first thing is the polymer residue on the graphene. Wet transfer method are a frequently used method for transferring the CVD graphene [4,12,16], but after transferring graphene by the this method, the PMMA residues remain on the surface of graphene [16]. However, completely removing the polymer residues is impossible [16], and not only the PMMA residues have a doping effect for the graphene [17], but also PMMA residues on the graphene can adsorb water molecules [18,19] so that the electrical properties of graphene can be effected. The second thing is that the CVD graphene intrinsically have various kinds of defects on it [20,21]. The defects on the graphene effects the adsorption of several gas molecules including water molecules [20,22–26]. Therefore, it is necessary to consider the amount of PMMA residues and defects the on the graphene when making the humidity sensor using graphene or the graphene devices under humidity changing conditions. In this study, we report the effect of the physical and chemical defects on the CVD graphene for humidity sensing. The physical defects on the graphene were created by reactive ion etching (RIE). To tailor the chemical defects on the graphene, we covered the PMMA on the graphene surface with different coverage areas and thicknesses using spin coating. To examine the humidity sensitivity, the resistance changes of the modified graphene were measured under the various humidity environment. Effects of the physical and chemical defects were demonstrated in terms of the enhancement of the humidity sensitivity of the graphene.
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
2. Experimental section To study the effect of physical defects on humidity sensitivity of graphene, defects in the graphene samples were controlled using a plasma etching process. The fabrication process of the graphene having defects is depicted in Fig. 1(a). CVD graphene was transferred onto a SiO2/Si substrate by a wet transfer method and a Cr/Au electrode (5 nm/30 nm thick) was deposited using an Ebeam evaporator. The size of the graphene sample exposed to air was 7 mm 10 mm. Subsequently, the graphene sample was etched with oxygen plasma at 20 W and an O2 gas flow of 20 standard cubic centimeters per minute (sccm) at 110 mTorr. The etching time of the samples was varied as 0, 1, 2, and 4 s for inducing defects on the graphene samples [27]. The graphene almost destroyed with further etching time over 4 s, so we didn’t tested. For a convenience, we call the graphene sample with 0 s etching time as the pristine graphene. The sample was then placed in a quartz tube and the electrodes were connected to a multimeter (Keithley 2002) for measuring the sample resistances. All the experiments were performed at room temperature. Through the tube inlet, air with 4% or 72% relative humidity (RH) was flown into the tube. The humidity was controlled using a mass flow controller and a bubbler. Initially, dry air (4% RH) was flown into the tube with a rate of 500 sccm for 1 h in order to stabilize the resistance of the sample. Then, humid air (72% RH) was passed at a rate of
207
500 sccm into the tube for 300 s followed by passing dry air at a rate of 500 sccm for 300 s. We repeated the airflow with different RH values 7 times, each with a duration of 300 s, and we repeated this humidity test for 3 times with the same samples. The resistance of the sample at time t is denoted as R(t). The initial resistance obtained in dry air after the first cycle is denoted as R0. The change in the resistance can be obtained as follows:
DR ðRðtÞ R0 Þ ¼ R0 R0 For convenience, the humidity sensitivity is defined as the average value of the resistance changes measured in 6 cycles under humid air and dry air conditions. To verify the effects of PMMA on the graphene samples, graphene was coated with PMMA layers of different thicknesses and coverage areas. The fabrication processes are shown in Fig. 1. After depositing the electrodes and connecting copper wire on the electrodes, we coated the PMMA layer on the graphene. To vary the thickness of the PMMA layer, we diluted the PMMA solution (MicroChem, 950 k, C4) with mono chlorobenzene, and coated the diluted solution on the graphene by spin coating. The concentrations of PMMA solutions we obtained were 0.5 wt%, 1 wt%, 2 wt %, and 4 wt% (not diluted). The condition of the spin coating was at 3000 rpm for 30 s. The samples coated with PMMA were heated on a hot plate at 180 °C for 1 min in order to cure PMMA on the
Fig. 1. Fabrication process of modified CVD graphene with RIE method, PMMA treatment (thickness and coverage area variation).
208
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
graphene. To control the PMMA are on the graphene, we partially dissolved the PMMA layer on the graphene in acetone. The details of the methods are as follows. After depositing the electrodes, PMMA was coated on the graphene samples by spin coating 4 wt % PMMA solution at 3000 rpm for 30 s. The samples coated with PMMA were heated on a hot plate at 180 °C for 1 min in order to cure PMMA on graphene. After coating the PMMA layer, we partially soaked the PMMA/graphene samples in acetone for more than 20 min to dissolve some portion of the PMMA coating or the entire PMMA on graphene. Thus, we obtained approximately 0%, 25%, 50%, and 75% PMMA covered graphene samples. Copper wires were attached to the electrodes using silver paste. We placed these samples in the quartz tube and tested the humidity sensitivity in the same way as we mentioned above. Then we compared the humidity sensitivity of these samples with the pristine graphene.
3. Results and discussions With RIE method, a two-dimensional (2D) mapping indicating the intensity ratio of D and G bands in the Raman spectra is shown in Fig. 2(a)–(d). The size of the mapping area is 30 lm 30 lm, and the number of measurement points is 30 30. The D/G intensity ratio is increased with an increase in the etching time, and the physically-created defects are uniformly distributed on the graphene surfaces. In this paper, the physically-created defects of the graphene means the empty spaces of the graphene. During RIE of graphene, the reactive ions directly bombards the surface of graphene and creates the physical defects of the graphene. Several groups already studied the defects of graphene made by oxygen RIE [27,28]. Fig. 2(a) shows that some defects already exist in
the pristine graphene, and a significant amount of physical defects are generated by RIE between 1 and 4 s as shown in Fig. 2(b)–(d). This is caused by the physical destruction of the carbon molecules by high energy. In addition, the surface of graphene was almost destroyed at over 4 s, so the experiment was limited to within 4 s. Fig. 2(e) shows the average D/G intensity ratio, which is proportional to the increase of the RIE time, and the maximum value is about 2. The numbers near the data points denote the initial resistances of the samples. The resistance were increased 417.55– 8415.27 X as RIE time increasing. Fig. 2(f) shows the sensitivity of the 0, 1, 2, and 4 s etched samples. The inset graph of Fig. 2f shows the results of second experiment. The baselines were subtracted to easily compare the sensitivity differences. Through the humidity tests, we could observe that the error ranges are relatively large, and the increasing or decreasing tendencies is not noticeable. During the RIE process, it is well known that the hole defects and sp3 defects would create on the graphene [22,27]. These defects can make graphene easily adsorb several gas molecules [22], but simply adding the defect sites on the graphene does not guarantee higher sensitivity on the gas detection such as NO2 and NH3 [22,23]. These studies reveals that there are some optimal point of an amount of defects for the highest performances to detect NO2 and NH3, and large amount of defects on the graphene cause the reducing the sensitivity due to low mobility and high resistance [29,30]. To detect H2O, the sensitivity of the graphene is several times lower than NO2 and NH3 [5], it may be hard to obtain the correlation between humidity sensitivity and the defects on the graphene. Nobuhiko et al. reported that only small amount of defects created by mild UV exposure on the graphene causes increased sensing ability for water molecules [30]. However, in this case the defects are only oxidized defects and there’s
Fig. 2. (a–d) 3D graphs of Raman mapping image for D/G intensity ratio according to the RIE time. (RIE time: upper left corner), (e) average D/G intensity ratio of defected CVD graphene and their initial resistances according to the RIE time, (f) sensitivity of defected CVD graphene according to the RIE time. Each experiments are marked as brown circle, blue rectangular, red triangle. The inset graph is the result of second experiment for 0, 1, 2, and 4 s RIE sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
no results for the change of humidity sensitivity according to the amount of defects. Salehi-Khojin et al. reveals that point defects have negligible contribution on the sensitivity of graphene towards molecule sensing, while a line defect that inherently exists on the surface of the graphene would significantly affect the sensing abilities [20]. Fig. 3(a) shows the resistance changes (DR/R0) of PMMA-coated graphene samples with different thicknesses. In Fig. 3(a), (c), the baseline is subtracted like the inset graph of Fig. 2(f). The resistance value measured over processing time is expressed as stable signals, and the resistance change value is inversely proportional to the thickness of the PMMA (the thickness of the PMMA layer: 11–430 nm). As shown in Fig. 3(b), the lowest sensitivity is observed with a PMMA coating in the thickness of 430 nm, and the graphene with less PMMA thickness tend to have higher sensitivity until the PMMA thickness reaches at 11 nm. Generally, it is well known that PMMA absorb small amount of water [31,32], therefore water molecules can reach through the PMMA layer. As the thickness of PMMA decrease, it may become easier for water molecules to reach the surface of graphene causing the increase of humidity sensitivity. Some research groups reported that a wet transferred graphene have PMMA residues are spread on the entire surface of the graphene and it forms a film that have 1–2 nm thickness [16]. Such a thin PMMA film on the wet transferred graphene can adsorb water molecules and enhance the humidity sensitivity of the graphene [19]. Here, we suppose that as the PMMA layer on the graphene becomes thinner and thinner, the humidity sensitivity of the graphene increases, and this thin layer
209
of PMMA have positive effect on sensing water molecules. As we can see in Fig. 3(c), we obtained the humidity sensitivity according to changes in the coverage area of PMMA. All the sample, as mentioned in experimental section, were coated with the 4 wt% PMMA, so the PMMA effect on the humidity sensing would be similar with the graphene with 430 nm PMMA. It shows that as the coverage of PMMA decreases with 100–25%, the humidity sensitivity tend to increase, and compare with 100% and 25%, the humidity sensitivity is increased as 3.78-fold (Fig. 3(d)). For the case of 0% PMMA covered graphene, it shows higher sensitivity than the graphene covered with 100% PMMA, and lower than the graphene partially covered with PMMA (25, 50, 75%). As the PMMA coverage decreases, and as the surface area of graphene exposed to the air increases, the sensitivity increases. When the surface of the graphene completely exposed to the air, the sensitivity decreases again. The variation range of humidity sensitivity due to the coverage area of PMMA is much wider than that of thickness control. This makes two possible outcomes. First, a 25% PMMA-coverage sample shows the highest sensitivity among the samples. From this result, it can be predicted that the sensing ability for water molecules is changed by the diffusion mechanism between the pristine graphene where the island-like PMMA is present and the PMMA interface that is covered. Water molecules can be absorbed and diffuse through the PMMA layer, [33,34] and finally can reach to the surface of the graphene. [35] Then the resistance of the graphene changes. The uncovered part of graphene has PMMA residues which have also effect the sensitivity of graphene, these residues have island-like shape and their height are 5–60 nm (Fig. 4). From
Fig. 3. (a) Resistance change of the PMMA-modified CVD graphene according to the different PMMA thickness, (b) the variation of sensitivity according to the PMMA thickness, (c) resistance change of the PMMA-modified CVD graphene according to the different PMMA covering area, (d) the variation of sensitivity according to the PMMA coverage. The upper right inset image is a photograph of 75% PMMA coverage sample. The images near the data points are the photographs of the corresponding samples partially covered with PMMA.
210
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
Fig. 4. AFM image of PMMA residues on the graphene surface of 25% PMMA sample (left) and line profiles (right).
the results, we speculate that the PMMA layer and the PMMA residues on the graphene can function as absorption site, and the coverage area of the PMMA on graphene have a strong factor for the water sensitivity. Second, we could speculate that the thicker PMMA layer may impede the adsorption of the water molecules on the graphene. The role of PMMA improves the performance of graphene sensing H2O, but the results are very different depending on the thickness and the ratio of graphene surface. From these results, we found that it is promising to improve the sensing ability of water molecules of graphene through chemical defects modified by using PMMA. In particular, we expect that it is possible to achieve improved-performance by more optimized thickness and surface shape of PMMA. 4. Conclusions In conclusion, we investigated the humidity sensitivity of modified graphene by creating defects. We described that the physical defects on the graphene by RIE method had negligible effects on the humidity sensing ability of graphene. The correlations between the amount of defects and the sensitivity of NO2 and NH3 are clear from the previous studies. Unlike the graphene’s sensitivity of NO2 and NH3, the graphene have relatively low sensitivity for water molecules. Additionally, the defects generated by RIE were point defects which have negligible effects for sensing gas molecules comparing with line defects. Therefore, the physically-defected graphene have almost no correlation between the humidity sensitivity. However, in terms of given chemical defects on graphene, the PMMA have considerable effects on the humidity sensitivity of the graphene. The thinnest PMMA coating (11 nm) on graphene shows the high sensitivity for humidity change. This may be because the PMMA residues on the graphene are in the form of a thin film, and the thin layer of PMMA adsorbs water molecules and increases the humidity sensitivity of the graphene. From the experiment of controlling the PMMA coverage area, we could infer that the PMMA coverage area have significant influences to the humidity sensitivity of the graphene. These results would be useful for the researches about the graphene-based humidity sensors and electronic devices. Acknowledgement This work was supported by Center for Advanced Soft Electronics, ERC project (2016R1A5A1010148) and Basic Science Research
Program funded by the Korea government (2015R1A2A2A01004751, 2015R1A2A1A01005931).
(MSIP)
References [1] J. Wei, Z. Zang, Y. Zhang, M. Wang, J. Du, X. Tang, Enhanced performance of light-controlled conductive switching in hybrid cuprous oxide/reduced graphene oxide Cu2O/rGO) nanocomposites, Opt. Lett. 42 (2017) 911–914. [2] Z. Zang, X. Zeng, M. Wang, W. Hu, C. Liu, X. Tang, Tunable photoluminescence of water-soluble AgInZnS–graphene oxide (GO) nanocomposites and their application in-vivo bioimaging, Sens. Actuators B: Chem. 252 (2017) 1179– 1186. [3] W. Yuan, G. Shi, Graphene-based gas sensors, J. Mater. Chem. A 1 (2013) 10078. [4] X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R.D. Piner, L. Colombo, R.S. Ruoff, Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Letters 9 (2009) 4359–4363. [5] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [6] C. Melios, A. Centeno, A. Zurutuza, V. Panchal, C.E. Giusca, S. Spencer, S.R.P. Silva, O. Kazakova, Effects of humidity on the electronic properties of graphene prepared by chemical vapour deposition, Carbon 103 (2016) 273–280. [7] F. Yavari, C. Kritzinger, C. Gaire, L. Song, H. Gulapalli, T. Borca-Tasciuc, P.M. Ajayan, N. Koratkar, Tunable bandgap in graphene by the controlled adsorption of water molecules, Small 6 (2010) 2535–2538. [8] L. Guo, H.-B. Jiang, R.-Q. Shao, Y.-L. Zhang, S.-Y. Xie, J.-N. Wang, X.-B. Li, F. Jiang, Q.-D. Chen, T. Zhang, H.-B. Sun, Two-beam-laser interference mediated reduction, patterning and nanostructuring of graphene oxide for the production of a flexible humidity sensing device, Carbon 50 (2012) 1667– 1673. [9] Y. Yao, X. Chen, H. Guo, Z. Wu, X. Li, Humidity sensing behaviors of graphene oxide-silicon bi-layer flexible structure, Sens. Actuators B: Chem. 161 (2012) 1053–1058. [10] O.C. Compton, S.T. Nguyen, Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials, Small 6 (2010) 711–723. [11] R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335 (2012) 442–444. [12] 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, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706–710. [13] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y.I. Song, Y.J. Kim, K.S. Kim, B. Ozyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Roll-toroll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578. [14] A.D. Smith, K. Elgammal, F. Niklaus, A. Delin, A.C. Fischer, S. Vaziri, F. Forsberg, M. Rasander, H. Hugosson, L. Bergqvist, S. Schroder, S. Kataria, M. Ostling, M.C. Lemme, Resistive graphene humidity sensors with rapid and direct electrical readout, Nanoscale 7 (2015) 19099–19109. [15] C. Meng-Chu, H. Cheng-Liang, H. Ting-Jen, Fabrication of humidity sensor based on bilayer graphene, IEEE Electron Dev. Lett. 35 (2014) 590–592. [16] J.W. Suk, W.H. Lee, J. Lee, H. Chou, R.D. Piner, Y. Hao, D. Akinwande, R.S. Ruoff, Enhancement of the electrical properties of graphene grown by chemical vapor deposition via controlling the effects of polymer residue, Nano Letters 13 (2013) 1462–1467.
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211 [17] A. Pirkle, J. Chan, A. Venugopal, D. Hinojos, C.W. Magnuson, S. McDonnell, L. Colombo, E.M. Vogel, R.S. Ruoff, R.M. Wallace, The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to SiO2, Appl. Phys. Lett. 99 (2011) 122108. [18] Y. Dan, Y. Lu, N.J. Kybert, Z. Luo, A.T. Johnson, Intrinsic response of graphene vapor sensors, Nano Letters 9 (2009) 1472–1475. [19] Y. Hwangbo, C.K. Lee, S.M. Kim, J.H. Kim, K.S. Kim, B. Jang, H.J. Lee, S.K. Lee, S.S. Kim, J.H. Ahn, S.M. Lee, Fracture characteristics of monolayer CVD-graphene, Sci. Rep. 4 (2014) 4439. [20] A. Salehi-Khojin, D. Estrada, K.Y. Lin, M.H. Bae, F. Xiong, E. Pop, R.I. Masel, Polycrystalline graphene ribbons as chemiresistors, Adv. Mater. 24 (2012) 53– 57, 52. [21] D. Van Lam, S.M. Kim, Y. Cho, J.H. Kim, H.J. Lee, J.M. Yang, S.M. Lee, Healing defective CVD-graphene through vapor phase treatment, Nanoscale 6 (2014) 5639–5644. [22] G. Lee, G. Yang, A. Cho, J.W. Han, J. Kim, Defect-engineered graphene chemical sensors with ultrahigh sensitivity, Phys. Chem. Chem. Phys. 18 (2016) 14198– 14204. [23] M.G. Chung, D.H. Kim, H.M. Lee, T. Kim, J.H. Choi, D.k. Seo, J.-B. Yoo, S.-H. Hong, T.J. Kang, Y.H. Kim, Highly sensitive NO2 gas sensor based on ozone treated graphene, Sens. Actuators B: Chem. 166–167 (2012) 172–176. [24] Y.H. Zhang, Y.B. Chen, K.G. Zhou, C.H. Liu, J. Zeng, H.L. Zhang, Y. Peng, Improving gas sensing properties of graphene by introducing dopants and defects: a first-principles study, Nanotechnology 20 (2009) 185504. [25] M.K. Kostov, E.E. Santiso, A.M. George, K.E. Gubbins, M.B. Nardelli, Dissociation of water on defective carbon substrates, Phys. Rev. Lett. 95 (2005) 136105. [26] B. Sanyal, O. Eriksson, U. Jansson, H. Grennberg, Molecular adsorption in graphene with divacancy defects, Phys. Rev. B 79 (2009).
211
[27] S.P. Surwade, S.N. Smirnov, I.V. Vlassiouk, R.R. Unocic, G.M. Veith, S. Dai, S.M. Mahurin, Water desalination using nanoporous single-layer graphene, Nat. Nanotechnol. 10 (2015) 459–464. [28] G. Xie, R. Yang, P. Chen, J. Zhang, X. Tian, S. Wu, J. Zhao, M. Cheng, W. Yang, D. Wang, C. He, X. Bai, D. Shi, G. Zhang, A general route towards defect and pore engineering in graphene, Small 10 (2014) 2280–2284. [29] J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Letters 8 (2008) 3137–3140. [30] N. Mitoma, R. Nouchi, K. Tanigaki, Enhanced sensing response of oxidized graphene formed by UV irradiation in water, Nanotechnology 26 (2015) 105701. [31] M. N’Diaye, F. Pascaretti-Grizon, P. Massin, M.F. Basle, D. Chappard, Water absorption of poly(methyl methacrylate) measured by vertical interference microscopy, Langmuir 28 (2012) 11609–11614. [32] M. Matsuguchi, S. Umeda, Y. Sadaoka, Y. Sakai, Characterization of polymers for a capacitive-type humidity sensor based on water sorption behavior, Sens. Actuators B: Chem. 49 (1998) 179–185. [33] M.C. Santos, B. Bendiksen, Y.A. Elabd, Diffusion of liquid water in free-standing polymer films using pressure-contact time-resolved fourier transform infrared attenuated total reflectance spectroscopy, Ind. Eng. Chem. Res. 56 (2017) 3464–3476. [34] P. Sutandar, D.J. Ahn, E.I. Franses, FTIR ATR analysis for microstructure and water uptake in poly(methyl methacrylate) spin cast and Langmuir-Blodgett thin films, Macromolecules 27 (1994) 7316–7328. [35] Z. Zhan, J. Sun, L. Liu, E. Wang, Y. Cao, N. Lindvall, G. Skoblin, A. Yurgens, Porefree bubbling delamination of chemical vapor deposited graphene from copper foils, J. Mater. Chem. C 3 (2015) 8634–8641.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Effects of chemical and physical defects on the humidity sensitivity of graphene surface Young Jun Son a,1, Kyoung-Yong Chun b,1, Jun-Sik Kim c, Jong-Heun Lee c, Chang-Soo Han a,b,⇑ a
School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea Development Group for Creative Research Engineers of Convergence Mechanical System, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea c Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 22 August 2017 In final form 12 October 2017 Available online 14 October 2017 Keywords: Graphene Humidity sensing Defect Poly(methyl methacrylate)
a b s t r a c t We investigate the effect of the chemical and physical defects on the humidity sensitivity of graphene. For this we apply reactive ion etching for physical defects and the poly(methyl methacrylate) (PMMA) coating for chemical defects to the CVD graphene. The tendency of humidity sensing is hardly found by the physical defects while the distinct changes are observed with chemical defects by control of the thickness and the coverage area of the PMMA on the graphene surface. The graphenes covered with thinner or smaller area of PMMA show an enhanced humidity sensitivity, indicating the possibility of H2O sensing. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction As one of the crystalline carbon structures, graphene is well known for its various kinds of applications [1–4]. Among them, one of the representative applications is the gas sensors. Electrical properties graphene are effected by a few molecules adsorbed on the surface of graphene. [5]. Among them, H2O molecules can be readily doped on graphene surfaces [6,7], implying that graphene could be used as a promising material in humidity sensing applications, but, it is still challenging to achieve high sensitivity for water molecule. In some reports, graphene derivatives like graphene oxide (GO) and reduced graphene oxide (rGO) had been chosen for the sensing material for humidity change [8,9]. GO and rGO are hydrophilic due to oxygen functional groups [10,11], so it is possible to obtain high humidity sensitivity. However, the structure of the GO layer on the substrate, is layer by layer structure of GO flakes [8,9], so it is hard to apply photolithography process and GO itself can have degradation in humid air because of its high solubility in water. The graphene grown by chemical vapor deposition (CVD) is favored in most of the graphene devices, because it is easy to obtain the mono layer of graphene in a large scale [12,13], so it is useful to apply mass production. There are already some reports about the
⇑ Corresponding author at: School of Mechanical Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea. E-mail address: [email protected] (C.-S. Han). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.cplett.2017.10.028 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
humidity sensor using CVD graphene [14,15]. Despite these advantages of the CVD graphene, there are some factors to carefully consider when we fabricate the humidity sensor using graphene. The first thing is the polymer residue on the graphene. Wet transfer method are a frequently used method for transferring the CVD graphene [4,12,16], but after transferring graphene by the this method, the PMMA residues remain on the surface of graphene [16]. However, completely removing the polymer residues is impossible [16], and not only the PMMA residues have a doping effect for the graphene [17], but also PMMA residues on the graphene can adsorb water molecules [18,19] so that the electrical properties of graphene can be effected. The second thing is that the CVD graphene intrinsically have various kinds of defects on it [20,21]. The defects on the graphene effects the adsorption of several gas molecules including water molecules [20,22–26]. Therefore, it is necessary to consider the amount of PMMA residues and defects the on the graphene when making the humidity sensor using graphene or the graphene devices under humidity changing conditions. In this study, we report the effect of the physical and chemical defects on the CVD graphene for humidity sensing. The physical defects on the graphene were created by reactive ion etching (RIE). To tailor the chemical defects on the graphene, we covered the PMMA on the graphene surface with different coverage areas and thicknesses using spin coating. To examine the humidity sensitivity, the resistance changes of the modified graphene were measured under the various humidity environment. Effects of the physical and chemical defects were demonstrated in terms of the enhancement of the humidity sensitivity of the graphene.
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
2. Experimental section To study the effect of physical defects on humidity sensitivity of graphene, defects in the graphene samples were controlled using a plasma etching process. The fabrication process of the graphene having defects is depicted in Fig. 1(a). CVD graphene was transferred onto a SiO2/Si substrate by a wet transfer method and a Cr/Au electrode (5 nm/30 nm thick) was deposited using an Ebeam evaporator. The size of the graphene sample exposed to air was 7 mm 10 mm. Subsequently, the graphene sample was etched with oxygen plasma at 20 W and an O2 gas flow of 20 standard cubic centimeters per minute (sccm) at 110 mTorr. The etching time of the samples was varied as 0, 1, 2, and 4 s for inducing defects on the graphene samples [27]. The graphene almost destroyed with further etching time over 4 s, so we didn’t tested. For a convenience, we call the graphene sample with 0 s etching time as the pristine graphene. The sample was then placed in a quartz tube and the electrodes were connected to a multimeter (Keithley 2002) for measuring the sample resistances. All the experiments were performed at room temperature. Through the tube inlet, air with 4% or 72% relative humidity (RH) was flown into the tube. The humidity was controlled using a mass flow controller and a bubbler. Initially, dry air (4% RH) was flown into the tube with a rate of 500 sccm for 1 h in order to stabilize the resistance of the sample. Then, humid air (72% RH) was passed at a rate of
207
500 sccm into the tube for 300 s followed by passing dry air at a rate of 500 sccm for 300 s. We repeated the airflow with different RH values 7 times, each with a duration of 300 s, and we repeated this humidity test for 3 times with the same samples. The resistance of the sample at time t is denoted as R(t). The initial resistance obtained in dry air after the first cycle is denoted as R0. The change in the resistance can be obtained as follows:
DR ðRðtÞ R0 Þ ¼ R0 R0 For convenience, the humidity sensitivity is defined as the average value of the resistance changes measured in 6 cycles under humid air and dry air conditions. To verify the effects of PMMA on the graphene samples, graphene was coated with PMMA layers of different thicknesses and coverage areas. The fabrication processes are shown in Fig. 1. After depositing the electrodes and connecting copper wire on the electrodes, we coated the PMMA layer on the graphene. To vary the thickness of the PMMA layer, we diluted the PMMA solution (MicroChem, 950 k, C4) with mono chlorobenzene, and coated the diluted solution on the graphene by spin coating. The concentrations of PMMA solutions we obtained were 0.5 wt%, 1 wt%, 2 wt %, and 4 wt% (not diluted). The condition of the spin coating was at 3000 rpm for 30 s. The samples coated with PMMA were heated on a hot plate at 180 °C for 1 min in order to cure PMMA on the
Fig. 1. Fabrication process of modified CVD graphene with RIE method, PMMA treatment (thickness and coverage area variation).
208
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
graphene. To control the PMMA are on the graphene, we partially dissolved the PMMA layer on the graphene in acetone. The details of the methods are as follows. After depositing the electrodes, PMMA was coated on the graphene samples by spin coating 4 wt % PMMA solution at 3000 rpm for 30 s. The samples coated with PMMA were heated on a hot plate at 180 °C for 1 min in order to cure PMMA on graphene. After coating the PMMA layer, we partially soaked the PMMA/graphene samples in acetone for more than 20 min to dissolve some portion of the PMMA coating or the entire PMMA on graphene. Thus, we obtained approximately 0%, 25%, 50%, and 75% PMMA covered graphene samples. Copper wires were attached to the electrodes using silver paste. We placed these samples in the quartz tube and tested the humidity sensitivity in the same way as we mentioned above. Then we compared the humidity sensitivity of these samples with the pristine graphene.
3. Results and discussions With RIE method, a two-dimensional (2D) mapping indicating the intensity ratio of D and G bands in the Raman spectra is shown in Fig. 2(a)–(d). The size of the mapping area is 30 lm 30 lm, and the number of measurement points is 30 30. The D/G intensity ratio is increased with an increase in the etching time, and the physically-created defects are uniformly distributed on the graphene surfaces. In this paper, the physically-created defects of the graphene means the empty spaces of the graphene. During RIE of graphene, the reactive ions directly bombards the surface of graphene and creates the physical defects of the graphene. Several groups already studied the defects of graphene made by oxygen RIE [27,28]. Fig. 2(a) shows that some defects already exist in
the pristine graphene, and a significant amount of physical defects are generated by RIE between 1 and 4 s as shown in Fig. 2(b)–(d). This is caused by the physical destruction of the carbon molecules by high energy. In addition, the surface of graphene was almost destroyed at over 4 s, so the experiment was limited to within 4 s. Fig. 2(e) shows the average D/G intensity ratio, which is proportional to the increase of the RIE time, and the maximum value is about 2. The numbers near the data points denote the initial resistances of the samples. The resistance were increased 417.55– 8415.27 X as RIE time increasing. Fig. 2(f) shows the sensitivity of the 0, 1, 2, and 4 s etched samples. The inset graph of Fig. 2f shows the results of second experiment. The baselines were subtracted to easily compare the sensitivity differences. Through the humidity tests, we could observe that the error ranges are relatively large, and the increasing or decreasing tendencies is not noticeable. During the RIE process, it is well known that the hole defects and sp3 defects would create on the graphene [22,27]. These defects can make graphene easily adsorb several gas molecules [22], but simply adding the defect sites on the graphene does not guarantee higher sensitivity on the gas detection such as NO2 and NH3 [22,23]. These studies reveals that there are some optimal point of an amount of defects for the highest performances to detect NO2 and NH3, and large amount of defects on the graphene cause the reducing the sensitivity due to low mobility and high resistance [29,30]. To detect H2O, the sensitivity of the graphene is several times lower than NO2 and NH3 [5], it may be hard to obtain the correlation between humidity sensitivity and the defects on the graphene. Nobuhiko et al. reported that only small amount of defects created by mild UV exposure on the graphene causes increased sensing ability for water molecules [30]. However, in this case the defects are only oxidized defects and there’s
Fig. 2. (a–d) 3D graphs of Raman mapping image for D/G intensity ratio according to the RIE time. (RIE time: upper left corner), (e) average D/G intensity ratio of defected CVD graphene and their initial resistances according to the RIE time, (f) sensitivity of defected CVD graphene according to the RIE time. Each experiments are marked as brown circle, blue rectangular, red triangle. The inset graph is the result of second experiment for 0, 1, 2, and 4 s RIE sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
no results for the change of humidity sensitivity according to the amount of defects. Salehi-Khojin et al. reveals that point defects have negligible contribution on the sensitivity of graphene towards molecule sensing, while a line defect that inherently exists on the surface of the graphene would significantly affect the sensing abilities [20]. Fig. 3(a) shows the resistance changes (DR/R0) of PMMA-coated graphene samples with different thicknesses. In Fig. 3(a), (c), the baseline is subtracted like the inset graph of Fig. 2(f). The resistance value measured over processing time is expressed as stable signals, and the resistance change value is inversely proportional to the thickness of the PMMA (the thickness of the PMMA layer: 11–430 nm). As shown in Fig. 3(b), the lowest sensitivity is observed with a PMMA coating in the thickness of 430 nm, and the graphene with less PMMA thickness tend to have higher sensitivity until the PMMA thickness reaches at 11 nm. Generally, it is well known that PMMA absorb small amount of water [31,32], therefore water molecules can reach through the PMMA layer. As the thickness of PMMA decrease, it may become easier for water molecules to reach the surface of graphene causing the increase of humidity sensitivity. Some research groups reported that a wet transferred graphene have PMMA residues are spread on the entire surface of the graphene and it forms a film that have 1–2 nm thickness [16]. Such a thin PMMA film on the wet transferred graphene can adsorb water molecules and enhance the humidity sensitivity of the graphene [19]. Here, we suppose that as the PMMA layer on the graphene becomes thinner and thinner, the humidity sensitivity of the graphene increases, and this thin layer
209
of PMMA have positive effect on sensing water molecules. As we can see in Fig. 3(c), we obtained the humidity sensitivity according to changes in the coverage area of PMMA. All the sample, as mentioned in experimental section, were coated with the 4 wt% PMMA, so the PMMA effect on the humidity sensing would be similar with the graphene with 430 nm PMMA. It shows that as the coverage of PMMA decreases with 100–25%, the humidity sensitivity tend to increase, and compare with 100% and 25%, the humidity sensitivity is increased as 3.78-fold (Fig. 3(d)). For the case of 0% PMMA covered graphene, it shows higher sensitivity than the graphene covered with 100% PMMA, and lower than the graphene partially covered with PMMA (25, 50, 75%). As the PMMA coverage decreases, and as the surface area of graphene exposed to the air increases, the sensitivity increases. When the surface of the graphene completely exposed to the air, the sensitivity decreases again. The variation range of humidity sensitivity due to the coverage area of PMMA is much wider than that of thickness control. This makes two possible outcomes. First, a 25% PMMA-coverage sample shows the highest sensitivity among the samples. From this result, it can be predicted that the sensing ability for water molecules is changed by the diffusion mechanism between the pristine graphene where the island-like PMMA is present and the PMMA interface that is covered. Water molecules can be absorbed and diffuse through the PMMA layer, [33,34] and finally can reach to the surface of the graphene. [35] Then the resistance of the graphene changes. The uncovered part of graphene has PMMA residues which have also effect the sensitivity of graphene, these residues have island-like shape and their height are 5–60 nm (Fig. 4). From
Fig. 3. (a) Resistance change of the PMMA-modified CVD graphene according to the different PMMA thickness, (b) the variation of sensitivity according to the PMMA thickness, (c) resistance change of the PMMA-modified CVD graphene according to the different PMMA covering area, (d) the variation of sensitivity according to the PMMA coverage. The upper right inset image is a photograph of 75% PMMA coverage sample. The images near the data points are the photographs of the corresponding samples partially covered with PMMA.
210
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211
Fig. 4. AFM image of PMMA residues on the graphene surface of 25% PMMA sample (left) and line profiles (right).
the results, we speculate that the PMMA layer and the PMMA residues on the graphene can function as absorption site, and the coverage area of the PMMA on graphene have a strong factor for the water sensitivity. Second, we could speculate that the thicker PMMA layer may impede the adsorption of the water molecules on the graphene. The role of PMMA improves the performance of graphene sensing H2O, but the results are very different depending on the thickness and the ratio of graphene surface. From these results, we found that it is promising to improve the sensing ability of water molecules of graphene through chemical defects modified by using PMMA. In particular, we expect that it is possible to achieve improved-performance by more optimized thickness and surface shape of PMMA. 4. Conclusions In conclusion, we investigated the humidity sensitivity of modified graphene by creating defects. We described that the physical defects on the graphene by RIE method had negligible effects on the humidity sensing ability of graphene. The correlations between the amount of defects and the sensitivity of NO2 and NH3 are clear from the previous studies. Unlike the graphene’s sensitivity of NO2 and NH3, the graphene have relatively low sensitivity for water molecules. Additionally, the defects generated by RIE were point defects which have negligible effects for sensing gas molecules comparing with line defects. Therefore, the physically-defected graphene have almost no correlation between the humidity sensitivity. However, in terms of given chemical defects on graphene, the PMMA have considerable effects on the humidity sensitivity of the graphene. The thinnest PMMA coating (11 nm) on graphene shows the high sensitivity for humidity change. This may be because the PMMA residues on the graphene are in the form of a thin film, and the thin layer of PMMA adsorbs water molecules and increases the humidity sensitivity of the graphene. From the experiment of controlling the PMMA coverage area, we could infer that the PMMA coverage area have significant influences to the humidity sensitivity of the graphene. These results would be useful for the researches about the graphene-based humidity sensors and electronic devices. Acknowledgement This work was supported by Center for Advanced Soft Electronics, ERC project (2016R1A5A1010148) and Basic Science Research
Program funded by the Korea government (2015R1A2A2A01004751, 2015R1A2A1A01005931).
(MSIP)
References [1] J. Wei, Z. Zang, Y. Zhang, M. Wang, J. Du, X. Tang, Enhanced performance of light-controlled conductive switching in hybrid cuprous oxide/reduced graphene oxide Cu2O/rGO) nanocomposites, Opt. Lett. 42 (2017) 911–914. [2] Z. Zang, X. Zeng, M. Wang, W. Hu, C. Liu, X. Tang, Tunable photoluminescence of water-soluble AgInZnS–graphene oxide (GO) nanocomposites and their application in-vivo bioimaging, Sens. Actuators B: Chem. 252 (2017) 1179– 1186. [3] W. Yuan, G. Shi, Graphene-based gas sensors, J. Mater. Chem. A 1 (2013) 10078. [4] X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R.D. Piner, L. Colombo, R.S. Ruoff, Transfer of large-area graphene films for high-performance transparent conductive electrodes, Nano Letters 9 (2009) 4359–4363. [5] F. Schedin, A.K. Geim, S.V. Morozov, E.W. Hill, P. Blake, M.I. Katsnelson, K.S. Novoselov, Detection of individual gas molecules adsorbed on graphene, Nat. Mater. 6 (2007) 652–655. [6] C. Melios, A. Centeno, A. Zurutuza, V. Panchal, C.E. Giusca, S. Spencer, S.R.P. Silva, O. Kazakova, Effects of humidity on the electronic properties of graphene prepared by chemical vapour deposition, Carbon 103 (2016) 273–280. [7] F. Yavari, C. Kritzinger, C. Gaire, L. Song, H. Gulapalli, T. Borca-Tasciuc, P.M. Ajayan, N. Koratkar, Tunable bandgap in graphene by the controlled adsorption of water molecules, Small 6 (2010) 2535–2538. [8] L. Guo, H.-B. Jiang, R.-Q. Shao, Y.-L. Zhang, S.-Y. Xie, J.-N. Wang, X.-B. Li, F. Jiang, Q.-D. Chen, T. Zhang, H.-B. Sun, Two-beam-laser interference mediated reduction, patterning and nanostructuring of graphene oxide for the production of a flexible humidity sensing device, Carbon 50 (2012) 1667– 1673. [9] Y. Yao, X. Chen, H. Guo, Z. Wu, X. Li, Humidity sensing behaviors of graphene oxide-silicon bi-layer flexible structure, Sens. Actuators B: Chem. 161 (2012) 1053–1058. [10] O.C. Compton, S.T. Nguyen, Graphene oxide, highly reduced graphene oxide, and graphene: versatile building blocks for carbon-based materials, Small 6 (2010) 711–723. [11] R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335 (2012) 442–444. [12] 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, Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706–710. [13] S. Bae, H. Kim, Y. Lee, X. Xu, J.S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H.R. Kim, Y.I. Song, Y.J. Kim, K.S. Kim, B. Ozyilmaz, J.H. Ahn, B.H. Hong, S. Iijima, Roll-toroll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574–578. [14] A.D. Smith, K. Elgammal, F. Niklaus, A. Delin, A.C. Fischer, S. Vaziri, F. Forsberg, M. Rasander, H. Hugosson, L. Bergqvist, S. Schroder, S. Kataria, M. Ostling, M.C. Lemme, Resistive graphene humidity sensors with rapid and direct electrical readout, Nanoscale 7 (2015) 19099–19109. [15] C. Meng-Chu, H. Cheng-Liang, H. Ting-Jen, Fabrication of humidity sensor based on bilayer graphene, IEEE Electron Dev. Lett. 35 (2014) 590–592. [16] J.W. Suk, W.H. Lee, J. Lee, H. Chou, R.D. Piner, Y. Hao, D. Akinwande, R.S. Ruoff, Enhancement of the electrical properties of graphene grown by chemical vapor deposition via controlling the effects of polymer residue, Nano Letters 13 (2013) 1462–1467.
Y.J. Son et al. / Chemical Physics Letters 689 (2017) 206–211 [17] A. Pirkle, J. Chan, A. Venugopal, D. Hinojos, C.W. Magnuson, S. McDonnell, L. Colombo, E.M. Vogel, R.S. Ruoff, R.M. Wallace, The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to SiO2, Appl. Phys. Lett. 99 (2011) 122108. [18] Y. Dan, Y. Lu, N.J. Kybert, Z. Luo, A.T. Johnson, Intrinsic response of graphene vapor sensors, Nano Letters 9 (2009) 1472–1475. [19] Y. Hwangbo, C.K. Lee, S.M. Kim, J.H. Kim, K.S. Kim, B. Jang, H.J. Lee, S.K. Lee, S.S. Kim, J.H. Ahn, S.M. Lee, Fracture characteristics of monolayer CVD-graphene, Sci. Rep. 4 (2014) 4439. [20] A. Salehi-Khojin, D. Estrada, K.Y. Lin, M.H. Bae, F. Xiong, E. Pop, R.I. Masel, Polycrystalline graphene ribbons as chemiresistors, Adv. Mater. 24 (2012) 53– 57, 52. [21] D. Van Lam, S.M. Kim, Y. Cho, J.H. Kim, H.J. Lee, J.M. Yang, S.M. Lee, Healing defective CVD-graphene through vapor phase treatment, Nanoscale 6 (2014) 5639–5644. [22] G. Lee, G. Yang, A. Cho, J.W. Han, J. Kim, Defect-engineered graphene chemical sensors with ultrahigh sensitivity, Phys. Chem. Chem. Phys. 18 (2016) 14198– 14204. [23] M.G. Chung, D.H. Kim, H.M. Lee, T. Kim, J.H. Choi, D.k. Seo, J.-B. Yoo, S.-H. Hong, T.J. Kang, Y.H. Kim, Highly sensitive NO2 gas sensor based on ozone treated graphene, Sens. Actuators B: Chem. 166–167 (2012) 172–176. [24] Y.H. Zhang, Y.B. Chen, K.G. Zhou, C.H. Liu, J. Zeng, H.L. Zhang, Y. Peng, Improving gas sensing properties of graphene by introducing dopants and defects: a first-principles study, Nanotechnology 20 (2009) 185504. [25] M.K. Kostov, E.E. Santiso, A.M. George, K.E. Gubbins, M.B. Nardelli, Dissociation of water on defective carbon substrates, Phys. Rev. Lett. 95 (2005) 136105. [26] B. Sanyal, O. Eriksson, U. Jansson, H. Grennberg, Molecular adsorption in graphene with divacancy defects, Phys. Rev. B 79 (2009).
211
[27] S.P. Surwade, S.N. Smirnov, I.V. Vlassiouk, R.R. Unocic, G.M. Veith, S. Dai, S.M. Mahurin, Water desalination using nanoporous single-layer graphene, Nat. Nanotechnol. 10 (2015) 459–464. [28] G. Xie, R. Yang, P. Chen, J. Zhang, X. Tian, S. Wu, J. Zhao, M. Cheng, W. Yang, D. Wang, C. He, X. Bai, D. Shi, G. Zhang, A general route towards defect and pore engineering in graphene, Small 10 (2014) 2280–2284. [29] J.T. Robinson, F.K. Perkins, E.S. Snow, Z. Wei, P.E. Sheehan, Reduced graphene oxide molecular sensors, Nano Letters 8 (2008) 3137–3140. [30] N. Mitoma, R. Nouchi, K. Tanigaki, Enhanced sensing response of oxidized graphene formed by UV irradiation in water, Nanotechnology 26 (2015) 105701. [31] M. N’Diaye, F. Pascaretti-Grizon, P. Massin, M.F. Basle, D. Chappard, Water absorption of poly(methyl methacrylate) measured by vertical interference microscopy, Langmuir 28 (2012) 11609–11614. [32] M. Matsuguchi, S. Umeda, Y. Sadaoka, Y. Sakai, Characterization of polymers for a capacitive-type humidity sensor based on water sorption behavior, Sens. Actuators B: Chem. 49 (1998) 179–185. [33] M.C. Santos, B. Bendiksen, Y.A. Elabd, Diffusion of liquid water in free-standing polymer films using pressure-contact time-resolved fourier transform infrared attenuated total reflectance spectroscopy, Ind. Eng. Chem. Res. 56 (2017) 3464–3476. [34] P. Sutandar, D.J. Ahn, E.I. Franses, FTIR ATR analysis for microstructure and water uptake in poly(methyl methacrylate) spin cast and Langmuir-Blodgett thin films, Macromolecules 27 (1994) 7316–7328. [35] Z. Zhan, J. Sun, L. Liu, E. Wang, Y. Cao, N. Lindvall, G. Skoblin, A. Yurgens, Porefree bubbling delamination of chemical vapor deposited graphene from copper foils, J. Mater. Chem. C 3 (2015) 8634–8641.