Effect of TiO2 nanoparticles on electrical properties of chemical vapor deposition grown single layer graphene

Effect of TiO2 nanoparticles on electrical properties of chemical vapor deposition grown single layer graphene

Synthetic Metals 256 (2019) 116155 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Effec...

2MB Sizes 0 Downloads 8 Views

Synthetic Metals 256 (2019) 116155

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Effect of TiO2 nanoparticles on electrical properties of chemical vapor deposition grown single layer graphene

T



Anand Kumar Singha, Vivek Chaudharyb, Arun Kumar Singhb, , S.R.P. Sinhaa a b

Institute of Engineering and Technology, Lucknow 226021, India Department of Physics, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, 211004, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Single layer graphene CVD graphene TiO2 nanoparticles (NPs) Doping TiO2 doped graphene

Modification of charge carriers is essential for better performance of electronic and optoelectronic devices based on graphene. In this article, we study the effect of different concentrations of titanium dioxide (TiO2) nanoparticles (NPs) on chemical vapor deposition (CVD) grown graphene by using Raman spectroscopy and electrical charge transport measurements. Large-area and single-layer graphene (SLG) film were grown on copper (Cu) foil by the CVD process and grown films were transferred onto the Si/SiO2 substrate for electrical measurements and other characterizations. The Raman spectra and electrical charge transport measurements show that TiO2 NPs change the electronic properties as well as the structure of the CVD-grown graphene. The drain current versus back gate voltage measurements revealed that TiO2 NPs imposed p-type doping in CVD-grown SLG. In Raman spectra, peak frequencies shifts were analyzed before and after the doping of TiO2 NPs on CVD graphene. The shifting of 2D peaks positions towards higher wavenumber also indicates the p-type doping in graphene by TiO2 NPs. We believe that our study may be useful to understand the charge transport phenomena in TiO2 doped graphene-based electronics devices.

1. Introduction Graphene is sp2 hybridized single layer of carbon atoms arranged in two-dimensional honeycomb lattice, that exhibits a unique set of electrical, mechanical and optical properties. The exceptional electrical and mechanical properties of graphene make it an auspicious material for various applications such as sensors, supercapacitors, transparent and flexible electrodes [1–5]. The realization of such device-level applications requires the adaptable making of uniform high-quality singlelayer graphene (SLG). Many methods such as mechanical exfoliation, epitaxial growth on silicon carbide (SiC) and chemical vapor deposition (CVD) have been developed for the synthesis of SLG [6–8]. Among these methods, mechanical exfoliation provides the best quality of graphene, however, this method is not applicable for large-area devices and also difficult to control the number of layers. Epitaxial growth of graphene on SiC introduced unintentional doping in graphene layer during the synthesis [9,10]. Nowadays, CVD method has been proven the most effective method for producing the high-quality and large area graphene for a variety of applications because it is inexpensive, high efficiency and easy transfer to any substrate [11,12]. Tuning of charge carrier in graphene is necessary for the successful operation of electronic devices [13,14]. It is already reported that



electrical properties of graphene can be tuned by charge impurities, defects and physisorbed molecules [15]. Several approaches like absorption of gaseous molecules, aromatic molecules, electrostatic and chemical doping have already been attempted to tailor the electronic properties of graphene [16–20]. Among them, the chemical doping is the most adequate and easiest way to tailor the electrical properties of graphene [14,21]. Single-layer graphene has high transparency (97.7%) for visible light (λ = 550 nm) [22]. The high mobility and transparency of graphene make suitable material for use as a transparent conductive electrode in photovoltaic cells, organic light-emitting diodes, and, touch screens displays [23,24]. The performance of these devices can be improved by Fermi level band alignment between the graphene electrode and active material [25,26]. Nowadays, titanium dioxide (TiO2) nanoparticles (NPs) is one of the most promising semiconducting material used in solar cells due to its magnificent properties such as excellent chemical stability, low cost, low toxic, and shows great photocatalytic performance for the degradation of organic contaminants [27,28]. In the recent time, TiO2 NPs layer has been deposited over the electrode (either anode side or cathode side depending on the nature of active material) to improve the performance of photovoltaic (solar cells, photodiodes, and photodetectors, etc.) devices. Graphene and TiO2 doped graphene have

Corresponding author. E-mail address: [email protected] (A.K. Singh).

https://doi.org/10.1016/j.synthmet.2019.116155 Received 14 June 2019; Received in revised form 20 August 2019; Accepted 21 August 2019 Available online 28 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.

Synthetic Metals 256 (2019) 116155

A.K. Singh, et al.

Fig. 1. Schematic diagram of TiO2 doped CVD grown SLG devices.

already been used as an electrode in photovoltaic devices [29–33]. However, the effect of TiO2 NPs on electrical charge transport properties of CVD grown SLG has not been much explored so far. In the present work, we study the effect of TiO2 nanoparticles on electrical charge transport properties of CVD grown SLG. The different concentrations of TiO2 nanoparticles are deposited on CVD grown SLG and investigated by using Raman spectroscopy and transport measurements. We have also studied the surface morphological changes in graphene after TiO2 doping via atomic force microscopy (AFM). Raman spectra and electrical transport measurements revealed that TiO2 nanoparticles impose p-doping in CVD grown SLG. The shift of 2D peaks and intensity ratio of 2D and G peaks are measured as a function of TiO2 doping fraction. The back gate voltage (Vbg) dependent drain current (ID) are also analyzed as a function of TiO2 NPs concentrations. The shifting of Dirac points (charge neutrality point) towards positive gate voltage confirmed the p-type doping (hole doping) in CVD grown SLG by TiO2 nanoparticles.

2. Experimental section 2.1. Synthesis and transfer of CVD graphene The synthesis of large-area graphene film on various conducting metal surfaces by the chemical vapor deposition (CVD) method is a very practical approach. High-quality single-layer graphene films were synthesized on 25 μm thick copper (Cu) (Alfa Aesar, Product code 13382) foils by inductively coupled plasma-enhanced chemical vapor deposition (PECVD) same as explained in our previous paper [14]. The foil was mounted in the chamber without any treatment. The Cu metal used as a catalyst substrate for preparation of the large area, high-quality SLG, and keeping the base pressure of the CVD chamber was about 10−7 Torr by using turbo-molecular-pump. The substrate temperature was increased to 830 0C and H2 gas flows into the chamber with the flow rate at 40 sccm [14,34]. It was discharged by high frequency (RF) power of 50 W for two minutes to remove surface oxides of Cu foil. During the synthesis of graphene, the chamber pressure was kept at 10 mTorr. RF plasma was generated for three minutes at a continuous flow of a mixture of argon (40 sccm) and methane (1 sccm) gases at 830 0C temperature. After growth, the sample was cooled down to room temperature. The CVD grown SLG on Cu substrate was transferred on Si/ SiO2 substrate. To transfer CVD grown SLG on Si/SiO2, first, a thin film of poly-methyl methacrylate (PMMA) polymer was coated (1000 rpm for 10 s, 2500 rpm for the 30 s) onto CVD grown Cu substrate. Then spin-coated PMMA film on CVD grown graphene/Cu was dipped in solution of ammonium persulfate (APS) to etch Cu foil for one day. Finally, PMMA coated CVD grown graphene film was transferred to the Si/SiO2 substrate. The CVD grown SLG on the Si/SiO2 substrate was kept in acetone to completely remove the PMMA layer from the graphene surface.

Fig. 2. (a) Normalized (by IG) Raman spectra of pristine and 0.1, 0.5, & 1.0 mg TiO2 doped CVD grown SLG. (b) Normalized (by IG) Raman G and 2D peaks of pristine and 0.1, 0.5, & 1.0 mg TiO2 doped CVD grown SLG. (c) Variation of peak intensity ratio ID/IG and I2D/IG with concentrations of TiO2 NPs.

2.2. TiO2 doping and characterization techniques The graphene films transferred over Si/SiO2 substrates were characterized by Raman spectroscopy (WiTec Alpha 300R Micro Spectrometer) with 532 nm laser with power of 1 mW and Surface morphology by atomic force microscope (AFM) (Asylum Research MFP3D). For electrical transport measurements gold (Au) electrodes were deposited over CVD graphene/SiO2/Si films via thermal coating using a shadow mask. The electrical transport properties of fabricated devices were measured by using Semiconductor Device Parameter Analyzer 2

Synthetic Metals 256 (2019) 116155

A.K. Singh, et al.

Table 1 Raman D, G and 2D peak position and width of pristine and TiO2 doped CVD grown graphene. Samples

Peak position (cm−1) D

Pristine 0.1 mg TiO2 0.5 mg TiO2 1.0 mg TiO2

1341 1341 1341 1341

Width (cm−1) G

± ± ± ±

0.8 0.5 1.0 0.7

1584 1584 1584 1584

2D ± ± ± ±

1.2 0.4 0.8 1.4

2676 2678 2679 2684

D ± ± ± ±

2.0 1.7 2.2 3.5

24.53 24.11 26.39 28.65

G ± ± ± ±

1.5 2.0 1.2 1.8

22.22 23.00 23.12 26.53

2D ± ± ± ±

2.0 1.6 1.3 2.0

33.63 32.22 32.81 39.81

± ± ± ±

2.5 0.8 1.7 3.0

Fig. 3. AFM height image of (a) pristine, (b) 0.1 mg TiO2 doped, (c) 0.5 mg TiO2 doped and (d) 1.0 mg TiO2 doped CVD grown graphene (inset shows height scale bar).

3. Results and discussion

(Keysight-B1500A). Further, TiO2 NPs with an average particle size 32 nm purchased from Alfa Aesar (Product code 39953, Phase- anatase) was suspended in distilled water with a concentration of 10 mg/ml. 10 μl, 50 μl, and 100 μl nanoparticle suspensions were then dropped over the channels of fabricated devices in order to dope the graphene by 0.1 mg, 0.5 mg, and 1.0 mg TiO2. The three different concentrations of TiO2 NPs were dropped on three different devices with the same device area by micropipette. We have adjusted TiO2 NPs drop such that drops were placed within the active area of the devices and most of TiO2 NPs were situated within the channel region of the devices. The doped CVD grown graphene was annealed at 60 °C for 1 h to remove the water and stored in a vacuum desiccator for one day to complete removal of water and further characterizations. The schematic device structure with TiO2 NPs is depicted in Fig. 1. In order to study the effect of TiO2 NPs on CVD grown single-layer graphene samples were again characterized by Raman spectroscopy, AFM, and electrical measurements under similar conditions. We used the van der Pauw measurement technique for all the electrical measurements.

3.1. Raman spectra of TiO2 NPs doped CVD graphene Fig. 2(a) displays the normalized (by intensity of G peak) Raman spectra of CVD grown SLG before and after treatment with TiO2 nanoparticles of different concentrations. The Raman spectra present the characteristic peaks D, G and 2D position at 1341 cm−1, 1584 cm−1, and 2676 cm−1, respectively, for pristine graphene. The D band generally signifies defect in the graphene layer and it occurs due to A1g mode vibration of six hexagonal sp2 carbon rings, which is absent in defect-free graphene. The D peak intensity of CVD graphene is increasing after treatment with TiO2 NPs with different concentrations. The increment in the intensity of the D peak may be due to the interaction to TiO2 NPs with CVD graphene. The G band is associated with the E2g vibration mode of sp2 hybridized bonds and the 2D peak is the second order of D peak [35,36]. The shift in the position of G and 2D peaks of graphene before and after TiO2 nanoparticles doping with different concentrations are shown in Fig. 2(b). In general, the G band position shift towards lower wavenumber or higher wave number arises 3

Synthetic Metals 256 (2019) 116155

A.K. Singh, et al.

Fig. 4. The back gate voltage (Vg) versus drain current (ID) characteristics of (a) pristine, (b) 0.1 mg TiO2 doped, (c) 0.5 mg TiO2 doped, (d) 1.0 mg TiO2 doped CVD grown graphene devices.

doping in graphene [46]. The peak intensity ratio I2D/IG is maximum for pristine graphene and decreases monotonically with the increasing concentration of TiO2 NPs as shown in Fig. 2(c). The decrease in intensity ratio I2D/IG may be due to the electron-electron scattering and also confirmed the interaction of TiO2 NPs with CVD grown graphene. The peak intensity ratio ID/IG of graphene initially increases after doping, due to chemical interaction of TiO2 NPs with graphene surface. The peak intensity ratio ID/IG further decreases, when doping concentration of TiO2 increases.

due to the effect of temperature or strain in material [37,38]. In our case, G band position shows almost no shift after TiO2 doping in CVD grown SLG. On the other way, the upward shifting of 2D band position with the increasing concentrations of TiO2 is observed by Raman spectroscopy, which is shown in Fig. 2(b). The maximum upshifting (∼8 cm−1) of 2D peak position is observer for highly doped (1 mg TiO2) graphene sample, similar observation had been reported by other in case of hole doping in graphene [39,40]. Wang et al. had reported hole doping in graphene by the deposition of Au and Ni metal film on SLG and showed shifting of Raman 2D peak towards higher wave number [40]. We have measured the Raman spectra of samples at different locations of the samples. The position and width of D, G and 2D peaks of pristine and TiO2 doped graphene are summarized in Table 1. It is already reported that the shifting of 2D band position toward upward and downward is the signature of p-type and n-type doping, respectively [41,42]. Previous DFT analysis shows that TiO2 imposed both n-type and p-type doping in graphene depending on which atoms (Ti or O) are closed to graphene surface [43]. The TiO2 layer in the g2 × 2-hex-TiO2 band structure can be seen as consisting of three atomic layers a titanium atom lies between two oxygen atoms, thus exposing oxygen atoms closest to graphene indicates p-type doping in graphene [43]. It was also reported that the charge transfer phenomenon occurs between graphene film and TiO2 guiding to efficient holes concentrations (p-type doping) in the graphene [44]. Fig. 2(c) shows the peak intensity ratio I2D/IG and ID/IG of graphene before and after doping as a function of TiO2 NPs concentrations. The intensity ratio I2D/IG also changes monotonically as a function of TiO2 NPs concentrations as similar reported by others [45]. The intensity of the 2D peak is generally more than two times of intensity of G peak, which is the trademark of single-layer graphene [36]. The 2D peak intensity over G peak intensity ratio (I2D/IG) is a sensitive parameter for

3.2. Surface morphology AFM is used to study the surface morphology of pristine and TiO2 NPs doped CVD grown graphene on SiO2 substrate. The AFM images of pristine and TiO2 doped graphene is shown in Fig. 3. Fig. 3 clearly shows a change in surface morphology after TiO2 NPs doping. As we increase the concentrations of TiO2 NPs on graphene, the surface morphology differs with more number of nanoparticles thereby increasing the surface roughness. In AFM analysis, the root mean square roughness (Rz) is the most frequently used parameter to characterize the surface morphology. The Rz of pristine, 0.1 mg TiO2 doped, 0.5 mg TiO2 doped and 1.0 mg TiO2 NPs doped CVD grown SLG are found to be 1.532 nm, 2.433 nm, 5.658 nm, and 1.646 nm, respectively. The lower Rz for highly doped (1.0 mg TiO2 NPs doped) graphene is may be due to smoother and denser TiO2 NPs on the graphene surface. The change in the surface roughness is due to the interaction of TiO2 NPs with the graphene layer [47]. 3.3. Electrical properties The electrical properties of the CVD grown graphene samples before and after TiO2 doping are measured under ambient condition. The drain 4

Synthetic Metals 256 (2019) 116155

A.K. Singh, et al.

samples and Vg is back gate voltage. The gate oxide capacitance (Cg) ∼115 AF/μm2 for Si/SiO2 substrate. The hole and electron mobility’s of pristine and TiO2 doped samples were calculated on the basis of the linear fitted slope of their respective drain current versus back gate voltage (ID -Vg) curve. We have also observed the change in the hole and electron mobility of TiO2 doped graphene as shown in Fig. 5(a).The carrier mobility degradation in TiO2 doped graphene may be attributed to charge impurities and scattering effect, the same was also reported by others [45]. The change in carrier density (Δn) of graphene was calculated using the relation Δn = Cg (VDt -VDp) /e [20], where e is the electronic charge, VDp is Dirac point of pristine SLG and VDt is the corresponding Dirac point of graphene doped with TiO2 NPs of different concentrations. The change in carrier density of graphene as a function of TiO2 NPs concentration is also shown in Fig. 5(b).The change in carrier density increases with increasing the concentration of TiO2 NPs and it is due to charge transfer between graphene and TiO2 NPs. The change in Fermi level of graphene is directly related to change in charge carrier density of graphene, therefore TiO2 NPs doping can modulate the Fermi level of CVD grown graphene. 4. Conclusion We have investigated the effect of TiO2 NPs on CVD-grown graphene by using Raman spectroscopy and electrical charge transport measurements. Single-layer graphene was grown on Cu foil by the CVD method and it was confirmed by Raman spectroscopy. Raman spectroscopy and transport measurements revealed that the TiO2 NPs impose p-type doping on CVD grown SLG. The shift of 2D peak frequencies and the intensity ratio of 2D and G peaks are analyzed in terms of the function of concentrations of TiO2 NPs. The shifting of Dirac points towards positive gate voltage confirmed the p-type doping on CVD grown SLG. The mobility of TiO2 doped CVD grown graphene was decreased with increasing concentrations of TiO2 NPs. Our study may be beneficial in the field of optoelectronic and photovoltaic devices. Acknowledgments All the authors and Dr. A. K. Singh greatly acknowledges to Prof. Jonghwa Eom, Sejong University, South Korea, for providing the CVD graphene. We are thankful to CIR, MNNIT-Allahabad, Prayagraj, India, for electrical measurements. Micro Raman spectroscopy measurement and atomic force microscopy were supported by the Center of Nanoscience and Material Science Engineering Department, IIT Kanpur, India, respectively. We are also acknowledged to DST (Project IFA-13 PH -53), India, and TEQIP-III for their supports.

Fig. 5. Variation of (a) mobility and (b) change in carrier density (Δn) as a function of concentrations of TiO2 NPs.

current (ID) as a function of applied back gate voltage Vg (ranging from -40 V to 40 V) at a fixed drain-source voltage (Vds = 1 V) for before and after TiO2 doped graphene is shown in Fig. 4. The Dirac point of pristine graphene was found to be +3 V (Fig. 4(a)), however, Dirac point for highly pure and defect-free graphene samples is 0 V. The Dirac point (VDP) or charge neutrality point is the gate voltage with the minimum drain-source current (IDS) for the samples. The shifting of Dirac point from 0 V to +3 V (our case) is maybe due to defects introduced during the transfer of CVD graphene on the substrate or atmospheric moisture. Fig. 4(b–d) display the shifting of Dirac point towards the positive gate voltage with increasing the concentration of TiO2 NPs on graphene film. The Dirac points for 0.1 mg, 0.5 mg, and 1.0 mg TiO2 doped graphene is found to be +7 V, +11.2 V, and +24 V, respectively. The shifting of Dirac point towards positive back gate voltage indicates charge transfer from graphene to TiO2 NPs, which revealed p-doping for pristine graphene [44]. Wu et al. reported shifting of Dirac point from 0 to +11 V for p-type doping in monolayer graphene by deposition of Au metal film [48]. It is also consistent with our Raman spectroscopy results. We have also calculated surface resistance for pristine, 0.1 mg, 0.5 mg, and 1.0 mg TiO2 doped CVD grown graphene is found to be 2.7kΩ, 9.09kΩ, 16.6kΩ, and 20kΩ, respectively. The carrier mobility of fabricated devices was calculated by

μ=

References [1] A. Geim, K. Novoselov, Nat. Mater. 6 (2007) 183. [2] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, Rev. Mod. Phys. 81 (2009) 109–162. [3] S.H. Bae, Y. Lee, B.K. Sharma, H.J. Lee, J.H. Kim, J.H. Ahn, Carbon 51 (2013) 236–242. [4] T. Das, B.K. Sharma, A.K. Katiyar, J.H. Ahn, J. Semicond. 39 (2018) 011007. [5] L.L. Zhang, R. Zhou, X.S. Zhao, J. Mater. Chem. 20 (2010) 5983–5992. [6] K. Novoselov, A. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Science 306 (2004) 666–669. [7] K.V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G.L. Kellogg, L. Ley, J.L. McChesney, T. Ohta, S.A. Reshanov, J. Röhrl, E. Rotenberg, A.K. Schmid, D. Waldmann, H.B. Weber, T. Seyller, Nat. Mater. 8 (2009) 203–207. [8] H. Park, J. Meyer, S. Roth, V. Skakalova, Carbon 48 (2010) 1088–1094. [9] A.K. Geim, Science 324 (2009) 1530–1534. [10] S.Y. Zhou, G.H. Gweon, A.V. Fedorov, P.N. First, W.A. de Heer, D.H. Lee, F. Guinea, A.H. Castro Neto, A. Lanzara, Nat. Mater. 6 (2007) 770. [11] X. Liang, B.A. Sperling, I. Calizo, G. Cheng, C.A. Hacker, Q. Zhang, Y. Obeng, K. Yan, H. Peng, Q. Li, X. Zhu, H. Yuan, A.R.H. Walker, Z. Liu, L. Peng, C.A. Richter, ACS Nano 5 (2011) 9144–9153. [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, Nature 457 (2009) 706–710. [13] B. Das, R. Voggu, C.S. Rout, C.N.R. Rao, Chem. Commun. 41 (2008) 5155–5157. [14] A.K. Singh, M.W. Iqbal, V.K. Singh, M.Z. Iqbal, J.H. Lee, S.H. Chun, K. Shin, J. Eom,

( )( ) [20], where σ (1/resistivity) is the conductivity of 1 Cg

∂σ ∂Vg

5

Synthetic Metals 256 (2019) 116155

A.K. Singh, et al.

[34] Y.S. Kim, J.H. Lee, Y.D. Kim, S.K. Jerng, K. Joo, E. Kim, J. Jung, E. Yoon, Y.D. Park, S. Seo, S.H. Chun, Nanoscale 5 (2013) 1221–1226. [35] A. Jorio, M.S. Dresselhaus, R. Saito, G. Dresselhaus, John Wiley & Sons. 2011. [36] M.W. Iqbal, A.K. Singh, M.Z. Iqbal, J. Eom, J. Phys. Condens. Matter 24 (2012) 335301. [37] Z. Ni, T. Yu, Y. Lu, Y.Y. Wang, Y.P. Feng, Z.X. Shen, ACS Nano 2 (2008) 2301–2305. [38] I. Calizo, W. Bao, F. Miao, C.N. Lau, A.A. Balandin, Appl. Phys. Lett. 91 (2007) 201904. [39] A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S.K. Saha, U.V. Waghmare, K.S. Novoselov, H.R. Krishnamurthy, A.K. Geim, A.C. Ferrari, A.K. Sood, Nat. Nanotechnol. 3 (2008) 210. [40] W.X. Wang, S.H. Liang, T. Yu, D.H. Li, Y.B. Li, X.F. Han, J. Appl. Phys. 109 (2011) 07C501. [41] H.J. Shin, W.M. Choi, D. Choi, G.H. Han, S.M. Yoon, H.-K. Park, S.-W. Kim, Y.W. Jin, S.Y. Lee, J.M. Kim, J.-Y. Choi, Y.H. Lee, J. Am. Chem. Soc. 132 (2010) 15603–15609. [42] S. Tongay, K. Berke, M. Lemaitre, Z. Nasrollahi, D.B. Tanner, A.F. Hebard, B.R. Appleton, Nanotechnology 22 (2011) 425701. [43] J. Sivek, O. Leenaerts, B. Partoens, F. Peeters, Cond. Mat. Mtrl. Sci. (2013) 1301–3654. [44] H. Li, Q. Zhang, C. Liu, S. Xu, P. Gao, ACS Nano 5 (2011) 3198–3203. [45] H. Medina, Y.C. Lin, D. Obergfell, P.W. Chiu, Adv. Funct. Mater. 21 (2011) 2687–2692. [46] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. [47] D. Naumenko, V. Snitka, B. Snopok, S. Arpiainen, H. Lipsanen, Nanotechnology 23 (2012) 465703. [48] Y. Wu, W. Jiang, Y. Ren, W. Cai, W.H. Lee, H. Li, R.D. Piner, C.W. Pope, Y. Hao, H. Ji, J. Kang, Small 8 (2012) 3129–3136.

J. Mater. Chem. 22 (2012) 15168–15174. [15] J.H. Chen, C. Jang, S. Adam, M.S. Fuhrer, E.D. Williams, M. Ishigami, Nat. Phys. 4 (2008) 377. [16] D.J. Late, A. Ghosh, K.S. Subrahmanyam, L.S. Panchakarla, S.B. Krupanidhi, C.N.R. Rao, Solid State Commun. 150 (2010) 734–738. [17] A.K. Singh, R.K. Pandey, R. Prakash, J. Eom, Appl. Surf. Sci. 437 (2018) 70–74. [18] X. Wang, X. Li, L. Zhang, Y. Yoon, P.K. Weber, H. Wang, J. Guo, H. Dai, Science 324 (2009) 768–771. [19] Z. Zhang, H. Huang, X. Yang, L. Zang, J. Phys. Chem. Lett. 2 (2011) 2897–2905. [20] A.K. Singh, M. Ahmad, V.K. Singh, K. Shin, Y. Seo, J. Eom, ACS Appl. Mater. Interfaces 5 (2013) 5276–5281. [21] H. Liu, Y. Liu, D. Zhu, J. Mater. Chem. 21 (2011) 3335–3345. [22] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselove, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Science 320 (2008) 1308. [23] F. Xia, T. Mueller, Y.M. Lin, A.V. Garcia, P. Avouris, Nat. Nanotechnol. 4 (2009) 839. [24] K.K. Kim, A. Reina, Y. Shi, H. Park, L.J. Li, Y.H. Lee, J. Kong, Nanotechnology 21 (2010) 285205. [25] A.K. Singh, C. Hwang, J. Eom, ACS Appl. Mater. Interfaces 8 (2016) 34699–34705. [26] F. Bonaccorso, Z. Sun, T. Hasan, A.C. Ferrari, Nat. Photon. 4 (2010) 611–622. [27] J. Zhang, Q. Xu, Z. Feng, M. Li, C. Li, Chem. Int. Ed. 47 (2008) 1766–1769. [28] M. Ni, M.K. Leung, D.Y. Leung, K. Sumathy, Renew. Sustain. Energy Rev. 11 (2007) 401–425. [29] L. Gomez De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson, C. Zhou, ACS Nano 4 (2010) 2865–2873. [30] Y. Un Jung, S.I. Na, H.K. Kim, S.J. Kang, J. Vac. Sci. Tech. A 30 (2012) 050604. [31] Y. Yang, L. Xu, H. Wang, W. Wang, L. Zhang, Mater. Des. 108 (2016) 632–639. [32] J. Suave, S.M. Amorim, R.F.P.M. Moreira, J. Environ. Chem. Eng. 5 (2017) 3215–3223. [33] M.A. Mousa, M. Khairy, H.M. Mohamed, J. Electron. Mater. 47 (2018) 6241–6250.

6