A facile method for the synthesis of transfer-free graphene from co-deposited nickel–carbon layers

Carbon 109 (2016) 154e162

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A facile method for the synthesis of transfer-free graphene from co-deposited nickelecarbon layers Sehoon An a, b, Geun-Hyuk Lee a, b, Seong Woo Jang a, b, Sehoon Hwang a, Sang Ho Lim b, Seunghee Han a, * a b

Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 May 2016 Received in revised form 19 July 2016 Accepted 29 July 2016 Available online 31 July 2016

We report a facile synthesis method for the preparation of transfer-free graphene on dielectric substrates from co-deposited nickelecarbon (NieC) films. To fabricate 75-nm-thick NieC layers on top of dielectric substrates, DC reactive magnetron sputtering was performed at a gas pressure of 2 mTorr by flowing a gas mixture of argon (Ar; flow rate: 5 sccm) and methane (CH4; flow rate: 1 sccm), while 200 W DC input power was applied to a Ni target. Then, the sample was annealed at 1000
C for 4 min at a pressure of 1 Torr and Ar flow rate of 200 sccm. The NieC layer was removed by a 0.5 M FeCl3 aqueous solution, and graphene was directly obtained on the dielectric substrate without any additional transfer process. The graphene quality was significantly influenced by annealing conditions, CH4 flow rate during codeposition, and NieC thin film thickness. Furthermore, temperature modulation was conducted to decrease the graphene sheet defect coverage down to 7.2%. Sheet resistance and transmittance (550 nm) of the transfer-free graphene obtained under optimal conditions were 1.9 kU sq1 and 93.9%, respectively. This facile non-transfer process will facilitate future research on graphene as well as its industrial applications. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, graphene, which consists of a single layer or a few layers of sp2-bonded carbon atoms, has attracted significant attention due to its outstanding electrical, mechanical, thermal, and optical properties [1e5]. Many research groups have focused on fabricating high-quality graphene on desired substrates by adopting different methods such as mechanical cleavage, chemical exfoliation, chemical vapor deposition (CVD), and SiC thermal decomposition [6e9]. Until now, among the aforementioned approaches, transition metal catalyzed CVD process has been considered as the most promising method for large-scale synthesis [10e13]. However, CVD requires a transfer process that not only increases the process complexity, but also causes a degradation of the graphene quality, due to inevitably induced defects, impurities, wrinkles, and cracks [14,15]. Furthermore, the direct synthesis of graphene on dielectric surfaces still appears as a premature field for

* Corresponding author. E-mail address: [email protected] (S. Han). http://dx.doi.org/10.1016/j.carbon.2016.07.066 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

practical applications. Therefore, the development of cost-effective and concise methods for the preparation of transfer-free graphene is critical to improve its commercialization. One of the routes to fabricate transfer-free graphene is a direct CVD process without using metallic catalysts. Xu et al. successfully developed a method for the direct synthesis of graphene through a two-temperature zone CVD method, and the measured properties of the asproduced graphene were as good as the conventional metalcatalyzed graphene [16e19]. The other route for the synthesis of transfer-free graphene synthesis involves a separately deposited catalytic metal thin film and a carbon source layer [20,21]. However, the synthesis of transfer-free graphene through nickelecarbon co-deposition has not been explored yet. Here, we report a facile method for the synthesis of transfer-free graphene on insulator substrates via nickel (Ni) and carbon (C) co-deposition, followed by annealing and subsequent etching process. The number of layers and defect density of graphene were tuned by varying process parameters such as annealing conditions, co-deposited Ni and C (NieC) layer thickness, and methane (CH4) flow rate at constant argon (Ar) flow rate. Moreover, temperature modulation was performed to effectively further reduce the graphene sheet resistance

S. An et al. / Carbon 109 (2016) 154e162

by minimizing the defect coverage. 2. Experimental 2.1. Growth of transfer-free graphene on dielectric substrate Fig. 1 shows a schematic procedure for the synthesis of transferfree graphene on a dielectric substrate. First, ultrasonically cleaned dielectric substrates (200 nm SiO2/Si or quartz) were placed in a homemade DC reactive magnetron sputtering system, and the chamber was evacuated down to 2  106 Torr. Then, the chamber was kept at a gas pressure of 2 mTorr while flowing a gas mixture consisting of Ar and CH4. The gas flow rate of Ar was fixed at 5 sccm, while that of CH4 was varied from 0.5 to 4 sccm. Subsequently, 200 W DC input power was applied to a Ni target (purity of 99.99%) to fabricate a NieC layer (thickness in the range of 10e100 nm) on top of the dielectric substrate. The deposition rate was varied from 7 to 9 nm/min depending on the CH4 flow rate. Afterwards, the sample was annealed at the pressure of 1 Torr by flowing Ar gas (flow rate: 200 sccm) at 600e1000
C for 1e4 min by employing a homebuilt rapid thermal annealing (RTA) equipment. After the annealing process, the samples were cooled to room temperature under the same Ar flow with a cooling rate of 20
C/min. During the RTA process, the carbon atoms diffused through the NieC film and deposited on both layer surfaces, forming graphene upon cooling. The remaining NieC layer and top-side graphene were removed by dipping the samples into a 0.5 M FeCl3 aqueous solution for 2 min. Thus, graphene was directly obtained on the dielectric substrate without any transfer processes. In addition, we conducted a temperature modulation procedure to improve the quality of our transfer-free graphene. A 75-nm-thick NieC thin film was deposited on a dielectric substrate under CH4 flow rate of 1 sccm; the film was pre-annealed at 300, 400, 500, and 600
C for 30 min. Then, post-annealing was conducted at 1000
C for 4 min, and the subsequent etching process was performed under the same conditions mentioned above. 2.2. Characterization To investigate the quality of the prepared graphene layer, Raman spectra and mapping analysis were performed by using a microRaman spectrometer (Renishaw inVia, Renishaw; excitation wavelength: 532 nm). The surface morphology of the graphene was characterized by using the non-contact mode of an atomic force microscope (AFM; XE-100, Park System). Furthermore, sheet resistance and transmittance were measured by a four-point probe tester (CMT-SR2000N, Changmin Tech.) and ultraviolet-visible (UV-

155

vis) spectroscopy (Lambda 20, Perkin Elmer), respectively. The asdeposited NieC layers were analyzed by X-ray photoelectron spectroscopy (XPS; PHI-5000 Versa Probe, ULVAC-PHI) to attain depth profile data. The surface morphology and crystalline structure of the NieC layer were investigated by using scanning electron microscopy (SEM; Nova 200, FEI) and X-ray diffraction (XRD; D/ max-2500, Rigaku), respectively. Transmission electron microscopy (TEM; Tecnai F20 G2, FEI) and selected area electron diffraction (SAED) analysis were conducted to investigate the morphology and the structure of the resulting transfer-free graphene film directly transferred onto a copper (Cu) TEM grid. 3. Results and discussion 3.1. Optimization of annealing conditions Different annealing temperatures and times were used to find the optimal annealing conditions necessary to obtain high-quality graphene. During NieC deposition, the Ar and CH4 gas flow rates were 5 and 0.5 sccm, respectively. The thickness of the NieC layer, deposited on a 200-nm-thick SiO2/Si substrate, was 40 nm. The annealing temperature was varied from 600 to 1000
C while keeping a constant time of 4 min; in addition, the annealing time was changed from 1 to 4 min while maintaining the temperature at 1000
C. The subsequent etching process was conducted to remove the remaining NieC and top-side graphene layers under the same conditions previously mentioned in the experimental section. Fig. 2(a) and (b) show the Raman spectra of transfer-free graphene layers grown on the substrates at different annealing temperatures and times. Three characteristic peaks located at approximately 1350 (D band), 1580 (G band), and 2700 cm1 (2D band) are clearly visible. The D peak typically indicates the presence of structural defects in graphene. The G peak identifies the formation of an sp2hybridized carbon network and originates from the doubly degenerated phonon vibrations at the Brillouin-zone center. The 2D peak is generated by second-order zone-boundary phonon scattering and is sensitive to the c-axis stacking of the graphene layers [22e24]. The Raman spectra in Fig. 2(a) and (b) indicate that annealing at higher temperature and longer time led to a decrease in the D peak intensity. These results can be elucidated by Fick's first law of diffusion. The Arrhenius-type equation is given by Ref. [25]:


Q Dc ¼ Dco exp  ID RT

(1)

where Dc is the diffusion coefficient, Dco is the maximum diffusion

Fig. 1. Schematic procedure for the synthesis of transfer-free graphene on dielectric substrate. (A colour version of this figure can be viewed online.)

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Fig. 2. Raman spectra of transfer-free graphene grown on 200-nm-thick SiO2/Si substrate at different annealing (a) temperatures and (b) times. (c) Raman spectra of synthesized graphene on the top and bottom surfaces of the NieC layer. (A colour version of this figure can be viewed online.)

coefficient at infinite temperature, QID is the activation enthalpy for the diffusion, R is the gas constant, and T is the absolute temperature. According to the equation, larger amounts of carbon atoms precipitate on the NieC layer top and bottom surfaces when the annealing process is conducted at a higher temperature for longer time, enabling to obtain high-quality graphene crystallites with lower D peak intensity. Fig. 2(c) illustrates the Raman spectra of synthesized graphene on the top and bottom surfaces of the NieC layer. The number of graphene layers and the quality of the synthesized graphene on the both surfaces of the NieC layer were almost identical, as determined by comparing I2D/IG (0.46 and 0.47, bottom and top of the NieC layer, respectively) and ID/IG (0.40 and 0.41, bottom and top of the NieC layer, respectively) intensity ratios. Hence, it can be concluded that the carbon atoms in the NieC layer diffuse out to both surfaces forming graphene layers of identical quality during the annealing process. 3.2. Influence of NieC layer thickness The NieC layer thickness has also a strong influence on the quality of the resulting transfer-free graphene. During the NieC cosputtering process, the Ar and CH4 gas flow rates were 5 and 0.5 sccm, respectively. The thickness of the NieC layer was varied from 10 to 100 nm. The subsequent annealing process was performed at 1000
C for 4 min, and the etching process was conducted under the same conditions mentioned above. Fig. 3(a) shows the Raman spectra of the resulting transfer-free graphene layers obtained on 200-nm-thick SiO2/Si substrates by using different NieC layer thicknesses. No graphene characteristic peaks could be observed for transfer-free graphene obtained from the 10nm-thick NieC layer, as the carbon concentration in the NieC layer was too low to form detectable graphitic carbon structures. Graphene Raman peaks appeared for graphene obtained from the 20nm-thick NieC layer. By increasing the thickness of the NieC layer from 10 nm to 75 nm, the sheet resistance of the resulting transfer-

free graphene layer on the substrate decreased from 64 to 7.5 kU sq1, with a reduction of the ID/IG peak intensity ratio from 0.87 to 0.28, as shown in Fig. 3(b). Fig. 4 shows the SEM images of the annealed NieC thin films before etching, for different NieC layer thicknesses. Island-shaped NieC particles could be observed for the sample with a 10-nmthick NieC layer; the particles became larger as the NieC layer thickness increased. The substrate surfaces were eventually fully covered by a NieC thin film when the thickness exceeded 40 nm. Metallic Ni rapidly agglomerates at high temperature, compared with other metals, owing to its high surface energy [26]. Several research groups have reported that graphene can be converted from amorphous carbon by metal-catalyzed crystallization [27e29]; in other words, graphene nucleation and growth occur at the periphery of the NieC particles. In order to reveal this phenomenon, a non-contact mode AFM analysis was carried on 1  1 mm2 area of the graphene surface on SiO2/Si substrates, which was produced using NieC layers of different thickness. Fig. 5(a) displays the image of transfer-free graphene from a 10 nm thick NieC layer, where isolated graphene platelets with a thickness of ~0.4 nm (mono-layer graphene) were observed. On the other hand, a relatively continuous graphene surface was seen for the 75 nm thick NieC sample. The thickness of the graphene film was about 0.7 nm corresponding to bi-layer graphene, which resulted from the increased supply of carbon atoms from the thicker NieC layer during the annealing process. Therefore, the graphene crystallites became larger with increasing the thickness of NieC layer as schematically illustrated in Fig. 5(c). Accordingly, the sheet resistance of graphene gradually decreased as the NieC layer thickness increased from 10 to 75 nm. This indicates that a relatively continuous graphene sheet with low defect density was formed on the substrate. However, upon further increase of the NieC layer thickness to 100 nm, the Raman spectrum gradually changed, assuming the shape of an amorphous carbon spectrum with relatively unseparated and broad D and G peaks, as shown in Fig. 3(a).

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3.3. Influence of CH4 flow rate during NieC co-deposition process

Fig. 3. (a) Raman spectra and (b) sheet resistance and D/G intensity ratio of transferfree graphene on 200-nm-thick SiO2/Si substrate for NieC layers of various thicknesses in the range of 10e100 nm. (A colour version of this figure can be viewed online.)

Notably, the residual carbon sources in thicker NieC layers caused the formation of the amorphous carbon structure after the annealing process.

The effect of the CH4 flow rate at constant Ar flow rate during the co-sputtering process was also investigated. The Ar flow rate was fixed as 5 sccm, while the CH4 flow rate was varied from 0.5 to 4 sccm. Fig. 6(a)e(d) illustrate the XPS depth profiles of asdeposited 75-nm-thick co-deposited NieC layers on 200-nmthick SiO2/Si at different CH4 flow rates. The average carbon concentration in the NieC layer linearly increased from ~1% to ~23% with the increase in the CH4 gas flow ratio from 0.5 to 4 sccm because co-sputter deposition is a non-equilibrium process without solid solubility limitation, as shown in Fig. 6(e) [30]. The as-deposited NieC samples were annealed at 1000
C for 4 min, and the subsequent etching process was conducted under the same conditions previously mentioned to fabricate transferfree graphene. Fig. 7(a) shows the Raman spectra of graphene directly grown on the substrates for different gas mixture ratios. When the CH4 gas flow rate increased from 0.5 to 4, the peak intensity ratio of 2D to G (I2D/IG) gradually decreased from 1.59 to 0.02. Thus, an increased amount of carbon feedstock during cosputtering resulted in a higher carbon atomic concentration in NieC thin films. After the annealing process, more carbon atoms precipitated on the surface of the NieC layer. Therefore, the number of graphene increased with higher CH4 flow rate. While varying the CH4 flow rate, the graphene sheet resistance decreased from 12 to 5 kU sq1 when the CH4 flow rate changed from 0.5 to 1 sccm, while it increased from 5 to 47 kU sq1 when the CH4 flow rate varied from 1 to 4 sccm (Fig. 7(b)). To clarify these opposite tendencies observed while increasing the CH4 flow rate, we conducted XRD analysis for the as-deposited and annealed NieC layers obtained at different CH4 flow rates, as shown in Fig. 7(d) and (e), respectively. As the CH4 flow rate of the as-deposited samples increased from 0.5 to 4 sccm, the fcc Ni peak intensity decreased, and peaks related to the hcp Ni3C phase appeared at 2 sccm (Fig. 7(d)). Eventually, the diffraction peaks disappeared at the highest CH4 flow rate of 4 sccm, owing to the increased portion of amorphous carbon [31]. In Fig. 7(e), the annealed NieC samples only exhibited metallic Ni diffraction peak features, as the NieC decomposition, which starts at ~400
C, occurred during the annealing process, and the carbon atoms mostly diffused out to both surfaces of the NieC layer [32,33]. Furthermore, the crystallite size, calculated by using the Ni (111) reflection and Scherrer's equation, varied from 32.4 to

Fig. 4. Top-view scanning electron microscopy images of annealed NieC thin films of various thicknesses: (a) 10 nm; (b) 20 nm; (c) 40 nm; (d) 75 nm; (e) 100 nm.

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Fig. 5. AFM images and line-scan profiles of transfer-free graphene from (a) 10 nm and (b) 75-nm-thick NieC layers. (c) Schematic illustration of the resulting transfer-free graphene from lower and thicker NieC layers. (A colour version of this figure can be viewed online.)

24.6 nm as the CH4 flow rate changed from 0.5 to 4 sccm (Fig. 7(f)). This can be explained by considering that the Ni grain growth was restricted, owing to encapsulation of Ni crystallites by amorphous carbon at higher carbon content in NieC films, as also reported by other groups [34e36]. Therefore, graphene of lower quality with higher Raman D peak intensity was obtained by increasing the CH4

flow rate, as graphene with smaller grains was formed, depending on the size of the Ni crystallites. Thus, the graphene sheet resistance increased from 5.8 to 35.4 kU sq1 when the CH4 flow rate increased in the range of 1e4 sccm. However, graphene formed at the CH4 flow rate of 0.5 sccm exhibited the highest sheet resistance (42.8 kU sq1), albeit it also had the largest Ni grain size. The NieC

Fig. 6. X-ray photoelectron spectroscopy depth profile data of as-deposited NieC layer on 200-nm-thick SiO2/Si substrates at different CH4 flow rates during the co-deposition process: (a) 0.5 sccm; (b) 1 sccm; (c) 2 sccm; (d) 4 sccm. (e) Average carbon concentration in the NieeC layer as a function of the CH4 gas flow rate (0.5, 1, 2, and 4 sccm). (A colour version of this figure can be viewed online.)

S. An et al. / Carbon 109 (2016) 154e162

159

Fig. 7. (a) Raman spectra and (b) sheet resistance of transfer-free graphene on 200-nm-thick SiO2/Si substrate, obtained from NieC layer prepared at various CH4 flow rates in the range of 0.5e4 sccm. (c) Transmittance of transfer-free graphene on quartz substrates, obtained from NieC layer prepared at various CH4 flow rates in the range of 0.5e4 sccm. The inset shows the transmittance of graphene samples at 550 nm. X-ray diffraction pattern of (d) as deposited NieC layers and (e) annealed NieC layers prepared at various CH4 flow rates in the range of 0.5e4 sccm. (f) Estimated grain size, obtained by Scherrer's equation and Ni (111) peaks, in annealed NieC layers prepared at various CH4 flow rates in the range of 0.5e4 sccm. (A colour version of this figure can be viewed online.)

layer obtained at the CH4 flow rate of 0.5 sccm did not have a sufficient amount of carbon atoms, which precluded the formation of a continuous graphene network. This was confirmed by the carbon atomic concentration calculated from XPS depth profile data. The atomic fraction of carbon, XC, can be converted to atomic density of carbon, NC, by using the relationship [37]:

xC ¼

NC NNi þ NC

(2)

where NNi is the Ni atomic density. The atomic concentration can

then be calculated by solving for NC (which is a function of the depth z) and integrating over the entire depth range: zfinal Z

∅¼

zfinal Z

NC ðzÞdz ¼ 0

0

xC ðzÞr dz 1  xC ðzÞ

(3)

The expected number of graphene layers on both sides of the NieC layer could be estimated by dividing the carbon atomic concentration by the carbon density of monolayer graphene, which corresponds to 3.6  1015 atoms cm2. Then, the calculated value

Fig. 8. (a) X-ray diffraction patterns of NieC layers pre-annealed at various annealing temperatures. (b) Estimated Ni (111) grain size from NieC layers pre-annealed at different temperatures; sheet resistance and transmittance at 550 nm of transfer-free graphene on quartz substrates, obtained from temperature-modulated NieC. (c) Raman spectra of NieC layers pre-annealed at different temperatures. (A colour version of this figure can be viewed online.)

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Fig. 9. Raman mapping of ID/IG over 100  100 mm2 areas of transfer-free graphene obtained from NieC films pre-annealed at various temperatures: (a) no pre-annealing; (b) 300
C; (c) 400
C; (d) 500
C; (e) 600
C. (f) Surface coverage of transfer-free graphene defects for each point with ID/IG > 0.1. (A colour version of this figure can be viewed online.)

should be divided by 2 to only represent the number of graphene layers directly formed on the dielectric substrate. The calculated number of graphene layers from the NieC thin film prepared at a CH4 flow rate of 0.5 sccm was 0.7, which was insufficient for the formation of a continuous graphene network. Fig. 7(c) displays the transmittance of transfer-free graphene fabricated on quartz substrate. The transmittances at 550 nm were 98.0%, 94.8%, 91.6%, and 88.8% at 0.5, 1, 2, and 4 sccm, respectively, depending on the amount of carbon atoms precipitated from the NieC layers, as shown in the inset of Fig. 7(c). 3.4. Influence of temperature modulation To further optimize the sheet resistance of graphene, temperature modulation was performed. First, 75-nm-thick NieC thin films were fabricated on 200-nm-thick SiO2/Si substrates at a CH4 flow rate of 1 sccm; the films were pre-annealed at 300, 400, 500, and 600
C for 30 min. Then, a post-annealing process was conducted at 1000
C for 4 min, while the following etching process was performed under the same conditions previously mentioned. Fig. 8(a) shows the XRD patterns of the NieC layers pre-annealed at different temperatures in the range of 300e600
C. The estimated grain size of the Ni (111) plane in the pre-annealed NieC layers increased with the annealing temperature, as shown in Fig. 8(b). The sheet resistance of the temperature-modulated transfer-free graphene pre-annealed at 400
C decreased to 1.9 kU sq1, as shown in Fig. 8(b). The larger domain size of catalytic metal

contributed to obtain higher quality graphene with lower defect density, as also confirmed by other research groups [38e40]. Fig. 9(a)e(e) show the Raman mapping of the ID/IG ratio over 100  100 mm2 areas to examine the quality of the obtained graphene. Additionally, Fig. 9(f) shows the surface coverage of graphene defects for each point with ID/IG > 0.1. Graphene obtained from the NieC film pre-annealed at 400
C exhibited the lowest sheet resistance of 1.9 kU sq1 as well as the smallest defect coverage of 7.2%. The variation in sheet resistance with the preannealing temperature depended on the surface defect coverage. However, the sheet resistance and defect coverage gradually increased above 400
C, although the NieC films pre-annealed at 500 and 600
C had larger grain sizes than those pre-annealed at lower temperatures (Fig. 9(f)). Fig. 8(c) shows the Raman spectra of NieC layers pre-annealed at different temperatures. The spectrum of the graphitic carbon structure appeared at the pre-annealing temperature above 400
C, owing to the precipitation of carbon via NieC decomposition, which started at temperatures above 400
C [32]. The prerequisite to obtain high-quality graphene is reducing the formation of nucleation sites while promoting the lateral growth of graphene [12,41e43]. However, pre-annealing conducted at temperatures above 400
C contributed to intensify the formation of additional nucleation sites after the postannealing process. Consequently, graphene of degraded quality and with higher defect coverage was obtained (Fig. 9(f)). The transmittance of graphene on quartz substrate gradually decreased with the pre-annealing temperature, and reached a value of 89%

Fig. 10. (a) TEM image of transfer-free graphene obtained from pre-annealed NieC film at 400
C. (b) SAED pattern of graphene from the region marked with the dotted red circle shown in Fig. 10(a). (c) Raman mapping of I2D/IG over a 100  100 mm2 area of transfer-free graphene obtained from NieC films pre-annealed at 400
C. (A colour version of this figure can be viewed online.)

S. An et al. / Carbon 109 (2016) 154e162

after pre-annealing the NieC film at 600
C (Fig. 8(b)), owing to the carbon atoms precipitated upon NieC decomposition. The morphology and crystallinity of the transfer-free graphene which is temperature modulated at 400
C were investigated by TEM and SAED as shown in Fig. 10. The graphene sample for the TEM analysis was prepared by placing the Cu TEM grid on the graphene/SiO2/Si sample and subsequently etching the SiO2/Si substrate by using a buffer oxide etchant (BOE). In this way, the graphene could be successfully transferred onto a Cu TEM grid. Fig. 10(a) displays the morphology of the graphene with areas having different contrast, arising due to the difference in graphene thickness. The morphological feature of the graphene is attributed to the precipitation of carbon atoms through the grain boundaries of the polycrystalline NieC layer and is typically observed in Niecatalyzed few-layer graphene [7]. Fig. 10(b) illustrates the electron diffraction patterns taken on a marked region and shows a typical hexagonally-arranged spot patterns. The ring-shaped diffraction spots indicate the existence of polycrystallinity in the as-produced graphene. In order to confirm the number of graphene layers, Raman mapping of the 2D and G peak intensity ratio (I2D/IG) over a 100  100 mm2 area was carried out on the transfer-free graphene pre-annealed at 400
C. As shown in Fig. 10(c), ~61% of the analysis region is covered with graphene having an I2D/IG ratio between 0.7 and 1.5 (bi-layer graphene). The rest of the regions have an I2D/IG intensity ratio of less than 0.7 (more than tri-layer graphene), which results in a transmittance of 93.9%, which is lower than the theoretical transmittance value of bi-layer graphene (95.4%) [44].

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4. Conclusion In conclusion, we developed a facile method to synthesize transfer-free graphene by using co-deposited NieC layer. The quality of graphene was significantly influenced by the annealing conditions, CH4 flow rate during co-deposition, and thickness of the NieC thin film. In addition, temperature modulation was conducted to decrease the defect coverage of the graphene sheet down to 7.2%. The transmittance at 550 nm and sheet resistance of graphene produced under optimal conditions were 93.9% and 1.9 kU sq1, respectively. Our method for the direct growth of graphene on dielectric substrates provides a promising approach for the commercial production of practical graphene-based applications by employing conventional semiconductor fabrication facilities.

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