Grain structures of nitrogen-doped graphene synthesized by solid source-based chemical vapor deposition

Accepted Manuscript Grain structures of nitrogen-doped graphene synthesized by solid source-based chemical vapor deposition Sachin M. Shinde, Emi Kano, Golap Kalita, Masaki Takeguchi, Ayako Hashimoto, Masaki Tanemura PII:

S0008-6223(15)30304-3

DOI:

10.1016/j.carbon.2015.09.086

Reference:

CARBON 10356

To appear in:

Carbon

Received Date: 24 April 2015 Revised Date:

21 September 2015

Accepted Date: 23 September 2015

Please cite this article as: S.M. Shinde, E. Kano, G. Kalita, M. Takeguchi, A. Hashimoto, M. Tanemura, Grain structures of nitrogen-doped graphene synthesized by solid source-based chemical vapor deposition, Carbon (2015), doi: 10.1016/j.carbon.2015.09.086. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Grain structures of nitrogen-doped graphene synthesized by solid

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source-based chemical vapor deposition

Sachin M. Shindea,1, Emi Kanob,c,1, Golap Kalitaa*, Masaki Takeguchib,c, Ayako

Department of Frontier Materials, Nagoya Institute of Technology, Gokiso-cho,

Showa-ku, Nagoya 466-8555, Japan b

Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba

305-8573, Japan

National Institute for Materials Science, Tsukuba 305-0047, Japan

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a

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Hashimotob,c, Masaki Tanemuraa

*Corresponding author: Tel: +81 52 735 5216. E-mail: [email protected]

These authors contributed equally

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(Golap Kalita)

Abstract

Doping of a foreign element in sp2 hybridized graphene lattice is of significant importance to tune the electrical and chemical properties. Here, we report on the grain structures of substitutional nitrogen-doped graphene synthesized by an atmospheric pressure (AP) solid source-based chemical vapor deposition (CVD) technique. Nitrogen-doped graphene was synthesized by mixing solid camphor and melamine as 1

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carbon and nitrogen source, respectively. The precursor materials quantity significantly affects the graphene growth on Cu foil and thereby the nitrogen doping and content.

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Transmission electron microscopy (TEM) analysis was performed to determine the nitrogen substitutional sites in the graphene. Dark-field (DF) TEM analysis was carried

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out to evaluate the graphene grain structure grown with introduction of nitrogen dopant.

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We obtained different grain orientations, where an individual grain size is more than 5 µm. Our findings show that graphitic nitrogen defects can be introduced in the large

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individual graphene grain by the developed solid source-based CVD technique.

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1. Introduction Graphene, the two dimensional honeycomb lattice of sp2 hybridized carbon atoms is

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considered to be next generation material for application in various electronic devices, such as transistors, optoelectronics, sensors, supercapacitors etc. [1-6]. However, the

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electrical, optical and chemical properties of pristine graphene based materials are

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limited by the absence of a bandgap [7, 8]. Incorporation of foreign atoms in the sp2 sites of graphene lattice can be an interesting prospect to tune the intrinsic properties of graphene [9, 10]. Theoretical studies have revealed that substitutional doping of graphene with a heteroatom like nitrogen, boron, etc., can change the Fermi energy and

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introduce a band gap [11-15]. The doping with heteroatoms creates charged sites in the graphene lattice, as a result the spin and charge densities are redistributed bringing new

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functionalities [16]. In this prospect, nitrogen incorporation in graphene by substitution

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of carbon atoms has been explored significantly to achieve high electron density, n-type doping, increased capacity of battery, high supercapacitance and oxygen reduction activities [16-22]. Thus, various studies have revealed that nitrogen doping can be exciting platform to tune or introduce novel properties in graphene. Considering the significant potential, control synthesis of high quality graphene with desired electronic and chemical properties can be the key for future applications. In the

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last few years, significant development has been made in large-area high quality and individual single crystal domain synthesized by the chemical vapor deposition (CVD)

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technique [23-27]. Synthesis of graphene with substitutional nitrogen doping by a CVD process has been also attracted significant interest [28-32]. CVD process can be the

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most effective approach to achieve substitutional doping without affecting the

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crystalline nature. The incorporation of foreign atoms in the graphene lattice site is quite significant to observe different properties depending on their concentration and structures. The pyridinic nitrogen doping in graphene can facilitate oxygen reduction reaction activity, while pyrrolic nitrogen contributes to enhance specific capacitance [19,

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33]. Again, it has been reported that a band gap can be observed in graphene with a nitrogen concentration of 2-12% [20, 34]. Transmission electron microscopy (TEM)

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analysis can provide significant information of the nitrogen dopant and other structures

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of graphene lattice [16]. Dark-field TEM (DF-TEM) and aberration-corrected annular dark-field scanning TEM (ADF-STEM) has been used to identify grain size and grain boundaries of CVD synthesized graphene in atomic scale [35]. These techniques provide insight of CVD synthesized graphene at atomic level to determine the induced defect structures. Recently, several solid and liquid precursor materials have been used for synthesis of nitrogen-doped graphene [36-40]. However, details of atomic level

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study of defects, substitutional doping and grain structure were not explored for the solid or liquid source-based CVD graphene. In contrast to previous reports, we

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demonstrate a solid source-based CVD process to synthesize nitrogen-doped graphene by mixing melamine and camphor solid precursors. We explore the nitrogen

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substitutional doping in graphene lattice and grain structures by TEM analysis. These

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results can be significant to correlate the atomic level structure of nitrogen-doped graphene derived by an unconventional solid source CVD technique, rather than using gaseous sources.

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2. Experimental details

2.1 Graphene synthesis process:

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The nitrogen-doped graphene was synthesized by the atmospheric pressure (AP) CVD

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(APCVD) process using a quartz tube of length 80 cm and diameter of 50 mm. Cu foil (Nilaco Corp.) of thickness 20 µm with 99.98% purity was taken as the catalytic substrate. The molecular structure of solid precursors and schematic of the APCVD process are presented in Figure 1. Cu foil was cleaned by acetone and placed in a quartz tube with a flow of 100 standard cubic centimeter per minute (sccm) hydrogen for annealing at 1015 °C. Solid camphor and melamine powders were taken with 1:3, 1:4

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and 1:5 ratios and changing their amount as the feedstock and placed in a ceramic boat inside the lower temperature zone. Once the high temperature zone reaches 1015°C, the

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lower temperature zone containing the feedstock was heated up to 600 °C. The growth was carried out with the flow of Ar and H2 (98:2.5 sccm) gas mixture. After 30 min of

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growth, the sample was cooled to room temperature at a control rate as reported

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previously [41].

Figure 1 Molecular structure of the solid precursors: camphor (carbon source) and melamine (nitrogen source). Schematic of the APCVD process for synthesis of nitrogen-doped graphene.

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2.2 Characterization of materials

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The synthesized materials were analyzed by Raman, X-ray photoelectron spectroscopy (XPS) and TEM studies. Raman spectra were obtained using NRS 3300 laser Raman

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spectrometer with a laser excitation energy of 532.08 nm. XPS data were acquired to

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determine the chemical composition of the graphene film on Cu foil using the Versaprobe photoelectron spectrometer with

photoemission stimulated by a

monochromated Al Kα radiation source (1486.6 eV).

To investigate the nitrogen-substitution site and the size of each grain, we used a

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transmission electron microscope (JEM-ARM200F, JEOL) with a probe aberration corrector for STEM and an image corrector for TEM. The microscope was operated at a

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relatively low voltage of 80 kV to reduce knock-on damage to the graphene sheet. The

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image-side aberration corrector is important to obtain high resolution TEM (HRTEM) images at 80 kV. Two defocus conditions of −4 nm (Scherzer defocus) and −12 nm (large underdefocus) were respectively used to obtain general atomic-resolution TEM images and identify the nitrogen-doped site in a graphene lattice. 2.3 Sample preparation for TEM analysis Graphene membranes synthesized by solid source APCVD technique on a Cu foil were

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transferred to Quantifoil holey carbon TEM grids (hole diameter is ~1 µm) as follows: First, graphene on Cu foil was coated with poly(methyl methacrylate) (PMMA) by

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spin-coating. Subsequently, Cu foil was etched by using an ammonium persulfate solution ((NH4)2S2O8; 0.02 g ml-1, Sigma Aldrich). The PMMA/graphene film was then

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rinsed in deionized water and treated with concentrated HCl solution to remove residual

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contaminants. Finally, the PMMA/graphene film was transferred to the Quantifoil grids, and PMMA was removed by dissolving in acetone for a few hours. Quantifoil grids are typically 10–20 nm thick, which is thin enough to allow DF- TEM imaging through the

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carbon support.

3. Results and discussion

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The overall quality of synthesized graphene was confirmed by Raman studies. Figure

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2a shows a Raman spectra of the synthesized graphene after transferring to a SiO2/Si substrate. The graphene was synthesized from solid camphor precursor without using the dopant precursor material. Raman spectra shows a small defect induced D peak, indicating high quality graphene growth. The graphitic G and second order 2D Raman peaks were observed at 1596 and 2698 cm-1, respectively. The higher intensity of 2D peak than that of G peak (I2D/IG ~3) confirm a single layer graphene domain. However,

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there are more than single layer graphene domains as observed by Raman and also TEM mapping analysis. Figure 2b shows a Raman spectra of the graphene synthesized with

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1:3 of camphor and melamine with similar growth conditions. We can observe significant difference in graphene structure with addition of melamine. The intensity of

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defect related D peak increased significantly irrespective of sample positions. The

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Raman studies confirm presence of defects in the sp2 hybridized graphene grown on Cu foil. The induced defects with the introduction of melamine can be attributed to the doping of nitrogen. Again, we observed a blue-shift (4 cm-1) of the G peak, whereas a red-shift (5 cm-1) for the 2D peak, considering the Si peak as base. Previously, Raman

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studies of nitrogen-doped graphene have been carried out in details to explain such type of red and blue shift of peak positions, corresponding to n-type doping and

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compressive/tensile strain in graphene [42]. The Raman analysis gives a brief idea of

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nitrogen incorporation in the graphene film. Graphene growth was explored by varying the mixture of two precursors to achieve higher amount of nitrogen doping. Good crystalline monolayer graphene was obtained with the 1:3 (1 and 3 mg) mg and 1:4 (0.5 and 2 mg) ratios of camphor and melamine, which allows us to study the nitrogen substitutional sites and grain structures. As follows, we explain detail structural morphology of these nitrogen-doped graphene samples.

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Figure 2 Raman spectra of graphene on SiO2/Si substrates synthesized from (a) solid

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camphor and (b) mixture of melamine: camphor with similar growth conditions.

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XPS analysis was performed to acquire an idea of elemental composition in the graphene samples. The survey spectra shows presence of C, O and small amount of N along with base Cu peaks. Figure 3a shows XPS C 1s spectra of the synthesized graphene. The C 1s XPS spectra shows a peak centered at 284.8 eV, corresponding to the sp2 hybridized carbon atoms. A small shoulder peak is observed in the higher binding energy (~288.4 eV). This can be attributed to presence of nitrogen containing 10

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C-N as well as oxygen containing C-O and C=O bonds. Further, the N 1s spectra was analyzed to evaluate the nitrogen content in the graphene samples as shown in Figure 3b.

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We observed two split N1s peaks with peak centered at ~398.7 and 406.3 eV, respectively. The quantitative analysis shows presence of around 2 at% of nitrogen.

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Previously, it has been reported that higher amount of nitrogen can be incorporated at a

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low growth temperature [17], however we have to contend with low crystalline quality of graphene. The intensity of N 1s spectra (Figure 3b) is quite low for the atomically thin layer, which makes it difficult to determine nitrogen structures. However, we obtained much pronounced N1s peak and higher nitrogen content by increasing the

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quantity of both camphor and melamine precursors to 7 and 21, respectively (supplementary information). The deconvoluted N1s peak shows presence of higher

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amount of pyridinic as well as smaller amount of graphitic and pyrrolic nitrogen atoms

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(Figure S1). The other peak at higher binding energy with a peak centered at 406.3 eV, signifies presence of NOx. [43]. The NOx related peak arises with presence of nitrogen atoms in amorphous carbon and other contaminant sites, which can easily react with surface oxygen in atmosphere. Thus, the synthesized graphene by the solid source-based CVD process shows presence of various nitrogen related components. In table 1, we have summarized the nitrogen content results (at %) with various ratios and amounts of

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the camphor and melamine. We have observed that the nitrogen incorporation in the graphene film significantly differ with amount and ratio. Incorporation of higher amount

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of nitrogen was not obtained by increasing only the melamine quantity. Furthermore, there was no good quality graphene formation with a ratio higher than 1:5. This shows

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the critical role of camphor and melamine molecules decomposition in the high

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temperature zone for the graphene growth on Cu surface. The formation of graphitic nitrogen was further analyzed at atomic level by the HRTEM analysis of individual

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graphene grain.

Figure 3 XPS (a) C1s and (b) N1s spectra of the synthesized graphene, confirming presence of carbon and nitrogen atoms. Splitting of N1s peak with peak centered at ~398.7 and 406.3 eV, are observed.

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Table 1 summarized results of nitrogen content (at %) in the synthesized graphene

Synthesis temperature

Ratio

Amount in mg

(0C)

1:3

1:3

1015

Graphene formation

Nitrogen content

types of nitrogen

(at %)

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Camphor:Melamine

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samples with various ratios and amounts of camphor and melamine precursors.

Higher amount

2

-

Monolayer and few-layer

0.7

-

Multi-layer

5.2

Higher pyridinic; Lower graphitic

of monolayer

1015

0.5:2.0

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7:21

and pyrrolic

1015

Higher amount of monolayer

1.8

-

3:12

1015

Monolayer and few-layer

1.7

-

1:5

1015

No graphene

-

-

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1:5

1015

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1:4

3:9

Figure 4a shows a Scherzer defocus HRTEM image of the nitrogen-doped graphene sheet. Under this condition, carbon atoms in the graphene sheet appeared as black and nitrogen substitution defects cannot be distinguished from them. Using larger-defocus 13

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condition and a low-pass filter, we explore the presence of graphitic nitrogen in the single-layer graphene lattice. Figure 4b shows a larger-defocus HRTEM image taken at

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same area as Figure 4a. The sample drifts in consecutive HRTEM images were corrected and then image contrast was averaged to improve the signal-to-noise ratio,

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using a Gatan imaging software (Digital Micrograph, Gatan Inc.). At larger-defocus

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image, contrast was inverted and carbon atoms appeared as white. The nitrogen positions appeared as wider dark contrast indicated by blue arrows in Figure 4c, when the graphene lattice was suppressed by a low-pass filter. Insets in Figure 4a-c show Fast Fourier Transform (FFT) of each HRTEM images. Low-pass filter, which applied on the

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FFT of Figure 4b, was used to remove the high-frequency pixel noise and suppress the graphene lattice contrast, in order to emphasize the nitrogen substitution sites. Meyer et

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al. explained these contrasts by density functional theory calculations and experimental

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HRTEM analysis [16]. There is a strong change in the charge density around the three carbon atoms next to the nitrogen substitution atom, and this charge redistribution due to C-N chemical bonds can appear as wider dark contrast in the large defocus HRTEM image. At least five nitrogen substitution sites were detected in Figure 4c. White contrast regions correspond to another type of defects such as vacancies and adatoms. The white contrast, indicated by red circle area in Figure 4d, seemed an adatom on the

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nitrogen substitution site. A few minutes after the Figure 4(c-d) was recorded, the adatom moved to the next nitrogen substitution site, and dark wider contrast appeared at

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that area. We investigated the concentration of graphitic nitrogen from the HRTEM images, which were taken from several different points of the sample. The number of

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graphitic nitrogen atoms was typically 1–6 in a ~16×16 nm2 area of graphene. Here, it

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should be noted that only the graphitic nitrogen sites are noticeable in the HRTEM images. The concentration of graphitic nitrogen in graphene lattice was low as obtained from the HRTEM images, while the other nitrogen atoms could be present in pyridinic or pyrrolic forms (correlating XPS results of Figure S1). Previous TEM studies have

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revealed substitutional nitrogen doping level in graphene of around 0.1-0.4 at% [44]. Nitrogen not only present in graphene lattice as a substitutional sites but also in the

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grain boundaries and defects [45].

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Figure 4 (a) Scherzer defocus HRTEM image (b) Large defocus TEM image (c)

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Low-pass filtered image of b. (d) Enlarged image of c. The nitrogen atom substitution position in the graphene lattice can be identified as the dark contrast, indicated by the blue arrows. White contrast corresponds to another type defect (vacancy or adatom on the graphene sheet). Insets of a-c are FFT of each TEM images.

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The grain structure of nitrogen-doped graphene synthesized from the solid precursor was investigated by DF-TEM analysis as shown in Figure 5a. The 1 µm circle of the

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image at the center is a hole in the carbon support, whereas 2.5 µm circle indicates the condenser aperture. Figure 5c shows a part of a diffraction pattern of the graphene

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membrane taken from a region in Figure 5a, which field of view is limited by the

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condenser aperture, i.e around 2.5 µm. DF-TEM images, for example, Figure 5a, were taken by using an objective aperture at the back focal plane to collect only electrons diffracted through a small range of angles, as shown by blue circles in Figure 5c. Electron diffraction of a single crystal graphene shows six fold symmetric spots,

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whereas the diffraction pattern with different families of spot indicates presence of more than one grain with different orientations, as shown in Figure 5d taken from a right

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upper area in Figure 5b. Figure 5b was created by coloring and overlaying several dark

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field images, taken around the region of Figure 5a, where the three colors correspond to the diffraction spots in Figure 5c-d. We observed three different grains orientation as presented by coloring the individual grain. The largest grain is observed to be more than 5 µm of the solid precursor-based nitrogen incorporated graphene. The sample prepared with higher amount of precursor (nitrogen 5.2 at%) has smaller grain size (~1 µm) and higher percentage of multilayered graphene domains (Figure S2). Comparing previous

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results of Huang et al., where the mapped grain size of CVD synthesized graphene is around 1-4 µm, the nitrogen-doped graphene synthesized from solid source contains

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larger grains [33]. The DF-TEM image also shows that there are single and few-layer graphene with similar grain orientations. Areas selected by white dashed line in Figure

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5a show moiré fringes, indicating presence of few-layers of graphene with

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closely-aligned crystallographic orientations. We cannot distinguish these grains, whose orientation difference was under 5°, even though we used the smallest aperture. The finding of grain structure of solid precursor-based CVD synthesized nitrogen-doped

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graphene can be significant to correlate their electronic and chemical properties.

Figure 5 (a) DF-TEM image. Grain structure of graphene is visible through the perforated amorphous-carbon Quantifoil support film. 1 µm circle at the center is a hole in the carbon support, whereas 2.5 µm circle indicates the condenser aperture. (b) 18

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Mapping of grain structure obtained by overlaying the several DF-TEM images. (c), (d) A part of a diffraction pattern of the graphene membrane from a selected region in the

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center of (b), and right upper of (b), respectively. The diffraction pattern in (d) shows different families of spot, indicating presence of more than one grain with different

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orientation. The color of each spot corresponds to the real-space image of (b).

4. Conclusions

We have demonstrated synthesis of nitrogen-doped graphene using camphor and melamine mixture as carbon and nitrogen source, respectively in an APCVD process.

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Raman and XPS analysis confirmed incorporation of nitrogen in the synthesized graphene on Cu foil. The nitrogen dopant induced defects and adatom in the graphene

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grain were detected by TEM analysis. Presence of graphitic nitrogen, i.e., nitrogen atom

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substitution position was confirmed by low-pass filtered HRTEM images under the larger defocus condition. The nitrogen atom substitution position in the graphene lattice can be identified as the dark contrast with charge redistribution due to chemical bonds. Comparing with the XPS result, the HRTEM images showed low concentration of graphitic nitrogen in graphene lattice, as other nitrogen atoms could be present in defect sites. Again, we explored the grain structure of the synthesized graphene with

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introduction of nitrogen dopant by DF-TEM analysis. We observed existence of different grain orientation, where an individual grain is more than 5 µm of the size. The

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DF-TEM mapping also confirmed presence of single layer and few-layer graphene with similar grain orientation. Our findings shows that graphitic nitrogen defects can be

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introduced in the large individual graphene grain by the developed solid source-based

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APCVD technique.

Acknowledgements

The work was supported by the funds for the development of human resources in

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science and technology, “Nanotechnology Platform Project” sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, and the Global

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(GREEN).

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Research Center for Environment and Energy based on Nanomaterials Science

Supplementary information Graphene growth with higher quantity of precursor (camphor and melamine), their Raman, XPS and TEM analysis data are included. Supplementary data associated with this article can be found in the online version.

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Supplementary Information Grain structures of nitrogen-doped graphene synthesized by solid

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source-based chemical vapor deposition

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Sachin M. Shinde1, Emi Kano2,3, Golap Kalita1, Masaki Takeguchi2,3, Ayako

1

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Hashimoto2,3, Masaki Tanemura1

Department of Frontier Materials, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan

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Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba

National Institute for Materials Science, Tsukuba 305-0047, Japan

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305-8573, Japan

*Corresponding author: Tel: +81 52 735 5216. E-mail: [email protected] (Golap Kalita)

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Synthesis of nitrogen-doped graphene: 1.1 Graphene synthesis process:

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The nitrogen-doped graphene samples were grown by the APCVD technique on Cu foils using solid camphor and melamine as precursors as discussed in the paper. The precursor materials compositions were changed to achieve high nitrogen doping in the

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graphene lattice. Solid camphor and melamine powder were taken with much higher

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quantity keeping the ratio constant (7 and 21 mg with 1:3). The annealing of Cu foil and growth duration were fixed for all the experiments. All the synthesized materials were characterized by Raman, X-ray photoelectron spectroscopy (XPS) measurements and

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transmission electron microscopy (TEM) analysis.

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1.2 Analysis of sample synthesized with higher amount of camphor and melamine: Nitrogen-doped graphene synthesis was investigated using larger amount of the

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precursor materials with a ratio of 1:3 (7 mg of camphor and 21 mg of melamine). Increasing the quantity of camphor and melamine, higher amount of nitrogen was confirmed by XPS analysis. Figure S1a shows a Raman spectra of the synthesized graphene sample. We can confirm a high intense D peak, as well as a shoulder peak at the graphitic G peak. These features of Raman spectra clearly show presence of induced defect structures in the synthesized graphene. The XPS analysis was performed to deter-

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Figure S1 (a) Raman spectra of the graphene sample with higher amount of solid

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camphor and melamine. (b) XPS N1s spectra and (c) deconvoluted N1s peak showing the presence of graphitic (400.8 eV), pyrrolic (400.5) and pyridinic (398.6 eV) N atoms

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(d) schematic diagram of the graphitic, pyrrolic and pyridinic nitrogen formation in a graphene.

-mine the nitrogen contents in the sample. Figure S1b shows the N1s spectra of the synthesized graphene. In this case also we obtained two split N1s peaks with peaks centered at ~398.8 and 406.6 eV. The peak at higher binding energy (peak centered at

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406.6 eV) signifies presence of NOx as observed in the previous case as well. It should be noted that the graphene samples on Cu foils were taken out to atmosphere after the

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CVD growth and then the XPS analysis was performed. The quantitative analysis shows 5.2 at % of nitrogen content in the sample. Figure S1c shows a deconvoluted N1s peak,

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corresponding to presence of graphitic (400.8 eV), pyrrolic (400.5) and pyridinic (398.6

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eV) nitrogen atoms. The deconvoluted spectra suggests that the pyridinic nitrogen content is much higher than that of graphitic and pyrrolic nitrogen. Figure S1d shows a schematic diagram of the graphitic, pyrrolic and pyridinic nitrogen structures in a graphene sheet. The TEM images showed only the graphitic nitrogen (encircled in the

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schematic diagram), as the other type of doped nitrogen such as pyridinic and pyrrolic

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sites are difficult to locate.

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The nitrogen substitution site was investigated by HRTEM and DF-TEM. However, we could not observe higher amount of nitrogen substituted graphitic nitrogen sites. We conclude that the nitrogen observed by XPS analysis is not only incorporated in graphene lattice, but also adsorbed on graphene as contaminant. Moreover, the sample prepared with higher amount of precursor has smaller grain size (~1 µm) and higher percentage of multilayered graphene domains. Figure S2a shows the mapping image

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created by coloring and overlaying the dark field images. The different colors correspond to the diffraction spots in Figure S2b. Figure S2c shows multilayered

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graphene structures at the position as indicated by red dotted line of mapping image. We observed such multilayered graphene structures at various positions with increase in

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investigation of nitrogen substitution sites.

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camphor and melamine quantity. The multilayered structures also hindered the

Figure S2 (a) Grain structure of the nitrogen-doped graphene prepared with higher amount of precursors. (b) A part of a diffraction pattern of the graphene membrane. (c) Increasing the quantity of precursor few-layers graphene formation was detected on

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various positions.