Synthesis of large-area monolayer and bilayer graphene using solid coronene by chemical vapor deposition

Accepted Manuscript Synthesis of large-area monolayer and bilayer graphene using solid coronene by chemical vapor deposition Haibin Sun, Junqi Xu, Chunlei Wang, Guixian Ge, Yonglei Jia, Jiangfeng Liu, Fengqi Song, Jianguo Wan PII:

S0008-6223(16)30599-1

DOI:

10.1016/j.carbon.2016.07.027

Reference:

CARBON 11154

To appear in:

Carbon

Received Date: 5 April 2016 Revised Date:

2 July 2016

Accepted Date: 13 July 2016

Please cite this article as: H. Sun, J. Xu, C. Wang, G. Ge, Y. Jia, J. Liu, F. Song, J. Wan, Synthesis of large-area monolayer and bilayer graphene using solid coronene by chemical vapor deposition, Carbon (2016), doi: 10.1016/j.carbon.2016.07.027. 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.

ACCEPTED MANUSCRIPT

Synthesis of large-area monolayer and bilayer graphene using solid coronene by chemical vapor deposition Haibin Suna,b,c,#,*, Junqi Xua, , Chunlei Wanga,#, Guixian Ged, Yonglei Jiaa, Jiangfeng Liuc, #

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Fengqi Songb, and Jianguo Wanb a

Key Laboratory of Advanced Micro/Nano Functional Materials, Department of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China b

National Laboratory of Solid State Microstructures, Department of Physics, Nanjing

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University, Nanjing 210093, China c

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Energy-Saving Building Materials Innovative Collaboration Center of Henan Province, Xinyang Normal University, Xinyang 464000, China

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Key Laboratory of Ecophysics and Department of Physics, College of Science, Shihezi University, Xinjiang 832003, China * Corresponding author. E-mail: [email protected] (Haibin Sun)

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Author Contributions: H. B. Sun, J. Q. Xu, and C. L. Wang contributed equally to this work. ABSTRACT: In this paper, large-area graphene films composed of mono- and bi-layer have been successfully synthesized by atmospheric pressure chemical vapor deposition (CVD) on

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Cu foils using solid coronene as the carbon precursor. The number of graphene layers was precisely controlled by adjusting the growth temperature range. In addition, the hexagonal lattice of graphene was shown using Raman spectroscopy, transmission electron microscopy

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(TEM) and selected area electron diffraction (SAED). The continuity and uniformity of graphene films were confirmed by optical microscopy and scanning electron microscopy (SEM). Further, the presence of weak localization was shown by magneto-transport measurements with the carrier mobility of 2120 cm2/Vs at room temperature. This solid-precursor-based atmospheric pressure CVD method provides a simple, inexpensive and safe route for fabrication of graphene films.

KEYWORDS: large area, graphene, coronene, temperature, chemical vapor deposition

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1. Introduction Graphene, a single atomic layer of sp2-bonded carbon atoms, has attracted significant attention as a perfect two-dimensional (2D) crystalline material [1].This 2D atomic crystal with carbon atoms has a unique band structure with the linear dispersion close to the Fermi

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level, which results in high carrier mobility [2, 3]. As pseudo-2D sp2-hybridized carbon structures, bi- and tri-layer graphene have a continuously tunable band gap under the perpendicular electric field because of a semimetallic band structure with parabolic-like bands

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[4-7]. In addition, when the number of graphene is more than three, few-layer graphene exhibits a large magnetoresistance effect under magnetic field [8]. Hence, the research

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described above clearly demonstrates that multilayer graphene films with the specific number of layers possess different physicochemical properties, and provides a great platform for exploration of high performance electronics.

Limiting to the graphene flakes of small size from micromechanical cleavage of highly

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oriented pyrolytic graphite (HOPG) [9], great efforts have been devoted to prepare large-area graphene with controlled thickness [10-12]. Due to the low solubility of carbon in Cu, large-area monolayer graphene has been successfully prepared by low pressure chemical

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vapor deposition (LPCVD) using methane, which has attracted widespread attention owing to

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the scalability and transferability [13-15]. However, due to the self-limiting growth process using LPCVD technology, the precise control of the number of layers in graphene growth is difficult. So far, many methods have been developed for preparing large-area graphene with the precise layer control on Cu [16]. For example, the precisely controlled total pressure and ratio of growth gases were performed to achieve large area graphene with precise thickness from 1 to 4 layers by the Tour’ group [17]. Another method for preparation multilayer graphene by Tu et al., was carried out by modulating CVD process conditions for the formation of 1-2 layers in LPCVD and 3-7 layers in atmospheric pressure chemical vapor 2

ACCEPTED MANUSCRIPT deposition (APCVD) [18]. However, the thickness of graphene is extremely vulnerable by the preparation conditions. Hence, a facile technological breakthrough for growing large-area graphene with precise layers control is urgently required for graphene electronics. In this work, a facile and versatile method for growing multilayer graphene is developed

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on Cu foils using solid carbon sources under APCVD conditions. It is environmentally friendly, low-cost, easily accessible and useful for the industrial scale. Moreover, under the APCVD condition, carbon source can be provided without spin-coating on catalyst surface or

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heating tape wrapped around the quartz tube [19-21]. Our method provides an integrated process for fabrication of graphene films, including the heating the solid coronene, annealing

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the surface of Cu foils, synthesizing the graphene films and cooling stages. The results show that the growth temperature plays a key role during the preparation of graphene with different layers.

2. Experimental section

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2.1 Synthesis of graphene with thickness of 1-2 layers by APCVD method In this study, the growth of graphene was performed on Cu foils (25 µm thick, 99.8% purity,

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Alfa Aesar 46986) using solid coronene as the carbon precursor. Before synthesis, Cu foils (1 cm × 1 cm) were cleaned for 10 min by diluted hydrochloric acid, acetone, ethanol and

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deionized (DI) water, respectively. The Cu foils were placed in the flat-temperature zone of the quartz tube, and annealed at 1000

in a forming gas rate at 400 sccm (10% H2 in Ar) for

30 min to make the surface non-adsorbent and smooth. The modified CVD growth equipment is depicted in Fig.1 (a). For preparation of graphene, the solid carbon source was sent to the certain position at the upstream end of the quartz tube. Further, to decrease the evaporation rate of carbon source, coronene was loaded in a small glass container, and wrapped in aluminum foil, in which an aperture with the diameter of only 0.5 mm was pricked. 3

ACCEPTED MANUSCRIPT During the preparation of the graphene, the desired growth temperature was tuned in the range of 1000

to 1050

, and the total gas flow rate was maintained at 400 sccm.

According to the thermogravimetric analysis (TGA) for weight loss of solid carbon sources

temperature in the range of 400

to 500

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during sublimation in Ar, the coronene carbon source was heated with the continuous ramping (Fig.1 (b)). It is shown that the coronene carbon

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source was heated naturally along with the quartz tube without a heating tape.

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Figure 1. (a) The setup for CVD growth of graphene. (b) The TGA curves of polystyrene and coronene in Ar at a heating rate of 10

/min. (c) Progress diagram of dependence of growth temperature of graphene

on time. (d) The UV-vis transmittance and sheet resistance of as-grown mono- and bi-layer graphene on a quartz substrate.

Each stage of graphene growth is shown in Fig. 1(c). At the growth temperature of 1030

, the quartz tube was maintained at atmospheric pressure, and the coronene carbon

source was heated and decomposed with a continuous ramping temperature naturally. Thus, 4

ACCEPTED MANUSCRIPT carbon radicals or small carbon fragments were continuously transported downstream and deposited onto the Cu surface. Large-area monolayer graphene can be synthesized after the growth time of 30 min. Then, the furnace turned off and opened for fast cooling. Similarly, the

1050

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preparation of bilayer graphene on Cu was carried out by tuning the growth temperature to under atmospheric pressure, and the decomposed temperature of coronene carbon

source was same as it for the monolayer synthesis.

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The coronene source is naturally heated by placing it adjacent to the furnace entrance. So

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the distance between the coronene and the furnace entrance is different for monolayer graphene (1 cm) or bilayer graphene (2 cm), along with the temperature variation from 1030 to 1050

. The specific location of coronene is drawn in Figure 1S. At the same time,

according to the thermogravimetric analysis (TGA), the coronene was heated with the no matter what the growth

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continuous ramping temperature in the range of 400 to 500

temperature. Usually large area mono- or bilayer graphene can be synthesized after 30 min of growth duration. The setup for preparing the samples is drawn in Figure 1S. We have

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reproducible.

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prepared the samples many times on this setup and make sure that the experiment is

2.2 Transfer of as-grown graphene films Graphene films were obtained on both sides of the foil. The graphene film on one side of Cu foil was transferred onto the target substrates such as SiO2/Si wafers or quartz slides by the PMMA-assisted wet-transfer method [14]. A thin layer of PMMA (J&K Chem 35 PMMA, 7% in anisole) was spin-coated on one side of the foil at 3000 rpm for a minute. Then, another layer was gently wiped off using a swab to remove the graphene. Cu was wet etched in FeCl3 5

ACCEPTED MANUSCRIPT solution for 30 min, and the suspended film was placed in deionized (DI) water to remove the residual etchant. Then, PMMA/graphene film was collected onto 300 nm SiO2/Si substrates. Finally, PMMA was removed by the hot acetone.

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2.3 Characterization of graphene films The synthesized graphene films were characterized by Raman spectroscopy (Renishaw inVia) using an Ar +laser (wavelength 532 nm) with a 1µm laser spot size, equipped with an optical

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microscope. Optical microscopy (OM) and Scanning electron microscopy (Sirion 200 SEM)

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with an operating voltage of 5 kV were used to characterize the morphology of the graphene films. Atomic force microscopy (Bruker Dimension Icon) was used to characterize uniformity of graphene on the SiO2/Si substrate. The optical transmission was measured using a UV-Vis NIR spectrophotometer (DH-2000, Mikropack) in the 400-800 nm range. Transmission

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electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were conducted in a Tecnai G2 T20 field emission gun TEM with an accelerating voltage of 200 kV. The transport measurements were carried out in a Quantum Design Physical Property

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Measurement System-9T system and a homemade MR measurement system. The electrical

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resistance of the graphene films was measured by in-line four-point probe with the dual-configuration procedure (RTS-9).

3. Results and discussion Coronene as a precursor is a significant advantage to other solid precursors such as PMMA, PS etc [19, 20]. Firstly, coronene have a wider and higher heat treatment up to 500-600

,

which give us a controllable range for process investigation [21]. Secondly, due to the low solubility of carbon in copper, mono- and bilayer graphene is easy to be prepared by low pressure CVD using CH4. However, graphene grown in APCVD processes is not self-limiting, 6

ACCEPTED MANUSCRIPT suggesting that controllable synthesis of graphene with different layer number on copper is greatly difficult and complicated. For CH4 as precursor, H2 atmosphere is required to keep the Cu surface reduced, but at the same time can etch as-grown graphene, which makes the CH4-based CVD process so delicate [22]. Hence, maintaining a balanced CH4/H2 ratio is

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critical. In contrast, coronene (C24H12) as precursor requires no reactive diluent, i.e., no delicate balance to be maintained, and consistently gives similar graphene quality at the high temperatures compared to CH4-based CVD.

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Based on the above wet-transfer method, the graphene films were transferred onto a

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quartz substrate. Fig 1(d) shows the transmittance values at 550 nm by the UV-Vis spectroscopy measurement, which indicates that incidence light are 97.1 ±0.2% and 95.2 ±0.5% for as-grown graphene films under different growth temperatures by APCVD method. Based on previous reports, the theoretical opacity of monolayer graphene is 2.3 ±0.1%, and each

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graphene layer adds another 2.3% in a linear attenuation of transmittance [23]. As expected, the transparency of bilayer graphene films decreases with the thickness of the film. Thus, the

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mono- and bi-layer graphene films with relevant large area and high uniformity were obtained. In addition, the transparencies of as-grown mono- and bi-layer graphene decrease slightly. It

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is lower than the theoretical value of graphene, which must be attributed to mechanical damages during the transferred process [24]. The sheet resistance of monolayer graphene is 1.8 kΩ/□ whereas that of bilayer graphene is 800 Ω/□. It is clear that the sheet resistance of our films decreases by stacking the films, suggesting that the cracks in one film are bridged by its neighboring films thus increasing the conductivity. The uniformity and topography of large-area graphene films were transferred onto the SiO2/Si substrates, and further investigated by OM and SEM. The low magnification OM and SEM images of graphene grown under different thickness films are shown in Fig. 2 (a-d). The 7

ACCEPTED MANUSCRIPT shallow color and uniform contrast except for some darker lines or lighter points with low coverage, indicating the uniform coverage of the graphene films. Owing to the graphene transfer process, some small cracks are formed between the graphene and SiO2/Si substrate surface (Fig. 2c, d) when the PMMA film is dissolved away.

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Furthermore, AFM images were used to characterize the surface profile of mono- and bi-layer graphene on the SiO2/Si substrates, which are shown in the Fig. 2S. A measurement of the root-mean-square (rms) surface roughness Rq showed that increased from 4.38 nm for

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monolayer graphene to 5.08 nm for bilayer graphene, indicating the uniform and continuous graphene films. While the intrinsic corrugations of graphene films are also observed in the

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AFM images.

Raman spectroscopy was performed to determine the typical features and quality of graphene films synthesized under different growth conditions. From seen in Fig. 2(e, g), two

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prominent graphene peaks appeared in the Raman spectra at ∼1580 and ∼2690 cm−1, corresponding to G and 2D bands. As the different growth temperature of graphene, the peak intensity ratio of I2D/IG decreased from 2.3 to 1.4, while the full width of half maximum

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(FWHM) of the 2D peak increased from 39 to 60 cm-1, indicating that graphene was

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corresponding from monolayer to bilayer [26]. A defect peak at

1345 cm-1 is very weak or

negligible in the samples, indicating that they are defect-free, high-quality films.

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Figure 2. The large-area OM images (a-b) and SEM images (c-d) of graphene films with different

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thickness grown under the optimized growth temperature, respectively. The characterization of mono-(left) and bi-layer (right) graphene films using coronene as the carbon source. (e, g) Raman spectra of mono- and bi-layer graphene films transferred onto SiO2/Si substrates. (f, h) Lorentzian curve fitted 2D bands of mono- and bi-layer graphene films in panel (e, g), respectively.

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Furthermore, the 2D peak of the monolayer graphene shows a sharp and symmetric single Lorentz curve (Fig. 2(f)). Meanwhile, we decomposed the asymmetric 2D peak of the bilayer graphene into four sub-bands with Lorentz distribution, as shown in Fig.2 (h). This is

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another sign of AB-stacked bilayer graphene [27]. These indicate that the bilayer graphene

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has a much broader and up-shifted 2D band compared with monolayer graphene. The results are in good agreement with the transmittance spectroscopy, which indicates that the graphene grown under different temperatures is an efficient way to control the number of graphene layers.

To characterize the stacking order of the bilayer graphene, we have further performed Raman spectroscopy measurements of bilayer graphene on more than 50 randomly chosen spots. The distributions of I2D/IG ratios and 2D band fwhm values of bilayer graphene were 9

ACCEPTED MANUSCRIPT shown in the supplementary material, Fig. 3S. The observed I2D/IG ratios are in the range of 0.7-1.6 for the AB-stacked bilayer and 2.9-3.6 for the disoriented bilayer. The fwhm of bilayer graphene is mostly in the range of 50-65 cm-1 for the AB-stacked bilayer graphene and

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31-37 cm-1 for the disoriented bilayer graphene. On the basis of the statistical results in Fig. 3S, the AB-stacked bilayer graphene consist of ∼90% coverage of the total bilayer area.

TEM images and SAED patterns can provide detailed information about the thickness

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and the stacking order of as-grown graphene films. The low- and high-resolution TEM images

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of graphene films transferred onto the TEM grid are shown in Fig. 3(a-c). According to the folded-edges of as-grown graphene films, the mono- and bi-layer graphene can be clearly determined [28]. Extensive SAED patterns exhibit only one set of hexagon diffraction spots (Figure 3(d-e)), indicating the single-crystal nature of the domains. Further, the diffraction

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intensities of the outer-order (1-210) peaks are equal to these of the inner diffraction (1-100) peaks for monolayer graphene. However, the intensity ratio of bilayer graphene increases to approximately twice (Fig. 3(e-f)). The corresponding SAED data show a single set of

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hexagonal diffraction spots without rotation, indicative of AB-stacked bilayer graphene [27].

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In addition, the disoriented bilayer graphene typically shows the combination of two sets of diffraction spots with a ～30o rotation (Fig. 3(f)). These observations almost agree well with the results of Raman spectra and light transmittance, further confirming the formation of mono- and bi-layer graphene.

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Figure 3. HRTEM and SAED characterization of mono- and bi-layer graphene. (a-c) High resolution TEM

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of randomly chosen edges of mono- and bi-layer graphene, respectively. (d-f) Representative SAED pattern of graphene from panels (b-c), respectively. (g-h) The intensity profile from the SAED patterns in panels (d-e), respectively.

Transport measurements were performed to evaluate the electronic property of our CVD-grown graphene. Magnetotransport measurements at room temperature were performed to characterize the electrical properties of graphene samples. Measurements were made in a van der Pauw geometry with solid indium at the edges of large-scale (several mm) graphene 11

ACCEPTED MANUSCRIPT sheets transferred to SiO2/Si substrates [29]. Figure 4(a-b) shows an optical micrograph of a completed device and the corresponding operation schematic diagram. The five electrodes were then applied onto the micro-flakes by a standard lift-off procedure. The width (W) and

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length (L) of the present sample are 1 mm and 4 mm respectively. In an ambient pressure, a perpendicular magnetic field of B= ± 5 T was swept along with various environmental temperatures from 3K to 290K. Further, using standard lock in techniques, the longitudinal

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resistance Rxx (B) was measured with a 14 µA current at 30Hz. Fig. 4(a) shows the typical

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magnetoconductance curve at 5K, indicating the presence of weak localizations in our samples. A normal parabolic magnetoresistance curve was obtained in the high-field range and the low temperature, but the sharp cusp was dominant near the zero field. It implies a negative contribution to magnetoconductance [30].

model

for

conduction

by

electron

and

hole

charge

puddles

[31]

VH L 1 2 ⋅ ⋅ , was founded to be 2120 cm /V s at room temperature (Fig. I x Bz W Rxx ( B = 0)

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µMR = RHσ =

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The magnetoresistance mobility µ MR, extracted from a fit of measured Rxx (B) to a simple

4(b)). Generally, the magnetoresistance mobility provides the equal evidence to the field

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effect mobility, indicating the high quality over large area of as-grown graphene films. Further, it is higher than that of graphene films grown with liquid precursor (ethanol, pentane) (～100 cm2/V s) [32], but lower than that of graphene grown with methane (～4000 cm2/V s) [14]. It is the intrinsic defects of high-density grain boundaries, and extrinsic doping during the graphene growth, transfer and the adsorption of contaminant during the measurement process [33].

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Figure 4. MR and carrier mobility measurements of coronene grown graphene. (a) The configuration of Hall bar-shaped field effect transistor. (b) Optical micrograph of graphene device. (c) Magnetoresistance curve depended on the magnetic field at 5 K. (d) The temperature-dependent carrier mobility.

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The narrow range of temperatures have been found to be the critical factor for forming uniform single or bilayer graphene on copper surface. We found that the graphene film could , in brief, no graphene film was produced if the preparation

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be only prepared at above 1030 was performed below 1030

.～For comparison, we performed the preparations of graphene

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film at a constant location of coronene for 1 cm, while the growth temperature changed from 1020 to 1030

.～According to the reference reported [25], the optical images of copper foil

show the different interference color after annealing in air at 160

for 6 min. Fig. 4S (a-b)

show the optical images of the as-prepared sample. It is clear that the pure Cu is oxidized to form Cu oxides to be the gray red while the Cu covered graphene shows little color changes. By this way, the growth of graphene is easily observed by the naked eyes directly on Cu foils. However, it is invalid for the growth temperature up to 1040 13

because of the Cu covered

ACCEPTED MANUSCRIPT graphene as well (Fig. 4S (c)). A wet transfer method is used to peel off graphene from Cu foil to the SiO2/Si substrate. The low magnification OM and SEM images of graphene grown under different thickness films are shown in Fig. 4S (d-e). There are two different visibly

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contrasting regions in the OM and SEM images. The corresponding Raman spectra of the different colored regions are also shown in Fig. 4S (f), which indicate that the mono- and bilayer graphene (red and blue circles in Fig. 4S (d)) is formed as well as the uniformity of

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graphene decreases.

Additional experiments were done to reveal the growth mechanism for the bilayer

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graphene films in the paper, as well as understanding the growth process (shown in Supporting Information, Fig. 5S). Fig. 5S (a) shows the SEM image of graphene domains after a short time-growth (10 min). Lots of small hexagon domains form when C atoms are supersaturated in the copper surface. Raman spectroscopy was employed to confirm the

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thickness of the graphene. In Fig. 5S (c), the G and 2D band centered at ～1580 and ～2690 cm-1, respectively. In particular, the intensity of I2D/IG is ～ 1, and the full width at half-maximum (FWHM) of 2D band is ～57 cm-1. The characteristic features in Raman

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spectra suggest that graphene domains are AB-stacked bilayer. Interestingly, the hexagonal

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domain typically shows the uniform color contrast, indicating that the graphene domain forms synchronously rather than layer-by-layer, as shown in Fig. 5S (b). The above discussion points to the fact that it is crucial to tune the growth temperature of graphene and make the surface as uniform as possible to obtain high-quality graphene. General speaking, for CH4-based CVD, the isolated C monomers will form carbon clusters containing defects such as pentagons that degrade the quality of synthesized graphene. However, the C−C bonds in coronene are unlikely to be shattered on Cu substrate due to the 14

ACCEPTED MANUSCRIPT weak interaction strength between C and metal surface [34]. Therefore, the graphene growth mechanism on Cu substrate from coronene mainly involves surface-mediated nucleation process of dehydrogenated coronene rather than segregation or precipitation process of small

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carbon species that decomposed from the precursors [35]. Thus, it is expected that the coronene might be easily grown graphene due to the low nucleation barrier compared to methane, resulting into the weaker coupling to the metal substrate [36, 37]. Further work to

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optimize growth conditions is to improve the carrier mobility and sheet resistance of our

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graphene films.

4. Conclusions

In this paper, a simple and effective method was presented to synthesize mono- and bilayer of large-area graphene films on Cu foils under APCVD conditions, with solid coronene as the

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precursor. Compared with methane-based graphene growth, graphene grown with coronene is more accessible because coronene contains six-fold rotational symmetry and planar configuration. Further, Raman spectroscopy, TEM imaging and the electrical characteristics of

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as-grown graphene by coronene confirm that the number of graphene layers have been

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precisely controlled by adjusting the growth temperature. This APCVD growth with solid coronene method provides a simple and safe route for the synthesis of large-area mono- and bi-layer graphene films.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51472113, 21403144, 11404277, 11464038), and Nanhu Scholars Program 15

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for Young Scholars of XYNU.

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