Growth of Uniform Monolayer Graphene Using Iron-Group Metals via

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Growth of Uniform Monolayer Graphene using IronGroup Metals via Forming Antiperovskite Layer Linfeng Chen, Zhizhi Kong, Shuanglin Yue, Jinxin Liu, Jingwen Deng, Yao Xiao, Rafael G. Mendes, Mark H. Rümmeli, Lianmao Peng, and Lei Fu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02788 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 28, 2015

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Chemistry of Materials

Growth of Uniform Monolayer Graphene using Iron-Group Metals via Forming Antiperovskite Layer Linfeng Chen,† Zhizhi Kong,† Shuanglin Yue,‡ Jinxin Liu,† Jingwen Deng,† Yao Xiao,† Rafael G. Mendes,§ Mark H. Rümmeli,§ Lianmao Peng,‡ and Lei Fu†,* †

College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P. R. China

‡

Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China

§

IFW Dresden, P.O. Box 270116, 01069 Dresden, Germany

ABSTRACT: It has been generally accepted that iron-group metals (Fe, Co, Ni) consistently show the highest catalytic activity for the growth of carbon nanomaterials, including carbon nanotubes (CNT) and graphene. However, it still remains a challenge for them to obtain uniform graphene because of their high carbon solubility, attributing to an uncontrollable precipitation in cooling process. The high-quality and uniformity of the graphene grown on low-cost irongroup metals determine whether graphene can be put into the mass productions or not. Here, we develop a novel strategy to form an antiperovskite layer using ambient-pressure chemical vapor deposition (APCVD), which is so far the only known way for iron-group metals to prepare uniform monolayer graphene with 100% surface coverage. Our strategy utilizes liquid metal (eg. Ga) to assist iron-group metals to form an antiperovskite layer which is chemically stable throughout the high-temperature growth process and then to seal the passageway of carbon segregation from the metal bulk during cooling. With the advantage of forming antiperovskite structure, the uniform monolayer graphene can always be obtained under the variations of experimental conditions. Our strategy solves the problem about how to get uniform graphene film on high-solubility carbon substrate, to utilize the high catalytic activity of low-cost iron-group metals and to realize low-temperature growth by chemical vapor deposition.

INTRODUCTION Iron-group metals (Fe, Co, Ni) are widely known for their ability to catalyze carbon nanomaterials, including carbon nanotubes (CNT) and graphene.[1-5] It has been generally accepted that these metals and their alloys consistently show the highest catalytic activity.[6-7] This is usually attributed to the high solubility of carbon in metal-solid solutions.[8] In addition, their relatively low prices as compared to noble metals (Cu[8], Au[9], Pt[10], Ru[11], Rh[12], Pd[13], or Ir[14] etc) imbue iron-group metals bright prospects for the catalytic growth of graphene in terms of commercial applications. Unfortunately, the very high carbon solubility of iron-group metals, such as Fe (~7.0 at.% at 1000 oC), and a sharp temperature dependency lead to massive carbon segregation from the bulk during cooling. As a result, it is generally considered impossible to achieve homogeneous growth of graphene on irongroup metals owing to the non-equilibrium precipitation during cooling.[15-17] Because of this most researchers have abandoned working on iron-group metals to synthesize uniform monolayer graphene films despite the high catalytic properties and low cost of iron-group metals. It is extremely important to obtain uniform graphene with controllable layer numbers for both fundamental

research and practical applications. The reason for this does not lie entirely with the widely varying characteristics of graphene that occur for different layer numbers but also with the urgent demands for reproducibility of device performance.[18] Many researchers have been working on improving the uniformity of graphene, in particular for the obtainment of uniform monolayer graphene.[19-20] However, attempts to grow monolayer graphene using iron-group metals which is characterized in high catalytic efficiency and low cost have not yet been fruitful. Their high carbon solubility seems to be an impenetrable barrier for the realization of strictly monolayer graphene fabrication.[21] In 2011, we embedded Ni islands in Mo foils that effectively suppresses the carbon precipitation process and activates a self-limited growth mechanism for homogeneous monolayer graphene.[22-23] The procedure to obtain the designed binary metal is relatively complex and the required temperature for graphene growth is high (1000 oC).[22-23] In 2012, Robert et al. produced Au−Ni catalysts by thermal evaporation to decorate the steps of Ni surface to grow continuous, uniform graphene under high vacuum.[24] A finite carbon solubility of the catalyst was considered as the key to obtain monolayer graphene, while it allowed the catalyst bulk to act as a mediating

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carbon sink. It remains a great challenge to fabricate uniform monolayer graphene using high carbon-soluble iron-group metals, especially Fe, which has extremely high carbon solubility, with a facile and mild method. To achieve the control of carbon diffusion, it is an effective way to establish a carbon barrier to suppress the precipitation of the undesired extra carbon. Here, we develop a novel strategy to establish a peculiar carbon barrier—antiperovskite layer[25-26] via introducing Ga to react with iron group metal, which is so far the only known way for iron-group metals to prepare uniform monolayer graphene with 100% surface coverage. Our strategy utilizes liquid metal (eg. Ga) to assist iron-group metals to form an antiperovskite layer which is chemically stable throughout the high-temperature growth process and then to seal the passageway of carbon segregation from the bulk during cooling on high dissolved carbon metals. With the advantage of antiperovskite structure, the uniform monolayer graphene can always be obtained under variations in experimental conditions. Our strategy solves the problem about how to get uniform graphene film on high-solubility carbon substrate, to utilize the high catalytic activity of low-cost iron-group metals and to realize low-temperature growth by chemical vapor deposition.

EXPERIMENTAL SECTION CVD growth of graphene. The iron-group metal foils (25~1000 μm, cut into 1 × 1 cm squares) were ultrasonicated and rinsed with acetone, ethanol and deionized water to remove the surface metal oxides and residues. A small Ga pellet (10 mg) was placed on the iron-group metal foils. The sample was positioned in the quartz tube furnace (HTF 55322C Lindberg/Blue M) under ambient pressure and was heated up to 600~1000 o C at a rate of 33–40 °C min−1. After the pre-annealing process, the CH4/H2 reaction gas mixture was introduced to synthesize graphene for 5–120 min. At last, the samples were cooled down to room temperature at the rate of 4– 250 °C min−1 under the protection of Ar/H2. Transferring the graphene to the target substrates. The graphene films synthesized on Ga‒Fe (Co, Ni) surfaces were transferred onto 300 nm SiO2/Si substrates and TEM grids using polymer supports, such as poly(methyl methacrylate) (PMMA), by etching away catalysts using Fe(NO3)3/HCl. After the etching of metal, polymer/graphene was delivered onto target substrates. The PMMA supporting films were effectively removed by acetone boiled up to 80°C.

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Characterization. Optical images were collected by an optical microscope (Olympus DX51, Olympus), and Raman spectra were obtained by a Laser Micro-Raman Spectrometer (inVia Plus, Renishaw) with an excitation wavelength of 532 nm. The AFM images were taken with an atomic force microscope (Ntegra Spectra, NT-MDT). All the graphene films for characterizations mentioned above were transferred onto the 300 nm SiO2/Si. The transmission electron microscopy (TEM) images were obtained by an aberration-corrected high-resolution TEM (AC-HRTEM, FEI Titan), in which the operating voltage was 80 kV and the graphene films were transferred onto a TEM grid. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi) depth profiling was performed by Ar ionic bombardment to gradually remove the surface layers until 7 nm downward into the bulk phase. The measuring spot size was 500 μm. The current (I)–voltage (V) data were taken in a probe station under ambient conditions using a Keithley 4200. X-ray diffraction (XRD) measurement was performed with LabX XRD-6000 using Cu-Kα radiation over the range of 2θ = 30–90°.

RESULTS After being deposited on iron-group metal foils (25 μm~1 mm, cut into 1 × 1 cm squares), the drop of Ga (10 mg) spreads spontaneously over the entire substrate at elevated temperature. Graphene films grown on pure iron-group metals are highly inhomogeneous, appearing as different colors in optical microscope (OM) images (Figure 1a−c). They show characteristic regions and islands corresponding to different layers. Figure 1d−f reveal the excellent uniformity and 100% surface coverage of the graphene on Ga covered Fe, Co, Ni foils, respectively. From the optical microscope image, we preliminarily confirmed that all areas of the graphene were monolayer, and no multilayer graphene was observed. Obviously, there was a marked improvement in the graphene uniformity on the Ga covered iron-group metal foils compared with the pure competitors. Raman spectroscopy was employed to further examine the graphene grown over Ga covered iron-group metals (Figure 1g−i). The typical Raman 2D bands exhibited a symmetric single Lorentzian line shape, and D to G peak intensity ratio was comparable to that reported on Ni.[27] The above characterization results indicate that the obtained films have the advantage of a monolayer graphene with a high uniformity and a relatively low defect density.


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Figure 1. Typical growth results on pure or Ga covered iron-group metals. (a−c) Optical microscope images of graphene grown on pure Fe, Co, and Ni, respectively, which indicate poor uniformity. (d−f) Optical microscope images of graphene grown on Ga covered Fe, Co, and Ni, respectively, which demonstrate the excellent uniformity of the monolayer graphene. All the graphene films were synthesized under ambient pressure and they were transferred onto 300 nm SiO2/Si substrates for characterization. The scale bars are 20 μm. (g−i) Typical Raman spectra of monolayer graphene grown on Ga covered iron-group metals (excitation wavelength: 532 nm).

It has been demonstrated that there exists a certain barrier during the dissociative chemisorption of hydrocarbon on the catalysts but the translational energy normal to the surface of Ni is effective in surmounting it, leading Ni to be an efficient catalyst for complete dehydrogenation of hydrocarbon precursors, which endows Ni the ability to catalyze the growth of graphene at low temperature.[28-29] As shown in Figure 2, we succeed in synthesizing homogeneous monolayer graphene under ambitious-pressure at 600 oC, which is the minimum temperature for the decomposition of methane. The markedly reduced growth temperature is beneficial for surpassing catalytic ability for complete dehydrogenation of hydrocarbon precursors. Figure 2a shows the picture of a transferred graphene film on a 300 nm SiO2/Si

substrate. The graphene films can be easily observed because of the light interference effect, which is a preliminary evidence to acknowledge the uniformity of the graphene film on a macroscopic scale. The monolayer feature and high crystallinity of the graphene grown on the Ga–Ni were examined by the low voltage aberrationcorrected, high resolution transmission electron microscopy (LVAC-HRTEM). The six-fold symmetry single-crystal nature of the graphene is highlighted by the HRTEM image in Figure 2b and the corresponding 2D fast Fourier transformation (FFT) pattern (inset in Figure 2b). In Figure 2c, a false 3D image from a Fourier enhanced TEM micrograph of the marked area in Figure 2b demonstrates



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Figure 2. Graphene films transferred from Ga-Ni foil at 600 °C. (a) Graphene film transferred onto a SiO2/Si substrate. (b) HRTEM image and its 2D FFT (inset) of the monolayer graphene. The scale bar is 1 nm. (c) A false 3D image originated from Fourier enhanced TEM micrograph of the marked area in Fig. 2b. (d) An AFM image of the transferred graphene on a SiO2/Si substrate. (e) Corresponding Raman maps (10 µm × 10 µm with step size of 1 µm) of the 2D/G peak intensity and (f) the FWHM of Raman 2D band.

the perfect atom-scale crystal structure of the graphene. The atomic force microscopy (AFM) image shows a uniform graphene film with area of 100 μm2.[30] The Raman mapping of the same area was also conducted to detect its layer number and quality, the spatial and spectral resolutions of the measurements are 1 μm and 1 cm-1 respectively. We found the distribution of intensity ratio (I2D/IG) fell into a range of 2.8–2.2 (Figure 2e) and the typical full width at half-maximum (FWHM) of 2D Raman band is < 38 cm−1, indicating that the samples are monolayer graphene. The conventional CVD growth of graphene has been shown to be sensitive to the growth parameters, which makes it difficult to obtain strictly monolayer graphene. Our method manifests a fascinating feature that is the extremely high tolerance to variations in experimental conditions (Figure 3; Supplementary Figure S1 and Supplementary Table S1). For instance, the growth time (Figure 3a) and the concentration of the carbon source (Figure 3b−c) dictate the total amount of carbon delivered into the reaction system. Generally, an excess carbon supply would result in the formation of thicker-layer graphene and even amorphous carbon deposits. Surprisingly, when we increased the carbon supply remarkably, we still obtained uniform monolayer

graphene with 100% surface coverage. No multilayer graphene spots were resolved in the highest resolution optical microscope images. Our method shows no dependence on temperature variations from 600 to 1000 °C (Figure 3d). To our best knowledge, it is the first time to achieve the homogeneous growth of monolayer graphene at as low as 600 °C using methane as carbon source under atmospheric conditions. As long as the surface of the iron-group metal foils is covered by a thin Ga layer to form antiperovskite layer, a monolayer graphene would be expected. Interestingly, when the thickness of iron-group metal foil ranged from 25 to 1000 μm, no changes were observed in the quality of the grown graphene (Figure 3e). Generally speaking, the cooling rate is one of the most crucial factors that significantly affect the growth, especially for the high-solubility carbon metals (such as Fe, Co, Ni). Astonishingly, no detectable changes are observed in the uniformity of graphene when the cooling rate varies from 4 to 250 °C min−1, because of the complete suppression of precipitation by antiperovskite layer (Figure 3f). In principle, our method ensures the growth of perfect monolayer graphene by amputating the precipitation process on the aspect of the growth kinetics. Our observations under various experimental conditions demonstrated that there was a


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large growth window for strictly uniform monolayer graphene on iron-group metals by our method (Figure 3).

This is undoubtedly an important advantage for practical applications over the conventional CVD process.

Figure 3. Typical optical microscope images of graphene films grown on Ga covered Fe (Co, Ni) foils under different growth conditions. The scale bars are 20 μm. All the graphene films were monolayer with 100% surface coverage, which demonstrates the good tolerance to various experimental conditions. All of the graphene samples were transferred to 300 nm SiO2/Si substrates for characterization.

DISCUSSION A comparison of the XPS depth profile between Ga-Ni foil and pure Ni foil provided more decisive information to understand the monolayer-growth feature and high tolerance of our method variations in experimental conditions. Figure 4a shows the changes of C 1s core-level spectra with argon ion sputtering on Ga-Ni foil. At the topmost surface the doublet C 1s band contains both graphitic peak centered at 284.6 eV originating from graphene overlayer and shouldered peak, C−H, around 285.3 eV. At the subsurface, the doublet C 1s band still contains graphitic peak at exactly the same position but its intensity drops, and the shouldered peak around 283.5 eV originates from the underlying carbide. However, as digging into the bulk, the carbidic C peak lies with stable intensity and width. This suggested that the carbon species dissolved in the bulk reacted with Ni and Ga to form stable compound. Similar XPS analysis can be done on pure Ni foil (Figures 4d). From the topmost surface to the bulk, the deconvolution C 1s spectrum always contains graphitic and C−H bands centered at 284.6 and 285.3 eV, respectively. No obvious carbidic C peak is detected, suggesting the absence of Ni carbides in accordance with the fact that carbide nickel can decompose during cooling.[31] The existence of the compound was confirmed by X-ray diffraction (XRD). As shown in Figure 5b, we found obvious signal of carbides, principally GaCNi3 after CVD growth. Ga1.4Ni2.5 was detected after annealing in 600 °C without introducing CH4 (Figure 5c). The unit cell of GaCNi3, an antiperovskite-type compound, is shown in the insert of Figure 5a. The structure of the obtained compound was illustrated through the comparison with the reported similar structure[32–33]. The characteristic peaks in the XRD pattern of reported structure (GaCNi3), such as (111), (200) and (220) can be found in the XRD pattern of as-obtained compounds.[33] The Ga atom is situated in the corner of the cube, C in the center, and Ni occupies the face

centers. The diffusion of the carbon atoms in the bulk phase are blocked by the compact compound GaCNi3 lattices, which are insoluble to carbon. Non-equilibrium precipitation paths are effectively suppressed by the formation of the compact structure in the Ga-Ni foil, allowing a self-limited CVD mechanism for the growth of perfect monolayer graphene. As shown in Figure 4b, the carbon concentration on the Ga-Ni foil remains almost unchanged in the bulk phase and the ratio of Ga, C and Ni content is ~1.0:1.1:2.6 in the bulk, which is close to the stoichiometric ratio of the mixture of GaCNi3 (67%) and Ga1.4Ni2.5 (33%). In the case of a pure Ni foil, the carbon content stays at a very high level and shows a monotonic decrease toward the bulk phase. For instance, the carbon concentrations at 0 and 7 nm from the surface are 89.12 and 79.62 atom %, respectively (Figures 4e). This is the result of surface absorption of carbon species and the extremely high carbon solubility in Ni. Figure 4c and 4f show the changes of C 1s core-level spectra. From the topmost surface to the bulk of the pure Ni foil, the doublet C 1s bands are always composed by graphitic peak centered at 284.6 eV and shouldered peak, C−H, around 285.3 eV (Figure 4f). We never discover the existence of carbidic C, indicating uncontrollable process upward segregation and precipitation of carbon species from the bulk metal onto the surface during cooling. As a result, it is hard to get uniform monolayer graphene using pure Ni foil as the substrate. For Ga-Ni foil, the doublet C 1s bands including graphitic peak and C−H peak are detected at its topmost surface (Figure 4c), which are similar to that at the topmost surface of the pure Ni foil. Apparent carbidic C peaks have always existed from the subsurface to the bulk around 283.5 eV originating from the underlying compound. The dissolution of carbons into metal bulk and their subsequent non-equilibrium precipitation are the crucial steps governing the uniformity of CVD graphene. This study demonstrates an alternative strategy for the CVD



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growth of monolayer graphene through forming antiperovskite layer, which is schematically shown in Figure 5a. The key of our strategy is to utilize antiperovskite layer to block the precipitation of the dissolved excess carbon species, in which the formed antiperovskite structure are chemically stable throughout the high-temperature growth process.[25-26] Our method utilizes antiperovskite layer to seal the passageway of carbon segregation from the bulk during cooling. All irongroup metals are all capable of forming stable and condensed antiperovskite structure. The antiperovskite structure can be measured by goldschmidt tolerance factor, which can be expressed by (1) where rA and rB are the radius of the cation and r0 is the radius of the anion. The goldschmidt tolerance factor of Fe, Co, Ni are 0.863, 0.864 and 0.864 so they are all antiperovskite structure.[32-33] Blocking and shielding all of the excess dissolved carbon species suppress the

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troublesome carbon precipitation process. In addition, the hydrocarbon precursor dissociates on the catalyst surface with Ga assistance to form a y smooth surface, and this flat molten surface favors a higher diffusion rate of absorbed carbon species.[34-35] The high quality of the monolayer graphene grown on iron-group metal foils was confirmed through electrical transport measurements. Back-gate field effect transistors were fabricated on 300-nm SiO2/ Si substrates using conventional electron beam lithography. The source and drain electrodes were deposited using Cr/Au (10/60 nm). The channel width was 15 μm and length was 6 μm, respectively. Supplementary Figure S2 demonstrated the excellent transfer characteristic (Ids–Vgs-Vdirac,gs) of the field effect transistors that were measured under ambient conditions. The V-shaped ambipolar property is typical for monolayer graphene with a zero bandgap. The extracted carrier mobility of electrons for this device is 3326 cm2V−1s−1, which is higher than the graphene grown from Ni surface (1000~2500 cm2V−1s−1).[36]

Figure 4. X-ray photoelectron spectroscopic analyses of graphene on a Ga-Ni foil (a−c) and a pure Ni foil (d−f) XPS C 1s peaks from surface and bulk on Ga-Ni foil (a) and pure Ni foil (d) after CVD growth. XPS composition profiles of elements along the surface normal direction on Ga-Ni foil (b) and pure Ni foil (e) after CVD growth (the relative composition of each element is represented by a specific colour for clarity). Changes of C 1s core-level signal with depth on Ga-Ni foil (c) and pure Ni foil (f) after CVD growth without normalization of core-level peak heights. The CVD growth was performed at 600 °C for 30 min and a gas flow composition of Ar: CH4 = 300:5 sccm at ambient pressure unless otherwise specified.


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Figure 5. Schematic illustration of our method and XRD spectrum of the Ga-Ni foils. (a) Schematic illustration of our method, which demonstrates the formation of the stable and intense compound GaCNi3 and suppressed upward precipitation of the carbons. XRD spectra of the Ga-Ni foils after (b) and before (c) the introducing of CH4. The CVD growth was performed at 600 °C for 30 min and a gas flow composition of Ar: CH4 = 300:5 sccm at ambient pressure.

CONCLUSION In summary, we developed a facile approach for growing strictly monolayer graphene with 100% surface coverage via forming the antiperovskite structure with introducing Ga to react with iron group metals. The excess dissolved carbon species are shielded and blocked by the compact and stable antiperovskite structure, such as GaCNi3. We succeed in effectively suppressing the troublesome precipitation process of dissolved carbons, presenting excellent tolerance to variations in experimental conditions. It is the first time to achieve the uniform graphene film on high-solubility carbon catalysts (such as Fe) and realize low-temperature (600 °C) growth under atmospheric pressure. The simplicity and scalability of using highest catalytic ability and low cost iron-group metals will greatly facilitate future graphene research and industrial applications.

ASSOCIATED CONTENT Supporting Information Optical microscope images of the graphene films grown on the Ga–Ni foil, the detailed growth conditions and the electrical property of single-layer graphene flake. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Author Contributions L.F. developed the concept and conceived the experiments. L.F.C. and Z.Z.K. carried out the experiments. L.F.C. wrote

the manuscript. L.F. revised the manuscript. All of the authors contributed to the data analysis and scientific discussion.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The research was supported by the Natural Science Foundation of China (Grants 51322209, 21473124), the SinoGerman Center for Research Promotion (Grants GZ 871) and the Ministry of Education (Grants 20120141110030).

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Growth of Uniform Monolayer Graphene using Iron-Group Metals via Forming Antiperovskite Layer It has been generally accepted that iron-group metals (Fe, Co, Ni) consistently show the highest catalytic activity for the growth of carbon nanomaterials, including carbon nanotubes (CNT) and graphene. However, it still remains a challenge for them to obtain uniform graphene because of their high carbon solubility, attributing to an uncontrollable precipitation in cooling process. The high-quality and uniformity of the graphene grown on low-cost iron-group metals determine whether graphene can be put into the mass productions or not. Here, we develop a novel strategy to form an antiperovskite layer using ambient-pressure chemical vapor deposition (APCVD), which is so far the only known way for iron-group metals to prepare uniform monolayer graphene with 100% surface coverage. Our strategy utilizes liquid metal (eg. Ga) to assist iron-group metals to form an antiperovskite layer which is chemically stable throughout the hightemperature growth process and then to seal the passageway of carbon segregation from the metal bulk during cooling. With the advantage of forming antiperovskite structure, the uniform monolayer graphene can always be obtained under the variations of experimental conditions. Our strategy solves the problem about how to get uniform graphene film on high-solubility carbon substrate, to utilize the high catalytic activity of low-cost iron-group metals and to realize lowtemperature growth by chemical vapor deposition.


Growth of Uniform Monolayer Graphene Using Iron-Group Metals via

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