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Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition
Accepted Manuscript Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition Wooseok Song, Cheolho Jeon, Soo Youn Kim, Yooseok Kim, Sung Hwan Kim, Su-Il Lee, Dae Sung Jung, Min Wook Jung, Ki-Seok An, Chong-Yun Park PII: DOI: Reference:
S0008-6223(13)00998-6 http://dx.doi.org/10.1016/j.carbon.2013.10.039 CARBON 8475
To appear in:
Carbon
Received Date: Accepted Date:
3 June 2013 14 October 2013
Please cite this article as: Song, W., Jeon, C., Kim, S.Y., Kim, Y., Kim, S.H., Lee, S-I., Jung, D.S., Jung, M.W., An, K-S., Park, C-Y., Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition, Carbon (2013), doi: http://dx.doi.org/10.1016/j.carbon.2013.10.039
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Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition
Wooseok Song1,2, Cheolho Jeon3, Soo Youn Kim1, Yooseok Kim1, Sung Hwan Kim1, Su-Il Lee1, Dae Sung Jung4, Min Wook Jung2, Ki-Seok An2, and Chong-Yun Park1,4,*
1
BK21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Republic of Korea
2
Thin Film Materials Research Group, Korea Research Institute of Chemical Technology (KRICT), Yuseong P. O. Box 107, Daejeon 305-600, Republic of Korea
3
Division of Materials Science, Korea Basic Science Institute, Daejeon, 305-333, Republic of Korea 4
Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
*
corresponding author. Tel.: +82-31-299-6501. Fax: +82-31-299-6505. E-mail address: [email protected] (Chong-Yun Park).
Abstract Here we provide evidence of two selective growth modes, namely the ‘surface adsorption (SA) mode’ and the ‘diffusion and precipitation (DP) mode’ for the synthesis of graphene on Cu foil by thermal chemical vapor deposition. Using acetylene feedstock, the number of graphene layers was controlled simply by adjusting the injection time, and the DP growth mode was clearly verified by the existence of a carbon-diffused Cu layer with expansion of the Cu lattice. With methane feedstock, either SA or DP growth modes could be selected for the growth of graphene at low or high partial pressure of carbon feedstocks, respectively. The critical pressure for switching the growth modes depends on reactivity of carbon feedstock to Cu substrate.
1. Introduction When thin films grow epitaxially on a crystal surface, the growth generally occurs through one of the three primary modes: Volmer-Weber (island formation), Frank-van der Merwe (layer-by-layer), or Stranski-Krastanov (layer-plus-island) growth [1]. The growth mode is determined by the competition between kinetics and thermodynamics. This fundamental understanding enables us to precisely control the growth and thus the properties of deposited films. For graphene growth on transition metal substrates using a thermal chemical vapor deposition (TCVD), there are two suggested growth modes: diffusion and precipitation (DP) and surface adsorption (SA). These modes have been considered to govern the growth of graphene on Ni and Cu due to the great difference in carbon solubility between these substrates. The high carbon solubility of Ni causes to dissolve into the bulk, and graphene subsequently grows by surface segregation of carbon upon cooling from a metastable carbon-metal solid solution (DP mode) [2, 3]. On the contrary, the extremely low carbon solubility of Cu leads to self-limited growth of graphene by simple thermal decomposition of hydrocarbons on the Cu surface (SA mode) [4]. As a result of these different growth modes, graphene grown on Ni has an inhomogeneous thickness distribution, whereas predominantly monolayer graphene is grown on Cu. This leads to an inherent problem; if graphene growth on any catalytic metal substrates obeys one of these two growth modes, then it is not possible achieve layer-controlled graphene growth, which will be in high demand for future applications. Fortunately, graphene growth on a Cu substrate can be a model system with which to study the possibility of selecting and manipulating the growth mode. Several experiments have been conducted with different working pressures and cooling rates, resulting in the growth of multilayer and bilayer graphene on Cu foil under atmospheric pressure with high
CH4 concentration [5] and using a slow cooling process [6, 7], respectively. These results indicate that the SA mode is not an exclusive growth mode for graphene on a Cu substrate. However, a detailed mechanism for the growth of a second graphene layer on a Cu substrate remains elusive. Since previous growth mechanism studies were conducted mostly using CH4 as a carbon feedstock [3-7], the results cannot be universally applied. Reference to previous research on carbon nanotubes (CNTs) provides a good understanding of the influence of the carbon feedstock on the growth of graphitic structures. It is generally accepted that CH4 feedstock is used mainly for the synthesis of highly crystalline single-walled CNTs, whereas C2H2 feedstock is employed for the high-yield growth of CNTs [8-11]. This feedstock specificity can be explained by the different reaction rate of carbon feedstocks with catalytic metals. It should be noted that considerable dependence on the hydrocarbon feedstock in TCVD growth of graphene has not been reported using Ni substrate, presumably because of its high carbon solubility. However, an in-depth study using different carbon feedstocks to elucidate the growth mechanism of graphene on a Cu substrate would be worthwhile. Here, we present evidence for two selective growth modes of graphene on Cu with various carbon feedstocks (C2H2 and CH4) and working pressures. The number of graphene layers grown by C2 H2 increased with increasing injection time. A combined secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) study revealed a carbon-diffused Cu layer created at the surface region of the Cu substrate with expansion of the Cu lattice. The graphene was grown on Cu by the DP mode rather than the SA mode, as evidenced by similar results in a graphene/Ni system. Interestingly, the carbon-diffused Cu layer was also observed after graphene growth under high CH4 pressure. Previous results and our data demonstrate two selective growth modes of graphene on a Cu substrate, and show that the desired mode
can be selected by tuning the working pressure according to the kind of carbon feedstock. We believe that this finding will shed light on the growth mechanism of graphene on a Cu substrate and will enable to grow layer-controlled graphene for its multifaceted applications.
2. Experimental Section Graphene was synthesized using a conventional TCVD system. We used 25-µm-thick Cu foil or 50-µm-thick Ni foil as the catalytic substrate for graphene synthesis. The catalytic substrate was heated to 950 °C inside the TCVD reactor with H2 gas flowing at 100 sccm. C2H2 (3 sccm) or CH4 (3 sccm) was introduced as a carbon feedstock with the H2 gas to synthesize the graphene under pressures of 0.9-500 Torr for 1-180 min. The feedstock was then turned off, after which the TCVD reactor was cooled to room temperature (~30 °C/min) with flowing H2 gas. The uniformity and number of graphene layers were investigated by resonant Raman spectroscopy (Renishaw, RM1000 inVia) and confocal Raman spectroscopy (WiTec, CRM200) at excitation wavelengths of 514 and 532 nm, respectively. Structural characterization of synthesized graphene was performed using transmission electron microscopy (TEM; JEOL, JEM2100F). The elemental depth profile of graphene on Cu was measured using a SIMS (ION-TOF, TOF-SIMS-5) equipped with a Bi+ ion analysis gun (operating energy: 25 keV, current: 1.0 pA) and a Cs+ ion sputter gun (operating energy: 0.5 keV, current: 30.0 nA). The analysis and sputter area were 49.8 × 49.8 and 200 × 200 µm2, respectively. The expansion of the Cu lattice constant by carbon diffusion was studied by XRD (Bruker, D8 Discover).
3. Results and Discussion
Fig. 1 – Raman spectra with an excitation wavelength of 514 nm for graphene synthesized on Cu using (a) C2H2 and (b) CH4 for 1, 10, and 30 min, respectively. (c) I2D/IG and (d) FWHM of 2D-band as a function of the feedstock injection time. I2D/IG maps (excitation wavelength: 532 nm) of the graphene synthesized using C2H2 for (e) 1, (f) 10, and (g) 30 min. (h) Optical transmittance of graphene synthesized using C2 H2 for 1, 10, and 30 min and CH4 for 30 min. (i) The transmittance at 550 nm and the sheet resistance for different carbon feedstocks as a function of feedstock injection time. HR-TEM images of the graphene synthesized using C2H2 for (j) 1, (k) 10, and (l) 30 min. (m) Electron diffraction pattern corresponding to (l). (n)
A photograph of the graphene grown using C2H2 for 30 min and transferred onto a polyethylene terephthalate film.
Graphene films were grown on Cu and Ni foils using TCVD in a quartz tube furnace at 950 °C with two carbon feedstocks (C2 H2 and CH4) under carefully selected growth conditions to allow for a direct comparison. Using Raman spectroscopy, optical microscopy, and TEM, the thickness of the graphene films was determined after observations of large areas [12, 13]. We first compared the growth behavior of graphene on Cu foils for different carbon feedstocks (C2H2 and CH4) with increasing injection times using Raman spectroscopy (Fig. 1a and b). Interestingly, Raman spectra taken from each graphene film transferred onto SiO2 substrates showed discernible variations in the intensity ratio of 2D- to G-bands (I2D/IG) and the full-width at half maximum (FWHM) of the 2D-band (Fig. 1c and d). Hereafter graphene films grown by C2 H2 and CH4 are denoted by A-graphene and M-graphene, respectively. For C2H2 injection, the I2D/IG decreased from 3.5, to approximately 1.0, to below 1.0 for injection times of 1, 10, and 30 min, respectively, whereas there was a relatively small decrease upon CH4 injection for up to 30 min. Coincidentally, the FWHM of A-graphene increased abruptly after injection for 10 min, whereas it only increased marginally for Mgraphene for injection up to 30 min. Since both the I2D/IG and FWHM represent the number of graphene layers on the SiO2 substrate, monolayer, bilayer, and multilayer A-graphene were grown after 1, 10, and 30 min using C2H2, respectively, which cannot be expected from the SA growth mode using CH4. In contrast, self-limited growth of monolayer graphene was also observed in this experiment with CH4 injected for up to 30 min, consistent with a previous report (SA mode) [4]. These results were sustained, with the exception of the decreasing Dband intensity after adjusting the growth temperature to that previously reported (1050 °C,
bottom of Fig. 1b). The uniformity and variations in the number of graphene layers according to the C2H2 injection time were evaluated by confocal Raman mapping (Fig. 1e-g), optical transmittances (Fig. 1h), and TEM observations (Fig. 1j-l). The Raman maps of I2D/IG exhibited excellent uniformity in the number of A-graphene layers over large areas except for some small imperfections. The decrease in optical transmittance with increasing C2H2 injection time directly coincided with the Raman mapping and TEM results, indicating that mono-, bi, and five-layer graphene were grown after the injection times of 1, 10, and 30 min, respectively. Additionally, the sheet resistances of graphene films were 920 ± 180, 324 ± 55, and 112 ± 68 Ω/sq., corresponding to growth time of 1, 10, and 30 min, respectively (Fig. 1i). The optical transmittances of both A-graphene grown for 1 min and M-graphene grown for 30 min are almost similar (~97.4%), indicating the synthesis of monolayer graphene (Fig. 1i). In addition, a single Lorentzian fit of all 2D-bands in the Raman spectra (Fig. 1a, yellow peaks) and electron diffraction pattern (Fig. 1m) of A-graphene indicated that stacked graphene has a turbostratic structure without ordering in the c-direction [14]. This is quite different stacking behavior from the AB stacking order of multilayer graphene grown on Ni foil. The stacking order does not seem to be related to the carbon feedstock because graphene grown on Ni foil with C2 H2 also exhibited an AB stacking order (Fig. S1 in the Supplementary data). Fig. 1n is a photograph of A-graphene film on polyethylene terephthalate (size = 3 × 3 cm2). In addition, typical optical microscopy and SEM images of graphene synthesized on Cu using C2H2 for 1, 10, and 30 min were shown, respectively (Fig. S2 in the Supplementary data). We emphasize that large-area, uniform, and layer-controllable graphene films were grown on Cu foil using C2H2, which cannot be explained by the SA growth mode.
Fig. 2 – (a) Schematics showing the strategy used for exploring growth modes, DP mode vs. SA mode, of graphene on Cu and the correlation between the existence of a C-diffused Cu layer and SIMS depth profiles of (b, c, d) graphene/Ni, (e, f, g) M-graphene, (h, i, j) Agraphene, and (k, l, m) A-graphene transferred onto a new Cu substrate for carbon feedstock injection times of 1, 10, and 30 min. (n) Plots of IG/I2D and tG as a function of C2H2 injection time. How could the growth of multilayer graphene on Cu foil be possible using C2 H2 gas? To
answer this question, we established applicable models for two proposed growth modes of graphene on metal substrates that are apparently incompatible. Schematics of graphene/Cu structures and expected SIMS depth profiling results for both growth modes are illustrated (Fig. 2a). Any difference would simply be distinguished in the SIMS results by the existence of a C-diffused Cu layer (tC) between graphene (tG) and the Cu foil for the DP mode, compared with an abrupt interface (“x”-shaped curve) for the SA mode. As representative references for each growth mode, we investigated graphene grown on Ni foil for the DP mode (Fig. 2b-d) and M-graphene for the SA mode (Fig. 2e-g) with increasing carbon feedstock injection time before examining A-graphene. The graphene/Ni samples exhibited three different regions in the vertical direction of the samples that were discernible by the relative intensities of carbon and Ni (Fig. 2b-d). These regions were divided by dotted lines. The first region (tG, graphene), which was close to the surface, expanded with increasing injection time. The second region (tC) consisting of both carbon and Ni also expanded with increasing injection time, and has been reported to be a Ni carbide layer [15]. The third region corresponded Ni foil, which was supposed to be minimally involved in graphene growth. The formation of these three regions clearly indicates carbon DP during graphene growth on the Ni substrate by the DP mode. On the contrary, M-graphene exhibited an “x”-shaped curve with carbon and Cu intensities without a noticeable increase in the thickness of the graphene layer (tG), which reflects the abrupt interface expected from the SA mode. In the context of these representative results for the two growth modes, we examined Agraphene to verify its growth mode. Surprisingly, three regions analogous to the graphene/Ni samples were clearly observed (Fig. 2h-j). The tG region (graphene) expanded with increasing C2H2 injection time, while the tC region was marginally increased. The expansion of the tG
region coincided with the increase in the number of graphene layers described in Fig. 1; the two plots of IG/I2D and tG for A-graphene showed a similar increase as a function of injection time (Fig. 2n). In addition, A-graphene was transferred to clean Cu foil to examine whether the tC region originated from graphene. The results clearly showed that the “x”-shaped interface without the tC region (Fig. 2k-m) and the wide tG region compared with that of Agraphene can be attributed to residual poly(methyl methacrylate) used for transfer. The differences in the SIMS results before and after the transfer indicate that the tC region was created in Cu due to heat-driven diffusion of carbon (C-diffused Cu layer) during TCVD growth using C2H2 at high temperatures. Hence, the growth mode for A-graphene with a controllable number of layers on Cu is unambiguously the DP mode under these growth conditions. There is strong evidence for the DP of carbon, even though Cu has extremely low carbon solubility [16], and the diffusion coefficient (D) of carbon in Cu is defined by the relationship:
where D0 is the temperature-independent pre-exponential (m2/s) and EA is the activation energy for diffusion. The value of D can be calculated as 3 × 10-11 m2/s at a growth temperature of 1143 K [17].
Fig. 3 – XRD patterns of Cu(100) of (a) pristine Cu foil, (b) annealed Cu, (c) M-graphene on Cu grown under LP for 30 min. XRD patterns of A-graphene on Cu grown under LP for (d) 30, (e) 60, and (f) 120 min and M-graphene on Cu grown under HP for (g) 60, (h) 120, and (i) 180 min. (j) A schematic of the heat-driven interstitial diffusion of carbon into Cu, resulting in lattice expansion.
Based on reports in the literature [17, 18], we expected some differences in the creation of the tC region in A-graphene/Cu samples compared with the graphene/Ni sample. The tC region of A-graphene remained nearly unchanged regardless of the injection time (Fig. 2h-j), whereas the tC region of graphene/Ni gradually increased with increasing injection time (Fig. 2b-d). Further investigation using XRD was conducted on A-graphene to observe an alteration in the Cu lattice constant due to creation of the tC region (Fig. 3). Cu(111) and
predominant Cu(100) diffraction peaks were visible in the overview of the XRD pattern (Fig. S3 in the Supplementary data). The Cu(100) diffraction peaks of pristine Cu foil before (Fig. 3a) and after annealing at 950 °C in a vacuum (Fig. 3b) were simply deconvoluted into a single component, indicating that annealing without introducing carbon feedstock had no influence on the Cu lattice. The Cu(100) peak of M-graphene on Cu grown for 30 min at 900 mTorr (relatively low pressure; LP) also showed a single component (Fig. 3c). In contrast, the peak of A-graphene on Cu grown for 30 min at LP was deconvoluted into two components (Fig. 3d). The blue peak was located at the same angle as Cu(100), but the yellow peak that emerged at a lower angle indicates expansion of the Cu lattice by 0.15 % (2.67 pm). The intensity of the yellow peaks increased gradually with increasing C2 H2 injection time to 60 (Fig. 3e) and 120 min (Fig. 3f), while that of the Cu(100) peaks (blue) decreased. Considering the small expansion of the Cu lattice and the fact that Cu does not facilitate the formation of any carbide phases, we can conclude that the lattice expansion of Cu was induced by interstitial diffusion of carbon (Fig. 3j) [19], corresponding to the tC region in the SIMS results (Fig. 2h-j). Hence, the difference in the expansion of tC is attributed to the different origin of the interstitially C-diffused Cu layer for A-graphene/Cu and the Ni carbide layer for graphene/Ni. Next, we investigated whether the interstitial diffusion of carbon in Cu occurred during graphene growth using CH4 with increasing injection time (Fig. 3g-h). There was no expansion of the Cu lattice under the same working pressure (LP) for 180 min (data not shown); however, lattice expansion was surprisingly observed after graphene growth for 180 min at 500 Torr (high pressure; HP) (Fig. 3h). Optical microscopy image and Raman spectra showed that multilayer graphene was partially grown, and the coverage of the multilayer graphene increased in proportion to the CH4 injection time (Fig. S4 in the Supplementary
data). This indicates that graphene can be grown on a Cu substrate by the DP mode, even using CH4 under different growth conditions.
Fig. 4 – Plot of the number of graphene layers vs. working pressure according to carbon feedstocks of CH4, C2H4, C6H14, and C2H2 (this work).
Using two different carbon feedstocks, we determined that the two growth modes can be selectively applied to graphene growth on Cu by controlling the working pressure, i.e., the partial pressure of the carbon feedstock. To reinforce these findings, data from previous studies performed using various carb on feedstocks under different pressures are displayed in Fig. 4 [3, 5, 20-28]. With CH4, monolayer graphene was grown at a working pressure of ~100 Torr, while multilayer graphene was grown at a higher working pressure of ~103 Torr. With C2H4, monolayer graphene was grown at an extremely low working pressure (ELP) than with CH4, but the pressure required for multilayer graphene growth was similar (~100 Torr). Interestingly, with C2H2, the growth of mono-, bi-, and multi-layer graphene was governed by the DP mode at the same working pressure, and the number of graphene layers was controlled
simply by varying the growth time. However, the SA mode for C2H2 was also observed in our recent study using angle-resolved photoemission spectroscopy, in which monolayer Agraphene was grown under working pressure of ~10-6 Torr (ELP) (not shown here). These results can be classified roughly into two types, those for CH4 and those for CxHy gases, illustrated by the lines in Fig. 4. The plots clearly show that adjusting the working pressure according to the carbon feedstock allows selection of the growth mode to be either SA or DP. The pressure differences for the feedstocks can be understood based on the different reaction trends of CxHy (higher reactive gases) and CH4 gases on the Cu surface. CxHy gases are easily tethered to the surface of Cu by π-π interactions, whereas SA of CH4 is thermodynamically unfavorable. According to a previous report, the adsorption and dissociation processes of C2H2 and CH4 are completely different (Fig. S5 in the Supplementary data). C2H2 adsorbs without dissociation, whereas CH4 can adsorb only by dissociative adsorption. Hence, the sticking coefficients on Cu at zero coverage of C2H2 and CH4 are 1 and 8.6 × 10 -9, respectively [29, 30]. A higher surface reactivity of C2H2 for graphene growth compared with CH4 was also observed in the growth of graphitic structures on SiO2 substrates (Fig. S6 in the Supplementary data). This means that the concentration of carbon adatoms on the Cu surface extracted from C2H2 is greater than that of extracted from CH4. Since atomic diffusion can be generated by a concentration gradient, this explains why the diffusion of carbon atoms dissociated from C2H2 is possible under the lower pressure than that of CH4.
Fig. 5 – (a) Depth profiles of relative carbon concentration for graphene synthesized on Cu and Ni using C2 H2 and CH4 under ELP, LP and HP. This relationship is calculated by Fick’s second law. C(x, t) is the concentration of carbon at depth x for time t. (b) Plot of initial concentration of carbon vs. feedstock partial pressure × injection time for graphene synthesized on Cu and Ni using C2H2 and CH4.
By solving Fick’s second law, the penetration of carbon is then described by the relationship:
where C(x, t) is the concentration of carbon at penetration distance x at time t, CS is the initial surface carbon concentration, and C0 is the intrinsic carbon concentration in Cu. D is the diffusion coefficient of the diffusing species. Gaussian error function (erf) is defined by
The erf values are given in Table S1 in the Supplementary data. Fig. 5a exhibits the concentration of diffused carbon (C(x,t)) as a function of penetration depth (x) of Cu for graphene synthesized on Cu and Ni using C2 H2 and CH4 under LP, ELP, and HP. C(x,t) values for all samples were normalized to C(x,t) for C2H2, Cu, 1 min, and LP (red line). In our system, C0 and D correspond to 0.01 %, 3 × 10-11 (Cu), and 1 × 10 -10 m2/s (Ni) with fixed t = 60 sec [17]. This result reveals that the C(x, t) decreased with increasing the depth of Cu regardless of the sample. Notably, however, a significant difference of Cs (C(x=0, t=0) for each sample, determined by the sticking coefficients, partial pressure of carbon feedstock, and feedstock injection time, seems to be the main contributor to the concentration of diffused carbon in Cu (Table S2 in the Supplementary data). Based on the well-established growth mode of graphene on Ni (green and brown lines), we classified that the growth mode of the graphene synthesized on Cu using C2 H2 under LP and using CH4 under HP could be governed by the DP mode, whereas that of the graphene synthesized on Cu using C2 H2 under ELP and using CH4 under LP seems to be the SA mode. In fact, thermal diffusion of carbon in Cu is unambiguously observed for all cases. However, the growth mode of graphene on Cu can be classified into the two types because the concentration of diffused carbon of the sample assigned as the SA mode seems to be below the detection limit. Fig. 5b reveals the plot of Cs as a function of feedstock partial pressure × injection time for graphene synthesized on Cu and Ni using C2H2 and CH4. This result suggested that Cs is proportional to feedstock partial pressure × injection time according to the types of feedstock and catalytic substrate, which is the crucial factor that determines the growth mode of graphene. Two trend lines have a similar slope, indicating that our suggestion could be universally acceptable. Jing Kong’s group presented the concentration of carbon feedstock was influenced by growth
pressure, determining the mass transport and surface reaction [5]. Thereby, the growth of graphene on Cu under atmospheric pressure with a high methane concentration is not selflimiting and they observed multilayer domains on a monolayer graphene. In an analogous manner, our results revealed that the number of graphene layers could be selected by tuning the growth pressure. Moreover, we suggested their detailed growth modes and the carbon feedstock dependency.
4. Summary We found that either SA or DP growth modes can be selected for the growth of monoand multi-layer graphene on Cu substrates by adjusting the partial pressure corresponding to the carbon feedstock. This finding is a significant breakthrough since it allows control of the number of graphene layers toward the development of graphene-based multifaceted applications. In addition, C2H2 might be a promising carbon feedstock because it yields rapid growth of high quality monolayer graphene and enables simple control of the number of graphene layers by adjusting only its injection time.
Acknowledgement This work was supported by Basic Science Research Program (NRF-2012R1A1A2041021) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) and by the Converging Research Center Program (2012K001301) and by a grant (2011-0031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea.
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Figure captions
Fig. 1 – Raman spectra with an excitation wavelength of 514 nm for graphene synthesized on Cu using (a) C2H2 and (b) CH4 for 1, 10, and 30 min, respectively. (c) I2D/IG and (d) FWHM of 2D-band as a function of the feedstock injection time. I2D/IG maps (excitation wavelength: 532 nm) of the graphene synthesized using C2H2 for (e) 1, (f) 10, and (g) 30 min. (h) Optical transmittance of graphene synthesized using C2 H2 for 1, 10, and 30 min and CH4 for 30 min. The inset compared the transmittance at 550 nm for different carbon feedstocks and injection times. HR-TEM images of the graphene synthesized using C2 H2 for (i) 1, (j) 10, and (k) 30 min. (l) Electron diffraction pattern corresponding to (k). (m) A photograph of the graphene grown using C2H2 for 30 min and transferred onto a polyethylene terephthalate film.
Fig. 2 – (a) Schematics showing the strategy used for exploring growth modes, DP mode vs. SA mode, of graphene on Cu and the correlation between the existence of a C-diffused Cu layer and SIMS depth profiles of (b, c, d) graphene/Ni, (e, f, g) M-graphene, (h, i, j) Agraphene, and (k, l, m) A-graphene transferred onto a new Cu substrate for carbon feedstock injection times of 1, 10, and 30 min. (n) Plots of IG/I2D and tG as a function of C2H2 injection
time.
Fig. 3 – XRD patterns of Cu(100) of (a) pristine Cu foil, (b) annealed Cu, (c) M-graphene on Cu grown under LP for 30 min. XRD patterns of A-graphene on Cu grown under LP for (d) 30, (e) 60, and (f) 120 min and M-graphene on Cu grown under HP for (g) 60, (h) 120, and (i) 180 min. (j) A schematic of the heat-driven interstitial diffusion of carbon into Cu, resulting in lattice expansion.
Fig. 4 – Plot of the number of graphene layers vs. working pressure according to carbon feedstocks of CH4, C2H4, C6H14, and C2H2 (this work).
Fig. 5 – (a) Depth profiles of relative carbon concentration for graphene synthesized on Cu and Ni using C2 H2 and CH4 under ELP, LP and HP. This relationship is calculated by Fick’s second law. C(x, t) is the concentration of carbon at depth x for time t. (b) Plot of initial concentration of carbon vs. feedstock partial pressure × injection time for graphene synthesized on Cu and Ni using C2H2 and CH4.
S0008-6223(13)00998-6 http://dx.doi.org/10.1016/j.carbon.2013.10.039 CARBON 8475
To appear in:
Carbon
Received Date: Accepted Date:
3 June 2013 14 October 2013
Please cite this article as: Song, W., Jeon, C., Kim, S.Y., Kim, Y., Kim, S.H., Lee, S-I., Jung, D.S., Jung, M.W., An, K-S., Park, C-Y., Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition, Carbon (2013), doi: http://dx.doi.org/10.1016/j.carbon.2013.10.039
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Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition
Wooseok Song1,2, Cheolho Jeon3, Soo Youn Kim1, Yooseok Kim1, Sung Hwan Kim1, Su-Il Lee1, Dae Sung Jung4, Min Wook Jung2, Ki-Seok An2, and Chong-Yun Park1,4,*
1
BK21 Physics Research Division, Sungkyunkwan University, Suwon 440-746, Republic of Korea
2
Thin Film Materials Research Group, Korea Research Institute of Chemical Technology (KRICT), Yuseong P. O. Box 107, Daejeon 305-600, Republic of Korea
3
Division of Materials Science, Korea Basic Science Institute, Daejeon, 305-333, Republic of Korea 4
Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
*
corresponding author. Tel.: +82-31-299-6501. Fax: +82-31-299-6505. E-mail address: [email protected] (Chong-Yun Park).
Abstract Here we provide evidence of two selective growth modes, namely the ‘surface adsorption (SA) mode’ and the ‘diffusion and precipitation (DP) mode’ for the synthesis of graphene on Cu foil by thermal chemical vapor deposition. Using acetylene feedstock, the number of graphene layers was controlled simply by adjusting the injection time, and the DP growth mode was clearly verified by the existence of a carbon-diffused Cu layer with expansion of the Cu lattice. With methane feedstock, either SA or DP growth modes could be selected for the growth of graphene at low or high partial pressure of carbon feedstocks, respectively. The critical pressure for switching the growth modes depends on reactivity of carbon feedstock to Cu substrate.
1. Introduction When thin films grow epitaxially on a crystal surface, the growth generally occurs through one of the three primary modes: Volmer-Weber (island formation), Frank-van der Merwe (layer-by-layer), or Stranski-Krastanov (layer-plus-island) growth [1]. The growth mode is determined by the competition between kinetics and thermodynamics. This fundamental understanding enables us to precisely control the growth and thus the properties of deposited films. For graphene growth on transition metal substrates using a thermal chemical vapor deposition (TCVD), there are two suggested growth modes: diffusion and precipitation (DP) and surface adsorption (SA). These modes have been considered to govern the growth of graphene on Ni and Cu due to the great difference in carbon solubility between these substrates. The high carbon solubility of Ni causes to dissolve into the bulk, and graphene subsequently grows by surface segregation of carbon upon cooling from a metastable carbon-metal solid solution (DP mode) [2, 3]. On the contrary, the extremely low carbon solubility of Cu leads to self-limited growth of graphene by simple thermal decomposition of hydrocarbons on the Cu surface (SA mode) [4]. As a result of these different growth modes, graphene grown on Ni has an inhomogeneous thickness distribution, whereas predominantly monolayer graphene is grown on Cu. This leads to an inherent problem; if graphene growth on any catalytic metal substrates obeys one of these two growth modes, then it is not possible achieve layer-controlled graphene growth, which will be in high demand for future applications. Fortunately, graphene growth on a Cu substrate can be a model system with which to study the possibility of selecting and manipulating the growth mode. Several experiments have been conducted with different working pressures and cooling rates, resulting in the growth of multilayer and bilayer graphene on Cu foil under atmospheric pressure with high
CH4 concentration [5] and using a slow cooling process [6, 7], respectively. These results indicate that the SA mode is not an exclusive growth mode for graphene on a Cu substrate. However, a detailed mechanism for the growth of a second graphene layer on a Cu substrate remains elusive. Since previous growth mechanism studies were conducted mostly using CH4 as a carbon feedstock [3-7], the results cannot be universally applied. Reference to previous research on carbon nanotubes (CNTs) provides a good understanding of the influence of the carbon feedstock on the growth of graphitic structures. It is generally accepted that CH4 feedstock is used mainly for the synthesis of highly crystalline single-walled CNTs, whereas C2H2 feedstock is employed for the high-yield growth of CNTs [8-11]. This feedstock specificity can be explained by the different reaction rate of carbon feedstocks with catalytic metals. It should be noted that considerable dependence on the hydrocarbon feedstock in TCVD growth of graphene has not been reported using Ni substrate, presumably because of its high carbon solubility. However, an in-depth study using different carbon feedstocks to elucidate the growth mechanism of graphene on a Cu substrate would be worthwhile. Here, we present evidence for two selective growth modes of graphene on Cu with various carbon feedstocks (C2H2 and CH4) and working pressures. The number of graphene layers grown by C2 H2 increased with increasing injection time. A combined secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) study revealed a carbon-diffused Cu layer created at the surface region of the Cu substrate with expansion of the Cu lattice. The graphene was grown on Cu by the DP mode rather than the SA mode, as evidenced by similar results in a graphene/Ni system. Interestingly, the carbon-diffused Cu layer was also observed after graphene growth under high CH4 pressure. Previous results and our data demonstrate two selective growth modes of graphene on a Cu substrate, and show that the desired mode
can be selected by tuning the working pressure according to the kind of carbon feedstock. We believe that this finding will shed light on the growth mechanism of graphene on a Cu substrate and will enable to grow layer-controlled graphene for its multifaceted applications.
2. Experimental Section Graphene was synthesized using a conventional TCVD system. We used 25-µm-thick Cu foil or 50-µm-thick Ni foil as the catalytic substrate for graphene synthesis. The catalytic substrate was heated to 950 °C inside the TCVD reactor with H2 gas flowing at 100 sccm. C2H2 (3 sccm) or CH4 (3 sccm) was introduced as a carbon feedstock with the H2 gas to synthesize the graphene under pressures of 0.9-500 Torr for 1-180 min. The feedstock was then turned off, after which the TCVD reactor was cooled to room temperature (~30 °C/min) with flowing H2 gas. The uniformity and number of graphene layers were investigated by resonant Raman spectroscopy (Renishaw, RM1000 inVia) and confocal Raman spectroscopy (WiTec, CRM200) at excitation wavelengths of 514 and 532 nm, respectively. Structural characterization of synthesized graphene was performed using transmission electron microscopy (TEM; JEOL, JEM2100F). The elemental depth profile of graphene on Cu was measured using a SIMS (ION-TOF, TOF-SIMS-5) equipped with a Bi+ ion analysis gun (operating energy: 25 keV, current: 1.0 pA) and a Cs+ ion sputter gun (operating energy: 0.5 keV, current: 30.0 nA). The analysis and sputter area were 49.8 × 49.8 and 200 × 200 µm2, respectively. The expansion of the Cu lattice constant by carbon diffusion was studied by XRD (Bruker, D8 Discover).
3. Results and Discussion
Fig. 1 – Raman spectra with an excitation wavelength of 514 nm for graphene synthesized on Cu using (a) C2H2 and (b) CH4 for 1, 10, and 30 min, respectively. (c) I2D/IG and (d) FWHM of 2D-band as a function of the feedstock injection time. I2D/IG maps (excitation wavelength: 532 nm) of the graphene synthesized using C2H2 for (e) 1, (f) 10, and (g) 30 min. (h) Optical transmittance of graphene synthesized using C2 H2 for 1, 10, and 30 min and CH4 for 30 min. (i) The transmittance at 550 nm and the sheet resistance for different carbon feedstocks as a function of feedstock injection time. HR-TEM images of the graphene synthesized using C2H2 for (j) 1, (k) 10, and (l) 30 min. (m) Electron diffraction pattern corresponding to (l). (n)
A photograph of the graphene grown using C2H2 for 30 min and transferred onto a polyethylene terephthalate film.
Graphene films were grown on Cu and Ni foils using TCVD in a quartz tube furnace at 950 °C with two carbon feedstocks (C2 H2 and CH4) under carefully selected growth conditions to allow for a direct comparison. Using Raman spectroscopy, optical microscopy, and TEM, the thickness of the graphene films was determined after observations of large areas [12, 13]. We first compared the growth behavior of graphene on Cu foils for different carbon feedstocks (C2H2 and CH4) with increasing injection times using Raman spectroscopy (Fig. 1a and b). Interestingly, Raman spectra taken from each graphene film transferred onto SiO2 substrates showed discernible variations in the intensity ratio of 2D- to G-bands (I2D/IG) and the full-width at half maximum (FWHM) of the 2D-band (Fig. 1c and d). Hereafter graphene films grown by C2 H2 and CH4 are denoted by A-graphene and M-graphene, respectively. For C2H2 injection, the I2D/IG decreased from 3.5, to approximately 1.0, to below 1.0 for injection times of 1, 10, and 30 min, respectively, whereas there was a relatively small decrease upon CH4 injection for up to 30 min. Coincidentally, the FWHM of A-graphene increased abruptly after injection for 10 min, whereas it only increased marginally for Mgraphene for injection up to 30 min. Since both the I2D/IG and FWHM represent the number of graphene layers on the SiO2 substrate, monolayer, bilayer, and multilayer A-graphene were grown after 1, 10, and 30 min using C2H2, respectively, which cannot be expected from the SA growth mode using CH4. In contrast, self-limited growth of monolayer graphene was also observed in this experiment with CH4 injected for up to 30 min, consistent with a previous report (SA mode) [4]. These results were sustained, with the exception of the decreasing Dband intensity after adjusting the growth temperature to that previously reported (1050 °C,
bottom of Fig. 1b). The uniformity and variations in the number of graphene layers according to the C2H2 injection time were evaluated by confocal Raman mapping (Fig. 1e-g), optical transmittances (Fig. 1h), and TEM observations (Fig. 1j-l). The Raman maps of I2D/IG exhibited excellent uniformity in the number of A-graphene layers over large areas except for some small imperfections. The decrease in optical transmittance with increasing C2H2 injection time directly coincided with the Raman mapping and TEM results, indicating that mono-, bi, and five-layer graphene were grown after the injection times of 1, 10, and 30 min, respectively. Additionally, the sheet resistances of graphene films were 920 ± 180, 324 ± 55, and 112 ± 68 Ω/sq., corresponding to growth time of 1, 10, and 30 min, respectively (Fig. 1i). The optical transmittances of both A-graphene grown for 1 min and M-graphene grown for 30 min are almost similar (~97.4%), indicating the synthesis of monolayer graphene (Fig. 1i). In addition, a single Lorentzian fit of all 2D-bands in the Raman spectra (Fig. 1a, yellow peaks) and electron diffraction pattern (Fig. 1m) of A-graphene indicated that stacked graphene has a turbostratic structure without ordering in the c-direction [14]. This is quite different stacking behavior from the AB stacking order of multilayer graphene grown on Ni foil. The stacking order does not seem to be related to the carbon feedstock because graphene grown on Ni foil with C2 H2 also exhibited an AB stacking order (Fig. S1 in the Supplementary data). Fig. 1n is a photograph of A-graphene film on polyethylene terephthalate (size = 3 × 3 cm2). In addition, typical optical microscopy and SEM images of graphene synthesized on Cu using C2H2 for 1, 10, and 30 min were shown, respectively (Fig. S2 in the Supplementary data). We emphasize that large-area, uniform, and layer-controllable graphene films were grown on Cu foil using C2H2, which cannot be explained by the SA growth mode.
Fig. 2 – (a) Schematics showing the strategy used for exploring growth modes, DP mode vs. SA mode, of graphene on Cu and the correlation between the existence of a C-diffused Cu layer and SIMS depth profiles of (b, c, d) graphene/Ni, (e, f, g) M-graphene, (h, i, j) Agraphene, and (k, l, m) A-graphene transferred onto a new Cu substrate for carbon feedstock injection times of 1, 10, and 30 min. (n) Plots of IG/I2D and tG as a function of C2H2 injection time. How could the growth of multilayer graphene on Cu foil be possible using C2 H2 gas? To
answer this question, we established applicable models for two proposed growth modes of graphene on metal substrates that are apparently incompatible. Schematics of graphene/Cu structures and expected SIMS depth profiling results for both growth modes are illustrated (Fig. 2a). Any difference would simply be distinguished in the SIMS results by the existence of a C-diffused Cu layer (tC) between graphene (tG) and the Cu foil for the DP mode, compared with an abrupt interface (“x”-shaped curve) for the SA mode. As representative references for each growth mode, we investigated graphene grown on Ni foil for the DP mode (Fig. 2b-d) and M-graphene for the SA mode (Fig. 2e-g) with increasing carbon feedstock injection time before examining A-graphene. The graphene/Ni samples exhibited three different regions in the vertical direction of the samples that were discernible by the relative intensities of carbon and Ni (Fig. 2b-d). These regions were divided by dotted lines. The first region (tG, graphene), which was close to the surface, expanded with increasing injection time. The second region (tC) consisting of both carbon and Ni also expanded with increasing injection time, and has been reported to be a Ni carbide layer [15]. The third region corresponded Ni foil, which was supposed to be minimally involved in graphene growth. The formation of these three regions clearly indicates carbon DP during graphene growth on the Ni substrate by the DP mode. On the contrary, M-graphene exhibited an “x”-shaped curve with carbon and Cu intensities without a noticeable increase in the thickness of the graphene layer (tG), which reflects the abrupt interface expected from the SA mode. In the context of these representative results for the two growth modes, we examined Agraphene to verify its growth mode. Surprisingly, three regions analogous to the graphene/Ni samples were clearly observed (Fig. 2h-j). The tG region (graphene) expanded with increasing C2H2 injection time, while the tC region was marginally increased. The expansion of the tG
region coincided with the increase in the number of graphene layers described in Fig. 1; the two plots of IG/I2D and tG for A-graphene showed a similar increase as a function of injection time (Fig. 2n). In addition, A-graphene was transferred to clean Cu foil to examine whether the tC region originated from graphene. The results clearly showed that the “x”-shaped interface without the tC region (Fig. 2k-m) and the wide tG region compared with that of Agraphene can be attributed to residual poly(methyl methacrylate) used for transfer. The differences in the SIMS results before and after the transfer indicate that the tC region was created in Cu due to heat-driven diffusion of carbon (C-diffused Cu layer) during TCVD growth using C2H2 at high temperatures. Hence, the growth mode for A-graphene with a controllable number of layers on Cu is unambiguously the DP mode under these growth conditions. There is strong evidence for the DP of carbon, even though Cu has extremely low carbon solubility [16], and the diffusion coefficient (D) of carbon in Cu is defined by the relationship:
where D0 is the temperature-independent pre-exponential (m2/s) and EA is the activation energy for diffusion. The value of D can be calculated as 3 × 10-11 m2/s at a growth temperature of 1143 K [17].
Fig. 3 – XRD patterns of Cu(100) of (a) pristine Cu foil, (b) annealed Cu, (c) M-graphene on Cu grown under LP for 30 min. XRD patterns of A-graphene on Cu grown under LP for (d) 30, (e) 60, and (f) 120 min and M-graphene on Cu grown under HP for (g) 60, (h) 120, and (i) 180 min. (j) A schematic of the heat-driven interstitial diffusion of carbon into Cu, resulting in lattice expansion.
Based on reports in the literature [17, 18], we expected some differences in the creation of the tC region in A-graphene/Cu samples compared with the graphene/Ni sample. The tC region of A-graphene remained nearly unchanged regardless of the injection time (Fig. 2h-j), whereas the tC region of graphene/Ni gradually increased with increasing injection time (Fig. 2b-d). Further investigation using XRD was conducted on A-graphene to observe an alteration in the Cu lattice constant due to creation of the tC region (Fig. 3). Cu(111) and
predominant Cu(100) diffraction peaks were visible in the overview of the XRD pattern (Fig. S3 in the Supplementary data). The Cu(100) diffraction peaks of pristine Cu foil before (Fig. 3a) and after annealing at 950 °C in a vacuum (Fig. 3b) were simply deconvoluted into a single component, indicating that annealing without introducing carbon feedstock had no influence on the Cu lattice. The Cu(100) peak of M-graphene on Cu grown for 30 min at 900 mTorr (relatively low pressure; LP) also showed a single component (Fig. 3c). In contrast, the peak of A-graphene on Cu grown for 30 min at LP was deconvoluted into two components (Fig. 3d). The blue peak was located at the same angle as Cu(100), but the yellow peak that emerged at a lower angle indicates expansion of the Cu lattice by 0.15 % (2.67 pm). The intensity of the yellow peaks increased gradually with increasing C2 H2 injection time to 60 (Fig. 3e) and 120 min (Fig. 3f), while that of the Cu(100) peaks (blue) decreased. Considering the small expansion of the Cu lattice and the fact that Cu does not facilitate the formation of any carbide phases, we can conclude that the lattice expansion of Cu was induced by interstitial diffusion of carbon (Fig. 3j) [19], corresponding to the tC region in the SIMS results (Fig. 2h-j). Hence, the difference in the expansion of tC is attributed to the different origin of the interstitially C-diffused Cu layer for A-graphene/Cu and the Ni carbide layer for graphene/Ni. Next, we investigated whether the interstitial diffusion of carbon in Cu occurred during graphene growth using CH4 with increasing injection time (Fig. 3g-h). There was no expansion of the Cu lattice under the same working pressure (LP) for 180 min (data not shown); however, lattice expansion was surprisingly observed after graphene growth for 180 min at 500 Torr (high pressure; HP) (Fig. 3h). Optical microscopy image and Raman spectra showed that multilayer graphene was partially grown, and the coverage of the multilayer graphene increased in proportion to the CH4 injection time (Fig. S4 in the Supplementary
data). This indicates that graphene can be grown on a Cu substrate by the DP mode, even using CH4 under different growth conditions.
Fig. 4 – Plot of the number of graphene layers vs. working pressure according to carbon feedstocks of CH4, C2H4, C6H14, and C2H2 (this work).
Using two different carbon feedstocks, we determined that the two growth modes can be selectively applied to graphene growth on Cu by controlling the working pressure, i.e., the partial pressure of the carbon feedstock. To reinforce these findings, data from previous studies performed using various carb on feedstocks under different pressures are displayed in Fig. 4 [3, 5, 20-28]. With CH4, monolayer graphene was grown at a working pressure of ~100 Torr, while multilayer graphene was grown at a higher working pressure of ~103 Torr. With C2H4, monolayer graphene was grown at an extremely low working pressure (ELP) than with CH4, but the pressure required for multilayer graphene growth was similar (~100 Torr). Interestingly, with C2H2, the growth of mono-, bi-, and multi-layer graphene was governed by the DP mode at the same working pressure, and the number of graphene layers was controlled
simply by varying the growth time. However, the SA mode for C2H2 was also observed in our recent study using angle-resolved photoemission spectroscopy, in which monolayer Agraphene was grown under working pressure of ~10-6 Torr (ELP) (not shown here). These results can be classified roughly into two types, those for CH4 and those for CxHy gases, illustrated by the lines in Fig. 4. The plots clearly show that adjusting the working pressure according to the carbon feedstock allows selection of the growth mode to be either SA or DP. The pressure differences for the feedstocks can be understood based on the different reaction trends of CxHy (higher reactive gases) and CH4 gases on the Cu surface. CxHy gases are easily tethered to the surface of Cu by π-π interactions, whereas SA of CH4 is thermodynamically unfavorable. According to a previous report, the adsorption and dissociation processes of C2H2 and CH4 are completely different (Fig. S5 in the Supplementary data). C2H2 adsorbs without dissociation, whereas CH4 can adsorb only by dissociative adsorption. Hence, the sticking coefficients on Cu at zero coverage of C2H2 and CH4 are 1 and 8.6 × 10 -9, respectively [29, 30]. A higher surface reactivity of C2H2 for graphene growth compared with CH4 was also observed in the growth of graphitic structures on SiO2 substrates (Fig. S6 in the Supplementary data). This means that the concentration of carbon adatoms on the Cu surface extracted from C2H2 is greater than that of extracted from CH4. Since atomic diffusion can be generated by a concentration gradient, this explains why the diffusion of carbon atoms dissociated from C2H2 is possible under the lower pressure than that of CH4.
Fig. 5 – (a) Depth profiles of relative carbon concentration for graphene synthesized on Cu and Ni using C2 H2 and CH4 under ELP, LP and HP. This relationship is calculated by Fick’s second law. C(x, t) is the concentration of carbon at depth x for time t. (b) Plot of initial concentration of carbon vs. feedstock partial pressure × injection time for graphene synthesized on Cu and Ni using C2H2 and CH4.
By solving Fick’s second law, the penetration of carbon is then described by the relationship:
where C(x, t) is the concentration of carbon at penetration distance x at time t, CS is the initial surface carbon concentration, and C0 is the intrinsic carbon concentration in Cu. D is the diffusion coefficient of the diffusing species. Gaussian error function (erf) is defined by
The erf values are given in Table S1 in the Supplementary data. Fig. 5a exhibits the concentration of diffused carbon (C(x,t)) as a function of penetration depth (x) of Cu for graphene synthesized on Cu and Ni using C2 H2 and CH4 under LP, ELP, and HP. C(x,t) values for all samples were normalized to C(x,t) for C2H2, Cu, 1 min, and LP (red line). In our system, C0 and D correspond to 0.01 %, 3 × 10-11 (Cu), and 1 × 10 -10 m2/s (Ni) with fixed t = 60 sec [17]. This result reveals that the C(x, t) decreased with increasing the depth of Cu regardless of the sample. Notably, however, a significant difference of Cs (C(x=0, t=0) for each sample, determined by the sticking coefficients, partial pressure of carbon feedstock, and feedstock injection time, seems to be the main contributor to the concentration of diffused carbon in Cu (Table S2 in the Supplementary data). Based on the well-established growth mode of graphene on Ni (green and brown lines), we classified that the growth mode of the graphene synthesized on Cu using C2 H2 under LP and using CH4 under HP could be governed by the DP mode, whereas that of the graphene synthesized on Cu using C2 H2 under ELP and using CH4 under LP seems to be the SA mode. In fact, thermal diffusion of carbon in Cu is unambiguously observed for all cases. However, the growth mode of graphene on Cu can be classified into the two types because the concentration of diffused carbon of the sample assigned as the SA mode seems to be below the detection limit. Fig. 5b reveals the plot of Cs as a function of feedstock partial pressure × injection time for graphene synthesized on Cu and Ni using C2H2 and CH4. This result suggested that Cs is proportional to feedstock partial pressure × injection time according to the types of feedstock and catalytic substrate, which is the crucial factor that determines the growth mode of graphene. Two trend lines have a similar slope, indicating that our suggestion could be universally acceptable. Jing Kong’s group presented the concentration of carbon feedstock was influenced by growth
pressure, determining the mass transport and surface reaction [5]. Thereby, the growth of graphene on Cu under atmospheric pressure with a high methane concentration is not selflimiting and they observed multilayer domains on a monolayer graphene. In an analogous manner, our results revealed that the number of graphene layers could be selected by tuning the growth pressure. Moreover, we suggested their detailed growth modes and the carbon feedstock dependency.
4. Summary We found that either SA or DP growth modes can be selected for the growth of monoand multi-layer graphene on Cu substrates by adjusting the partial pressure corresponding to the carbon feedstock. This finding is a significant breakthrough since it allows control of the number of graphene layers toward the development of graphene-based multifaceted applications. In addition, C2H2 might be a promising carbon feedstock because it yields rapid growth of high quality monolayer graphene and enables simple control of the number of graphene layers by adjusting only its injection time.
Acknowledgement This work was supported by Basic Science Research Program (NRF-2012R1A1A2041021) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) and by the Converging Research Center Program (2012K001301) and by a grant (2011-0031636) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea.
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Figure captions
Fig. 1 – Raman spectra with an excitation wavelength of 514 nm for graphene synthesized on Cu using (a) C2H2 and (b) CH4 for 1, 10, and 30 min, respectively. (c) I2D/IG and (d) FWHM of 2D-band as a function of the feedstock injection time. I2D/IG maps (excitation wavelength: 532 nm) of the graphene synthesized using C2H2 for (e) 1, (f) 10, and (g) 30 min. (h) Optical transmittance of graphene synthesized using C2 H2 for 1, 10, and 30 min and CH4 for 30 min. The inset compared the transmittance at 550 nm for different carbon feedstocks and injection times. HR-TEM images of the graphene synthesized using C2 H2 for (i) 1, (j) 10, and (k) 30 min. (l) Electron diffraction pattern corresponding to (k). (m) A photograph of the graphene grown using C2H2 for 30 min and transferred onto a polyethylene terephthalate film.
Fig. 2 – (a) Schematics showing the strategy used for exploring growth modes, DP mode vs. SA mode, of graphene on Cu and the correlation between the existence of a C-diffused Cu layer and SIMS depth profiles of (b, c, d) graphene/Ni, (e, f, g) M-graphene, (h, i, j) Agraphene, and (k, l, m) A-graphene transferred onto a new Cu substrate for carbon feedstock injection times of 1, 10, and 30 min. (n) Plots of IG/I2D and tG as a function of C2H2 injection
time.
Fig. 3 – XRD patterns of Cu(100) of (a) pristine Cu foil, (b) annealed Cu, (c) M-graphene on Cu grown under LP for 30 min. XRD patterns of A-graphene on Cu grown under LP for (d) 30, (e) 60, and (f) 120 min and M-graphene on Cu grown under HP for (g) 60, (h) 120, and (i) 180 min. (j) A schematic of the heat-driven interstitial diffusion of carbon into Cu, resulting in lattice expansion.
Fig. 4 – Plot of the number of graphene layers vs. working pressure according to carbon feedstocks of CH4, C2H4, C6H14, and C2H2 (this work).
Fig. 5 – (a) Depth profiles of relative carbon concentration for graphene synthesized on Cu and Ni using C2 H2 and CH4 under ELP, LP and HP. This relationship is calculated by Fick’s second law. C(x, t) is the concentration of carbon at depth x for time t. (b) Plot of initial concentration of carbon vs. feedstock partial pressure × injection time for graphene synthesized on Cu and Ni using C2H2 and CH4.