Article pubs.acs.org/JPCC
Size Dependence of Compressive Strain in Graphene Flakes Directly Grown on SiO2/Si Substrate Yuqing Song,† Jinyang Liu,†,‡ Lin Quan,† Nan Pan,§,∥ Hong Zhu,† and Xiaoping Wang*,†,§,∥ †
Department of Physics, University of Science and Technology of China, Hefei 230026, P. R. China College of Physics and Energy, Fujian Provincial Key Laboratory of Quantum Manipulation and New Energy Materials, Fujian Normal University, Fuzhou 350108, P. R. China § Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, P. R. China ∥ Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡
ABSTRACT: Graphene flakes with various sizes are prepared directly on SiO2/Si substrate using a remote catalytic method. The flakes are characterized by Raman spectra, and the peak of the 2D band shows a drastic blue shift due to the in-plane compressive strain induced by the different coefficients of thermal expansion between the graphene and the substrate. More importantly, the compressive strain of the flake is found to increase with flake size. The behavior can be understood by the strain releasing through the defects and the edge of graphene flakes. Additionally, we find that the defects contribute more than 75% of strain relaxation for all graphene samples, and the width of the edge region for the strain relaxation increases from ∼50 to ∼110 nm for 2 and 5 h grown flakes. Our finding indicates that the compressive strain inevitably exists in the as-grown graphene on SiO2/Si, which may be eliminated through a new preparation method.
1. INTRODUCTION Graphene, a single flat layer of carbon atoms arranged in a honeycomb lattice, has attracted great attention because of its fundamental properties of two-dimensional nanostructures and wide potential applications.1 Recent development of large-scale graphene synthesis by chemical vapor deposition (CVD) opened the possibility of a wide range of applications of graphene in electronics.2−4 However, complicated and skilled post-treatment techniques have to be employed to remove the metallic catalysts and transfer the graphene to the substrate of interest (such as SiO2/Si, quartz), which will inevitably result in the degradation of materials in many cases due to various contaminations, wrinkles, and unintentional doping.5−7 In order to diminish the above drawbacks, many efforts have been made to directly grow graphene on the target insulated substrates,8−12 including the remote catalytic method13 and the copper-vapor-assisted CVD,14 in which the catalyst metal as a vapor reacts with the carbon precursor either in the gas or on the surface of the substrate. Due to the distinctly different coefficients of thermal expansion (CTE) between graphene and the SiO2/Si, the substrate-induced compressive strain always exists in the graphene film directly grown on the SiO2/Si substrates during the CVD process.8,13,15 The compressive strain can be well observed and characterized by the blue shift in the Raman peak of the 2D band of graphene.8,13,15−18 However, the reported peak positions of 2D bands show clear discrepancy in previous © XXXX American Chemical Society
works; the reason may be ascribed to the different unintentional doping or the inhomogeneous strain in the graphene.8,12,19 Moreover, most of the graphene films grown on SiO2/Si substrate are coalesced by an amount of flakes, while the dependence of the compressive strain on the flake size has not yet been investigated. In this work, we grew the graphene flakes with various sizes directly on the SiO2/Si substrate through the remote catalytic method. We find that the compressive strain in the graphene flake increases with its size and reaches saturation as the flakes fully cover the substrate. A model based on the strain relaxation through the defect and edge region of the graphene flake has been proposed, and the fitting result is in good agreement with our experimental observation.
2. EXPERIMENTAL SECTION The graphene flakes were directly grown onto the SiO2/Si substrates using the remote catalytic growth. As illustrated in Figure 1a, a clean SiO2/Si substrate (300 nm thick SiO2) was inserted into the center of a horizontal tube and a porous curled Cu foil (Alfa Aesar, item no.46365) was placed in the upstream, approximately 3 cm away from the substrate. In contrast to previous reports,13 a porous curled Cu foil was used here to Received: February 23, 2014 Revised: May 8, 2014
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dx.doi.org/10.1021/jp501897a | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic diagram of the remote catalytic growth of graphene on SiO2/Si substrate. (b−-e) AFM phase images of graphene flakes with different growth times of (b) 2, (c) 3, (d) 4, and (e) 5 h. (f) SEM image of the graphene flakes. All scale bars are 500 nm. (g) Average size of the flakes as a function of growth time.
enhance the sublimation of Cu atoms and improve its catalysis ability in the CVD process. After the tube was inserted into the CVD furnace, the system was heated to 1045 °C in a 200 sccm H2 flow; then graphene flakes could be directly formed on the substrate at 1045 °C with a flowing gas mixture of CH4 (10 sccm), H2 (50 sccm), and Ar (70 sccm) at ambient pressure. The growth time varied from 2 to 5 h. After growth, the sample was cooled rapidly to room temperature by simply open the furnace. The morphology and size of the graphene flakes were characterized by field-emission SEM (FEI Sirion 200) and atomic force microscopy (AFM) (Seiko Instrument Industry Co.). Raman spectra were obtained by a LABRAM-HR Raman spectrometer with an excitation wavelength of 514.5 nm generated by an Ar+ laser.
3. RESULTS AND DISCUSSION The morphology of the graphene flakes is shown in Figure 1b− f. Because the surface roughness of the SiO2 is comparable to the thickness of graphene flakes, it is difficult to distinguish the flake from the underlying substrate with the AFM topological images. However, the difficulty can be overcome using the phase images due to the evident difference in the stiffness between the graphene and SiO2; hence, the graphene flake with a clearly round-shaped outline can be well observed. On the basis of the above characterization, as shown in Figure 1g, the average size of the flakes is found to increase drastically with the growth time in the first 4 h; then the flakes coalesce with each other and fully cover the surface when the growth time reaches 5 h. Raman spectra were used to investigate the quality of the asgrown graphene.20 As seen in Figure 2a, the G and 2D band
Figure 2. Raman spectra (514.5 nm laser excitation) of the graphene formed directly on SiO2/Si substrate with different growth times (a) before and (b) after transfer. (Inset) Enlarged 2D peak of graphene with 3 h growth (black line), and corresponding single Lorentz curve fitting (red dotted line). Vertical dashed line shows the position of the 2D band for mechanical exfoliation graphene. Spectra are vertically shifted for clarity.
peaks, corresponding, respectively, to the first-order scattering of the E2g mode and a double-resonance electron phonon scattering process in graphene,21 can be clearly observed at ∼1580 and ∼2700 cm−1 from all samples under investigation. The results, well consistent with previous works,8,13,15 indicate that the graphene is synthesized successfully on the SiO2/Si substrate. In addition, we also find the defect-activated D peak at ∼1350 cm−1 in the spectra, implying that disorders and B
dx.doi.org/10.1021/jp501897a | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. Results of Raman spectra for graphene before and after the transfer process: (a) I2D/IG, (b) position of the 2D band, (c) position of the G band, and (d) fwhm of the 2D band as a function of growth time.
defects may exist in the as-grown samples.8,21 In order to know the stacking layer number of the graphene, the intensity ratio of the 2D (I2D) to G band (IG) was calculated, and the result is about 1.5 for all samples, as shown in Figure 3a. Moreover, each 2D peak can be well fitted with a single Lorentz curve, and a typical result is shown in the inset of Figure 2a. Both of the above results demonstrate that all as-grown samples are monolayer graphene.20−22 From Figure 2a it can also be seen that the 2D peak blue shifts obviously with growth time, while the G peak only shows a slight shift. Compared to the reported 2D peak position of mechanical exfoliation graphene (∼2687 cm−1, as shown with a vertical dashed line in Figure 2),20 the blue shift of the 2D peak of our graphene sample varies remarkably from 14.3 to 42.6 cm−1 when the growth time increases from 2 to 5 h (Figure 3b), while the shift of the G peak is kept only at ∼10 cm−1 (Figure 3c). Generally, either the strain or the charge doping in graphene can result in the apparent shift of the Raman peaks.16,23 On one hand, it is well known that the compressive strain can be created by the mismatch of CTE between the graphene and the underlying SiO2/Si substrate.16 In this case, the shift in the 2D peak is much more sensitive than that of the G peak because the strain responses are about ∼144 and ∼58 cm−1/% for the 2D and G modes, respectively.24,25 On the other hand, the G band is much more sensitive to the charge doping and the shift is about 3−10-fold that of the 2D band.23,26 Considering that the shift of the G peak shows a very weak dependence on growth time, we attribute the dominating reason for the observed Raman shift to the substrate-induced compressive strain. To further certify the above consideration, the as-grown samples were transferred to another clean SiO2/Si substrate by the developed method similar to the transfer of graphene from Cu substrate.27,28 Raman spectra of the transferred graphene were also characterized. As shown in Figure 3a and the inset of Figure 2b, I2D/IG is kept about 1.5 and the 2D band can still be fitted well with a single Lorentz curve, indicating that the monolayer character of graphene flake can be reserved after the
transfer process. However, as compared to the Raman spectra of the as-grown graphene (Figure 2a), the blue shift of the 2D band of the transferred graphene (Figure 2b) is suppressed drastically, which can be observed more clearly in Figure 3b. Besides, we find that the full width at half-maximum (fwhm) of the 2D peak of the as-grown graphene narrows down after transfer (Figure 3d). Both of the above features demonstrate that the compressive strain in the as-grown graphene can be released after the transfer process. It has been aforementioned that the considerable difference in CTE between graphene and SiO2/Si substrate will trigger expansion of the graphene and contraction of the substrate during the cooling stage of the growth process, thus leading to the compressive strain in the graphene. Due to fact that the SiO2 film is only 300 nm thick and has excellent adhesion to Si substrate while the thickness of Si is about 0.5 mm, it is reasonable to adopt the CTE of Si to estimate the compressive strain originated from the thermal mismatch between graphene and the substrate, similar to the case of graphene grown on Ni films (300 nm) on SiO2/Si substrate.29 Therefore, the strain ε0 can be estimated by30 ε0 = 1 − exp[−
Ts
∫RT Δα(T )dT ]
(1)
where RT is the room temperature and Ts is the growth temperature and Δα(T) = αgra(T) − αSi(T)25,31 is the difference of CTE between graphene and Si substrate. Accordingly, with our growth parameters of Ts = 1045 °C and RT = 25 °C, the calculated compressive strain ε0 is −0.66%. This strain will cause the Raman blue shifts in the 2D band of graphene, while the mode-dependent relationship between the peak shift Δω and the strain tensor εij is further described by30,32 Δω = −ω0γ2Dtrεij
(2)
where ω0 is the position of the 2D peak without any strain (∼2687 cm−1) and γ2D = 2.7 is the Grüneisen parameter for the 2D mode.24 With eq 2 and the blue shift data of the 2D peak in C
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Figure 4. (a) Compressive strain of graphene with different growth times as a function of flake size. (b) Schematic diagram of the compressive strain distribution in the graphene flake (R = D/2). The core region has the remaining strain of Δε, while the edge region has decreasing strain.
In order to understand the relevant contributions of the defect and edge to the strain relaxation, we first suppose that εdefect is simply proportional to ID‑d/IG (see Table 1)
Figure 3b we can further estimate the strain in the graphene with various flake sizes. As seen from Figure 4a, the strain in the graphene flakes increases with flake size, i.e., a size dependence of the compressive strain for the graphene flake directly grown on the SiO2/Si substrate is clearly manifested. Also from Figure 4a, however, we find that the measured compressive strain ε is much smaller than the calculated compressive strain ε0. This difference is probably ascribed to the strain relaxation taking place at defects and edges of flakes. In this regard, we first estimate the intensity ratio of the D peak to the G peak ID/IG from the Raman spectra. The result shows that ID/IG decreases as the size of graphene flake increases, as seen in Table 1. On the other hand, based on the Tuinstra−
εdefect = C1·ID − d /IG
where C1 is a coefficient. As for the strain releasing through the edge of the graphene flake, we propose a model in which the strain in the flake possesses the core−edge distribution, as shown in the schematic of Figure 4b. On the basis of above model, the strain releasing through the edges can be expressed as εedge =
Table 1. Experimental Results of ID/IG and Relevant Contributions from Defects and Edges
experiment of ID/IG edge contribution of ID‑e/IG defect contribution of ID‑d/IG
3
4
5
1.43 0.10 1.33
1.09 0.05 1.04
0.87 0.02 0.85
0.73
