Strain Relaxation of Graphene Layers by Cu Surface Roughening

Sep 14, 2016 - The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, wh...
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Strain Relaxation of Graphene Layers by Cu Surface Roughening Jin Hyoun Kang,†,‡ Joonhee Moon,†,§ Dong Jin Kim,∥ Yooseok Kim,§ Insu Jo,† Cheolho Jeon,§ Jouhahn Lee,*,§ and Byung Hee Hong*,†,∥ †

Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University, Seoul 08826, Republic of Korea § Advanced Nano-Surface Research Group, Korea Basic Science Institute, Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea ∥ Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, Seoul 08826, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The surface morphology of copper (Cu) often changes after the synthesis of graphene by chemical vapor deposition (CVD) on a Cu foil, which affects the electrical properties of graphene, as the Cu step bunches induce the periodic ripples on graphene that significantly disturb electrical conduction. However, the origin of the Cu surface reconstruction has not been completely understood yet. Here, we show that the compressive strain on graphene induced by the mismatch of thermal expansion coefficient with Cu surface can be released by forming periodic Cu step bunching that depends on graphene layers. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. We found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. We believe that our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene. KEYWORDS: graphene, Cu step bunching, Raman spectroscopy, AFM, strain relaxation

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phenomenon of moving Cu atom under graphene layers,22,23 dynamic instability of the Cu−C phase,24 stress relaxation between Cu and graphene,25 or lattice mismatch.26 However, the interplay between graphene and corrugated Cu surface still has not been understood yet, which is one of great important factors for the electrical conductivity of graphene. Moreover, the control of the ripple density of graphene is a key parameter due to scattered phonons in the rippled structures.21,27 Therefore, understanding the origin of the Cu texture formation during or after graphene growth is crucial to study electrical properties of graphene. Graphene was synthesized using 25 μm thick polycrystalline Cu foils (99.7%) as a catalytic substrate in a 1000 °C quartz reactor with 5 sccm H2 flow for 1 h under a pressure of 4.5 × 10−2 Torr. Next, methane was introduced into the chamber at a flow rate of 30 sccm, and the total pressure inside the chamber was maintained at 5.4 × 10−1 Torr during the growth stage. After 30 min graphene growth, the furnace was cooled to room

t is well known that the chemical and mechanical properties of graphene considerably depend on the number of layers. For example, surface oxidation1 or hydrogenation2,3 on monolayer graphene is much easier compared to those on bi-/trilayer graphene. In addition, the morphology of Au deposited on graphene4 and the surface-enhanced Raman spectroscopy (SERS) enhancement factors are strongly affected by the number of graphene layers.5 On the other hand, Raman spectroscopy has been utilized as a powerful tool10 to characterize the layer number,6 doping level,7,8 and defect density.9 Recently, Raman study was introduced to separate the mixed effects of doping and strain,11 which has been utilized in our study. Graphene can be synthesized by chemical vapor deposition (CVD) using various metal catalysts, such as Ni,12 Ru,13 Ir,14 Fe,15 and Cu.16,17 Among them, a Cu foil has been considered the most promising material owing to the low cost and the suitable carbon solubility to synthesize monolayer graphene.18 However, the corrugated morphology of Cu surface formed during the graphene growth has been a problem for the direct transfer of graphene to a counter substrate. In previous reports,19−21 it was claimed that the corrugated Cu steps with nanoscale periodicity and height were formed by the pinning © 2016 American Chemical Society

Received: April 16, 2016 Revised: August 16, 2016 Published: September 14, 2016 5993

DOI: 10.1021/acs.nanolett.6b01578 Nano Lett. 2016, 16, 5993−5998

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Nano Letters temperature (cooling rate ∼3.6 °C/s for rapidly cooled samples) without methane flow, as shown in Figure S1. The dark-field (DF) optical microscope image in Figure 1a shows

controlled the growth time of graphene to study the origin of corrugated Cu surface (Figure S1). The optical microscope and AFM images show the formation of Cu step bunching underneath partially grown graphene, whereas no noticeable change on Cu surface is obtained without graphene grown region (Figure S5). This supports that large step bunching can be induced by graphene growth rather than by impurity atoms on a Cu foil. In addition, high-quality Cu foil (99.999%, Alfa Aesar) was used to synthesize graphene and it shows that the smooth Cu surface is obtained before graphene synthesis, whereas the corrugated Cu surface with longer step bunching appears after graphene growth (Figures S6,S7). To characterize the graphene grown on Cu, Raman spectroscopy (Alpha 300M, WITec Gmbh) was performed using a 532 nm laser. The dark-field image in Figure 2a includes the same area shown in Figure 1c, which represents a distinctive periodicity and height of the Cu step bunching in the 4-fold region. Point Raman spectra were collected at narrow Cu step bunching (white circle), broadened Cu step bunching in the 4fold region (red circle), and the center of the 4-fold area (blue circle) in Figure 2a. There are two strong Raman G peak at 1590 cm−1 and 2D peak at 2700 cm−1, which come from the bond stretching and ring breathing modes, respectively,31 and a D peak related to defect sites (∼1340 cm−1) is negligible in all spectra (Figure 2b). The Raman spectrum collected at the white circle (black in Figure 2b) exhibits the narrow 2D peak fitted with a single Lorentzian (Figure 2c, full width at half maximum (FWHM) ∼30.8 cm−1) and an intensity ratio I(G)/ I(2D) of ∼0.51, which typically indicates monolayer graphene. On the other hand, the Raman spectra taken at the red and blue circles show bilayer graphene (fwhm ∼50.8 cm−1 for a single Lorentzian, and I(G)/I(2D) ∼ 1.73) and graphene with more than three layers (fwhm ∼61.9 cm−1 for a single Lorentzian, and I(G)/I(2D) ∼ 2.65); splitting of the 2D peak into multiple Lorentzian components clearly indicates graphene with two and more than three layers (Figure 2d,e, respectively).10 Spatially resolved Raman data on graphene-grown Cu were analyzed to study the correlation between the Cu step bunching and the graphene layers. Figure 3 shows Raman map images plotted using the fwhm of the 2D peak and the intensity ratio I(G)/I(2D). Normally, outside of the 4-fold area (blue region in Figure 3a), the average FWHM of the 2D peak is ∼30 cm−1, indicating that monolayer graphene is uniformly grown on Cu foil. On the other hand, average FWHM of the 2D peak increases as the probe position moves from the boundary to the center of the 4-fold region. The increase in the fwhm of the 2D peak originates from 2D band splitting owing to interlayer coupling.32 Specifically, the pink region represents bilayer graphene (av. FWHM !∼48 cm−1), and the red region at the center indicates more than tri-layer graphene (av. FWHM (∼53 cm−1). The data plotted in Figure 3d show that the average fwhm of the 2D peak broadens with increasing graphene layers. Additionally, the average I(G)/I(2D) values of monolayer graphene, bilayer graphene, and graphene with more than three layers are ∼0.7, ∼ 1.6, and ∼2.2, respectively, showing a gradual increase from outside of the 4-fold region to the center of the area (Figure 3b). This matches the graph of the intensity ratio I(G)/I(2D) vs the graphene layers in Figure 3e, indicating that the intensity of the 2D peak decreases and broadens with increasing graphene layers, leading to the increased I(G)/I(2D) for multilayer graphene.10,33 The Raman maps are consistent with the dark-field optical microscope image (Figure 3c). The

Figure 1. Optical microscope and atomic force microscope (AFM) images of graphene-grown Cu surface. (a) Dark-field image of optical microscope shows 4-fold region with bright periodic lines. (b) Phase AFM image was taken at the yellow dashed box in image (a) and the inset is topographic AFM image. (c) Magnified AFM image at the boundary (white dashed box) of 4-fold region shown in inset. The white dashed line shows the boundary between monolayer and bilayer graphene. Cu morphology (direction b) is formed with nonparallel direction to normal Cu step bunching (direction a) under multilayer graphene. (d) AFM image of 4-fold region across the grain boundary of Cu.

that corrugated texture with broadened 4-fold regions appeared on Cu surface. The shape of these regions is similar to that of bilayer graphene grown by low-pressure CVD reported previously.28 To study the changed Cu morphology, atomic force microscopy (AFM) was applied to the area in the yellow dashed box in Figure 1a; the result shows that the Cu step bunching appears periodically over the Cu surface (Figure 1b). Unexpectedly, Cu step bunching of 4-fold region with increased period and height is shown in the AFM image, which is consistent with the DF optical image. The magnified AFM image represents that the Cu step bunching broadens notably in 4-fold region (direction a) and the other Cu facets in 4-fold region are formed with periodicity (direction b) in Figure 1c. However, the smooth morphology of Cu was observed in the AFM image in Figure S2 when Cu was annealed in hydrogen atmosphere. The increased periodicity of ripples under bilayer graphene after transfer process was reported by Ni et al. (Figure S3),21 which possibly resulted from the widened Cu step bunching. Interestingly, the broadened Cu step bunching with 4-fold region was observed across the grain boundary of Cu, indicating that the broadened 4-fold area could be grown across the Cu grain boundaries (Figures 1d and S4). The step bunching of Cu surface could be dominantly affected by impurity in Cu foil29 or surface stress.25,30 Here we 5994

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Figure 2. Optical microscope image and Raman spectra of the graphene on the reconstructed Cu surface. (a) Dark field optical microscope image. (b) Raman spectra from the white circle (black), red circle (red), and blue circle (blue), respectively. (c−e) 2D peaks from the white, red, and blue circles, indicating mono-, bi-, and trilayers.

Figure 3. (a,b) Spatially resolved Raman mapping plotted with fwhm(2D) and I(G)/I(2D), respectively. The peaks were fitted with single Lorentzian. (c) Dark field optical microscope image overlapped with (b). Scale bars, 1 μm. (d,e) Distribution of fwhm(2D) and I(G)/I(2D) values with respect to the number of graphene layers, respectively. The fwhm(2D) of exfoliated graphene and CVD graphene shown in Figure S8.

bilayer graphene with the shape of 4-fold region is obviously

The pixel-to-pixel variation in the Raman G and 2D peaks represents the correlation between the periodicity of the Cu step bunching and graphene layers, which is possibly related to the strain on graphene lattice. Previous reports used the Raman shift to explain the strain and charge doping effects,7,34,35 and those effects can be distinguished using the ΔωG − Δω2D plot

associated with the broadened Cu step bunching, as observed via optical microscope and AFM. Our results suggest that surface reconstruction of Cu depends strongly on the number of graphene layers. 5995

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Figure 4. (a) ΔωG − Δω2D correlation of the monolayer/bilayer/trilayer graphene. The dashed line indicates a charge neutral line assuming ∂ωG/∂ε ∼ −57.3 and ∂ω2D/∂ε ∼ −160.3 (ref 34). (b) Compressive strain map derived from the spatially resolved Raman spectra.

Figure 5. (a) A topographic AFM image of graphene on Cu corresponding to Figure 3 and 4. (b) AFM height profiles along the red and blue dotted lines in a. The surface areas marked with the yellow and green dotted boxes are increased by 0.77% and 1.41% for monolayer and bilayer graphene, respectively.

as suggested by Ryu et al. (Figure 4a).11 The ωG and ω2D values of unstrained graphene were experimentally deduced from exfoliated monolayer (ωG = 1581.9 cm−1, ω2D = 2680.3 cm−1), bilayer (ωG = 1581.2 cm−1, ω2D = 2694.4 cm−1), and trilayer (ωG = 1581.6 cm−1, ω2D = 2697.1 cm−1) graphene, which agree with values in previous report.33 Figure 4a shows the plot of ΔωG − Δω2D graph; the dashed line represents ΔωG/Δω2D without charge doping effect, which has a slope of 2.8.34 Neither G nor 2D peaks exhibited noticeable broadening or splitting, indicating that biaxial strain is dominant in all the graphene layers.36,37 The data points for monolayer graphene are located to the upper right of those for bilayer and trilayer graphene, indicating that monolayer graphene is more compressed than the others. On the other hand, the data points are not exactly on the dashed line but are scattered on the right side of it, because of the charge doping effects possibly induced by either charge transfer between Cu and graphene (ntype doping)38,39 or adsorbates from ambient air (p-type doping).40 Figure 4b shows a compressive strain map obtained from the Raman data. As inferred from the ΔωG − Δω2D plot, the biaxial compressive strain on monolayer graphene is higher than that of bilayer and trilayer graphene, which is consistent with the AFM image of Cu step bunching. The biaxial strain varies from −0.1% to −0.2% for monolayer graphene and from −0.05% to −0.1% for bi-/trilayer graphene. A comparison of the compressive strain map in Figure 4b and the surface

morphology in Figure 1c reveals that the periodicity of the Cu step bunching is related to the strain on the graphene. The step bunching is narrower under strongly compressed monolayer graphene, whereas they are broadened under less compressed bilayer and trilayer graphene. Stress-induced step bunching is possible explanation for roughened Cu surface. During cooling step of graphene growth, graphene can serve as a strained layer on Cu foil due to thermal expansion coefficient mismatch between Cu and graphene, which applies biaxial lateral force on Cu surface.41 Tersoff et al. suggested that the stress exerted by vicinal layer makes attractive interaction between steps or step bunches.42 In our study, the magnified AFM image and line profiles exhibit approximately 3 times longer and 5 times higher step bunching of Cu underneath multilayer graphene than that underneath monolayer graphene (Figure 5). Moreover, the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, which indicates that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. To confirm the formation of Cu step bunching under different conditions, graphene was synthesized at the same growth condition with slow cooling rate. The cooling rate was about 0.26 °C/s, which is about 15 times slower than the rapidly cooled sample (Figure S1). The AFM image exhibits that the surface of slowly cooled sample shows significantly 5996

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Nano Letters longer and higher step bunches under multilayer graphene, which is similar behavior to that of the rapidly cooled sample (Figures S9 and S10). The Raman maps of fwhm of the 2D peak, I(G)/I(2D) ratio, and the peak shift of the G and the 2D explain that the broadened Cu step bunching more effectively relaxes strain on multilayer graphene than that on monolayer graphene. The ΔωG − Δω2D plot obtained from several points of Raman spectra from both the rapidly cooled sample (Figure S11a) and the slowly cooled sample (Figure S11b) shows the similar tendency between graphene layers and strain relaxation. On the other hand, Cu step bunching in different crystal orientation of Cu foil after graphene growth was investigated by electron back scattering diffraction (EBSD), DF optical microscope and Raman spectroscopy, as shown in Figure S12. The longer periodic Cu step bunching under multilayer graphene appeared on Cu surfaces with different orientations, indicating that the compressive strain on multilayer graphene can be effectively released regardless of the crystal orientation. In summary, we demonstrate that a graphene layer and its strain relaxation are closely related to the periodicity of Cu step bunches. Atomic force microscopy (AFM) images and the Raman analysis show the noticeably longer and higher step bunching of Cu surface under multilayer graphene and the weaker biaxial compressive strain on multilayer graphene compared to monolayer. The Raman G and 2D peaks of multilayer graphene are generally less blue-shifted than monolayer graphene, which indicates that biaxial strain of multilayer graphene are more relaxed than that of monolayer graphene. We also found that the surface areas of Cu step bunches under multilayer and monolayer graphene are increased by ∼1.41% and ∼0.77% compared to a flat surface, respectively, indicating that the compressive strain on multilayer graphene can be more effectively released by forming the Cu step bunching with larger area and longer periodicity. Because the graphene from a flatter area shows higher charge mobility and conductivity than that of graphene from a rippled area (Figure S13), our finding on the strain relaxation of graphene layers by Cu step bunching formation would provide a crucial idea to enhance the electrical performance of graphene electrodes by controlling the ripple density of graphene in the future.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the Korean Basic Science Institute (KBSI) research Grant No. E36800 and Korea Evaluation Institute of Industrial Technology (KEIT) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (10063400), by the Basic Science Research Program (2012M3A7B4049807) through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future, Korea, by InterUniversity Semiconductor Research Center (ISRC) and Graphene Research Center at Advanced Institute of Convergence Technology, Seoul National University.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01578. Additional experimental details, AFM images, bright- and dark-field optical microscope images, surface morphology, fwhm analysis, ΔωG − Δω2D correlation, crystal orientation, electric characteristics. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-2-882-6569. *E-mail: [email protected]. Phone: +82-42-865-3540. Author Contributions

(J.H.K. and J.M.) These authors contributed equally to this work. Notes

The authors declare no competing financial interest. 5997

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