Strain in Graphene Sheets Attached to a Porous Alumina Membrane

Jul 15, 2013 - 2D-nanomaterials for controlling friction and wear at interfaces. Jessica C. Spear , Bradley W. Ewers , James D. Batteas. Nano Today 20...
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Strain in Graphene Sheets Attached to a Porous Alumina Membrane Takayuki Kase and Toshio Ogino* Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan ABSTRACT: We attached monolayer graphene sheets grown by chemical vapor deposition to porous alumina membranes with hexagonal pore arrays to introduce strain distribution. We have demonstrated by the dynamic force mode of atomic force microscopy that graphene sheets on porous alumina membranes with different diameters (60, 90, and 170 nm) were deformed according to the shape of the membrane surface. As a result, locally triangular strains are introduced into the graphene sheets in a wide area. We also confirmed the strain introduction by the G- and 2D-peak shift fashions of the Raman spectra. Porous alumina membranes are useful for strain engineering of graphene sheets because introduction of periodic, triangular strain is possible.

1. INTRODUCTION Graphene, 2-D hexagonal network of carbon atoms, exhibits excellent properties, such as high electron mobility and high mechanical strength,1−3 and is expected to be a key material for novel devices in the near future. However, monolayer graphene is a zero-gap semiconductor,4 and bandgap opening is required for its application to switching devices. To obtain the bandgap of monolayer, a variety of approaches has recently been proposed, such as fabrication of graphene nanoribbons5 or nanomesh,6,7 application of a perpendicular electric field to bilayer graphene,8 and formation of a strain field in graphene sheets.9−11 We focus on strain engineering of graphene sheets because it has two advantages: one is nonformation of graphene edges that generally behave as defects and the other is use of monolayer graphene sheets, which exhibit the highest electron mobility. Recently, a considerable amount of research has been reported to open the bandgap by introducing periodic9,10 or uniaxial11 strain to graphene sheets. Although the periodic strain is theoretically predicted to be effective for this purpose, an appropriate method for incorporation of the periodic strain into a large area of graphene sheets has not been found. In a recent study, porous alumina was used as a hexagonal nanohole template to fabricate graphene nanomesh.7 Porous alumina membranes can be fabricated in a large area at low cost by anodic oxidation of aluminum sheets, and hexagonally arrayed nanohole structures are self-organized.12−15 The nanohole diameter can be varied from several tens of nanometers to submicrometer depending on the applied voltage during the oxidation.13,15 We demonstrate a new method for incorporation of a periodic strain field using a porous alumina membrane as a substrate for graphene sheets. Graphene sheets were grown by chemical vapor deposition (CVD) and transferred to the porous alumina substrate surfaces. By this method, we have achieved formation of strain-modulated graphene sheets. We have also investigated effects of the introduced strain on lattice © 2013 American Chemical Society

vibrational properties of the graphene sheets attached to the porous alumina membranes.

2. EXPERIMENTAL SECTION Graphene sheets were grown on polycrystalline copper substrates (99.99%, 100 μm thick, Furuuchi Chemical) by atmospheric pressure CVD. First, a copper substrate was loaded into a CVD furnace and heated to 1000 °C under a 1000 sccm Ar flow. Next, the sample was annealed at 1000 °C for 30 min under a 1000 sccm Ar and 50 sccm H2 mixed gas flow to remove the native oxide on the substrate surface. Graphene sheet growth was carried out at 1000 °C for 10 min under a 1000 sccm Ar, 15 sccm H2, and 50 sccm CH4 mixed gas flow. After the growth, the sample was cooled to 500 °C at a rate of ∼11 °C/min and to room temperature at a rate of 50 °C/min. We confirmed growth of monolayer or bilayer graphene sheets on the copper substrate by Raman spectroscopy.16,17 We fabricated porous alumina membranes with different nanohole diameters by the two-step anodic oxidation process.12 An aluminum substrate (99.99%, 0.5 × 10 × 25 mm, Furuuchi Chemical) was rinsed with a phosphoric acid to clean its surface. We performed first anodic oxidation in a 0.3 M oxalic acid at an applied voltage of (a) 20, (b) 40, or (c) 60 V, (samples A−C, respectively) for 1 h. Then, the sample was etched by an 8 wt % phosphoric acid until the barrier layer of the porous structure appeared. After the first anodization and etching, we carried out the second anodization, where the samples A−C were anodized in a 0.3 M oxalic acid at the same voltage as the first anodization for 5 h. Finally, we etched the porous alumina membranes by an 8 wt % phosphoric acid to Received: February 7, 2013 Revised: July 2, 2013 Published: July 15, 2013 15991

dx.doi.org/10.1021/jp4013834 | J. Phys. Chem. C 2013, 117, 15991−15995

The Journal of Physical Chemistry C

Article

In the AFM (DFM) image of Figure 2a, graphene sheets attached to the porous alumina membrane A with a pore diameter of 60 nm are deformed according to the shape of the membrane surface. Figure 2d shows a DFM phase image of the same area as Figure 2a, where two areas with different profiles are observed: inside the dotted line and outside them. Figure 2b,c shows magnified images of a bare porous alumina area and a graphene-covered area, respectively. Deep holes and sharp protrusions are observed on the bare alumina areas shown in Figure 2b, whereas the graphene surface around the holes are gentle, as shown in Figure 2c. Therefore, we can distinguish the bare and graphene-covered areas by magnified images. Figure 2e shows a cross-sectional height profile of an area indicated by the blue circle in Figure 2a, whose magnified image is shown in Figure 2f, where a graphene sheet is suspended above a membrane hole. The height difference between the center and the edge areas of the hole is ∼16 nm. Because the graphene sheets are subject to the porous alumina membrane surface, a strain should be generated in the graphene sheets. Figure 3 shows 3-D views of the AFM (DFM) images of graphene sheets attached to the porous alumina membranes A−C with pore diameter of (a) 60, (b) 90, and (c) 170 nm. The junctions of the hexagonal sides of the porous alumina are higher than the sides. Therefore, the graphene sheets are strongly deformed with triangular symmetry around the junctions. In Figure 3b, the center area is deformed according to the shape of the porous alumina surface, and the difference in heights of the graphene sheets between the center and the edge of the porous alumina hole is ∼21 nm. However, in some areas, for example, an area indicated by the white circle, the graphene sheet is detached from the substrate surface. In Figure 3c, the graphene sheets are closely attached to the porous alumina membrane surface and reproduce the shape of the membrane surface. The difference in heights of the graphene sheet between the center and the edge of the porous alumina hole is ∼26 nm. The larger the pore diameter, the deeper the graphene sheet at the center of the pore. The bandgap opening is related to strain itself that is applied at three nonadjacent sides according to one of the theoretical predictions.9 The triangular deformation is formed by attachment of the graphene

expand their pore diameters for (a) 2 h and 30 min, (b) 3 h and 10 min, or (c) 5 h for the samples A−C, respectively. To attach the CVD-grown graphene sheets to the porous alumina substrates, we used the standard polymethyl methacrylate (PMMA) transfer technique.18 PMMA (average molecular weight of ∼996 000, Sigma-Aldrich product no.182265) was dissolved in anisole with a concentration of 4%, drop-casted on the graphene/Cu substrate, and spin-coated at 3000 rpm. Then, the Cu film was etched away by an iron trichloride solution for more than 12 h. The PMMA/graphene sheets were rinsed with deionized water and attached to the porous alumina substrates (samples A−C) or sapphire substrates (sample D for comparison). Finally, the samples with the graphene sheets were annealed at 400 °C under a 500 sccm Ar and 500 sccm H2 mixed gas flow to remove the PMMA layer. We observed graphene sheets attached to the porous alumina membranes by the dynamic force mode (DFM) of atomic force microscopy (AFM) and characterized them by Raman spectroscopy equipped with a 532 nm excitation.

3. RESULTS Figure 1a−c shows scanning electron microscope (SEM) images of porous alumina membranes fabricated by the two-

Figure 1. SEM images of the porous alumina membranes fabricated by the two-step anodic oxidation at (a) 20, (b) 40, and (c) 60 V.

step anodic oxidization at (a) 20, (b) 40, and (c) 60 V, respectively, followed by phosphoric acid etching. All samples exhibit hexagonal pore arrays, and the pore diameters are 60, 90, and 170 nm, respectively. In this way, we prepared the porous alumina membranes with the largest pore diameter at each voltage.

Figure 2. (a) AFM (DFM) image of graphene sheets attached to the porous alumina membrane anodized at 20 V, (b) its magnification in a bare alumina area, (c) that in a graphene area, (d) phase image in the same area as panel a, (e) cross-sectional view of an area indicated by the blue circle in panel a, and (f) magnification of the area of the cross-sectional view, where the cross-sectional profile was taken along the white arrow. 15992

dx.doi.org/10.1021/jp4013834 | J. Phys. Chem. C 2013, 117, 15991−15995

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Figure 3. Three-dimensional AFM (DFM) images of graphene sheets attached to porous alumina membrane surfaces of (a) sample A, (b) sample B, and (c) sample C.

sheets to the junctions of the hexagonal sides of the porous alumina membrane. Therefore, porous alumina membranes can basically be applied to bandgap engineering of graphene films. However, poor uniformity of graphene films is often observed, as shown in Figure 3b, although a periodic area is clearly observed in Figure 3c. This is owing to defective surfaces of the porous alumina membranes, and their fabrication procedures should be improved to obtain uniform strain distributions for a bandgap without fluctuation. Figure 4 shows Raman spectra of graphene sheets attached to the porous alumina membranes A−C with pore diameters of

Figure 5. Raman shifts of the G- and 2D-peaks of (a) monolayer graphene sheets attached to the porous alumina membranes and the sapphire substrate (r-face) and (b) bilayer graphene sheets.

Figure 4. Raman spectra of the monolayer graphene sheets attached to the porous alumina membranes A−C with pore diameters of (a) 60, (b) 90, and (c) 170 nm and (d) those that attached to a sapphire substrate (r-face).

2688 cm−1, respectively. We can observe a clear difference in Raman shift fashions between the porous alumina membranes and the sapphire substrate, whereas the dispersion among the porous alumina membranes is not large. In Figure 5b, the Gand 2D-peak positions of the bilayer graphene sheets were not widely distributed on the respective substrates. (Note that the axis scales are different between panels a and b.)

(a) 60, (b) 90, and (c) 170 nm and (d) those that attached to a sapphire substrate (r-face) as a control. Intensity ratios of the G- and 2D-peaks clearly indicate that monolayer graphene is obtained,16,17 and we also observed Raman spectra of bilayer graphene sheets in other areas (not shown). However, because the D- and D′-peaks are observed in every spectrum, these graphene sheets are defective.17 The intensities of the D-, G-, and 2D-peaks in the graphene sheets on porous alumina membranes are up to 19 times enhanced compared with that on the sapphire substrate. The largest enhancement occurred on the membrane with the pore diameter of 90 nm. Figure 5 shows dispersions of the G- and 2D-peak positions when Raman spectra were obtained from 10 different points. In Figure 5a, the G- and 2D-peak positions of the monolayer graphene sheets on the sapphire substrate appear at 1587−1596 and 2685−2695 cm−1, respectively, whereas, those on the porous alumina membranes appear at 1576−1585 and 2670−

4. DISCUSSION We have demonstrated that graphene sheets are deformed according to the hexagonal nanostructures on the porous alumina substrate. To open the bandgap of monolayer graphene by strain, it should be tensile.9−11,19,20 In Figure 3, it is obvious that tensile strains are introduced to the attached graphene sheets from the porous alumina substrates. In the theoretical predictions,9 bandgap is created by strain itself, where stresses are applied at three nonadjacent sides and along ⟨100⟩ axes. Therefore, the pore size is related only to a magnitude of deformation and bandgap opening is expected to 15993

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with triangular symmetry are introduced to them at the apexes and the sides of the hexagonal pores. The strains affect the carbon−carbon bond vibration so that the G- and especially 2D-peaks shift to higher wavenumbers.11,28−30 As clearly shown in Figure 5a, distribution of the G-peak positions is small, whereas the 2D-peak positions are widely distributed. Because the local strain in the graphene sheets is affected by the local morphology of the underlying substrate, tensile, or occasionally compressive, strain is introduced into the graphene sheets according to the shape of the porous alumina membranes. Furthermore, the G- and 2D-peak positions of the bilayer graphene sheets are not widely dispersed. It has been reported that the strain in multilayer graphene sheets is more easily relaxed than that in the monolayer ones.32 Bilayer graphene sheets attached to the porous alumina membranes exhibit a similar tendency. From the above discussion, we can conclude that the G- and 2D-peak behaviors on the porous alumina membranes are attributed to the strain introduced from the substrate, whereas, on sapphire surfaces, they are attributed to the chemical doping. However, dependence of the deformation on the pore diameters was not clearly observed in the Raman spectra. This is because the surface morphology of our sample was not perfect enough to distinguish the difference between the samples. Finally, we discuss a possible cause other than water for the Raman shift observed on the sapphire surface. In a previous paper, we reported that water layers between a graphene film and a sapphire surface remain confined even in high vacuum.31 Moreover, it was reported that water reconstruction occurs at the interface and form a strong hydrogen bond.33 Therefore, part of water molecules may remain at the interface after annealing on the flat sapphire-graphene interfaces. Another possible cause for the Raman spectral changes is contamination introduced during the graphene film transfer process. In our experiments, the CVD-grown graphene films were placed on the porous alumina surfaces or sapphire substrates using the standard PMMA transfer technique, in which contamination still remain on the graphene surfaces after annealing.34 Although we cannot exclude the contamination as a possible cause of the Raman shifts, the important point in this experiment is a difference in the Raman shift fashions between graphene films on the porous alumina membranes and those on the sapphire surfaces.

occur in every sample with a different diameter. However, we also observed areas where the graphene sheets did not attach to the substrate surface under our experimental conditions. We have to consider that the total interaction between the graphene sheets and the porous alumina membrane is weak in comparison with attachment to a flat substrate because the contact area is smaller on the nanohole surface than on the flat one. The main factor for the attachment failure is irregularly formed protrusions on the porous alumina substrate that interfere with the graphene attachment. Because we obtained graphene sheets that are at least partially attached to the porous alumina membranes, fabrication of uniform defect-free membranes is a key technique for device applications of the present method. Raman intensity of the graphene sheets attached to porous alumina membranes is largely enhanced. The Raman signal intensity of single-walled carbon nanotubes on the suspended part is generally larger than that on the substrate.21 It has also been reported by Zhang et al. that Raman intensity of carbon nanotubes is enhanced on a porous alumina membrane and that the phenomenon is a kind of surface-enhanced Raman scattering (SERS) attributed to the presence of nanohole arrays.22,23 We observed the largest enhancement with the pore diameter of 90 nm in comparison among 60, 90, and 170 nm. Because our sample surfaces were not perfect enough for the quantitative discussion, however, further study is required to distinguish the factors of the observed enhancement of the Raman signals. The G- and 2D-peaks of the graphene sheets attached to the porous alumina membranes appear at lower wavenumbers than that attached to the sapphire substrate. The blue shift of the Gand 2D-peaks is attributed to chemical doping16,24−27 or strains.11,28−30 Strictly, Raman spectra of the suspended part and the attached part of the graphene films on the porous alumina membranes should be separately compared with suspended graphene without strain and attached graphene to a flat surface, respectively. In the experiments, we measured averaged signals and cannot compare them separately. However, peak shift fashions of the G-band and 2D-band are different between the strain-driven shift and the dopingoriginated one, as shown in Figure 5. In the present discussion, we focus only on the correlation between shifts of the G- and 2D-bands and not on the absolute values of the shifts. Chemical doping is generally induced by remaining water layers under the graphene sheets24 and enhanced with an increase in the amount of the interfacial water. The G- and 2D-peaks of graphene sheets attached to the sapphire substrate are simultaneously blue-shifted, but their amounts are dispersed. It is because the amount of the remaining water between the graphene sheets and the sapphire substrate varies with location on the substrate.31 The graphene sheets attached to the porous alumina membranes were free-standing just above the nanoholes. Therefore, chemical doping into the graphene sheets from the porous alumina membranes is smaller than that from the sapphire substrate. As a result, the blue shift in the graphene sheets attached to the sapphire substrate is larger than that to the porous alumina membranes. When the blue shift is induced by chemical doping, the G-peak shift is generally larger than that of the 2D-peak.25,28−30 The shift of the 2D-peak in the monolayer graphene sheets on the porous alumina membranes, however, is larger (∼20 cm−1) than that of the G-peak (∼10 cm−1). Because the attached graphene sheets are deformed according to the membrane surface morphology, tensile strains

5. CONCLUSIONS We have demonstrated that graphene sheets can be deformed according to the shape of the substrate on a nanometer scale; therefore, it is possible to introduce triangular strains into graphene sheets by using porous alumina membranes. We have also found that Raman spectra of graphene sheets attached to porous alumina membranes exhibit unique properties. The enhancement of Raman signals was observed, which is attributed to suspended graphene areas or a kind of SERS related to a porous alumina membrane. The variation of the Gand 2D-peak positions can be interpreted by strains introduced from the morphology of the substrate surface, although chemical doping from the remaining water or another contamination at the graphene/substrate interface is a main contribution on flat substrates. The use of porous alumina membranes as substrates is a promising technique to introduce triangular strain distribution into graphene films in a large area, 15994

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by which bandgap opening of graphene is theoretically predicted.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-45-339-4147. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology. The sapphire wafers were provided from Namiki Precision Jewel Co. Ltd.

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REFERENCES

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