Article pubs.acs.org/JPCC
Structural Instability of Transferred Graphene Grown by Chemical Vapor Deposition against Heating Satoru Suzuki,* Carlo M. Orofeo, Shengnan Wang, Fumihiko Maeda, Makoto Takamura, and Hiroki Hibino NTT Basic Research Laboratories, NTT Corporation, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan S Supporting Information *
ABSTRACT: A heating treatment is often used in graphene research to remove adsorbates and resist materials from graphene. Heating graphene followed by air exposure is also known to result in heavy hole doping in graphene, although the role of heating has been unclear. Here, we demonstrate that a practical graphene sample fabricated using the commonly used growth and transfer techniques is unstable against heating in a high vacuum. Structural disorder likely due to defect formation is induced by heating, and the disorder is accompanied by hole doping. Our analysis shows that the main cause of the defect formation is graphene reacting with O2 and H2O molecules inserted between graphene and the substrate. The hole doping caused by air exposure after heating is explained by gas adsorption at the defect sites.
temperatures from 300 to 500 °C followed by exposure to air causes heavy hole doping in graphene.11−15 There is a consensus that exposing graphene to air causes adsorption of O2 or H2O molecules and that the adsorbate supplies graphene with a carrier. However, the role of heating prior to the exposure is still unclear. In previous reports, the possibility that heating leads to the creation of defects has not been considered.11−15 Heavy hole doping has similarly been observed when heating is performed in oxygen atmosphere.16 In this specific case, doping was observed at a lower temperature of 200−250 °C, but defect formation at such a low temperature was not considered. Evaluating heatinginduced effects is also very important in terms of practical sample preparation in graphene research. A graphene device is often annealed to remove adsorbates in a vacuum prior to measurements. Without annealing, the Fermi level is often located quite far from the Dirac point because of unintentional doping caused by adsorbates.17 A PMMA film, which is commonly used as a protective layer during the transfer process and as a resist for electron beam lithography, is also often removed by annealing at 400 °C or above in H2 atmosphere.18 In this paper, we show that a practical graphene sample obtained by the commonly used growth and transfer techniques is not stable against heating in a high vacuum. Structural disorder is induced by heating and is likely due to defect formation caused by reactions with O2 and H2O molecules underneath graphene. The disorder is found to be strongly correlated with the hole doping. We propose that the role of
1. INTRODUCTION Graphene is generally considered to be thermally and chemically stable because of its robust sp2 bonding with no dangling bonds. This stability, as well as the high mobility at room temperature, has been one of the important factors that make graphene a very attractive material for various applications. On the other hand, it has been theoretically predicted that a purely two-dimensional crystal is structurally unstable and cannot exist at a finite temperature because of large thermal fluctuations of atoms.1,2 The three-dimensionality induced by corrugation has been suggested to be essential to stabilize graphene.3 Practically, the situation is further complex. In a graphene sample, graphene often contacts a substrate, metal electrodes, and gas molecules in the environment. Poly(methyl methacrylate) (PMMA) is commonly used as a protective layer when a graphene film grown by the chemical vapor deposition (CVD) method is transferred to a substrate. However, PMMA residue is known to remain insistently on the graphene surface even after a PMMA removal procedure.4−7 Transfer of CVD-graphene is usually performed in liquid water, and thus, H2O molecules are considered to be inserted between the graphene layer and the substrate.8 Even for exfoliated graphene, a non-negligible number of inserted H2O molecules would remain at the interface because a substrate surface is often hydrophilic. In fact, the origin of hysteresis often observed in the gate characteristics of a graphene device is considered to be H2O molecules at the graphene/substrate interface.9 Residue of the adhesive tape would also remain on the graphene surface.10 Under a certain condition, graphene would chemically react with these surroundings and would become defective. It has been experimentally well established that heating graphene in a vacuum or inert gas atmosphere at moderate © XXXX American Chemical Society
Received: August 2, 2013 Revised: September 26, 2013
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3. RESULTS A Raman spectrum of a transferred graphene film is shown at the bottom of Figure 1a. The relatively small intensity of the G
heating prior to air exposure is to create defects and that the hole doping is caused by adsorbates at the defect sites.
2. EXPERIMENTAL SECTION In this study, we only used very common techniques for both CVD growth and transfer unless otherwise mentioned. Singlelayer graphene was grown on Cu foil (Alfa Aesar) at 1000 °C by the low-pressure CVD method as described in a previous report.19 Methane (15 standard cubic centimeters per minute, sccm) was the carbon feedstock, and the growth time was typically 15 min. Hydrogen (2 sccm) was also provided before, during, and after the growth. The pressure during the growth was about 20 Pa. After growth, the methane supply was stopped, and the sample was naturally cooled to room temperature in a H2 atmosphere. The grain size of graphene was measured to be about a few micrometers as evaluated by low-energy electron microscopy. BN-doped graphene was also grown on Cu foil at 1000 °C by the low-pressure CVD method. For this growth, both methane (35 sccm) and borane ammonia (∼0.1 sccm including hydrogen of a thermal decomposition product) gases were introduced in the furnace.20−22 The pressure during the growth was about 90 Pa, and the growth time was 15 min. For Raman and electric measurements, a graphene film (∼1 cm2) was transferred onto a SiO2 (285 nm)/Si substrate by using the commonly used method as follows. A protective PMMA layer (∼100 nm) was formed on the graphene film grown on the Cu foil by spin-coating an anisole solution of PMMA (MicroChem Corp.) after which the sample was heated in air at 170 °C to remove the solvent. The PMMA/graphene film was isolated by dissolving the Cu foil in a water solution of 1 M FeCl3 and by rinsing in deionized water. The floating PMMA/graphene film in water was picked up on a SiO2/Si substrate. One sample (Figure 5, isopropyl alcohol, IPA) was rinsed in IPA just after being picked up in water. After the transfer, the sample was naturally dried at room temperature. Finally, the PMMA film was removed with acetone at room temperature. For X-ray photoelectron spectroscopy (XPS) measurements, a graphene film was transferred to a Au plate using essentially the same technique. We also used commercially obtained graphene/SiO2/Si samples, which were fabricated by transferring a CVD-grown graphene film to a SiO2/Si substrate using similar transfer techniques. The heating-induced structural disorder was observed for both samples. Sample heating was performed in a high vacuum of ∼10−4 Pa (∼10−7 Pa in XPS) at a certain temperature for 30 min. Micro-Raman (Renishaw, Invia) measurements were performed in air at the excitation wavelength of 532 nm. The laser power density was about 1 mW/μm2. The illumination did not cause any detectable change in the Raman spectrum. For each measurement, spectra were obtained from more than 200 points. The mobility and carrier density were evaluated in air at room temperature using the van der Pauw method. Silver paste was used as electrodes. X-ray photoelectron spectroscopy measurements were performed by using a monochromatized Al Kα source (1486.6 eV) and a hemispherical photoelectron analyzer. The takeoff angle of photoelectrons from the surface was set at a relatively small angle of 25° so that the measurements were surface-sensitive. The measurements were performed at room temperature at a pressure below 3 × 10−7 Pa.
Figure 1. (a) Raman spectra of a graphene film on SiO2 before and after heating in a vacuum at several temperatures. The spectrum of a PMMA/SiO2/Si sample after heating is shown for reference. (b) Magnified spectra of the G and D band regions before and after heating.
band compared to the 2D band (IG/I2D = 0.29) and the symmetric shape of the 2D band are characteristics of singlelayer graphene.23 The graphene sample was heated in a vacuum (∼10−4 Pa) at a certain temperature for 30 min. After heating, Raman spectra were measured at room temperature in air. The evolution of Raman spectra induced by heating is also shown in Figure 1a. With increasing temperature, the G band is upshifted and the IG/I2D ratio increases. These spectral changes are consistent with previously reported hole doping by heating followed by air exposure.11−15 In addition, a very broad spectrum appears with heating. In Figure 1b, magnified spectra of graphene before annealing and after annealing at 600 °C are shown. The G band tails extend extraordinarily to about 1490 and 1750 cm−1, although the peak near the center is still sharp. The broad spectrum covers the entire region of both the G and the D bands and further extends down to ∼1000 cm−1 (not shown). Such a broad spectrum is similar to that observed in amorphous carbon (a-C).24 The G band peak height does not decrease but increases. Thus, the integrated spectral intensity in the G and D band regions largely increases with heating. The spectral broadenings mean that graphene is disordered by heating and that the Raman selection rule is considerably relaxed. Considering that the cross sections of originally allowed transitions (the initially observed sharp G, D, and 2D bands) are not largely decreased, however, the deformation is not very severe. The extent of the selection rule relaxation is larger in the order of D, G, and 2D band. In this study, we used PMMA as a protective layer during transfer. It is known that PMMA residue remains on graphene even after the acetone treatment.4,7 PMMA residue might be partially crystallized by heating. Thus, one may think that the spectra after heating are explained by a simple superposition of the spectra of graphene (almost unchanged by heating) and partially crystallized PMMA residue, which would show an a-Clike spectrum. To check this possibility, we prepared a 100-nmthick PMMA film deposited on essentially the same SiO2/Si substrate. Optical microscope observation showed that most of the PMMA film disappeared after heating in a vacuum at 400 °C, although invisible residue would still exist on the surface. Nonetheless, no distinct peak was observed in the entire B
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Figure 2. (a) Raman spectra obtained from an intentionally damaged (dotted lines) and undamaged (solid lines) part of the same graphene sample. The damage was caused by 20 keV electron irradiation. Spectra before and after heating at 400 and 600 °C are shown. The blue broken lines denote background spectral intensities. (b) Raman spectra of BN-doped graphene before and after heating at several temperatures.
Figure 3. (a) Evolution of the G and 2D peak positions with heating. (b) Evolution of the G peak position and the intensity of the heating-induced broad spectrum.
spectral region of 1300−2800 cm−1 after heating at 400 and 600 °C as shown in Figure 1a. We also prepared a residue/ SiO2/Si sample by immersing a PMMA-coated SiO2/Si substrate in acetone. The sample was heated at 400 and 600 °C, and again, no distinct peak was observed after the heat treatments. One may still think that crystallization of PMMA residue would be largely enhanced on graphene or that the spectrum of partly crystallized PMMA residue would be largely enhanced by the graphene-enhanced Raman scattering effect, which has been reported for some molecules25,26 and nanoparticles27 placed on graphene. However, even if such effects are considered, the following results are difficult to explain by the simple spectral superposition of graphene and partially crystallized residue. Figure 2a shows Raman spectra of graphene intentionally damaged by electron irradiation (dotted lines).28−30 For comparison, spectra obtained from an unirradiated part of the same sample are also shown (solid lines). The irradiation was performed in an electron beam lithography machine at the electron energy of 20 keV. The irradiation dose was set to 8 × 1016 cm−2. A sharp and intense D band appeared at about 1350 cm−1 because of the irradiation damage. The sharp D band almost completely disappeared after heating at 400 °C, and a broad spectral component appeared. The broad component is much larger than that of unirradiated graphene heated at the same temperature. (The disappearance of the sharp D band is not due to recovery from the damage but to extensive relaxation of the Raman selection rule.)
Similar results are also observed after heating at 600 °C. Another example is shown in Figure 2b. In this case, boron nitride (BN) was doped in graphene during CVD growth as described in refs 28−30. Because of the BN doping, a sharp and large D band appeared. With heating treatments, the intensity of the sharp D band decreases and the peak almost completely disappears at 600 °C. Instead, the broad component more quickly increases than in usual graphene samples. These results cannot be explained by the simple superposition of the spectra of the initial graphene and contaminants on it because the sharp D peak disappeared after heating. Instead, the results mean that the graphene films are disordered by heating, which results in the extensive relaxation of the Raman selection rule. The results also show that more severely defective graphene is more easily disordered. The heating-induced disorder was also observed when heating was performed in Ar at 7 × 104 Pa. We also would like to point out that the disorder was observed when nitric acid was used as a Cu etchant (see Figure S1 of the Supporting Information). This means that Fe contaminants that might originate from the FeCl3 etchant are not the cause of the disorder. Now, let us see whether there is a correlation between doping concentration and the structural disorder. Figure 3a shows the heating temperature dependence of the G and 2D band position obtained from Raman mapping measurements (the specimen was the same as in Figure 1). Both the G and the 2D band positions are upshifted by the heat treatments. The G band shift is larger than that of the 2D band (Δω2D/ΔωG ≅ C
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0.6), meaning that the shifts are predominantly due to hole doping and not to mechanical strain.15,31−34 These results are fully consistent with previously reported hole doping by heating followed by air exposure.11−15 Although it is not so easy to distinguish electron doping from hole doping by only Raman spectroscopy, electric measurements show that our graphene films are hole-doped as shown below. Figure 3b shows a correlation between the G band position and the broad disorder-induced spectrum intensity. Here, the disorder-induced intensity was determined by the difference between the spectral intensities at 1490 and 1800 cm−1. The results are mostly independent of the choice of wavenumber as long as one is set inside the broad D band region and the other one is set outside the broad G band tail. As mentioned above, the upshift of the G band in this case mostly corresponds to hole doping. Figure 3b clearly shows that the hole doping is accompanied by an increase in the disorder-induced component. This result was also confirmed by electric measurements. In Figure 4, we show the heating temperature
Figure 5. Raman spectra before and after heating of essentially the same graphene films prepared by different transfer procedures. One was transferred in water as usual; the other one was rinsed in IPA just after transfer in water.
growth. Thus, the growth conditions for the two films were considered to be essentially the same. Although the disorder-induced component appears for both samples after heating, the intensity is systematically smaller for the sample rinsed in IPA than for the typical one. Similarly, a relatively small disorder-induced component is observed for the graphene transferred onto a hydrophobic substrate as shown in Figure S2 of the Supporting Information. These results strongly suggest that the structural instability is mainly related to some molecules, such as water, inserted between the graphene and the substrate during transfer. Direct X-ray photoelectron spectroscopy (XPS) observation of oxygen-containing molecules inserted between graphene and a SiO2 substrate would be very difficult because the small oxygen signal would be hidden in the intense signal from the substrate. For XPS measurements, a graphene film was transferred to a Au plate using essentially the same transfer process as above. Measurements were performed under a more surface-sensitive geometry than conventional XPS by setting the photoelectron takeoff angle from the surface to be small (25°). Figure 6a shows C 1s spectra of a graphene/Au sample before and after heating. The spectrum of the pristine sample unambiguously consists of the main peak (sp2-C) at about 284.8 eV and at least three additional peaks at binding energies about 0.8, 2.3, and 4.4 eV higher. The three peaks can be ascribed to PMMA residue,4,5,7,35 which was not removed by the acetone treatment. The 4.4 and 2.3 eV higher components can be assigned to CO and C−O in PMMA residue, respectively (see the inset of Figure 6a). The 0.8 eV higher component corresponds to the other three types of C atoms in PMMA, all of which have the sp3-like character. (Although the three types of C atoms, of course, should have different binding energies,35 they could not be clearly resolved because their binding energies are close to one another.) The PMMAoriginated components disappeared relatively quickly by heating. A large decrease of the PMMA components is observed even at 300 °C. After heating at 400 °C, the spectrum can be well fitted by a single Doniach-Sunjic line shape.36−38 We did not observe any significant spectral change at 500 °C and higher temperatures (up to 700 °C). Thus, we think that the PMMA residue that initially existed on the graphene surface was mostly removed by the heat treatments at 400 °C. The XPS results are consistent with a previous report.7
Figure 4. Heating temperature dependence of hole density and mobility. The error in the data is within 2% for both hole density and mobility.
dependence of mobility and hole density. The mobility decreases with temperature, which is consistent with the structural disorder observed in Raman. The hole density largely increased at 400 °C. This temperature is the same as that at which the disorder-induced component increased (Figure 3b). Thus, we conclude that the hole doping by heating followed by air exposure is highly correlated to the structural disorder. Next, we consider the origin of the structural disorder induced by heating in a high vacuum. All the graphene films used in this study were grown by the CVD method at 1000 °C and were naturally cooled down to room temperature. Thus, they should have been stable at the growth temperature. We therefore believe that the origin of the structural instability dwells somewhere in the transfer procedure. Figure 5 shows Raman spectra of a typical graphene film transferred in water (H2O) and a graphene film rinsed in isopropyl alcohol (IPA) just after transfer in water. In the latter case, we think that most of the water initially inserted between the substrate and the graphene during the transfer was replaced by IPA. (We tried to transfer in IPA but failed because the PMMA/graphene film sank because of the small surface tension.) The two graphene films were initially grown on the same Cu foil and were obtained by cutting the foil into two pieces after the CVD D
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Figure 6. (a) C 1s XPS of a graphene/Au sample before and after heating at several temperatures. The circles are experimental data, and the solid lines are curve-fitting results. The red line is the graphene component, and the blue, green, and orange lines are PMMA components. The inset shows the chemical formula of PMMA. (b) Raman spectra of a graphene/Au sample before and after heating at several temperatures. (c) O 1s XPS of a graphene/Au sample before and after heating at several temperatures. The circles are experimental data, and the solid lines are curve-fitting results. The red and blue lines can be assigned to H2O and O2 components, respectively. The spectrum of a residue/Au sample after heating at 500 °C is also shown for reference. (d) The O/C atomic ratios evaluated from integrated XPS intensities. The static error is within 1%.
Figure 6b shows the heating temperature dependence of Raman spectra of graphene on Au. The large disorder-induced component appeared at 400 °C again and further increased at 500 °C. At these temperatures, PMMA residue should have been mostly removed from the graphene surface as shown in Figure 6a. These results strongly suggest that PMMA residue on graphene is not the main cause of the heating-induced disorder. On the other hand, oxygen-related components remain much more insistently. Figure 6c shows the evolution of O 1s spectrum with heating. The spectra consist of at least two components. The O 1s intensity largely decreases with heating at 300 °C probably because of desoption of physisorbed H2O and O2 molecules and thermal decomposition of PMMA residue (Figure 6a). However, at 400 °C and higher temperatures, little spectral change is observed, and the integrated spectral intensity almost stays constant. In a PMMA residue/Au sample measured for reference, much less oxygen is observed, and the O 1s signal is barely visible after heating at 500 °C as also shown in Figure 6c. The results strongly suggest that oxygen-containing molecules are inserted between the graphene layer and the substrate. The binding
energies of the two components are about 532.0 and 534.0 eV after heating. These values are close to reported binding energies of physisorbed H2O39 and O240 molecules, respectively. Such an interfacial water layer has been directly observed by atomic force microscopy (AFM) on mica41,42 and sapphire43 substrates at ambient conditions at room temperature. Similarly, insertion of H2O and O2 molecules between graphene and SiO2 is often supposed from transport properties of a graphene device.9,44 The O/C atomic ratios evaluated from integrated XPS intensities are shown in Figure 6d. The O/C ratio reaches ∼10% even after heating in an ultrahigh vacuum. It would be possible that a larger amount of water is inserted between graphene and a SiO2 substrate because SiO2 is much more hydrophilic than Au (water contact angles have been reported to be about 0° on SiO245 and about 77° on Au46,47). Our results show that molecules inserted underneath graphene cannot be easily eliminated even in an ultrahigh vacuum and at a high temperature. The results also mean that the heatinginduced disorder occurs regardless of the substrate if O2 or H2O exists at the interface. E
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4. DISCUSSION We think that the heating-induced structural disorder is due to defects created by reactions with O2 and H2O molecules underneath graphene at the elevated temperatures. According to a previous report on graphene oxidation with heating in O2 atmosphere, an indication of etching (formation of pits) was clearly observed at 450 °C for single-layer graphene.16 An upshift of the G band was observed at a much lower temperature of 200−250 °C. Although in our study, heating was performed in a high vacuum, and the non-negligible amount of O2 and H2O molecules inserted beneath graphene would attack graphene and would create defects by chemical reactions. Once a defect is created, it is likely that an O2 or H2O molecule beneath graphene chemisorbs to the defect site. However, in the C 1s spectrum in Figure 6a, we did not observe CO or C−O bonding after heating. We think that this is because the defect density is too small to detect the peak clearly in XPS. If we assume that the hole density (2.4 × 1013 cm−2 at 600 °C; see Figure 4) corresponds to the defect density (see below), ∼0.6% of carbon atoms would have a CO or C−O bond after heating. This value would be too small to clearly detect in XPS. In a C 1s spectrum of graphene oxide, a large C−O (CO) component is generally observed.48,49 Thus, the defect density after heating in a vacuum is much smaller than that of graphene oxide as expected from the limited amount of O2 and H2O. We did not observe a sharp D band after heating in a vacuum but a broad spectrum. On the other hand, we observed a sharp D peak with a very weak broad spectrum, when we heated a graphene sample in air (the spectrum was similar to that observed after heating in O2 atmosphere16). In an O2 abundant environment, chemical reactions occur more severely, and the most prominent feature caused by heating is etching.16 If a defective or amorphous part is formed, it will be preferentially etched away quickly. As a result, relatively well crystallized graphene with many pits (edges) remains, and a graphene edge shows a sharp D peak without a broad spectrum.50,51 On the other hand, the etching effect is much less prominent in a vacuum because of the limited amount of O2 and H2O. In fact, we did not observe any indication of etching in the C 1s XPS (Figure 6a) or in optical microscope observations up to 700 °C. Thus, defects or defected parts are left in the graphene network. The absence of long-range order without etching would largely relax the Raman selection rule and would result in the broad Raman spectrum. We think that one of the reasons the heatinginduced instability has not been recognized so far is the absence of the sharp D band. In previous reports on heating-induced doping, adsorption of O2 and H2O molecules has been assumed to be responsible for the doping, although the role of heating has not been clear.11−15 We also think that the hole doping is caused by the adsorbates. Once a defect is formed in graphene, it is very likely that the defect acts as a doping center in an ambient atmosphere because the adsorption energy often increases at a defect site and the adsorbate would supply graphene with a carrier. In fact, enhanced gas sensitivity has been reported for intentionally defected graphene sensors.52,53 Previous reports have also shown that once graphene is heavily p-doped by heating followed by air (O2, H2O) exposure, a considerably high temperature of ∼500 °C is necessary to dedope graphene.12,14 The temperature is much higher than necessary
to remove molecules physisorbed on an ideal graphene surface. This also suggests that the adsorbates, which are responsible for the hole doping, are bound to defect sites. We have shown that a CVD-grown graphene sample prepared by commonly used growth and transfer techniques is unstable against heating in a vacuum. Also, the hole doping by heating followed by air exposure has been observed not only in CVD-grown graphene 14 but also in exfoliated graphene.11−13,15 This suggests that the heating-induced defect formation also more or less occurs in exfoliated graphene. Although in the mechanical exfoliation method, liquid water is not used, and H2O (and O2) molecules would still be inserted at the graphene/substrate (usually SiO2) interface,9,11,54 and the molecules would attack graphene at an elevated temperature. In fact, in ref 12, a slight symptom of disorder (broad spectrum) can be seen in a Raman spectrum of exfoliated graphene after heating at 500 °C in a vacuum, although the disorder-induced component is much smaller than that observed in our study. Practically, it is important to note that the heating temperature of ∼400 °C, at which a large disorder-induced component was observed in Raman, is close to the temperature often used for removing adsorbates and PMMA.18 The heatinginduced disorder has to be a concern when a graphene sample is heated. Our results also suggest that fabricating a heatingtolerant graphene sample leads to high-performance graphene. The molecules below graphene would also degrade the physical properties, such as the mobility.9
5. CONCLUSION We showed that CVD-grown graphene transferred to a SiO2 substrate using the commonly used transfer technique is unstable against heating in a high vacuum. Heating in a vacuum causes structural disorder that largely relaxes the Raman selection rule and thus broadens the peaks. The disorder can be ascribed to defect formation caused mainly by graphene reacting with O2 and H2O molecules beneath it. The structural disorder was found to coincide with heating-induced hole doping, which has been commonly observed. Heating-induced hole doping is considered to be due to adsorption of molecules at defects created by heating. Fabricating a heating-tolerant graphene will lead to high performance because higher heating tolerance means cleaner graphene that is less affected by surroundings.
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ASSOCIATED CONTENT
S Supporting Information *
Raman spectra before and after heating of a graphene sample prepared using nitric acid as a Cu etchant and Raman spectra before and after heating of a graphene film transferred onto a HMDS layer on a SiO2/Si substrate. This information is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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