Article pubs.acs.org/cm
Crack-Free Growth and Transfer of Continuous Monolayer Graphene Grown on Melted Copper Ye Fan,† Kuang He,† Haijie Tan,† Susannah Speller,† and Jamie H. Warner*,† †
Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom ABSTRACT: Monolayer graphene with large domain sizes can be grown by chemical vapor deposition using a Cu catalyst in its molten state. However, extending this to fully continuous sheets of graphene on the centimeter scale is challenging, because of cracks, rips, and tears that are induced upon rapid cooling. The various issues that prohibit fully continuous graphene sheets are identified and solutions presented. These include (i) developing a novel two-stage CVD growth process that fills in the cracks and holes formed upon cooling; (ii) appropriate choice of underlying wetting substrate of W, instead of Mo, which causes holes; and (iii) a new electrochemical transfer method that removes W and then Cu to enable the efficient transfer of crack-free graphene sheets onto silicon wafers. Our results provide important solutions to challenges related to the synthesis and transfer of high-quality monolayer graphene grown on molten Cu catalysts for electronic applications.
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scale),27−30 it is not easy to extend these same growth recipes to produce large-area (centimeter-scale) uniform continuous graphene sheets with concomitant millimeter domain sizes, because of various challenges in the growth approaches. Largearea continuous graphene sheets have been extensively synthesized, but with smaller domain sizes on the order of 20 μm.31−33 Multilayer regions of graphene can be found even in graphene synthesized under low pressure.34 Impurities, surface roughness, and grain boundaries of Cu are suspected to be responsible for the multilayered regions.35−37 In contrast, largearea single-layer graphene domains are easily synthesized when the Cu catalyst is in a molten state rather than solid form.38−41 With longer growth time, graphene domains floating on melted Cu are expected to grow larger and finally meet and match with each other, forming a continuous film. Nevertheless, synthesis and transfer of graphene film on melted Cu is not trivial. A high-melting-point metal is used to “wet” the liquid Cu during synthesis to prevent balling, but it may induce unwanted alloying. In addition, the chemical inertia of the supporting metal makes conventional wet-etching transfer methods difficult. Furthermore, graphene stores a huge amount of elastic energy during Cu solidification and may finally result in cracks. In this paper, we study the growth and transfer process of fully continuous graphene sheets on melted Cu by CVD, as opposed to prior work that focused on just growing large single-crystal domains on the order of few hundreds of micrometers. We find that macrodefects (e.g., cracks and holes) arise from tensions introduced in the growth and transfer processes of graphene. To address that problem, we
INTRODUCTION Graphene is expected to play versatile roles in various electronic and spintronic applications including, but not limited to, radiofrequency field-effect transistors,1−4 spin valves,5,6 solar cells,7,8 and organic light-emitting diodes (OLEDs).9,10 Despite its huge potential, graphene-based devices remain far from industrialization, due in part to challenges associated with the synthesis and transfer processing of uniform large-area monolayer graphene. Physical methods such as mechanical exfoliation were the first approach that successfully isolated single-layer graphene from graphite.11 Although mechanical exfoliated graphene exhibits good electronic performance,12−14 mechanical exfoliation is too is labor-intensive and low yield to supply graphene for industrial-scale device fabrication. Another route to isolate graphene is via wet-chemical methods. Chemical exfoliation15 or chemical reduction of graphite oxide16,17 produce large numbers of small graphene fragments18 that are favored for certain applications such as molecular sensing19,20 and pollutant processing,21 but are not desirable for electronic applications, because of their small size and poor conductivity. Obtaining graphene from graphite is a “top-down” approach, whereas fabricating graphene from carbon-based precursors provides a “bottom-up” approach that is scalable. The “bottom-up” method mainly includes thermal annealing of silicon carbide22 and chemical vapor deposition (CVD). Epitaxial graphene on thermal annealed silicon carbide exhibits excellent electronic properties,1,23 but its applications are limited by the challenges in controlling graphene layer number and transferring it to alternative substrates. On the other hand, CVD graphene can match epitaxially grown graphene on silicon carbide in its electronic properties,24−26 while being easier to transfer onto an arbitrary substrate. Although the record size of individual isolated graphene domains continues to increase (now in the millimeter © XXXX American Chemical Society
Received: May 26, 2014 Revised: July 19, 2014
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developed a novel two-stage “regrowth” method. A tension-free electrochemical transfer method is also investigated for the efficient transfer of centimeter-scale continuous graphene without introducing any major cracks.
The second key factor determining the continuity of graphene is the cooling process. We compare two cooling processes: the conventional “fast cooling”, and a novel “regrowth cooling”. “Fast cooling” refers to a cooling process where the sample is quickly removed from the heating zone immediately after graphene growth to rapidly cool to room temperature. “Regrowth cooling” refers to a cooling process where the temperature is gradually reduced from 1090 °C to 1060 °C for the last 30 min of graphene growth before removing the sample out of the heating zone and cooling to room temperature. Numerous cracks are found in the fast-cooled graphene and can be classified into three types. The first type of cracks are the long ones that originate from the edge of the entire graphene sheet, propagating up to a millimeter across until reaching a Cu grain boundary, as shown in Figure 2a. This type of crack is the result of the stress between graphene and Cu during Cu solidification, as illustrated in Figure 2l. Notches in the edge of the graphene film, which may be etched by the nanoparticles,42,43 are commonly found near these long cracks, as shown in Figure 2b. Stress between graphene and Cu concentrates around the notches and leads to some of them further developing into larger cracks, as illustrated in Figure 2l. Smaller cracks were found propagating out sideways from the long main cracks, as shown in Figures 2j and 2k. These shorter off-shoot side cracks are of higher density but are narrower than the main long cracks. The second type of crack includes the crack that spreads across the dimple in Cu along the thermal grooves. This type of crack spreads hundreds of micrometers and has widths up to several tens of micrometers, as shown in Figure 2d. The drastic deformation of Cu during solidification introduces a huge amount of stress on the graphene film and finally tears this type of crack in the film, as illustrated in Figure 2e. Secondary cracks running parallel with the thermal grooving also commonly appear, as pointed out by yellow arrows in Figure 2d. The third type of cracks form around the Cu grain boundary and we call them “boundary tears”. They are parallel with each other and intersect the Cu grain boundary at 45°, as shown in Figures 2f and 2h. In contrast with the stress-induced cracks from thermal grooving, shear stress is responsible for “boundary tears”. Cu grains rotate to accommodate stress during solidification,44 which shears graphene near the Cu grain boundaries. Cracks then form on the Cu grain boundary to release the tensile stress. Occasionally, some cracks of this type can develop into larger lightning-shaped cracks, as shown in Figure 2g. In order to eliminate these cracks, graphene was grown by a novel “regrowth” process, hereafter referred to as “regrown” graphene. This resulted in substantially better continuity than “fast-cooled” graphene. Slowly decreasing the temperature from 1090 °C to 1060 °C enables enough time for Cu to solidify and release the thermal stress between graphene and Cu. Fewer cracks are found as a result (see Figures 3a and 3e). Furthermore, by continuing to supply CH4 during this period, it allows graphene to continue to grow and fill the vacant space left by cracks in the film, actively repairing the cracks. The “regrown” graphene is more continuous, and when the sample is baked in air, there is no sign of oxidation of the Cu in the main central region (see Figures 3a and 3e). Although the growth of graphene by CVD can lead to highquality material, it is the transfer stage that is a major ratelimiting step in the progress of graphene for electronics. Transferring a single graphene layer from one substrate to
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RESULTS AND DISCUSSION Graphene is grown by ambient pressure chemical vapor deposition, as reported in the Methods section. The catalyst substrate, cooling rate, and transfer method are found to be the key factors in determining the quality of the final graphene product. Refractory metals like W or Mo are used as substrates, because they have higher melting points than that of Cu and also “wet” the molten Cu, producing flat Cu surfaces for graphene growth. Although W and Mo share similar chemical and mechanical properties, they have different effects on the morphology of the CVD-grown graphene. Hexagonal particles decorate the as-grown graphene on Mo-supported Cu, as shown in Figure 1. Despite their similar shape with graphene
Figure 1. SEM and EDX images of graphene grown by CVD on Cu with a Mo or W wetting substrate: (a) SEM image of graphene grown on molten Cu with a Mo wetting substrate by CVD, where a hexagonal domain comprised of Mo−C−O is observed to disrupt the graphene in the local neighborhood, as noted by the arrow; (b) SEM image of a region of graphene grown by CVD on Cu:Mo used for energy-dispersive X-ray spectroscopy (EDX) maps in panels c−f; (c) Mo Lα1 EDX map; (d) Cu Kα1 EDX map; (e) O Kα1 EDX map; (f) C Kα1_2 EDX map; (g) SEM image showing a large number of holes in graphene caused by the Mo−C−O domains; and (h) continuous graphene film obtained by CVD on molten Cu with a W substrate (inset shows a typical high-magnification image of graphene on Cu supported by W).
domains, Raman spectroscopy shows no peaks associated with graphene, while energy-dispersive X-ray spectroscopy (EDX) reveals that the particles are actually composed of Mo−C−O (see Figures 1c−f). The density of Mo−C−O particles increases with reaction time (i.e., the time when the sample is exposed to CH4), but does not change with preannealing time (i.e., the time when the sample is annealed in hydrogen before introducing CH4). Therefore, the Mo−C−O particles are likely to be formed by metal-catalyzed carbonization. In contrast to graphene grown on Cu supported by Mo, graphene grown on Cu supported by W achieves continuity. No secondary phase particles are found over the entire sample, as shown in Figure 1h. B
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Figure 2. Cracks in “fast-cooled” graphene samples: (a) SEM image of long cracks across graphene; originating from graphene edges, cracks propagate through graphene until reaching a Cu grain boundary. (b) SEM image of nanoparticles etching tracks in graphene edges. (c) SEM image of the cracks in graphene resulting from thermal grooving of the Cu; a dimple, indicated by an arrow, shows the deformation of Cu during solidification, which tears the graphene film. (d) High-magnification SEM image of the tail of the crack in graphene induced by thermal grooving; secondary cracks running parallel to the main crack are indicated. (e) Schematic illustration of the formation of cracks in graphene from thermal grooving and its secondary cracks. (f) SEM image of “boundary tears” in graphene from a Cu grain boundary. (g) SEM image of a millimeter-scale lightning-shaped crack. Some cracks among the “boundary tears” occasionally develop into large lightning-shaped cracks. (h) Histogram of the intersection angles between “boundary tears” and the copper grain boundary. 95 cracks over 6 different Cu grain boundaries are counted. (i) Schematic illustration of the formation of “boundary tears”. (j) SEM image of the large cracks which reach the edge of graphene (as in panel (a)), along with the offshoot side cracks. (k) Higher-magnification SEM image of the offshoot side cracks stemming from a main crack. (l) Schematic illustration of the formation of long cracks that reach the edge of graphene and its offshoot side cracks.
The transfer process influences the quality of graphene on the silicon substrate, in terms of the continuity and surface contamination. In our experiment, transferring CVD graphene grown on molten Cu by the bubbling method destroys its integrity, as shown in Figure 5a−5c. Turbulence caused by hydrogen bubbles twists and cracks the graphene−PMMA film during the transfer process. However, we have noticed that some isolated hexagonal domains of monolayer graphene with widths up to 100 μm retain better continuity than the continuous film after transfer by the bubbling method, because of their smaller size and reduced tension. A substantial amount of surface contamination remains on the graphene even after the cleaning process, described in the Methods section. The root-mean-square surface roughness of graphene transferred by the bubbling method reaches as high as 7.2 nm, which is the highest among all methods we examine here. Compared to the bubbling method, the side etching method retains better integrity of the graphene. Only few cracks are found across the graphene, as shown in Figures 5e and 5f. However, because of the chemical inertness of W, common etchants such as FeCl3 or ammonium persulfate cannot dissolve it efficiently. In this way, the etchant can only approach Cu from the space between graphene and the W substrate, which results in a slow etching speed. The etchant process generally takes 3−5 days to complete. Moreover, W anchors to the bottom of the liquid and distorts the graphene−PMMA film during etching causing further cracks. The bright blue contrast in Figure 5d indicates
another without inducing cracks is not trivial. Although several transfer methods for graphene grown on Cu foil,34,45 or metal thin film46,47 have been reported, there is still a lack of detailed studies on the transfer process of graphene grown on melted Cu to silicon substrates with an oxide surface layer. Here, we compare the effect of different transfer methods on the quality of graphene, in terms of continuity and surface residues. PMMA is spin-coated onto graphene as a protective supporting layer for all methods discussed. Figure 4 schematically illustrates the three transfer methods, and two Cu etchants used, studied in this report. In the “side etching” method, the sample floats on the FeCl3 solution. Because of the chemical inertness of W, the etchant can only gradually dissolve Cu from the narrow space between graphene and W substrate accessible through the side of the sample. When Cu is totally consumed, the W substrate sinks, leaving the graphene−PMMA film floating on the etchant. In the “bubbling” method, hydrogen bubbles lift the graphene−PMMA film off the Cu substrate.48,49 These hydrogen bubbles come from the electrolysis of a 1 M sodium hydroxide solution with the sample used as the cathode. Apart from the two methods above, we have developed a novel “2-step” transfer method specifically for graphene grown on molten Cu with a W substrate. First, the W substrate is etched by the electrochemical method. This etching step is referred as “anodic etching” hereafter. Following the anodic etching of W, Cu is dissolved with either FeCl3 or ammonium persulfate, followed by successive rinsing in water to finish the transfer. C
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Figure 3. Morphology of graphene treated by different cooling processes: (a) Optical microscopy image of fast-cooled graphene on Cu (sample is baked in air at 150 °C for 15 min to oxidize uncovered Cu, in order to increase the optical contrast); oxidized Cu appears dark red, while graphene protected Cu appears light orange. (b) SEM image of a large crack in fast-cooled graphene from thermal grooving. (c) SEM image of “boundary tears” on the fast-cooled graphene. (d) SEM image of long cracks from the graphene edge. (e) Optical microscopy image of “regrown” graphene on Cu; sample is partially oxidized in order to increase the image contrast. (f) SEM image of crack-free “regrown” graphene covering the Cu grain boundary. (g) SEM image of crack-free “regrown” graphene edge; no long cracks develop from the edge of graphene. (h) Higher-magnification SEM image of the fast-cooled sample, showing a large crack. (i) Higher-magnification SEM image of “regrown” graphene showing no cracks.
the FeCl3−PMMA residue on graphene. The AFM measurement (Figure 5f) confirms the discontinuity of graphene by finding cracks with widths of ∼600 nm. The surface roughness is 2.4 nm, which is similar to previous reported values of transferred CVD graphene. Both “side etching” and “bubbling” transfer methods stretch or distort the graphene−PMMA film at a certain stage. The “2step” etching method, on the other hand, is stress-free and therefore preserves the continuity of graphene. The first step of transfer involves oxidizing and dissolving W by anodic etching in a sodium hydroxide solution. The reaction requires a high pH solution environment, which is provided by the 2 mol/L sodium hydroxide solution in our experiment. W with a lateral size of 1 cm2 and thickness of 100 μm is found to dissolve
within 30 min by anodic etching. We further study the effect of two Cu etchantsFeCl3 and ammonium persulfateon the graphene. When Cu is dissolved by FeCl3, a similar amount of FeCl3−PMMA residue is found on the graphene, as shown in Figure 5g. On the other hand, not much surface residue appears on the graphene when it is transferred by ammonium persulfate (see Figure 5j). Regardless of which etchant is used, the graphene remains crack-free over a centimeter-scale region. We also used AFM to measure the surface roughness of the 2-step transferred graphene. When Cu is etched by FeCl3, the surface roughness is 3.4 nm. When Cu is etched by ammonium persulfate, a flatter surface is observed, with a root-mean-square roughness as low as 1.6 nm. It is clear that using ammonium D
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Figure 4. Schematic illustrations of different transfer processes: (a) Side etching and H2 bubbling methods to remove graphene−PMMA film from the Cu−W substrate. In the “side etching” method, Cu is gradually etched by FeCl3 solution from sample side. In the “bubbling” method, graphene is detached from the Cu by delamination by hydrogen bubbles. (b) Anodic etching method; W is etched by an electrochemical process, and, subsequently, the remaining Cu is dissolved by either FeCl3 or ammonium persulfate solution.
obtained similar sheet resistance values. The field-effect mobility of the graphene measured in air with this geometry was 1600 cm2/(V s), as shown in Figure 6e. With a smaller length-to-width ratio, we find an average FET mobility of ∼3000 cm2/(V s). Further cleaning processes, such as ultrahigh vacuum annealing, using BN as a substrate, and performing measurements under vacuum should help to further increase the mobility values in our devices.
persulfate leads to a cleaner graphene surface than FeCl3 as the Cu etchant. To further quantify the graphene grown and transferred following our procedure, we examined its electronic properties within different length scales. Graphene is patterned and trimmed into 2-μm ribbons by a combination of electron beam lithography and oxygen plasma etching. Arrays of Cr/Au contacts are then deposited by evaporation onto the ribbon, so that the channel length is 200 μm × N, where N is an integer, as shown in Figure 6a. The large aspect ratio of the graphene ribbon ensures the conductivity is sensitive to the continuity of the graphene. Raman mapping indicates a 2D/G ratio of ∼2, confirming the single layer nature of graphene, as shown in Figure 6c. A weak D peak is also observed, which is from the edges of the 2-μm ribbon sampled by the Raman spectroscopy measurement. The conductivity of graphene is measured over different channel lengths. The plot of resistance versus length intercepts the y-axis at 240 Ω, as shown in Figure 6d, which is from the contact resistance according to previous reports.50,51 The resistance of the graphene ribbons in our geometry is 422.5 Ω/μm; thus, we can derive the sheet resistance as 825 Ω/ □. We also measured the sheet resistance directly on other transferred samples using the Van der Pauw geometry and
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CONCLUSION The results in this paper provide a solution for large-area singlelayer graphene grown on a liquid Cu catalyst and its transfer onto silicon substrates. Our findings clarify the influence of each step in the process on the final quality of graphene. Three critical factors, e.g., metal substrate, cooling process, and transfer method, are used to determine the quality of the graphene. The metal substrate should be carefully chosen so as to balance its wettability by the melted Cu and chemical inertness. Secondary phase particles caused by the Mo substrate break holes in graphene, while a W substrate provided a continuous graphene sheet. Different cooling processes change the thermal stresses in graphene. A conventional fast-cooling process tears graphene, because of drastic deformation of Cu E
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Figure 5. Continuity and surface roughness of transferred graphene: (a) Optical microscope, (b) SEM, and (c) AFM images of graphene transferred by “bubbling” method (graphene is shredded and large amounts of surface contamination are found); (d) Optical microscope, (e) SEM, and (f) AFM images of graphene transferred by the “side etching” method (some cracks and holes are found); (g) Optical microscope, (h) SEM, and (i) AFM images of graphene transferred by the “2-step” method with Cu etched by FeCl3 (graphene maintains continuity over a large region but some surface residues are found; and (j) Optical microscope, (k) SEM, and (l) AFM map of graphene transferred by the “2-step” method with Cu etched by ammonia persulfate (graphene is clean and continuous over a large region). The brightness/contrast of the optical microscope images are adjusted to reveal any surface contamination that might be present. 30 min before growth. Graphene is grown on liquid Cu with a flow of 200 sccm of argon, 80 sccm of hydrogen (25%), and 10 sccm of methane (1%) for 90 min. Two cooling processes are studied in this work. In the cooling process referred as “fast cooling”, samples are quickly moved out of the heating zone and cooled to room temperature directly after the synthesis. In the cooling process referred as “regrowth cooling”, a secondary 30-min growth process of graphene at 1060 °C is carried out without changing the flow rate of each gas. The sample then is removed from the heating zone and cooled to room temperature. Transfer. A 500-nm PMMA film is spin-coated onto graphene as a protection layer right after synthesis. Three transfer methods (e.g., “bubbling transfer”, “side etching”, “2-step etching”) and two etchants (e.g., iron chloride (FeCl3) and ammonia persulfate) are compared in this work. In the “bubbling” method, the sample is used as the cathode in the electrolysis of a 1 M sodium hydroxide solution. The current is maintained as 0.1 A, to provide a stable generation of hydrogen bubbles from the sample. Hydrogen bubbles peel the graphene− PMMA film off from the Cu substrate gradually. In the “side etching” method, the sample is floated on the FeCl3 solution. Cu is slowly consumed by the 1 M FeCl3 solution from the side until the graphene−PMMA film separates from the substrate. The underlying
during solidification. We have addressed that problem by developing the “regrowth” cooling process, during which graphene continues to grow from the edge of the cracks and stitches them. In addition to the graphene growth, the choice of transfer process also influences the continuity of graphene. Distortion caused by bubbles or heavy substrates sinking tears cracks into graphene grown on Cu. A stress-free wet transfer method is developed to minimize the transfer-induced cracks in graphene. These results will help the further development of large-area continuous sheets of high-quality monolayer graphene for electronic applications.
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METHODS
Graphene Preparation. Graphene is grown via chemical vapor deposition (CVD). Both the substrate (W or Mo) and the catalyst (Cu) are cut into 1 cm2 pieces and then thoroughly cleaned in acetone and IPA. An addition step of rinsing Cu in hydrochloric acid (HCl) is applied to eliminate the oxide layer. Cu is then mounted on W and placed into the furnace. Samples are melted and annealed at 1090 °C with a flow of 100 sccm of hydrogen (25%) and 200 sccm of argon for F
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Figure 6. Electrical characterization of graphene: (a) Optical microscope image of an array of graphene FETs on a silicon wafer with 300 nm oxide (2-μm-wide graphene ribbons pass under contacts spaced 200 μm apart). (b) Optical microscope image of metal contacts on graphene ribbon; the yellow square marks the region for Raman mapping in panel (c). (c) D, G, and 2D Raman mapping of graphene ribbon crossing a contact. The ratio between 2D and D peak is ∼2. (d) Resistance of graphene ribbons, as a function of different channel lengths. Graphene ribbons remain conductive even when the length-to-width ratio exceeds 800 and for lengths of 1.6 mm. (e) Gate-dependent field effect of graphene measured under atmospheric conditions. The channel width and length are 2 and 400 μm, respectively. W substrate prevents the access of FeCl3 to the Cu from beneath and only permits etching from the side. In the “2-step etching” method, the W substrate is electrochemically etched in a 2 M sodium hydroxide solution. The sample is linked with the anode, and a 2.4 V voltage drop is applied between the sample and the cathode (Cu foil). Hydrogen bubbles are generated from the cathode while no gas is released from the sample. Cu is then etched by a 1 M solution of either FeCl3 or ammonium persulfate. For graphene transferred with all methods, cleaning steps are carried immediately after separating the graphene−PMMA film from the catalyst. The film is first rinsed in DI water for 30 min before a second rinse in 1 M HCl for another 15 min. Etchant residues are removed by this step. The graphene−PMMA film is again rinsed in deionized (DI) water for three times with at least 1 h per rinse.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.H.W. thanks the Royal Society and Balliol College for support. Y.F. thanks the Clarendon Fund for support. H.T. thanks the Merdeka Scholarship for support.
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REFERENCES
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dx.doi.org/10.1021/cm501911g | Chem. Mater. XXXX, XXX, XXX−XXX
