Selectively Patterned Regrowth of Bilayer Graphene for the Self

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Functional Nanostructured Materials (including low-D carbon)

Selectively Patterned Regrowth of Bilayer Graphene for the SelfIntegrated Electronics by Sequential Chemical Vapor Deposition Donggi Yi, Sangheon Jeon, and Suck Won Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11902 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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ACS Applied Materials & Interfaces

Selectively Patterned Regrowth of Bilayer Graphene for the Self-Integrated Electronics by Sequential Chemical Vapor Deposition Donggi Yi,† Sangheon Jeon,† and Suck Won Hong*,†,§ †

Department of Cogno-Mechatronics Engineering, Pusan National University, Busan 46241,

Republic of Korea §

Department of Optics and Mechatronics Engineering, College of Nanoscience and

Nanotechnology, Pusan National University, Busan 46241, Republic of Korea *Correspondence should be addressed to S.W.H. (email: [email protected]) KEYWORDS: graphene, regrowth, chemical vapor deposition, transparent electrode, transistor

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ABSTRACT There is a critical demand for the highly qualified synthesis of graphene with precisely controlled thickness over the large coverage area. Selective growth can be considered as one method in preparing a vertically stacked graphene, but it usually requires elaborately alloyed substrates for chemical vapor deposition (CVD). Here, we report on a newly developed synthesis strategy for a selectively patterned grown graphene sheet in spatially defined multi-thickness scale, exhibiting single and bilayer graphene produced by conventional CVD process. In particular, a sequential CVD growth technique on a single Cu substrate was used to produce highly ordered and alternatively patterned single and bilayer graphene maintaining its continuous configuration in a simplified and scalable manner. Our regrowth process did not require multiple transfer procedures or alloying catalytic substrate to satisfy the properties of graphene associated with the needs for various applications. We also thoroughly investigated the most valid mechanisms for our regrowth CVD process, which suggests that it is useful for the cost-effective synthetic approach into a built-in-heterostructured single and bilayer graphene. Finally, we demonstrated the possible accesses of transparent flexible electrodes and monolithically self-integrated allgraphene-based thin film transistors to fully utilize regrown graphene.


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Introduction The intriguing physical properties of graphene and its enormous potential for countless successful applications triggered many basic and technical studies.1-3 Graphene is a twodimensional carbon isotope with a single-atom-thick layer of a hexagonal structure in which one carbon atom forms a covalent bond with three different carbon atoms. This unique bond structure gives rise to the charge carriers of graphene to behave as Dirac fermions, resulting in extraordinary electrical properties with the theoretical electron mobility of up to 200,000 cm2 Vs1

at room temperature and atmospheric pressure.4 In addition to these electrical properties,

graphene has an elastic modulus of ~1 TPa and a tensile strength of 1,100 GPa, which does not lose its electrical conductivity even when it is expended or bent by more than 10% of its area as experimentally revealed.5-6 Moreover, the one-atomic carbon layer is an optically transparent material with a transmittance of over 97% in the range of visible light and show also excellent in thermal stability and chemical resistance.7-8 These unique properties of graphene have proved to be an ideal material for next-generation high performance electronic and optoelectronic devices. At the beginning of the research, mechanical exfoliation was used to peel off the monolayer of graphene from the layered-graphite in order to discover its natural properties.9 However, rapid studies on the artificial synthesis methods have been extensively explored and progressed to date for immediate practical applications of graphene. Among the valued approaches to synthesize graphene, chemical vapor deposition (CVD) process has been garnered much attention because of the great advantages of uniform thickness and relatively highthroughput.1 Using a low-pressure gas mixture of carbon-feeding methane and hydrogen flowing over the catalytic metal substrate, single or few-layered graphene can be formed on the metal



surface with a limited carbon solubility by elevating the temperature slightly below the melting point of the substrate.10 Graphene synthesis via CVD is the most promising, inexpensive and easily accessible method for synthesizing reasonably high-quality graphene in a large scale-up to present for electronic applications. Although chemical exfoliation routes to synthesize grapheneplatelets competitively provide some advantages rather different applications such as coating or composite reinforced materials by solution process, the intrinsic properties of multiscale thin films derived from the colloid suspensions of graphene flakes have not yet been achieved in the field of electronics.11 Thus, many efforts have been made to apply high quality CVD grown graphene to an active material for field-effect transistors (FET) or transparent conductive electrodes. This can be realized by some efficient methods of large-scale synthesis and subsequent transfer process onto desired receiving substrates such as glass, silicon wafer, or other flexible and transparent plastics, depending on the required applications. For example, as reported previously, to improve the electrical conductivity of graphene, an impressive progress was achieved on the roll-to-roll transfer-printing process in a large-scale with high uniformity at a moderate speed, utilizing a conventional polymeric supporting layer (i.e., thermal release tape) by the repeated layup stacking of separately grown CVD graphene on a single receiving substrate; this only requires a precise conformal contact of polymer-supportive graphene onto the desired substrates under optimal temperature condition.12 Similarly, another graphene-transfer method was introduced using soft elastomeric transfer-medium, which is to transfer the patterned graphene to a target substrate with a soft-mold, controlling the relative differences in the interfacial adhesive forces that are advantages of soft lithography.13 In the case of above two examples, however, it is still necessary to modify the surface energy of the supporting layer or elastomeric mold by engineering the graphene adhesion to transfer at a desired position with high


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resolution and fidelity because the multiple deposition process of monolayer graphene sheets may degrade the original quality of CVD grown graphene by repetitive contacts with unwanted polymer residues or other chemicals in the sequential processing. Besides this, there is an alternative way to control the layer numbers of graphene by tuning the carbon solubility of the catalytic metal substrate. In general, Cu and Ni are catalytic metal species mainly used for the synthesis of graphene using CVD process. Since Cu has a limited carbon solubility, graphene growth is usually suppressed on the catalytic surface, leading to the formation of monolayer graphene. On the other hand, in case of Ni, carbon molecules are sufficiently soluble inside of the metal surface in the middle of carbon feeding process and precipitated on the Ni surface in the relatively fast cooling process that induces the formation of graphene, naturally growing multilayered graphene.10 Due to the similar atomic properties (i.e., diameter) of Cu and Ni, conventional Cu-Ni alloys could be designed by simple annealing process at an elevated temperature to utilize difference in carbon solubility of Cu-Ni, which involved some metallurgical parametric studies such as sequence of deposited thickness of Cu and Ni, a distribution of Cu in Ni matrix or vice versa, and the separated defined area (e.g., patterning) of Cu and Ni in one alloying system.14 Notably, Cu-Ni alloy-based graphene synthesis is advantageous on the controlling of the numbers of graphene layers with a singlecycle CVD process without tedious multiple transfer printing procedures even for the patterned surface area.15-16 However, the delicate deposition-based alloying of Cu and Ni with an appropriate atomic weight percentage in a few micron film thickness distributions is essentially required, in which CVD conditions also should be critically optimized such as an amount of carbon feedstock, temperature, and cooling speed.17 In addition, some interesting research on bilayer graphene on a single Cu substrate has been reported, but most of the results have focused



on mechanism analysis based on the growth mode of graphene domains or stacking sequences in the grains with only partial coverage areas. The spatially defined multi-thickness scale representing the coexisting mono- and bilayer graphene films with high uniformity have not yet been reported.18-20 Herein, we report a simple route to synthesize the controlled layer-numbers of graphene by performing a sequential CVD process with precisely selective growth for multi-thickness scale, exhibiting single and bilayer graphene lattice structure within the patterned regions. Our newly developed graphene synthesis method relies on two separate CVD growth cycles on a single Cu substrate to produce self-junctioned single and bilayer graphene in a scalable and reproducible manner. This technique involves slightly optimized growth conditions for each process with exceptionally high levels of synthetic sophistication to avoid the alloying of catalytic metal substrate or multiple transfer processes that are commonly used to match the needs for various types of applications. We also investigated possible mechanisms for the regrowth CVD process, which could be important for scientific study and useful for the development of the cost-effective synthesis strategies. In addition, we implemented a built-inheterostructured single and bilayer graphene for the first time through a sequential CVD method and successfully verified its ability by realizing ultrathin-film type devices such as transparent flexible electrodes without any significant loss in performance compared to otherwise similar devices. Furthermore, as an extended step toward all‐graphene‐based electronics, we fabricated the arrays of transistors that incorporate selectively grown graphene regions for single (i.e., active channel) and bilayer graphene (i.e., source and drain) with high fidelity because selfintegration occurs in a subsequent growth step.21


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Results and discussion As illustrated schematically in Figure 1, the first step involved with a conventionally used thermal CVD process using the methane carbon feedstock with the mixture of Ar and H2 gas (see the details in Experimental Section and Supporting Information, Figure S1). Next, photolithography and mild oxygen plasma readily define the firstly grown graphene on Cu foil protected by photoresist in micron scale (Figure.1b-d), on which the monolayer graphene was patterned in line/space configuration (typical widths are 10 to 30 µm), providing the exposed catalytic surface regions (i.e., patterned bare Cu surface, Figure 1e) after stripping the photoresist off.22 This patterned graphene on Cu foil was subsequently used as a “regrowth substrate” for the slightly modified CVD process (Figure 1f). For the second growth, the conditions for the temperature and gas feeding must be carefully selected to avoid the unwanted reactions with the pre-existing graphene (see Figure S2). Surprisingly, after the second CVD growth cycle, the single grown patterned graphene layer was fully covered with newly grown graphene layer, which confirmed by Cu etching and transfer process on Si/SiO2 substrate using a supporting polymer film as shown in the inset in Figure 1f. For an immediate measurement of the results from the scheme of Figure 1, optical microscope and Raman spectroscopy were used. Figure 2a shows an optical micrograph for the stripe-patterned surface of the firstly grown graphene on Cu foil after the photolithography followed by mild oxygen plasma, corresponding to the step of Figure 1e; the dashed box in blue and green indicates the pristine monolayer and patterned removal of graphene in narrow surface regions (i.e., free Cu surface), respectively. Thus, the stripe-patterned active sites next to the



originally grown single layer of graphene were exposed and readily reacted with the carbon species, resulting in the formation of newly grown monolayer of graphene by the second CVD process (Figure 1f). Then, the regrown sample was transferred on the Si/SiO2 (300 nm) substrate after the Cu foil etching (typical sample size was 2.5 x 2.5 cm2, inset in Figure 1f). The different contrast of optical micrograph appeared in Figure 2b clearly represent a completely regrown single and few-layer of graphene across all the surface area where the regrown area is corresponding to the single grown graphene regions (blue dashed box in Figure 2a). Due to the unique optical properties of graphene on Si/SiO2 substrate, the stripe-patterned graphene with different numbers of layers were apparently measured, resulting from the increased thickness of graphene compared to the single grown area; the optical contrast was darker in the regrown graphene (red dashed box) than the single-grown graphene regions (orange dashed box).23 To examine the quality of regrown patterned graphene, the surface of the samples was explored by Raman spectroscopy. The collected Raman spectra in Figure 2c displays the characteristic 2D (~ 2687 cm-1), G (~ 1603 cm-1), and D (~ 1384 cm-1) peaks for patterned single grown graphene and two different regrown defined regions, colored in blue, red, green and orange, respectively; the colors of peaks denote the measured each region shown in Figure 2a and 2b. It has been commonly used in the graphene research field that the number of graphene can be estimated by the ratio of 2D peak and G peak intensity as metrics to distinguish monoand few-layer graphene.24 The intensity ratio of the G peak to the 2D peak (i.e., I2D/IG) colored in blue resulted from the patterned single grown graphene, where the area protected by the photoresist in the patterning and mild oxygen plasma etching process, was ~1.82, which shows the characteristics of the monolayer graphene.3,17,24-25 The peak colored in green was measured from the unprotected regions that showed the high intensity of D peak (i.e., a degree of defect in


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the structure) and a loss of observable intensity of 2D peak, indicating the plane state was disappeared in the graphene lattice. This result substantiates that the graphene etching process by O2 plasma was clearly performed on Cu foil substrate; CuO and Cu2O can be reduced by H2 annealing, and the reduced Cu surface was to be almost identical to the original surface state as reported previously.26,27 After the complete synthesis of sequential CVD growth, two different peaks were simultaneously observed on a single-substrate as appeared in red and orange, respectively (Figure 2c). Notably, in case of the peaks collected from the regrown region on the exposed bare Cu surface colored in orange, the value of the I2D/IG was increased to ~1.39 by remarkably arising in 2D band peak again after the second growth, which confirmed newly synthesized monolayer graphene. In contrast, the I2D/IG located at regrowth areas was somewhat lower than other cases, reaching values of ~1.04 over the entire regrown regions on a patterned substrate; this value well matches with a property of the “bilayer graphene” based on the meticulous arguments presented previously.3,17,24-25 For more information, Raman mapping analysis was performed to clarify the dimension of the defined graphene surface areas (120 x 120 µm2). Figure 2d and 2e show the mapping images corresponding to the I2D/IG and the full width at half maximum (FWHM) extracted by 2D mode from the second grown graphene sample on Si/SiO2 substrate. As presented in Figure 2d, the Raman mapping layout clearly provides the discrete colored areas, in which mono- and bilayer graphene was determined, fairly yielding the I2D/IG of ~1.3 (green pixels, monolayer graphene) and ~1.01 (blue pixels, bilayer graphene), respectively. The relatively uniform color-contrast distributions reveal the successful regrowth CVD process spatially guided by the patterned Cu/graphene surface (i.e., firstly grown patterned graphene substrate, Figure 1e). In addition, the 2D peak line shape can be used to analyze the number of graphene layer in Raman spectroscopy analysis since the line shape of the 2D peak is



usually shifted as the number of graphene layers increases.24-25 Thus, the FWHM value of the 2D peak has been suggested to identify the number of graphene layers. As shown in Figure 2e, the Raman mapping layout of the FWHM values consists of ~60 (green pixels) and ~47 (blue pixels) due to the differences in graphene thickness, and each region displayed with the green pixels can be expected to contain a higher number of graphene layers than the blue pixel regions as reported in the previous studies.28 Therefore in this study, the specification of the number of graphene layers in each patterned region might be confirmed with respect to the combination of the Raman peak and the FWHM analysis (i.e., the defined mono- and bilayer graphene in a single substrate). To demonstrate the versatility of the above results, we designed the micropattern-shapes of masks for photolithography such as star, circle, and square arrays performing the sequentially regrown self-junctioned graphene. As a result, we obviously found that the shape of the patterns was not limited to the regrowth approach and various patterns can be specifically implemented, thereby expanding the possible applications of graphene (see Supporting Information, Figure S3).29-30 For more structural analysis in detail, the higher-resolution images of graphene formed by the second growth were obtained by the measurement from scanning electron microscopy (SEM) and atomic force microscopy (AFM), and transmission electron microscopy (TEM) as collected in Figure 3. Figure 3a provides typical SEM images of the surface of graphene after a regrowth process, wherein the first and second grown regions are indicated separately with dashed lines. By the different contrast, differences in thickness on the regrown graphene substrate clearly appeared. However, the brighter field was not detected due to the absence of the topographic contrast at the edges of the adjacent area in-between the mono- and bilayer graphene on the Si/SiO2 substrate, which means that it constitutes a uniform continuous graphene film


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although the thickness is slightly varied at two different locations.31 Especially, the inset image in Figure 3a show more detailed boundaries of mono- and bilayer graphene that include the wrinkles across the borderline, exhibiting the unique crease structure usually observed from the CVD grown graphene,32 which was caused by the different thermal expansion coefficients between the graphene and Cu foil during the cooling process of the conventional thermal CVD process.33 Hence, this result explicitly supports our main scheme that the continuous stripepatterned graphene film can be synthesized on a single metal catalytic substrate, and it is considered that independent behavior presumably was not occurred of one another to form a connected single-graphene layer during the second growth CVD process. To measure the height difference between the mono- and bilayer graphene, AFM was used (Figure 3b). The height difference was measured to be ~0.86 nm. This value was slightly higher than the expected theoretical height difference (i.e., 0.335nm).34 This could be due to the existed ripples and wrinkles on the graphene surface or other possible contaminants that were not completely removed during the processing,34-35 preventing the accurate AFM measurements; in this observation, we only have confirmed that the regrown graphene consists of two different layers on a single substrate. TEM examination provides a most accurate way to measure the number of layers for the graphene films. The quality and number of layers of the regrown graphene were more clearly identified by the high-resolution TEM images and the diffraction patterns as described in Figure 3c and 3d. TEM image in upper panel in Figure 3c and the selected area diffraction (SAED) pattern image in left panel of Figure 3d shows a typical property of monolayer graphene.36 Especially, the SAED pattern image reveals the perfect hexagonal crystalline nature of graphene domain (marked by red hexagon in Figure 3d). On the other hand, TEM image from the regrown



region and the SAED pattern image (lower panel in Figure 3c and right panel in Figure 3d) suggest that the regrown stripe-regions has been selectively transformed to an excellent bilayer graphene throughout the sequential CVD process, and the corresponding SAED pattern differs from that of monolayer, in which two hexagonal crystal structures was appeared to be the property of overlapping of graphene domains, resulting in 12 diffraction points (marked in red and orange hexagons in right panel in Figure 3d).36 We observed two types of domain orientations, that is, almost perfect 30° apart (i.e., between red and orange arrows). On the basis of this work, the above outcomes were obviously consistent with the results obtained from optical and Raman analysis data and demonstrate that high-quality graphene films were successfully produced by optimized sequential growth condition (Supporting Information, Table S2). Figure 4 schematically illustrates the possible mechanisms for the sequential growth of graphene resulting in this study. Notably, by applying photolithographically patterned catalytic surface with graphene growth-mask, which has not attempted with conventional approaches, resulted in unprecedent results. In most reports, the exposed surface at the top surface of the Cu foil was considered the primary origin for the continuous growth of a graphene sheet under typical CVD growth conditions.37-38 However, the samples in our cases involve specific molecular regions of as-grown graphene (i.e., edges of the micropatterned monolayer graphene) and other adjacent exposed surface regions of Cu foil for the second growth as illustrated in Figure 4a. Our current hypotheses are as follows: at the initial stage of regrowth, the second layer starts to grow all over the surface area when the carbon species constantly provided in the reaction chamber at the elevated temperature because the nucleation of the second layer may occur both at the adjacent exposed surface regions and underneath the micropatterned graphene


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layer.39-41 Then, the mono- and bilayer graphene simultaneously continues to grow larger and expand their domains at the limited growth rate below the patterned graphene with the incoming carbon active species by diffusing from the edges of the patterned monolayer of graphene and the free Cu surface regions underlying mechanism of hydrogen in CVD graphene growth (left panel in Figure 4a). At this stage, it is noteworthy that we finely tuned the hydrogen ratio from 450 to 100 sccm (the standard growth condition, a flow of hydrogen is 60 sccm in our CVD system) in the fixed feedstock of argon (50 sccm) and methane (20 sccm) for the second growth (e.g., 8 step-growth, Table S2) to control the overlapped grain boundaries inducing the hydrogen-terminated edges for the newly growing graphene domains (middle panel in Figure 4a). Importantly, hydrogen-terminated edges could play an important role in passivating the edges of graphene with high-pressure hydrogen (i.e., 450 sccm), which is well known to be advantageous in terms of thermodynamic energy stabilization as the hydrogen partial pressure increases while lowering the process temperature during CVD graphene growth.39-40 We were able to determine the hydrogen effect on the multiple step-growth by varying the flow rate of the hydrogen gas as described in Figure S4. It should be acknowledged that the continuous bi- and monolayer graphene was synthesized only with the multiple step-growth by linearly decreasing the hydrogen flow rate from 450 to 200 sccm (each step: 50 sccm) to minimize the etching of graphene during the second growth. At the extremely high hydrogen flow condition (i.e., 500 sccm), the growth of monolayer graphene from the second CVD was prohibited and represented a fragmented low-quality graphene as shown in Figure S5. In contrast, at the lower hydrogen partial pressure condition (400 sccm), the first grown patterned graphene lost their shapes with a misalignment configuration, which was presumably due to the randomly grown graphene domains by the weakened blocking effect of hydrogen against the carbon species at the edges of



pre-existing graphene, regarding the lowered hydrogen concentration (Figure S6). Meanwhile, in the case of the regrowth without patterned graphene surface, the graphene was not uniformly covered all over the surface areas, instead, partially grown bilayer graphene was dominant (Figure S7). When only the patterned graphene was provided in microscale, the highly uniform patterned arrays of bilayer graphene could be formed in confined regions adjacent to monolayer graphene. With the optimized regrowth-window (Figure S2 and Table S2), the second growth CVD yielded substantially to form the continuous reconstructed mono- and bilayer graphene in a single catalytic substrate over the entire growth area (right panel in Figure 4a). From a previous report,42 an interesting study was conducted roughly to utilize small exfoliated pieces of graphitic flakes as a seed layer on a Cu foil to grow a lateral homoepitaxial graphene by CVD process, allowing the local growth of multilayered graphene that was surrounded with newly grown graphene. Similarly, to clarify our aforementioned hypothesis, we performed a sequential growth of graphene using the partially etched graphene (i.e., graphene seeds were provided) as shown in Figure 4b. Figure 4c shows a case in which the second growth yielded the surrounded bilayer graphene with the pre-existing monolayer observed in the boundary region, where the overlapped graphene domains are dominant. As with the previous study results, the first grown graphene acted as a seed layer providing some defect sites by plasma etching, where hydrogen etching might proceed along with the defects during the CVD process with comparatively highpressure hydrogen condition. It is interesting to note that overlapped grain boundaries were simultaneously appeared with various configurations when carbon species actively diffuse on the exposed Cu surface as shown in the SEM image separately on the left and right regions by the white dotted line (Figure 4c). From the one example in the partially etched region, it was observed that the second grown graphene preferred to surround the first grown graphene (marked


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by a blue arrow).42 On the other hand, the other regions grown on the pre-existed graphene was to form atomically abrupt interdomain structures by up-rising the first grown graphene (see inset illustration, i.e., underlayer growth) as described previous reports.39-41 Thus, continuous fewlayer graphene was also observed in local-growth areas (marked by red (trilayer) and yellow (bilayer) arrows) containing stitched crystalline structures together with adjacent domains in the process. Particularly, the wrinkled surface structure denoted by the red arrows clearly shows that the top-surface full-coverage of graphene film is uniformly connected in one layer. The height level was confirmed by the AFM measurement as shown in Figure 4d. Figure 5 illustrates for the more detailed regrowth process in multiple step-growth (Table S2) with corresponding SEM images at the borders to support our hypothesis. At the initial stage of regrowth, the graphene edge is attached to the Cu catalyst surface (i.e., metalpassivated graphene edge), which may block the diffusion of active carbon species (Figure 5a). However, under the high pressure hydrogen condition, the hydrogen-terminated edges of graphene become less effective against the Cu catalyst surface (i.e., detachment from the catalyst surface), thereby the active carbon species rapidly can diffuse beneath the pre-existed graphene to nucleate at the multiple locations and form the adlayer graphene (Figure 5b). As reported previously,40 the barrier energy of the diffusive active carbon species through the hydrogenterminated graphene edge (~0.84 eV) is slightly lower than that of the free Cu surface (~1.15 eV) at the regrowth temperature, that is, carbon atoms will be allowed to easily diffuse underneath the pregrown graphene continuously during the regrowth process. Additionally, the diffusion rate of the active carbon species can be thermodynamically beneficial and significantly faster on the graphene covered Cu surface than the free Cu surface because of a great reduction in the size of the activation barrier of migration of active carbon species (~0.19 eV).40 Surface morphologies



of regrown bilayer graphene and monolayer graphene was observed as shown in Figure 5c, which indicate the reconstructed polycrystalline bilayer (left) and newly grown monolayer graphene (right) simultaneously on a single substrate resulted from 10 min growth. On this stage, to clarify the quality of bi- and monolayer graphene at this stage, we performed the grain size estimation by SEM measurement; Figure S8 shows the sub-monolayer graphene (i.e., individual islands), where the graphene islands cover the substrate surface right before the complete polycrystalline graphene layer; the typical grain size of graphene was ~8-12 µm and ~5-10 µm for mono- and bilayer graphene, respectively (Figure S8). Such differences can be rationalized as follows: the nucleation of the second layer occurred beneath the micropatterned graphene layer with the incoming carbon active species by allowing the diffusion from the hydrogen terminated graphene edges. Under such a circumstance, the nucleation of graphene domains is a random event and therefore there may be less control over the nucleation and crystal orientations. The coalescence of each graphene domain can be achieved by forming covalently bonded grain boundaries with the disruption between the periodic hexagonal lattices resulting in a linear defect, which is identifiable through an increased D-peak in Raman spectra (Figure 2c). Meanwhile, we finely tuned the hydrogen ratio from 450 to 100 sccm during the regrowth; at a low hydrogen pressure, the graphene edges tend to be passivated by metal surface that e‐ectively stop the migration of the active carbon species beneath the pre-existed graphene region and thereby hydrogen terminated graphene edge yields a complete graphene layer by the coalescence of graphene islands to form a continuous reconstructed mono- and bilayer graphene, respectively, on the exposed Cu surface and beneath the patterned graphene layer over the entire growth area (Figure 3 and Figure 5d). It was clearly confirmed by Raman analysis and TEM measurement as presented in Figure 2 and 3. Conclusively, our suggested mechanism is possibly reliable because


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the diffusion of active carbon species at the underneath of the patterned graphene or the exposed Cu surface is energetically favorable than the adsorption at the hydrogen-terminated graphene edges because it is highly endothermic to break the C-H covalent bonds at the hydrogensaturated edges in the regrowth process as expected based on the meticulous arguments presented previously.40 Graphene sheet has excellent transparency, absorbing light evenly in the wide wavelength range because its thickness is only one layer of carbon atoms and the absence of a bandgap. To date, much effort has been made in many applications such as graphene-based flexible transparent electrode exhibiting excellent mechanical properties with high flexibility around 25%.43-46 For example, when applied to flexible-type touch panels or displays that are subjected to continuous mechanical stress, currently used indium tin oxide (ITO) can cause significant durability problems. Thus, it is timely important to progress a graphene technology for transparent electrode with such flexibility and a low level of sheet resistance. Figure 5 presents the summarized characterization of the electrical responses that reveals the key properties providing some preliminary insights into materials and physical aspects of newly developed graphene applications by regrowth approach. Figure 5a shows a photograph of transferred regrown graphene with a size of 2.5 x 2.5 cm2 on a transparent flexible polyethylene terephthalate (PET) substrate. We measured the transparency in the visible light frequency range to confirm the usefulness of regrown graphene as a flexible transparent conductive film. Figure 5b plots the transmittance of regrown graphene samples with different line pattern intervals (50, 25, and 10 µm: monolayer-bilayer-monolayer configuration) compared to single grown graphene, in which the slight variations were observed as ~97.6, ~97.4, ~ 96.9, and ~ 96.7% at 550 nm wavelength, respectively, showing a superior transparency. To verify the enhanced sheet



resistance of the regrown patterned graphene, Van der Pauw method was used. Fairly, the single grown graphene indicated ~648.1 Ω sq-1 that can be directly compared to the minimum sheet resistance of regrown graphene, ~116.9 Ω sq-1 (median: ~403.3 Ω sq-1) in case of 25 µm width of bilayer graphene. To within measurement uncertainty, the lowest value with moderately reduced resistance for bilayer graphene result from variations in some nonlinear structural defects due to spatial non‐uniformities in density of grain boundaries, which suggest that the sheet resistance would be improved explicitly by the optimized growth condition. In addition, a flexible nature of graphene was confirmed by bending-test on PET substrate. As shown in Figure 5c, the resistance changes were measured during 1000 times of repeated bending and relaxing under a radius curvature of 7.5 mm, indicating that a significant change in the resistance value (i.e., less than ~0.2) was not detected for all samples. Based on these results, the graphene sheet transferred onto a plastic substrate produced by regrowth process can provide some options for determining transparency and sheet resistance depending on the end use, such as a transparent touch panel or other possible applications. For a more diverse approach to demonstrate a unique capability of multiple-growth strategies for devices, we built a bottom-gated field-effect transistor (FET) using the heterostructured configuration (i.e., bilayer-monolayer-bilayer graphene) on a highly doped Si with SiO2 (300 nm) gate dielectric substrate. Figure 5d shows a representative optical micrograph of such devices, where the alternatively patterned array of mono- and bilayer graphene (see Figure 2b) was horizontally isolated by photolithographically defined mask and subsequent O2 plasma etching providing a pair of electrical probing access points as the source/drain electrodes (i.e., regrown bilayer graphene, Figure S9) for a monolayer graphene active channel with the length/width of 30 and 25 µm, respectively. Notably, this all-graphenebased FET offers a novel design concept for advanced synthetic integration without the need for


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additional metal-electrode deposition and liftoff process. Raman mapping image clearly exhibits the parallel self-junctioned layout with side-by-side structures (Figure 5e). Similar to the previous Raman results, I2D/IG (i.e., ~1.01, blue pixels) determined a bilayer graphene, yielding the I2D/IG of ~1.6 (green pixels) for a monolayer graphene. The unseen colors of the black pixels represent the etched surface regions due to the extremely weak intensity of 2D peak. Finally, electrical responses of a representative set of devices were characterized in ambient condition. Figure 5f presents a plot of the drain/source current (Ids) as a function of gate voltage (Vg) at forward sweeps between −80 and 80 V for a source/drain bias (Vds) of 0.1 V for devices (Figure S9). The device exhibits Dirac points in the positive Vg region and represents the bipolar transport behavior centered on this charge neutron point and can be operated in the hole and electron accumulation regime. The current modulation ratio (Ion/Ioff) was approximately 2-3 that incorporates the values from the lithographically patterned micro-ribbon-based graphene FETs.47 This can be improved by bandgap engineering because the graphene bandgap can be opened with lateral charge carrier confinement and edge effects by narrowing the width of the graphene channel into nanoscale as previously reported.48 The inset shows typical output characteristics (Vg: -80 to 40 from the top, 40 V step). From the transfer curve, the linear-regime mobilities, µ can be calculated using the peak transconductance and typical model. Here, we define the effective field-effect mobility, µ, as ∆ μ = ∆σ/( ∆ ) = /( ∆ )

Where L and W are respectively the channel length and width, Cg is the relative dielectric constant of the gate dielectric, and Ids, Vds, and Vg are the drain-source currents, the drain-source voltage, and the gate voltage, respectively. A maximum hole and electron mobility were



extracted as ~123.37 and ~33.01 cm2 V−1 s−1 for each device. The edge of graphene has known to be easily oxidized and influenced by surface adsorbates in the process, resulting in the dominant p-doping effect of graphene channel.49 In addition, although the median effective mobility of the micro-ribbon-based FETs built with conventional metal electrodes (e.g., Ti/Au) was ~400 cm2 V−1 s−1 in our system, the decrease in mobility observed here may attribute to the nonnegligible role of contacts in the device operation, that is, corresponding relatively ultrathin contact access points (i.e., ~2 nm, bilayer graphene). In addition, the regrown graphene film on a single substrate composed of mono- and bilayer graphene showed a slight amount of a disorder by the number of defects in the structure as well as chemical contamination or structural damages during the lithography, plasma etching, and transferring procedures in the process of tailoring the graphene films into the desired configuration. More accurate designing of the manufacturing process will be required to overcome the spatial non-uniformities for real device applications in near future. However, our results suggest that regrown graphene films by sequential CVD process can be effectively integrated to yield functioning devices, useful for basic study of graphene synthesis and with some potential for viable applications. Moreover, the built-in-heterostructured graphene devices can exploit the optically transparent and mechanically stable properties without any significant loss in performance compared to other conventional devices. CONCLUSIONS In summary, we have developed a robust strategy to generate a selectively patterned graphene sheet in spatially defined multi-thickness scale, exhibiting coexisted mono- and bilayer graphene films produced by slightly tuned conventional CVD process. Especially, the sequential


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growth technique on a single Cu substrate was used to produce self-junctioned continuous graphene structures with high fidelity in a simplified and scalable manner. Mechanisms of graphene-regrowth were also systematically studied by integrating previous information. To fully utilize regrown graphene, we demonstrated the possible accesses of transparent flexible electrodes and all-graphene-based thin film transistors that were monolithically self-integrated with heterostructured bilayer graphene electrodes. Looking to the future, graphene-based transparent conductive film synthesized by our sequential CVD process may present a viable route toward many other potential applications in transparent and flexible electronic devices such as touch screens, organic electronics, and other optoelectronics. We envision that possible alternative advanced techniques may also be applicable to complex multilayer configurations, stacking junctions, or cross-layouts that incorporate graphene and other two-dimensional (2D) materials in integrated forms that are more comprehensive than those described here.

EXPERIMETAL SECTION Synthesis of pristine monolayer graphene. Cu foil was prepared for conventional thermal CVD graphene synthesis. Prior to graphene growth, the Cu foil was located inside the quartz reaction tube, then annealed for 50 min under H2/Ar (20 sccm/50 sccm) flow from room temperature to 1050°C at a pressure of 100 mTorr or less. CH4/H2/Ar (20 sccm/60 sccm/50 sccm) reaction gas mixture was then introduced into the reaction tube for 20 min at the same temperature. The reaction gas mixture was maintained while the quartz tube was cooled down at 3 °C/s until it reached room temperature (Figure 1a). Secondary CVD process for the synthesis of regrown graphene. Parallel to the first CVD



process, sequential graphene synthesis was carried out at low pressure in a quartz tube reactor. The first grown patterned graphene on Cu foil was readily reacted with carbon species after photolithography and etching process (Figure 1b-f). The Cu foil was located inside the tube furnace, then annealed from room temperature to 950°C. Elevating the temperature, H2/Ar (20 sccm/50 sccm) flowed for 35 minutes at a pressure of 100 mTorr or less. To passivate the preexisting graphene edges during the growth, CH4/H2/Ar (20 sccm/450 sccm/50 sccm) reaction gas mixture was introduced into the tube for 10 min at the same temperature. For the regrowth at the edges of passivated graphene, the hydrogen partial pressure was gradually lowered step-by-step from 450 sccm to 100 sccm and then reacted additionally for 30 minutes. Finally, reaction gas mixture was maintained while the quartz tube was cooled down at 3 °C s-1 until it reached room temperature. Transfer of graphene onto Si/SiO2 substrate and TEM grid. After the synthesis of the graphene sheet, polymethyl methacrylate (PMMA) solution (average MW ~ 120,000, Sigma Aldrich) in toluene (10 wt %) was casted onto the CVD graphene-grown Cu foil, then spincoated at 2500 RPM for 40 s. The uncoated side with PMMA of Cu is treated by floating it on a bath of optimized etchant (etchant Type I, Transene) for 1 hr to remove the unwanted Cu, leaving a graphene/PMMA film. Deionized water baths were used to rinse the sample by floating it on the liquid surface. The graphene/PMMA film was moved between baths and etchants using a flat glass to scoop the sample. Following rinse at three times, the sample was scooped onto the desired substrate (i.e., Si/SiO2 or TEM grid). The transferred sample was placed on the hot plate with sufficient time to remove the moisture, followed by the dissolving of the PMMA layer with acetone. Photolithography and oxygen plasma treatment for graphene etching. For photolithography,


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AZ-GXR 601(AZ Electronic Materials Co., Wiesbaden, Germany), a type of photoresist (PR), was spin-coated on the graphene placed on the substrate (i.e., Cu foil or Si/SiO2 substrate) at 3500 rpm for 40 s. The Cu foil was firmly fixed to a rigid and flat supporting layer to avoid unwanted folds or wrinkles. After that, the soft backing was performed on a hot plate at 110 ° C for 90 s in order to evaporate the solvent in PR for increase the process accuracy. PR is a substance that hardens or softens in response to light, it exposes the photoresist only to the desired part through a prefabricated photomask to react. The AZ-GXR 601 used in this experiment exposes only the regions that needs to be removed as a positive PR that softens in response to light. The exposure process was carried out with an UV exposure (power of 20 mW cm-2 for 5 s). Prior to the final development process, the solvent was evaporated again through the hard backing to increase the adhesion and hardness of the photoresist to improve process accuracy. The hard backing was performed on a hot plate at 110 °C for 90 s. At the end of the process, the PR was developed to the desired patterns for 60 s. Since the rate at which PR reacts to the developer depends on the size of the pattern, the reaction time may differ. AZ-300MIF (Merck, Darmstadt, Germany) was selected as a suitable developer for this development process. A mild oxygen plasma etching was optimized with power (100 W), the flow of the oxygen gas (100 sccm) for ~1 min to remove the graphene (i.e., uncovered area). Because high power of the oxygen plasma process affects the exposed Cu foil, the graphene etching process on Cu foil of mild oxygen plasma was performed with the reduced power and flow of oxygen gas (50 W and 50 sccm, respectively) with slightly increased process time. Electrical measurements. All-graphene-based thin film transistor measurement was carried out using a semiconductor parameter analyzer (Agilent 4156A) in ambient condition. For the preparation of bottom-gated transistor geometry, SiO2 (gate dielectric, 300 nm) was wet-oxidized



on a heavily p-doped Si substrate. Raman, TEM, and SEM measurements. The graphene grown on the Cu foil was transferred to 300 nm SiO2/Si substrates and TEM grids by the polymer-assisted transfer method. (See supporting information) The transferred graphene samples were characterized by optical microscopy(Olympus BX50), Raman spectroscopy (UniNanoTech UniRAM-II, λ = 532 nm), SEM (Carl Zeiss AG - SUPRA 40VP, 5-10 kV), AFM (Park System NX10), and TEM (TALOS F200X, operated at 200 kV).


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Figure 1. Stepwise representation of bilayer graphene synthesis in the selective area by multiple CVD process. (a) Side view of typical single-layer graphene synthesis via CVD process. CVD growth yields a graphene sheet grown on Cu foil. (b,c) Process for synthesizing regrown graphene begins with lithographical patterning of pregrown graphene on a Cu surface. (d) Patterned removal of uncovered graphene on Cu foil with O2 plasma. (e) Patterned pregrown monolayer graphene after the removal of photoresist. Exposed catalytic Cu surface regions can be parallelly formed next to the pregrown graphene, followed by reduction of Cu surface and deposition of carbon species produces a new collection of regrown graphene. (f) Schematic illustration shows a newly synthesized regrown graphene, in which the unetched pregrown graphene regions further grow into additionally bilayer graphene and the other Cu exposed regions are selectively grown as a monolayer graphene. Inset show a digital image of a typical sample (2.5 x 2.5 cm2)



Figure 2. Optical micrographs and Raman analysis on the selectively patterned regrown graphene. (a) Optical micrograph of the patterned monolayer graphene before regrowth on the Cu foil. The dashed box in blue and green indicates the pregrown monolayer and patterned removal of graphene in narrow surface regions (i.e., the exposed Cu foil). (b) Optical micrograph of regrown patterned graphene transferred on the Si/SiO2 substrate. The optical contrast was darker in the regrown graphene (dashed box in red) than the single-grown graphene regions (dashed box in orange). (c) The Raman spectra from four different points marked in (a) and (b), and their respective I2D/IG. (d,e) Raman mapping images of I2D/IG and FWHM (2D) provides the discrete colored regions, in which mono- and bilayer graphene was determined.


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Figure 3. Structural characterization of regrown graphene. (a) SEM image of the surface of graphene after the regrowth process; by the different contrast, differences in thickness on the regrown graphene substrate clearly appeared. The inset indicates more detailed boundaries of mono- and bilayer graphene that include the wrinkles across the borderline. (b) AFM height image showing boundaries between different layers of regrown graphene. The height profile indicates that the thickness difference at the boundary (i.e., white dotted line in (a)). (c) Highresolution TEM image of regrown graphene clearly identifies monolayer and bilayer graphene. (d) SAED pattern images obtained from (c).



Figure 4. Sequential growth mechanism for mono- and bilayer graphene formed by first and second growths. (a) A graphical diagram of the graphene regrowth mechanism. A monolayer patterned graphene before the process begins (left panel). The disordered aperiodic graphene is represented by a green block, and the patterned graphene is described in gray lattice. During the regrowth process (middle panel), the newly growing graphene domains (blue colored block) expand their areas, where the saturated hydrogen colored in red plays an important role in the sequential growth. Bilayer graphene can be formed and surrounded with newly grown graphene by diffusion and overlapping grain boundaries. At the end of the process (right panel) (b) SEM image of pregrown graphene and partially etched graphene seeds after the controlled O2 plasma etching process. (c) SEM image of the second growth from partially etched graphene-seeding that shows overlapped grain boundaries and stitched crystalline structures. The inset illustrates the interdomain regions grown on the pre-existing graphene (i.e., underlayer growth) to form atomically abrupt structures by up-rising the first grown graphene. (d) AFM image and height profile data collected from (c).


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Figure 5. Schematic illustration of graphene regrowth mechanism and corresponding SEM images in the sequential CVD process. (a) The first-grown graphene with metal-passivated edges can be open as active sites (i.e., free Cu surface) at the beginning of the second CVD process. (b) During the regrowth, the active carbon species readily diffuse underneath pre-existing graphene through hydrogen-terminated graphene edge under the high-pressure hydrogen condition. (c) The diffusion of carbon monomers is faster underneath the pregrown graphene surface than free Cu surface. At the later stage of the regrowth, pregrown graphene with hydrogen-terminated edges returns to the state with the metal-passivated edges due to lowering hydrogen partial pressure (i.e., step-growth) (d) The graphene with the metal-passivated edges tends to form covalent bonds between adjacent graphene domains; it eventually grows into a bilayered graphene following the inverse wedding cake configuration.



Figure 5. Optical and electrical properties of regrown graphene. (a) Transferred regrown graphene on a flexible and transparent PET substrate. (b) Optical transmittance of regrown graphene on PET substrate. The blue curve indicates the transmittance of the graphene synthesized by single CVD growth. (c) A set of plots on the resistance changes measured during 1000 times of repeated bending and relaxing under a radius curvature of 7.5 mm. (d) Optical micrograph of all-graphene-based FET that incorporates to an active channel (dashed box) of monolayer graphene and bilayer graphene electrodes (i.e., source and drain). (e) Raman mapping image captured from (d) exhibiting a parallel self-junctioned layout with side-by-side structures. (f) Drain current (Ids) as a function of gate voltage (Vg) for a source/drain bias (Vds) of 0.1 V, measured on a bottom-gated transistor (LC = 10 µm, W = 10 µm). The inset shows typical output characteristics of all-graphene-based thin film transistor (Vg: -80 to 40 V from the top, 40 V step).


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ASSOCIATED CONTENT Supporting Information. Additional figures and captions for graphs, tables, and schematic illustration of regrowth steps and expected nanostructures; Optical micrographs, SEM, and Raman mapping images resulted from the control experiments in detail can be found. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions D.Y. and S.W.H. conceived the idea and designed the experiments. D.Y. and S.J. performed the experiments and characterization. D.Y., S.J., and S.W.H. wrote the paper. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Research Foundation (NRF) of Korea under the auspices of the Ministry of Science and ICT, Republic of Korea (Grant no. NRF2017R1A4A1015627, NRF-2017R1A2B4007483).



REFERENCES 1.

Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.;

Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. 2.

Stankovich, S.; Dikin, D. A.; Dommett, G. H.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E.

A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-Based Composite Materials. Nature 2006, 442, 282−286. 3.

Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec,

S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. 4.

Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.;

Stormer, H. L. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146, 351−355. 5.

Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, P.;

Choi, J. Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films for Stretchable Transparent Electrodes. Nature 2009, 457, 706−710. 6.

Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the Elastic Properties and

Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385−388. 7.

Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N.

Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902−907. 8.

Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.;

Peres, N. M.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308. 9.

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;

Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. 10.

Li, X.; Cai, W.; Colombo, L.; Ruoff, R. S. Evolution of Graphene Growth on Ni and Cu

by Carbon Isotope Labeling. Nano Lett. 2009, 9, 4268−4272. 11.

Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.;

Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets Via Chemical


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558−1565. 12.

Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim,

H. R.; Song, Y. I.; Kim, Y. J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J. H.; Hong, B. H.; Iijima, S. Rollto-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574−578. 13.

Kim, H.; Jung, M.-w.; Myung, S.; Jung, D.; Lee, S. S.; Kong, K.-j.; Lim, J.; Lee, J.-H.;

Park, C. Y.; An, K.-S. Soft Lithography of Graphene Sheets Via Surface Energy Modification. J. Mater. Chem. C 2013, 1, 1076−1079. 14.

Liu, X.; Fu, L.; Liu, N.; Gao, T.; Zhang, Y.; Liao, L.; Liu, Z. Segregation Growth of

Graphene on Cu–Ni Alloy for Precise Layer Control. J. Phys. Chem. C 2011, 115, 11976−11982. 15.

Choi, J. S.; Choi, H.; Kim, K. C.; Jeong, H. Y.; Yu, Y. J.; Kim, J. T.; Kim, J. S.; Shin, J.

W.; Cho, H.; Choi, C. G. Facile Fabrication of Properties-Controllable Graphene Sheet. Sci. Rep. 2016, 6, 24525. 16.

Yoo, M. S.; Lee, H. C.; Lee, S.; Lee, S. B.; Lee, N. S.; Cho, K. Chemical Vapor

Deposition of Bernal-Stacked Graphene on a Cu Surface by Breaking the Carbon Solubility Symmetry in Cu Foils. Adv. Mater. 2017, 29, 1700753. 17.

Chen, S.; Cai, W.; Piner, R. D.; Suk, J. W.; Wu, Y.; Ren, Y.; Kang, J.; Ruoff, R. S.

Synthesis and Characterization of Large-Area Graphene and Graphite Films on Commercial CuNi Alloy Foils. Nano Lett. 2011, 11, 3519−3525. 18.

Zhao, P.; Kim, S.; Chen, X.; Einarsson, E.; Wang, M.; Song, Y.; Wang, H.; Chiashi, S.;

Xiang, R.; Maruyama, S. Equilibrium Chemical Vapor Deposition Growth of Bernal-Stacked Bilayer Graphene. ACS Nano 2014, 8, 11631–11638. 19.

Kalbac, M.; Frank, O.; Kavan, L. The Control of Graphene Double-Layer Formation in

Copper-Catalyzed Chemical Vapor Deposition. Carbon 2012, 50, 3682–3687. 20.

Liu, W.; Li, H.; Xu, C.; Khatami, Y.; Banerjee, K. Synthesis of High-Quality Monolayer

and Bilayer Graphene On Copper Using Chemical Vapor Deposition. Carbon 2011, 49, 4122– 4130. 21.

Hong, S. W.; Du, F.; Lan, W.; Kim, S.; Kim, H. S.; Rogers, J. A. Monolithic Integration

of Arrays of Single-Walled Carbon Nanotubes and Sheets of Graphene. Adv. Mater. 2011, 23, 3821−3826. 22.

Hong, S. W.; Banks, T.; Rogers, J. A. Improved Density in Aligned Arrays of Single-



Walled Carbon Nanotubes by Sequential Chemical Vapor Deposition on Quartz. Adv. Mater. 2010, 22, 1826−1830. 23.

Roddaro, S.; Pingue, P.; Piazza, V.; Pellegrini, V.; Beltram, F. The Optical Visibility of

Graphene: Interference Colors of Ultrathin Graphite on SiO2. Nano Lett. 2007, 7, 2707−2710. 24.

Malard, L. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Raman

Spectroscopy in Graphene. Phys. Rep. 2009, 473, 51−87. 25.

Yan, K.; Peng, H.; Zhou, Y.; Li, H.; Liu, Z. Formation of Bilayer Bernal Graphene:

Layer-by-Layer Epitaxy Via Chemical Vapor Deposition. Nano Lett. 2011, 11, 1106−1110. 26.

Kim, J. Y.; Rodriguez, J. A.; Hanson, J. C.; Frenkel, A. I.; Lee, P. L. Reduction of CuO

and Cu2O with H2: H Embedding and Kinetic Effects in the Formation of Suboxides. J. Am. Chem. Soc. 2003, 125, 10684–10692. 27.

Chen, X.; Zhao, P.; Xiang, R.; Kim, S.; Cha, J.; Chiashi, S.; Maruyama, S. Chemical

Vapor Deposition Growth of 5 mm Hexagonal Single-Crystal Graphene from Ethanol. Carbon 2015, 94, 810–815. 28.

Kim, E.; Kim, Y. S.; Park, J.; Hussain, S.; Chun, S.-H.; Kim, S. J.; An, K.-S.; Lee, W.-J.;

Lee, W.-G.; Jung, J. Graphene Film Growth on Sputtered Thin Cu–Ni Alloy Film by Inductively Coupled Plasma Chemical Vapor Deposition. RSC Adv. 2014, 4, 63349−63353. 29.

Kim, D. H.; Lu, N.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S.; Wu, J.; Won, S. M.; Tao,

H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.; Huang, Y.; Coleman, T.; Rogers, J. A. Science 2011, 333, 838−843. 30.

Jang, K. I.; Han, S. Y.; Xu, S.; Mathewson, K. E.; Zhang, Y.; Jeong, J. W.; Kim, G. T.;

Webb, R. C.; Lee, J. W.; Dawidczyk, T. J.; Kim, R. H.; Song, Y. M.; Yeo, W. H.; Kim, S.; Cheng, H.; Rhee, S. I.; Chung, J.; Kim, B.; Chung, H. U.; Lee, D.; Yang, Y.; Cho, M.; Gaspar, J. G.; Carbonari, R.; Fabiani, M.; Gratton, G.; Huang, Y.; Rogers, J. A. Rugged and Breathable Forms of Stretchable Electronics with Adherent Composite Substrates for Transcutaneous Monitoring. Nat. Commun. 2014, 5, 4779. 31.

Xie, J.; Spallas, J. P. Different Contrast Mechanisms in SEM Imaging of Graphene.

Agilent Technologies, lnc., USA 2012. 32.

Deng, S.; Berry, V. Wrinkled, Rippled and Crumpled Graphene: An Overview of

Formation Mechanism, Electronic Properties, and Applications. Mater. Today 2016, 19, 197−212.


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33.

Yoon, D.; Son, Y. W.; Cheong, H. Negative Thermal Expansion Coefficient of Graphene

Measured by Raman Spectroscopy. Nano Lett. 2011, 11, 3227−3231. 34.

Shearer, C. J.; Slattery, A. D.; Stapleton, A. J.; Shapter, J. G.; Gibson, C. T. Accurate

Thickness Measurement of Graphene. Nanotechnology 2016, 27, 125704. 35.

Luo, B.; Chen, B.; Wang, A.; Geng, D.; Xu, J.; Wang, H.; Zhang, Z.; Peng, L.; Xu, Z.;

Yu, G. Chemical Vapor Deposition of Bilayer Graphene with Layer-Resolved Growth through Dynamic Pressure Control. J. Mater. Chem. C 2016, 4, 7464−7471. 36.

Tu, Z.; Liu, Z.; Li, Y.; Yang, F.; Zhang, L.; Zhao, Z.; Xu, C.; Wu, S.; Liu, H.; Yang, H.;

Richard, P. Controllable Growth of 1–7 Layers of Graphene by Chemical Vapour Deposition. Carbon 2014, 73, 252−258. 37.

Wang, H.; Wang, G.; Bao, P.; Yang, S.; Zhu, W.; Xie, X.; Zhang, W. J. Controllable

Synthesis of Submillimeter Single-Crystal Monolayer Graphene Domains on Copper Foils by Suppressing Nucleation. J. Am. Chem. Soc. 2012, 134, 3627−3630. 38.

Zhao, L.; Rim, K. T.; Zhou, H.; He, R.; Heinz, T. F.; Pinczuk, A.; Flynn, G. W.;

Pasupathy, A. N. Influence of Copper Crystal Surface on the CVD Growth of Large Area Monolayer Graphene. Solid State Commun. 2011, 151, 509−513. 39.

Nie, S.; Wu, W.; Xing, S.; Yu, Q.; Bao, J.; Pei, S.-s.; McCarty, K. F. Growth from Below:

Bilayer Graphene on Copper by Chemical Vapor Deposition. New J. Phys. 2012, 14, 093028. 40.

Zhang, X.; Wang, L.; Xin, J.; Yakobson, B. I.; Ding, F. Role of Hydrogen in Graphene

Chemical Vapor Deposition Growth on a Copper Surface. J. Am. Chem. Soc. 2014, 136, 3040−3047. 41.

Abidi, I. H.; Liu, Y.; Pan, J.; Tyagi, A.; Zhuang, M.; Zhang, Q.; Cagang, A. A.; Weng, L.-

T.; Sheng, P.; Goddard, W. A.; Luo, Z. Regulating Top-Surface Multilayer/Single-Crystal Graphene Growth by “Gettering” Carbon Diffusion at Backside of the Copper Foil. Adv. Funct. Mater. 2017, 27, 1700121. 42.

Wang, H.; Wang, G.; Bao, P.; Shao, Z.; Zhang, X.; Yang, S.; Zhu, W.; Deng, B. Lateral

Homoepitaxial Growth of Graphene. Cryst. Eng. Comm. 2014, 16, 2593-2597. 43.

Lee, S. K.; Kim, B. J.; Jang, H.; Yoon, S. C.; Lee, C.; Hong, B. H.; Rogers, J. A.; Cho, J.

H.; Ahn, J. H. Stretchable Graphene Transistors with Printed Dielectrics and Gate Electrodes. Nano Lett. 2011, 11, 4642−4646. 44.

Guo, C.; Kong, X.; Ji, H. Hot-Roll-Pressing Mediated Transfer of Chemical Vapor



Deposition Graphene for Transparent and Flexible Touch Screen with Low Sheet-Resistance. J. Nanosci. Nanotechnol. 2018, 18, 4337−4342. 45.

Li, X.; Xie, D.; Park, H.; Zeng, T. H.; Wang, K.; Wei, J.; Zhong, M.; Wu, D.; Kong, J.;

Zhu, H. Anomalous Behaviors of Graphene Transparent Conductors in Graphene-Silicon Heterojunction Solar Cells. Adv. Energy Mater. 2013, 3, 1029−1034. 46.

Shi, J.; Li, X.; Cheng, H.; Liu, Z.; Zhao, L.; Yang, T.; Dai, Z.; Cheng, Z.; Shi, E.; Yang,

L.; Zhang, Z.; Cao, A.; Zhu, H.; Fang, Y. Graphene Reinforced Carbon Nanotube Networks for Wearable Strain Sensors. Adv. Funct. Mater. 2016, 26, 2078−2084. 47.

Yan, Z.; Peng, Z.; Sun, Z.; Yao, J.; Zhu, Y.; Liu, Z.; Ajayan, P. M.; Tour, J. M. Growth of

Bilayer Graphene on Insulating Substrates. ACS Nano 2011, 5, 8187−8192. 48.

Kang, S. H.; Hwang, W. S.; Lin, Z.; Kwon, S. H.; Hong, S. W. A Robust Highly Aligned

DNA Nanowire Array-Enabled Lithography for Graphene Nanoribbon Transistors. Nano Lett. 2015, 15, 7913−7920. 49.

Wang, S.; Ang, P. K.; Wang, Z.; Tang, A. L.; Thong, J. T.; Loh, K. P. High Mobility,

Printable, and Solution-Processed Graphene Electronics. Nano Lett. 2010, 10, 92−98.


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Table of Contents Graphic and Synopsis:


Selectively Patterned Regrowth of Bilayer Graphene for the Self

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