Ultrafast Transition of Nonuniform Graphene to High-Quality Uniform

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Functional Nanostructured Materials (including low-D carbon)

Ultrafast Transition of Non-Uniform Graphene to High-Quality Uniform Monolayer Film on Liquid Cu Xing Xin, Chuan Xu, Dingdong Zhang, Zhibo Liu, Wei Ma, Xitang Qian, Maolin Chen, Jinhong Du, Hui-Ming Cheng, and Wencai Ren ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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ACS Applied Materials & Interfaces

Ultrafast Transition of Non-Uniform Graphene to High-Quality Uniform Monolayer Film on Liquid Cu Xing Xin1,2‡, Chuan Xu1‡, Dingdong Zhang1,3, Zhibo Liu1, Wei Ma1,3, Xitang Qian1,3, Mao-Lin Chen1,3, Jinhong Du1,3, Hui-Ming Cheng1,3,4*, Wencai Ren1,3* 1 Shenyang

National Laboratory for Materials Science, Institute of Metal Research,

Chinese Academy of Sciences, Shenyang 110016, P. R. China. 2 University

3 School

of Chinese Academy of Sciences, Shenyang 110016, P. R. China.

of Material Science and Engineering, University of Science and Technology

of China, Shenyang 110016, P. R. China. 4 Tsinghua-Berkeley

Shenzhen Institute (TBSI), Tsinghua University, 1001 Xueyuan

Road, Shenzhen 518055, P. R. China. *Correspondence to: [email protected], [email protected] ‡These authors contributed equally to this work.

KEYWORDS: graphene, 2D material, film, chemical vapor deposition, number of layers.

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ABSTRACT: It is essentially important to synthesize uniform graphene films with controlled number of layers since their properties strongly depend on the number of layers. Although chemical vapor deposition (CVD) on Cu has been widely used to synthesize large-area graphene films, the growth on solid and liquid Cu suffers from poor thickness uniformity with a great number of adlayers and difficulty in forming continuous film even after a long growth time of hours, respectively. Here, we found that non-uniform graphene film initially grown on solid Cu foil can rapidly transform into continuously uniform monolayer graphene film on liquid Cu within 3 min. Moreover, the films obtained show larger grain size, higher quality, better optical and electrical properties and better performance in organic light-emitting diode (OLED) applications than the original films grown on solid Cu foil. By using carbon isotope labeling, we revealed that the multilayer-to-monolayer transition of graphene on liquid Cu experiences etching-‘self-aligning’-coalescence processes. This two-step CVD method not only opens up a new way for the rapid growth of uniform monolayer graphene films, but also provides helpful information for the controlled growth of uniform monolayers of other 2D materials such as monolayer h-BN.

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INTRODUCTION Graphene, a perfect two-dimensional crystal that consists of a single layer of sp2hybridized carbon atoms, has attracted numerous attention due to its unusual electronic, optical, thermal and mechanical properties.1-4 Currently, chemical vapor deposition (CVD) is the most commonly used method to synthesize large-area high-quality graphene films. In particular, polycrystalline Cu foil has shown a great potential to grow monolayer graphene at a low cost because of the low carbon solubility, which allows self-limited surface adsorption growth of graphene.5 In fact, however, the graphene grown on solid Cu (S-Cu) foil usually has a great number of adlayers6 because of the non-uniformity of the S-Cu surfaces such as defects and grain boundaries. In addition, it has been found that extra carbon supply also can easily lead to the formation of multilayers.7 Several strategies have been developed to reduce the formation of adlayers on S-Cu, such as electropolishing, long-time annealing and monocrystallization of Cu foils, using single-crystal Cu as substrate, and optimizing growth parameters.8-15 Alternatively, liquid Cu (L-Cu) provides a uniform surface free of defects and grain boundaries, which enables the growth of self-aligned, uniform, hexagonal monolayer single-crystal graphene domains.16,17 However, these graphene domains are difficult to join together on L-Cu to form a continuous film even after a very long growth period of hours.18-20 It has been reported that the uniformity of the graphene grown on S-Cu can be significantly improved after a long-time treatment on L-Cu, but there are still many exposed Cu areas without graphene.21 Therefore, efficient growing strictly monolayer graphene film by CVD is still very challenging. 3



Here, we developed a two-step CVD method, which can rapidly transform nonuniform graphene film initially grown on S-Cu into continuously uniform monolayer graphene film within 3 min on L-Cu. This two-step CVD method overcomes the problems of inevitable inhomogeneous growth of graphene on S-Cu foil and difficulty in coalescence of graphene with extremely long time on L-Cu. Compared with the original graphene grown on S-Cu foil, both the grain size and quality of graphene films are improved after the treatment on L-Cu in the second step. As a result, graphene films obtained by two-step CVD method show better optical and electrical properties and better performance in organic light-emitting diode (OLED) applications. By using carbon isotope labeling, we revealed that the multilayer-to-monolayer transition of graphene on L-Cu experiences etching-‘self-aligning’-coalescence processes. RESULTS AND DISCUSSION Figure 1a and Figure S-1 illustrate our two-step CVD process for the growth of continuously uniform monolayer graphene films, which involves the growth of continuously non-uniform graphene films on S-Cu foils (Step 1) and following shorttime treatment on L-Cu surface (Step 2). A 25-µm-thick Cu foil sitting on a 50-µmthick W foil was used as the growth substrate. After the growth temperature reached 1,070 °C under hydrogen (H2) and argon (Ar) flow, methane (CH4) was introduced to initial graphene grown on S-Cu foil (Step 1). As the continuous graphene film was formed, we raised the growth temperature above the melting point of Cu (1,084 °C) to 1,090 °C while keeping all the other parameters constant (Step 2). According to the WCu phase diagram,22 W and Cu are totally immiscible and retain their own chemical 4


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and physical properties without forming an alloy. The good wettability of liquid Cu on W enables a smooth L-Cu layer on the surface of W foil. After 3 min, the L-Cu substrate was cooled down slowly to 1,000 °C and then quickly removed from the hightemperature zone (see Methods). Figure 1b shows the as-grown graphene film on the re-solidified Cu surface. For comparison, we also studied the direct growth of graphene on L-Cu with the same conditions as those used in the second step of the two-step CVD. After growth, the graphene films were transferred to SiO2/Si substrates, polyethylene terephthalate (PET) or transmission electron microscopy (TEM) grids by chemical etching method with poly(methyl methacrylate) (PMMA) as protecting layer for further characterizations and applications (see Methods).

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Figure 1. The two-step CVD process and comparison of the graphene films grown by different methods. (a) Schematic of the multilayer-to-monolayer transition in the twostep CVD. We used S+L-Cu to represent the substrate used in the two-step CVD. (b) Photo of a graphene film grown by two-step CVD. (c–e) High-magnification SEM images of graphene films grown on (c) S-Cu, (d) S+L-Cu, and (e) L-Cu. (f–h) Optical images of graphene films grown on (f) S-Cu, (g) S+L-Cu, and (h) L-Cu, which have been transferred onto SiO2/Si substrates. The blue arrows show the graphene adlayers, while the red arrows show the exposed Cu without graphene. We first used scanning electron microscopy (SEM) and optical microscopy to study the coverage, thickness and uniformity of the graphene grown by different methods. As shown in Figure 1c, f, the graphene grown on S-Cu foil in the first step forms a continuous film, but has a great number of randomly distributed adlayers with different thicknesses and sizes. Very surprisingly, after 3-min-treatment on L-Cu in the second step, the adlayers completely disappear and a continuous strictly monolayer graphene film is obtained without any uncovered Cu area (Figure 1d, g). Although the graphene directly grown on L-Cu is more uniform than those grown on S-Cu, there still exist many exposed Cu areas without graphene even after growth for 2 hours (Figure 1e, h), similar to those reported previously.18-20 Moreover, multilayers can be easily observed at the edges of graphene film, as indicated by the blue arrow in Figure 1h. The lowmagnification SEM images of these graphene films across large area show the same results (Figure S-2). The roughness of graphene films was also measured by using atomic force microscope (AFM) (Figure S-3). It can be seen that the graphene films 6


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grown on S+L-Cu and L-Cu are more uniform and have less wrinkles and PMMA residues than that on S-Cu, and consequently show a smaller surface roughness. This method can also be used to transform previously synthesized non-uniform graphene films on S-Cu just by high temperature annealing over the melting point of Cu for a very short time. We first synthesized graphene film with many multilayer domains on S-Cu foil (Figure S-4a). Then, we put the sample on the top of a W foil and placed the stack in the low temperature zone of a CVD chamber. Once the temperature reached 1,090 °C, the stack was quickly pushed into the chamber center. Surprisingly, after 3 min annealing on L-Cu, continuously uniform monolayer graphene film was obtained as shown in Figure S-4b. Therefore, this method provides a general strategy for ultrafast growth of uniform monolayer graphene films.

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Figure 2. Strucutre characterizations of graphene films grown by different methods. (a–c) Typical Raman spectra of graphene grown on (a) S-Cu, (b) S+L-Cu, and (c) LCu. (d–f) Raman maps of IG/I2D of graphene films grown on (d) S-Cu, (e) S+L-Cu, and (f) L-Cu. (g–i) Raman maps of D peak intensity of graphene films grown on (g) S-Cu, (h) S+L-Cu, and (i) L-Cu. The graphene films in (a-i) have been transferred onto 8


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SiO2/Si substrates. (j, k) HRTEM images of (j) the edge and (k) the interior of a graphene film grown by two-step CVD method. Raman spectroscopy provides a versatile tool for studying the detailed structure of graphene materials such as the number and orientation of layers, the quality and the types of defects, and the effects of perturbations.23 We further used Raman spectroscopy to study the uniformity and quality of the graphene films grown by different methods (Figure 2). As shown in Figure 2a–c, the graphene films grown on both S-Cu and L-Cu show large variation in the intensity ratio of G peak to 2D peak (IG/I2D), indicating the non-uniformity of the number of layers. In contrast, the graphene films grown by two-step CVD show very uniform Raman spectra with IG/I2D of ~0.5, a typical value for monolayer graphene. The IG/I2D Raman maps (Figure 2d–f) further show the presence of a great number of adlayers on the graphene grown on S-Cu, exposed Cu areas and multilayers around them on the graphene grown on L-Cu, and the uniform monolayer graphene grown by two-step CVD. The Raman map of IG/I2D across an area of 140 μm × 120 μm (Figure S-5) confirms that the graphene film obtained by two-step method is very uniform and continuous over large area. Furthermore, the graphene film obtained by two-step CVD method shows much higher quality than those grown on S-Cu in the first step. Importantly, the two-step growth makes the transfer of graphene grown on L-Cu much easier. Note that it usually takes double time to transfer the graphene directly grown on L-Cu than that on S+LCu. The greatly shortened etching time of Cu reduces the damages on graphene during transfer. As a result, the graphene grown on S+L-Cu shows better quality than that 9



directly grown on L-Cu after transfer although the as-grown samples have similar quality (Figure S-6). As shown in Figure 2a–c, g–i, and Figure S-6, no visible defectrelated D peak is observed for the transferred graphene grown on S+L-Cu , while strong D peaks are frequently seen in both the as-grown and transferred graphene grown on SCu (less etching time) and the transferred graphene directly grown on L-Cu. Highresolution TEM (HRTEM) measurements on the graphene films grown by two-step CVD show a single lattice fringe from the edge and the six-fold symmetry of the graphene lattice without any vacancies, topological defects, carbon adatoms and dislocations (Figure 2j, k), further confirming that the samples are high-quality monolayer graphene .

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Figure 3. Grain structure evolution of graphene films during two-step CVD. (a, b) SEM images of individual graphene grains that are obtained by etching the graphene films grown (a) on S-Cu and (b) by two-step CVD with H2 (200 sccm, 2 min). The etching was performed after the substrate was cooled down to 1,000 °C. (c, d) Histograms of (c) grain size and (d) orientation angle distribution obtained from 40 hexagonal graphene domains in the etched samples for each case. The diagonal length of the hexagonal domains was used as the grain size, and the angle of the diagonal line relative to the horizontal direction, θ, was used as the orientation angle (inset of d). More importantly, the grain size is largely increased and the grain orientations become more aligned after the second-step-treatment on L-Cu (Figure 3). As we known, the etching of graphene film starts from the grain boundaries and defective sites because of their high reactivity,24,25 which allows for the identification of grain structure and defects in the film. To show the grain structure evolution, the graphene films grown after the first and second step were etched under the same hydrogen flow and etching time. As shown in Figure 3a and 3b, individual graphene domains are exposed after etching, which show hexagonal shape, indicating that they are single crystals. We then measured the grain size and orientation angles relative to the horizontal direction of the individual domains, and Figure 3c and 3d present the corresponding histograms obtained from 40 domains for each case. It is clearly seen that the grains in the graphene films grown on S-Cu in the first step are dominantly 15–20 m in size and randomly oriented, while those in the graphene films grown by two-step CVD are dominantly 40– 50 m in size and have small relative orientation angles. Because of the ordered grains, 11



it is expected that low-tilt-angle grain boundaries should be dominated in the graphene films obtained by two-step CVD. To further confirm the above observations, we studied the structural evolution of graphene domains grown on S-Cu with the treatment time on L-Cu in the second step (Figure S-7). It can be seen that the graphene domains initially grown on S-Cu are very small (~10 µm) and randomly oriented. Interestingly, these domains gradually expand and their orientations become more ordered with prolonging the treatment time on LCu. After 20 min, the domain size increases by 8 times to ~90 µm and the relative orientation angles are dominantly distributed below 15º. These results give further evidence that the treatment on L-Cu can greatly increase the grain size and improve the grain alignment of the graphene originally grown on S-Cu in the two-step CVD process. The increase in the grain size is attributed to the coalescence of perfectly aligned domains. Different from S-Cu, L-Cu surface is movable and very uniform without grain boundaries. Such movable uniform substrate plays a key role in the aligning of neighboring grains to form larger grains by merging.16,17,20 Selective area electron diffraction (SAED) patterns confirm the formation of larger single-crystal grains in the graphene film grown on S+L-Cu than the graphene film grown on S-Cu (Figure S-8 and S-9).

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Figure 4. Optical and electrical properties of graphene films and their applications in OLEDs. (a) Transmittance of graphene films grown on S-Cu and S+L-Cu. (b, c) Sheet resistance (Rs) map of the graphene film grown on (b) S+L-Cu and (c) S-Cu. (d) Device structure of green OLEDs. (e) The OLEDs operated at different voltages from 2.6 to 6.6 V. (f) Normalized electroluminescence spectra obtained at different voltages corresponding to (e). (g) Current efficiency (CE) and power efficiency (PE) versus luminance of OLEDs made with graphene films grown on S-Cu and S+L-Cu as anodes. GF in (a) and (g) represents graphene film. We then compared the optical and electrical properties of the graphene films grown on S-Cu in the first step and those obtained by two-step CVD after transferring them onto PET and SiO2/Si substrates. The uniform monolayer graphene film grown by twostep CVD shows a higher transmittance (96.9%) than the non-uniform graphene with 13



adlayers grown on S-Cu (96.0%) at 550 nm wavelength (Figure 4a). The small decrease in transmittance compared to ideal monolayer graphene (97.7%) is attributed to the PMMA residues introduced in the transfer process. It has been reported that the electronic transport properties of graphene strongly depend on the grain size and grain orientation. Normally, the electrical conductivity decreases with decreasing grain size and increasing the tilt angle of grain boundaries.26-29 Because of the large grain size and small relative orientation angle mentioned above, the graphene grown by the two-step CVD shows uniform sheet resistance from 607 to 642 Ω per square on PET (Figure 4b), while that grown on S-Cu shows higher and non-uniform sheet resistance from 610 to 832 Ω per square (Figure 4c). The electronic transport properties of graphene films transferred on SiO2/Si were studied by fabricating back-gate field effect transistors (FETs). The channel width and length for the FET devices were 200 μm and 100 μm, respectively (Figure S-10a). Note that such devices are much larger than those reported previously, typically smaller than 10 μm  10 μm.6,9,30,31 Under ambient conditions at room temperature, the hole mobility obtained for the graphene film grown on S+L-Cu is ~4,489 cm2V-1s-1, which is higher than the value (~3,417 cm2V-1s-1) obtained for the graphene film grown on S-Cu (Figure S-10b). The uniform and better optical and electrical properties give further evidence of the advantages of our two-step CVD method for growing high-quality uniform monolayer graphene films and suggest their potential for electronic and optoelectronic applications. To demonstrate the optoelectronic applications, we fabricated flexible phosphorescent green OLED devices with a typical lighting area of 0.4  0.4 cm2 by 14


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using the uniform monolayer graphene films obtained by two-step CVD method as anodes (Figure 4d–g). It can be seen that with increasing the voltage, the brightness of the device significantly increases while keeping the identical electroluminescence spectra (Figure 4e, f), indicate the good stability of our devices. Furthermore, the maximum current efficiency (56 cd A-1) and power efficiency (66 lm W-1) are much higher than those (48 cd A-1 and 55 lm W-1) of OLEDs fabricated using graphene films grown on S-Cu as anode (Figure 4g). In order to understand the formation mechanism of uniform monolayer graphene, we systematically studied the influence of growth parameters on the formation of graphene films during the two-step CVD process. First, it is found that continuous graphene film grown on S-Cu is the prerequisite for the formation of continuously uniform monolayer graphene film on L-Cu in the second step. Only discontinuous graphene film is obtained when the graphene formed in the first step is discontinuous (Figure S-11). Second, the uniformity and continuity of the graphene films obtained by two-step CVD strongly depends on the treatment time on L-Cu in the second step. Note that there are still some irregular adlayers after 1 min treatment, 100% continuous monolayer is obtained after 3 min, while some areas are etched away if further extending the treatment time to 5 min (Figure S-12). Third, continuous CH4 supply is essential for the formation of uniform monolayer graphene. If stopping the feeding of methane in the second step, the graphene films are intensively etched, leaving a large portion of Cu surface exposed (Figure S-13). This means that the continuously fed CH4 provides carbon for the formation of monolayer graphene on L-Cu. 15



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The structural evolution of graphene films during the two steps was further studied by

13C

isotopic tracing experiments, which have been widely used to monitor the

growth process and reveal the growth mechanism of graphene. After 13CH4 and 12CH4 are fed into the CVD reactor with pre-designed time sequences to grow graphene, the isotope compositions of each region in graphene can be detected by Raman spectroscopy since Raman mode frequency strongly depends on the atomic fractions of 12C

and

13C.5,32-34

If the carbon atoms for graphene growth consist of

13C

and

12C,

a

mixture of isotopes can be found in Raman G peak. If the carbon atoms for graphene growth come from only one type of isotope, Raman G peak characteristic of 13C or 12C will appear. By relating the isotope composition to the precursor feeding sequence, the growth process of graphene can be obtained. For example, Li et al. studied the growth mechanism of graphene on Ni and Cu by sequentially feeding 13CH4 and 12CH4.5 The graphene film grown on Ni shows Raman G peak characteristic of a homogeneous mixture of 13C and 12C, indicating that the sequentially fed isotopic carbon diffuses into the Ni first, mixes, and then segregates and precipitates at the surface of Ni forming graphene. In contrast, the graphene film grown on Cu shows alternative

13C

and

12C

isotopic rings following the precursor feeding sequence, suggesting the surface adsorption growth mechanism of graphene on Cu. The schematic diagram of our isotopic tracing experiments is shown in Figure S14. We alternated 13CH4 and 12CH4 three times for periods of 5 min in the first step and then kept 12CH4 supply for 5 min in the second step, and used Raman spectroscopy to map the isotopic composition of the grown graphene. As shown in Figure 1, continuous 16


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monolayer graphene film with many adlayers was formed on Step 1 and these adlayers disappeared after Step 2. As reported previously, the graphene grown on Cu is expected to show alternative

13C

and

12C

isotopic rings because of the surface adsorption

mechanism,5 but mixed isotopes are observed in the uniform monolayer graphene film obtained after treatment on L-Cu in Step 2 (Figure 5a-d). Considering that only 12CH4 was supplied in Step 2,

13C

in the mixed isotopes should come from the fed

13CH

4

during Step 1. Therefore, the presence of mixed isotopes and disappearance of adlayers observed in the obtained monolayer graphene indicate that (1) the pre-formed monolayer on S-Cu experiences etching and regrowth on L-Cu, and (2) the adlayers containing 13C were etched on L-Cu to form carbon atoms, which act as carbon source together with the fed 12CH4 for the regrowth of survived graphene domains. In addition, the joint area of two neighboring graphene domains initially grown on S-Cu also shows mixed isotopes (Figure 5c, d), indicating that these small grains underwent etching and regrowth as well, which provide the possibility for self-aligning to form more ordered large grains (Figure 5c and Figure 3).

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Figure 5. The formation process of monolayer graphene film by two-step CVD. (a) Raman map of the overall G13 and G12 intensity acquired from the continuously uniform monolayer graphene film. (b) Raman spectra taken from the four different positions marked in (a), showing the spectra of pure 13C or 12C and a mixture of them. Note that the regions marked with blue dots show isotopic rings, while the regions marked with red dots which correspond to the etched regions in Figure S-13 show mixed isotopes. (c) Raman map of the overall G13 and G12 intensity acquired from the edge of the uniform monolayer graphene film. (d) Raman spectra taken from the five different positions marked in (c). Note that the regions between two neighboring grains show mixed isotopes (red dots), indicating that etching-regrowth process occurred in these regions. In (a and c), the dark brown and yellow represents 13C and 12C, respectively, while the light brown represents the mixed isotopes. (e–h) Schematic of the formation mechanism of uniform monolayer graphene films by two-step CVD, which includes 18


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etching-‘self-aligning’-coalescence processes. The light grey polygons represent the grains of monolayer graphene film, the dark grey and white areas represent the graphene adlayers and exposed Cu, respectively. According to the above results, the formation mechanism of the continuously uniform monolayer graphene films by two-step CVD can be summarized as follows (Figure 5e–h). First, non-uniform continuous graphene films with many adlayers underneath were formed on S-Cu (Figure 5e) by the surface adsorption mechanism.5 It is well known that etching and attachment of carbon atoms from/onto the edges are two competitive processes for the growth of graphene domains. Note that direct growth of graphene on L-Cu with even much higher flow rate of CH4 as carbon source only leads to discontinuous film (Figure S-15), indicating much stronger etching capability of LCu than S-Cu. Therefore, when the solid copper transform into liquid phase, the highly active sites, such as defects, grain boundaries and adlayers with abundant edges,24,35-37 are preferentially and strongly etched (Figure 5f). On the one hand, the etching of grain boundaries results in separated graphene domains. As reported,16-20 these domains tend to be self-aligned on the movable L-Cu surface. On the other hand, the etching of adlayers leads to the formation of uniform monolayers (Figure 5g, h). Moreover, the etched adlayers play a key role to provide sufficient carbon, together with those produced from the decomposition of CH4, to enable the coalescence of self-aligned adjacent graphene domains to form bigger domains20,38,39 (Figure 5h). As reported previously, this etching-regrowth process can also heal the defects.18 This is another reason why the graphene films obtained by two-step CVD have much better quality and 19



properties than those grown on S-Cu in the first step. CONCLUSION In summary, we demonstrated that continuously non-uniform graphene film grown on S-Cu can be rapidly transformed into continuously uniform monolayer graphene films on L-Cu in 3 min. The obtained films show larger grain size, higher quality, better optical and electrical properties and better performance in OLED applications than the original films grown on S-Cu. Moreover, we revealed that the multilayer-to-monolayer transition on L-Cu experiences etching-‘self-aligning’-coalescence processes by using carbon isotope labeling. This two-step CVD method not only opens up a new way for the rapid growth of uniform monolayer graphene films, but also provides helpful information for the controlled growth of uniform monolayers of other 2D materials such as monolayer h-BN. METHODS CVD growth of continuously uniform monolayer graphene films. A 25-µm-thick Cu foil with a purity of 99.999 wt.% was placed onto a 50-µm-thick W foil with a purity of 99.5 wt.% (Alfa Aesar China Co., Ltd) as the substrate. Before use, the W foil and Cu foil were ultrasonicated in acetone and ethanol for 10 min each, then dried under a gentle nitrogen stream. The CVD growth of the uniform monolayer graphene films was carried out in a fused-silica reaction tube (inner diameter: 22 mm) under ambient pressure. As shown in Figure S-1, the growth process contains four steps: (1) heating the S-Cu substrate to 1,070 °C at a rate of 15–40 °C min-1 under H2 flow (200 sccm) 20


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and Ar flow (500 sccm); (2) introducing CH4 (1.5 sccm) as the carbon source, while keeping other parameters constant, to grow graphene film with many adlayers on S-Cu for 20 min; (3) heating the S-Cu substrate over the melting point of Cu (1,084 °C) to 1,090 °C in 2 min to make Cu change from solid to liquid phase, while keeping other parameters constant, and then keeping the reaction for 3 min on L-Cu to form uniform monolayer graphene; (4) cooling the substrates to room temperature at a rate of 15– 30 °C min-1 under the protection of Ar (100 sccm). Transfer of the graphene films. We used a wet etching method to transfer the graphene films from Cu substrates to 300-nm-thick SiO2/Si wafers, PET or TEM grids. A thin layer of PMMA (996 kDa molecular weight, 4 wt.% in ethyl lactate) was first spin coated on the surface of graphene films at 2,000–2,500 r.p.m. for 60 s and dried at 160 °C for 30 min. The PMMA-coated substrate was then immersed in a 0.2 M (NH4)2S2O8 solution at 70 °C for ~10 min (graphene grown on S-Cu), ~20 min (graphene grown on S+L-Cu) and 40 min (graphene grown on L-Cu) to etch the Cu substrate. Finally, the separated PMMA/graphene bilayer was collected on a target substrate, and cleaned with warm acetone (50 °C) to remove the PMMA layer. Fabrication of OLED devices. First, a 2.0 × 2.0 cm2 graphene transferred onto PET were patterned by covering with a shadow mask and subsequently rubbing away the uncovered areas. Then, the obtained graphene-based anodes were loaded into a high vacuum chamber of 8 × 10-4 Pa for the deposition of a 5–7 nm MoO3 layer. After that, phosphorescent green OLEDs were fabricated by subsequently depositing a 60 nm 1,121



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di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) hole transportation layer, a 8 nm

4,4′,4″-tris(N-carbazolyl)triphenylamine

(TCTA)

layer

and

a

8

nm

bathophenanthroline (Bphen) light emission layer doped with 8% bis(2-phenylpyridine) (acetylacetonate)iridium(Ш) [Ir(ppy)2(acac)], a 60 nm Bphen electron transportation layer and a 0.5 nm Li/130 nm Al cathode. The active area defined by the cathode is 0.4 × 0.4 cm2. Fabrication of FET devices. The graphene films were first transferred onto highly doped Si substrates with a 100 nm-thick SiO2 layer, then a 5nm Ti/50 nm Au layer was deposited by electron beam evaporation as source and drain electrode, and finally the graphene films were patterned by photolithography and O2 plasma. The channel width and length were 200 μm and 100 μm, respectively. Characterization. Optical microscope (Nikon LV100D), SEM (Nova NanoSEM 430, acceleration voltage of 10 kV) and Raman spectrometer (LabRAM HR800, 532 nm laser) were used to characterize the morphology and structure of the graphene films on Cu and SiO2/Si substrates. For Raman measurements, the laser spot size was about 1 μm with a laser power below 2 mW to avoid laser-heating-induced sample damage. AFM (Bruker Multimode 8) was used to characterize the roughness of the graphene films. SAED measurements were performed in FEI Tecnai T12 (120 kV) to identify the grain structure of the graphene films. HRTEM (FEI Tecnai G2 F20, acceleration voltage of 200 kV) and aberration-corrected TEM (FEI Titan Cube Themis G2 300, 60 kV) were used to characterize the detailed structure of the graphene films transferred 22


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onto a TEM grid. The sheet resistance and transmittance of the graphene films transferred onto PET were measured by a 4-probe resistivity measurement system (RTS-9, Guangzhou, China) and UV-vis-NIR spectrometer (Agilent Model Cary 5E), respectively. Electrical properties of FETs were measured under ambient conditions at room temperature using an Agilent semiconductor parameter analyzer (4155C Semiconductor Parameter Analyzer). Current-brightness-voltage characteristics of the unencapsulated OLEDs were characterized by Keithley source measurement units (Keithley 2450 and Photo Research, Inc. PR-655) with a calibrated silicon photodiode in air. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Schematic of the two-step CVD process, SEM image, AFM image, Raman data, TEM image, SAED pattern and the back-gate FET device. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (W.C.R). *E-mail: [email protected] (H.M.C).

23



Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (No. 2016YFA0200101), National Natural Science Foundation of China (Nos. 51325205, 51290273, 51521091, 51572265 and 51861135201), Chinese Academy of Sciences (Nos. KGZD-EW-303-1, and KGZD-EW-T06), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB30000000) and the Youth Innovation Promotion Association of Chinese Academy of Sciences. ABBREVIATIONS S-Cu, solid Cu; L-Cu, liquid Cu; IG/I2D, the intensity ratio of G peak to 2D peak; GF, graphene film; Rs, sheet resistance REFERENCES (1)

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Ultrafast Transition of Nonuniform Graphene to High-Quality Uniform

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