Controlling Fundamental Fluctuations for Reproducible Growth of

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Controlling Fundamental Fluctuations for Reproducible Growth of Large Single-Crystal Graphene Wei Guo, Bin Wu, Shuai Wang, and Yunqi Liu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Controlling Fundamental Fluctuations for Reproducible Growth of Large Single-Crystal Graphene Wei Guo†,‡, Bin Wu*,†, Shuai Wang*,‡, Yunqi Liu*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,

Institute of Chemistry, Chinese Academy of Science, Beijing 100190, P. R. China E-mail: [email protected]; [email protected]

‡

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of

Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China E-mail: [email protected]

KEYWORDS: graphene, single-crystal, fluctuation, gaseous oxidants, ppm level

ABSTRACT The controlled growth of graphene by chemical vapor deposition method is vital for its various applications, however, the reproducibility remains a great challenge. Here, using single-crystal graphene growth on a Cu surface as a model system, we demonstrate that trace

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amount of H2O and O2 impurity gases in reaction chamber is a key for the large fluctuation of graphene growth. By precisely controlling their ppm-levelled concentrations, centimeter-sized single-crystal graphene is obtained in a reliable manner with a maximum growth rate up to 190 µm min–1. The roles of oxidants are elucidated as an effective modulator for both graphene nucleation density and growth rate. This control is more fundamental for reliable growth of graphene beyond previous findings, and is expected to be useful for the growth of various 2D materials that are also sensitive to trace oxidant impurities. Fluctuation is an intrinsic and universal nature in the equilibrium and non-equilibrium macroscopic systems. With the increment of the degree of freedom of the specific system, the fluctuation tends to be more prominent, leading to a dramatic variation of final results beyond the thermodynamic fluctuation under a similar initial preparation conditions. The phenomenon origins largely from the existence of multiple interacted/coupled non-linearity processes, in which a small change of process controlling factors can result in a significant change in a final state of a dynamic system. Understanding the processes and controlling parameters to suppress the fluctuation in scientific or technological interesting systems have been a long-standing effort. Graphene growth on a Cu surface by chemical vapor deposition (CVD) is such a complex system, in which graphene is formed on a supporting substrate from atomistic building blocks by a phase transition under a certain temperature, pressure and the flow of various gases. This method provides a promising platform for the synthesis of large-scale high quality graphene film or single-crystal.1–4 Especially, previous extensive studies have found that the fluctuation in graphene growth is dramatic. For example, graphene growth results appear to be very sensitive to the tiny changes of experimental parameters and the similar conditions results in a large scatter


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of the results from run to run. The reproducibility is especially poor when graphene nucleation density is required to be controlled to be one or two within the desired area and growth time. The growth of single-crystal graphene (SCG) that is important for various applications, is a model system for exploring the control of fluctuation. In the ideal case for the efficient production of large-area SCG, two extreme approaches can be envisioned. The first is to merge well-aligned SCG into a continuous single-crystal film.5–10 The second is to locally grow one SCG nucleus into a large one.11–15 In principle, both cases allow the independent controls over graphene growth rate to achieve fast growth without compromising the single-crystal nature of the final graphene. However, in practice, it is essential to tune the balance of nucleation density of graphene on surface and growth rate in the latter case. Several methods have been developed for this purpose by either tuning the substrate’s properties such as geometry, type, surface structure and oxygen contents in bulk Cu foils or by passivating the active graphene nucleation sites.12–15 While more examples of balancing size of SCG and growth rate were demonstrated, the fluctuation still remains and the fundamental issue of fluctuation origin and control is still poorly understood. Here we demonstrate that the variation of trace residual water or oxygen gases in CVD system is a key to the big fluctuation of graphene growth. Using SCG growth on a Cu surface as a model system, we show that the precisely-controlled concentrations (ppm-level) of water or oxygen gases can largely eliminate the fluctuation in terms of graphene nucleation density and sizes. This approach is capable to reliably grow centimeter-sized SCG with a growth rate of 190 µm min–1 that is faster than those of the most previous results. RESULTS AND DISCUSSION


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Our idea of exploring the fluctuation of graphene growth involves a set of changes of conventional CVD setup including the direct measurement and controls of H2O vapor and O2 concentrations as shown in Figure S1. The existence of trace amounts of H2O vapor and O2 is possibly from the inherent impurities in gas precursors and any possible leakage and adsorption in gas lines. To get informed of the purity of gas source before growth, a trace moisture analyzer is optionally added through series connection with gas control system. It is a powerful tool to precisely measure concentration of trace amount of H2O vapor, reflecting the cleanliness and tightness of CVD gas line, as depicted in supporting information and Figure S2. It is clear that H2O concentrations dramatically increase with extended gas paths and joint points. On the other hand, H2O vapor impurity can be immediately condensed into white frost to be “visible” when flowing across a cold trap, as shown in Figure S3. After improving the CVD facility by using gas source with ultrahigh purity or minimizing water vapor with cold trap bathed in a low temperature solution (–110 °C) in gas path, the concentration of H2O vapor in CVD chamber was decreased from the fluctuation within 30–70 ppm to 10 ppm, approaching the coherent concentration of gas source. It is worth noting that the improvement will also diminish most of O2 impurity from any leakage in gas line. Three typical cases of graphene growth influenced by different amounts of impurities are schematically illustrated in Figure 1a–c. The repeated results of grown graphene always show wide difference although the recipe of reactive ingredients (CH4, H2, Ar) remains constant, due to the fluctuation of impurity content (10~200 ppm), almost as much as that of CH4 (50–150 ppm). In general, there are two ways to eliminate the fluctuation of these impurities, thus making graphene growth reproducible. The first is to lower the fluctuation of gaseous oxidant concentration, and the second is to introduce additional O2 or H2O in the CVD chamber to an


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amount that is much larger than the fluctuation of impurity concentration from various sources and random environment changes. Figure 1d–g shows optical images of four typical growth cases with different amount of H2O vapor under other similar conditions (~100 ppm CH4 and ~3% H2 mixed in Ar). The results illustrate several important points. Firstly, the result in each case indeed shows exceptionally reproducible growth from run to run. Secondly, the distribution of graphene nucleation density and size is relatively uniform for all cases. Finally, average SCG sizes show a clear monotonic increase with gradually lowering the H2O vapor concentrations in the range of 25 to 140 ppm (Figure 1h). More careful statistical analysis (Figure 1i) shows that the change of nucleation density as a function of H2O vapor amount is characterized with a nonmonotonic curve and a peak. Considering that nucleation density usually changes with thermodynamic parameters such as temperature and C source concentration in a monotonic manner in classical nucleation theory, this observation is anomalous, indicating two competing roles of oxidant-promoted and oxidant-etching played by H2O vapor. Except the above multi-effects of oxidants on graphene growth reproducibility, nucleation and growth process, we discovered a distinct phenomenon that correlates with the role of oxidants. Figure 2a–c shows the typical results of large SCGs (grown from ~100 ppm CH4, ~3% H2 and Ar mixture) centered around a population of small graphene nuclei formed on Cu surface when existing a relatively high concentration of oxidants. Some dominated features can be easily identified. For example, as-formed SCGs are surrounded with a large number of small graphene nuclei that essentially adopt a non-regular geometric shape compared with hexagon for SCGs (Figure 2a). In addition, small nuclei appear to be new centers surrounded by another group of much smaller nuclei (Figure 2b). Moreover, the dynamic evolution of SCG growth can be further understood. Evidences come from observations that part of small “son” nuclei merge


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to edges of the central “mother” SCGs, leading to a non-straight edges resembling to the morphology of small nuclei for SCGs (Figure 2c). Therefore, we can reasonably conclude that as the central “mother” SCGs continue growing, the already or newly-formed small “sons” nuclei are swallowed up. This process occurs repeatedly until a complete film is formed (Figure S4). Our subsequent etching experiments for as-formed SCGs also support the above growth process. Briefly, as-grown graphene was placed in a fixed H2/Ar atmosphere for 1 to 10 min immediately after turning off CH4 flow while keeping temperature unchanged. This situation resulted in a H2-dominated anisotropic etching on graphene. If small nuclei are etched pieces of “mother” SCGs, etched edges of “mother” SCGs will deviate from the original zigzag edge directions for hexagonal graphene.16 We found that edge orientations of etched hexagonal holes having zigzag directions within SCGs is parallel to edges of SCGs (Figure S5), demonstrating that there is no etching process occurred in the original graphene growth process. As a result, the merging event possibly occur during the growth, which will cause the possible appearance of domain boundaries due to the inter-merging of small nuclei and SCGs with different orientations, thus limiting the size and quality of SCGs. However, a merging event occurred on edges of a SCG can be easily identified as edges of merged SCGs appear rough as shown in Figure S4. Remarkably, the quantity and distribution of small “son” nuclei around central “mother” SCG are closely related with oxidant concentrations. We define a circle occupied by small “son” domains as the fringe region. Figure 2d–f shows the marginal area of the central “mother” domain grown with different concentrations of oxidants. Along with the vertical direction of the edge of central “mother” domain toward the outside, the numbers of small “son” nuclei gradually become less and finally disappear (Figure 2g–i). This distance can be regarded as the width of fringe region, as indicated in the model in Figure 2g. The width of fringe region and number of


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small “son” domains increase along with the increase of oxidants concentration within a certain range. In addition, the round shape is associated with high C flux around growing graphene grains based on the result of phase-field theory simulations,20 which can be used to explain shape difference found in round “son” nuclei and hexagonal “mother” SCGs. In the case of graphene growth with a large C flux, graphene grain edges are surrounded by equally supersaturated C species (CHx, x = 0 to 3, that are dissociated from CH4). The C flux is quite large for the growth of small “son” nuclei but relatively not adequate for “mother” SCGs. Therefore, small “son” nuclei are in round shape while “mother” SCGs remain to be hexagonal. To elucidate the effects of oxidants on graphene nucleation and growth, we analyzed the dominated microscopic processes (Figure 3a) and performed the density functional theory (DFT) calculations for adsorption energies of C on Cu and Cu-OH surface sites (Figure 3b, c and Figure S6). Our DFT calculations show that the adsorption energy of C on Cu-OH surface (8.01 eV) is more energetically stable than that of C on pure surface (5.83 eV), which is consistent with a previous experimental results and theoretical calculations that CH4 prefers to adsorb on O preadsorbed Cu surface than clean Cu surface because of strong interaction between C-H and Cu-O bonds.17–21 Therefore, the presence of O2 or H2O favors the graphene nucleation. On the other hand, the existence of oxidants plays important roles in modulating the graphene growth. Firstly, H2O and O2 molecules adsorb onto Cu surface, leading to cleavage of O-O and O-H bonds to decompose into O species (O and OH radicals) at high growth temperature via coupled reactions:22,23 O2 → O + O H2O + O → OH + OH


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We consider O and OH radicals as O-containing species because graphene growth is highly sensitive to them and the coupled reactions are more energetically favorable.24–26 Disassociated O species on Cu surface essentially have two possibilities of existence that they either diffuse on Cu surface or dwell at energetically favorable sites via the formation of stable chemical bonds with surface Cu atoms. On average, the former is responsible for etching C from graphene lattice by random collision, and the latter is associated with the promoted nucleation of graphene. The above simplified idea can be applied to the analysis of our observations. For example, when the population of O species is much less than that of C species, the collision between O and C species has little influence on etching graphene or lowering the net growth rate. In contrast, except from the promoted effect for graphene nucleation, the immobile O or OH species have a role in improving the disassociation of CH4 due to the interaction between C-H and Cu-OH bonds in heterogeneous catalysis.21 Consequently, the combined effects for reduced graphene nucleation, enhanced CH4 disassociation and minimized graphene etching provided by low amount of oxidants, lead to a relatively small graphene nucleation density and fast growth rate that is desired for the synthesis of large-sized SCGs. Figure 3d shows a typical result of a ∼1.3 cm sized SCG formed at low oxidant concentration obtained by the use of ~50 ppm CH4, 1.25% H2 and Ar mixtures, and another example is shown in Figure S7a. These results are otherwise not achieved by using high oxidant concentration. It should be mentioned that the production is highly reproducible when oxidant concentration is well controlled in our case. Furthermore, when surface population of O species reaches a substantial value relative to that of C species, the possibility of collision between active diffusing C and O species will be enhanced in an exponential manner according to statistical theory, thus leading to enhanced etching for the grown graphene and the formed nuclei.14 In contrast, the number of sites for


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adsorbed O on Cu surface is expected to be increased in a linear manner with the amount of O species. The net effect of these two factors results in a peak in the curve of nucleation density as a function of oxidant concentration as shown in Figure 1d. Finally, when too much oxidants exist in the system, the etching effect is dominated, resulting in a drop of both graphene nucleation density and net growth rate (Figure 1h and 1i). This picture can also explain the formation of irregular small nuclei around the central “mother” graphene qualitatively. The formed halo consists of numbers of small “son” nuclei, and it can be widened by increased amount of oxidant concentration within certain ranges. The adsorbed O species around the “mother” crystal can promote CH4 dissociation and adsorption of active C species, which makes it more possible for small “son” nuclei emerging. In general, as oxidant concentrations are beyond the turning point of Figure 1i, surface population of active C species will be dramatically decreased, leading to tremendous drop of the possibility of C species accumulation and formation of stable nucleus, as well as deceleration of subsequent lattice attachment growth. In this case, a graphene domain should experience a series of episodes including long-time incubation of stable islands, nucleus formation and slow expansion growth. We further investigated the dynamic growth of SCGs in cases of near oxidant-free and oxidant-rich conditions as shown in Figure 3e and S8. Moreover, the corresponding plot of average grain size as a function of synthesis time (including incubation time, nucleation and growth time) is shown in Figure 3f. The slopes of fitted solid and dashed lines indicate apparent growth rates for synthesis time and growth time, respectively. Compared to the oxidant-rich case, the near oxidant-free case is featured by a less incubation time for graphene nucleation and high growth rate that is desired for the synthesis of large-sized SCGs. While growth rate is essentially constant within short growth time, it decreases with longer growth time. In practice, centimeter-


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and millimeter-sized SCGs can be achieved within 8 h and 10 min (Figure S7), respectively, and the maximum growth rate is up to 190 µm min−1 (Figure S9). The comparison of efficiency for the synthesis of large SCGs is shown in Figure S10. In addition, it is worth noting that H2 plays an important role in regulating the effect of oxidants on graphene growth. H2 is a strong reducing agent that can weaken the oxidative etching effect through dissociated H adatoms.28–30 Similar with C species desorption interacted with H adatoms, O species can be taken away from Cu surface by H adatoms as well. By modulating H2 concentrations, we observed similar trends with changing oxidative amounts as shown in Figure S11. As high H2 concentration greatly impedes the decomposition of CH4, the optimal condition for the synthesis of large SCGs would be using small H2 and near oxidant-free situation. Consequently, low H2 concentration under near oxidant-free condition results in highly concentrated and equally supersaturated C species around every part SCG edges, leading to a round shape and fast growth rate.20 However, H2 concentration should be high enough to balance the effect of oxidant under the oxidant-rich condition, which restrains the concentration of C species to produce SCGs with hexagonal shape. We performed Raman spectroscopy, transmission electron microscopy (TEM) and electrical measurements to evaluate the crystal quality of graphene. Figure 4a shows an optical image of a transferred graphene crystal on SiO2/Si wafer and the corresponding Raman spectrum. The Raman D band and G band intensity (ID/IG) is ~0.045, and the ratio of 2D band and G band (I2D/IG) is ~4, indicating the low-defect quality and monolayer nature. The selected area electron diffraction patterns with the rotation of less than 1° prove the single-crystal structure of the hexagonal graphene domains (Figure 4b and Figure S12).31 A series of SAED patterns are also collected over a round shape SCG to testify the single-crystal nature (Figure


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S13). To further confirm the film quality and uniformity, we perform the Raman mapping measurement toward the corner of a hexagonal graphene domain (Figure 4c). The original maps of D, G and 2D band are shown in Figure S14. The uniform color contrast in Figure 4d, e matches well with graphene region in Figure 4c, demonstrating the uniformity of sample. We further performed transport measurements on graphene field-effect transistors as shown in Figure 4f. The extracted electron mobility is 4232 cm2 V–1 s–1, which is comparable with recently reported values of SCGs.20 In consequence, the graphene synthesized by our method possesses reliable quality and uniformity for current industrial demand. CONCLUSION In summary, we demonstrate that oxidant impurities such as H2O and O2 play vital roles in graphene growth fluctuation and regulating graphene nucleation density and growth rate. By using near oxidant-free conditions, centimeter-sized SCGs can reliably be produced with a fast growth rate. This control is more fundamental for reliable growth of graphene beyond previous findings, and is expected to be useful for the growth of various 2D materials that are also sensitive to trace oxidant impurities. METHODS Graphene growth and transfer: First, 0.1-mm-thick Cu foil (Sinopharm Chemical Reagent Co., Ltd, 51008360) was loaded into a 1-inch quartz tube in the middle of the heating zone in the furnace and heated to 1050 °C in 240 sccm Ar (6N, 99.9999%) within 20 min and then heated to 1080 °C with introducing 3–12 sccm H2 (6N, 99.9999%). The actual melting point of Cu in our furnace is 1090 °C. Next 2.0–5.0 sccm CH4 (0.5% diluted in Ar) was introduced into the chamber for graphene growth. For near oxidant-fee condition growth, the CH4 and H2 flow rates


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are 2.0–5.0 sccm and 2–7 sccm, and the corresponding concentrations are 50–150 ppm and 1.25%–3% diluted in Ar, respectively. For oxidant-rich condition growth, an additional amount of H2O vapor is introduced into the CVD chamber by 2 sccm Ar bubbling in deionized H2O at room temperature. After growth, the furnace was cooled down to room temperature with the supply of CH4. The transfer process was conducted by the poly(methyl, methacylate) (PMMA) assisted method. For microscopy and spectroscopy characterizations, graphene samples were transferred by conventional wet transfer process. For electrical transport measurement, graphene sample were transferred by a dry transfer method. The field-effect transistor devices were fabricated by photolithography and thermal evaporation of Ti/Au (5 nm/50 nm) electrodes. Characterizations: Optical characterizations of graphene were conducted by SEM (JEOL 6510), TEM (JEOL F2011), optical microscope (PSM-1000), and Raman spectroscopy (Renishaw inVia Plus, with laser excitation at 532 nm). Electrical measurements were performed by semiconductor characterization system (Keithley 4200-SCS). The mobility calculation is based on the equation:

µ=

L dI ⋅ d CgVdW dVg

(1)

where Vd (50 mV) and Vg is source-drain and gate voltage respectively; L and W is the channel length and width, respectively; Cg (11 nF cm–2) is the capacitance of the back-gate dielectric layer; and Id is the source-drain current.


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Figure 1. (a–c) Schematic of graphene growth results with different amounts of gaseous oxidants (H2O and O2). (d–g) Photographs of graphene domains grown on Cu with different concentrations of H2O vapor (the upper) and the corresponding size distribution statistics (the lower). The growth recipe is ~100 ppm CH4 and ~3% H2 mixed in Ar. (h) Graphene domain size and (i) nucleation density as a function of H2O vapor concentration.


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Figure 2. (a) Scanning electron microscopy (SEM) images of large graphene domains and surrounding small nuclei. (b) An enlarged SEM image of a corner of a hexagonal graphene domain with many small domains emerging nearby. (c) A typical view of edge of a graphene domain during growth. (d–f) SEM images showing the distribution of small domains in the fringe region of central graphene domain in three different growth cases for 40 min. The growth condition is ~100 ppm CH4 and ~3% H2 mixed in Ar. (g–i) Statistical diagrams of the number of small domains per every 100 µm distance as a function of width. The insets show the model of central graphene domain and surrounding fringe region with different width under different concentration of H2O, which is ~70 ppm in (d, g), ~40 ppm in (e, h) and ~20 ppm in (f, i), respectively.


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Figure 3. (a) Schematic showing microscopic processes such as graphene nucleated on Cu-O (blue ball) site, C attachment/detachment to graphene lattice and adsorbed O and C (grey ball) species on surface. (b, c) Schematic showing C adatom adsorbed on clean and OH radical preadsorbed Cu surface with different adsorption energy, respectively. (d) Photograph of a centimeter-size graphene domain grown on Cu from ~50 ppm CH4, 1.25% H2 and Ar mix for 8h. (e) SEM images of graphene domain growth with time under oxidant-assisted (1–3) and near oxidant-free (4–6) condition, respectively. The scale bars are 0.5 mm. The growth recipe is ~150 ppm CH4, ~2% H2 in Ar for oxidant-free condition and ~150 ppm CH4, ~5% H2 in Ar for oxidant-rich condition. (f) Variation of average domain size in different growth condition as a function of total synthesis time, where the slope of dashed line and solid line represent graphene synthesis rate and growth rate, respectively.


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Figure 4. (a) An optical image (inset) and a typical collected Raman spectrum of large graphene domains transferred on a Si/SiO2 wafer. (b) Summarized rotation angles of selected area electron diffraction (SAED) patterns on a graphene domain with variation less than 2°. The inset is a typical SAED pattern marked with rotation angle. (c) An optical image of the corner region of a hexagonal graphene domain on Si/SiO2 wafer for Raman mapping. (d) The corresponding Raman ID/IG and (e) I2D/IG maps. (f) Transfer curve measured at the source-drain voltage of 50 mV. The Dirac voltage is ~15 V, and the extracted electron and hole mobility are 4.25 × 103 cm2 V–1 s–1 and 4.1 × 103 cm2 V–1 s–1, respectively. The inset shows the fabricated graphene fieldeffect transistor device, in which the scale bar is 10 µm.


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ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge. Figures S1–S13 as referred to in the text. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (2016YFA0200101), the National Natural Science Foundation of China (21633012, 60911130231, 51233006 and 61390500), Beijing Municipal Science & Technology Commission (Z161100002116025), and Chinese Academy of Sciences and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030100). REFERENCES (1)

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Controlling Fundamental Fluctuations for Reproducible Growth of

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