Liquid Metal: An Innovative Solution to Uniform Graphene Films

Subscriber access provided by UNIV OF WATERLOO

Article

Liquid Metal: An Innovative Solution to Uniform Graphene Films Mengqi Zeng, Lifang Tan, Jiao Wang, Linfeng Chen, Mark H. Rümmeli, and Lei Fu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm501571h • Publication Date (Web): 22 May 2014 Downloaded from http://pubs.acs.org on May 26, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 10

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

Chemistry of Materials

Liquid Metal: An Innovative Solution to Uniform Graphene Films Mengqi Zeng,† Lifang Tan,† Jiao Wang,† Linfeng Chen,† Mark H. Rümmeli,§ and Lei Fu†,* †

College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P. R. China

§

IFW Dresden, P.O. Box 270116, 01069 Dresden, Germany

KEYWORDS Uniform graphene, liquid metal catalyst, growth mechanism. ABSTRACT: The self-limited chemical vapor deposition of uniform single-layered graphene on Cu foils generated significant interest when it was initially discovered. Soon after, the fabrication of real uniform graphene was found to need extremely precise control of the growth conditions. Slight deviations terminate the self-limiting homogeneous growth, inevitably leading to multilayer graphene formation. Here we propose an innovative way to utilize liquid metals to resolve this thorny problem. In stark contrast to the low carbon solubility found in solid metals (e.g. Cu), catalytically-decomposed carbon atoms are embedded in liquid metals. During cooling, homogeneous solidified surface from the quasi-atomic smooth liquid surface, and carbon precipitation is blocked by the frozen metal lattices, which are insoluble to carbon. The underlying liquid bulk acts as a container to buffer the excess carbon supply, which normally would lead to the formation of multilayer graphene in the conventional CVD process. As a result, the growth of graphene becomes governed by a selflimiting surface catalytic process and is robust to variations in growth conditions. With simplicity, scalability and a large growth window, the use of liquid metals provide an attractive solution to obtain uniform graphene.

INTRODUCTION Graphene, a fantastic carbon nanomaterial full of numerous excellent properties, has drawn researchers’ attention for nearly ten years. Graphene shows quite different characteristics with various layers and it is widely accepted that graphene films with different thicknesses have diverse applications due to their distinct properties. For example, single-layer graphene has a zero bandgap, and its conduction and valence bands are shaped like an inverted pair of cones that meet in a single point at E = 0 in momentum space.1 While as for bilayer graphene, it has a continuously tunable bandgap under an electric field2, 3 and shows unique quantum-mechanical behavior.2, 4 In the case of trilayergraphene, it behaves like a semimetal due to its controllable band overlap.5 Once the layer number exceeds ten, graphene films share a band structure similar to graphite.6 It is thus important to synthesize graphene with controllable layers for both fundamental research and practical applications. Many researchers have been working on improving the uniformity of graphene, especially for the obtainment of uniform singlelayer graphene.7 The solubility of C in a metal determines the depth of carbon diffusion into the metal bulk and hence growth mechanism.8 It had been proposed that chemical vapor deposition (CVD) growth of graphene on Ni occurs by a C segregation or precipitation process whereas graphene on Cu grows by a surface adsorption process. The only significant difference is that the solubility of C in Cu is much lower than that in Ni. Since only a small amount of car-

bon could be dissolved in Cu, the source for graphene formation is mainly from the methane that is catalytically decomposed at the Cu surface with minimal carbon diffusion into the Cu. These films obtained at low pressure were characterized as ∼95 % single-layer with small regions of bi- and multilayer graphene.9 However, because of microstructural defects, predominantly grain boundaries, it is very difficult to obtain fully uniform graphene films due to precipitation of extra C during the cool-down process. Since the initial discovery by Zhu et al. that graphene films can be grown on Cu electrodes patterned on a highly n-doped silicon wafer (with a thermally oxidized SiO2 dielectric layer) under ambient pressure,10 graphene growth on Cu foils has rocketed both under low and ambient pressure conditions.11 In fact, one usually obtains graphene films with inhomogeneous growth under ambient pressure. The rate-limiting step was changed from that of growth to the transport of gas molecules from the bulk region to surface.12 Therefore, since growth is no longer “self-limiting”, the excess carbon supply leads to the formation of multilayer graphene. As a result, the uniformity of graphene films is very sensitive to the growth conditions. Various complicated pretreatments are involved to improve the uniformity of graphene films. When electro-polished Cu foils were used, 95 % singlelayer coverage could be obtained under low methane concentrations, which could be attributed to the reduction in graphene nucleation sites on a smoother metal surface.13 In addition, the design of alloy catalysts also offers an efficient approach for the growth of uniform graphene films. Growth of strictly single-layer graphene with 100%

ACS Paragon Plus Environment


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

surface coverage has been achieved by a Ni-Mo alloy.14, 15 This is because carbon dissolved in the bulk is trapped and forms as stable molybdenum carbide. Thus the graphene formation proceeds via a self-limited surface catalyzed process rather than carbon segregation, which leads to an excellent tolerance to variations in growth conditions. Geng et al. reported the synthesis of uniform singlelayered, large-size (up to 10,000 μm2) graphene films on liquid Cu surfaces.16 We noticed that all their presented films synthesized under various growth conditions were all single-layer graphene. Very recently, we also successfully prepared highly uniform single-layer films over pblock liquid metal (e.g. Ga, In) surface using ambientpressure CVD (APCVD), which also showed good tolerance to experimental synthesis variations.17 Hence, regarding the formation of uniform graphene, the question arises are liquid metals intrinsically superior to solid metals? Here we systematically investigate the growth behavior of graphene with all accessible liquid metals and we have successfully grown strictly single-layer graphene over each various liquid metal, that is to say, the growth of uniform graphene depends only on the liquid nature instead of the metals intrinsic properties, such as carbon dissolubility, lattice mismatch and the surface morphology. The liquid surface offers a quasi-atomic smooth plane to support the graphene with the lowest possible defects. More importantly, liquid metals possess an “amorphous” atomic structure, which is truly homogenous. As we now demonstrated with three respective liquid metal catalysts (Ga, In, Cu at 1120 °C), strictly single-layer graphene films are obtained easily along with an excellent tolerance to variations in synthesis conditions. What is more, we succeed in transforming non-uniform graphene with a considerable proportion of multilayer grown at solid Cu into strictly uniform graphene by increasing the temperature to reach the melting-point of Cu. We believe that the unique atomic structure of liquid metals will open up an innovative solution to for uniform graphene films.

Page 2 of 10

process mainly consisted of three steps: (1) heating the substrates to the target temperature at a rate of 30–40 °C/min under the H2 and Ar atmosphere; (2) exposure of substrates to a carbon source at proper melting point for 5–120 min under the flow of CH4, Ar and H2; (3) cooling the substrates to room temperature at a rate of 20 °C/min under Ar, H2. The growth procedure is illus-trated in Supplementary Fig. S1–S3 and Supplementary Table S1–S3. Transferring the Graphene to SiO2/Si Substrates. The process of transferring involved spin-coating a poly(methyl methacrylate) (PMMA) film on the graphene-grown substrates, and releasing the PMMA/graphene film by etching out the metals. In or Ga was etched in a diluted hydrogen chloride (1:1) and Cu was etched in FeCl3 (1 M) for 2–3 h. This was followed by a rinse in ultrapure water to remove the metal ions. The PMMA layer was dissolved in an acetone bath at 100 °C for 5 min after the PMMA/graphene film was transferred onto SiO2/Si substrates for further characterization. Characterization. Optical images were taken with an optical microscopy (Olympus DX51, Olympus), and Raman spectroscopy was performed with a laser microRaman spectrometer (Renishaw in Via, Renishaw, 532 nm excitation wavelength). The AFM images were taken with a NT-MDT Ntegra Spectra with graphene transferred onto the 300-nm SiO2/Si. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi) depth profiling was performed by Ar ionic bombardment to gradually remove the surface layers until 9 nm downward into the bulk phase. The measuring spot size was 500 μm. Scanning electron microscopy (SEM) images were obtained by Hitachi-S4800. The current (I)–voltage (V) data were collected in a probe station under ambient conditions using an Agilent 4155C. The transmission electron microscopy (TEM) images were taken with a double aberrationcorrected, high-resolution TEM (AC-HRTEM, JEOL 2010F) operating at 80 kV with graphene samples directly transferred onto a lacey carbon copper TEM grid.

RESULTS

EXPERIMENTAL SECTION CVD Growth of Graphene on Liquid Metals. Three representative metals were employed to act as a liquid catalyst at high temperature suitable for graphene growth. 25-μm thick Cu, W and Mo foils with a purity of >99.99 wt.%, Ga and In pellets with a purity of >99.99 wt.% were obtained from Alfa Aesar. Before loading liquid metals, the W or Mo foils were ultrasonically cleaned and rinsed with diluted hydrochloric acid, acetone, ethanol and ultrapure water prior to being dried under nitrogen. The CVD growth of graphene was carried out in a quartz tube furnace (HTF 55322C Lindberg/Blue M) under ambient pressure. A single piece of Cu foil or Ga (In) pellet was placed onto a W (Mo) foil, which was near fully covered by the liquid metal during the high-temperature annealing process due to the wetting nature between them. The growth

Cu, In and Ga are utilized as representative liquid metals to demonstrate the distinct advantages with regard to the uniformity of graphene films. All of them exist in the liquid state at graphene growth temperature implemented. In addition to Cu which is a well-known catalyst for graphene CVD growth, In and Ga also show excellent catalytic ability as demonstrated in a previous report of ours17. The adopted approach for graphene grown over liquid metals is to support the catalyst over Mo or W foil. Graphene growth on In or Ga was carried out at 950–1030 °C under 300–600 sccm Ar, 20–50 sccm H2 and 5–20 sccm CH4 for 15–120 min in an APCVD system, while for liquid Cu 1080–1120 °C under 300 sccm H2 and 6–10 sccm CH4 for 5–30 min was used. The liquid metals spread over the entire substrate by itself at elevated temperature after


Page 3 of 10

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


Figure 1. Typical growth results on liquid or solid metal substrates. (a−c) Optical microscope images of graphene grown on liquid Cu, In and Ga respectively, which demonstrates the excellent uniformity of the single-layer graphene. (d) Optical microscope image of graphene grown on solid Cu foil under ambient pressure, which indicates poor uniformity. All the graphene films were transferred onto 300-nm SiO2/Si substrates for characterization. The scale bars are 10 μm. (e−g) Layer distribution determined by RGB color analysis of the corresponding optical microscope images (a-d), respectively. The layer thickness is represented by different colors as seen from the bottom color codes.

introducing CH4. Solid Cu foil was also utilized to grow graphene below its melting point for comparison purposes with liquid Cu. Fig. 1 shows the optical microscope (OM) images of the grown graphene films after transfer onto 300 nm SiO2/Si substrates grown over liquid Cu (Fig. 1a), liquid In (Fig. 1b), liquid Ga (Fig. 1c) and solid Cu (Fig. 1d). One can easily observe the graphene films due to the light interference effect.1 They show excellent uniformity of the graphene films grown on over the liquid metals at a macroscopic scale. By analyzing the green channel contrast of graphene films with respect to the underlying SiO2/Si in the OM image, one can easily obtain the layer number distribution in the graphene film.14 As can be seen in the color code of Fig. 1e–h, the black color represents the bare SiO2/Si substrate, and the other colors ranging from red to purple represent monolayer, bilayer, and multilayer, respectively. The visual false 3D images further reveal the unexceptionable uniformity of the single-layer graphene grown on liquid metals. Fig. 1d, h show the typical growth results on solid Cu foil under ambient pressure,18 which are highly inhomogeneous (i.e., characteristic of regions and islands with different layer numbers). Using the popular solid Cu foils, the best growth results still contain about 5 % of thicker-layer graphene.9 By comparison, with a liquid Cu surface it is easy to obtain strictly single-layer graphene with 100 % surface coverage. The marked improvement in the growth uniformity over the liquid metal surfaces as compared with the conventional solid Cu foil

in APCVD is obvious. Although these three representative liquid metals have different properties, such as different carbon dissolubility and catalytic behaviors, they all enable the growth of uniform single-layer graphene. In addition, it has been reported that the average mobility values of the graphene grown on the liquid Cu fell into the range (1,000−2,500 cm2V-1s-1),16 which is consistent with that of the graphene grown on a solid Cu surface.7, 19-22 To confirm the quality of the graphene film grown on Ga, the I-V characteristics of the graphene was determined by fabricating field-effect transistors (FETs, Fig. S4) using conventional electron beam lithography. Fig. S4a shows the SEM image of the graphene FETs. The extracted carrier mobility of holes for this device was 2107 cm2V-1s-1, which can match single-crystal graphene grown over liquid Cu surfaces. The graphene grown over liquid In was further examined by Raman spectroscopy, transmission electron microscopy (TEM), selected-area electron diffraction and atomic force microscopy (AFM) (Fig. 2a–c). The Raman 2D bands, which were collected over 100 sampling points on the transferred graphene, exhibited a symmetric single Lorentzian line shape with a full width at half-maximum (FWHM) of < 36.5 cm-1 and the intensity ratio of G to 2D bands fell into a range of 0.3–0.5, which was the feature of single-layer graphene with high uniformity. A more indepth inspection of the graphene films using fine Raman mapping confirmed the uniformity and the high quality of continuous films transferred on 300-nm SiO2/Si, shown



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

Page 4 of 10

Figure 2. Characterization of single-layer graphene grown on liquid In and illustration of tolerance to variations in experimental conditions. (a) A typical Raman spectrum of single-layer graphene grown on liquid In (excitation wavelength: 532 nm). (b) A TEM image and the selected-area electron diffraction pattern (inset) reveal the excellent crystallinity of single-layer graphene grown on the Indium. The scale bar is 4 nm. (c) An AFM image of the graphene with wrinkles shows the single-layer feature. The scale bar is 400 nm. (d) Typical optical microscope images of graphene films grown on liquid In under different growth conditions. The scale bars are 10 μm. All the graphene films were single layered with 100 % surface coverage, which demonstrates the good tolerance to various experimental conditions. The growth conditions were 1000 °C, ambient pressure with a gas flow of -1 H2+Ar: CH4 = 22:1 sccm and a cooling rate of 20 °C min on In for 90 min, unless otherwise specified. The other growth parame-1 ters from left to right in (d) were H2+Ar: CH4 = 42:1 sccm, 90 min, 1020 °C, 10 °C min . All of the graphene samples were transferred to 300-nm SiO2/Si substrates for characterization.

in Fig. S5. The single-layer feature and high crystallinity of the graphene grown on liquid In were also supported by the Low voltage aberration-corrected, high resolution transmission electron microscopy (LVAC-HRTEM) and selected-area electron diffraction studies (Fig. 2b and inset), which highlighted the six-fold symmetry singlecrystal nature of the graphene. The AFM image shows the typical wrinkles over the single-layer graphene surface.Based on the above characterization results and analysis, we demonstrate that large-area, high-quality and single-layer graphene can be grown on liquid In surfaces. As is well-known, stringent control of the growth conditions is needed to prepare single-layer graphene by APCVD over conventional solid metals, which is a big challenge for industrial application. Generally, an excess carbon supply23 or improper cooling rate1 results in the

formation of thick-layer graphene and even deposition of amorphous carbon on the metal. The most fascinating feature of the presented liquid metal CVD (LMCVD) approach is its high tolerance to variations in experimental conditions (Fig. 2c; Supplementary Fig. S1–S3 and Supplementary Table S1–S3). As demonstrated by In, when we changed the carbon supply (H2+Ar: CH4) ranging from 1:16~1:64 and growth times ranging from 15~120 min, we always obtained uniform single-layered graphene with almost 100 % surface coverage (Fig. 2c). No multilayer graphene spots were resolved in the highest resolution optical microscope images. The CVD growth of graphene on solid Cu and Ni has been shown to be sensitive to the growth temperature, which makes it difficult to control the thickness and uniformity. Our LMCVD approach showed no dependence to temperature variations from


Page 5 of 10

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


Figure 3.Schematic illustration of the carbon distribution in liquid Cu, solid Cu and the solidified surface of liquid Cu during the phase transition. (a) The catalytic-decomposed carbon atoms was randomly mixed in the bulk phase of liquid Cu. (b) The solidified surface and the bulk of liquid Cu in transitional state exhibited different behaviors for dissolving carbon. The carbon atoms were arranged in a definite principle at the solidified surface, whereas in the underlying bulk the carbon atoms remained randomly distribution. (c) The frozen metal lattices would block the carbon precipitation process. (d) XPS composition profiles of elements along the surface normal direction on Cu substrate after LMCVD growth (the relative composition of each element is represented by a specific color for clarity), which demonstrated a larger amount of carbon dissolved in the bulk and “broke” the -1 insolubility in Cu. The CVD growth was performed at 1120 °C for 30 min, with a cooling rate of 20 °C min and a gas flow composition of H2:CH4 = 300:6 sccm at ambient pressure.

950 to 1030 °C (Fig. 2c). Below 950 °C a noticeable defect band was observed in the Raman spectrum (Fig. S6). This we postulate is due to insufficient graphitization and the deposition of amorphous carbon, which is consistent with previous studies on solid metal CVD (SMCVD). Moreover, the cooling rate is one of the crucial factors that significantly affect the growth quality of graphene. Slower cooling facilitates an equilibrium precipitation and leads to more uniform graphene films with SMCVD. On a liquid metal surface, however, variations in the cooling rate from 5 to 20 °C min-1 did not influence the growth of perfect single-layer graphene since the bulk liquid metal substrate serve as a kind of buffer for the precipitation of dissolved carbons.

DISCUSSION A comparison of the X-ray photoelectron spectroscopy (XPS) depth profile between LMCVD and SMCVD highlights a distinctly different dissolution behavior of carbons into bulk solid and liquid metals. Fig. S7 and Fig. 3d

show the composition profiles of elements along a surface normal to the direction (cross-section) on solid Cu and liquid Cu, respectively (the relative composition of each element was represented by a specific color for clarity). In the case of a solid Cu substrate, the carbon content showed a rapid monotonic decrease towards the bulk phase (Fig. S7). For instance, the carbon concentrations at 1 and 9 nm from the surface were 8.35 and 0.38 at.%, respectively. This is the result of surface absorption of carbon species and the very low carbon solubility in Cu. The decomposed carbon species stick to the surface of solid Cu substrate and self-assemble into graphene islands, and then the neighboring domains collide with each other to splice a graphene quilt.8 Due to its carbon-insolubility, the supersaturated carbon species stacked in the shallow surface of solid Cu cause the non-equilibrium precipitation which deteriorates the uniformity of the resultant graphene. In contrast, the carbon concentration in the liquid Cu substrate changed slowly towards the bulk phase and remained at a quite high level (Fig. 3d). For instance, the carbon concentrations at 1 and 9 nm from



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

the surface were 66.68 and 43.70 at.%, respectively. These results suggest that vast carbon species was embedded in the liquid Cu substrate and ‘break’ the so-called insolubility of C. The atomic structure of solid metals is periodic, where the layout of atomic elements shows repeating patterns over an extended range (Fig. 3c). This atomic structure is called "crystalline" and limits the overall performance of solid metals, such as the solubility of C. Liquid metals possess a non-periodic or an "amorphous" atomic structure, which is truly uniform. By contrast to the crystalline structure, no discernable patterns exist in the atomic structure of the liquid metals. As such, properties superior to the limits of conventional solid metals can be achieved. Fig. 3a−c shows the schematic illustration of the carbon distribution in Cu during cooling down. There is a short-range order of the atomic configuration in liquids, though there is no long-range order. Liquid metals are two-component systems which consist of ions and conduction electrons. The high energy ions lead to intense thermal motion that makes it possible to escape the original atomic (ionic) clusters and join others. The atomic (ionic) arrangement is time-dependent in the case of liquid. As a result, the space between the atomic (ionic) clusters fluctuates, which could allow them to act as carbon atom containers. As shown in Fig. 3a, the vast catalytically-decomposed carbon atoms are randomly mixed or embedded in the space between the atomic (ionic) clus ters in the bulk phase of liquid Cu. When the temperature drops near the freezing point, the surface atoms start to arrange in a periodic structure, whereas in the underlying bulk the carbon atoms remain randomly distributed, because they are still in a liquid phase (Fig. 3b). The solidified surface is derived from a quasi-atomic smooth and homogeneous liquid surface, which could benefit to a reduction of graphene nucleation sites and thus form uniform graphene films. The diffusion of the carbon atoms in the bulk phase are blocked by the frozen metal lattices, which are insoluble to carbon. The underlying liquid bulk acts as a container to buffer the excess carbon supply, preventing the formation of multilayer graphene as found in the conventional Cu-CVD process. As a result, the growth of graphene over liquid metals is governed by a self-limiting surface catalytic process and is robust to variations in growth conditions. In order to highlight the buffer ability for the extra absorbed carbon on the metal surface, controlled experiments with different carbon supplies were conducted. Fig. 4a−c provides the statistical analysis of the IG/I2D intensity ratio from randomly selected 400 sampling points taken on samples with different carbon supplies, i.e. 10 sccm, 15 sccm, 20 sccm, respectively. The intensity ratio of G to 2D bands were 0.39 ± 0.02, 0.41 ± 0.03, 0.43 ± 0.03, respectively, which was characteristic of single-layer graphene with high uniformity.23 The corresponding XPS depth profiles

Page 6 of 10

(Fig. 4e−f) on the In after growth show the composition profiles of In and C along the surface normal direction. When we doubled the carbon supply, the In/C ratio remained relatively stable. In essence, the liquid metal acts as a buffer to stabilize the excess carbon supply and provides a big growth window for uniform single layer graphene. This is undoubtedly an important advantage for practical applications over the conventional SMCVD process.

Figure 4.Characteristic Raman spectra of various graphene films on 300 nm SiO2/Si substrates and corresponding XPS depth profiles on liquid In after CVD growth under a series of CH4 flow. (a−c) Statistical analysis of Raman spectra on IG/I2D taken from randomly selected 400 points on samples under CH4 flow of 10 sccm, 15 sccm, 20 sccm respectively, which demonstrated the single-layer feature of grown graphene. (d−f) The XPS depth profiles on liquid In after CVD growth corresponding to (a−c) and showed composition profiles of elements along the surface normal direction, which exhited similar composition of elements. The growth conditions were 1000°C, ambient pressure with a gas flow of 30 sccm H2 and 300 sccm Ar and a cooling rate of 4 °C/min.

A comparison of representative scanning electron microscope (SEM) images between LMCVD and SMCVD highlight the different graphene growth behaviors found for solid and liquid Cu substrates. Fig. 5a–d show the typical morphology of grown graphene with a series of different growth times, i.e. 1 min, 2 min, 10 min and 30 min, respectively, while Fig. 5e–h correspond to graphene grown on liquid Cu with the same time intervals. With the solid catalyst, due to the low solubility of carbon in Cu, all the


Page 7 of 10

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


Figure 5.The different growth behaviors of graphene on liquid Cu and solid Cu foil. SEM images of graphene grown on (a−d) solid Cu foils and (e–h) liquid Cu at growth times of 1 min, 2 min, 10 min and 30 min, respectively. The SEM characterizations were directly carried out on the Cu surface without transferring and the scale bars are 3 μm. The growth condition was 1120 °C, ambient pressure with a gas flow of 6 sccm CH4, 300 sccm H2 and a cooling rate of 20 °C min-1, while for solid Cu foil was 1000 °C.

Figure 6.The inhomogeneous graphene grown on solid Cu turn to uniform after melting the Cu substrate. (a) Typical SEM image showed the multilayer graphene islands on the Cu foil obtained after CVD growth on solid Cu foil for 5 min. (c) and (e) Typical SEM images indicated the second growth for another 25 min on liquid Cu and solid Cu respectively. (b), (d), (e) Schematic drawings for corresponding growth results, respectively.



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

decomposed carbon species stacked on the surface and it was hard for the C species diffuse to the bulk or over the surface to accomplish a uniform graphene film. This means once the nucleation sites are formed, they are fixed at their birthplace (Fig. 5a–c), which results in the accumulation and magnification of defects in the produced graphene (Fig. 5d). The situation in liquid Cu was quite different. During the initial nucleation stage, the multilayer spots are found as the growth time was not long enough to complete the diffusion of carbon species in the liquid metal (Fig. 5e–f). The multilayer spots gradually disappear with longer reaction times. The neighboring graphene domains rapidly expand outward (Fig. 5g), meeting with each other, and finally forming a continuous uniform graphene layer (Fig. 5h). The distinctly different growth behaviors of graphene on solid and liquid metals are due to the large space between the atomic (ionic) clusters, which can trap the excess carbon atoms, enabling the formation of uniform graphene films. As previously demonstrated, the use of liquid metals allowed us to grow uniform graphene. An even more surprising finding was that the inhomogeneous graphene grown on solid Cu could be transformed to uniform film after melting the Cu substrate. Fig. 6a–b show the typical morphology of grown graphene on the Cu foil for 5 min under ambient pressure. The growth was not "selflimiting", and thus excess carbon leads to the formation of multilayer islands. However, by keeping the growth parameters constant, except the temperature which was increased to 1120 °C (sufficient for the Cu foil to melt and form liquid Cu) for another 25 min. It is obvious that the uniformity was significantly improved by the second growth step on liquid Cu in that almost all the multilayer islands disappeared (Fig. 6c–d). A control experiment was conducted on the Cu foil keeping the temperature at 1000 °C for another 25 min as the second growth step. The corresponding data are shown in Fig. 6e–f, where graphene islands of varying numbers of layers with lateral growth that sometimes meet each other are observed, viz the graphene films grown on solid Cu foil were still highly inhomogeneous. The implementation of liquid metals make it possible to not only fabricate uniform graphene in a convenient way but also recover existing multilayer graphene to uniform single layer graphene.

CONCLUSION In summary, the use of liquid metals allows us to easily grow strictly single-layer graphene. Compared to the low carbon solubility in some solid phases (e.g. Cu), the nonperiodic and fluctuant atomic configurations in liquids metals allow it to act as a buffer to hold carbon atoms. Upon cooling down, the frozen metal lattice near the surface where the graphene has formed, block carbon precipitation from the liquid deeper within during the phase transition. As a result, the growth of graphene becomes governed by a self-limited surface catalytic process and is robust to variations in growth conditions. This method takes advantage of a liquids quasi-atomically smooth surface to avoid defects or grain boundaries as found with

Page 8 of 10

solid metals. The presence of liquid metals removes the need for various complex pretreatments as often required with solid catalysts. It also reduces restrictions on the need to finely control the growth conditions. The simplicity and scalability of using liquid metals will greatly facilitate future graphene research and industrial applications.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

Author Contributions L.F. developed the concept and conceived the experiments. M.Q.Z. carried out the experiments. L.F. and M.Q.Z. wrote the manuscript. L.F., M.Q.Z., L.F.T., J.W., L.F.C. and M.H.R contributed to data analysis and scientific discussion.

Funding Sources The Natural Science Foundation of China (Grants 51322209), the Sino-German Center for Research Promotion (Grants GZ 871) and the Ministry of Education (Grants 20120141110030).

ACKNOWLEDGMENT The research was supported by the Natural Science Foundation of China (Grants 51322209), the Sino-German Center for Research Promotion (Grants GZ 871) and the Ministry of Education (Grants 20120141110030). We thank Qiang Fu for his assistance with SEM characterizations and thank Zhangyuan Zhang and Prof. Lei Liao for their assistance with electrical transport measurements.

REFERENCES (1) Katsnelson, M. I. Mater. Today 2007, 10, 20-27. (2) Zhang, Y. B.; Tang, T. T.; Girit, C.; Hao, Z.; Martin, M. C.; Zettl, A.; Crommie, M. F.; Shen, Y. R.; Wang, F. Nature 2009, 459, 820-823. (3) Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Science 2006, 313, 951-954. (4) Hofmann, S.; Kleinsorge, B.; Ducati, C.; Ferrari, A. C.; Robertson, J. Diam. Relat. Mater. 2004, 13, 1171-1176. (5) Craciun, M. F.; Russo, S.; Yamamoto, M.; Oostinga, J. B.; Morpurgo, A. F.; Tarucha, S. Nat. Nanotechnol. 2009, 4, 383-388. (6) Partoens, B.; Peeters, F. M. Phys. Rev. B 2006, 74, 075404. (7) Yan, K.; Fu, L.; Peng, H. L.; Liu, Z. F. Acc. Chem. Res. 2013, 46, 2263-2274. (8) Li, X. S.; Cai, W. W.; Colombo, L.; Ruoff, R. S. Nano Lett. 2009, 9, 4268-4272. (9) Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312-1314. (10) Di, C. A.; Wei, D. C.; Yu, G.; Liu, Y. Q.; Guo, Y. L.; Zhu, D. B. Adv. Mater. 2008, 20, 3289–3293. (11) Mattevi, C.; Kim, H.; Chhowalla, M. J Mater. Chem. 2011, 21, 3324-3334. (12) Bhaviripudi, S.; Jia, X. T.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2010, 10, 4128-4133. (13) Luo, Z. T.; Lu, Y.; Singer, D. W.; Berck, M. E.; Somers, L. A.; Goldsmith, B. R.; Johnson, A. T. C. Chem. Mater. 2011, 23, 1441-1447. (14) Dai, B. Y.; Fu, L.; Zou, Z. Y.; Wang, M.; Xu, H. T.; Wang, S.; Liu, Z. F. Nat. Commun. 2011, 2, 522. (15) Rummeli, M. H.; Zeng, M. Q.; Melkhanova, S.; Gorantla, S.; Bachmatiuk, A.; Fu, L.; Yan, C. L.; Oswald, S.; Mendes, R. G.;


Page 9 of 10

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


Makarov, D.; Schmidt, O.; Eckert, J. Chem. Mater. 2013, 25, 38803887. (16) Geng, D. C.; Wu, B.; Guo, Y. L.; Huang, L. P.; Xue, Y. Z.; Chen, J. Y.; Yu, G.; Jiang, L.; Hu, W. P.; Liu, Y. Q. Proc. Natl. Acad. Sci. USA 2012, 109, 7992-7996. (17) Wang, J.; Zeng, M. Q.; Tan, L. F.; Dai, B. Y.; Deng, Y.; Rummeli, M.; Xu, H. T.; Li, Z. S.; Wang, S.; Peng, L. M.; Eckert, J.; Fu, L. Sci. Rep. 2013, 3, 2670. (18) Shin, Y. C.; Kong, J. Carbon 2013, 59, 439-447. (19) Wu, B.; Geng, D. C.; Guo, Y. L.; Huang, L. P.; Xue, Y. Z.; Zheng, J.; Chen, J. Y.; Yu, G.; Liu, Y. Q.; Jiang, L.; Hu, W. P. Adv. Mater. 2011, 23, 3522-3525.

(20) Wu, W.; Jauregui, L. A.; Su, Z. H.; Liu, Z. H.; Bao, J. M.; Chen, Y. P.; Yu, Q. K. Adv. Mater. 2011, 23, 4898-4903. (21) Sun, Z. Z.; Yan, Z.; Yao, J.; Beitler, E.; Zhu, Y.; Tour, J. M. Nature 2010, 468, 549-552. (22) Ji, H. X.; Hao, Y. F.; Ren, Y. J.; Charlton, M.; Lee, W. H.; Wu, Q. Z.; Li, H. F.; Zhu, Y. W.; Wu, Y. P.; Piner, R.; Ruoff, R. S. ACS Nano 2011, 5, 7656-7661. (23) Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 3087-3087.



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

Page 10 of 10

Liquid Metal: An Innovative Solution to Uniform Graphene Films The self-limited chemical vapor deposition of uniform single-layered graphene on Cu foils gener-ated significant interest when it was initially discovered. Soon after, the fabrication of real uniform gra-phene was found to need extremely precise control of the growth conditions. Slight deviations terminate the self-limiting homogeneous growth, inevitably leading to multilayer graphene formation. Here we propose an innovative way to utilize liquid metals to resolve this thorny problem. In stark contrast to the low carbon solubility found in solid metals (e.g. Cu), catalytically-decomposed carbon atoms are em-bedded in liquid metals. During cooling, homogeneous solidified surface from the quasi-atomic smooth liquid surface, and carbon precipitation is blocked by the frozen metal lattices, which are inso-luble to carbon. The underlying liquid bulk acts as a container to buffer the excess carbon supply, which normally would lead to the formation of multilayer graphene in the conventional CVD process. As a result, the growth of graphene becomes governed by a self-limiting surface catalytic process and is ro-bust to variations in growth conditions. With simplicity, scalability and a large growth window, the use of liquid metals provide an attractive solution to obtain uniform graphene.


Liquid Metal: An Innovative Solution to Uniform Graphene Films

Recommend Documents