View Article Online View Journal
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Cho, M. Kim, M. Kim, K. Kim, H. Kim, D. Lee, S. Lee and K. Kim, Nanoscale, 2015, DOI: 10.1039/C5NR03352G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/nanoscale
Page 1 of 7
Journal Name
Nanoscale
Dynamic Article Links ► View Article Online
DOI: 10.1039/C5NR03352G
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
www.rsc.org/xxxxxx
Seong-Yong Cho, a Min-Sik Kim,a Minsu Kim, a Ki-Ju Kim, a Hyun-Mi Kim, a Do-Joong Lee, b Sang-Hoon Lee a and Ki-Bum Kim* a 5
10
15
20
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Graphene growth on liquid Cu has gathered a great interest owing to self-assembly behavior of hexagonal graphene flakes with aligned orientation and to possibility of forming a single grain of graphene through a commensurate growth of these graphene flakes. Here, we propose and demonstrate a two-step growth process which allows a formation of self-assembled, completely continuous graphene on the liquid Cu. After the formation of full coverage on the liquid Cu, grain boundaries are revealed via selective hydrogen etching and the original grain boundaries were clearly resolved. This result indicates that, while the flakes self-assembled with a same orientation, there still remain structural defects, gaps, and voids that were not resolved by optical microscopy or scanning electron microscopy. To overcome this limitation, the two-step growth process is employed by consisting a sequential process of (a) normal single layer graphene growth and self-assembly process with a low carbon flux followed by (b) final stage of graphene growth at high degree of supersaturation with a high carbon flux. Continuity of the flakes is verified via hydrogen etching and NaCl-assisted oxidation process as well as by measuring electrical properties of the graphene grown by the two-step process. Two-step growth can provide continous graphene layer, but commensurate stithicng should be further studied.
1 Introduction
25
30
35
40
45
50
Motivated by its superior electrical,1-3 optical4 and mechanical5 properties, various synthesis methods for graphene growth have been proposed for a formation of high quality, large scale graphene.6-11 Among those, graphene growth on a solid Cu surface has been extensively studied due to a readiness of a large scale synthesis with monolayer dominant graphene growth.12-14 However, during the growth, grain boundaries are inevitably formed between adjacent graphene grains as a result of the nature of graphene synthesis and those grain boundaries are known to significantly degrade the quality of graphene.15-18 While various methods have been proposed to grow relatively large-sized graphene domains (up to ~ mm) either by controlling a nucleation density19-25 or by aiming for commensurate growth of graphene domains,26,27 synthesizing a high quality, large domain size graphene within a short period of process time still remains as a great challenge. In this respect, recent publications by Geng et al., provided several intriguing aspects of graphene growth on liquid Cu by chemical vapour deposition (CVD) as compared to that on solid Cu.28-31 Firstly, one does not have to worry about various heterogeneous nucleation sites that inevitably formed on solid Cu foil as a result of Cu melting. Secondly, it appears that a uniformity of the size of hexagonal graphene flakes can be easily controlled by tuning process parameters. Thirdly, the graphene flakes self-assemble in a close-packed manner when the relative size of graphene flakes is uniform. Each hexagonal graphene flakes appears to be aligned to have the same crystallographic orientation relationship. Lastly and most importantly, they claimed that graphene flakes are merged with each other to This journal is © The Royal Society of Chemistry [year]
55
60
65
70
75
80
possibly form a single grain when the process is continued to have the full coverage growth, based on observations of optical and scanning electron microscopies (SEM). Despite of the limitations of needing to use a high melting point and good wettability substrate like W or Mo, which does not intermix with Cu, and a drawback of large thermal expansion mismatch between the liquid Cu and the graphene during cooling,32 this process certainly provides one of the unique opportunities to form a large size single crystal with predominantly monolayer of graphene and warrants further expedition.33 Not only graphene, but h-BN can be grown on liquid Cu according to recent report.34 Even though their reports clearly suggest advantages and impacts of using the liquid Cu as a growing substrate, understandings on its growth mechanism is still lacking and there are more rooms to improve a quality of the graphene by amending grain boundaries between the hexagonal graphene flakes. Specifically, we focused on the last stage of graphene growth on the liquid Cu, namely, the continuous growth of each graphene flake. To pursue this purpose, we first grew hexagonal graphene flakes on the liquid Cu, as was suggested by Geng et al.,28 and confirmed the self-assembly of those graphene flakes by SEM and transmission electron microscopy (TEM). Then, the continuous growth of graphene flakes on liquid Cu was investigated by selective hydrogen etching35,36 and NaCl-assisted oxidation of the underlying Cu37 to reveal defect sites. Our results showed that the stitched domain boundaries were well resolved by these processes and indicated that, even in a fully grown graphene on the liquid Cu, there still exists a high density of defects, voids, and small gaps. Such results proved that the continuous growth is not perfect by itself. The discontinuous growth of graphene on liquid Cu has already been reported by [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
Self-assembly and continuous growth of hexagonal graphene flakes on liquid Cu
Nanoscale
Page 2 of 7 View Article Online
DOI: 10.1039/C5NR03352G
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
20
25
30
35
40
45
50
55
60
65
metal shadow mask. Sheet resistance and Hall mobility was measured by the VDP method. For Hall measurement, magnetic field was 0.6 T. (HL 5500PC, BIO-RAD) For transmission line measurement (TLM) patterning after transferring the graphene onto a 285-nm-thick SiO2/Si wafer, photoresist (AZ 5214) was uniformly coated and was patterned using an MA 6-II aligner. Then, Ti/Au (5 nm / 80 nm) electrode was evaporated and lift-off process was done to leave desired patterns. For two points of current/voltage measurements, an Agilent 4156C system was used.
2 Experimental Section Graphene growth: Electropolished Cu foils (Alfa Aesar #13382) were put on top of 1 inch W foil (also, Alfa Aesar) and loaded into a quartz tube type furnace. (Lindberg, blue) Then, sequentially, the furnace was pumped down to ~10-3 Torr, hydrogen flowed at 500 sccm until the chamber reaches atmospheric pressure, and heating was started. After reaching a target temperature of 1100 oC, which is above the melting point of Cu, CH4 was introduced at 3.5 sccm as a carbon precursor. Growth time was varied from 30 minutes for a partial coverage graphene and longer than 2 hours for a full coverage one. To reveal grain boundaries of the graphene grwon on the liquid Cu, hydrogen etching was performed in-situ by shutting CH4 flux off and by flowing only 500 sccm H2 after cooling down the sample to 1050 oC for 5 to 10 minutes (to solidifify the underlying Cu). For the two-step growth, once the growth was proceded on the liquid Cu with the conditions addressed above, CH4 flux was further increased to 12 sccm either on the liquid Cu (at 1100 oC) or after solidifying Cu (at 1050 oC). For comparison, graphene was also grown on solid Cu at 1030 oC using a typical low pressure CVD process. CH4 flux was maintained to 0.5 sccm with flowing 12 sccm of H2 and CH4 flux was finally increaed up to 2 sccm at the final step for full coverage. Grain size of graphene grown on the solid Cu by low pressure CVD was approximately 50 µm, which is a similar size as the graphene grown on the liquid Cu in this work. Transfer and pattern fabrication: Graphene was transferred by a well-known process of poly methyl methacrylate (PMMA) coating, and subsequent etching of underlying W and Cu layers. In particular, W etching was performed by an anodic etching, as proposed by Y. Fan et al. 38 using a bare Cu foil as a cathode in 2 M NaOH solution. Then, PMMA/graphene/Cu stack was floated in ammonium persulfate solution (0.1 M) for overnight to completely etch the Cu foil. After etch, floated PMMA/graphene layer was scooped on a TEM grid (Quantifoil, Ted Pella) or on a SiO2 (285 nm)/Si substrate for patterning and for electrical transport measurement. Then, PMMA layer was removed by boiling acetone or direct heating at 380 oC flowing Ar and H2. For the preparation of VDP patterns, Ti/Au (5 nm / 80 nm) electrode patterns was deposited by e-beam evaporation using a 2 | Journal Name, [year], [vol], 00–00
Fig. 1. Characterization of graphene grown on liquid Cu. (a) Schematic illustration of the graphene growth process on the liquid Cu, (b) SEM image of as-grown graphene islands on the liquid Cu for 30 min of growth time showing a self-assembled growth behavior and (c) optical microscope image of a graphene grown on the liquid Cu for 50 min followed by oxidation of the Cu substrate. Inset figure in (c) shows a representative Raman spectrum of a single layer of graphene grown on the liquid Cu after transferred onto a SiO2/Si substrate. 70
3 Results and discussion
75
80
85
90
95
Fig. 1 (a) shows a schematic picture of the graphene growth on liquid Cu via a single step process. The self-assembled graphene was grown at 1100 oC with CH4 and H2 flow rates of 3.5 and 500 sccm, respectively. Fig. 1 (b) and (c) show representative SEM (b) and optical microscope images (c) showing the formation of hexagonal-shaped graphene flakes and their self-assembly behavior on a partially grown sample. The inset in Fig. 1 (c) shows the corresponding Raman spectra which clearly indicate a monolayer graphene growth after the transfer to a SiO2/Si substrate. A process condition was tuned to have roughly uniform size of graphene flakes at CH4 and H2 flow rates of 3.5 and 500 sccm, respectively, to match with the results of Geng et al.,28 in the aspect of graphene flake shape, size and its uniformity. In order to induce self-assembly, the formation of hexagonal graphene flakes and its size uniformity are important factors while an average size does not take a critical role. Our preliminary experimental results showed that a rigorous selection of the process condition is required; for instance, when the CH4 flow rate is increased above 3.8 sccm, multiple graphene layers grew, while with a slightly lower CH4 flow rate of 3.3 sccm than the optimum flow, size of graphene flakes were obviously nonuniform. Further reduction of CH4 flow rate below 3.0 sccm flow resulted in no graphene growth (Supplementary Fig. S1). Certainly, it is important to determine what the driving force of this assembly is. We have noted that the graphene flakes move to the liquid area to assemble during cooling when the overall This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
5
Fan et al.38 Their work was focused on healing cracks which formed during solidification of underlying liquid Cu catalyst. To accomplish a better continuous growth with minimal defects and grain boundaries, we employed a two-step growth process by additionally introducing a high degree of supersaturation growth condition, with increased carbon flux, at the last stage of growth. Surprisingly, samples grown by the two-step process were neither etched by hydrogen nor oxidized by NaCl. Electrical resistance of the graphene grown by the two-step process shows much lower value compared to graphene grown on the conventional solid Cu and even to that on the liquid Cu via a single-step growth. Intragrains (within a single grain) and inter-grain (between two grains through the grain boundaries) resistances were also measured and compared by preparing electrode17 atop the graphene to prove excellent self-assembly of the graphene flakes. These results comprise outstanding quality and properties of a full coverage graphene layer grown by the newly proposed two-step process within a short period of growth time.
Page 3 of 7
Nanoscale View Article Online
5
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
coverage is low. This result indicates that the capillary force is one of the driving forces for this assembly.38 However when the overall coverage is relatively high; the flakes also self-assemble with each other on liquid Cu, which indicates that Van der Waals interaction between the flakes is another driving force. Our interest, then, is how well the adjacent graphene flakes commensurately grows and form continuous layer when the growth process continues on the liquid Cu. It is well known that grain boundary is formed by grains merging that have different crystallographic orientations. In the case of graphene, nonhexagon type carbon arrangements, such as heptagon and pentagon are formed in the grain boundaries to accommodate lattice mismatch.15 By a series of TEM images and selected area electron diffraction patterns, we confirmed that individual hexagonal graphene flake is composed of a single grain and the graphene flake is crystallographically well oriented through the same directions (Fig. S2 and S3). However, these results are not still sufficient to show the commensurate growth and single crystallinity of these flakes.
40
45
50
55
60
65
70
Fig. 2. Grain boundaries revealed by hydrogen etching and NaCl-assisted oxidation. SEM images of graphene islands (a) after 2 h of growth time (as-grown) and (b) the corresponding sample after the hydrogen etching for 5 min. Optical microscopy images of (c) graphene after 2 h of growth time and (d) the corresponding sample after the NaCl-assisted oxidation for 24 h. Scale bar indicates 30 µm. 20
25
30
35
To check the continuity of hexagonal graphene flakes, we first grew graphene to full coverage for 2 hours, with the optimum conditions addressed above, as is shown in Fig. 2 (a). This sample was then cooled to 1050 oC and H2 etching was carried out without breaking vacuum. The hydrogen-induced etching of graphene has already been reported35,36 and is known to etch selectively defect sites of graphene such as the grain boundary (Fig. 2 (b)). Surprisingly, the original hexagonal flakes are well delineated by hydrogen etching as are the nucleation centers at the center of the flakes. This result clearly demonstrates that, while the graphene flakes are self-assembled, the flakes do not continuously grow with each other at all and there still remain defect sites which are prone to etching by hydrogen. Similar etching results were obtained with extended growth time such as 3 and 4 hours. Indeed, closer examination of the fully grown samples often shows small voids (Fig. S4 (a)) and gaps (Fig. S4 (b)) between the flakes that are not even filled by the extended growth time up to 4 hours. NaCl-assisted oxidation was also employed to observe grain boundaries, as previously proposed by This journal is © The Royal Society of Chemistry [year]
75
80
85
90
95
Ly et al. 37 The Graphene/Cu/W samples prepared by the single step process were dipped into NaCl (6 wt. %) solution and left for 24 hours. Grain boundaries of the graphene grown on liquid Cu by the single step were obviously revealed by the NaCl-assisted oxidation as shown in Fig. 2 (d). (Fig. S5 also) Contrary to the grain boundaries image with well-arranged and delineated by H2 etching as shown in Fig. 2 (b), NaCl-assisted oxidation does not give perfect alignment. We believe that this is difference between in-situ H2 etching and ex-situ NaCl-assisted oxidation. H2 etching gives more accurate grain boundaries image since H2 etching could be carried out without cooling Cu and it can contribute to the visualization of grain boundaries as it formed. On the other hands, NaCl-assisted oxidation inevitably accompanies cooling and solidification of Cu, which can damage to graphene. Also, severe oxidation environment can also deteriorate the graphene film and NaCl precipitate might be formed in the 1M solution. Here, a difference between the CVD growth of graphene and the conventional CVD process of other materials should be noted. The conventional CVD process is the method of continuously depositing films by using decomposition and reaction of gas precursors on a substrate. Thus, a deposition rate is constant with respect to a deposition time at a given process condition. On the contrary, the CVD graphene growth is a process critically relying on a catalytic effect of underlying Cu substrate for decomposition of carbon precursors and supply/diffusion of carbon monomers. Thus, as a graphene coverage increases, an overall growth rate of graphene decreases definitely due to a limited supply of carbon monomers, which should be generated by the exposed Cu area. This type of growth mode is well explained by the twodimensional Johnson-Mehl-Avrami (JMA) type growth. The question that brought out here is whether the limited monomer supply at a final stage of the growth is still enough to fill the gaps between adjacent grains. These narrow gaps between graphene grains should be distinguished with merged neighboring grains (For instance, Fig. 5 (a) and (c)) at the initial stage when carbon attachment was facile due to larger exposed area. The two grains can stitch each other easily. Obviously, to fill the gaps at the last stage, an overall process should be executed at a high degree of supersaturation, namely at a high flow rate of carbon precursors for acquisition of sufficient carbon monomer in-flux. However, high flow rate condition can simultaneously resulted in a high degree of nucleation rate which is not preferable for growth of large grain sized graphene. To increase a grain size, one has to critically control the degree of supersaturation to minimize the nucleation rate at the initial stage of growth. However, in this case, one often finds out that the small gaps between the grains and little voids are not well filled by the continuous growth even though one successfully grows large-sized graphene grains. A possible mechanism of the voids formation during the graphene growth has already been reported by a few researchers. For instance, Li et al., 40 suggested that carbon in-flux should be supersaturated for obtaining a continuous graphene layer. They claimed that a small amount of carbon monomer is insufficient to continuously drive carbon attachment to the island edges and thereby a Cu surface is only partially covered with the graphene islands. Kim et al.,41 also suggested models describing graphene growth kinetics and a coverage as a function of a growth time, where they described a final coverage can be expressed as a
Journal Name, [year], [vol], 00–00 | 3
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR03352G
Nanoscale
Page 4 of 7 View Article Online
5
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
function of a supersaturation ratio. Additionally, Eres et al.,42 found out that voids can exist under a low carbon feeding rate. In our growth condition, the graphene growth was not observed at CH4 flow rate less than 3.0 sccm. This means that our growth condition (3.5 sccm of CH4) is just above the minimum feeding rate for the growth but is not enough to fill the gaps between graphene flakes at the final stage of growth. However a higher carbon flux condition (3.8 and 4 sccm) at the initial stage of graphene growth on the liquid Cu resulted in the size discrepancy of flakes and the formation of multilayer graphene, as shown in Fig. S1.
40
45
50
55
60
Fig. 3. (a) Schematic illustration of two-step graphene growth on liquid Cu. CH4 flux was increased for continuous stitching of graphene flakes and further growth was also carreid out after cooling to solid. (b) SEM image of a continuous graphene layer by the two-step method after experiencing the hydrogen etching process which shows still continuous film, and (c) optical microscope image of the graphene layer on the liquid Cu by the two-step growth and no damages were observed after the NaClassisted oxidation. Scale bar indicates 30 µm.
15
20
25
30
35
Based on these arguments, our conjecture here is that narrow gaps still exist between graphene flakes grown on the liquid Cu due to the small feeding rate of carbon monomers and the resultant low supersaturation ratio. In this regard, we propose a two-step growth recipe for the formation of continuous, gap-free graphene, as illustrated in Fig. 3 (a). (Fig. S6 for reaction scheme) A full coverage graphene can also be prepared by using a high CH4 flow rate at the initial stage, but the over-flux condition definitely results in either the multilayer formation (supporting information, Fig. S1) or a large size discrepancy in graphene flakes. In this reason, we design a two-step process which uses a low carbon flux at the initial stage for a sufficiently low nucleation rate while introduces a high carbon flux (high degree of supersaturation) at the final stage for filling the gaps between adjacent graphene flakes. Carbon flux was maintained after cooling to solid to fill the possible gaps which might be formed by thermal stress due to cooling. Cracks were often observed after cooling the sample down to solid. Such cracks are suspected to be formed because of a thermal stress induced by Cu solidification,43 which inevitably results in volume shrinkage, as identified SEM images. (Supporting information, Fig. S7) A recent study also emphasized the crack formation in a graphene layers grown on the liquid Cu.38 To overcome the limitation described above, two-step growth for stitching process was carried out after cooling to solid (1050 oC), which showed lower crack formation. Also, the crack formation was significantly suppressed by slow cooling rate and maintaining high carbon flux 4 | Journal Name, [year], [vol], 00–00
65
on solid Cu even though two-step process was applied on liquid Cu. We admit that for better commensurate stitching between graphene islands, two-step growth without cooling to solid is desirable because self-assembly can occur easily on liquid surface. We believe that continuous graphene with well stitching (hexagon carbon rings) can be grown on liquid Cu by two-step carbon feeding, but continuous graphene on liquid Cu can be easily torn due to thermal stress between graphene and Cu. It might be reason for low carrier mobility for continuous graphene layer which will be discussed. To examine the continuous growth of graphene, hydrogen etching was applied to two-step grown graphene in this study. Fig. 3 (b) shows a continuous graphene layer that was formed by two-step after hydrogen etching (Higher magnification SEM image in Fig. S8). We could not observe empty area even at higher magnifications. The result shows that grain boundaries were not etched by hydrogen etching at all, contrary to the result in the single step graphene in Fig. 2 (b). We do not believe that this continuous graphene shows no grain boundaries with wellstitching because two-step growth was carried out after cooling the Cu catalyst, but continuity of graphene was achieved. NaClassisted oxidation was also employed to check the continuity of the film. Surprisingly, the sample grown by the two-step growth did not show any remarkable oxidation behavior at grain boundaries despite of 24 hours of oxidation time as shown in Fig. 3 (c) (see Fig. S9 also for the detail information) which is quite different from behavior of single step sample (Fig. 2 (d)). These results clearly suggest that the graphene grown by the two-step recipe in this study is completely continuous and has no gaps between the neighboring graphene flakes at all, which were not achievable in the single-step growth.
Fig. 4. (a) Sheet resistances and (b) Hall mobilities of graphene grown on the solid and the liquid Cu. 70
75
80
85
Not only imaging grain boundaries via etching, but electrical transport properties of the two-step grown graphene were evaluated and compared to those of a graphene grown on a conventional solid Cu. For the characterization, the graphene grown on the solid Cu was transferred to 285nm-SiO2/Si substrate using a polymer coating and subsequent wet-etching technique.10,11 Meanwhile, in the case of our two-step grown sample on the liquid Cu, an underlying W foil was etched by anodic etching suggested by Fan et al.38 After transferring the graphene onto a target substrate, Ti/Au electrode was formed to prepare Van der Pauw (VDP) geometry. The detailed VDP sample preparation method is described in supporting information, Fig. S6. The results are shown in Fig. 4 (a) and (b). Remarkably, the electrical sheet resistance of the graphene grown on the liquid Cu (two-step) measured by VDP configuration (298.1 ± 93.6 Ω/sq.) was much lower compared to the graphene grown on the solid Cu (700.7 ± 472.2 Ω/sq.) (Fig. 4 (a)). On the other hands, This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR03352G
Page 5 of 7
Nanoscale View Article Online
DOI: 10.1039/C5NR03352G
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
20
40
45
50
55
60
65
Fig. 5. Comparison of commensurate and incommensurate stitching. (a) Optical microscope image of two adjacent graphene islands aligned in the same orientation and Ti/Au electrodes were deposited by the lift-off process, (b) current vs. applied bias curve from electrode #1 to various electrodes including intra- (1-2, 1-3, and 1-4) and inter-grain (1-5), (c) optical microscope image of two adjacent graphene islands having misaligned orienetation and (d) current vs. Applied bias curve from electrode #7 to various electrodes including intra- (7-6 and 7-5) and intergrain (7-4, 7-3, 7-2 and 7-1). Scale bars indicate 50 µm.
25
30
35
Moreover, in order to examine effects of grain boundaries on electrical properties of the commensurate, gap-free graphene in this study, the electrode patterns were prepared on top of two adjacent graphene grains using a photolithography and a lift-off process, as shown in Fig. 5 (a). These grain boundaries were formed in the initial stage of graphene growth where we do not have to worry about stitching issue due to low carbon flux. Large domain size of our graphene (~100 µm) allowed to easily fabricate the electrode patterns by the photolithography. The representative current-voltage curves are shown in Fig. 5 (b) with various lengths of current paths. We did not observe any degradation in the electrical performance due to the grain boundaries (across the electrode #4 in Fig. 5 (a)), and the resistance value was only dependent on the length of current path (distance between the electrodes) as shown in Fig. 5 (b). This journal is © The Royal Society of Chemistry [year]
Specifically, there was no abrupt increase of the resistance when current flowed across the grain boundary of the graphene (dotted line (1-5) in Fig. 5 (b)). The resistance of the pattern from position a to c can be calculated from the following equation,17 b c dx dx (1) Ra −c = ρ a −b ∫ + ρb −c ∫ W x W ( ) ( x) a b Here, Ra-c refers a resistance value between positions a and c, ρ means a resistivity (dimension is Ω/sq. for a graphene since it has two-dimensional structure), and W is a width of the graphene. Based on the equation (1), the resistivity values within left and right grains (intra-grain) were calculated and listed in Table 1. The resistivity between electrode 3 and 5 (inter-grain through the grain boundary) was also calculated as 1260 Ω/sq. And this value is almost in the same regime with the intra-grain resistivities obtained from the left and right grains. In addition, the resistance value between electrode 3 and 5 (R3-5) was 840 Ω, which is close to the summation of R3-4 and R4-5, both from the intra-grains, calculated from the resistivity of each grain and sample geometry. For comparison, on the misaligned grain boundareis as shown in Fig. 5 (c), the same electrode pattern was deposited and currentvoltage measurment was carried out. The abrupt increase of resistance was verified near the grain boundary. (between electrode 4 and 5) The resistivity between electrode 2 and 3 (left grain) was 1360 Ω/sq. and the resistivity between electrode 5 and 6 (right grain) was 1455 Ω/sq. after geometry consideration in eq. (1). However, the resisvity between electrode 4 and 5 (inter-grain in Fig. 5 (c)) was 5082 Ω/sq. which shows degradation of electrical connection throguh incommensurate stitching in grain boundaries. Also, Yu et al.,17 already made 6 samples to measure inter-grain resistance of the graphene grown on a solid Cu, and all the samples showed higher resistance when crossing grain boundaries. Again, these results clearly prove that the aligned graphene grains show negligible degradation of the electrical transport properties across the commensurate grain boundary. Electrodes
Resistance (Ω)
Resistivity (Ω/sq.)
1-2 (left grain) 1-3 (left grain) 1-4 (left grain) 1-5 (inter-grain) 3-5 (inter-grain) 3-6 (inter-grain) 4-6 (right grain) 5-6 (right grain)
307 412 735 1048 840 885 441 356
1458 939 1009 1002 1260 1254 843 1566
Table 1. 2-point resistance values measured from the TLM (transfer length method) patterning between intra- and inter-grain electrodes of two adjacent graphene grains in Fig. 5 (a) 70
Electrodes
Resistance (Ω)
Resistivity (Ω/sq.)
2-3 (left grain) 4-5 (inter-grain) 5-6 (right grain)
120 847 213
1360 5082 1455
Table 2. 2-point resistance values measured from the TLM patterning between intra- and inter-grain electrodes of two adjacent graphene grains in Fig. 5 (c) 75
Journal Name, [year], [vol], 00–00 | 5
Nanoscale Accepted Manuscript
5
single-step graphene shows higher resistance (1409.36 ± 273.1 Ω/sq.) than two-step sample. This might be from void area and reduction of current path. The carrier mobility was also extracted from Hall measurements, as shown in Fig. 4 (b). The hole mobility which indicates natural p-type doping during transfer of graphene grown on solid Cu was 504.9 ± 304.2 cm2/V·sec, probably due to existence of grain boundaries and random orientation. However, graphene grown on the liquid Cu (two-step) gave higher Hall mobility of 1063.4 ± 259.7 cm2/V·sec (also, ptype). Singe-step grown graphene shows low carrier mobility which also indicates degradation of conducting path due to gaps between islands. Both of these electrical properties clearly show that the graphene grown on liquid Cu by the two-step process can have a superior electrical transport behavior than that grown on the solid Cu. In the case of sheet resistance measurement by our Van der Pauw geometry in Fig. 4, overall film resistance was evaluated through the wafer scale which includes all the possible grain boundaries across millimeter length. It should not be directly compared to the high mobility values of single crystal graphene grain of micrometer scale.12,14,26
Nanoscale
Page 6 of 7 View Article Online
2
4 Conclusions
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
5
10
15
20
25
30
Although the hexagonal graphene flakes growth on the liquid Cu were reported to show the self-assembly behavior between the flakes, a the degree of continuous growth of the graphene flakes at the final stage of growth has not been reported. In this work, we applied hydrogen etching to the fully grown graphene sample to resolve the structural defects in the grain boundaries of the graphene. Our result showed that the hexagonal graphene flakes did not continuously grow with a single growth conditions because of the limited supply of carbon monomers at the final stage. To induce the continuous growth, the two-step growth was suggested by additionally introducing a step with increased the CH4 flux (from 3.5 to 12 sccm) to provide a high degree of supersaturation at the final stage of growth. Gaps and voids were completely filled by the two-step growth and continuity between the graphene flakes was confirmed by both the NaCl-assisted oxidation and the H2 etching. Due to solidification of liquid Cu catalyst, commensurate stitching is not fully carried out. We believe that the potential grain boundaries might be composed of non-hexagon carbon rings which are the result of incommensurate stitching. However, our graphene grown on the liquid Cu shows significantly improved electrical transport behaviors in terms of both sheet resistance and Hall mobility on a wafer scale. The intra- and inter-grain resistance measurement clearly demonstrated the excellent capability of controlling the commensurate grain boundaries in this study. The demonstration and development of self-assembled, continuous, and gap-free graphene with the improved electrical properties will accelerate introduction of the two-step growth recipe on the liquid Cu into many other growth studied and applications.
60
3 4 5 6
65
7
70
8
75
9
80
10 11 85
12
13 90
14 15 95
16
Acknowledgement 35
40
This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (20120009563). This research was also supported by the Global Frontier Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014M3A6B206301).
100
17
18 105
19 20
110
21 22
Notes and references 45
50
23 115
24
a
Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea; Fax: 81-2-885-5820; Tel: 82-2-8807465; E-mail: [email protected] b School of Engineering, Brown University, Providence, Rhode Island 02912, United States † Electronic Supplementary Information (ESI) available: See DOI: 10.1039/b000000x/
25 120
26
27 1 55
S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proc. Nati. Acad. Sci. 2005, 102, 10451.
6 | Journal Name, [year], [vol], 00–00
125
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science 2004, 306, 666. A. K. Geim and K. S. Novoselov, Nat. Mater. 2007, 6, 183. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. Peres and A. K. Geim, Science 2008, 320, 1308. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science 2008, 321, 385. C. Berger, Z. M. Song, X. B. Li, X. S. Wu, N. Brown, C. Naud, D. Mayo, T. B. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First and W. A. de Heer, Science 2006, 312, 1191. W. A. de Heer, C. Berger, X. S. Wu, P. N. First, E. H. Conrad, X. B. Li, T. B. Li, M. Sprinkle, J. Hass, M. L. Sadowski, M. Potemski and G. Martinez, Solid State Commun. 2007, 143, 92. J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science 2011, 331, 568. Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun'Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari and J. N. Coleman, Nat Nanotech. 2008, 3, 563. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi and B. H. Hong, Nature 2009, 457, 706. Y. Lee, S. Bae, H. Jang, S. Jang, S. E. Zhu, S. H. Sim, Y. I. Song, B. H. Hong and J. H. Ahn, Nano Lett. 2010, 10, 490. X. Li, C. W. Magnuson, A. Venugopal, R. M. Tromp, J. B. Hannon, E. M. Vogel, L. Colombo and R. S. Ruoff, J. Am. Chem. Soc. 2011, 133, 2816. X. S. Li, W. W. Cai, J. H. An, S. Kim, J. Nah, D. X. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo and R. S. Ruoff, Science 2009, 324, 1312. X. Li, W. Cai, L. Colombo and R. S. Ruoff, Nano Lett. 2009, 9, 4268. P. Y. Huang, C. S. Ruiz-Vargas, A. M. van der Zande, W. S. Whitney, M. P. Levendorf, J. W. Kevek, S. Garg, J. S. Alden, C. J. Hustedt, Y. Zhu, J. Park, P. L. McEuen and D. A. Muller, Nature 2011, 469, 389. K. Kim, Z. Lee, W. Regan, C. Kisielowski, M. F. Crommie and A. Zettl, ACS Nano 2011, 5, 2142. Q. Yu, L. A. Jauregui, W. Wu, R. Colby, J. Tian, Z. Su, H. Cao, Z. Liu, D. Pandey, D. Wei, T. F. Chung, P. Peng, N. P. Guisinger, E. A. Stach, J. Bao, S.-S. Pei and Y. P. Chen, Nat. Mater. 2011, 10, 443. S. Y. Cho, K. J. Kim, H. M. Kim, D. J. Lee, M. H. Lee and K. B. Kim, RSC Adv. 2013, 3, 26376. H. Zhou, W. J. Yu, L. Liu, R. Cheng, Y. Chen, X. Huang, Y. Liu, Y. Wang, Y. Huang and X. Duan, Nat. Commun. 2013, 4, 2096. Y. Hao, M. S. Bharathi, L. Wang, Y. Liu, H. Chen, S. Nie, X. Wang, H. Chou, C. Tan, B. Fallahazad, H. Ramanarayan, C. W. Magnuson, E. Tutuc, B. I. Yakobson, K. F. McCarty, Y. W. Zhang, P. Kim, J. Hone, L. Colombo and R. S. Ruoff, Science 2013, 342, 720. L. Gan and Z. Luo, ACS Nano 2013, 7, 9480. Z. Yan, J. Lin, Z. Peng, Z. Sun, Y. Zhu, L. Li, C. Xiang, E. L. Samuel, C. Kittrell and J. M. Tour, ACS Nano 2012, 6, 9110. G. H. Han, F. Güneş, J. J. Bae, E. S. Kim, S. J. Chae, H.-J. Shin, J.-Y. Choi, D. Pribat and Y. H. Lee, Nano Lett. 2011, 11, 4144. Z. Luo, Y. Lu, D. W. Singer, M. E. Berck, L. A. Somers, B. R. Goldsmith and A. T. C. Johnson, Chem. Mater. 2011, 23, 1441. H. Wang, G. Wang, P. Bao, S. Yang, W. Zhu, X. Xie and W.-J. Zhang, J. Am. Chem. Soc. 2012, 134, 3627. A. W. Tsen, L. Brown, M. P. Levendorf, F. Ghahari, P. Y. Huang, R. W. Havener, C. S. Ruiz-Vargas, D. A. Muller, P. Kim and J. Park, Science 2012, 336, 1143. V. L. Nguyen, B. G. Shin, D. L. Duong, S. T. Kim, D. Perello, Y. J. Lim, Q. H. Yuan, F. Ding, H. Y. Jeong, H. S. Shin, S. M. Lee, S. H. Chae, Q. A. Vu, S. H. Lee and Y. H. Lee, Adv. Mater. DOI: 10.1002/adma.201404541
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR03352G
Page 7 of 7
Nanoscale View Article Online
DOI: 10.1039/C5NR03352G
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
20
25
30
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
5
28 D. Geng, B. Wu, Y. Guo, L. Huang, Y. Xue, J. Chen, G. Yu, L. Jiang, W. Hu and Y. Liu, Pro. Nat. Acad. Sci. 2012, 109, 7992. 29 D. Geng, B. Luo, J. Xu, Y. Guo, B. Wu, W. Hu, Y. Liu and G. Yu, Adv. Funct. Mater. 2014, 24, 1664. 30 B. Wu, D. Geng, Z. Xu, Y. Guo, L. Huang, Y. Xue, J. Chen, G. Yu and Y. Liu, NPG Asia Mater. 2013, 5, e36. 31 D. Geng, L. Meng, B. Chen, E. Gao, W. Yan, H. Yan, B. Luo, J. Xu, H. Wang, Z. Mao, Z. Xu, L. He, Z. Zhang, L. Peng and G. Yu, Adv. Mater. 2014, 26, 6423. 32 J. A. Cahill and A. D. Kirshenbaum, J. Phys. Chem. 1962, 66, 1080. 33 Y.A. Wu, Y. Fan, S. Speller, G.L. Creeth, J.T. Sadowski, K. He, A.W. Robertson, C.S. Allen, J.H. Warner, ACS Nano, 2012, 6, 5010. 34 M. H. Khan, Z. Huang, F. Xiao, G. Casillas, Z. Chen, P. J. Molino and H. K. Liu, Sci. Rep. 2015, 5, 7743. 35 I. Vlassiouk, M. Regmi, P. Fulvio, S. Dai, P. Datskos, G. Eres and S. Smirnov, ACS Nano 2011, 5, 6069. 36 Y. Zhang, Z. Li, P. Kim, L. Zhang and C. Zhou, ACS Nano 2011, 6, 126. 37 T. H. Ly, D. L. Duong, Q. H. Ta, F. Yao, Q. A. Vu, H. Y. Jeong, S. H. Chae and Y. H. Lee, Adv. Funct. Mater. 2013, 23, 5183. 38 Y. Fan, K. He, H. Tan, S. Speller and J. H. Warner, Chem. Mater. 2014, 26, 4984. 39 N. Bowden, F. Arias, T. Deng and M. Whitesides, Langmuir 2001, 17, 1757. 40 X. Li, C. W. Magnuson, A. Venugopal, J. An, J. W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, L. Fu, E. M. Vogel, E. Voelkl, L. Colombo and R. S. Ruoff, Nano Lett. 2010, 10, 4328. 41 H. Kim, C. Mattevi, M. R. Calvo, J. C. Oberg, L. Artiglia, S. Agnoli, C. F. Hirjibehedin, M. Chhowalla and E. Saiz, ACS Nano 2012, 6, 3614. 42 G. Eres, M. Regmi, C. M. Rouleau, J. Chen, I. N. Ivanov, A. A. Puretzky and D. B. Geohegan, ACS Nano 2014, 8, 5657. 43 J. J. Hoyt, M. Asta and A. Karma, Materials Science and Engineering: R: Reports 2003, 41, 121.
Journal Name, [year], [vol], 00–00 | 7
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Cho, M. Kim, M. Kim, K. Kim, H. Kim, D. Lee, S. Lee and K. Kim, Nanoscale, 2015, DOI: 10.1039/C5NR03352G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/nanoscale
Page 1 of 7
Journal Name
Nanoscale
Dynamic Article Links ► View Article Online
DOI: 10.1039/C5NR03352G
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
www.rsc.org/xxxxxx
Seong-Yong Cho, a Min-Sik Kim,a Minsu Kim, a Ki-Ju Kim, a Hyun-Mi Kim, a Do-Joong Lee, b Sang-Hoon Lee a and Ki-Bum Kim* a 5
10
15
20
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Graphene growth on liquid Cu has gathered a great interest owing to self-assembly behavior of hexagonal graphene flakes with aligned orientation and to possibility of forming a single grain of graphene through a commensurate growth of these graphene flakes. Here, we propose and demonstrate a two-step growth process which allows a formation of self-assembled, completely continuous graphene on the liquid Cu. After the formation of full coverage on the liquid Cu, grain boundaries are revealed via selective hydrogen etching and the original grain boundaries were clearly resolved. This result indicates that, while the flakes self-assembled with a same orientation, there still remain structural defects, gaps, and voids that were not resolved by optical microscopy or scanning electron microscopy. To overcome this limitation, the two-step growth process is employed by consisting a sequential process of (a) normal single layer graphene growth and self-assembly process with a low carbon flux followed by (b) final stage of graphene growth at high degree of supersaturation with a high carbon flux. Continuity of the flakes is verified via hydrogen etching and NaCl-assisted oxidation process as well as by measuring electrical properties of the graphene grown by the two-step process. Two-step growth can provide continous graphene layer, but commensurate stithicng should be further studied.
1 Introduction
25
30
35
40
45
50
Motivated by its superior electrical,1-3 optical4 and mechanical5 properties, various synthesis methods for graphene growth have been proposed for a formation of high quality, large scale graphene.6-11 Among those, graphene growth on a solid Cu surface has been extensively studied due to a readiness of a large scale synthesis with monolayer dominant graphene growth.12-14 However, during the growth, grain boundaries are inevitably formed between adjacent graphene grains as a result of the nature of graphene synthesis and those grain boundaries are known to significantly degrade the quality of graphene.15-18 While various methods have been proposed to grow relatively large-sized graphene domains (up to ~ mm) either by controlling a nucleation density19-25 or by aiming for commensurate growth of graphene domains,26,27 synthesizing a high quality, large domain size graphene within a short period of process time still remains as a great challenge. In this respect, recent publications by Geng et al., provided several intriguing aspects of graphene growth on liquid Cu by chemical vapour deposition (CVD) as compared to that on solid Cu.28-31 Firstly, one does not have to worry about various heterogeneous nucleation sites that inevitably formed on solid Cu foil as a result of Cu melting. Secondly, it appears that a uniformity of the size of hexagonal graphene flakes can be easily controlled by tuning process parameters. Thirdly, the graphene flakes self-assemble in a close-packed manner when the relative size of graphene flakes is uniform. Each hexagonal graphene flakes appears to be aligned to have the same crystallographic orientation relationship. Lastly and most importantly, they claimed that graphene flakes are merged with each other to This journal is © The Royal Society of Chemistry [year]
55
60
65
70
75
80
possibly form a single grain when the process is continued to have the full coverage growth, based on observations of optical and scanning electron microscopies (SEM). Despite of the limitations of needing to use a high melting point and good wettability substrate like W or Mo, which does not intermix with Cu, and a drawback of large thermal expansion mismatch between the liquid Cu and the graphene during cooling,32 this process certainly provides one of the unique opportunities to form a large size single crystal with predominantly monolayer of graphene and warrants further expedition.33 Not only graphene, but h-BN can be grown on liquid Cu according to recent report.34 Even though their reports clearly suggest advantages and impacts of using the liquid Cu as a growing substrate, understandings on its growth mechanism is still lacking and there are more rooms to improve a quality of the graphene by amending grain boundaries between the hexagonal graphene flakes. Specifically, we focused on the last stage of graphene growth on the liquid Cu, namely, the continuous growth of each graphene flake. To pursue this purpose, we first grew hexagonal graphene flakes on the liquid Cu, as was suggested by Geng et al.,28 and confirmed the self-assembly of those graphene flakes by SEM and transmission electron microscopy (TEM). Then, the continuous growth of graphene flakes on liquid Cu was investigated by selective hydrogen etching35,36 and NaCl-assisted oxidation of the underlying Cu37 to reveal defect sites. Our results showed that the stitched domain boundaries were well resolved by these processes and indicated that, even in a fully grown graphene on the liquid Cu, there still exists a high density of defects, voids, and small gaps. Such results proved that the continuous growth is not perfect by itself. The discontinuous growth of graphene on liquid Cu has already been reported by [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
Self-assembly and continuous growth of hexagonal graphene flakes on liquid Cu
Nanoscale
Page 2 of 7 View Article Online
DOI: 10.1039/C5NR03352G
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
20
25
30
35
40
45
50
55
60
65
metal shadow mask. Sheet resistance and Hall mobility was measured by the VDP method. For Hall measurement, magnetic field was 0.6 T. (HL 5500PC, BIO-RAD) For transmission line measurement (TLM) patterning after transferring the graphene onto a 285-nm-thick SiO2/Si wafer, photoresist (AZ 5214) was uniformly coated and was patterned using an MA 6-II aligner. Then, Ti/Au (5 nm / 80 nm) electrode was evaporated and lift-off process was done to leave desired patterns. For two points of current/voltage measurements, an Agilent 4156C system was used.
2 Experimental Section Graphene growth: Electropolished Cu foils (Alfa Aesar #13382) were put on top of 1 inch W foil (also, Alfa Aesar) and loaded into a quartz tube type furnace. (Lindberg, blue) Then, sequentially, the furnace was pumped down to ~10-3 Torr, hydrogen flowed at 500 sccm until the chamber reaches atmospheric pressure, and heating was started. After reaching a target temperature of 1100 oC, which is above the melting point of Cu, CH4 was introduced at 3.5 sccm as a carbon precursor. Growth time was varied from 30 minutes for a partial coverage graphene and longer than 2 hours for a full coverage one. To reveal grain boundaries of the graphene grwon on the liquid Cu, hydrogen etching was performed in-situ by shutting CH4 flux off and by flowing only 500 sccm H2 after cooling down the sample to 1050 oC for 5 to 10 minutes (to solidifify the underlying Cu). For the two-step growth, once the growth was proceded on the liquid Cu with the conditions addressed above, CH4 flux was further increased to 12 sccm either on the liquid Cu (at 1100 oC) or after solidifying Cu (at 1050 oC). For comparison, graphene was also grown on solid Cu at 1030 oC using a typical low pressure CVD process. CH4 flux was maintained to 0.5 sccm with flowing 12 sccm of H2 and CH4 flux was finally increaed up to 2 sccm at the final step for full coverage. Grain size of graphene grown on the solid Cu by low pressure CVD was approximately 50 µm, which is a similar size as the graphene grown on the liquid Cu in this work. Transfer and pattern fabrication: Graphene was transferred by a well-known process of poly methyl methacrylate (PMMA) coating, and subsequent etching of underlying W and Cu layers. In particular, W etching was performed by an anodic etching, as proposed by Y. Fan et al. 38 using a bare Cu foil as a cathode in 2 M NaOH solution. Then, PMMA/graphene/Cu stack was floated in ammonium persulfate solution (0.1 M) for overnight to completely etch the Cu foil. After etch, floated PMMA/graphene layer was scooped on a TEM grid (Quantifoil, Ted Pella) or on a SiO2 (285 nm)/Si substrate for patterning and for electrical transport measurement. Then, PMMA layer was removed by boiling acetone or direct heating at 380 oC flowing Ar and H2. For the preparation of VDP patterns, Ti/Au (5 nm / 80 nm) electrode patterns was deposited by e-beam evaporation using a 2 | Journal Name, [year], [vol], 00–00
Fig. 1. Characterization of graphene grown on liquid Cu. (a) Schematic illustration of the graphene growth process on the liquid Cu, (b) SEM image of as-grown graphene islands on the liquid Cu for 30 min of growth time showing a self-assembled growth behavior and (c) optical microscope image of a graphene grown on the liquid Cu for 50 min followed by oxidation of the Cu substrate. Inset figure in (c) shows a representative Raman spectrum of a single layer of graphene grown on the liquid Cu after transferred onto a SiO2/Si substrate. 70
3 Results and discussion
75
80
85
90
95
Fig. 1 (a) shows a schematic picture of the graphene growth on liquid Cu via a single step process. The self-assembled graphene was grown at 1100 oC with CH4 and H2 flow rates of 3.5 and 500 sccm, respectively. Fig. 1 (b) and (c) show representative SEM (b) and optical microscope images (c) showing the formation of hexagonal-shaped graphene flakes and their self-assembly behavior on a partially grown sample. The inset in Fig. 1 (c) shows the corresponding Raman spectra which clearly indicate a monolayer graphene growth after the transfer to a SiO2/Si substrate. A process condition was tuned to have roughly uniform size of graphene flakes at CH4 and H2 flow rates of 3.5 and 500 sccm, respectively, to match with the results of Geng et al.,28 in the aspect of graphene flake shape, size and its uniformity. In order to induce self-assembly, the formation of hexagonal graphene flakes and its size uniformity are important factors while an average size does not take a critical role. Our preliminary experimental results showed that a rigorous selection of the process condition is required; for instance, when the CH4 flow rate is increased above 3.8 sccm, multiple graphene layers grew, while with a slightly lower CH4 flow rate of 3.3 sccm than the optimum flow, size of graphene flakes were obviously nonuniform. Further reduction of CH4 flow rate below 3.0 sccm flow resulted in no graphene growth (Supplementary Fig. S1). Certainly, it is important to determine what the driving force of this assembly is. We have noted that the graphene flakes move to the liquid area to assemble during cooling when the overall This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
5
Fan et al.38 Their work was focused on healing cracks which formed during solidification of underlying liquid Cu catalyst. To accomplish a better continuous growth with minimal defects and grain boundaries, we employed a two-step growth process by additionally introducing a high degree of supersaturation growth condition, with increased carbon flux, at the last stage of growth. Surprisingly, samples grown by the two-step process were neither etched by hydrogen nor oxidized by NaCl. Electrical resistance of the graphene grown by the two-step process shows much lower value compared to graphene grown on the conventional solid Cu and even to that on the liquid Cu via a single-step growth. Intragrains (within a single grain) and inter-grain (between two grains through the grain boundaries) resistances were also measured and compared by preparing electrode17 atop the graphene to prove excellent self-assembly of the graphene flakes. These results comprise outstanding quality and properties of a full coverage graphene layer grown by the newly proposed two-step process within a short period of growth time.
Page 3 of 7
Nanoscale View Article Online
5
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
coverage is low. This result indicates that the capillary force is one of the driving forces for this assembly.38 However when the overall coverage is relatively high; the flakes also self-assemble with each other on liquid Cu, which indicates that Van der Waals interaction between the flakes is another driving force. Our interest, then, is how well the adjacent graphene flakes commensurately grows and form continuous layer when the growth process continues on the liquid Cu. It is well known that grain boundary is formed by grains merging that have different crystallographic orientations. In the case of graphene, nonhexagon type carbon arrangements, such as heptagon and pentagon are formed in the grain boundaries to accommodate lattice mismatch.15 By a series of TEM images and selected area electron diffraction patterns, we confirmed that individual hexagonal graphene flake is composed of a single grain and the graphene flake is crystallographically well oriented through the same directions (Fig. S2 and S3). However, these results are not still sufficient to show the commensurate growth and single crystallinity of these flakes.
40
45
50
55
60
65
70
Fig. 2. Grain boundaries revealed by hydrogen etching and NaCl-assisted oxidation. SEM images of graphene islands (a) after 2 h of growth time (as-grown) and (b) the corresponding sample after the hydrogen etching for 5 min. Optical microscopy images of (c) graphene after 2 h of growth time and (d) the corresponding sample after the NaCl-assisted oxidation for 24 h. Scale bar indicates 30 µm. 20
25
30
35
To check the continuity of hexagonal graphene flakes, we first grew graphene to full coverage for 2 hours, with the optimum conditions addressed above, as is shown in Fig. 2 (a). This sample was then cooled to 1050 oC and H2 etching was carried out without breaking vacuum. The hydrogen-induced etching of graphene has already been reported35,36 and is known to etch selectively defect sites of graphene such as the grain boundary (Fig. 2 (b)). Surprisingly, the original hexagonal flakes are well delineated by hydrogen etching as are the nucleation centers at the center of the flakes. This result clearly demonstrates that, while the graphene flakes are self-assembled, the flakes do not continuously grow with each other at all and there still remain defect sites which are prone to etching by hydrogen. Similar etching results were obtained with extended growth time such as 3 and 4 hours. Indeed, closer examination of the fully grown samples often shows small voids (Fig. S4 (a)) and gaps (Fig. S4 (b)) between the flakes that are not even filled by the extended growth time up to 4 hours. NaCl-assisted oxidation was also employed to observe grain boundaries, as previously proposed by This journal is © The Royal Society of Chemistry [year]
75
80
85
90
95
Ly et al. 37 The Graphene/Cu/W samples prepared by the single step process were dipped into NaCl (6 wt. %) solution and left for 24 hours. Grain boundaries of the graphene grown on liquid Cu by the single step were obviously revealed by the NaCl-assisted oxidation as shown in Fig. 2 (d). (Fig. S5 also) Contrary to the grain boundaries image with well-arranged and delineated by H2 etching as shown in Fig. 2 (b), NaCl-assisted oxidation does not give perfect alignment. We believe that this is difference between in-situ H2 etching and ex-situ NaCl-assisted oxidation. H2 etching gives more accurate grain boundaries image since H2 etching could be carried out without cooling Cu and it can contribute to the visualization of grain boundaries as it formed. On the other hands, NaCl-assisted oxidation inevitably accompanies cooling and solidification of Cu, which can damage to graphene. Also, severe oxidation environment can also deteriorate the graphene film and NaCl precipitate might be formed in the 1M solution. Here, a difference between the CVD growth of graphene and the conventional CVD process of other materials should be noted. The conventional CVD process is the method of continuously depositing films by using decomposition and reaction of gas precursors on a substrate. Thus, a deposition rate is constant with respect to a deposition time at a given process condition. On the contrary, the CVD graphene growth is a process critically relying on a catalytic effect of underlying Cu substrate for decomposition of carbon precursors and supply/diffusion of carbon monomers. Thus, as a graphene coverage increases, an overall growth rate of graphene decreases definitely due to a limited supply of carbon monomers, which should be generated by the exposed Cu area. This type of growth mode is well explained by the twodimensional Johnson-Mehl-Avrami (JMA) type growth. The question that brought out here is whether the limited monomer supply at a final stage of the growth is still enough to fill the gaps between adjacent grains. These narrow gaps between graphene grains should be distinguished with merged neighboring grains (For instance, Fig. 5 (a) and (c)) at the initial stage when carbon attachment was facile due to larger exposed area. The two grains can stitch each other easily. Obviously, to fill the gaps at the last stage, an overall process should be executed at a high degree of supersaturation, namely at a high flow rate of carbon precursors for acquisition of sufficient carbon monomer in-flux. However, high flow rate condition can simultaneously resulted in a high degree of nucleation rate which is not preferable for growth of large grain sized graphene. To increase a grain size, one has to critically control the degree of supersaturation to minimize the nucleation rate at the initial stage of growth. However, in this case, one often finds out that the small gaps between the grains and little voids are not well filled by the continuous growth even though one successfully grows large-sized graphene grains. A possible mechanism of the voids formation during the graphene growth has already been reported by a few researchers. For instance, Li et al., 40 suggested that carbon in-flux should be supersaturated for obtaining a continuous graphene layer. They claimed that a small amount of carbon monomer is insufficient to continuously drive carbon attachment to the island edges and thereby a Cu surface is only partially covered with the graphene islands. Kim et al.,41 also suggested models describing graphene growth kinetics and a coverage as a function of a growth time, where they described a final coverage can be expressed as a
Journal Name, [year], [vol], 00–00 | 3
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR03352G
Nanoscale
Page 4 of 7 View Article Online
5
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
function of a supersaturation ratio. Additionally, Eres et al.,42 found out that voids can exist under a low carbon feeding rate. In our growth condition, the graphene growth was not observed at CH4 flow rate less than 3.0 sccm. This means that our growth condition (3.5 sccm of CH4) is just above the minimum feeding rate for the growth but is not enough to fill the gaps between graphene flakes at the final stage of growth. However a higher carbon flux condition (3.8 and 4 sccm) at the initial stage of graphene growth on the liquid Cu resulted in the size discrepancy of flakes and the formation of multilayer graphene, as shown in Fig. S1.
40
45
50
55
60
Fig. 3. (a) Schematic illustration of two-step graphene growth on liquid Cu. CH4 flux was increased for continuous stitching of graphene flakes and further growth was also carreid out after cooling to solid. (b) SEM image of a continuous graphene layer by the two-step method after experiencing the hydrogen etching process which shows still continuous film, and (c) optical microscope image of the graphene layer on the liquid Cu by the two-step growth and no damages were observed after the NaClassisted oxidation. Scale bar indicates 30 µm.
15
20
25
30
35
Based on these arguments, our conjecture here is that narrow gaps still exist between graphene flakes grown on the liquid Cu due to the small feeding rate of carbon monomers and the resultant low supersaturation ratio. In this regard, we propose a two-step growth recipe for the formation of continuous, gap-free graphene, as illustrated in Fig. 3 (a). (Fig. S6 for reaction scheme) A full coverage graphene can also be prepared by using a high CH4 flow rate at the initial stage, but the over-flux condition definitely results in either the multilayer formation (supporting information, Fig. S1) or a large size discrepancy in graphene flakes. In this reason, we design a two-step process which uses a low carbon flux at the initial stage for a sufficiently low nucleation rate while introduces a high carbon flux (high degree of supersaturation) at the final stage for filling the gaps between adjacent graphene flakes. Carbon flux was maintained after cooling to solid to fill the possible gaps which might be formed by thermal stress due to cooling. Cracks were often observed after cooling the sample down to solid. Such cracks are suspected to be formed because of a thermal stress induced by Cu solidification,43 which inevitably results in volume shrinkage, as identified SEM images. (Supporting information, Fig. S7) A recent study also emphasized the crack formation in a graphene layers grown on the liquid Cu.38 To overcome the limitation described above, two-step growth for stitching process was carried out after cooling to solid (1050 oC), which showed lower crack formation. Also, the crack formation was significantly suppressed by slow cooling rate and maintaining high carbon flux 4 | Journal Name, [year], [vol], 00–00
65
on solid Cu even though two-step process was applied on liquid Cu. We admit that for better commensurate stitching between graphene islands, two-step growth without cooling to solid is desirable because self-assembly can occur easily on liquid surface. We believe that continuous graphene with well stitching (hexagon carbon rings) can be grown on liquid Cu by two-step carbon feeding, but continuous graphene on liquid Cu can be easily torn due to thermal stress between graphene and Cu. It might be reason for low carrier mobility for continuous graphene layer which will be discussed. To examine the continuous growth of graphene, hydrogen etching was applied to two-step grown graphene in this study. Fig. 3 (b) shows a continuous graphene layer that was formed by two-step after hydrogen etching (Higher magnification SEM image in Fig. S8). We could not observe empty area even at higher magnifications. The result shows that grain boundaries were not etched by hydrogen etching at all, contrary to the result in the single step graphene in Fig. 2 (b). We do not believe that this continuous graphene shows no grain boundaries with wellstitching because two-step growth was carried out after cooling the Cu catalyst, but continuity of graphene was achieved. NaClassisted oxidation was also employed to check the continuity of the film. Surprisingly, the sample grown by the two-step growth did not show any remarkable oxidation behavior at grain boundaries despite of 24 hours of oxidation time as shown in Fig. 3 (c) (see Fig. S9 also for the detail information) which is quite different from behavior of single step sample (Fig. 2 (d)). These results clearly suggest that the graphene grown by the two-step recipe in this study is completely continuous and has no gaps between the neighboring graphene flakes at all, which were not achievable in the single-step growth.
Fig. 4. (a) Sheet resistances and (b) Hall mobilities of graphene grown on the solid and the liquid Cu. 70
75
80
85
Not only imaging grain boundaries via etching, but electrical transport properties of the two-step grown graphene were evaluated and compared to those of a graphene grown on a conventional solid Cu. For the characterization, the graphene grown on the solid Cu was transferred to 285nm-SiO2/Si substrate using a polymer coating and subsequent wet-etching technique.10,11 Meanwhile, in the case of our two-step grown sample on the liquid Cu, an underlying W foil was etched by anodic etching suggested by Fan et al.38 After transferring the graphene onto a target substrate, Ti/Au electrode was formed to prepare Van der Pauw (VDP) geometry. The detailed VDP sample preparation method is described in supporting information, Fig. S6. The results are shown in Fig. 4 (a) and (b). Remarkably, the electrical sheet resistance of the graphene grown on the liquid Cu (two-step) measured by VDP configuration (298.1 ± 93.6 Ω/sq.) was much lower compared to the graphene grown on the solid Cu (700.7 ± 472.2 Ω/sq.) (Fig. 4 (a)). On the other hands, This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR03352G
Page 5 of 7
Nanoscale View Article Online
DOI: 10.1039/C5NR03352G
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
20
40
45
50
55
60
65
Fig. 5. Comparison of commensurate and incommensurate stitching. (a) Optical microscope image of two adjacent graphene islands aligned in the same orientation and Ti/Au electrodes were deposited by the lift-off process, (b) current vs. applied bias curve from electrode #1 to various electrodes including intra- (1-2, 1-3, and 1-4) and inter-grain (1-5), (c) optical microscope image of two adjacent graphene islands having misaligned orienetation and (d) current vs. Applied bias curve from electrode #7 to various electrodes including intra- (7-6 and 7-5) and intergrain (7-4, 7-3, 7-2 and 7-1). Scale bars indicate 50 µm.
25
30
35
Moreover, in order to examine effects of grain boundaries on electrical properties of the commensurate, gap-free graphene in this study, the electrode patterns were prepared on top of two adjacent graphene grains using a photolithography and a lift-off process, as shown in Fig. 5 (a). These grain boundaries were formed in the initial stage of graphene growth where we do not have to worry about stitching issue due to low carbon flux. Large domain size of our graphene (~100 µm) allowed to easily fabricate the electrode patterns by the photolithography. The representative current-voltage curves are shown in Fig. 5 (b) with various lengths of current paths. We did not observe any degradation in the electrical performance due to the grain boundaries (across the electrode #4 in Fig. 5 (a)), and the resistance value was only dependent on the length of current path (distance between the electrodes) as shown in Fig. 5 (b). This journal is © The Royal Society of Chemistry [year]
Specifically, there was no abrupt increase of the resistance when current flowed across the grain boundary of the graphene (dotted line (1-5) in Fig. 5 (b)). The resistance of the pattern from position a to c can be calculated from the following equation,17 b c dx dx (1) Ra −c = ρ a −b ∫ + ρb −c ∫ W x W ( ) ( x) a b Here, Ra-c refers a resistance value between positions a and c, ρ means a resistivity (dimension is Ω/sq. for a graphene since it has two-dimensional structure), and W is a width of the graphene. Based on the equation (1), the resistivity values within left and right grains (intra-grain) were calculated and listed in Table 1. The resistivity between electrode 3 and 5 (inter-grain through the grain boundary) was also calculated as 1260 Ω/sq. And this value is almost in the same regime with the intra-grain resistivities obtained from the left and right grains. In addition, the resistance value between electrode 3 and 5 (R3-5) was 840 Ω, which is close to the summation of R3-4 and R4-5, both from the intra-grains, calculated from the resistivity of each grain and sample geometry. For comparison, on the misaligned grain boundareis as shown in Fig. 5 (c), the same electrode pattern was deposited and currentvoltage measurment was carried out. The abrupt increase of resistance was verified near the grain boundary. (between electrode 4 and 5) The resistivity between electrode 2 and 3 (left grain) was 1360 Ω/sq. and the resistivity between electrode 5 and 6 (right grain) was 1455 Ω/sq. after geometry consideration in eq. (1). However, the resisvity between electrode 4 and 5 (inter-grain in Fig. 5 (c)) was 5082 Ω/sq. which shows degradation of electrical connection throguh incommensurate stitching in grain boundaries. Also, Yu et al.,17 already made 6 samples to measure inter-grain resistance of the graphene grown on a solid Cu, and all the samples showed higher resistance when crossing grain boundaries. Again, these results clearly prove that the aligned graphene grains show negligible degradation of the electrical transport properties across the commensurate grain boundary. Electrodes
Resistance (Ω)
Resistivity (Ω/sq.)
1-2 (left grain) 1-3 (left grain) 1-4 (left grain) 1-5 (inter-grain) 3-5 (inter-grain) 3-6 (inter-grain) 4-6 (right grain) 5-6 (right grain)
307 412 735 1048 840 885 441 356
1458 939 1009 1002 1260 1254 843 1566
Table 1. 2-point resistance values measured from the TLM (transfer length method) patterning between intra- and inter-grain electrodes of two adjacent graphene grains in Fig. 5 (a) 70
Electrodes
Resistance (Ω)
Resistivity (Ω/sq.)
2-3 (left grain) 4-5 (inter-grain) 5-6 (right grain)
120 847 213
1360 5082 1455
Table 2. 2-point resistance values measured from the TLM patterning between intra- and inter-grain electrodes of two adjacent graphene grains in Fig. 5 (c) 75
Journal Name, [year], [vol], 00–00 | 5
Nanoscale Accepted Manuscript
5
single-step graphene shows higher resistance (1409.36 ± 273.1 Ω/sq.) than two-step sample. This might be from void area and reduction of current path. The carrier mobility was also extracted from Hall measurements, as shown in Fig. 4 (b). The hole mobility which indicates natural p-type doping during transfer of graphene grown on solid Cu was 504.9 ± 304.2 cm2/V·sec, probably due to existence of grain boundaries and random orientation. However, graphene grown on the liquid Cu (two-step) gave higher Hall mobility of 1063.4 ± 259.7 cm2/V·sec (also, ptype). Singe-step grown graphene shows low carrier mobility which also indicates degradation of conducting path due to gaps between islands. Both of these electrical properties clearly show that the graphene grown on liquid Cu by the two-step process can have a superior electrical transport behavior than that grown on the solid Cu. In the case of sheet resistance measurement by our Van der Pauw geometry in Fig. 4, overall film resistance was evaluated through the wafer scale which includes all the possible grain boundaries across millimeter length. It should not be directly compared to the high mobility values of single crystal graphene grain of micrometer scale.12,14,26
Nanoscale
Page 6 of 7 View Article Online
2
4 Conclusions
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
5
10
15
20
25
30
Although the hexagonal graphene flakes growth on the liquid Cu were reported to show the self-assembly behavior between the flakes, a the degree of continuous growth of the graphene flakes at the final stage of growth has not been reported. In this work, we applied hydrogen etching to the fully grown graphene sample to resolve the structural defects in the grain boundaries of the graphene. Our result showed that the hexagonal graphene flakes did not continuously grow with a single growth conditions because of the limited supply of carbon monomers at the final stage. To induce the continuous growth, the two-step growth was suggested by additionally introducing a step with increased the CH4 flux (from 3.5 to 12 sccm) to provide a high degree of supersaturation at the final stage of growth. Gaps and voids were completely filled by the two-step growth and continuity between the graphene flakes was confirmed by both the NaCl-assisted oxidation and the H2 etching. Due to solidification of liquid Cu catalyst, commensurate stitching is not fully carried out. We believe that the potential grain boundaries might be composed of non-hexagon carbon rings which are the result of incommensurate stitching. However, our graphene grown on the liquid Cu shows significantly improved electrical transport behaviors in terms of both sheet resistance and Hall mobility on a wafer scale. The intra- and inter-grain resistance measurement clearly demonstrated the excellent capability of controlling the commensurate grain boundaries in this study. The demonstration and development of self-assembled, continuous, and gap-free graphene with the improved electrical properties will accelerate introduction of the two-step growth recipe on the liquid Cu into many other growth studied and applications.
60
3 4 5 6
65
7
70
8
75
9
80
10 11 85
12
13 90
14 15 95
16
Acknowledgement 35
40
This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (20120009563). This research was also supported by the Global Frontier Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2014M3A6B206301).
100
17
18 105
19 20
110
21 22
Notes and references 45
50
23 115
24
a
Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea; Fax: 81-2-885-5820; Tel: 82-2-8807465; E-mail: [email protected] b School of Engineering, Brown University, Providence, Rhode Island 02912, United States † Electronic Supplementary Information (ESI) available: See DOI: 10.1039/b000000x/
25 120
26
27 1 55
S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, Proc. Nati. Acad. Sci. 2005, 102, 10451.
6 | Journal Name, [year], [vol], 00–00
125
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science 2004, 306, 666. A. K. Geim and K. S. Novoselov, Nat. Mater. 2007, 6, 183. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. Peres and A. K. Geim, Science 2008, 320, 1308. C. Lee, X. Wei, J. W. Kysar and J. Hone, Science 2008, 321, 385. C. Berger, Z. M. Song, X. B. Li, X. S. Wu, N. Brown, C. Naud, D. Mayo, T. B. Li, J. Hass, A. N. Marchenkov, E. H. Conrad, P. N. First and W. A. de Heer, Science 2006, 312, 1191. W. A. de Heer, C. Berger, X. S. Wu, P. N. First, E. H. Conrad, X. B. Li, T. B. Li, M. Sprinkle, J. Hass, M. L. Sadowski, M. Potemski and G. Martinez, Solid State Commun. 2007, 143, 92. J. N. Coleman, M. Lotya, A. O’Neill, S. D. Bergin, P. J. King, U. Khan, K. Young, A. Gaucher, S. De, R. J. Smith, I. V. Shvets, S. K. Arora, G. Stanton, H.-Y. Kim, K. Lee, G. T. Kim, G. S. Duesberg, T. Hallam, J. J. Boland, J. J. Wang, J. F. Donegan, J. C. Grunlan, G. Moriarty, A. Shmeliov, R. J. Nicholls, J. M. Perkins, E. M. Grieveson, K. Theuwissen, D. W. McComb, P. D. Nellist and V. Nicolosi, Science 2011, 331, 568. Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. McGovern, B. Holland, M. Byrne, Y. K. Gun'Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari and J. N. Coleman, Nat Nanotech. 2008, 3, 563. K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J. H. Ahn, P. Kim, J. Y. Choi and B. H. Hong, Nature 2009, 457, 706. Y. Lee, S. Bae, H. Jang, S. Jang, S. E. Zhu, S. H. Sim, Y. I. Song, B. H. Hong and J. H. Ahn, Nano Lett. 2010, 10, 490. X. Li, C. W. Magnuson, A. Venugopal, R. M. Tromp, J. B. Hannon, E. M. Vogel, L. Colombo and R. S. Ruoff, J. Am. Chem. Soc. 2011, 133, 2816. X. S. Li, W. W. Cai, J. H. An, S. Kim, J. Nah, D. X. Yang, R. Piner, A. Velamakanni, I. Jung, E. Tutuc, S. K. Banerjee, L. Colombo and R. S. Ruoff, Science 2009, 324, 1312. X. Li, W. Cai, L. Colombo and R. S. Ruoff, Nano Lett. 2009, 9, 4268. P. Y. Huang, C. S. Ruiz-Vargas, A. M. van der Zande, W. S. Whitney, M. P. Levendorf, J. W. Kevek, S. Garg, J. S. Alden, C. J. Hustedt, Y. Zhu, J. Park, P. L. McEuen and D. A. Muller, Nature 2011, 469, 389. K. Kim, Z. Lee, W. Regan, C. Kisielowski, M. F. Crommie and A. Zettl, ACS Nano 2011, 5, 2142. Q. Yu, L. A. Jauregui, W. Wu, R. Colby, J. Tian, Z. Su, H. Cao, Z. Liu, D. Pandey, D. Wei, T. F. Chung, P. Peng, N. P. Guisinger, E. A. Stach, J. Bao, S.-S. Pei and Y. P. Chen, Nat. Mater. 2011, 10, 443. S. Y. Cho, K. J. Kim, H. M. Kim, D. J. Lee, M. H. Lee and K. B. Kim, RSC Adv. 2013, 3, 26376. H. Zhou, W. J. Yu, L. Liu, R. Cheng, Y. Chen, X. Huang, Y. Liu, Y. Wang, Y. Huang and X. Duan, Nat. Commun. 2013, 4, 2096. Y. Hao, M. S. Bharathi, L. Wang, Y. Liu, H. Chen, S. Nie, X. Wang, H. Chou, C. Tan, B. Fallahazad, H. Ramanarayan, C. W. Magnuson, E. Tutuc, B. I. Yakobson, K. F. McCarty, Y. W. Zhang, P. Kim, J. Hone, L. Colombo and R. S. Ruoff, Science 2013, 342, 720. L. Gan and Z. Luo, ACS Nano 2013, 7, 9480. Z. Yan, J. Lin, Z. Peng, Z. Sun, Y. Zhu, L. Li, C. Xiang, E. L. Samuel, C. Kittrell and J. M. Tour, ACS Nano 2012, 6, 9110. G. H. Han, F. Güneş, J. J. Bae, E. S. Kim, S. J. Chae, H.-J. Shin, J.-Y. Choi, D. Pribat and Y. H. Lee, Nano Lett. 2011, 11, 4144. Z. Luo, Y. Lu, D. W. Singer, M. E. Berck, L. A. Somers, B. R. Goldsmith and A. T. C. Johnson, Chem. Mater. 2011, 23, 1441. H. Wang, G. Wang, P. Bao, S. Yang, W. Zhu, X. Xie and W.-J. Zhang, J. Am. Chem. Soc. 2012, 134, 3627. A. W. Tsen, L. Brown, M. P. Levendorf, F. Ghahari, P. Y. Huang, R. W. Havener, C. S. Ruiz-Vargas, D. A. Muller, P. Kim and J. Park, Science 2012, 336, 1143. V. L. Nguyen, B. G. Shin, D. L. Duong, S. T. Kim, D. Perello, Y. J. Lim, Q. H. Yuan, F. Ding, H. Y. Jeong, H. S. Shin, S. M. Lee, S. H. Chae, Q. A. Vu, S. H. Lee and Y. H. Lee, Adv. Mater. DOI: 10.1002/adma.201404541
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C5NR03352G
Page 7 of 7
Nanoscale View Article Online
DOI: 10.1039/C5NR03352G
Published on 25 June 2015. Downloaded by Freie Universitaet Berlin on 04/07/2015 07:19:52.
10
15
20
25
30
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
5
28 D. Geng, B. Wu, Y. Guo, L. Huang, Y. Xue, J. Chen, G. Yu, L. Jiang, W. Hu and Y. Liu, Pro. Nat. Acad. Sci. 2012, 109, 7992. 29 D. Geng, B. Luo, J. Xu, Y. Guo, B. Wu, W. Hu, Y. Liu and G. Yu, Adv. Funct. Mater. 2014, 24, 1664. 30 B. Wu, D. Geng, Z. Xu, Y. Guo, L. Huang, Y. Xue, J. Chen, G. Yu and Y. Liu, NPG Asia Mater. 2013, 5, e36. 31 D. Geng, L. Meng, B. Chen, E. Gao, W. Yan, H. Yan, B. Luo, J. Xu, H. Wang, Z. Mao, Z. Xu, L. He, Z. Zhang, L. Peng and G. Yu, Adv. Mater. 2014, 26, 6423. 32 J. A. Cahill and A. D. Kirshenbaum, J. Phys. Chem. 1962, 66, 1080. 33 Y.A. Wu, Y. Fan, S. Speller, G.L. Creeth, J.T. Sadowski, K. He, A.W. Robertson, C.S. Allen, J.H. Warner, ACS Nano, 2012, 6, 5010. 34 M. H. Khan, Z. Huang, F. Xiao, G. Casillas, Z. Chen, P. J. Molino and H. K. Liu, Sci. Rep. 2015, 5, 7743. 35 I. Vlassiouk, M. Regmi, P. Fulvio, S. Dai, P. Datskos, G. Eres and S. Smirnov, ACS Nano 2011, 5, 6069. 36 Y. Zhang, Z. Li, P. Kim, L. Zhang and C. Zhou, ACS Nano 2011, 6, 126. 37 T. H. Ly, D. L. Duong, Q. H. Ta, F. Yao, Q. A. Vu, H. Y. Jeong, S. H. Chae and Y. H. Lee, Adv. Funct. Mater. 2013, 23, 5183. 38 Y. Fan, K. He, H. Tan, S. Speller and J. H. Warner, Chem. Mater. 2014, 26, 4984. 39 N. Bowden, F. Arias, T. Deng and M. Whitesides, Langmuir 2001, 17, 1757. 40 X. Li, C. W. Magnuson, A. Venugopal, J. An, J. W. Suk, B. Han, M. Borysiak, W. Cai, A. Velamakanni, Y. Zhu, L. Fu, E. M. Vogel, E. Voelkl, L. Colombo and R. S. Ruoff, Nano Lett. 2010, 10, 4328. 41 H. Kim, C. Mattevi, M. R. Calvo, J. C. Oberg, L. Artiglia, S. Agnoli, C. F. Hirjibehedin, M. Chhowalla and E. Saiz, ACS Nano 2012, 6, 3614. 42 G. Eres, M. Regmi, C. M. Rouleau, J. Chen, I. N. Ivanov, A. A. Puretzky and D. B. Geohegan, ACS Nano 2014, 8, 5657. 43 J. J. Hoyt, M. Asta and A. Karma, Materials Science and Engineering: R: Reports 2003, 41, 121.
Journal Name, [year], [vol], 00–00 | 7
