Visualization of Grain Structure and Boundaries of Polycrystalline

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Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides Hiroki Ago, Satoru Fukamachi, Hiroko Endo, Pablo Solís-Fernández, Rozan Mohamad Yunus, Yuki Uchida, Vishal Panchal, Olga Kazakova, and Masaharu Tsuji ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b05879 • Publication Date (Web): 04 Mar 2016 Downloaded from http://pubs.acs.org on March 6, 2016

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Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides Hiroki Ago,†,‡,§,* Satoru Fukamachi,† Hiroko Endo,† Pablo Solís-Fernández, † Rozan Mohamad Yunus,‡ Yuki Uchida,‡ Vishal Panchal,# Olga Kazakova,# and Masaharu Tsuji||

†

Institute for Materials Chemistry and Engineering (IMCE), Kyushu University, Kasuga,

Fukuoka 816-8580, Japan ‡

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga,

Fukuoka 816-8580, Japan §

PRESTO, Japan Science and Technology Agency (JST), Saitama 332-0012, Japan

#

National Physical Laboratory, Hampton Road, Teddington TW11 0LW, U.K.

||

Research and Education Center of Carbon Resources, Kyushu University, Kasuga, Fukuoka

816-8580, Japan

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ABSTRACT

The presence of grain boundaries in two-dimensional (2D) materials is known to greatly affect their physical, electrical, and chemical properties. Given the difficulty in growing perfect large single-crystals of 2D materials, revealing the presence and characteristics of grain boundaries becomes an important issue for practical applications. Here, we present a method to visualize the grain structure and boundaries of 2D materials by epitaxially growing transition metal dichalcogenides (TMDCs) over them.

Triangular single-crystals of molybdenum disulfide

(MoS2) epitaxially grown on the surface of graphene allowed us to determine the orientation and size of the graphene grains.

Grain boundaries in the polycrystalline graphene were also

visualized reflecting their higher chemical reactivity than the basal plane. The method was successfully applied to graphene field-effect transistors, revealing the actual grain structures of the graphene channels.

Moreover, we demonstrate that this method can be extended to

determine the grain structure of other 2D materials, such as tungsten disulfide (WS2). Our visualization method based on van der Waals epitaxy can offer a facile and large-scale labeling technique to investigate the grain structures of various 2D materials, and it will also contribute to understand the relationship between their grain structure and physical properties.

KEYWORDS: Graphene, transition metal dichalcogenide, epitaxy, grain boundaries, fieldeffect transistors


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Graphene, a single-atom thick sheet of carbon atoms arranged in a hexagonal lattice, has attracted a great interest because of its unique and excellent physical properties, such as extraordinary high carrier mobility, high Young’s modulus, mechanical flexibility, and optical transparency.1 These properties make graphene an attractive material for electronic devices, including transparent electrodes, field-effect transistors (FETs), and chemical/biochemical sensors which can be expected to develop into flexible/wearable devices and integrated circuits.2–5 For these applications, the synthesis of large-area and high-quality graphene is essential. Recently, chemical vapor deposition (CVD) using Cu foil has been widely used to grow large-area graphene, because a uniform film of single-layer graphene can be easily obtained due to the self-limiting growth on Cu.6–9 However, the single-layer graphene grown by CVD is usually polycrystalline, composed of randomly oriented grains with limited size.10–14 Grains in polycrystalline graphene are separated by grain boundaries (GBs), deteriorating the physical properties of graphene. This ultimately leads to a low carrier mobility, high sheet resistance, low mechanical strength, and low thermal conductivity.11,13,15–18 Therefore, knowing the grain structure of graphene is essential for the development of applications requiring high-quality graphene, and consequently a great effort has been devoted to determine it.11–14,19 Small isolated graphene grains can be obtained for short CVD growth time, and when hexagonal grains are obtained, knowing the orientation of the graphene grains becomes straightforward.15,20–22 However, when the shape of the graphene grains is irregular or the CVD growth time is extended to completely cover the Cu surface, limited methods are available for the investigation of the graphene’s grain structure. Dark-field (DF) imaging of transmission electron microscope (TEM) has been frequently used to determine the grain structure of graphene.11,12 However, this method requires the transfer of graphene to a


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TEM grid, which is a delicate process that can damage the sample before the analysis. This method is also spatially limited by the size of the TEM grid, which can impede the examination of samples with relatively large graphene grains that are being actually synthesized.22,23 It is also difficult to apply this technique to processed samples, such as graphene devices already fabricated on Si wafers.

Dark-field low-energy electron microscope (LEEM) is another

technique employed to observe the grain structure of graphene. However, the availability of LEEM equipment is limited and measurements require a metallic substrate to avoid charging effects.10,13,19,24 Lastly, selected liquid crystalline molecules adsorbed on graphene surface align in certain direction, enabling the observation of graphene’s grain structure by polarized optical microscopy (POM).14,25 This is a comparatively simple method as a confocal optical microscope is widely available, but it is employed only for relatively large graphene grains due to the spatial resolution constrains imposed by the optical microscope employed to analyze the macroscopic orientation of the aligned molecules. In this paper, we present a novel method to visualize the grain structure of CVD-grown polycrystalline graphene by the post-growth of transition metal dichalcogenides (TMDCs). We epitaxially grew triangular grains of molybdenum disulfide (MoS2) on the graphene using CVD technique.

Based on the epitaxial relationship between both structures, by observing the

distribution in the orientation of the MoS2 grains along the surface we could determine the grain structure of the underlying graphene lattice. Furthermore, the direct observation of the graphene GBs was also possible due to a higher nucleation density of MoS2 at the GBs compared to areas within the grains. This method was also employed in a practical case to reveal the grain structure of graphene FETs fabricated on SiO2/Si substrates. Finally, we demonstrated the possibility of extending this method to other layered two-dimensional (2D) materials for which epitaxial


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growth is realizable. For this, the grain structure of tungsten disulfide (WS2) was visualized from the orientation of MoS2 grains grown over it. As far as we are concerned, this is the first method that can determine the grain structure of a TMDC in a relatively large scale.

RESULTS AND DISCUSSION The relationship between the orientation of the CVD-grown MoS2 grains and the underlying graphene used as a template was first established. For this purpose, isolated hexagonal graphene grains were synthesized by ambient-pressure CVD and transferred onto a SiO2/Si substrate. Then, triangular grains of single-layer MoS2 were grown on the graphene by reacting MoO3 and S at 880-900 ºC in a pure Ar flow.26 Detailed experimental conditions of the CVD processes are described in the Experimental Section. Figure 1a shows a scanning electron microscope (SEM) image of a single hexagonal graphene grain collected after the MoS2 growth. Triangular features with lateral sizes of 0.5-1 µm, can be seen on the hexagonal graphene grain. The inset of Figure 1a is a conductive atomic force microscope (C-AFM) image of one of such triangles, which shows the different electronic properties of the triangular islands and the graphene. The triangles were identified as MoS2 by Raman spectroscopy (see Supporting Information, Figure S1) and by energy dispersive x-ray spectroscopy (EDX) mapping (Figure 1b inset). The separation between the Raman E12g and A1g modes (~20 cm-1) indicates that most of the MoS2 grains are singlelayer.27,28 This was also confirmed by AFM topography images (Figure S2a,b). Because we measured single-layer MoS2, the EDX intensity of the Mo peak was low, so that the background of EDX mapping image also shows light green contrast.

This is however related to the

resolution of the method and does not mean the formation of MoS2 particles around the MoS2


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grain. As we previously reported,26 the triangular grains of MoS2 are aligned in two preferential orientations, which are highlighted by the yellow triangles of Figure 1a. Interestingly, every side of the MoS2 grains is always aligned parallel to two of the sides of the underlying hexagonal graphene grain. This indicates that there is an epitaxial relationship between the MoS2 and graphene, reflecting van der Waals epitaxy. Figure 1b shows a TEM image of the MoS2/graphene heterostructure after being transferred to a TEM grid (Quantifoil). The selected area diffraction (Figure 1c) showed two sets of hexagonal diffraction patterns completely aligned, with the inner and outer diffraction spots corresponding to MoS2 and graphene, respectively. This indicates that the sides of the MoS2 triangular grains are parallel to zigzag directions of the graphene, which is consistent with recent works on direct growth of TMDCs on graphene.26,29 The van der Waals interaction allows the epitaxial growth of MoS2 on graphene in spite of the large lattice mismatch (a(MoS2) = 0.312 nm, a(graphene) = 0.246 nm). A schematic model of the heterostructure is depicted in Figure 1d, showing one of the two equivalent orientations of the MoS2 grain. This epitaxial relation between the MoS2 and graphene can be applied to visualize the grain structure of polycrystalline graphene, as illustrated in Figure 2a. During the graphene growth on a Cu foil, graphene grains with random orientations nucleate at different parts of the Cu surface and develop until they coalescence each other to fully cover the Cu surface. Given the random orientations of the graphene grains, the alignment of MoS2 synthesized on graphene will vary depending on the orientation of the underlying graphene grain. Thus, measuring the relative orientation of the MoS2 grains will allow us to determine the grain structure of the CVD-grown polycrystalline graphene. Figure 2b shows a SEM image of the polycrystalline graphene with MoS2 grains grown on top. The grain structure of the underlying CVD graphene is depicted in


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Figure 2c, where the orientations were determined from the orientations of the MoS2 grains in high resolution SEM images (Figure 2d). A wider area SEM image is shown in Figure S3. We note that as there is no preferred orientation for the growth of graphene on polycrystalline Cu, the indicated angles show the relative orientations of each grain with respect to an arbitrary direction (set as 0º). Figure S4 shows another example of the graphene grain structure visualized based on orientations of triangular MoS2 grains. From Figures 2c and S4, the grain size of CVD graphene was found to be 5-50 µm, which is consistent with our experiment of growing isolated graphene grains on Cu foils (see Figure S5a). We also confirm that the orientation and grain structure of graphene are not affected by the MoS2 growth process. Figure S5 shows the optical micrographs of graphene grains taken before and after the MoS2-CVD. It can be seen that the size, shape, and orientation of the graphene grains are generally unaltered. The only observed difference is the darker contrast after the CVD growth due to the presence of MoS2 grains. We would like to point out here that the high temperatures used for the growth of the MoS2 may damage the graphene, and removal of the MoS2 can be difficult. Thus, the present method is expected to be useful only for post-growth analysis. In addition to the oriented MoS2 grains found within graphene grains, we found that irregular shaped MoS2 grains are preferentially formed along the graphene GBs, as presented in Figure 3 (highlighted by red arrows). We observed that those MoS2 grains are relatively small and linearly arranged along most of the GBs between adjacent graphene grains. Shapes of the MoS2 grains formed along the GBs are not as perfect as those of their intra-grain counterparts, and usually consist of two or more coalescing smaller grains which originate from either side of the adjacent graphene grains (see Figures 3b and S6). In view of the SEM images, it is likely that MoS2 precursors diffusing from both sides of the GB are being trapped. The presence of GBs


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reduces the diffusion length of the MoS2 precursor, which accounts for the smaller MoS2 grains formed at the GBs. These results suggest that the GBs of the graphene are more reactive than the basal plane, as previously reported in the literature.30,31 Most likely, at the high temperatures employed during the CVD growth the MoS2 precursors are trapped at the GBs, which act as nucleation sites.

Therefore, the MoS2-labeling is useful not only to visualize the grain

orientation but also to directly visualize the GB distribution. Although oxidative treatments have been previously used to visualize GBs of graphene,31 our method is capable to determine both the grain orientation and the grain boundaries. Some linear arrays of small MoS2 grains can be also found within the graphene grains, indicated by the blue arrows in Figures 3 and S6. We speculate that these linear arrays originate in wrinkles formed in the graphene during the CVD growth and the transfer process. These wrinkles are expected to trap the intermediates diffusing across the surface more efficiently, thus facilitating the MoS2 nucleation. The density of MoS2 grains found along the graphene GBs varies depending on the presence of wrinkles around the GBs, and sometimes GBs free from MoS2 grains can be found. We speculate that when wrinkles exit the nearest graphene GB, MoS2 precursors tend to be trapped at the wrinkles due to their height (0.3-7 nm),16 preventing the formation of MoS2 grains at graphene GBs. There are also other possible nucleation sites of MoS2 in addition to GBs and wrinkles. Point defects as well as surface impurities such as polymethyl methacrylate (PMMA) residuals (PMMA is used at the graphene transfer) can also affect the MoS2 nucleation. In graphene heavily contaminated with residual PMMA, well-faceted MoS2 triangular grains were hardly observed, while disordered or round-shaped MoS2 were produced instead. This suggests that the PMMA residue also suppresses the effective surface diffusion of MoS2 precursors, preventing


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the growth of MoS2 grains with high crystallinity. Therefore, the density and shape of MoS2 on the basal plane of graphene (within the graphene grains) may become a good indicator of the graphene’s purity and crystallinity. As noted above, one of the advantages of our TMDCs-based visualization method is that it can be used to directly study graphene transistors without transferring them to TEM grids or other substrates.

Figure 4a shows an optical microscope image of a back-gated graphene FET

fabricated on a SiO2 substrate. The length and width of the graphene channels are 50 and 10 µm, respectively. To demonstrate the effectiveness of our visualization method, we synthesized two types of single-layer graphene sheets with different grain sizes. In addition to graphene sheets with a grain size of 5-50 µm (shown in Figures 2b-d and S4), we synthesized millimeter-sized single-crystalline graphene grains (Figure S7). Such large graphene grains were synthesized by oxidizing the Cu foil before introducing CH4 in CVD, as pre-oxidized Cu foil is known to strongly suppress the nucleation of graphene enabling the growth of millimeter-sized grains.22,23 In both cases, the same Cu foil purchased from Alfa Aesar was used, and more detailed growth procedures are described in Experimental Section. Both FET samples were electrically characterized (Figure 4e,f), and then MoS2 was grown on top of the graphene channel. Figure 4b-d shows SEM images of the graphene FETs obtained after the MoS2 growth. These images are labeled with the relative orientation of the MoS2 grains (see details in Figures S8 to S10), showing clear differences between the multi-grain (Figure 4b,c) and the single-grain (Figure 4d) channels. Multi-grain FET channels consist of many rotated graphene grains (Figure 4b,c), which is expected as the channel size is of the same order of magnitude of the graphene grains (see Figure 2c).

In the case of the millimeter-grain


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graphene, the orientation of the MoS2 crystals is maintained within the whole FET channel (Figure 4d). This clearly proves the efficiency of the TMDC-based visualization method in practical cases to discriminate the quality of the graphene. While in previous works the analysis of the grain structure required to transfer the FET channel to a TEM grid,32 this method avoids such step and allows a direct study of the grain structure.

The obvious advantages of the technique presented here are its simplicity and

robustness together with the significantly decreased failure rate, as the transfer is a delicate step that can damage the sample before its inspection.

Our results also show that the device

fabrication processes (i.e. photolithography, O2 plasma etching, metal evaporation and lift-off), did not significantly influence the growth of MoS2. As already noted in Figure S5, the CVD graphene was stable during the high temperatures involved in the MoS2 growth process, and acted as a good template to grow MoS2 triangular crystals. However, the high temperature damaged the Au electrodes, which changed their morphology and formed small Au particles during the MoS2 growth. The electronic properties of the graphene GBs were studied by collecting the transfer curves of multi- and single-grain FET channels for over 25 different devices on each sample.

The

representative examples of transfer curves for each sample are shown in Figure 4e,f. The fieldeffect hole mobility distributions of all the FETs are plotted in Figure 4g. Statistically, the FETs with a multi-grain channel showed lower hole mobility (average values of 2,410 cm2/Vs) than those of single-grain FET (4,200 cm2/Vs). A more pronounced decrease in the mobility was also seen for the electron conductance, as shown in Figure S11. These results strongly suggest that graphene GBs reduce the carrier mobility due to the carrier scattering.16 We also observed that the Dirac points of the multi-grain FETs are shifted to more positive voltage than those of single-


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grain FETs (41 V and 20 V respectively) (see Figure S12 for a detailed distribution plot). This is probably attributed to an increase of residual PMMA and other adsorbed impurities on the GBs, which lead to the observed pronounced p-type doping.33 Effects of rotation angles between neighboring grains as well as the total length of GBs (or the number of grains) in multi-grain channel are still open questions requiring further studies. Our TMDC-labeled visualization method will enable to analyze these issues, so that further study will give us deeper understanding of the transport phenomenon in graphene sheets. The TMDC-based visualization method presented here has a potential to become a versatile tool for observation of the grain structure and boundaries not only of graphene, but also in other atomically-thick 2D materials. For this, it is necessary to grow a second layered material over them by van der Waals epitaxy. To demonstrate this possibility, we grew MoS2 on merged WS2 grains, as presented in Figure 5. It was previously determined that under certain circumstances there is an epitaxial relation between WS2 and MoS2 layers, producing heterostructures with a 2H stacking order.34 In our case of MoS2/WS2 heterostructures, Raman bands of both MoS2 and WS2 can be found across the surface of the WS2 grains (Figure 5d). This is due to the relatively large laser spot used in our Raman setup as well as the high density of MoS2 grains. However, the SEM images showed a sufficient number of isolated MoS2 grains, allowing to determine their orientation (Figure 5c). The false colors of Figure 5b indicate the different WS2 grains, with their relative orientation obtained both from their sides and from the small triangular MoS2 grains grown on them. As can be seen, there is a good correlation between the orientation of the WS2 and MoS2 grains, indicating epitaxial growth. This result opens up the possibility of applying this method to determine the grain structure of a wide variety of 2D materials. As the transport and mechanical properties of such materials are strongly related to the grain structure, this can


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become an important way to assert the quality of 2D heterostructure for a wide variety of applications.

CONCLUSIONS We have introduced a novel method to determine the grain structure of polycrystalline 2D layered materials by the epitaxial growth of MoS2 single-crystals.

The grain structures of

polycrystalline graphene and WS2 were determined from the crystal orientation of the CVDgrown MoS2, by exploiting the epitaxial relation existing between two crystal structures. Apart from determining the grain orientation and size, it was possible to directly visualize the graphene GBs given their higher reactivity towards the nucleation of small MoS2 grains. As a practical example of this, we have shown the possibility to determine the grain structure of graphene-FET channels. Our method based on van der Waals heteroepitaxy offers a new route to visualize the grain orientations and boundaries of graphene and related 2D materials, which are important for future development of applications various 2D materials.


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EXPERIMENTAL SECTION 1. Graphene growth Polycrystalline, single-layer graphene was grown by ambient pressure CVD using Cu foil (Alfa Aesar (99.8% purity)) at 1075 ºC.26,35 After the annealing in H2 flow at high temperatures, CH4 (30 ppm) and H2 gases diluted with Ar were flowed into the CVD chamber. After a reaction time of 30 min, the substrate was quickly taken out from the heating zone. This CVD process produced polycrystalline graphene whose grain size is around 5-50 µm. To grow large single-crystalline graphene, the Cu foil was pre-baked in air at 250 ºC and heated up in the flow of pure Ar, before introducing CH4 at 1075 ºC.22,23 Graphene was then grown by introducing CH4 (20 ppm) and H2 gases at 1075 ºC for 60 min. Then, the CVD grown graphene was transferred onto a SiO2/Si substrate (SiO2 thickness: 300 nm) using PMMA and an etching solution (ammonium persulfate). 2. MoS2 (WS2) growth MoS2 was synthesized using MoO3 and sulfur powder as precursors. We used a three-zone furnace with an extra heating belt to precisely control the temperatures of MoO3 and S as well as that of the graphene substrate. The temperature of graphene substrates was 880-900 ºC. The growth time and the distance from the graphene substrate and feedstock were controlled so as to obtain isolated MoS2 grains for the visualization of graphene grain structure.

Detailed

experimental condition of MoS2 growth is described in ref. 26. The single-layer WS2 was synthesized by ambient-pressure CVD using WO3 and S feedstock.36 After the growth of WS2, the WS2 sheet was transferred onto a SiO2/Si substrate for the successive MoS2 growth process.


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3. Characterizations SEM (HITACHI S-4800) with an accelerated voltage of 1 kV was used to observe the orientation of MoS2 grains.

The grain orientation of the underlying graphene and WS2 was

colored based on the color table shown in Figure S13. In case of the uniform single-layer graphene sheets, we chose an arbitrary grain and assigned it to 0º orientation. Because we used polycrystalline graphene, only the relative angle of each graphene grain is important. For the graphene channels in FETs, we set 0º for the graphene whose zigzag edge is parallel to the edge of the electrodes. TEM was measured with JEOL 2100F with an accelerating voltage of 200 kV. Conductive AFM was performed by scanning a Pt/Ir coated conductive probe across the surface of the sample in contact mode using the Bruker Dimension Icon scanning probe microscope. The current maps were generated by voltage biasing the sample at 200 mV and recording the current flowing through the probe at each pixel of the scan area. The I-V curves on the MoS2 grain and the plain graphene surface were obtained by sweeping the sample bias voltage and recording the current flowing through the probe. Although the graphene grains on the SiO2 substrate are electrically isolated, a reliable electrical contact was achieved with silver paste. All conductive AFM measurements were performed at room temperature under ambient conditions. Raman spectra were measured with a Nanofinder30 (Tokyo Instruments) using a 532 nm excitation. FETs were made by photolithography, Au metal evaporation, and lift-off processes and measured by semiconductor parameter analyzer (B1500A, Keysight Technologies) at room temperature in vacuum (below 5 × 10−4 Pa). Before measuring the FET, the devices were annealed at 200 ºC for 16 hours in vacuum. The channel length and width of all the FETs were set 50 and 10 µm, respectively.


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ASSOCIATED CONTENT Supporting Information available: Raman data of MoS2 grown on graphene, conductive AFM images, other grain structure images, transport properties of graphene FETs, and color map. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by PRESTO-JST and KAKENHI (#15H03503, 15K13304).


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Figure 1. (a) SEM image of a MoS2/graphene heterostructure. The yellow triangles indicate the orientations of MoS2 grains. Inset shows a current distribution image of one of the MoS2 grains measured by a conductive AFM. (b) TEM image of MoS2 supported by single-layer graphene. Inset shows the EDX mapping image of the Mo L peak. (c) Selected-area diffraction from the MoS2 grain of (b). (d) Atomic image schematic of the epitaxial MoS2 grown on graphene. Blue, yellow, and gray represent Mo, S, and C atoms, respectively.


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Figure 2. (a) Schematic of MoS2-labeled visualization of grain structure of polycrystalline graphene. (b) SEM image of uniform graphene sheet with MoS2 grains. (c) Grain structure of the graphene determined from the orientation of MoS2 grains. The angles show the relative orientation with respect to the white-colored grain. (d) High-magnification SEM images of the areas marked in (c).


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Figure 3. (a) SEM image of merged graphene grains which are labeled by MoS2. Red arrows indicate the linear MoS2 arrays align along graphene’s GBs, while blue arrows suggest the MoS2 grown along wrinkles present in graphene. (b) Magnified image of the red square shown in (a). Irregular shaped MoS2 grains are observed along the graphene GBs.


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Figure 4. (a) Optical micrograph of a graphene FET. (b-d) Grain structures of the graphene channels are visualized by MoS2 growth and successive SEM measurement. (b,c) and (d) present multi-grain and single-grain channels, respectively. Transfer curves of multi-grain (e) and single-grain (f) channel FETs. (g) Histogram comparing the hole mobility distributions for the graphene FETs with multi- (blue) and single-grain (red) channels. For making this histogram, 49 multi-grain and 28 single-grain devices were compared.


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Figure 5. (a) Schematic of the epitaxial MoS2 growth over a large WS2 grain to visualize the grain orientation of the WS2. (b) Orientations of the large WS2 grains determined from the orientation of surface-grown small MoS2 grains. (c) Magnified image of squared region in (b) with orientations determined from MoS2 grains marked by red and blue. The large triangles seen at the center of WS2 are double-layer WS2 grains which were formed at the first WS2 growth process. (d) Raman spectra of the original WS2 (black spectrum) and MoS2/WS2 heterostructure (red).


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Table of Contents Graphic


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Visualization of Grain Structure and Boundaries of Polycrystalline

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