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hexagonal boron nitride heterostructure on SiO2 substrate without metal catalyst
Carbon 138 (2018) 76e80
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Carbon journal homepage: www.elsevier.com/locate/carbon
In situ direct growth of graphene/hexagonal boron nitride heterostructure on SiO2 substrate without metal catalyst Qinke Wu a, 1, Joohyun Lee a, 1, Jia Sun b, Young Jae Song a, c, d, e, * a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, Hunan, China c Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea d Department of Physics, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea e Department of Nano-Engineering, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2018 Received in revised form 23 May 2018 Accepted 30 May 2018 Available online 7 June 2018
Here, we report the in situ direct growth of a graphene/hexagonal boron nitride (hBN) heterostructure on a SiO2 substrate without a metal catalyst by chemical vapor deposition (CVD). The hBN could be grown easily on a SiO2 substrate, while graphene growth was difficult and time-consuming as graphite could be grown only partially on the dielectric substrate, even after 5 h. Graphene was grown directly on this hBN/ SiO2 substrate sequentially, which demonstrated easy and quick growth of a fully covered and highquality graphene multilayer film within 40 min. The effect of hydrogen on the direct growth of hBN on a SiO2 substrate was also studied, and it was found that when a higher flow rate of hydrogen was used, the domain size was larger and higher quality of hBN could be grown. The quality of the grown hBN and graphene/hBN samples were confirmed by UVevis, Raman, and atomic force microscopy (AFM). This new method can be used for graphene multilayer coating on dielectric substrates, on which it is difficult to grow graphene directly, for industrial or scientific applications. © 2018 Published by Elsevier Ltd.
Keywords: Direct growth Graphene Hexagonal boron nitride Free metal-catalytic CVD
Graphene, which has a two-dimensional structure, has many outstanding properties like high mobility, high strength and chemical inertness [1e5]. It has, therefore, many potential applications [3,5e8]. To date, many methods have been used to fabricate a graphene film like mechanical exfoliation by a tape, chemical exfoliation by sonication in solution, or chemical vapor deposition, etc. [1,4,9,10]. Of all the methods, chemical vapor deposition (CVD) is one with the maximum potential to obtain a large-area and highquality graphene with a variable and controllable number of layers [1,4,9,10]. For CVD graphene growth, there are many substrates that can be used to grow graphene. There are roughly two kinds of those substrates; they can be divided into two groups, metallic substrates with a catalytic effect and dielectric substrates without a catalytic effect. Metallic substrates with a catalytic effect (e.g., copper, nickel, platinum etc.) can be used to grow graphene rapidly; however, they need the post transfer technique which can introduce chemical
* Corresponding author. SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea. E-mail address: [email protected] (Y.J. Song). 1 QW and JL contributed to this work equally. https://doi.org/10.1016/j.carbon.2018.05.072 0008-6223/© 2018 Published by Elsevier Ltd.
residues and cause mechanical damage during the transfer process [11]. For the case of direct graphene growth on dielectric substrates (e.g., SiO2 and Si3N4), although finally we can obtain a fully covered graphene film without an additional transfer process, it would require a few to tens of hours to grow it, which leads to excessive power consumption [12,13]. So, it is important that we find a new way to grow graphene quickly on a dielectric substrate. Here, we present a new method to grow graphene directly on a dielectric substrate like SiO2 within a very short growth time (40 min) by introducing an interstitial hBN film grown by CVD, yielding a fully covered graphene film. First, we grow hBN on the SiO2 substrate. When the hBN covers the whole surface, its boundaries will be used as nucleation sites for the subsequent growth of graphene. The growth mechanism is similar to our previous work with the hydrogen etching effect: the graphene can easily form nucleation seeds on the hBN [10]. In this way, we could grow graphene very quickly and fully cover the dielectric substrates. To grow graphene and hBN on a SiO2 (285-nm thick) substrate, we cleaned the substrate surface as the supporting information describes. Then, the substrate was loaded into the growth chamber
Q. Wu et al. / Carbon 138 (2018) 76e80
for growth. Initially, Ar (30 sccm) and H2 (70 sccm) gases were introduced into the growth chamber to protect the substrate during the annealing process until the temperature reached 1000 C. We changed the gas flow to H2 (40 sccm) and CH4 (40 sccm) for graphene direct growth. But here, we found that the graphene was very difficult to grow on a SiO2 substrate for the lack of a catalytic effect (unlike the copper substrate case). Even after 5 h of growth, only a partial multilayer graphene on a SiO2 substrate was obtained, as supporting information 1 shows. As the 2D/G ratio of the Raman spectrum is < 1, we can see that the grown graphene is a very thick multilayer. In conclusion, the direct growth of graphene on a SiO2 substrate is difficult. Consequently, growing graphene faster on a SiO2 substrate is a critical issue. Here, we found that if we grow hBN on a SiO2 substrate first, and then grow graphene in situ on it sequentially, then the full cover of graphene can be very easily and quickly achieved. Fig. 1a is a schematic diagram of the chemical vapor deposition (CVD) technique used to grow graphene and hBN. For hBN growth, we used borazine as the growth source. N2 was used to carry the source into the growth chamber, as the schematic diagram shows. After the hBN growth, the borazine source was stopped and CH4 admitted to the growth chamber for sequential graphene growth. Fig. 1b is a schematic diagram of the in situ growth process of graphene/hBN on a SiO2 substrate. Unlike the graphene-only growth (as supporting information 1 shows) case, we can obtain the fully covered multilayer graphene very quickly (40 min) under the same graphene growth conditions. Fig. 1c is a schematic diagram of hBN growth on a SiO2 substrate by using a borazine source and Fig. 1d, the photographic image of the bare SiO2 substrate, hBN/SiO2 and Graphene/hBN/SiO2 samples after growth, for comparison. First, we studied hBN growth on the SiO2 substrate in detail with different growth times (as shown in Fig. 2a to e) and investigated the formation of nucleation sites. Fig. 2a shows the AFM image of a bare SiO2 substrate, which was cleaned before hBN growth. Fig. 2b shows the AFM image of the SiO2 substrate after 3 min of hBN growth; we can see some triangular hBN flakes on the surface. From the line profile shown in Fig. 2f, according to the white dashed line in Fig. 2b, we can see that those triangular flakes now have various thicknesses from a monolayer of ~0.35 nm (the red-dashed circle in
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(f)) to a multilayer of 3.2 nm. This means that the growth behavior of hBN on a SiO2 substrate is a kind of Volmer-Weber growth, unlike the case of hBN grown on copper [14]. One interesting feature is that all those flakes share the same orientations even at the beginning of the growth process, which was visualized and confirmed with a symmetrical pattern in the corresponding FFT image. Fig. 2c shows the AFM image of the SiO2 substrate surface after 5 min of hBN growth. Here, a larger area of the SiO2-substrate surface has been covered by triangular hBN flakes and there are many nucleation dots for additional growth of hBN. When the growth time reaches 20 min, the whole SiO2 surface would be covered by a hBN film. From the AFM image in Fig. 2e, we can see that all the hBN flakes have a very clear triangular shape and are aligned in parallel. The FFT image in the inset shows that those flakes are aligned, which is an indication that all those domains may have the same crystal orientation [14]. As supporting information 2 shows, with the growth time increasing, we can clearly see that the SiO2 surface color changes from light purple to deep blue, which means that the hBN grown on the SiO2 substrate is getting thicker. Fig. 2g shows the plot of the areal coverage of hBN depending on the growth time. We can see that initially the hBN grows slowly, but when the growth time is over 3 min, the SiO2 substrate surface will be very quickly covered by the hBN film. This is because with the growth time increasing, more and more hBN domains will be formed while the domains grow larger. Fig. 2h shows the OM image of the hBN grown on a SiO2 substrate after 20 min of growth, which shows very good uniformity. We also performed XPS on the hBN sample and supporting information 3b and 3c show the B 1s (at 190.7 eV) and N 1s (at 398.3 eV) lines of the XPS spectrum, which was measured on the sample in Fig. 2h. This XPS spectrum corresponds well with previous work [14]. After understanding the growth details of the hBN on a SiO2 substrate with growth-time dependence in Fig. 2, we studied the hydrogen flow effect on the hBN growth while keeping other growth parameters the same. Fig. 3a to d are the atomic force microscopy (AFM) images of hBN that was grown on a SiO2 substrate with hydrogen flows of 1 sccm, 3 sccm, 5 sccm and 10 sccm. The images in Fig. 3e to h are the corresponding FFT images. Initially, when the hydrogen flow is very low (1e3 sccm), the hBN film is
Fig. 1. (a) Is the schematic diagram of the chemical vapor deposition technique for graphene and hBN growth. (b) Is the schematic diagram to show the growth process of a graphene/hBN heterostructure sample on a SiO2 substrate. (c) Is the schematic diagram for the hBN growth with a borazine source. (d) Is a photograph of the SiO2 substrate before and after the sample growth. (A colour version of this figure can be viewed online.)
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Fig. 2. (a) (e) are the AFM images of the hBN directly grown on SiO2 substrates with the optimized hydrogen flow for different growth times of 0 min, 3 min, 5 min, 10 min and 20 min. Each inset image is the FFT data of each corresponding AFM image. (f) Is the line profile according to the dashed-white line in (b). (g) Is the plot of the areal coverage of hBN depending on the growth time. (h) Is the OM image of the fully covered hBN film on the SiO2 substrate after growth. All the scale bars are 2 mm for (a) (d) and is 6 mm for (h). (A colour version of this figure can be viewed online.)
Fig. 3. (a) (d) are the AFM images of hBN directly grown on SiO2 substrates with different hydrogen flows of 1 sccm, 3 sccm, 5 sccm and 10 sccm. (e) (h) are the corresponding FFT images of (a) (d) respectively. (i) Is the plot to show that the hBN domain size varies with hydrogen flow. (j) Is the Raman spectra of the hBN grown on SiO2 substrates with different hydrogen flows. (k) Is the UVevis spectra of the hBN film grown under different hydrogen flows on SiO2 substrates, after being transferred onto quartz substrates. (l) Is the plot of the band gap of hBN extracted from (k). All the scale bars in (a) (d) are 2 mm. (A colour version of this figure can be viewed online.)
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constructed from very small particles and the FFT image is random. However, when a higher flow rate of hydrogen (over 5 sccm) is used, the hBN domains have a very clear triangular shape with all the edges parallel. Fig. 3c and d also show that those flakes have a very clear shape with all the edges parallel and their corresponding FFT images in Fig. 3g and h shows very good symmetry. One interesting feature is that by increasing the hydrogen flow, the FFT image will show better symmetry with 60 , unlike the case in Fig. 3g, which is a little tortured. The plot in Fig. 3i shows that the domain size of the hBN grown on SiO2 is largely affected by the hydrogen flow used during the growth process. When the hydrogen flow is low (1 sccm), the domain size of the hBN is very small, which is less than a few nanometers. But with the increase in the hydrogen flow, the domain size increases to around 200 nm. However, later, even though we increased the hydrogen flow, we could not increase the domain size. This kind of phenomenon is similar to the case of hBN grown on a copper substrate [14], where the hydrogen has the etching effect and can help decrease the density of hBN domains and make them larger during subsequent growth. Fig. 3j shows the plot of the Raman spectra measured on the hBN grown with different hydrogen flows. Clearly, from the Raman spectra we can see that the hBN grown under the low hydrogen flow has a lower and broader hBN peak (~1350 nm), but when the hydrogen flow increases to 10 sccm, we have a higher and sharper hBN peak, which means the hBN has better quality. This shows that the hBN quality can be affected by the hydrogen flow and a higher flow rate of hydrogen can help grow high quality hBN on a SiO2 substrate. The reason may be that at a high flow rate of hydrogen, the domain size of hBN is growing and the density of boundaries is less (meaning fewer defects). As shown in Fig. 3k, we measured the UVevis spectra of the grown hBN sample, after it was transferred onto quartz substrates. Fig. 3l shows the corresponding band gaps calculated from Fig. 3k. From the band gap of the hBN, we can see that by increasing the hydrogen flow, we obtain a larger band gap in the hBN film, which means that a high flow rate of hydrogen will grow a better quality hBN film on a SiO2 substrate. But, using 30 sccm of hydrogen, the hBN cannot be grown on the SiO2 substrate; this is because the etching effect of hydrogen at this flow rate is too strong and the hBN domains cannot be formed on the SiO2 surface.
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After understanding the hBN growth on the SiO2 substrate and optimizing the hydrogen flow, we used the hBN/SiO2 sample (grown under the optimized conditions, 10 sccm of hydrogen) as the growth substrate for subsequent graphene growth. When the hBN growth is complete, we turn off the N2 carrier gas and then introduce CH4 for in situ graphene growth. Under the CH4 (40 sccm) and H2 (40 sccm) conditions, the graphene growth process lasts 40 min. Fig. 4a shows the OM image of the as-grown graphene/hBN sample on a SiO2 substrate, which has a very high uniformity. The inset image is the corresponding Raman spectrum, from the peak ratio of 2D/G < 1, we can see that the graphene on top is multilayer graphene and the high peak (~1336 cm 1) comes from the combination of the hBN peak and the graphene D peak. Fig. 4b shows the AFM image of the graphene/hBN sample surface, where we can see that even after the graphene growth, the sample surface has good symmetry (as the inset of the corresponding FFT image shows). Fig. 4c and d are the Raman mapping of G/D and 2D/G, which shows high uniformity and completeness of the fully covered graphene film. This helps confirm that the graphene fully covers the hBN surface. Compared to the case of graphene directly grown on a SiO2 substrate, which obtained only partial coverage in 5 h, we can see that the pre-grown hBN helps improve the graphene growth speed. The reason may be that the hBN can help improve the graphene growth speed because, in the bare SiO2 surface case, there is a lower density of nucleation sites for graphene growth, thus less of a catalytic effect (comparing with copper substrate). However, when hBN is pre-grown on the SiO2 substrate, the high density of boundaries of hBN can be used as nucleation sites for graphene growth. Further, when graphene is growing on the hBN film, the hydrogen will etch the underside of the hBN film producing more defects for subsequent graphene growth [10]. Thus, graphene can be very quickly grown with a full cover. Although, during the later graphene growth, the hydrogen will etch the hBN to a certain degree, but the growth time is short, the hBN quality will not be changed much. After obtaining the fully covered graphene/hBN heterostructure sample on a SiO2 substrate, we transferred it onto quartz substrates. Fig. 4e shows the UVevis spectrum, which shows both the graphene peak (~265 nm) and hBN peak (~200 nm). Again, we confirmed the successful growth of graphene and hBN on the SiO2
Fig. 4. (a) Is the OM image of the graphene/hBN sample on a SiO2 substrate after growth. (b) Is the AFM image of the sample in (a). The inset is the corresponding FFT image. (c) (d) are the Raman mapping of G/D and 2D/G of the sample in (a). (e) Is the UVevis spectra of the grown graphene/hBN sample, after being transferred onto a quartz substrate. (f) (h) are the XPS spectra of C 1s, N 1s and B 1s of the graphene/hBN sample on a SiO2 substrate. The scale bars are 6 mm for (a), 2 mm for (b) and 6 mm for (c) (d). (A colour version of this figure can be viewed online.)
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substrate. Fig. 4f to h presents the B 1s (190.9 eV), C 1s (284.8 eV) and N 1s (398.5 eV) XPS spectra, measured on the sample in Fig. 4a. All these data correspond well with previous work [2]. And this again proved that our sample has its graphene and hBN with high quality, even after the later graphene growth. In summary, we developed a new method to grow a graphene film directly on a dielectric substrate of SiO2, by introducing a hBN interface in situ grown by CVD. First, we optimized the hBN growth on a SiO2 substrate and found that the hydrogen flow has a significant influence on the hBN domain size and quality. Under the optimized hBN growth conditions, we grew hBN first, and then we grew graphene in situ on the same substrate, sequentially. Compared to graphene growth on a bare SiO2 substrate (which needs tens of hours), we could obtain full-cover graphene film very quickly (~40 min). The reason may be that hBN has a large density of boundaries suitable for graphene nucleation sites, and, during the graphene growth process, the hydrogen etches the underside of the hBN film creating more defects for subsequent graphene growth. And this method even has very high possibility for preparing stable n-doped graphene sheets directly on the dielectric substrates without transferring process [15e17]. With this technique, we offer a new method to easily and quickly grow graphene on dielectric substrates, without a catalytic effect, for industrial and scientific applications. Acknowledgements This research was supported by the Basic Science Research Program (Grant No. 2015R1A1A1A05027585, 2015M3A7B4050455), the SRC Center for Topological Matter (Grant No. 2011-0030046), the Pioneer Research Center Program (Grant No. NRF-2014M3C1A3053029) and Institute for Basic Science (Grant No. IBS-R011-D1) through the National Research Foundation of Korea funded by the Ministry of Science and ICT (MSIT), Korea. Appendix A. Supplementary data Supplementary data related to this article can be found at
https://doi.org/10.1016/j.carbon.2018.05.072.
References [1] Y. Zhu, et al., Graphene and graphene oxide: synthesis, properties, and applications, Adv. Mater. 22 (2010) 3906e3924. [2] Q. Wu, et al., In situ synthesis of a large area boron nitride/graphene monolayer/boron nitride film by chemical vapor deposition, Nanoscale 7 (2015) 7574e7579. [3] K.S. Kim, et al., Large-scale pattern growth of graphene films for stretchable transparent electrodes, Nature 457 (2009) 706e710. [4] S. Bae, et al., Roll-to-roll production of 30-inch graphene films for transparent electrodes, Nat. Nanotechnol. 5 (2010) 574e578. [5] A.A. Balandin, Thermal properties of graphene and nanostructured carbon materials, Nat. Mater. 10 (2011) 569e581. [6] C. Qian, L. Kong, J. Yang, Y. Gao, J. Sun, Multi-gate organic neuron transistors for spatiotemporal information processing, Appl. Phys. Lett. 110 (2017) 083302. [7] C. Zou, et al., Polymereelectrolyte-gated nanowire synaptic transistors for neuromorphic applications, Appl. Phys. A 123 (2017). [8] L. Kong, et al., Spatially-correlated neuron transistors with ion-gel gating for brain-inspired applications, Org. Electron. 44 (2017) 25e31. [9] G. Lu, et al., Synthesis of high-quality graphene and hexagonal boron nitride monolayer in-plane heterostructure on Cu-Ni alloy, Adv. Sci. 4 (2017) 1700076. [10] Q. Wu, et al., Controllable poly-crystalline bilayered and multilayered graphene film growth by reciprocal chemical vapor deposition, Nanoscale 7 (2015) 10357e10361. [11] J.W. Suk, et al., Transfer of CVD-grown monolayer graphene onto arbitrary substrates, ACS Nano 5 (2011) 6916e6924. [12] J. Chen, et al., Two-stage metal-catalyst-free growth of high-quality polycrystalline graphene films on silicon nitride substrates, Adv. Mater. 25 (2013) 992e997. [13] J. Chen, et al., Oxygen-aided synthesis of polycrystalline graphene on silicon dioxide substrates, J. Am. Chem. Soc. 133 (2011) 17548e17551. [14] Q. Wu, et al., Single crystalline film of hexagonal boron nitride atomic monolayer by controlling nucleation seeds and domains, Sci. Rep. 5 (2015) 16159. [15] W. Xu, et al., N-doped graphene field-effect transistors with enhanced electron mobility and air-stability, Small 10 (2014) 1999e2005. [16] Y.-F. Lu, et al., Nitrogen-doped graphene sheets grown by chemical vapor deposition: synthesis and influence of nitrogen impurities on Carrier transport, ACS Nano 7 (2013) 6522e6532. [17] J. Wu, et al., Nitrogen-doped graphene with pyridinic dominance as a highly active and stable electrocatalyst for oxygen reduction, ACS Appl. Mater. Interfaces 7 (2015) 14763e14769.
Contents lists available at ScienceDirect
Carbon journal homepage: www.elsevier.com/locate/carbon
In situ direct growth of graphene/hexagonal boron nitride heterostructure on SiO2 substrate without metal catalyst Qinke Wu a, 1, Joohyun Lee a, 1, Jia Sun b, Young Jae Song a, c, d, e, * a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, Hunan, China c Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea d Department of Physics, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea e Department of Nano-Engineering, Sungkyunkwan University (SKKU), Suwon, 440-746, Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 May 2018 Received in revised form 23 May 2018 Accepted 30 May 2018 Available online 7 June 2018
Here, we report the in situ direct growth of a graphene/hexagonal boron nitride (hBN) heterostructure on a SiO2 substrate without a metal catalyst by chemical vapor deposition (CVD). The hBN could be grown easily on a SiO2 substrate, while graphene growth was difficult and time-consuming as graphite could be grown only partially on the dielectric substrate, even after 5 h. Graphene was grown directly on this hBN/ SiO2 substrate sequentially, which demonstrated easy and quick growth of a fully covered and highquality graphene multilayer film within 40 min. The effect of hydrogen on the direct growth of hBN on a SiO2 substrate was also studied, and it was found that when a higher flow rate of hydrogen was used, the domain size was larger and higher quality of hBN could be grown. The quality of the grown hBN and graphene/hBN samples were confirmed by UVevis, Raman, and atomic force microscopy (AFM). This new method can be used for graphene multilayer coating on dielectric substrates, on which it is difficult to grow graphene directly, for industrial or scientific applications. © 2018 Published by Elsevier Ltd.
Keywords: Direct growth Graphene Hexagonal boron nitride Free metal-catalytic CVD
Graphene, which has a two-dimensional structure, has many outstanding properties like high mobility, high strength and chemical inertness [1e5]. It has, therefore, many potential applications [3,5e8]. To date, many methods have been used to fabricate a graphene film like mechanical exfoliation by a tape, chemical exfoliation by sonication in solution, or chemical vapor deposition, etc. [1,4,9,10]. Of all the methods, chemical vapor deposition (CVD) is one with the maximum potential to obtain a large-area and highquality graphene with a variable and controllable number of layers [1,4,9,10]. For CVD graphene growth, there are many substrates that can be used to grow graphene. There are roughly two kinds of those substrates; they can be divided into two groups, metallic substrates with a catalytic effect and dielectric substrates without a catalytic effect. Metallic substrates with a catalytic effect (e.g., copper, nickel, platinum etc.) can be used to grow graphene rapidly; however, they need the post transfer technique which can introduce chemical
* Corresponding author. SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon, 440-746, Korea. E-mail address: [email protected] (Y.J. Song). 1 QW and JL contributed to this work equally. https://doi.org/10.1016/j.carbon.2018.05.072 0008-6223/© 2018 Published by Elsevier Ltd.
residues and cause mechanical damage during the transfer process [11]. For the case of direct graphene growth on dielectric substrates (e.g., SiO2 and Si3N4), although finally we can obtain a fully covered graphene film without an additional transfer process, it would require a few to tens of hours to grow it, which leads to excessive power consumption [12,13]. So, it is important that we find a new way to grow graphene quickly on a dielectric substrate. Here, we present a new method to grow graphene directly on a dielectric substrate like SiO2 within a very short growth time (40 min) by introducing an interstitial hBN film grown by CVD, yielding a fully covered graphene film. First, we grow hBN on the SiO2 substrate. When the hBN covers the whole surface, its boundaries will be used as nucleation sites for the subsequent growth of graphene. The growth mechanism is similar to our previous work with the hydrogen etching effect: the graphene can easily form nucleation seeds on the hBN [10]. In this way, we could grow graphene very quickly and fully cover the dielectric substrates. To grow graphene and hBN on a SiO2 (285-nm thick) substrate, we cleaned the substrate surface as the supporting information describes. Then, the substrate was loaded into the growth chamber
Q. Wu et al. / Carbon 138 (2018) 76e80
for growth. Initially, Ar (30 sccm) and H2 (70 sccm) gases were introduced into the growth chamber to protect the substrate during the annealing process until the temperature reached 1000 C. We changed the gas flow to H2 (40 sccm) and CH4 (40 sccm) for graphene direct growth. But here, we found that the graphene was very difficult to grow on a SiO2 substrate for the lack of a catalytic effect (unlike the copper substrate case). Even after 5 h of growth, only a partial multilayer graphene on a SiO2 substrate was obtained, as supporting information 1 shows. As the 2D/G ratio of the Raman spectrum is < 1, we can see that the grown graphene is a very thick multilayer. In conclusion, the direct growth of graphene on a SiO2 substrate is difficult. Consequently, growing graphene faster on a SiO2 substrate is a critical issue. Here, we found that if we grow hBN on a SiO2 substrate first, and then grow graphene in situ on it sequentially, then the full cover of graphene can be very easily and quickly achieved. Fig. 1a is a schematic diagram of the chemical vapor deposition (CVD) technique used to grow graphene and hBN. For hBN growth, we used borazine as the growth source. N2 was used to carry the source into the growth chamber, as the schematic diagram shows. After the hBN growth, the borazine source was stopped and CH4 admitted to the growth chamber for sequential graphene growth. Fig. 1b is a schematic diagram of the in situ growth process of graphene/hBN on a SiO2 substrate. Unlike the graphene-only growth (as supporting information 1 shows) case, we can obtain the fully covered multilayer graphene very quickly (40 min) under the same graphene growth conditions. Fig. 1c is a schematic diagram of hBN growth on a SiO2 substrate by using a borazine source and Fig. 1d, the photographic image of the bare SiO2 substrate, hBN/SiO2 and Graphene/hBN/SiO2 samples after growth, for comparison. First, we studied hBN growth on the SiO2 substrate in detail with different growth times (as shown in Fig. 2a to e) and investigated the formation of nucleation sites. Fig. 2a shows the AFM image of a bare SiO2 substrate, which was cleaned before hBN growth. Fig. 2b shows the AFM image of the SiO2 substrate after 3 min of hBN growth; we can see some triangular hBN flakes on the surface. From the line profile shown in Fig. 2f, according to the white dashed line in Fig. 2b, we can see that those triangular flakes now have various thicknesses from a monolayer of ~0.35 nm (the red-dashed circle in
77
(f)) to a multilayer of 3.2 nm. This means that the growth behavior of hBN on a SiO2 substrate is a kind of Volmer-Weber growth, unlike the case of hBN grown on copper [14]. One interesting feature is that all those flakes share the same orientations even at the beginning of the growth process, which was visualized and confirmed with a symmetrical pattern in the corresponding FFT image. Fig. 2c shows the AFM image of the SiO2 substrate surface after 5 min of hBN growth. Here, a larger area of the SiO2-substrate surface has been covered by triangular hBN flakes and there are many nucleation dots for additional growth of hBN. When the growth time reaches 20 min, the whole SiO2 surface would be covered by a hBN film. From the AFM image in Fig. 2e, we can see that all the hBN flakes have a very clear triangular shape and are aligned in parallel. The FFT image in the inset shows that those flakes are aligned, which is an indication that all those domains may have the same crystal orientation [14]. As supporting information 2 shows, with the growth time increasing, we can clearly see that the SiO2 surface color changes from light purple to deep blue, which means that the hBN grown on the SiO2 substrate is getting thicker. Fig. 2g shows the plot of the areal coverage of hBN depending on the growth time. We can see that initially the hBN grows slowly, but when the growth time is over 3 min, the SiO2 substrate surface will be very quickly covered by the hBN film. This is because with the growth time increasing, more and more hBN domains will be formed while the domains grow larger. Fig. 2h shows the OM image of the hBN grown on a SiO2 substrate after 20 min of growth, which shows very good uniformity. We also performed XPS on the hBN sample and supporting information 3b and 3c show the B 1s (at 190.7 eV) and N 1s (at 398.3 eV) lines of the XPS spectrum, which was measured on the sample in Fig. 2h. This XPS spectrum corresponds well with previous work [14]. After understanding the growth details of the hBN on a SiO2 substrate with growth-time dependence in Fig. 2, we studied the hydrogen flow effect on the hBN growth while keeping other growth parameters the same. Fig. 3a to d are the atomic force microscopy (AFM) images of hBN that was grown on a SiO2 substrate with hydrogen flows of 1 sccm, 3 sccm, 5 sccm and 10 sccm. The images in Fig. 3e to h are the corresponding FFT images. Initially, when the hydrogen flow is very low (1e3 sccm), the hBN film is
Fig. 1. (a) Is the schematic diagram of the chemical vapor deposition technique for graphene and hBN growth. (b) Is the schematic diagram to show the growth process of a graphene/hBN heterostructure sample on a SiO2 substrate. (c) Is the schematic diagram for the hBN growth with a borazine source. (d) Is a photograph of the SiO2 substrate before and after the sample growth. (A colour version of this figure can be viewed online.)
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Fig. 2. (a) (e) are the AFM images of the hBN directly grown on SiO2 substrates with the optimized hydrogen flow for different growth times of 0 min, 3 min, 5 min, 10 min and 20 min. Each inset image is the FFT data of each corresponding AFM image. (f) Is the line profile according to the dashed-white line in (b). (g) Is the plot of the areal coverage of hBN depending on the growth time. (h) Is the OM image of the fully covered hBN film on the SiO2 substrate after growth. All the scale bars are 2 mm for (a) (d) and is 6 mm for (h). (A colour version of this figure can be viewed online.)
Fig. 3. (a) (d) are the AFM images of hBN directly grown on SiO2 substrates with different hydrogen flows of 1 sccm, 3 sccm, 5 sccm and 10 sccm. (e) (h) are the corresponding FFT images of (a) (d) respectively. (i) Is the plot to show that the hBN domain size varies with hydrogen flow. (j) Is the Raman spectra of the hBN grown on SiO2 substrates with different hydrogen flows. (k) Is the UVevis spectra of the hBN film grown under different hydrogen flows on SiO2 substrates, after being transferred onto quartz substrates. (l) Is the plot of the band gap of hBN extracted from (k). All the scale bars in (a) (d) are 2 mm. (A colour version of this figure can be viewed online.)
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constructed from very small particles and the FFT image is random. However, when a higher flow rate of hydrogen (over 5 sccm) is used, the hBN domains have a very clear triangular shape with all the edges parallel. Fig. 3c and d also show that those flakes have a very clear shape with all the edges parallel and their corresponding FFT images in Fig. 3g and h shows very good symmetry. One interesting feature is that by increasing the hydrogen flow, the FFT image will show better symmetry with 60 , unlike the case in Fig. 3g, which is a little tortured. The plot in Fig. 3i shows that the domain size of the hBN grown on SiO2 is largely affected by the hydrogen flow used during the growth process. When the hydrogen flow is low (1 sccm), the domain size of the hBN is very small, which is less than a few nanometers. But with the increase in the hydrogen flow, the domain size increases to around 200 nm. However, later, even though we increased the hydrogen flow, we could not increase the domain size. This kind of phenomenon is similar to the case of hBN grown on a copper substrate [14], where the hydrogen has the etching effect and can help decrease the density of hBN domains and make them larger during subsequent growth. Fig. 3j shows the plot of the Raman spectra measured on the hBN grown with different hydrogen flows. Clearly, from the Raman spectra we can see that the hBN grown under the low hydrogen flow has a lower and broader hBN peak (~1350 nm), but when the hydrogen flow increases to 10 sccm, we have a higher and sharper hBN peak, which means the hBN has better quality. This shows that the hBN quality can be affected by the hydrogen flow and a higher flow rate of hydrogen can help grow high quality hBN on a SiO2 substrate. The reason may be that at a high flow rate of hydrogen, the domain size of hBN is growing and the density of boundaries is less (meaning fewer defects). As shown in Fig. 3k, we measured the UVevis spectra of the grown hBN sample, after it was transferred onto quartz substrates. Fig. 3l shows the corresponding band gaps calculated from Fig. 3k. From the band gap of the hBN, we can see that by increasing the hydrogen flow, we obtain a larger band gap in the hBN film, which means that a high flow rate of hydrogen will grow a better quality hBN film on a SiO2 substrate. But, using 30 sccm of hydrogen, the hBN cannot be grown on the SiO2 substrate; this is because the etching effect of hydrogen at this flow rate is too strong and the hBN domains cannot be formed on the SiO2 surface.
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After understanding the hBN growth on the SiO2 substrate and optimizing the hydrogen flow, we used the hBN/SiO2 sample (grown under the optimized conditions, 10 sccm of hydrogen) as the growth substrate for subsequent graphene growth. When the hBN growth is complete, we turn off the N2 carrier gas and then introduce CH4 for in situ graphene growth. Under the CH4 (40 sccm) and H2 (40 sccm) conditions, the graphene growth process lasts 40 min. Fig. 4a shows the OM image of the as-grown graphene/hBN sample on a SiO2 substrate, which has a very high uniformity. The inset image is the corresponding Raman spectrum, from the peak ratio of 2D/G < 1, we can see that the graphene on top is multilayer graphene and the high peak (~1336 cm 1) comes from the combination of the hBN peak and the graphene D peak. Fig. 4b shows the AFM image of the graphene/hBN sample surface, where we can see that even after the graphene growth, the sample surface has good symmetry (as the inset of the corresponding FFT image shows). Fig. 4c and d are the Raman mapping of G/D and 2D/G, which shows high uniformity and completeness of the fully covered graphene film. This helps confirm that the graphene fully covers the hBN surface. Compared to the case of graphene directly grown on a SiO2 substrate, which obtained only partial coverage in 5 h, we can see that the pre-grown hBN helps improve the graphene growth speed. The reason may be that the hBN can help improve the graphene growth speed because, in the bare SiO2 surface case, there is a lower density of nucleation sites for graphene growth, thus less of a catalytic effect (comparing with copper substrate). However, when hBN is pre-grown on the SiO2 substrate, the high density of boundaries of hBN can be used as nucleation sites for graphene growth. Further, when graphene is growing on the hBN film, the hydrogen will etch the underside of the hBN film producing more defects for subsequent graphene growth [10]. Thus, graphene can be very quickly grown with a full cover. Although, during the later graphene growth, the hydrogen will etch the hBN to a certain degree, but the growth time is short, the hBN quality will not be changed much. After obtaining the fully covered graphene/hBN heterostructure sample on a SiO2 substrate, we transferred it onto quartz substrates. Fig. 4e shows the UVevis spectrum, which shows both the graphene peak (~265 nm) and hBN peak (~200 nm). Again, we confirmed the successful growth of graphene and hBN on the SiO2
Fig. 4. (a) Is the OM image of the graphene/hBN sample on a SiO2 substrate after growth. (b) Is the AFM image of the sample in (a). The inset is the corresponding FFT image. (c) (d) are the Raman mapping of G/D and 2D/G of the sample in (a). (e) Is the UVevis spectra of the grown graphene/hBN sample, after being transferred onto a quartz substrate. (f) (h) are the XPS spectra of C 1s, N 1s and B 1s of the graphene/hBN sample on a SiO2 substrate. The scale bars are 6 mm for (a), 2 mm for (b) and 6 mm for (c) (d). (A colour version of this figure can be viewed online.)
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substrate. Fig. 4f to h presents the B 1s (190.9 eV), C 1s (284.8 eV) and N 1s (398.5 eV) XPS spectra, measured on the sample in Fig. 4a. All these data correspond well with previous work [2]. And this again proved that our sample has its graphene and hBN with high quality, even after the later graphene growth. In summary, we developed a new method to grow a graphene film directly on a dielectric substrate of SiO2, by introducing a hBN interface in situ grown by CVD. First, we optimized the hBN growth on a SiO2 substrate and found that the hydrogen flow has a significant influence on the hBN domain size and quality. Under the optimized hBN growth conditions, we grew hBN first, and then we grew graphene in situ on the same substrate, sequentially. Compared to graphene growth on a bare SiO2 substrate (which needs tens of hours), we could obtain full-cover graphene film very quickly (~40 min). The reason may be that hBN has a large density of boundaries suitable for graphene nucleation sites, and, during the graphene growth process, the hydrogen etches the underside of the hBN film creating more defects for subsequent graphene growth. And this method even has very high possibility for preparing stable n-doped graphene sheets directly on the dielectric substrates without transferring process [15e17]. With this technique, we offer a new method to easily and quickly grow graphene on dielectric substrates, without a catalytic effect, for industrial and scientific applications. Acknowledgements This research was supported by the Basic Science Research Program (Grant No. 2015R1A1A1A05027585, 2015M3A7B4050455), the SRC Center for Topological Matter (Grant No. 2011-0030046), the Pioneer Research Center Program (Grant No. NRF-2014M3C1A3053029) and Institute for Basic Science (Grant No. IBS-R011-D1) through the National Research Foundation of Korea funded by the Ministry of Science and ICT (MSIT), Korea. Appendix A. Supplementary data Supplementary data related to this article can be found at
https://doi.org/10.1016/j.carbon.2018.05.072.
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