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Ni substrate for hot filament chemical vapor deposition growth of high quality graphene films
Diamond & Related Materials 72 (2017) 7–12
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Ingenious design of Cu/Ni substrate for hot filament chemical vapor deposition growth of high quality graphene films Cheng Chen, Zhiyong Zhang ⁎, Manzhang Xu, Junfeng Yan, Jiangni Yun, Wu Zhao School of Information Science and Technology, Northwest University, Xi'an 710069, PR China.
a r t i c l e
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Article history: Received 20 October 2016 Accepted 16 December 2016 Available online xxxx Keywords: Graphene Thin film Cu/Ni substrate HFCVD RF-magnetron sputtering
a b s t r a c t The Cu/Ni substrates were fabricated at low defect densities of the Ni substrates by RF magnetron sputtering and the composition of Cu atoms on Ni substrates were modulated by the sputtering time. The high-quality graphene films were fabricated on the Cu/Ni substrates by hot filament chemical vapor deposition system (HFCVD). The ID/ IG ratios were very low (bb0.1) for all samples, which reflected the high-quality graphene films were obtained. The composition of Cu atoms played an important role in the growth of graphene films. Based on the dissolution precipitation mechanism of Ni and the absorption and self-limiting mechanism of Cu on the Cu/Ni substrates, it has been found that the layers of graphene was restrained by the composition of Cu atoms. Different layers of high-quality graphene films were obtained. A new growth mechanism of fabrication method was proposed by this investigation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a two-dimensional material deriving from sp2 hybridized carbon atoms [1]. By employing vdW-corrected density functional theory (DFT) calculations, the structure, interaction energy, and electronic properties of graphene have been investigated [2]. Hence, it has been stimulating wide-ranging interests due to its excellent chemical and physical properties [3]. It has highly transparent (97.4%), with a relatively low resistance (125/sp) [4], and a high electron mobility with a surface area of 2630 m2/g [5]. These properties have being developed and proposed for high speed electronic devices, biological and chemical sensors, etc. [6]. Many approaches have been used to fabricate graphene including mechanical cleavage, chemical vapor deposition (CVD) of hydrocarbon gas and graphite oxide reduction, etc. [7]. The growth of thin solid films from gas phase precursors is a versatile technique that has been widely exploited to obtain materials with exceptional properties [8]. Therefore, the CVD technique has been proved that it is widely used to fabricate graphene. HFCVD technique can represent advantage over other techniques for industrial scaling at low cost [9]. This technique has been done on the fabrication of graphene using vertical flow of hydrocarbon gas [10]. Compared with the CVD method, the research on the fabrication of CVD graphene is based on the horizontal flow hydrocarbon gas [11]. It has been proved that, the graphene fabricated using thermal-CVD techniques contained a high defect density [12]. Their differences are not only in deposition way, but also in the substrate temperature (TS) and the vacuum degree. Graphene films ⁎ Corresponding author. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.diamond.2016.12.013 0925-9635/© 2016 Elsevier B.V. All rights reserved.
deposited at low TS have a lot of defects in comparison to those deposited at higher TS [13]. Hence, the TS in the CVD system will be reached to 1000 °C in order to resolve the carbon source and improve the quality of graphene films [14]. However, the TS was limited by the melting point of metal substrate in the CVD system. In HFCVD system, hydrocarbon gas was disassociated at a very high filament temperature (~2200 °C) rather than in vicinity of substrate, so, it is more likely to grow high-quality graphene than other CVD processes [15]. Therefore, HFCVD technique was chosen to grow high-quality graphene films. Cu, Ni, Ir, Ru all have been used as catalyst to fabricate graphene films, and large-scale synthesis methods such as CVD and epitaxial growth on these substrates have been developed [16–18]. In general, Cu and Ni are most adopted [19]. However, the graphene films on Ni are grown as a mixture of various layers, which can be ascribed to the imbalanced precipitation of carbon from Ni substrate especially at grain boundaries [20]. The Cu is a suitable growth substrate for uniform graphene, large area of monolayer graphene with b 5% of that having bilayer or trilayers has been produced by CVD process on Cu substrate [21]. However, the growth of the second layer would stop once the catalytic Cu surface is fully covered by one layer graphene, this mechanism was called self-limiting effect [22]. In order to resolve the problem of the precise control of the layers of films, some experimental studies have found that a ingenious designed binary alloy metal can effectively overcome the defects of pure metals and activate the self-limited growth of monolayer graphene, monolayer and bilayer graphene films have been prepared by changing only the atomic percentage of Ni and Cu in Cu–Ni alloy [23]. In consequence, substrate alloying provides a new way of CVD-produced high-quality graphene films and rationalizes the catalyst design during growth. In general, the Cu-Ni substrate was obtained by
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purchase or fabrication of state fusion method. Therefore, it is difficult to regulate and control the composition of the Cu or Ni expediently. By the background, we propose a quick, simple and economic method to design a binary alloy catalyst called Cu/Ni. In this work, we introduce the binary alloy catalyst by using RF magnetron sputtering. We changed the sputtering time to adjust the composition of Cu atoms on the surface of substrate, and modulate the layers of the graphene films. Based on the systematic experiment, a reasonable growth mechanism was proposed, and the different layers of high-quality graphene films were obtained.
substrates were polished for 10 min, and the optical photographs were shown in Fig. 1b–d. The Cu atoms were sputtered on Ni substrates by RF sputtering using Cu target (the distance between target and substrate was 8 cm). In the RF magnetron sputtering system, the working pressure was adjusted to 2.0 Pa, the RF power was maintained at 30 W and the argon gas maintained at 20 sccm. Finally, a series of the Cu/Ni substrates with different composition of Cu atoms were fabricated by different sputtering time.
2. Experimental methods
The graphene films were fabricated on Cu/Ni substrates in a selfmade HFCVD system (the base pressure of system was 3.0 × 10−4 Pa). The internal structure was shown in Fig. 1a. The tungsten filaments (99.9% purity, 0.8 mm diameter) were clamped above the substrate. Among them, the distance between the filament and the substrate was 7 mm, and the three roots of tungsten filaments were arranged side by side with 8 mm spacing. A thermocouple was placed under the substrate to monitor the substrate temperature and a tungsten lamp heater was fixed under the substrate table. Substrate and filament temperature were maintained at 800 °C and 1760 °C respectively. The C2H2/H2 (10sccm/75sccm) mixture gas was passed into the airway
2.1. Preparation of Cu/Ni substrates Firstly, the Ni substrates (20 ∗ 20 ∗ 0.5 mm, 99.99% purity) were disposed by the mechanical and electrochemical polishing process before the sputtering and deposition process. The process of electric mechanical polishing was polished by sand paper (1500#) for 12 h. In the electrochemical system, the electrolyte was composed of deionized water (50 ml), polyethylene glycol (50 ml), phosphoric acid (60 ml) and vitriol (40 ml), and the distance of cathode and anode was 4 cm. Then, the
2.2. Preparation of graphene films
Fig. 1. (a) The schematic of HFCVD equipment. The polishing results based on the metallographic microscope, (b) unpolished of Ni substrate, (c) polishing for 10 min and magnified 100 times, (d) magnified 200 times.
C. Chen et al. / Diamond & Related Materials 72 (2017) 7–12
and the deposition time was 10 s. In the cooling stage, the TS was reduced for 10 °C/min during 800 °C ~ 600 °C, and the cooling rate was set at TS 25 °C/min during 600 °C ~ 20 °C. 2.3. Characterization The quality of polishing process was characterized using metallographic microscope (Nikon Eclipse ME600). The material phase analyses of Cu/Ni substrate were characterized by X-ray diffractometer (XRD, Shimadzu 6100). The layers of graphene films were characterized by Raman spectroscopy (Renishaw inVia) with 514 nm laser excitation (~ 2 μm spot size) and atomic force microscope (Shimadzu, SPM9500 J3). The surface morphology of Cu/Ni substrates and graphene films were characterized by scanning electron microscope (SEM, Phenom ProX). 3. Result and discussion Fig. 2 shows the Raman spectra of graphene films, which fabricated on different Cu/Ni substrate, every sample was in the same condition of HFCVD process. The electronic structure of graphene is uniquely captured in its Raman spectrum, that clearly evolves with the number of layers [24]. In general, Raman spectra showed prominent characteristic peaks of graphene at about ~ 1350 cm− 1 (D), ~ 1580 cm− 1 (G) and ~ 2700 cm−1 (2D) [25], it just has a single 2D peak, and the G band is produced by vibrations of E2g and the D band is caused by the defect of films [26]. The ratio of the intensity of the G band and 2D band (IG/ I2D) has been used to characterize the number of graphene layers, while the ratio of the intensity of the D band and G band (ID/IG) has been used to characterize the degree of crystalline quality and disorder in the carbon network [27]. Besides, the change of graphene layers can be also proved by the full width at half maximum (FWHM) of 2D peaks [28]. As shown in the Fig.2, firstly, the D band cannot be observed in the figure, and the ID/IG ratios were very low (bb0.1) for all the films, it suggests that the crystalline quality should be better, and the highquality graphene films were obtained by this technique. Secondly, the value of IG/I2D for every spectral line was calculated, through a systematic comparison, it has been found that the layers of graphene was decreased with the increase of sputtering time, in other words, the composition of Cu atoms will restrain the layers of grapheme films. According to the report, monolayer, bilayer and trilayer graphene films have totally different properties [29], controlling the number of graphene layer is very important in graphene synthesis for potential
Fig. 2. 514-nm laser excitation Raman spectra of graphene deposited on (a) Ni substrate and (b–d) Cu/Ni substrate with different sputtering times. (b) 5 min, (c) 15 min, (d) 30 min and (e) 45 min.
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applications. In the same condition of HFCVD process, the graphene films were fabricated with about 20 layers on the surface of Ni substrate, and the result was even closed to graphite. However, the monolayer graphene film was found on the surface of Cu/Ni substrate, which was sputtered for 30 min. In order to analyze the inhibitory action of Cu atoms on the surface of Cu/Ni substrate for the graphene films, it is necessary to analyze the sample, which was sputtered for 30 min. The layers of graphene films were characterized by Raman spectrum and atomic force microscope (AFM). As shown in Fig.3a, through the analyses of the surface of the substrate, it can be found that the graphene films were fabricated within four layers. Meanwhile, we found a gradient and used the AFM to characterize the layers graphene films. It can be seen that the layers of graphene were changed from one to four, as shown in Fig.3b. By the way, the noise was disposed by system signal in Fig.3. However, when we want to extend the sputtering time to 45 min, a different result appeared. According to the mechanism analysis, firstly, on the surface of Cu/Ni substrate, the Cu atoms will restrain the catalyzing of Ni for the hydrocarbon, and in cooling stage, the carbon atoms will dissolve out to the surface of Cu/Ni substrate, meanwhile, the Cu atoms will absorb some part of C atoms and will form a stable metal carbide in the precipitation stage, this process will limit the precipitation of C atoms and restrain the layers of graphene. It can be prove that, in a certain range, the increase of the composition of the Cu atoms have obvious effect on the formation of graphene. But, if the sputtering time is too long, this effect will disappear, because the Cu atoms on the surface that may be covered on the surface of Ni completely, at this time, the dissolving precipitation mechanism would no longer work together with the mechanism of absorption and self-limit in growth stage of graphene films. In the other words, the effect of Ni is very weak at this time, the surface of Cu atoms cannot effectively catalytic the carbon source in just 10 s, a survey of the literature available shows that carbon atoms are difficult to dissolve into the Cu substrate by using the principle of absorption, which is difficult to grow graphene films in a very short period of time. Therefore, when the sputtering time was extended to 45 min, it was difficult to find an ideal result on this experimental sample. Because of the Cu films were too thick to cover on the surface of Ni substrate. Fig.4 shows the SEM images of graphene films grown on Cu/Ni substrates with different sputtering times. Based on the effects of the composition of Cu atoms on the graphene layers, a reasonable growth mechanism was proposed. Therefore, it is necessary to research the different sputtering conditions of Cu/Ni substrates. By the investigation of the growth mechanism of graphene on Ni, the solubility of carbon atoms is favourable in Ni, and it was able to dissolve into Ni substrate in deposition stage, and will dissolve out to the surface of substrate plentifully during cooling stage [30]. The growth mechanism of graphene on Cu was different from the mechanism on Ni, the solubility of carbon atoms in Cu is very small [31], it just has ability to absorb carbon atoms. Using transition metal catalysts (such as Cu and Ni) have been demonstrated to effectively produce large-area monolayer and multilayer graphene, it is difficult to control the number of graphene layers owing to some serious obstacles [32]. The binary alloy surface state of Cu/Ni was combined two kinds of metal catalytic mechanism, in the process of substrate heating, the surface of Ni and Cu will be formed in the surface state of binary alloy [33]. In the cooling stage, the surface of substrate of Cu atom will turn into stable metal carbide and restrain the excessive precipitation of carbon atoms [34]. The surface of Cu/Ni substrates were characterized by SEM, Fig.5a–d show the distribution situation of Cu atoms after RF sputtering stage. Fig.5e–h show that the surface of Cu/Ni after the process of heating and cooling stage. In order to do a systematic analysis for the coverage condition of Cu atoms, the SEM was used to observe the surface of Cu/Ni substrate, which was sputtered the Cu atoms by RF-magnetron sputtering system and the surface of Cu/Ni, which was put into the HFCVD system to do the process of heating and cooling. The process of heating and cooling was similar to the process of the fabrication of the grapheme films, the
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Fig. 3. (a) The Raman spectrum of the graphene films on the Cu/Ni substrate, which was sputtered for 30 min. (b) The atomic force microscope images of the layers of graphene film.
only difference was that, there have not any gases were put into the chamber throughout this process. It can be found by comparison of Fig. 5a–d and Fig.5e–h, the surface appearance of substrate was changed. Because the heating and cooling process could eliminate some defects, adjust the structure and refined the grain. Finally, it can be clearly to see the cover condition of Cu atoms. As shown in Fig.5h, the coverage ratio is very large, it is also explains why we have not to find a good result in the sample of 45 min. Through the comparison of Fig.5a–h, it can be proved that, the fold which in the Fig.4 is caused by the deposition of carbon atoms, through the surface of fold, on the one hand, the area of graphene can be characterized by SEM
through the fold, on the other hand, through the dense degree of the fold, it is clear to distinguish from every sputtering condition. Fig.6 shows the diffraction spectrum of x-ray of Cu/Ni substrate in the different sputtering time. According to the standard spectrum of Cu and Ni, the diffraction peak of Cu and Ni were very close at (111), (200), (220) crystal orientation. The position of diffraction peaks of Ni were at 44.5, 51.8, 76.3, and the position of diffraction peaks of Cu were at 43.3, 50.4, 74.1, respectively. By comparing with the (220) diffraction peak of Cu/Ni, the position of diffraction peaks were still close to Ni. However, we compared the peak at (220), the position of peak
Fig. 4. SEM images of graphene grown on Cu/Ni substrates with different sputtering times (a) 0 min (Ni), (b) 5 min, (c) 15 min, (d) 30 min.
C. Chen et al. / Diamond & Related Materials 72 (2017) 7–12
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shift to the left and close to the Cu with the increase of sputtering time. This suggests that, the Cu atoms affect the surface state of the substrate. It has been found that, the increase of the composition of Cu will influence the layers of graphene films within a certain range. The technology of binary alloy catalytic has been found that an alloy catalyst can dramatically increase the quality of graphene films even at low temperatures [35]. According to the report, a reasoningly designed binary alloy metal can effectively overcome the defects of pure metals and stimulate the self-limited symmetrical monolayer graphene films [36]. As mentioned earlier, some experimental studies have found that use the CuNi alloy substrate as catalyst and change the atomic percentage of Cu or Ni can control the number of the graphene layers. It is obviously that, changing the composition of Cu on the surface by RF magnetron sputtering is simpler and more economic than change the composition of Cu in the Cu-Ni alloy. Finally, high-quality graphene films have been obtained and we could control the layers of films.
4. Conclusion In summary, the Cu/Ni substrates were fabricated on the Ni substrates by RF magnetron sputtering. The composition of Cu atoms on Cu/Ni substrates were modulated by the sputtering time. The Cu/Ni substrates were used as a catalytic to fabricate the high-quality graphene films via a simple and efficient HFCVD method. The surface of Ni substrate could decompose the C2H2 effectively, and made the carbon atoms reformed to the graphene films by using the principle of binary catalyst. The Cu atoms on the surface of Ni substrate limited the ability of surface absorption and the surface blooming of carbon atoms. Through the investigation of Cu/Ni substrate, we found that the HFCVD process will change the surface state of Cu/Ni substrate. By the systematic experiments, we obtained many different layers of highquality graphene films by the catalytic of Cu/Ni substrate in this selfmade HFCVD system. A growth mechanism was proposed to explain the growth of graphene by using this technique, and this approach is expected to realize the controllable growth of graphene films.
Acknowledgements Fig. 5. The figures of a-d show the SEM image of Cu/Ni substrate with different sputtering times after RF-magnetron sputtering process. (a) 5 min, (b) 15 min, (c) 30 min, (d) 45 min. The figures of e–h show the SEM image of Cu/Ni substrate with different sputtering time after process of heating and cooling. (e) 5 min, (f) 15 min, (g) 30 min, (h) 45 min.
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 61306009 and 61405159), the Key Project of Natural Science Foundation of Shaanxi Province (Grants 2014JZ2-003), the Natural Science Foundation of Shaanxi Province (Grants 2014JM8339 and 2015JM6274), the Science and Technology Star Project of Shaanxi Province (Grants 2013KJXX24) and the NWU Graduate Innovation and Creativity Funds (YZZ15010). Reference
Fig. 6. XRD pattern of the Cu/Ni substrate with different sputtering time.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Ingenious design of Cu/Ni substrate for hot filament chemical vapor deposition growth of high quality graphene films Cheng Chen, Zhiyong Zhang ⁎, Manzhang Xu, Junfeng Yan, Jiangni Yun, Wu Zhao School of Information Science and Technology, Northwest University, Xi'an 710069, PR China.
a r t i c l e
i n f o
Article history: Received 20 October 2016 Accepted 16 December 2016 Available online xxxx Keywords: Graphene Thin film Cu/Ni substrate HFCVD RF-magnetron sputtering
a b s t r a c t The Cu/Ni substrates were fabricated at low defect densities of the Ni substrates by RF magnetron sputtering and the composition of Cu atoms on Ni substrates were modulated by the sputtering time. The high-quality graphene films were fabricated on the Cu/Ni substrates by hot filament chemical vapor deposition system (HFCVD). The ID/ IG ratios were very low (bb0.1) for all samples, which reflected the high-quality graphene films were obtained. The composition of Cu atoms played an important role in the growth of graphene films. Based on the dissolution precipitation mechanism of Ni and the absorption and self-limiting mechanism of Cu on the Cu/Ni substrates, it has been found that the layers of graphene was restrained by the composition of Cu atoms. Different layers of high-quality graphene films were obtained. A new growth mechanism of fabrication method was proposed by this investigation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Graphene, a two-dimensional material deriving from sp2 hybridized carbon atoms [1]. By employing vdW-corrected density functional theory (DFT) calculations, the structure, interaction energy, and electronic properties of graphene have been investigated [2]. Hence, it has been stimulating wide-ranging interests due to its excellent chemical and physical properties [3]. It has highly transparent (97.4%), with a relatively low resistance (125/sp) [4], and a high electron mobility with a surface area of 2630 m2/g [5]. These properties have being developed and proposed for high speed electronic devices, biological and chemical sensors, etc. [6]. Many approaches have been used to fabricate graphene including mechanical cleavage, chemical vapor deposition (CVD) of hydrocarbon gas and graphite oxide reduction, etc. [7]. The growth of thin solid films from gas phase precursors is a versatile technique that has been widely exploited to obtain materials with exceptional properties [8]. Therefore, the CVD technique has been proved that it is widely used to fabricate graphene. HFCVD technique can represent advantage over other techniques for industrial scaling at low cost [9]. This technique has been done on the fabrication of graphene using vertical flow of hydrocarbon gas [10]. Compared with the CVD method, the research on the fabrication of CVD graphene is based on the horizontal flow hydrocarbon gas [11]. It has been proved that, the graphene fabricated using thermal-CVD techniques contained a high defect density [12]. Their differences are not only in deposition way, but also in the substrate temperature (TS) and the vacuum degree. Graphene films ⁎ Corresponding author. E-mail address: [email protected] (Z. Zhang).
http://dx.doi.org/10.1016/j.diamond.2016.12.013 0925-9635/© 2016 Elsevier B.V. All rights reserved.
deposited at low TS have a lot of defects in comparison to those deposited at higher TS [13]. Hence, the TS in the CVD system will be reached to 1000 °C in order to resolve the carbon source and improve the quality of graphene films [14]. However, the TS was limited by the melting point of metal substrate in the CVD system. In HFCVD system, hydrocarbon gas was disassociated at a very high filament temperature (~2200 °C) rather than in vicinity of substrate, so, it is more likely to grow high-quality graphene than other CVD processes [15]. Therefore, HFCVD technique was chosen to grow high-quality graphene films. Cu, Ni, Ir, Ru all have been used as catalyst to fabricate graphene films, and large-scale synthesis methods such as CVD and epitaxial growth on these substrates have been developed [16–18]. In general, Cu and Ni are most adopted [19]. However, the graphene films on Ni are grown as a mixture of various layers, which can be ascribed to the imbalanced precipitation of carbon from Ni substrate especially at grain boundaries [20]. The Cu is a suitable growth substrate for uniform graphene, large area of monolayer graphene with b 5% of that having bilayer or trilayers has been produced by CVD process on Cu substrate [21]. However, the growth of the second layer would stop once the catalytic Cu surface is fully covered by one layer graphene, this mechanism was called self-limiting effect [22]. In order to resolve the problem of the precise control of the layers of films, some experimental studies have found that a ingenious designed binary alloy metal can effectively overcome the defects of pure metals and activate the self-limited growth of monolayer graphene, monolayer and bilayer graphene films have been prepared by changing only the atomic percentage of Ni and Cu in Cu–Ni alloy [23]. In consequence, substrate alloying provides a new way of CVD-produced high-quality graphene films and rationalizes the catalyst design during growth. In general, the Cu-Ni substrate was obtained by
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purchase or fabrication of state fusion method. Therefore, it is difficult to regulate and control the composition of the Cu or Ni expediently. By the background, we propose a quick, simple and economic method to design a binary alloy catalyst called Cu/Ni. In this work, we introduce the binary alloy catalyst by using RF magnetron sputtering. We changed the sputtering time to adjust the composition of Cu atoms on the surface of substrate, and modulate the layers of the graphene films. Based on the systematic experiment, a reasonable growth mechanism was proposed, and the different layers of high-quality graphene films were obtained.
substrates were polished for 10 min, and the optical photographs were shown in Fig. 1b–d. The Cu atoms were sputtered on Ni substrates by RF sputtering using Cu target (the distance between target and substrate was 8 cm). In the RF magnetron sputtering system, the working pressure was adjusted to 2.0 Pa, the RF power was maintained at 30 W and the argon gas maintained at 20 sccm. Finally, a series of the Cu/Ni substrates with different composition of Cu atoms were fabricated by different sputtering time.
2. Experimental methods
The graphene films were fabricated on Cu/Ni substrates in a selfmade HFCVD system (the base pressure of system was 3.0 × 10−4 Pa). The internal structure was shown in Fig. 1a. The tungsten filaments (99.9% purity, 0.8 mm diameter) were clamped above the substrate. Among them, the distance between the filament and the substrate was 7 mm, and the three roots of tungsten filaments were arranged side by side with 8 mm spacing. A thermocouple was placed under the substrate to monitor the substrate temperature and a tungsten lamp heater was fixed under the substrate table. Substrate and filament temperature were maintained at 800 °C and 1760 °C respectively. The C2H2/H2 (10sccm/75sccm) mixture gas was passed into the airway
2.1. Preparation of Cu/Ni substrates Firstly, the Ni substrates (20 ∗ 20 ∗ 0.5 mm, 99.99% purity) were disposed by the mechanical and electrochemical polishing process before the sputtering and deposition process. The process of electric mechanical polishing was polished by sand paper (1500#) for 12 h. In the electrochemical system, the electrolyte was composed of deionized water (50 ml), polyethylene glycol (50 ml), phosphoric acid (60 ml) and vitriol (40 ml), and the distance of cathode and anode was 4 cm. Then, the
2.2. Preparation of graphene films
Fig. 1. (a) The schematic of HFCVD equipment. The polishing results based on the metallographic microscope, (b) unpolished of Ni substrate, (c) polishing for 10 min and magnified 100 times, (d) magnified 200 times.
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and the deposition time was 10 s. In the cooling stage, the TS was reduced for 10 °C/min during 800 °C ~ 600 °C, and the cooling rate was set at TS 25 °C/min during 600 °C ~ 20 °C. 2.3. Characterization The quality of polishing process was characterized using metallographic microscope (Nikon Eclipse ME600). The material phase analyses of Cu/Ni substrate were characterized by X-ray diffractometer (XRD, Shimadzu 6100). The layers of graphene films were characterized by Raman spectroscopy (Renishaw inVia) with 514 nm laser excitation (~ 2 μm spot size) and atomic force microscope (Shimadzu, SPM9500 J3). The surface morphology of Cu/Ni substrates and graphene films were characterized by scanning electron microscope (SEM, Phenom ProX). 3. Result and discussion Fig. 2 shows the Raman spectra of graphene films, which fabricated on different Cu/Ni substrate, every sample was in the same condition of HFCVD process. The electronic structure of graphene is uniquely captured in its Raman spectrum, that clearly evolves with the number of layers [24]. In general, Raman spectra showed prominent characteristic peaks of graphene at about ~ 1350 cm− 1 (D), ~ 1580 cm− 1 (G) and ~ 2700 cm−1 (2D) [25], it just has a single 2D peak, and the G band is produced by vibrations of E2g and the D band is caused by the defect of films [26]. The ratio of the intensity of the G band and 2D band (IG/ I2D) has been used to characterize the number of graphene layers, while the ratio of the intensity of the D band and G band (ID/IG) has been used to characterize the degree of crystalline quality and disorder in the carbon network [27]. Besides, the change of graphene layers can be also proved by the full width at half maximum (FWHM) of 2D peaks [28]. As shown in the Fig.2, firstly, the D band cannot be observed in the figure, and the ID/IG ratios were very low (bb0.1) for all the films, it suggests that the crystalline quality should be better, and the highquality graphene films were obtained by this technique. Secondly, the value of IG/I2D for every spectral line was calculated, through a systematic comparison, it has been found that the layers of graphene was decreased with the increase of sputtering time, in other words, the composition of Cu atoms will restrain the layers of grapheme films. According to the report, monolayer, bilayer and trilayer graphene films have totally different properties [29], controlling the number of graphene layer is very important in graphene synthesis for potential
Fig. 2. 514-nm laser excitation Raman spectra of graphene deposited on (a) Ni substrate and (b–d) Cu/Ni substrate with different sputtering times. (b) 5 min, (c) 15 min, (d) 30 min and (e) 45 min.
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applications. In the same condition of HFCVD process, the graphene films were fabricated with about 20 layers on the surface of Ni substrate, and the result was even closed to graphite. However, the monolayer graphene film was found on the surface of Cu/Ni substrate, which was sputtered for 30 min. In order to analyze the inhibitory action of Cu atoms on the surface of Cu/Ni substrate for the graphene films, it is necessary to analyze the sample, which was sputtered for 30 min. The layers of graphene films were characterized by Raman spectrum and atomic force microscope (AFM). As shown in Fig.3a, through the analyses of the surface of the substrate, it can be found that the graphene films were fabricated within four layers. Meanwhile, we found a gradient and used the AFM to characterize the layers graphene films. It can be seen that the layers of graphene were changed from one to four, as shown in Fig.3b. By the way, the noise was disposed by system signal in Fig.3. However, when we want to extend the sputtering time to 45 min, a different result appeared. According to the mechanism analysis, firstly, on the surface of Cu/Ni substrate, the Cu atoms will restrain the catalyzing of Ni for the hydrocarbon, and in cooling stage, the carbon atoms will dissolve out to the surface of Cu/Ni substrate, meanwhile, the Cu atoms will absorb some part of C atoms and will form a stable metal carbide in the precipitation stage, this process will limit the precipitation of C atoms and restrain the layers of graphene. It can be prove that, in a certain range, the increase of the composition of the Cu atoms have obvious effect on the formation of graphene. But, if the sputtering time is too long, this effect will disappear, because the Cu atoms on the surface that may be covered on the surface of Ni completely, at this time, the dissolving precipitation mechanism would no longer work together with the mechanism of absorption and self-limit in growth stage of graphene films. In the other words, the effect of Ni is very weak at this time, the surface of Cu atoms cannot effectively catalytic the carbon source in just 10 s, a survey of the literature available shows that carbon atoms are difficult to dissolve into the Cu substrate by using the principle of absorption, which is difficult to grow graphene films in a very short period of time. Therefore, when the sputtering time was extended to 45 min, it was difficult to find an ideal result on this experimental sample. Because of the Cu films were too thick to cover on the surface of Ni substrate. Fig.4 shows the SEM images of graphene films grown on Cu/Ni substrates with different sputtering times. Based on the effects of the composition of Cu atoms on the graphene layers, a reasonable growth mechanism was proposed. Therefore, it is necessary to research the different sputtering conditions of Cu/Ni substrates. By the investigation of the growth mechanism of graphene on Ni, the solubility of carbon atoms is favourable in Ni, and it was able to dissolve into Ni substrate in deposition stage, and will dissolve out to the surface of substrate plentifully during cooling stage [30]. The growth mechanism of graphene on Cu was different from the mechanism on Ni, the solubility of carbon atoms in Cu is very small [31], it just has ability to absorb carbon atoms. Using transition metal catalysts (such as Cu and Ni) have been demonstrated to effectively produce large-area monolayer and multilayer graphene, it is difficult to control the number of graphene layers owing to some serious obstacles [32]. The binary alloy surface state of Cu/Ni was combined two kinds of metal catalytic mechanism, in the process of substrate heating, the surface of Ni and Cu will be formed in the surface state of binary alloy [33]. In the cooling stage, the surface of substrate of Cu atom will turn into stable metal carbide and restrain the excessive precipitation of carbon atoms [34]. The surface of Cu/Ni substrates were characterized by SEM, Fig.5a–d show the distribution situation of Cu atoms after RF sputtering stage. Fig.5e–h show that the surface of Cu/Ni after the process of heating and cooling stage. In order to do a systematic analysis for the coverage condition of Cu atoms, the SEM was used to observe the surface of Cu/Ni substrate, which was sputtered the Cu atoms by RF-magnetron sputtering system and the surface of Cu/Ni, which was put into the HFCVD system to do the process of heating and cooling. The process of heating and cooling was similar to the process of the fabrication of the grapheme films, the
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Fig. 3. (a) The Raman spectrum of the graphene films on the Cu/Ni substrate, which was sputtered for 30 min. (b) The atomic force microscope images of the layers of graphene film.
only difference was that, there have not any gases were put into the chamber throughout this process. It can be found by comparison of Fig. 5a–d and Fig.5e–h, the surface appearance of substrate was changed. Because the heating and cooling process could eliminate some defects, adjust the structure and refined the grain. Finally, it can be clearly to see the cover condition of Cu atoms. As shown in Fig.5h, the coverage ratio is very large, it is also explains why we have not to find a good result in the sample of 45 min. Through the comparison of Fig.5a–h, it can be proved that, the fold which in the Fig.4 is caused by the deposition of carbon atoms, through the surface of fold, on the one hand, the area of graphene can be characterized by SEM
through the fold, on the other hand, through the dense degree of the fold, it is clear to distinguish from every sputtering condition. Fig.6 shows the diffraction spectrum of x-ray of Cu/Ni substrate in the different sputtering time. According to the standard spectrum of Cu and Ni, the diffraction peak of Cu and Ni were very close at (111), (200), (220) crystal orientation. The position of diffraction peaks of Ni were at 44.5, 51.8, 76.3, and the position of diffraction peaks of Cu were at 43.3, 50.4, 74.1, respectively. By comparing with the (220) diffraction peak of Cu/Ni, the position of diffraction peaks were still close to Ni. However, we compared the peak at (220), the position of peak
Fig. 4. SEM images of graphene grown on Cu/Ni substrates with different sputtering times (a) 0 min (Ni), (b) 5 min, (c) 15 min, (d) 30 min.
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shift to the left and close to the Cu with the increase of sputtering time. This suggests that, the Cu atoms affect the surface state of the substrate. It has been found that, the increase of the composition of Cu will influence the layers of graphene films within a certain range. The technology of binary alloy catalytic has been found that an alloy catalyst can dramatically increase the quality of graphene films even at low temperatures [35]. According to the report, a reasoningly designed binary alloy metal can effectively overcome the defects of pure metals and stimulate the self-limited symmetrical monolayer graphene films [36]. As mentioned earlier, some experimental studies have found that use the CuNi alloy substrate as catalyst and change the atomic percentage of Cu or Ni can control the number of the graphene layers. It is obviously that, changing the composition of Cu on the surface by RF magnetron sputtering is simpler and more economic than change the composition of Cu in the Cu-Ni alloy. Finally, high-quality graphene films have been obtained and we could control the layers of films.
4. Conclusion In summary, the Cu/Ni substrates were fabricated on the Ni substrates by RF magnetron sputtering. The composition of Cu atoms on Cu/Ni substrates were modulated by the sputtering time. The Cu/Ni substrates were used as a catalytic to fabricate the high-quality graphene films via a simple and efficient HFCVD method. The surface of Ni substrate could decompose the C2H2 effectively, and made the carbon atoms reformed to the graphene films by using the principle of binary catalyst. The Cu atoms on the surface of Ni substrate limited the ability of surface absorption and the surface blooming of carbon atoms. Through the investigation of Cu/Ni substrate, we found that the HFCVD process will change the surface state of Cu/Ni substrate. By the systematic experiments, we obtained many different layers of highquality graphene films by the catalytic of Cu/Ni substrate in this selfmade HFCVD system. A growth mechanism was proposed to explain the growth of graphene by using this technique, and this approach is expected to realize the controllable growth of graphene films.
Acknowledgements Fig. 5. The figures of a-d show the SEM image of Cu/Ni substrate with different sputtering times after RF-magnetron sputtering process. (a) 5 min, (b) 15 min, (c) 30 min, (d) 45 min. The figures of e–h show the SEM image of Cu/Ni substrate with different sputtering time after process of heating and cooling. (e) 5 min, (f) 15 min, (g) 30 min, (h) 45 min.
The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants 61306009 and 61405159), the Key Project of Natural Science Foundation of Shaanxi Province (Grants 2014JZ2-003), the Natural Science Foundation of Shaanxi Province (Grants 2014JM8339 and 2015JM6274), the Science and Technology Star Project of Shaanxi Province (Grants 2013KJXX24) and the NWU Graduate Innovation and Creativity Funds (YZZ15010). Reference
Fig. 6. XRD pattern of the Cu/Ni substrate with different sputtering time.
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