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A systematic study of the growth and characterization of few-layer graphene on Ni foils
Materials Today: Proceedings xxx (xxxx) xxx
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A systematic study of the growth and characterization of few-layer graphene on Ni foils Vishakha Kaushik a,⇑, Sachin Pathak b, H. Sharma c a
Department of Physics, DIT University Dehradun, Uttarakhand 248009, India Department of Physics, School of Engineering, University of Petroleum and Energy Studies, Dehradun 248007, Uttarakhand, India c Department of Physics, Doon University, Dehradun, Uttarakhand 248009, India b
a r t i c l e
i n f o
Article history: Received 17 January 2020 Accepted 23 January 2020 Available online xxxx Keywords: Graphene Chemical vapor deposition Raman spectroscopy SEM EDAX
a b s t r a c t Graphene, a two-dimensional material, has gained ample interest in today’s society owing to its remarkable properties such as high conductivity, thermal stability, mechanical strength. Up to date, various graphene synthesis methods are available and out of them, atmospheric pressure chemical vapor deposition (APCVD) is one of the best synthesis due to a very low diffusivity coefficient and a critical step for graphene-based device fabrication. In this work, we have optimized the growth conditions for the synthesis of graphene and its layers on Ni foil using APCVD. It is observed that the temperature i.e., heating and cooling rate, plays an essential role in the growth process. It is also found that annealing of substrate also plays a vital role in the growth of graphene and its layers and the annealing time has been varied from 30 to 120 min the substrate. From XRD and surface roughness analysis, the optimized time for annealing is optimized at 60 min. This led to continuous graphene layers comparable to those achieved with hightemperature CVD with full surface coverage and excellent quality. Our APCVD method is expected to allow the direct growth of graphene in device manufacturing processes for practical applications while keeping underlying devices intact. From TEM analysis, the layers can be seen in the size range from 50 to 100 nm. The presented method in this work provides an integrated device technology for wafer scale deposition with a controllable number of graphene layers. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
1. Introduction Two-dimensional materials such as graphene and MoS2 have received tremendous attention and high expectations due to their extraordinary electronic, mechanical, and chemical properties[1– 4].Several research groups showed theoretical and experimental investigations on the performance of graphene in electronics and demonstrated that they respond well to expectations [5,6]. Mechanical exfoliation was the first isolation of graphene from highly oriented pyrolytic graphite (HOPG) in 2004, but still the uniform large area production has been an exigent subject for realizing practical graphene-based applications [7]. There are various well-developed methods continuously used for the production of graphene in several forms as required or demanded. Each method has its own limitations; for example, in mechanical exfoliation, we cannot achieve large area coverage, ⇑ Corresponding author.
while in case of low-pressure chemical vapor deposition large area coverage is possible but only monolayers can be produced [8–10]. Wet chemical ways also present promising synthesis tools, but again, the quality of as produced graphene is an issue to be resolved [11]. The atmospheric pressure chemical vapor deposition (APCVD) method has been considered the most suitable method for the production of uniform graphene layers on a large scale. In this method, reactive carbon species are being produced, e.g., at the metal surface by decomposing hydrocarbon gas, a concentration gradient results, causing carbon atoms to diffuse into the metal, normal to the surface. The solubility of carbon in a metal, if nonzero, increases with temperature as pure nickel dissolves 1.3 atom % of carbon at 1000 °C. Some of the carbon atoms dissolved in metal at a high temperature can precipitate as a graphite film upon cooling, for relatively thin pieces of metal. While, in the case of using Cu foil, due to low carbon solubility in Cu, large-area monolayer graphene can be synthesized on Cu by self-limiting surface deposition.
E-mail address: [email protected] (V. Kaushik). https://doi.org/10.1016/j.matpr.2020.01.468 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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On the other hand, thin Ni films and fast-cooling processes have been used to suppress the amount of precipitated carbon. However, these methods yield non-uniform graphene films in a wide thickness range from 1 to 12 graphene layers, with monolayer regions up to 20 lm in lateral size, which was affected by the relatively small grain size of Ni film. Especially, Zhang et al. observed that monolayer graphene grows inside the dimension boundaries of the metal grains while multilayer graphene preferentially forms at the metal grain boundaries [10]. APCVD growth of graphene layers is desirable in order to reduce the manufacturing cost and increase throughput. Moreover, severe evaporation of metal catalysts in low-pressure conditions, which may lead to deterioration of the graphene quality, can be significantly suppressed by increasing the working pressure. However, oxygen and water molecules in ambient air should be avoided to obtain high-quality graphene layers. In this study, we have used an APCVD system equipped with a halogen lamp to heat up or anneal the Ni foils to synthesize graphene layers. It is an incandescent lamp with tungsten filament sealed into a compact transparent tube and based on the energy transfer between a radiant heat source and an object with short processing times, i.e., in minutes or seconds. The obvious benefit of this process is the fast cycle times for heating substrates to their required deposition temperature. Here we report, the growth of uniform graphene layers in the absence of residual oxidizing species that may influence the formation of amorphous and oxidized carbon layers. We can quickly transfer the graphene layers to arbitrary substrates by etching the Ni foil, without damaging the graphene. Our employed growth strategy is the ultralow-loss of the substrate and minimal to no damage to the fabricated graphene. We believe that our research will open new pathways for large areas and high-quality graphene growth and as such, promote its practical application. Using this method, large-area continuous graphene layers with excellent quality were synthesized on Ni foils at 900 °C.
2. Materials and methods In this work, we have used APCVD equipment to determine suitable experimental parameters for the uniform growth of large-area graphene layers high quality and the system set up is shown in Fig. 1. The graphene synthesis was done on Ni foil (Size 100 100 mm). The thickness of the Ni foil was 0.25 mm, with 99.9%. The Ni foil was used as received and done many cleaning processes also to see the effect on the growth of graphene layers and its uniformity. Before the whole CVD processes, the quartz tube furnace was evacuated with a rotary pump and refilled with Ar and the pumping and refilling process was repeated three times, and the tube was finally kept in an Ar environment under atmospheric pressure. In the APCVD process a mixture of gases which have a carrier gas and precursor gases, the key is to maintain a high ratio between hydrogen and methane. As a deposition process step, after loading the substrate into the APCVD quartz tubular furnace, a constant Ar flow was established to purge the atmospheric gases from the quartz tube by a few minutes. Subsequently, the sample was heated at the processing temperature of 1000 °C, in the presence of Argon and Hydrogen. Due to the halogen lamp, the required temperature reaches within a few minutes which saves our time as well. We have varied the range of annealing time from 30 min to 120 min with an interval of 30 min. The variation is carried out to check the effect of it on the growth as annealing reduces the roughness and cleans the surface of the material, trying to get a single crystalline nickel phase. Next step is to improve a mixture with hydrogen and methane, leaving a few seconds for the reaction and deposition of carbon on nickel, then cools it decreasing temperature to 400 °C, keeping only the hydrogen flow and when the temperature descended to room temperature, the hydrogen flow was closed. Controlling deposition time and the amount of methane, we can control layer numbers on the substrate. The as-grown samples were baked on a hot plate at 160 °C under ambient conditions.
Fig. 1. (a) The schematic process showing the temperature profile and gas flow for hydrogen annealing. (b) Photograph of Ni foil in etchant (c) The schematic diagram of Atmospheric pressure chemical vapor deposition system used for the growth graphene layers.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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3. Characterization of graphene Ni foils, and graphene/Ni foil samples were characterized by atomic force microscopy (AFM, copy (AFM, Seiko Instruments Inc., Chiba, Japan, SI), scanning electron microscopy (SEM, Zeiss EVO 50, Cambridge, UK). For Raman and electrical characterizations, the graphene samples were transferred to 285 nm SiO2/Si substrate by the typical polymerassisted method. Raman spectra were obtained using a Raman microscope (Nanobase) at the excitation wavelength of 532 nm. The laser power was set at 3 mW. 4. Results and discussion 4.1. Optimization of the temperature Initially, we have optimized the growth parameters such as temperature regime to obtain the uniform large-area graphene layers. The temperature is set from outside temperature controller and monitor inside chamber temperature using a thermocouple placed in the middle of the quartz tube, where the substrate will be placed for the growth of graphene layers. It was found that there is a temperature gradient between the set outside temperature and the achieved inside temperature at the center part of the quartz tube. Fig. 2 shows the plots for all temperature gradients. So, up to now, whatever temperature is mentioned in the previous reports is questionable. We have varied the temperature range from 900 to 1050°Cin the interval of 25 °C.It is observed that there is a temperature gradient of almost 75 °C between outside settled temperature and the observed inside quartz tube temperature. This is an important and useful finding, so that we came to know the exact temperature profile for the growth of graphene, and it is interesting to note that the temperature is comparatively low as compared to the previously reported values. This gives a useful insight that the graphene can be synthesized below 1000°Ctemperature using APCVD setup.
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Another useful area that needs to be concern is the cooling rate. It is again a crucial factor because the diffusion depends on this factor. If cooling is high enough, i.e., not to give the time to diffuse back from the substrate, then only the uniformity could be achieved while if the cooling rate is slow, then it is hard to achieve uniformity. In the present case, the cooling rate was 18 °C=s. 4.2. Growth and characterization of few-layer graphene by APCVD To study the influence of annealing treatment on the growth of graphene, Ni foils were first heated up to growth temperature (977 °C) in 15 min and annealed under an Ar/H2 environment in the APCVD system with a fixed gas flow rate (250/25sccm) for different durations (0, 15, 30, 60, and 90, 120 min). The surface profiler and SEM were used to see the surface morphology and roughness of Ni foils. The topographic evolution of Ni foils after annealing treatments is shown in Fig. 3. It is clearly seen that the preheating step yielded the surface reconstruction of Ni foil to generate step bunching (Fig. 3a), which is evidence of the presence of surface oxygen-containing impurities, which are further confirmed by EDX(Fig. 3b).As can be seen from the SEM images that without annealing, there are no grains in Ni foil; also the residual oxygen is present as confirmed from EDX (as shown in Fig. 3b) while after annealing grains can be clearly seen as well as the oxygen peak is absent. The optimum annealing time is kept 60 min fixed for the substrates or Ni foils for further studies as not much change is observed after that much annealing time period. It is important to notice that after annealing period, the width of such terraces becomes small, and the surface fluctuation is degraded. After 60 min annealing, the oxygen coverage seems sufficiently low to favor atomic steps (shown in EDX image Fig. 3b) and, as obtained flat catalyst surface could be beneficial for the suppression of carbon nucleation in the form of graphene. All growth experiments presented here were performed at atmospheric pressure. A typical growth procedure can be defined
Fig. 2. Temperature profile set for the inside temperature analysis during the graphene layers growth process in APCVD system equipped with halogen lamp.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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Fig. 3. (a–e) Annealing of Ni foil before growth process for different annealing period varied from 0 min to 120 min (f) corresponding TEM image (g-h) SEM and corresponding EDAX spectra for without annealing and annealed Ni foils showing that after annealing the Oxygen species are absent.
Fig. 4. Photograph showing large area graphene layers samples.
in 4 distinct parts: temperature reaching up, annealing period, growth duration, and cooling rate (Fig. 1a). For our experiments, the typical growth temperature was 1050 °C (actual temp. 950 °C), and annealing period was 60 min with a pure hydrogen/ argon atmosphere. The growth duration was15 minutes under a mixture of gas species (250 sccm Ar, 25 sccm H2, and 5 sccm CH4).
The cooling was performed in Ar/H2 atmosphere (250 sccm/25 sccm) [1,2]. The photograph of large-size graphene layers after 15 min grown on the Ni foil is shown in Fig. 4. The other growth and cooling parameters were kept also same. The growth is observed on back surface of Ni foil which is consistent with the in previous studies. We have also tried the annealing in the presence of Ar only
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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Fig. 5. (a) Raman spectra of graphene layers prepared by APCVD. (b-c) optical images of transferred graphene on SiO2/Si substrate.
Fig. 6. Schematic diagram of the processes used for transfer graphene layers from Ni foils to SiO2/Si substrates and flexible films like PMMA.
atmosphere and observed that the uniform growth does not occur due to the passivation of the catalyst by the oxide layer on the top of Ni foil. Further, we have used H2 + Ar annealing for all samples and observed the smoother surface with a suitable and optimized condition in this case. During the annealing process, the oxygen species desorbs from the surface and leaves the Ni foil areas as active nucleation sites for the growth of graphene layers. Raman Studies has been carried out to investigate the quality, uniformity, and layer number of as-grown graphene samples. Fig. 5(b-c) shows optical images of a transferred graphene domain on a 285 nm SiO2/Si substrate.The representative Raman plots are shown in Fig. 5a. The positions and relative intensities of the 2D (2700 cm 1) and G bands (1590 cm 1) agree with previous reports on the typical Raman characteristics of a few layer graphene. However, the full width at half maximum (FWHM) of the 2D Raman band is observed to vary from 77 to 80.4 cm 1. The shape, 2D to G peak ratio (I2D/IG), and FWHM of the 2D band was used to determine the number of graphene layers because the 2D band originates from the two phonon double resonance process and is closely related to the band structure of graphene layers.
The D peak has negligible intensity except for some of the defective regions in the form of cracks and folds, which may have been created during the transfer process. The weak D peak observed on edge could contribute to the chirality nature of the graphene edge. 4.3. Optimized transfer process Transfer of graphene also done in different ways, as reported in earlier studies. Before etching the Ni foil, we need to first clean the back surface of as-grown carbon on it. It is an important step to be carried out; otherwise, this back grown carbon will deteriorate the quality of our graphene layers sample during the transfer process by attaching with it. We have carefully clean the as-grown carbon from the back surface of Ni foil using scotch tape/abrasive paper. After that, we have tried to etch out the Ni foil using three different solutions HCl, HNO3, and FeCl3 with two concentrations i.e., 1 and 3 M at room temperature and at high temperature (80 °C). Initially, we kept the Ni foil in a higher concentration for 5 min and then transferred it to the low concentration FeCl3 solution. This process is adopted to etch fast and avoid the chemical impurities to get
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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incorporated in graphene during the transfer process. The schematic of the detailed transfer process is shown in Fig. 6. It is observed that not only concentration but the high temperature also favors the fast etching of the Ni foil. It was observed that the FeCl3 (3 M) at 80 °C was much more effective and took less time to etch out Ni foil as compared to others. Fig. 1c shows the typical photograph of the FeCl3 solution containing graphene-coated Ni foil. 5. Conclusions In summary, the effects of various parameters were studied through the experimental method. It has been shown that graphene layers can be synthesized by means of a halogen lamp equipped APCVD system with the optimized synthesis parameters. Suitable conditions give a uniform deposition of graphene layers over large areas and a reduction in defects. At these conditions, the residence time is reduced preventing the formation of complex hydrocarbons and the boundary layer is thinner, which is favorable for higher quality graphene deposits. It is concluded that APCVD plays an essential role for graphene growth. The large area uniform graphene layers open up the possibility of applications for electronic devices such as solar cells and touch panels. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement All authors are thankful to DIT University for providing funds to carry out this work. References [1] K. Kanahashi, M. Ishihara, M. Hasegawa, H. Ohta, T. Takenobu, Giant power factors in p- and n-type large-area graphene films on a flexible plastic substrate, Npj 2D Mater. Appl. 3 (2019) 44. [2] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Ultrasensitive photodetectors based on monolayer MoS2, Nat. Nanotechnol. 8 (2013) 497– 501. [3] M.A. Alrefae, A. Kumar, P. Pandita, A. Candadai, I. Bilionis, T.S. Fisher, Process optimization of graphene growth in a roll-to-roll plasma CVD system, AIP Adv. 7 (2017) 115102. [4] T. Das, B.K. Sharma, A.K. Katiyar, J.-H. Ahn, Graphene-based flexible and wearable electronics, J. Semicond. 39 (2018) 011007. [5] K.A. Ritter, J.W. Lyding, The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons, Nat. Mater. 8 (2009) 235–242. [6] J.-A. Yan, L. Xian, M.Y. Chou, Structural and electronic properties of oxidized graphene, Phys. Rev. Lett. 103 (2009) 086802. [7] K.S. Novoselov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [8] A. Khan, S.M. Islam, S. Ahmed, R.R. Kumar, M.R. Habib, K. Huang, M. Hu, X. Yu, D. Yang, Direct CVD growth of graphene on technologically important dielectric and semiconducting substrates, Adv. Sci. 5 (2018) 1800050. [9] A. Martinez, K. Fuse, S. Yamashita, Mechanical exfoliation of graphene for the passive mode-locking of fiber lasers, Appl. Phys. Lett. 99 (2011) 121107. [10] M. Zhang, B.-C. Huang, Y. Wang, J.C.S. Woo, Selective LPCVD growth of graphene on patterned copper and its growth mechanism, Appl. Phys. Lett. 109 (2016) 253109. [11] Y.L. Zhong, Z. Tian, G.P. Simon, D. Li, Scalable production of graphene via wet chemistry: progress and challenges, Mater. Today 18 (2015) 73–78.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
A systematic study of the growth and characterization of few-layer graphene on Ni foils Vishakha Kaushik a,⇑, Sachin Pathak b, H. Sharma c a
Department of Physics, DIT University Dehradun, Uttarakhand 248009, India Department of Physics, School of Engineering, University of Petroleum and Energy Studies, Dehradun 248007, Uttarakhand, India c Department of Physics, Doon University, Dehradun, Uttarakhand 248009, India b
a r t i c l e
i n f o
Article history: Received 17 January 2020 Accepted 23 January 2020 Available online xxxx Keywords: Graphene Chemical vapor deposition Raman spectroscopy SEM EDAX
a b s t r a c t Graphene, a two-dimensional material, has gained ample interest in today’s society owing to its remarkable properties such as high conductivity, thermal stability, mechanical strength. Up to date, various graphene synthesis methods are available and out of them, atmospheric pressure chemical vapor deposition (APCVD) is one of the best synthesis due to a very low diffusivity coefficient and a critical step for graphene-based device fabrication. In this work, we have optimized the growth conditions for the synthesis of graphene and its layers on Ni foil using APCVD. It is observed that the temperature i.e., heating and cooling rate, plays an essential role in the growth process. It is also found that annealing of substrate also plays a vital role in the growth of graphene and its layers and the annealing time has been varied from 30 to 120 min the substrate. From XRD and surface roughness analysis, the optimized time for annealing is optimized at 60 min. This led to continuous graphene layers comparable to those achieved with hightemperature CVD with full surface coverage and excellent quality. Our APCVD method is expected to allow the direct growth of graphene in device manufacturing processes for practical applications while keeping underlying devices intact. From TEM analysis, the layers can be seen in the size range from 50 to 100 nm. The presented method in this work provides an integrated device technology for wafer scale deposition with a controllable number of graphene layers. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
1. Introduction Two-dimensional materials such as graphene and MoS2 have received tremendous attention and high expectations due to their extraordinary electronic, mechanical, and chemical properties[1– 4].Several research groups showed theoretical and experimental investigations on the performance of graphene in electronics and demonstrated that they respond well to expectations [5,6]. Mechanical exfoliation was the first isolation of graphene from highly oriented pyrolytic graphite (HOPG) in 2004, but still the uniform large area production has been an exigent subject for realizing practical graphene-based applications [7]. There are various well-developed methods continuously used for the production of graphene in several forms as required or demanded. Each method has its own limitations; for example, in mechanical exfoliation, we cannot achieve large area coverage, ⇑ Corresponding author.
while in case of low-pressure chemical vapor deposition large area coverage is possible but only monolayers can be produced [8–10]. Wet chemical ways also present promising synthesis tools, but again, the quality of as produced graphene is an issue to be resolved [11]. The atmospheric pressure chemical vapor deposition (APCVD) method has been considered the most suitable method for the production of uniform graphene layers on a large scale. In this method, reactive carbon species are being produced, e.g., at the metal surface by decomposing hydrocarbon gas, a concentration gradient results, causing carbon atoms to diffuse into the metal, normal to the surface. The solubility of carbon in a metal, if nonzero, increases with temperature as pure nickel dissolves 1.3 atom % of carbon at 1000 °C. Some of the carbon atoms dissolved in metal at a high temperature can precipitate as a graphite film upon cooling, for relatively thin pieces of metal. While, in the case of using Cu foil, due to low carbon solubility in Cu, large-area monolayer graphene can be synthesized on Cu by self-limiting surface deposition.
E-mail address: [email protected] (V. Kaushik). https://doi.org/10.1016/j.matpr.2020.01.468 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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On the other hand, thin Ni films and fast-cooling processes have been used to suppress the amount of precipitated carbon. However, these methods yield non-uniform graphene films in a wide thickness range from 1 to 12 graphene layers, with monolayer regions up to 20 lm in lateral size, which was affected by the relatively small grain size of Ni film. Especially, Zhang et al. observed that monolayer graphene grows inside the dimension boundaries of the metal grains while multilayer graphene preferentially forms at the metal grain boundaries [10]. APCVD growth of graphene layers is desirable in order to reduce the manufacturing cost and increase throughput. Moreover, severe evaporation of metal catalysts in low-pressure conditions, which may lead to deterioration of the graphene quality, can be significantly suppressed by increasing the working pressure. However, oxygen and water molecules in ambient air should be avoided to obtain high-quality graphene layers. In this study, we have used an APCVD system equipped with a halogen lamp to heat up or anneal the Ni foils to synthesize graphene layers. It is an incandescent lamp with tungsten filament sealed into a compact transparent tube and based on the energy transfer between a radiant heat source and an object with short processing times, i.e., in minutes or seconds. The obvious benefit of this process is the fast cycle times for heating substrates to their required deposition temperature. Here we report, the growth of uniform graphene layers in the absence of residual oxidizing species that may influence the formation of amorphous and oxidized carbon layers. We can quickly transfer the graphene layers to arbitrary substrates by etching the Ni foil, without damaging the graphene. Our employed growth strategy is the ultralow-loss of the substrate and minimal to no damage to the fabricated graphene. We believe that our research will open new pathways for large areas and high-quality graphene growth and as such, promote its practical application. Using this method, large-area continuous graphene layers with excellent quality were synthesized on Ni foils at 900 °C.
2. Materials and methods In this work, we have used APCVD equipment to determine suitable experimental parameters for the uniform growth of large-area graphene layers high quality and the system set up is shown in Fig. 1. The graphene synthesis was done on Ni foil (Size 100 100 mm). The thickness of the Ni foil was 0.25 mm, with 99.9%. The Ni foil was used as received and done many cleaning processes also to see the effect on the growth of graphene layers and its uniformity. Before the whole CVD processes, the quartz tube furnace was evacuated with a rotary pump and refilled with Ar and the pumping and refilling process was repeated three times, and the tube was finally kept in an Ar environment under atmospheric pressure. In the APCVD process a mixture of gases which have a carrier gas and precursor gases, the key is to maintain a high ratio between hydrogen and methane. As a deposition process step, after loading the substrate into the APCVD quartz tubular furnace, a constant Ar flow was established to purge the atmospheric gases from the quartz tube by a few minutes. Subsequently, the sample was heated at the processing temperature of 1000 °C, in the presence of Argon and Hydrogen. Due to the halogen lamp, the required temperature reaches within a few minutes which saves our time as well. We have varied the range of annealing time from 30 min to 120 min with an interval of 30 min. The variation is carried out to check the effect of it on the growth as annealing reduces the roughness and cleans the surface of the material, trying to get a single crystalline nickel phase. Next step is to improve a mixture with hydrogen and methane, leaving a few seconds for the reaction and deposition of carbon on nickel, then cools it decreasing temperature to 400 °C, keeping only the hydrogen flow and when the temperature descended to room temperature, the hydrogen flow was closed. Controlling deposition time and the amount of methane, we can control layer numbers on the substrate. The as-grown samples were baked on a hot plate at 160 °C under ambient conditions.
Fig. 1. (a) The schematic process showing the temperature profile and gas flow for hydrogen annealing. (b) Photograph of Ni foil in etchant (c) The schematic diagram of Atmospheric pressure chemical vapor deposition system used for the growth graphene layers.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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3. Characterization of graphene Ni foils, and graphene/Ni foil samples were characterized by atomic force microscopy (AFM, copy (AFM, Seiko Instruments Inc., Chiba, Japan, SI), scanning electron microscopy (SEM, Zeiss EVO 50, Cambridge, UK). For Raman and electrical characterizations, the graphene samples were transferred to 285 nm SiO2/Si substrate by the typical polymerassisted method. Raman spectra were obtained using a Raman microscope (Nanobase) at the excitation wavelength of 532 nm. The laser power was set at 3 mW. 4. Results and discussion 4.1. Optimization of the temperature Initially, we have optimized the growth parameters such as temperature regime to obtain the uniform large-area graphene layers. The temperature is set from outside temperature controller and monitor inside chamber temperature using a thermocouple placed in the middle of the quartz tube, where the substrate will be placed for the growth of graphene layers. It was found that there is a temperature gradient between the set outside temperature and the achieved inside temperature at the center part of the quartz tube. Fig. 2 shows the plots for all temperature gradients. So, up to now, whatever temperature is mentioned in the previous reports is questionable. We have varied the temperature range from 900 to 1050°Cin the interval of 25 °C.It is observed that there is a temperature gradient of almost 75 °C between outside settled temperature and the observed inside quartz tube temperature. This is an important and useful finding, so that we came to know the exact temperature profile for the growth of graphene, and it is interesting to note that the temperature is comparatively low as compared to the previously reported values. This gives a useful insight that the graphene can be synthesized below 1000°Ctemperature using APCVD setup.
3
Another useful area that needs to be concern is the cooling rate. It is again a crucial factor because the diffusion depends on this factor. If cooling is high enough, i.e., not to give the time to diffuse back from the substrate, then only the uniformity could be achieved while if the cooling rate is slow, then it is hard to achieve uniformity. In the present case, the cooling rate was 18 °C=s. 4.2. Growth and characterization of few-layer graphene by APCVD To study the influence of annealing treatment on the growth of graphene, Ni foils were first heated up to growth temperature (977 °C) in 15 min and annealed under an Ar/H2 environment in the APCVD system with a fixed gas flow rate (250/25sccm) for different durations (0, 15, 30, 60, and 90, 120 min). The surface profiler and SEM were used to see the surface morphology and roughness of Ni foils. The topographic evolution of Ni foils after annealing treatments is shown in Fig. 3. It is clearly seen that the preheating step yielded the surface reconstruction of Ni foil to generate step bunching (Fig. 3a), which is evidence of the presence of surface oxygen-containing impurities, which are further confirmed by EDX(Fig. 3b).As can be seen from the SEM images that without annealing, there are no grains in Ni foil; also the residual oxygen is present as confirmed from EDX (as shown in Fig. 3b) while after annealing grains can be clearly seen as well as the oxygen peak is absent. The optimum annealing time is kept 60 min fixed for the substrates or Ni foils for further studies as not much change is observed after that much annealing time period. It is important to notice that after annealing period, the width of such terraces becomes small, and the surface fluctuation is degraded. After 60 min annealing, the oxygen coverage seems sufficiently low to favor atomic steps (shown in EDX image Fig. 3b) and, as obtained flat catalyst surface could be beneficial for the suppression of carbon nucleation in the form of graphene. All growth experiments presented here were performed at atmospheric pressure. A typical growth procedure can be defined
Fig. 2. Temperature profile set for the inside temperature analysis during the graphene layers growth process in APCVD system equipped with halogen lamp.
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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Fig. 3. (a–e) Annealing of Ni foil before growth process for different annealing period varied from 0 min to 120 min (f) corresponding TEM image (g-h) SEM and corresponding EDAX spectra for without annealing and annealed Ni foils showing that after annealing the Oxygen species are absent.
Fig. 4. Photograph showing large area graphene layers samples.
in 4 distinct parts: temperature reaching up, annealing period, growth duration, and cooling rate (Fig. 1a). For our experiments, the typical growth temperature was 1050 °C (actual temp. 950 °C), and annealing period was 60 min with a pure hydrogen/ argon atmosphere. The growth duration was15 minutes under a mixture of gas species (250 sccm Ar, 25 sccm H2, and 5 sccm CH4).
The cooling was performed in Ar/H2 atmosphere (250 sccm/25 sccm) [1,2]. The photograph of large-size graphene layers after 15 min grown on the Ni foil is shown in Fig. 4. The other growth and cooling parameters were kept also same. The growth is observed on back surface of Ni foil which is consistent with the in previous studies. We have also tried the annealing in the presence of Ar only
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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Fig. 5. (a) Raman spectra of graphene layers prepared by APCVD. (b-c) optical images of transferred graphene on SiO2/Si substrate.
Fig. 6. Schematic diagram of the processes used for transfer graphene layers from Ni foils to SiO2/Si substrates and flexible films like PMMA.
atmosphere and observed that the uniform growth does not occur due to the passivation of the catalyst by the oxide layer on the top of Ni foil. Further, we have used H2 + Ar annealing for all samples and observed the smoother surface with a suitable and optimized condition in this case. During the annealing process, the oxygen species desorbs from the surface and leaves the Ni foil areas as active nucleation sites for the growth of graphene layers. Raman Studies has been carried out to investigate the quality, uniformity, and layer number of as-grown graphene samples. Fig. 5(b-c) shows optical images of a transferred graphene domain on a 285 nm SiO2/Si substrate.The representative Raman plots are shown in Fig. 5a. The positions and relative intensities of the 2D (2700 cm 1) and G bands (1590 cm 1) agree with previous reports on the typical Raman characteristics of a few layer graphene. However, the full width at half maximum (FWHM) of the 2D Raman band is observed to vary from 77 to 80.4 cm 1. The shape, 2D to G peak ratio (I2D/IG), and FWHM of the 2D band was used to determine the number of graphene layers because the 2D band originates from the two phonon double resonance process and is closely related to the band structure of graphene layers.
The D peak has negligible intensity except for some of the defective regions in the form of cracks and folds, which may have been created during the transfer process. The weak D peak observed on edge could contribute to the chirality nature of the graphene edge. 4.3. Optimized transfer process Transfer of graphene also done in different ways, as reported in earlier studies. Before etching the Ni foil, we need to first clean the back surface of as-grown carbon on it. It is an important step to be carried out; otherwise, this back grown carbon will deteriorate the quality of our graphene layers sample during the transfer process by attaching with it. We have carefully clean the as-grown carbon from the back surface of Ni foil using scotch tape/abrasive paper. After that, we have tried to etch out the Ni foil using three different solutions HCl, HNO3, and FeCl3 with two concentrations i.e., 1 and 3 M at room temperature and at high temperature (80 °C). Initially, we kept the Ni foil in a higher concentration for 5 min and then transferred it to the low concentration FeCl3 solution. This process is adopted to etch fast and avoid the chemical impurities to get
Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468
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incorporated in graphene during the transfer process. The schematic of the detailed transfer process is shown in Fig. 6. It is observed that not only concentration but the high temperature also favors the fast etching of the Ni foil. It was observed that the FeCl3 (3 M) at 80 °C was much more effective and took less time to etch out Ni foil as compared to others. Fig. 1c shows the typical photograph of the FeCl3 solution containing graphene-coated Ni foil. 5. Conclusions In summary, the effects of various parameters were studied through the experimental method. It has been shown that graphene layers can be synthesized by means of a halogen lamp equipped APCVD system with the optimized synthesis parameters. Suitable conditions give a uniform deposition of graphene layers over large areas and a reduction in defects. At these conditions, the residence time is reduced preventing the formation of complex hydrocarbons and the boundary layer is thinner, which is favorable for higher quality graphene deposits. It is concluded that APCVD plays an essential role for graphene growth. The large area uniform graphene layers open up the possibility of applications for electronic devices such as solar cells and touch panels. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Please cite this article as: V. Kaushik, S. Pathak and H. Sharma, A systematic study of the growth and characterization of few-layer graphene on Ni foils, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.468