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Effect of Cu surface treatment in graphene growth by chemical vapor deposition
Accepted Manuscript Effect of Cu surface treatment in graphene growth by chemical vapor deposition Seong-Yong Cho, MinSu Kim, Min-Sik Kim, Min-Hyun Lee, Ki-Bum Kim PII: DOI: Reference:
S0167-577X(18)31714-2 https://doi.org/10.1016/j.matlet.2018.10.134 MLBLUE 25177
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
28 September 2018 17 October 2018 22 October 2018
Please cite this article as: S-Y. Cho, M. Kim, M-S. Kim, M-H. Lee, K-B. Kim, Effect of Cu surface treatment in graphene growth by chemical vapor deposition, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet. 2018.10.134
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Cu surface treatment in graphene growth by chemical vapor deposition
Seong-Yong Cho,a,* MinSu Kim,b Min-Sik Kim,b and Min-Hyun Lee,c and Ki-Bum Kimb,*
a
Department of Materials Science and Engineering, College of Engineering, Myongji University,
Yongin, Gyeonggi 17058, Korea b
Department of Materials Science and Engineering, College of Engineering, Seoul National
University, Seoul 08826, Korea c
Samsung Advanced Institute of Technology, Suwon, Gyeonggi 16678, Korea
*e-mail: [email protected]; [email protected]
Keywords Graphene; Cu foil; electropolishing; surface roughness; heteronucleation
Abstract Cu foils with various initial roughness were prepared and graphene was grown on each Cu foil. Graphene nuclei density was highly related to the initial surface roughness of Cu foil. Smoothing was done on Cu foil and initial roughness was relieved during 3 hours annealing, but Cu surface with high surface roughness and impurity atoms provides larger nuclei density for the graphene growth. Heteronuclei effect of size variations of impurity atoms on multilayer graphene formation was calculated. Electropolishing was carried out to suppress the heteronuclei effect and the nuclei formation on Cu surface as well. For electropolished Cu substrate, nuclei density of graphene was extremely low even without extended pre-annealing step.
INTRODUCTION Graphene is a single layer of hexagonal carbon arrangement. Since the first discovery of graphene by Manchester group in 2004,[1,2] it has attracted a great attention due to its extraordinary electrical and optical properties.[2–4] At the very beginning of graphene research, many groups tried to get a single layer of graphene with limited area by repeating exfoliation of highly oriented pyrolytic graphite using scotch tape. Researches of graphene have been accelerated by developing various synthesis techniques such as sublimation of SiC substrate[5], reduction of graphene oxide[6] and chemical vapor deposition (CVD).[7,8] These kinds of synthesis are industry-compatible in terms of reproducibility and scaling up.[9,10] Accordingly, many potential applications of graphene have been proposed such as transparent electrode, photodetector, and photocatalyst.[11-13] Thus, providing high quality graphene is highly required for potential applications. Among many synthetic routes for graphene material, CVD provides large-area synthesis and compatible to thin film processing techniques. Dissociation of hydrocarbon source on catalytic metal proceeds nucleation and growth of 2D graphene lattice.[14] Graphene can be grown various catalytic metals, but Cu foil is typically chosen as a substrate since single layer is easily grown on Cu due to limited carbon solubility.[15] In terms of minimizing root-mean-square (RMS) roughness of catalytic Cu surface, preparing Cu thin films on the wafer scale would be beneficial, but high surface energy of metal induces agglomeration and break-up of the film during high temperature annealing[16] which is essential for dissociation of hydrocarbon source and followed graphene growth. In this aspect, Cu foil is used as a template for the graphene growth, but systematic studies on the roughness of Cu surface have been limited.[17-19] Cu foil is typically manufactured by mechanical rolling which results in scratches along the rolling direction. Cr2O3 is typically coated on the surface of Cu as a protection layer from the oxidation, but impurity atoms are believed to act as a center for the heteronucleation. Graphene lattice grows when hydrocarbon source decomposes by catalytic effect of metal, and generally, polycrystalline graphene lattice is achieved by random orientation of each nucleus. Suppressing heteronucleation site has been studied by several groups in this aspect. Han et al., adopted chemical mechanical planarization (CMP) process to minimize surface roughness of Cu
foil.[20] However, additional CMP step generates process complexity and may induce mechanical damages to Cu foil. Here, we prepared Cu foils with different RMS roughness and measured RMS roughness after annealing. Graphene was grown on each Cu foil and image analysis was carried out to understand nucleation mechanism on the Cu surface. Shape factor for the heteronucleation was calculated to explain multilayer graphene formation with impurities on the Cu surfaces. Also, Cu foil with initial high RMS roughness was electropolished. This verified the fact that graphene nuclei density was extremely suppressed according to previous studies.[17,21] The graphene nuclei density is mainly from surface roughness of Cu foil, and the nuclei density of graphene can be suppressed by electropolishing and adequate smoothing by heat treatment according to our study.
EXPERIMENTAL Cu foils with different roughness and purity were prepared from Alfa Aesar. RMS roughness of each Cu foil was measured by atomic force microscopy (AFM; Surface Imaging Systems, NANO Station II). Foil A (#13382) has Cu purity 99.8 % and measured RMS roughness was 237 nm. Foil B (#10950) has 99.999 % Cu purity with initial RMS roughness of 74 nm. Foil C (#42972) also has 99.999 % Cu purity and initial RMS roughness of 38.3 nm. (Note that Foil C has thickness of 50 μm, but Foil A and B are 25-μm thick) Pre-annealing (1050 oC, 3 hours) and growth condition were all the same. Atmospheric pressure (AP) CVD was used to provide enough time for pre-annealing and smoothing Cu surface. 30 ppm of CH4 gas was injected with 300 sccm (standard cubic centimeter per minute) of Ar gas flow. Hydrogen flow rate was 10 sccm. Scanning electron microscopy (SEM; Hitachi, SU70) analysis was done to understand nucleation and growth behavior of graphene on each Cu foil. Electropolishing of Cu foil was done using lab-made equipment. Both anode and cathode were connected to Cu foil by alligator clip, and 5 A of current was supplied by power supply. Electrolyte solution was prepared by 2:1:1 ratio of deionized water, orthophosphoric acid and ethanol. Small amount of isopropyl alcohol and urea were added. Graphene was grown by Low pressure (LP) CVD on electropolished Cu since pre-annealing was not necessary. For the observation of graphene
grown on electropolished Cu, selective oxidation method was used to observe graphene islands at lower magnifications.[22,23] Cu foil was oxidized at 100 oC after partial coverage of graphene growth and observed via optical microscopy. (OM; Olympus BX50) RESULTS AND DISCUSSION Figure 1 clearly illustrates initial roughness of various Cu foils measured by AFM. Foil A (Figure 1 (a)) is typical Cu foil which is dominantly used by researchers for a decade. The measurement area of AFM analysis was 80 μm by 80 μm. After 3 hours of heat treatment at 1050 oC, smoothing effect is clearly shown in Figure 1 (d). Roughness was decreased to 60 % of initial value. As-received foil B shows 74.0 nm of initial surface roughness and the roughness decreased to 30 nm after annealing. Foil C has the smallest initial surface roughness (38.3 nm) and it reaches 24.6 nm by smoothing effect during heat treatment. We are not aware of details of manufacturing process of each Cu foil, but foil A has larger density of scratch lines presumably produced by mechanical rolling directions along the lines. Heat treatment relieves scratches and surface defects significantly. However, the larger initial roughness has, the more surface defects remains after heat treatment. We also observed large grooving effect in foil A as shown in Figure 1 (d). The effect of using various Cu substrates in the graphene growth is clearly shown in SEM images (Figure 2 (a) to (f)). The largest graphene nuclei density was observed when foil A was used (low purity and high roughness). Note that pre-annealing temperature and time are all the same. Even, multilayer graphene formation was also observed in Figure 2 (d) which is described using heteronuclei formation and simple calculations. One will notice that graphene nuclei grows along the rolling direction of Cu foil which is observed in low magnification SEM image (Figure 2 (a)). Despite of heat treatment and smoothing of Cu surface, mechanical scratch lines do not recover completely and act as a nucleation sites for the graphene seeds. Graphene grown on top of foil B and C does not show significant multilayer formation and lower nuclei density compared with graphene grown on foil A. Since we observed bilayer graphene formation on foil A, and small particles in the center for bilayer graphene, simple calculation was carried out to understand heteronuclei effect for the graphene
growth. Nuclei density of graphene on foil B and C decreased notably compared with graphene grown on A. Since foil A has higher surface roughness and lower purity (99.8 %), higher nucleation density was observed. Details of nuclei density is shown in Figure 2 (g). P. Braeuninger-Weimer et al., used secondary ion mass spectrometry (SIMS) techniques to get the impurity information, and they revealed the role of impurities in graphene growth on Cu.[21] To understand effect of heteronuclei which can be either impurity atoms or roughness of the surface on the graphene growth, we need to calculate shape factor (which is related to free energy) of the nucleation. For the heterogeneous nucleation of graphene, spherical shape of impurity atom was considered for simple calculation in Figure 3 (a). Free energy of nuclei formation in the homogeneous nucleation is expressed by surface energy and volume free energy of nuclei as shown in Equation 1 below.[25] (1)
Here,
is radius of nuclei,
is volume free energy of nuclei, and
is interfacial energy
between nuclei and substrate. For heterogeneous nucleation, the equation is expressed by shape factor as shown in Equation (2). (2)
Here,
can be expressed by equation (3).
(3)
is wetting angle of heteronuclei, but for graphene layer, the wetting angle,
can be related to
impurity atom size and the number of graphene layers as shown in Figure 3 (a). The bigger particle provides smaller shape factor, which means that the driving force for nucleation is much larger. The effect of particle diameter and the number of graphene layers are summarized in Figure 3 (b). In the same impurity particle size, the effect on shape factor of the number of graphene layers can be simply calculated by contact angle.
Heteronuclei effect could be a result of either scratch and rolling damage or impurity atoms in the surface of Cu. This can explain why larger nuclei density was observed in Foil A with high surface roughness. Heteronuclei can be formed by initial impurity atoms on the Cu surface, but initial scratch lines can also be ledges or topological heterogeneous nucleation sites even after annealing. Finally, electropolishing was applied to reduce surface roughness of Cu foil. Current level was set to 5 A during electropolishing to minimize high current damage to Cu foil. Electropolishing (chemical etching of Cu) was done at anode, and recycling cathode Cu did not show any noticeable change for electropolished Cu at anode. OM image of as-received Cu foil in Figure 4 (a) also reveals mechanical rolling damage during foil manufacturing process. OM image of Cu foil after electropolishing removes scratch lines at some extent, but surface undulations were observed (Figure 4 (b)). However, when surface RMS roughness values were measured by AFM, it was 116 nm which is the half value of initial surface roughness of foil A (237 nm). The effect of electropolishing is clearly shown in Figure 5. Figure 5 (a) shows graphene seeds grown on bare Cu foil A. For this sample, pre-annealing time was just 30 minutes and LPCVD was done. For LPCVD growth of graphene, longer pre-annealing time produces extensive amount of Cu vapor and results in surface roughness.[26, 27] When electropolishing was done on top of Cu foil A, nuclei density was extremely suppressed as shown in Figure 5 (b). We believe that surface defects were removed by electropolishing and followed etching of the Cu surface. Similar effect was reported by X. Wu et al.[28] It is apparent that some of the Cu surface which contains coating material was removed by electropolishing. Also, smoother Cu surface provides lower nucleation sites for the graphene, and there will be lower density of impurities due to surface etching. Generally, surface of Cu foil is treated by manufacturers and certain amounts of impurities. P. Braeuninger-Weimer et al., already reported that impurities are significantly important factor for the graphene nucleation.[24] Selective oxidation was carried out for Figure 5 (a) and (b). Cu foil was oxidized after graphene growth at 100 oC. Thus, graphene-covered Cu area is clearly shown due to protection from the oxidation.
CONCLUSION Surface roughness of as-received Cu foil can be decreased by pre-annealing process prior to graphene growth, but larger initial surface roughness results in higher degree of graphene nuclei density due to larger surface roughness even after pre-annealing step. Heteronucleation is believed to be a main factor for large nucleation density, which is result of scratch and rolling damages on Cu surface. Heteronuclei effect was dominant along the scratch lines produced by Cu foil manufacturing process, which was simply calculated by free energy model. Electropolishing significantly reduced surface roughness and heteronucleation of graphene seeds, and nuclei density of graphene after electropolishing was extremely suppressed.
ACKNOWLEDGEMENT This work was supported by 2018 Research Fund of Myongji University.
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Figure 1. AFM measurements of various Cu foils when as-received and after heat treatment. (80 80 μm) (a) AFM image of as-received Cu foil A, (b) Cu foil B, (c) Cu foil C, (d) Cu foil A after 3 hours of annealing at 1050 oC, (e) Cu foil B after 3 hours of annealing at 1050 oC, and (f) Cu foil C after 3 hours of annealing at 1050 o C. Z color axis scales are μm in (a) and (d), and all other Z color scales are nm.
Figure 2. SEM images of graphene grown on various Cu foils. Graphene islands grown on (a) foil A, (b) foil B, and (c) foil C at low magnifications, graphene islands at higher magnification on (d) foil A, (e) foil B, and (f) foil C. Red circles in (d) indicate bilayer formation in the center of graphene islands and (g) Nuclei density of different Cu foils used in this work.
Figure 3. Impurity atom effect in heterogeneous nucleation of graphene. (a) Wetting angle is expressed as a function of particle size and (b) calculated shape factor of graphene formation as a function of particle diameter and number of graphene layers.
Figure 4. (a) OM image of as-received Cu foil A, and (b) electropolished Cu foil A. Scale bar indicates 100 μm in both figures.
Figure 5. (a) OM image of graphene grown on as-received Cu foil A after selective oxidation. (b) OM image of graphene grown on electropolished Cu foil A. Scale bar indicates 200 μm in both photographs.
Graphene growth behavior was studied on Cu foils with various roughness. Heteronuclei effect was thermodynamics model
calculated
based
on
classical
Electropolishing suppressed graphene nuclei density significantly.
S0167-577X(18)31714-2 https://doi.org/10.1016/j.matlet.2018.10.134 MLBLUE 25177
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
28 September 2018 17 October 2018 22 October 2018
Please cite this article as: S-Y. Cho, M. Kim, M-S. Kim, M-H. Lee, K-B. Kim, Effect of Cu surface treatment in graphene growth by chemical vapor deposition, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet. 2018.10.134
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Cu surface treatment in graphene growth by chemical vapor deposition
Seong-Yong Cho,a,* MinSu Kim,b Min-Sik Kim,b and Min-Hyun Lee,c and Ki-Bum Kimb,*
a
Department of Materials Science and Engineering, College of Engineering, Myongji University,
Yongin, Gyeonggi 17058, Korea b
Department of Materials Science and Engineering, College of Engineering, Seoul National
University, Seoul 08826, Korea c
Samsung Advanced Institute of Technology, Suwon, Gyeonggi 16678, Korea
*e-mail: [email protected]; [email protected]
Keywords Graphene; Cu foil; electropolishing; surface roughness; heteronucleation
Abstract Cu foils with various initial roughness were prepared and graphene was grown on each Cu foil. Graphene nuclei density was highly related to the initial surface roughness of Cu foil. Smoothing was done on Cu foil and initial roughness was relieved during 3 hours annealing, but Cu surface with high surface roughness and impurity atoms provides larger nuclei density for the graphene growth. Heteronuclei effect of size variations of impurity atoms on multilayer graphene formation was calculated. Electropolishing was carried out to suppress the heteronuclei effect and the nuclei formation on Cu surface as well. For electropolished Cu substrate, nuclei density of graphene was extremely low even without extended pre-annealing step.
INTRODUCTION Graphene is a single layer of hexagonal carbon arrangement. Since the first discovery of graphene by Manchester group in 2004,[1,2] it has attracted a great attention due to its extraordinary electrical and optical properties.[2–4] At the very beginning of graphene research, many groups tried to get a single layer of graphene with limited area by repeating exfoliation of highly oriented pyrolytic graphite using scotch tape. Researches of graphene have been accelerated by developing various synthesis techniques such as sublimation of SiC substrate[5], reduction of graphene oxide[6] and chemical vapor deposition (CVD).[7,8] These kinds of synthesis are industry-compatible in terms of reproducibility and scaling up.[9,10] Accordingly, many potential applications of graphene have been proposed such as transparent electrode, photodetector, and photocatalyst.[11-13] Thus, providing high quality graphene is highly required for potential applications. Among many synthetic routes for graphene material, CVD provides large-area synthesis and compatible to thin film processing techniques. Dissociation of hydrocarbon source on catalytic metal proceeds nucleation and growth of 2D graphene lattice.[14] Graphene can be grown various catalytic metals, but Cu foil is typically chosen as a substrate since single layer is easily grown on Cu due to limited carbon solubility.[15] In terms of minimizing root-mean-square (RMS) roughness of catalytic Cu surface, preparing Cu thin films on the wafer scale would be beneficial, but high surface energy of metal induces agglomeration and break-up of the film during high temperature annealing[16] which is essential for dissociation of hydrocarbon source and followed graphene growth. In this aspect, Cu foil is used as a template for the graphene growth, but systematic studies on the roughness of Cu surface have been limited.[17-19] Cu foil is typically manufactured by mechanical rolling which results in scratches along the rolling direction. Cr2O3 is typically coated on the surface of Cu as a protection layer from the oxidation, but impurity atoms are believed to act as a center for the heteronucleation. Graphene lattice grows when hydrocarbon source decomposes by catalytic effect of metal, and generally, polycrystalline graphene lattice is achieved by random orientation of each nucleus. Suppressing heteronucleation site has been studied by several groups in this aspect. Han et al., adopted chemical mechanical planarization (CMP) process to minimize surface roughness of Cu
foil.[20] However, additional CMP step generates process complexity and may induce mechanical damages to Cu foil. Here, we prepared Cu foils with different RMS roughness and measured RMS roughness after annealing. Graphene was grown on each Cu foil and image analysis was carried out to understand nucleation mechanism on the Cu surface. Shape factor for the heteronucleation was calculated to explain multilayer graphene formation with impurities on the Cu surfaces. Also, Cu foil with initial high RMS roughness was electropolished. This verified the fact that graphene nuclei density was extremely suppressed according to previous studies.[17,21] The graphene nuclei density is mainly from surface roughness of Cu foil, and the nuclei density of graphene can be suppressed by electropolishing and adequate smoothing by heat treatment according to our study.
EXPERIMENTAL Cu foils with different roughness and purity were prepared from Alfa Aesar. RMS roughness of each Cu foil was measured by atomic force microscopy (AFM; Surface Imaging Systems, NANO Station II). Foil A (#13382) has Cu purity 99.8 % and measured RMS roughness was 237 nm. Foil B (#10950) has 99.999 % Cu purity with initial RMS roughness of 74 nm. Foil C (#42972) also has 99.999 % Cu purity and initial RMS roughness of 38.3 nm. (Note that Foil C has thickness of 50 μm, but Foil A and B are 25-μm thick) Pre-annealing (1050 oC, 3 hours) and growth condition were all the same. Atmospheric pressure (AP) CVD was used to provide enough time for pre-annealing and smoothing Cu surface. 30 ppm of CH4 gas was injected with 300 sccm (standard cubic centimeter per minute) of Ar gas flow. Hydrogen flow rate was 10 sccm. Scanning electron microscopy (SEM; Hitachi, SU70) analysis was done to understand nucleation and growth behavior of graphene on each Cu foil. Electropolishing of Cu foil was done using lab-made equipment. Both anode and cathode were connected to Cu foil by alligator clip, and 5 A of current was supplied by power supply. Electrolyte solution was prepared by 2:1:1 ratio of deionized water, orthophosphoric acid and ethanol. Small amount of isopropyl alcohol and urea were added. Graphene was grown by Low pressure (LP) CVD on electropolished Cu since pre-annealing was not necessary. For the observation of graphene
grown on electropolished Cu, selective oxidation method was used to observe graphene islands at lower magnifications.[22,23] Cu foil was oxidized at 100 oC after partial coverage of graphene growth and observed via optical microscopy. (OM; Olympus BX50) RESULTS AND DISCUSSION Figure 1 clearly illustrates initial roughness of various Cu foils measured by AFM. Foil A (Figure 1 (a)) is typical Cu foil which is dominantly used by researchers for a decade. The measurement area of AFM analysis was 80 μm by 80 μm. After 3 hours of heat treatment at 1050 oC, smoothing effect is clearly shown in Figure 1 (d). Roughness was decreased to 60 % of initial value. As-received foil B shows 74.0 nm of initial surface roughness and the roughness decreased to 30 nm after annealing. Foil C has the smallest initial surface roughness (38.3 nm) and it reaches 24.6 nm by smoothing effect during heat treatment. We are not aware of details of manufacturing process of each Cu foil, but foil A has larger density of scratch lines presumably produced by mechanical rolling directions along the lines. Heat treatment relieves scratches and surface defects significantly. However, the larger initial roughness has, the more surface defects remains after heat treatment. We also observed large grooving effect in foil A as shown in Figure 1 (d). The effect of using various Cu substrates in the graphene growth is clearly shown in SEM images (Figure 2 (a) to (f)). The largest graphene nuclei density was observed when foil A was used (low purity and high roughness). Note that pre-annealing temperature and time are all the same. Even, multilayer graphene formation was also observed in Figure 2 (d) which is described using heteronuclei formation and simple calculations. One will notice that graphene nuclei grows along the rolling direction of Cu foil which is observed in low magnification SEM image (Figure 2 (a)). Despite of heat treatment and smoothing of Cu surface, mechanical scratch lines do not recover completely and act as a nucleation sites for the graphene seeds. Graphene grown on top of foil B and C does not show significant multilayer formation and lower nuclei density compared with graphene grown on foil A. Since we observed bilayer graphene formation on foil A, and small particles in the center for bilayer graphene, simple calculation was carried out to understand heteronuclei effect for the graphene
growth. Nuclei density of graphene on foil B and C decreased notably compared with graphene grown on A. Since foil A has higher surface roughness and lower purity (99.8 %), higher nucleation density was observed. Details of nuclei density is shown in Figure 2 (g). P. Braeuninger-Weimer et al., used secondary ion mass spectrometry (SIMS) techniques to get the impurity information, and they revealed the role of impurities in graphene growth on Cu.[21] To understand effect of heteronuclei which can be either impurity atoms or roughness of the surface on the graphene growth, we need to calculate shape factor (which is related to free energy) of the nucleation. For the heterogeneous nucleation of graphene, spherical shape of impurity atom was considered for simple calculation in Figure 3 (a). Free energy of nuclei formation in the homogeneous nucleation is expressed by surface energy and volume free energy of nuclei as shown in Equation 1 below.[25] (1)
Here,
is radius of nuclei,
is volume free energy of nuclei, and
is interfacial energy
between nuclei and substrate. For heterogeneous nucleation, the equation is expressed by shape factor as shown in Equation (2). (2)
Here,
can be expressed by equation (3).
(3)
is wetting angle of heteronuclei, but for graphene layer, the wetting angle,
can be related to
impurity atom size and the number of graphene layers as shown in Figure 3 (a). The bigger particle provides smaller shape factor, which means that the driving force for nucleation is much larger. The effect of particle diameter and the number of graphene layers are summarized in Figure 3 (b). In the same impurity particle size, the effect on shape factor of the number of graphene layers can be simply calculated by contact angle.
Heteronuclei effect could be a result of either scratch and rolling damage or impurity atoms in the surface of Cu. This can explain why larger nuclei density was observed in Foil A with high surface roughness. Heteronuclei can be formed by initial impurity atoms on the Cu surface, but initial scratch lines can also be ledges or topological heterogeneous nucleation sites even after annealing. Finally, electropolishing was applied to reduce surface roughness of Cu foil. Current level was set to 5 A during electropolishing to minimize high current damage to Cu foil. Electropolishing (chemical etching of Cu) was done at anode, and recycling cathode Cu did not show any noticeable change for electropolished Cu at anode. OM image of as-received Cu foil in Figure 4 (a) also reveals mechanical rolling damage during foil manufacturing process. OM image of Cu foil after electropolishing removes scratch lines at some extent, but surface undulations were observed (Figure 4 (b)). However, when surface RMS roughness values were measured by AFM, it was 116 nm which is the half value of initial surface roughness of foil A (237 nm). The effect of electropolishing is clearly shown in Figure 5. Figure 5 (a) shows graphene seeds grown on bare Cu foil A. For this sample, pre-annealing time was just 30 minutes and LPCVD was done. For LPCVD growth of graphene, longer pre-annealing time produces extensive amount of Cu vapor and results in surface roughness.[26, 27] When electropolishing was done on top of Cu foil A, nuclei density was extremely suppressed as shown in Figure 5 (b). We believe that surface defects were removed by electropolishing and followed etching of the Cu surface. Similar effect was reported by X. Wu et al.[28] It is apparent that some of the Cu surface which contains coating material was removed by electropolishing. Also, smoother Cu surface provides lower nucleation sites for the graphene, and there will be lower density of impurities due to surface etching. Generally, surface of Cu foil is treated by manufacturers and certain amounts of impurities. P. Braeuninger-Weimer et al., already reported that impurities are significantly important factor for the graphene nucleation.[24] Selective oxidation was carried out for Figure 5 (a) and (b). Cu foil was oxidized after graphene growth at 100 oC. Thus, graphene-covered Cu area is clearly shown due to protection from the oxidation.
CONCLUSION Surface roughness of as-received Cu foil can be decreased by pre-annealing process prior to graphene growth, but larger initial surface roughness results in higher degree of graphene nuclei density due to larger surface roughness even after pre-annealing step. Heteronucleation is believed to be a main factor for large nucleation density, which is result of scratch and rolling damages on Cu surface. Heteronuclei effect was dominant along the scratch lines produced by Cu foil manufacturing process, which was simply calculated by free energy model. Electropolishing significantly reduced surface roughness and heteronucleation of graphene seeds, and nuclei density of graphene after electropolishing was extremely suppressed.
ACKNOWLEDGEMENT This work was supported by 2018 Research Fund of Myongji University.
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Figure 1. AFM measurements of various Cu foils when as-received and after heat treatment. (80 80 μm) (a) AFM image of as-received Cu foil A, (b) Cu foil B, (c) Cu foil C, (d) Cu foil A after 3 hours of annealing at 1050 oC, (e) Cu foil B after 3 hours of annealing at 1050 oC, and (f) Cu foil C after 3 hours of annealing at 1050 o C. Z color axis scales are μm in (a) and (d), and all other Z color scales are nm.
Figure 2. SEM images of graphene grown on various Cu foils. Graphene islands grown on (a) foil A, (b) foil B, and (c) foil C at low magnifications, graphene islands at higher magnification on (d) foil A, (e) foil B, and (f) foil C. Red circles in (d) indicate bilayer formation in the center of graphene islands and (g) Nuclei density of different Cu foils used in this work.
Figure 3. Impurity atom effect in heterogeneous nucleation of graphene. (a) Wetting angle is expressed as a function of particle size and (b) calculated shape factor of graphene formation as a function of particle diameter and number of graphene layers.
Figure 4. (a) OM image of as-received Cu foil A, and (b) electropolished Cu foil A. Scale bar indicates 100 μm in both figures.
Figure 5. (a) OM image of graphene grown on as-received Cu foil A after selective oxidation. (b) OM image of graphene grown on electropolished Cu foil A. Scale bar indicates 200 μm in both photographs.
Graphene growth behavior was studied on Cu foils with various roughness. Heteronuclei effect was thermodynamics model
calculated
based
on
classical
Electropolishing suppressed graphene nuclei density significantly.