IMPROVING QUALITY OF GRAPHENE GROWN ON THE COPPER FOIL BY PHYSICAL POLISHING ELECTROPOLISHING AND THERMAL ANNEALING PRETREATMENT PROCESSES BY LOW PRESSURE CHEMICAL VAPOR DEPOSITION

Objective: This study aimed to investigate the influence of chemical treatments and thermal annealing on the quality of graphene films grown on copper foils using low-pressure chemical vapor deposition (LPCVD) with cyclohexane as the precursor and N 2 as the carrier gas. Method: Cu foils were subjected to physical and electropolishing with varying phosphoric acid concentrations (30-60%) and etching times (60, 90, 120 seconds), followed by thermal annealing at temperatures from 860-940 °C for 6 minutes and consistent graphene growth at 920 °C for 10 minutes. The study employed Raman spectroscopy and microscopy analyses to assess the impact of pretreatment processes, annealing temperature, and cyclohexane flow rates on graphene film quality. Results and Discussion: Optimal conditions were identified at a 45% phosphoric acid concentration with a 90-second etching time, paired with an annealing temperature of 900 °C. This setup produced a high I 2D /I G intensity ratio of 2.79, resulting in the formation of predominantly monolayer graphene films, while varying conditions led to multilayer graphene. Experimental observations also revealed that adjusting growth time and cyclohexane flow rates further enhanced the formation of monolayer graphene film. Research Implications


INTRODUCTION
Graphene possesses unique properties, including high thermal conductivity, low resistivity, and high transparency which is applied to various applications such as sensing materials, flexible optoelectronics, high-frequency electronic devices, and energy storage [ (Singh et al. 2017), (Rana et al. 2014), (Han et al. 2014), (Zhang et al. 2020)].We are looking for the methods available for fabricating large scale area graphene films, chemical vapor deposition (CVD) stands out as a promising approach due to its rapid growth process and the ease of controlling growth parameters, leading to high-quality graphene films [ (Torres et al. 2014), (Xu et al. 2014), (Yazdi et al. 2016), (Zhou et al. 2013)].The first parameter is substrate that crucial factor for ensuring good film quality.Metal substrates like Ni, Pt, and Cu have been employed in CVD-based graphene growth, with Cu foils being particularly favored due to their low cost, ease of fabrication, and ability to produce high-quality graphene films [ (Rybin et al. 2018), (Qian et al. 2017), (Lee, K., and Ye, J. 2016)].Furthermore, growing graphene films on Cu foils via CVD enables the production of large-scale graphene with high quality.
However, a major challenge in growing graphene films on Cu foils via CVD is the occurrence of grain boundaries on the Cu foils surface due to the high-temperature graphene growth process.Large grain sizes are desired to minimize defects in graphene.Therefore, the Cu foils' surface is necessary to remove contamination by Pre-polishing and electropolishing processes, reduce surface roughness resulting from the manufacturing process, and address issues such as line defects and native oxide presence, while thermal annealing is utilized to reduce residual strain and dislocations in the foils (Zhan, L., Wang, Y., Chang, H. et al. 2018, Aranha et al. 2023, Bueno et al. 2023).Cyclohexane stands out as a liquid precursor with several benefits, including ease of flow rate control and the ability to produce monolayer graphene films (Gan et al. 2017).Additionally, nitrogen was utilized as a carrier gas, providing an inert atmosphere, stable flow, enhanced safety, and cost-effectiveness for the CVD process.
The advantage of LPCVD lies in its operation at low pressure, which allows for controlled precursor flow and homogeneous deposition.Compared to atmospheric pressure chemical vapor deposition (APCVD) and plasma-enhanced chemical vapor deposition (PECVD) methods (Wang et al. 2016, Woehrl et al. 2014), LPCVD offers better control over the growth process and results in high-quality graphene films.In the case of graphene synthesis, there are various precursors that have been investigated (Guermoune rt al. 2011, Yang et al. 2017).
In this study, our objectives were to prepare smooth Cu foils and investigate the impact of annealing temperature on graphene films.Using LPCVD with cyclohexane as the precursor and nitrogen as the carrier gas, we grew graphene films on polished copper foils.The Cu foils were optimized through pre-polishing and electropolishing techniques, confirmed for surface smoothness with optical microscopy.By varying the annealing temperature and graphene growth time, we optimized the graphene growth process.The resulting graphene films on Cu foils were characterized using X-ray diffraction and micro-Raman spectroscopy to assess quality and determine the number of layers.

EXPERIMENTS
Cu foil with a thickness of 25 µm and a high purity of 99.99%, which was procured from MTI Corporation, was used as a substrate for LPCVD growth of graphene.To prepare the Cu foils measuring 4 x 5 cm², the elimination of surface contaminants and reduction of roughness resulting from rolling lines that occurred during the fabrication process of Cu foil were achieved using metal polish liquid (Brasso).Subsequently, the Cu foils underwent a series of cleaning steps.They were immersed in acetone, ethanol, and deionized (DI) water for 3 minutes each using an ultrasonic bath.After blowing the foils with nitrogen gas, electropolishing (EP) was conducted.The electrolytes for EP were optimized using various concentrations (30%, 40%, 45%, 50%, and 60%) and durations (60s, 90s, and 120s) of a dilute 85% aqueous solution of phosphoric acid H3PO4 (QRëC).Following the EP process, the Cu foils were rinsed with DI water and then subjected to a 5-minute ultrasonic bath in DI water, acetone, and isopropyl alcohol (IPA).The foils were blown with nitrogen gas once more.The optimized EP conditions were determined to be 45% H3PO4 concentration with 2.5 V voltage and a 90s dipping time, as a result of smooth Cu foils ready for transfer into the quartz chamber of the CVD system.
Graphene growth on Cu foils was achieved through LPCVD using an Annealsys MC-050 machine equipped with a direct liquid injection (DLI) vaporizer for precise control of the precursor flow rate.The CVD system featured a 2-inch horizontal quartz tube, and the graphene growth experiments were conducted at a pressure of 2 mbar (1.5 Torr).Cyclohexane, C6H12 (QRëC) served as the precursor, while nitrogen (99.999%) gas acted as the carrier gas.
Prior to growth, the Cu foils were subjected to an annealing process in nitrogen gas flow at a fixed annealing time of 10 minutes, gradually increasing the annealing temperature at a rate of 1 ºC/s.The annealing temperatures ranged from 860 ºC to 940 ºC.Subsequently, the graphene was grown on the Cu foils using a cyclohexane concentration flow rate of 0.5 g/min, an operating frequency of 0.5 Hz, and a response time (tON) of 1 ms.The growth temperature was set at 920 ºC, and various growth times were employed.Finally, the temperature was cooled down to room temperature at a rate of 1 ºC/s.
Optical microscopy (Seek) was employed to assess the surface roughness of the copper foils after EP process.The surface images were captured using a UCM1300 USB2.0 Digital color camera equipped with a 1.3-megapixel CMOS sensor.X-ray diffractometer (Bruker AXS Model D8 Discover) was inspected to diffraction signal of x-ray after annealing process as well as growth process.The detector (VÅNTEC-1 Detector (Super Speed Detector)) was detected to an angle 20-100 degree, increment step by 0.02 degree/step under step tine 48.1 sec.Raman spectra were acquired using a Raman spectrometer (Renishaw) at room temperature, with laser excitation at a wavelength of 473 nm.The laser beam was focused to a diameter of less than 1 µm using a x50 objective lens.All Raman spectra were collected and processed using the WIRE TM 4.2 software.

RESULTS AND DISCUSSION
In order to prepare the Cu foils with a smooth surface for the graphene growth process, various polishing methods were employed.The EP process was conducted using different concentrations of phosphoric acid (H3PO4) ranging from 30% to 60%, with a dipping time of 90 seconds to optimize the surface roughness of the Cu foils.Optical images in Figure 1 illustrate the surface morphology of the Cu foils after EP treatment.Cu foils subjected to EP with 30% and 40% H3PO4 concentrations exhibited a rough surface with small grain sizes.
However, the Cu foils treated with a 40% H3PO4 concentration displayed smaller grain sizes and greater smooth surface compared to those treated with a 30% H3PO4 concentration.On the other hand, Cu foils treated with 40% and 50% H3PO4 concentrations showed minimal observation of small rolling lines, with only slight residual line scratches from the fabrication process.Among the EP processes using different H3PO4 concentrations, the Cu foil treated with a 50% H3PO4 concentration exhibited the smoothest surface.
However, it is important to note that using high H3PO4 concentrations for EP can pose issues with the Cu foil thickness.Specifically, when using a 60% H3PO4 concentration for a dipping time of 90 seconds, there were instances where the Cu foils broke after the EP process.
Therefore, caution must be exercised in selecting the appropriate H3PO4 concentration to balance surface roughness and foil integrity.

Figure 1
Optical images of copper foils after electropolishing with different concentrations of phosphoric acid (H3PO4) (30%, 40%, 50%, and 60%) for 90 seconds On the Other hand, it should be noted that the EP process alone may not completely eliminate all rolling lines from the surface of the Cu foils.Therefore, additional optimization of the polishing process is required to achieve truly smooth Cu foils for effective graphene growth.
This is because the surface roughness of the Cu foils significantly impacts the quality and properties of the resulting graphene films.
To achieve smoother Cu foil surfaces, a physical polishing process using Brasso solvent was employed prior to the EP process.The EP process was then adjusted using H3PO4 concentrations of 45% and 50%, with varying dipping times of 60, 90, and 120 seconds.Figure 7 2 presents optical images of Cu foils obtained from the EP process with 45% H3PO4 concentration, comparing the results with and without Brasso polishing after EP.
The findings revealed that pre-polishing the Cu foils with Brasso solvent before the EP process resulted in smoother surfaces for all dipping times.Furthermore, with a dipping time of 90 seconds, the rolling lines of the Cu foils decreased significantly.However, caution must be exercised with a dipping time of 120 seconds, as it can lead to excessive etching, potentially damaging the Cu foil.Additionally, when pre-polished Cu foils were subjected to the EP process with 50% H3PO4 concentration, similar rolling lines to that of 45% H3PO4 concentration was observed, but the Cu foils exhibited a thinner thickness.Based on these results, Cu foils that underwent pre-polishing and the EP process with 45% H3PO4 concentration for a dipping time of 90 seconds were chosen as the optimal substrate for graphene growth in this study.The integration of pre-polishing and EP processes led to enhanced surface smoothness, a critical factor in facilitating the growth of high-quality graphene films on Cu foils.These findings underscore the importance of meticulous substrate preparation to achieve desirable surface characteristics, ultimately enhancing the quality and properties of the resultant.8 The X-ray diffractions were obtained for the graphene samples immediately after copper foil annealing, revealing different intensities with increasing annealing ranging from 860 to 940°C in a N2 atmosphere for 10 minutes revealed XRD diffraction peaks corresponding to Cu(111), Cu(200), Cu(220), and Cu(311), indicating a polycrystalline structure.The impact of the annealing process on graphene films grown using LPCVD with a cyclohexane flow rate of 0.5 g/min and a constant growth temperature of 920 °C for 6 minutes was investigated like-signal diffraction peaks, as show in Figure 3a and b.
After the growth of graphene, the same five diffraction peaks remained present, indicating a Cu(111) orientation at 860, 880 and 920 °C has changed that annealing temperature at 880 and 920 °C has been increasing.Additionally, at 920 °C, a Cu(222) orientation was obtained, suggesting a family number of (111) Cu plane.This annealing temperature may be optimized for producing template hexagonal structure for graphene films.By the way, at 860 °C shown to a decreasing of diffraction peak.
The graphene films were grown on Cu foils using LPCVD with cyclohexane as the precursor and nitrogen as the carrier gas.Precise control of the cyclohexane flow rate was achieved using a direct liquid injection vaporizer.The effect of annealing temperature on graphene film quality was investigated, while the annealing time for all samples remained fixed at 10 minutes.Micro Raman spectroscopy was employed at room temperature, utilizing a 473nm excitation laser wavelength, to analyze the samples.The intensity of the dominant Raman peaks at the D, G, and 2D bands was measured to assess the impact of annealing temperature for a fixed annealing time of 10 minutes.The D peak intensity is related to defects in carbon materials, while the G peak corresponds to in-plane vibrational modes of sp 2 carbon atoms.The 2D band reflects the stacking order of graphene layers, and the intensity ratio of the 2D peak to the G peak (I2D/IG) indicates the number of graphene layers.A value above 2.0 suggests a monolayer, while a value below 2.0 indicates a few layers or multilayers.The intensity ratio of the D peak to the G peak (ID/IG) provides information about disorder or defects in graphene (Ferrari et al. 2018).Within the annealing temperature range of 880 ºC to 900 ºC, the intensity ratio of I2D/IG exceeded 2.0, indicating the presence of monolayer graphene films.At other annealing temperatures, the intensity ratio of I2D/IG suggested the formation of graphene with few layers or multiple layers.However, the intensity ratio of ID/IG, which remained below 1.0, was not significantly influenced by the annealing temperature prior to graphene growth.This observation may be attributed to defect formation or disorder occurring during the nucleation of graphene.Thus, the annealing temperature plays a crucial role in controlling the preparation of Cu foil surfaces for the growth of monolayer graphene films using LPCVD.12 These results indicate that a substantial difference between the annealing temperature and growth temperature can result in the production of multilayer graphene films, as indicated by an I2D/IG ratio lower than 2.0 after 4 minutes of growth.However, at shorter growth times, approximately 1-2 minutes, the characteristics of the graphene films seemed to exhibit greater variability.It is important to note that these results were obtained using a cyclohexane precursor with a flow rate of 0.5 g/min, which likely resulted in a higher growth rate and increased likelihood of obtaining few layers of graphene.

Figure 6
Intensity ratio of 2D to G peak in graphene films on Cu foils as a function of growth time (1 to

minutes) at different annealing temperatures
To optimize the number of graphene layers, the flow rate of the cyclohexane precursor was carefully adjusted.It is known that a higher flow rate can lead to the production of graphene films with multiple layers and a higher density of defects (Pham et al. 2019).In order to investigate this issue, graphene films were grown at a growth temperature of 920 ºC for 10 minutes, with an annealing temperature of 900 ºC for 10 minutes, to observe the effect of cyclohexane flow rate.The flow rates of cyclohexane were varied from 0.5 to 0.1 g/min.Since lower flow rates are expected to result in a slower growth rate, a longer growth time of 10 minutes was chosen for this study.

Figure 2
Figure 2 Optical images of Cu foils obtained from the electropolishing (EP) process with 45% H3PO4 concentration, comparing the results with and without Brasso polishing after EP, showing different dipping times of 60, 90, and 120 seconds

Figure 3 (
Figure 3 (a) Diffraction X-ray of Cu foil annealed from thermal temperature 860-940  C in N2 atmosphere (b) Diffraction X-ray of graphene on the Cu foil annealed from thermal temperature 860-940  C with cyclohexane carbon source under 920  C growth temperature in N2 atmosphere

Figure 4 (Figure 5
Figure 4 (a) Optical image of graphene film on Cu foil illustrating Raman spot measurement positions (1-red solid lines and 2-blue dotted lines) and a table displaying the temperature and time duration of the annealing and graphene growth.(b) Normalized Raman spectra of graphene obtained at positions 1 and 2

ImprovingFigure 6
Figure6presents the intensity ratio of I2D/IG for graphene films grown at a constant growth temperature of 920 ºC.The growth time varied from 1 to 6 minutes, and different annealing temperatures ranging from 860 ºC to 940 ºC were applied for a fixed duration of 10 minutes.The purpose was to observe the impact of growth time on the layer's amount of graphene.The results revealed distinct trends in the intensity ratio of I2D/IG for different annealing temperatures.Annealing temperatures of 860 ºC and 940 ºC displayed a decreasing trend as the growth time extended, indicating a reduction in the monolayer character of the graphene films.Conversely, annealing temperatures of 880 ºC, 900 ºC, and 920 ºC exhibited an increasing trend in the intensity ratio of I2D/IG with longer growth times.Notably, at an annealing temperature of 900 ºC, the graphene films appeared to be the most stable in terms of maintaining a monolayer structure compared to the other temperatures.This stability can be attributed to the direct influence of annealing temperature on the grain size and crystal orientation of the Cu foil surface.

Figure 5
Figure 5Intensity ratios of I2D/IG and ID/IG for graphene films on Cu foils at various annealing temperatures Figure7(a) demonstrates that reducing the cyclohexane flow rate can improve the number of layers and the quality of the grown graphene films.The full width at half maximum (FWHM) decreased from 59.67 to 46.49 cm -1 , indicating improved crystallinity, and the number of graphene layers was adjusted from multilayers (I2D/IG = 0.97) to a monolayer (I2D/IG = 2.79) when the cyclohexane flow rate was reduced from 0.5 to 0.1 g/min.However, the intensity ratio of ID/IG, which is related to defects, showed a less pronounced effect with varying cyclohexane flow rate, consistently remaining below 1.0.The optical images of graphene films on Cu foils with different cyclohexane flow rates are presented in Figure7(b).Higher cyclohexane flow rates resulted in a dark brown color, while lower flow rates produced a light brown color.The optical observations, combined with the Raman scattering results, indicate that reducing the cyclohexane flow rate is highly effective in controlling the number of graphene layers.Unfortunately, our study did not confirm a reduction in defect formation during graphene growth, as the intensity ratio of ID/IG consistently remained below 1.0 and showed minimal influence from the synthesis process.

Figure 7 (
Figure 7 (a) Raman spectra of graphene films on Cu foils at various flow rates (0.1, 0.3 and 0.5 g/min) of cyclohexane.(b) Optical images of graphene films on Cu foils corresponding to different flow rates (0.1, 0.3 and 0.5 g/min) of cyclohexane Improving Quality of Graphene Grown on the Copper Foil by Physical Polishing Electropolishing and Thermal Annealing Pretreatment Processes by Low Pressure Chemical Vapor Deposition ___________________________________________________________________________ Rev. Gest.Soc.Ambient.| Miami | v.18.n.3 | p.1-17 | e07042 | 2024.