Effect of copper pretreatment on optical and electrical properties of camphor-based graphene by chemical vapour deposition

Polycrystalline copper (Cu) foil is widely used as catalytic substrate for graphene growth in chemical vapor deposition (CVD) technique. The surface properties of the Cu foil strongly affect the growth behavior and final quality of CVD-grown graphene. The effect of pretreatment of Cu foil using four different solutions (acetone, acetic acid, HCl and HNO3) on the graphene growth held in atmospheric pressure CVD and its subsequent impact on electrical and optical properties are investigated. Natural camphor is used as the solid carbon precursor. The surface characteristics before and after the growth are studied using scanning electron microscopy and atomic force microscopy. The pretreatment conditions of Cu and the growth of graphene from camphor were correlated using Raman spectroscopy, optical and electrical characteristics. Our findings suggest that HCl-pretreated Cu foil exhibited large domain, uniform coverage of the transferred graphene with excellent optical (> 93% at 550 nm) and electrical properties (sheet resistance of 861 ± 40 Ω/sq), with promisingly low RMS value of roughness (38 nm). The pretreatment process improved the quality of graphene by removing the surface impurity particles and surface native oxides. A Schottky junction diode of graphene/n-silicon is fabricated by transferring the graphene to SiO2/Si substrate under dark and illuminated conditions is also demonstrated to establish its potential in micro- and opto-electronics.


Introduction
Single-Layer Graphene (SLG), a one-atom-thick sheet of hexagonally arrayed sp 2 -bonded carbon atoms, has received much attention due to its interesting electrical, mechanical, optical, and thermal properties [1][2][3][4][5][6]. Electronic and optoelectronic applications, such as transparent conducting electrode, solar cells, photodetectors and field-effect transistors, demand graphene film with very high-quality and large domain size in a larger area [1,7,8]. The graphene can be produced by various methods such as micromechanical cleavage [1], chemical vapor deposition (CVD) [9,10], epitaxial growth on Silicon Carbide (SiC) [11] and reduction of graphene oxide [12,13]. Among these techniques, CVD method is emerged as a simple, scalable and straight forward for high-quality, large-area graphene. In this technique, the carbon precursors are decomposed on the catalytic metal surface such as Cu, Ni, Pt and Ir [9,10,14,15]. The resulting graphene on metal substrate can be easily transferred to any desirable substrates by employing simple transferring process without affecting much of the properties of the graphene. Gaseous hydrocarbon like CH 4 , C 2 H 4 , or C 2 H 2 is introduced to a chamber and dissociated on metal surfaces (Cu, Pt or Ni) at elevated temperature to form active carbon species, which constitute the long chain of graphene [10,16,17]. Moreover, liquid carbon sources, like benzene, toluene, methanol, ethanol, propanol and pentane have been used for highquality graphene growth [18,19]. Recently, solid carbon source such as camphor has taken much attraction for the synthesis to form a large-area monolayer graphene by very facile and novel approach [20][21][22][23]. Kalita et al. has performed the growth of mono/bi-layer graphene by employing the camphor as a carbon source on different metal substrates [20][21][22][23][24].
Polycrystalline Cu foils are most widely used catalyst in CVD technique to grow monolayer graphene sheets due to its low-cost, scalability, and compatibility with roll-to-roll processing [25,26]. The electrical and physical properties of CVD-grown graphene sheets are limited by polycrystalline nature and smaller grain size of Cu foil. It is well-known that Cu is having low carbon solubility and thus the graphene growth is surface-catalyzed process which limits to monolayer graphene [27,28]. We understood that the surface properties of Cu foil are very important as it strongly influence the growth of graphene [29,30]. In particular, the graphene growth can be obstructing for instance due to Cu surface roughness, contamination, crystallographic orientations and grain size. These properties can influence the nucleation density and growth rate of the graphene or control the alignment of graphene domains [10,26,31]. As received Cu foils are known to have surface contamination particles, striation due to the cold-rolling process during manufacturing and native oxide on the surface. Also, some of the Cu foils are supplied with oxidative protective coating. These impurities and surface defects are reported to have significant effect on the final properties of graphene by acting as hetero-nucleation centers. The Cu foil pretreatment is prerequisite for preparing highquality graphene by CVD technique [32][33][34][35][36][37][38][39][40][41]. Different pretreatment procedures for Cu foils are reported; among them (1) acetic acid (CH 3 COOH) [42], (2) dilute HCl [37] (3) dilute HNO 3 [32], (4) ferric chloride [34], (5) KOH [36], (6) potassium peroxodisulfate [33] and (7) electropolishing [43] are explored.
Despite of significant efforts made in recent years, the growth of high-quality CVD-graphene from camphor and its applications are not addressed with respect to the pretreatment effect on Cu foil surfaces. In the present work, we have critically addressed a systematic analysis of the surface roughness and morphology of the pretreated Cu foil for the growth of graphene from camphor. Atmospheric pressure CVD (APCVD) is used to grow the graphene and corroborated the optical and electrical properties with different pretreated Cu foil in detail. A graphene/n-Si schottky junction photodiode by applying the optimally grown graphene is also fabricated in above mentioned scheme and demonstrated its device potential.

Experimental section 2.1 Pretreatment of copper foil
The 25 lm copper foil (13,382) from Alfa Aaser, USA was used as a metal substrate for graphene growth. The Cu foil was cut into the dimensions of 2.5 cm 9 2.5 cm. Then, they were cleaned by different cleaning procedures. The detailed pretreatment procedures for the Cu foils are given as follows: (1) The Cu foil was ultrasonically cleaned in acetone for 10 min. (2) Cu foil was cleaned with acetone, DI water and then immersed in glacial acetic acid at room temperature for 10 min. After that the Cu foil was rinsed with DI water three times and finally washed with isopropanol (IPA). (3) Cu foil was cleaned with acetone, DI water and then immersed in 2:1 ratio of DI water:HCl at room temperature for 5 min. Hereafter, the Cu foil was rinsed with DI water three times and finally washed with isopropanol (IPA). (4) Cu foil was cleaned with acetone, DI water and then immersed in 5 wt% nitric acid at room temperature for 30 s. Finally, Cu foil was rinsed with DI water three times and washed with isopropanol (IPA). At the final stage, Cu foils were dried under the stream of Ar gas before loading into the furnace.

Synthesis of graphene by atmospheric pressure chemical vapor deposition (APCVD)
A quartz tube with a diameter of 55 mm and length of 1250 mm was placed in a horizontal split furnace. The pretreated Cu foils were loaded into the quartz tube and placed in the high temperature zone. The Cu foil was kept * 10°of inclination position, facing towards the gas flow. The camphor was placed in a quartz boat and kept at an optimized distance from the heating zone in order to avoid the unwanted sublimation during pre-annealing and annealing step. The Cu foil was heated from room temperature to 1020°C at the heating rate of * 15°C/min under the flow of 200 sccm high purity Ar flow followed by annealing for 15 min under the flow of100 sccm H 2 . For growth of graphene, the gas composition was changed to Ar:H 2 in 98:2 ratio. The camphor was evaporated at the heating rate of 10°C/min to 200°C. The total growth time was nearly 10-15 min. After the growth, the operation was switched off and chamber was allowed to cool naturally under the flow of Ar and H 2 gas.

Transfer of graphene to SiO 2 /Si and glass substrates
The graphene covered Cu was coated with PMMA by spin coating at 3000 rpm for 60 s and 1000 rpm for 60 s, followed by baking of PMMA/graphene/copper at 180°C for 2 min. Then the copper was etched by placing the samples over 0.5 M (NH 4 ) 2 S 2 O 8 for 6 h. The graphene grown on the back side of the Cu was removed by washing under the stream of DI water and placed again on the etching solution. The PMMA/graphene was repeatedly cleaned in DI water for 5 to 6 times. Then the PMMA/graphene was transferred on to the glass or SiO 2 /Si substrates. The substrates were kept overnight for the complete adhesion to the substrate. Finally, the PMMA layer was removed by dipping in acetone for 30 min and after that repeatedly washed with DI water and IPA.

Characterization
FE-SEM analysis was carried out using a SEM microscope (Zeiss, Ultra-55) using 5.0 kV acceleration voltage. The elemental composition of the pretreated Cu foils was investigated using X-ray photoelectron spectroscope (XPS, PHI5000 VersaProbe II, ULVAC-PHI, INC, USA) with a monochromated Al-Ka X-ray source operating at 14 kV and 220 W. The XPS data were processed using PHI's Multipak software. The binding energy was referenced to the C 1 s peak at 284.8 eV. The sheet resistance of all samples was measured using a four-point resistance meter with silver paste as the contact pad. Optical transmittance of graphene films was measured using a UV-Vis spectrophotometer (UV-2600, Shimadzu). Raman measurements were carried out to study the identification of graphene using a 532 nm laser source with a Micro Raman microscope (inVia, Renishaw, UK).
The measurements were carried out on graphene transferred on to SiO 2 /Si substrate. The laser power used was 2.5 mW, with accumulation of 30 s.

Graphene/n-silicon schottky junction diode
In order to establish the device potential of the graphene, simple schottky diode was fabricated using the graphene as metal with n-type silicon as the semiconductor counterpart. The device was fabricated as follows. A 150 nm SiO 2 was coated on n-Si (100) (1 cm 9 1 cm pieces) by thermally annealing the substrates at 1000°C for 4 h under O 2 atmosphere. The buffer oxide etch solution (BOE, NH 4 F/ HF; 6:1) was used for SiO 2 etching and to expose the n-Si with a window size of 0.3 cm 9 0.3 cm (* 0.09 cm 2 ). The wafers were then repeatedly cleaned with DI water. Immediately, PMMA-coated graphene was transferred on to the SiO 2 -coated n-Si on to the active area window. The PMMA was removed by acetone. The contacts were made using silver paste on front side connected with SiO 2 /graphene and similarly on the back side of the n-Si. In this configuration, graphene sheet serves as the metal semi-transparent upper electrode and the antireflection layer for schottky junction. The dark and light behavior of the schottky diode was tested under the solar simulator integrated with a source-measure unit (Agilent) by applying the bias of -5 to ? 5 V.
3 Results and discussion

Effect of pretreatment on the Cu foil
The quality of graphene strongly depends on the nucleation process which gets affected by the surface properties of Cu foils [36,39,43]. Thus, the Cu foil cleaning process is very important to get reproducible and high-quality large domain graphene because the as received Cu foils usually contains contamination particles, native oxides, protective coating, striation lines and surface defects [35,39]. The SEM images of the acetone, AA, HCl and HNO 3pretreated Cu foils are shown in Fig. 1a-d. The acetone-treated foil contains surface inhomogeneity and striation lines which are usually formed during the rolling of the Cu foils. On the other hand, AA, HCl and HNO 3 -pretreated Cu foils have shown a reduction in the surface inhomogeneity and impurity particles. Among these pretreatment conditions, HCltreated Cu foils exhibited smooth surface. Acetone and IPA cleaning is not sufficient for the removal of the particles, even though this step is important in order to get rid of the organic containments on the Cu foils. The particles are partially removed from the Cu foil when it is treated with AA. However, complete removal of the particles is required to get the highquality graphene. It is seen that, HCl and HNO 3 pretreatments remove the particle contamination by more than 95% from the Cu foil. Further we have carried out AFM analysis to understand the Cu topology after different pretreatment conditions (Fig. 1e-h). The influence of the pretreatment process with different pretreatment conditions can be seen from the RMS values calculated from the AFM analysis. The RMS values of the roughness estimated from the AFM analysis for the acetone-pretreated Cu foil was found to be 96 nm. The acetone is expected to remove any organic contaminants on the Cu foil but it does not reduce the protective coating, striation lines and oxide layers. The calculated RMS values of the Cu foils pretreated with AA, HCl and HNO 3 was found to reduce to 60, 38 and 45 nm, respectively. Moreover, the acid pretreatment process reduces the striation lines and removes the surface oxide layers. The HCl pretreatment reduced the RMS value to 38 nm. A slight increase in the RMS value in the HNO 3 -treated Cu foil was observed which was due to the formation of pits and it was confirmed from the high magnification SEM images. This clearly shows the reduction in the RMS values because of the treatment process which will modify the graphene nucleation process.
In order to identify the effect of pretreatment on Cu foils, we carried out XPS analysis on all pretreated samples. Figure 2a shows the XPS survey spectra of the all samples which reveal the presence of Cu, C and O. Apart from these elements, we can clearly see that the acetone cleaned Cu foils contains various other elements such as P, Ca, Mg, Fe and Si. In general, when compared to the acetone-treated Cu foils, other acid treatments resulted in the reduction of the impurities in the Cu foils. Importantly, we observed that the Cu foil pretreated with HCl resulted in the removal of the almost all the impurity particles. Figure 2b shows the magnified region between 150 and 60 eV. From Fig. 2a, which reveals the treatment effectively removed the Si and P. This study confirms the effectiveness of the pretreatment protocols that has been adopted for the complete removal of impurities from the Cu foils. Especially, HCl pretreatment leads to the best results by reducing the surface impurities, thereby, decreasing the nucleation sites which helps in the growth of the uniform graphene over the Cu foil.

APCVD graphene growth
Camphor is a botanical origin, low-cost, solid carbon precursors with the molecular structure consisting of hexagonal and pentagonal rings fused together. When camphor is evaporated slowly around 200°C and pushed to the growth region near the Cu substrate, the camphor molecules get dissociated to the smaller carbon fragments and deposited on the Cu substrate. Subsequently, the carbon nucleates and starts to grow from the further supply from the carbon precursor into 2D graphene. The camphor molecules decomposed on the surface of the Cu foil and the active carbon species forms the nucleation and further supply leads to the uniform growth of the graphene. The pretreated Cu foils were loaded inside the quartz tube furnace and graphene was grown as per the procedure mentioned in the experimental section. Figure 3 shows the low and high magnification SEM images of the graphene-coated Cu foils at different pretreatment conditions. Low magnification images of Cu foils in all pretreatments show the rolling lines even after annealing and graphene growth. HCl-pretreated Cu foil showed large graphene domains when compared to other pretreated Cu foils. This clearly revealed that the nucleation density of HCl-pretreated Cu foil is reduced and thus leads to the growth of larger graphene domain. The high magnification image of graphene grown on acetone-pretreated Cu foil, as displayed in Fig. 3a and e. The graphene does not grow uniformly on the Cu foil after acetone pretreatment. Figure 3f-h shows graphene grown on AA, HCl and HNO 3 -pretreated Cu foils where we can see the reduction in impurity particles. However, HCl-pretreated graphene/Cu foil showed reduced roughness and uniform coverage of the graphene. AA and HNO 3 -pretreated graphene/ Cu foils showed rougher surfaces which is due toan increase in the nucleation density on the Cu foils.
In order to observe the coverage of graphene on the Cu foils with respect to the pre-treatment conditions, we oxidized the graphene/Cu foils for 10 min at 200°C in a muffle furnace. The direct optical observation on the evidence of graphene coverage is possible in this approach as the graphene-coated Cu foil remains protected whereas the exposed Cu foil get oxidized and displays reddish color. Figure 4 shows the digital photograph of oxidized graphene on different pretreated Cu foils. We can see clearly the difference in the graphene coverage with respect to the Cu pretreatment. Acetone and AA pretreated Cu foils show non-uniform graphene coverage. The HClpretreated Cu foil shows excellent protection towards the oxidation whereas HNO 3 -pretreated sample shows small grains and incomplete protection.

Properties of transferred graphene
The APCVD graphene were transferred by PMMAassisted technique to the glass and * 150 nm SiO 2 / Si substrate. The SEM micrographs of the graphene on SiO 2 /Si substrate are shown in Figure S1. Raman spectroscopy is a very useful technique to study the quality of the CVD-graphene [44,45]. It provides the information on number of layers and defect structure in the CVD-graphene. Figure 6a shows the Raman spectra of the graphene transferred to SiO 2 /Si substrate grown at various pretreated Cu foils. The Raman spectrum of the graphene exhibits peaks at * 2700 cm -1 (2D peak), * 1580 cm -1 (G peak) and * 1350 cm -1 (D peak) for all the variations, respectively. The intensity ratios of 2D to G (I 2D /I G ) and D to G (I D / I G ) provide information on the number of graphene layers and defect structure, respectively, as graphically described in Fig. 6b. The value of I 2D /I G greater than 1 generally shows the single-layer graphene, value around 1 is bi-layer and value less than 1 shows the multi-layer in the CVDgraphene [44]. Also, increase in the I D /I G ratio shows the increase in the defect in the graphene layer. In our case, an optimum formation of mono-bi-layer graphene with almost no defect states of D peak is observed only in HCl-treated Cu foil. It is to be noted that, Raman analysis has been carried out at various regions of the as-grown graphene-coated substrates.
The UV-visible transmittance and sheet resistance have been studied to understand the effect of Cu foil pretreatment conditions on the graphene on its optical and electrical properties, respectively. Figure 7a and b shows the UV-visible transmittance spectra and sheet resistance of the graphene grown on the Cu foils pretreated at different conditions. The effect of the Cu pretreatment is having a strong influence on the transmittance and electrical properties of the graphene. The acetone and AA pretreated Cu foil derived graphene show low transmittance which may be due to the formation of the multilayer on the Cu foils. We have observed that the graphene grown on HCl and HNO 3 -pretreated Cu foils show high  Fig. 7). Moreover, the sheet resistance of the graphene from the acetone and AA-pretreated Cu foils shows 1571 ± 37 and 1197 ± 23 X/sq, respectively. The HCl and HNO 3 samples show the value of 861 ± 40 and 1044 ± 29 X/sq, respectively. The observed behavior can be explained by the formation of multilayer and domains structure of the graphene layer due to different nucleation densities as reported in our previous article [46]. Recently, Chamoli et al. and his co-workers have developed low defect density graphene nanosheets (GNs) have via chemical reduction of exfoliated graphite (EG) in the presence of a green reducing agent for transparent conductive film [47,48]. They have achieved the transmittance value of * 71.5% and sheet resistance of * 1KX/sq. Comparing both the results, we have achieved a very  high values of transmittance and moderate sheet resistance values. This shows that the Cu pretreatment helps to remove the native copper oxide and impurity particles on the Cu foil.
Finally, we have fabricated and characterized the graphene/n-Si schottky junction diode using graphene grown on HCl-pretreated Cu foil because it showed excellent transmittance and optimum sheet resistance values. Schottky diodes are formed by making contact between the metal and semiconductor [49]. These diodes are nowadays providing a platform for a variety of optoelectronic devices such as solar cells, photodetectors, RF attenuators and chemical sensors. [51][52][53]. Graphene/semiconductor schottky junction diodes are widely studied because the Fermi-level of the graphene can be varied with the applied potential [53,54]. Figure 8a shows a schematic diagram of the fabricated graphene/metal device on SiO 2 /Si substrate with Ag as contact electrode. The J-V characteristic of the as chosen HClpre-treated Cu foil derived camphor-based graphene is investigated. The device showed the rectifying behavior which can be seen from the current-voltage characteristics. The pretreated Gr/n-Si shows a dramatic change in the rectification behavior of 5.5 lA/ cm 2 at -0.5 V, which is found to be more than the as-grown Gr/n-Si device with J SC B 5 lA/cm 2 reported in our previous article [55]. Table 1 demonstrates the performances parameters of various devices developed under different conditions to deliver a prominent currant different desired bias voltages. It is observed that, at V b = 0 V, the J SC values are not as high as it is expected for all the devices. Our present work also shows a similar results when the Cu foils are pretreated with HCl. The Ag electrodes make an ohmic contact to the graphene, while the n-Si/graphene interface forms a Schottky barrier. The typical dark and light J-V characteristic of HCl-treated graphene/n-Si heterojunction device is shown in Fig. 8b. The mono/bi-layer graphene on the top of n-Si demonstrates the photodiode like behavior for the fabricated device. The increase in the photocurrent under reverse bias condition has been observed under light due to large number of accessible states of collection of holes in Si that inject into the graphene [54]. In addition, the inset shows the light chopped J-V curve under reverse bias condition for V bias typically from -5 to 0 V. The excellent increment in the photocurrent with increase in the bias voltage has been observed for a typical photodiode with a very high sensitivity. Thus, the J-V characteristic and the rectifying nature confirms the formation of a Schottky junction at the graphene/n-Si.

Conclusion
In summary, we have investigated different pretreatment conditions of Cu foils, such as acetone, AA, HCl and HNO 3 on the graphene growth and final properties of graphene grown from APCVD technique using camphor as a solid carbon precursor. Our results suggested that the HCl pretreatment improved the surface properties of the Cu foil which reduces the graphene nucleation density and thus the large domain can be obtained. SEM and AFM studies revealed that the Cu foil pretreated with HCl shown better smoothness when compared to other pretreatment conditions. It was found that the reduction in the surface impurity particles and RMS after HCl pretreatment. The oxidation study showed the uniform coverage of the graphene on the Cu foil obtained from HCl pretreatment. The graphene transferred on to the SiO 2 /Si studied using Raman spectroscopy showed that HCl-pretreated Cu foil sheet resistance of graphene grown on pretreated Cu foils transferred on to glass substrate exhibited I 2D /I G ratio of 1.35 and I D /I G ratio of 0.05 which showed the high-quality of graphene grown from camphor. The optical transmittance of * 91.5% and sheet resistance of 861 ± 40 X/sq were achieved using HCl-pretreated Cu foil. Further, we transferred the graphene on to the SiO 2 /Si and investigated the graphene/n-Si schottky diode characteristics.