Synthesis and characterization of CVD-grown graphene on copper: influence of the synthesis conditions

Graphene, an extraordinary two dimensional carbon material with a honeycomb structure, has been the focus of many researches due to its characteristics and its extraordinary mechanical, electronic and optical properties. It can be synthesized by a variety of methods such as discharge, epitaxial growth on SiC, unzipping carbon nanotubes, reduction of CO, chemical conversion, self assembly of surfactant and Chemical Vapor Deposition (CVD)[1]. Among them, CVD method has been shown to produce large-area and high quality graphene [2]. In CVD, Ni and Cu are normally used as substrates although other transition metals are also used, but less frequently[3]. Raman spectroscopy has been shown to be the most potential method for non-destructive and quick structural and electronic characterization of graphene and graphite materials[4]. In the present work, different studies have been performed with the goal of optimizing CVD synthesis of graphene on copper. In order to achieve that objective, the influence of different synthesis conditions such as temperature (900-1050 oC), reaction time (5-45 min), CH4/H2 ratio (3%-45%) and total flow rate of gases involved in the reaction (40-140 ml/min) have been studied. Raman spectroscopy was used to characterize the obtained products. Influence of the reaction time at different synthesis temperatures. Table 1 shows the most important characterization parameters obtained with RAMAN spectroscopy for the best reaction temperature (1050oC) and the influence of different temperatures at the same reaction time. In general, ID/IG ratio (related with the amount of defects in the sample) tended to increase and, I2D/IG ratio (related to the number of graphene layers) tended to decrease with the reaction time at 1050oC and with the increase of temperature at the same reaction time. By its part, FWHM (Full Width at Half Maximum) parameter varied between 62 and 76 for 1050oC and varied between 56 and 67 for the synthesis performed at different temperatures for 25 minutes reaction time. 2D position in the RAMAN spectra is considered an important parameter in graphene characterization because, in the case of graphene, this peak should be displaced to values around 2700 cm -1 if compared with graphite, which is around 2740 cm -1 [5]. In Figure 1 it could be observed the optimum sample of the study, obtained at 1050oC and 10 minutes reaction time, observing that the defects in the sample are very low. 2D peak deconvulation of sample synthesized at 1050 oC during 10 minutes originated four different peaks which is a common feature of bilayer graphene [6]. According to the obtained results, a synthesis temperature of 1050 oC and a reaction time of 10 minutes were selected as the optimum values to carry out the following experiments. Influence of the CH4/H2 ratio. Figure 2 shows RAMAN spectrum and, Table 2 shows the most important RAMAN parameters obtained in the study carried out varying the CH4/H2 ratio in the range 3-45%. As observed, D peak increased and 2D peak even disappeared with the increase of CH4/H2 value. According to the obtained results, the ID/IG and I2D/IG ratio values were the lowest and the highest respectively for a CH4/H2 ratio value of 7%, indicating that the obtained graphene had the lowest amount of both, defects and layers. FWHM value was for this sample of 61 cm -1 and, the 2D peak appeared at 2706 cm1 which was in the range generally accepted for graphene. Again, the 2D peak deconvolution showed the presence of the 4 different peaks characteristics in bilayer graphene [6]. Influence of the total gas flow rate. Figure 3 shows RAMAN spectrum and, Table 3 shows the most important RAMAN parameters obtained in the study carried out varying the total gas flow rate in the range 40-140 ml/min. As observed, D peak was practically non-existing for a total flow rate of 60 ml/min and, 2D peak position was displaced to better positions (2706 cm-1) As observed, the lowest ID/IG ratio value was obtained using a total flow rate of 60 ml/min although the I2D/IG ratio was not the higher one. The values obtained for the parameters FWHM and 2D position were not determinant in this study due to they were maintained practically constant.