Conference-ISSS-7-Growth Characteristics of Graphene Film by Chemical Vapor Deposition Method Using Nozzle Gas Injection

Graphene has attracted much attention due to its many unique properties and potential applications. It is considered that a chemical vapor deposition (CVD) method is a promising for growth method of graphene. We have examined growth characteristics of graphene by means of a low-pressure alcohol catalytic CVD (LP-ACCVD) method. Here nozzle gas injection is used to supply carbon source gas to a substrate. This method enables efficient supply of carbon source gas to a substrate for the growth of graphene. Therefore, an efficient growth and lowering of the growth temperature of graphene is expected. [DOI: 10.1380/ejssnt.2015.265]


INTRODUCTION
Graphene, a two-dimensional material composed of carbon atoms in a honeycomb lattice, has recently attracted much attention because of its many unique properties, such as large specific surface area [1], high charge carrier mobility [2] and high thermal conductivity [3].Several methods for synthesis of graphene have been proposed [4][5][6].Among those methods, a chemical vapor deposition (CVD) method is the most promising one [7].However controllability of the graphene growth by the CVD methods, e.g.control of the number of layer, enlargement of the domain size, enhancement of growth rate, are not sufficient at present and are still to be improved.
In this study, a low-pressure alcohol catalyst CVD (LP-ACCVD) method was used for the graphene growth.This method enables prompt removal of by-product that is produced by the decomposition of precursor.The by-product inhibits the growth of graphene and its prompt elimination is desirable for the efficient graphene growth.We have confirmed that a nozzle gas injection LP-ACCVD method, which uses a nozzle with small diameter (< 1 mm) for efficient supply of the precursor to a substrate, enables efficient growth of single-wall carbon nanotubes [8].In addition, iron (Fe) was used as a catalyst film.In CVD growth of graphene, copper or nickel is usually used as a catalyst layer [9].On the other hand, it is considered that efficient graphene growth is possible by Fe catalyst layer owing to its high solubility of carbon [10].We have examined the growth characteristics of graphene in the condition of different growth temperatures by the LP-ACCVD method, using ethanol gas as a carbon source and Fe as a catalyst layer.

II. EXPERIMENTAL
P-type Si (100) wafers with SiO 2 layer (300 nm in thickness) were used as substrates for the graphene growths.
The catalyst film was deposited onto the substrate by vacuum evaporation method.Fe film of 60 nm in thickness was used as a catalyst layer.The substrate after the catalyst deposition was introduced into a cold-wall type high vacuum chamber shown in Fig. 1.This chamber was evacuated by the turbo molecular pump and the base pressure was < 1 × 10 −4 Pa.The substrate was placed on the substrate heater and the carbon source gas (ethanol) was irradiated onto the substrate via the stainless steel nozzle with a 0.50 mm inner diameter.The CVD process was carried out with the process timing chart shown in Fig. 2. Then the temperature was raised to growth temperature of 750 • C. The irradiation of ethanol gas to the substrate carried out in a pressure of 0.5 Pa for 30 min.After the irradiation, the temperature of the substrate was lowered with a constant cooling rate of 50 • C/min.
The graphene grown in this study was characterized by an optical microscope (OM) and Raman scattering spectrometer.The excitation wavelength and the power of the laser were 532 nm and 2.5 mW, respectively.The spot size of the laser on the sample was about 1 µm.

III. RESULTS AND DISCUSSION
The dependence of the growth of the graphene on the CVD temperature was first examined.Here the CVD processes were carried out at various the temperatures between 600 • C and 800 • C and the growth characters of the graphene were examined.Figure 3(a) shows Raman spectra on the sample substrates after the CVD processes.The Raman spectra were acquired on randomly selected twelve different positions on the sample substrates.The spectra shown here are the representative ones on the samples.In these spectra, D, G and 2D band peaks are confirmed.The peaks observed at ∼ 1350 cm −1 originate from defects in the graphene and called D-band, and the peaks observed at ∼ 1590 cm −1 originate from a graphitic structure with sp 2 carbon bond and called G-band.The peaks appear at ∼ 2700 cm −1 , called 2D-band, is also a specific one to the graphene structure [11].It can be confirmed that the peak intensity of G-band depends on the CVD temperature and decreases at the higher CVD temperature.The D-band peaks are also found in the spectra.Those intensities are large at the lower temperature and decreases as the temperature increases.Enlarged 2D band peaks are shown in Fig. 3  sitions on the sample surface.The CVD temperature of 750 • C shows the lowest Raman shift (2697 ± 3 cm −1 ) and the smallest FWHM (32 ± 6 cm −1 ).These value increases at the higher and lower CVD temperature than 750 • C.This indicates that the CVD temperature of 750 • C gives the smallest number of the graphene layer on the surface [12]. of Surface Science and Nanotechnology Volume 13 (2015) where L a is domain size of graphene and λ laser is the wavelength of the laser for excitation.Thus, the domain size of the graphene can be estimated from the intensity ratio I G /I D .The values of I 2D /I G and L a for the each CVD temperatures are plotted on Figs. 5.The both values increases as the CVD temperature rises.The value of I 2D /I G reaches the maximum value at 750 • C, and then decreases at 800 • C.These changes correspond to that of the peak positions and the FWHMs of 2D spectra shown in Table 1, and also indicate that the number of the graphene layer decreased at 750 • C. In these graphs, steep increases are seen at the temperature between 700 • C and 750 • C, indicating that a drastic change occurred in the growth mode of the graphene.
These results can be explained by the behavior of the carbon on/in the Fe film.It is considered that the formation of the graphene film on the transition metals with higher carbon solubility limit, such as Fe, Ni, and Co, occurs by the solution of the carbon atoms into the metals and the precipitation of the carbon on the surface of the metals [13,14,15].In this study, the carbon atoms are produced by the decomposition of the ethanol on the Fe films.Those carbon atoms solve into the Fe film during heating at the CVD temperature and precipitate again on the surface during the cooling process.In this process, the decomposition and the solution of carbon atoms are more promoted at higher temperature.Thus the higher CVD temperatures are preferable.Based on the phase diagram of Fe-C system, austenite phase (γ-Fe) changes into a mixed phase of ferrite (α-Fe) and cementite (Fe 3 C)  phases at 723 • C [16].The drastic changes of I 2D /I G and La seen at the temperature between 700 • C and 750 • C are probably due to this phase transformation.
An austenite phase can contain much carbon atoms than ferrite phase.Thus the austenite phase precipitates more carbon atoms than the ferrite phase on surface because few solution of carbon, resulting in promotion of graphene growth.Therefore, growth temperature ≥ 750 • C gives larger domain size.The I 2D /I G ratio also showed the maximum value at 750 • C, that is, the number of the graphene layers became minimum at 750 • C.This is probably due to the enlargement of the domain size of the graphene at this temperature.At this condition, the carbon atoms were consumed for the formation of the thin and large graphene layer because of the higher catalyst activity and the active solution/precipitation of the Fe film at the higher temperature.As a result, the thin graphene was uniformly formed on the Fe films.In the case of the lower CVD temperature, the active solution/precipitation behavior occurred at the limited small area on the Fe film.This resulted in the formation of thicker graphene with the small domain size.
Finally, we demonstrated exfoliation of the graphene film from the substrate and transfer it to the SiO 2 /Si(100) substrate.The graphene film grown at 750 • C was used for the exfoliation process.The exfoliation was carried out by immersing the substrate after the graphene growth in FeCl 3 aqueous solution (1 M/l) for 24 hours.The graphene exfoliated and floating on the water is shown in Fig. 6 (a).The graphene film was then scooped up on the SiO 2 /Si(100) substrate [Fig.6 (b)].Figure 6 (c) is Raman spectra of the graphene films before and after the exfoliation.It can be confirmed that the decrease of the I 2D /I G ratio and increase of the D-band after the transfer.These indicate that the crystallinity of the graphene degraded after the transfer.

IV. CONCLUSIONS
We have examined growth characteristics of graphene by means of a low-pressure alcohol catalytic CVD (LP-ACCVD) method using the adopted nozzle injection of ethanol gas to supply carbon source gas to a substrate.The dependences of the growths of the graphene on the CVD temperature showed that the optimum CVD temperature existed.In this study, the CVD temperature of 750 • C gave the thinner and more uniform graphene with the largest domain size.The exfoliation of the graphene film from the Fe film and its transfer to the SiO 2 /Si(100) substrate were successfully demonstrated.

FIG. 5 .
FIG. 5.The values of (a) I2D/IG ratios and (b) graphene domain sizes (La) plotted as a function of the CVD temperature.These values were obtained from the Raman spectra shown in Fig. 3.

FIG. 6 .
FIG. 6.(a) Graphene exfoliated from the substrate and floating on the water.(b) Graphene transferred on the SiO2/Si(100) substrate.(c)The Raman spectra of the graphene before and after the transfer.

TABLE I
. Peak positions and FWHMs of 2D spectra graphene films grown at each CVD temperature.CVD temperature ( • C)Peak position (cm ) FWHM (cm −1 )