Deposition of vertical carbon nanostructures by microwave plasma source on nickel and alumina

Vertical carbon nanostructures on metal and ceramic substrates are deposited successfully using planar microwave plasma source at frequency of 2.45 GHz. The PECVD (Plasma Enhanced Chemical Vapor Deposition) with microwave surface wave discharge is applied because it produces a dense plasma providing efficient decomposition of the methane and creation of a large number of reactive particles which results in lower substrate temperature for graphene deposition compared to CVD method. Optimization of the gas mixture of H2, Ar and CH4, and the gas pressure in the chamber results in a homogeneous graphene structures deposition on the substrates of Ni-foil, Ni-foam and alumina ceramics at substrate temperatures ∼600 °C. The plasma parameters of surface wave discharge such as gas temperature, electron temperature and density are obtained by measuring OH-band and Ar-lines using optical emission spectroscopy. The morphology of the vertical carbon structures is obtained using SEM analysis and the characteristics of the graphene layers were determined by Raman spectroscopy.


Introduction
Carbon and its structures are an area of high research interest.Factors that lead to this are its abundance and ability to take a variety of forms including zero-dimensional (0D), one-dimensional (1D), twodimensional (2D), and-three dimensional (3D).Different carbon nanostructures have been shown to have many properties of interest such as high mechanical strength and flexibility and high conductivity (both thermal and electric) which in turn leads to possible applications in supercapacitors, DNA and gas sensors, nanomedicine and others [1].
The method presented in this study is plasma enhanced chemical vapor deposition (PECVD), as it allows relatively low substrate temperatures (as low as 600 qC for Ni) during deposition avoiding defect formation and degradation of the substrate being coated [2].The structures can also be formed at shorter times than the standard CVD techniques due to higher deposition rate.With this method we can achieve control over the morphology of the dispositions by controlling the experiment parameters such as the precursor gas (carbon-containing gas, in our case methane), diluent gas, gas pressure, power applied to the discharge, chosen substrate and substrate temperature.
The PECVD results for carbon nanostructures demonstrated in this article are obtained using a microwave plasma reactor at low pressure.The substrates used for the film growth are nickel (Ni) and alumina (Al2O3).The setup consists of a vacuum chamber where the substrates are placed on top of a holder with a heating element to which voltage is applied through a transformer.The temperature of the substrate surface is monitored with a thermocouple which allows for precise control of the temperature.The plasma is ignited by a surface wave at 2.45 GHz traveling on the surface between the plasma and a flat quartz window.The surface wave reflection from the metal wall around the window causes the appearance of a standing wave in the discharge.The microwave power generated by a SAIREM GMP20 Microwave generator with 2 kW power (including a water cooling amplifier) is fed to the system.The matching of the load impedances to provide the maximum power transfer between the generator and the load is done with the use of a triple stub and a sliding metal piston.The gas pressure in the chamber is maintained by a dry pump and measured by a Thermovac TM 20 Vacuum Gauge Controller.Additional cooling of the microwave source is done by an air cooling system.
The main working gas used in the discharge is argon (Ar) while hydrogen (H2) and methane (CH4) are added for production of the reactants needed for deposition of carbon nanostructures.The precise gas mixture is controlled by a system of four Bronkhorst flow controllers.
The light from the plasma column is collected by an optical cable connected to a collimator and the emission spectra of the plasma are measured by Horiba iHR550 and HR Ocean Optics spectrometers.These spectra are used to determine the plasma parameters via spectral methods.The electron density (݊ ୣ ) and the electron temperature (ܶ ୣ ) are determined from the Line Ratio Method (LRM) [3,4] using the argon (Ar) spectral lines.By varying ݊ ୣ and ܶ ୣ , a number of points are found for which the line intensity ratio between two selected spectral lines is equal to the experimental results.The same procedure is applied for a second pair of spectral lines and both ݊ ୣ (ܶ ୣ ) curves are plotted together.The point of intersection shows the actual values for the electron density ݊ ୣ and the electron temperature ܶ ୣ .An important step for obtaining these results is the selection of a ratio of spectral lines sensitive to the electron temperature and a ratio sensitive to the density change [3].

Results
The Line Ratio Method is applied for simultaneous determination of plasma parameters from Ar spectral lines recorded at gas pressure ‫‬ = 0.6 ÷ 4 Torr (figure 2).The gas temperature ܶ is estimated to be in the range of 600 ÷ 700 K by optical emission spectroscopy methods, similar to our previous work on PECVD at atmospheric pressure [5].The spectral lines selected for application of this method are: 696.5 nm ‫2(‬ ଶ → ‫ݏ1‬ ହ ), 706.72 nm ‫2(‬ ଷ → ‫ݏ1‬ ହ ) and 727.22 nm ‫2(‬ ଶ → ‫ݏ1‬ ସ ).The results for the intensity ratios of the 706.72/727.22 lines and the 706.72/696.5 lines) are shown in figure 3. The results for the electron density and the electron temperature taken from the plot in the above gas pressure range show that ݊ ୣ ≈ 3 ÷ 4 × 10 11 cm -3 and ܶ ୣ ≈ 4 ÷ 5 eV.With increasing the pressure ܶ ୣ decreases while ݊ ୣ increases.
The deposition time is minimum 90 mins and maximum 180 mins.The gas in the chamber is a mixture of Ar/H2/CH4 or H2/CH4 and the gas pressure is varied between 0.6 and 4 Torr.It was shown that increasing the pressure results in greater amount of deposited material and increased homogeneity of the deposition.The used substrates (Ni-foil, Ni-foam and alumina) are heated to temperatures in the range between 600 and 800 qC.Applying higher voltage to the heating element raises the substrate temperature and also increases the amount of the deposited material, as well as the amount of the carbon nanostructures relative to the carbon layers.
SEM imaging is used to investigate the morphology of the deposited carbon nanostructures.The images show successful deposition of carbon nanotubes on Ni foam and Ni plates with diameter around 200 nm, carbon nanowalls on Ni foam with length around 500 nm and thickness below 50 nm and thick carbon layers on alumina ceramics (figure 4).The deposited on the Ni substrates carbon nanostructures are investigated with Raman spectroscopy (figure 5).The Raman spectrum includes a G (1580 cm −1 ) peak usually observed for graphene-based materials, distinguished D (1320 cm −1 ) and for nanowalls D′ peaks that could be associated with the lattice defects of different nature.The 2D peak that corresponds to the second order of the D mode is also characteristic for graphene-based materials.The first-order D peak itself is not visible in pure graphene because of crystal symmetries.However, as the amount of disorder in the graphene increases, so does the Raman intensity I(D).For our spectrum the I(D)/I(G) ratio is around 1.12 for nanotubes and around 1.21 for nanowalls which is in agreement with other measurements for carbon nanostructures [6][7][8].

Conclusion
An experimental setup for PECVD of three types of carbon nanostructures at low pressure is developed in this paper.Controlling the substrate temperature and the Ar/H2/CH4 ratios allows control over the deposition rate and the morphology of the deposition on Ni substrates (carbon layers or structures).It is also shown that the setup can be successfully used for simultaneous deposition of carbon layers on ceramics.The results from SEM imaging and Raman spectroscopy confirm the type and the characteristics of the produced graphene nanostructures.

Figure 2 .
Figure 2. Argon spectral lines from a mixture of Ar/H2/CH4 used for the Line Ratio Method at ‫‬ = 3.3 Torr, ܶ = 600 K.

Figure 4 .
Figure 4. SEM images of: (a) carbon nanowalls on Ni foam, (b) carbon nanotubes on Ni foam, (c) carbon nanotubes on Ni plate, (d) carbon layers on alumina.

Figure 5 .
Figure 5. Raman spectroscopy results for: (a) carbon nanowalls on Ni foam achieved at substrate temperature T > 650 qC, (b) carbon nanotubes on Ni foam achieved at substrate temperature T < 650 qC.