The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films

NOC-18245; No of Pages 7 Journal of Non-Crystalline Solids xxx (2017) xxx–xxx

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The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films Liutauras Marcinauskas a,b,⁎, Marius Černauskas a,c, Mindaugas Milieška b, Denis Reso d a

Department of Physics, Kaunas University of Technology, Studentų 50, LT-51368 Kaunas, Lithuania Plasma Processing Laboratory, Lithuanian Energy Institute, Breslaujos 3, LT-44403 Kaunas, Lithuania Department of Semiconductor Physics, Vilnius University, Saulėtekio str. 9, LT-10222 Vilnius, Lithuania d Otto-von-Guericke-University Magdeburg, Institute of Micro and Sensor Systems, Universitatsplatz 2, 39106 Magdeburg, Germany b c

a r t i c l e

i n f o

Article history: Received 14 December 2016 Received in revised form 14 March 2017 Accepted 15 March 2017 Available online xxxx Keywords: Carbon films Argon-hydrogen-acetylene plasma Optical emission spectroscopy Microstructure Atmospheric pressure

a b s t r a c t Amorphous carbon films were formed on nickel/silicon (100) substrates by plasma jet chemical vapor deposition (PJCVD). The carbon films were deposited at atmospheric pressure using an argon-acetylene and argon-hydrogen-acetylene plasma. The plasma composition was analyzed by optical emission spectroscopy (OES). The results indicated that the dominant species in the argon-acetylene and argon-hydrogen-acetylene plasmas were the CH radical and C2 particles. The emission intensities of CH and C2 lines depended on the hydrogen amount in plasma and the distance. Scanning electron microscopy analysis demonstrated that the surface roughness of the coatings decreased with the addition of hydrogen and the increase of distance from 5 mm to 7 mm. The oxygen concentration increased with the increase of the deposition distance. The Raman spectroscopy results indicated that the addition of the hydrogen in the argon-acetylene plasma lead to the formation of nano-crystalline graphite films at 5 mm distance. The increase of the distance from 5 mm up to 7 mm stipulated the formation of multiphase SiC/ carbon films when the argon-hydrogen-acetylene ratios were 100:2.4:1 and 100:1.2:1. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Amorphous carbon films and carbon nanostructures have attracted considerable interest due to their wide range of excellent properties [1–10]. Diamond-like carbon (DLC) films due to high sp3 C\\C content demonstrates a high hardness values, corrosive resistivity, biocapability, chemical inertness, low coefficient of friction, etc. [1,3–5]. Graphite-like carbon (GLC) films mainly consist of sp2 hybridized carbon atoms with a low fraction of randomly embedded sp3 carbon atoms and are characterized by chemical inertness, low friction and internal stress, high surface area, conductivity, etc. [1,8–9]. The carbon nanostructures (nanotubes, nanoonions, graphene, graphene oxide) have high electrical and thermal conductivities, hardness values, plasmonic resonance, electron emission and etc. [2,6–7]. Their properties make carbon films and nanostructures suitable for a large variety of application fields such as: biomedicine, tribology, microelectronics, energy, optics, and etc. [1–10]. Various plasma techniques are used to produce high quality amorphous carbon films with desirable physical properties under conditions relating to low pressure [1,3–5]. However, the low pressure deposition costs are sufficiently high because of the employment of ⁎ Corresponding author at: Department of Physics, Kaunas University of Technology, Studentų 50, LT-51368 Kaunas, Lithuania. E-mail address: [email protected] (L. Marcinauskas).

expensive pumping equipment and a time-consuming vacuum process. So, in recent times a special attention is paid to the deposition of amorphous carbon films or nanostructures at atmospheric pressure conditions [11–23]. The main advantage of such process is the possibility to replace a high cost vacuum system by a new plasma equipment which can be operated simply at air conditions. It also has an easy installation process and in-line process capabilities and allows the treatment of substrates comprising of large areas and various geometrical shapes [14, 18]. Acetylene and methane gases containing plasma are widely used in the formation of various carbon films and nanostructures at atmospheric pressure [11–18,20–24]. T. Sakata et al. [11] deposited the amorphous carbon films on a steel substrate under atmospheric pressure from acetylene – helium gases. W.J. Liu et al. [12] found that the structure of DLC films depended on the C2H2 gas flow rate and the substrate temperature. However, the deposition at air conditions lead to other complications such as contamination of the films by air. It was found that the oxygen contamination is very sensitive to substrate temperature [11,13–17]. Our previous studies demonstrated that with the increase of deposition distance the oxygen concentration in the carbon films increases [15–16]. M. Noborisaka et al. [17] demonstrated that aC:H and a-C:N:H films prepared from C2H2/He and C2H2/N2 gases have hardness values in the range of 0.6–1.1 GPa. A.H.R. Castro et al. [24] obtained that the deposition rate of polymer films of about 330 nm/min could be achieved when the argon/air/acetylene mixture was used.

http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021 0022-3093/© 2017 Elsevier B.V. All rights reserved.

Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021

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The nano-scale DLC films were successfully deposited using Ar-CH4 plasma at atmospheric pressure and it was demonstrated that the CH3 radical has a main role in the film growth [20]. It was demonstrated that the carbon nanotubes could be synthesized at atmospheric pressure from acetylene gas [22]. It should be noted that the plasma composition, process parameters and the deposition equipment play a crucial role in the yield of the final properties of carbon films synthesized at atmospheric pressure [11–21]. It should also be mentioned that the chemical composition of the plasma has huge effect on the properties of the carbon films [25–30]. The chemistry of methane and acetylene gases containing plasma is very complicate and depending on the deposition conditions (pressure, distance, acetylene or methane gas flow etc.) various species are found [18,25–30]. G. Le Du et al. [25] found that CH, CH+, C2, C3, Hβ, Hγ and H2 species exist in the acetylene plasma created by the RF source. Other authors also indicated that the CH radical, Hβ, and the C2 molecule were the dominant species in plasmas containing acetylene or methane [27–30]. It should be mentioned that the most of optical emission spectroscopy (OES) investigations were done at reduced pressure or vacuum conditions. However, the OES measurements of acetylene-argon or argon-hydrogen-acetylene plasmas at atmospheric pressure conditions were hard to find in the scientific literature. The aim of this work was firstly, deposit amorphous carbon films at atmospheric pressure, secondly to determine the influence of the addition of hydrogen into the plasma and thirdly find a relationship between plasma composition and carbon films structures.

2. Experimental setup The amorphous carbon films were deposited on silicon substrates at atmospheric pressure from argon-acetylene and argon-hydrogen-acetylene gas mixtures by plasma jet chemical vapor deposition. Argon (Ar) was the plasma forming gas, with a constant flow rate of 6.6 l/min. Acetylene gas (C2H2) was used as carbon source with flow rates of 0.066 and 0.143 l/min. Hydrogen gas (H2) flow rates of 0.08 and 0.16 l/min were used. An experimental set up consisted of power generator, gas feeding (flow meters) and water cooling systems and plasma torch. Linear direct current plasma torch (PT) was used for the deposition of amorphous carbon films. The PT consists of a hot hafnium cathode, the gas injecting ring containing three blowholes of tangential gas supply for the arc stabilization, water-cooled copper anode with a blowhole for C2H2 gas introduction. The working gas (argon or argonhydrogen mixture) passes through the arc column affecting by dissociation and ionisation until it forms plasma. The geometry of the copper anode forces acceleration of rapidly expanding gas and results the formation of a high velocity and high-temperature jet. More details about the deposition system is given in Ref. [31]. The formation of carbon films was performed on Si substrates on which a thin nickel layer was deposited. The substrates were chemically cleaned with acetone and then with an argon-hydrogen plasma for 30 s before initiating the formation process. The distances between the plasma torch nozzle exit and the samples were 5 mm and 7 mm, while the process duration was 300 s. Additional details about the films formation are given in Table 1. Table 1 Deposition conditions of amorphous carbon films. C2H2, Sample Power, Distance, Ar, H2, W mm l/min l/min l/min Ni1 Ni2 Ni3 Ni4 Ni5

850 1000 1000 980 980

5 5 7 7 7

6.6 6.6 6.6 6.6 6.6

– 0.16 0.16 0.08 0.08

0.066 0.066 0.066 0.066 0.143

Ar:H2:C2H2 volume gas ratio

Plasma temperature, °C

100:1 100:2.4:1 100:2.4:1 100:1.2:1 46:0.56:1

970 1020 850 850 850

Surface morphology was characterized by scanning electron microscope (SEM) of model JEOL JSM–5900. The bonding structure of carbon films was analyzed by Raman scattering (RS) spectroscopy (Ivon Jobin spectrometer) and Fourier transform infrared (FTIR) spectrometer. RS was investigated using an Nd:YAG laser (532.3 nm, 50 mW, spot size 0.32 mm, in the range of 400–2000 cm−1) as an excitation source. The RS spectra were fitted by Lorentzian shape lines in the spectral range of (1000–1800) cm−1 using the Microcal Origin software. FTIR reflectance measurements were performed in the range of 670–4000 cm−1. Plasma flow temperature was measured with a chromel-aliumel (XA) thermocouples. Three thermocouples were inserted in the ceramic tubes of 1 mm diameter and placed into the steel cell. The steel cell was placed perpendicularly to the plasma flow and kept for 60 s. The temperature of plasma was calculated as arithmetical mean value of the three thermocouples (Table 1). Then steel cell was replaced by identical height steel cell with the Ni/Si substrate. The energy-dispersive spectrometry (EDS, Bruker AXS Microanalysis GmbH) analysis was used to determine the elemental composition of formed carbon films. The initial substrate composition was: nickel (10.3 at%), oxygen (3.2 at%) and silicon (86.5 at%). Nanoindentation tests were performed by MTS-Agilent G200 nanoindenter with a Berkovich diamond tip using the continuous stiffness measurement technique. Argon-acetylene and argon-hydrogen–acetylene plasma analysis were studied in the range of 250–800 nm wavelength using an acousto-optic emission spectrometer IFU AOS4, which operates on the basis of a tuneable acousto-optical filter (AOTF). The spectrometer had a solid state monochromator/grating (spectral resolution: 0.05 nm at 250 nm and 0.5 nm at 800 nm). The optical lens collimator of 5 mm diameter and a 2 m long optical fiber was used to focus at the axial center of the plasma jet. The OES spectra were recorded at 5 mm and 7 mm from the torch exit nozzle perpendicular to the plasma flow. The focal distance was 0.05 m. The fiber directs emission from the plasma into the optical interface located at the pinhole entrance of the grating spectrometer. 3. Results and discussion The spectra and assignment of different lines of argon-acetylene and argon-hydrogen-acetylene plasmas are shown in Fig. 1. Fig. 1 shows the emission spectra at the position of the carbon film substrate for the argon-acetylene and the argon-hydrogen-acetylene plasmas. The line observed at 431.2 nm corresponds to the CH radical, i.e. specifically the Q-branch of the (0, 0) transition in the A2Δ–X2Π system. The main emission lines in the argon-acetylene and argon-hydrogen-acetylene plasmas are assigned to C2 swan D3Πg–A3Πu, Δv = 0, 516.4 nm, Δv = −1, 473.7 nm, Δv = +1, 563.4 nm and Δv = + 2, 618.9 nm [25–27]. The band at 473.0 nm is assigned to the emissive radiation of the H2 (0,0) transition in the Q1Πg–B1Σ+ u system. The bands at 468.1 nm and 542.1 nm correspond to the emission of molecular hydrogen. These bands overlap with the bands of C2 emission lines [25]. The dominant emission lines of both plasma mixtures were of C2 spectra belonging to the Swan system and CH emission. The emission of both C2 and CH lines changed with the introduction of hydrogen gas and the increase of distance. It should be noted that the intensity of the CH emission line increased with the introduction of hydrogen gas. It was obtained that the CH peak intensity increased 1.65 times when the hydrogen flow changed from 0 to 0.16 l/min (from Ar:C2H2-100:1 to Ar:H2:C2H2-100:2.4:1). Meanwhile, the intensity of C2 lines started to decrease, the intensity of the peak at 516.4 nm decreased by ~ 30% when 0.16 l/min of hydrogen was added into argon-acetylene plasma (Fig. 1). The CH/C2 ratio was 0.11 and 0.26 for Ar-C2H2 and Ar-H2C2H2 plasma, respectively. The increase of distance (from 5 mm to 7 mm) had no effect on the nature of plasma spectra, but had influenced changes on the intensities of characteristic emission lines. The intensities of the CH line were 1.35 and 1.50 times higher, while the intensities of the C2 line decreased 1.74 and 1.82 times with the increase of the hydrogen flow rate from 0.08 l/min to 0.16 l/min. It was obtained that the

Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021

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Fig. 1. The OES spectra of the argon-acetylene and the argon-hydrogen-acetylene plasma measured at the distance of 5 mm.

CH/C2 ratios were 0.23 and 0.25 for the hydrogen flow rates of 0.08 l/min and 0.16 l/min, respectively. It was demonstrated that with the increase of emission intensities of CH radicals, typically, the deposition rate of the carbon films increased [26]. The emission intensities of C2 Swan system steeply dropped with the increase of hydrogen gas or the reduction of argon gas content in the plasma [27]. Similar variations of C2 emission intensities were demonstrated in the performed measurements. SEM surface images of the carbon films are presented in Fig. 2. As it can be seen, the coating produced using argon-acetylene plasma is very rough and has a columnar structure. The diameters of the columns are in the range of 20–50 μm (Fig. 2a). The columns consist of the microsize interlinked granules (Fig. 2b). The addition of hydrogen gas (0.16 l/min) into the plasma drastically changed the surface morphology. The addition of the hydrogen in the argon-acetylene plasma lead to the intensive etching of the sp2 C_C, C_O and C\\O sites [30]. Thus, the columnar structure disappeared, the growth rate decreased and the surface became more uniform (Fig. 2c). The film surface consisted of micrometer size particles composed of 200–500 nm fragments. The increasing of distance from 5 mm to 7 mm yields a lowering of the plasma flow (substrate) temperature from 1020 °C to 850 °C. The temperature of the plasma flow decreases with the increase of the distance due to the interaction and mixing with the ambient air. The plasma flow spreads into inert air and the drag force between causes the reduction of plasma flow velocity and temperature [15,31]. Thus, the surface become quite homogenous, but is covered by various size particles in the range of 0.5–5 μm (Fig. 2d). The decrease of the hydrogen flow rate from 0.16 l/min to 0.08 l/min influenced the growth of a smooth film without any micrometer size particles (Fig. 1e). The disappearances of the particles indicate that the arriving hydrogen effectively etches the graphitic phase and terminates the films growth [3]. The increase of the acetylene gas flow rate up to 0.143 l/min stipulated the formation of a non-uniform, rough and quite a porous film (Fig. 2f). The branches are about 10 μm in size and there are deep holes between individual branches that are formed. The EDS measurements (see Table 2) indicated that the coating deposited without the hydrogen consisted of the carbon (95.1 at%) and oxygen (4.9 at%). The addition of hydrogen reduced the growth rate and increased the oxygen concentration within the films. The C/O ratio decreased from ~ 19 down to ~5. The increase of the deposition distance from 5 mm to 7 mm decreased the C/O ratio down to 0.8, while the

reduction of the hydrogen flow lead to the lowest (~0.59) carbon-oxygen ratio. The increase of the acetylene flow almost twice, lead to the reduction of the oxygen concentration and caused quite a high C/O ratio ranging up to ~9. It should be noted that the nickel layer fraction was reduced up to 10–20 at% for Ni2, Ni3 and Ni4 films compared to the initial substrate layer. Such result indicated that the hydrogen effectively etched not only the carbon phase, but also caused the evaporation of the nickel from the substrate. C. Wei et al. indicated that as the hydrogen content increased, the carbon film thickness and the surface roughness were reduced [3]. The quite high concentration of the oxygen in the Ni3 and Ni4 coatings is probably due to the oxidation of nickel layer and the formation of the nickel oxides. The surface morphology and EDS measurements indicated that the content of oxygen increased with the decrease of carbon fraction. The energies of C_O (8.11 eV) and C_C (6.36 eV) bonds are higher than that of C\\H (4.28 eV), C\\C (3.60 eV) and C\\O (3.57 eV) [32]. It is well-known fact that the additional appearance of hydrogen stipulates the etching of the graphite phase and prevents the formation of sp2 carbon sites [3,30]. The etching rate of pure graphite (sp2 bond) is about 10 times higher compared to the diamond (sp3 bond) [1]. The high substrate temperature and the presence of hydrogen break most of C\\H, C\\O, C\\C, C_C or even the sp2 C_O sites. The carbon content would drastically decrease with the increase of hydrogen content in the plasma. The oxygen arriving from the atmosphere will bond with an unbound carbon and would mostly create C\\O or C_O sites [11, 13]. The results indicated that the influence of the hydrogen etching started to dominate over the growth of films at higher distances when the acetylene flow was 0.066 l/min (samples Ni3 and Ni4). It has been reported that hydrogen desorption from the hydrogenated amorphous carbon films occurred often at relatively low temperatures in the range of 350–400 °C [10,15]. The increase of the acetylene flow rate from 0.066 l/min (Ni4) to 0.143 l/min (Ni5) increased the concentration of carbon species in the plasma. As a result, the etching process is partly terminated and a well-defined columnar structure is formed. Fig. 3 shows the Raman spectra of carbon films deposited at different conditions. For quantitative analysis, Raman spectra were decomposed into two Lorentz peaks which are denoted as G (graphite) band and D (disorder) bands, respectively [1,33–38]. The Raman spectra were fitted using the same method in order to determine the characteristic parameters. The obtained parameters are presented in Fig. 3 and Table 2. The Raman spectrum of amorphous carbon films typically shows a D peak

Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021

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Fig. 2. Surface morphology of the coatings: (a–b) Ni1, (c) Ni2, (d) Ni3, (e) Ni4 and (f) Ni5.

Table 2 EDS results and Raman spectra characteristic parameters of the films. Sample C, at%

O, at%

SSi + at%

Ni1 Ni2 Ni3 Ni4 Ni5

4.9 11.8 5.9 8.1 9.9

– 29.2 89.3 87.1 –

95.1 59.0 4.8 4.8 90.1

Ni,

D, G, ΔD, cm−1 cm−1 cm−1

ΔG, cm−1

ID/IG ratio

AD/AG ratio

1352 1332 – – 1351

90 65 – – 105

0.80 0.98 – – 0.96

2.39 1.52 – – 1.87

1589 1599 – – 1589

264 108 – – 205

centered at 1360 cm−1 and a G peak centered at around 1550 cm−1 [1]. The Raman spectrum of the film deposited without the hydrogen (Ni1) has two separated D (1352 cm−1) and G (1589 cm−1) peaks (Fig. 3). The full width at half-maxima (FWHM) of the D and G peaks are 264 cm−1 and 90 cm−1, respectively. The relative intensity of the Dband and the G-band (ID/IG) ratio is 0.80, while the relative area of the D-band and the G-band (AD/AG) ratio is 2.39. The broad peak appears at 810 cm−1 indicating the formation of SiC compound during the deposition process [15]. It was demonstrated that for the DLC films, the ID/IG ratio should be less than one or low as possible [10]. The shift of G peak to a higher wavenumbers indicates the increase of the sp2 carbon

Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021

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Fig. 3. The Raman spectra of the carbon films deposited at various conditions.

fraction in the films [1,8]. The wide D band indicates the presence of high sp2 carbon bond angle distortion [1,10]. The separation of D and G peaks is attributed to an existence of nano-crystalline graphite or/ and a glass carbon phase [1,33–36]. However the D peak is higher than G peak for the typical glassy carbon films [1]. The obtained results indicate that the film is a mixture of sp3/sp2 sites and the sp2 carbon sites are the dominant bond type in the film [33]. The Raman spectra of Ni2 carbon film demonstrated two separate and similar intensity peaks; D (1332 cm−1) and G (1599 cm−1) (Fig. 3). The narrow ΔD (108 cm−1) and ΔG (65 cm−1) band values, and a high ID/IG ratio (0.98) and AD/AG ratio (1.52) indicated that the micro/ nano-crystalline graphite was formed. The appearance of the peaks at 1060 cm−1 indicated the formation of nano-crystallites [30]. The high deposition temperature leads to the intensive inter-diffusion of silicon and carbon atoms and thus the formation of silicon carbides (810 cm−1) occurs [33]. E. Capelli et al. [33] demonstrated that a higher ID/IG ratio and a narrower ΔG are associated with the carbon configurational changes from an amorphous mixture containing sp3/sp2 configuration to a locally ordered sp2 nano-graphite structure. Meanwhile the narrowing of the D band is related to the presence of less sp2 carbon bond angle distortion [1,3]. The upward shift of the G peak position indicates the sp2 carbon aggregation into the larger clusters with the shorter nearest neighbour distances [1]. The greater distance between the D and G band positions demonstrate the higher sp2 content [10]. These results implied that the sp2 grain size and bond angle order increased with the hydrogen gas addition. The increase of the distance drastically changed the nature of the amorphous carbon films. The typical D and G peaks disappeared in the Raman spectrum of Ni3 coating. The G peak is very low and wide, indicating a low amount of the sp2 amorphous phase. The peak at 810 cm−1 is attributed to the SiC phase, while the peaks at 1060 cm− 1 and 1180 cm−1 indicated a nano-crystalline phase. The peak at 1060 cm−1 is attributed to a resonant enhancement of the σ states and directly probes the sp3 bonding. It was obtained that the peak centered around

1180 cm− 1 represented the trans-polyacetylene bonding [30]. The Raman spectrum shape is similar to the one of the film deposited without the hydrogen (Ni1 coating), when the acetylene gas flow (0.143 l/min) is higher than that of hydrogen (0.08 l/min) (Fig. 3). The spectrum of Ni5 film has two separated peaks almost of the same intensity; D (1351 cm−1) and G (1589 cm− 1). The Δ D and Δ G values are 205 cm−1 and 105 cm−1, respectively. Moreover the ID/IG and AD/AG ratios are 0.96 and 1.87, respectively. The increase of the G band value is attributed to the higher disordered degree of the sp2 carbon sites [1,3]. The widening of the G peak demonstrates a higher bond length and angle disorder and higher sp3 carbon fraction [1,10]. The structure of the film deposited under such condition can be described as small size sp2 clusters in ring-like configurations embedded in an amorphous carbon matrix. The increase of the acetylene flow rate from 0.066 l/min to 0.143 l/min increases the concentration of carbon containing species in the plasma. It was demonstrated that the excess C2-species landing on the surface of grown films would simultaneously promote deposition of the sp2 bonded phase [30]. The hydrogen effectively etches the graphitic phase [3]; however the film growth dominates over the etching process when argon-hydrogen-acetylene ratio is 46:0.56:1. The etching process starts to dominate with the increase of hydrogen gas fraction (or reduction of acetylene flow) in the plasma. The graphitic net is destroyed and various volatile species are formed. As a result, a thin carbon films are formed when the Ar:H2:C2H2 ratios were 100:2.4:1 (Ni3) and 100:1.2:1 (Ni4). It is a well-known fact that the hydrogen influences the growth of carbon nanostructures [2,39]. The decrease of Ni concentration in the Ni3 and Ni4 coatings indicate that a part of the nickel evaporated due to high deposition temperature. It could also correspond to local delamination of the Ni layer and formation of nano-size nucleation centers on the surface. These locally formed nucleation centers may serve as a catalyst for the growth of various carbon nanostructures [2]. The FTIR reflectance spectra of the carbon films are shown in Fig.4. The film deposited from the argon-acetylene plasma has a low

Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021

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Fig. 4. The FTIR spectra of the coatings prepared at various conditions.

absorption peaks at 2800–3000 cm−1 region attributed to the sp3 CH3–2 group [1,15]. The wide peaks at 1720 cm−1 and 1580 cm−1 are assigned to the sp2 C_O stretching and sp2 C_C bond vibrations [1,34]. The broad band in the 1050–1450 cm−1 range indicates the existence of sp2/sp3 carbon sites, C\\O and C\\H bonds in the film [15,24]. The similar absorption bands (1730 cm−1, 1590 cm−1, 1300 cm−1) indicated the presence of C_O, C_C and C\\C sites observed for the Ni2 coating (Fig. 4). However the peaks were more distinct and separated. The band at ~810 cm−1 represents the SiC sites [40]. The films deposited at higher distances (Ni3 and Ni4) demonstrated the different natures of the bonding sites (Fig. 4). The main absorption peaks situated at 1390 cm−1, 1150 cm−1, 1010 cm− 1, and 720 cm− 1 are attributed to the CHx, C\\O vibrations and/or to the Si\\O bonds, SiH and SiC/SiCH3 vibrations [1,24,34,40]. The broad band situated at 3300 cm−1 is due to the O\\H stretching mode [16,24]. The hardness of the Ni4 sample prepared at 7 mm distance using Ar:H2:C2H2 ratio of 100:1.2:1 was measured at different randomly chosen points as shown in the fig. 5. The hardness values of the Ni4 sample changed in wide range observed from 0.48 GPa up to 41.2 GPa. Meanwhile the Young's modulus values range from 40 GPa to 1160 GPa (Fig. 5). Such microhardness and Young's modulus values indicated that the hydrogen distribution in the plasma jet was uneven. The higher fraction of atomic hydrogen usually causes the formation of C\\C sp3 sites and the growth of diamond phase [1]. The measurements indicated that due to various hydrogen concentrations in the plasma, different phase compositions in the carbon films were found. The carbon film

Fig. 5. Nanohardness (a) and Young's modulus (b) of the Ni4 film measured at different places.

was a mixture of nano-crystallites, silicon carbide (high hardness values) and graphite/polymer phases (low hardness). It was demonstrated that the CH radical influences the growth of crystalline phase coatings, whereas C2 particle the amorphous carbon [30,39]. The increase of CH/C2 ratio increases the diamond (111) phase fraction in the coatings [26–27]. F. Sohbatzadeh et al. [18] found that the intensity of the CH radical and the C2 species was reduced with the increase of the distance from the plasma jet exit. The intensity of the CH radical in the plasma decreases less than that of C2 carbon species [39]. W.H. Liao et at [30]. demonstrated that the CH/C2 ratio decreased from 1.4 to 0.85 with the increase of the CH4 concentration from 0.5 to 5% in the argon-hydrogen plasma. The intensity of C2 emission lines are higher and thus the CH/C2 ratio is lower when argon-acetylene plasma was used. Raman spectroscopy demonstrated that the amorphous carbon phase is dominant for the Ni1 coating. The addition of hydrogen gas reduces the emission intensities of the CH and C2 species at 7 mm distance. However, the decrease of the intensity of the CH radical is much low compared to the C2 species. Thus, the CH/C2 ratio increases from 0.11 to 0.26 with the addition of hydrogen into argon-acetylene plasma or at higher distances. The increase of the CH/ C2 ratio lead to higher fractions of the nano-crystalline graphite phase

Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021

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in the coatings (at 5 mm) or the formation of multi-componential SiC/ nanocrystaline carbon films at 7 mm distance. 4. Conclusions The amorphous carbon films were deposited from argon-acetylene and argon-hydrogen-acetylene plasma at atmospheric pressure. The effect of hydrogen content in the argon-acetylene plasma and the deposition distance (temperature) on the structure of carbon films was studied. OES measurements indicated that the C2 and CH species dominate in the Ar-C2H2 and Ar-H2-C2H2 plasmas. The introduction of the hydrogen into the plasma increases the CH/C2 ratio from 0.11 to 0.26. The film deposited without the hydrogen was amorphous carbon film with nanocrystalline/glassy carbon phase. The addition of hydrogen flow rate of 0.16 l/min at 5 mm induced the etching of graphitic domains, caused the increase of oxygen concentration about 4 times and initiated the structural transition from the amorphous carbon film to nano-crystalline graphite. The increase of distance decreases the intensity of C2 emission band and reduces the plasma flow temperature from 1020 °C to 850 °C, while the variation of hydrogen flow rate from 0.08 l/min to 0.16 l/min changed the CH/C2 ratio from 0.23 to 0.25. As a result, the etching of the film started to dominate over the growth, the growth rate was reduced and oxygen content become higher than carbon concentration. Such process conditions influenced the growth of multicomponential SiC/carbon films with the microhardness values ranging from 0.48 GPa to 41.2 GPa. The increase of the acetylene flow rate from 0.066 l/min to 0.143 l/min (at H2 flow of 0.08 l/min) led to the growth of rough and branched morphology micro-nanocrystalline graphite/amorphous carbon film of hardness ~ 0.30 GPa. It should be noted that the content of the highly disordered sp2 carbon sites was drastically reduced with the increase of hydrogen gas fraction in argon-acetylene plasma. Acknowledgments This article was prepared under partial support of the European Social Fund Agency implementing measure VP1-3.1-SMM-08-K of the Human Resources Development Operational Programme of Lithuania 2007–2013 3rd priority “Strengthening of capacities of researchers and scientists” (project No. VP1-3.1-SMM-08-K-01-013). The authors thank Dr. R. Zabels from the Institute of Solid State Physics, University of Latvia for the hardness measurements. References [1] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. B 37 (2002) 129–281. [2] M. Terrones, Science and technology of the twenty-first century: synthesis, properties, and applications of carbon nanotubes, Annu. Rev. Mater. Res. 33 (2003) 419–501. [3] C. Wei, K.S. Peng, M.S. Hung, The effect of hydrogen and acetylene mixing ratios on the surface, mechanical and biocompatible properties of diamond-like carbon films, Diam. Relat. Mater. 63 (2016) 108–114. [4] S. Neuville, New application perspective for tetrahedral amorphous carbon coatings, QScience Connect 8 (2014) 1–27. [5] G. Dearnaley, J.H. Arps, Biomedical applications of diamond-like carbon (DLC) coatings: a review, Surf. Coat. Technol. 200 (2005) 2518–2524. [6] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Rev. Mod. Phys. 81 (2009) 109–162. [7] J.R. Gong, Graphene - synthesis, Characterization, Properties and Applications, InTech, Croatia, 2011. [8] M. Huang, X. Zhang, P. Ke, A. Wang, Graphite-like carbon films by high power impulse magnetron sputtering, Appl. Surf. Sci. 283 (2013) 321–326. [9] R. Olivares, S.E. Rodil, H. Arzate, Osteoinduction properties of graphite-like amorphous carbon films evaluated in-vitro, Diam. Relat. Mater. 16 (2007) 1858–1867. [10] I. Solomon, M. Bhatnagar, K. Shukla, B. Sarma, M. Ranjan, A. Sarma, Correlation of structural and optical properties of PVD grown amorphous carbon thin films, Diam. Relat. Mater. 75 (2017) 69–77. [11] T. Sakata, H. Kodama, H. Hayashi, T. Shimo, T. Suzuki, Effects of substrate temperature on physical properties of amorphous carbon film synthesized under atmospheric pressure, Surf. Coat. Technol. 205 (2010) S414–S417.

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Please cite this article as: L. Marcinauskas, et al., The effect of hydrogen addition in argon-acetylene plasma on the structure of amorphous carbon films, J. Non-Cryst. Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.03.021