Oxygen doping effect on wettability of diamond-like carbon films

In recent years, researchers have been interested in developing the deposition using particle flux enhancement for various applications such as space ion thruster and plasma accelerators. Figure 1 presents a cross-section image of the anode layer source. As shown in this figure, the source of the anode layer includes SmCo permanent magnet, gas inlet, inner and outer cathodes, magnetpoles, and anode with the water-cooled channel. A DC high-voltage source reinforces the source of the anode layer. As an advantage, there is no electron source using this method, so that one can use gases such as oxygen, nitrogen, or other reactive gases. The gas is injected directly into a discharge channel. The suspension of the electrons and keeping a strong electric field in the discharge plasma are both the result of a strong magnetic field between the inner and outer poles.

Zoom In Zoom Out Reset image size

DLC layers were prepared using the direct ion beam method with 120 sccm of CH₄ (99.999%) on Ge substrates with the same thickness (2 mm) and the same diameter (34.8 mm). These films were deposited at room temperature. First, the Ge substrates were cleaned using deionized water, soap, acetone, and ethanol in an ultrasonic bath and then dried by pure nitrogen gas (99.999%). The fixed substrates distance from the ion source was approximately 15 cm. DLC thin layers were doped by oxygen flow rates (5, 10, 20 and 40 sccm). The deposition time was 23 min Samples were named as O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC, respectively. The working voltage of the ion source to clean the substrate in the chamber prior to deposition was 1.8 kV, while argon (purity 99.999%) by the flow rate of 25 sccm was flown via the gun. For deposition of methane, 1kV discharge voltage was applied between cathode and anode of the ion gun. For all the processes, the pressure in the vacuum system was pumped down to 6 × 10⁻⁶ mbar and pressure during the deposition process was 3 × 10⁻³ mbar.

Field-emission scanning electron microscopy (FE-SEM, Tescan Mira 3) and energy-dispersive X-ray spectrometry (EDAX) were employed to determine the microstructure, thickness, and composition of the coatings. The structural properties of DLC samples were also investigated by Raman analysis (Takram P50C0R10 model, excitation wavelength of 532 nm). The water contact angles of the films were measured by an optical contact angle measurer instrument (Data Physics OCA 15 plus). Each measurement was repeated three times and the average value was reported. The measurement accuracy was 0.1°.

The EDAX analysis was applied to evaluate the elements in the O-DLC films. EDAX spectra for O-DLC samples are illustrated in figure 2. This figure indicates that there are no impurities in as-deposited films. The purity of the film is mainly due to conducting the complete experiment in a high vacuum with highly pure CH₄ and O₂ gases. This result shows that by increasing the O₂ flow rate from 5 to 40 sccm, the O content increases from 1.1 to 3.9 wt%.

Zoom In Zoom Out Reset image size

The FESEM was applied to evaluate the thickness and study the morphological properties of O-DLC films. Figure 2 also represents cross-section and surface views of the films deposited at various Oxygen flow rates (5, 10, 20, and 40 sccm). The thickness of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples was calculated to be 134, 132, 130, and 121nm, respectively. The cross-section pictures indicate that all films are uniform and compact with proper adhesion to the substrate, which may be due to employing methane as the carbon precursor [20, 21]. The surface FESEM images of O-DLC samples shows that the surface of layers is smooth in general. By increasing the O₂ content (flow rate of 20 sccm and more), several tiny particles of about 5–10 nm in size are generated on the surface. Devriendt et al., while sputtering graphite in the presence of Ar and C₂H₂ gases, observed the generation of nanoparticles. In the direct ion beam method, the oxygen radicals in the gas plasma, which greatly prevent the formation of carbon particles due to etching of the graphite component, transported to the substrate [22]. When O₂ and CH₄ are introduced to the anode layer ion source that generates the gas plasma and transports it to the substrate, O₂ radicals in the plasma are highly reactive and hinder the formation of carbon particles due to etching of the graphite component.

The Raman spectroscopy technique was applied for analyzing the structural properties of O-DLC films. Raman analysis is a standard and non-destructive tool to characterize the nature of DLC bonding and microstructural information [23]. The Raman peaks of O-DLC were fitted by two peaks with Gaussian line shape. A broad peak appeared in the Raman spectra of O-DLC films in the wavenumber range 900–1800 cm⁻¹. The broad Raman peak of DLC films is usually an overlap of two peaks: 1) D (disorder) peak around wavenumber of 1370 cm⁻¹ and 2) G (graphite) peak around wavenumber of 1500–1650 cm⁻¹. D peak is a breathing mode of symmetry phonons arising from sp² carbon atoms in the ring. This mode becomes active only in the disorder, and its peak intensity is strictly connected to the presence of a six-fold aromatic ring [24]. G peak originates from the stretching vibration of any pair of sp² sites in both rings and chains [25]. From fitting the Raman spectra, it is possible to extract G peak position, the intensity ratio of D peak to G peak (I_D/I_G), full width at half maximum (FWHM) of G peak, and graphite cluster size (La). Raman spectra of all the O-DLC samples are presented in figure 3. In this figure, two peaks of 1367 cm⁻¹ and 1576 cm⁻¹ are observed, which are related to D and G peak, respectively, indicating the formation of a typical DLC structure. Figure 4 presents the fitting of two Gaussian peaks of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples. One can basically observe G peak moves to the higher values and increment in the intensity of D peak upon increasing O₂ gas flow rate in the figure. FWHM of G peak values of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples were estimated 192, 207, 123, and 114 cm⁻¹, respectively. FWHM of D peak values of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples were estimated 186, 548, 150, and 120 cm⁻¹, respectively. G peak position values of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples were estimated 1530, 1522, 1576, and 1586 cm⁻¹, respectively. D peak position values of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples were estimated 1298, 1318, 1419, and 1376 cm⁻¹, respectively. I_D/I_G ratio values of the O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples were estimated 0.55, 0.51, 0.8, and 1.17, respectively. For the O₁₀-DLC film, the I_D/I_G ratio and G-peak position decrease while FWHM (G) increases, indicating that oxygen increases the sp³ content, C-C bonding, disorder, and reduces the sp² cluster size [26]. If the oxygen concentration is higher, for instance, O20-DLC, and O40-DLC samples, the I_D/I_G ratio and G-peak position increase again and FWHM (G) decrease indicating an ordering of sp² clusters. FWHM is a measure of the bond length disorder, and the I_D/I_G ratio is a measure of the size of the sp² phase organized in rings. The reduction in the I_D/I_G ratio means the increase in sp³ content. The large value of FWHM indicates disorder due to the larger angle and bond length [27]. FWHM and peak position of G peak of the O- DLC samples versus O₂ gas flow rate are shown in figure 5. As can be seen, by increasing O₂ content to 10 sccm, FWHM of G increased, G peak position, and I_D/I_G ratio decreased from 1530 to 1522 cm⁻¹. These results indicate that by an increase in the oxygen incorporation, the sp² content decreases while the C-C bonding disorders and sp³ content increase [28]. If the oxygen content is higher than 40 sccm, the FWHM of G peak decreases. In this regard, an increase in G peak position and I_D/I_G ratio indicates an ordering of sp² clusters [29]. Chain-like sites compared to aromatic sites possess more C-H bonds; hence, by an increase in the O₂ content of DLC films, the hydrogen content of the films will be reduced. Dwivede et al. [30] reported that by increasing O₂ plasma in DLC films, I_D/I_G ratio, G peak position, and sp² clustering decreased. Mckindra et al. [31] deposited O₂ doped a-C films using magnetron sputtering and reported a decrease in I_D/I_G ratio followed by an increase in O₂ content.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

The La value is calculated using equation (1) [32]:

$$\begin{eqnarray}&&\displaystyle \frac{{{\rm{I}}}_{{\rm{D}}}}{{{\rm{I}}}_{{\rm{G}}}}={\rm{C}}.{{\rm{L}}}_{{\rm{a}}}^{2}\end{eqnarray} \tag{ 1 }$$

where C is ∼0.0055. L_a values and I_D/I_G ratio versus O₂ gas flow rate for all specimens are shown in figure 6. As one can see, the size of graphite clusters increased with an increase in the O₂ content. The I_D/I_G ratio is found to increase from 0.55 to 1.17, followed by an increase in the graphite cluster size from 1 to 1.45 Å.

Zoom In Zoom Out Reset image size

In the present study, the electrical resistivity (ρ) of O-DLC samples was measured using equation (2) [33] with the help of a four-point probe instrument. For this purpose, R_s, which is calculated from the I–V characteristic curve, was considered.

$$\begin{eqnarray}&&\rho =\displaystyle \frac{\pi t}{\mathrm{ln}\,2}{R}_{s}\end{eqnarray} \tag{ 2 }$$

where t is the thickness of the O-DLC film. Figure 7 depicts the variation of electrical resistivity as a function of the oxygen flow rate. As can be seen, an increase in the oxygen flow rate causes a significant increment in the electrical resistivity of O-DLC films. Electrical resistivity depends on the carrier density, cluster size, and mobility [34]. I_D/I_G ratio and cluster size increase as the number of rings per cluster increases, and the number of π states per cluster decreases. It may be deduced that the increase in electrical resistivity is due to decreasing the number of π states per cluster [34]. Yari et al. reported that the increase in the electrical resistivity could be related to the increment of the sp² bond fraction or crystallization of DLC films. Some authors believe that the formation of C-H bonds can significantly affect the electrical resistivity of DLC films [35, 36]. the resistivity of the DLC film undergoes a radical increased from 0.83 × 10² to 1 × 10⁵ ohm.cm for the DLC to O₄₀-DLC samples. This can be explained by the fact that the energetic ion bombardment on the surface of growing films is one of the major parameters that control the ad-atom mobility on the film surface and thereby their structure. Therefore, the self-bias of plasma changes with increase in the oxygen flow rate, so that the change of oxygen flow rate can cause the energy change of ions, which are bombarding on the film surface.

Zoom In Zoom Out Reset image size

The internal stress of the O-DLC samples was measured from the curvature of the Ge substrate. The internal stress was calculated using film curvature after deposition on one side via the Stoney equation [37]:

$$\begin{eqnarray}&&\sigma =\displaystyle \frac{{E}_{s}}{6\left(1-{\nu }_{s}\right)}\left(\displaystyle \frac{{t}_{s}^{2}}{{t}_{f}}\right)\left(\displaystyle \frac{1}{{R}_{2}}-\displaystyle \frac{1}{{R}_{1}}\right)\end{eqnarray} \tag{ 3 }$$

where $\sigma$ is the internal stress, ${\nu }_{s}$ is the Poisson ratio of the substrate, R₁ and R₂ are respectively the curvatures of the substrate before and after deposition, E_s is Young's modulus of the substrate, and t_f and t_s are respectively the thicknesses of the film and substrate. The values of ${\nu }_{s}$=0.2 and E_s = 115 GPa were adopted for Ge substrate [38]. The radius of curvatures of the substrate before and after the deposition was measured through Newton's rings method using optical interferometry. The radius of the substrate curvature is given by:

$$\begin{eqnarray}&&R=\left(\displaystyle \frac{{d}_{m}^{2}}{4m\lambda }\right)\,\,\,\left(m=1,2,3,\ldots \right)\end{eqnarray} \tag{ 4 }$$

where d_m is the diameter of the mth dark interference fringe and λ is the wavelength of the light (λ=589.3 nm) used as a light source. The residual stress of the O-DLC films versus oxygen flow rate is shown in figure 8. Based on the figure, it is obvious that the internal stress decreases upon increasing the oxygen flow rate. By increasing the oxygen flow rate from 0 to 40 sccm, the internal stress reduced from 2.7 to 0.11 GPa. The mechanical properties of the DLC thin layers depend on the sp²/sp³ fraction and H content in the film's structure [26, 39]. It has been reported that the DLC (a-C:H) films contained both sp² and sp³ sites, and sp² sites are separated into clusters embedded in an sp³ bonded matrix [40]. Increasing the hydrogen content in the DLC film structure causes fracturing the sp³ C-C bonds, which are bonded clusters, and sp2 C = C aromatic, which are inside of clusters. When the fracture of sp³ C-C bonds and formation sp³ C-H bonds happens, a discontinuity is created in the films' network followed by a decrease in their residual stress. As reported in [26], by an increase in the hydrogen content in O-DLC films, the residual stress increases first and then decrease abruptly.

Zoom In Zoom Out Reset image size

A microscope equipped with CCD-video was utilized to measure the surface contact angle of the O-DLC samples, a stand with a tiltable plane, and a prism. Images taken using this method were analyzed by digimizer software. Figure 9 shows the oxygen flow rate dependence of the water contact angle for DLC, O₅-DLC, O₁₀-DLC, O₂₀-DLC, and O₄₀-DLC samples. The results showed that the water contact angle of O-DLC decreased from 82.9°±2.1° to 50°±3° with the increase in oxygen flow rate. The boundary between hydrophilicity and hydrophobicity is the contact angle of 65° [41]. The hydrophilicity of DLC films is sensitive to the sp²/sp³ bonding ratio on the surface [42]. Reports from others show that the sp²-rich surfaces exhibit a higher contact angle than sp³-rich surfaces [43, 44]. Decrease of water contact angle is the result of the increase in surface energy. The incorporation of oxygen into the film's structure led to increase the surface energy and consequently the wettability. the surface energy includes two important components, i.e., dispersion and polar components. Polar component represents the surface interactions, which are related to dipoles, while dispersive component represents the surface interactions, which are based on temporary variation in the electron density. The π bond electrons of sp² carbon sites and dangling bond electrons have more potential for polarization than σ bond electrons of sp3 carbon sites. Thus, the polar component of the surface energy of the oxygen doped films is raised by increasing the incorporation of oxygen into the film's structure. Hence, as shown by Raman analysis performed in this study, the decrease in the contact angle of O-DLC films upon increasing the oxygen flow rate may be attributed to the increase in ordering sp³ clusters and sp³ content. Shen et al. [45] reported a decrease in water contact angle from 98.5 ± 2.3° to 26.3 ± 4° with the increase in oxygen flow rate. Nakamura et al. [14] reported a decrease in the water contact angle of DLC samples from 80° to 51° for the oxygen flow rate increase from 0 to 50%.

Zoom In Zoom Out Reset image size