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Microwave plasma induced surface modification of diamond-like carbon films To cite this article: Shyamala Rao Polaki et al 2017 Surf. Topogr.: Metrol. Prop. 5 045005

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Surf. Topogr.: Metrol. Prop. 5 (2017) 045005

https://doi.org/10.1088/2051-672X/aa806f

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18 October 2016

Microwave plasma induced surface modification of diamond-like carbon films

RE VISED

11 July 2017 ACCEP TED FOR PUBLICATION

18 July 2017 PUBLISHED

9 November 2017

Shyamala Rao Polaki1 , Niranjan Kumar1, Nanda Gopala Krishna2, Kishore Madapu1, Mohamed Kamruddin1, Sitaram Dash1 and Ashok Kumar Tyagi1 1 2

Materials Science Group, Indira Gandhi Centre for Atomic Research—HBNI, Kalpakkam, Tamilnadu 603102, India Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research—HBNI, Kalpakkam, Tamilnadu 603102, India

E-mail: [email protected] (N Kumar) and [email protected] (S R Polaki) Keywords: diamond-like carbon film, chemical structure, surface chemistry, contact angle Supplementary material for this article is available online

Abstract Tailoring the surface of diamond-like carbon (DLC) film is technically relevant for altering the physical and chemical properties, desirable for useful applications. A physically smooth and sp3 dominated DLC film with tetrahedral coordination was prepared by plasma-enhanced chemical vapor deposition technique. The surface of the DLC film was exposed to hydrogen, oxygen and nitrogen plasma for physical and chemical modifications. The surface modification was based on the concept of adsorption–desorption of plasma species and surface entities of films. Energetic chemical species of microwave plasma are adsorbed, leading to desorbtion of the surface carbon atoms due to energy and momentum exchange. The interaction of such reactive species with DLC films enhanced the roughness, surface defects and dangling bonds of carbon atoms. Adsorbed hydrogen, oxygen and nitrogen formed a covalent network while saturating the dangling carbon bonds around the tetrahedral sp3 valency. The modified surface chemical affinity depends upon the charge carriers and electron covalency of the adsorbed atoms. The contact angle of chemically reconstructed surface increases when a water droplet interacts either through hydrogen or van dear Waals bonding. These weak interactions influenced the wetting property of the DLC surface to a great extent.

1. Introduction Most of the research work on diamond-like carbon (DLC) films is centered around its mechanical and tribological properties [1–3]. Recently, surface modifications of this film have become more important in self-cleaning and anti-fogging applications [4–6]. All these characteristics are derived from the surface chemistry of the film. DLC films are ideal for bio transplants because of their biocompatibility and inherent chemical inertness, which prevents side effects in cell growth [7]. Their properties of mechanical stability, anti-wear in bioaqueous media and anti-corrosion are important for sustainable functionality in bio-fluidic environments [8]. The increasing demand for DLC films in bio medical devices and bio sensors is due to its unique tunable wetting characteristics derived by surface chemical modification [9–11]. Developing DLC/ diamond films with the desired wettability is essential for several applications. Wettability is an essential © 2017 IOP Publishing Ltd

property of materials and is influenced by both surface chemistry as well as topography [12]. It alters the surface energy in order to achieve desirable wetting characteristics. Generally, the energy of diamond surfaces is tuned by hydrogenation, oxidation and surface termination by functional groups [8, 13–15]. This basically works on the principle of modified reconstruction and termination of dangling bonds (DBs). However, the long-term sustainability of these surfaces in fluidic media has not yet become a reliable proposition. Surface modification through doping is another important method by which to modify surface chemistry and bonding [10, 13]. Doping with atomic species alters bonding configurations as it chemically reconstructs the surface [8, 16]. On the other hand, hierarchical structures created by micro- or nano-texturing on the surface also alters the wetting characteristics of the materials [17]. Advanced lithography techniques are useful to generate nanoscale patterns for reducing surface energy. However, these are expensive and involve complex processing


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methods [18]. Plasma etching is another promising method by which to modify the surface both physically and chemically. This process creates a hierarchical rough surface of high energy which further enables functionalization of the surface [13, 17, 19]. Plasma treatment of DLC films is a useful method by which to modify the surface chemistry in order to achieve the desired wetting characteristics. The wetting characteristics of plasma-exposed DLC films mostly depend on properties of gases, plasma energy, and duration of exposure. Earlier studies showed that plasma-exposed DLC films are functionalized with hydroxyl, carboxylic, carbonyl and nitrile groups which in turn influence the wetting characteristics [19, 20]. The hydrophobic and hydrophilic characteristics of the DLC films have applications in the textile industry. This is useful in the development of both highly wetting as well as highly water repellent clothes [20]. This offers potential features such as ultra fast drying, anti-fogging and self-cleaning to the clothes, which can be used in textile industries and medical applications. Detailed microstructure and surface chemistry analysis are significant for an understanding of the wetting characteristics of plasma-exposed films. In the present studies, a novel post-deposition modification method of a DLC film surface in hydrogen, oxygen and nitrogen plasma for various exposure times and microwave power was quantitatively investigated. Surface modification by both chemical and physical changes in plasma-exposed DLC films were investigated by atomic force microscopy, Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). The water contact angle (CA) of plasmaexposed DLC film was correlated with the modification of the chemical and physical nature of surfaces. A model of CA variation of plasma-exposed surfaces is proposed on the basis of surface chemical interaction of adsorbed chemical species and water droplets.

The films were designated as DX-1-1, DX-1-2, DX-1-3, DX-3-3, and DX-5-3, where X indicates the type of plasma, i.e. hydrogen (H), oxygen (O) and nitrogen (N). The first digit indicates the microwave power (1 is 100 W, 3 is 300 W, and 5 is 500 W) and the second digit indicates exposure time (1 is 10 min, 2 is 20 min, 3 is 30 min).

2. Experimental section

3. Results and discussion

2.1. Film deposition and post-modification of film surface DLC films were deposited by plasma-enhanced chemical vapor deposition (PECVD) technique. Details of the experimental set-up are reported elsewhere [21]. For surface modification, the DLC films were further exposed in hydrogen, oxygen and nitrogen microwave plasma. The plasma exposure studies carried out in the present study were based on post-deposition surface modification. In this process, all the films were exposed to the ambient conditions prior to surface modification. Plasma exposure studies were carried out under two sets of conditions. In the first, the plasma exposure was carried out with 100 W of microwave power and with varying exposure times of 10, 20 and 30 min. In the second set of experiments, the exposure time was fixed at 30 min and the microwave power was varied, at 100, 300 and 500 W.

3.1. Basic characterization and AFM analysis of DLC films The surface topography, chemical structure, and water CA of untreated DLC films are shown in figure 1. It is evident that the as-deposited DLC film is uniform, with rms roughness around 0.2 nm and a thickness of 450 nm (figure 1(a)). A broad band in Raman spectra of as-deposited DLC was deconvoluted by four individual peaks, with the two designated as ν1 and ν2 corresponding to a trans-polyacetylene (TPA) peak [19, 21] and the other two designated as D and G bands of carbon states [1] (figure 1(b)). The I(D)/I(G) ratio in this film is 0.24, indicating a high fraction of sp3 carbon. The significance of these bands will be further discussed below, in more detail. The surface chemical composition of the film is shown in the survey spectra of XPS, which showed adsorption of oxygen (figure 1(c)). This might be due to contamination on

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2.2. Characterization techniques A multi-mode scanning probe microscope (Ntegra Aura, NT-MDT, the Netherlands) was used to analyze topography and rms roughness of these films in a non-contact or tapping mode with a Si cantilever. The thicknesses of the films were analyzed by taking crosssectional images of the freshly cleaved DLC/Si surface using a field emission secondary electron microscope (FESEM) (Supra 55, Carl Zeiss, Germany). The energy of the electron beam was maintained at 5 kV during the cross-section imaging of these films. The chemical structure of the films was investigated by laser Raman spectroscopy by a micro-Raman spectrometer (inVia, Renishaw, UK), at a wavelength of 514.5 nm operated at 2 mW power. An x-ray photoelectron spectrometer (XPS, SPECS, Germany), equipped with monochromatic Al Kα radiation of energy 1486.74 eV and resolution of 0.47 eV, was used to study the surface chemistry of the films. The water CA at the surface of these films was measured by sessile drop method with a Kruss Easy Drop CA instrument (EasyDrop DSA 100). In this measurement, the volume of the water droplets was kept as ~1 µl. These measurements were carried out at room temperature and at atmospheric pressure. Standard deviations in CA measurements were typically around ±2°. Elastic recoil detection analysis was carried out for analyzing the hydrogen content in the untreated film and it was observed to be around 22%. The details of the measurement have been given elsewhere [21].


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Figure 1. (a) Surface topography; (b) Raman spectroscopy; (c) XPS survey spectra; and (d) water CA of as-deposited DLC film.

exposure to ambient atmosphere. The CA of this film showed hydrophobic behavior that could be associated with the combined effect of the chemical structure and the topography of the film (figure 1(d)). The influence of different plasma exposure on the topography of DLC film is illustrated in figure 2. The vertically stacked images of DH, DO, and DN refer to hydrogen, oxygen and nitrogen plasma with increasing exposure time and microwave power, respectively. At low microwave power, the topography shows fine features of the DLC surface, which transforms into nano-granular morphology at higher microwave power and longer exposure time (figure 2). Moreover, morphological changes in oxygen and nitrogen plasma-exposed films are significant at lower microwave power, as illustrated in figure 2 DO and figure 2 DN. This is attributed to the selective reactive plasmainduced etching of the carbon network present in DLC films [8, 20]. The etching seems to be more pronounced in the case of oxygen in comparison with nitrogen and hydrogen plasmas. Moreover, oxygen plasma caused a differential etching. It is also evident that the change in surface topography becomes a noteworthy feature upon plasma exposure at a higher microwave power, irrespective of the gaseous constituent present in the plasma. At high power, the chemical species present in microwave plasma diffuses further towards the inter

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ior and interacts with the subsurface region. Films exposed for 30 min in nitrogen plasma at 500 W power showed the highest value of roughness. The corre sponding cross-section of the FESEM image showed a reduction of film thickness with increases in exposure time and plasma power. This is shown in the supplementary information (SI) (figure S1 (stacks.iop.org/ STMP/5/045005/mmedia)). Plasma exposure mainly involves recombination, etching and diffusion on the film surface [19, 20]. Surface diffusion reserves the surface roughness and it does not cause much reduction in film thickness. However, recombination and chemical etching alter the morphology and microstructure, and bring about a chemical transformation that causes a significant reduction in film thickness [19]. Figure 3 illustrates the reduction in film thickness with plasma exposure time and microwave energy. Film thickness reduces with exposure time as well as microwave energy, irrespective of the gas used. Figure 3 depicts the film thickness measured by FESEM of plasma-exposed DLC films. It is evident that the reduction in film thickness is low in hydrogen plasma, moderate in nitrogen plasma, and intensive in oxygen plasma (figure SI-1). This is attributed to the chemical etching caused by the plasma–surface interaction [15, 19]. Chemical etching is more severe in the case of oxygen plasma than in the hydrogen and nitrogen plasmas. This reduction follows the


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Figure 2. Surface topography of plasma-exposed DLC films. The films were designated as DX-1-1, DX-1-2, DX-1-3, DX-3-3, DX-53, where X indicates type of plasma, i.e. hydrogen (H), oxygen (O) and nitrogen (N). The first digit indicates microwave power (1 is 100 W, 3 is 300 W, 5 is 500 W) and the second digit indicates exposure time (1 is 10 min, 2 is 20 min and 3 is 30 min).

same trend as topographical changes. At low microwave power and short exposure times, the etching is insignificant. However, high energy leads to higher etching, irrespective of the plasma gas used. At high microwave power the ions are energetic, so it causes resputtering from the film surface, and in addition chemical etching exhibited by the ion–surface recombination. Moreover, the chemical reactivity of energetic oxygen is always high and it easily adsorbs on the film surface, thereby initiating a reaction with the carbon atoms present on the film surface. Therefore, the reactivity of the gaseous ions and their energy determines the topography of the film surface upon plasma exposure.

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3.2. Raman and XPS The detailed chemical structure of the DLC films after plasma exposure was studied, using laser Raman spectroscopy. The important parameters such as the I(D)/I(G) ratio and G peak positions are plotted in figure 4. Moreover, the original curves of the spectra are illustrated in figure SI-2. A broad band present in the Raman spectra is deconvoluted, using a combination of Lorentzian and Gaussian functions, into four major peaks positioned around 1200, 1320, 1430 and 1540 cm−1 [22]. Two bands around 1200 and 1430 cm−1 correspond to the TPA peak [19, 21]. The two peaks mentioned as D and G bands are fingerprints of tetrahedral amorphous carbon (t-aC)


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Figure 3. Film thickness of plasma-exposed DLC films using: (a) hydrogen; (b) oxygen, and; (c) nitrogen plasma; 1-1, 1-2, 1-3, 3-3 and 5-3 corresponds to 100 W for 10 min, 100 W for 20 min, 100 W for 30 min, 300 W for 30 min and 500 W for 30 min, respectively; DH, DO and DN in figures (a)–(c) correspond to hydrogen, oxygen and nitrogen plasma-exposed DLC films, respectively.

[1]. It was shown that the peak shape and broadening of the curves are quite similar, with no significant change in the Raman spectra of plasma-exposed films. However, figure 4 showed that the I(D)/I(G) ratio of the DLC films exposed to hydrogen plasma continuously increased up to 0.37, compared to 0.24 for as-deposited DLC film, indicating graphitization. The G peak position shifts with exposure time and microwave power of hydrogen plasma. This trend is nearly similar to the alteration in the I(D)/I(G) ratio, thereby establishing graphitization of the t-aC structure. In general, hydrogen passivates the DBs at the surface and stabilizes sp3 bonding [15]. Thus, the ratio of I(D)/I(G) is low when film is exposed for a short time in low-energy plasma. As the ion energy 5

increases, the hydrogen becomes more energetic and percolates into the subsurface. Hydrogen ions also interact with sp2 and C–H bonds. In the former case, it breaks the C=C bonds in graphite and stabilizes the sp3 bonding, whereas in the later case it knocks out the H atom attached to carbon atoms and causes formation of an sp2 phase, a process commonly known as hydrogen abstraction [1]. This is attributed to the increase in the I(D)/I(G) ratio at high microwave energy. Conversely, oxygen plasma exposure always results in a continuous reduction in the I(D)/I(G) ratio and a shift in the G peak position. This is indicative of an increase in sp3 bonding with increasing exposure time and microwave power. This is further attributed to preferential etching of sp2 bonding, leading to enhancement of the sp3 content. Nitrogen plasma exposure results in an increase in the I(D)/I(G) ratio and a shift in the G peak position, which is a similar trend to that depicted in hydrogen plasma. However, at higher microwave power, the I(D)/I(G) ratio and G peak position are reduced. At low plasma energy, C–N bonds are formed by substituting C–H bonds in DLC film. This leaves an excess electron in the nitrogen atom, which further bonds with electrons available in the π bonding configuration of carbon atoms. This results in higher sp2 bonding [1]. This is affirmed by the increase in the G peak position, which is a signature of a clustering of sp2 bonding in DLC film. However, nitrogen ions or radicals also cause chemical etching of DLC films. Etching increases with the ion energy and this is confirmed by the observations in the cross-sectional FESEM image (figure SI-1). This results in an enhanced sp3 bonding fraction when the film is exposed to higher energy of microwave plasma and the governing mechanism is the same as explained above. The above analysis reveals insignificant changes in the chemical structure of films upon plasma exposure. Plasma exposure not only incorporates the reactive species into the film, but at the same time these species interact with the film surface and chemically etches it out [20]. Raman spectroscopy cannot be limited to the surface chemical changes. Hence, the compositional changes in the film surface induced by plasma exposure on surface were probed by XPS. Figure 5 shows XPS survey spectra of three major peaks of plasma-exposed DLC films, i.e. C1s (~285 eV), N1s (~400 eV) and O1s (~530 eV) [20]. Surface modification is an obvious outcome of microwave plasma exposure [19]. Oxygen concentration on the surface is negligible and the C/O ratio is as high as in as-deposited DLC film. However, at low microwave power, the oxygen content is high at the DLC film surface upon hydrogen plasma exposure (figure 5(a)). This systematically decreases with increases in microwave power and exposure time. At low microwave power and a short exposure time, the density of the carbon DBs is high on the surface. Under this condition, hydrogen plasma interacts with the surface carbon atoms of the DLC film. This results in a substantial


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Figure 4. I(D)/I(G) ratio and G peak width of plasma exposed DLC films using: (a) hydrogen; (b) oxygen; and (c) nitrogen plasma; 1-1, 1-2, 1-3, 3-3 and 5-3 corresponds to 100 W for 10 min, 100 W for 20 min, 100 W for 30 min, 300 W for 30 min and 500 W for 30 min, respectively; DH, DO and DN in figures (a)–(c) correspond to hydrogen, oxygen and nitrogen plasma-exposed DLC films, respectively.

desorption of carbon by hydrogen radicals present in the plasma species. The energetic radical knocks off carbon atoms and creates an active center for the trapping of atmospheric oxygen [19]. In this circumstance, re-passivating the dangling carbon bonds through hydrogen atoms during plasma exposure is also a possible consequence. However, at high microwave power, hydrogen atoms do not interact with the DLC surface because it diffuses into the subsurface, which in turn restricts the generation of fresh DBs at surface. This prevents the formation of oxygen trapping centers. Therefore, the chemical affinity of the DLC film surface for atmospheric oxygen species decreases. Oxygen concentration on the film surface increases when the surface is exposed to oxygen and nitrogen

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plasmas (figures 5(b) and (c)). In both these cases, oxygen concentration increases with increases in microwave power and exposure time. Reactive species generated from oxygen and nitrogen plasma interact with carbon and knock them out of the DLC film surface. This is evident from the observed reduction in film thickness (figure 3). The desorption of carbon atoms leaves behind fresh DBs at the film surface. This increases the susceptibility of the surface and the further adsorption of oxygen in exposure to ambient atmosphere (figure 5(b)). In addition, oxygen chemisorptions on the DLC surface are also possible during exposure in oxygen plasma. The nitrogen concentration decreases on the DLC film surface when the microwave power and exposure time of nitrogen


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Figure 5. XPS survey spectra of DLC films exposed by: (a) hydrogen; (b) oxygen; and (c) nitrogen plasma with the variation of microwave power and exposure time; 1-1, 1-2, 1-3, 3-3 and 5-3 correspond to 100 W for 10 min, 100 W for 20 min, 100 W for 30 min, 300 W for 30 min and 500 W for 30 min, respectively; DH, DO and DN in figures (a)–(c) correspond to hydrogen, oxygen and nitrogen plasmaexposed DLC films, respectively.

plasma are increased (figure 5(c)). At high microwave power, nitrogen desorbs from the surface while interacting with incoming energetic nitrogen. It is clear that oxygen adsorption of DLC films exposed to oxygen and nitrogen plasma is considerably higher compared to the film exposed to hydrogen plasma alone. The high resolution C1s, N1s and O1s spectra are considered to understand the bonding environment of the elements at the DLC film surface exposed in hydrogen, oxygen and nitrogen environments. The results were compared with the as-deposited DLC film (figure 6). The deconvoluted C1s spectra consists of two major 7

peaks at 284.5 and 285.3 corresponding to C-sp2 and C-sp3 bonding in DLC-0, hydrogen, oxygen and nitrogen plasma-exposed films, respectively [23–25]. The ratio of C-sp3/C-sp2 was increased in oxygen plasmaexposed films and it could be related to defect formation due to plasma etching. In these spectra, other minor peaks at higher binding energies are related to oxygen functional groups (C–O/C–OH, C=O and O–C=O) [23–25]. In these films, a broad O1s photoelectron shift is deconvoluted into three bands that correspond to C=O, O–C–O and C–OH photoelectron shifts, respectively. The negative and positive photoelectron shifts in binding energy were noticed, which correspond to two factors: (a) oxidation state, and; (b) residual static surface charge. The oxygen functional groups in hydrogen plasma-exposed films could be explained by contamination during deposition and plasma treatment. This is also related to the exposure of films to ambient conditions. There was no noticeable increase in oxygen functional groups in C1s spectra while it was exposed in oxygen plasma. The trend was similar to hydrogen plasma-exposed DLC films. However, the sp3/sp2 ratio in oxygen plasma-exposed DLC films was increased noticeably. The disordered sp3 carbon state could be functionalized by oxygen groups, mainly by C–O [23, 24]. The nitrogen plasma-exposed DLC film showed five well resolved peaks in deconvoluted C1s spectra. These correspond to C-sp2, C-cp3, C–O/C–OH, C=O and N–C=O, respectively [23, 25]. Oxygen contamination is observed in the O1s photoelectron shift. A broad peak in DN-5-3 was deconvoluted by only two peaks, i.e. related to O–C–O and C–OH chemical bonding. Further, the three peaks in N1s correspond to C=N–C (N-sp2), C–N (N-sp3) and N–O bonds, respectively. These deconvoluted bands are broadened in the DN-5-3 film and this could be explained by the presence of more functional groups. The important parameters of photoelectron shifts of C1s, O1s and N1s spectra of plasma-exposed and asdeposited DLC films are given in table SI-1. It is clearly evident that the functional groups are higher at higher plasma powers, irrespective of type of the plasma, and it is more evident in oxygen plasma-exposed DLC films. A systematic trend of surface roughness and C/O ratio is observed in plasma-exposed DLC films (figure 7). Changes in roughness value and C/O ratio are related to the reactivity of plasma species. In hydrogen plasma, surface roughness does not vary much due to the absence of significant surface modification, thereby resisting defect generation and carbon atom desorption. In contrast, hydrogen atoms passivate the reactive carbon DBs [19]. This is the reason that the oxygen content on the surface is low and the C/O ratio is the highest in the hydrogen plasma-exposed DLC film surface. Therefore, the roughness slightly changes with C/O ratio (figure 7(a)). However, the concentration of oxygen species increases significantly in the oxygen and nitrogen plasma-exposed DLC films. This also causes the increase in surface roughness (figures


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Figure 6. C1s, O1s, N1s spectra of as-deposited DLC film and hydrogen, oxygen and nitrogen plasma-exposed DLC films; 1-1, 5-3, correspond to 100 W for 10 min and 500 W for 30 min, respectively; DLC-0, DH, DO and DN correspond to as-deposited DLC film, and hydrogen, oxygen and nitrogen plasma-exposed films, respectively. (fO and fO,N are oxygen and oxygen, nitrogen functional groups, respectively).

7(b) and (c)). These effects are governed by the reactivity of oxygen and nitrogen plasma that produces the surface defects via the desorption of carbon atoms, along with simultaneous adsorption of oxygen and nitrogen species [20]. High roughness leads to high surface energy, and rougher asperities acts as oxygen trapping centers for adsorption [12, 20]. 3.3. CA analysis CA measurements of plasma-exposed DLC films are shown in figure 8. It is clear that the water CA of the DLC film surface increases with increases in microwave power and exposure time. At lower exposure durations, 8

all three plasma-exposed films become hydrophilic as compared to the as-deposited DLC films (figures 8(a)–(c)). Especially, oxygen and nitrogen plasma exposure renders the film surface more hydrophilic (figures 8(b) and (c)). However, increases in exposure time and microwave power increases the water CA, and this value become close to that of the as-deposited DLC film. Upon hydrogen plasma exposure, dangling carbon bonds of the DLC surface interact with hydrogen atoms/ions/radicals present in the plasma, and undergo surface modification [19, 20]. Alongside hydrogen chemisorptions, trace levels of oxygen ingress also occur, as seen in the XPS analysis.


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Figure 7. Comparison of C/O ratio and roughness of DLC surface exposed in: (a) hydrogen; (b) oxygen, and; (c) nitrogen plasma; 1-1, 1-2, 1-3, 3-3 and 5-3 corresponds to 100 W for 10 min, 100 W for 20 min, 100 W for 30 min, 300 W for 30 min and 500 W for 30 min, respectively; DH, DO and DN in figures (a)–(c) correspond to hydrogen, oxygen and nitrogen plasma-exposed DLC films, respectively.

3.4. Water droplet interaction mechanism at DLC film surfaces A possible interaction mechanism of water droplets with the DLC film surface is schematically shown in figure 9. The droplet interacts with the DLC surface, forming a hydrogen–oxygen network through hydrogen bonding. This happens when carbon DBs of the DLC surface are passivated by hydrogen atoms forming a covalent bond and the hydrogen atom interacts with the oxygen of water molecules (figure 9(a)). The hydrogen atoms of water molecules may also interact with hydrogen atoms, which saturate the 9

carbon DBs in the DLC surface. This interaction is weak and electrostatic in nature and hinders wetting by water droplet. Further, a covalent bond is formed between the adsorbed oxygen and the carbon DBs of the DLC surface. The interaction induces asymmetric polarization, thereby promoting hydrogen bonding with water molecules. The negatively charge centers created by oxygen atoms can coordinate with several hydrogen bonds belonging to water molecules. The strength of hydrogen bonding is stronger than the weak electrostatic force that brings in hydrophilicity of the surface [26, 27]. The XPS analysis shows reduced


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Figure 8. CA of: (a) hydrogen; (b) oxygen; and (c) nitrogen plasma-exposed DLC film surface with the variation of microwave power and exposure time; 1-1, 1-2, 1-3, 3-3 and 5-3 corresponds to 100 W for 10 min, 100 W for 20 min, 100 W for 30 min, 300 W for 30 min and 500 W for 30 min, respectively; DH, DO and DN in figures (a)–(c) correspond to hydrogen, oxygen and nitrogen plasma-exposed DLC films, respectively.

oxygen content in the DLC film surface. Hydrogen bonding causes enhanced hydrophilicity and restricts oxygen chemisorptions. Such interaction is expected to be negligible because there is less adsorbed oxygen on the hydrogen plasma-exposed DLC films. The surface roughness of the hydrogen exposed DLC surface does not change much and the effect of roughness on the CA can be ignored. Oxygen and nitrogen plasmaexposed films are found to be much more hydrophilic than hydrogen plasma-exposed DLC films (figures 9(b) and (c)). However, its characteristic of resistance 10

to water wetting increases with increasing microwave power and exposure time. This is attributed to oxygen and nitrogen adsorption on the surface. In the case of oxygen plasma exposure, the oxygen content on the DLC surface increases with increasing microwave power and exposure time, as is evident from XPS analysis. Moreover, increases in the sp3 carbon state and oxygen functional groups are noticed in high resolution XPS analysis. This indicates the adsorption of more oxygen that establishes hydrogen bonding between adsorbed oxygen, thereby forming a C–O bond and water molecules on the DLC film surface (figure 9(b)). Therefore, it brings in the −C–O–H2O bonding network. Adsorbed oxygen atoms might form covalent bonding with dangling carbon bonds [19]. Implantation at high microwave power leads to more energetic oxygen bombardment on the DLC surface, which causes desorption of surface carbon atoms [20]. The DBs formed under this processes causes a carbon– oxygen covalent network to be established (figure 9(b)). A covalent chemical bond possibly forms between water molecules and unsaturated carbon atoms of the DLC surface when the adsorbed oxygen content is less on the surface. This occurs when microwave power and plasma exposure duration are low. A plasmaexposed DLC surface containing unsaturated carbon DBs has a tendency to physisorb or chemisorb, either forming H2O or O2 molecules from ambience, leading to saturation. The sorption phenomenon more or less inactivates the surface, leading to a reduction in CA, which was indeed observed in this case. Nitrogen plasma exposure of the DLC film surface showed an increase in oxygen and a decrease in nitrogen species with increases in microwave power and exposure time. In this condition, the proposed operating interaction mechanisms are shown schematically in figure 9(c). Nitrogen adsorption on the film surface dominates at low microwave power and exposure time. This leads to the formation of covalent bonding between adsorbed nitrogen and dangling carbon bonds of the DLC network. Here, the negative polarity on the nitrogen atom become high and interacts covalently with negatively-charged oxygen atoms of water molecules that leads to hydrophilic behavior (figure 8(c)). This explains that chemical affinity of the surface becomes stronger when the nitrogen content is more on the surface, leading to an enhanced water-wetting characteristic. A hydrogen bonding network is possible when covalently adsorbed nitrogen in the DLC network interacts with hydrogen atoms of water molecules, forming a N–H coordination (figure 9(c)) [27]. However, oxygen content is high at higher microwave power and exposure time. The dominant mode of interaction involves the formation of hydrogen bonding between water molecules and adsorbed oxygen atoms on the DLC surface. This interaction is weaker than the existing covalent interaction between adsorbed nitrogen and oxygen atoms of water molecules. Thus, the wetting characteristic is influenced by


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Figure 9. Schematic representation of plasma-exposed DLC film surface using: (a) hydrogen; (b) oxygen; and (c) nitrogen plasma.

the adsorbed oxygen content on the DLC surface. At high microwave power, and irrespective of the gaseous constituent used, surface defect density increases. The chemical etching and diffusion of implanted species are predominant in increasing in defect density [20]. Hydrogen is a lighter atom than carbon and it does not cause high surface roughness. This normally diffuses and passivates the carbon DBs on the surface and in the subsurface region [19, 20]. Here, the masses of oxygen and nitrogen are comparable to the carbon. With increases in energy, the heavier species collides with the surface and transfers energy and momentum, leading to resputtering of the film surface [1]. Moreover, the carbon atom becomes unstable and it is etched away from the surface when oxygen and nitrogen like reactive species form a covalent bonding with dangled carbon atoms. The surface thinning and etching is evident from the cross-sectional images of the film acquired at differing microwave power and exposure times (figure 3). This is the main reason for the significant increase in surface roughness in nitrogen and oxygen plasma-exposed DLC films. Surface roughness can also be considered to influence the CA. High roughness acts as a cavity on the surface, where air pressure is more effective. These pressurized centers repel the water droplets. This is explained by the Cassie and Baxter model [28]. However, this model

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is more reliable in explaining the CA of a water droplet in microscopic scale. Here, the relationship of the CA with chemical modification of the DLC surface is explained more reliably on the basis of bonding configuration.

4. Conclusion Ultra-smooth DLC films were prepared by PECVD technique by optimizing the microwave power and methane concentration. Hydrogen, oxygen and nitrogen ions were used at various microwave powers and exposure times for post-modification of DLC film surfaces. An ultra-smooth as-deposited DLC film surface became significantly rougher in oxygen and nitrogen plasma. The origin of this roughness is explained by the adsorption–desorption of implanted species and carbon atoms from the tetrahedral DLC network, leading to modification of the surface through chemical etching. However, resistance to desorption of carbon atoms from the DLC surface in hydrogen plasma modification showed limited physical and chemical etching. This was explained by the covalency of hydrogen atoms, which form a chemically stable covalent bond with carbon atoms where the electronic charge is compensated within the bond. The CA of a water droplet increased when the surface was highly reconstructed by oxygen and nitrogen plasma.


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This feature was described by forming the hydrogen bonding and a weak electrostatic network between the water droplet and adsorbed species. However, wetting properties were enhanced when adsorbed atoms interacted with water molecules.

Acknowledgments The authors acknowledge Dr K Ganesan for acquiring topography of the films by AFM. The authors also acknowledge Dr G Amarendra, Director, Materials Science Group, IGCAR and Dr A K Bahaduri, Director, IGCAR, for their support.

Supporting information Cross-sectional SEM images and Raman spectra of DLC films deposited in hydrogen, oxygen and nitrogen plasma; parameter of chemical properties of DLC films analyzed by XPS.

ORCID iDs Shyamala Rao Polaki 6344-4472

https://orcid.org/0000-0002-

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