Morphological and Chemical Evolution of Gradually Deposited

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Morphological and Chemical Evolution of Gradually Deposited Diamond-Like Carbon Films on Polyethylene Terephthalate: From Subplantation Processes to Structural Reorganization by Intrinsic Stress Release Phenomena Alberto Catena,† Qiaochu Guo,‡ Michael R. Kunze,§ Simonpietro Agnello,∥ Franco M. Gelardi,∥ Stefan Wehner,† and Christian B. Fischer*,† †

Department Department § Department ∥ Department ‡

of of of of

Physics, University Koblenz-Landau, 56070 Koblenz, Germany Physics, The Ohio State University, Columbus, Ohio 43210, United States Chemistry, University Koblenz-Landau, 56070 Koblenz, Germany Physics and Chemistry, University of Palermo, 90100 Palermo, Italy

S Supporting Information *

ABSTRACT: Diamond-like carbon (DLC) films on polyethylene terephthalate (PET) are nowadays intensively studied composites due to their excellent gas barrier properties and biocompatibility. Despite their applicative features being highly explored, the interface properties and structural film evolution of DLC coatings on PET during deposition processes are still sparsely investigated. In this study two different types of DLC films were gradually deposited on PET by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) using acetylene plasma. The surface morphology of the deposited samples has been analyzed by atomic force microscopy (AFM). Their chemical composition was investigated by diffusive reflectance infrared Fourier transform (DRIFT) and Raman spectroscopy analysis and the surface wettability by contact angle measurements. Subplantation processes and interface effects are revealed through the morphological and chemical analysis of both types. During plasma deposition processes the increasing carbon load causes the rise of intrinsic film stress. It is proven that stress release phenomena cause the transition between polymer-like to a more cross-linked DLC network by folding dehydrogenated chains into closed 6-fold rings. These findings significantly lead to an enhanced understanding in DLC film growth mechanism by RF-PECVD processes. KEYWORDS: interlayer formation, amorphous hydrogenated carbon (a-C:H), grain analysis, film dehydrogenation, DRIFT, Raman, RF-PECVD

1. INTRODUCTION Plastic materials have become an essential resource in modern society.1 This success is due to their unique properties such as remarkable deformability, chemical- and light-resistance, a wide range of working temperatures, and the possibility to be manipulated as hot melt.1,2 Plastics present also disadvantages such as low hardness, low abrasion resistance, and reduced mechanical qualities, limiting their usability in certain manufacturing applications, such as the biomedical and packaging industry.3 Diamond-like carbon (DLC) coatings on plastic materials represent nowadays the most widespread approach to overcome such limitations.4−7 DLC is an amorphous carbonaceous material containing π-bonded sp2 carbon clusters arranged in plane. The sp2 clusters are limited to single 6-fold rings and short chains embedded in a sp3 matrix of carbon and hydrogen atoms.8,9 By varying the sp3/sp2 ratio as well as the H content in the DLC chemical composition, the physical properties can be controlled to achieve enhanced material properties such as high hardness, low friction coefficients, and chemical inertness among others.8 DLC © XXXX American Chemical Society

coated polyethylene terephthalate (PET) is one of the most studied composite due to its numerous and promising industrial applications.5,10−13 PET is a common thermoplastic polymer resin belonging to the polyester family.14 It consists of polymerized units of the monomer ethylene terephthalate.14 Hubácě k et al.5 showed that carbon coatings on PET increase the sample wettability and have a positive effect to the biocompatibility of the polymer. Kodama et al.10 demonstrated that enhanced gas barrier properties of DLC films on PET synthesized by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique are achievable at atmospheric pressure compared to films synthesized at low pressure. Zhang et al.11 showed that a higher sp3 content leads to better barrier properties of DLC films on PET foils, pointing out that deposition process parameters have an important effect on the performance of the Received: February 19, 2016 Accepted: April 8, 2016

A

DOI: 10.1021/acsami.6b02113 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces resulting films. Boutroy et al.12 found an intermediate layer (thickness of 500 × 1015 cm−2 atoms) for 100 nm deposited DLC film on PET characterized by a lower H and O content compared to the PET substrate, providing a 50-fold reduction of the oxygen permeation rates. Wang et al.13 reported a reduced bacterial adhesion on DLC films parallel to a decreased sp3/sp2 ratio of the carbon atoms in the coating since the adhesion became energetically unfavorable. Beside applicative purposes of such compounds, it is doubtlessly very important to understand the growth mechanism of such carbon films on plastic materials. A pronounced interlayer formation was already observed between DLC films and polyethylene (PE) substrate.15 It was shown that DLC gradually intermixes with the plastic, resulting in a stable combination of both materials.15 Successively, two different DLC films have been grown on high-density polyethylene (HDPE) by RF-PECVD in acetylene plasma with two different deposition methods.16 During deposition grains appear on the film surface and their evolution was studied to characterize the carbon layer growth.16 This hereby observed behavior is overall similar to the grains’ characterization of DLC films deposited on a harder substrate like silicon (100).17 For high DLC loads on HDPE a growth of the sp2 carbon clusters parallel to a reduction of the hydrogen content in the sample was revealed.16,17 In the present study two different types of DLC films deposited on common PET material are investigated. Accordingly to previous works,15−17 these two types are named respectively f-DLC (with f- meaning more flexible) and r-DLC (with r- meaning more robust), due to the different deposition mechanism used during the deposition process. The films were produced by RF-PECVD technique with acetylene plasma. The surface morphology of these deposited samples was studied ex-situ by atomic force microscopy (AFM). Diffusive reflectance infrared Fourier transform (DRIFT) and Raman spectroscopy techniques were used to investigate the chemical composition of the realized coatings.

profilometer (Veeco Instruments Inc., Dektak3 surface profile measurement system, Plainview, NY, USA) on additional coated silicon wafers partially covered by aluminum foil on the aluminum sample holder.15−17 2.2. Surface Characterization by AFM. The surface morphologies of all samples were recorded by atomic force microscopy (AFM, Omicron Nano Technology GmbH, Taunusstein, Germany). AFM images were acquired in contact mode using standard silicon nitride PNP-TR cantilevers (Nano And More GmbH, Wetzlar, Germany) in air and at room temperature. Throughout this study, the scanned area was a square of 5 μm × 5 μm. The microscope was regularly calibrated by a cellulose acetate replica (Pelco, calibration specimen for atomic force microscopy, 607-AFM) of a 2160 lines/mm waffle pattern diffraction grating to guarantee the accuracy of the measurements. As done in earlier studies,15−17 a minimum of three different positions spread all over the sample were examined and images from every position were repeated in continuous scans until the accuracy of the measurement was ensured. 2.3. AFM Image Analysis. Images are recorded in the size of 5 μm × 5 μm and analyzed using the commercial Scanning Probe Image Processor software (SPIP version 4.6.1, Image Metrology A/S, Hørsholm, Denmark).16,17 First the images were plane corrected using an LMS fit of degree 3, and a median filter was used to remove the worst horizontal noise. A convolution smooth mean filter and a convolution smooth low pass filter were used to reduce the low frequency noise. If necessary, the values outside the color boundaries were cut off to remove the long waves. Finally, the grains on top of the surface were analyzed using a threshold method. All the parameters of the filters and threshold method were set in order to guarantee the recognition of appropriate surface features. 2.4. DRIFT Spectroscopy and Analysis. Infrared (IR) investigations were performed via DRIFT spectroscopy. A Shimadzu Fourier transform infrared spectrophotometer (IRPrestige-21, Kyoto, Japan) was used. The instrument was equipped with the diffuse reflectance measuring apparatus DRS-8000.23,24 The measurements were performed at room temperature in a flow of argon gas to reduce contaminations from the air environment. Every final spectrum was recorded with 300 repetition scans in the relevant spectral range of 3050−2800 cm−1, i.e., the C−H stretching zone.25,26 A minimum of three different positions on each sample were acquired to probe samples’ homogeneity and correctness of the measurements. It was checked that no useful information is extractable from others spectral ranges (surveys between 4000 and 500 cm−1 are available in the Supporting Information). Spectra were analyzed with a commercial IR Solution − FTIR Control Software (software version 1.30, Shimadzu Corporation, Kyoto, Japan). First the spectra were line corrected using the baseline-multipoint manipulation tool and then fine-tuned with the smoothing manipulation tool. The manipulation steps do not alter the relevant information on the analysis. 2.5. Raman Spectroscopy and Analysis. A Bruker-Senterra Micro-Raman spectrometer with a laser excitation light at 532 nm in confocal mode and 50× magnification was used to acquire the Raman spectra. Use of input laser power at the minimum of 0.2 mW prevents structure damages of the samples.16,17 The acquisition time for each measurement was 5 s × 300 repetitions in the spectral range 50−4478 cm−1. The maximum spectral resolution was 9−15 cm−1. As for IR measurements, a minimum of three spectra at different positions all over the sample were acquired. Spectra were analyzed with OriginPro software by OriginLab Corporation. The spectrum of the highest rdepositions was corrected using a baseline to remove the photoluminescence (PL) background. The resulting spectrum was then normalized to the maximum G band intensity value and fitted with two Gaussian curves simulating the D and G band to investigate the film structures and enable comparisons with previous results.16,17 The whole analytical procedure is already a proven approach of earlier studies.16,17 2.6. Contact Angle Investigation. Surface wettability was examined by contact angle measurements with water (HPLC grade). Conditions were the sessile drop technique with the OCA 15 plus contact angle goniometer (DataPhysics Instruments GmbH, Filder-

2. EXPERIMENTAL DETAILS 2.1. Sample Preparation and Coating. PET material of 1 mm thickness was obtained in the best commercially available quality (ES303010, Goodfellow GmbH, Bad Nauheim, Germany). Circular samples with diameters of 10 mm were cut out, cleaned with isopropanol and dried in ambient air at room temperature. Subsequently the samples were glued on a homemade aluminum sample holder with commercial carbon pads (Plano G3347, Plano GmbH, Wetzlar, Germany). Two types of DLC films were deposited on the samples in various thicknesses: 2, 5, 10, 20, 50, 100, and 200 nm for the f-type and 2, 5, 10, 20, 50, 100, 200, 500, and 1000 nm for the r-type, via a RF-PECVD process. The deposition of both types was operated with the RF-driven (13.6 MHz) plasma source (COPRA DN 400, CCR GmbH, Troisdorf, Germany).18,19 Briefly, to enable a proper surface coating the plastic material is initially cleaned with oxygen plasma (65 sccm/min, 1 Pa, 200 W).15−17 The coating process is then obtained by exposing the cleaned plastics to acetylene plasma in a high vacuum chamber at room temperature (65 sccm/min, 0.65 Pa, 107 W).15−17 In the direct r-deposition method, more ion species impinge the sample surface and a higher degree of cross-linked sp3 carbon centers in the coating is expected.19−21 In the indirect fdeposition instead the sample surface is not in line of sight to the plasma source and mainly radical species are present and diffuse on it, producing an enriched sp2 carbon configuration for the films.21,22 Different DLC thicknesses were obtained via different duration of the plasma process. The deposition rate is 2 nm/min for the f-type and 10 nm/min for the r-one. The obtained thicknesses were checked using a B


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ACS Applied Materials & Interfaces stadt, Germany) under ambient air conditions and 20 °C. A dispensing needle drops the volume of 1 μL automatically on the sample’s surface. For each sample a minimum of five drops at different positions was analyzed and corresponding contact angles were measured on both sides of the single drop.

representations are available in the Supporting Information). All of the AFM images show the normal force signal (FN) in a representative 2.5 μm × 2.5 μm area of a single measurement. The raw PET material (Figure 1a) shows overall a coarse surface with small-scale irregularities. After the f-type oxygen plasma cleaning process (Figure 1b) the sample surface appears less contrasted and displays more blurred bumps in bigger scale. For the 10 nm f-deposition (Figure 1c) the contrast is enhanced and the surface appears more flat. After 20 nm f-DLC are deposited (Figure 1d) well-formed grains are first visible with different sizes spread all over the surface. By increasing deposition, the size of grains increases, as depicted in the 200 nm f-deposition image (Figure 1e). The series of r-DLC deposition (Figure 2b−e) shows a comparable trend as the f-type (Figure 1c−e), but corresponding f- and r-depositions show different morphologies. The rtype oxygen plasma cleaned surface (Figure 2a) is dissimilar to the f-type cleaned one (Figure 1b) and compared to the raw PET material (Figure 1a). Bigger and higher asperities besides some related particles are produced with the direct cleaning process (Figure 2a) than the untreated sample (Figure 1a), resulting in enhanced image contrast. A comparable surface morphology to the pretreated PET surface (Figure 2a) is found for the 10 nm r-DLC deposition (Figure 2b), showing similar features as the cleaned PET sample. Features characteristic of the oxygen plasma cleaning process disappear with increasing deposition, as visible for 50 nm r-deposition (Figure 2c). For this r-deposition the surface morphology of the sample is characterized by a grain-like structure composed by small bulges that start to appear on the surface. Similar to the 20 nm f-deposition (Figure 1d), well-formed grains of different sizes spread all over the surface show up with 200 nm r-deposition (Figure 2d). The growth of the grains with increasing carbon load results in a coalescence phenomenon for the 1000 nm rdeposition (Figure 2e). Big grains as well as multiple ones are found for this high r-deposition (Figure 2e). For the 1000 nm rtype the grains appear uneven compared to smaller rdepositions and reveal certain graininess or intrinsic rugosity forming a structured texture on the surface. The same morphological features are also detectable on the surface of the bottom layer (Figure 2e). In Table 1 surface roughness values (Rq) of raw PET, O2 plasma cleaned samples (f- and r-type), and the f- and rdeposited ones are displayed. The values are calculated as the mean of the corresponding 5 μm × 5 μm AFM measurements

3. RESULTS 3.1. Atomic Force Microscopy. Figure 1 shows the surface morphology of raw PET material (Figure 1a), PET

Figure 1. 2.5 μm × 2.5 μm AFM images (FN) showing (a) raw PET material, (b) PET cleaned with f-type oxygen plasma and PET covered with (c) 10, (d) 20, and (e) 200 nm of f-DLC respectively (full series and 3D representations are available in the Supporting Information).

cleaned with indirect-type oxygen plasma (Figure 1b), and PET covered with 10, 20, and 200 nm f-DLC (Figure 1c−e). Figure 2 shows accordingly the surface morphology of PET cleaned with indirect-type oxygen plasma (Figure 2a), and PET covered with 10, 50, 200, and 1000 nm r-DLC (Figure 2b−e). Here only images from samples with significant changes in the surface morphology by increasing deposition are shown. Accordingly, no additional images of other analyzed f- and rdepositions are presented since the respective surface morphologies display unchanged features (full series and 3D

Table 1. Roughness Values (Rq) of the Raw PET Material, PET Material Cleaned by f- and r-Type Oxygen Plasma and PET Material Covered with Various Amounts of f- and rDLC Rq/nm 0.8 ± 0.2

raw PET O2 cleaned 10 nm 20 nm 50 nm 100 nm 200 nm 500 nm 1000 nm

Figure 2. 2.5 μm × 2.5 μm AFM images (FN) showing (a) PET cleaned with r-type oxygen plasma and PET covered with (b) 10, (c) 50, (d) 200, and (e) 1000 nm of r-DLC respectively (full series and 3D representations are available in the Supporting Information). C

f-DLC

r-DLC

± ± ± ± ± ±

1.1 ± 0.1 1.2 ± 0.2 1.1 ± 0.1 1.3 ± 0.2 1.7 ± 0.4 3.1 ± 1.0 3.2 ± 1.4 12.3 ± 1.2

0.8 1.0 1.6 1.7 2.5 3.4

0.1 0.2 0.4 0.3 0.5 0.5


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ACS Applied Materials & Interfaces Table 2. Average Height and Area of Grains for the f- and r-Deposition on PETa f-DLC height/nm 20 nm 50 nm 100 nm 200 nm

a

6.4 ± 0.9 12.8 ± 2.1 12.6 ± 3.3 12.7 ± 2.3

r-DLC area/μm2 0.014 0.018 0.020 0.022

± ± ± ±

0.005 0.001 0.003 0.002

200 nm 500 nm 1000 nm

height/nm

area/μm2

7.0 ± 1.0 6.8 ± 1.2 11.6 ± 1.2

0.026 ± 0.008 0.025 ± 0.009 0.058 ± 0.007

The values are calculated from each individual 5 μm × 5 μm AFM measurements by the SPIP software analysis, as described in the text.

In Figure 3 DRIFT spectra in the range of 3050−2800 cm−1 for the raw PET material (green solid lines) with respect to the

for each sample. The roughness values of 2 and 5 nm f- and rdeposited samples are not shown here since their AFM images resulted in poor contrast and high noise. The presence of the substantial noise precludes an appropriate revelation of roughness values. Even if the images of raw PET and both f- and r-type cleaned samples clearly show different surface morphologies (Figures 1a,b and 2a), Table 1 displays that the roughness of these samples stays in the same range. Likewise, depositions of 10 nm f-type as well as 10, 20, and 50 nm r-type still have comparable roughness values. The surface roughness starts to increase only for f- and r-depositions higher than 10 and 50 nm, respectively. Above all, a much higher roughness value is found for 1000 nm r-deposition. 3.2. Grain Analysis. Detected grains on top of the surfaces of both f- and r-depositions were analyzed according to section 2.3. The analysis was done for the entire 5 μm × 5 μm AFM image of each sample. Table 2 summarizes the obtained average grain height and area. The given errors are calculated as the deviation of individual AFM measurement values.16,17 For the f-DLC samples the average grain height increases from about 6.4 nm for the 20 nm f-deposition to about 12.8 nm for the 50 nm one. Thereafter, the height stays nearly constant for higher f-depositions. The average grain area instead increases from about 0.014 μm2 for the 20 nm f-deposition up to about 0.022 μm2 for the 200 nm one. The r-DLC samples show a different behavior. As previously discussed in section 3.1 (see Figures 1 and 2), no grains are detectable before 200 nm r-DLC is deposited. For the 200 nm r-deposition the average grain height is found around 7.0 nm and remains approximately constant for the 500 nm r-one. Successively, the average grain height significantly increases to about 11.6 nm for the 1000 nm r-deposition. This average grain height is close to the values of f-depositions between 50 and 200 nm. As for the height, the average grain area is nearly the same about 0.025 μm2 for the 200 and 500 nm r-depositions. The area then increases to about 0.058 μm2 for the 1000 nm rdeposition. 3.3. DRIFT Spectra. DRIFT analysis was performed to examine differences between the chemical compositions of raw PET material and cleaned ones and to investigate the structure of the f- and r-depositions. All the f- and r-deposition measurements were respectively acquired using the f- and rtype cleaned PET material as background, since the respective films are deposited on such pretreated samples. Spectra for all samples are presented in arbitrary units (a.u.) of absorbance. The assignment of the peaks for all DRIFT measurements was done referring to fundamental IR spectroscopy knowledge for material science25 and by comparison with previous results of related works.26−29

Figure 3. Comparison between the DRIFT spectra of raw PET material (in green) with (a) f- and (b) r-type oxygen plasma cleaned samples (in red). The two cleaned surfaces were respectively used as a background for the measurements.

f- (Figure 3a) and r-type (Figure 3b) cleaned samples spectra (red dashed lines) are shown. In both Figure 3a and b the PET material cleaned by oxygen plasma treatments obviously shows a quite flat curve, since the measurements are recorded on the same sample used as background. Figure 3 clearly shows that by cleaning the raw PET material in the indirect (Figure 3a) and direct (Figure 3b) oxygen plasma process, a different chemical structure on the sample surface is obtained. No differences are recognizable between the spectrum of raw PET and f-type cleaned sample (Figure 3a). The r-type cleaning process (Figure 3b) instead displays a different situation. The raw PET shows two peaks centered at about 2851 and 2920 cm−1 (Figure 3b). These two peaks are related to the symmetric (s) and antisymmetric (a) vibrations of CH2 molecular groups, respectively.25−29 After cleaning the PET material with the direct oxygen plasma process these two peaks are not present anymore (Figure 3b). Figures 4 and 5 show the DRIFT spectra in the range of 3050−2800 cm−1 for the f- and r-deposited samples, respectively. Figure 4 shows that no peaks related to bond stretching vibrations are visible in the spectral range of 3050−2800 cm−1 for f-depositions from 2 nm up to 20 nm. Instead starting from D


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Figure 4. DRIFT spectra of all f-DLC depositions from 2 to 200 nm on PET. The f-type oxygen plasma cleaned PET sample was used as a background for all the measurements.

Figure 5. DRIFT spectra of all r-DLC depositions from 2 to 1000 nm on PET. The r-type oxygen plasma cleaned PET sample was used as a background for all the measurements.

depositions (Figure 5). All vibrational modes for 200 nm rdeposition are present in the spectrum of 500 nm r-one, but the peaks of the symmetric (s) CH2 vibrations and the antisymmetric (a) CH2 ones seem to merge together by forming a unique bigger band. This combined band appears especially more intense in the region of antisymmetric (a) CH2 vibrational mode at 2920 cm−1. The same IR vibrational modes of 200 nm r-deposition are found separated again for the 1000 nm r-one (Figure 5). 3.4. Raman Spectra. Raman spectroscopy was used to investigate the carbon bonding structures of the f- and rdepositions on PET material. Two bands characterize the Raman spectra of DLC depositions: the D band originated from the breathing modes of sp2 carbon cluster (at about 1355 cm−1) and the G band originated from the bond stretching of all sp2 bonded carbon atoms (at about 1500−1630 cm−1).30 In the present analysis Raman signals for D and G band were detected only for 500 and 1000 nm r-deposition. The PL background is strongly distorted by the scattering from the plastic substrate, masking any possible signal from D and G bands for smaller depositions.16,17 Different to the same f- and

50 to 200 nm f-depositions two peaks at about 2951 and 3014 cm−1 are clearly detectable. These peaks are associated with the symmetric (s) and antisymmetric (a) vibrations of CH2 molecular groups, respectively.25−29 Figure 5 displays that the scenario of stretching group’s evolution for r-deposited samples is clearly different (compare to Figure 4). Two peaks related to the symmetric (s) and antisymmetric (a) vibrations of CH2 groups25,29 are found again between 2 and 20 nm, for r-depositions at about 2851 and 2920 cm−1 respectively, according to Figure 3b for the r-type oxygen cleaned sample. As soon as 50 nm r-DLC are deposited an additional peak related to the symmetric (s) vibrations of  CH2 groups appears at approximately 2960 cm−1. The 100 nm r-DLC spectrum is very similar to the 50 nm one. For the 200 nm r-deposition the symmetric (s) CH2 vibrational mode shifts to 2955 cm−1 compared to the peak position of 50 and 100 nm r-ones. Furthermore, a small peak related to the symmetric (s) vibrations of CH3 end-groups at about 2880 cm−1 is also visible.25−29 The position of this small peak is determined within a confidence of 5 cm−1 by comparing the position of the same peak found for 500 and 1000 nm rE


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ACS Applied Materials & Interfaces r-depositions on HDPE16 the detection of Raman signals for depositions thinner than 500 nm on PET was not possible. The nondetectability of Raman signals for 200 nm f- and rdeposition on PET compared to the same carbon loads on HDPE16 is most likely an issue of the different chemical composition and formed interlayer of these two diverse polymers. To compare the spectra of these two r-deposited samples a normalization to the maximum intensity value of G band is done for both. A double Gaussian fit is used only to deconvolute the spectrum of 1000 nm r-deposition as sum of the D and G band intensities, enabling comparison with previous studies.16,17 Since the penetration depth of the laser used for Raman analysis is of about 1 μm,16 dissimilar PL background effects from the diverse plastic substrates and different interlayer formations may deform the line-shape of the 500 nm r-deposition spectrum leading to incomparable ID/IG ratios. Consequently, no deconvolution is done for the 500 nm r-DLC spectrum. Figure 6 shows the Raman spectra of the 1000 nm (red solid line) and 500 nm (blue solid line) r-deposition. The

4. DISCUSSION 4.1. Surface Morphologies and Grain Analysis. PET samples after oxygen plasma cleaning process (Figures 1b and 2a) present a different surface morphology than the untreated material (Figure 1a). The surface morphology of raw PET (Figure 1a) is obviously altered by the oxygen plasma cleaning process (Figures 1b and 2a) since most of the impurities originated from the industrial production process and short hydrocarbon chain fragments of the plastic should be removed completely.32 By forming polar groups on the plastic surface, the cleaning process makes PET suitable for DLC depositions.33,34 From Table 1 it is clear that the surface roughness of the two cleaned samples (Figures 1b and 2a) is the same with respect to the raw PET (Figure 1a). Junkar et al.34 reported that the surface roughness of PET increases of about 1 order of magnitude by pretreating the sample with oxygen plasma. This process was operated in an inductively coupled RF generator at a frequency of 27.12 MHz with an output power of about 200 W.34 The present f- and r-depositions were previously studied on HDPE where the surface roughness of the two oxygen plasma treated samples was also found higher compared to the raw material.16 Therefore, the final surface morphology of oxygen cleaned samples depends both on the chemical structure of plastic material and on the specific experimental settings used for the cleaning process. When f-DLC is deposited, the resulting surface morphology of coated samples (Figure 1c−e) differs from the f-type cleaned PET surface, changing with increasing deposition. The surface of 10 nm f-deposition (Figure 1c) presents enhanced contrast indicating the presence of carbon on top. Well-formed DLC grains are clearly visible for the 20 nm f-deposition (Figure 1d), spread all over the surface. By increasing f-deposition up to 200 nm (Figure 1e) the grains grow in both height and area (Table 2). For the r-DLC deposition the required amount of carbon deposition needed to observe similar DLC grains is higher than for f-DLC (Table 2). This is in line with previous studies of fand r-deposition on HDPE and silicon (100).16,17 Thin rdepositions like 10 nm (Figure 2b) show a morphology close to the r-type cleaned sample (Figure 2a). With increasing rdeposition up to 50 nm (Figure 2c) the surface starts to show a more defined grain-like structure, but only for the 200 nm rdeposition (Figure 2d) well-formed grains are first visible. In contrast, grains were already visible for 20 nm f-deposition (Figure 1d). As for the f-type, the grains evolve in height and size with increasing r-deposition (Table 2). For the 1000 nm rone (Figure 2e) they are found merged together in a coalescence phenomenon, which was observed as well for the 1000 nm r-deposition on HDPE.16 In a previous study the interlayer formation between PE and f- and r-DLC was proven with the same experimental conditions as in this work.15 Boutroy et al.12 found a transitional layer between PET and 100 nm deposited DLC, with a different chemical structure and density compared to the PET substrate. As discussed for f- and r-depositions on HDPE,16 the absence of grains for f-depositions thinner than 20 nm (Figure 1c) and r-ones thinner than 200 nm (Figure 2b,c) is attributed to this intermixing process. The interlayer formation induces a density rearrangement for the top layers of the plastic material.15,16 The chemical and morphological reorganization of the sample is due to the intermixing of carbon plasma species with the topmost material, preventing an epitaxial DLC growth for thin depositions.15,16 Only when the

Figure 6. Normalized Raman spectra of 1000 nm (red solid line) and 500 nm (blue solid line) r-DLC deposition. The inset illustrates the deconvolution of the 1000 nm r-deposition spectrum by using two Gaussian curves simulating the D and G bands (green dashed lines) and their resultant sum (green open circles).

deconvolution of the 1000 nm r-deposition spectrum is illustrated in the inset, where the D and G band components (green dashed lines) and their sum (green open circles) are presented. Both r-depositions display the typical Raman spectrum shape of DLC coatings confirming the amorphous nature of the deposited coating.31 The PL background for 1000 nm rdeposition is linear in the Raman shift region between 1000 and 1800 cm−1, whereas between 1200 and 1800 cm−1 for the 500 nm r-one. Although the D band of 500 nm r-deposition detectability extends in a smaller Raman shift region due to PL background effects,16 it appears unequivocally less intense than the respective D band of 1000 nm r-deposition. Furthermore, the G bands of 500 and 1000 nm r-depositions are found exactly with the same shape. This fact clearly confirms that the D band for the 500 nm r-deposition is less intense than the 1000 nm r-deposition one despite the existence of PL background effects for 500 nm r-DLC sample. F


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(Figure 4) may be justified, as in the case of f-type oxygen plasma pretreatment, because the involved plasma species have less energy for this deposition type with reduced interlayer effects on PET.15,16 As observed in section 3.3, the chemical structures of rdeposited samples (Figure 5) are different from the f ones (Figure 4). Figure 3b shows that after r-type oxygen plasma treatment the signals for CH2 molecular groups on raw PET material disappear. Considering the chemical PET composition, this means that the CH2 groups of the untreated PET are clearly modified by the oxygen plasma exposure. Besides other possible modifications of PET material, the H bonded C atoms in CH2 groups are expelled by the oxygen plasma process, leaving dangling C atoms in the cleaned PET structure.15,21 When the r-DLC deposition starts (Figure 5) it can be clearly seen that the two CH2 signals, as detected in the raw PET material, (Figure 3b) appear again. Consequently, for rdepositions between 2 and 20 nm this CH2 bonding is restored (Figure 5). As previously discussed, a more pronounced intermixing processes is expected between rDLC and PET material.12,15,17 Therefore, the new CH2 groups are formed during the interlayer formation. The generation of interlayer can be explained in terms of subplantation processes.15,21 Three processes are dominant for subplantation: surface etching from plasma species (with a subsequent readjustment of the chemical composition of the sample), insertion of C ions and H+ ions into the sample structure, and absorption of radical species to form new bonds at surface sites via saturation of dangling bonds.8,36 From Figure 5 it is visible that during the first stages of r-DLC deposition, there is a continuous creation of new CH2 groups and etching of H atoms from groups of the same nature, since only signals of CH2 are present. The formation of new CH2 groups can be due to the absorption of plasma species such as CH and CH2 radicals on the sample surface,37 and/or insertion of H+ ions from plasma in the inner sample structure.8,36 The etching takes place because of CH bond breaks by H+ ions from plasma up to 20 nm (Figure 5), with the successive creation of desorbing H2 molecules.8,36 No other peaks are visible in the spectra of rdepositions from 2 nm up to 20 nm (Figure 5) because there is a continuous remodeling of the chemical structure of the sample during the interlayer formation without producing any other CH bonded groups. In this reading key, the interlayer formation is explained in terms of subplantation processes of DLC on plastic materials. Furthermore, the plastic is a soft material compared to DLC and the interlayer formation between these unequal materials is present up to higher amounts of r-depositions, i.e., between 20 and 50 nm (Figure 5). It is assumed that the intermix of plasma species and PET material ends at a certain deposition thickness less than 50 nm since in the corresponding spectrum another peak at 2960 cm−1 related to CH2 vibrations is observable (Figure 5). The presence of this new signal is indicative of a different chemical composition of the coating and the ending of interlayer formation. Lopez-Santos et al.37 have demonstrated that the species involved in the DLC growth are mostly CH and the CH2 radicals. After a certain r-deposition higher than 20 nm (Figure 5), some of these radicals remain absorbed on the PET surface. The epitaxial DLC growth starts. The presence of CH2 groups for the 50 nm r-deposition (Figure 5) reveals a polymerlike DLC configuration, and the contemporary existence of  CH2 groups for the same r-deposition (Figure 5) suggests that the polymer-like DLC network is composed also by sp2 carbon

intermixing between plastic material and carbon species ends the epitaxial growth of a DLC film starts.15,16 The density reorganization is more pronounced for the r-type deposition than for the f-one,15,16 accordingly for the f-deposition grains are found already starting at 20 nm (Figure 1d) whereas for the r-one only starting at 200 nm (Figure 2d). With increasing f-deposition up to 20 nm (Figure 1d) and rdeposition up to 200 nm (Figure 2d) the surface roughness keeps to values close to the respective f- and r-type oxygen plasma treated surface (Table 1) and no grains are detectable. For thin f- and r-depositions on HDPE instead a decrease in the surface roughness value with respect to the two cleaned samples was observed.16 The reason could be found in the difference of surface roughness for the f- and r-type cleaned PET (0.8 nm ± 0.1 nm and 1.1 nm ± 0.1 nm respectively, see Table 1) and the respective HDPE values (7.2 nm ± 1.6 nm and 7.8 nm ± 1.6 nm, see ref 16). The surface roughness of both pretreated PET samples is less than the respective values found for HDPE. The decrease of roughness at the first steps of DLC deposition on HDPE was attributed to the smoothing effect due to the preferred DLC growth on the surface asperities or valleys left by the oxygen plasma cleaning processes.16,35 No decrease of surface roughness is detected for thin DLC depositions on PET since the surfaces of f- and rtype cleaned samples (Figures 1b and 2a) are already flat in the micrometer scale and possess no deep surface corrugations. For higher f- (Figure 1d,e) and r-deposition (Figure 2e), instead, the surface roughness increases parallel with the increasing granular structure of these depositions (Table 1). This trend is in line with previous studies.16,17 The growth behavior of average grain height and area for fand r-deposition on PET (Table 2) is completely in line with the evolution pattern of same depositions on HDPE.16 However, the values of average height of grains for singular fand r-deposition are overall smaller for PET (12.7 nm ± 2.3 nm for 200 nm f-DLC, and 11.6 nm ± 1.2 nm for 1000 nm r-DLC, see Table 2) than for HDPE (19.2 nm ± 3.5 nm for 200 nm fDLC, and 21.1 nm ± 2.6 nm for 1000 nm r-DLC).16 This fact indicates that the epitaxial DLC growth mechanism is the same for both PET and HDPE substrates, and it depends only slightly on the different chemical structure of these two plastics and thicknesses of interlayer formation. 4.2. Chemical Structure of the Coatings. Figure 3a shows that indirect-type oxygen plasma treatment does not produce any difference in the CH group’s structure of the cleaned PET material compared to the untreated one. Therefore, the chemical composition of these groups remains unchanged. The energy of oxygen plasma species for the f-type cleaning process15,16 may not be enough to chemically modify the PET structure. Figure 4 displays that no further IR features in the region of 3050−2800 cm−1 are detectable for f-DLC deposition between 2 and 20 nm. Only after 50 nm f-deposition CH vibrational modes rise (Figure 4). As discussed in section 3.3, these modes are due to symmetric (s) and antisymmetric (a) vibrations of CH2 groups. The same peaks are found as well for the 100 and 200 nm f-deposition (Figure 4). For the f-type coating mostly radical species are involved into deposition processes, leading to an enriched sp2 carbon configuration of the resulting film.21,22 The findings of both the symmetric (s) and antisymmetric (a) vibrational modes of CH2 groups for high f-depositions (Figure 4) is totally in line with earlier reports.21,22 Absent CH stretching modes in thin f-depositions G


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the CH2 groups starting at 200 nm r-deposition (Figure 5) suggests that the hydrogenation of the coating is mainly due to unbound H atoms from these groups. Hydrogen atoms can leave the coating structure as H2 molecules.28 Once those hydrogen atoms are ejected the unpaired sp2 bonded carbon atoms link together forming 6-fold rings to minimize their internal energy. These 6-fold rings are then developed by folding of dehydrogenated sp2 chain parts by structural readjustment of the chain and rearrangement of the entire coating structure. During this structural modification of the DLC network other dehydrogenation phenomena from CH2 groups and additional chemical rearrangements of the entire coating may occur to enable the formation of 6-fold rings from sp2 chain parts. Also Thiry et al.38 revealed a branched polymerlike structure with a bonded 6-fold ring DLC network grown on p-doped silicon substrates. The film coating was obtained from a semi-industrial working chamber by graphite sputtering in argon and acetylene gas mixtures. Veres et al.39 instead observed a development of an amorphous carbon network formed by distorted or open six-folds rings as well as intact ones from a polymer-like DLC configuration by increasing the film thickness between 60 and 500 nm. The deposition was operated on crystalline silicon wafers by a radio frequency glow discharge method in a capacitively coupled discharge chamber with benzene plasma.39 The results from the here presented study provide a deeper insight into the DLC evolution process with increasing deposition from a polymer-like to a more diamond-like configuration. The calculated ID/IG ratio from Raman spectrum of 1000 nm r-deposition is 0.33 ± 0.02. This value is in line with previous ratios for 1000 nm r-deposition on HDPE (resulted 0.35 ± 0.03) and on silicon (100) (resulted 0.34 ± 0.01).16,17 The similar ID/IG ratios found for 1000 nm r-deposition on these three different materials, and the comparable results of dehydrogenation of the coating with the increasing deposition parallel to the growth of 6-fold ring structures, suggest that for high carbon load the DLC films grown by RF-PECVD processes from acetylene plasma evolve in the same way as illustrated in this study also for different substrates than PET. 4.3. Contact Angle Measurement Analysis. The surface wettability of raw PET material, oxygen plasma cleaned samples, f- and r-depositions is investigated by contact angle measurements with water. The wetting angle is determined by interactions between wetting liquid and surface chemical bonds,40−42 as well as by the specific morphology of the surface.43 Figure 7 shows the results for raw plastic material (black filled square), f-type oxygen plasma cleaned surface (blue open circle, indicated and encircled in green), r-type cleaned sample (red open triangle, indicated and encircled in green), and for the various f- (blue filled circles) and r-depositions (red filled triangles) (data table is available in the Supporting Information). Representative images of characteristic drops on the samples’ surface are displayed. In Figure 7 it is visible that contact angles reduce after the fand r-type oxygen plasma treatments of raw PET material from about 86° to about 73° and 72°, respectively. As observed in AFM analysis, the cleaned PET samples (Figures 1b and 2a) show different surface morphologies compared to the raw PET (Figure 1a), clearly indicating that surface modifications occur. The removal of surface contaminations accompanied by alterations in the chemical structure of the plastic substrate by oxygen plasma exposures (more pronounced in Figure 3b

bonded hydrogen structures, such as short sp2 chains or open 6-fold rings. Differently than the 50, 100, and 200 nm f-deposition (Figure 4) no peak related to the antisymmetric (a) vibrations of CH2 groups is found for the 50 nm and higher rdepositions (Figure 5). Its nondetectability for r-DLC samples is most likely due to noise effects in the measurements, hiding the signal. Its detectability for the f-DLC samples is instead a consequence of their higher sp2 carbon amount resulting in more pronounced signals for both the symmetric (s) and antisymmetric (a) vibrations of CH2 groups. Furthermore, this point addresses a different chemical composition in the total amount of sp2 carbon bonded atoms of the two coatingtypes. With increasing r-deposition, the same vibrational modes as for 50 nm are detected for 100 nm (Figure 5) and a similar rDLC configuration is obtained. The observation of a polymerlike DLC structure additionally supports previous results of a polymer-like configuration for thin r-depositions.16,17 For 200 nm and higher r-depositions, a small signal related to CH3 groups is present (Figure 5). This peak is characteristic for chain end-groups, pointing out that a number of chains in the forming DLC network stop growing. The different peak position between 200, 500, and 1000 nm r-deposition originates from baseline correction and smoothing effects because its signal is very small and noisy in the raw DRIFT spectrum. The deviation of the signal in a range of 5 cm−1 at the reported position of 2880 cm−1 (Figure 5) do not weaken the above interpretation, since vibrations related to CH3 groups are usually found within this range.27 Figure 5 shows that for 200 nm r-deposition the peak position relative to CH2 vibrations back-shifts from 2960 to 2955 cm−1. This frequency decrease, with increasing r-deposition, is indicative of a minor carbon−hydrogen bonding energy in the CH2 groups,25 and consequently of a longer carbon−hydrogen bonding distance.25 The DRIFT spectrum of 500 nm r-deposition (Figure 5) shows again the same IR features as the 200 nm r-one, but as discussed in section 3.3, a higher amount of CH2 groups is revealed, since peaks related to symmetric (s) CH2 and antisymmetric (a) CH2 vibrations are more intense, forming a unique merged band. The composite band character for 500 nm r-deposition as sum of two bands was ensured. The growing network of DLC reaches the highest polymer-like character for the 500 nm r-deposition. The Raman spectrum of 500 nm rdeposition (Figure 6) displays that closed 6-fold rings are formed in the DLC structure, since D band is clearly detectable. The DRIFT spectrum of 1000 nm r-deposition is again similar to the 200 nm r-one (Figure 5): the two peaks related to the symmetric (s) CH2 and antisymmetric (a) CH2 vibrations are once again distinguishable (Figure 5). The Raman spectrum of 1000 nm r-deposition shows a higher D band intensity than 500 nm r-one (Figure 6). As described in details in previous works,16,17 the increased intensity of D band between these two r-deposited samples is due to a dehydrogenation effect in the coating by increasing the deposition from 500 to 1000 nm. The dehydrogenation goes parallel with the increase of 6-fold ring structures in the coating.16,17 Tomasella et al.28 also revealed that loss of hydrogen leads to a graphitization DLC network. The decreasing of hydrogen content by increasing r-deposition between 500 and 1000 nm (Figure 6) is completely in line with the observations of DRIFT spectra (Figure 5). Since the hydrogen content is less for 1000 nm r-deposition, the peaks relative to CH2 and CH2 groups are found unmerged again (Figure 5). Furthermore, findings of a back-shifted position for H


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deposition process cause a successive decline in the contact angle value for the 1000 nm r-deposition.16,42 4.4. Chemical and Morphological Correlation of 1000 nm r-DLC Features. Lackner et al.44 demonstrated that intrinsic stress for inorganic films (such as DLC) on polymeric materials rises with the increasing thickness during deposition processes. The authors have shown that spontaneously buckling patterns develop for strongly adhered coatings depending on the internal stress and film thickness, and that the produced structures are morphological features stress-related due to relaxation of the whole compound system.44 With increasing deposition from 500 nm up to 1000 nm rDLC a transition from a polymer-like to a more cross-linked DLC network occurs. The existing interlayer between PET material and DLC makes the coating strongly attached to the polymeric substrate.12,15,16 As discussed in section 3.1, it is evident from Figure 2e that both grains and bottom layer surface of 1000 nm r-DLC deposition result structured by forming a texture. This structured texture may be formed by stress relaxation processes occurring during the increasing rdeposition. The revealed transition between 500 and 1000 nm r-depositions would then be a stress-related phenomenon. Stress relaxation is conceivably due to the decreased hydrogen content of the coating between 500 and 1000 nm r-depositions, and to the consequent readjustment of the whole DLC network. Therefore, intrinsic stress is enhanced by increasing rdeposition from 500 nm up to 1000 nm. The produced stress is released through a transition from a polymer-like to a more cross-linked DLC by dehydrogenation of the network and subsequent readjustment of the entire film structure.

Figure 7. Contact angle measurements of raw PET (black filled square), f-type (blue open circle) and r-type (red open triangle) O2plasma cleaned PET (both indicated and encircled in green), and fDLC (blue filled circles) up to 200 nm and r-DLC (red filled triangles) up to 1000 nm depositions. Inserts show water drops for the raw PET (indicated in black) and for the r-DLC series (indicated in red) at important points. The gray band summarizes a region of samples with similar values for wettability in comparison to the raw PET material and the diametrically opposed points in the r-DLC series for smaller depositions (e.g., 10 nm) and for 500 nm.

for the r-type) result in smaller contact angles equivalent for more hydrophilic surfaces than the raw material.16,40 With rising f-DLC depositions, contact angles slightly decrease with respect to the f-type oxygen plasma cleaned sample. Furthermore, these f-depositions remain in a band between 60° and 70° indicated in Figure 7 as gray layer. For the r-deposition, a different development of contact angles is observed. For the 2 nm r-deposition the value increases slightly to about 74° compared to the r-type oxygen plasma cleaned one (Figure 7). Afterward, the contact angle rapidly drops to about 45° for the 10 nm r-deposition via 55° for the 5 nm one. For r-depositions between 20 and 200 nm the contact angle returns and oscillates again between 60° and 70° (indicated as gray band in Figure 7). The present r-DLC series reaches a maximum for 500 nm at about 80° and decreases again to approximately 64° for 1000 nm (Figure 7). As confirmed for f- and r-depositions on HDPE16 and in agreement with results shown above, the changing of contact angle values in the r-DLC series shows clearly a pronounced reorganization on the sample’s surface during deposition. For the 2 nm r-deposition CH2 bonds are formed again with respect to the r-type oxygen plasma cleaned sample (compare Figures 3b and 5). At this stage dangling carbon bonds get more and more saturated and the hydrogenated surface becomes more hydrophobic with a higher contact angle than the r-type oxygen plasma cleaned one.40 With increasing r-deposition, the enhanced interlayer formation between r-DLC and PET strongly changes the chemical structure and morphology. Especially the first steps of the r-deposition process lead to more hydrophilic surfaces (compare 10 nm in Figure 7). Afterward, the progressive rearrangement of the sample surface increases the contact angle value for r-depositions between 20 and 200 nm.16 The maximum at 500 nm in the r-DLC series (Figure 7) is characteristic for the highest polymer-like character of the present coating.40,41 The branched polymerlike configuration of this r-deposition leads to a contact angle value more similar to the raw PET one. Dehydrogenation processes as well as an increasing sp2 content during the

5. CONCLUSIONS Different DLC films have been grown on PET by RF-PECVD with acetylene plasma via an indirect (f-DLC) and a direct (rDLC) deposition method. The resulting composites were analyzed by AFM, contact angle measurements, DRIFT and Raman spectroscopy. The surface morphologies of the two differently deposited coatings show similar grain growth patterns, but f-DLC samples need smaller deposition amounts to develop the same features as the r-DLC ones. For the f-type the average grain height increases up to a deposition of 50 nm, remaining constant thereafter. The average grain area increases continuously. For the r-type both average grain height and area are the same for 200 and 500 nm, while they increase for the 1000 nm r-deposition parallel to the starting of a coalescence phenomenon appearance. The absence of grains for f- and rdepositions thinner than 20 and 200 nm respectively is attributed to the different interlayer formation between PET and the two DLC-types. The epitaxial f- and r-DLC growth mechanisms are found to be independent from different polymeric substrates as PET and HDPE. From the DRIFT analysis no differences were detected between raw PET, f-type cleaned sample and f-depositions up to 20 nm. The nondetectability of CH stretching signals is related to less subplantation processes and minor interlayer effects during fdeposition on PET. From 50 nm f-deposition up to 200 nm both the symmetric and antisymmetric peaks of CH2 stretching modes are revealed, confirming an enhanced sp2 composition of the f-DLC coatings compared to the r-DLC ones. The r-type cleaning process causes instead the loss of CH2 groups from the plastic material surface. The same groups reappear between 2 and 20 nm r-depositions. This observation is described in terms of enhanced subplantation processes and I


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(7) Rohrbeck, M.; Fischer, C. B.; Wehner, S.; Meier, J.; Manz, W. DLC-Coated Pure Bioplastic Foil: Effect of Various Sterilization Methods on the Surface Morphology. Vak. Forsch. Prax. 2014, 26, 42− 47. (8) Robertson, J. Diamond-Like Amorphous Carbon. Mater. Sci. Eng., R 2002, 37, 129−281. (9) Couderc, P.; Catherine, Y. Structure and Physical Properties of Plasma-Grown Amorphous Hydrogenated Carbon Films. Thin Solid Films 1987, 146, 93−107. (10) Kodama, H.; Nakaya, M.; Shirakura, A.; Hotta, A.; Hasebe, T.; Suzuki, T. Synthesis of Practical High-Gas-Barrier Carbon Films at Low and Atmospheric Pressure for PET Bottles. New Diamond Front. Carbon Technol. 2006, 16, 107−119. (11) Zhang, Z.; Song, R.; Li, G.; Hu, G.; Sun, Y. Improving Barrier Properties of PET by Depositing a Layer of DLC Films on Surface. Adv. Mater. Sci. Eng. 2013, 2013, 1. (12) Boutroy, N.; Pernel, Y.; Rius, J. M.; Auger, F.; von Bardeleben, H. J.; Cantin, J. L.; Abel, F.; Zeinert, A.; Casiraghi, C.; Ferrari, A. C.; Robertson, J. Hydrogenated Amorphous Carbon Film Coating of PET Bottles for Gas Diffusion Barriers. Diamond Relat. Mater. 2006, 15, 921−927. (13) Wang, J.; Huang, N.; Pan, C. J.; Kwok, S. C. H.; Yang, P.; Leng, Y. X.; Chen, J. Y.; Sun, H.; Wan, G. J.; Liu, Z. Y.; Chu, P. K. Bacterial Repellence from Polyethylene Terephthalate Surface Modified by Acetylene Plasma Immersion Ion Implantation-Deposition. Surf. Coat. Technol. 2004, 186, 299−304. (14) Cowie, J. M. G.; Arrighi, V. Polymers: Chemistry and Physics of Modern Materials; Taylor & Francis Group: Boca Raton, FL, 2007. (15) Fischer, C. B.; Rohrbeck, M.; Wehner, S.; Richter, M.; Schmeißer, D. Interlayer Formation of Diamond-Like Carbon Coatings on Industrial Polyethylene: Thickness Dependent Surface Characterization by SEM, AFM and NEXAFS. Appl. Surf. Sci. 2013, 271, 381−389. (16) Catena, A.; Agnello, S.; Rösken, L. M.; Bergen, H.; Recktenwald, E.; Bernsmann, F.; Busch, H.; Cannas, M.; Gelardi, F. M.; Hahn, B.; Wehner, S.; Fischer, C. B. Characteristics of Industrially Manufactured Amorphous Hydrogenated Carbon (a-C:H) Depositions on HighDensity Polyethylene. Carbon 2016, 96, 661−671. (17) Catena, A.; McJunkin, T.; Agnello, S.; Gelardi, F. M.; Wehner, S.; Fischer, C. B. Surface Morphology and Grain Analysis of Successively Industrially Grown Amorphous Hydrogenated Carbon Films (a-C:H) on Silicon. Appl. Surf. Sci. 2015, 347, 657−667. (18) Weiler, M.; Lang, K.; Li, E.; Robertson, J. Deposition of Tetrahedral Hydrogenated Amorphous Carbon Using a Novel Electron Cyclotron Wave Resonance Reactor. Appl. Phys. Lett. 1998, 72, 1314−1316. (19) Morrison, N. A.; Muhl, S.; Rodil, S. E.; Ferrari, A. C.; Nesládek, M.; Milne, W. I.; Robertson, J. The Preparation, Characterization and Tribological Properties of TA-C:H Deposited Using an Electron Cyclotron Wave Resonance Plasma Beam Source. Phys. Status Solidi A 1999, 172, 79−90. (20) Kleinen, L.; Böde, U.; Schenk, K.; Busch, H.; Bradenahl, J.; Müller, S. C.; Hillebrands, B.; Laube, N. Amorphous Carbon Coatings Inhibit Crystalline Biofilm Formation on Urological Implants. Plasma Processes Polym. 2007, 4, S386−S391. (21) Neyts, E.; Bogaerts, A.; van de Sanden, M. C. M. Reaction Mechanisms and Thin a-C:H Film Growth from Low Energy Hydrocarbon Radicals. J. Phys.: Conf. Ser. 2007, 86, 1−15. (22) Liu, D.; Benstetter, G.; Lodermeier, E.; Vancea, J. Influence of the Incident Angle of Energetic Carbon Ions on the Properties of Tetrahedral Amorphous Carbon (ta-C) Films. J. Vac. Sci. Technol., A 2003, 21, 1665−1670. (23) Armaroli, T.; Bécue, T.; Gautier, S. Diffuse Reflection Infrared Spectroscopy (DRIFTS): Application to the In Situ Analysis of Catalysts. Oil Gas Sci. Technol. 2004, 59, 215−237. (24) D’Souza, L.; Devi, P.; Kamat, T.; Naik, C. G. Diffuse Reflectance Infrared Fourier Transform Spectroscopic (DRIFTS) Investigation of E.Coli, Staphylococcus Aureus and Candida Albicans. Indian J. Mar. Sci. 2009, 38, 45−51.

is in line with the more pronounced interlayer formation between r-DLC and PET. Between the 50 and 500 nm rdeposited coatings a polymer-like DLC network is developed. By comparing DRIFT and Raman results of increasing rdeposition between 500 and 1000 nm a transition from a polymer-like to a more cross-linked DLC network occurs. This transition is clearly related to the folding of sp2 chain parts in the r-DLC network through intrinsic stress release phenomena caused by the dehydrogenation of CH2 groups. Contact angle measurements are in line with these findings. It is furthermore suggested that the film growth mechanisms for both DLC-types by RF-PECVD technique with acetylene plasma develop in the same way also for different substrates such as silicon (100) and HDPE.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02113. Additional experimental details as described in the text. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: chrbfi[email protected]. Phone: + 49 261 287 2345. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C. and C.F. gratefully acknowledge the financial support provided by the German Research Foundation DFG (Deutsche Forschungsgemeinschaft) through the project FI 1802/2-1. Q.G. thanks the DAAD RISE program that gave her the opportunity to be a trainee student working abroad at the surface science research group of Koblenz-Landau University in Koblenz. All authors thank Heinz Busch and Falk Bernsmann (NTTF Coatings GmbH, Rheinbreitbach, Germany) for the industrial coating of the samples and Joachim Scholz for the DRIFT equipment. The authors acknowledge the scientific collaborations between the Department of Physics of the University of Koblenz-Landau, the Department of Physics and Chemistry of the University of Palermo, and the Department of Chemistry of the University of Koblenz-Landau.

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REFERENCES

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Morphological and Chemical Evolution of Gradually Deposited

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