ARTICLE pubs.acs.org/JPCC
High Superhydrophobicity Achieved on Poly(ethylene terephthalate) by Innovative Laser-Assisted Magnetron Sputtering C. Becker,*,† J. Petersen,†,‡ G. Mertz,† D. Ruch,† and A. Dinia‡ † ‡
Department of Advanced Materials and Structures, CRP Henri Tudor, 66 rue de Luxembourg, L4221 Esch sur Alzette, Luxembourg Institut de Physique et Chimie des Materiaux de Strasbourg, (IPCMS) UMR 7504 CNRS-UdS (ECPM), 23 rue du L!ss, BP 43, 67034 Strasbourg Cedex 2, France ABSTRACT: The main development described in this paper consists of combining magnetron sputtering with a Nd:YAG laser to provide additional properties to the deposited thin films. The combination of both of these techniques has been revealed as an innovative methodology by the complementary effects able to respond to all the technical requirements of the desired ultrathin film properties. Plasma sputter-deposited fluoropolymers have been deposited on poly(ethylene terephthalate) (PET) substrates by this innovative Nd:YAG laser-assisted magnetron sputtering technique from a poly(tetrafluoroethylene) (PTFE) target. Laser irradiation of the substrate, occurring directly in the deposition chamber simultaneous to the first step of the deposition process, exhibits the advantage of inducing a micropatterning of the polymer surface by a photoablation process and then affords a high control of the roughness and microstructure. Deposition parameters of the fluoropolymer thin film have been optimized to obtain the maximum F/C atomic ratio to optimally functionalize the structure with fluoro groups; the F/C ratio was evaluated by X-ray photoelectron spectroscopy. Finally, the combination of the optimized sputtering parameters and the Nd:YAG laser treatment of the PET substrates led to superhydrophobic surfaces with a high water contact angle (θ = 160°). The films' morphology and topography were studied by scanning electron microscopy and atomic force microscopy, respectively.
’ INTRODUCTION Fluoropolymer films are widely investigated due to specific requirements in the development of new technologies and their numerous fields of application, such as superhydrophobicity, selfcleaning,1 antifogging, and antisticking,25 mainly due to their extremely low surface energies. More recently, fluoropolymer thin films have shown a particular interest in the field of FEDs for their low dielectric constant (k = 1.52.5) and, particularly, as a permeation barrier for oxygen and/or water vapor. Numerous methods have been reported in the past decade to process superhydrophobic surfaces by various chemical68 or physical approaches.914 Indeed, wet chemical reactions represent a smart and easy way to obtain rough surfaces on polymeric substrates and to modify their chemical composition.15,16 However, deposition of organic fluorocarbon thin films by radio frequency (rf) magnetron sputtering from a polymeric solid PTFE target on diversified substrates received considerable attention and has been extensively studied by Biederman et al.17,18 and more recently by Zhang et al.19 Magnetron sputtering still represents the most promising way to obtain dense ultrathin organic films. The advantage of this pure physical process concerns its high-energy environment, which induces strong bond formation between the thin coating species and the polymeric substrate, leading to a high adherence and a perfect homogeneity of the organic coating. r 2011 American Chemical Society
Two basic approaches can be considered to increase the surface hydrophobicity: (i) the decrease of the surface energy by modifying the surface chemistry and (ii) the increase of the surface roughness, leading to the improvement of the effective surface area, according to Wenzel20 and CassieBaxter21 theories. Only the combination of both approaches can lead to superhydrophobicity properties.2224 The Wenzel model represents the apparent water contact angle θW of a droplet in a wet-contact regime with a rough surface, for which the liquid remains in contact with the whole solid surface. Hydrophobicity can be also evaluated quantitatively by the Wenzel equation cos θW ¼ r cos θFLAT
ð1Þ
where θW is the observed angle on the rough surface, θFLAT is the equilibrium contact angle on the flat surface of the same chemical character, and r is the roughness ratio, which can be defined as the actual and projected solid surface area (r = 1 for a flat surface and > 1 for a rough surface). This relation implies that, for a welldefined chemical surface, hydrophophobic properties are enhanced by the roughness. Received: January 18, 2011 Revised: April 5, 2011 Published: May 05, 2011 10675
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In the CassieBaxter state, the droplet sits on top of the structure's asperities. Indeed, water cannot penetrate and wet the asperities; thus, air is trapped under the droplet, which is finally in contact with a composite surface formed by the solid substrate and air. The contact angle obtained (θC) for such heterogeneous solid surfaces can be evaluated by the CassieBaxter equation cos θC ¼ f SOLID cos θSOLID þ f AIR cos θAIR
ð2Þ
. With where the air area fraction is described as f = 1 f θAIR = π for a flat surface, the WCA (water contact angle) of the solid fraction (θSOLID) is identical to that of the flat surface, θFLAT; eq 2 can be considered as AIR
SOLID
cos θC ¼ f SOLID cos θFLAT ð1 f SOLID Þ
ð3Þ
Therefore, according to eq 3, by decreasing the solidliquid contact area, the hydrophobic properties are enhanced. Water contact angle hysteresis (WCAH) values, being equal to the difference between advancing and receding contact angles, reflect the surface heterogeneities as well as surface chemistry and microstructure of the material. The CassieBaxter wetted regime is characterized by low WCAH values, whereas for the Wenzel state associated to more random or irregular microstructures,25,26 higher WCAH values are observed. Numerous methods have been developed to control the roughness and especially the micro- and nanoscale combined structure, which represents the most promising engineered surfaces related to superhydrophobic properties. To prepare regular microstructures, the deposition or grafting of nanoparticles, such as SiO2, is envisaged, followed by chemical vapor deposition of a fluorinated adsorbate.27 To achieve a perfect regular microstructure, a photolithographic approach is generally preferred,28,29 followed by fluoroplasma treatments or silica coating deposition using a PECVD process. Surfaces with a random patterning, close to the real lotus leaf microstructure, can be easily elaborated by etching processes by significantly increasing the roughness by plasma or chemical30 treatments. Plasma treatments31 represent considerable advantages to attaining superhydrophobic performances, especially due to the ability, in the same process, to increase the roughness and to modify the surface free energy by grafting specific functional groups.3234 Only a few studies35,36 to our knowledge report the preparation of superhydrophobic surfaces exhibiting a static contact angle higher than 150°, using rf magnetron sputtering from a PTFE target, allowing the control of the coating roughness by working under limit sputtering pressure conditions (argon pressure > 5 101 mbar). In a recent interesting paper,37 it has been demonstrated the possibility to obtain superhydrophobic surfaces by rf magnetron sputtering of PTFE by considerably increasing the substrate/target distance. In this paper, we clearly show that the wetting behavior is governed by the chemical surface composition of the coating that is directly linked to the magnetron sputtering parameters, such as the rf power, the sputtering gas composition, the sputtering pressure, and the target/substrate distance. To deal with the superhydrophobic surface by rf magnetron sputtering in standard pressure conditions, the innovative approach of this study is to control the surface sample roughness and microstructure by focusing a Nd:YAG laser directly on the semicrystalline PET (χ = 35%) substrate simultaneously to the first step of the fluoropolymer film growth, which can be finally considered as an in situ sequential irradiation process. Indeed, this promising method allows achieving topographic modifications in a
Figure 1. Three-dimensional cross-sectional representation of the laser-assisted magnetron sputtering reactor: (1) quartz crystal microbalance, (2) magnetron device (with the plasma representation), (3) rotary sample holder, (4) fused silica window mounted on an adjustable flange (with laser beam representation), (5) pumping system, and (6) sputtering gas introduction.
short treatment time and in controlled conditions and consists of the photoablation of the PET substrate by pulsed laser irradiation at 266 nm of the fourth harmonic of a Nd:YAG laser.
’ EXPERIMENTAL DETAILS Plasma Reactor and Operating Conditions. All films have been processed in a custom-made high-vacuum reactor (Figure 1), equipped with an rf unbalanced planar magnetron. A dual flange has been especially developed to directly focus a pulsed Nd:YAG laser (266 nm and 10 Hz) on the sample polymer surface without affecting the deposition process. The angle of the laser flange, equipped with a fused silica window, can be adjusted to improve focusing accuracy. A 1 in. pristine PTFE (99%) disk (1/4 in. thick) from Goodfellow was used as a target and backside-bonded to a conductive elastomer disk and fixed on the magnetron head. Substrates have been fixed on a rotary sample holder with a substrate/target distance of 100 mm. The vacuum chamber was initially pumped down to a pressure below 1 107 mbar. Sputtering gas was then introduced into the chamber to the working pressure by a leak valve at a constant flow rate. A laser has been used under this high-vacuum condition to pattern the PET surface. Laser and magnetron devices have been switched on simultaneously, and after 5 s, the laser was stopped and only sputtering of PTFE occurred. A top-hat beam shaper optic has been used to collimate the Gaussian incident beam. This shaper, associated with the optical pathway necessary to focus the laser beam in the deposition chamber, finally applied a 10 mm2 uniform intensity well-defined spot on the PET substrate. 10676
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Figure 2. Comparison of the Nd:YAG (266 nm) laser irradiated PET with a fluence of 150 and 300 mJ/cm2 and evaluation of the roughness by white light interferometer (WLI) measurements.
Figure 3. SEM observations of the PET substrate after laser irradiation with a fluence of 300 mJ/cm2 cross-sectional observation.
The sputtering chamber was equipped with a quartz crystal microbalance (QCM) to monitor the deposition rates, which is calibrated by measuring the final thickness of the coating by SEM observation in the sample cross section. All of the deposition times were finally adjusted to obtain fluoropolymer thin coatings with a target thickness of 80 nm. Surface Characterization. FTIR-ATR (attenuated total reflectance) analyses have been performed with a TENSOR 27 from Br€uker on sputtered films to investigate the chemical structure obtained in the different deposition conditions. X-ray photoelectron spectroscopy (XPS) measurements have been performed with a SPECS Phobios 150 equipment employing a monochromatic Al KR radiation operating at 200 W with an anode voltage of 12.69 kV and a pressure of 1 109 mbar. The
XPS spectra were referenced with respect to the 284.6 eV C 1s to eliminate charging effects. C 1s spectra were decomposed by fitting a GaussianLorentzian mixture function. Morphology and Topography. The effect of the laser fluence on the PET samples' topography was evaluated by a Quanta FEG environmental scanning electron microscope (ESEM) from FEI (Quanta-FEG-200). The global roughness was assessed by a white light interferometer (WLI) method using a Wyko NT 3300 Optical 3D Profiling System. Nanoroughness of the ultrathin fluoropolymer coatings has been investigated with a Nanoscope Dimension 3100 atomic force microscope in tapping mode. Water Contact Angle. Contact angles were measured on a OCA 15þ contact angle measuring instrument from Dataphysics, equipped with a CCD camera. Static water contact angles 10677
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were measured with a 4 μL deionized water droplet. The dynamic contact angle (advancing and receding contact angles) was determined automatically during growth and shrinkage of the droplet of distilled water by a drop-shape analysis. Three measurements were made on each surface. The WCAH, which is equal to the difference between advancing and receding contact angles, was then determined.
’ RESULTS AND DISCUSSION The study of PET irradiation by a pulsed Nd:YAG laser at 266 nm showed a partial surface destruction, which is principally due to photochemically initiated electron excitation, followed by the polymer surface decomposition into monomer and gases with subsequent thermal decomposition of these fragmented molecules to form the plume. Several excellent studies have demonstrated the increase of polymer roughness by a laser photoablation process,3840 and some physical models for microscopic simulation of laser ablation of organic solids have been proposed.41 The main changes observed on the ablated polymers' surface have been an increase of the surface roughness, modification of the crystalilinity, and changes in the surface oxidation state. Formation of conical structures on the photoablated PET substrates can be explained by the so-called impurity shielding theory. This is mainly due to formation of small spherical carbon islands on the polymer substrate during its photoablation, which will finally become the cone tips, shielding the original polymer from the next incident photons. For strongly absorbing polymers, such as PET, photoablation phenomena and cone formation are sharply dependent on the laser fluence and the polymer crystallinity. An increase in the polymer crystallinity provides a more regular structural cone distribution. The obtained results showed that the PET roughness increases with the ablation fluence. However, with a fluence up to 300 mJ/cm2, the Ra and the Rms values evaluated by the white light interferometer from Veeco (Figure 2) were 2.01 and 2.43 μm, respectively. These parameters have been optimized and adapted to produce new flexible PET substrates with controlled surface micropatternings inside the deposition chamber under low vacuum pressure, previous to the deposition process (Figure 3). rf magnetron sputtering is of a high interest in the processing of polymer thin coatings, especially due to the numerous deposition parameters that can be modulated. It can be clearly determined that the deposition rate of the sputtered fluoropolymer films is of the order of several nanometers per minute and is highly dependent on the sputtering parameters using a PTFE target (Figure 4). Argon plasma gave rise to the highest deposition rate. Moreover, the increase of the deposition rate was noticed by increasing the power and decreasing the working pressure and the targetsubstrate distance for both gases used (Ar and N2). These results are totally in accordance with those previously mentioned by Biederman et al.; however, in our reactor configuration, deposition performed with an argon working pressure higher than 4 101 mbar failed to lead to any significant deposition in the mentioned conditions. This result can be explained by an etching effect that occurred simultaneously for such a pressure. Although all the above cited parameters and their effect on the thin coating surface chemical composition has been investigated, only the effects of the rf power and the sputtering gas are presented in detail in this paper. It is well known that FTIR bands detected between 700 and 800 cm1 are related to the amorphous phase of fluoropolymers (Figure 5). The presence of an amorphous phase is characteristic of a plasma polymer structure. In all sputtered fluoropolymer films,
Figure 4. Deposition rate of fluorocarbon plasma polymer films as a function of (a) rf magnetron power with a sputtering pressure of 2 102 mbar and (b) sputtering pressure with an rf magnetron power of 200 W and for both Ar (blue) and N2 (red) gases.
Figure 5. FTIR-ATR spectra of fluoropolymer ultrathin films deposited with Ar and N2 as the sputtering gas.
the vibration bands observed at 1240 and 1125 cm1 are assigned to asymmetric CF2 and CF3 stretching, respectively. The band observed at 920 cm1 is assigned to CF stretching. The CH stretching region showed that both CH2 and CH3 are present in the coating because peaks have been observed at 28502920 and 2960 cm1, respectively. These analyses evidenced that fluoropolymer films obtained from a PTFE target were constituted by only carbon, hydrogen, and fluoro elements. The C 1s spectra were resolvable into five regions: CF3, CF2, CF, CCF, and CC bonding at BE = 294.1 eV, BE = 292.0 eV, BE = 290.0 eV, BE = 287.4 eV, and BE = 285.0 eV, respectively (Figure 6a). Data obtained after peak deconvolutions and 10678
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integrations allowed determining the effect of sputtering parameters on the samples' surface chemistry, especially to evaluate the fluorine abundance at the surface through the [F]/[C] ratio by the following equation:
50 to 200 W and then became constant up to 300 W with argon as the sputtering gas. This ratio can be correlated with the crosslinked structure, the cross-linking increasing for higher sputtering power. The obtained results regarding the effect of the argon pressure were in accordance with those reported by Biederman et al.42 with these parameter variations. Moreover, an increase of the [F]/[C] ratio has been observed with the pressure of argon. This result is in accordance with the expected results. Indeed, it is well known that, by increasing the sputtering gas pressure or by decreasing the rf power, the mean energy of the plasma ions decreases and less fragmentation of the polymeric target occurs. Hydrophobic and superhydrophobic properties were first evaluated by means of static water contact angle measurements using the sessile drop method at 25 °C and constant relative humidity. Contact angle results can be directly correlated with the [F]/[C] ratio obtained on sputtered films. The highest contact angle values have been obtained for low power and high sputtering gas pressure. Unfortunately, for such sputtering conditions, the deposition rate was very low (Figure 4) and combining both effects could not be achievable. The maximum value of the static contact angle for an efficient deposition rate has not overcome 125.4° from the PTFE target with argon. In a previous study,42 the influence of N2 as a sputtering gas has been evaluated by high-resolution XPS and mainly by analyzing the plasma composition during the sputtering, revealing that CN and CNF fragments are produced by gas-phase reaction in the discharge and also showing changes in CFx species concentration in the plasma and on the substrate surface. In the case of N2 sputtering of the PTFE target, we
½F 3 CF3% þ 2 CF2% þ CF% ¼ ½C 100 XPS results highlight the effect of power and sputtering gas on the surface chemical composition at a pressure of 2 102 mbar of Ar and N2 and for the maximum [F]/[C] ratio at a pressure of Ar of 4 101 mbar (Table 1). It has been shown that the [F]/[C] ratio decreased with increasing the sputtering power from
Figure 6. XPS C 1s core level of (a) the sputtered film from PTFE with Ar as the sputtering gas and (b) the sputtered film from PTFE with N2 as the sputtering gas.
Figure 7. 3D AFM image of the fluoropolymer ultrathin coating on a PET substrate with optimized deposition conditions.
Table 1. Power, Gas, and Pressure Dependence of Surface Composition and [F]/[C] Ratio of the Coating, Determined by XPS Measurements from the PTFE Target, Correlated with Static Contact Angle Values Obtained by the Sessile Drop Method
PTFE target
power (W)
CF3 (%)
CF2 (%)
CF (%)
CCF (%)
4 101 mbar Ar
50
24.6
25.3
29.4
2 102 mbar Ar
50
21.2
26.2
30.9
100
17.5
25.6
150
17.3
200 300 2 102 mbar N2
CN (%)
CC (%)
[F]/[C]
contact angle values
15.4
5.3
1.54
125.4 ( 1.5
16.2
5.5
1.47
119.3 ( 1.9
31.0
20.3
5.6
1.35
118.1 ( 1.7
25.8
28.2
23.5
5.1
1.32
114.3 ( 1.4
14.9 18.9
28.4 19.4
26.3 36.3
24.8 19.6
5.5 5.7
1.28 1.32
115.2 ( 1.7 110.7 ( 1.7
50
5.4
27.1
11.2
21.3
35.0
0.81
92.6 ( 1.0
100
14.4
28.2
20.1
21.8
15.5
1.19
108.2 ( 1.2
200
28.6
12.0
22.4
24.6
12.4
1.32
112.4 ( 1.2
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Figure 8. SEM observations of the microstructure obtained (laser spot) for a fluence of 300 mJ/cm2 on a PET surface and corresponding contact angles obtained on both areas after the deposition of a 80 nm thick fluoropolymer from the PTFE target with Ar as the sputtering gas at a pressure of 4 101 mbar and a sputtering power of 50 W.
Figure 9. Water contact angle in dynamic mode measured on the fluoropolymer from the PTFE target with Ar as the sputtering gas at a pressure of 4 101 mbar and a sputtering power of 50 W.
Figure 10. Image of a water droplet deposited on superhydrophobic PET films.
evidenced that CFx are in high concentration (Figure 6b). We observed an important increase of CF3 species (Table 1) by increasing the sputtering power, which led to an evaluated [F]/[C] ratio of 1.32 obtained with such a power at a pressure of 2 102 mbar. Obtained results with N2 led to different reactions in the plasma than those obtained with Ar sputtering gas, mainly due to the fragments emitted from the target, which are completely different. With nitrogen, depending of the sputtering pressure and power, we evidenced the higher capacity to maintain CF3 species in the discharge than with argon. This can be mainly explained by the differences in the species' energy and density in the plasma, which are highly influenced by the gas mixture and generally evaluated lower with N2 than with argon. Naturally, XPS measurements performed on these samples showed the presence of additional CN fragments increasing with the N2 sputtering pressure and rf power. The FTIR spectrum for the sputtered thin film from PTFE deposited with N2 (Figure 5) showed additional bands at 1340 and 1750 cm1 assigned to the vibration of the CN groups
incorporated in the plasma polymer. Contact angle values obtained on the N2 sputtered films were lower than those obtained for sputtered films with argon mainly due to the presence of nitrogen, suggesting the presence of amine functions. The surface roughness of the 80 nm thick fluoropolymer ultrathin coatings was evaluated by AFM measurement, according to the optimized rf magnetron deposition parameters (Ar as the sputtering gas at a pressure of 4 101 mbar, a sputtering power of 50 W, and a deposition duration of 10 min), which were determined from the sample exhibiting the higher [F]/[C] ratio. Ra and Rms values obtained were 4.8 and 2.6 nm, respectively (Figure 7). The Nelumbo nucifera lotus leaf microstructure has been, therefore, perfectly mimicked by the combination of both treatments. Indeed, the laser effect produces the single microroughness on the PET substrate and the fluoropolymer ultrathin layer covered this microstructure with a nanoroughness to mimic a real lotus leaf surface with a final dual roughness. PET surface micropatterning optimization by laser irradiation combined with the fluoropolymer coating deposition, leading to an important 10680
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The Journal of Physical Chemistry C decrease of the surface energy and then introducing a nanoscale surface roughness, achieves a maximum static contact angle value of 160.8° ( 2.7 (Figure 8). According to Wenzel and CassieBaxter theories, the role of the roughness is essential to reach superhydrophobic properties. The water contact angle in dynamic mode has been evaluated for the optimized deposition conditions associated with the optimized laser treatment (Figure 9): an advancing (θA) contact angle of 157.9° ( 0.4° and a receding (θR) contact angle of 147.7° ( 0.7°. A WCAH value of 10.2° ( 0.8° has been evaluated, which corresponds to the limit between a Wenzel and a Cassie regime. Low contact angle hysteresis has been obtained on hierarchical surfaces both by minimizing the solid area fraction and by controlling continuity of the solid component according to C. Priest et al.43 The WCAH value observed in our case is mainly associated with the presence of topographic heterogeneities observed on the PET surface microstructure initiated during the laser patterning process.
’ CONCLUSION In this paper, we reported that innovative laser-assisted magnetron sputtering represents a smart technique to achieve superhydrophobic properties on a PET substrate (Figure 10). By this novel method, hierarchical surfaces have been elaborated with a good control of the surface topographic microstructure and chemical composition in the same vacuum chamber. However, we demonstrated the necessity of parameter optimization for both laser irradiation and the deposition process. A maximum static contact angle of 160.8° ( 2.7 has been finally obtained under optimized conditions. The WCAH value of 10.2 ( 0.8 evaluated in dynamic mode can be correlated with the presence of topographic heterogeneities on the PET surface, which are inevitably generated during the photoablation process. A significant advantage of such a method is certainly due to the versatility of the laser treatment, which represents an interesting combination with the magnetron sputtering deposition technique and allows one to foresee a good potential with numerous other substrates. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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