Annealing of gold nanolayers sputtered on polyimide and polyetheretherketone

Thin Solid Films 616 (2016) 188–196

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Annealing of gold nanolayers sputtered on polyimide and polyetheretherketone Ondřej Kvítek a,⁎, Dominik Fajstavr a, Alena Řezníčková a, Zdeňka Kolská b, Petr Slepička a, Václav Švorčík a a b

Department of Solid State Engineering, University of Chemical Technology, Technicka 5, Prague, 166 28, Czech Republic Materials Centre of Usti nad Labem, Faculty of Science, J.E. Purkyne University, Usti nad Labem, 400 96, Czech Republic

a r t i c l e

i n f o

Article history: Received 10 November 2015 Received in revised form 24 June 2016 Accepted 10 August 2016 Available online 10 August 2016 Keywords: Polymers Gold Nanostructures Sputtering Annealing Atomic force microscopy Thermal dewetting

a b s t r a c t Annealing of thin Au films sputtered on polyimide (PI) and polyetheretherketone (PEEK) polymer substrates was carried out to study influence of substrate on surface morphology transformation and possible “dewetting”, which was observed in the past on glass substrates. Thermal stability of substrates was studied by differential scanning calorimetry, PI substrate was found to be stable up to 400 °C, PEEK undergoes crystallization at circa 170 °C and melts at circa 320 °C. Therefore, annealing temperatures of 200 °C and 300 °C were chosen for PEEK and PI respectively. Surface morphology of the samples was studied by atomic force microscopy. Annealing of the PEEK substrate leads to significant changes of its surface structure, a rugged structure is formed. A sufficiently thin Au layer is then broken into islands, thicker layers however cover the substrate structure. Annealing of Au layers on PI substrate leads to formation of coarser Au nanoislands with narrower size distribution, but the layer remains continuous. The UV–Vis absorption spectra show a rise of surface plasmon resonance peak after annealing, which documents the formation of uniform nanostructure. The peak is most evident at the thinnest Au layers. The results suggest the PI is an interesting substrate capable of supporting very thin metal films and preventing their dewetting during annealing. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Thin metal films on polymer substrates are subject of many studies recently [1–4] thanks to recent development of elastic electronics for displays [5], electronic textiles [6] or flexible solar cells [7]. For these applications, stable components are necessary to achieve a required endurance of the device. One of the most important properties in this respect is thermal stability. While thermal stability of polymers [8,9] and thin metal films [10,11] are well known separately, the effects of heating a material created by the combination of those components are studied just recently [12–14]. Surface structure of polymer materials is determined by their preparation process. The surface morphology of a polymer foil that was prepared by rolling will be different from a foil prepared by blowing or a sheet prepared by extrusion or casting of either a melted polymer or its solution. The method used to prepare a sheet of a specific polymer has to account for its properties. Certain polymers cannot be melted, others are insoluble in most solvents. Therefore for example polytetrafluoroethylene cannot be mechanically formed after its polymerization and its sheets are prepared by cutting a block of the material [15]. The chemical structure of the polymer chain has also a significant ⁎ Corresponding author. E-mail address: [email protected] (O. Kvítek).

http://dx.doi.org/10.1016/j.tsf.2016.08.025 0040-6090/© 2016 Elsevier B.V. All rights reserved.

impact on the surface morphology. For example a semicrystalline polymer can show specific surface features [16]. The chemical composition has also influence on the manufacturing properties of the polymer such as its viscosity, formability or gelation. Therefore different polymers show various surface structures that cannot be controlled as precisely as in the case of crystalline materials. If we specifically focus on processes occurring in polymers at elevated temperatures, secondary phase transitions have significant influence on the morphology as well. At temperatures higher than the glass transition point (Tg), segments of the polymer chain are able to shift leading in some cases to secondary crystallization. If the melting temperature (Tm) is exceeded, polymer chains are able to relax the strain in the material and a new surface structure is formed [15]. All these processes can influence a morphology of a metal film prepared on the polymer substrate assuming it is thin enough. Moreover, annealing of thin metal films on flat surfaces was found to lead to their morphology transformation as well [11,17]. With annealing, thin films deposited on glass and silicon substrates were found to transform from continuous coverage to island-like structure in a process called “solid-state dewetting” [18–21]. If an appropriate metal is used in the process, localized surface plasmon resonance band can be observed in its absorption spectrum due to the annealing induced nanostructure evolution [11,17]. This process is based on a solid state diffusion of the metal atoms over the surface of the substrate at an elevated

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their supposedly sufficient thermal stability polyetheretherketone (PEEK) and polyimide (PI) were chosen as substrates. The thermal properties of the substrates were determined by differential scanning calorimetry (DSC) measurements. The Au films were prepared on the substrates by sputtering and the samples were subsequently annealed at 200 °C (PEEK) and 300 °C (PI) for 1 h. The surface morphology evolution was studied by atomic force microscopy (AFM), changes of optical absorption were determined by UV–Vis spectroscopy and the surface chemistry and polarity by electrokinetic analysis. 2. Experimental details 2.1. Sample preparation

Fig. 1. DSC curves of the PEEK and PI polymers. Glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures of PEEK are labeled.

temperature and it leads to minimization of the surface free energy. The equilibrium on the interface of the metal and the substrate is believed to be the driving force of the process [18,22]. Therefore the interactions between the metal film and the substrate are important in this matter and it is expected the composition and surface morphology of the substrate has an influence in this [23]. In the case of glass and silicon substrates, the “dewetting” process of Au was observed at temperatures of about 300 °C, where most polymers are not stable. To study these interactions on polymer materials we must therefore carefully choose appropriately thermally stable polymers. In previous works, our group studied the “dewetting” process of thin Au films on glass substrate [11,22]. In this work, the effect of annealing on properties of thin Au films on polymer substrates was studied. For

Thermal stability of PEEK and PI substrates (films 50 μm thick, supplied by Goodfellow Ltd., UK) was tested by annealing 2 × 2 cm2 samples in Binder FED 23 oven for 1 h at 150–300 °C. On the basis of these measurements and supplier information 200 °C and 300 °C annealing temperatures were chosen for thin Au films on PEEK and PI respectively. Thin Au films were prepared on circular polymer substrates (3 cm diameter, cut out of the supplied films with a steel stamper) by cathode sputtering method in a Balzers sputter coater SCD 050 device (BalTec AG, CH). Au target (99.95% purity, Safina a.s., CZ) was placed together with samples in a chamber evacuated to circa 2 Pa. Then, Ar work gas (99.99%, SIAD Czech s.r.o., CZ) was introduced at a pressure of circa 4 Pa and at 360 V a sputtering current of 20 mA occurred. Different thicknesses of the Au film were achieved by different deposition times (10–300 s). No damage to the substrate caused by landing Au atoms was observed during the sputter deposition. Samples with PEEK substrate were then annealed at 200 °C for 1 h, samples with PI substrate at 300 °C for 1 h. 2.2. Analytical methods Differential scanning calorimetry (DSC) measurements were performed in DSC-2920 TA calorimeter (Thermal Support Inc., US) in nitrogen atmosphere to determine the thermal properties of the substrates. The DSC curves were obtained for temperature range of 20–400 °C. Samples prepared for this analysis weighed 4.2 mg and the heating rate was 10 °C·min−1. The crystallinity of PEEK samples was

Fig. 2. UV–Vis absorption spectra of PEEK and PI substrates at room temperature (RT) and annealed at 150–300 °C.

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Fig. 3. AFM images of PI substrate (A) unannealed and (B) annealed at 300 °C and PEEK substrate (C) unannealed and (D) annealed at 200 °C.

determined by DSC analysis taking into account theoretical heat of fusion of PEEK foil with 100% crystallinity (ΔHf0 = 130 J·g−1) [24]. Crystallinity portion of studied PEEK foil was 28% [16]. Thickness of the deposited Au films was determined by gravimetry. Samples of polymer substrates were repeatedly weighed on UMX2 Ultra-microbalance device (Mettler Toledo Inc., US) before and after the Au deposition. From the weight difference Δm, sample radius r and Au density ρAu, effective thickness h of the Au layer was determined: h = Δm / (ρAuπ r2). Standard deviation of the weighing did not exceed 3 μg. Gravimetry was also employed to determine decrease of weight of the samples after annealing. The UV–Vis spectroscopy was employed to determine the changes of the substrates after annealing at different temperatures and to study absorption of the thin Au films before and after annealing. To obtain the spectra, Lambda 25 UV/Vis Spectrophotometer (Perkin-Elmer Inc., US) was used. Spectra were captured in range of 400–1000 nm using a halogen lamp at scanning rate of 240 nm·min−1 with 1 nm data collecting interval. Surface morphology of the substrates and the thin Au films were examined by atomic force microscope Digital Instruments CP II (Bruker Corp., US). Tapping mode was employed with silicon P-doped probes RTESPA-CP with a spring constant of 20–80 N·m−1. To represent the surface morphology as well as possible, scans of various size (1 × 1, 2 × 2 and 10 × 10 μm2) were captured where appropriate. Sheet electrical resistance of the samples was determined by a twopoint method using KEITHLEY 487 picoammeter. For this measurement additional Au contacts, about 50 nm thick, were prepared by sputtering. The electrical measurements were performed at a pressure of about 10 Pa in a shielded chamber to minimize influence of atmospheric humidity and stray current. Electrokinetic analysis (zeta potential) of polymer substrates and samples with thin Au nanolayers was performed on SurPASS device (Anton Paar GmbH, AT). Samples were placed in a cell with adjustable gap in contact with the electrolyte (0.001 mol·L−3 KCl). For each measurement a pair of samples was fixed to sample holders (with a cross section of 20 × 10 mm2 and a gap of 100 μm). All samples were measured four times at a constant pH value with relative error of 10%. For determination of the zeta potential streaming current method was

used and Helmholtz-Smoluchowski (HS) equation was applied to calculate zeta potential [25]. 3. Results and discussion In previous works, our group studied the “thermal dewetting” process of thin Au films on glass substrate. To study the influence of the substrate on the process, polymer foils were employed in the present

Fig. 4. Gravimetrically determined dependence effective Au layer thickness on sputtering time. Weight decrease of samples after annealing in inset.

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work as substrates for the Au nanolayers. Because the dewetting takes place at an elevated temperature, polymers with sufficient thermal stability had to be chosen. The supplier information suggests that PEEK and PI polymers should have high enough upper working temperature of 250 and 250–320 °C respectively. To evaluate this fact, DSC measurements were performed on the as supplied polymer foils and the results can be seen in Fig. 1. The DSC curve of PEEK polymer obtained from the measurement is similar to those in literature [16]. The endothermic process occurring at 144.4 °C can be attributed to glass temperature transition, which allows segments of the polymer chains to rotate and form crystallites, which is represented by the exothermic peak at 169.5 °C (the secondary crystallization temperature). The endothermic peak at 319.6 °C corresponds to the melting point of the crystalline part of the polymer. The supplier therefore suggests the polymer can be worked with even at temperatures higher than its glass transition point. The DSC curve of the PI polymer does not show such prominent features. The broad endothermic peak on the beginning of the curve suggests gradual evaporation of residual polymerization initiator agents. The rest of the curve then suggests no degradation of the polymer and the PI can be therefore identified as stable at temperatures up to 400 °C. Thermal stability of PI and PEEK was also studied on samples heated in oven to 150–300 °C for 1 h in Petri dishes. The PEEK foil was visibly deformed after heating at 250–300 °C and was difficult to extract from the dish, while the PI foil appeared stable and unchanged after annealing even at the highest used temperatures. The UV–Vis spectra of the polymer foils annealed at different temperatures are in Fig. 2. An overall

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increase of optical absorbance of PEEK is apparent after annealing at 200 °C and gets even more pronounced at higher annealing temperatures. This is probably connected to the secondary crystallization of the polymer. The spectra of PI on the other hand are unchanged with annealing, the character and overall absorption of the material remains the same after annealing at even the highest used temperatures. For all the following measurements the annealing temperatures of 200 and 300 °C for PEEK and PI were chosen respectively, based on the results of the thermal stability measurements and the supplier information on upper working temperature. AFM images of the substrates before and after annealing are in Fig. 3. It is apparent the surface of PI (Fig. 3A, B) is very smooth and the annealing has no significant impact on its character. The PEEK foil on the other hand changes its surface morphology with annealing significantly (Fig. 3C, D). Parallel valleys are dominating the structure of the unannealed substrate surface. After annealing the corrugated structure is disrupted and the surface reforms into a broken rugged morphology. This dramatic change is probably caused by the recrystallization that takes place in the polymer at temperatures higher than circa 170 °C. Gravimetry was employed to determine the relation between deposition time and effective thickness of the sputtered Au layers. Weight changes of the samples with Au layers were also studied after the annealing. Results of the gravimetry measurements in Fig. 4 suggest a linear dependence of the layer thickness on the deposition time. The longest deposition time of 300 s corresponds to layer thickness of circa 35 nm and the shortest deposition time of 10 s leads to a layer

Fig. 5. AFM images of Au layers on glass. Layer of effective thickness 2 nm (A) unannealed and annealed at (B) 200 and (C) 300 °C and a layer of effective thickness 20 nm (D) unannealed and annealed at (E) 200 and (F) 300 °C.

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thickness of circa 1 nm. The sputtering yield is for both substrates the same within the measurement error. Weight decrease of the samples with PEEK substrate after the annealing was around 3.6 μg·cm−2 regardless of the thickness of the Au layer. Weight decrease of the samples with PI substrate on the other hand was inversely proportional to the thickness of the Au layer and ranged from 40 μg·cm−2 for the thinnest Au layer to 20 μg·cm−2 for the Au layer with 300 s deposition time. The weight decrease in this case is probably caused by evaporation of residual polymerization agents with the Au layer protecting the surface and preventing the evaporation. To get a reference point for samples with polymer substrates, surface morphology measurements of a well explored [26] system of thin Au film on glass substrate were performed. The main advantage of the glass substrate is its “very low” surface roughness enabling detail observations of the metal film morphology evolution. In Fig. 5 AFM scans of pristine and annealed Au films of 2 and 20 nm effective thickness on glass substrate can be seen. A structure of randomly distributed Au

grains can be seen on pictures of the unannealed samples. In the case of the thicker layer, it was documented by electrical resistance measurements an underlying continuous Au sheet is present [11]. Annealing at 200 °C does not lead to a significant increase of the grain size, only in the case of the thinner layer, more Au grains are formed. A much more significant change of morphology takes place at 300 °C annealing temperature. The dewetting process leads to formation of much larger Au grains, which affects UV–Vis absorption of the structure rather substantially [11]. The UV–Vis absorption spectrum typical for bulk Au can be observed in the case of the thicker films. This spectrum is dominated by the absorption minimum at circa 500 nm, which is between the onset of plasmon absorption at the higher wavelengths and interband transition absorption at the lower wavelength end of the spectrum (see Fig. 6). With decreasing size of the structure elements of the material, number of modes of plasmon absorption decreases and, depending on the structure and grain size distribution, a single surface plasmon resonance absorption peak can develop. Thus the UV–Vis absorption

Fig. 6. UV–Vis absorption spectra of Au layers of effective thickness 1–21 nm on PEEK and PI before (RT) and after (200 resp. 300 °C) annealing.

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Fig. 7. AFM images of Au layers on PEEK substrate. Layer of effective thickness 2 nm (A) before and (B) after annealing and layer of effective thickness 20 nm (C) before and (D) after annealing.

spectrum can be a valuable indicator of the morphological changes in the thin Au layer [27]. In the case of the lower annealing temperature employed for the PEEK the annealing conditions were not sufficient to induce dewetting of the Au on the glass substrate and the according surface morphology changes. However, a noticeable change in the character of the UV–Vis spectra of the PEEK sample occurs (Fig. 6). The surface plasmon resonance band of the nanostructured Au gets more pronounced especially on the samples with the lower effective Au thicknesses. This is related to change of the Au film morphology induced by crystallization of the polymer substrate (Fig. 7). Annealing of the PEEK at 200 °C leads to its recrystallization and therefore its surface roughness increases

dramatically as seen in Fig. 3D. This process causes cleavage of the Au layer influencing its nanostructure and coherency. Embedding of the Au grains could play role in this process as well, as the glass transition temperature of the substrate was exceeded [28,29]. On the other hand, thicker Au films cover the surface irregularities of the PEEK substrate influencing the surface morphology significantly. These layers prevent the transformation of the substrate surface during annealing which remains mostly unchanged and the UV–Vis spectra preserve their character in this case as well. Surface morphology evolution of the thin Au films due to dewetting is more readily observed on the smoother and thermally more stable PI substrate. In this case as well the UV–Vis spectra show an increase of the

Fig. 8. AFM images of Au layers on PI substrate. Layer of effective thickness 2 nm (A) before and (B) after annealing and layer of effective thickness 20 nm (C) before and (D) after annealing.

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SPR absorption peak (Fig. 6). Compared to the PEEK substrate the maximum of the SPR absorption is at lower wavelengths, suggesting a finer nanostructure, but the peak is weaker and wider, which means the structure is not as well developed with broader distribution of the grain size. This corresponds well with the AFM images (Fig. 8). Only a subtle change of surface morphology can be seen after annealing of the thinner layer as the grains of the structure get slightly coarser. With the thicker layer the structural difference is more visible, bigger grains are formed in the film, but in comparison to the annealing of Au films on the glass substrate, no significant dewetting takes place and the Au islands do not get well separated. The structure of the Au layer during deposition and annealing is determined by both growth kinetics and thermodynamic state of the thin film. The growth kinetics influence the morphology of the thin Au film especially during the deposition. The sputtering, taking place at high deposition rates and relatively low temperatures of both the substrate and the deposited material, leads to evolution of a film far from equilibrium state as the deposited atoms possess little momentum to diffuse over the surface and occupy low energy sites. Therefore, directly after the deposition the morphology usually corresponds to a metastable state driven by the kinetics of the deposition process. During the post-deposition annealing the diffusion rate of surface atoms increases significantly and the morphology of the film is transformed to a more thermodynamically stable state [30–32]. Generally, two processes to transform morphology can occur – surface diffusion and evaporation/condensation. Given the properties of the Au and the annealing conditions, the evaporation/condensation mechanism can be excluded to have influence on the studied system [33]. In the case of ultrathin discontinuous films the metal atoms have to diffuse over the substrate surface and as such the interfacial energy has a great influence on the diffusion coefficient and the surface irregularities set obstacles to their motion. The equilibrium structure of metal layer on an amorphous substrate is determined by the surface free energy of the phases present on the surface, which can be described by the Young equation: γs = γi + γmcosΘ. Depending on the ratio between the surface energy of the metal γm and the substrate γs and the interfacial energy γi the stable structure is either continuous metal coverage or isolated metal islands according to the contact angle Θ between the substrate and the metal [34–36]. In the case of Au layer on glass substrate the values of the surface energies are γm(Au) = 1.36 J·m−2, γs(glass) = 0.31 J·m−2 and γi = 1.17 J·m−2 [37]. These values produce the contact angle between Au and glass of about 130° in the state of equilibrium, which corresponds well with the experiment where isolated Au islands are formed on the glass surface. However, the AFM method is not suitable for precise evaluation of the contact angle as in ideal conditions only contact angles lower than 90° can be viewed theoretically and the observable maximum of the contact angle is further lowered by the shape and properties of the probe. It is therefore complicated to determine the energy of the Au/PI interface. The AFM images on the other hand can be used for evaluation of the average size of the Au nanoislands. The values of grain size obtained from measurement of 30 prominent grains on the surface as viewed by the AFM are shown in Table 1. It can be concluded from these values and the AFM images the Au layer is much more stable at the same annealing conditions on the PI substrate than on the glass. The much bigger grain size in the case of the glass substrate suggests a great portion of the Au from the continuous layer is used up to form the islands. The AFM images show a separation between the islands as well. In the case of PI the grains are considerably smaller (excluding the thinnest layer, where the structure is influenced by the growth on the different substrate surface). The very similar grain size of 7 nm and 21 nm thick Au layer on PI indicates decreasing dewetting rate with increasing film thickness as was reported [38]. The Au island separation is less apparent for the PI substrate, the Au layer remains continuous as was confirmed by sheet electrical resistance measurements (Fig. 9). A graph of resistance dependence on effective layer thickness typical for thin metal

Table 1 Average size of Au nanoislands on glass and PI substrates of the samples annealed at 300 °C. Substrate

Au layer thickness (nm)

Average grain size (nm)

Glass

2 7 21 2 7 21

40 ± 10 250 ± 30 400 ± 100 50 ± 20 110 ± 40 100 ± 20

PI

layers on isolating substrate can be seen. A very high resistance for initial stage of film growth can be attributed to the resistance of the substrate as the film is not continuous yet. Then, during a narrow increase of effective film thickness (percolation threshold) the resistance values drop significantly, when the film becomes continuous and the sample now shows the resistance of the metal [39]. During the postdeposition annealing the percolation threshold shifts to higher effective thicknesses of the metal film, if dewetting takes place, as the layer becomes discontinuous [15]. In the case of PI substrate this shift of the percolation threshold is very mild compared to the glass substrate. This means the dewetting of Au is strongly suppressed. The differences between the dewetting process observed on glass substrate and PI can be attributed to two effects: (i) a different surface roughness of the substrates, and (ii) a different interfacial energy between the Au film and the substrates. These effects influence both kinetic and thermodynamic aspects of the system. While the influence of surface features was described quite in detail for crystalline substrates [40–43], the amorphous glass and PI surface contain irregular features that complicate a rigorous description of the system and such a theory would reach out of scope of this study. The surface energy circumstances suggest much lower interfacial energy between Au and PI substrate than between Au and glass, since the surface energy of the PI is γs(PI) = 0.047 J·m−2, which has important role during thin film growth [44]. This, with the above mentioned Young equation in mind, suggests much higher adhesion between Au and PI, which was confirmed by peel tests with scotch tape, where Au/glass samples show were weak adhesion. Results of electrokinetic analysis are shown in Fig. 10. Both of polymers exhibit different results for samples before and after annealing. Pristine PI (Fig. 10 right, value for 0 s of deposition) shows no significant changes of the zeta potential after annealing, while values in case of PEEK substrate (Fig. 10 left) change significantly due to changes in chemistry and morphology of the surface after annealing. The zeta potential of samples with Au layers on PI and PEEK has different progress.

Fig. 9. Sheet electrical resistance of thin Au layers on PI substrate before and after annealing.

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Fig. 10. Dependence of zeta potential measured by streaming current method on deposition time for Au layers on PEEK and PI substrates for unannealed (RT) and annealed samples.

In case of PI values of zeta potential show similar progress with, the curves for PEEK differ significantly, especially for lower deposition times. In the case of nonannealed (RT) samples the zeta potential increases slightly for the Au layers sputtered for the shortest time. Then the value steadily increases with the growing thickness of the Au layer up to the deposition time of about 100 s. This suggests that at lower deposition times only small nanoislands of Au are created on the substrate and zeta potential value is a combination of an uncovered flat polymer surface and rough Au nanoislands. This corresponds well with the AFM and UV–Vis measurements. Above 100 s the curve shows a slow increase as most of the substrate is already covered by the Au. The values of zeta potential for annealed PEEK samples the curve exhibit strong fluctuations probably due to high roughness of the polymer surface and the nonhomogeneous coverage of the sputtered Au layer. The curve gets smoother at circa 150 s of deposition followed by a slow increase of zeta potential. The annealed PI samples exhibit a steady increase of zeta potential which suggests a homogeneous coverage of the surface by Au. These results are in agreement with the AFM and UV–Vis absorption measurements discussed above. 4. Conclusions Thermal stability of PI a PEEK substrates for thin Au films was investigated. DSC measurements confirmed phase transitions in PEEK in the range of 140–180 °C. PI substrate was found to be stable in the whole range up to 400 °C, which was confirmed by UV–Vis measurements. For further measurements annealing temperatures of 200 °C and 300 °C were chosen for PEEK and PI substrate respectively. Surface structure of PI (as observed by AFM) was found to remain unchanged after annealing, the PEEK substrate on the other hand undergoes recrystallization which is accompanied by surface reconstruction and transformation to a rugged morphology. The polymer surfaces were used as substrates for sputtered Au nanolayers. Gravimetry confirmed the effective thickness of Au layer is linearly dependent on deposition time. The UV–Vis spectra of the samples of thin Au films on polymer substrates show slight amplification of SPR absorption in the spectra after annealing for both polymers. While the samples with PEEK substrate show stronger bands of SPR absorption, their spectra change character to that of bulk Au at lower effective thicknesses than those of the samples with PI substrate. This can be attributed to lower annealing temperatures and different structures forming on the surface. The surface morphology of the samples was

studied by AFM. The layers deposited on PI showed uniform coverage. After annealing the nanostructure got slightly coarser, but with no apparent dewetting and island separation, which was also supported by sheet electrical resistance measurements. The Au layers on PEEK showed formation of larger round grains unevenly spread over the surface. The annealing of samples with very thin Au layer leads cleavage of the Au layer into isolated parts. The thicker Au layers on PEEK on the other hand form a continuous cover over the substrate structure and annealing leads to no significant structural changes. Electrokinetic analysis confirmed different behaviour of PEEK and PI during deposition and annealing. The measurements suggest PI is covered continuously, while PEEK exhibits Au nanoislands at first and the continuous coverage develops after higher deposition times. Of the studied substrates the PEEK showed underwhelming resistance to elevated temperatures. Annealing even at temperatures as low as 200 °C lead to significant changes of its surface structure, which is unsuitable for preparation of continuous thin metal films. The PI substrate on the other hand was able to withstand temperatures comparable to amorphous SiO2 substrates and even the surface of this substrate is sufficiently smooth. Moreover, the PI substrate showed a good affinity to sputtered Au layers even without surface modification or adhesion layers which are necessary in the case of Si or SiO2 substrates. The PI suppressed solid state dewetting of the Au layer preserving continuous metal coverage under annealing conditions, where annealing of Au on other substrate leads to dewetting. This further broadens the possible applications of PI in flexible electronics construction as it is able to support very thin continuous metal films even if elevated temperatures are necessary during the fabrication processes. Acknowledgements This work was supported by Grantová Agentura České Republiky projects 14-18149P and P108/12/G108. References [1] G. Kaune, M.A. Ruderer, E. Metwalli, W. Wang, S. Couet, K. Schlage, R. Röhlsberger, S.V. Roth, P. Müller-Buschbaum, In situ GISAXS study of gold film growth on conducting polymer films, Appl. Mater. Interfaces 1 (2009) 353–360. [2] F. Ruffino, V. Torrisi, G. Marletta, M.G. Grimaldi, Growth morphology of nanoscale sputter-deposited Au films on amorphous soft polymeric substrates, Appl. Phys. A Mater. Sci. Process. 103 (2011) 939–949. [3] S.V. Roth, H. Walter, M. Burghammer, C. Riekel, B. Lengeler, C. Schroer, M. Kuhlmann, T. Walther, A. Sehrbrock, R. Domnick, P. Müller-Buschbaum,

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