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Laser-Induced Periodic Surface Structures Nanofabricated on Poly(trimethylene terephthalate) Spin-Coated Films I. Martín-Fabiani,† E. Rebollar,‡ S. Pérez,‡ D. R. Rueda,† M. C. García-Gutiérrez,† A. Szymczyk,§ Z. Roslaniec,∥ M. Castillejo,‡ and T. A. Ezquerra*,† †

Instituto de Estructura de la Materia, IEM-CSIC and ‡Instituto de Química Física Rocasolano, IQFR-CSIC, Serrano 119-121, 28006 Madrid, Spain § Institute of Physics and ∥Institute of Materials Science and Engineering, West Pomeranian University of Technology, Szczecin, Poland ABSTRACT: Here we present a precise morphological description of laser-induced periodic surface structures (LIPSS) nanofabricated on spin-coated poly(trimethylene terephthalate) (PTT) films by irradiation with 266 nm, 6 ns laser pulses and by using a broad range of fluences and number of pulses. By accomplishing real and reciprocal space measurements by means of atomic force microscopy and grazing incidence wide- and small-angle X-ray scattering respectively on LIPSS samples, the range of optimum structural order has been established. For a given fluence, an increase in the number of pulses tends to improve LIPSS in PTT. However, as the pulse doses increase above a certain limit, a distortion of the structures is observed and a droplet-like morphology appears. It is proposed that this effect could be related to a plausible decrease of the molecular weight of PTT due to laser-induced chain photo-oxidation by irradiation with a high number of pulses. A concurrent decrease in viscosity enables destabilization of LIPSS by the formation of droplets in a process similar to surface-limited dewetting.

1. INTRODUCTION The controlled structuring of polymer surfaces at length scales ranging from micrometers to nanometers has appeared as a promising approach to create functional substrates or to mimic some interesting properties present in natural surfaces. Among the most popular examples, the hierarchical structure of the gecko foot or of the lotus leaf are paradigmatic cases of an almost universal natural adherence1 and of a natural superhydrophobic2 behavior respectively. Different processes are currently being investigated in order to produce superficial nanostructures on polymers, most of them based on advanced soft lithographic techniques.3−5 Alternative lithographic procedures are also attracting a lot of interest as a complement to standard nanolithography aiming to avoid the necessity of demanding experimental conditions like clean rooms, high vacuum, or complex mask fabrication. Electrical,6 chemical,7,8 mechanical,9 block copolymer self-assembly,10 templates,11,12 and laser-induced13−15 patterning of polymer surfaces are some versatile strategies in order to obtain functional polymer materials. Formation of laser-induced periodic surface structures (LIPSS) is a phenomenon that may occur by illumination of solid surfaces by intense laser pulses. When this happens, spontaneous periodic surface structures with periodicities closely related to the wavelength of the irradiating laser are detected.16 Most of the related studies have been performed on metals and semiconductors with lasers of different pulse © XXXX American Chemical Society

duration from nanosecond (ns) to femtosecond (fs) and with different wavelengths from ultraviolet to infrared.17−19 In the case of polymers, several reports have shown that irradiation by a polarized laser beam may also induce self-organized ripple structure formation within a narrow fluence range well below the ablation threshold.14,18−20 The period of the ripples (L) produced by a laser beam of wavelength λ reaching the surface of a material of refraction index n at an incidence angle θ can be described21 by the relation L = λ/(n − sin θ). Although LIPSS are thought to develop by interference between the incoming and the surface scattered wave, resulting in an intensity modulation on the material surface,22−24 the whole mechanism is complex and still not fully understood. Poly(trimethylene terephthalate) (PTT) is a linear aromatic polyester based on terephthalic acid like poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT). PTT combines the outstanding properties of PET and the processing characteristics of PBT, making PTT highly suitable for uses in fiber, film, and engineering thermoplastic applications. In particular, PTT possesses strong flexibility and more than 90% elastic recovery, relatively large refractive index, low dielectric losses at room temperature,25 and good Received: February 27, 2012 Revised: April 17, 2012

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dx.doi.org/10.1021/la300833x | Langmuir XXXX, XXX, XXX−XXX

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by averaging the results of three different areas in each sample. Values of the period were derived from the FFT analysis (spatial frequency in the direction perpendicular to the LIPSS). Irradiated areas were also analyzed by grazing incidence X-ray scattering at small and wide angles, GISAXS and GIWAXS, respectively, using the facilities of the BW4 beamline at HASYLAB [Deutsches Elektronen Synchrotron (DESY), Hamburg, Germany]. The experimental setup for GISAXS has been previously described.29−31 Let us remember here that the sample is positioned horizontal and that the incoming and reflected beams define the vertical scattering plane. Both the scattering plane and the sample plane intersect the detector along the meridian and the horizon lines, respectively, which are the reference to measure the angles ω and α. The information can be interpreted on the basis of the two orthogonal scattering vectors qz = (2π/λ)(sin αi + sin α) and qy = (2π/λ)sin ω cos α, which provide information about structural correlations perpendicular and parallel to the film plane, respectively. GISAXS images were analyzed by using the software Fit2D. An X-ray wavelength of λ = 0.13808 nm, with a beam size of 40 × 20 μm2, was used in our experiments. Scattered intensity was recorded by a Mar CCD detector of 2048 × 2048 pixels with a resolution of 79.1 μm per pixel, and a sample-to-detector distance of 2.211 m for GISAXS. For GIWAXS we used a similar setup32,33 with a sample-to-detector distance of 0.105 m. An incidence angle of αi = 0.4° was typically selected. Samples were positioned to ensure that the beam was parallel to the direction of the LIPSS, and acquisition times between 40 and 600 s were used. Raman spectroscopy experiments were performed by using a Renishaw Raman InVia Reflex spectrophotometer, with excitation at 785 nm (diode laser) and a resolution of 2 cm−1.

transparency for visible to near-infrared light. These properties make PTT very attractive for applications in photonic devices.26 In particular, superficial gratings can be created on PTT by LIPSS, opening the possibility for nanofabrication of functional surfaces on this polymer.14 In this paper, we present a precise morphological description of LIPSS on PTT by accomplishing real and reciprocal space measurements by means of atomic force microscopy and grazing incidence X-ray scattering both, at small (GISAXS) and wide angle (GIWAXS), respectively on LIPSS samples prepared over a broad range of fluences and number of pulses. Here we show that by choosing certain conditions of fluence and number of pulses, the degree of structural order of LIPSS, as determined by the analysis of X-ray scattering patterns, can be optimized. For a given fluence, an increase in number of pulses tends to improve the quality of LIPSS in PTT. However, as the pulse doses increase above a certain limit, the structures get distorted. Supported by Raman spectroscopy, we propose that at high pulse doses, laser-induced photo-oxidation may lead to a reduction of PTT molecular weight. A concurrent decrease in viscosity enables destabilization of LIPSS by surface-limited dewetting.

2. EXPERIMENTAL SECTION 2.1. Materials. PTT was synthesized by polycondensation as previously described27,28 using dimethyl terephthalate (DMT, Elana S.A., Torun, Poland) and 1,3-propanediol (PDO, Shell Chemicals Europe B.V., The Netherlands). Tetrabutyl orthotitanate [Ti(OBu)4, Fluka] and Irganox 1010 (Ciba-Geigy, Switzerland) were used as catalyst and as antioxidant, respectively. Molecular weight, as determined by size exclusion chromatography (SEC), is Mn = 31 294 g/mol with a polydispersity of Mw/Mn = 2.22. PTT is a semicrystalline polymer, with a melting temperature Tm = 229 °C, which can be quenched from the molten state to render a fully amorphous state with a glass transition temperature Tg = 44 °C, as determined by calorimetry. Polymer thin films were prepared by spincoating on silicon wafers (100) (Wafer World Inc.) polished on both surfaces. The wafers were previously cleaned with a piranha solution (H2SO4:H2O2, 3:1). PTT was solved in trifluoroacetic acid (SigmaAldrich, reagent grade ≥98%) with a concentration of 20 g/L. A fixed amount of 0.1 mL of polymer solution was instantly dropped by a syringe on a square (typically 2 × 2 cm2) silicon substrate placed in the center of a rotating metallic horizontal plate. A rotation speed of 2380 rpm was kept during 30 s. Spin-coated polymer films with a thickness of about 157 ± 24 nm and extremely flat surface (mean surface roughness Ra ≤ 1 nm), as measured by AFM, are typically obtained under these conditions. 2.2. Laser Irradiation. Laser irradiation was carried out in ambient air under normal incidence, with the linearly polarized laser beam of a Q-switched Nd:YAG laser (Quantel Brilliant B, pulse duration τ = 6 ns full width half-maximum) at a repetition rate of 10 Hz. The fourth harmonic at 266 nm was used for the experiments, since at this wavelength PTT absorbs efficiently14 with an absorption coefficient of 26 000 cm−1. The fluences of irradiation were determined by measuring the laser energy in front of the sample with a joulemeter (Gentec-E, QE25SP-H-MB-D0) and by calculating the area of the irradiated spots after delimitating the beam with an iris of 0.45 cm diameter. The spin-coated polymer films were irradiated with a constant number of pulses in the range of fluences of 1−15 mJ/cm2 and as a function of the number of pulses (up to several thousands) at a constant fluence. 2.3. Surface Characterization and Analysis. Morphology of the spin-coated polymer films was examined by atomic force microscopy (AFM, Nanoscope IIIA Multimode, Veeco) in tapping mode, and images were analyzed with the software Nanoscope Analysis 1.10, obtaining the fast Fourier transform (FFT) of AFM images. The periods and heights of LIPSS were determined from the AFM analysis

3. RESULTS 3.1. Dependence on Number of Pulses. As prepared spin-coated films of PTT present rather smooth surfaces (with mean surface roughness Ra ≤ 1 nm, as mentioned) without morphological indications of crystallization, as revealed by AFM (Figure 1a). However, GIWAXS patterns (Figure 2 Inset) of the spin-coated films reveal a small degree of crystallinity, as revealed by the broad rings observed in the pattern. An estimate of the overall crystallinity can be accomplished by integrating azimuthally the GIWAXS pattern. Figure 2 shows the integrated intensity, after subtraction of the primary beam intensity background, as a function of the scattering vector q =

Figure 1. AFM height images (10 × 10 μm2) of PTT: (a) nonirradiated and (b−f) irradiated at 266 nm with a fluence of 7 mJ/cm2 as a function of number of pulses as labeled in the upper left corner. The height profile along a 2 μm line perpendicular to the ripples is shown below every image. The arrows indicates the polarization vector of the laser. B


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Figure 2. Corrected intensity profile of the GIWAXS pattern (inset) as a function of the q-vector for a spin-coated PTT film. The deconvolution according to the expected reflections of the triclinic unit cell of PTT34 is shown.

4π/λ(sin θ) being 2θ the scattering angle (in GIWAXS experiments ω = 2θ). The deconvolution of the profile of Figure 2 was performed by considering the expected reflections of the triclinic unit cell of PTT.34 The separated reflections and their Miller indexes together with the broad amorphous halo are shown in Figure 2. By this procedure crystallinity was estimated to be lower than 15 ± 3%. Considering the lack of crystal orientation as revealed by the isotropy of the Bragg rings, we can attribute this slight crystallinity to the formation of small crystal nuclei during solvent evaporation. After laser irradiation, under certain fluence conditions well below the ablation threshold, ripples, parallel to the polarization vector of the laser, are observed. As an example, Figure 1 shows, together with the AFM image of the initial nonirradiated sample, selected height images of different PTT film surfaces irradiated at 7 mJ/cm2 with a varying amount of pulses. As shown in Figure 1, upon irradiation with 7 mJ/cm2, surface modification starts to be detected below the onset of LIPSS formation at around 300 pulses. Ripples are parallel and well-defined in the range between 300 and 600 pulses and get distorted upon additional pulse irradiation. At high pulse doses, around 6000 pulses (Figure 1f), the ripple morphology almost disappears and the AFM image reveals signs of droplet formation. The dependence of the LIPSS period on the number of pulses is shown in Figure 3a. The LIPSS period increases with the number of pulses, N, up to ≈1000 and reaches a steady level for higher values of N. In relation to the heights of LIPSS, no clear trend is observed with increasing fluence or number of pulses. Typical values vary between 40 and 100 nm. As regards X-ray scattering analysis of the irradiated films, Figure 4a shows characteristic GISAXS patterns of PTT irradiated at 7 mJ/cm2 for different numbers of pulses. The horizontal sample was aligned with the main ripple axis parallel to the direction of the X-ray beam. It is worth to mention that, for grating-like structures, small misalignment of the ripple axis with the beam gives rise to strongly anisotropic GISAXS patterns.35,36 Scattering maxima out of the meridian (i.e., for angles ω ≠ 0) are clearly visible in the range of pulses in which LIPSS formation is observed. This effect is emphasized in Figure 4b by the corresponding intensity profile (black lines) along the horizontal direction (at a fixed exit angle α = 0.2°). In a first approach, the period L of the nanostructures can be determined through the expression L = 2π/qMAX , where qMAX is y y the reciprocal scattering vector corresponding to the first intensity maximum31 next to ω = 0. L values derived from GISAXS patterns of the irradiated polymers have been included

Figure 3. (a) Period length of the LIPSS as a function of the number of pulses at 7 mJ/cm2 and (b) period length of the LIPSS as a function of fluence for a fixed amount of pulses N = 600. Circles and triangles indicate values derived from AFM and GISAXS analysis, respectively.

Figure 4. (a) Experimental GISAXS patterns of PTT irradiated at 7 mJ/cm2 for different amount of pulses as labeled on the upper left corner. (b) Experimental (black lines) and simulated (red lines) intensity profiles taken at a fixed exit angle α = 0.2°. (c) Corresponding simulated GISAXS patterns.

in Figure 3a for comparison with those taken by AFM. A good correlation between AFM and GISAXS results is observed, although the values obtained by GISAXS are systematically lower than the ones obtained by AFM. It is known that for nonhomogeneous systems, the comparison between AFM Fourier transformed images, which produces essentially a two-dimensional power spectral density function, and diffraction patterns can exhibit slight mismatches31 since GISAXS C


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intensity profiles as a function of the scattering angle ω extracted from the GISAXS patterns at an exit angle α = 0.2°. As for the previous case, the period L of the nanostructures was determined from the first intensity maximum and the obtained values have been included in Figure 3b. Again very good correlation between AFM and GISAXS results is observed. 3.3. GISAXS Modeling. In general, a precise analysis of Xray scattering patterns requires more sophisticated tools than the mere application of Bragg’s law (L = 2π/qmax) to the observed intensity maxima in order to extract the underlying structural features of the investigated sample.37,38 In our case, aiming for a deeper description of the LIPSS obtained on the spin-coated PTT films, simulations of the GISAXS patterns were accomplished by using the software package IsGISAXS.39 The distorted wave Born approximation (DWBA) was applied, assuming a simple nanostructure consisting of polymer boxes standing on the polymer film. In this case the values of the refraction indexes for both box and substrate were considered to be similar: δ = 3.3 × 10−6 and β = 5.1 × 10−9. The dimensions of the boxes used to model the nanostructures are those determined by AFM, i.e., width (2R), height (H), length (W), and period (L). Box dimensions used to simulate GISAXS patterns shown in Figures 4 and 6 are listed in Table 1.

averages over a sample volume usually larger than that visualized by AFM. 3.2. Dependence on Fluence. Figure 5 displays AFM height images of selected areas of PTT samples irradiated with

Figure 5. AFM height images (10 × 10 μm2) of PTT irradiated at 266 nm with 600 pulses at fluences of (a) 4 mJ/cm2, (b) 5 mJ/cm2 and (c) 13 mJ/cm2. The height profile along a 2 μm line perpendicular to the ripples is shown below every image. The arrows indicate the polarization vector of the laser.

600 pulses at different fluences. For low fluences (F < 4 mJ/ cm2), no morphological changes are induced on the polymer surface and the initial roughness remains essentially constant. For fluences above 4 mJ/cm2, ripples start to develop. Ripples are parallel and well-defined in the range of 6−13 mJ/cm2. The dependence of the period of LIPSS with the fluence at 600 pulses, as derived from AFM analysis, is represented in Figure 3b. As observed, the period increases in the fluence range of 4−6 mJ/cm2 and remains practically constant afterward. Above 14 mJ/cm2 the ripples start to disappear. Regarding the height of ripples, and similarly to what is observed in the dependence with number of pulses, there is not a clear dependence on the irradiation fluence. However, it is observed that the height of the structures is systematically larger in the upper fluence range of LIPSS formation. Typical height values vary between 40 and 100 nm. Representative GISAXS patterns corresponding to these samples are shown in Figure 6a. Scattering maxima out of the meridian (ω ≠ 0) are clearly visible in the fluence range of LIPSS formation, as likewise shown in the dependence of patterns with number of pulses (Figure 4a). In Figure 6b we present the corresponding

Table 1. Geometric Parameters of the Box Used for the Simulations of GISAXS Patterns Shown in Figures 4 and 6: Width (2R), Height (H), Box Length (W), Period (L), Paracrystalline Disorder Parameter (g) and Correlation Length (Δ0)a PTT F (mJ/cm2)

pulses

2R (nm)

H (nm)

W (nm)

L (nm)

g

Δ0 (nm)

7 7 7 5 13

100 300 6000 600 600

105 120 85 136 172

12 20 48 58 50

500 1000 425 1000 1000

208 220 203 201 264

0.071 0.040 0.190 0.099 0.041

1500 1400 1500 1500 1800

a A Gaussian distribution of width 0.1R was used for the R and of 0.1H for the H parameters.

Typically, Gaussian distributions for R and H with σ = 0.1 R were assumed, as derived from the AFM calculations. Recently, a similar approach was applied by Meier et al. to the structural characterization of conducting polymer films microstructured by channel-type master structures.36 In this case, successful simulations were obtained by using IsGISAXS considering channels as anisotropic pyramids. In our case, in order to assess the level of order, a one-dimensional paracrystalline approach has been considered, where the long-range order can disappear gradually in a probabilistic way.38,40−42 The probability of finding a particle at a distance L is defined by a function p(x) that is considered as a Gaussian in the present simulation: p(x ) =

⎡ (x − L)2 ⎤ 1 exp⎢ − ⎥ σ 2π ⎣ 2σ 2 ⎦

(1)

As the paracrystalline distortion parameter g = σ/L increases, structural disorder increases, while for very small g values a 1D crystalline lattice is obtained. Finite size effects of the 1D paracrystal are introduced through the correlation length, Δ0, of the 1D paracrystal. Figures 4c and 6c show the IsGISAXS simulated patterns with the geometrical parameters 41

Figure 6. (a) Experimental GISAXS patterns of PTT irradiated with 600 pulses at fluences of 5 and 13 mJ/cm2 as labeled on the upper left corner. (b) Experimental (black lines) and simulated (red lines) intensity profiles taken at a fixed exit angle α = 0.2°. (c) Corresponding simulated GISAX patterns. D


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point at fluences in the range from 7 to 9 mJ/cm2. As will be mentioned in the discussion, Raman spectroscopic measurements have shown a slight broadening of the band at 1720 cm−1, characteristic of the vibration of the CO bond, suggesting a certain level of laser-induced chain scission.

of Table 1 and the g value considered for the modeling. The intensity values have been normalized to that of the first correlation maximum. The corresponding intensity profiles as a function of the scattering angle ω extracted from the IsGISAXS patterns at an exit angle of α = 0.2° have been included in Figure 4b and Figure 6b (red lines). These simulations strongly suggest that LIPSS can be well-described considering a quasione-dimensional paracrystalline lattice and that the irradiation parameters (i.e., number of pulses and fluence) have an influence on the structural order of such a lattice. Considering that the structural parameters used in the simulations are taken directly from those estimated by AFM, it is clear that both techniques are consistent with each other in providing the period length of LIPSS. As the simulation yields the order factor of the structures, it is possible to derive, for each material and laser wavelength, the optimal number of pulses and fluence that optimize LIPSS formation. Figure 7a shows the depend-

4. DISCUSSION The mechanism for LIPSS formation in polymers using nanosecond pulses has been discussed previously.14,43 It is generally accepted that two main effects contribute to the LIPSS formation. First, there is an optical effect responsible for the interference between the incident laser beam and the surface scattered wave, which produces a modulation of laser intensity on the irradiated surface. Second, a physical process converts the intensity modulation into a structural modification of the surface. Both effects are interconnected by a feedback process, in the sense that a surface structure originates intensity modulation which, in turn, affects further structural development. Laser irradiation causes the heating of the upper layer of the polymer film,20,22,43 inducing temperature gradients in the region close to the irradiated area and allowing the diffusion of polymeric chains. Accumulation of laser pulses on the irradiated spot induces subsequent cycles of heating and cooling, eventually leading to LIPSS formation. Following the approach described in the literature,22 the corresponding temperature increase is estimated by solving the one-dimensional heat conduction equation as a function of space (x) and time (t) coordinates: ∂ 2T (x , t ) ∂T (x , t ) α = − P(t ) exp( −αx)F0 − a2 ∂T κ ∂x 2

(2)

a = ρc/κ, where ρ is the density, c the specific heat, and κ the thermal conductivity. P(t) describes the temporal shape of the laser pulse, which is approximated by a modified Gaussian beam: 2

P(t ) =

⎡ ⎛ t ⎞2 ⎤ 2t ⎢−⎜ ⎟ ⎥ exp τ2 ⎣ ⎝τ⎠ ⎦

(3)

where the t factor ensures that intensity vanishes at t = 0 and τ is the pulse duration. Using the appropriate Green’s function, T(x,t) can be calculated assuming an initial temperature of 23 °C. For the calculation in PTT, values of ρ = 1350 kg/m3, c = 1359 J/(kg K) and 0.22 W/(m K) have been selected by considering the κ values of poly(ethylene terephthalate) and poly(butylene terephthalate).14 The results are illustrated in Figure 8, where temperature profiles are shown for the surface

Figure 7. Paracrystalline distortion parameter, g, estimated by modeling, as a function of (a) the number of pulses for fluence of 7 mJ/cm2 and (b) fluence for a constant number of pulses (600 and 1200). In part a the continuous line is a visual guide.

ence of the paracrystalline distortion parameter, g, as a function of the number of pulses for a fluence of 7 mJ/cm2 as estimated by the modeling. In agreement with the AFM observations (Figure 1), it appears that for this fluence there is a certain pulse dose, between 300 and 600, where the optimum structural order of LIPPS is achieved. The g-parameter increases for higher number of pulses, indicating an increased structural disorder. It is worth emphasizing that for values of the g-parameter around 0.25, only a first neighbor order exists, essentially providing only one correlation maximum in scattering.40 This is practically the case for the sample irradiated with 6000 pulses (Figure 1). On the other hand, Figure 7b shows the dependence of the g-parameter as a function of fluence for two series with a constant number of pulses (600 and 1200) as estimated by modeling. Again, there is a good agreement with AFM observations (Figure 5), since it seems that the degree of structural order improves when the fluence increases. It is worth mentioning the existence of an inflection

Figure 8. Time dependence of the temperature reached on the surface (solid line) of the PTT sample and 70 nm deeper (dashed line) for irradiation at 266 nm at the indicated fluences. E


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1f (corresponding to irradiation at 7 mJ/cm2 with 6000 pulses). The characteristic droplet morphology, which appears in addition to a weak periodic structuring effect, can be better visualized in Figure 9.

and at a depth of 70 nm. It is clear that the maximum temperature reached tends to decrease with depth. It is important to remark that, for the sake of simplification, the temperature dependence of the polymer physical parameters was not considered in the calculation. Additionally, in a real case, the possible changes in the thermal properties of the surface during the thermal and chemical transformation caused by the previous laser pulses (i.e., incubation effects) have to be considered. Moreover, the thermal dissipation effect of the silicon substrate on the temperature increase has not been taken into account. From the values of the linear absorption coefficient of PTT (α = 26 000 cm−1), the irradiation depth can be calculated to be a few hundred nanometers. This implies that a negligible amount of laser radiation reaches the silicon substrate, and therefore, the contribution of silicon in the spatial and temporal temperature evolution can be disregarded. In any case, according to the thermal properties of silicon and, in particular, its thermal conductivity [149 W/(m K)], the substrate is expected to contribute to reduce the heating of the closest polymer surface. This effect can also moderate the heating of the outer polymer layers, which in turn would favor LIPSS formation. The figure shows that the temperature at the surface of the irradiated PTT sample reaches maximum values of around 87 and 203 °C upon irradiation with a single laser pulse of 5 and 14 mJ/cm2, respectively, fluences that correspond to the lowest and highest values at which LIPSS are observed. According to these estimations, the temperature increase is well above the Tg (44 °C) but significantly below the melting point (229 °C) of PTT. These calculations suggest that in order to obtain LIPSS in PTT, a minimum fluence value, which depends on the thermal properties of the polymer, is necessary to ensure surface heating above Tg. In our case, for fluences above 4 mJ/cm2, the surface temperature meets this requirement and therefore is high enough to induce surface devitrification and allows polymer segmental and chain dynamics. Temperature increase above Tg is expected to rise surface roughness caused by capillary waves,44 consequently enhancing surface inhomogeneities and facilitating the feedback mechanism involved in LIPSS formation. The estimation of temperature increase also allows discussing the dependence of the degree of structural order of LIPSS with fluence (Figure 7b). In fact, if the number of pulses applied to the sample is kept within the stability region described in Figure 7a, N < 1000, the increase of fluence leads to the increase of superficial temperature, to the softening of the material, and to the reduction of superficial viscosity.45 This sequence allows the formation of additional ordered structures characterized by a higher degree of structural order and by larger periods. Extension of the calculation shown in Figure 8 to longer times reveals that the initial sample temperature (23 °C) is recovered in about 14 μs. Considering the laser repetition rate (10 Hz), successive pulses reach the sample in periods of 100 ms and therefore no significant cumulative temperature increase is expected by repetitive irradiation. The results presented in Figure 7a indicate that, for a given fluence, there is an optimum number of pulses above which the degree of structural order of the periodic structures diminishes. Since increase of temperature by repetitive pulse irradiation is not expected, standard dewetting of the film from the substrate can be excluded, in a first approach, as the origin of the loss of structural order. However, as mentioned before, samples subjected to irradiation with a high number of pulses exhibit morphologies similar to those shown in AFM images of Figure

Figure 9. (Top) AFM topographic image of PTT irradiated at λ = 266 nm with a fluence of 7 mJ/cm2 and 6000 pulses (1 pulse = 6 ns). (Bottom) Height profiles corresponding to the white lines indicated in the images along characteristic ripple zone (a) or through a characteristic droplet line (b). The arrow indicates the polarization vector of the laser.

Dewetting in thin polymer films heated above their Tg values can be initiated by several processes that may typically lead to disruption of the film, either by nucleation and growth of holes46 or by spinodal decomposition.47 In the latter case, amplification of the fluctuations on the free surface of the film, like those incited by capillary waves,48 leads to the rupture of the thin film and the formation of droplets. Although the results presented here are insufficient to elucidate the precise dewetting mechanism, the AFM topographic profiles shown at the bottom of Figure 9 are indicative of a destabilizing process that is competing with LIPSS formation for irradiation with a high number of pulses. In this figure, the drawn white lines show the presence of droplet-like motives (profile b), with heights on the order of about 40 nm, over a significantly smoother ripple background (profile a). In order to understand this effect, other processes, including laser-induced photofragmentation of polymer chains, could be invoked.43 In fact, laser-induced photo-oxidation leads to reduction of the polymer molecular weight as reported for poly(methyl methacrylate),49 polystyrene,50 and aromatic polyesters and polycarbonates.43 Additionally, it is worth mentioning the phenomenon of incubation, by which under repetitive irradiation ablation begins below the single pulse ablation threshold. A permanent increase of absorption after repetitive irradiation due to chemical changes in the polymer is a wellF


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established phenomenon.51 Evidence of such effects in our case can be found in the Raman spectra shown in Figure 10.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from Ministry of Science and Innovation of Spain (MICINN) under Projects CTQ2010-15680, MAT200907789, and MAT2011-23455 is acknowledged. E.R. thanks Juan de la Cierva Program for a contract. I.M-F. and S.P. acknowledge MICINN for “Formación de Personal Investigador” fellowship. The experiments performed at BW4 in HASYLAB (DESY) were supported by the European Community [Contract RII3-CT-2004-506008 (IA-SFS)]. Beamline assistance provided by Dr. Jan Perlich is gratefully acknowledged.

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Figure 10. Raman spectra of nonirradiated and irradiated PTT at a fluence of 7 mJ/cm2 with 600 and 1200 pulses. The inset shows magnification of the region around 1720 cm−1.

Figure 10 compares the Raman spectrum of the nonirradiated sample with those of the samples irradiated at 7 mJ/ cm2 with 600 and 1200 pulses. The absence of new bands in the Raman spectra of the irradiated samples is a clear indication of the chemical stability of PTT under these irradiation conditions. However, magnification of the spectra in the region characteristic of the stretching vibration of the CO bond (band at 1720 cm−1, inset of Figure 10) reveals a slight broadening of this band upon irradiation. This is an indication of a certain level of oxidation and supports the hypothesis of laser-induced chain scission. In such a case, shorter polymer chains would be more affected by temperature, since the Tg value is expected to decrease as the molecular weight is reduced. It is thus conceivable that, at a given fluence and for high number of pulses, the lower molecular weight material reaches a sufficiently low viscosity as to enable the destabilization of LIPSS by the formation of droplets, in a process similar to a surface-limited dewetting.

5. CONCLUSIONS LIPSS have been successfully obtained on spin-coated PTT films (≈150 nm thick) by laser irradiation using a broad range of fluences and number of pulses. The irradiation conditions, in terms of fluence and number of pulses, to obtain the optimum degree of structural order in PTT at 266 nm have been established by accomplishing real and reciprocal space measurements by means of atomic force microscopy and grazing incidence X-ray scattering, respectively. For a given fluence, an increase in the number of pulses tends to improve the quality of LIPSS in PTT. However, as the number of pulses increase above a certain limit, distortion of the LIPSS is observed and a droplet-like morphology emerges. GISAXS patterns can be satisfactorily simulated by considering a quasione-dimensional paracrystalline lattice in which the number of pulses and fluence have a strong influence on the paracrystalline disorder parameter. It is proposed that at a high number of pulses, laser-induced photo-oxidation decreases the polymer molecular weight. A concurrent decrease of viscosity enables destabilization of LIPSS by formation of droplets in a process similar to a surface-limited dewetting.

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