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We present the light-controlled hierarchical mechanical properties of optically patterned azobenzene thin films through a nanoindentation study. In this study, we inscribed holographic surface relief grating (SRG) of azopolymers by two-beam coupling-based light interference lithography. The resultant morphological profile of azopolymers was monitored by atomic force microscope (AFM), followed by the nanoindentation study. From the load-displacement curve of the indentation procedure, photomechanical changes of the azopolymers along grating patterns were evaluated in terms of hardness and modulus at the crest and trough of the SRG, respectively. The results revealed that the surface height as well as the mechanical properties was modulated according to the light interference pattern.
© 2016 Optical Society of America
1. Introduction
Holographic lithography using photo-addressable azopolymers has great advantages in fabricating multi-scale structures from micro- to nano-sized patterns in large scale without the help of an expensive photo-mask [1]. Regularly arrayed patterns of azopolymer gratings can manipulate light propagation precisely; therefore, many efforts have been made to implement azopolymer gratings in advanced applications of optofluidics, 3-dimensional data storage, waveguide filters and holographic optical elements [2–5]. To do this, optical patterning through mass transfer phenomena of azopolymers upon light irradiation has been studied for the past several decades, whereas the underlying mechanism that explains the photo-triggered fluidic behavior of azopolymers has not been fully explained.
In an effort to understand light induced-fluidization, much research based on nanoindentation studies have explored the photomechanical property changes of azopolymers. The Barett group, for example, showed that the azo-component is the key ingredient of the photo-softening effect on azopolymers upon light irradiation below the glass transition temperature (Tg) [6]. With well-designed experiments, they demonstrated that mechanical properties, including the hardness and modulus, were significantly influenced by the molar ratio of azo-moieties where the thermal-driven softening effect was almost the same regardless of the amount of azo-moiety. Karageogiev and associates also observed the photo-softening effect of azopolymers upon light exposure by in situ monitoring force-displacement of tip penetration [7]. This revealed that azopolymers behaved like elastic solids without light irradiation and became viscoelastic under illumination. Interestingly, they also found that anisotropic photofluidization of azopolymers occurred along the polarization direction of incident light.
Meanwhile reversible photomechanical change with light irradiation was described by those researchers. The Barret group found a photo-expansion effect that was reversible as well as irreversible changes in the mechanical properties of azopolymers [8]. Photo-expansion based on repeated cis-trans photo-isomerization generates free volume in the surrounding polymer matrix, which then increased the total volume of azopolymer films. However, a temporarily increased volume by configurational change of azo-moieties recovers with lights off, whereas newly generated free volume showed limited volume recovery under Tg of azopolymers, resulting in irreversible photomechanical properties. These reversible/irreversible mechanical changes of azopolymers have been exploited further in numerous studies in terms of hardness, stiffness, creep elastic modulus and creep coefficient, where glassy solid azopolymers became rubber-like plastic during light irradiation with various azo-derivative-polymers [9–11].
While those studies focused on analyzing the single beam assisted photomechanical property change of azopolymers, the optically patterned SRG of azopolymers, fabricated by two-beam coupled holographic lithography, has rarely been researched [12,13]. In this study, we investigated the mechanical changes in azopolymers in local areas with texturing holographically inscribed SRG with exposure time using a nanoindentation method. From this, we report ordered mechanical properties aligning with the SRG patterns of azopolymers. We believe that understanding the mechanical changes of azopolymers in local areas provides insights for implementing SRG to develop advanced patterning techniques.
2. Experimental details
2.1 Preparation of azopolymer-coated thin film
Poly (Disperse Red 1 methacrylate) (PDR1) was purchased from Sigma-Aldrich as a photosensitive azopolymer and used without further purification, and the chemical structure of PDR1 is shown in Fig. 1(a). PDR1 was dissolved in chloroform by 3 wt% with vigorous stirring. The prepared PDR1 solution was filtered with a PTFE filter with a 0.45 um pore size to remove undissolved particles. The glass substrates for spin coating were cleaned by sonication in acetone and IPA for 20 and 10 minutes, respectively. To provide good wettability between the glass substrates and the PDR1 solution, Ultraviolet/Ozone treatment was carried out on a glass surface just before spin coating. The filtered PDR1 solution was spin coated at 1000 rpm for 30 seconds. The spin coated PDR1 thin film was annealed at 95 °C for 24 hours to remove residual solvent. The thickness of the azo-thin film was 320 nm, measured by alpha-step (alpha-step IQ, KLA Tencor). The UV-Visible absorption property of PDR1 was measured using a UV-Visible spectrophotometer (UV-1800, SHIMADZU), with the maximum absorption wavelength at 464 nm as shown in Fig. 1(b).
Fig. 1 (a) Chemical structure of Poly (Disperse Red 1 methacrylate) (PDR1) and (b) UV-Visible absorption spectrum of PDR1
2.2 Fabrication of surface relief grating of azopolymer-coated thin film
Construction of the surface relief grating was conducted through an LIP assisted photo-fabrication process with two p-polarized laser beams with equal intensity of 33 mW/cm2, as shown in Fig. 2. A laser with a wavelength of 532 nm was used as a light source. The incident angle of the laser beam pair was 11.5°, and we expected a grating period of 1350 nm as calculated from Bragg’s law. As LIP-induced optical gradient force supplied the sinusoidal oscillation of light intensity along the x-axis, also known as k-vector of LIP, 1-dimensional linear grating was inscribed on the surface of the azopolymer thin film. The expected texture of the SRG is presented in forms of cross section and bird-eye-view image in Fig. 2. The diffraction efficiency of the SRG was evaluated by measuring the light intensities of the transmission and diffraction beams from a red-visible laser source (633 nm) with a photodetector (OP-2 VIS, COHERENT) upon SRG inscription. After laser irradiation, the resultant surface topology was characterized by atomic force microscope (AFM, XE-7 from Park Systems) using the non-contact mode.
Fig. 2 Schematic presentation of the holographic optical apparatus for fabricating surface relief grating (PBS: polarizing beam splitter, HWP: half wave plate, POL: polarizer, PD: photo detector).
2.3 Nanoindentation study of fabricated SRG
Nanoindentation analysis was studied by using Berkovich type tip (PPP-NCHR) with 23.3° of effective cone angle and 12.1° of semi angle, as shown in Fig. 3(a). The procedure was performed by applying the maximum load of 6 μN on penetrating the indentation tip with a loading/unloading speed of 0.1 μm/s. The obtained load versus displacement data, shown as Fig. 3(b), was analyzed by the Oliver-and-Pharr model [14], and the definitions of geometrical values are shown in Fig. 3(c). Briefly, the indentation tip penetrated the object surface until it reached the maximum depth of hmax with the maximum load of Pmax. Meanwhile, the actual contact area Ac and the contact depth hc were less than hmax because of the pile-in (or pile-up) surface deformation induced residual depth hr. After the tip is unloaded, there was a certain amount of permanent surface deformation with a final depth of hf because of the elastic deformation. With those values, the hardness was calculated by following Eq.
Ac is the projected contact area which could be evaluated by the measured contact depth hc and the tip constant (0.239 for PPP-NCHR, calculated from the semi angle of the indentation tip) when Pmax was applied.The contact depth was calculated as described in Eq. (2) where hmax is the maximum penetration depth when Pmax is applied. ε is geometry constant of the indentation tip (0.75 for Berkovich type indentation tip). Stiffness (S) was determined by the slope of the unloading curve at the hmax, defined by Oliver-and-Pharr as the following Eq.:From the above contact stiffness, a reduced Young’s modulus was determined as follows.β is the geometrical constant of the indentor (1.034 for Berkovich indentor).Fig. 3 (a) Scanning electron microscope image of the indentation tip (PPP-NCHR) and geometrical definitions for evaluating tip coefficients (b) Schematic presentation of the indentation curve of load-displacement and (c) surface profile behavior before/after indentation (hmax: maximum penetration depth, hc: contact depth, hf: final depth, hr: residual depth, S: stiffness, Pmax: maximum load, Ac: projected contact area).
3. Results and discussion
3.1 Analyzing the mechanical properties of SRG
To examine the changes in mechanical properties with optical pattern formation, nanoindentation was performed on the surface of azopolymer thin film before and after laser irradiation for 1200 seconds as shown in Fig. 4(a). Furthermore, we also investigated the photomechanical effect in terms of the peak and valley positions of SRG, corresponding to trough/crest of LIP. Figure 4(b) shows the results of load-displacement curves obtained from nanoindentation experiments. No samples restored their initial surface state after unloading where the maximum penetration depth and final depth, meaning the degree of permanent deformation, was highest in the case of the valley position of SRG and vice versa with a bare PDR1 sample. From this, the real contact depth was evaluated based on the above Eqs. as shown in Fig. 4(c). It is noteworthy that mechanical property of PDR1 was reduced with laser irradiation and the degree of reduction in stiffness was most significant at the valley position of the SRG. According to our interpretation, the newly formed free volume in azopolymers by an irreversible photo-expansion effect weakened the hardness of the azopolymer matrix. In addition, the lowest mechanical property of the azopolymers at the valley position of SRG (equals to crest of LIP) is reasonable because the degree of photo-expansion directly depends on the light intensity. Finally, profiles of the contact depth and corresponding modulation height along the x-axis are summarized in Fig. 4(d). It reveals that the contact depth profile had sinusoidal curves matching the LIP profile, whereas the inverse profile of the grating depth was observed.
Fig. 4 (a) The schematic presentation of the indentation process for a bare and laser irradiated PDR1 sample. Each indentation point refers to a random point of PDR1 without laser irradiation, peak and valley of SRG (marked as 1, 2 and 3, respectively) (b) The obtained load versus displacement curves from indentation experiments, (c) The calculated contact depth variation with different indentation points and (d) The modulation height with the corresponding contact depth profile.
As the photomechanical property variation upon light irradiation was demonstrated in Fig. 4, the effect of light exposure dose on the degree of reduction in mechanical properties was also investigated as shown in Fig. 5. Two beam coupled LIP irradiated PDR1 for different irradiation times from 0 to 1200 seconds and their surface morphology was monitored by AFM, where the resultant images are shown in Figs. 5(a)-5(f). From these AFM images, light propagation was simulated by a fast Fourier transform (FFT) as presented in Figs. 5(g)-5(l) and surface profiles along the x-axis are summarized in Fig. 5(m). The results show that SRG grows with respect to exposure dose and eventually, highly diffracted beams are observed in Fig. 5(l). The mean modulation height is also summarized in Fig. 5(n) with measured diffraction efficiency as a function of irradiation time. The diffraction efficiency shows an analogous tendency with the FFT results, resulting in the highest diffraction efficiency of 10.39% at an irradiation time of 1200 seconds. We interpreted the results to indicate that the incident red-visible probe beam was strongly diffracted by well-developed SRG with a high aspect ratio.
Fig. 5 (a-f) AFM images of PDR1 with different exposure doses from 0 to 1200 s and (g-l) their corresponding FFT results. (m) Growth behavior of the modulation height profile with respect to exposure the dose and (l) mean modulation height and diffraction efficiency as a function of the exposure dose.
Finally, we evaluated the mechanical properties, including the contact depth, hardness and reduced Young’s modulus, with the measured nanoindentation data using the above Eqs. Figure 6(a) shows the summarized results of the contact depth behavior with an increasing exposure dose while Figs. 6(b) and 6(c) present the calculated hardness and reduced Young’s modulus results, respectively. Regardless of the amount of the exposure dose, laser irradiated PDR1 sample showed reduced mechanical properties compared to bare azopolymer thin film, which confirms that even a very short irradiation could cause a sufficient softening effect on the azopolymer matrix. Similarly, the mechanical properties at the valley position of SRG are always lower than those at peak position. This could be also explained by the irreversible photo-expansion effect as described in Fig. 4. More in detail, the hardness as well as a reduced Young’s modulus of PDR1 was decreased exponentially and was saturated at an irradiation time of 100 seconds. It is noteworthy that reduction in mechanical properties caused by the photo-expansion effect was saturated whereas the modulation depth of SRG continuously increased until an irradiation time of 1200 seconds. This is because the pressure gradient force induced by photo-expansion behavior is saturated in the early stage of laser irradiation as reported previously. Meanwhile, SRG was continuously modulated by the combined effects of the optical gradient and pressure gradient forces. Compared to the limited volume change by the photo-expansion effect, the optical gradient force is based on a temporary configurational change of cis-trans isomerization that was continuously supplied by photo-energy.
Fig. 6 (a) Contact depth, (b) hardness and (c) reduced Young’s modulus curves at peak and valley position of SRG with respect to exposure dose. (d) Schematic illustration of photo-expansion effect on mechanical property change. (1) Before LIP irradiation. Trans-form of azobenzene components are predominantly exists in azopolymers due to its structural thermal stability (2) during LIP irradiation. Photoisomerization from trans- to cis-form vigorously occur at the crest of LIP whereas transformation rather weakly occur at trough of LIP due to light intensity dependency of photoisomerization (3) After LIP irradiation. With the mass transfer along the light intensity gradient, newly formed free volume through the photo-expansion also described at trough of SRG where the biggest amounts of free volumes were generated. Note that the dashed line of orange color indicates initial film surface before light irradiation.
Figure 6(d) illustrates the possible mechanism of change in mechanical properties of azopolymers upon light irradiation. (1) Before laser irradiation, azopolymer film has uniform mechanical property with trans-form of azobenzene groups. (2) Upon LIP exposure, however, photo-isomerization of azobenzene continuously occurs while differently distributed degree of photo-expansion effect is generated with respect to LIP. Eventually, the amount of photo-expansion induced free volume is expected to have sinusoidal distribution. (3) After light irradiation, patterning of SRG as well as mechanical property is formed along k-vector of LIP. It is interpreted as mechanical property was most softened by photo-expansion induced additional free volume at crest of LIP.
4. Conclusions
In summary, we investigated the behavior of the photomechanical property of azopolymers upon SRG formation through holographic lithography. The mechanical properties, including hardness and the reduced Young’s modulus, were quantitatively studied using a nanoindentation method with respect to the light exposure dose. Our study revealed that photo-softening of azopolymers occurred very fast with a short light irradiation time, and a reduced mechanical property was not recovered after light off. We interpreted this irreversible change of the photomechanical property as the newly formed free volume by the photo-expansion effect weakening the azopolymer matrix below Tg. Finally, patterning of the surface morphology as well as mechanical property was demonstrated along the k-vector of LIP.
Funding
KITECH ((Korea Institute of Industrial Technology) (JG160005, ‘Development of multi-lay-up method and fine pattern technique for the highly integrated PCB’)); MOTIE (Ministry of Trade, Industry & Energy (project number 10051918)) and KDRC (Korea Display Research Consortium) support program for the development of future devices technology for display industry.
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