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Sample title 4no. of active NN unitsno. of active NN units4000300020001000400030002000100000 1 2 3 4 50 1 2 3 4 5 6 7 8 9 10E2ZZ2Etotalrmsd / nmrmsd / nm1.61.20.80.40.0-0.4-0.80 1 2 3 4 5 61.61.20.80.40.0-0.40 1 2 3 4 5 6 7 8 9 10 11xyztotxyztot00 1 2 3 4 5t / ns0 1 2 3 4 5 6 7 8 9 10t / ns-0.80 1 2 3 4 5 6t / ns0 1 2 3 4 5 6 7 8 9 10 11t / nsFIG. 6. Time evoultion of photoexcitation of azo chromophore unitsfor the four different MD setups.by their center of mass (COM) in the ’bright’ and ’dark’ regionfor the four different setups. Generally, it is seen thatinitially the rmsd in the bright region is dominated by the z-component (blue line) for all four setups, implying an initialheight increase. Later on, the growth in z slows down, whilex and y-components rise more rapidly. the In the case of the’yx’ and ’yy’ setups, movement starts to be dominated by they-component from about 1 ns onwards.For the horizontal setups ’xx’ and ’xy’ (Fig. 7b,d), themolecules are seen to travel much more slowly, which is inline with their reduced temperature (Fig. 4). The general behaviourof the x, y, and z components is similar to the verticalsetups, except for the fact that the x and y components neverdiverge meaning that there is no preferred direction of masstransport.All four setups have in common that mass transport occursfrom the bright to the dark region. However, in contrast toour expectation and seemingly at odds with many previousworks, molecular displacement is practically independent ofthe polarisation direction. As it is known that the influence ofpolarisation depends on many factors including the chemicalnature of the material and the light intensity, we aimed to verifyour computational observations by performing matchingexperiments using the same material (see below).B. ExperimentalIn order to experimentally verify the computational results,we illuminated a PDO3MA azopolymer film with differentintensity patterns. These consisted of bright stripes of varyingdistance, while their width remained the same. This way, theposition of the peaks of the SRG, whether in the illuminatedor dark regions, can be determined. Moreover, we varied thepolarization of the light to investigate its impact on the SRGformation.For fabrication of the azopolymer samples, 3 wt% ofFIG. 7. Time evolution of average COM root mean square displacement(RMSD) split into individual cartesian components anddark/bright regions for the four different MD setups. The curves forthe dark region are plotted with negative RMSD for better distinction.poly-disperse-orange3-methyl-methacrylate (PDO3M)(Sigma-Aldrich) were added to tetrahydrofuran (THF) andstirred for 2 hours. In the meantime, glass substrates werecleaned via ultrasonication in acetone and isopropyl alcoholfor 10 min each and subsequently treated in UV/ozone toimprove wettability. The azopolymer solution was spincoatedat 1000 rpm for 10 s on the glass substrates and finallyannealed on a hotplate at 110 ◦ C for 1 h.To generate stripe patterns with different distances of thebright stripes we utilized the setup shown in Fig. 8. Lightfrom a laser diode with a wavelength of 405 nm is modified bya half-wave plate to ensure a polarization parallel to the longside of the employed LETO phase-only spatial light modulator(SLM) (HOLOEYE). The SLM is based on a reflectiveLCOS microdisplay with a full high definition (1920 × 1080pixel) resolution and a pixel pitch of 6.4 µm. It is used to tailorthe transverse intensity distribution of the reflected wave.Therefore a blazed grating consisting of a linear phase rampbetween 0 and 2π with a grating period of ten pixels and a tiltangle of 51 ◦ is addressed to the SLM 34 . The reflected beam isfocused with a lens on a pinhole to separate the first diffractionorder containing the desired pattern from residual light andimaged with a second lens on the sample, resulting in a 10×demagnification of the pattern. Using an infinity-corrected objectiveand a lens, the generated light pattern can be observedwith a CMOS camera. After illumination, the azopolymersample surfaces were examined with the atomic force microscope(AFM) NaioAFM from Nanosurf with the Naio ControlSoftware (version 3.6), operated at ambient conditions. Usingthe sharp tip in the AFM, high resolution topography mapscan be obtained, see Fig. 9. On the left, the different illuminationpatterns used to inscribe the SRGs in the films. Next tothem, images of the respective AFM measurements of the topographiesand the corresponding line profiles are shown. Ineach case, the bright stripes have a width of 12.8 µm. The dis-
Sample title 5FIG. 8. Scheme of the experimental setup. λ/2: half-wave plate,SLM: phase-only spatial light modulator, M: mirror, L: lens, P: pinhole,S: sample, MO: infinity-corrected microscope objective andCam: camera.tance between the stripes varies between 25.6 µm (Fig. 9a),31.4 µm (Fig. 9b) and 37.2 µm (Fig. 9c). For all of them,the polarization of the light is perpendicular to the stripes andthe illumination time is 20 h. The intensity varies slightly between158 mWcm −2 (Fig. 9a), 164 mWcm −2 (Fig. 9b) and151 mWcm −2 (Fig. 9c).The line profiles extracted from the topography images canprovide insight into the roughness of the surface, which can bedirectly correlated to the polymer motion. Line profiles for thesample in Fig. 9a show a valley of approximately 12 µm wideand a plateau which is 26 µm wide. The average depth/heightof these features is between 60 nm and 100 nm. The line profilesof the other two samples show valleys of the same width,while the width of the plateaus varies. Thus, comparing thedimensions of the topographical features in these images tothe respective illumination patterns, we can conclude that theazopolymer has the tendency to move away from the brightregions in the illumination pattern and accumulate in the darkregions.Moreover, the influence of the polarization on the SRG formationis investigated. While in Fig. 9 the polarization is perpendicularto the stripes, in Fig. 10 it is parallel. Otherwisethe illumination parameters are the same as in Fig. 9a with anillumination time of 20 h and an intensity of 158 mWcm −2 .Again the width of the valleys correspond to the width of thebright stripes and the plateaus to the dark regions in between.Also the depth of the structures is again in the range of 60 nmto 100 nm as for the illumination with perpendicular polarization.The only difference is the appearance of small bead-likestructures of 10 nm to 20 nm height in the valleys.FIG. 9. Illumination patterns with different distances between thebright stripes and the respective AFM topography images and lineprofiles: (a) 25.6 µm, (b) 31.4 µm and (c) 37.2 µm. The line profilesare extracted across the whole width of the respective scan image atpoints indicated by the colored arrows. Laser light polarized perpendicularto the striped pattern is used.C. Merging theory and experimentThe simulation predicts that during illumination maximaform in the exposed regions and then migrate from the sidesinto the dark regions, as has been observed similarly earlierby Yadavalli et al. 35 . Double-peak substructures similar tothose formed in our simulation after cooling have also beenFIG. 10. Illumination pattern with 12.8 µm wide bright stripes with25.6 µm in between and respective AFM topography images and correspondingline profiles of the illuminated azopolymer sample. Theline profiles are extracted across the whole width of the scan image atpoints indicated by the colored arrows. The polarization of the laserbeam is parallel to the illumination pattern.
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