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J. Phys. Chem. C 2008, 112, 16042–16045
Photoinduced Surface Relief Grating Formation on a (100) Surface of a Single Crystal of 4-(Dimethylamino)azobenzene Hideyuki Nakano* Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity, Yamadaoka, Suita, Osaka 565-0871, Japan ReceiVed: June 2, 2008; ReVised Manuscript ReceiVed: August 5, 2008
Surface relief grating (SRG) can be fabricated on a (100) surface of a single crystal of 4-(dimethylamino)azobenzene by irradiation with coherent laser beams. The SRG formation depended upon both orientation of the crystal and polarization direction of the writing beams. It was suggested that the photoinduced SRG formation took place by mass transport from the bright region toward the dark region near the surface of the crystal. In addition, the molecules existing in the convex region of the resulting SRG were found to be oriented according to the underlying crystal. Introduction
CHART 1: Molecular Structure of DAAB
Recently, photomechanical effects observed for photochromic materials, such as bending of liquid-crystalline azo-polymer network films1 and reversible shape change of molecular crystals of diarylethene derivatives,2 have attracted attention. Phenomena of photoinduced mass transport, especially surface relief grating (SRG) formation of azo-polymer films upon irradiation with coherent laser beams, are also subjects of interest from both viewpoints of academic fundamental science such as elucidation of the mechanism of photoinduced mass transport and practical applications for photonics, e.g., erasable and rewritable holographic memory and waveguide coupler.3-15 We have been performing studies of photoinduced SRG formation using azobenzene-based photochromic amorphous molecular materials, namely, small photochromic molecules that readily form amorphous films above room temperature.16 Related phenomena of photoinduced mass transport, such as relief formation upon irradiation through the patterned mask by using azo-polymer liquid crystals17 and by using poly(methyl methacrylate) films doped with spiropyran and diarylethene derivatives,18 SRG formation using a photo-cross-linkable polymer liquid crystal,19 photoinduced surface deformation of an azo-polymer film using monodispersed polystyrene spheres,20 near-field optical patterning of azobenzene-based photochromic sol-gel films,21 and spontaneous patterning of hexagonal structure produced on an azo-polymer film by uniform laser beam irradiation,22 have also been reported. Photoinduced SRG formation of azobenzenebased films is believed to be due to mass transport induced by the trans-cis and cis-trans isomerizations of azobenzene chromophore. SRG formation depends upon the polarization direction of writing beams. Several models for the mechanism of photoinduced SRG formation have been proposed;5-8,14,23 however, the details have not been clear yet. In contrast to amorphous materials, it is of interest to examine whether fabrication of SRG on an “organic single crystal” is possible or not by irradiation with two coherent laser beams. If possible, the study of photoinduced SRG formation may provide information about the mechanism of photoinduced mass transport. In addition, such studies may offer new insights into * To whom correspondence should be addressed. E-mail: nakano@ chem.eng.osaka-u.ac.jp.
organic solid-state physical chemistry concerned with the behavior of molecules existing near the surface of the organic crystal. Under these considerations, we have demonstrated photoinduced SRG formation on a (001) surface of a single crystal of 4-(dimethylamino)azobenzene (DAAB) (Chart 1) by irradiation with two coherent laser beams.24 In addition, the SRG formation was found to depend upon both orientation of the crystal and the polarization of writing beams.24 As the photoinduced SRG formation may be affected by the orientation of the molecules, the study of photoinduced SRG formation on a (100) surface of the DAAB crystal is expected to provide more clear and consequently more detailed information than the study on the (001) surface because of larger dichroism for the (100) surface than for the (001) surface of DAAB single crystal as described below. In the present study, photoinduced SRG formation on the (100) surface of DAAB single crystal has been investigated. Experimental Section DAAB was purchased commercially (Wako Pure Chemical Industries, Ltd.). A single crystal of DAAB (composed of the trans isomers; melting point 117 °C) was grown by gradual evaporation of the solvent of an ethanol solution of DAAB at room temperature in the dark. The sample crystal with dimensions of ca. 0.8 × 0.3 × 0.2 mm3 was prepared by cutting the as-grown crystal with a razor’s edge to have smooth (100) surface and was fixed on a glass substrate by an epoxy-type adhesive agent. Photoinduced SRG formation was carried out by using a compact CW laser (488 nm, CYAN-488-50NH-W, Spectra Physics) as a source of writing beams at room temperature (20-22 °C). Atomic force microscopy (AFM) was performed by means of Scanning Probe Microscope (JSTM-4200D, JEOL Ltd.) with a microcantilever (OMCL-AC160T-C2, OLYMPUS). Results and Discussion A DAAB single crystal could be obtained by gradual evaporation of the solvent of an ethanol solution of DAAB.
10.1021/jp804853q CCC: $40.75 2008 American Chemical Society Published on Web 09/20/2008
Photoinduced SRG Formation on a Single Crystal
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16043
Figure 1. Crystal structure of DAAB25 projected along the a axis. Hydrogen atoms are omitted.
Figure 3. AFM image of SRG formed on the (100) surface of DAAB single crystal.
TABLE 1: Modulation Depths of SRGs Obtained by Irradiation with the Writing Beams under Different Experimental Conditionsa no.
orientation of the crystal
polarization of writing beams
1 2 3 4
V V H H
s p s p
modulation depth (nm) 200 - 400 b b
70 - 150
Intensity of the writing beams, 50 µW × 2; irradiation time, 10 min. b SRG formation was not observed. a
Figure 2. (a) Schematic experimental setup for SRG formation. S, sample; P, polarizer; M, mirror; W, wave plate; B, beam splitter. (b) Schematic illustration of the sample orientation.
Figure 1 shows the crystal structure of DAAB projected along the a axis drawn by using the data reported in the literature,25 suggesting relatively large dichroism for the (100) surface. Dichroic ratios of the crystal for the (100) surface, defined here as the ratio of the optical density for polarized light perpendicular to the b axis to that parallel to the b axis, is equal to the square of the ratio of the c axis component to the b axis component of the transition moment of DAAB molecule lying in the crystal. By assumption that the transition moment of DAAB molecule was parallel to the line connecting 4-carbon with 4′-carbon of the azobenzene moiety, the dichroic ratio was estimated to be ca. 106 for the (100) surface, being considerably larger than that for the (001) surface (ca. 17). Thus the study for the (100) surface of the DAAB single crystal is expected to provide more clear information about the SRG formation than that for the (001) surface. However, it is too difficult to investigate the SRG formation on (100) surface using as-grown crystals because the DAAB crystal tends to grow from the solution to form rather elongated hexagonal columnar shape with its longest axis parallel to the a axis and both ends of the crystal do not have smooth (100) surface generally. Thus, the sample crystal was prepared by cutting the as-grown crystal with a razor’s edge to have smooth (100) surface with an appropriate thickness (ca. 0.2 mm). The schematic experimental setup for SRG formation is illustrated in Figure 2a. Two coherent laser beams (488 nm) with incident angles of +10° and -10° to the normal of the sample surface and with either p or s polarization were irradiated as writing beams toward the (100) surface of the sample crystal, which was fixed on a glass substrate to be oriented as its c axis was either perpendicular (H, horizontal) or parallel (V, vertical) to the s-polarization direction of the writing laser beams (Figure 2b). The intensities of the writing beams were 50 µW (ca. 400 µW cm-2) each. As observed for the (001) surface,24 SRG formation was observed by irradiation of the (100) surface of DAAB single crystal with the writing beams, confirmed by AFM. Figure 3
shows AFM image of the resulting SRG formed on the (100) surface after irradiation of the V-oriented sample with the s-polarized writing beams for 10 min. The SRG with a modulation depth of ca. 300 nm was observed. SRG formation on the DAAB crystal was found to depend upon both the orientation of the crystal and the polarization of the writing beams. The modulation depths of the SRG obtained by irradiation with the writing beams for 10 min under four different experimental conditions were summarized in Table 1. When the V-oriented crystal was irradiated with the p-polarized writing beams (condition 2) and when the H-oriented crystal was irradiated with the s-polarized writing beams (condition 3), no SRG formation was observed. On the other hand, SRG formation was observed when the V-oriented crystal was irradiated with the s-polarized writing beams (condition 1) and when the H-oriented crystal was irradiated with the p-polarized writing beams (condition 4). Modulation depth of the resulting SRG obtained under the condition 1 was found to be larger than that obtained under the condition 4. As for conditions 2 and 3, absorption of the writing beams by DAAB molecules near the surface was not so effective since the transition moment of DAAB molecules was almost perpendicular to the polarization direction of the writing beams, and therefore no SRG formation took place under the conditions 2 and 3. On the other hand, absorption of writing beams by DAAB molecules near the surface was more effective to induce a photochromic reaction for conditions 1 and 4, resulting in SRG formation. Such dependence of an SRG-forming property upon experimental conditions was more prominent than that observed for the (001) surface,24 which was suggested to be due to larger dichroic ratio for the (100) surface than for the (001) one as described above. It is noteworthy that the modulation depth of the SRG obtained under condition 1 was found to be different from that obtained under condition 4 although the absorption of the writing beams near the surface was expected to be almost similar between these conditions. The fact suggested that neither ablation nor degradation of the molecules existing near the surface of the crystal upon photoirradiation was dominant for the SRG formation observed in the present study. Thus, the
16044 J. Phys. Chem. C, Vol. 112, No. 41, 2008
Nakano
Figure 5. AFM image of surface relief pattern fabricated by superposition of orthogonal gratings on the (100) surface of DAAB single crystal.
Figure 4. (a) Schematic experimental setup for irradiation with three coherent writing beams. S, sample; P, polarizer; W, wave plate; M, mirror; B, beam splitter. (b) Calculated intensity profile of the interference light of the three writing beams with the same intensities at the surface of the sample. (c) AFM profile of the (100) surface of DAAB single crystal irradiated with three s-polarized writing beams (25 µW × 3) for 5 min.
transport of the molecules existing near the surface of the crystal induced by trans-cis and cis-trans isomerizations of azobenzene chromophore was suggested to take place to form SRG. It has been reported that mass transport is induced in the direction from the bright region toward the dark region for azobenzene-functionalized amorphous polymer systems.8a We have reported that the photoinduced mass transport was also suggested to be induced from the bright region toward the dark region for photochromic amorphous molecular materials.16e To gain information about the direction of photoinduced mass transport for the SRG formation on DAAB single crystal, the surface relief profile formed on the (100) surface by irradiation with coherent three laser beams16e was investigated as follows. The sample was irradiated with three coherent writing beams with incident angles of +10, 0, and -10° to the normal of the sample surface as shown in Figure 4a. An approximate intensity profile at the sample surface produced by interference of the three writing beams with the same intensities was shown in Figure 4b, showing that the intensities at the local maxima change alternately. It is noted that the ratio of light intensities at a local maximum and at the neighboring one depends upon phases of incident three beams. Since the rate of mass transport is expected to increase with the increasing intensity gradient, a characteristic surface relief profile reflected the intensity gradient pattern and hence the direction of mass transport is expected to be observed in an early stage of irradiation with the three writing beams. Figure 4c shows the surface relief profile of the V-oriented sample irradiated with the three s-polarized writing beams (25 µW × 3) for 5 min. It was found that the depths of valleys of the resulting relief change alternately (Figure 4c) corresponding to the light intensity profile (Figure 4b). Although the result could not be discussed quantitatively because phase
matching of the three beams was not carried out, the result can be explained as follows. The molecules moved away from the bright region toward the dark region, and faster mass transport took place from the brighter region than from the less-bright region at the early stage according as the intensity gradient. Therefore, the number of molecules moved away was larger for the brighter region than for less-bright region at the early stage, producing the surface relief pattern where the depths of valleys change alternately. Thus, the mass transport was suggested to be induced from the bright region toward the dark region, being similar to that observed for amorphous systems.8a,16e It has been reported that superposition of two gratings to form an “egg-crate”-like relief structure was possible using amorphous polymers8a and amorphous molecular materials.16e It is of interest to examine whether similar superposition of two gratings to fabricate the “egg-crate”-like relief structure on the surface of the single crystal is possible or not. In addition, such a study is expected to provide further information about the photoinduced mass transport near the surface of the crystal. Thus the superposition of two gratings was examined on the (100) surface of DAAB single crystal as follows. At the first stage, the V-oriented crystal was irradiated with two writing beams with s-polarization (50 µW × 2) for 5 min. After a 5-min interval, the sample rotated by 90° to H-orientation was irradiated with writing beams with p-polarization (50 µW × 2) for 5 min at the second stage. As a result, the egg-cratelike structure was observed as shown in Figure 5 although the resulting relief structure seemed to be rather imperfect. The result suggested that a variety of surface relief structures may be fabricated by photoirradiation under appropriate conditions. It is noteworthy that such an egg-crate-like structure was not obtained and only a simple SRG being similar to that shown in Figure 3 was found when the s-polarized beams were used as the writing beams at the second stage instead of the p-polarized beams. The direction of the resulting SRG indicated that the SRG formation took place at the first stage. These facts suggested that the molecules existing in the convex region of resulting SRG by irradiation of two writing beams were oriented according to the underlying crystal. In addition, no SRG formation was observed for the V-oriented sample irradiated with two p-polarized writing beams after 5-min irradiation with a single uniform s-polarized beam. The result also suggested that the orientation of molecules near the surface of the single crystal was not randomized so much by photoirradiation. By consideration of the results described above, the phenomena of photoinduced SRG formation on the (100) surface of
Photoinduced SRG Formation on a Single Crystal DAAB single crystal can be explained as follows. When the sample is irradiated with the two coherent writing beams, molecules existing near the surface at the bright region absorb the light to isomerize to cis isomers, making the molecules easy to migrate due to the reduction of the melting point caused by increasing the content of the cis isomers and/or due to the repetition of trans-cis and cis-trans isomerizations. Under such a situation, a certain number of molecules may migrate toward the dark region, and the others stay at the bright region. When the cis isomers go back to trans isomers photochemically or thermally, such reproduced trans isomers are allowed to recrystallize according to the underlying crystal. The molecules recrystallized in the dark region will stay at the place whereas those in the bright region can absorb the writing beams again to migrate. In this manner, SRG was formed by the mass transport from the bright region toward the dark region. It has been reported that the irradiation with the p-polarized writing beams is more effective for SRG formation using amorphous azo-polymers and azobenzene-based photochromic amorphous molecular materials.8,16 However, that is not the case with SRG formation using DAAB single crystal. That is, modulation depth of SRG formation formed on the V-oriented crystal by irradiation with the s-polarized writing beams (condition 1) was larger than that formed on the H-oriented crystal by irradiation with the p-polarized writing beams (condition 4) as described above (Table 1). The results suggested that an anisotropic environment of the crystal affected the polarization dependence of photoinduced SRG formation. Since the orientation of molecules was not randomized so much by photoirradiation as described above, it is conceivable that the side-by-side intermolecular interaction which act in the direction parallel to the b axis makes the molecules easier to migrate parallel to the b axis than to the c axis, and hence larger SRGs could be obtained under the condition 1 than the condition 4. Conclusions Photoinduced SRG formation on a (100) surface of DAAB single crystal has been observed. It was suggested that SRG formation took place by mass transport from the bright region toward the dark region induced by the trans-cis and cis-trans isomerizations. The SRG formation depended upon both orientation of the crystal and polarization direction of writing beams and the dependence of polarization direction of the writing beams on SRG formation was different from that observed for amorphous systems. In addition, the molecules existing in the convex region of resulting SRG were suggested to be oriented according to the underlying crystal. In contrast to amorphous systems, the present study suggested that an anisotropic environment of the crystal play an important role in the photoinduced SRG formation. Acknowledgment. This work was partly supported by a Grant-in-Aid for Science Research in Priority Areas “New
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JP804853Q
