© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 39, OCTOBER 5, 2000
LETTERS Optical Near Field Induced Change in Viscoelasticity on an Azobenzene-Containing Polymer Surface Taiji Ikawa,* Takuya Mitsuoka, Makoto Hasegawa, Masaaki Tsuchimori, and Osamu Watanabe Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan
Yoshimasa Kawata, Chikara Egami, Okihiro Sugihara, and Naomichi Okamoto Faculty of Engineering, Shizuoka UniVersity, Johoku, Hamamatsu 432-8561, Japan ReceiVed: March 28, 2000; In Final Form: June 1, 2000
Phase imaging during tapping mode atomic force microscopy (TMAFM) has revealed that an optical near field caused a change in the viscoelastic property on the surface of an urethane-urea copolymer film containing donor-acceptor substituted azobenzenes. Monolayers of polystyrene microspheres with 100 nm diameter and 19 nm diameter were fabricated on the surface of the copolymer film and exposed to a 488 nm wavelength laser beam coincident with the absorption band of the azobenzene derivatives. After removal of the monolayer, the phase image of the film’s surface was obtained by TMAFM. The phase shift of a cantilever oscillation (the shift was induced by a tip-sample interaction) indicated that the area affected by the optical near field of the microsphere became relatively softer (the phase shift was smaller) and the vicinal area became harder (the phase shift was larger). These results suggested that the optical near field produced a change in the density on the surface of the copolymer in nanometric dimensions. The copolymer was capable of transcribing the optical near field within the resolution of 20 nm on the basis of the viscoelastic feature.
Introduction Azobenzene-containing polymers have attracted much interest in applications for electrooptic modulation1 and optical data storage.2 According to many researchers, it has been reported that irradiation with an interference pattern from the coherent superposition of laser beams generates a topographic relief structure on the material (i.e., surface relief grating, SRG).3-6 The SRG is considered to be a photodriven mass transport,4,5 and several models for the origin of the driving force, such as a light intensity gradient,5 intermolecular interaction,7 and * Corresponding author e-mail:
[email protected] internal pressure,4 are proposed. However, neither the mass transport nor its origin have been directly confirmed. Recently, our research group demonstrated that the intensity distribution of the optical near field8,9 in the subwavelength region could be converted to a topographical change on the surface of the azobenzene polymer.10 The procedure for the experiment is as follows. A monolayer of polystyrene (PS) microspheres was placed on the surface of the material and irradiated with the laser beam. After removal of the monolayer, the surface was observed using atomic force microscopy (AFM). The topographic image in the surface provided a map of the interaction between the optical near field of the monolayer and
10.1021/jp001172l CCC: $19.00 © 2000 American Chemical Society Published on Web 09/13/2000
9056 J. Phys. Chem. B, Vol. 104, No. 39, 2000
Figure 1. Chemical structure of the urethane-urea copolymer.
the azobenzene polymer. We have succeeded in observing the optical near field of polystyrene (PS) microspheres with 28 nm diameter.11 We have proposed nonoptically probing near-field microscopy (NOM) using both this material and AFM. However, the mechanism of the deformation induced by the optical near field has not yet been clarified. In addition, when the sample is much smaller and/or the intensity of the optical near field is weaker, the topographical change on the azobenzene polymer became smaller and more difficult to detect. Thus, a more sensitive change in the physical property of the material and/or more sensitive detecting system is required for NOM. Under other circumstances, the recent development of tapping mode atomic microscopy (TMAFM) allows one to clarify various properties of the surface.12 In TMAFM, the cantilever oscillates vertically near its resonance frequency. As the tip is brought close to the sample’s surface, the vibrational characteristics of the cantilever (e.g., the amplitude, resonance frequency, and phase angle of oscillation) vary due to the tipsample interaction. The factors influencing the shift in phase angle (hereafter referred to as phase shift) are capillary force, van der Waals force, and viscoelasticity. With the tip sufficiently in contact with the sample, the phase shift mainly occurs due to the viscoelasticity of the surface,13 and the phase image provides a map of the variation in the viscoelastic property. Thus far, the viscoelastic property of several polymers has been investigated using the phase imaging in TMAFM;13-16 however, there is limited information about the viscoelastic behavior of the azobenzene polymers,17 and the viscoelasticity of the optically modified azobenzene polymer has not yet been studied using the phase imaging. In this letter, we report that the optical near field induced viscoelastic transformation on the surface of the azobenzene polymer in nanometric dimensions. The phase imaging in TMAFM revealed a change in the viscoelasticity on the surface of the material. We demonstrated that the material is capable of transcribing the optical near field of PS microspheres with 100 and 19 nm diameters onto the surface using the viscoelastic feature. In addition, we discuss the mechanism of viscoelastic transformation and surface deformation of the azobenzene polymer. Experimental Section The urethane-urea copolymer containing the donor-acceptor-substituted azobenzene shown in Figure 1 was used. Details of the synthesis are described in reference 18. The polymer had a molecular weight of 170 000 (relative to PS) and a glass transition temperature (Tg) of 145 °C. In the UV-vis spectrum, the maximum absorption wavelength due to the azobenzene derivative appeared at around 475 nm. The film of the copolymer was prepared by spin coating from a pyridine solution. For the purpose of releasing any residual stresses, the film was heated in a vacuum at 80 °C for 24 h and at 140 °C for 2 h, then gradually cooled to room temperature. The
Letters thickness of the film was about 0.5 µm. A monolayer of PS microspheres on the film was prepared using the following experimental procedure.10,19 A disk with a 4 mm diameter hole was placed on the film. The aqueous solution of the monodispersed PS microspheres was dropped into the hole, and the water was evaporated from the solution. The microspheres were arranged by a self-organization process. Two kinds of PS microspheres, with diameters of 100 and 19 nm, were used. The aqueous solutions of the PS microspheres were purchased from the Duke Scientific Corp. The monolayer was then irradiated with a coherent 488 nm light from Ar+ laser. The circular polarization states of the beam were achieved by inserting a properly oriented quarter-wave plate. The laser power at the sample was 500 mW/cm2 and the irradiation time was 5 min. After removal of the microspheres from the surface by immersion in water and/or by eluting in benzene, the topographic and phase images of the surface of the film were obtained by TMAFM using a Nanoscope IIIa (Digital Instruments, Inc). A commercial cantilever, which consists of silicon (the tip radius of curvature was 5 nm, the cantilever length was 125 µm, the spring constant was 50 N/m, and the free resonant frequency was 298 kHz) was used. Results and Discussion The topographic and phase images of the sample prepared using the 100 nm diameter microsphere are shown in Figure 2a,b, respectively. The images were obtained during the same scanning. The contrast in Figure 2a is proportional to the surface height while the contrast in Figure 2b is proportional to the phase shift displayed in degrees. Figure 2a shows that dents (darker area) have been formed on the surface. The diameters of the dents were about 100 nm and the depths were about 40 nm. The diameter of the dent almost corresponded with the diameter of the microsphere used for the experiment. The dents were formed just under the microspheres. We see from Figure 2 that the phase image was similar to the topographic image. The insides of the dents were assigned to the darker areas in the phase image, and the margins of the dents correspond to the brighter one. As noted above, the phase shift is related to the viscoelastic property on the surface. Therefore, Figure 2 indicates that the change in the viscoelastic property on the surface occurred simultaneously with the change in the surface height. The following experiments revealed that the contribution of opt-thermal effect to these changes was low. When the film with the monolayer was heated on the hot plate to 140 °C without irradiation, the substrate surface showed no dent. In the case that the monolayer on the film was irradiated with a 0.01 W/cm2 laser beam for 250 min, the topographic and phase images of the substrate surface were practically identical to Figure 2. According to the calculation based on the Mie scattering theory,20,21 the optical near field of the microsphere irradiated with the laser beam was enhanced several times larger than the incident field.10 These facts led us to conclude that the optical near field of the microsphere caused the change in the surface height and the viscoelastic property. The topographic and phase images of the sample prepared using the 19 nm diameter microspheres are shown in Figure 3a,b, respectively. In contrast to Figure 2, these images differed from each other. The topographic image in Figure 3a shows that the surface was almost planar. The undulation in the surface height was less than 2 nm. We reported that the dent depth decreased depending on the microsphere diameter under the same conditions, and a several-nanometer depth was obtained using the 28 nm diameter microsphere.11 Thus, we considered
Letters
J. Phys. Chem. B, Vol. 104, No. 39, 2000 9057
Figure 2. TMAFM images of the sample surface prepared using PS microspheres with a 100 nm diameter. (a) Two-dimensional image of surface shape. (b) Two-dimensional image of the phas-e shift. Figures (a) and (b) were obtained at the same time.
Figure 3. TMAFM images of the sample surface prepared using PS microspheres with a 19 nm diameter. (a) Two-dimensional image of surface shape. (b) Two-dimensional image of the phase shift. Figures (a) and (b) were obtained at the same time.
that no noticeable deformation was caused using the 19 nm diameter microsphere. On the other hand, a periodic contrast was observed in the phase image in Figure 3b. The period of the contrast was about 20 nm and was regarded as corresponding to the diameter of the microsphere used in this experiment. The calculation indicates that the intensity of the optical near field of the 19 nm diameter microsphere is weaker than that of the 100 nm diameter microspheres. Therefore, it is reasonable to suppose that the optical near field of the 19 nm diameter microsphere caused the change in viscoelasticity, though the intensity of the field was not enough to induce the surface deformation. In our view, the phase image in these experiments provided a map of stiffness variation on the surface such that a stiffer region had a more positive phase shift, and hence, appeared brighter in a phase image. The reason for this is that the phase shift ∆Φ0 is approximately described as
∆Φ0 ∝ x〈A〉E* (Q/k)
(1)
where 〈A〉 is the time averaged value of the contact area, E* is the effective modulus, Q is the quality factor of a cantilever, and k is the spring constant of the cantilever.13 E* is proportional to the stiffness when the tip is much harder than the sample. Equation 1 implies that the phase shift is more positive in the
stiffer region. It should be noted that when the rubber-like material is observed by TMAFM, the phase shift is greater on a softer region than on a harder region, because a softer material leads to a larger contact area 〈A〉. In our experiments, we operated the TMAFM at room temperature, much lower than the glass transition temperature of the azobenzene polymer, and at moderate tapping (large A0 and somewhat small rsp).13 Assuming that 〈A〉 was constant under these conditions,13,14 we could determine that the phase shift was dominated by the stiffness on the surface such that a stiffer region had a greater phase shift. As a similar example, for the low- and high-density parts of a microlayered polyethylene sample, the phase shifts were substantially larger on the harder than on the softer part of the surface.13 Hence, we concluded that the area affected by the optical near field became relatively softer and the vicinal area became harder. Bleaching, polymer degradation, and polymer chain migration were regarded as the origins of the change in the viscoelastic property. We consider that the change in the viscoelasticity at lower intensity of the incident light (0.01 W/cm2) is direct evidence for the migration of the azobenzene polymer chain. In the case of the SRG formation, many researchers have pointed out that the azobenzene polymer chains were considered to move in the direction from the stronger optical field area to the weaker one.3,5 The phenomenon in the present experiments was
9058 J. Phys. Chem. B, Vol. 104, No. 39, 2000 explained in the same way: the azobenzene polymer chains affected by the optical near field of the microsphere migrated and, accordingly, the area where the microsphere had located took on a lower density (lower elasticity) and the margin of that area became more dense (higher elasticity). When the intensity of the optical near field was sufficiently strong, a dent was formed as a result of the chain migration. A surface relief grating was also formed on the surface of the azobenzene polymer used in the experiments,10 and the contrast with respect to the grating was observed in the phase image. The detailed results will be reported elsewhere. Conclusion We have shown that the viscoelastic property on the surface of the azobenzene polymer varied with the optical near field in the 20 nm region. We used the phase imaging in TMAFM for the analysis of the surface viscoelasticity. The results suggested that the density on the surface was changed by the optical near field and the polymer chain migration was responsible for the change. From the standpoint of using this material for NOM, we could observe the optical near field of the specimen by taking advantage of both the viscoelastic feature of the material and the phase imaging in the TMAFM. A resolution within 20 nm could be achieved by this sensitive procedure. References and Notes (1) Sawodny, M.; Schmidt, A.; Stamm, M.; Knoll, W.; Urban, C.; Ringsdorf, H. Polym. AdV. Technol. 1991, 2, 127. (2) Rochon, P.; Gosslin, J.; Natansohn, A. Appl. Phys. Lett. 1992,60, 4.
Letters (3) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 9. (4) Barrett, C. J.; Natansohn, A. L.; Rochon, P. L. J. Phys. Chem. 1996, 100, 8836. (5) Viswanathan, N. K.; Balasubramanian, S.; Li, L.; Kumar, J.; Tripathy, S. K. J. Phys. Chem. B. 1998, 102, 6064. (6) Holme, N. C.; Nikolova, L.; Norris, T. B.; Hvilsted, M.; R. H. Pedersen.; Berg, R. H.; Rasmussen, P. H.; Ramanujam, P. S. Macrolol. Symp. 1999, 137, 83. (7) Pedersen, T.; Johansen, P. M.; Holme, C. R.; Ramanujam, P. S.; Hvilsted, S. Phys. ReV. Lett. 1998, 80, 89. (8) Ohtsu, M.; Hori, H. Near-Field Nano-Optics; Kluwer Academic: New York, 1999. (9) Shinya, A.; Fukui, M. Opt. ReV. 1999, 6, 215. (10) Kawata, Y.; Egami, C.; Nakamura, O.; Sugihara, O.; Okamoto, N.; Tsuchimori, M.; Watanabe, O. Optics Comm. 1999, 161, 6. (11) Watanabe, O.; Ikawa, T.; Hasegawa, M.; Tsuchimori, M.; Kawata, Y.; Egami, C.; Sugihara, O.; Okamoto N. Mol. Cryst. Liq. Cryst., in press. (12) Zhong, Q; Innis, D.; Kjoller, K.; Elings, V. B. Surf. Sci. 1993, 290, L688. (13) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. 1997, 375, L385. (14) Bar, G.; Thomann, Y.; Brandisch, R.; Cantow, H.-J. Langmuir 1997, 13, 3807. (15) Lecle`re, Ph.; Lazzaroni, R.; Bre´das, J. L.; Yu, J. M.; Dubois, P.; Je´roˆme, R. Langmuir 1996, 12, 4317. (16) Zhong, Q.; Innis, D.; Kjoller, K.; Elings, V. B. Surf. Sci. 1993, 290, L688. (17) Barrett, C. J.; Natansohn, A. L.; Rochon, P. L. J. Phys. Chem. 1996, 100, 8836. (18) Watanabe, O.; Tsuchimori, M.; Okada, A. J. Mater. Chem. 1997, 6, 1487. (19) Dushkin, C. D.; Nagayama, K.; Miwa, T.; Kralchevsky, P. A. Langmuir 1993, 9, 3695. (20) Born, M.; Wolf, E., Principles of Optics, 5th ed.; Pergamon Press: Oxford, U.K., 1975; Chapter 13. (21) Mie, G. Ann. D. Physik. 1908, 25, 377.
