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Effects of Optical Trapping and Liquid Surface Deformation on the Laser Microdeposition of a Polymer Assembly in Solution Yu Nabetani,† Hiroyuki Yoshikawa,*,† Andrew C. Grimsdale,‡ Klaus Mu¨llen,§ and Hiroshi Masuhara*,† Department of Applied Physics, Graduate School of Engineering, Osaka UniVersity, Suita, Osaka 565-0871, Japan, Bio21 Institute, UniVersity of Melbourne, 30 Flemington Road, 3010 Victoria, Australia, and Max-Planck-Institute for Polymer Research, Ackermannweg 10, Postfash 3148, D-55021 Mainz, Germany ReceiVed NoVember 15, 2006. In Final Form: March 21, 2007 A polymer microassembly is formed by focusing a near-infrared (NIR) laser beam in a thin film of a polymer solution. We have investigated the mechanism of laser microdeposition of a polyfluorene assembly by measuring the surface deformation of the solution film and the morphology of the deposited assembly. It is clearly observed that a rupture is formed at the laser focus in the solution film by using laser interferometric imaging. The time necessary for the rupture formation and the volume of the deposited microassembly are analyzed as a function of laser power. Experimental results suggest that the solution surface deformation induced by local laser heating and optical trapping effects determined the volume of the laser microdeposition. By combining this method with multiple optical trapping, a polymer microassembly with a polygonal morphology is formed on the glass substrate.
Introduction Optical trapping by a focused laser beam is going to be one of the key techniques to support and advance the present microand nanochemistry.1,2 By focusing an intense laser beam tightly in the colloidal solution, a microparticle can be trapped in the laser focus and be manipulated three-dimensionally.2-6 Now, targets of optical trapping are expanding to nanomaterials such as metallic nanoparticles,2-4 latex nanoparticles,2,3,5,6 polymers,7,8 molecular J-aggregates,9 and so on. Since the size of nanomaterials is smaller than that of a laser focus, a number of nanomaterials can be trapped in a single focal spot and a microassembly of nanomaterials can be formed. This optical assembly phenomenon has been confirmed in various nanomaterials. In the case of some polymers, it is found that a gel-like microassembly is formed by focusing a near-infrared laser beam in a polymer solution. However, the formed microassembly does not remain permanently, because it disappears as the laser beam is turned off. On the other hand, the characteristic assembly of nanoparticles and organic molecules in solution under thermodynamically nonequilibrium conditions, such as a solvent evaporation process, has been studied for a long time.10 Various periodical and * To whom correspondence should be addressed. Telephone: +81-66879-7839. Fax: +81-6-6879-7840. E-mail:
[email protected] (H.M.);
[email protected] (H.Y.). † Osaka University. ‡ University of Melbourne. § Max-Planck-Institute for Polymer Research. (1) (a) Masuhara, H.; De Schryver, F. C.; Kitamura, N.; Tamai, N. Microchemistry: Spectroscopy and Chemistry in Small Domains; Elsevier: Amsterdam, 1994. (b) Prasad, P. N. Introduction to biophotonics; Wiley-Interscience: New York, 2003. (2) Ashkin, A. IEEE J. Sel. Top. Quantum Electron. 2000, 6, 841. (3) Smalyukh, I. I.; Kachynski, A. V.; Kuzmin, A. N.; Prasad, P. N. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18048. (4) Yoshikawa, H.; Matsui, T.; Masuhara, H. Phys. ReV. E 2004, 70, 061406. (5) Hosokawa, C.; Yoshikawa, H.; Masuhara, H. Phys. ReV. E 2004, 70, 061410. (6) Hosokawa, C.; Yoshikawa, H.; Masuhara, H. Phys. ReV. E 2005, 72, 021408. (7) Juodkazis, S.; Mukai, N.; Wakaki, R.; Yamaguchi, A.; Matsuo, S.; Misawa, H. Nature 2000, 408, 178. (8) Masuo, S.; Yoshikawa, H.; Asahi, T.; Masuhara, H.; Sato, T.; Jiang, D.-L.; Aida, T. J. Phys. Chem. B 2002, 106, 905. (9) Tanaka, Y.; Yoshikawa, H.; Masuhara, H. J. Phys. Chem. B 2006, 110, 17906.
dissipative structures can be fabricated on the substrate by choosing the appropriate solvent and controlling the surrounding conditions (such as temperature and vapor pressure). From this point of view, a focused near-infrared (NIR) laser beam also gives thermodynamically nonequilibrium conditions in solution, so that optical trapping must compete with the dissipative process. However, optical trapping is barely affected by the nonequilibrium effect in the case of aqueous solutions because the temperature elevation and the gradient induced by the focused NIR laser beam are small due to the high thermal conductivity. However, convection and surface deformation are easily induced in a volatile solution with low thermal conductivity. Therefore, optical assembly of nanomaterials is attributed to the cooperation of optical trapping and dissipative effects in such a system. We have recently developed a method for the laser micro- and nanodeposition of a polymer assembly by a focusing NIR laser beam in a tetrahydrofuran (THF) solution. As we reported in ref 11, the size of the deposited polymer assembly seems to be determined by the solution surface deformation, while the polymer is deposited in the laser focus due to optical trapping. In this paper, we discuss the laser microdeposition mechanism on the basis of the liquid surface deformation and the volume of the deposited microassembly estimated from interferometric and atomic force microscope (AFM) images. Experimental Section A schematic of the experimental setup is shown in Figure 1. A near-infrared laser (Coherent, Compass 1064-4000M, CW-Nd:YVO4, 1064 nm) was used as the optical trapping beam, whose power is adjusted by a polarization beam splitter and a half-wave plate. The laser beam was introduced to the inverted microscope (Olympus, IX70) and focused on the upper surface of the glass substrate with the oil immersion objective lens (Olympus, UPlanFl, 100×, NA 1.30). To focus the laser beam in multiple spots, the focal position can be quickly transferred by scanning with a pair of galvano scanning (10) Smalyukh, I. I.; Zribi, O. V.; Butler, J. C.; Lavrentovich, O. D.; Wong, G. C. L. Phys. ReV. Lett. 2006, 96, 177801. (11) Nabetani, Y.; Yoshikawa, H.; Grimsdale, A. C.; Mu¨llen, K.; Masuhara, H. Jpn. J. Appl. Phys., Part 1 2007, 46, 449.
10.1021/la063341k CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007
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Figure 1. Schematic of the experimental setup for optical trapping. The AFM head on the microscope is changed to He-Ne laser illumination and a CCD setup when liquid surface deformation is observed. DM, dichroic mirror; OF, optical filter; GM, galvano scanning mirrors; L1 and L2, lenses; BS, beam splitter; WP, λ/2 plate; SP, mirror for the microscope side port. mirrors (GSI Lumonics, VM1000). A He-Ne laser was also introduced to monitor the thickness of the solution film on the glass substrate. An atomic force microscopy unit (Digital Instruments, Nanoscope IIIa, Bioscope) was installed on the microscope to measure the surface morphology of the sample substrates around the laser focus after complete solvent evaporation. The AFM unit equipped on the microscope stage was replaced by a charge coupled device (CCD) camera when liquid surface deformation was measured, as shown in Figure 1. Another He-Ne laser beam was expanded and illuminated on the large area of the liquid surface through a Petri dish to prevent quick evaporation. Poly[2,7-(9,9-bis(2-ethylhexyl)fluorene)] (PEHF) (Mw ) 212 000) was used as the polymer sample because deposited PEHF is easily confirmed by fluorescence measurements with mercury lamp excitation (365 nm). PEHF was dissolved in tetrahydrofuran (THF) (1.0 × 10-3, 2.0 × 10-2, and 1.8 mg/mL) (Nacalai Tesque, Guaranteed Reagent, H2O < 50 ppm). A constant amount (200 µL) of the solution was cast all over the glass substrate (22 × 22 mm2) to make a solution thin film, and a near-infrared laser beam was focused on the solution/substrate interface when the thickness of the solution film was reduced to ∼100 µm. The topographic and fluorescence images of the PEHF microassemblies deposited around the laser focus were measured by atomic force and fluorescence microscopy.
Results and Discussion The surface deformation of the liquid film induced by a focused NIR laser beam was measured with a CCD camera with He-Ne laser illumination. Since the sample solution is colorless and transparent, surface deformation is undetectable by optical transmission imaging. That is why a He-Ne laser is illuminated on the top surface of the solution film and the interferometric image produced by the laser beams reflected by the top and bottom of the solution film is measured. Figure 2a shows a series of CCD camera images of the thin THF film when a 400 mW laser beam was focused on the liquid/substrate interface. The surface deformation of the THF film was induced just after introducing a focused laser beam (Figure 2a (i)), and the large circular depression, whose size was ∼2 mm in diameter, occurred within a few seconds (Figure 2a (iii)). The liquid surface reached the glass substrate, and a rupture formed around the focal point.
Figure 2. (a) Interferometric images of the liquid surface deformation on the thin THF film induced by focusing a NIR laser at (i) 0.03, (ii) 1.5, (iii) 2.5, (iv) 4.0, (v) 5.0, and (vi) 7.0 s after introducing a laser beam. The 400 mW laser beam is focused on the liquid/substrate interface. (b) Laser power dependence of the time duration, tr, between the start of the laser focus and rupture formation (open circle) and the averaged drying rate derived by dividing the solvent film thickness (100 µm) by tr (open square).
Eventually, the wide area, whose size was >3 mm in diameter, was dried over several seconds (Figure 2a (vi)). The surface deformation of the thin THF film was caused by the reduction of the surface tension at the laser focus. This thermocapillary effect has been studied for a long time.12,13 The surface tension, that is, the surface free energy, is dependent upon the reciprocal of temperature and becomes lower at the laser focus than that in the surrounding area. Therefore, the shape of the surface is deformed according to the local gradient of the surface tension around the laser focus. In our experiment, a focused NIR laser beam produced the temperature gradient on the surface of the THF film because of the light absorption of the THF molecules. We estimated this on the basis of the absorption coefficient of THF and some previous reports that the local temperature at the laser focus increases by 2.4 K with a laser power of 400 mW.14,15 According to the approximate twodimensional model reported by Da Costa et al., the temperature rise is needed for the surface deformation to reach the glass substrate; that is, rupture formation is estimated to be at 0.2 K under our experimental conditions.13 Therefore, this temperature gradient induced by focusing the laser beam is enough to cause a rupture in a 100 µm thick THF film.11 Generally, optical trapping (12) Da Costa, G.; Calatroni, J. Appl. Opt. 1978, 17, 2381. (13) Da Costa, G.; Calatroni, J. Appl. Opt. 1979, 18, 233. (14) Wurlitzer, S.; Lautz, C.; Liley, M.; Duschl, C.; Fischer, Th. M. J. Phys. Chem. B 2001, 105, 182. (15) Masuo, S.; Yoshikawa, H.; Nothofer, H.-G.; Grimsdale, A. C.; Scherf, U.; Mu¨llen, K.; Masuhara, H. J. Phys. Chem. B 2005, 109, 6917.
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Figure 3. (a) Schematic illustrations of the laser power dependence of the deposited assembly volume. (b) (left) Laser power dependence of the deposited volumes of the PEHF assemblies obtained from the AFM topographic images shown on the right. (right) AFM 3D topographic images of the PEHF assemblies deposited at different laser powers: (i) 200, (ii) 300, (iii) 400, (iv) 500, (v) 600, and (vi) 700 mW.
has been used under static conditions free from rapid solvent evaporation. This rupture formation is characteristic of the laser microdeposition process under the present nonequilibrium conditions. To reveal the mechanism in more detail, the liquid surface deformation around the laser focus was measured at different laser powers. Figure 2b shows the relationship between laser power and time for the disappearance of the liquid (rupture formation) around the laser focus. In this experiment, the backscattering intensity of the focused NIR laser beam clearly increases, as the liquid at the laser focus disappears, because the environment of the focal spot changes from a liquid/glass interface to an air/glass interface. We observed this backscattering intensity using the CCD camera to determine the time at which a rupture forms. The time necessary for rupture formation, tr, was defined as the period between the time when the laser beam starts to irradiate and the time when the liquid surface reaches the glass substrate. The average drying rate of the liquid layer at the laser focus derived by dividing the solvent film thickness (100 µm) by tr is also shown in Figure 2b. Figure 2b indicates that tr almost linearly decreases as the laser power increases. In the present case, a polymer microassembly grows at the laser focus only during the time period tr between the start of the laser focus and rupture formation. Therefore, in the case of high laser power, the growth of the polymer assembly is stopped within a shorter amount of time because of rapid solvent depletion. On the other hand, the optical assembly rate, Jtrap, namely the number of nanoparticles trapped per unit of time, is proportional to the laser power, P, at the beginning stage of growth, as demonstrated in ref 4 by using polymer nanoparticles. As a result, the volume, V, of the deposited assembly would be represented by the multiplication of Jtrap and tr, namely V ) Jtraptr. This laser power dependence of the deposited volume is schematically illustrated in Figure 3a.
As the laser power increases, Jtrap increases and tr decreases, as experimentally demonstrated in Figure 2b. Multiplication of these dependences leads to the laser power dependence of the deposited volume with a parabolic shaped curve as illustrated in Figure 3a. Actually, we can confirm this estimation by analysis of the AFM data obtained in the previous work.11 Figure 3b shows the volume of the deposited assembly estimated from AFM images plotted as a function of laser power. These assemblies were obtained from the 1.0 × 10-3 mg/mL PEHF solution. These experimental results are consistent with the estimation illustrated in Figure 3a, suggesting that the volume of the deposited assembly is mainly determined by the surface deformation speed and optical assembly rate. To discuss the optical trapping effect apart from another conceivable mechanism derived from thermodynamically nonequilibrium conditions, the laser-induced deposition of a PEHF assembly was observed in the high concentration solution while suppressing the solvent evaporation effect. In this case, a large amount (0.5 mL) of the sample solution was dropwise added onto a glass substrate and then covered with a Petri dish, so that the concentration increase induced by solvent evaporation was negligible during laser irradiation. Figure 4 shows the bright field CCD camera images when a laser beam of 400 mW was focused on a 1.8 mg/mL PEHF solution. The assembly was deposited at the laser focus just after introducing a focused laser beam to the solution. The deposited assembly grew while the laser beam was focused, and the size of the assembly became ∼5 µm in lateral width after 10 s. However, the assembly that formed in the solution disappeared within a few seconds when the laser was turned off; that is, the laser deposition process is reversible in quasi-equilibrium conditions free from solvent evaporation. Such laser deposition was not observed for
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Figure 4. Transmitted CCD images of PEHF deposition at the laser focus. The images taken just after turning the laser on and off are shown in (a) and (i), respectively. (b-h) Deposition images taken at 0.1, 0.2, 0.5, 1, 2, 5, and 10 s after introducing a NIR laser beam, respectively. The laser power and the concentration of the solution are 400 mW and 1.8 mg/mL, respectively.
concentrations lower than 1.0 × 10-3 mg/mL. As mentioned above, the temperature elevation at the laser focus is estimated to be 2.4 K. However, the temperature elevation makes the solubility higher and gives a negative contribution to the molecular deposition. Therefore, it is reasonable that the deposition of a PEHF assembly is induced by the optical trapping effect. The optical trapping potential, U, for a nanoparticle with polarizability R is expressed as follows:
1 U ) - RE2 2
(1)
where E is the electric field of light, so that E2 is proportional to the laser power. The polarizability of a single polymer is so small that it cannot be trapped in the laser focus for a long time. However, the local concentration, CL, increases as compared to the surrounding (original) concentration, C, as follows:16
CL ) C exp
(|U| kT )
(2)
Since |U| , kT (k, Boltzmann constant; T, temperature) in the present case, the increase of the local concentration is estimated to be less than 1%. Therefore, if the laser beam is focused in the dilute solution, the local concentration CL does not exceed the saturation concentration, so that the deposition assembly does not occur at the laser focus. On the other hand, in the case of such high concentration as near-saturation or supersaturation, the deposition of the polymer assembly will be induced by a slight increase in the local concentration, as shown in Figure 4. In addition, it is expected that polymer aggregates, possessing larger polarizability than individual polymer molecules, naturally form in a high concentration solution. Since the optical trapping potential U is proportional to the polarizability, such aggregates enhance the laser microdeposition. Although the laser-induced deposition is not permanent in the static solution, as shown in Figure 4, we could leave the deposited assembly after turning off the laser beam, as shown in Figure 3b, by focusing a laser beam in the solvent evaporation process. It is noteworthy that (16) Svoboda, K.; Block, S. M. Opt. Lett. 1994, 19, 930.
Figure 5. Schematic explanation of the laser microdeposition mechanism. (i) A solution film (thickness: 100 µm) is made on a glass substrate. (ii) Surface deformation occurs around the laser focus when a near-infrared (NIR) laser beam is focused on the surface of a substrate. (iii) Polymer molecules form the aggregates during deformation and evaporation of the solution film due to the increase of concentration. (iv) Polymer aggregates are trapped and/or deposited at the laser focus by optical trapping. (v) Polymer microassembly is fixed on the glass substrate, and the other molecules in the solution are removed from the laser focus by the rupture expansion.
the size of the deposited assembly in Figure 3 is smaller than that deposited in a static solution (Figure 4). As mentioned above, the deposition and growth of the polymer assembly are stopped by the rupture of the solution film in Figure 2. The experimental results suggest the following mechanism of laser microdeposition (Figure 5). A slight temperature elevation occurs around the laser focus, and the temperature gradient induces the surface depression. Since the solvent evaporation proceeds, the molecular concentration around the laser focus becomes close to saturation or supersaturation before the deformed solution surface reaches the substrate. Therefore, the deposition of the polymer assembly occurs at the laser focus, as shown in Figure 4. However, the solution surface reaches the substrate and soon ruptures. At that time, the polymer assembly deposited at the laser focus is fixed on the glass substrate. Since the laser deposition can proceed over 10 s in a static solution, as shown in Figure 4, the size of the polymer assembly fixed on the substrate is
Optical Trapping and Surface Deformation Effects
Figure 6. Topographic and fluorescence images of the PEHF assemblies deposited by multiple optical trapping techniques. The laser beam was focused on (a and b) three and (c and d) four positions (indicated by white circles in (a) and (c)). The fluorescence images (a) and (c) correspond to the topographic images (b) and (d), respectively. The fluorescence images were measured by mercury lamp excitation (wavelength 365 nm).
determined by the time necessary for rupture formation. This surface deformation plays another important role. As shown in Figure 3b, there is no assembly in the surrounding area of the polymer microassembly produced at the laser focus. When the solution is allowed to evaporate on the glass substrate without focusing the laser beam, the substrate is covered by many polymer assemblies. However, the quick rupture of the solution around the laser focus suppresses such natural deposition and convection flow (rupture expansion) sweeps out the residual polymers. Therefore, a polymer microassembly is formed only at the laser focus. To study the potential characteristics, laser microdeposition was performed by focusing a laser beam in multiple spots. Three
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and four spots of the laser focus are produced in the focal plane by scanning the laser beam with a pair of galvano scanning mirrors. This laser scanning procedure is explained elsewhere.11 Figure 6 shows the topographic and CCD fluorescence images of the PEHF assemblies deposited by focusing a laser beam on three or four spots. The 400 mW laser beam was focused on three or four spots in a 2.0 × 10-2 mg/mL solution. It is interesting that polygonal microassemblies whose vertices are located near the focused laser spots are formed. This indicates that the polymer assemblies deposited at the multiple laser focus spots fused together and a large assembly became trapped at the position between the three or four laser spots. Since the fused assembly is sustained by three or four laser spots focused near the peripheral part of the assembly, it is extended by the laser focus, whereas it shrinks when accompanied by solvent evaporation. Therefore, a triangle and a square shape, whose vertices are located at the laser focus, were formed. The formation of such polygonal PEHF assemblies suggests that a gel-like polymer assembly is trapped in the solution. It was reported previously that the gel-like polymer assembly is soft and is easily deformed by optical trapping.7,8 Various polygonal polymer structures may be fabricated by changing the number and position of the focal spots.
Conclusions We revealed that optical trapping and liquid surface deformation play critical roles in laser microdeposition. It is interesting that a combination of a conservative optical force and a dissipative thermocapillary force functionally works under the nonequilibrium and dynamic conditions of a solvent evaporation process. This laser microdeposition is expected to be a nondestructive and effective method for the micropatterning of soft materials such as polymers, supramolecules, and biomaterials. Acknowledgment. This work is supported by a Grant-inAid for Scientific Research (KAKENHI) (S) (14103006 and 18106002) from the Japan Society for the Promotion of Science. H.Y. is very grateful for financial support from the Sekisui Chemical Co. Ltd. Research-Aid Program on “The Nature-Guided Materials Processing”. LA063341K
