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“Smart” Surface Dissymmetrization of Microparticles Driven by Laser Photochemical Deposition Emmanuel Hugonnot,† Anne Carles,†,‡ Marie-He´le`ne Delville,‡ Pascal Panizza,† and Jean-Pierre Delville*,† Centre de Physique Mole´ culaire Optique et Hertzienne, UMR CNRS/Universite´ No 5798, Universite´ Bordeaux I, 351 Cours de la Libe´ ration, F-33405 Talence Cedex, France; and Institut de Chimie de la Matie` re Condense´ e de Bordeaux, UPR CNRS No 9048, 87 Avenue du Docteur A. Schweitzer, F-33608 Pessac Cedex, France Received June 21, 2002. In Final Form: September 9, 2002 Dissymmetric nano/microsized spheres are very appealing because controlled dissymmetry brings an additional degree of freedom for the synthesis of a new generation of materials with spatially separated chemical properties. We explore this aspect by extending to spherical surfaces the application field of lithographic techniques that was up to now essentially limited to planar and cylindrical substrates. The method proposed uses a strongly focused laser beam to generate dissymmetric coatings on microparticles by micro-photochemical deposition in a reactive solution. This is experimentally illustrated by considering the photochemical reduction of chromate ions induced by a continuous Ar+ laser wave to “nucleate” and grow a dissymmetry on the surface of silica beads dispersed in a chromate solution. When properly rescaled, the coating growth laws measured at different laser excitations are reduced to a single master behavior that implies a simple strategy to control and predict the desired dissymmetry from its dynamics. The versatility of the technique is then demonstrated by scanning the beam (i) to tailor microscale patterning on one hemisphere and (ii) to assemble beads into ordered structures. Owing to its flexibility, the method can easily be extended to the coating of different types of particles and various photochemical reactions.
Introduction Organization,1 manipulation,2 and functionalization3 of nano/microsized objects are playing an increasingly significant role in scientific and technologic endeavors. While most efforts have been devoted to methods for preparing isotropically coated4 or functionalized5 colloidal particles, much less attention focused on the dissymmetrization of their chemical properties. Since dissymmetry represents an additional degree of freedom that can be used to build complexity in tailored micro/nanomaterials, such a strategy is very challenging in areas as different as self-assembling,6 chemical sensoring,7 drug delivery,8 and optics.9 However, conventional10 and advanced11,12 lithographic techniques are essentially limited to planar and cylindrical13 substrates. The first attempts at particle dissymmetrization were therefore prompted by partial silanization after protecting one hemisphere with a * E-mail:
[email protected]. † Centre de Physique Mole ´ culaire Optique et Hertzienne. ‡ Institut de Chimie de la Matie ` re Condense´e de Bordeaux. (1) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (2) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56. (3) Fan, H.; et al. Nature 2000, 405, 56. (4) Cohen, I.; Li, H.; Hougland, J. L.; Mrksich, M.; Nagel, S. R. Science 2001, 292, 265. (5) Caruso, F. Adv. Mater. 2001, 13, 11. (6) Lehn, J. M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (7) Himmelhaus, M.; Takei, H. Sensors Actuators B 2000, 63, 24. (8) Mathiowitz, E.; et al. Nature 1997, 386, 410. (9) Ito, Y.; Bleloch, A. L.; Brown, L. M. Nature 1998, 394, 49. (10) Handbook of Microlithography, Micromachining, and Microfabrication, Vol. 1; Rai-Choudhury, P., Ed.; SPIE Optical Engineering Press: Bellingham, WA, 1997. (11) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (12) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (13) Jackman, R. J.; Wilbur, J. L.; Whitesides, G. M. Science 1995, 269, 664.
varnish,14 chemisorption in directional fluxes,15 and differential nanoclusters adsorption at interfaces.16,17 Although promising, these methods do not allow a “smart” control over either the extension of the modified area (dissymmetrization mainly concerns one hemisphere) or its coverage density. To overcome these limitations, we present here a general and convenient alternative combining micro-photochemical deposition with strongly localized laser excitation, to “nucleate” and grow a dissymmetry on the surface of a microsphere. With appropriate scaling, we show that the kinetics of dissymmetrization can be reduced to a single master curve. The method is then extended to tailor dissymmetric microscale patterning on microspheres, and several appealing applications are anticipated. Laser chemical processing is a powerful noncontact tool for micromachining and microprocessing of materials,18 and photochemically stimulated deposition represents nowadays an appealing alternative for direct material writing without using mask-based photolithography. The technique uses UV or visible photons to efficiently break molecular bonds and induce rapid thin film deposition with very moderate beam intensities. Furthermore, laser light offers strong localization in excitation, spectral selectivity in molecule activation, and ease of manipulation. While most experiments are realized in gas phases, photochemical deposition in liquids is very appealing19 because (i) it can be applied to a broad range of precursors, (14) Casagrande, C.; Fabre, P.; Raphae¨l, E.; Veyssie´, M. Europhys. Lett. 1989, 9, 251. (15) Takei, H.; Shimizu, N. Langmuir 1997, 13, 1865. (16) Nakahama, K.; Kawaguchi, L.; Fujimoto, K. Langmuir 2000, 16, 7882. (17) Petit, L.; Manaud, J. P.; Mingotaud, C.; Ravaine, S.; Duguet, E. Mater. Lett. 2001, 51, 478. (18) Ba¨uerle, D. Laser Processing and Chemistry, 3rd ed.; Springer: Berlin, 2000. (19) Weiss, V.; Friesem, A. A.; Peled, A. J. Imaging Sci. Technol. 1997, 41, 355.
10.1021/la0261085 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/19/2002
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Figure 1. Schematic diagram of the experimental setup implemented for formation of laser-induced dissymmetric microparticles by photochemical microdeposition. A side view of a dissymmetrized hemisphere is illustrated on the 10-µm silica bead centered in the beam.
including molecular compounds, (ii) light energy conversion is high because the material density is larger than that of a gas phase, and (iii) production is experimentally easy. Experimental Section On the basis of the well-known photoreduction of Cr(VI) ions into Cr(III) ones, used in dichromated gelatins for optical recording,20 the experiment relies on the photochemical deposition of chromium oxide layers. It is performed at room temperature on silica beads dispersed in a stable liquid mixture composed of potassium chromate (10 wt %), ethanol (8 wt %), 1 N hydrochloric acid (10 wt %), and ultrapure water (72 wt %). We used 10-µm NUCLEOSIL silica beads for liquid-phase chromatography purchased from Macherey-Nagel. Depending on the acid-base equilibrium, HCrO4- h CrO42- + H+ (pK ) 6.49),20 different forms of Cr(VI) can be found in aqueous solutions. We chose to work at pH ) 3 to have HCrO4- as the major species in solution, since it is this hexavalent form of chromium which is lightactivated in the blue-green wavelength range of a continuous Ar+ laser. Furthermore, the addition of an organic compound, here an alcohol (ROH), enhances the photoreduction of HCrO4to a Cr(III) species of very low solubility; X-ray photoelectron spectroscopy and X-ray diffraction show that the photodeposited film of Cr(III) is mainly composed of amorphous Cr(OH)3.21 The major pathway from Cr(VI) to Cr(III) starts with a light-induced excitation of Cr(VI) followed by a reduction to a Cr(V) intermediate and a dark reaction from Cr(V) to Cr(III):20
{
Cr(VI) + hν h Cr(VI)* Cr(VI)* + ROH f “Cr(V)” f ... f Cr(III)
(1)
where hν represents the energy of the absorbed photon. The photochemical reaction is driven by a linearly polarized TEM00 Ar+ laser (wavelength in a vacuum: λ0 ) 5145 Å). Since the experiment involves a resonant liquid, the sample is enclosed in a very thin tight homemade cell composed of a glass slide and a cover slip separated by 30-µm-thickness Mylar spacers. Disturbing thermal effects induced by the light absorption are then prevented. The cover slip was also silanized to prevent any photochemical deposition on the entrance face of the cell. As illustrated in Figure 1, this cell is horizontally mounted to keep the beads at rest on the bottom plate. A computer controls the translation stages, with a step accuracy of 0.5 µm, to move the holder in the three directions. To observe in situ the growth of the dissymmetry induced, the sample is illuminated with a white light source and placed between two beam splitters. One of them collinearly injects the beam in the sample while the other one rejects it to protect the white light source from laser radiation. (20) Keinonen, T.; Grzymala, R. Appl. Opt. 1999, 38, 7222. (21) Unpublished results.
Figure 2. Growth of a dissymmetry photodeposited on a 10µm silica bead by laser-assisted photochemical deposition; the beam is left stationary. The incident beam power is P ) 28 µW, and the bar scale is 10 µm. The incident beam power P is adjusted with a rotating half-wave plate (not shown) located before the first beam splitter. Laserassisted micro-photochemical deposition is monitored by a long working distance, ×50 microscope lens (numerical aperture N.A. ) 0.5) that focuses the beam on the top hemisphere of a bead; at the focus, the beam radius is ω0 ) 0.6 µm. Note that the relative position of the focus versus the surface of the microsphere intercepted by the beam does not represent a critical parameter because the wave symmetry is almost cylindrical around the propagation axis for axial distances |z| e zd ) πnω02/λ0 ≈ 3 µm from the focus; n is the index of refraction of the liquid mixture. Imaging of the induced dissymmetrization is made by the conjugation of the ×50 microscope lens with a second lens (f ) 20 cm) on the CCD video camera which is coupled to a computer for the frame acquisition.
Results and Discussion According to eq 1, the concentration in Cr(III) is an increasing function of the exciting intensity. The nucleation of a Cr(III) precipitate occurs as soon as the solubility limit is reached, and a photodeposit starts to coat the hemisphere of the particle intercepted by the laser beam, as illustrated in Figure 2. The side view of the central bead shown in Figure 1 clearly evidences that dissymmetry is preserved until the complete capping of the concerned hemisphere. Considering the regularity of the induced deposit, the image processing approximates the measured cross section of the covered surface by the area of a circular domain of mean radius R(t). Measurements, carried out over almost one order of magnitude in incident beam power (3.5 e P e 28 µW), are illustrated in the inset of Figure 3. They show that the growth rate is an increasing function of the optical excitation. An analytical formulation of the observed growth laws is difficult to theoretically retrieve due to the lack of simple symmetry in the laser-particle interaction (i.e. spherical surface intercepted by a strongly focused Gaussian wave). However, bearing in mind the fact that the beam divergence is weak for axial distances |z| e 3 µm from the focus, an interpretation can be presented by assuming a direct-write induced by a cylindrical laser wave on a flat substrate (i.e. for a radius of the deposit much smaller than that of the bead). Under these conditions, the photodeposit growth rate driven by a reaction-diffusion process involving a one-photon photochemical reaction can be derived according to the following scheme.22 Using eq 1 and assuming that the kinetics of Cr(VI) excitation/ relaxation is much faster than any mass diffusion involved in the dynamics of deposition, we find that the production of Cr(III) in solution is described by a field-modified (22) Hugonnot, E.; Mu¨ller, X.; Delville, J. P. Accepted for publication in J. Appl. Phys.
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Figure 3. Plot in reduced time Pt of the growth of the dissymmetry presented in the inset for increasing incident beam power excitations for a stationary beam. The dashed line corresponds to a power law fit of the set of rescaled data for R > ω0. We find R(t) ∝ (Pt)0.57(0.03, in close agreement with the growth law Rh(t) ∝ (Pt)1/2 expected from the model when R . ω0. Spots slightly smaller than 1 µm are measured.
reaction-diffusion equation. Moreover, since the measured deposit is incomparably larger than any molecular size involved in the process, we assimilate its shape to a spherical cap of radius R on the substrate and build a “droplet growth” model driven by the diffusion and the adsorption of Cr(III) particles. The growth rate is then obtained by equating the flux in Cr(III) at the surface of the deposit with its volume change. We find22
d(R/ω0) d(Pt/ω02)
∝
∞J0(xx2R/ω0)
∫0
exp(-x2/4) x dx x + σω0/x2
(2)
where J0 is the 0th order Bessel function and σ ) 1.3 × 104 m-1 is the optical absorption of the chromate solution. The proportionality constant, which is not given here, involves the initial concentrations in Cr(VI) and alcohol, the reaction rates associated with eq 1, and a saturation intensity associated with the optical transition between Cr(VI) and Cr(VI)*.22 Note that the deposit radius R as well as the time t have been rescaled in eq 2 to illustrate the pertinent variables of the process. In the present situation, σω0/x2 ) 6 × 10-3 can be neglected, leading to the following expression:
d(R/ω0)/d(Pt/ω02) ∝ exp(-R2/ω02)I0(R2/ω02)
(3)
where I0 is the 0th order modified Bessel function. As expected for a one-photon chemical reaction, this equation shows that photochemical deposition is driven by the light energy deposited per surface unit Pt/ω02. Since the beamwaist ω0 is held constant in the experiment, a simple rescaling of time by the incident beam power should therefore point out a single-scale dynamics. This data reduction on a master curve is illustrated in Figure 3. Moreover, I0(R2/ω02) ≈ exp(R2/ω02)/(2πR2/ω02)1/2 for R2/ω02 . 1, which leads to R(t) ∝ (Pt)1/2, in close agreement with the behavior deduced from a power law fit of the rescaled data set (i.e. R(t) ∝ (Pt)0.57(0.03). Agreement is also observed for R < ω0, since the data are consistent with R(t) ∝ Pt predicted from eq 3 when R , ω0, that is, as long as the deposit does not feel the transverse Gaussian shape of the
Figure 4. Dissymmetric micropatterning of silica beads induced by laser-assisted photochemical deposition via a monitoring of the cell holder. (a) Patterns generated by continuously moving the holder in the x-y plane for P ) 42 µW and a scanning rate v ) 1 µm/s; features slightly smaller than 1 µm are observed. (b) Series of numbers deposited on different beads by a computed matrix addressing of the (x, y) translation stages for P ) 28 µW and an exposition time ∆t ) 5 s; the dot size is ∼1 µm. The bar scales are 10 µm.
exciting beam. Finally, considering our spherical cap model, the temporal behavior of the thickness h on top of the chromate layer can be directly deduced from the radius growth rate by the relation h ) [(1 - cos θ)/sin θ]R, where θ is the “contact angle” at the perimeter. We expect h/R j 0.1, as already measured in photodeposited surface relief gratings.19,23 This is qualitatively confirmed by the side view of the beam-centered bead presented in Figure 1. Thus, despite some approximations, a unified behavior of the dynamics is obtained. As a consequence, the spot growth is controlled by the light energy deposited per surface unit and the induced dissymmetry can be dynamically monitored in a very smart way. Analogous features are then expected in experiments involving either high or low absorption photosensitive mixtures by a simple modification of the incident beam power and a further adjustment of the exposition time. An independent control of these two parameters prevents possible alterations usually generated by thermal effects. The proportionality constant between R(t) and (Pt)1/2 is the only quantity which varies from one mixture to another, because it depends on the photochemical reaction rates and the adsorption properties on the substrate. Besides the soft tailoring of microspheres coating, laserassisted photochemical deposition can easily be extended to dissymmetric microscale patterning via a monitoring of the cell holder translation stages.24 Another advantage of this method is that the beam can be considered as cylindrical over a few microns on both sides of the beamwaist location. Then, as illustrated in Figure 4, writing distortions induced by defocusing effects at the edge of particles are eliminated for diameters smaller than 10 µm. To draw micropatterns on the intercepted hemisphere, a first procedure consists of focusing the laser beam on the top of a bead and in continuously moving the holder in the x-y perpendicular plane. Figure 4a presents different patterns created this way. Due to the beam scanning, the regularity in continuous deposition requires powers slightly larger than those used for stationary beams. The laser can also operate in a discontinuous way to build a variety of micropatterns by a computed matrix addressing of the x and y translation stages. Figure 4b exhibits a series of numbers deposited on different beads using this method. Laser-assisted microdeposition is thus (23) Hugonnot, E.; Delville, J. P. Appl. Phys. Lett. 2002, 80, 1523. (24) Lachish-Zalait, A.; Zbaida, D.; Klein, E.; Elbaum, M. Adv. Funct. Mater. 2001, 11, 218.
“Smart” Surface Dissymmetrization of Microparticles
Figure 5. (a) Dynamics of a dissymmetry deposited on a 4.7µm glass bead when the beam is left stationary. The incident beam power is P ) 100 µW. (b) Assembling by interbead connection using localized photochemical deposition (P ) 35 µW and ∆t ) 5 s). (c) Coating of the hemisphere of a 1.6-µm silica bead (P ) 28 µW). The bar scales are 5 µm.
highly promising to deposit and dynamically tailor surface dissymmetry on micrometric silica beads. The approach is general, and analogous behaviors are expected for any photochemical deposition driven by a one-photon absorption. Applications of these asymmetrically patterned microspheres can be devised in numerous emerging areas. For example, the use of microlenses in optoelectronics devices has tremendously increased and many efforts are actually devoted to improve their performances by tailoring the phase and the amplitude of coatings.25 By dissymmetrizing one hemisphere of a 5 µm glass bead, we show in Figure 5a that laser-assisted microdeposition can successfully be applied to control the patterning of microlenses. Note that the incident beam power used is larger than that required to dissymmetrize a silica bead because the decrease in porosity of the substrate makes the adsorption of Cr(III) less efficient. Another important feature is the chemical anisotropy which can result from this dissymmetrization. Indeed, the induced coating can be further functionalized or used as a mask to chemically modify the uncoated surface. In a second step, the deposit can be removed, allowing a further different functionalization of the previously cov(25) Stern, M. B. Microelectron. Eng. 1997, 34, 299.
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ered surface. Unsymmetrical microspheres can as well be conceived as “rectified” elemental bricks which can selfassemble in a privileged fashion to build organized architectures, such as photonic crystals,26 with multiple functionalities. As illustrated in Figure 5b, mixed functional devices can also be created in situ by controlling the spatial pattern with localized photochemical deposition. In this preliminary experiment, which clearly deserves further investigations, we injected a low power continuous He-Ne laser (wavelength in a vacuum λ0 ) 6328 Å, where the chromate solution does not absorb) collinearly to the Ar+ pump to transversally trap the beads and draw them together using the translation stages. When in contact, we deposited small dots of Cr(III) between the beads. They seem to be irreversibly stuck by the Cr(III) “solder” because attempts to separate them with the trapping beam failed. Attempts were also performed to operate dissymmetrical coatings on smaller particles (Figure 5c). Therefore, the flexibility of this approach (photochemical deposition of noble metals27 or semiconductors28 can easily be implemented) and its ability to coat particles of different types (SiO2, Al2O3, or TiO2, for instance) offer a valuable level of development for applications as different as microlens coating, microsphere functionalization,29 or micrometric self-assembling with multiple functionalities. Acknowledgment. We are grateful to F. Adamietz and X. Mu¨ller for technical assistance. C. Labruge`re and E. Lebrau are acknowledged for respectively X-ray photoelectron spectroscopy and X-ray diffraction. This work was partly supported by the CNRS and the Conseil Re´gional d’Aquitaine. LA0261085 (26) van Blaarderen, A.; Ruel, R.; Wiltzius, P. Nature 1997, 385, 321. (27) Wehner, M.; Legewie, F.; Theisen, B.; Beyer, E. Appl. Surf. Sci. 1996, 106, 406. (28) Ichimura, M.; Goto, F.; Ono, Y.; Arai, E. J. Cryst. Growth 1999, 198/199, 308. (29) Kawaguchi, H. Prog. Polym. Sci. 2000, 24, 1171.
