Spatial Control of Urea Crystal Growth by Focused Femtosecond Laser Irradiation Hiroshi Y.
Yoshikawa,†
Yoichiroh
Hosokawa,*,†,‡
and Hiroshi
Masuhara*,†,‡
Department of Applied Physics, Handai Frontier Research Center and Venture Business Laboratory of Center for AdVanced Science and InnoVation, Osaka UniVersity, Suita, Osaka 565-0871, Japan, and CREST JST, 4-7-8 Honcho, Kawaguchi, Saitama 332-0032, Japan
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 1 302-305
ReceiVed April 28, 2005; ReVised Manuscript ReceiVed June 24, 2005
ABSTRACT: Growth of urea crystals in a supersaturated solution of urea was spatially controlled by irradiation with a femtosecond laser pulse. When a needlelike urea crystal was irradiated by a single femtosecond laser pulse with wavelength of 800 or 1470 nm through an objective lens, a new needlelike crystal grew from the laser focal spot (approximately micrometer sized). The number of the laser-induced subsequent crystals increased with laser energy. However, such crystal growth could not be induced by 1480 nm continuous wave (CW) laser irradiation. By application of the new phenomenon, patterning of the crystals was successfully demonstrated. Finally, the mechanism of the femtosecond laser-induced crystallization was discussed in terms of the crystal fragmentation caused by photomechanical effect. Introduction Femtosecond laser ablation of organic materials has received much attention as a promising fabrication technique, and its potential has been demonstrated in clear laser etching patterns,1,2 discrete etching,2,3 multistep etching,4 and material transfer5 with reduced thermal damage. The mechanism of the femtosecond laser ablation of organic polycrystalline thin films has been investigated using the method of time-resolved spectroscopy and imaging.2,3 In a typical experimental result using a copper phthalocyanine thin film, ultrafast temperature elevation was observed to be due to the rapid nonradiative relaxation (∼20 ps) of highly excited electronic states formed by femtosecond laser excitation, and the heating rate was estimated to be dT/dt ≈ 100 °C/10 ps > 1013 °C/s. This implies that the intramolecular and lattice vibrations are enhanced tremendously on a time scale of 10 ps, while the molecules are unable to change their positions from their equilibrium condition on this time scale. As the result, mechanical stress accumulated in the film, leading to a transient high pressure, will induce a characteristic ejection of fragments. On the other hand, nanosecond laser ablation of the copper phthalocyanine thin film is considered to be induced by relatively slow heating due to cyclic excitation.6 On the basis of these results, we consider that a photomechanical effect, based on the transient pressure, will be a key to the understanding of material fragmentation and ejection of femtosecond laser ablation of organic materials. When this photomechanical phenomenon of femtosecond laser ablation is induced in a supersaturated solution of organic and biological molecules, it will be a useful perturbation in their crystallization, the control of which is highly desirable in the field of the organic device and protein crystal engineering. Indeed, we have succeeded in triggering the nucleation of organic molecules and proteins by focusing the femtosecond laser into their supersaturated solutions.7,8 The crystallization was possible even when the concentration of the solution was so low that spontaneous nucleation never occurred. It is * To whom correspondence should be addressed. E-mail addresses:
[email protected];
[email protected]. † Handai Frontier Research Center and Venture Business Laboratory of Center for Advanced Science and Innovation. ‡ CREST JST.
important to note that the crystallization efficiency was higher under femtosecond excitation than with continuous wave (CW) or nanosecond laser irradiation. Furthermore, we suggest that the photomechanical effect will be an efficient perturbation not only for nucleation but also for crystal growth. In this paper, we propose a novel way to induce subsequent crystal growth from the crystal surface in a supersaturated solution where the femtosecond laser pulse is focused to a crystal. In this way, subsequent crystal growth from the irradiated area has been found above a threshold laser energy at which surface etching of crystals occurs, and the number of laser-induced crystals has been controlled by laser energy. As a sample, urea was chosen, being a representative organic molecule in the studies of crystal growth using a laser.9 The growth process was investigated as a function of laser wavelength and pulse duration (femtosecond pulse or CW light). By optimization of the laser irradiation conditions, urea crystal patterning by femtosecond laser irradiation has been demonstrated. Finally, we concluded that the photomechanical effect of femtosecond laser irradiation induces crystal growth. Although some researchers have reported nucleation and crystal growth control due to photophysical9-11 and photochemical12,13 reactions with nanosecond laser irradiation in solution, such spatial growth control of crystals due to the photomechanical effect of femtosecond laser irradiation has not been conducted, as far as is known. Experimental Section A sample of 12.0 M aqueous urea was prepared at 60 °C, and a 20 µL droplet was placed on a cover glass set on an inverted microscope (Olympus, IX71). Because the cover glass was at 25 °C, the droplet was cooled, and the solution became supersaturated (solubility of urea at 25 °C is approximately 10.5 M). Spontaneous nucleation usually took place within 1 min, and needlelike crystals grew in the droplet. Then, about 1 min after the spontaneous nucleation, laser beams were focused and irradiated on the side face of the needle axis through an objective lens, and the subsequent crystallization behavior was monitored by a CCD camera through the same objective lens. In all, three lasers, the wavelength and pulse duration of which were different, were used, and crystallization phenomena induced by these lasers were compared. First, an 800 nm femtosecond laser pulse generated by a regeneratively amplified Ti:sapphire laser system (Spectra Physics, Hurricane, 120 fs, 1000 Hz) was used. Since urea single crystals are
10.1021/cg050190v CCC: $33.50 © 2006 American Chemical Society Published on Web 10/06/2005
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Figure 2. Femtosecond laser energy vs (O) etched area on the surface of urea single crystal and (2) probability of laser-induced crystal growth at (a) 800 and (b) 1470 nm by one shot irradiation through a 20× objective lens. The dashed line represents fitting to eq 1.
Figure 1. Subsequent crystal growth of urea from the irradiated area marked with an open circle by a single shot of (a) an 800 nm femtosecond laser pulse at 0.12 µJ/pulse through a 10× objective lens (NA ) 0.4), (b) an 800 nm pulse at 0.030 µJ/pulse through a 20× objective lens (NA ) 0.46), (c) an 800 nm pulse at 0.21 µJ/pulse through a 10× objective lens, and (d) a 1470 nm pulse at 0.26 µJ/ pulse through a 20× objective lens. Delay time before observation after the laser irradiation is indicated in the photographs, and the scale is shown below. transparent at 800 nm, multiphoton absorption will occur and highly excited electronic states of urea molecules will be formed under intense excitation. Second, a 1470 nm femtosecond laser pulse was created using an optical parametric amplifier (OPA) system (Spectra Physics, OPA-800CF), in which the 800 nm femtosecond laser pulse was modulated. Last, a 1480 nm CW laser beam was produced by a semiconductor laser (The Furukawa Electric Co., FOL 1402P). The urea crystal exhibits weak absorption coefficients of about 10 cm-1 at 1470 and 1480 nm, which are associated with molecular vibrational excited states, resulting in efficient heating when the samples were irradiated with the 1470 nm pulse or the 1480 nm CW light. To investigate size and shape of ablated fragments and the etched area of a urea single crystal by 800 nm laser irradiation, ablated fragments were deposited on a glass substrate in air by the laser ablation transfer method.5 Namely, a glass substrate with a 100 µm spacer was overlaid on a second glass substrate to which a urea single crystal was adhered, and the urea single crystal was ablated by a single shot laser pulse. The deposited fragments on the glass substrate and etched area of the urea single crystal were observed using an atomic force microscope (AFM) (Digital Instruments, Nanoscope IIIa).
Results and Discussion When a urea crystal was irradiated with a single 800 nm femtosecond pulse with an energy of 0.12 µJ/pulse through a 10× objective lens or with 0.030 µJ/pulse through a 20× objective lens, subsequent crystal growth was observed in the irradiated area. As shown in Figure 1a,b, the shape of the crystal is similar to a needle, which is characteristic of urea crystals. The subsequent crystals tended to grow perpendicular (to within
∼30°) to the original urea crystals, and the orientation of the needle axis of the subsequent crystal did not depend on polarization of the laser pulse. At the higher laser energy, as shown in Figure 1c, several urea crystals grew near the irradiated area. When the 1470 nm laser pulse was used, similar subsequent crystal growth was also observed as shown in Figure 1d. The number of urea crystals induced by these pulsed laser irradiations increased with both the laser energy and the number of pulses. To investigate the origin of the crystal growth due to femtosecond laser irradiation, the threshold energy of femtosecond laser ablation of urea single crystals was estimated in air by observing the etched area on the surface of urea single crystals. The etched area of the crystal surface was increased with laser energy because of the radial Gaussian distribution of the spatial intensity of the laser pulse.5 Because laser ablation of urea single crystals takes place only in the irradiated area where the laser energy exceeds a certain threshold energy, Eth, of n-photon ablation, the relation between etched area S and Eth can be written as
( ( )) ( ( ))
Eth ) E exp -
R2 a xn
2
) E exp -
S a π xn
2
(1)
where E is the laser energy, R is the radius of etched area S, and a is the laser beam radius at the crystal surface. Figure 2 shows the relation between the laser energy, E, and the etched area, S, of the crystal surface, as well as crystal growth probability by a single shot femtosecond laser irradiation, where etched area S is well-fitted by eq 1. Here, the pulse energy thresholds, Eth, for etching by 800 and 1470 nm femtosecond laser irradiation through a 20× objective lens were determined as 0.026 and 0.20 µJ/pulse, respectively. The values of (a/xn) are also estimated from eq 1 to be 2.0 and 2.4 µm for 800 and 1470 nm laser pulses, respectively. Above the threshold energy, the probability of femtosecond laser-induced crystal growth is dramatically increased. This strongly suggests that the crystal growth is initiated by the etching of the crystal due to the femtosecond laser ablation, which will lead to the ejection of urea crystal fragments. Because the surrounding urea solution
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Figure 3. The dissolution and recrystallization of urea crystals by 1480 nm CW laser irradiation for 2.0 s at 73 mW through a 50× objective lens (NA ) 0.8). Observation time is given in the photographs. The laser irradiated the open circle area from 0 to 2 s.
Figure 4. AFM images of the ablated fragments on a glass substrate (right) and the corresponding etched area of a urea single crystal (left) by one shot irradiation of an 800 nm laser pulse with (a) 0.031 µJ/ pulse and (b) 0.056 µJ/pulse through a 20× objective lens. Each image shows a 10 × 10 µm2 region.
is supersaturated, the ejected fragments can act as seed crystals and grow in the solution. Generally, the number of fragments ejected by femtosecond laser ablation is increased with laser energy,5 so the number of subsequent crystals formed by femtosecond laser irradiation was similarly increased. On the other hand, when the samples were irradiated with a 1480 nm CW laser, dissolution of the urea crystal was observed in the irradiated area. Figure 3 shows the optical images of the urea crystal before (1), during (2), and after the laser irradiation (3). The area of the dissolution was increased with irradiation time. After the laser irradiation, urea crystals grew parallel to the needle axis of the original urea crystal, and growth in the other directions, as in case of pulsed laser irradiation as shown in Figure 1, was never observed. In case of the 1480 nm CW laser irradiation, the dissolution of the crystal is due to temperature elevation, following thermal diffusion from the laser irradiated area of the urea crystal. After the cessation of laser irradiation, the urea crystals began to grow again because the solution in the irradiated area became cooled and supersaturated. In this case, because no urea fragments will be ejected from the crystal surface into the supersaturated solution, the subsequent crystal growth as in the pulsed laser irradiation case does not occur with CW laser irradiation. The relation between the subsequent crystal growth by femtosecond laser irradiation and the ablated fragments was confirmed by the laser ablation transfer method.5 Figure 4 shows AFM images of ejected crystal fragments and the etched surface after irradiating a urea single crystal with a femtosecond laser pulse in air. A dot-like crystal fragment with a size of 3 µm was observed on the glass substrate on irradiation with laser energy near the ablation threshold, and the deposited area of
Yoshikawa et al.
the fragment corresponded well with the etched area. At higher laser energy, many fragments with sizes ranging from a few tens of nanometers to a few micrometers, were observed, and they spatially dispersed on the substrate. Such laser energy dependence of the urea crystal fragments was quite similar to the results from several samples of phthalocyanine thin film that we have reported previously.5 In case of copper phthalocyanine thin film, for example, a dot-like fragment, which corresponded well with the etched area, was deposited by femtosecond laser ablation although no material deposition was realized with nanosecond laser irradiation under similar conditions. Near the threshold of laser ablation, the temperature estimated by comparing femtosecond time-resolved and temperature difference spectra was only about 150 °C, which is quite low compared with melting and boiling points. Furthermore, critical plasma formation and photochemical reactions, which lead to the onset of laser ablation, were not confirmed in the laser energy range used.2,3 The results indicate that photomechanical ablation of the copper phthalocyanine film is induced by mechanical stress accumulated in the film, so that a large fragment (approximately micrometer sized) will be ejected from the surface without any chemical decomposition.5,14 In the case of nanosecond laser ablation of a copper phthalocyanine film, however, molecularly dispersed clusters (approximately nanometer sized) will be ejected by explosive boiling and sublimation,5,14 which can be attributed to photothermal laser ablation. Thus, the fragmentation of the urea crystal is considered to be assigned to increasing stress by such a photomechanical effect. Alternatively, because urea is easy to decompose chemically, there is another possibility that the stress increase is induced by the gaseous products due to photochemical reactions or plasma formation in urea crystals. The molecular C2 axis of a urea crystal is parallel to the crystallographic [001] direction, which is also parallel to the needle axis of urea crystals grown from the aqueous solution,9 so the orientation of the ejected crystal fragments is inferred from the needle axis of the subsequent urea crystals produced by the femtosecond laser irradiation. For example, the needle axis of the ejected fragments, as shown in Figure 1a,b,d, will be almost perpendicular to that of the original urea crystal. In the ablation process, the crystal fragments ejected by laser ablation seem to be randomly oriented. The subsequent crystals, however, tended to grow perpendicular (to within ∼30°) to the original urea crystals when the crystal was irradiated with a laser pulse near the threshold. Although the subsequent crystal orientation is novel and needs detailed study, at this present stage of the investigation, we consider it to be the result of the inclusion of crystal fragments within the original crystal. The original crystal also grows, as well as the ejected crystal fragments, in a supersaturated solution and can include the crystal fragments with orientations that are similar to the original crystal. As a result, the subsequent crystal the needle axis of which is nearly perpendicular to that of the original crystal will grow independently of the original crystal. As another possibility of the origin of the orientation, friction between the supersaturated solution and ejected fragment may be considered. When a solution is irradiated by a laser pulse, a shock wave will be generated and accompanied by liquid flow. The ejected crystal fragments will begin to grow immediately after ejection from the crystal surface and will take on the needle shape, so the crystal fragments might minimize friction with the supersaturated solution by orienting their needle axis parallel to the direction of ejection or of liquid flow.
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achieved. This technique can be applied to isolation and purification of a single crystal of organic and biological materials. To analyze three-dimensional structure of protein by means of X-ray diffraction, it is important to produce high quality single crystals. Additionally, the dissolution by 1480 nm CW laser irradiation is also interesting as method of recrystallizing a particular local area of crystals. We foresee that such crystal growth techniques by laser irradiation will be extended to be very fruitful in the crystal engineering of organic molecules and proteins, as well as in crystal nucleation control by femtosecond laser irradiation Acknowledgment. We thank Drs. H. Kitano, H. Adachi, Y. Mori, and T. Sasaki for technical support of the CW laser system, and Dr. H. Ichida for technical support of the OPA system. This work has been partly supported by CREST of JST (Japan Science and Technology Corporation) and by “BioMedical Cluster in Saito (Northern Part of Osaka Prefecture)”, which is promoted by Ministry of Education, Culture, Sports, Science, and Technology. Figure 5. (a) Crystal patterning procedure for urea. The first pulse was irradiated to the open circle area numbered 1, the second pulse was to the circle 2, and so on. A single shot of the 800 nm femtosecond laser pulse with 0.12 µJ/pulse was irradiated through a 10× objective lens. (b) The ladder-like spatial pattern of urea crystals obtained by successive single shot laser irradiation of the five open circle areas each under the same irradiation conditions.
The crystal growth with femtosecond laser irradiation can be repeated with a crystal previously prepared by an earlier femtosecond laser irradiation. As shown in Figure 5a, the first laser-induced crystal growth was initiated from a crystal nucleated spontaneously from the supersaturated solution, and the second laser-induced crystal growth was then induced from the first crystal. This suggests that subsequent crystals were not decomposed and dissolved, and the crystallinity was thus almost retained. With repetition of the same irradiation procedure, the subsequent urea crystals could be also patterned on a glass substrate as demonstrated in Figure 5b, where six needlelike crystals were connected in a ladder formation by successive single shot femtosecond laser irradiation of each open circle area. Conclusion In this paper, we have demonstrated arbitrary spatial control of crystal growth by utilizing a focused laser beam. With femtosecond laser irradiation, the crystal growth of urea was spatially controlled and a patterning of needlelike crystals was
References (1) Shrinivasan, R.; Stucliffe, E.; Braren, B. Appl. Phys. Lett. 1987, 51, 1285. (2) Hosokawa, Y.; Yashiro, M.; Asahi, T.; Masuhara, H. J. Photochem. Photobiol. A: Chem. 2001, 142, 197. (3) Hosokawa, Y.; Yashiro, M.; Asahi, T.; Fukumura, H.; Masuhara, H. Appl. Surf. Sci. 2000, 154-155, 192. (4) Hosokawa, Y.; Yashiro, M.; Asahi, T.; Masuhara, M.; Kadota, T.; Shirota, Y. Jpn. J. Appl. Phys. 2001, 40, L1. (5) Asahi, T.; Yoshikawa, H. Y.; Yashiro, M.; Masuhara, H. Appl. Surf. Sci. 2002, 197-198, 777. (6) Fujiwara, H.; Ishii, H.; Ishikawa, T.; Hayashi, T.; Fukumura, H.; Masuhara, H. Bull. Chem. Soc. Jpn. 2003, 76, 1075. (7) Adachi, H.; Takano, K.; Hosokawa, Y.; Inoue, T.; Mori, Y.; Matsumura, H.; Yoshimura, M.; Tsunaka, Y.; Morikawa, M.; Kanaya, S.; Masuhara, H.; Kai, Y.; Sasaki, T. Jpn. J. Appl. Phys. 2003, 42, L798. (8) Hosokawa, Y.; Adachi, H.; Yoshimura, M.; Mori. Y.; Sasaki, T.; Masuhara, H. Cryst. Growth Des. 2005, 5, 861. (9) Garetz, B. A.; Aber, J. E.; Goddard, N. L.; Young, R. G.; Myerson, A. S. Phys. ReV. Lett. 1996, 77, 3475. (10) Zaccaro, J.; Matec, J.; Myerson, A. S.; Garetz, B. A. Cryst. Growth Des. 2001, 1, 5. (11) Garetz, B. A.; Matec, J.; Myerson, A. S. Phys. ReV. Lett. 2002, 89, 175501. (12) Okutsu, T.; Nakamura, K.; Haneda, H.; Hiratsuka, H. Cryst. Growth Des. 2004, 4, 113. (13) Okutsu, T.; Isomura, K.; Kakinuma, N.; Horiuchi, H.; Unno, M.; Matsumoto, H.; Hiratsuka, H. Cryst. Growth Des. 2005, 5, 461. (14) Hosokawa Y.; Yashiro M.; Asahi T.; Masuhara H. Proc. SPIE 2001, 78, 4247.
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