Control of Organic Crystal Shape by Femtosecond Laser Ablation

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Control of Organic Crystal Shape by Femtosecond Laser Ablation Daiki Suzuki, Seiichiro Nakabayashi, and Hiroshi Y Yoshikawa Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00697 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 11, 2018

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Crystal Growth & Design

Control of Organic Crystal Shape by Femtosecond Laser Ablation Daiki Suzuki, Seiichiro Nakabayashi, Hiroshi Y. Yoshikawa* Department of Chemistry, Saitama University, Shimo-okubo 255, Sakura-ku, Saitama 338-8570, Japan. AUTHOR INFORMATION Corresponding Author *[email protected].

ABSTRACT Control of organic crystal shape is crucial for various scientific and industrial fields, while it is still very challenging even with systematic optimization of environmental parameters such as temperature and concentration. Here we report an innovative approach for spatiotemporal control of organic crystal growth by directly modifying local crystal structures via femtosecond laser ablation. We found that a crystal face that is locally ablated only with a single laser pulse shows enhanced growth without the loss of crystal quality. The underlying mechanism can be explained by the generation of energetically favorable crystal growth mode (spiral growth mode), which is

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a fundamental growth mode for various organic crystals. We demonstrated that various crystal shapes can be achieved by femtosecond laser ablation. The fine-tuned, spatiotemporal cue given by femtosecond laser ablation will provide a facile means to obtain organic crystals with desired shape.

KEYWORDS Spatiotemporal control of crystal growth, Spiral growth, Crystal shape, Femtosecond laser ablation


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MAIN TEXT Control of organic crystal shape is crucial for various scientific and industrial fields1 such as X-ray/Neutron crystallographic studies,2 pharmaceutics,3,

4

optoelectronic devices,5 food

industries,6, 7 because these factors significantly influence quality, functionality, handing, and storage of end-products. In general, researchers try to control organic crystal growth by adjusting environmental parameters such as concentration, solvents, temperature, and additives.1 However, due to relatively weak interaction natures of organic materials, it is still very challenging to obtain crystals with desired shape, even with the systematic optimization of such environmental parameters. In contrast, one of the authors, Yoshikawa, have recently developed an innovative method for the promotion of crystal growth of proteins by locally destroying the crystal structures via laser ablation.8 The paper reported that local etching (several micrometers in diameter) of protein crystal surfaces by femtosecond laser ablation enhances crystal growth rates without deterioration of crystal quality.8 The further measurement by optical and X-ray imaging techniques showed that the laser ablation can generate spiral growth,8 which is known as thermodynamically favored growth mode compared with the spontaneous two-dimensional nucleation growth mode at low supersaturation.9 This paper is the first demonstration of the control of protein crystal growth by actively switching crystal growth modes, which is in contrast to the conventional methods based on adjusting the environmental parameters. It should be also noted that the approach with femtosecond laser ablation of crystals is also different from the other light-based crystallization methods via photoelectric fields10-12 or photochemical reactions13, 14 where light is used to control concentration, structure, and/or alignment of solutes (not crystals).



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Based on the previous demonstration with protein crystals, we now consider that femtosecond laser ablation may provide a new approach for control of shape of various organic crystals without complicated optimization of environmental parameters. Actually, spiral growth is a fundamental growth mode and potentially appears for crystals of not only proteins but also various organic materials.1, 15 Furthermore, considering the following physical characteristics, femtosecond laser ablation is promising for spatiotemporal control of organic crystal growth without significant damages. First, focused irradiation with a femtosecond laser to transparent materials can result in ablation via multiphoton excitation,16 which enables three-dimensional nanoprocessing of organic crystals in a closed chamber with a supersaturated solution.8, 17, 18 By contrast, laser excitation via one-photon absorption (e.g., in ultraviolet region) cannot induce ablation of organic crystals in a supersaturated solution, owing to light absorption by solutes. Second, at the energy close to ablation threshold, femtosecond laser is known to induce ablation via a photomechanical process where less heat generation is accompanied compared to ablation induced by other lasers with longer pulse duration.19, 20 Such photomechanical ablation should lessen thermal damage to organic crystals that are generally less tolerant to temperature elevation compared to inorganic crystals. In this work, we demonstrated control of shape of organic crystals by femtosecond laser ablation. We here used small organic compounds, amino acids, which are basic components of proteins and used for various scientific and industrial fields such as pharmaceutics and foods. We systematically investigated the influence of laser ablation on shape and quality of crystals. In addition, the underlying mechanism was investigated by using a newly developed microscopy system (Figure S1), which allows in-situ monitoring of nanometerscale crystal growth dynamics induced by femtosecond laser ablation. Detailed experimental protocols are provided as Supporting Information.


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Figure 1 shows a representative result of glycine crystal growth induced by femtosecond laser ablation. We first prepared α-glycine crystals by spontaneous nucleation and then adjusted the concentration, c, to ~ 235 mg/mL of water where no further spontaneous nucleation occurs. The solubility of glycine in our experimental condition was estimated to be ~ 200 mg/mL of water (Figure S2 and Figure S3), which is close to the previously reported solubility value at 20 C (~ 199 mg/mL of water)21. Then, a single femtosecond laser pulse (Δτ ~ 120 fs) with energy at 0.35 J, which is just same as the ablation threshold (Eth ~ 0.35 J), was focused to a (010) face of a plate-like glycine crystal. The focal radius calculated by the Rayleigh’s criterion with numerical aperture of the used objective lens (0.5) is ~ 1 m. The locally irradiated crystal immediately started growing at the ablated face and finally became a pyramidal shape in 6 min (Movie S1). Such crystal shape change by femtosecond laser ablation could be induced in solutions with the concentration larger than ~ 200 mg/mL of water. Figure 2 shows the dependence of laser energy on the laser-induced crystal shape change. Here, the concentration was adjusted close to equilibrium (~ 205 mg/mL of water) to clearly distinguish influence of laser ablation on crystal growth. In case of a laser shot with energy at 0.28 J (below Eth), crystals did not grow after the laser irradiation and their shape remained plate-like even after 2 hours (Figure 2a). The laser shot with energy at 1.2 J, approximately three times larger than Eth, induced a crystal shape change from plate-like to pyramid-like (Figure 2b) like the case of Figure 1. The laser shot with much larger energy (1.8 J) fragmented crystals, which led to polycrystallization (Figure 2c). To assess the damage of glycine crystals by femtosecond laser ablation, crystals were observed by Crossed Nicols conditions (Figure S4). In case of laser energy at 0.48 J (close to Eth), the entire crystal showed same extinction angles as those before laser ablation, proving that single crystallinity and crystal orientation were kept after



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laser ablation. In addition, no crystal defects were detected also by the careful investigation with the magnified view of the irradiated region, indicating that the damaged area was at least smaller than optical resolution (< 1 m). Even in case of larger laser energy at 1.2 J, the damaged area was limited to be below 6 m in length (Figure S5). We also confirmed that the crystal structure (α form) was not changed after femtosecond laser ablation by Raman spectroscopy. These results strongly suggest that femtosecond laser ablation with energy close to Eth can promote growth of irradiated faces of glycine crystals with the minimum damage to crystal structures, which agrees with the case of protein crystals investigated by X-ray diffraction measurement.8 In order to reveal the mechanism of the change in glycine crystal shape induced by laser ablation with a single laser pulse, we have developed a new experimental system by introducing a femtosecond laser to laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM),22,

23

which allows in-situ observation of laser-induced

behavior of molecular steps on crystal surfaces (Figure S1). LCM-DIM images clearly showed that crystal growth steps were generated and propagated from the ablated area immediately after the laser ablation, while there were no visible steps before the laser ablation (Figure 3a, movie S2). Within several minutes after the laser irradiation, spiral-like growth step patterns appeared and fully covered the irradiated crystal surface. The spiral growth continued to grow for more than one hour (Figure 3b, movie S3). On the other hand, the corresponding bright field images showed that the crystal shape started to change from plate-like to pyramid-like after the generation of the crystal growth steps. To more quantitatively assess the change of the crystal shape after laser ablation, crystal thickness was estimated from a z-stack (0.2 m / z- frame) of LCM-DIM images (Figure 3c). The result clearly showed that the crystal started growing at the


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irradiated face just after laser ablation, and its thickness almost became double (from 3.8 m to 8.0 m) at 90 min. These results clearly indicate that the change in glycine crystal shape by laser ablation is attributed to the generation of spiral growth. Actually, one of the authors, Yoshikawa, previously found the generation of such laser-induced spiral growth also for protein crystals and explained the underlying mechanism from a thermodynamic viewpoint.8, 9 Briefly, at low supersaturation, defectless crystals usually grow according to 2D nucleation growth mode where molecularly flat islands with the size beyond a critical radius are formed on a crystal terrace by overcoming an energy barrier for 2D nucleation.8,

9

Hence, crystals cannot show continuous growth in the

solution of which supersaturation gets lower than that required for 2D nucleation. On the other hand, spiral hillocks continuously produce nonvanishing steps with kink positions and thus can exhibit three-dimensional growth at very low supersaturation where 2D nucleation cannot occur.8, 9

In relation to this, X-ray topography imaging previously revealed that femtosecond laser

ablation can promote protein crystal growth by generating screw dislocations, which lead to produce spiral growth hillocks.8 Moreover, in the present study, we have found that the spiral growth mode can be induced for crystals of other amino acids via femtosecond laser ablation induced only with a single laser pulse (Figure 4, movie S4). Therefore, the results in this work clearly indicate that crystal growth of not only proteins but also smaller organic compounds can spatiotemporally be controlled by the generation of spiral growth via femtosecond laser ablation. Finally, we also investigated the influence of laser ablation on glycine crystals with different shapes. Figure 5a shows a glycine crystal grown in water with sodium acetate (24.6 mg/mL of water), which leads to generation of crystals with more shape varieties such as



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hexagonal and lozenge-like shapes.24 First, a one side face of a hexagonal glycine crystal was irradiated with a femtosecond laser pulse (~ 5 J). After that, the ablated face started growing immediately and formed a sharp edge in 40 s. Then, a second pulse was shot to another face of the crystal, which also induced the generation of a sharp edge. Afterwards, the crystal shape became lozenge-like after the second shot (movie S5). We also tested similar sequential laser shots (~ 1 J) to a lozenge-like glycine crystal as shown Figure 5b. The ablated crystal face selectively grew and became protuberant within ten minutes after laser ablation. After a second pulse, the crystal shape became star-like, which apparently cannot be achieved by spontaneous growth. In conclusion, our results clearly indicated that laser ablation with energy close to Eth can enhance growth rate of the irradiated crystal face with the minimum damage to crystal structure, which allows the shape control of single crystals of organic molecules without complicated optimization of environmental parameters. Furthermore, it was previously reported that nucleation and microseeding of various organic compounds can also be controlled by femtosecond laser ablation.25 Hence, we foresee that the spatiotemporal control of nucleation and crystal growth by femtosecond laser ablation will provide a new approach toward obtaining functional crystals that are not achieved by conventional methods.

ASSOCIATED CONTENT Experimental protocols (PDF) Movie S1: Crystal shape change induced by femtosecond laser ablation (Figure 1a) (AVI)


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Movie S2: LCM-DIM and bright field images of glycine crystal growth induced by femtosecond laser ablation (Figure 3a) (AVI) Movie S3: LCM-DIM images of glycine crystal growth at about 50 min after the laser ablation (Figure 3b) (AVI) Movie S4: Generation of spiral growth of a L-histidine crystal (Figure 4a) (AVI) Movie S5: Shape changes of glycine crystals induced by femto-second laser ablation with sodium acetate. (Figure 5a) (AVI)

AUTHOR INFORMATION Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was partly supported by grants from the Japan Society for the Promotion of Science (JSPS) KAKENHI (No. 16K12868, 17H02774) and the JGC-S Scholarship foundation. We thank Prof. Katsuo Tsukamoto (Tohoku University) for insightful discussions about Crossed Nicols images.



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Figure captions Figure 1. (a) Bright field images of a crystal shape change from plate-like to pyramid-like induced by femtosecond laser ablation. The red arrow represents a focal point. (b) Schematic illustration of the crystal shape change.

Figure 2. Dependence of laser energy on glycine crystal growth in supersaturated solutions (c ~ 205 mg/mL of water). A single laser pulse with energy at (a) 0.28 J, (b) 1.2 J J or (c) 1.8 J was shot to the points indicated by red arrows.

Figure 3. (a) LCM-DIM and bright field images of glycine crystal growth before and after laser ablation at 0.80 J in a supersaturated solution (c ~ 205 mg/mL of water). The red arrows represent focal points. (b) A magnified image of spiral growth of the ablated crystal at about 50 min after the laser ablation. (c) Crystal thickness vs time after laser ablation. The vertical dashed line represents 0 min.

Figure 4. LCM-DIM images of surfaces of (a) L-histidine or (b) L-glutamic acid crystals before and after laser ablation with energy at 0.51 J and 0.44 J, respectively. The red arrows represent focal points.



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Figure 5. Shape changes of α-glycine crystals induced by femtosecond laser ablation in supersaturated solutions with sodium acetate. The red arrows represent focal points. A single laser pulse was shot at t = 0 s and t’ = 0 s.


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“For Table of Contents Use Only” Control of Organic Crystal Shape by Femtosecond Laser Ablation Daiki Suzuki, Seiichiro Nakabayashi, Hiroshi Y. Yoshikawa* Department of Chemistry, Saitama University, Shimo-okubo 255, Sakura-ku, Saitama 338-8570, Japan.

TOC GRAPHIC

Control of organic crystal shape by femtosecond laser ablation is reported. A crystal face that is locally ablated only with a single laser pulse shows enhanced single crystal growth. The mechanism is attributed to the generation of energetically favorable, spiral growth mode. The fine-tuned, spatiotemporal cue will provide a facile means to obtain organic crystals with various shapes.


Control of Organic Crystal Shape by Femtosecond Laser Ablation

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