飛秒雷射誘發氯酸鈉掌性結晶化之動態特征與機制

Dynamics and mechanism of chiral crystallization of sodium chlorate by femtosecond laser irradiation

葉育豪(Yu-Hau Ye)

指導教授 ： 杉山輝樹

國立交通大學/理學院/應用化學系所/碩士(2020年)

摘要

本研究旨在通過飛秒雷射輻射實現氯酸鈉的結晶並控制其晶體掌性。飛秒雷射被歸類為超快雷射，並且通過光與物質相互作用，有可能引發連續波雷射輻射無法實現的各種物理和化學現象。此外，從飛秒雷射輻射引起的各種現象的角度出發，討論結晶和掌性控制的動力學和機制也是本研究的主要目的之一。當將具有高頻率（八千萬赫茲）的飛秒雷射光束聚焦在氯酸鈉水溶液之空氣與溶液介面時，在雷射聚焦處會觀察到氯酸鈉結晶。從有關氯酸鈉結晶的許多文獻中，最早生成的晶體被確定為熱力學不穩定的非掌性晶體。當雷射能量密度相對較高（大於270 mJ/cm2）時，在輻射後立即觀察到空蝕氣泡，並在此後立即誘發非掌性晶體。 另一方面，當雷射能量密度相對較低（小於270 mJ/cm2）時，在雷射輻射一段時間後會產生空蝕氣泡，然後觀察到結晶。此外，我們確認輻射點的濃度在產生空蝕氣泡的同時不斷增加。亦即，是否產生空蝕氣泡由雷射焦點處的溶質濃度和雷射能量密度來決定。足夠高能量密度的雷射通過溶質分子的多光子吸收立即產生空蝕氣泡，由於所產生空蝕氣泡表面上分子濃度的增加最終誘發了結晶。該機制與飛秒雷射輻射具有低頻率（十赫茲）引起的常規結晶機制是一致的。另一方面，當雷射強度相對較低時，溶液中的分子和簇首先被捕陷並聚集在雷射焦點處，在該焦點處濃度升高到足以產生空蝕氣泡後，最終誘發結晶。接下來，當使用聚焦的飛秒雷射光束連續輻射所產生的介穩態晶體後引起相轉變。所得之晶體被鑑定為熱力學上最穩定的掌性氯酸鈉晶體。我們在這裡考慮從非掌性晶體到掌性晶體相轉變有兩種可能的機制。首先是在非掌性晶體的界面上首先生成掌性晶體的結晶核外延形成的機制。第二種機制是飛秒雷射輻射作用於首先生成的非掌性晶體上的外部應力，從而導致相轉變。為了進一步討論後者的機制，我們在空氣條件下製備了介穩態晶體，並用聚焦飛秒雷射光束直接輻射到晶體上。結果，我們在範圍很大的雷射能量密度（150-550 mJ/cm2）內觀察到掌性晶體的相轉變。這一結果可能支持後一種機制。但是，必須仔細檢查以下實驗結果。眾所周知，可以通過簡單的方法立即區分氯酸鈉掌性晶體的掌性。在使用圓偏振光作為光源時，溶液中生成的晶體的掌性與圓偏振光的偏振方向無關，而在空氣中的實驗結果顯示，掌性晶體的掌性在很大程度上取決於圓偏振光的偏振方向。此外，我們還確認，即使在我們儀器的最大雷射能量密度下，將聚焦飛秒雷射光束輻射在非掌性晶體內也沒有顯示出任何相轉變。從這些結果，我們認為結晶機制主要由前一種機制決定。也就是說，通過用飛秒雷射光束連續輻射非掌性晶體，從雷射焦點形成空蝕氣泡，並且在停滯點（空蝕氣泡，玻璃基板和溶液之間的界面）的溶質濃度增加。結果，在非掌性晶體的表面上外延形成掌性結晶核。
通過飛秒雷射輻射控制氯酸鈉晶體的掌性也是一個有趣的研究主題。我們在這裡使用球形金奈米粒子的聚集體的局部表面電漿共振來控製掌性氯酸鈉晶體的掌性，球形金奈米粒子的大小在10到250 nm之間，因為僅通過飛秒雷射輻射觀察不到晶體對映體過量(CEE)。將金奈米粒子添加到氯酸鈉飽和水溶液中，並將飛秒雷射輻射到金奈米粒子的聚集體中，其雷射能量密度範圍為0.27-0.67 J/cm2。 電荷耦合裝置影像上氯酸鈉的結晶動力學與無金奈米粒子時相同。但晶體對映體過量值與沒有金奈米粒子時不同。 晶體對映體過量值隨雷射能量密度和金奈米粒子直徑的增加而增加，最終在實驗條件（雷射功率密度：0.67 J/cm2，粒徑250 nm）下達到40％。這些結果表明金奈米粒子的聚集體在確定氯酸鈉晶體的掌性方面起到至關重要的作用。我們提出了兩種可能的機制來解釋對稱性破壞的現象。首先，在金奈米粒子之間的奈米間隙處產生極大的增強電磁場，此增強電磁場根據圓偏振光的偏振方向來選擇性地捕陷或穩定其中一種掌性氯酸鈉團簇（左旋或右旋團簇）。最後，具有任一掌性的團簇聚集，產出高的晶體對映體過量值。其次，左旋團簇或右旋團簇的圓二色性具有較高的晶體對映體過量值。通過圓偏振飛秒雷射輻射的多光子吸收，可防止任一簇導致結晶。我們相信這項工作的結果將加速許多研究領域，例如雷射捕陷，雷射燒蝕，表面電漿共振捕陷，結晶和掌性。

關鍵字

飛秒雷射 ； 氯酸鈉 ； 結晶化機制 ； 掌性

並列摘要

This study aims to realize the crystallization of NaClO3 and control of its crystal chirality by femtosecond laser irradiation. The femtosecond laser is categorized into an ultra-short pulse laser. It induces various physical and chemical phenomena via light-matter interactions that are never achieved by a continuous-wave laser. Moreover, it is also one of the primary purposes of this study to discuss the dynamics and mechanism of crystallization and chirality control from the viewpoint of their various phenomena induced by femtosecond laser irradiation.
When a femtosecond laser beam with a high repetition rate (80 MHz) is focused on a NaClO3 aqueous solution, NaClO3 crystallization is observed at the laser focus. From many reports on the crystallization of NaClO3, the firstly-generated crystal is its thermodynamically unstable achiral crystal. When the laser fluence is relatively high (>270 mJ/cm2), cavitation bubbles are observed immediately after the irradiation, and achiral crystals are induced immediately after that. On the other hand, when the laser fluence is relatively low (<270 mJ/cm2), cavitation bubbles are generated after laser irradiation for a while, and then crystallization is observed. Furthermore, we confirm that the concentration of the condensing point continuously increases while the cavitation bubbles are generated. That is, whether or not the cavitation bubbles are generated is determined by the solute concentration at the laser focus and the laser fluence. The sufficiently-high power laser immediately produces cavitation bubbles via the multiphoton absorption of the solute molecules, eventually triggering the crystallization due to an increase in the molecular concentration on the surface of the generated bubble. This mechanism is consistent with the conventional crystallization mechanism induced by femtosecond laser irradiation with a low repetition rate (10 Hz). On the other hand, when the laser light intensity is relatively low, molecules and clusters in the solution are first trapped and collected at the laser focus, where the concentration is increased high enough to generate cavitation bubbles, eventually inducing crystallization.
Next, when a focused femtosecond laser beam continuously irradiates the generated metastable crystal, a phase transition to a chiral stable is induced. The resulting crystal is the thermodynamically most stable chiral NaClO3 crystal. We here consider two possible mechanisms for the phase transition from achiral to chiral crystal. The first is the mechanism that nuclei of chiral crystal epitaxially form at the interface of the achiral crystal firstly generated. The second is a mechanism that femtosecond laser irradiation acts as external physical forces on the firstly-generated achiral crystal, leading to the phase transition. To further discuss the latter mechanism, we prepare the metastable crystals under air conditions and directly irradiate to the crystals with a focused femtosecond laser beam. As a result, we observe the phase transition to the chiral crystal in a wide range of laser fluence (0.14-0.54 mJ/cm2). This result may support the latter mechanism; however, the following experimental result must be carefully examined. It is well known that the chirality of NaClO3 chiral crystal can be instantly discriminated by a straightforward method. We confirm that upon using circularly polarized light (CPL) as a light source, the chirality of the crystal generated in solution is independent of the handedness of CPL, while the experimental results in the air show the chirality of the chiral crystals strongly depends on it.
Furthermore, we also confirm that the focused laser irradiation inside the achiral crystal shows no transformation even under the maximum laser fluence of our instrument. From these results, we consider that the crystallization mechanism is dominated by the former mechanism. That is, by continuously irradiating the achiral crystal with a femtosecond laser beam, cavitation bubbles are formed from the laser focus, and the solute concentration at the stagnation point (the interfaces among bubble, a glass substrate, and solution) increases. As a result, a chiral crystal nucleus is epitaxially formed at the surface of the achiral crystal. Then, the chiral crystal grows, and the achiral crystal dissolves due to the difference in solubility between the achiral and the chiral crystals. Interestingly, we confirm that the chirality of the chiral crystal in solution is independent of the handedness of CPL.
It is also a fascinating research theme to control the chirality of chiral NaClO3 crystal by femtosecond laser irradiation. We here control the chirality of chiral NaClO3 crystal using localized surface plasmon resonance of the aggregates of spherical gold nanoparticles (Au NPs) with different sizes ranging from 10 to 250 nm because no crystal enantiomeric excess (CEE) is observed just by the femtosecond laser irradiation. The Au NPs are added into the saturated aqueous solution of NaClO3, and the femtosecond laser pulses with a laser fluence ranging from 0.27-0.67 J/cm2 is introduced into the aggregates of Au NPs. The crystallization dynamics of NaClO3 on the CCD image is the same as that without Au NPs; however, the CEE value is different from that without Au NPs. The CEE value increases with an increase in the laser fluence and the diameter of Au NPs, eventually achieving to be 40 % under an experimental condition (Laser fluence: 0.67 J/cm2 and particle size 250 nm). These results indicate that the aggregates of Au NPs play an essential role in determining the handedness of chiral NaClO3 crystal. We believe that the results obtained in this work would accelerate many research fields such as laser trapping, laser ablation, plasmon trapping, crystallization, and chirality.

並列關鍵字

femtosecond laser ； sodium chlorate ； crystallization ； chiral crystal ； chirality ； dynamics and mechanism

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