雷射捕陷控制無機分子結晶化: 結晶形態與掌性

Abstract

我們演示了利用雷射捕陷技術控制無機化合物的結晶形態及掌性結晶化。我們選擇氯化鉀和氯酸鈉作為捕陷的化學物，並且使用1064奈米波長之連續雷射作為雷射捕陷的光源。 在控制氯化鉀結晶形的結晶化實驗中，雷射聚焦在溶液表面，幾分鐘過後，可以觀察到三種結晶的形態，針狀結晶、平面結晶及方塊結晶出現在焦點處。根據不同的雷射強度和極性，可以使出現特定結晶形的機率改變，針狀結晶通常出現較低的雷射強度;而產生方塊結晶的機率隨著雷射增強而增加。另外，線性雷射的結晶化機率比圓性雷射來的高。我們更發現雷射捕陷會在結晶核產生以前，先形成一個高濃度團簇的區域。控制結晶型的機制將會由雷射強度及極性對於高濃度區域以及團簇影響的角度出發。 控制氯酸鈉的掌性結晶化實驗在由三個三角形組成金奈米結構的基板下完成。在圓性雷射照射下，在此奈米結構的20-30奈米的中心點產生局域表面電漿共振熱點。此實驗的焦點雷射強度估計在1 kW/cm2的等級下進行，僅是前面KCl實驗的千分之一。持續的照射雷射，非掌性結晶會透過固態間相轉變生成具有掌性的結晶。右旋或左旋雷射的實驗重複20組，我們得到相較於之前文獻很高的晶體對映體過量，估計為30-40 %。這樣的現象我們推測可能是氯酸鈉團簇與局域表面電漿之間的強耦合。

We present laser trapping-controlled crystal morphology and chirality for inorganic compounds. A 1064-nm continuous-wave laser was used as a trapping source, potassium chloride (KCl) and sodium chlorate (NaClO3) were selected as solutes for experiments of control of crystal morphology and chirality, respectively. For KCl/D2O saturated solution, the trapping laser was focused at its solution layer. After a few minutes, single KCl crystal with three different crystal morphologies, cubic-like, rectangle-like, and needle-like, was formed consistently at the laser focus and the observation probability of three morphologies depended on laser power and polarization. The needle-like crystal was observed just at relatively lower laser powers, while the probability of the cubic-like crystal formation was increased with the input laser power. Moreover, the crystallization probability upon linear polarization was higher than that upon circular polarization. It was also found that laser trapping created a dense domain consisting of KCl clusters prior to crystal nucleation. The dynamics and mechanism of control of the crystal morphology are discussed from the view point of the domain size and/or the cluster structure in the domain, depending on laser power and polarization. NaClO3/D2O saturated solution was poured into a glass substrate with a trimer structure consisting of triangle gold nanostructures. By irradiating circularly-polarized laser to the gold nanostructure, localized surface plasmon resonance was generated at the center spot of the trimer with the size of 20-30 nm in diameter. When the density of the laser power at the focus was set to the order of 1 kW/cm2, which is 103 times smaller than the case of KCl, NaClO3 metastable crystal was instantly generated. The continuous irradiation to the generated crystal led to solid-solid phase transition, resulting in chiral crystal formation. Such an experiment was repetitively carried out for 20 times using right- or left-handed circularly polarized laser. As a result, the crystal enantiomorph excess was evaluated to 30-40 %, which is considerably high and possibly due to the strong coupling between localized surface plasmon resonance and NaClO3 clusters.