顯微光譜探討雷射捕陷聚苯乙烯奈米粒子於溶液表面之 光學拓展聚集化

Abstract

我們探討雷射捕陷下，聚苯乙烯奈米粒子(208 nm)於溶液表面聚集成類結晶狀的聚集體之形成過程。連續波雷射(1064 nm)照射下，奈米粒子聚集至大小約一立方微米的焦點內，之後逐漸長成直徑20微米的圓形聚集體。在此研究，我們藉由穿透/反射影像和反射顯微光譜來瞭解聚集體於液面成形的行為。成形的聚集體於白光照明下呈現結構光，表示組成的奈米粒子以週期性秩序排列，結構光顯現過程於反射影像上更為清楚。我們利用反射顯微光譜來檢視聚集體的成形與動態過程，所測得的光譜可對應出粒子間距。聚集體上所有位置皆可偵測到單一反射帶，代表整個聚集體內的奈米粒子都呈現週期性排列，雷射捕陷下此反射帶藍移且帶寬變窄，表示奈米粒子逐漸聚集且其結構變得緊密和均一，另外，來回調整雷射功率會造成波峰反覆位移變化，根據這些成果，我們認為雷射捕陷所形成的聚集體可重整成週期性秩序排列的結構，且聚集體內的奈米粒子在固定濃度下處於光學捕陷吸引力與粒子間靜電斥力的動態平衡下。 為了更深入地探討聚集體形成的機制，我們檢視不同的粒子濃度和鹽離子濃度下，雷射捕陷形成的聚集體與濃度的相關性。在高粒子濃度下，聚集體成長快速，飽和尺寸較大且結構較為緊密。添加鹽離子於溶液中，延伸團粒會從焦點生成，1064奈米的捕陷雷射光從聚集體中心沿團粒方向向外散射；然而，沒有鹽離子添加的溶液中，聚集體以圓形的形狀成長且1064奈米雷射光均勻對稱向外散射。我們認為雷射捕陷奈米粒子聚集化過程包含奈米粒子間的多重散射和沿著延伸團粒的方向性散射，這兩者的差異導因於奈米粒子間的光學親合關係，這樣的動態現象我們稱之為光學拓展聚集化。另外，我們推論穩定狀態的聚集體和其周遭溶液間奈米粒子反覆地捕陷與釋放，這反覆的過程最終會達到光學和化學平衡狀態。

We demonstrated the formation of a colloidal crystal-like assembly of 208 nm polystyrene nanoparticles by irradiating a continuous-wave near-infrared laser beam (1064 nm) into the air/solution interfacial layer. Upon the irradiation, nanoparticles were gathered around the focal volume (about 1 μm3), and eventually one circular assembly was grown up to the size of 20 μm in diameter. In this work, we have studied the assembly formation behavior in the solution surface layer by transmission/reflection microscopy and reflection microspectroscopy. The large assembly under halogen lamp illumination showed structural color, meaning that constituent nanoparticles are arrayed in a periodic manner. The structural color from the nanoparticle assembly was observed more clearly in reflection image than in transmission image. We applied reflection microspectroscopy to this assembly formation and examined its dynamics by measuring the time evolution of reflection spectra relating to particle distance. In the position-dependent reflection spectra of the backscattered light from the assembly, one reflection band was observed anywhere over the assembly. The peak position of the reflection band was shifted to short wavelength, accompanied by the decrease in the bandwidth. The observed spectral change clearly indicated that nanoparticles were gradually accumulated during laser irradiation and their packing structure became tight and homogeneous. The peak position in a reflection band was dynamically varied in a repetitive manner by changing the laser power. We explain these results under a given concentration from the viewpoint that nanoparticles are gathered and gradually rearranged into a periodic array due to dynamic balance between the attractive optical trapping force and the electrostatic repulsive force among nanoparticles. In order to understand its formation mechanism generally, it is important and indispensable to examine the assembly formation with various concentrations of particle and salt. Initial nanoparticle and salt concentration dependences of the colloidal crystal-like assembly formation induced by optical trapping were examined and considered. In the high concentration of nanoparticles, the assembly formation exhibits fast growth, gives large saturation size, and leads to dense packing structure. In the presence of salt, one assembly with the elongated aggregates was generated from the focal spot and 1064 nm trapping light was scattered outwardly with certain directions, while a circular assembly and symmetrical scattering of the 1064 nm light were found without salt. The nanoparticle assembling in optical trapping is driven through isotropic scattering in gathered nanoparticles and directional scattering along the elongated aggregates derived from optical association of nanoparticles, which dynamic phenomenon is called optically evolved assembling. Repetitive trapping and release processes of nanoparticles between the assembly and the surrounding solution always proceed, and the steady state of the resultant assembly formed by laser trapping is determined under optical and chemical equilibrium.