Fan Guanghua, Han Yanhua, Luo Suilian, Li Yutong, Qu Shiliang, Wang Qiang, Gao Renxi, Chen Minrui, Han Min
National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China.
Department of Optoelectronic Science, Harbin Institute of Technology at Weihai, Weihai 264209, China.
Phys Chem Chem Phys. 2016 Apr 7;18(13):8993-9004. doi: 10.1039/c5cp04382d.
Both fabrication of Au nano-objects and the nonlinear optical properties of Au nano-objects are the focus of research. In the present work, Au nanoparticles with different mean sizes (18, 32, 42, and 70 nm) are controllably fabricated in ethanol by changing the concentration of poly(vinylpyrrolidone) (PVP) and HAuCl4, as well as the power of continuous wave UV light at 365 nm. PVP acts as both reducing and protective agent. The mechanism of photoreduction of PVP to HAuCl4 is proposed. PVP undergoes a series of chemical reactions which include the attack of the hydrogen atom on the tertiary carbon atom at the α-position of the nitrogen atom, production of a hydroxyl radical, and chain scission. The hydroxyl radical combines with the hydrogen atom produced through the dissociation of HAuCl4, which facilitates the decomposition of HAuCl4. The fabrication mechanism of Au nanoparticles is discussed. The nonlinear absorption of these Au nanoparticles is investigated; all of them exhibit saturable absorption, and the saturable absorption dominates the nonlinear absorption with the increase of laser energy. The dominance of saturable absorption in the nonlinear absorption is due to the stronger single-photon absorbed intraband absorption from the ground state to the first excited state in the conduction band, the weaker excited state absorption in the conduction band, and the weaker two-photon absorption from the d band to the conduction band.
金纳米物体的制备及其非线性光学性质都是研究的重点。在本工作中，通过改变聚乙烯吡咯烷酮（PVP）和氯金酸（HAuCl4）的浓度以及365nm连续波紫外光的功率，在乙醇中可控地制备了具有不同平均尺寸（18、32、42和70nm）的金纳米颗粒。PVP兼具还原剂和保护剂的作用。提出了PVP对HAuCl4的光还原机理。PVP经历了一系列化学反应，包括氢原子对氮原子α位叔碳原子的攻击、羟基自由基的产生以及链断裂。羟基自由基与通过HAuCl4离解产生的氢原子结合，这促进了HAuCl4的分解。讨论了金纳米颗粒的制备机理。研究了这些金纳米颗粒的非线性吸收；它们都表现出饱和吸收，并且随着激光能量的增加，饱和吸收主导了非线性吸收。非线性吸收中饱和吸收的主导地位是由于从基态到导带第一激发态的单光子吸收带内吸收更强、导带中的激发态吸收较弱以及从d带向导带的双光子吸收较弱。
