Liaw Jiunn-Woei, Liu Guanting, Ku Yun-Cheng, Kuo Mao-Kuen
Department of Mechanical Engineering, Chang Gung University, Taoyuan 33302, Taiwan.
Department of Mechanical Engineering, Ming Chi University of Technology, New Taipei City 243303, Taiwan.
Nanomaterials (Basel). 2020 Sep 20;10(9):1881. doi: 10.3390/nano10091881.
Plasmon-enhanced photothermal and optomechanical effects on deforming and reshaping a gold nanoparticle (NP) are studied theoretically. A previous paper (Wang and Ding, ACS Nano 13, 32-37, 2019) has shown that a spherical gold nanoparticle (NP) irradiated by a tightly focused laser beam can be deformed into an elongated nanorod (NR) and even chopped in half (a dimer). The mechanism is supposed to be caused by photothermal heating for softening NP associated with optical traction for follow-up deformation. In this paper, our study focuses on deformation induced by Maxwell's stress provided by a linearly polarized Gaussian beam upon the surface of a thermal-softened NP/NR. We use an elastic model to numerically calculate deformation according to optical traction and a viscoelastic model to theoretically estimate the following creep (elongation) as temperature nears the melting point. Our results indicate that a stretching traction at the two ends of the NP/NR causes elongation and a pinching traction at the middle causes a dent. Hence, a bigger NP can be elongated and then cut into two pieces (a dimer) at the dent due to the optomechanical effect. As the continuous heating process induces premelting of NPs, a quasi-liquid layer is formed first and then an outer liquid layer is induced due to reduction of surface energy, which was predicted by previous works of molecular dynamics simulation. Subsequently, we use the Young-Laplace model to investigate the surface tension effect on the following deformation. This study may provide an insight into utilizing the photothermal effect associated with optomechanical manipulation to tailor gold nanostructures.
从理论上研究了等离激元增强的光热效应和光机械效应在金纳米颗粒（NP）变形和重塑过程中的作用。之前的一篇论文（Wang和Ding，《美国化学会纳米》13卷，32 - 37页，2019年）表明，被紧聚焦激光束照射的球形金纳米颗粒（NP）可变形为细长的纳米棒（NR），甚至被切成两半（二聚体）。其机制被认为是由光热加热使NP软化，并伴随光学牵引力导致后续变形。在本文中，我们的研究重点是由线偏振高斯光束在热软化的NP/NR表面产生的麦克斯韦应力所引起的变形。我们使用弹性模型根据光学牵引力数值计算变形，并使用粘弹性模型从理论上估计温度接近熔点时的后续蠕变（伸长）。我们的结果表明，NP/NR两端的拉伸牵引力会导致伸长，而中间的挤压牵引力会导致凹陷。因此，由于光机械效应，较大的NP可以被拉长，然后在凹陷处被切成两块（二聚体）。随着连续加热过程导致NP预熔，首先形成准液层，然后由于表面能降低诱导形成外层液层，这是先前分子动力学模拟工作所预测的。随后，我们使用杨 - 拉普拉斯模型研究表面张力对后续变形的影响。本研究可能为利用与光机械操纵相关的光热效应来定制金纳米结构提供见解。
