Inasawa Susumu, Sugiyama Masakazu, Yamaguchi Yukio
Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan.
J Phys Chem B. 2005 Mar 3;109(8):3104-11. doi: 10.1021/jp045167j.
Relatively large gold nanoparticles (mean diameter of major axis 38.2 nm, mean aspect ratio 1.29) in aqueous solution were found to undergo shape transformations from ellipsoids to spheres at ca. 940 degrees C, which is much lower than their melting point, ca. 1060 degrees C. The shape transformation of gold nanoparticles induced by a single pulse of a Nd:YAG laser (lambda = 355 nm, pulse width = 30 ps) was directly observed by a transmission electron microscope (TEM). Analysis of the experimental data showed that the threshold energy for photothermally induced shape transformation was on the order of 40 fJ for a particle, which is smaller than the energy, 67 fJ, required for its complete melting. Estimations based on the heat balance and surface melting model revealed that the temperature which particles reach after a single laser pulse was about 940 degrees C, with the thickness of the liquid layer on the surface of the solid core being 1.4 nm. We also examined thermally induced shape transformation of gold nanoparticles on Si substrates; above 950 degrees C they changed their shapes to spheres, which supported our estimation. Due to the surface melting of particles, their shape transformation occurs at a temperature much lower than their melting point.
研究发现，水溶液中的相对较大的金纳米颗粒（长轴平均直径为38.2 nm，平均纵横比为1.29）在约940℃时会发生从椭球体到球体的形状转变，该温度远低于其熔点（约1060℃）。通过透射电子显微镜（TEM）直接观察到了由Nd:YAG激光的单个脉冲（波长λ = 355 nm，脉冲宽度 = 30 ps）诱导的金纳米颗粒的形状转变。对实验数据的分析表明，光热诱导形状转变的阈值能量对于单个颗粒约为40 fJ，这比其完全熔化所需的能量67 fJ要小。基于热平衡和表面熔化模型的估算表明，单个激光脉冲后颗粒达到的温度约为940℃，固体核表面的液层厚度为1.4 nm。我们还研究了硅衬底上金纳米颗粒的热诱导形状转变；在950℃以上它们会转变为球形，这支持了我们的估算。由于颗粒的表面熔化，它们的形状转变发生在远低于其熔点的温度下。
