Ihm Yungok, Cho Do Hyung, Sung Daeho, Nam Daewoong, Jung Chulho, Sato Takahiro, Kim Sangsoo, Park Jaehyun, Kim Sunam, Gallagher-Jones Marcus, Kim Yoonhee, Xu Rui, Owada Shigeki, Shim Ji Hoon, Tono Kensuke, Yabashi Makina, Ishikawa Tetsuya, Miao Jianwei, Noh Do Young, Song Changyong
Department of Chemistry, Pohang University of Science and Technology, Pohang, 37673, Korea.
Department of Physics, Pohang University of Science and Technology, Pohang, 37673, Korea.
Nat Commun. 2019 Jun 3;10(1):2411. doi: 10.1038/s41467-019-10328-4.
Despite more than a century of study, the fundamental mechanisms behind solid melting remain elusive at the nanoscale. Ultrafast phenomena in materials irradiated by intense femtosecond laser pulses have revived the interest in unveiling the puzzling processes of melting transitions. However, direct experimental validation of various microscopic models is limited due to the difficulty of imaging the internal structures of materials undergoing ultrafast and irreversible transitions. Here we overcome this challenge through time-resolved single-shot diffractive imaging using X-ray free electron laser pulses. Images of single Au nanoparticles show heterogeneous melting at the surface followed by density fluctuation deep inside the particle, which is directionally correlated to the polarization of the pumping laser. Observation of this directionality links the non-thermal electronic excitation to the thermal lattice melting, which is further verified by molecular dynamics simulations. This work provides direct evidence to the understanding of irreversible melting with an unprecedented spatiotemporal resolution.
尽管经过了一个多世纪的研究，但在纳米尺度下，固体熔化背后的基本机制仍然难以捉摸。强飞秒激光脉冲辐照材料时产生的超快现象，重新唤起了人们对揭示令人困惑的熔化转变过程的兴趣。然而，由于难以对经历超快且不可逆转变的材料内部结构进行成像，各种微观模型的直接实验验证受到限制。在此，我们通过使用X射线自由电子激光脉冲的时间分辨单次衍射成像克服了这一挑战。单个金纳米颗粒的图像显示，表面存在非均匀熔化，随后颗粒内部深处出现密度波动，且这种波动与泵浦激光的偏振方向相关。对这种方向性的观察将非热电子激发与热晶格熔化联系起来，分子动力学模拟进一步证实了这一点。这项工作以前所未有的时空分辨率为理解不可逆熔化提供了直接证据。
