Collaborative Research: Ultrafast Laser-Driven Phase Transitions in Nanoparticles near their Melting

尽管固体的熔化是最普遍的相变，但它的原子机制仍不是很清楚。实验观察表明，熔化在表面和扩展的缺陷处形核。除了它们的技术应用，纳米颗粒还为研究熔融相变提供了有趣的机会，因为它们的性质可以由它们的表面决定。熔化的研究因转变的超快性质而变得复杂。在这个项目中，超快电子衍射被用来解决纳米粒子在短激光脉冲加热下的原子动力学。电子衍射图描绘了纳米粒子在熔化时的结构演变。使用原子成键的精确描述进行建模，模拟了电子衍射结果。提高对纳米粒子对激光激发响应的了解，可能会对生物医学、纳米电子学和传感应用的发展产生重大影响。一名博士后、两名博士生和本科生积极参与了这项研究，并为面向高中生和普通公众的推广活动做出了贡献。技术摘要本项目的主要目标是了解飞秒激光照射下金属纳米颗粒快速相变直至熔化的机制。超快电子衍射(UED)被用来描绘激光辐照的纳米粒子的结构动力学。分子动力学(MD)模拟真实地描述了激光能量在纳米颗粒中的耦合和分配，补充了UED的研究，并提供了对激光诱导相变的原子水平的见解。由于熔化和无扩散的固体-固体相变可以在皮秒时间尺度上发生，因此需要超快测量来探测沿转变路径的暂态。超快激光熔化良好的、尺寸选择的金属纳米粒子(In、Pb和Bi)将被研究。这些纳米粒子的低蒸汽压使得能够在不影响其大小的情况下研究其熔点附近的结构。这些实验是在超高真空中制备在具有弱表面范德华力的衬底上的纳米颗粒上进行的。这些实验旨在研究相变发生的结构途径及其对升温速率的依赖；电子激发效应；电子-声子耦合；过热和过冷的极限；以及界面和衬底对熔化和固态转变的影响。分子动力学模拟包括激发电子热压的影响，它是基于从头计算的预测和UED结果而被参数化的。

Non-Technical AbstractAlthough the melting of solids is the most ubiquitous of phase transitions, its atomistic mechanism is still not well understood. Experimental observations show that melting nucleates at surfaces and extended defects. Besides their technological applications, nanoparticles provide interesting opportunities to study the melting phase transition since their properties can be dominated by their surfaces. The study of melting is complicated by the ultrafast nature of the transition. In this project, ultrafast electron diffraction is used to resolve the atomistic dynamics for nanoparticles heated with a short laser pulse. The electron diffraction maps the structural evolution of the nanoparticles while they melt. Modeling using an accurate description of atomic bonding simulates the electron diffraction results. Improved understanding of nanoparticle response to the laser excitation is likely to make a strong impact on the development of biomedical, nano-electronics, and sensing applications. A post-doc, two Ph.D. students, and undergraduates are actively involved in this research and contribute to outreach activities to high school students and the general public.Technical AbstractThe main objective of this project is to understand the mechanisms of the rapid phase transitions in metal nanoparticles driven up to their melting by femtosecond laser irradiation. Ultrafast electron diffraction (UED) is used to map the structural dynamics of the laser-irradiated nanoparticles. Molecular dynamics (MD) simulations with a realistic description of the laser energy coupling and partitioning in the nanoparticles complements the UED studies and provide atomic-level insights into the laser-induced phase transitions. Since melting and diffusionless solid-solid phase transitions can occur on a picosecond time scale, ultrafast measurements are needed to probe transient states along the transition pathways. Ultrafast laser melting of well-characterized, size-selected metal nanoparticles (In, Pb, and Bi) will be studied. The low vapor pressure of these nanoparticles enables structural studies near their melting point without affecting their size. The experiments are conducted on nanoparticles fabricated in an ultrahigh vacuum on substrates with weak surface van der Waals forces. The experiments are designed to study the structural pathways through which the phase transitions occur and their dependence on heating rate; electronic excitation effects; electron-phonon coupling; limits of superheating and supercooling; and interface and substrate effects on melting and solid-state transitions. MD simulations include the effect of the thermal pressure from the excited electrons, which is parameterized based on the predictions of ab initio calculations and the UED results.