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

合作研究：纳米颗粒熔化附近的超快激光驱动相变

基本信息

批准号：
1708486
负责人：
Leonid Zhigilei
金额：
$ 9万
依托单位：
University of Virginia Main Campus
依托单位国家：
美国
项目类别：
Continuing Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-08-01 至 2021-07-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1708486&HistoricalAwards=false
关键词：
Collaborative Research Ultrafast Laser Driven

项目摘要

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.

非技术摘要虽然固体的熔化是最普遍的相变，但其原子机制仍不清楚。实验观察表明，熔化在表面和扩展缺陷处成核。除了其技术应用之外，纳米颗粒还为研究熔融相变提供了有趣的机会，因为它们的特性可以由其表面决定。由于转变的超快性质，熔化的研究变得复杂。在该项目中，超快电子衍射用于解析用短激光脉冲加热的纳米颗粒的原子动力学。电子衍射图描绘了纳米颗粒熔化时的结构演变。使用原子键的准确描述进行建模模拟电子衍射结果。提高对纳米粒子对激光激发响应的理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。学生和本科生积极参与这项研究，并为高中生和公众的推广活动做出贡献。技术摘要该项目的主要目标是了解金属纳米颗粒快速相变直至熔化的机制飞秒激光照射。超快电子衍射（UED）用于绘制激光照射纳米颗粒的结构动力学图。分子动力学 (MD) 模拟对纳米粒子中的激光能量耦合和分配进行了真实的描述，补充了 UED 研究，并提供了对激光诱导相变的原子级见解。由于熔化和无扩散固-固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瞬态。将研究特征明确、尺寸选定的金属纳米颗粒（In、Pb 和 Bi）的超快激光熔化。这些纳米颗粒的低蒸气压使得能够在其熔点附近进行结构研究，而不会影响其尺寸。该实验是在超高真空中在具有弱表面范德华力的基底上制造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加热速率的依赖性；电子激发效应；电子声子耦合；过热和过冷的极限；以及界面和基材对熔化和固态转变的影响。 MD 模拟包括激发电子的热压力的影响，该影响是根据从头计算的预测和 UED 结果进行参数化的。