Synthesis of high-entropy alloy nanoparticles by laser ablation in liquids: scalability and monodispersity control by beam shaping

高熵合金（HEA）的研究活动开始不到十年，主要集中在块状材料上，忽视了HEA纳米颗粒（NPs）。这主要是由于缺乏一种完全建立的、可扩展的和直接的合成这种NPs的方法。液体脉冲激光烧蚀（PLAL）已被证明是一种简单、灵活和高效的合成各种材料（包括合金NPs）胶体NPs的技术。PLAL方法可以合成稳定的胶体合金NPs，而无需使用任何配体进行稳定，并且具有其他合成方法难以实现的性能。特别是，PLAL能够合成元素分布均匀、成分可控的二元合金NPs，这是（湿）化学合成方法由于元素偏析而难以完成的任务。此外，最近还证明了扩大PLAL生产与工业应用有关的以克每小时生产率为单位的NPs的可能性。然而，即使对于二元合金，在PLAL下形成NP的机制仍然知之甚少，而对于许多应用来说，单分散NP的产生仍然是一个相当悬而未决的问题。对于多组分HEA NPs，其形成机制更为复杂且几乎未被探索，控制NP的大小以及高NP生产率是一个真正的挑战。控制和调整激光烧蚀过程的最有效和最灵活的方法之一是在空间和时间上对所利用的激光脉冲进行整形。双脉冲辐照技术在材料加工、快速离子生成、纳米结构设计和纳米粒子生成等方面具有重要的应用价值。利用各种非高斯激光束的实验也证明了空间脉冲整形可以提高激光加工的质量，并且可以有效地控制激光产生的纳米颗粒的尺寸分布。从本质上讲，在PLAL中对这些技术的深入研究代表了一种实现尺寸分布控制的有趣方法。在本项目中，我们提出了在液体中激光烧蚀制备HEA纳米颗粒的综合研究。重点讨论了激光脉冲整形对纳米胶体颗粒尺寸和产率的影响。将研究使用平顶脉冲和甜甜圈形脉冲以及使用两个脉冲之间具有可变延迟的辐照制度。将揭示多组分材料在皮秒激光烧蚀下在液体中形成纳米颗粒的机理，并有望找到对纳米颗粒尺寸和产率的有效控制。将研究扩大已发现的体系以提高合成效率和生产每小时克HEA纳米粒子的可能性。

The research activities on high-entropy alloys (HEA) started less than ten years ago and focused primarily on bulk materials, ignoring HEA nanoparticles (NPs). This is mainly due to the absence of a fully established, scalable and straightforward synthesis method for such NPs. Pulsed laser ablation in liquids (PLAL) has proven to be a simple, flexible and efficient technique for the synthesis of colloidal NPs of various materials including alloy NPs. The PLAL method allows synthesizing stable colloidal alloy NPs without the use of any ligands for stabilization and with properties difficult to achieve by other synthesis methods. In particular, PLAL is capable of synthesizing binary alloy NPs with a homogeneous elemental distribution of controllable composition, a difficult task for (wet) chemical synthesis methods due to element segregation. Besides, the possibility of scaling up PLAL to produce NPs in gram per hour productivities relevant for industrial applications was recently demonstrated.However, even for binary alloys, the mechanisms of NP formation under PLAL are still poorly understood and the generation of monodisperse NPs, desirable for many applications, is a rather unsettled problem. For multicomponent HEA NPs, whose formation mechanisms are more complicated and virtually unexplored, controlling the NP size, as well as a high NP productivity, are a real challenge. One of the most efficient and flexible way to control and tune the laser ablation process is shaping of the utilized laser pulses, both in space and time. The use of double-pulse irradiation with two ultrashort pulses separated in time was demonstrated to be advantageous for material processing, fast ion generation, designing of nanoscale structures, and nanoparticle generation. It was also demonstrated with various non-Gaussian laser beams that spatial pulse shaping improves the quality of laser machining and enables efficient control over the size distribution of laser-generated nanoparticles. Essentially an in depth study of these techniques in PLAL represents an interesting approach for achieving a size distribution control. In this project, we propose a comprehensive study on the generation of HEA nanoparticle by laser ablation in liquids. The main attention will be given to the effects of laser pulse shaping on the size and the yield of the produced colloidal nanoparticles. Irradiation regimes using flat-top and doughnut-shaped pulses as well as using two pulses with a variable delay between them will be investigated. The mechanisms of nanoparticle formation under picosecond laser ablation of multicomponent materials in liquids will be revealed and efficient control over the nanoparticle size and yield is expected to be found. The possibilities of scaling-up the found regimes to increase the synthesis efficiency and to produce HEA nanoparticles in gram per hour amounts will be investigated.