Self-Assembly of Janus Patchy Particles: A Dissipative Particle Dynamics Simulation Study

人们对胶体粒子的自组装越来越感兴趣，因为它可以制造纳米晶体，为磁性、光伏、传感、生物医学和催化应用提供独特的材料性能。具有方向依赖相互作用的粒子（斑状粒子，Janus粒子）特别有趣，因为它们允许更广泛的自组装结构。使用它们的一个主要障碍是对单个粒子的结构和它们自组装成的结构之间关系的理解有限。分子模拟是阐明自组装机制的有力工具。虽然大多数现有的胶体片状颗粒自组装的模拟研究使用简化的隐式溶剂模型，但我们在资助期1开发了一个更详细的溶液中三块Janus颗粒模型。Janus粒子被建模为交联聚苯乙烯球体，在两极有烷基补丁（吸引），在赤道区域带负电荷（排斥）。模型中包括显性溶剂、反离子和在其表面发生二维结晶的衬底。用多体耗散粒子动力学方法计算了流体力学和多体相互作用。为了样品的成核和相变，发展了一种新的元动力学变体，并将其应用于Kagome晶格和六边形晶格的形成。在第二资助期，我们将使用我们的现实模型进一步研究三块Janus粒子的相平衡和组装过程。在基本相图已知的情况下，我们将通过允许模拟更自由地搜索替代装配路径：在步骤1中，过渡路径采样将用于查找（或排除）已知相之间的其他装配路线。在第二步中，无偏差模拟将探索其他有序结构的可能存在。在这两个步骤中，我们将研究粒子结构（吸引和排斥区域的大小和形状）对组装结构和它们之间的路径的影响。同时，我们将通过自由的、无偏的模拟来探索晶核的演化。为此，我们将在粒子溶液中播种不同大小和形状的晶核，并观察它们是否以及通过何种机制生长或收缩。此外，我们将扩展我们的研究三维自组装。我们将遵循为二维装配建立的路线，从有偏差的模拟（元动力学）到过渡路径采样和自由模拟。重点将放在开放的三维结构上，即任何不紧密排列的结构，如钙钛矿或焦绿石晶格。我们将研究哪种粒子结构有利于有序的开放结构以及它们的组装途径是什么。

There is a growing interest in the self-assembly of colloidal particles toward the fabrication of nanocrystals that offer unique material properties for magnetic, photovoltaic, sensing, biomedical, and catalytic applications. Particles with orientation-dependent interactions (patchy particles, Janus particles) are of special interest, as they allow a wider variety of self-assembled structures. One major stumbling block for their use is the limited understanding of the relation between the architecture of the individual particles and the structures, into which they self-assemble. Molecular simulations are a powerful tool for elucidating the mechanisms of self-assembly. While most existing simulation studies of the self-assembly of colloidal patchy particles use simplified, implicit-solvent models, we have developed in funding period 1 a much more detailed model for triblock Janus particles in solution. The Janus particles were modeled as crosslinked polystyrene spheres, capped at the poles with alkyl patches (attractive) and in the equatorial region with negative charges (repulsive). Explicit solvent, counterions and a substrate, on whose surface the two-dimensional crystallization takes place, were included in the model. Hydrodynamic and many-body interactions were accounted for by the many-body dissipative particle dynamics method. To sample nucleation and phase transitions, a new variant of metadynamics has been developed and applied to the formation of Kagome and hexagonal lattices. In funding period 2, we will use our realistic model to further study the phase equilibria and assembly processes for triblock Janus particles. With the basic phase diagram known, we will search for alternative assembly pathways by allowing the simulations successively more freedom: In step 1, transition-path sampling will be used to find (or rule out) other assembly routes between the known phases. In step 2, bias-free simulations will explore the possible existence of other ordered structures. In both steps, we will investigate the influence of the particle architecture (size and shape of attractive and repulsive regions) on the assembled structures and the pathways between them. Simultaneously, we will explore the evolution of crystal nuclei by free, unbiased simulations. To this end, we will seed particle solutions with crystal nuclei of different sizes and shapes and observe, whether and by which mechanism they grow or shrink. Moreover, we will extend our investigations to three-dimensional self-assembly. We will follow the route established for two-dimensional assembly and proceed from biased simulations (metadynamics) to transition-path sampling and free simulations. A strong focus will be on open three-dimensional structures, i.e. any structure that is not close-packed, such as perovskite or pyrochlore lattices. We will investigate which particle architecture favours ordered open structures and what are the pathways for their assembly.