Field-Directed Self-Assembly of Mutually Polarizable Nanoparticles.
Field-Directed Self-Assembly of Mutually Polarizable Nanoparticles.
复制标题
相互极化纳米粒子的场定向自组装。
作者：
Z. Sherman;Dipanjan Ghosh;J. Swan
介电和顺磁性纳米粒子的定向组装可用于合成响应于外部施加的电场或磁场而发生电致变色的多种功能材料。然而，能够预测自组装状态的理论是缺乏的。在拟议的工作中，我们开发了一个完整的热力学描述这样的组装球形纳米粒子。我们展示了这些类型的粒子的一个重要物理特征，相互极化，如何塑造自由能景观，并对自组装态的性质产生了非常强烈的影响。纳米粒子间的相互极化模型需要求解粒子偶极矩的多体问题。通常，通过忽略相互极化并假设集中分散体中的每个粒子获得与单个孤立粒子相同的偶极矩，可以避免这种计算上昂贵的任务。虽然有效的限制小的介电或渗透性的颗粒和溶剂之间的对比，这种恒定的偶极子假设导致定性不正确的预测共存相平衡在大的介电或渗透性的对比。正确地考虑相互极化，使热力学理论，描述的平衡相图的极化分散体的实验可控变量。我们的理论预测与我们在这些分散体的动态模拟中观察到的相行为以及在场导向结构转变的实验中观察到的相行为一致。与恒定偶极子模型的预测相反，我们发现具有不同介电常数或磁导率的粒子的分散体表现出质的不同的相行为。这个新的模型还预测了一个共晶点的存在，在该共晶点处，两个结晶相和纳米颗粒的无序相同时共存。
Directed assembly of dielectric and paramagnetic nanoparticles can be used to synthesize diverse functional materials that polarize in response to an externally applied electric or magnetic field. However, theories capable of predicting the self-assembled states are lacking. In the proposed work, we develop a complete thermodynamic description of such assemblies for spherical nanoparticles. We show how an important physical feature of these types of particles, mutual polarization, sculpts the free energy landscape and has a remarkably strong influence on the nature of the self-assembled states. Modeling the mutual polarization among nanoparticles requires solving a many-bodied problem for the particle dipole moments. Typically, this computationally expensive task is avoided by neglecting mutual polarization and assuming that each particle in a concentrated dispersion acquires the same dipole moment as a single, isolated particle. Although valid in the limit of small dielectric or permeability contrasts between particles and solvent, this constant dipole assumption leads to qualitatively incorrect predictions for coexisting phases in equilibrium at large dielectric or permeability contrasts. Correctly accounting for mutual polarization enables a thermodynamic theory that describes the equilibrium phase diagram of polarizable dispersions in terms of experimentally controllable variables. Our theoretical predictions agree with the phase behavior we observe in dynamic simulations of these dispersions as well as that in experiments of field-directed structural transitions. In contrast to predictions of a constant dipole model, we find that dispersions of particles with different dielectric constants or magnetic permeabilities exhibit qualitatively different phase behavior. This new model also predicts the existence of a eutectic point at which two crystalline phases and a disordered phase of nanoparticles all simultaneously coexist.