Van der Waals versus dipolar forces controlling mesoscopic organizations of magnetic nanocrystals
Van der Waals versus dipolar forces controlling mesoscopic organizations of magnetic nanocrystals
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DOI：
10.1038/nmat1054
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发表时间：
2004-02-01
期刊：
影响因子：
41.2
通讯作者：
Pileni, MP
中科院分区：
文献类型：
--
作者：
Lalatonne, Y;Richardi, J;Pileni, MP
许多物理和化学系统的结构、热力学和动力学是由短程各向同性和长程各向异性力相互作用决定的。分散在溶液中的磁性纳米粒子是研究这种相互作用的理想模型系统，因为它们同时受到各向同性范德华力和各向异性偶极力的作用。实验表明，当纳米颗粒溶液在磁场作用下蒸发时，磁铁矿纳米晶体结构从链状结构突然转变为无序结构。布朗动力学模拟用范德华相互作用强度随粒子接触距离的变化来解释这一点，粒子接触距离由包裹粒子的分子的长度来调节。磁铁矿颗粒之间的弱偶极-偶极相互作用通常不足以导致在这里观察到的链的形成。然而，由于范德华相互作用，当纳米晶体接触距离足够短时，在蒸发过程中会形成纳米晶团簇。与单个粒子相比，这些团簇表现出更大的偶极矩，这解释了链状结构的形成。相反，当纳米晶接触距离过长时，不会发生纳米晶的聚集，而得到随机分布的磁铁矿纳米晶。
The structure, thermodynamics and dynamics in many physical and chemical systems are determined by interplay of short-range isotropic and long-range anisotropic forces. Magnetic nanoparticles dispersed in solution are ideal model systems to study this interplay, as they are subjected to both isotropic van der Waals and anisotropic dipolar forces. Here we show from experiment an abrupt transition of maghemite nanocrystal organization from chain-like to random structures when nanoparticle solutions are evaporated under a magnetic field. This is explained by brownian dynamics simulations in terms of a variation of the strength of van der Waals interactions with the particle contact distance, which is tuned by the length of the molecules coating the particles. The weak dipole-dipole interactions between the maghemite particles are usually not sufficient to result in the chain formation observed here. However, due to the van der Waals interactions, when the nanocrystal contact distance is short enough, clusters of nanocrystals are formed during the evaporation process. These clusters exhibit large dipole moments compared with a single particle, which explains the formation of chain-like structures. Conversely, when the nanocrystal contact distance is too long, no nanocrystal aggregation occurs, and a random distribution of maghemite nanocrystals is obtained.