Hierarchically self-assembled hexagonal, honeycomb, and kagome superlattices of binary 1D nanoparticles

The packing behavior of 1D nanoparticles of different diameters into a hexagonally-packed cylindrical micellar system was investigated. By tuning particle diameter and number ratios, highly ordered binary 1D nanoparticle superlattices of two different symmetries are obtained. The maximization of free volume entropy is considered as the main driving force for the formation of superlattices. Our approach may be applicable for fabricating binary superlattices of metallic, semiconducting, or magnetic 1D nanoparticles.

Article  |  Spring 2018

Synthesis of binary or multicomponent nanoparticle superlattices, which may exhibit new emerging properties through synergetic coupling between different types of nanoparticles, has attracted attention for a broad spectrum of potential applications as well as its scientific merit.

The fabrication of binary nanoparticle superlattices with different symmetries (e.g., NaCl, AlB2, AuCu3, and NaZn13-types, mimicking the atomic crystals) has been intensively investigated using an interplay of entropy and van der Waals, electrostatic, and other interactions. However, these approaches have been limited to the spherical nanoparticle systems, and the synthesis of binary 1D nanoparticles superlattices are very challenging.

Theoretical and experimental studies have shown that the binary colloidal mixtures of 1D particles with different diameters exhibit an entropically driven de-mixing transition, separating the two types of 1D particles into two co-existing subphases, respectively. This inherently prevents the cooperative self-assembling of two types of 1D nanoparticles into highly-ordered superlattices. To achieve highly-ordered binary superlattices of 1D nanoparticles, therefore, new approaches that can overcome the phase separation of two types of 1D nanoparticles should be designed and explored.

The image on the top shows the self-assembly process of binary 1D nanoparticle superlattice by replacing one 1D nanoparticle in hexagonal lattice with other 1D nanoparticle.

 

The new approach designed by Professor Sung-Min Choi’s group is to investigate how 1D nanoparticles are self-assembled when they are added into a hexagonally-packed cylindrical-micellar system, depending on the diameter and number ratios between the 1D nanoparticles and cylindrical micelles (Figure 1). Here, the hexagonally packed cylindrical micellar system can be considered as a pre-formed highly-ordered assembly of 1D nanoparticles, which allows avoiding the phase separation.

 

 

To understand the packing behavior of 1D nanoparticles and cylindrical micelles, Professor Sung-Min Choi’s group performed small angle x-ray and neutron scattering measurements (SAXS and SANS, respectively, Figure 2). SAXS and contrast-matched SANS measurements show that when the 1D nanoparticles are added to a hexagonally packed cylindrical micellar system, the two types of new hierarchical superlattices without the phase separation are formed depending on the diameter and number ratios between the 1D nanoparticles and cylindrical micelles. The hexagonal arrays of 1D nanoparticles are embedded in a honeycomb lattice of cylindrical micelles (AB2 type binary superlattice) or a kagome lattice of cylindrical micelles (AB3 type binary superlattice) (Figure 3). It is understood that the formation of AB2 and AB3 type binary superlattice is driven by free volume entropy maximization of the system.

This is the first demonstration that binary 1D nanoparticle superlattices with different symmetries can be formed by controlling the diameter and number ratios between 1D nanoparticles of two different diameters. Furthermore, this new approach designed in the present study may be applicable for the synthesis of highly-ordered metallic, semiconducting, or magnetic 1D nanoparticle binary superlattices of different symmetries and functionalities, which are of great current interests for possible applications in photovoltaics, catalysts, and molecular sensing.

Reference: Sung-Hwan Lim, Taehoon Lee, Younghoon Oh, Theyencheri Narayanan, Bong June Sung, and Sung-Min Choi, “Hierarchically self-assembled hexagonal honeycomb and kagome superlattices of binary 1D colloids” Nature Communications, 8, 360 (2017)

Additional links for more information:
https://www.nature.com/articles/s41467-017-00512-9
http://neutron.kaist.ac.kr