Assembly of reconfigurable one-dimensional colloidal superlattices due to a synergy of fundamental nanoscale forces

Abstract

We report that triangular gold nanoprisms in the presence of attractive depletion forces and repulsive electrostatic forces assemble into equilibrium one-dimensional lamellar crystals in solution with interparticle spacings greater than four times the thickness of the nanoprisms. Experimental and theoretical studies reveal that the anomalously large d spacings of the lamellar superlattices are due to a balance between depletion and electrostatic interactions, both of which arise from the surfactant cetyltrimethylammonium bromide. The effects of surfactant concentration, temperature, ionic strength of the solution, and prism edge length on the lattice parameters have been investigated and provide a variety of tools for in situ modulation of these colloidal superstructures. Additionally, we demonstrate a purification procedure based on our observations that can be used to efficiently separate triangular nanoprisms from spherical nanoparticles formed concomitantly during their synthesis.anisotropic | tunable | small angle X-ray scattering | depletion interaction T he ability to form ensembles of inorganic nanoparticles with a high degree of control has become one of the main areas of focus in nanoscience research (1). This interest stems from the fact that nanocrystal superlattices often exhibit electronic (2), optical (3), and magnetic (4) properties that are distinct from both the corresponding individual particles and the bulk solid as a result of the interactions between the excitons, surface plasmons, or magnetic moments of the assembled particles (5). Superlattices composed of spherical building blocks have been extensively studied, and researchers now have the ability to synthesize a wide variety of structures (6). However, as new techniques are developed to synthesize high-quality anisotropic nanoparticles with new physical properties that cannot be obtained with spheres alone, researchers are increasingly interested in the rich assembly behavior of particles with reduced symmetry (7-9). Indeed, periodic arrays of these anisotropic building blocks have been shown to possess unique collective properties with applications in various fields including plasmonics (10) and photonics (11). However, to take full advantage of these collective properties, it is necessary to understand the relationship between the architectural parameters of the ensemble and the emergent physical properties. For this purpose, it is crucial to be able to "engineer" the various interactions that exist between nanoparticle building blocks to produce a desired structure (12, 13). The assembly of nanocrystals into ordered arrays can be induced via the manipulation of interparticle interactions including van der Waals (14), electrostatic (15), entropic (16-21), and through highly specific biological interactions (22-24). Herein, we report the assembly of colloidal triangular gold nanoprisms protected by a cetyltrimethylammonium bromide (CTAB) bilayer in a solution of CTAB micelles into highly ordered 1D crystals with unexpected structural f...

Using nanoscale and mesoscale anisotropy to engineer the optical response of three-dimensional plasmonic metamaterials

“…1a depicts a variety of solution synthesized noble metal HAR-NPs including nanorods 47,48 , cylindrical disks 49 and triangular nanoprisms 50,51 . Because of this interest, a variety of methods have been used to assemble HAR-NPs into arrays 12,14,15,17,19 . Importantly, all of these methods orient the NPs in at least one dimension, allowing for a polarizationdependent optical response.…”

“…This so-called localized surface plasmon resonance (LSPR) excitation is highly sensitive to the size, shape, composition and local environment of the nanostructure 10 . In addition, shape anisotropy provides a powerful handle for the control of plasmonic electric field enhancement 10 , polarization-dependent response in metamaterial biosensors 11 and for the directed assembly of noble metal nanoparticles [12][13][14][15][16][17][18][19] . By arranging anisotropic plasmonic nanoparticles into complex ordered structures, the resulting optical properties can be intelligently designed to vary with polarization, orientation and spectral location, a feat that cannot be reproduced with homogeneous materials.…”

A Directional Entropic Force Approach to Assemble Anisotropic Nanoparticles into Superlattices

“…[9][10][11] Herein, we introduce a directional entropic force approach (DEFA) for controlling the assembly of anisotropic nanoparticles into crystalline lattices. The method relies on surfactant micelle-induced depletion interactions [12] to assemble anisotropic gold nanoparticles into reconfigurable, nonclose-packed (open) superlattices in solution. The anisotropic nanoparticles align along their flat facets to maximize entropy, and therefore minimize the free energy of the system, leading to assemblies with long-range order.…”

Dynamically self-assembled silver nanoparticles as a thermally tunable metamaterial

“…DSA would unlock the opportunities of an active engineering of aggregate structures and associated properties as lately evidenced by theoretical modelling 29,30 . Recently, a handful of DSA methods have been reported [31][32][33][34] , but most of these techniques rely unfortunately on solvated environments, which result in very low NP-filling fractions, or are limited to twodimensional systems 35,36 . Therefore, the application of these methods in metamaterials research is limited.…”