Abstract
Using a three-phase system, centimeter-scale monolayer gold nanoparticle (Au NP) films have been prepared that have long-range order and hydrophobic ligands. The system contains an interface between an aqueous phase containing Au NPs and an oil phase containing one of various types of amine ligands, and a water/air interface. As the Au NPs diffuse to the water/oil interface, ligand exchange takes place which temporarily traps them at the water/oil interface. The ligand-exchanged particles then spontaneously migrate to the air/water interface, where they self-assemble, forming a monolayer under certain conditions. The spontaneous formation of the NP film at the air/water interface was due to the minimization of the system Helmholtz free energy. However, the extent of surface functionalization was dictated by kinetics. This decouples interfacial ligand exchange from interfacial self-assembly, while maintaining the simplicity of a single system. The interparticle center-to-center distance was dictated by the amine ligand length. The Au NP monolayers exhibit tunable surface plasma resonance and excellent spatial homogeneity, which is useful for surface-enhanced Raman scattering. The “air/water/oil” self-assembly method developed here not only benefits the fundamental understanding of NP ligand conformations, but is also applicable to the manufacture of plasmonic nanoparticle devices with precisely designed optical properties.
Highlights
In the past decade, significant efforts have been dedicated to NP synthesis[12]
The scanning electron microscopy (SEM) image at low magnification (e.g. 10,000×), without resolving individual Au NPs, exhibits moiré interference patterns, indicating the Au NP film has long range HCP “crystal” order[31]. We propose that these moiré patterns are due to interference between the incident electron beam and that reflecting off the silicon substrate, much like an optical moiré pattern can be formed by a single pattern held near a mirror
A fast, efficient self-assembly method has been developed whereby aqueous nanoparticles form ordered Au NP monolayers of long-range ordering in an air/water/oil three-phase system
Summary
Significant efforts have been dedicated to NP synthesis[12]. different colloidal synthetic routes have been developed, leading to an accurate control over NP size[13], shape[14], and surface chemistry[15]. The Langmuir-Blodgett (LB) technique allows large-scale, two-dimensional (2D) NP monolayers to be transferred from an interface[19] One requirement of this strategy is to float hydrophobic ligand-capped NPs at the air/water interface to form monolayer NP films[21]. At the water/air interface, additional rearrangement occurs resulting in an ordered centimeter-scale film The beauty of this technique is that the ligand exchange reaction and superlattice self-assembly are largely decoupled due to their occurring at separate interfaces. Unlike our previous publication[27], the monolayer structures that form are much less sensitive to the ligand concentration in the oil phase This is due to the fact that the extent of ligand exchange is controlled by kinetics (the particles appear to reside at the water/oil interface for a mere fraction of a second), whereas monolayer formation at the water/air interface achieves equilibrium. These findings are promising in opening up a new avenue to fabricate photonic and plasmonic devices based on NP superlattices in a scaled-up manner
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