Synthesis of Homogeneous Plasmonic Nanostructures for Generating Uniform and Reproducible Photonic Environments

The preserved bosonic nature of surface plasmon polaritons from incident photons allows plasmonic nanomaterials to serve as effective photonic platforms. The strong light–matter interaction occurring at the surface concentrates light energy within a narrow region, thereby altering the local density of optical states. This modified photonic environment is typically expressed as near-field enhancement and improves the transition probability of nearby molecules or quantum emitters. However, despite the potential of plasmonic nanostructures to act as signal-transducers, issues in generating reproducible and consistent photonic responses with these platforms hinder their wide use for practical applications. As the parameters of plasmonic modulation are highly sensitive to even minor differences in surface morphology, a key origin of fluctuation in optical responses is the physical heterogeneity of the constituent nanostructures themselves. Therefore, although statistical and analytical techniques can obtain optically consistent signals from plasmonic nanostructures, the necessity of synthesizing uniform plasmonic nanostructures to achieve identical optical signals is becoming ever-more evident.

This Account focuses on synthetic approaches to ensure structural uniformity of plasmonic nanostructures, which in turn produces precisely tunable and reproducible optical signals. The discussion begins with methods to realize monodispersity in simple forms of plasmonic nanostructures whose geometric homogeneity can be acquired during the nucleation and growth stages of synthesis. As heterogeneous nanoparticles of different shapes and their corresponding nonuniform plasmonic responses originate primarily from differences in seed crystallinity, nucleation─the point at which the crystallinity of the nanoparticle emerges─plays a crucial role. By gaining an understanding of how reduction rate, surface ligands, and etching control the formation of seeds with different crystallographic structures, strategies for obtaining homogeneous seed structures are explained. This is followed by discussions about the growth stage where the final morphology of nanostructures is determined. The main task of this subsequent growth stage is the preservation of initial crystallinity by balancing the interplay between deposition and surface diffusion. Lastly, shape-selective purification strategies are investigated, particularly depletion-induced flocculation, as they remove impurities and improve overall yields.

Meanwhile, the advanced functionality of plasmonic nanostructures has been achieved through the synthesis of more complex architectures. Accordingly, this Account then discusses approaches using the monodisperse basic nanostructures as building blocks, either by acting as templates on which secondary structures can be grown or by their self-assembly into multidimensional superlattices. Template synthesis maintains the uniformity of initial structures while endowing nanostructures with enhanced functionality via compositional or structural complexity. In addition, the assembly of building blocks into superlattices can strengthen their optical responses via coupling between constituent plasmonic nanostructures. The degree of coupling is correlated to their interparticle distance and spatial orientation, reinforcing the necessity of uniform plasmonic nanostructures beyond the particle level. In closing, we examine how the uniformity of the synthesis is reflected in plasmonic platforms by investigating representative case studies, involving plasmonic scattering and photoluminescence signals. By evaluating plasmonic nanostructures based on their uniformity, we address the current need for standardization toward widespread use and imminent commercialization of plasmonic platforms and highlight their importance in practical applications.