We present ensembles of surface-ordered nanoparticle arrangements, which are formed by template-assisted self-assembly of monodisperse, protein-coated gold nanoparticles in wrinkle templates. Centimeter-squared areas of highly regular, linear assemblies with tunable line width are fabricated and their extinction cross sections can be characterized by conventional UV/vis/NIR spectroscopy. Modeling based on electrodynamic simulations shows a clear signature of strong plasmonic coupling with an interparticle spacing of 1–2 nm. We find evidence for well-defined plasmonic modes of quasi-infinite chains, such as resonance splitting and multiple radiant modes. Beyond elementary simulations on the individual chain level, we introduce an advanced model, which considers the chain length distribution as well as disorder. The step toward macroscopic sample areas not only opens perspectives for a range of applications in sensing, plasmonic light harvesting, surface enhanced spectroscopy, and information technology but also eases the investigation of hybridization and metamaterial effects fundamentally.

We present a plasmon-active hybrid nanomaterial design with electrochemical tunability of the localized surface plasmon resonances. The plasmonic-active nanostructures are composed of silver nanocube aggregates embedded into an electrochromic polymer coating on an indium tin oxide electrode with the nanocube aggregation controlled by the surface pressure. Such polymer-nanocube hybrid nanomaterials demonstrated unique tunable plasmonic behavior under an applied electrochemical potential. A significant reversible experimental peak shift of 22 nm at an electrical potential of 200 mV has been achieved in these measurements. Finite-difference time-domain (FDTD) simulations show that, under full oxidation potential, a maximal spectral shift of ca. 80 nm can be potentially achieved, which corresponds to a high sensitivity of 178 nm per refractive index unit. Furthermore, FDTD modeling suggests that the electrochemically controlled tunability of plasmonic peaks is caused by reversible changes in the refractive index of the electrochromic polymer coating caused by oxidation or reduction reactions under external electrical potential. Consequently, we define the orthogonal plasmonic resonance shift as a shift that is orthogonal to the redox process responsible for the refractive index change. On the basis of these results, we suggest that the combination of anisotropic nanostructures and electrochromic matrix has the potential to reversibly electrically tune plasmonic resonances over the full visible spectrum.

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Metallic nanostructures exhibit strong interactions with electromagnetic radiation, known as the localized surface plasmon resonance. In recent years, there is significant interest and growth in the area of coupled metallic nanostructures. In such assemblies, short‐ and long‐range coupling effects can be tailored and emergent properties, e.g., metamaterial effects, can be realized. The term “plasmonic metasurfaces” is used for this novel class of assemblies deposited on planar surfaces. Herein, the focus is on plasmonic metasurfaces formed from colloidal particles. These are formed by self‐assembly and can meet the demands of low‐cost manufacturing of large‐area, flexible, and ultrathin devices. The advances in high optical quality of the colloidal building blocks and methods for controlling their self‐assembly on surfaces will lead to novel functional devices for dynamic light modulators, pulse sharpening, subwavelength imaging, sensing, and quantum devices. This progress report focuses on predicting optical properties of single colloidal building blocks and their assemblies, wet‐chemical synthesis, and directed self‐assembly of colloidal particles. The report concludes with a discussion of the perspectives toward expanding the colloidal plasmonic metasurfaces concept by integrating them with quantum emitters (gain materials) or mechanically responsive structures.

Colloidal particles show interaction with electromagnetic radiation at optical frequencies. At the same time clever colloid design and functionalization concepts allow for versatile particle assembly providing monolayers of macroscopic dimensions. This has led to a significant interest in assembled colloidal structures for light harvesting in photovoltaic devices. In particular thin-film solar cells suffer from weak absorption of incoming photons. Consequently light management using assembled colloidal structures becomes vital for enhancing the efficiency of a given device. This review aims at giving an overview of recent developments in colloid synthesis, functionalization and assembly with a focus on light management structures in photovoltaics. We distinguish between optical effects related to the single particle properties as well as collective optical effects, which originate from the assembled structures.Colloidal templating approaches open yet another dimension for controlling the interaction with light. We focus in this respect on structured electrodes that have received much attention due to their dual functionality as light harvesting systems and conductive electrodes and highlight the impact of interparticle spacing for templating.

In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC61BM and PTB7:PC71BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies.

We present a versatile approach to produce macroscopic, substrate-supported arrays of plasmonic nanoparticles with well-defined interparticle spacing and a continuous particle size gradient. The arrays thus present a “plasmonic library” of locally noncoupling plasmonic particles of different sizes, which can serve as a platform for future combinatorial screening of size effects. The structures were prepared by substrate assembly of gold-core/poly(N-isopropylacrylamide)-shell particles and subsequent post-modification. Coupling of the localized surface plasmon resonance (LSPR) could be avoided since the polymer shell separates the encapsulated gold cores. To produce a particle array with a broad range of well-defined but laterally distinguishable particle sizes, the substrate was dip-coated in a growth solution, which resulted in an overgrowth of the gold cores controlled by the local exposure time. The kinetics was quantitatively analyzed and found to be diffusion rate controlled, allowing for precise tuning of particle size by adjusting the withdrawal speed. We determined the kinetics of the overgrowth process, investigated the LSPRs along the gradient by UV–vis extinction spectroscopy, and compared the spectroscopic results to the predictions from Mie theory, indicating the absence of local interparticle coupling. We finally discuss potential applications of these substrate-supported plasmonic particle libraries and perspectives toward extending the concept from size to composition variation and screening of plasmonic coupling effects.

For many photonic applications, it is important to confine light of a specific wavelength at a certain volume of interest at low losses. So far, it is only possible to use the polarized light perpendicular to the solid grid lines to excite waveguide–plasmon polaritons in a waveguide-supported hybrid structure. In our work, we use a plasmonic grating fabricated by colloidal self-assembly and an ultrathin injection layer to guide the resonant modes selectively. We use gold nanoparticles self-assembled in a linear template on a titanium dioxide (TiO2) layer to study the dispersion relation with conventional ultraviolet–visible–near-infrared spectroscopic methods. Supported with finite-difference in time-domain simulations, we identify the optical band gaps as hybridized modes: plasmonic and photonic resonances. Compared to metallic grids, the observation range of hybridized guided modes can now be extended to modes along the nanoparticle chain lines. With future applications in energy conversion and optical filters employing these cost-efficient and upscalable directed self-assembly methods, we discuss also the application in refractive index sensing of the particle-based hybridized guided modes.