Plasmon-Modulated Photoluminescence of Individual Gold Nanostructures

In this work, we performed a systematic study on the photoluminescence and scattering spectra of individual gold nanostructures that were lithographically defined. We identify the role of plasmons in photoluminescence as modulating the energy transfer between excited electrons and emitted photons. By comparing photoluminescence spectra with scattering spectra, we observed that the photoluminescence of individual gold nanostructures showed the same dependencies on shape, size, and plasmon coupling as the particle plasmon resonances. Our results provide conclusive evidence that the photoluminescence in gold nanostructures indeed occurs <i>via</i> radiative damping of plasmon resonances driven by excited electrons in the metal itself. Moreover, we provide new insight on the underlying mechanism based on our analysis of a reproducible blue shift of the photoluminescence peak (relative to the scattering peak) and observation of an incomplete depolarization of the photoluminescence

Image Dipole Method for the Beaming of Plasmons from Point Sources

A point dipole source models the electrical excitation of surface plasmon polaritons (SPPs), and is promising as a compact source for the beaming of plasmons in optical nanocircuits. However, conventional design approaches rely on iterative numerical simulations to achieve beaming from dipole sources, and they consume significant computing resources. Here, we introduce a universal semianalytical approach to solve the reflection of dipole-excited SPPs from the edge of a metal film by developing the image dipole method for SPPs. This approach achieves the directional propagation of SPPs through a multielement dipole array formed by a single dipole source and its reflections. Doing so mitigates the challenges in integrating and achieving coherent excitation among independently driven electrical sources. In addition, we provide design parameters for tuning the amplitude and phase of the image dipoles to engineer the directivity of SPP propagation. The configurations discussed can be readily implemented in the setting of tunnel-junction-based plasmon sources

Nanoplasmonics: Classical down to the Nanometer Scale

We push the fabrication limit of gold nanostructures to the exciting sub-nanometer regime, in which light–matter interactions have been anticipated to be strongly affected by the quantum nature of electrons in metals. Doing so allows us to (1) evaluate the validity of classical electrodynamics to describe plasmonic effects at this length scale and (2) witness the gradual (instead of sudden) evolution of plasmon modes when two gold nanoprisms are brought into contact. Using electron energy-loss spectroscopy and transmission electron microscope imaging, we investigated nanoprisms separated by gaps of only 0.5 nm and connected by conductive bridges as narrow as 3 nm. Good agreement of our experimental results with electromagnetic calculations and LC circuit models evidence the gradual evolution of the plasmonic resonances toward the quantum coupling regime. We demonstrate that down to the nanometer length scales investigated classical electrodynamics still holds, and a full quantum description of electrodynamics phenomena in such systems might be required only when smaller gaps of a few angstroms are considered. Our results show also the gradual onset of the charge-transfer plasmon mode and the evolution of the dipolar bright mode into a 3λ/2 mode as one literally bridges the gap between two gold nanoprisms

Directed Self-Assembly of sub-10 nm Particles: Role of Driving Forces and Template Geometry in Packing and Ordering

By comparing the magnitude of forces, a directed self-assembly mechanism has been suggested previously in which immersion capillary is the only driving force responsible for packing and ordering of nanoparticles, which occur only after the meniscus recedes. However, this mechanism is insufficient to explain vacancies formed by directed self-assembly at low particle concentrations. Utilizing experiments, and Monte Carlo and Brownian dynamics simulations, we developed a theoretical model based on a new proposed mechanism. In our proposed mechanism, the competing driving forces controlling the packing and ordering of sub-10 nm particles are (1) the repulsive component of the pair potential and (2) the attractive capillary forces, both of which apply at the contact line. The repulsive force arises from the high particle concentration, and the attractive force is caused by the surface tension at the contact line. Our theoretical model also indicates that the major part of packing and ordering of nanoparticles occurs before the meniscus recedes. Furthermore, utilizing our model, we are able to predict the various self-assembly configurations of particles as their size increases. These results lay out the interplay between driving forces during directed self-assembly, motivating a better template design now that we know the importance and the dominating driving forces in each regime of particle size

Large-Aperture and Grain-Boundary Engineering through Template-Assisted Metal Dewetting for Resonances in the Short Wave Infrared

We extend the fabrication method of template-assisted metal dewetting (TeAMeD) to create near-infrared resonant nanostructures in an Au film without the need for etching or lift-off. TeAMeD has previously been used to generate high aspect-ratio sub-10 nm apertures, but struggles to generate larger apertures (>100 nm). In this work, we introduce a method to create larger apertures using templates consisting of fin-like patterns with radial symmetry. We also report evidence of grain boundary engineering, through the template pinning effect. Our three-dimensional phase field model of TeAMeD predicts both the grain-boundary pinning and aperture opening effects that agree well with experiments. Combined with simulation design, TeAMeD can be established as a grain engineering platform, allowing grain shape and boundary position to be controlled. Variations of template motif produce larger grains and numerous possible outcomes, including suspended Au nanodisks and triangular apertures

Large Area Plasmonic Color Palettes with Expanded Gamut Using Colloidal Self-Assembly

Optical resonances in metallic nanostructures are promising in enabling high-resolution plasmonic color prints, color filters, and in rendering colors for plastic consumer products. However, nanostructure patterning approaches have relied on charged-particle beam lithography, with limited throughput. For the purpose of visually evaluating colors spanning a large parameter space, it is important to develop a rapid and cost-effective approach to patterning large areas. The speed at which the parameter space is explored experimentally needs to be comparable to the time it takes to run full electromagnetic simulations. Here, we used a bottom-up approach to cost-effectively create periodic nanostructures on centimeter-scale samples. Upon further processing, this approach produced more complex geometries, such as rings and domes, compared to the standard structures consisting of metal disks. By adjusting various geometric parameters, vivid colors with an expanded gamut were obtained

Anomalous Shift Behaviors in the Photoluminescence of Dolmen-Like Plasmonic Nanostructures

Localized surface plasmon resonance (LSPR) on metallic nanostructures is able to enhance photoluminescence (PL) emission significantly. However, the mechanism for anomalous blue-shifted peak of PL emission from metallic nanostructures, relative to the corresponding scattering spectra, is still unclear so far. In this paper, we presented the detailed investigations on both the Lorentz-like PL profile with blue-shifted peak and Fano-like one with almost unshifted dip, as observed on dolmen-like metallic nanostructures. Such anomalous PL emission profile is the product of the density of plasmon states (DoPS) with Lorentz-/Fano-like profile and the population distribution of the relaxed collective free electrons during relaxation. To be more specific, the fast relaxation process of these collective free electrons contributes to the PL shifting characteristics of both Lorentz-like and Fano-like emission profiles. We believed that our results provide a general solid foundation and guidance for analyzing and manipulating the physical processes of the PL emission from various plasmonic nanostructures

Template-Induced Structure Transition in Sub-10 nm Self-Assembling Nanoparticles

We report on the directed self-assembly of sub-10 nm gold nanoparticles confined within a template comprising channels of gradually varying widths. When the colloidal lattice parameter is mismatched with the channel width, the nanoparticles rearrange and break their natural close-packed ordering, transiting through a range of structural configurations according to the constraints imposed by the channel. While much work has been done in assembling ordered configurations, studies of the transition regime between ordered states have been limited to microparticles under applied compression. Here, with coordinated experiments and Monte Carlo simulations we show that particles transit through a more diverse set of self-assembled configurations than observed for compressed systems. The new insight from this work could lead to the control and design of complex self-assembled patterns other than periodic arrays of ordered particles

Directed Self-Assembly of Densely Packed Gold Nanoparticles

Directing the self-assembly of sub-10-nm nanoparticles has been challenging because of the simultaneous requirements to achieve a densely packed monolayer and rearrange nanoparticles to assemble within a template. We met both requirements by separating the processes into two steps by first forming a monolayer of gold nanoparticles on a suitable liquid subphase of anisole and then transferring it edgewise onto a silicon substrate with a prepatterned template comprising nanoposts and nanogratings. Doing so resulted in nanoparticles that assembled in commensuration with the template design while exhibiting appreciable template-induced strain. These dense arrays of nanostructures could either be directly applied or used as lithographic masks in applications for light collection, chemical sensing, and data storage

Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution

Directed self-assembly of nanoparticles (DSA-n) holds great potential for device miniaturization in providing patterning resolution and throughput that exceed existing lithographic capabilities. Although nanoparticles excel at assembling into regular close-packed arrays, actual devices on the other hand are often laid out in sparse and complex configurations. Hence, the deterministic positioning of single or few particles at specific positions with low defect density is imperative. Here, we report an approach of DSA-n that satisfies these requirements with less than 1% defect density over micrometer-scale areas and at technologically relevant sub-10 nm dimensions. This technique involves a simple and robust process where a solvent film containing sub-10 nm gold nanoparticles climbs against gravity to coat a prepatterned template. Particles are placed individually into nanoscale cavities, or between nanoposts arranged in varying degrees of geometric complexity. Brownian dynamics simulations suggest a mechanism in which the particles are pushed into the template by a nanomeniscus at the drying front. This process enables particle-based self-assembly to access the sub-10 nm dimension, and for device fabrication to benefit from the wealth of chemically synthesized nanoparticles with unique material properties