Metallic nanoparticles have significance in various fields, including biosensing, photothermal therapy, multimodal imaging, modulation of molecular properties, and photovoltaics. The interaction between light and matter at the nanoscale creates specific light-driven electrical and optical properties, including coherent delocalized electron oscillations at the dielectric-metal interface via light stimulation and localized surface plasmon resonances (LSPRs).
Several techniques make these properties accessible at the nanoscale, including photoemission electron microscopy (PEEM), photoinduced near-field electron microscopy (PINEM), cathodoluminescence microscopy, and electron microscopy in electron energy loss mode (EELS).
Photoemission Electron Microscopy (PEEM)
Photoemission electron microscopy follows a "photon in/electron out" interaction pattern based on photoemission yield increment in resonant nanoparticle's electromagnetic near-field in which higher near-field causes higher photoemission yield.
Photoemission electron microscopy demonstrates solid capabilities in single particles' investigation of different geometries such as stars, triangles, cubes, and nanorods. Researchers have pursued the investigations of nanoparticle assemblies since these are promising materials.
Photoemission electron microscopy can be utilized to conduct multiscale research of controlled nanoparticle assemblies from a single object up to large aggregates.
The near-field couplings are optimized by controlling assembly processes. In recent years various techniques have been established to improve patterning, such as smectic oily streaks, solvent-assisted self-assembly, funneled traps prepared by lithography, and surface functionalization. These techniques are efforts to balance a microscaled two-dimensional area patterning and nanoscaled interparticle distance control, both imperative for future meta-material applications.
How Experiments Were Conducted
Gold nanorods synthesis
In this study, the first step of the experimentation was synthesizing gold nanorods via Murray's synthesis. For this purpose, growth and seed solutions were mixed for this purpose, and a binary surfactant mixture was used to better control the aspect ratio.
Gold nanorods functionalization
Gold nanorods were functionalized by sodium polystyrenesulfonate (PSS) to drive their organization on a silicon wafer. For this purpose,1.34 mL of PSS solution and 6 mL of gold nanoparticle solution were mixed and kept undistributed at room temperature for three-quarters of an hour, allowing electrostatic interactions between polyelectrolytes and GNRs, and then the functionalized gold nanorods were redispersed on 6 mL of water.
Gold nanorod depositions
Selfassembled monolayers (SAMs) were prepared using classical methods. Silicon wafer substrates were immersed in the silane precursor solution. Photopatterning of self-assembled monolayers was conducted through deep-UV irradiation, and binary masks of metal lines on fused silica substrates were used. Gold nanorods were deposited on these functionalized surfaces via spin-coating and droplet methods. After that, photoemission electron microscopy measurements were carried out.
Electromagnetic calculations
Electromagnetic calculations were conducted to compare experimental and theoretical results. The researchers used the Metallic Nanoparticles Boundary Element Method (MNPBEM) Matlab toolbox43 for metallic nanoparticles' simulation. This method calculates the photoemission response from the field component normal to the object surface. For each surface element, the scalar product between the electromagnetic field and the surface normal vector was computed, the magnitude of the result was raised six times, and all the surface contributions were added.
Key Takeaways and Future Outlook
The researchers developed a novel technique to organize nanoparticles on a substrate, specifically GNRs on a silicon wafer. The study demonstrated that the GNRs' adsorption on the hydrophilic strips of the modified surface is primarily driven by the polyelectrolyte's ability to functionalize the gold nanorods. Two deposit methods: spin-coating and droplet evaporation, were investigated, leading to ordered gold nanorod assemblies. Spin-coating maximized the monolayer nature and selectivity of the oligomers.
Researchers can take gold nanorod aspect ratio and size compared to hydrophilic strip width into account for future studies. For instance, a reduction in hydrophilic bands' width can result in finer aggregates leading to GNRs' better end-to-end and side-by-side relative orientation. Gold nanorod ordering is the central point to produce effective metamaterials for future applications, which will display remarkable features.
The aggregate ordering has an immediate and direct effect on the near-field electromagnetic behavior. Researchers explored gold nanorod aggregates, gold nanorod dimers, and the response of single gold nanorods via electromagnetic simulation and photoemission electron microscopy to characterize the near-field optics of gold nanorod assemblies.
Investigation of the fundamental building blocks contributes to the justification of the appearance of electric field hot spots at distinctive contact locations in the aggregate. In such complicated gold nanorod assemblies, further control of the total near-field coupling recommends that further development of deposition technologies should be pursued.
Reference
Céline Jégat, Edouard Rollin, Ludovic Douillard, Olivier Soppera, Keitaro Nakatani, and Guillaume Laurent (2022) Patterning Gold Nanorod Assemblies by Deep-UV Lithography. The Journal of Physical Chemistry C. https://pubs.acs.org/doi/10.1021/acs.jpcc.2c03047
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