New method to pattern gold nanorods using deep UV lithography

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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-focused electrical and optical properties, including coherent delocalized electronic oscillations at the dielectric-metal interface via light stimulation and surface plasmon resonances localized (LSPR).

Several techniques make these properties accessible at the nanoscale, including photoemission electron microscopy (PEEM), near-field photoinduced electron microscopy (PINEM), cathodoluminescence microscopy, and electron energy loss mode electron microscopy ( EELS).

Photoemission Electron Microscopy (PEEM)

Photoemission electron microscopy follows a “photon in/electron out” interaction model based on the increased photoemission efficiency in the electromagnetic near field of the resonant nanoparticle in which a higher near field results in a higher photoemission efficiency. raised.

Photoemission electron microscopy demonstrates strong capabilities in studying single particles of different geometries such as stars, triangles, cubes, and nanorods. The researchers continued the investigations on the assemblies of nanoparticles because they are promising materials.

Photoemission electron microscopy can be used to conduct multiscale investigations of controlled nanoparticle assemblies, from a single object to large aggregates.

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, funnel traps prepared by lithography, and surface functionalization. These techniques are efforts to balance microscale two-dimensional zone structuring and nanoscale interparticle distance control, both imperative for future metamaterial applications.

How the Experiments Were Conducted

Synthesis of gold nanorods

In this study, the first step of the experiment was to synthesize gold nanorods via Murray synthesis. To this end, growth and seed solutions were mixed for this purpose, and a binary mixture of surfactants was used to better control the aspect ratio.

Functionalization of gold nanorods

Gold nanorods were functionalized with sodium polystyrenesulfonate (PSS) to control 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, then the functionalized gold nanorods were redispersed in 6 mL of water.

Deposits of gold nanorods

Self-assembled monolayers (SAM) were prepared using standard methods. Silicon wafer substrates were immersed in the silane precursor solution. Photopatterning of self-assembled monolayers was achieved by deep UV irradiation, and binary masks of metallic lines on fused silica substrates were used. Gold nanorods were deposited on these functionalized surfaces via spin-coating and droplet methods. Then, photoemission electron microscopy measurements were performed.

Electromagnetic calculations

Electromagnetic calculations were performed to compare experimental and theoretical results. The researchers used the Metal Nanoparticle Boundary Element Method (MNPBEM) Matlab43 toolkit for the simulation of metal nanoparticles. This method calculates the photoemission response from the field component normal to the surface of the object. For each surface feature, the dot product between the electromagnetic field and the surface normal vector was calculated, the magnitude of the result was multiplied by six, and all surface contributions were added.

Key Takeaways and Looking Ahead

The researchers developed a new technique to organize nanoparticles on a substrate, in particular GNRs on a silicon wafer. The study demonstrated that the adsorption of GNRs on the hydrophilic bands of the modified surface is mainly driven by the ability of the polyelectrolyte to functionalize the gold nanorods. Two deposition methods, spin-coating and droplet evaporation, have been studied, leading to ordered assemblies of gold nanorods. Spin-coating maximized the monolayer nature and selectivity of the oligomers.

Researchers can consider the aspect ratio and size of gold nanorods against the width of the hydrophilic band for future studies. For example, a reduction in the width of hydrophilic bands can result in finer aggregates leading to better end-to-end and side-to-side relative orientation of GNRs. The ordering of gold nanorods is the focal point to produce efficient metamaterials for future applications, which will exhibit remarkable characteristics.

The global order has an immediate and direct effect on the electromagnetic behavior in the near field. Researchers explored gold nanorod aggregates, gold nanorod dimers, and the response of single gold nanorods using electromagnetic simulation and photoemission electron microscopy to characterize the near-field optics of gold nanorod assemblies. gold.

The study of the fundamental building blocks contributes to the substantiation of the occurrence of electric field hotspots at distinctive contact locations in the aggregate. In these assemblies of complex gold nanorods, a more thorough control of the total coupling in the near field recommends to pursue the development of the deposition technologies.

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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