Preparation of plasmonic Au nanostructures for transmission electron microscopy investigations

Au triangular nanostructures are known to generate localised surface plasmon resonances when excited by electromagnetic radiations, leading to significant enhancement of electromagnetic fields at specific locations. The ability to produce these structures over large-area surfaces is important for advancement in the field of optics and nanophotonics. In this work a large-area self-assembly approach based on colloidal lithography using the Langmuir-Blodgett technique is presented. Colloidal particles were first deposited as a monolayer on a water surface and then transferred onto a solid substrate to create a spherical mask. A thin layer of Au was deposited over this mask, and once coated, the colloidal particles were removed, leaving behind gold nanotriangles. To further characterise their morphology at the nanoscale, conventional transmission electron microscopy sample preparation techniques were used, ensuring that the samples were thin enough to be electron-transparent. Electron energy-loss spectroscopy together with scanning transmission electron microscopy were employed to examine the localised surface plasmon resonances of the nanotriangles. The energy-loss spectra and the resulting localised surface plasmonic maps revealed four distinct optical responses at 1.15, 1.65, 1.85, and 2.30 eV. The experimental findings were validated through simulations performed using the open-source ADDA software, which is based on the discrete dipole approximation . This work presents a simple and reproducible method for fabricating plasmonic gold nanotriangles and provides detailed results into their plasmonic properties. The findings offer valuable contributions to the field of plasmonics, with potential for further optimisation and applications in nanophotonics and sensing technologies.