Ultra-thin h-BN substrates for nanoscale plasmon spectroscopy

Probing plasmonic properties of surface deposited nanoparticles with high spatial resolution requires the use of a low absorption support. In this work, ultra-thin hexagonal boron nitride (h-BN) flakes are employed as substrates for scanning transmission electron microscopy. The thicknesses of only a few atomic layers, the flat surface, and the large bandgap provide a unique set of properties, which makes h-BN ideally suitable for high resolution plasmon spectroscopy by means of electron energy loss spectroscopy (EELS), especially for small nanoparticles. A facile fabrication process allows the production of h-BN substrates with a thickness of only a few atomic layers. The advantages of h-BN, especially for the low-loss energy region of EEL spectra, are shown in a direct comparison with a silicon nitride substrate. Furthermore, results of the investigation of localized surface plasmon resonances (LSPRs) of Ag and Ag-Au core-shell nanoparticles in the sub-20 nm size regime are presented, confirming the advantages of the fabricated substrate for LSPR mapping. The plasmonic nanoparticles were assembled utilizing the helium nanodroplet synthesis approach, which allows for a very soft deposition and the preservation of the integrity of the ultra-thin substrate. Moreover, it provides a completely solvent and surfactant free environment for the assembly of tailored nanoparticles.