Tuning the Plasmonic Response up: Hollow Cuboid Metal Nanostructures

Abstract

We report the fine-tuning of the localized surface plasmon resonances (LSPRs) from ultraviolet to near-infrared by nanoengineering the metal nanoparticle morphologies from solid Ag nanocubes to hollow AuAg nanoboxes and AuAg nanoframes. Spatially resolved mapping of plasmon resonances by electron energy loss spectroscopy (EELS) revealed a homogeneous distribution of highly intense plasmon resonances around the hollow nanostructures and the interaction, that is, hybridization, of inner and outer plasmon fields for the nanoframe. Experimental findings are accurately correlated with the boundary element method (BEM) simulations demonstrating that the homogeneous distribution of the plasmon resonances is the key factor for their improved plasmonic properties. As a proof of concept for these enhanced plasmonic properties, we show the effective label free sensing of bovine serum albumin (BSA) of single-walled AuAg nanoboxes in comparison with solid Au nanoparticles, demonstrating their excellent performance for future biomedical applications.

Hollow metal nanostructures for enhanced plasmonics: synthesis, local plasmonic properties and applications

“…Figure 7A shows the spectra and abundance maps of three plasmonic components of a single-walled AuAg nanobox (STEM image with red borders) which is 50 nm in size and has about 7 nm thick walls, confirming the postulation that the hollow nanostructures would generate homogeneously distributed plasmon resonances [127,128]. These maps revealing the homogeneous spatial distribution of plasmon resonances in such a hollow nanostructure may be the explanation of their enhanced plasmonic properties for different applications such as sensing, as all the surface of the nanobox acts like a continuous "hotspot" with intense plasmon excitations [133]. Spectra and abundance maps of three plasmonic components of an AuAg nanoframe (STEM image with green borders) which is 48 nm in size and has about 7 nm thick walls are shown in Figure 7B.…”

“…As it is clearly seen in Figure 6D, plasmon resonances shift to lower energies with increasing void size even though there is no alloying, as suggested by the plasmon hybridization mechanism. The amount of the shift is higher for the thinner walls, where plasmon resonances barely shift for the 5 nm void but they shift about 0.8 eV as the void size increases from 40 to 45 nm due to strong hybridization [133]. Figure 7A shows the spectra and abundance maps of three plasmonic components of a single-walled AuAg nanobox (STEM image with red borders) which is 50 nm in size and has about 7 nm thick walls, confirming the postulation that the hollow nanostructures would generate homogeneously distributed plasmon resonances [127,128].…”

“…Experimental studies of the plasmonic properties of hollow metal nanostructures at the nanoscale by EELS proving the coupling of inner and outer modes, i.e. hybridization are conducted only during recent years [73,133]. These results are discussed in more detail in the following paragraphs.…”

“…Although, EELS has been used intensively for the imaging of plasmon resonances in solid nanostructures, there are only a few studies dealing with the application of EELS for the plasmonic properties of hollow metal nanostructures [73,133]. Prieto et al [73] investigated the evolution of plasmon resonances in spherical hollow gold nanoparticles and gold nanorings by using EELS and related DDA simulations and have reported that as the aspect ratio between the wall thickness and diameter of the nanorings increases, plasmon resonances shift to lower energies in accordance with the plasmon hybridization mechanism and the scheme shown in Figure 1B [39].…”

“…By reporting the ultralocal distribution of plasmon resonances, we have provided experimental insights about their enhanced plasmonic properties [133]. Figure 6 shows the structural and optical evolution of various cuboid nanostructures from solid Ag nanocubes to AuAg nanoframes, where it is clearly seen that plasmon resonances shift to lower energies with increasing void size.…”