Plasmon Resonances in Self-Assembled Two-Dimensional Au Nanocrystal Metamolecules

Abstract

We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer "metamolecules" as the number of nanocrystals is systematically varied. Precise, hexagonally ordered Au nanocrystal oligomers with 1-31 members are assembled via capillary forces into polygonal topographic templates defined using electron-beam lithography. The visible and near-infrared scattering response of individual oligomers is measured by spatially resolved, polarized darkfield scattering spectroscopy. The response is highly sensitive to in-plane versus out-of-plane incident polarization, and we observe an exponentially saturating red shift in plasmon resonance wavelength as the number of nanocrystals per oligomer increases, in agreement with theoretical predictions. Simulations further elucidate the modes supported by the oligomers, including electric dipole and magnetic dipole resonances and their Fano interference. The single-oligomer sensitivity of our measurements also reveals the role of positional disorder in determining the wavelength and character of the plasmonic response. The progression of oligomer metamolecule structures studied here advances our understanding of fundamental plasmonic interactions in the transition regime between few-member plasmonic clusters and extended two-dimensional arrays.

Gold Nanoparticle Plasmonic Superlattices as Surface-Enhanced Raman Spectroscopy Substrates

“…These higher order cluster modes, which have been discussed in previous works, 28,38 can shift the peak to λ~560 nm. 22,39 The second region at higher wavelengths displays the most intense extinction peak and clearly red-shifts when increasing the lattice parameter of the assembly. The strong correlation of the latter extinction peak with the lattice parameter indicates that this resonance originates from the hybridization of the plasmonic cluster modes with the diffractive lattice mode.…”

“…17,18 Strategies for preparing homogeneously structured nanoparticle films by template-assisted self-assembly have been improved significantly over the last decade, now having the potential to solve the above mentioned issues. [19][20][21][22] These substrates featuring both a high density of hot spots and improved uniformity have been employed, for example, in the detection of gases like carbon monoxide or pyrene, 23,24 as well as in monitoring the expression of bacterial quorum sensing molecules via SERS. 25 The occurrence of collective plasmon resonances is directly connected to the presence of hot spots in metal nanoparticle assemblies, which often dominate the optical response of nanoparticle films.…”

Recent advances in gold nanoparticles for biomedical applications: from hybrid structures to multi-functionality

“…24 For prolonged blood circulation halflife, the NPs should be within the size window of 10-150 nm. In cases where rapid renal clearance is desirable, the NPs should be smaller than 10 nm, which is the effective cut-off size of the 47 (b) single crystal nanowire, 52 (c) chiral NPs, 31,45 (d) core-shell NPs, 27 (e) Janus NPs, 23 (f) alloyed NPs, 142,174 (g) plasmonic meta-molecules, 175 (h) DNA-origami templated assemblies, 25 (i) large-scale assemblies of NPs. 99,176 Adapted by permission from the corresponding references.…”

Latest Novelties on Plasmonic and Non-Plasmonic Nanomaterials for SERS Sensing

“…Nevertheless, the reproducibility of the SERS signal is very weak with this type of substrates [ 65 ]. However, improvements have emerged as the template-assisted self-assembly suppressing the issue of signal reproducibility [ 66 , 67 ]. In addition, alternative materials such as metal oxides (different of zinc oxide as MoO 3 molybdenum trioxide, WO 3− x tungsten oxide or CoFe 2 O 4 cobalt ferrite) emerged for SERS application [ 68 , 69 ].…”