Optical response and SERS properties of individual large scale supracrystals made of small silver nanocrystals

Abstract

There is a considerable interest for producing and understanding the optical and spectroscopic properties of ordered nanoparticles assemblies. Herein, we describe and interpret the optical absorbance and Raman properties of silver 5.9  0.3 nm diameter nanocrystals coated with dodecanethiol organized in highly ordered 3D superlattices of different heights. Each superlattice could be studied individually. A model based on Maxwell-Garnett formulation reproduces qualitatively the height and wavelength dependence of the absorbance. Importantly, because of their small size compared to traditional nanoparticles used in SERS, the large 3D distribution of hot spots generated by the silver superlattices allows to obtain easily SERS spectra of the surrounding ligands despite their intrinsic low Raman cross section. Accordingly, traces of thiophenol can be very easily detected.

Tunable SERS Platforms from Small Nanoparticle 3D Superlattices: A Comparison between Gold, Silver, and Copper

“…We demonstrated very recently that silver NPs (5.9±0.3) nm in diameter, self‐organized by evaporation of a colloidal solution on a solid substrate, lead to important EFs ( around 10 4 ) even for surrounding alkyl ligands characterized by a low Raman cross‐section. Furthermore, the ligands were shown to not prevent the detection of other SERS‐active molecules, such as thiophenol …”

“…Note that the optical properties of the samples depend on the dielectric properties of both the NP SLs and the substrate. The substrate effect was thus taken into account by using the Tomlin Model . Furthermore, this model allowed us to take into account the interference phenomena.…”

“…Regardless of the nature of the NPs, we observed a redshift and a broadening of the main absorbance bands with the SL thickness in the simulated and experimental absorbance spectra. However, the broadening of the absorbance band was more important in the experimental spectra than that in the simulated ones, which suggested possible additional interparticle plasmon couplings that are not taken into account in the model . The small interparticle distance in the NP SLs (inferior to the NP diameter and deduced from XRD experiments) is known to favor electromagnetic coupling between NPs .…”

“…Knowing the probed SL thickness and the thiolate coverage for silver and gold NPs, the SERS EFs of the SLs as compared with pure dodecanethiolate were determined for the C−H bands . Figure a and b shows the calculated EFs at λ exc =473, 532, and 633 nm for C 12 SAu 5nm and C 12 SAg 5nm NP SLs.…”

Plasmon‐enhanced inelastic scattering by 2D and 3D superlattices made of silver nanocrystals

“…The optical properties of similar supercrystals of AgNCs have been recently examined, and it has been shown that a collective behavior is reflected in plasmonic properties due to long‐range dipolar interaction between neighboring NCs. The localized surface plasmon resonance (SPR), localized at 420 or 410 nm for isolated spherical nm‐sized AgNCs in colloidal suspensions or embedded in silica, is splitted in transverse and longitudinal SPR by their mutual coupling.…”