Dispersion in the SERS Enhancement with Silver Nanocube Dimers

Abstract

The SERS phenomenon was studied using a large set of silver nanocube dimers programmed to self-assemble in preset locations of a patterned substrate. This SERS substrate made it possible to demonstrate the dependence of the SERS enhancement on the geometry of the silver nanocube dimers and to quantify the dispersion in the SERS enhancement obtained in an ensemble of dimers. In addition to the effects of the gap distance of the dimer and the orientation of the dimer axis relative to the laser polarization on SERS enhancement, the data reveal an interesting dependence of the site-to-site variations of the enhancement on the relative orientation of the nanocubes in the dimer. We observed the highest heterogeneity in the SERS signal intensity with face-to-face dimers and a more robust SERS enhancement with face-to-edge dimers. Numerical calculations indicate that the plasmon resonance frequencies of face-to-face dimers shift considerably with small changes in gap distance. The resonance frequency shifts make it less likely for many of the dimers to satisfy the matching condition between the photon frequencies and the plasmon resonance frequency, offering an explanation for the large site-to-site variations in SERS signal intensity. These results indicate that plasmonic nanostructure designs for SERS substrates for real-world applications should be selected not only to maximize the signal enhancement potential but also to minimize the heterogeneity of the substrate with respect to signal enhancement. The latter criterion poses new challenges to experimentalists and theorists alike.

SERS Substrates by the Assembly of Silver Nanocubes: High-Throughput and Enhancement Reliability Considerations

“…The orientation of the nanocubes of the dimer relative to each other has a more subtle influence on the SSEF [23]. Consequently, our study on the effect of the azimuth angle is fairly insensitive to and unobstructed by variations in the nanocube dimer configuration.…”

“…Single-crystal silver nanocubes were synthesized by the polyol reduction method in 1,5-pentanediol [23,38]. The as-prepared nanocubes are terminated by the silver crystal 100 planes and are coated with a passivation layer of polyvinylpyrrolidone (PVP).…”

“…Several experimental results and theoretical calculations with nanocube dimers have predicted the largest plasmonic enhancements for nanocubes arranged face-toface with a gap of few nanometers between them [23,41]. Calculations show that strong fields are predicted to extend over the entire face of the nanocube forming the narrow gap.…”

Localized surface-plasmon resonances on single and coupled nanoparticles through surface integral equations for flexible surfaces

“…This is indeed a critical issue upon comparing theoretical calculations with experimental measurements, for real life nanoparticles often differ from perfect mathematical shapes [42]. In order to generate a cube with the SS, we have to choose n 1 = n 2 = n 3 ≡ n > 4, a = b = 1, and m = 4.…”

“…5, we show the scattering cross section of three sort of dimer: namely, sphere-sphere (ss), cube-cube (cc), and cube-sphere (cs). Dimers have been intensely used in Plasmonics and Nanophotonics due to the large fields generated in the gap, and their behaviour as optical nanoantennas [12,42]. The scatterers are made of silver with r = 15 nm in the case of the sphere, and L = 30 nm in the case of the cubes, with the gap width g = 1.5 nm; in all cases the polarization of the incident field is along the dimer axis.…”