SERS of Individual Nanoparticles on a Mirror: Size Does Matter, but so Does Shape

Abstract

Coupling noble metal nanoparticles by a 1 nm gap to an underlying gold mirror confines light to extremely small volumes, useful for sensing on the nanoscale. Individually measuring 10 000 of such gold nanoparticles of increasing size dramatically shows the different scaling of their optical scattering (far-field) and surface-enhanced Raman emission (SERS, near-field). Linear red-shifts of the coupled plasmon modes are seen with increasing size, matching theory. The total SERS from the few hundred molecules under each nanoparticle dramatically increases with increasing size. This scaling shows that maximum SERS emission is always produced from the largest nanoparticles, irrespective of tuning to any plasmonic resonances. Changes of particle facet with nanoparticle size result in vastly weaker scaling of the near-field SERS, without much modifying the far-field, and allows simple approaches for optimizing practical sensing.

Ideal Dimers of Gold Nanospheres for Precision Plasmonics: Synthesis and Characterization at the Single‐Particle Level for Identification of Higher Order Modes

“…Strikingly, the position of the longitudinal bonding dipolar plasmon coupling (LBDP) mode in the ideal dimers is highly uniform (735 ± 6.7 nm). In the region 500–650 nm, the ideal dimers exhibit a relatively broad band, which may be indicative of overlapping contributions from transverse and higher order longitudinal coupling modes such as the longitudinal bonding quadrupolar coupling (LBQP) mode . We therefore performed polarization‐resolved DF scattering spectroscopy on single ideal dimers ( Figure a) for an unambiguous mode assignment .…”

“…For predicting the properties of AuNP dimers, computer simulations usually employ a pair of gold spheres separated by a constant gap distance . In contrast to theory, the experimental realization of such an ideal dimer of spheres is challenging because of structural inhomogeneities: (i) uncontrolled gap distance at the scale of Ångströms; (ii) AuNPs prepared by conventional reduction chemistry are faceted, nonspherical nanocrystals; and (iii) multiple gap morphologies, i.e., different configurations of the opposing crystal facets in the gap region . The combination of all these structural inhomogeneities leads to plasmonic inhomogeneities of the corresponding dimers.…”

“…The second issue, the nonperfect sphericity at the monomer level, has been solved in pioneering colloidal chemistry work using chemical etching yielding a smooth metal surface (Figure S2, Supporting Information) . The third issue, gap morphology, has recently been recognized as a decisive factor influencing the plasmonic properties of dimers or model system of dimers such as a particle on a mirror . However, it is currently not possible to experimentally control either gap morphology or to eliminate the influence of gap morphology in metal nanoparticle dimers.…”

Advances in surface-enhanced vibrational spectroscopy at electrochemical interfaces

“…Whilst electrochemical roughening is easy to implement, its main limitation lies in poor control over surface morphology, uniformity and particle geometry, which are known to strongly influence both electrochemical behaviour and SERS activity [32,33]. Improved control can be gained by assembling pre-synthesised metal nanoparticles onto an electrode surface.…”

“Hot” in Plasmonics: Temperature‐Related Concepts and Applications of Metal Nanostructures

“…The most widely used plasmonic materials are Ag and Au. The frequency of the LSPRs also depends on the shape of the nanoparticle; in a nanorod, the LSPR along the main axis of the rod red‐shifts with increasing length . Moreover, plasmonic electron density fluctuations can be dipolar, quadrupolar, or octupolar in nature and the shape of the nanoparticle plays a role in selecting that nature.…”