Ultrashort, Angstrom-Scale Decay of Surface-Enhanced Raman Scattering at Hot Spots

Anisotropic plasmonic nanostructures are known to exhibit large enhancements of surface-enhanced Raman scattering (SERS) of adsorbed molecules at their sharp tips or edges, where the near-field is intense. We show that the SERS enhancement at such field hot spots decays over a distance of ca. 4 Å, much shorter than the typical decay length reported for SERS. The finding is made in SERS sensors constructed from chemically synthesized triangular nanoprisms with azobenzene reporter molecules linked to the nanoprism surface using variable chain length alkanethiol spacers. With the aid of electrodynamic simulations, the ultrashort decay length, the shortest reported to date, is explained by solely an electromagnetic field effect. Our work provides a key design consideration for the use of hot spots of anisotropic nanostructures for SERS. The angstrom-scale effect may also allow the achievement of intramolecular spatial resolution in SERS probing

Low-Temperature Synthesis of Magic-Sized CdSe Nanoclusters: Influence of Ligands on Nanocluster Growth and Photophysical Properties

We present a low-temperature (68–70 °C) synthesis of green light-emitting, trioctylphosphine oxide-capped magic-sized CdSe nanoclusters from the reaction of trioctylphosphine oxide–cadmium acetate precursors with trioctylphosphine selenide. We observed continuous growth of these magic-sized nanoclusters, which displayed a first absorption peak at 422 nm and broad luminescence covering the entire visible region. The diameter of the nanoclusters determined by transmission electron microscopic measurement was ∼1.8 nm. Powder X-ray diffraction analysis showed a sharp peak at low angle (2θ = 5.3°), confirming the formation of ultrasmall, magic-sized nanoclusters. The nanocluster formation was also studied using different purities of trioctylphosphine oxide. The synthetic protocol was extended to the preparation of oleylamine-, ethylphosphonic acid-, lauric acid-, and trioctylamine-stabilized magic-sized CdSe nanoclusters. Importantly, the investigation showed that the nature of the cadmium precursors plays a crucial role in the nanocluster growth mechanism. The applicability of the trioctylphosphine oxide-capped nanoclusters was further investigated through a ligand exchange reaction with oleylamine, which displayed an extremely narrow absorption peak at 415 nm (full width at half-maximum of 14 nm) and a band edge emission peak at 456 nm with a shoulder at 438 nm