RESUMEN
We demonstrate that the silica shell on nanoparticles formed by a typical Stöber method is inhomogeneous in nature. The outer layer of the shell is chemically more robust than the inner layer, which can be selectively etched by hot water. Methods are developed to "harden" the soft silica shells. These new understandings are exploited to develop versatile and template-free approaches for fabricating sophisticated yolk-shell nanostructures.
RESUMEN
We report a nanowire growth that is highly unconventional: (1) nanowires can grow from substrate-bound seeds but cannot from colloidal seeds under otherwise the same conditions; (2) the nanowires grow from only one side of the seeds, with their diameter independent of the size of the seeds; and (3) vertically aligned ultrathin nanowires are obtained on substrates, using aqueous solution and ambient conditions. With carefully designed experiments, we propose and test a new mechanism that can explain these unusual phenonmena. It turns out that the strong binding of ligands in this system forces selective deposition of Au at the ligand-deficient interface between Au seeds and oxide substrates. This means of promoting anisotropic growth of nanocrystals into nanowires is previously unknown in the literature. We are able to pinpoint the site of active growth and explain the control of nanowire width. The sustained growth at the active site and the inhibited growth at its parameter push the nanocrystals upward into wires; their diameter is dependent on the dynamic competition of the two processes. The site-specific growth from substrate-anchored seeds provides a rare means to create substrate-nanowire hierarchical structures in aqueous solution under ambient conditions. Rendering a surface conductive, particularly one with complex surface morphology, is now made easy.
RESUMEN
The most studied effect of surface-enhanced Raman scattering (SERS) hotspots is the enormous Raman enhancement of the analytes therein. A less known effect, though, is that the formation of hotspots may cause the trapped analytes to change molecular orientation, which in turn leads to pronounced changes in SERS fingerprints. Here, we demonstrate this effect by creating and characterizing hotspots in colloidal solutions. Anisotropically functionalized Au nanorods were synthesized, whereby the sides were specifically encapsulated by polystyrene-block-poly(acrylic acid), leaving the ends unencapsulated and functionalized by a SERS analyte, 4-mercaptobenzoic acid. Upon salt treatment, these nanorods assemble into linear chains, forming hotspots that incorporate the SERS analyte. Enormous SERS enhancement was observed, particularly for some weak/inactive SERS modes that were not present in the original spectrum before the hotspots formation. Detailed spectral analysis showed that the variations of the SERS fingerprint were consistent with the reorientation of analyte molecules from nearly upright to parallel/tilted conformation on the Au surface. We propose that the aggregation of Au nanorods exerts physical stress on the analytes in the hotspots, causing the molecular reorientation. Such a hotspot-induced variation of SERS fingerprints was also observed in several other systems using different analytes.