(753b) Growth of Gold Nanoshells On Colloidal Silica Characterized By in Situ Second-Harmonic Generation

Metal nanoparticles and metal composite nanoparticles can have unique optical properties due to localized surface plasmon resonances and are therefore of particular interest in various applications. The resonance frequency of such particles is a function of their composition and their geometry. Gold nanoshells are comprised of a dielectric core and a thin gold shell. The optical properties of such particles can be readily tuned over a wide range by changing the ratio of the core diameter to the shell thickness.

In order to impart better control over the optical properties of gold nanoshells a detailed understanding of the shell formation is of great importance. For that purpose it is necessary to monitor changes in surface composition and morphology in situ and in real time. However, most experimental methods lack the desired surface sensitivity or cannot be applied in situ under the required experimental conditions. For that   reason we have applied second-harmonic generation (SHG) spectroscopy as a powerful surface sensitive technique which shows great potential to probe surfaces of nanoparticles under relevant experimental conditions^1-3.

In our contribution, we present results from synchronized SHG and UV/Vis/NIR spectroscopies that were obtained on-line during the seeded growth of gold shells on silica core particles. UV/Vis/NIR spectroscopy allows the monitoring of the coverage of gold and to estimate whether the shells are closed or not, while SHG shows a remarkable sensitivity to changes in the surface morphology which has not been accessible by other in situmethods so far. Notably, at intermediate conditions where a still incomplete shell exists on the core particle during shell growth, we observe first a steep increase in the second-harmonic intensity that is followed by a subsequent rapid decrease in SHG intensity. As the SHG signal is a function of the fundamental electric field to the fourth power, we propose that the observed maximum in the temporal evolution of the SHG intensity is caused by a local field enhancement due to crevices that are being formed in between individual gold islands. As shell growth proceeds, the surface coverage of these crevices decreases and closed shells are formed which results in a substantially decreased SHG signal. This hypothesis is corroborated by additional angle-resolved SHG experiments and ex-situ electron microscopy of gold shells with different gold coverages and surface morphologies.

[1]   Schürer, B., Wunderlich, S., Sauerbeck, C., Peschel, U., Peukert, W.,Phys. Rev. B, 82 (2010) 241404

[2]   Schürer, B., Elser, M. J.; Sternig, A., Peukert, W., Diwald, O., J. Phys. Chem. C, 115 (2011) 12381–12387

[3]   Schürer, B., Hoffmann, M., Wunderlich, S., Harnau, L., Peschel, U., Ballauff, M., Peukert, W., J. Phys. Chem. C, 115 (2011) 18302–18309