Revealing the Mechanism of Photoluminescence from Single Gold Nanospheres†

Abstract

Using defocused imaging technique, we investigate the photoluminescence (PL) properties of gold nanospheres on a single-particle level. Photo-blinking and photo-bleaching phenomena are both observed, similar to that reported previously for gold nanoclusters (∼2 nm). We notice that although the surface plasmon resonance (SPR) of these nanospheres (15-20 nm in diameter) is at ∼515 nm, they can emit at ∼630 nm (excited by 532 nm), which is ∼100 nm separated from the SPR peak. The observed defocused images (DIs) exhibit isotropy first, and then either transform into anisotropy, or vanish rapidly. Surprisingly, the DIs can change their emission pattern within one single nanosphere during the tracing time. All these PL properties suggest a multiple-dipole-emission model. High-resolution transmission electron microscopy images demonstrate that these nanospheres are polycrystalline containing multiple small crystal domains (∼3 nm). We believe that these small domains divide the particle into different clusters, and give rise to the photo-blinking behavior and rotary DIs. The presented mechanism provides a link in fluorescence between gold nanoclusters and single-crystal nanorods, which could be helpful on understanding the origins of photo-blinking and the luminescent properties from metallic nanoparticles. In addition, these water-soluble gold nanospheres provides new opportunities for biological labels and light-emitting sources in nanophotonics.