Revealing the Mechanism of Photoluminescence from Single Gold Nanospheres by Defocused Imaging

The mechanism for the photoluminescence (PL) emission from gold nanoparticles has attracted considerable attention for many years. However, there is an important gap between small nanoclusters (∼2 nm) and larger plasmonic particles (∼50 nm). In this work, using defocused imaging technique, we investigate the PL properties of gold nanospheres (15–20 nm in diameter) on a single-particle level. Photoblinking and photobleaching phenomena are both observed. We notice that although these nanospheres can support surface plasmon resonance at ∼515 nm, they emit at ∼630 nm (excited by 532 nm), which is obviously plasmon-independent. 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 photoblinking behavior and rotary DIs. The presented PL mechanism links gold nanoclusters and single-crystal nanorods, which could be helpful in understanding the origins of photoblinking and the luminescent properties from metallic nanoparticles. In addition, these water-soluble gold nanospheres provide new opportunities for biological labels and light-emitting sources in nanophotonics.