Light Induced Further Agglomeration of Metal Particles

The observation of an unusual light-induced agglomeration phenomenon that occurs besides the trapping of the gold nanoparticles aggregates (GNAs) has been observed. The observed agglomerate has a 60-100 μm donut-shaped metal microstructure with the rate of formation dependent on the laser power used. In this paper, the forces involved and the mechanism of this further agglomeration phenomenon are analyzed in detail. The observed trapping can partially be explained by a model including the optical radiation force and radiometric force. However, the lightinduced agglomeration cannot be explained by optical trapping alone as the size of the agglomerate is much greater than the waist of the Gaussion beam used in the optical trapping. Hydrodynamic drag force induced by the laser heating is also considered to play a role. Besides these forces, the mechanism of light-induced agglomeration is attributed to ion detachment from the surface of the nanoparticles/aggregates due to light illumination or heating. This is supported by the observation of reversible conductivity changes in the nanoparticle/aggregate solution upon laser illumination or direct heating. Light-induced agglomeration can be useful in the design and fabrication of microstructures from nanomaterials for various device applications.