Self-assembly of silver nanoparticles illuminated by a visible-near-infrared lamp

Silver nanoparticles (NPs) with diameters of 5-10 nm were synthesized by using formaldehyde (HCHO) to reduce silver nitrate (AgNO3) and polyethylenemine (PEI) as a stabilizer. It was found that on the carbon-supported cupper grid (Cu/polyvinylformal (PVF)/C) and under a visible-near-infrared (VIS-NIR, 400-860 nm) lamp illuminating at about 75 degrees C for six minutes, the spherical Ag NPs self-assembled into bigger NPs, triangular nanoprisms, or structures like clock hands. On the glass slides, the NPs firstly self-assembled into spherical particles with 3-6 microm in sizes, then assembled into chain structures comprised of 3-20 spherical particles after being illuminated by the VIS-NIR lamp for about nine minutes. The violent thermal motion of the NPs after being illuminated by the VIS-NIR lamp leads to the self-assembly and the shapes of the self-assembled particles are related to the interactions between the substrates and the samples.