Novel method for controlling nano thin coatings on particulate materials

Particulate coatings have wide ranging applications in several new technologies such as flat-panel displays, sintering of advanced ceramics, rechargeable batteries, etc. In this paper, we show the feasibility of the pulsed laser ablation technique to make very thin, uniformly distributed and discrete coatings in particulate systems so that the properties of the core particles can be suitably modified. Presently, laser ablation techniques have been primarily applied to deposit thin films on flat substrate materials. To deposit discontinuous particulate coatings, the laser induced plume from the target comes in contact with an agitated bed of core particles. The pressure and nature of the background gas (inert or active) controls the cluster size of the nano particles in the laser plume. Experiments were conducted for laser deposition of Ag nano particles on Al₂O₃ and SiO₂ core particles by pulsed excimer laser (wavelength = 248 nm and pulse duration = 25 nanosecond) irradiation of a Ag sputtering target. The surface coverage and coating uniformities of the film were found to depend on the synthesis parameters (energy density, # laser pulses, gas pressure backfill gas, molecular weight) as well as the residence time of the core particles in the plume regime. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), wavelength dispersive x-ray analysis (WDX), scanning transmission electron microscopy (STEM), and x-ray photoelectron spectroscopy (XPS).