Much research has been carried out to produce nanoparticles of various materials due to excellent mechanical, chemical, electrical, and optical properties of nanoparticles. However, it is difficult to deposit and transform nanoparticles into large two-dimensional and three-dimensional structures, such as thin films and discrete arrays, in a controlled manner. This award supports fundamental research to enable a new additive manufacturing process to deposit nanoparticles into thin films or discrete nanodot arrays (a nanodot is a cluster of nanoparticles). The new process is versatile and scalable, and uses less materials and energy. It can promote roll-to-roll manufacturing of a variety of energy and electronic devices such as conformal solar cells, sensors, and actuators. It can also be used to fabricate masks for nanolithography, nanopillar arrays for photonic crystals, and nanodot arrays for plasmonic surfaces.
The new additive manufacturing process involves injecting silicon nanoparticles (contained inside electric field-driven water droplets) into a hollow laser beam. The size of nanoparticles ranges from 5 to 100 nm. The laser beam will heat the droplets, causing the water to evaporate and the nanoparticles to sinter and form microlayers on flexible substrates such as polyimide plastic (Kapton) or stainless steel foil. For this process to work, the laser beam needs to be focused so that its diameter is smaller than its wavelength. The droplet will serve as both a nanoparticle carrier, and a superlens that focuses a laser beam to subwavelength diameters. The dropsize has to be within an upper and lower critical limit. The first research objective is to determine these limits, and will be achieved through laser beam propagation modeling and droplet deposition experiments. The model involves numerical solution of Maxwell equations, and the solution yields the two limits. Several dropsizes will be chosen within these limits to conduct experiments. The size and feed rate of droplets will be measured using high speed photography; particle concentration using colloidal testing meter; the laser power, diameter, and pulse repetition rate using laser beam analyzer; the speed and temperature of substrate using substrate feed controller and thermography respectively. The grain size of the microlayer (determined by process variables) affects the electrical conductivity of the microlayer. The second research objective is to establish the relationship between the grain size and conductivity. It will be achieved by measuring the grain size using atomic force microscopy and scanning electron microscopy, and the conductivity using four-probe technique.
