Several types of 3-D printers are commercially available. A direct print method uses a micrometer sized nozzle to deposit a paste that immediately hardens into the final form. With this method, it is possible to print 3-D electrodes for lithium ion batteries.
A laser may be used to modify the atomic configuration of a material (e.g. convert a crystalline material into an amorphous one). This modification could subsequently be selectively etched or guarded. This is the principal behind the PolyMUMPs process so widely used with micro-electromechanical systems (MEMS). Similar processes may also be applied to create 3-D nanostructures.
Grey masking is a modified method to create 3D structures, or multiple layers of structures simultaneously. Electon beam lithography with modulated intensity can create grayscale features smaller than 200nm.
Crystal structures can grow atom by atom and plane by plane. The variations of resulting surface energies and the influence of environmental conditions may lead to unique nano- and micro-architectures. One example is a field of flowers, synthesized by dissolving barium chloride and sodium silicate in water. Carbon dioxide influences the precipitation of barium carbonate crystals, which also lowers the pH surrounding the crystals and in turn adds a layer of silica to the growing structures. Varying the carbon dioxide concentrations and the pH gradient creates a variety of leaf geometry.
Magnesiothermic reduction uses magnesium at an elevated temperature to reduce SiO2 into Si nanostructures. By using common table salt as a heat scavenger during production, the collapse of the nanostructures is prevented and the salt may be reused afterwards. Salt solutions are also found to increase the yield of oxide nanoparticles by mixing potassium superoxide (KO2). Several grams of various nanoparticles across the periodic table are quickly synthesized.
Many different nanostructures may be synthesized by this method. The key is to control the molecular self-assembly of atoms and molecules. For example, vertically oriented Si nanorods may be grown with the assistance of Au liquid droplets on the Si surface. As the droplets collect the vaporized particles in its liquid state, they begin to saturate the system and precipitate out to form the solid wire. Liquid nitrogen cooling helps to better define the deposition of the vapour particles. In addition, rotating the substrate can create a helix nanostructure.
There is also a sub class of Liquid-Solid transitions. A single fs laser pulse can generate a single amorphous Si NP and have it stick to a collector plate. That same NP could be irradiated with a second pulse to change its phase from amorphous to crystal. Moreover, the size is highly controllable which gives specific optical responces.
||Wafer-Scale Nanograting Template
Nanowires are long linear structures with a maximum width of 100 nm, which possess undiscovered thermal, electric and mechanical properties. The traditional technology required to produce them has issues of nanowire alignment, low productivity, long manufacturing time and material synthesis restrictions. However, these issues have been resolved by switching from chemical synthesis to a semiconductor technique which applies the sputtering process to a wafer-scale nanograting template. With this new technology, nonawires can be mass produced at any length and with a variety of materials for applications in biodevices, semiconductors, and high performance sensors.