Nanofabrication uses scalable, high-throughput processes for the commercial production of nanoscale materials, structures and devices. This means characteristics with dimensions between one and 100 nanometers.
At the lower end, it approximates the size of individual atoms – a hydrogen atom is 0.1nm in diameter while lead is 0.4nm. Nanoscale materials include powders and carbon nanotubes. Nanoscale structures include many semiconductor devices such as computer chips, as well as fuel cells, batteries, and filters.
Nanofabrication is performed in a clean room and typically involves methods such as thin-film material deposition, modeling, and etching, as developed by the semiconductor industry. Additional methods such as quantum dots, nanowires and self-assembly have also been added.
Semiconductor fabrication first deposits layers of metallic material on a semiconductor substrate, and then the layers are patterned and etched. Thin film deposition methods include vacuum evaporation, sputtering deposition, and chemical vapor deposition. Patterning involves the selective removal of regions of layered material using techniques such as photolithography, electron beam lithography, and nanoimprint lithography.
Other layers can then be removed by etching. Wet chemical etching uses reactive liquids such as acids, bases and solvents. Dry etching uses reactive gases in processes including reactive ion etching and conduction coupled plasma etching.
Nanofabrication is the most widely used for the production of integrated circuits. Transistors are formed at the intersections of metal fins in a grid. The fins are produced using a lithography process in which a mask creates shadows in a laser so that a pattern can be burnt out.
The state-of-the-art “5nm process” involves producing metal fin grids at 20nm pitches. The lines at this scale are much smaller than the wavelength of visible light, and therefore extreme ultraviolet lasers are used, with wavelengths almost in the x-ray spectrum. Similar nanofabrication methods are also used to produce semiconductor lasers.
Nanostructured materials can have a range of properties that make them well suited for energy storage. These include a large surface area, favorable support properties, and high electrical and thermal conductivity. Energy storage applications include battery and supercapacitor electrodes, thermal energy storage, and hydrogen storage.
While lithium-ion batteries with large area nanostructured electrodes have the potential to dramatically increase energy storage capacity, there are issues of efficiency, density, and cost.
Perhaps the most promising application of nanomaterials in energy storage is supercapacitors. Researchers at the University of Washington have demonstrated a low-cost evolutionary process to produce supercapacitors with nanostructured surfaces.
Lightly charged batteries
Researchers at the University of Cambridge are developing photo-rechargeable batteries – solar cells that use light to store charge, then release it as needed. Such devices could be extremely useful in overcoming intermittency, one of the greatest limitations of renewables. Combining solar panels with batteries is very expensive and it is hoped that by combining their capabilities into one device the cost could be reduced.
Lithium-ion photo-batteries have already been demonstrated, using a structured light collecting layer in perovskite and a capacity of 100 mAh / g. Zinc-ion photo-batteries have now been developed to achieve longer life and lower cost. These use cathodes containing vanadium pentoxide nanofibers, allowing direct charging of light as well as scalable fabrication.
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