At the lower end, this is approaching the size of individual atoms – a hydrogen atom has a diameter of 0.1nm 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 carried out in a clean room and typically involves methods such as thin-film material deposition, patterning and etching, as developed by the semiconductor industry. Additional methods such as quantum dots, nanowires and self-assembly have also been added.
Semiconductor manufacturing first deposits layers of metallic material on a semiconductor substrate, and then the layers are patterned and etched. Methods of thin-film deposition include vacuum evaporation, sputter deposition and chemical vapour deposition. Patterning involves selectively removing regions of layered material using techniques such as photolithography, electron-beam lithography and nano-imprint lithography.
Further layers may then be removed by etching. Chemical wet etching uses reactive liquids such as acids, bases and solvents. Dry etching uses reactive gases in processes including reactive ion etching and conductively coupled plasma etching.
Nanofabrication is 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 burned away.
The state-of-the-art ‘5nm process’ involves producing grids of metal fins at pitches of 20nm. 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 to energy storage. These include large surface area, favourable carrier properties and high electrical and thermal conductivity. Energy storage applications include electrodes for batteries and supercapacitors, thermal energy storage, and hydrogen storage.
Although lithium-ion batteries with large surface area nanostructured electrodes have the potential to greatly increase energy storage capacity, there are issues with efficiency, density and cost.
Perhaps the most promising application for nanomaterials in energy storage is in supercapacitors. Researchers at the University of Washington have demonstrated a scaleable low-cost process to produce supercapacitors with nanostructured surfaces.
Light charging batteries
Researchers at the University of Cambridge are developing photo-rechargeable batteries – solar cells that use light to store charge and then release it as required. Such devices could be enormously useful in overcoming intermittency, one of the greatest limitations of renewable energy. The combination of solar panels with batteries is very expensive and it is hoped that by combining their capabilities into a single device the cost could be reduced.
Lithium-ion photo-batteries have already been demonstrated, using a perovskite-structured light-harvesting layer and a capacity of 100mAh/g. Zinc-ion photo-batteries have now been developed in order to achieve greater cycle life and lower cost. These use cathodes containing vanadium pentoxide nanofibres, enabling direct light charging as well as scaleable fabrication.
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