Carbon Nanotubes I Focus
Current techniques for separating SWNTs use high power sonication in a surfactant solution to individualise the SWNTs, and they’re kept in solution by wrapping them in surfactant. However, sonication separates the SWNTs by creating a series of small explosions which damage and shorten the tubes thus harming the inherent electronic properties and leaving surfactant in the films made from the solution.
Although TCFs made from these sonicated nanotube solutions can produce films good enough for some applications, in most cases, the required transparency and conductivity of the film is simply not high enough to meet performance needs. This is because the conductivity of a TCF made from SWNTs depends on a number of scaling factors, such as Purity: the purer the SWNT sample, the
more paths to conduction and the higher the conductivity of the film at a given transparency Bundle size: the smaller the bundle size, the more conductivity you can achieve at a given transparency Length: the longer the SWNTs, the fewer junctions between tubes are needed and the higher the conductivity.
The future of displays Linde’s salt enhanced electrostatic repulsion (SEER) technology address all of these factors and produces SWNT inks which can be used to make ‘best in class’ TCFs which meet current performance needs - and with the right partner - could be directly incorporated into current touch screens. SEER technology uses the power of
solvated electrons to produce negatively charged SWNTs which will repel one another and dissolve into solution without using any sonication, surfactant,
ultracentrifugation or functionalisation. When using SEER technology to produce SWNT inks the SWNTs remain the same length from start to finish. This means that if you start with long nanotubes you end up with a solution of long nanotubes which can be used to make exceptional TCFs.
How different? The SEER process begins when ammonia is condensed onto a mixture of SWNTs and an alkali metal such as sodium. In the reaction tube,
cooled to -79°C, the ammonia turns from gas to liquid and the alkali metal dissolves into the liquid ammonia, losing its outer shell electron to produce a strongly reducing solution of solvated electrons. Hydrogen bonding takes place in liquid ammonia which creates cavities within the ammonia structure where the solvated electrons can reside; these electrons can then be seen as an intense blue colour within the solution. Over time these solvated electrons are
transferred to the SWNTs as the metal intercalates between the SWNTs, giving
them a negative charge and causing the SWNT powder to expand within the liquid ammonia. When the reaction is completed the liquid ammonia is removed leaving behind an expanded nanotubide salt. This material contains negatively charged SWNTs and is extremely air sensitive; it must be handled in an air and moisture free environment. After addition of a polar aprotic solvent to the nanotubide salt electrostatic repulsion between the charged nanotubes causes them to spontaneously dissolve, resulting in individual SWNTs in solution. This is then sold by Linde nanomaterials as SEER Ink and can be used to deposit best in class TCFs as it contains long, individual and pure SWNTs.
What does this mean for the industry? The TCFs made from SEER Ink can be used in many different applications such as displays, touch screens, solar cells, thin film transistors
and electro chromic windows. The charge on the SWNTs within the SEER Ink has great potential for further functionalisation, for many potential applications including composites, sensors and in a biological context. Although SEER Ink is air sensitive and must be deposited in an inert environment, it can be deposited using almost any deposition technique including printing, spin coating and spray coating.
Although Linde nanomaterials is
currently focused on partnering with one of the big display manufacturers with the intention of developing products using our SEER Ink, we also recently launched the ink into the R&D market to foster innovation and the creation of exciting new applications. For example, a transparent GPS device embedded in the windscreen of a car; a screen for a mobile phone that can roll up to look like a pencil; or (one of my personal favorite ideas) a contact lens containing a display which would be completely see though when not active and partially see through when being used to surf the internet.
In addition to the exciting new technologies that SEER Ink will help to facilitate there is an emerging market for which SEER Ink is a great fit - smart phones that have the look and feel of high end phones but are produced and sold at a greatly reduced cost due to cheaper components and reduced functionality. This is something the market leaders are currently developing. The lower cost of SEER Ink compared to its counterparts makes it an ideal TCF material for use in these phones. With such great advancements being made in this space, the potential for devices being made using SEER Ink is limited only by your own imagination.
Linde Electronics |
www.linde-electronics.eu
Dr Sian Fogden is Market and Technology Development Manager for Linde Electronics' nanomaterials unit
www.cieonline.co.uk
Components in Electronics
October 2013 11
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