Modified Ferritin For Gold and Silver Nanoparticle Synthesis
Printing Electronics
Printing electronics is a recent development whereby metal nanoparticles are mixed with organic solvents to form an ink that can be deposited in thin layers on various surfaces with inkjet printers or other printing techniques.
After printing the nano ink gets reduced to elemental metal structures by either heating at relatively low temperatures or applying intense light.
The printing of metal nanoparticles allows for a quick prototyping and in combination with the special properties of metal nanoparticles very thin layers made of moderately heat stable materials becomes possible.
To show that nanoparticles can indeed be made conductive at relatively low temperatures we ordered copper nanoparticles and either processed them further for creating a printable ink or used them directly for the production of conductive lines without further additives.
Since copper NP are known to oxidise when coming in contact with air all experiments needed to be done under N2 atmosphere.
Figure 1: Christoph working under a N2 atmosphere to protect the copper naoparticle from oxidising.
Only a few metals have a conductivity high enough to be practical for use in printed electronics. Copper (Cu) is cheap and widely available but has the disadvantage of oxidizing in ambient conditions. Cu also tends to agglomerate, which is unfavorable when used in NP ink.
Silver has the highest conductivity and does not oxidize making it a popular choice for flexible electronics. Gold on the other hand is very expensive but has the advantage of not reacting with biological systems.
Beside the advantage of being easily processed into metallic inks and printed into any 2D shape the high surface energy of metal NP leads to a much lower melting point compared to metallic particles with micrometer scales. When NPs are heated to around 200°C they melt and fuse together to form a solid surface (Sunho Jeong et al., 2011).
For Cu-based nano inks, better conductivity is achieved when elemental Cu NPs are used. Copper oxide (CuO) NPs are formed when the NP are produced under aerobic conditions. CuO NPs have lower conductivity therefore a protective coating layer applied to the Cu NPs is often used to prevent oxidation prior to sintering.
The use of commercial inkjet printers for printing NP based ink allows for a very low hurdle of entry into the area of do-it-yourself (DIY) electronics (Yoshihiro Kawahara. et al., 2013). Our tests also showed that it is possible to use a commercially available household printers for printing different solvents which can be used to create copper inks. But the print head of standard printer easily clogs when copper nanoparticles (40 nm) are added to the ink. Therefore we recommend more traditional printing methods like screen printing for the low cost production of electronics made from metal nanoparticles.
Figure 1: Different probes made from copper ink or pure copper nanoparticles printed via the screen print technique and heated to 340°C.
Figure 1: Our printed circuits under the TEM. The structure after heating the 40 nm particles to 340°C shows clusters of combined copper nanoparticles.
Figure 1: Measuring the electrical resistance of our fragile copper nanoparticle prints. Display shows an resistance of 0.9 mOhm.
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