Desktop Printing of Conductive Nanostructures

Printed electronics is an emerging field, wherein the aim is to use "additive" technologies to add material as required and form multi-layered functional electronic devices/systems.

A key process in fabricating such devices is that of fabricating electrodes for contacting the devices. Several companies, such as CimaNanotech, Cambrios, offer solutions based on complex nano-colloid formulations for fabricating electrodes in an additive step. These formulations are aimed at the flexible and transparent conductive electrode markets, also there have been several reports on the use of nanoparticle inks for fabricating desired patterns of electrodes on paper using inkjet printing, screen printing etc.

To overcome the complexities of formulating colloidal inks for passing through printer heads and nozzles without clogging, we adapted our room temperature tannic acid based protocol to inkjet print the precursor solutions onto paper in desired patterns and form the nanoparticles in situ on paper by reaction. We also played with this process to form secret/latent information on paper that could be developed later etc.

Sintering of the nanoparticles is an important additional step required to obtain acceptable conductivity levels. Several groups have come up with different techniques such as laser sintering, IR sintering, Salt-induced sintering to achieve this. However, this adds to the complexity and increases manufacturing costs.

Recently, we showed that highly conductive (sheet resistance of a few Ohms/Sq.) films can be obtained using a simple hp1010 desktop inkjet printer, a Tungsten halogen lamp, and standard photographic developer (The way photographs were made before the digital storage era). In essence, the process is an adaptation of a 19th century precursor of the 20th century silver halide photographic process called as "Salt Printing".

Electroadhesion is an interesting phenomena, wherein electrically switchable adhesive forces can be generated between a pliable interdigitated capacitor pattern and a nearby surface by applying a very high voltage (~few kVs) between the two leads of the capacitor. Very little current is drawn, however, and so the net power consumption is negligible. This phenomena is being explored for applications ranging from office solutions (Justick) to Industrial applications (GrabIT) and even being considered for combat/surveillance operations (SRI).

As the essential component of all these technologies is the fabrication of closely spaced (~1 mm or lower) interdigitated electrodes on a compliant surface, we decided to test electrodes fabricated using our print-develop process. The results are promising as seen in this video, where two glass slides (coloured black for visualisation) can be seen to adhere onto the paper (standard copier paper with electrodes printed onto it). The electrode spacing was ~0.7 mm and the resolution is limited by the resolution of inkjet printing on paper.

Note: the high kV source looks bulky, but that was the one available handily, High Voltage modules for attaching onto PCB's are available in the market.