Fabrication and characterization of Cu-based nanoink for the application in printed electronics

Abstract

The printed electronics technology is a solution process for various functional ink materials for direct printing ink using the process technique to produce a variety of electronic devices as an attractive alternative to conventional patterning techniques. Electronic device is manufactured through a printing process, when compared to conventional lithography process has various advantages. First, a variety of processes without expensive vacuum equipment can dramatically lower the cost of the process, toxic waste, and continuous process which through the process speed can be also increased. Second, the energy consumption used to maintain the process can be reduced, and only the desired portion of the electronic device can be produced selectively so that the emission of unwanted waste of chemical may be minimized. The functional electronic ink printing processes for the development of a variety of electronic devices can be said to be a key factor. The most important requirements of materials are as follows

High solubility in various solvents and stable dispersion. Also have the necessary electrical properties, hold the manufacturing cost must be low, and for the application to a flexible electronic device, sintering temperature should be relatively low. Therefore, in my doctoral thesis, nanoparticles with low oxidation properties were synthesized to obtain high conductivity with low cost. In Chapter two, Ag-Cu bimetallic nanoparticles were synthesized using a two pot thermal decomposition process to reduce the oxidation properties of Cu nanoparticles. Galvanic displacement was used as main mechanism for synthesizing bimetallic nanoparticles. As a results, Ag-Cu nanoparticles with a diameter of 13.9 nm was synthesized and analyzed by TEM, XRD, XPS and SEM. To see the resistance to oxidation, experimental and theoretical researches were performed and bimetallic nanoparticles show better properties when compared to monometallic Cu nanoparticles. For the application in printing process, ink based on synthesized Ag-Cu nanoparticles is fabricated and electric conductivity is measured after thermal and photonic sintering process. In Chapter three, nanosized inks were fabricated with Ag-Cu nanoparticles varying the surface ligand materials. Sintering temperature and electrical conductivity are important factors for using nanoinks in flexible electronic devices. Therfore, decrease of sintering temperature was tried by surface modification of nanoparticles with tetramethyl ammonium hydroxide solution. The synthesized nanoparticles capping by oleylamine are sonicated in 1wt % of tetramethyl ammonium hydroxide solution. Then, surface modified nanoparticles are used to fabricate nanoink and electrical properties were examined by thermal sintering process. In order to evaluate the electrical characteristics of the nanoparticles, electrical contact between particles must be derived, therefore surface morphology of thermally sintered ink is analyzed by SEM. The electrical conductivity of surface modified nanoparticle based ink show better conductivity at low sintering temperature approximately 200 ˚C. This result can be used on plastic substrate such as polyimide. In Chapter four, finally Cu only nanoparticles without oxidation are synthesized by aqeuous synthetic methods. Reducing agent, L-ascorbic acid, is not a very strong reductant, however, other surface capping molecule successfully protect Cu particles from oxidation. Also, synthesized Cu particles are bigger compared to the Ag-Cu particles explained in chapter two, therefore handling was easier during the synthesis and printing procedure. Cu particles with a diameter of 450 nm were used to fabricate highly viscous ink and printed in multidimensional structure. After thermal sintering process for 60 min at 200, 250, 300 ˚C, Cu film show electrical conductivity of 108, 10.4 and 8.8 $\mu \Omega$·cm, respectively. And the conductivity was stable for more than 30 days in ambient condition. These certainly show the Cu ink can be used in many electrode part in flexible or multidimensional structure applications.