The research objective of this GOALI project is to create new low temperature, nanoparticle-based printing processes for inorganic semiconductor nanoparticle inks in order to enable fabrication of low cost, all-printed devices for wireless applications, such as WiFi and cellular communications. Although inorganic semiconductor nanoparticles may sinter at lower temperatures than their bulk counterparts, these temperatures are generally significantly higher than 150C, the maximum processing temperature allowable with low cost polyester or paper substrates. New low temperature sintering processes will be explored to improve the semiconducting performance of the printed nanoparticle films without increasing the maximum temperature during processing above this target temperature. The proposed approach includes preparation of nanoscale films from model semiconductor nanoparticle inks using non-proprietary benchmark printing platforms. The nanostructural characteristics of the starting powders and resulting films will be related quantitatively to device performance and film mobility. The processing conditions necessary to increase particle-particle contact area will be explored using two new processing methods: precipitation sintering for oxide semiconductors and metal nanofilm mediated sintering for core-shell structured elemental nanopowders. This project will focus on inorganic semiconductor nanoparticle ink systems with low toxicity, whose elemental constituents have not been banned in electronics by the European Union. (Recent EU legislation (WEEE and RoHS) bans the use of Cd, Pb, and Hg in electrical and electronic devices.) By combining theory and in-situ and ex-situ electron microscopy experiments with measurements of the semiconducting performance of nanoparticle printed films, the proposed research will seek a fundamental understanding of how nanoparticle structure and the resulting structure of the nanoparticle-based printed films limit the semiconducting properties in the films as compared that observed in bulk single crystals. This research project will contribute to the understanding of size- and shape-dependence of nanoparticle sintering, to the understanding of interface processes for the proposed innovative processing techniques, and to the establishment of materials science-based guidelines for the design of nanoparticles, nanoparticle inks, and printing processes for current and next generation printed electronics. The impact of this research is expected to be significant, allowing the use of printing technologies in a wide range of applications, including but not limited to, emissive flexible displays, all-printed mobile devices (e.g., a paper cellphone) and pervasive use of high performance sensors. This is far beyond what is currently achievable through non-sintered inorganic ink systems or organic printed electronics. Such low temperature sintering processes might be adaptable, as well, to the fabrication of inorganic nanoparticle coatings and thin films for a wide range of non-electrical applications.
