Self Assembled Monolayers on Inkjet-Printed ITO

and Introduction: Ink-jet printing allows for cost efficient roll-to-roll manufacture of thin-film transistors (TFTs) over large areas, enabling next-generation transparent displays, touch panels, opto-electronics, and radio frequency identification [1-4]. TFT conductance and cutoff frequency scale inversely with gate length. However sub-micron gate lengths are incompatible with traditional printing techniques, which have ~ 20 μm resolution [5]. New methods have been investigated to make sub-micron gate length organic TFTs without advanced photolithography [4]. Here, we expanded that approach to treat inkjet-printed indium tin oxide (ITO) electrodes with hydrophobic octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) to repel away a second printed ITO electrode, yielding sub-micron electrode gaps. By combining this process with a solution-processed semiconductor, a cost-effective short channel TFT can be made. In this project, we refined inkjet printing of ITO nanoparticle ink to obtain uniform printed ITO patterns, which erre annealed to minimize sheet resistance. We used OTS to selectively manipulate the wettability of printed ITO versus various substrates. The effect of SAMs was quantified by contact angle measurements. Preliminary experiments show that it may be possible to use this process to fabricate narrow gaps between ITO electrodes. Experimental Procedure: A carbon-water ink was used to optimize printing parameters on a DMP-2800 Series Dimatix Inkjet printer (Fujifilm, Santa Clara, CA). An aqueous ITO nanoparticle (NP) dispersion with 18% wt ITO (In2O3 and SnO2) and an average particle size of 18 nm (US Research Nanomaterials, Houston, TX) was printed on sodalime glass, SiO2, Si3N4, and Parylene-C substrates. To optimize printed ITO resistance, 2 mm × 0.1 mm one-layer printed ITO patterns on glass were rapid thermal annealed (RTA) on a JetFirst 150 RTP for four minutes at temperatures of 400-600°C in O2, N2, or N2:O2 4:1. Resistance measurements were performed using applied voltages from -5V to 5V. Selected samples were treated with a 0.2% OTS/toluene solution for 20-25 minutes followed by sequential rinsing in toluene, acetone and isopropanol. Measurements of advancing contact angle were made with a VCA Optima series contact angle instrument (AST Products, Inc., Billerica, MA). Results and Discussion: The printing waveform was optimized to obtain uniform ITO lines with 150 nm thickness per printed layer by tuning Figure 1: Printed ITO sheet resistance vs. temperature of 4-min RTA in various ambient environments. Figure 2: Contact angle of water on ITO and other substrates before and after OTS-SAM treatment.