Networks of Conjugated Polymer-Wrapped Single-Walled Carbon Nanotubes through Controlled Drop-Dispensing for Thin-Film Transistors

Despite the simplicity of the drop-casting method todeposit filmsof single-walled carbon nanotubes (SWNTs) for applications such asprinted electronics, this technique presents many challenges and uncertaintiesthat are yet to be addressed. The coffee-ring effect is a known example,which results in an accumulation of solids around the edge of thedrop. This can be mitigated by increasing the temperature of the substrateto induce Marangoni flows, but it is unknown what the repercussionsof this change would be on dispersions of conjugated polymer-wrappedSWNTs and resulting thin-film transistor devices. In this study, wedemonstrate that the drop-casting method benefits greatly from theuse of an automated drop-dispenser coupled with a desktop robot, leadingto more consistent and improved charge-carrier mobilities (mu)and threshold voltages. We then present the effect of drop size andsubstrate temperature on the surface coverage, linear density, andappearance of the resulting SWNT networks. A reduction in surfacecoverage and linear density was noted with increasing temperatureabove 50 degrees C, in addition to increased bundling of the SWNTs,as observed in atomic force microscopy images. Raman spectroscopydemonstrated that drops cast at higher temperatures allow for a moreeven distribution of the SWNTs throughout the drop due to the formationof deposits during transitions between constant contact radius andconstant contact angle modes. The devices resulting from drop-castingat higher temperatures also exhibit decreased mobilities (mu)at smaller drop volumes.