Fabrication of Highly Dense Carbon Nanotubes with Improved Conductivity Using Shrinkable Polymer: Towards 4D Printing

Abstract

Carbon nanotubes (CNTs) are a form of carbon that is allotropic with a high electric conductivity, stability, and mechanical flexibility making them ideal for application in electronics, sensors, thin-film transistors, and storage devices.1 More specifically, CNTs are a promising channel material in thin-film transistors (TFT) with high-performance, high mobility, and low-cost processing due to their one-dimensional nature and excellent electrical properties.2-3 Various methods have been reported to coat/deposit CNTs on various substrates, such as 1) filtration which can form uniform films but using a relatively complicated process, 2) dip coating which is a simple process but lacks controllability, 3) transfer printing but the process is also complicated due to the CNTs small diameters and high adherence to the substrate, 4) ink-jet printing which produces a lot of material waste, 5) spray coating/spin coating which are simple processes but lack uniformity and generate material waste, and 6) drop-casting which is nonuniform and requires stamping to create dense and uniformly distributed CNTs.1-3 In this work, we propose a new CNTs-solution process flow based on drop casting on a shrinkable polymer which allows us to create a dense CNTs mesh with reduced porosity, and thus improve the resulting film conductivity using a single droplet of the solution. Thus, there will be no need to use multiple coating steps or a stamping mold to achieve higher density of CNTs, which suggests lower material waste and lower cost. Figure 1a shows the process flow which was performed on a 1 cm by 1 cm plastic heat-shrinkable plastic. First, a Kapton tape-based mask was applied and patterned using a CO2 laser to create square shaped holes for contacts deposition using sputtering. Next, CNTs were mixed with Isopropyl Alcohol (IPA) and sonicated to disperse the CNTs in the solution. Using a pipette, one drop (4 µL) of the solution is placed on the masked shrinkable paper and left to dry at room temperature. Finally, the mask is removed and heat is applied to create the shrinking effect. The approach is promising towards 4D printing as the shrinkable polymer and CNTs can be 3D printed and an external stimulus can induce the shrinkage effect.Using different temperatures for different durations, the effect of the substrate shrinking on the CNTs mesh is analyzed. Figure 1b shows that as the heating duration is increased at a fixed temperature, the resistance of the CNTs channel decreases. The reduction in the resistance is due to the decrease in the porosity of the mesh as confirmed by scanning electron microscopy (SEM) images depicted in Figure 1c. In fact, using SEM images, the sheet is found to shrink by 15% and 19% when heated at 110°C for 1 min and 130°C for 3 min, respectively. In addition, as a result of the shrinking of the substrate, CNTs experience compressive strain which increases the charge density in the material and thus further improves the mesh conductivity.4 The compression effect is observed using Raman spectroscopy using a 532 nm laser, where the initial G peak of the CNTs mesh4 was located at ~1571 cm-1 and after 1 min of heating at 110°C, the peak shifted to ~1575 cm-1 as shown in Figure 1d.In conclusion, we have demonstrated a new fabrication technique that results in high conductivity with a single drop in a single step by using the shrinking effect of the polymer. The results show that the conductivity will increase with increasing the time for a specific temperature due to stress and reduction of porosity. This will open the door for the fabrication of devices using less amount of material, and less waste at a lower cost.