Solution Processable Carbon Nanotubes on Shrinkable Polymer for Enhanced Porosity and Conductivity for Sensing Application

Carbon nanotubes (CNTs) have a huge potential in electronic devices applications such as sensors and transistors due their high electric conductivity and stability. More specifically, a wide range of carbon-based nanomaterials, including carbon nanotubes (CNTs), have been used as humidity sensing medium due to their high conductivity, electrical sensitivity, and chemical stability. In terms of fabrication, various methods for coating/depositing CNTs on various substrates have been previously demonstrated, however, they are either complicated with a lot of material waste, or they require stamping to achieve dense and uniformly dispersed CNTs. In this work, we report a new process flow for achieving a porous, yet more conductive mesh of solution processable CNTs with minimized waste based on drop casting on a shrinkable polymer. Thus, even when a single droplet of the solution is used, the shrinking effect allows improving the resulting film conductivity. Metrology showed that by increasing the heating time at a specific temperature (e.g., 110°C), the CNTs channel area decreases (e.g., 1.3% after 1 min), however, the volume overall increases. This was confirmed using the profilometer where the thickness of the film was increased by 42% at 110°C after 1 min. Moreover, the overall increase in the volume was verified using the porosity test which showed an increasing in the CNTs surface area ~10 times when heated at 130°C for 3 min. Due to the shrinking effect, the CNTs experience a compressive stress which was confirmed by Raman spectroscopy, which showed a shifting of the G peak from 1571 to 1575 after 1 min of heating at 110°C. Moreover, Hall measurements using the physical property measurement system (PPMS) showed that the conductivity of the CNTs film increased by 92.4% after shrinking while the mobility decreased by 44.9%. In fact, the charge density in the CNTs increases under the experienced compressive strain which makes its effect dominant on the overall increase in the film conductivity, despite the degradation in the charge mobility. As an application, the CNTs mesh was used in humidity sensing application. A micro probe system was used to test the device at different humidity levels. The system was connected to the humidity control system, N₂supply, DI water, and Keithley 4200A-SCS system. The resistive based sensors were tested with and without the shrinking effect. The humidity level was changed from 10% to 90% with a step of 10% and the devices showed a higher change in resistance after shrinking and thus a higher sensitivity compared to the initial intact devices. For instance, with a 30% relative humidity (RH), the resistance increased by 16.3% after 3 min of heating to achieve the shrinking effect compared to 8.7% for the initial device, while at 70% RH, these numbers change to 25.4% and 20.7%, respectively.