Joseph B. Andrews

Improving Contact Interfaces in Fully Printed Carbon Nanotube Thin-Film Transistors

Single-walled carbon nanotubes (CNTs) printed into thin films have been shown to yield high mobility, thermal conductivity, mechanical flexibility, and chemical stability as semiconducting channels in field-effect, thin-film transistors (TFTs). Printed CNT-TFTs of many varieties have been studied; however, there has been limited effort toward improving overall CNT-TFT performance. In particular, contact resistance plays a dominant role in determining the performance and degree of variability in the TFTs, especially in fully printed devices where the contacts and channel are both printed. In this work, we have systematically investigated the contact resistance and overall performance of fully printed CNT-TFTs employing three different printed contact materials-Ag nanoparticles, Au nanoparticles, and metallic CNTs-each in the following distinct contact geometries: top, bottom, and double. The active channel for each device was printed from the dispersion of high-purity (>99%) semiconducting CNTs, and all printing was carried out using an aerosol jet printer. Hundreds of devices with different channel lengths (from 20 to 500 μm) were fabricated for extracting contact resistance and determining related contact effects. Printed bottom contacts are shown to be advantageous compared to the more common top contacts, regardless of contact material. Further, compared to single (top or bottom) contacts, double contacts offer a significant decrease (>35%) in contact resistance for all types of contact materials, with the metallic CNTs yielding the best overall performance. These findings underscore the impact of printed contact materials and structures when interfacing with CNT thin films, providing key guidance for the further development of printed nanomaterial electronics.

Noninvasive Material Thickness Detection by Aerosol Jet Printed Sensors Enhanced Through Metallic Carbon Nanotube Ink

Demand for cheaper and more functional sensors continues to rise in an era when data can be used to improve health, safety, and efficiency in daily lives. In this paper, we present a fully printed sensor capable of noninvasive material thickness detection. By applying an oscillating signal between two millimeter-scale electrodes, the fringing electric field is measurably perturbed by a material placed directly on top of the electrodes, leading to a linearly varying capacitance with change in the material’s thickness. We simulate this electric field perturbation and experimentally demonstrate the linear correlation between capacitance and overlying material thickness. Various parameters, from sensor size and structure to substrate and ink materials, are studied to optimize the performance of the printed sensors. Sensors made of metallic carbon nano-tube ink yield the best sensitivity, exhibiting a capacitance change of 26 fF per mm thickness of rubber—ten times more sensitive than devices composed of silver nanoparticle ink. Finally, we demonstrate an effective application of the sensors in automobile tires. By applying the sensors directly beneath the tread (within the tire), mm changes in the tread depth are able to be detected in a 99% confidence interval. These findings provide a straightforward, low-cost approach for monitoring mm changes in material thickness using noninvasive, printed sensors applicable to innumerable Internet-of-Things (IoT) applications.

Patterned Liquid Metal Contacts for Printed Carbon Nanotube Transistors

Flexible and stretchable electronics are poised to enable many applications that cannot be realized with traditional, rigid devices. One of the most promising options for low-cost stretchable transistors are printed carbon nanotubes (CNTs). However, a major limiting factor in stretchable CNT devices is the lack of a stable and versatile contact material that forms both the interconnects and contact electrodes. In this work, we introduce the use of eutectic gallium-indium (EGaIn) liquid metal for electrical contacts to printed CNT channels. We analyze thin-film transistors (TFTs) fabricated using two different liquid metal deposition techniques-vacuum-filling polydimethylsiloxane (PDMS) microchannel structures and direct-writing liquid metals on the CNTs. The highest performing CNT-TFT was realized using vacuum-filled microchannel deposition with an in situ annealing temperature of 150 °C. This device exhibited an on/off ratio of more than 104 and on-currents as high as 150 μA/mm-metrics that are on par with other printed CNT-TFTs. Additionally, we observed that at room temperature the contact resistances of the vacuum-filled microchannel structures were 50% lower than those of the direct-write structures, likely due to the poor adhesion between the materials observed during the direct-writing process. The insights gained in this study show that stretchable electronics can be realized using low-cost and solely solution processing techniques. Furthermore, we demonstrate methods that can be used to electrically characterize semiconducting materials as transistors without requiring elevated temperatures or cleanroom processes.

Fully printed memristors from Cu-SiO 2 core-shell nanowire composites

An area of printed electronics in which additional development is necessary is printable non-volatile memory, which will be essential for the development of fully printed RFID tags and sensors with integrated data storage.[1] An approach to making a printable memory is to utilize materials that exhibit resistive switching; devices based on this mechanism are often referred to as memristors.[2] However, existing fully-printed memories using memristors have properties that do not allow for practical application, with inadequate cycling endurance ( 10 μs), or short retention times ( 2 NWs). When coated from solution, these devices have modest switching voltages (2 V), a fast switching speed (50 ns), good endurance (>104 cycles), and data retention times (4 days) comparable to other fully printed memristors.[6] This work reports the fabrication and characteristics of a Cu-SiO 2 NW/ethylcellulose composite that is aerosol printed in a fully-printed memristor array.

Correction to Improving Contact Interfaces in Fully Printed Carbon Nanotube Thin-Film Transistors

It has come to our attention that the contact resistance (Rc) data reported in Figure 4d and Table 1 of our original paper did not take into consideration the width of the carbon nanotube thin-film transistors (CNT-TFTs). Contact resistance was extracted using the transfer length method (TLM) using a plot of the total resistance measured under a certain voltage bias condition. Extrapolation from the linear fit of a TLM plot (as in Figure 3f) yielded 2Rc, but the units are kΩ. In the original paper, these units were inadvertently converted to kΩ·μm (a normalization to the device width) without modifying the actual Rc value. The proper approach is to multiply the extracted value (in kΩ) by the CNT-TFT width, which was 160 μm for all devices in this work. Hence, the originally reported Rc values were off by a factor of 160. This has been corrected in the updated Figure 4d and Table 1 values below. Note that Figure 4d was also a part of the table of contents graphic in the original paper; hence, the updated plot below should also serve to replace that graphic.