Semiconducting carbon nanotubes (CNTs) printed into thin films offer high electrical performance, significant mechanical stability, and compatibility with lowtemperature processing. Yet, the implementation of lowtemperature printed devices, such as CNT thin-film transistors (CNT-TFTs), has been hindered by relatively high process temperature requirements imposed by other device layersdielectrics and contacts. In this work, we overcome temperature constraints and demonstrate 1D− 2D thin-film transistors (1D−2D TFTs) in a low-temperature (maximum exposure ≤80 °C) full print-in-place process (i.e., no substrate removal from printer throughout the entire process) using an aerosol jet printer. Semiconducting 1D CNT channels are used with a 2D hexagonal boron nitride (h-BN) gate dielectric and traces of silver nanowires as the conductive electrodes, all deposited using the same printer. The aerosol jet-printed 2D h-BN films were realized via proper ink formulation, such as utilizing the binder hydroxypropyl methylcellulose, which suppresses redispersion between adjacent printed layers. In addition to an ON/ OFF current ratio up to 3.5 × 10 5 , channel mobility up to 10.7 cm 2 •V −1 •s −1 , and low gate hysteresis, 1D−2D TFTs exhibit extraordinary mechanical stability under bending due to the nanoscale network structure of each layer, with minimal changes in performance after 1000 bending test cycles at 2.1% strain. It is also confirmed that none of the device layers require high-temperature treatment to realize optimal performance. These findings provide an attractive approach toward a cost-effective, direct-write realization of electronics.

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 10 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.

A water-based silver nanowire ink for the room temperature printing of highly conductive traces onto biological and nonplanar surfaces.

No abstract

The Internet of Things (IoT) is the concept of a ubiquitous computing ecosystem in which electronics of custom form factors are seamlessly embedded into everyday objects. At the heart of the IoT are electronic sensors capable of detecting physical/environmental phenomena, translating these measurements into electrical signals, and wirelessly transmitting the data for remote computing. Critical to the development of IoT sensors and systems are low-cost materials, robust enough to sustain stable electrical performance over medium to long periods of time, yet sensitive enough to detect small changes in the surrounding environment. Such materials should be mechanically flexible and amenable to solution-based processing to facilitate large scale production methods, such as roll-to-roll printing. Carbon nanotubes (CNTs) are one of the leading material candidates to satisfy these requirements because of their unique electrical and mechanical properties, which enable robust and versatile devices, in combination with their chemical properties, which allow for the processing of CNTs from solution. These advantages have enabled demonstration of a myriad of printed CNT-based electronics and sensors on diverse substrates with wide ranges of functionality, spanning from simple sensors based on passive devices to complex multi-stage circuitry and display electronics. In this review, we provide a comprehensive summary of the CNT-based electronics and sensor space with an emphasis on applications aligned with the IoT. Primary coverage is devoted to devices consisting of randomly oriented CNT networks; however, the advantages and capabilities of single-nanotube devices will also be discussed. Key works across various types of sensors will be reviewed and a summary of the remaining challenges for CNT-based sensor technologies will be presented.

Semiconducting carbon nanotube (CNT) networks exhibit electrical, mechanical, and chemical properties attractive for thin-film applications, and printing allows for scalable and economically favorable fabrication of CNT thin-film transistors (TFTs). However, device-to-device variation of printed CNT-TFTs remains a concern, which largely stems from variations in printed CNT thin-film morphology and resulting properties. In this work, we overcome the challenges associated with printing uniformity and demonstrate an aerosol jet printing process that yields devices exhibiting a hole mobility of μ h = 12.5 cm 2 /V•s with a relative standard deviation as small as 4% (from over 38 devices). The enabling factors of such high uniformity include control of the CNT ink bath temperature during printing, ink formulation with nonvolatile and viscosifying additives, and a thermal treatment for polymer removal. It is discovered that a low CNT ink temperature benefits aerosol jet printing uniformity and stability in both shortterm (∼1 min) and long-term (∼1 h) printing settings. These findings shed light on the effect of a commonly overlooked dimension of CNT aerosol jet printing and provide a practical strategy for large-scale, high-consistency realization of CNT-TFTs.

Interest in flexible, stretchable, and wearable electronics has motivated the development of additive printing to fabricate customizable devices and systems directly onto virtually any surface. However, progress has been limited by the relatively high temperatures (>200 °C) required to sinter metallic inks and time-consuming process steps, many of which require removal of the substrate from the printer for coating, washing, or sintering. In this work, we addressed these challenges and demonstrate carbon nanotube thin-film transistors (CNT-TFTs) that are fabricated by aerosol jet printing with the substrate never leaving the printer. The full in-place printing approach, from first step to last, used a maximum process temperature of only 80 °C on the printer platen. Silver nanowire (Ag NW) ink was found to be most viable for low-temperature, in-place sintering while still yielding good electrical interfaces to the CNT thin-film channels. These aerosol-jet printed Ag NW films were conductive immediately after fabrication, which is the key component enabling rapid and sequential in-place printing. The devices exhibit on-currents as high as 80 μA/mm, effective mobilities of 12 cm 2 /(V•s), and on/off current ratios exceeding 10 5 . These findings provide a promising path forward toward the additive manufacture of flexible and stretchable electronics in a low-cost, highly customizable, and agile manner.

Silver nanoparticles (NPs) are the most widely used conductive material throughout the printed electronics space due to their high conductivity and low cost. However, when interfacing with other prominent printed materials, such as semiconducting carbon nanotubes (CNTs) in thin-film transistors (TFTs), silver is suboptimal when compared to more expensive or less conductive materials. Consequently, there would be significant value to improving the interface of printed silver to CNT films. In this work, the impact of nanostructure morphology on the electrical properties of printed silver and nanotube junctions in CNT-TFTs is investigated. Three distinct silver morphologies (NPs, nanoflakes -NFs, and nanowires -NWs) are explored with top-and bottom-contact configurations for each. The NF morphology in a top-contact configuration is found to yield the best electrical interface to CNTs, resulting in an average contact resistance of 1.2 MΩ ⋅ µm. Beyond electrical performance, several trade-offs in morphology selection are revealed, including print resolution and process temperature. While NF inks produce the best interfaces, NP inks produce the smallest features, and NW inks are compatible with low processing temperatures (<80 °C). These results outline the trade-offs between silver contact morphologies in CNT-TFTs and show that contact morphology selection can be tailored for specific applications.