Addressing Signal Drift and Screening for Detection of Biomarkers with Carbon Nanotube Transistors.

Electrical biosensors, including transistor-based devices (i.e., BioFETs), have the potential to offer versatile biomarker detection in a simple, low-cost, scalable, and point-of-care manner. Semiconducting carbon nanotubes (CNTs) are among the most explored nanomaterial candidates for BioFETs due to their high electrical sensitivity and compatibility with diverse fabrication approaches. However, when operating in solutions at biologically relevant ionic strengths, CNT-based BioFETs suffer from debilitating levels of signal drift and charge screening, which are often unaccounted for or sidestepped (but not addressed) by testing in diluted solutions. In this work, we present an ultrasensitive CNT-based BioFET called the D4-TFT, an immunoassay with an electrical readout, which overcomes charge screening and drift-related limitations of BioFETs. In high ionic strength solution (1X PBS), the D4-TFT repeatedly and stably detects subfemtomolar biomarker concentrations in a point-of-care form factor by increasing the sensing distance in solution (Debye length) and mitigating signal drift effects. Debye length screening and biofouling effects are overcome using a poly(ethylene glycol)-like polymer brush interface (POEGMA) above the device into which antibodies are printed. Simultaneous testing of a control device having no antibodies printed over the CNT channel confirms successful detection of the target biomarker via an on-current shift caused by antibody sandwich formation. Drift in the target signal is mitigated by a combination of: (1) maximizing sensitivity by appropriate passivation alongside the polymer brush coating; (2) using a stable electrical testing configuration; and (3) enforcing a rigorous testing methodology that relies on infrequent DC sweeps rather than static or AC measurements. These improvements are realized in a relatively simple device using printed CNTs and antibodies for a low-cost, versatile platform for the ongoing pursuit of point-of-care BioFETs.

Solution Processed Schottky Diodes Enabled by Silicon Carbide Nanowires for Harsh Environment Applications.

Silicon carbide nanowires (SiC NWs) exhibit promising features to allow solution-processable electronics to be deployed in harsh environments. By utilizing a nanoscale form of SiC, we were able to disperse the material into liquid solvents, while maintaining the resilience of bulk SiC. This letter reports the fabrication of SiC NW Schottky diodes. Each diode consisted of just one nanowire with an approximate diameter of 160 nm. In addition to analyzing the diode performance, the effects of elevated temperatures and proton irradiation on the current-voltage characteristics of SiC NW Schottky diodes were also examined. The device could maintain similar values for ideality factor, barrier height, and effective Richardson constant upon proton irradiation with a fluence of 1016 ion/cm2 at 873 K. As a result, these metrics have clearly demonstrated the high-temperature tolerance and irradiation resistance of SiC NWs, ultimately indicating that they may provide utility in allowing solution-processable electronics in harsh environments.

Flexible, Print-in-Place 1D-2D Thin-Film Transistors Using Aerosol Jet Printing.

Semiconducting carbon nanotubes (CNTs) printed into thin films offer high electrical performance, significant mechanical stability, and compatibility with low-temperature processing. Yet, the implementation of low-temperature 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 × 105, channel mobility up to 10.7 cm2·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.

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.

Poly(oligo(ethylene glycol) methyl ether methacrylate) Brushes on High-κ Metal Oxide Dielectric Surfaces for Bioelectrical Environments.

Advances in electronics and life sciences have generated interest in "lab-on-a-chip" systems utilizing complementary metal oxide semiconductor (CMOS) circuitry for low-power, portable, and cost-effective biosensing platforms. Here, we present a simple and reliable approach for coating "high-κ" metal oxide dielectric materials with "non-fouling" (protein- and cell-resistant) poly(oligo(ethylene glycol) methyl ether methacrylate (POEGMA) polymer brushes as biointerfacial coatings to improve their relevance for biosensing applications utilizing advanced electronic components. By using a surface-initiated "grafting from" strategy, POEGMA films were reliably grown on each material, as confirmed by ellipsometric measurements and X-ray photoelectron spectroscopy (XPS) analysis. The electrical behavior of these POEGMA films was also studied to determine the potential impact on surrounding electronic devices, yielding information on relative permittivity and breakdown field for POEGMA in both dry and hydrated states. We show that the incorporation of POEGMA coatings significantly reduced levels of nonspecific protein adsorption compared to uncoated high-κ dielectric oxide surfaces as shown by protein resistance assays. These attributes, combined with the robust dielectric properties of POEGMA brushes on high-κ surfaces open the way to incorporate this protein and cell resistant polymer interface into CMOS devices for biomolecular detection in a complex liquid milieu.

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.