Enrichment of Semiconducting Single-Walled Carbon Nanotubes with Indigo-Fluorene-Based Copolymers and Their Use in Printed Thin-Film Transistors and Carbon Dioxide Gas Sensors

Synergistic p-Doping of Polymer-Wrapped Small-Diameter Single-Walled Carbon Nanotubes by Tris(pentafluorophenyl)borane

Despite its comparatively low electron affinity, tris(pentafluorophenyl)borane (BCF) has been widely explored as an efficient molecular p-dopant for semiconducting polymers through the formation of Brønsted acidic complexes as well as its high affinity toward Lewis-basic nitrogen moieties. Many conjugated polymers that are used for selective wrapping and dispersion of semiconducting single-walled carbon nanotubes (SWCNTs) such as poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(6,6'-(2,2'-bipyridine))] (PFO-BPy) contain nitrogen moieties that should promote interaction with BCF. Here, we demonstrate that BCF indeed efficiently p-dopes even small-diameter (6,5) SWCNTs that are wrapped with large-bandgap PFO-BPy as corroborated by bleaching of the main absorption peaks and the appearance of red-shifted trion absorption and emission. In contrast, SWCNTs that are wrapped with poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) without any Lewis-basic nitrogen moieties are only mildly doped. UV-Vis-NIR absorption, 19F NMR, and 11B NMR spectra confirm that BCF dopes the bipyridine-containing PFO-BPy but not PFO, thus leading to a proposed doping mechanism that relies on the unique interactions between BCF, the bipyridine moieties in PFO-BPy, and the nanotubes. Since BCF doping of PFO-BPy-wrapped (6,5) SWCNTs is more efficient than doping with F4TCNQ and more stable than doping with AuCl3, it provides a reliable alternative for spectroscopic studies of the interactions of charge carriers and excitons in SWCNTs.

Separation of Highly Pure Semiconducting Single-Wall Carbon Nanotubes in Alkane Solvents via Double Liquid-Phase Extraction

This study delves into the distinctive selective property exhibited by a non-conjugated cholesterol-based polymer, poly(CEM11-b-EHA7), in sorting semiconducting single-walled carbon nanotubes (s-SWCNTs) within isooctane. Comprised of 11 repeating units of cholesteryloxycarbonyl-2-hydroxy methacrylate (CEM) and 7 repeating units of 2-ethylhexyl acrylate (EHA), this non-conjugated polymer demonstrates robust supramolecular interactions across the sp2 surface structure of carbon nanotubes and graphene. When coupled with the Double Liquid-Phase Extraction (DLPE) technology, the polymer effectively segregates s-SWCNTs into the isooctane phase (nonpolar) while excluding metallic SWCNTs (m-SWCNTs) in the water phase (polar). DLPE proves particularly efficient in partitioning larger-diameter s-SWCNTs (0.85-1.0 nm) compared to those dispersed directly in isooctane by poly(CEM11-b-EHA7) using direct liquid-phase exfoliation (LPE) techniques for diameters ranging from 0.75 to 0.95 nm. The DLPE method, bolstered by poly(CEM11-b-EHA7), successfully eliminates impurities from s-SWCNT extraction, including residual metallic catalysts and carbonaceous substances, which constitute up to 20% of raw HiPCO SWCNTs. DLPE emerges as a scalable and straightforward approach for selectively extracting s-SWCNTs in nonpolar, low-boiling-point solvents like alkanes. These dispersions hold promise for fabricating fast-drying s-SWCNT inks, which are ideal for printed and flexible thin-film transistors.

1D and 2D Field Effect Transistors in Gas Sensing: A Comprehensive Review

Rapid progress in the synthesis and fundamental understanding of 1D and 2D materials have solicited the incorporation of these nanomaterials into sensor architectures, especially field effect transistors (FETs), for the monitoring of gas and vapor in environmental, food quality, and healthcare applications. Yet, several challenges have remained unaddressed toward the fabrication of 1D and 2D FET gas sensors for real-field applications, which are related to properties, synthesis, and integration of 1D and 2D materials into the transistor architecture. This review paper encompasses the whole assortment of 1D-i.e., metal oxide semiconductors (MOXs), silicon nanowires (SiNWs), carbon nanotubes (CNTs)-and 2D-i.e., graphene, transition metal dichalcogenides (TMD), phosphorene-materials used in FET gas sensors, critically dissecting how the material synthesis, surface functionalization, and transistor fabrication impact on electrical versus sensing properties of these devices. Eventually, pros and cons of 1D and 2D FETs for gas and vapor sensing applications are discussed, pointing out weakness and highlighting future directions.

Amine-Functionalized Black Phosphorus Nanosheets toward Ultrafast and Room-Temperature Trace Carbon Dioxide Sensing

Carbon dioxide (CO₂) poses a significant effect on global climate, indoor activity, and crop yield, thus necessitating real-time and high-performance detection. Traditional CO₂-sensing materials always suffer from weak and sluggish reaction, elevated operation temperature, and poor detection limit. To surmount these obstacles, in this work a series of amine-rich polymer functionalized black phosphorus nanosheets (BP) were prepared for room-temperature CO₂ detection. Superior to TMMAP or 3-DEAPTES modified counterparts, the BP-10% APTES sensor delivered a response of 28.5% and ultrafast response/recovery time of 4.7 s/4.8 s toward 10 ppm of CO₂ under 36% RH at 22 °C, a lowest detection limit of 5 ppm, as well as excellent selectivity. Also, a nice repeatability and long-term operation stability were demonstrated. Thus, BP-APTES composites offer a promising strategy for high-performance CO₂ detection in terms of high sensitivity, low power-consumption, and convenient fabrication, and showcase brilliant prospects in portable optoelectronic detection systems and the Internet of Things.

Networks of as-dispersed, polymer-wrapped (6,5) single-walled carbon nanotubes for selective Cu2+ and glyphosate sensing

Networks of semiconducting single-walled carbon nanotubes (SWNTs) can be used as the transducing layer for sensors based on water-gated transistors. To add specific sensing capabilities, SWNTs are often functionalized with additional moieties or selective membranes are applied, thus increasing the complexity of the fabrication process. Here we demonstrate that drop-cast networks of monochiral (6,5) SWNTs, which are commonly dispersed in organic solvents with the polyfluorene-bipyridine copolymer PFO-BPy, can be employed directly and without additional functionalization or ion-selective membranes to detect Cu²⁺ ions over a wide range of concentrations in aqueous solutions. The observed voltage shifts of water-gated transistors with these (6,5) SWNT networks directly correlate with the cupric ion concentration. They result from induced n-doping due to the complexation of positive copper ions to the bipyridine units of the wrapping polymer. Furthermore, the competitive binding of Cu²⁺ to the herbicide glyphosate as well as to biologically relevant pyrophosphates can be used for the direct detection and quantification of these molecules at nano- to micromolar concentrations.

Surface acoustic wave sensor based on Au/TiO2/PEDOT with dual response to carbon dioxide and humidity

A surface acoustic wave (SAW) gas sensor with an Au/TiO₂/poly(3,4-ethylenedioxythiophene) (PEDOT, which is a conductive polymer with photoelectric conversion function) sensing film was constructed for the quantitative detection of water vapor and CO₂. The Au/TiO₂/PEDOT sensing film was assembled on the delayed region of the 204 MHz SAW delay line, which was used as the base device for the gas sensor. The center frequency of the sensor decreases with an increase in relative humidity (RH), and the center frequency increases with increasing CO₂ concentration, so that not only can the two gases be identified, but quantitative analysis can also be performed. The SAW sensor has a response range of 5%-90% for RH and a response range of 500-2000 ppm for CO₂ gas. The shifts in center frequency varied linearly with the concentrations, giving rise to the sensitivities of -0.0068 and -0.1880 kHz %^-1 for RH and ∼0.003 kHz ppm^-1 CO₂. The response/recovery time is 9 s/9.2 s for 700 ppm CO₂ and 15 s/14 s for 70% RH. The experimental results show that the SAW sensor offers excellent selectivity, wide response range, rapid response, and good stability and repeatability. The mechanism of humidity and CO₂ sensing is attributed to the hydrophilic porous structure of the Au/TiO₂/PEDOT sensing film, and also to the reversible variation of its viscoelasticity under illumination conditions. The sensor, combined with the communication function of its own SAW device, has several prospective applications in the monitoring of atmospheric conditions.

Strategies for the performance enhancement of graphene-based gas sensors: A review

Gas sensors have aroused much attention in recent years for the important effect in modern society. Graphene, with unique structure and characteristic properties, has been considered as a promising candidate for fabricating high-performance gas sensor. Great efforts in current research are directed towards exploiting various graphene-based gas sensors, but the core of gas sensing study is how to enhance the gas sensing performance. Herein, we propose a perspective that focuses on the strategies for sensing performance enhancement of graphene-based gas sensors. Several strategies are reviewed such as the modification of graphene with organic molecules, functionalization by metal oxide or noble metals, and nanostructural engineering. Particular emphasis is also provided to clarify the mechanism for the gas sensing enhancement. Further, the sensor device design is also concerned for the significant effect on reaching full potential of the gas sensing materials and realizing multifunctional integration. Finally, the opportunities and challenges for the development of gas sensors are pointed out.

Large-Area Flexible Printed Thin-Film Transistors with Semiconducting Single-Walled Carbon Nanotubes for NO2 Sensors

Development of large-area, low-cost, low-voltage, low-power consumption, flexible high-performance printed carbon nanotube thin-film transistors (TFTs) is helpful to promote their future applications in sensors and biosensors, wearable electronics, and the Internet of things. In this work, low-voltage, flexible printed carbon nanotube TFTs with a large-area and low-cost fabrication process were successfully constructed using ultrathin (∼3.6 nm) AlO_x thin films formed by plasma oxidation of aluminum as dielectrics and screen-printed silver electrodes as contact electrodes. The as-prepared bottom-gate/bottom-contact carbon nanotube TFTs exhibit a low leakage current (∼10^-10 A), a high charge carrier mobility (up to 9.9 cm² V^-1 s^-1), high on/off ratios (higher than 10⁵), and small subthreshold swings (80-120 mV/dec) at low operation voltages (from -1.5 to 1 V). At the same time, printed carbon nanotube TFTs showed a high response (ΔR/R = 99.6%) to NO₂ gas even at 16 ppm with a faster response and recovery speed (∼8 s, exposure to 0.5 ppm NO₂), a lower detection limit (0.069 ppm NO₂), and a low power consumption (0.86 μW, exposure to 16 ppm NO₂) at a gate voltage of 0.2 V at room temperature. Moreover, the printed carbon nanotube devices exhibited excellent mechanical flexibility and bias stress stability after 12,000 bending cycles at a radius of 5 mm and a bias stress test for 7200 s at a gate voltage of ±1 V, which originated from the ultrathin and compact AlO_x dielectric and the super adhesion force between screen-printed silver electrodes and polyethylene terephthalate substrates.