Carbon Nanotube Thin-film Transistors Research Articles

Poly(ionic liquid) dielectric for high performing P- and N-type single walled carbon nanotube transistors

There is an increasing demand for low-cost and high-performance electronics which has stimulated a need for new high-performance dielectric materials. We have developed a facile synthesis of poly(2-(methacryloyloxy)ethyl trimethylammonium bis(trifluoromethylsulfonyl)azanide-ran-methyl methacrylate) (P(METATFSI-MMA)), a polymeric ionic liquid that can be used as a high-performance dielectric for semiconducting single walled carbon nanotube (SWCNTs) thin film transistors (TFTs). The P(METATFSI-MMA) polymer was synthesized at both 35 and 62 mol% of 2-(methacryloyloxy)ethyl trimethylammonium bis(trifluoromethylsulfonyl)azanide and produced p- and n-type devices that functioned under ambient conditions. These TFTs were then used to study the impact of electrochemical doping on the performance of SWCNT TFTs when switching from n-type, where an electrical double layer is formed, to p-type, where the TFSI− anions are free to interact with the SWCNTs. The TFTs operating in p-type had higher current on/off ratios and a larger transconductance than those operating in n-type, which is characteristic of electrochemically doped transistors. Furthermore, we tested the impact of operating frequency on device performance and discovered that decreasing the operating frequency of the TFTs resulted in a decreased hysteresis. The decrease in hysteresis was also observed to be more significant for the 35 mol% polymer.

Highly sensitive SWIR photodetector using carbon nanotube thin film transistor gated by quantum dots heterojunction

Low-dimensional semiconductors have been considered excellent materials to construct photodetectors for infrared detection with an easy process and excellent compatibility but suffer from low detectivity mainly owing to the poor light absorption of the ultra-thin body. Here, we demonstrate a thin film transistor (TFT) based short-wave infrared photodetector consisting of a carbon nanotube (CNT) TFT gated by a PbS colloidal quantum dots (CQDs) based heterojunction. The thick PbS CQDs' film efficiently absorbs infrared light and then excites and separates electron–hole pairs to generate a photovoltage at the pn heterojunction of the PbS CQDs/ZnO film. The photovoltage is further amplified and transduced in situ by the CNT TFT under the heterojunction, and then the detector featured a specific detectivity of 5.6 × 1013 Jones under 1300 nm illumination and a fast response of the sub-ms level (0.57 ms). The CQDs based heterojunction gating TFT represents a universal architecture for highly sensitive low-dimensional semiconductor based infrared photodetectors, competitive with state-of-the-art epitaxial semiconductors and enabling monolithic integration technology.

Low-temperature supercritical activation enables high-performance detection of cell-free DNA by all-carbon-nanotube transistor

Biosensors based on carbon nanotube (CNT) thin-film transistors (TFTs) have drawn intense interests for their exhibiting great potential in ultrasensitive and label-free DNA detection. However, the inevitable defects in the dielectric and channel of CNT-based TFTs may induce noisy or unreliable output signals. Here, we propose a low-temperature supercritical carbon dioxide (SCCO2) fluid activation method to enhance the performance of transistor-based biosensors. An activation model of SCCO2 treatment is proposed to elaborate that the SCCO2 treatment can repair the dangling bonds of SiO2 dielectric facilely and passivate the hydroxyl groups effectively. Furthermore, a cell-free DNA detection platform is built up based on all-carbon-nanotube TFTs (AC-TFTs) modified by the CW-Peptide probe. By adopting the SCCO2 treatment, the response of AC-TFT biosensors to AKT2 gene related to triple-metastatic negative breast cancer can be enhanced by more than 114% with high uniformity, and a broad detection range of six orders of magnitude is yielded with a theoretical limit of detection of 42 fM. Additionally, the response has a linear correlation with the length of DNA targets. The nondestructive and eco-friendly SCCO2 activation method provides a promising and universal strategy for achieving highly accurate and sensitive transistor-based biosensors in future clinical applications.

Highly temperature-tolerant p-type carbon nanotube transistor doped with 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile

The development of chemical doping methods for carbon nanotubes (CNTs) is essential for various electronic applications. However, typical p-doping methods for CNT thin-film transistors (TFTs), using oxygen and water from the atmosphere, are quite sensitive to changes in the surrounding environment, and thus, their poor temperature tolerance is a critical problem during device fabrication. As a p-dopant for CNT–TFTs, we used 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN), which is a strong electron acceptor aromatic compound. The HATCN-doped CNT–TFTs exhibited p-type characteristics after exposure to a high-temperature environment of 200 °C, and prolonged heating did not degrade the p-doping performance of HATCN. In addition, stable p-type characteristics even under ambient conditions were obtained by encapsulating the surface of the device with a Parylene–Al2O3 bilayer.

Highly sensitive and selective H2S sensors with ultra-low power consumption based on flexible printed carbon-nanotube-thin-film-transistors

Flexible printed carbon-nanotube-thin-film-transistors (CNT-TFTs) with characteristics of low cost, low operating voltage, low power consumption, and signal amplification have great potential for emerging electronics powered by self-powered systems or low-power density thin-film solar cells. In this work, we have fabricated flexible printed CNT-TFTs using the printing process to selectively deposit semiconducting CNTs in device channels as channel materials, printed silver electrodes as side gates, and printed ion gel as the dielectric layer. The as-prepared TFTs exhibited an extremely high sensitivity to H2S (up to 1565% ppm-1) at room temperature (25 ℃) and the relative humidity (RH) of 5%, excellent signal amplification (response increased from 1% to 4045.5% towards 2 ppm H2S), ultra-low working power consumption (1.03 nW under 2 ppm H2S), excellent selectivity, fast response/recovery time (10 s/26 s) and great mechanical flexibility (stable after 10,000 times bending with 5 mm radius). The outstanding improvement of sensor characteristics was related to the enrichment and absorption of H2S by ionic liquids, and the resistor-mode device's response to 2 ppm H2S increased from ~1% to 80% after the device channel was covered by ion gel. To the best of our knowledge, our H2S sensors demonstrated the recorded low power consumption (1.03 nW) and high response (4045.4%) when exposed to 2 ppm H2S at room temperature, making our sensors a promising candidate for portable applications in gas detection.

PMMA/Al2O3 bilayer passivation for suppression of hysteresis in chemically doped carbon nanotube thin-film transistors

Hysteresis is usually present in carbon nanotube thin-film transistors exposed to air due to adsorbed water and oxygen molecules. Thus, it is desirable to passivate the device from these environmental effects and provide an air-stable platform for chemical doping to tune the threshold voltages. Here, we demonstrate p- and n-doped carbon nanotube transistors with suppressed hysteresis using bilayer stacking of poly(methyl methacrylate) and aluminum oxide (Al2O3) passivation layers using a low-temperature process suitable for flexible substrates. The results show that the bilayer passivation layers achieved reduced hysteresis to be 2.25% of applied gate voltage at low operation voltage as 2 V.

Hysteresis Suppression of Carbon Nanotube Thin-Film Transistor Using Laminated HfO₂/Al₂O₃ by ALD

Laminated hafnium oxide (HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> )/alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) fabricated by atomic layer deposition (ALD) process is employed as an encapsulation to improve the performance of carbon nanotube thin-film transistor (CNT-TFT). The outstanding hysteresis suppression ability of laminated HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is demonstrated in the transfer characteristic of thin-film transistor (TFT) devices, with the threshold voltage variation ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta{V}_{\text {th}}$ </tex-math></inline-formula> ) of different scanning directions decreasing from ~11.06 to ~0.48 V after encapsulation, which is mainly attributed to the removal of water and oxygen molecules adsorption on the carbon nanotubes (CNTs) backchannel surface and the passivation effect to the defects at the interface between the CNTs and the gate insulator. It appears that the CNT-TFT with laminated HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with an optimized 5 nm/5 nm structure exhibits excellent hysteresis suppression ability and performance improvement. In addition, the TFT devices with the optimal laminated HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layers also show great reliability and gate bias stress stability.

Enhanced current path by circularly and periodically-aligned semiconducting single-walled carbon nanotubes for logic circuit device

Improving the performance of solution-processed single-walled carbon nanotube thin film transistors (SWCNT TFTs) is essential to their wide usage in next generation large-area electronic devices. However, uncontrollable tube-tube junction and random network formation from conventional solution processes of SWCNTs has limited mobility and on-current level of SWCNT TFTs. Herein, we demonstrate a facile method by switching idea of reducing coffee-ring of the conventionally solution-processed or inkjet-printed thin films. Spontaneous coffee-ring formation of the inkjet-printed droplets is found to enhance directional alignment of SWCNTs in the outer rim of the coffee-rings. The evaporation-driven capillary flow toward the rim inside induces migration of SWCNT and thus forms densely aligned SWCNT rings. Periodic connection of such rings can provide high-current path at a given voltage. Therefore, by additionally forming the periodically connected rings on a pre-established random network of SWCNT in channel area of TFTs, we significantly improved the mobility and I on/I off ratio of SWCNT TFTs without degradations in other electrical parameters such as threshold voltage and subthreshold swing. We also demonstrated all-solution-processed inverters with higher voltage-gain in comparison with conventional ones.

Printed carbon nanotube thin film transistors based on perhydropolysilazane-derived dielectrics for low power flexible electronics

Electric double layer (EDL) electrolyte dielectrics have attracted great interest for low voltage and low power consumption portable thin film transistor (TFT) devices and circuits due to their high gate modulation efficiency. In this work, fully printed single-walled carbon nanotube (SWCNT) TFTs are fabricated using proton conducting inorganic perhydropolysilazane (PHPS) derivatives as the EDL dielectrics at low frequencies (<1 kHz) that can be processed in humidity even at quite low temperature (<100 °C) and relatively short reaction time (∼10 min). Fabricated fully printed SWCNT TFTs exhibit excellent electrical properties with low operating voltage (±1 V), high on/off ratio (∼106), small subthreshold swing (70–110 mV/dec), good stability and the average carrier mobility of 6.3 cm2 V−1 s−1. In addition, the precise control of the SWCNT TFTs’ threshold voltage and polarity (from depleted p-type to enhanced p and n-type, and well-balanced ambipolar) is successful demonstrated by adjusting the printable sc-SWCNT ink concentrations and electron doping technology (using ethanolamine as the electron doping agent). Furthermore, fully printed flexible complementary metal oxide semiconductor (CMOS) and CMOS-like inverters based on as-prepared SWCNT TFTs represent extremely low static power consumption (0.1 pW/μm and 0.05 pW/μm, respectively), high voltage gain (up to 45 at VDD = 0.7 V) and relatively large noise margins.

Ionic dielectrics for fully printed carbon nanotube transistors: impact of composition and induced stresses.

Printed carbon nanotube thin-film transistors (CNT-TFTs) are candidates for flexible electronics with printability on a wide range of substrates. Among the layers comprising a CNT-TFT, the gate dielectric has proven most difficult to additively print owing to challenges in film uniformity, thickness, and post-processing requirements. Printed ionic dielectrics show promise for addressing these issues and yielding devices that operate at low voltages thanks to their high-capacitance electric double layers. However, the printing of ionic dielectrics in their various compositions is not well understood, nor is the impact of certain stresses on these materials. In this work, we studied three compositionally distinct ionic dielectrics in fully printed CNT-TFTs: the polar-fluorinated polymer elastomer PVDF-HFP; an ion gel consisting of triblock polymer PS-PMMA-PS and ionic liquid EMIM-TFSI; and crystalline nanocellulose (CNC) with a salt concentration of 0.05%. Although ion gel has been thoroughly studied, e-PVDF-HFP and CNC printing are relatively new and this study provides insights into their ink formulation, print processing, and performance as gate dielectrics. Using a consistent aerosol jet printing approach, each ionic dielectric was printed into similar CNT-TFTs, allowing for direct comparison through extensive characterization, including mechanical and electrical stress tests. The ionic dielectrics were found to have distinct operational dependencies based on their compositional and ionic attributes. Overall, the results reveal a number of trade-offs that must be managed when selecting a printable ionic dielectric, with CNC showing the strongest performance for low-voltage operation but the ion gel and elastomer exhibiting better stability under bias and mechanical stresses.

Study of Gate Induced Sensitivity Amplification in Carbon Nanotube Thin Film Transistor Based Ammonia Sensor

Study of Gate Induced Sensitivity Amplification in Carbon Nanotube Thin Film Transistor Based Ammonia Sensor

Stretchable Carbon Nanotube Thin-Film Transistor Arrays Realized by a Universal Transferable-Band-Aid Method

Stretchable electronics offer new possibilities for humans and artificial intelligent robots, in which stretchable transistors are the core components. Either geometric engineering or intrinsic material integration has its own limitations for realizing high-performance stretchable transistors. In this work, by combining the advantages of both approaches, we propose a novel transferable-Band-Aid method to realize stretchable carbon nanotube (CNT) thin-film transistor (TFT) arrays with high electrical performance and stretchability simultaneously. This method uses polyimide (PI) tapes to transfer devices to elastomer substrates. The mobility and ON/OFF-current ratio of the stretchable CNT-TFTs reach 24 cm2/ $\text{V}\cdot \text{s}$ and $1.1\times10$ 5, respectively. After stretched by 50% and 100% for 2000 cycles, the device performance is maintained on both polydimethylsiloxane and Eco-flex substrates. Neither obvious degradation on electrical properties nor displacement between the PI and the elastomer is observed during the stretching. The mechanical performance of the stretchable devices has also been verified and analyzed by simulation using ANSYS. Young’s modulus ratio between the stiff island and the elastomer substrate influences stretchability and stress distribution of the devices. The simulation result provides a guideline for realizing highly robust stretchable devices. The Band-Aid structure could generally be stuck to most elastomer substrates, providing a new solution to convert flexible devices into stretchable devices universally. The demonstrations of the design exhibit its excellent transferability, conformal capability, and versatility, showing its application potentials in wearable devices, biological-human interfaces and Internet of Things.

Efficient charge carrier control on single walled carbon nanotube thin film transistors using water soluble polymer coatings

Efficient charge carrier control on single walled carbon nanotube thin film transistors using water soluble polymer coatings

Flexible Light-to-Frequency Conversion Circuits Built with Si-Based Frequency-to-Digital Converters via Complementary Photosensitive Ring Oscillators with p-Type SWNT and n-Type a-IGZO Thin Film Transistors.

In this study, as system-level photodetectors, light-to-frequency conversion circuits (LFCs) are realized by i) photosensitive ring oscillators (ROs) composed of amorphous indium-gallium-zinc-oxide/single-walled carbon nanotube (a-IGZO/SWNT) thin film transistors (TFTs) and ii) phase-locked-loop Si circuits built with frequency-to-digital converters (PFDC). The 3-stage ROs and logic gates based on a-IGZO/SWNT TFTs successfully demonstrate its performance on flexible substrates. Herein, along with the advantage of scalability, a-IGZO films are used as photosensitive n-type TFTs and SWNTs are employed as photo-insensitive p-type TFTs for better photosensitivity in circuit level. Through the controlling a post-annealing condition of a-IGZO film, responsivities and detectivities of a-IGZO TFTs are obtained as 36 AW-1 and 0.3 × 1012 Jones for red, 93 AW-1 and 3.1 × 1012 Jones for green, and 194 AW-1 and 11.7 × 1012 Jones for blue. Furthermore, as an advanced demonstration for practical application of LFCs, a unique circuit (i.e., PFDC) is designed to analyze the generated oscillation frequency (fosc ) from the LFC device and convert it to a digital code. As a result, the designed PFDC can exactly count the generated fosc from the flexible a-IGZO/SWNT ROs under light illumination with an outstanding sensitivity and assign input frequencies to respective digital code.

Excess Polymer in Single-Walled Carbon Nanotube Thin-Film Transistors: Its Removal Prior to Fabrication Is Unnecessary.

Ultrapure semiconducting single-walled carbon nanotube (sc-SWNT) dispersions produced through conjugated polymer sorting are ideal candidates for the fabrication of solution-processed organic electronic devices on a commercial scale. Protocols for sorting and dispersing ultrapure sc-SWNTs with conjugated polymers for thin-film transistor (TFT) applications have been well refined. Conventional wisdom dictates that removal of excess unbound polymer through filtration or centrifugation is necessary to produce high-performance TFTs. However, this is time-consuming, wasteful, and resource-intensive. In this report, we challenge this paradigm and demonstrate that excess unbound polymer during semiconductor film fabrication is not necessarily detrimental to device performance. Over 1200 TFT devices were fabricated from 30 unique polymer-sorted SWNT dispersions, prepared using two different alternating copolymers. Detailed Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) studies of the random-network semiconductor films demonstrated that a simple solvent rinse during TFT fabrication was sufficient to remove unbound polymer from the sc-SWNT films, thus eliminating laborious polymer removal before TFT fabrication. Furthermore, below a threshold polymer concentration, the presence of excess polymer during fabrication did not significantly impede TFT performance. Preeminent performance was achieved for devices prepared from native polymer-sorted SWNT dispersions containing the "original" amount of excess unbound polymer (immediately following enrichment). Lastly, we developed an open-source Machine Learning algorithm to quantitatively analyze AFM images of SWNT films for surface coverage, number of tubes, and tube alignment.

Semi analytical model for electrical transport in single wall carbon nanotube thin film transistors

With the recent advances in carbon nanotube (CNT) electronics, transistors made of thin films of single wall CNTs (SWNTs) are gaining great attention due to their high quality electrical characteristics. With such improvements it has become essential to develop suitable models to accurately predict the electrical transport in such devices which in the current scenario remains scarce. This work presents a numerical simulation of current transport in SWNT thin film-based transistors (TFTs). The developed model considers both Ohmic and Schottky type contacts that are exhibited by such field effect transistors based on which two separate modes of transport for electrons and holes are described. The electron transport is described through a tunneling transmission process while the hole transport is described by a thermionic transmission process where the model treats each SWNT individualistically and determines the overall current through a novel iterative process. The accuracy of the proposed model is verified by comparing it with multiple experimental data.

Low-voltage carbon nanotube complementary electronics using chemical doping to tune the threshold voltage

Simultaneous controlling of the threshold voltage of both p- and n-type transistors, comprising complementary integrated circuits, is required to develop low-voltage and low-power flexible electronics. In this study, we report tuning the threshold voltage of carbon nanotube thin-film transistors with organic and metal-ion complex salts as dopants, and using device passivation to secure air-stability. Chemical doping affords simple yet precise control of the dopant level and enables the threshold voltages to be finely tuned. Complementary inverters were fabricated on a plastic substrate. Operation at a low supply voltage of 0.5 V was achieved with fairly high gain and noise margins.

In-Place Printing of Flexible Electrolyte-Gated Carbon Nanotube Transistors with Enhanced Stability.

Ion gel-based dielectrics have long been considered for enabling low-voltage operation in printed thin-film transistors (TFTs), but their compatibility with in-place printing (a streamlined, direct-write printing approach where devices never leave the printer mid- or post-process) remains unexplored. Here, we demonstrate a simple and rapid 4-step in-place printing procedure for producing low-voltage electrolyte-gated carbon nanotube (CNT) thin-film transistors at low temperature (80 °C). This process consists of the use of polymer-wrapped CNT inks for printed channels, silver nanowire inks for printed electrodes, and imidazolium-based ion gel inks for printed gate dielectrics. We find that the efficacy of rinsing CNT films and printing an ion gel in-place is optimized using an elevated platen temperature (as opposed to external rinsing or post-process annealing), where resultant devices exhibited on/off-current ratios exceeding 103, mobilities exceeding 10 cm2V-1s-1, and gate hysteresis of only 0.1 V. Additionally, devices were tested under mechanical strain and long-term bias, showing exceptional flexibility and electrochemical stability over the course of 14-hour bias tests. The findings presented here widen the potential scope of print-in-place (PIP) devices and reveal new avenues of investigation for the improvement of bias stress stability in electrolyte-gated transistors.

Printed Carbon Nanotube Thin Film Transistors Based on Perhydropolysilazane-Derived Dielectrics for Low Power Flexible Electronics

Printed Carbon Nanotube Thin Film Transistors Based on Perhydropolysilazane-Derived Dielectrics for Low Power Flexible Electronics

Printed solid state electrolyte carbon nanotube thin film transistors for sub-1 V fully printed flexible CMOS inverters

Flexible fully-printed single-walled carbon nanotube CMOS inverters with low operating voltage that exhibit excellent voltage gains, noise margins and static power consumption, and good mechanical flexibility have been achieved.