Carbon Nanotube Thin-film Transistors Research Articles

Printed, Sub-3V Digital Circuits on Plastic from Aqueous Carbon Nanotube Inks

Printing electronic components on plastic foils with functional liquid inks is an attractive approach for achieving flexible and low-cost circuitry for applications such as bendable displays and large-area sensors. The challenges for printed electronics, however, include characteristically slow switching frequencies and associated high supply voltages, which together impede widespread application. Combining printable high-capacitance dielectrics with printable high-mobility semiconductors could potentially solve these problems. Here we demonstrate fast, flexible digital circuits based on semiconducting carbon nanotube (CNT) networks and high-capacitance ion gel gate dielectrics, which were patterned by jet printing of liquid inks. Ion gel-gated CNT thin-film transistors (TFTs) with 50 microm channel lengths display ambipolar transport with electron and hole mobilities >20 cm(2)/V.s; these devices form the basis of printed inverters, NAND gates, and ring oscillators on both polyimide and SiO(2) substrates. Five-stage ring oscillators achieve frequencies >2 kHz at supply voltages of 2.5 V, corresponding to stage delay times of 50 micros. This performance represents a substantial improvement for printed circuitry fabricated from functional liquid inks.

A low voltage operational single-walled carbon nanotube thin-film transistor containing a high capacitance gate dielectric layer produced by layer-by-layer deposition

This paper presents the characteristics of a low voltage driven thin-film transistor (TFT) containing an active layer coated with a single-walled carbon nanotube (SWCNT) network. To make the high capacitance gate dielectrics layers, poly(ethylene imine) and the titanium oxide precursor, titanium(IV) bis(ammonium lactate) dihydroxide, were coated by a layer-by-layer deposition method. Through this process we obtained a capacitance of 118 nF/cm 2 and a total thickness of about 70 nm. The SWCNT active layer was deposited by spray-coating onto the layered-gate dielectric using a solution containing purified SWCNTs and a polystyrene additive. We used polystyrene in the coating solution to increase the dispersion of SWCNTs in the 1-methyl-2-pyrrolidone solution and decrease current leakage though the TFT channels. The resulting TFT showed a mobility of 6.7 cm 2/V s, a threshold voltage of −0.88 V, and an on/off ratio of about 500 at operating voltages less than 2 V, which is suitable for the operation of various portable electronic devices. The output characteristics showed a good linear character and well-saturated behavior at elevated drain voltage.

Low-Voltage Operation of Ink-Jet-Printed Single-Walled Carbon Nanotube Thin Film Transistors

We demonstrate a low-voltage and less hysteresis operation of single-walled carbon nanotube thin film transistors (SWCNT-TFTs) using an ionic liquid gate dielectric layer. We fabricated the density controlled SWCNT-TFTs using the inkjet printing technique, where both source/drain electrodes and the semiconducting transistor channel were made from SWCNT films. As the gate dielectric, we have adopted a printable ionic liquid for future all-printable processes and achieved marked improvements in operating voltages and hysteretic response.

Self-Sorted Nanotube Networks on Polymer Dielectrics for Low-Voltage Thin-Film Transistors

Recent exploitations of the superior mechanical and electronic properties of carbon nanotubes (CNTs) have led to exciting opportunities in low-cost, high performance, carbon-based electronics. In this report, low-voltage thin-film transistors with aligned, semiconducting CNT networks are fabricated on a chemically modified polymer gate dielectric using both rigid and flexible substrates. The multifunctional polymer serves as a thin, flexible gate dielectric film, affords low operating voltages, and provides a platform for chemical functionalization. The introduction of amine functionality to the dielectric surface leads to the adsorption of a network enriched with semiconducting CNTs with tunable density from spin coating a bulk solution of unsorted CNTs. The composition of the deposited CNT networks is verified with Raman spectroscopy and electrical characterization. For transistors at operating biases below 1 V, we observe an effective device mobility as high as 13.4 cm(2)/Vs, a subthreshold swing as low as 130 mV/dec, and typical on-off ratios of greater than 1,000. This demonstration of high performance CNT thin-film transistors operating at voltages below 1 V and deposited using solution methods on polymeric and flexible substrates is an important step toward the realization of low-cost flexible electronics.

Ink-Jet Printing of a Single-Walled Carbon Nanotube Thin Film Transistor

Single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) were fabricated using ink-jet printing. We printed both thick source–drain electrodes and thin active semiconducting films using N,N-dimethylformamide (DMF)-based SWCNT dispersion. Despite the presence of metallic SWCNTs, the device exhibited field-effect behavior, with an effective mobility of 2.99 cm2 V-1 s-1 and an on/off current ratio of up to 75. The method used in this study is promising for the fabrication of large-scale high-performance SWCNT-TFTs.

Carbon nanotube thin film transistors based on aerosol methods

We demonstrate a fabrication method for high-performance field-effect transistors(FETs) based on dry-processed random single-walled carbon nanotube networks(CNTNs) deposited at room temperature. This method is an advantageous alternativeto solution-processed and direct CVD grown CNTN FETs, which allows usingvarious substrate materials, including heat-intolerant plastic substrates, and enablesan efficient, density-controlled, scalable deposition of as-produced single-walledCNTNs on the substrate directly from the aerosol (floating catalyst) synthesisreactor. Two types of thin film transistor (TFT) structures were fabricated toevaluate the FET performance of dry-processed CNTNs: bottom-gate transistors onSi/SiO2 substrates and top-gate transistors on polymer substrates. Devices exhibited on/off ratios up to105 and field-effectmobilities up to 4 cm2 V−1 s−1. The suppression of hysteresis in the bottom-gate device transfer characteristics by meansof thermal treatment in vacuum and passivation by an atomic layer depositedAl2O3 film was investigated.A 32 nm thick Al2O3 layer was found to be able to eliminate the hysteresis.

All ink-jet-printed carbon nanotube thin-film transistor on a polyimide substrate with an ultrahigh operating frequency of over 5 GHz

We report a flexible carbon nanotube (CNT) thin-film transistor (TFT) fabricated solely by ink-jet printing technology. The TFT is top gate configured, consisting of source and drain electrodes, a carrier transport layer based on an ultrapure, high-density (>1000 CNTs/μm2) CNT thin film, an ion-gel gate dielectric layer, and a poly(3,4-ethylenedioxythiophene) top gate electrode. All the TFT elements are ink-jet printed at room temperature on a polyimide substrate without involving any photolithography patterning or surface pretreatment steps. This CNT-TFT exhibits a high operating frequency of over 5 GHz and an on-off ratio of over 100. Such an all-ink-jet-printed process eliminates the need for lithography, vacuum processing, and metallization procedures and thus provides a promising technology for low-cost, high-throughput fabrication of large-area high-speed flexible electronic circuits on virtually any desired flexible substrate.

Preparative Ultracentrifuge Method for Characterization of Carbon Nanotube Dispersions

Quantitative characterization of carbon nanotube (CNT) dispersions is critical for developing various CNT-based applications, e.g., transparent conducting thin films, CNT thin film transistors, CNT-reinforced composites, etc. Using both of the preparative ultracentrifuge and absorption spectrum measurements, this paper presents a simple, easy-to-use, and reproducible method to experimentally determine the sedimentation function of carbon nanotube dispersions. This information can then be used for the semiquantitative characterization of the dispersion of CNT in various liquid media. Theoretical analysis of the sedimentation function based upon the conventional ultracentrifugation model allows for the determination of the apparent sedimentation and diffusion coefficients for a specific SWNT dispersion. As demonstrated in the examples, this method, along with dynamic light scattering, was used for studying the processing−structure relationship of SWNT/H2O dispersions. In brief: (1) size reduction of SWNT bundles due to exfoliation (diameter reduction) and shortening/cutting (length reduction) occurs simultaneously during the sonication process; power−law dependence of the diameter and length reduction on the sonication duration were observed; (2) size reduction of SWNT bundles reflected by the sedimentation function strongly depends upon the amount of dispersion being processed; for the same sonication conditions, the greater the amount of the dispersion, the less efficient the exfoliation of the SWNT bundles is; (3) under the same sonication conditions, the sedimentation coefficient of sodium dodecylbenzenesulfonate (SDBS) assisted SWNT/H2O is significantly smaller than that of Triton X-100, which indicates the superior dispersing capability of the former surfactant.

Bias-induced doping engineering with ionic adsorbates on single-walled carbon nanotube thin film transistors

Doping control on carbon nanotubes (CNTs) is a crucial step for implementation in future nanodevices. Chemical doping of CNTs is typically done via functionalization. Controlling the dopant types and position on the CNT-based thin film transistors (TFTs) and their stability has been a considerable challenge. Here, we report a CNT doping strategy utilizing ionic adsorbates controlled via gate and drain bias. Dopant type was controlled by the polarity of the gate bias, and positioning of the adsorbates was expedited by applying drain and gate bias. This enabled us to control n- or p-type doping that leads to drain-to-source or source-to-drain direction of Schottky diodes. The combined adsorbate-associated biasing method also allowed a selective dissociation of metallic channels to improve TFT on/off performance. Our approach demonstrates a practical means of fabricating CNT-based random network TFTs with deliberate doping and high stability under ambient conditions.

Improved electrical performance of carbon nanotube thin film transistors by utilizing composite networks

This work presents a simple scheme of using composite carbon nanotube networks (c-CNNs) to significantly improve the electrical performance of long-channel thin film transistors based on single-walled carbon nanotubes (SWCNTs). Such c-CNNs comprise two sets of SWCNTs. A primary set consists of dense arrays of perfectly aligned long SWCNTs along the transistor channel direction. A secondary set is composed of short SWCNTs either randomly orientated or perpendicularly aligned with respect to the channel. While retaining a high on/off current ratio, the drive current in such c-CNNs is much higher than that in currently studied systems with single CNNs or SWCNT arrays.

Field-effect modulation of contact resistance between carbon nanotubes

Local transport properties of a carbon nanotube (CNT) thin-film transistor (TFT) have been investigated by conducting atomic force microscopy. The current in a CNT bundle is almost constant, whereas it drastically decreases at the contacts between CNTs. Current drops at the contacts are reduced with increasing negative gate voltage VG. The results show that the contact resistance between CNTs can be modified by VG, and the operation of CNT-TFT is mainly governed by the modulation of contact resistance.

Ink-jet printing of carbon nanotube thin film transistors

Ink-jet printing is an important process for placing active electronics on plastic substrates. We demonstrate ink-jet printing as a viable method for large area fabrication of carbon nanotube (CNT) thin film transistors (TFTs). We investigate different routes for producing stable CNT solutions (“inks”). These consist of dispersion methods for CNT debundling and the use of different solvents, such as N-methyl-2-pyrrolidone. The resulting printable inks are dispensed by ink-jet onto electrode bearing silicon substrates. The source to drain electrode gap is bridged by percolating networks of CNTs. Despite the presence of metallic CNTs, our devices exhibit field effect behavior, with effective mobility of ∼0.07 cm2/V s and ON/OFF current ratio of up to 100. This result demonstrates the feasibility of ink-jet printing of nanostructured materials for TFT manufacture.

Printable high-speed thin-film transistor on flexible substrate using carbon nanotube solution

A printable high-speed thin-film transistor (TFT) fabricated on a regular plastic transparency film is proposed. The carrier transport layer of the TFT is an ultrapure carbon nanotube (CNT) thin film of high density (>1000 CNTs per µm2) formed at room temperature by dispensing a tiny droplet of an electronic-grade CNT solution, which does not contain any surfactant. This CNT–TFT exhibited a high-modulation speed of 312 MHz and a large current-carrying capacity beyond 20 mA. A unique ink-jet printing compatible process demonstrated herein would enable mass production of large-area electronic circuits on any virtually desired flexible substrate at low cost and high throughput.

Complementary carbon nanotube-gated carbon nanotube thin-film transistor

We introduce, a complementary carbon nanotube (CNT)-gated CNT thin-film field effect transistor (FET). By using two perpendicularly crossed single-wall CNT (SWNT) bundles as the gate and the channel interchangeably, a sub-50nm complementary CNT-FET is demonstrated. It is found that the new CNT-FET shows acceptable FET characteristics by interchanging the roles of the gate and the channel. The unique dual functionality of the device will open up a new possibility and flexibility in the design of future complementary CNT electronic circuits.

Design Criteria for Transparent Single-Wall Carbon Nanotube Thin-Film Transistors

A study based on two-dimensional percolation theory yielding quantitative parameters for optimum connectivity of transparent single-wall carbon nanotube (SWNT) thin films is reported. Optimum SWNT concentration in the filtrated solution was found to be 0.1 mg/L with a volume of 30 mL. Such parameters lead to SWNT fractions in the films of approximately Phi = 1.8 x 10(-3), much below the metallic percolation threshold, which is found to be approximately PhiC = 5.5 x 10(-3). Therefore, the performance of transparent carbon nanotube thin-film transistors is limited by the metallic SWNTs, even below their percolation threshold. We show how this effect is related to hopping or tunneling between neighboring metallic tubes.

Nanotransfer printing of organic and carbon nanotube thin-film transistors on plastic substrates

A printing process for high-resolution transfer of all components for organic electronic devices on plastic substrates has been developed and demonstrated for pentacene (Pn), poly (3-hexylthiophene) and carbon nanotube (CNT) thin-film transistors (TFTs). The nanotransfer printing process allows fabrication of an entire device without exposing any component to incompatible processes and with reduced need for special chemical preparation of transfer or device substrates. Devices on plastic substrates include a Pn TFT with a saturation, field-effect mobility of 0.09cm2(Vs)−1 and on/off ratio approximately 104 and a CNT TFT which exhibits ambipolar behavior and no hysteresis.

High-mobility carbon-nanotube thin-film transistors on a polymeric substrate

We report the development of high-mobility carbon-nanotube thin-film transistors fabricated on a polymeric substrate. The active semiconducting channel in the devices is composed of a random two-dimensional network of single-walled carbon nanotubes (SWNTs). The devices exhibit a field-effect mobility of 150cm2∕Vs and a normalized transconductance of 0.5mS∕mm. The ratio of on-current (Ion) to off-current (Ioff) is ∼100 and is limited by metallic SWNTs in the network. With electronic purification of the SWNTs and improved gate capacitance we project that the transconductance can be increased to ∼10–100mS∕mm with a significantly higher value of Ion∕Ioff, thus approaching crystalline semiconductor-like performance on polymeric substrates.