Single-walled Carbon Nanotube Thin-film Transistors Research Articles

Highly flexible printed carbon nanotube thin film transistors using cross-linked poly(4-vinylphenol) as the gate dielectric and application for photosenstive light-emitting diode circuit

In this work, cross-linked poly(4-vinylphenol) (PVP) was utilized in printed single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) as the gate dielectric layer to achieve low operating voltage, environmental stability and high flexibility. The printed top-gate TFTs on polyethylene terephthalate (PET) substrates exhibited effective mobility up to 8.4 cm2V−1s−1 (based on device capacitances measured at 0.1 Hz), on/off ratio of 106 and subthreshold swing (ss) of 63.8 mV/dec at operation voltage of only 1 V. The printed TFTs exhibited excellent mechanical flexibility and stability under ambient conditions due to the use of cross-linked PVP as dielectric layers. No obvious discrepancies in electrical properties were observed after bending 15000 cycles at the radius of 5 mm. A photosensitive light-emitting diode (LED) circuit was constructed, which was composed of two commercial LEDs and the printed SWCNT TFTs. One LED was mounted on top of the gate electrode, acting as a photon sensor to modulate the gate voltage of TFT. The second LED was connected to the source electrode to act as an illuminating device. The circuit could drive the illuminating LED at Vdd of −0.75 V and Vds = −2 V with lighting intensity varying from 0 to 8.96 μW/cm2.

Improvement in interfacial characteristics of low-voltage carbon nanotube thin-film transistors with solution-processed boron nitride thin films

In this article, we demonstrate the potential of solution-processed boron nitride (BN) thin films for high performance single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) with low-voltage operation. The use of BN thin films between solution-processed high-k dielectric layers improved the interfacial characteristics of metal-insulator-metal devices, thereby reducing the current density by three orders of magnitude. We also investigated the origin of improved device performance in SWCNT-TFTs by employing solution-processed BN thin films as an encapsulation layer. The BN encapsulation layer improves the electrical characteristics of SWCNT-TFTs, which includes the device key metrics of linear field-effect mobility, sub-threshold swing, and threshold voltage as well as the long-term stability against the aging effect in air. Such improvements can be achieved by reduced interaction of interfacial localized states with charge carriers. We believe that this work can open up a promising route to demonstrate the potential of solution-processed BN thin films on nanoelectronics.

Selective Conversion from p-Type to n-Type of Printed Bottom-Gate Carbon Nanotube Thin-Film Transistors and Application in Complementary Metal-Oxide-Semiconductor Inverters.

The fabrication of printed high-performance and environmentally stable n-type single-walled carbon nanotube (SWCNT) transistors and their integration into complementary (i.e., complementary metal-oxide-semiconductor, CMOS) circuits are widely recognized as key to achieving the full potential of carbon nanotube electronics. Here, we report a simple, efficient, and robust method to convert the polarity of SWCNT thin-film transistors (TFTs) using cheap and readily available ethanolamine as an electron doping agent. Printed p-type bottom-gate SWCNT TFTs can be selectively converted into n-type by deposition of ethanolamine inks on the transistor active region via aerosol jet printing. Resulted n-type TFTs show excellent electrical properties with an on/off ratio of 106, effective mobility up to 30 cm2 V-1 s-1, small hysteresis, and small subthreshold swing (90-140 mV dec-1), which are superior compared to the original p-type SWCNT devices. The n-type SWCNT TFTs also show good stability in air, and any deterioration of performance due to shelf storage can be fully recovered by a short low-temperature annealing. The easy polarity conversion process allows construction of CMOS circuitry. As an example, CMOS inverters were fabricated using printed p-type and n-type TFTs and exhibited a large noise margin (50 and 103% of 1/2 Vdd = 1 V) and a voltage gain as high as 30 (at Vdd = 1 V). Additionally, the CMOS inverters show full rail-to-rail output voltage swing and low power dissipation (0.1 μW at Vdd = 1 V). The new method paves the way to construct fully functional complex CMOS circuitry by printed TFTs.

Inkjet printed circuits based on ambipolar and p-type carbon nanotube thin-film transistors

Ambipolar and p-type single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) are reliably integrated into various complementary-like circuits on the same substrate by inkjet printing. We describe the fabrication and characteristics of inverters, ring oscillators, and NAND gates based on complementary-like circuits fabricated with such TFTs as building blocks. We also show that complementary-like circuits have potential use as chemical sensors in ambient conditions since changes to the TFT characteristics of the p-channel TFTs in the circuit alter the overall operating characteristics of the circuit. The use of circuits rather than individual devices as sensors integrates sensing and signal processing functions, thereby simplifying overall system design.

Electronically Pure Single Chirality Semiconducting Single-Walled Carbon Nanotube for Large Scale Electronic Devices

Abstract Carbon nanotube thin film transistors (TFTs) with characteristics resembling those of TFTs constructed on amorphous silicon, low-temperature polycrystalline silicon and metal oxides were fabricated on (6,5) single chirality single-walled carbon nanotube (SWCNT) thin film deposited from electronically pure semiconducting (6,5) single chirality single-walled carbon nanotube (SWCNT) ink. This ink was extracted in industrial scale from raw SWCNTs produced using high pressure carbon monoxide conversion, and deposited on pretreated substrates to form uniform and consistent (6,5) HiPCO SWCNT thin film using solution process. The (6,5) HiPCO SWCNT thin films were characterized as pure semiconductor without metallic impurities showing classic nonlinear current-bias curves in Schottky-type diodes. Both N-type and P-type (6,5) HiPCO SWCNT TFTs were fabricated with femto Ampere off-current and ION/IOFF ratio of 108 by depositing SiNx and HfO2 dielectrics on the top of (6,5) HiPCO SWCNT thin films, respectively. The (6,5) HiPCO SWCNT inverter with voltage gain of 52 was also demonstrated by wire-bonding one P-type HiPCO SWCNT TFT to one N-type HiPCO SWCNT TFT.

Pressure-sensing properties of single-walled carbon nanotubes covered with a corona-poled piezoelectric polymer

Single-walled carbon nanotubes (SWNTs) have been studied extensively as sensing elements for chemical and biochemical sensors because of their excellent electrical properties, their ultrahigh ratio of surface area to volume, and the consequent extremely high sensitivity of their surface to the surrounding environment. The extremely high sensitivity indicates that SWNTs can operate as excellent transducers when combined with piezoelectric materials. In this paper, we present a touch sensor based on SWNT thin-film transistors (SWNT-TFTs) covered with a thin film of the piezoelectric polymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). Devices were fabricated by spin-coating a P(VDF-TrFE) layer on an SWNT-TFT, which was followed by in situ corona poling to polarize the P(VDF-TrFE) layer. We studied the effect of the corona polarity on the device characteristics and revealed that poling with a negative corona discharge induced a large amount of hole doping in the SWNTs and improved the touch-sensing performance of the devices, while a positive discharge had a negligible effect. The poled devices exhibited regular, stable, and positive drain current modulation in response to intermittent pressing, and the response was proportional to the magnitude of the applied pressure, suggesting that it was caused by the piezoelectric effect of the polarized P(VDF-TrFE) layer. Furthermore, we also fabricated a device using horizontally aligned SWNTs with a lower SWNT density as an alternative transducer to an SWNT thin film, which demonstrated sensitivity as high as 70%/MPa.

Evaluation of interface trap densities and quantum capacitance in carbon nanotube network thin-film transistors

The interface trap density in single-walled carbon nanotube (SWNT) network thin-film transistors (TFTs) is a fundamental and important parameter for assessing the electronic performance of TFTs. However, the number of studies on the extraction of interface trap densities, particularly in SWNT TFTs, has been insufficient. In this work, we propose an efficient technique for extracting the energy-dependent interface traps in SWNT TFTs. From the measured dispersive, frequency-dependent capacitance–voltage (C–V) characteristics, the dispersive-free, frequency-independent C–V curve was obtained, thus enabling the extraction and analysis of the interface trap density, which was found to be approximately 8.2 × 1011 eV−1 cm−2 at the valence band edge. The frequency-independent C–V curve also allows further extraction of the quantum capacitance in the SWNT network without introducing any additional fitting process or parameters. We found that the extracted value of the quantum capacitance in SWNT networks is lower than the theoretical value in aligned SWNTs due to the cross point of SWNTs on the SWNT network. Therefore, the method proposed in this work indicates that the C–V measurement is a powerful tool for obtaining deep physical insights regarding the electrical performance of SWNT TFTs.

Controlled n-Type Doping of Carbon Nanotube Transistors by an Organorhodium Dimer.

Single-walled carbon nanotube (SWCNT) transistors are among the most developed nanoelectronic devices for high-performance computing applications. While p-type SWCNT transistors are easily achieved through adventitious adsorption of atmospheric oxygen, n-type SWCNT transistors require extrinsic doping schemes. Existing n-type doping strategies for SWCNT transistors suffer from one or more issues including environmental instability, limited carrier concentration modulation, undesirable threshold voltage control, and/or poor morphology. In particular, commonly employed benzyl viologen n-type doping layers possess large thicknesses, which preclude top-gate transistor designs that underlie high-density integrated circuit layouts. To overcome these limitations, we report here the controlled n-type doping of SWCNT thin-film transistors with a solution-processed pentamethylrhodocene dimer. The charge transport properties of organorhodium-treated SWCNT thin films show consistent n-type behavior when characterized in both Hall effect and thin-film transistor geometries. Due to the molecular-scale thickness of the organorhodium adlayer, large-area arrays of top-gated, n-type SWCNT transistors are fabricated with high yield. This work will thus facilitate ongoing efforts to realize high-density SWCNT integrated circuits.

High-Performance Single-Walled Carbon Nanotube-Based Thin-Film Transistors by Reducing Charge Transfer

We demonstrate high-performance single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) using the simple and robust method of an interacting fluoropolymer encapsulation with thermal annealing in vacuum to reduce the charge transfer. With an optimized combination of the fabrication process and the device configuration, drop-casted SWCNT-TFTs exhibit a field-effect mobility of $\sim 18$ cm2/ $\text{V}\,\cdot \, \text{s}$ in the linear regime, an ON–OFF-current ratio of $\sim 10^{6}$ , a threshold voltage of around 0 V, and a subthreshold swing of 97 mV/decade, which is among the best results that have been achieved in 2-D random networks of the SWCNTs. We also investigate the instability characteristics in the SWCNT-TFTs encapsulated with fluoropolymer during the prolonged electrical bias-stress. Interestingly, the drop in drain current of the SWCNT-TFTs was partially recovered with a second phenomenon that increases the drain current after a certain time. The hydrophobic surface that the fluoropolymer film possesses, reduced the chemical interaction of charge carriers with the hydroxyl (–OH) functional groups which act as defects in the SWCNT-TFTs.

The Effect of Interface States on Single-Walled Carbon Nanotube Transistors

Different from p-type single-walled carbon nanotubes (SWCNT) thin film transistors (TFTs) with SiO2 dielectrics, air-stable top-gated n-type SWCNT TFTs are reported with amorphous silicon nitride (SiNx) dielectrics exhibiting the strong dependence of deposition methods and conditions, and metal contacts. Amorphous silicon TFTs-like characteristics was observed from semiconducting single-chirality (6,5) SWCNT TFTs with SiNx deposited at 225°C and appropriate NH3/SiH4 feeding ratio of 10 sccm/5.3 sccm using plasma enhanced chemical vapor deposition (PECVD). These results are interpreted as the effect of interface states between (6,5) SWCNT and SiNx gate dielectrics, which provide the avenue to alter the ratio of free electrons and trapped electrons along applied gate potential.

Stretchable Active Matrix Temperature Sensor Array of Polyaniline Nanofibers for Electronic Skin.

A stretchable polyaniline nanofiber temperature sensor array with an active matrix consisting of single-walled carbon nanotube thin-film transistors is demonstrated. The integrated temperature sensor array gives mechanical stability under biaxial stretching of 30%, and the resultant spatial temperature mapping does not show any mechanical or electrical degradation.

Gram‐scale carbon nanotubes as semiconducting material for highly versatile route of integration in plastic electronics

A versatile chemical functionalization of single‐walled carbon nanotubes (SWNT) is developed to eliminate the conductivity of metallic SWNTs in pristine SWNT mixtures, without sorting. Thanks to the high selectivity of the diazoether reagent, metallic SWNTs can be functionalized while preserving the transport properties of semiconducting SWNTs, even in nonindividually dispersed SWNT solutions. In this way, liters of aqueous semiconducting ink for printing or spray can be prepared at the laboratory scale and used for the fabrication of thin‐film transistors (TFT) by spraying. Diazoether grafting also improved SWNT dispersion by preventing rebundling. Consequently, while less conductive than pristine SWNTs, diazoether‐treated SWNTs provided higher TFT transconductance thanks to a more homogeneous percolation in the film. SWNT TFTs made on wafer and plastic with pristine and diazoether‐treated SWNTs were compared. Sprayed films of diazoether treated, unsorted SWNTs provided TFTs with ION/IOFF around 500, about 2 orders of magnitude higher than pristine SWNT TFTs. Mobilities were similar, up to 1 cm2/V s. Interestingly, diazoether‐treated SWNT TFTs kept a high ION/IOFF in a wide range of SWNT density and channel length, where pristine SWNT films became metallic.

Effects of Ambient Air and Temperature on Ionic Gel Gated Single-Walled Carbon Nanotube Thin-Film Transistor and Circuits.

Single-walled carbon nanotube thin-film transistor (SWCNT TFT) and circuits were fabricated by fully inkjet printing gold nanoparticles as source/drain electrodes, semiconducting SWCNT thin films as channel materials, PS-PMMA-PS/EMIM TFSI composite gel as gate dielectrics, and PEDOT/PSS as gate electrodes. The ionic gel gated SWCNT TFT shows reversible conversion from p-type transistor behavior in air to ambipolar features under vacuum due to reversible oxygen doping in semiconducting SWCNT thin films. The threshold voltages of ionic gel gated SWCNT TFT and inverters are largely shifted to the low value (0.5 V for p-region and 1.0 V for n-region) by vacuum annealing at 140 °C to exhausively remove water that is incorporated in the ionic gel as floating gates. The vacuum annealed ionic gel gated SWCNT TFT shows linear temperature dependent transconductances and threshold voltages for both p- and n-regions. The strong temperature dependent transconductances (0.08 μS/K for p-region, 0.4 μS/K for n-region) indicate their potential application in thermal sensors. In the other hand, the weak temperature dependent threshold voltages (-1.5 mV/K for p-region, -1.1 mV/K for n-region) reflect their excellent thermal stability.

Solution-processed carbon nanotube thin-film complementary static random access memory.

Over the past two decades, extensive research on single-walled carbon nanotubes (SWCNTs) has elucidated their many extraordinary properties, making them one of the most promising candidates for solution-processable, high-performance integrated circuits. In particular, advances in the enrichment of high-purity semiconducting SWCNTs have enabled recent circuit demonstrations including synchronous digital logic, flexible electronics and high-frequency applications. However, due to the stringent requirements of the transistors used in complementary metal-oxide-semiconductor (CMOS) logic as well as the absence of sufficiently stable and spatially homogeneous SWCNT thin-film transistors, the development of large-scale SWCNT CMOS integrated circuits has been limited in both complexity and functionality. Here, we demonstrate the stable and uniform electronic performance of complementary p-type and n-type SWCNT thin-film transistors by controlling adsorbed atmospheric dopants and incorporating robust encapsulation layers. Based on these complementary SWCNT thin-film transistors, we simulate, design and fabricate arrays of low-power static random access memory circuits, achieving large-scale integration for the first time based on solution-processed semiconductors.

Tunable threshold voltage in solution-processed single-walled carbon nanotube thin-film transistors

We have investigated tunable threshold voltage in solution-processed single-walled carbon nanotube thin-film transistors (SWCNT TFTs) employing a simple and reproducible method of chemical encapsulation. Compared to a pristine one, SWCNT TFTs encapsulated with ammonium hydroxide (NH4OH) and nitric acid (HNO3) exhibit a negative shift and a positive shift in threshold voltage, respectively. Such results can be explained by the modification of the energy band at the interface between the source metal electrode and the SWCNT network. By using the Y-function method, we also characterized electrical properties such as field-effect mobility, threshold voltage, and contact resistance for TFTs treated with NH4OH or HNO3. The technique to favorably tune threshold voltage in solution-processed SWCNTs is significant for constituting CNT-based nanoelectronics.

High-performance carbon nanotube thin-film transistors on flexible paper substrates

Single-walled carbon nanotubes (SWCNTs) are promising materials as active channels for flexible transistors owing to their excellent electrical and mechanical properties. However, flexible SWCNT transistors have never been realized on paper substrates, which are widely used, inexpensive, and recyclable. In this study, we fabricated SWCNT thin-film transistors on photo paper substrates. The devices exhibited a high on/off current ratio of more than 106 and a field-effect mobility of approximately 3 cm2/V·s. The proof-of-concept demonstration indicates that SWCNT transistors on flexible paper substrates could be applied as low-cost and recyclable flexible electronics.

Highly uniform carbon nanotube nanomesh network transistors

A new type of single-walled carbon nanotube (SWNT) thin-film transistor (TFT) structure with a nanomesh network channel has been fabricated from a preseparated semiconducting nanotube solution and simultaneously achieved both high uniformity and a high on/off ratio for application in large-scale integrated circuits. The nanomesh structure is prepared on a high-density SWNT network channel and enables a high on/off ratio while maintaining the excellent uniformity of the electrical properties of the SWNT TFTs. These effects are attributed to the effective elimination of metallic paths across the source/drain electrodes by forming the nanomesh structure in the high-density SWNT network channel. Therefore, our approach can serve as a critical foundation for future nanotube-based thinfilm display electronics.

Accurate extraction of mobility in carbon nanotube network transistors using C-V and I-V measurements

The mobility of single-walled carbon nanotube (SWNT) network thin-film transistors (TFTs) is an essential parameter. Previous extraction methods for mobility encountered problems in extracting accurate intrinsic mobility due to the uncertainty of the SWNT density in the network channel and the existence of contact resistance at the source/drain electrodes. As a result, efficient and accurate extraction of the mobility in SWNT TFTs is challenging using previous methods. We propose a direct method of extracting accurate intrinsic mobility in SWNT TFTs by employing capacitance-voltage and current-voltage measurements. Consequently, we simply obtain accurate intrinsic mobility within the ink-jet printed SWNT TFTs without any complicated calculations.

Low-frequency noise in carbon nanotube network thin-film transistors

Bottom-gate single-walled carbon nanotube (SWCNT) network thin-film transistors (TFTs) are fabricated and their low-frequency noise characteristics evaluated. Clear 1/f (f: frequency) and (IDS: drain current) dependences are observed in the drain current noise power densities. The A/R (A: noise coefficient and R: resistance) was proportional to (LG: gate length). The experimental results for TFTs are discussed on the basis of Hooge’s empirical law. A simple model is proposed to explain why the A/R in our TFTs are smaller than those reported in individual CNT field-effect transistors (FETs). These results demonstrate that the 1/f noise in SWCNT TFTs originates not with the CNT–CNT junctions but rather with the CNTs themselves.

Highly stable carbon nanotube top-gate transistors with tunable threshold voltage.

Carbon-nanotube top-gate transistors with fluorinated dielectrics are presented. With PTrFE as the dielectric, the devices have absent or small hysteresis at different sweep rates and excellent bias-stress stability under ambient conditions. Ambipolar single-walled carbon nanotube (SWNT) transistors are observed when P(VDF-TrFE-CTFE) is utilized as a topgate dielectric. Furthermore, continuous tuning of the threshold voltages of both unipolar and ambipolar SWNT thin-film transistors (TFTs) is demonstrated for the first time.