P-type Single-walled Carbon Nanotube Research Articles

Thermoelectric generator and temperature sensor based on polyamide doped n-type single-walled nanotubes toward self-powered wearable electronics

Due to its ability to convert body heat into electricity, organic thermoelectric material is considered a promising and smart maintenance-free power source to charge wearable electronics. However, developing flexible n-type organic thermoelectric materials and wearable p/n junction thermoelectric devices remains challenging. In this work, two insulated polyamides (PA6 and PA66) that have been widely used as fiber materials are employed as novel dopants for converting p-type single-walled carbon nanotubes (SWCNTs) to n-type thermoelectric materials. Because of the electron transferability of the amide group, polyamide-doped SWCNTs exhibit excellent thermopower values as large as −56.0 µV K−1 for PA66, and −54.5 µV K−1 for PA6. Thermoelectric devices with five p/n junctions connected in series are fabricated. The testing device produces a thermoelectric voltage of 43.1 mV and generates 1.85 µW thermoelectric power under temperature gradients of approximately 80 K. Furthermore, they display charming capability for temperature recognition and monitoring human activities as sensors. These promising results suggest that the flexible polyamide-doped SWCNT composites herein have high application potential as wearable thermoelectric electronics.

Ultra-long air-stability of n-type carbon nanotube films with low thermal conductivity and all-carbon thermoelectric generators

This report presents n-type single-walled carbon nanotubes (SWCNT) films with ultra-long air stability using a cationic surfactant and demonstrates that the n-type Seebeck coefficient can be maintained for more than two years, which is the highest stability reported thus far to the best of our knowledge. Furthermore, the SWCNT films exhibit an extremely low thermal conductivity of 0.62 ± 0.08 W/(m·K) in the in-plane direction, which is very useful for thin-film TEGs. We fabricated all-carbon-nanotube TEGs, which use p-type SWCNT films and the n-type SWCNT films developed, and their air-stability was investigated. The TEGs did not degrade for 160 days and exhibited an output voltage of 24 mV, with a maximum power of 0.4 µW at a temperature difference of 60 K. These results open a pathway to enable the widespread use of carbon nanotube TEGs as power sources in IoT sensors.

Doping of Carbon Nanotubes with Encapsulated Phosphorus Chains

Single-walled carbon nanotubes (SWCNTs) are a perfect host for the formation of one-dimensional phosphorus structures and to obtain hybrid materials with a large P-C ratio. This work presents a procedure for high-yield phosphorus filling of commercial Tuball SWCNTs and efficient removal of phosphorus deposits from the external nanotube surface. We probed white and red phosphorus as precursors, varied the synthesis temperature and the ampoule shape, and tested three solvents for sample purification. High-resolution transmission electron microscopy and Raman spectroscopy indicated crystallization of interior phosphorus in a form resembling fibrous red phosphorus. An aqueous sodium hydroxide solution allowed removing the majority of external phosphorus particles. Thermogravimetric analysis of the product determined ∼23 wt % (∼10 atom %) of phosphorus, and the X-ray photoelectron spectroscopy (XPS) data showed that ca. 80% of it is in the form of elemental phosphorus. Externally purified SWCNTs filled with phosphorus were used to study the interaction between the components. Raman spectroscopy and core-level XPS revealed p-type SWCNT doping. Valence-band XPS data and density functional theory calculations confirmed the transfer of the SWCNT electron density to the encapsulated phosphorus.

Fully roll-to-roll gravure printed 4-bit code generator based on p-type SWCNT thin-film transistors

Current Si-based technologies have reached their intrinsic limits in meeting the demands of flexible electronics where free-form factors and low cost are critical for successful applications. For this reason, roll-to-roll (R2R) gravure printing has been considered a way to achieve the free-form factor and the low cost. However, the R2R gravure systems (servomechanism, electronic ink, printing process, and device design) could not integrate a number of thin-film transistors (TFTs) with small threshold voltage (V th) variations. Therefore, we designed a 4-bit code generator by combining one ring oscillator, six NAND gates, and one OR gate based on 37 p-type single-walled carbon nanotube (SWCNT) TFTs as a concept devices to test the R2R gravure system. First, ring oscillators with different physical dimensions were printed on a poly (ethylene terephthalate) roll using the R2R gravure. Then, we extracted important factors (channel length, channel width, and SWCNT network density) to optimize the V th variation and demonstrated a 4-bit code generator integrated with 37 p-type TFTs. This work will be further extended in the near future to develop R2R gravure printed near-field communication labels for smart packaging.

Integrated, Highly Flexible, and Tailorable Thermoelectric Type Temperature Detectors Based on a Continuous Carbon Nanotube Fiber.

As possible alternatives to traditional thermoelectric (TE) materials, carbon nanomaterials and their hybrid materials have great potential in the future application of flexible and lightweight temperature detection. In this work, an integrated, highly flexible, and tailorable TE temperature detector with high performance has been fabricated based on a continuous single-walled carbon nanotube (SWCNT) fiber. The detector consists of more than one pairs of thermocouples composed of p-type SWCNT fiber and n-type SWCNT hybrid fiber in situ doped by polyethylenimine. Due to the node contact mechanism of the detection, the sensitivity of the detector can be improved with the increase of the number of p-n thermocouples, independent of the length of the thermocouple. The temperature detection process of the detector has been studied in detail. In particular, the integrated and flexible detector can be divided into several sub-detectors easily by cutting, illustrating the prospect of large-scale preparation of this kind of novel temperature detectors. Its high flexibility ensures the detector to maintain excellent detection performance after 15000 bending circles. Furthermore, the as-designed TE type temperature detector demonstrates a great application promise for real-time temperature detection and temperature change sensing even in complex surface and harsh environment.

Photo-Detectivity Modulation in Complementary Light-to-Frequency Conversion Circuits via Oxygen Vacancy Controlled Amorphous Indium–Gallium–Zinc Oxide Films

Herein, light-to-frequency conversion circuits (LFCs) using n-type amorphous indium-gallium-zinc oxide thin film transistors (a-IGZO TFTs) and p-type single-walled carbon nanotube (SWNT) TFTs are proposed as circuit level photodetector. The photoresponse of a-IGZO TFTs under visible spectrum is adjusted with control of annealing temperature condition, which is closely related with oxygen vacancy level, validated by X-ray photoelectron spectroscopy (XPS) and band diagrams in the TFTs. With optimized annealing temperature for a-IGZO TFTs, LFCs are successfully demonstrated by photosensitive complementary inverters and their 3-stage ring oscillators with power and wavelength dependency of light. This result is expected to be applicable to the newly conceived internet-of-things (IoT) sensor systems including interactive displays.

Oxygen-Rich Polymer Polyethylene Glycol-Functionalized Single-Walled Carbon Nanotubes Toward Air-Stable n-Type Thermoelectric Materials.

It is crucial for thermoelectric (TE) devices to obtain both p-type and n-type materials and control charge carrier density. However, n-type thermoelectric materials are quite deficient and have lower thermoelectric properties. We report one oxygen-rich polymer named polyethylene glycol (PEG) for converting p-type single-walled carbon nanotubes (SWCNTs) to air-stable n-type thermoelectric materials. When pristine SWCNTs were doped with 2 mg·mL-1 PEG in an ethanol solution, the optimal Seebeck coefficient of PEG/SWCNT composites reached -50.8 μV·K-1. The result of ultraviolet photoelectron spectroscopy demonstrated that the lone pair of oxygen atoms in the PEG chain has electron transferability to SWCNTs. According to the hard and soft acid and base theory, sodium hydroxide (NaOH) was further introduced to improve air stability and thermoelectric performance of doped SWCNTs. As a result, PEG/NaOH/SWCNT composites achieved the highest power factor of 173.8 μW·m-1·K-2 at 300 K. Meanwhile, their final changes in electrical conductivity and the Seebeck coefficient are less than 8% in the investigation of air stability over two months. Inspired by this finding, we fabricated the TE generator composed of the pristine p-type SWCNTs and n-type PEG/NaOH/SWCNT composites. The maximum output power of this robust TE device reached 5.3 μW at a temperature gradient of 76 K, which is superior to many reported TE devices. Moreover, the experimental procedure is attractive as a sustainable process for materials preparation. Our study has indicated that the oxygen-rich polymer-functionalized SWCNTs have huge potential for developing air-stable n-type carbon-based thermoelectric materials.

Fine tuning of the Fermi level of single-walled carbon nanotubes with onium salts and application for thermoelectric materials

Fermi level is one of the most important parameters for control of semiconductor polarity and improvement of thermoelectric performances. Although single-walled carbon nanotubes (SWCNTs) are promising candidates for thermoelectric materials, control of the Fermi level of SWCNTs with chemical dopants is still challenging. Reported here is the fine-tuning of the Fermi level of SWCNTs with onium salts. The Fermi level of SWCNTs was very sensitive to substituent size on the cation and nucleophilicity of the anion, and the Seebeck coefficients markedly changed between −50 μV K−1 and 68 μV K−1. The optimum Fermi level of p-type SWCNTs that exhibits the maximum power factor was realized by ammonium chloride or pyridine hydrochloride, and the power factor (PF) was improved by about 7 times as compared with non-doped SWCNTs. SWCNTs doped with onium salts were stable in both p-type and n-type states, and thermoelectric performances did not change more than 100 days in the air.

Overcoming Electrochemical Instabilities of Printed Silver Electrodes in All-Printed Ion Gel Gated Carbon Nanotube Thin-Film Transistors.

Silver ink is the most widely used conductive material for printing electrodes in the fabrication of all-printed ion gel gated transistors because of their high conductivity and low cost. However, electrochemical instability of printed silver electrodes is generally one of the biggest issues, whether it is in air where silver gets oxidized or in a moisture environment where electrochemical migration occurs. Notwithstanding, the electrochemical stability of printed silver electrodes in ion gel medium has not been studied so far. In this work, we studied the electrochemical instabilities of printed silver electrodes in fully printed ion gel gated single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs) and developed some strategies to overcome these issues. All-printed ion gel-based p-type SWCNT TFTs were employed to investigate the impact of electrochemical instabilities on the electrical behavior of printed SWCNT TFTs. The results have demonstrated that printed silver was unstable at anodic and cathodic polarization because of the corrosion by the ionic liquid. Besides, anodic corrosion of silver source/drain electrodes was shown to be responsible for the electrical failure of printed SWCNT TFTs in both the linear and saturated regime. These issues were completely resolved when preventing printed silver electrodes from coming into direct contact with ion gels. For example, ion gels were partially printed in device channels to avoid contacting the printed silver source and drain electrodes. At the same time, silver side-gate electrodes were replaced by inkjet-printed PEDOT:PSS electrodes to avoid gate electrode-related instabilities. Consequently, all-printed electrochemically stable SWCNT TFTs fabricated were obtained with enhanced performance of higher ION/IOFF ratios (105 to 106), smaller subthreshold slopes (∼70 mV/dec), and smaller hysteresis (ΔV = 0.025 V) at gate voltages from 1.2 to -0.5 V. Additionally, the polarity of all-printed SWCNT TFTs was converted from the p-channel to ambipolar while achieving lower leakage currents.

Significantly Enhanced Power Factors of p-Type Carbon Nanotube-Based Composite Films by Tailoring the Peripheral Substituents in Porphyrin

Considering that the development of p-type single-walled carbon nanotube (SWCNT)/organic small molecule (OSM)-based thermoelectric (TE) materials with high performance is lagging behind and only a ...

Conductivity Properties of Single-Walled Carbon Nanotubes Upon the Encapsulation of TTF and TCNQ.

N-type and p-type single-walled carbon nanotubes (SWNTs) were formed via the encapsulation of tetrathiafulvalene (TTF) and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) inside SWNTs, respectively. Raman, near-infrared, and X-ray photoelectron spectrometer were used to confirm the encapsulation. From measurements of the current-voltage curves in a vacuum, it was revealed that current of TTF-encapsulated SWNTs decreased and TCNQ-encapsulated SWNTs increased comparing with that of pristine SWNTs. This was resulted from electron-donating (TTF) and withdrawing (TCNQ) character into SWNTs.

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.

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.

Fully gravure printed complementary carbon nanotube TFTs for a clock signal generator using an epoxy-imine based cross-linker as an n-dopant and encapsulant.

Printed p-type single walled carbon nanotube (SWCNT) based circuits exhibit high power dissipation owing to their thick printed dielectric layers (>2 μm) and long channels (>100 μm). In order to reduce the static power dissipation of printed SWCNT-base circuits while maintaining the same printing conditions and channel lengths, complementary metal-oxide-semiconductor (CMOS) based circuits are more ideal. These circuits, however, have not been successfully implemented in a scalable printing platform due to unstable threshold voltages of n-doped SWCNT based thin film transistors (TFTs). In this work, a thermally curable epoxy-imine-based n-doping ink is presented for achieving uniform doping and sealing of SWCNT layers by gravure printing. After printing the n-doping ink, the ink is cured to initiate a cross-linking reaction to seal the n-doped SWCNT-TFTs so that the threshold voltage of the n-doped SWCNT-TFTs is stabilized. Flexible CMOS ring oscillators using such n-doped SWCNT-TFTs combined with the intrinsically p-type SWCNT-TFTs can generate a 0.2 Hz clock signal with significantly lower power consumption compared to similarly printed p-type only TFT based ring oscillators. Moving forward, this CMOS flexible ring oscillator can be practically used to develop fully printed inexpensive wireless sensor tags.

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.

Origin of Open Circuit Voltage in Solar Cells Based on Polymer Wrapped Carbon Nanotubes and Fullerenes

Thin films of semiconducting p-type single-walled carbon nanotubes (SWCNTs) have been recently used both for photo-absorption and for hole transport in the quest for all-carbon solar cells [1-6]. Quantum efficiency as high as 43% at the peak absorption wavelength of the nanotubes has been reported using single-chirality tubes [3]. These devices were of bilayer type, with efficiencies around 1%. More recently, bulk heterojunction (BHJ) type solar cells were also reported with a certified efficiency of 3.1% [2]. With the major focus on the interface between SWCNTs and fullerenes where the excitons generated by photo-absorption get dissociated, varying values of open circuit voltage (Voc ) have been obtained. Common to all these devices is the use of semiconducting polymers such as polyfluorene and polythiophene along with the nanotubes. During the separation process for single chirality tubes, the excess polymer is usually removed to obtain a polymer-nanotube mass ratio of 1:1. However, along with the SWCNT interfaces, there will still exist polymer-fullerene and polymer-electrode interfaces, which can influence the device performance. For example, S. Na et al [7] have shown that a polyfluorene derivative interfacial layer on the cathode side can in a BHJ solar cell with poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61(PCBM) can increase its efficiency. We report the effect of polymers on the performance of two single chirality SWCNT-C60 bilayer solar cells using: (a) poly(9,9-dioctylfluorene-2,7-diyl) (PFO) separated (7,5) tubes and (b) 9,9-di-octylfluorenyl-2,7-diyl and bipyridine copolymer (PFO-BPy) separated (6,5) tubes. We show that along with polymer-SWCNT weight ratio, the absolute weight of the polymer is also a critical parameter affecting the device operation. Performance of devices with and without SWCNTs both having the same polymer content is compared. In devices without SWCNTs, the polymer alone acts as interfacial layer between C60 and the anode. Polymer concentration dependence is also studied in these devices. It is found that the polymer layer, even at very low concentrations, increases the device efficiency by nearly an order of magnitude. While the reference cell with only C60 gave an efficiency of 0.07%, the devices with PFO interfacial layer had efficiencies up to 0.6% with an increase in all three efficiency determining parameters. The Voc of devices with PFO increased in the range of 0.2 -0.5 V over that of the C60 reference device, increasing with PFO layer thickness. In addition, while the PFO-SWCNT-C60 device gave a Voc of 0.40 V, PFO-C60 device gave a Voc of 0.56 V even though both devices contain equal concentration of PFO. These results indicate that the origin of Voc may be the PFO-C60 interface and that the presence of SWCNTs has a negative effect on achievable Voc . With PFO-SWCNT-C60 device working like a ternary system with PFO concentration dependent efficiency, optimization of this system should take into account the PFO-C60 interface as well.

Enhancement of ambipolar characteristics in single-walled carbon nanotubes using C60 and fabrication of logic gates

We demonstrate a technique to convert p-type single-walled carbon nanotube (SWNT) network transistor into ambipolar transistor by thermally evaporating C60 on top. The addition of C60 was observed to have two effects in enhancing ambipolar characteristics. First, C60 served as an encapsulating layer that enhanced the ambipolar characteristics of SWNTs. Second, C60 itself served as an electron transporting layer that contributed to the n-type conduction. Such a dual effect enables effective conversion of p-type into ambipolar characteristics. We have fabricated inverters using our SWNT/C60 ambipolar transistors with gain as high as 24, along with adaptive NAND and NOR logic gates.

Large-area, low-voltage, antiambipolar heterojunctions from solution-processed semiconductors.

The emergence of semiconducting materials with inert or dangling bond-free surfaces has created opportunities to form van der Waals heterostructures without the constraints of traditional epitaxial growth. For example, layered two-dimensional (2D) semiconductors have been incorporated into heterostructure devices with gate-tunable electronic and optical functionalities. However, 2D materials present processing challenges that have prevented these heterostructures from being produced with sufficient scalability and/or homogeneity to enable their incorporation into large-area integrated circuits. Here, we extend the concept of van der Waals heterojunctions to semiconducting p-type single-walled carbon nanotube (s-SWCNT) and n-type amorphous indium gallium zinc oxide (a-IGZO) thin films that can be solution-processed or sputtered with high spatial uniformity at the wafer scale. The resulting large-area, low-voltage p-n heterojunctions exhibit antiambipolar transfer characteristics with high on/off ratios that are well-suited for electronic, optoelectronic, and telecommunication technologies.

Direct Measurements of In-plane Thermal and Electrical Transport in P-type Single-walled Carbon Nanotube Thin Films

In order to optimize next-generation thermoelectrics for use in converting waste heat into useable energy, the close correlation between the thermal conductivity, electrical conductivity, and Seebeck coefficient or thermopower in these materials must be decoupled. As promising alternatives to traditional inorganic semiconductor materials, nanocomposites constructed of conducting polymers with organic inclusions such as single-walled carbon nanotubes (SWCNTs) offer simple fabrication techniques, reduced cost, and low toxicity. They also offer the possibility of decoupling the thermal and electrical transport pathways which would enable more efficient organic composites for thermoelectric conversion. However, in order to develop high-performance organic thermoelectric devices, it is necessary to develop a detailed fundamental understanding of the factors governing directionally dependent thermal and electrical transport through these materials in reduced geometries such as thin films. In this talk, we describe our suspended membrane technique for directly measuring the in-plane thermal and electrical transport through thin films and present results for several different thin film samples. We present our approach to developing p-type materials with tunable transport behavior, by fabricating composites consisting of SWCNTs dispersed in a polymer matrix. Finally, we discuss fabrication and post-fabrication treatments of the SWCNT thin films and the benefits offered by nanostructuring these architectures to optimize the thermoelectric dimensionless figure-of-merit, ZT.