Carbon Nanotube Thin Films Research Articles

Isotropic Seebeck coefficient of aligned single-wall carbon nanotube films

How the morphology of a macroscopic assembly of nanoobjects affects its properties is a long-standing question in nanomaterials science and engineering. Here, we examine how the thermoelectric properties of a flexible thin film of carbon nanotubes depend on macroscopic nanotube alignment. Specifically, we have investigated the anisotropy of the Seebeck coefficient of aligned and gated single-wall carbon nanotube thin films. We varied the Fermi level in a wide range, covering both the p-type and n-type regimes, using electrolyte gating. While we found the electrical conductivity along the nanotube alignment direction to be several times larger than that in the perpendicular direction, the Seebeck coefficient was found to be fully isotropic, irrespective of the Fermi level position. We provide an explanation for this striking difference in anisotropy between the conductivity and the Seebeck coefficient using Mott's theory of hopping conduction. Our experimental evidence for an isotropic Seebeck coefficient in an anisotropic nanotube assembly suggests a route toward controlling the thermoelectric performance of carbon nanotube thin films through morphology control.

Acoustic response characterization of thermoacoustic CNT thin film arrays

A new model for a thermoacoustic transducer array with high acoustic beam directivity applying devised carbon nanotube (CNT) thin films is developed. Analytical exact solutions of acoustic pressure field for linear and planar arrays are derived by constructing a new spherical thermoacoustic model. The beamwidth and side-to-main lobe ratios are defined and used as the criteria for determining the optimal array design parameters. The key influencing factors including input frequency, element number, element length and inter-element spacing on the acoustic performance and characteristics are extensively investigated. A comparison between theoretical prediction and experiment is presented to verify the analytical model and very good agreement is reported. The numerical result indicates that increasing the input frequency, element length and inter-element spacing improves the main lobe sharpness while at the same time it lowers the energy efficiency for a fixed element number. It is recommended the largest possible element length, inter-element spacing and element number should be adopted to produce a higher acoustic pressure and better acoustic directivity while meeting the energy efficiency requirement.

Thiophene-containing polymer on sorting semiconducting single-walled carbon nanotubes

Two new thiophene-containing polymers are designed and synthesized successfully for selectively dispersion of high purity (>99%) semiconducting single-walled carbon nanotubes. But the morphologies of carbon nanotube thin films based on the two dispersions are of large difference. We find that alkyl chain not only improved the solubility of polymer-wrapping CNTs but also served as steric hindrance preventing bundling in thin film fabrication process. The obtained thin film transistors are also showing largely different performance, which indirectly reflects the important effect of polymer structure on sorting s-SWCNTs.

Effect of constructive rehybridization on transverse conductivity of aligned single-walled carbon nanotube films

Alignment of densely packed single-walled carbon nanotubes (SWNTs) largely preserves the extraordinary electronic properties of individual SWNTs in the alignment direction, while in transverse direction the films are very resistive due to large energy barriers for tunneling between adjacent SWNTs. We demonstrate that chromium atoms inserted between the sidewalls of parallel SWNTs effectively coordinate to the benzene rings of the nanotubes via hexahapto bonds that preserve the nanotube-conjugated electronic structure and serve as a conduit for electron transfer. The atomically interconnected aligned SWNTs exhibit enhanced transverse conductivity, which increases by ∼2100% as a result of the photoactivated organometallic functionalization with Cr. The hexahapto mode of bonding the graphitic surfaces of carbon nanotubes with transition metal atoms offers an attractive route to the reversible chemical engineering of the transport properties of aligned carbon nanotube thin films. We demonstrate that a device fabricated with aligned SWNTs can be reversibly switched between a state of high electrical conductivity (ON) by light and low electrical conductivity (OFF) by applied potential. This study provides a route to the design of novel nanomaterials for applications in electrical atomic switches, optoelectronic and spintronic devices.

Unusual conductivity temperature dependence of multiwalled carbon nanotube thin film

The Multiwalled carbon nanotube (MWCNT) thin films has been formed on PVC substrate using Langmuir-Blodgett technique as highly promising material for optoelectronics. The layers were annealed at 367 K which has led to significant electrical resistance decrease. The resistance of the obtained layer was investigated in wide temperature range showing the dependence significantly different from that observed for the layers created using CNTs of smaller diameter. The reason of the unusual conductivity temperature dependence is most probably the difference in thermal expansion coefficients of MWCNTs and the polymer combined with relatively high stiffness of the nanotubes with such large diameter.

Fabrication, electrical characterization and mechanical flexibility test of back gated carbon nanotube thin film transistors on polyimide substrate

This work demonstrates the scalable method of fabricating back gated carbon nanotube thin film transistors (CNTTFTs) on a polyimide (kapton) substrate using photo lithography and lift-off process for patterning gate dielectrics and metals deposited for electrodes , and dip coating method for single walled carbon nanotubes deposition on functionalized dielectric surface. The CNTTFTs of varying dimensions are fabricated and electrical characterizations are done at room temperature and the CNTTFTs have exhibited p-type behaviour. The CNTTFT of channel length(L) 3 μm, width(W) 10 μm exhibits on-off current ratio of 9.219 × 105, carrier mobility of 4.9 cm2 V−1 s−1, trans conductance of 26 μS and on current of 472 μA when the substrate is kept flat (not bent). The mechanical flexibility test of CNTTFTs is done by conducting electrical characterization of CNTTFTs with the substrate bent to different diameters. Irrespective of the channel length, CNTTFTs have exhibited minimal change in the electrical characteristics when bent to 14 mm diameter. However, short channel CNTTFT (L = 3 μm) has shown considerable change in electrical characteristics when bent to 8 mm diameter compared to long channel length CNTTFT (L = 75 μm). The fabricated back gated flexible CNTTFTs work as CNTTFTs when bent up to the diameter of 8 mm, demonstrates that the CNTTFTs can be used for practical applications in the area of flexible electronics.

Characterization of the spatial elastoresistivity of inkjet-printed carbon nanotube thin films

Next-generation lightweight-designed structures shall be able to perform self-state assessment via integrated health monitoring systems. In this article a carbon nanotube-embedded polymeric thin film is applied via inkjet-printing to perform spatial strain sensing in conjunction with using electrical impedance tomography. To gain an advanced understanding of the thin film’s spatial strain sensitivity, the elastoresistivity matrix, a fourth-order tensor correlating the strain state of a conductor into its normalized change in resistivity state, is characterized. The Montgomery method is adopted to derive the planar resistivity coefficients of the thin film, and a digital image correlation system is used to measure the planar strains. A validation test suggests that the calculated determinant of the correlated change in anisotropic resistivity shows a fairly similar result to the measured isotropic EIT reconstruction results.

Functionalized Carbon Nanotube Sensors for the Detection of Sub-ppm Nitric Oxide Gas

In this paper, we demonstrate the highly sensitive carbon nanotube (CNT) sensors for the detection of sub-ppm nitric oxide (NO) gas operating at room temperature. Such achievement can be realized by functionalizing CNT thin films with amine-based polymers through a solution-process technology at low temperature. In addition to high sensitivity, functionalized CNT sensors exhibit high selectivity towards NO gas, which is an effective and practical factor for health-care monitoring nano-electronics. We also investigated the effect of a post-cleaning treatment on the sensing performance of functionalized CNT thin films for sub-ppm NO gas sensors. We believe that this work can open-up new routes to realize high performance human-interactive electronics for respiratory diseases detection in exhaled air.

Thermionic cooling efficiency of functionalized carbon nanotube thin films

A large thermionic cooling effect is observed from a functionalized carbon nanotube cathode surface, and the cooling efficiency, defined as thermionic cooling power as a percentage of total input heating power to the surface, is reported. The cathode consists of a thin tungsten ribbon covered with low work function barium strontium oxide coated carbon nanotubes. It has both a low work function surface and a large Schottky field effect induced by the carbon nanotubes. The combination of the two leads to a strong thermionic emission and hence a relatively large thermionic cooling effect and high cooling efficiency. Detailed measurements of the cathode temperature profiles are presented together with the analysis of the cooling efficiency. The highest cooling efficiency obtained in this study is 5.9% at 1099 °C of cathode temperature and 114.5 mA of thermionic emission current. The dependencies of the cooling efficiency on the cathode temperature and on the emission current are also presented. Prospects of further improvements are discussed as well.

Lowering interface state density in carbon nanotube thin film transistors through using stacked Y2O3/HfO2 gate dielectric

Wafer-scale fabrication of transistors is the prerequisite for practical applications of carbon nanotube (CNT) based electronics. In this work, we fabricated top-gated thin film transistors (TFTs) based on solution-derived CNT film prepared on a 2 in. substrate through a photolithography based process. In particular, we improved the gate dielectric layer in CNT TFTs through using a thin thermal oxidized Y2O3 film as a buffer layer before the growth of high-κHfO2 layer. The introduction of the Y2O3 film significantly enhanced the performance of CNT TFTs, including the improved on-state current and transconductance, lowered threshold voltage and subthreshold swing, and drastically enhanced carrier mobility, owing to the reduction of the interface state density and scattering centers. Quantitative extraction of the interface state density based on either capacitance-voltage measurements or subthreshold swing data further demonstrates that the introduction of the Y2O3 interlayer reduces the interface state density from 9.24 × 1012 cm−2 to 4.63 × 1012 cm−2 in the gate insulator.

Thickness-dependent thermoelectric power factor of polymer-functionalized semiconducting carbon nanotube thin films

ABSTRACTThe effects of polymer structures on the thermoelectric properties of polymer-wrapped semiconducting carbon nanotubes have yet to be clarified for elucidating intrinsic transport properties. We systematically investigate thickness dependence of thermoelectric transport in thin films containing networks of conjugated polymer-wrapped semiconducting carbon nanotubes. Well-controlled doping experiments suggest that the doping homogeneity and then in-plane electrical conductivity significantly depend on film thickness and polymer species. This understanding leads to achieving thermoelectric power factors as high as 412 μW m−1 K−2 in thin carbon nanotube films. This work presents a standard platform for investigating the thermoelectric properties of nanotubes.

Biosensor response from target molecules with inhomogeneous charge localization

We analyzed the sensor response from aptamer-modified carbon nanotube thin film transistors that adsorb immunoglobulin E (IgE) with an inhomogeneous charge distribution. The net charge of the target molecules within the Debye length reflects the polarity of the electrical sensor signal. The complicated sensor response, which depends on the Debye length, can be interpreted by two kinds of adsorption states considering the influence of steric hindrance by previously adsorbed IgE molecules. These results explain the anomalous behavior of the sensor response in previous reports and pave the way to control the dynamic range.

High Modulation Speed, Depth, and Coloration Efficiency of Carbon Nanotube Thin Film Electrochromic Device Achieved by Counter Electrode Impedance Matching

AbstractImproving the modulation depth and switching speed of electrochromic devices is important for expanding the field of electrochromic functional materials applications. The previous study demonstrates that semiconducting (SC‐) single‐wall carbon nanotube (SWNT) thin film based electrochromic cells with ionic liquid as the electrolyte and metallic (MT‐) SWNT counter electrode can operate with fast switching times in the millisecond range. However, achieving a high modulation depth requires an increasing thickness of the electrochromically active SC‐SWNT layer resulting in a slowdown of the switching time by more than order of magnitude. Here it is reported that milliseconds range switching time can be restored by increasing the thickness of MT‐SWNT thin film counter electrode thus matching the electrochemical capacitances of the two sides of the electrochromic cell while reaching a high modulation depth of 20 dB and high coloration efficiency exceeding 1800 cm2 C−1 at an infrared wavelength of 1770 nm. The results are interpreted in terms of considering the SWNT cell as a supercapacitor with two connected in series electric double layer 3D capacitors associated with two opposing SWNT electrodes.

On/Off Ratio Improvement of Carbon Nanotube Thin Film Transistors By Combining Asymmetric Gate Design and Chemical Doping

Carbon nanotubes are regarded promising building blocks for the future nanoelectronics. High performance transistors and integrated circuits based on semiconducting CNT thin films, e.g., CMOS type 4-bit adder, CNT computer, flexible sensor etc., have been reported. These progresses demonstrate the CNT transistor technology is being pushed to practical application. For real circuits applications, high Ion/Ioff ratio with low Ioff for a transistor are required in high speed and low power consumption integrated circuits. However, the switching behavior of most reported CNT thin film transistors are not good enough. Due to the intrinsic ambipolarity of CNTs, the Ioff values are usually higher than hundreds of nano Ampe and the Ion/Ioff ratio is decreased to less than 102 at relatively high source/drain bias voltages (e.g. 1V). Though the switching behavior of the FETs based on individual CNTs can be improved by either special design of the device geometry or chemical doping, these methods do not work well in CNT thin film transistors. In this work, we improved the performance of CNT thin film transistors by combining the asymmetric gate design and chemical doping. In this way, the Ioff was suppressed while the Ion was increased at the mean time. Thus the Ion/Ioff ratio was increased by ~2 orders of magnitude when compared with normally designed top-gate device. Moreover, the devices showed better subthreshold swing (SS) and the threshold voltage (Vth) was closer to zero than normal devices. These improvements are benefit to more complicated circuits design, and our method is applicable for the emerging nanodevices.

(Invited) Carbon Nanotube Transistors for Flat Panel Display Application

Carbon nanotubes are promising materials which can provide more functions at low cost for macroelectronics application, such as backplane electronics for display, flexible electronics, bio-chemical sensors, solar cells and others. Among these macroelectronics applications, carbon nanotube thin film transistors (CNT-TFTs) are expected to be used soon for backplanes electronics in flat panel displays (FPDs). This is due to the performance of CNT-TFTs surpasses the current commercialized TFT technologies, e.g., high mobility, high on/off ratio and high on-current surpass. Moreover, the fabrication process of CNT-TFTs is regarded facile, scalable, and compatible with the current TFT manufacturing process. Prototype AMOLED devices driven by CNT-TFTs have been demonstrated. However, most of the efforts on the application of CNT-TFTs for the backplane electronics are in research laboratories, and there are still many challenges before it is commercialized. In this presentation, we will report our progress on CNT-TFTs for FPD application, which includes the following aspects: high efficient fabrication of highly uniform carbon nanotube films on large area substrates (from 4 inch silicon wafers to 2.5th generation backplane glasses (370mm × 470mm)), manufacturing compatible fabrication process of CNT-TFTs, contact effect on the performance and scaling behavior of CNT-TFTs, hysteresis and ambipolarity suppression. Our results indicate practical application of CNT-TFTs in FPD products is expectable in a near future.

SnO2 Nanowires on Carbon Nanotube Film as a High Performance Anode Material for Flexible Li-ion Batteries

Today, Li-ion batteries (LIBs) are the most common rechargeable batteries used in electronic devices. SnO2 with theoretical specific capacity of 782 mAh/g is among the best anode materials for LIBs. In this report, Three-dimensional SnO2 nanowires (NWs) on carbon nanotube (CNT) thin film (SnO2 / CNT) is fabricated using a combination of vacuum filtration and thermal evaporation techniques. The resulting 3D heterostructure SnO2/CNT was characterized by X-ray diffraction, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). This fabricated SnO2/CNT electrode has been tested as a flexible and binder-free anode for LIB, which exhibits high initial discharge/charge capacity of 4.8/2.25 mAh/cm2 at a current density of 0.25 A/g, much larger than discharge/charge capacity of bare CNT film (2.2/0.3 mAh/cm2). Relatively high areal capacity of 1.23 mAh/cm2 has been achieved for the fabricated LIB with SnO2/CNT electrode after 20 cycles, proposing this material as a high performance flexible LIB anode material.

Measurement of in-plane sheet thermal conductance of single-walled carbon nanotube thin films by steady-state infrared thermography

A useful and rapid method of measuring the in-plane sheet thermal conductance of single-walled carbon nanotube (SWNT) thin films by steady-state infrared thermography is proposed in this study. The SWNT thin films are suspended between the free ends of two cantilevered silicon thin plates, and the two anchored ends are kept at different but constant temperatures to achieve steady-state heat transport through this silicon–SWNT–silicon bridge in vacuum. The temperature gradient along the bridge is captured with an infrared camera. A silicon plate with known thermal conductivity serves as a reference for calculating the heat flux, so that the sheet thermal conductance of SWNT thin films can be calculated using Fourier’s law after deducting the influence of thermal radiation. The thermal conductivity of the 50-nm-thick SWNT thin film is 68.1 W m−1 K−1.

Evaluation of Thin Film Surface Shape as a Function of Dispersant Concentration in Carbon Nanotube Thin Film Fabrication via the Electrospray Method

Evaluation of Thin Film Surface Shape as a Function of Dispersant Concentration in Carbon Nanotube Thin Film Fabrication via the Electrospray Method

Thermodynamics and kinetics of polyoxyethylene alkyl ether evaporation from inkjet-printed carbon nanotube thin films by vacuum annealing

Carbon nanotubes (CNTs) dispersed in water by sonication with surfactants are used as printing inks. However, surfactants are usually electrical insulators that remain on the prepared CNT films, reducing their conductivity. While evaporation by annealing is an effective removal process, it is usually performed at high temperatures (180 °C) in air, making it inapplicable for printing on common plastic substrates. This work demonstrates a vacuum annealing process for evaporating surfactants. Poly(oxyethylene)4 lauryl ether (Brij L4)—a nonionic surfactant—was utilized. Its evaporation behaviors (thermodynamics and kinetics) were analyzed by thermogravimetric (TG) analysis and infrared (IR) spectroscopy. The TG measurements confirmed that the evaporation onset temperature under vacuum (51.0 °C) was lower than that in air (148.7 °C). The IR results revealed that ∼100% Brij L4 was removed by annealing under vacuum at 130 °C. The desorption energy of the surfactant from CNTs was calculated from the activation energy results to be about 10 kJ mol−1; this value is about a half to one order of magnitude smaller than those reported for other surfactants. This energy would allow adequate adsorption interactions for the surfactant to stably disperse the nanotubes, but is sufficiently small to allow easy removal of the surfactant by thermal treatment. The electrical resistance of the vacuum-annealed CNT thin film dramatically decreased compared with that of the film annealed in air. Therefore, this process can be used for fabricating flexible printed electronics with CNTs.

Ultraviolet/ozone and oxygen plasma treatments for improving the contact of carbon nanotube thin film transistors

Carbon nanotube thin film transistor (CNT-TFT) is an emerging technology for future macroelectronics, such as chemical and biological sensors, optical detectors, and the backplane driving circuits for flat panel displays. The mostly reported fabrication method of CNT-TFT is a lift-off based photolithography process. In such fabrication process, photoresist (PR) residue contaminates the interface of tube-metal contact and deteriorates the device performance. In this paper, ultraviolet ozone (UVO) and oxygen plasma treatments were employed to remove the PR contamination. Through our well-designed experiments, the UVO treatment is confirmed an effective way of cleaning contamination at the tube-metal interface, while oxygen plasma treatment is too reactive and hard to control, which is not appropriate for CNT-TFTs. It is determined that 2–6 min UVO treatment is the preferred window, and the best optimized treatment time is 4 min, which leads to 15% enhancement of device performance.