Surfactant Micelle-Driven High-Efficiency and High-Resolution Length Separation of Carbon Nanotubes for Electronic Applications.

High-efficiency extraction of long single-wall carbon nanotubes (SWCNTs) with excellent optoelectronic properties from SWCNT solution is critical for enabling their application in high-performance optoelectronic devices. Here, a straightforward and high-efficiency method is reported for length separation of SWCNTs by modulating the concentrations of binary surfactants. The results demonstrate that long SWCNTs can spontaneously precipitate for binary-surfactant but not for single-surfactant systems. This effect is attributed to the formation of compound micelles by binary surfactants that squeeze the free space of long SWCNTs due to their large excluded volumes. With this technique, it can readily separate near-pure long (≥500 nm in length, 99% in content) and short (≤500 nm in length, 98% in content) SWCNTs with separation efficiencies of 26% and 64%, respectively, exhibiting markedly greater length resolution and separation efficiency than those of previously reported methods. Thin-film transistors fabricated from extracted semiconducting SWCNTs with lengths >500 nm exhibit significantly improved electrical properties, including a 10.5-fold on-state current and 14.7-fold mobility, compared with those with lengths <500 nm. The present length separation technique is perfectly compatible with various surfactant-based methods for structure separations of SWCNTs and is significant for fabrication of high-performance electronic and optoelectronic devices.

Length-Dependent Enantioselectivity of Carbon Nanotubes by Gel Chromatography.

High-purity enantiomer separation of chiral single-wall carbon nanotubes (SWCNTs) remains a challenge compared with electrical type and chirality separations due to the limited selectivities for both chirality and handedness, which is important for an exploration of their properties and practical applications. Here, we performed length fractionation for enantiomer-purified SWCNTs and found a phenomenon in which the enantioselectivities were higher for longer nanotubes than for shorter nanotubes due to length-dependent interactions with the gel medium, which provided an effective strategy of controlling nanotube length for high-purity enantiomer separation. Furthermore, we employed a gentler pulsed ultrasonication instead of traditional vigorous ultrasonication for preparation of a low-defect long SWCNT dispersion and achieved the enantiomer separation of single-chirality (6,5) SWCNTs with an ultrahigh enantiomeric purity of up to 98%, which was determined by using the linear relationship between the normalized circular dichroism intensity and the enantiomeric purity. Compared with all results reported previously, the present enantiomeric purity was significantly higher and reached the highest level reported to date. Due to the ultrahigh selectivity in both chirality and handedness, the two obtained enantiomers exhibited perfect symmetry in their circular dichroism spectra, which offers standardization for characterizations and evaluations of SWCNT enantiomers.

A High-Temperature Accelerometer with Excellent Performance Based on the Improved Graphene Aerogel.

A high-temperature accelerometer plays an important role for ensuring normal operation of equipment in aerospace, such as monitoring and identifying abnormal vibrations of aircraft engines. Phase transitions of piezoelectric crystals, mechanical failure and current leakage of piezoresistive/capacitive materials are the prominent inherent limitations of present high-temperature accelerometers working continuously above 973 K. With the rapid development of aerospace, it is a great challenge to develop a new type of vibration sensor to meet the crucial demands at high temperature. Here we report a high-temperature accelerometer working with a contact resistance mechanism. Based on the improved graphene aerogel (GA) prepared by a modulated treatment process, the accelerometer can operate continuously and stably at 1073 K and intermittently at 1273 K. The developed sensor is lightweight (sensitive element <5 mg) and has high sensitivity (an order of magnitude higher than MEMS accelerometers) and wide frequency response range (up to 5 kHz at 1073 K) with marked stability, repeatability and low nonlinearity error (<1%). These merits are attributed to the excellent and stable mechanical properties of the improved GA in the range of 299-1073 K. The accelerometer could be a promising candidate for high-temperature vibration sensing in space stations, planetary rovers and others.

Preparing high-concentration individualized carbon nanotubes for industrial separation of multiple single-chirality species.

Industrial production of single-chirality carbon nanotubes is critical for their applications in high-speed and low-power nanoelectronic devices, but both their growth and separation have been major challenges. Here, we report a method for industrial separation of single-chirality carbon nanotubes from a variety of raw materials with gel chromatography by increasing the concentration of carbon nanotube solution. The high-concentration individualized carbon nanotube solution is prepared by ultrasonic dispersion followed by centrifugation and ultrasonic redispersion. With this technique, the concentration of the as-prepared individualized carbon nanotubes is increased from about 0.19 mg/mL to approximately 1 mg/mL, and the separation yield of multiple single-chirality species is increased by approximately six times to the milligram scale in one separation run with gel chromatography. When the dispersion technique is applied to an inexpensive hybrid of graphene and carbon nanotubes with a wide diameter range of 0.8-2.0 nm, and the separation yield of single-chirality species is increased by more than an order of magnitude to the sub-milligram scale. Moreover, with present separation technique, the environmental impact and cost of producing single-chirality species are greatly reduced. We anticipate that this method promotes industrial production and practical applications of single-chirality carbon nanotubes in carbon-based integration circuits.

Chirality-dependent electrical transport properties of carbon nanotubes obtained by experimental measurement.

Establishing the relationship between the electrical transport properties of single-wall carbon nanotubes (SWCNTs) and their structures is critical for the design of high-performance SWCNT-based electronic and optoelectronic devices. Here, we systematically investigated the effect of the chiral structures of SWCNTs on their electrical transport properties by measuring the performance of thin-film transistors constructed by eleven distinct (n, m) single-chirality SWCNT films. The results show that, even for SWCNTs with the same diameters but different chiral angles, the difference in the on-state current or carrier mobility could reach an order of magnitude. Further analysis indicates that the electrical transport properties of SWCNTs have strong type and family dependence. With increasing chiral angle for the same-family SWCNTs, Type I SWCNTs exhibit increasing on-state current and mobility, while Type II SWCNTs show the reverse trend. The differences in the electrical properties of the same-family SWCNTs with different chiralities can be attributed to their different electronic band structures, which determine the contact barrier between electrodes and SWCNTs, intrinsic resistance and intertube contact resistance. Our present findings provide an important physical basis for performance optimization and application expansion of SWCNT-based devices.

Simultaneously enhanced tenacity, rupture work, and thermal conductivity of carbon nanotube fibers by raising effective tube portion.

Although individual carbon nanotubes (CNTs) are superior to polymer chains, the mechanical and thermal properties of CNT fibers (CNTFs) remain inferior to synthetic fibers because of the failure of embedding CNTs effectively in superstructures. Conventional techniques resulted in a mild improvement of target properties while degrading others. Here, a double-drawing technique is developed to rearrange the constituent CNTs. Consequently, the mechanical and thermal properties of the resulting CNTFs can simultaneously reach their highest performances with specific strength ~3.30 N tex-1 (4.60 GPa), work of rupture ~70 J g-1, and thermal conductivity ~354 W m-1 K-1 despite starting from low-crystallinity materials (IG:ID ~ 5). The processed CNTFs are more versatile than comparable carbon fiber, Zylon and Dyneema. On the basis of evidence of load transfer efficiency on individual CNTs measured with in situ stretching Raman, we find that the main contributors to property enhancements are the increasing of the effective tube contribution.

Recent Advances in Structure Separation of Single-Wall Carbon Nanotubes and Their Application in Optics, Electronics, and Optoelectronics.

Structural control of single-wall carbon nanotubes (SWCNTs) with uniform properties is critical not only for their property modulation and functional design but also for applications in electronics, optics, and optoelectronics. To achieve this goal, various separation techniques have been developed in the past 20 years through which separation of high-purity semiconducting/metallic SWCNTs, single-chirality species, and even their enantiomers have been achieved. This progress has promoted the property modulation of SWCNTs and the development of SWCNT-based optoelectronic devices. Here, the recent advances in the structure separation of SWCNTs are reviewed, from metallic/semiconducting SWCNTs, to single-chirality species, and to enantiomers by several typical separation techniques and the application of the corresponding sorted SWCNTs. Based on the separation procedure, efficiency, and scalability, as well as, the separable SWCNT species, purity, and quantity, the advantages and disadvantages of various separation techniques are compared. Combined with the requirements of SWCNT application, the challenges, prospects, and development direction of structure separation are further discussed.

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 15 000 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.

Submilligram-scale separation of near-zigzag single-chirality carbon nanotubes by temperature controlling a binary surfactant system.

Mass production of zigzag and near-zigzag single-wall carbon nanotubes (SWCNTs), whether by growth or separation, remains a challenge, which hinders the disclosure of their previously unknown property and practical applications. Here, we report a method to separate SWCNTs by chiral angle through temperature control of a binary surfactant system of sodium cholate (SC) and SDS in gel chromatography. Eleven types of single-chirality SWCNT species with chiral angle less than 20° were efficiently separated including multiple zigzag and near-zigzag species. Among them, (7, 3), (8, 3), (8, 4), (9, 1), (9, 2), (10, 2), and (11, 1), were produced on the submilligram scale. The spectral detection results indicate that lowering the temperature induced selective adsorption and reorganization of the SC/SDS cosurfactants on SWCNTs with different chiral angles, amplifying their interaction difference with gel. We believe that this work is an important step toward industrial separation of single-chirality zigzag and near-zigzag SWCNTs.

High-precision solid catalysts for investigation of carbon nanotube synthesis and structure.

The direct growth of single-walled carbon nanotubes (SWCNTs) with narrow chiral distribution remains elusive despite substantial benefits in properties and applications. Nanoparticle catalysts are vital for SWCNT and more generally nanomaterial synthesis, but understanding their effect is limited. Solid catalysts show promise in achieving chirality-controlled growth, but poor size control and synthesis efficiency hampers advancement. Here, we demonstrate the first synthesis of refractory metal nanoparticles (W, Mo, and Re) with near-monodisperse sizes. High concentrations (N = 105 to 107 cm-3) of nanoparticles (diameter 1 to 5 nm) are produced and reduced in a single process, enabling SWCNT synthesis with controlled chiral angles of 19° ± 5°, demonstrating abundance >93%. These results confirm the interface thermodynamics and kinetic growth theory mechanism, which has been extended here to include temporal dependence of fast-growing chiralities. The solid catalysts are further shown effective via floating catalyst growth, offering efficient production possibilities.

Quantitative analysis of the effect of reabsorption on the Raman spectroscopy of distinct (n, m) carbon nanotubes.

We quantitatively analyze the effect of reabsorption on the Raman spectroscopy of (10, 3) and (8, 3) single-chirality single-wall carbon nanotube (SWCNT) solutions by varying the detection depth in confocal micro-Raman measurements and SWCNT concentration the in sample solution. The increase of the detection depth and concentration of SWCNTs enhances the reabsorption effect and decreases the intensities of the Raman features. More importantly, reabsorption exhibits different effects on different Raman features such as the radial breathing mode (RBM) and G+ band, strongly depending on the resonance degree of the scattered light energy and the interband transition of SWCNTs. When (10, 3) SWCNTs are excited with a 633 nm laser, the scattered light from RBM has stronger resonance with the interband transition of the SWCNTs than that from the G+ band, leading to a faster reduction in the RBM intensity and a lower intensity ratio of RBM to the G+ band. In contrast, when (8, 3) SWCNTs are excited with a 633 nm laser, reabsorption has the same effect on the RBM and G+ band intensities and thus maintains a constant intensity ratio of RBM to the G+ band. Furthermore, we precisely establish a quantitative relationship of the intensities of the Raman features such as RBM, the G+ band and their intensity ratio as a function of the focal depth and SWCNT concentration by theoretical calculations and numerical simulation, which reproduces the experimental results well. These results are very useful in the precise analysis of the Raman spectroscopy of SWCNTs and thus their applications in molecular detection and imaging.

Transparent and Freestanding Single-Walled Carbon Nanotube Films Synthesized Directly and Continuously via a Blown Aerosol Technique.

Single-walled carbon nanotube (SWCNT) films are promising materials as flexible transparent conductive films (TCFs). Here, inspired by the extrusion blown plastic film technique and the SWCNT synthesis approach by floating catalyst chemical vapor deposition (FCCVD), a novel blown aerosol chemical vapor deposition (BACVD) method is reported to directly and continuously produce freestanding SWCNT TCFs at several hundred meters per hour. The synthesis mechanism, involving blowing a stable aerosol bubble and transforming the bubble into an aerogel, is investigated, and a general phase diagram is established for this method. For the SWCNT TCFs via BACVD, both carbon conversion efficiency and SWCNT TCF yield can reach three orders of magnitude higher than those with the conventional FCCVD. The film displays a sheet resistance of 40 ohm sq-1 at 90% transmittance after being doped, representing the record performance based on large-scale SWCNT films. Transparent, flexible, and stretchable electrodes based on BACVD films are demonstrated. Moreover, this high-throughput method of producing SWCNT TCFs can be compatible with the roll-to-roll process for mass production of flexible displays, touch screens, solar cells, and solid-state lighting, and is expected to have a broad and long-term impact on many fields from consumer electronics to energy conversion and generation.

All-Carbon Pressure Sensors with High Performance and Excellent Chemical Resistance.

An all-carbon pressure sensor is designed and fabricated based on reduced graphene oxide (rGO) nanomaterials. By sandwiching one layer of superelastic rGO aerogel between two freestanding high-conductive rGO thin papers, the sensor works based on the contact resistance at the aerogel-paper interfaces, getting rid of the alien materials such as polymers and metals adopted in traditional sensors. Without the limitation of alien materials, the all-carbon sensors demonstrate an ultrawide detecting range (0.72 Pa-130 kPa), low energy consumption (≈0.58 µW), ultrahigh sensitivity (349-253 kPa-1 ) at low-pressure regime (<1.4 Pa), fast response time (8 ms at 1 kPa), high stability (10 000 unloading-loading cycles between 0 and 1 kPa), light weight (<10 mg), easily scalable fabrication process, and excellent chemical stability. These merits enable them to detect real-time human physiological signals and monitor the weights of various droplets of not only water but also hazardous chemical reagents including strong acid, strong alkali, and organic solvents. This shows their great potential applications in real-time health monitoring, sport performance detecting, harsh environment-related robotics and industry, and so forth.

Designing hybrid gate dielectric for fully printing high-performance carbon nanotube thin film transistors.

The electrical characteristics of carbon nanotube (CNT) thin-film transistors (TFTs) strongly depend on the properties of the gate dielectric that is in direct contact with the semiconducting CNT channel materials. Here, we systematically investigated the dielectric effects on the electrical characteristics of fully printed semiconducting CNT-TFTs by introducing the organic dielectrics of poly(methyl methacrylate) (PMMA) and octadecyltrichlorosilane (OTS) to modify SiO2 dielectric. The results showed that the organic-modified SiO2 dielectric formed a favorable interface for the efficient charge transport in s-SWCNT-TFTs. Compared to single-layer SiO2 dielectric, the use of organic-inorganic hybrid bilayer dielectrics dramatically improved the performances of SWCNT-TFTs such as mobility, threshold voltage, hysteresis and on/off ratio due to the suppress of charge scattering, gate leakage current and charge trapping. The transport mechanism is related that the dielectric with few charge trapping provided efficient percolation pathways for charge carriers, while reduced the charge scattering. High density of charge traps which could directly act as physical transport barriers and significantly restrict the charge carrier transport and, thus, result in decreased mobile carriers and low device performance. Moreover, the gate leakage phenomenon is caused by conduction through charge traps. So, as a component of TFTs, the gate dielectric is of crucial importance to the manufacture of high quality TFTs from the aspects of affecting the gate leakage current and device operation voltage, as well as the charge carrier transport. Interestingly, the OTS-modified SiO2 allows to directly print horizontally aligned CNT film, and the corresponding devices exhibited a higher mobility than that of the devices with the hybrid PMMA/SiO2 dielectric although the thickness of OTS layer is only ∼2.5 nm. Our present result may provide key guidance for the further development of printed nanomaterial electronics.

High-performance and compact-designed flexible thermoelectric modules enabled by a reticulate carbon nanotube architecture.

It is a great challenge to substantially improve the practical performance of flexible thermoelectric modules due to the absence of air-stable n-type thermoelectric materials with high-power factor. Here an excellent flexible n-type thermoelectric film is developed, which can be conveniently and rapidly prepared based on the as-grown carbon nanotube continuous networks with high conductivity. The optimum n-type film exhibits ultrahigh power factor of ∼1,500 µW m-1 K-2 and outstanding stability in air without encapsulation. Inspired by the findings, we design and successfully fabricate the compact-configuration flexible TE modules, which own great advantages compared with the conventional π-type configuration modules and well integrate the superior thermoelectric properties of p-type and n-type carbon nanotube films resulting in a markedly high performance. Moreover, the research results are highly scalable and also open opportunities for the large-scale production of flexible thermoelectric modules.

Carbon nanotube thin film transistors fabricated by an etching based manufacturing compatible process.

Carbon nanotube thin film transistors (CNT-TFTs) have been regarded as strong competitors to currently commercialized TFT technologies. Though much progress has been achieved recently, CNT-TFT research is still in the stage of laboratory research. One critical challenge for commercializing CNT-TFT technology is that the commonly used device fabrication method is a lift-off based process, which is not suitable for mass production. In this paper, we report an etching based fabrication process for CNT-TFTs, which is fully manufacturing compatible. In our process, the CNT thin film channel was patterned by dry etching, while wet etching was used for patterning the layers of metal and insulator. The CNT-TFTs were successfully fabricated on a 4 inch wafer in both top-gate and buried-gate geometries with low Schottky barrier contact and pretty uniform performance. High output current (>1.2 µA µm-1), high on/off current ratio (>105) and high mobility (>30 cm2 V-1 s-1) were obtained. Though the fabrication process still needs to be optimized, we believe our research on the etching fabrication process pushes CNT-TFT technology a step forward towards real applications in the near future.

Spin-exciton interaction and related micro-photoluminescence spectra of ZnSe:Mn DMS nanoribbon.

For their spintronic applications the magnetic and optical properties of diluted magnetic semiconductors (DMS) have been studied widely. However, the exact relationships between the magnetic interactions and optical emission behaviors in DMS are not well understood yet due to their complicated microstructural and compositional characters from different growth and preparation techniques. Manganese (Mn) doped ZnSe nanoribbons with high quality were obtained by using the chemical vapor deposition (CVD) method. Successful Mn ion doping in a single ZnSe nanoribbon was identified by elemental energy-dispersive x-ray spectroscopy mapping and micro-photoluminescence (PL) mapping of intrinsic d-d optical transition at 580 nm, i.e. the transition of 4 T 1(4 G) â†’ 6 A 1(6 s),. Besides the d-d transition PL peak at 580 nm, two other PL peaks related to Mn ion aggregates in the ZnSe lattice were detected at 664 nm and 530 nm, which were assigned to the d-d transitions from the Mn2+-Mn2+ pairs with ferromagnetic (FM) coupling and antiferromagnetic (AFM) coupling, respectively. Moreover, AFM pair formation goes along with strong coupling with acoustic phonon or structural defects. These arguments were supported by temperature-dependent PL spectra, power-dependent PL lifetimes, and first-principle calculations. Due to the ferromagnetic pair existence, an exciton magnetic polaron (EMP) is formed and emits at 460 nm. Defect existence favors the AFM pair, which also can account for its giant enhancement of spin-orbital coupling and the spin Hall effect observed in PRL 97, 126603(2006) and PRL 96, 196404(2006). These emission results of DMS reflect their relation to local sp-d hybridization, spin-spin magnetic coupling, exciton-spin or phonon interactions covering structural relaxations. This kind of material can be used to study the exciton-spin interaction and may find applications in spin-related photonic devices besides spintronics.

Hydro-actuation of hybrid carbon nanotube yarn muscles.

Hybrid hydro-responsive actuators are developed by infiltrating carbon nanotube yarns using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). These actuators demonstrate impressive rotation and contraction in response to water due to volumetric expansion of the helical arrangement of carbon nanotubes. The total torsional stroke is 3720 revolutions per m and the simultaneously generated contractive strain reaches 24% at a paddle-to-yarn mass ratio of 350. The contraction output can furthermore be significantly enhanced by constraining the rotational motion and it reaches 68% with an applied stress of 1 MPa. Additionally, hybrid yarns exhibit an approximately linear response to humidity changes and show extra capability of electrical actuation, which, combined with the excellent hydro-actuation performance, endow them with great potential for a variety of applications including artificial muscles, hydro-driven generators, moisture switches and microfluidic mixers.

Ultrahigh-Power-Factor Carbon Nanotubes and an Ingenious Strategy for Thermoelectric Performance Evaluation.

An ingenious strategy is put forward to evaluate accurately the thermoelectric performance of carbon nanotube (CNT) thin films, including thermal conductivity, electrical conductivity, and Seebeck coefficient in the same direction. The results reveal that the as-prepared CNT interconnected films and CNT fibers possess enormous potential of thermoelectric applications because of their ultrahigh power factors.

Ethanol-assisted gel chromatography for single-chirality separation of carbon nanotubes.

Surfactants or polymers are usually used for the liquid processing of carbon nanotubes for their structure separation. However, they are difficult to remove after separation, affecting the intrinsic properties and applications of the separated species. Here, we report an ethanol-assisted gel chromatography for the chirality separation of single-walled carbon nanotubes (SWCNTs), in which ethanol is employed to finely tune the density/coverage of sodium dodecyl sulfate (SDS) on nanotubes, and thus the interactions between SWCNTs and an allyl dextran-based gel. Incrementally increasing the ethanol content in a low-concentration SDS eluent leads to successive desorption of the different structure SWCNTs adsorbed on the gel, and to achieve multiple distinct (n, m) single-chirality species. The use of ethanol enables the working concentration of SDS to be reduced dramatically and also avoids the introduction of other surfactants or chemical reagents. More importantly, ethanol can be easily removed after separation. The ability of ethanol to tune the interactions between SWCNTs and the gel also gives a deeper insight into the separation mechanism of SWCNTs using gel chromatography.