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.

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.

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.

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.

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.

A repeated halving approach to fabricate ultrathin single-walled carbon nanotube films for transparent supercapacitors.

Ultrathin SWCNT transparent and conductive films on flexible and transparent substrates are prepared via repeatedly halving the directly grown SWCNT films and flexible and transparent supercapacitors with excellent performance were fabricated.

Controllable two-stage droplet evaporation method and its nanoparticle self-assembly mechanism.

Bottom-up self-assembly is able to constitute a variety of structures and has been thought to be a promising way for advanced nanofabrication. Droplet evaporation, as the simplest method, has been used in various self-assemblies. However, the assembled area is not large enough and the order is still not well controlled. Here we show a facile and controllable two-stage droplet evaporation method by adjusting the humidity and temperature of the evaporating droplet. Taking the highly monodispersed gold nanorods (GNRs) as an example, large-area, self-assembly monolayer arrays are reproducibly achieved. To understand the self-assembly mechanism, we adopted simplified models to analyze the interactions between the nanorods. The results show that a metastable state of secondary-energy-minimum exists, especially in the latter stage of the assembly process, leading to the ordered arrays. A large electrostatic barrier between the assembled arrays prevents the formation of the multilayer structures and thereby leads to the preferential monolayers. Moreover, we predict possibilities of different types of assemblies of the nanorods, and a schematic phase diagram is finally given. The results here may offer a way toward high-quality self-assembled nanoparticles superlattices for use in enhanced spectroscopy, sensors, or nanodevices.

Growth of single crystal zinc oxide beaded nanowires.

Reported here is the growth of single crystal zinc oxide beaded nanowires. The beaded nanowires are composed of nanobeads and ultrafine joint nanorods, which alternatively epitaxially grow along [001] direction of zinc oxide wurtzite structure. The growth mechanism was discussed and a zinc-rich process was proposed. Due to the size confinement, the bandgap along the beaded nanowires is modulated by their diameter and like multiple quantum well structure. Different from traditional multiple quantum well structures through alternate heterogeneous epitaxial growth of semiconductors, this kind of quasi-one-dimensional homogeneous multiple quantum well structures is composed of same substance. Their bandgap is modulated through alternately adjusting their diameter along single crystal nanowires. Further studies exhibit that they are of special properties on photoluminescence and Raman spectra. The demonstration would open the route to synthesize homogeneous multiple quantum well structures, study their fundamental physical phenomena, and exploit their applications.

Optically powered ZnO nanowires with symmetric and asymmetric contacts.

In this paper, we show large photovoltage and photocurrent obtained in individual ZnO nanowire devices with symmetrical and asymmetrical contacts. Ti/Au terminal electrodes were deposited for symmetrical contacts, while Ti/Au and Au electrodes were employed for asymmetrical contacts. Photovoltage and photocurrent measurements were carried out on individual ZnO nanowires devices without external bias. The symmetric nanowire devices exhibited -3 mV photovoltage and -10 nA photocurrent powered by UV laser illumination, while -60 mV photovoltage and -10 nA photocurrent were obtained for asymmetric contacts. Furthermore, the photovoltage were noticeably dependent on the local excitation position. Further calculation showed that the laser illumination reduced the Schottky barrier heights. The above results can be understood on the basis of illumination controlled Schottky barrier heights at nanowire-metal contacts and surface oxygen adsorption-desorption. We demonstrate that the simple two-terminal devices can be useful for self-powered and position sensitive photodetection, optically powered circuit, and nanowire based integrated nanophotonics.

Indium tin oxide nanowires grown by one-step thermal evaporation-deposition process at low temperature.

Indium tin oxide (ITO), as one of the most important transparent conducting oxide, is widely used in electro-optical field. We have developed a simple one-step method to synthesize ITO nanowires at low temperature of 600 degrees C. In detail, mixtures of InN nanowires and SnO powder, with the molar ratio of 10:1, have been used as precursors for the thermal evaporation-deposition of ITO nanowires on silicon/quartz slices. During the growth process, the evaporation temperature is maintained at 600 degrees C, which favors the decomposition of InN and oxidation of In, with a limited incorporation of Sn in the resulting compound (In:Sn approximately 11:1 in atomic ratio). As far as we know, this is the lowest growth temperature reported on the thermal deposition of ITO nanowires. The diameters of the nanowires are about 120 nm and the lengths are up to tens of micrometers. XRD characterization indicates the high crystallization of the nanowires. HRTEM results show the nanowires grow along the [200] direction. The transmittance of the nanowire film on quartz slice is more than 75% in the visible region. Based on photolithography and lift-off techniques, four-terminal measurement was utilized to test the resistivity of individual nanowire (6.11 x 10(-4) omega x cm). The high crystallization quality, good transmittance and low resistivity make as-grown ITO nanowires a promising candidate as transparent electrodes of nanoscale devices.

Temperature dependent resistance of as-grown and chemical treated single walled carbon nanotubes films.

Single Walled Carbon Nanotube (SWCNT) films were directly synthesized via Floating Catalyst Chemical Vapor Deposition (FCCVD) method. Temperature dependent resistance measurements were carried out on the as-grown and chemical treated SWCNTs films. A "U" shaped curve was obtained for each sample, with a significant variation in the crossover temperatures between the as-grown and treated samples. A heterogeneous model was adopted to interpret the experimental data, revealing the coexistence of anisotropic 1D metallic conduction, conventional metallic conduction and fluctuation assisted tunneling. Our results implied very low barriers, verifying the good intertube and interbundle contacts in the directly synthesized SWCNTs films. We speculated that oxidization and acid treatments would affect the overall configuration of the films, leading to the changes in the temperature dependence of resistance. In addition, Raman and absorption spectra indicated that oxidization and acid process would cause moderate changes in the hole carrier concentration of the films.

In-situ Raman spectra of single-walled carbon nanotube/epoxy nanocomposite film under strain.

In-situ Raman spectra under continuous strain have been carried out to follow the load transfer efficiency in a single-walled carbon nanotube/epoxy nanocomposite film, which was fabricated by a facile and filter-free infiltration method using directly synthesized SWNT film as skeleton. It was found that in the low elastic range of 1%, with the increase of applied strain, the G- and G+ Raman bands downshift nearly lineally with corresponding ultrahigh rates up to -12 and -17 cm(-1)/% strain, respectively. As the applied strain decrease, these Raman bands upshift to their original frequencies with almost the same rates. These precise relationships between the Raman band shift and the applied strain enable this nanocomposite film promising stress or stain sensors. Furthermore, some Raman radial breathing modes (RBMs) disappear (appear) under larger strain (1.6%) and recover (vanish) when the strain is released, also verify the effective deformation of micro SWNTs induced by extra macro strain applied on the nanocomposite film. More interestingly, these frequency shifts and intensity vibrations are recoverable and repeatable, indicating the high elasticity of deformation in this strain region of 2%.

A facile method to fabricate ultrathin vertical ZnO nanowall arrays.

We demonstrate a facile method to synthesize ultrathin vertical ZnO nanowall arrays only through a one-step catalyst fabrication procedure. After being coated with uniformly dispersed gold particles, the substrates were polished gently to create discrete gold nanoparticle alignment as catalyst. The commercial ZnO powder and activated carbon powder with an atom ratio of 1:1 were utilized as source materials, and the whole growth process occurred in a conventional tube furnace chemical vapor deposition (CVD) system. The SEM images confirm that the ZnO nanowall arrays grow vertically along certain tracks made by polishing. The walls are about 17 nm wide, 90 nm high and 1 microm long after 90 min growth. After second growth, the nanowalls become 20 nm wide, 205 nm high, and 2 microm long. AFM images indicate that the alignment of discrete Au particles with certain separations due to polishing plays the key role in realizing vertical ZnO nanowall arrays.

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.

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.

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.

Electrophoretic build-up of alternately multilayered films and micropatterns based on graphene sheets and nanoparticles and their applications in flexible supercapacitors.

Graphene nanosheets and metal nanoparticles (NPs) have been used as nano-building-blocks for assembly into macroscale hybrid structures with promising performance in electrical devices. However, in most graphene and metal NP hybrid structures, the graphene sheets and metal NPs (e.g., AuNPs) do not enable control of the reaction process, orientation of building blocks, and organization at the nanoscale. Here, an electrophoretic layer-by-layer assembly for constructing multilayered reduced graphene oxide (RGO)/AuNP films and lateral micropatterns is presented. This assembly method allows easy control of the nano-architecture of building blocks along the normal direction of the film, including the number and thickness of RGO and AuNP layers, in addition to control of the lateral orientation of the resultant multilayered structures. Conductivity of multilayered RGO/AuNP hybrid nano-architecture shows great improvement caused by a bridging effect of the AuNPs along the out-of-plane direction between the upper and lower RGO layers. The results clearly show the potential of electrophoretic build-up in the fabrication of graphene-based alternately multilayered films and patterns. Finally, flexible supercapacitors based on multilayered RGO/AuNP hybrid films are fabricated, and excellent performance, such as high energy and power densities, are achieved.

Superfast-response and ultrahigh-power-density electromechanical actuators based on hierarchal carbon nanotube electrodes and chitosan.

Here we report a novel single-walled carbon nanotube (SWNT) based bimorph electromechanical actuator, which consists of unique as-grown SWNT films as double electrode layers separated by a chitosan electrolyte layer consisting of an ionic liquid. By taking advantage of the special hierarchical structure and the outstanding electrical and mechanical properties of the SWNT film electrodes, our actuators show orders-of-magnitude improvements in many aspects compared to previous ionic electroactive polymer (i-EAP) actuators, including superfast response (19 ms), quite wide available frequency range (dozens to hundreds of Hz), incredible large stress generating rate (1080 MPa/s), and ultrahigh mechanical output power density (244 W/kg). These remarkable achievements together with their facile fabrication, low driving voltage, flexibility, and long durability enable the SWNT-based actuators many applications such as artificial muscles for biomimetic flying insects or robots and flexible deployable reflectors.