Strain-induced semiconductor to metal transition in the two-dimensional honeycomb structure of MoS2

The electronic properties of two-dimensional honeycomb structures of molybdenum disulfide (MoS₂) subjected to biaxial strain have been investigated using first-principles calculations based on density functional theory. On applying compressive or tensile bi-axial strain on bi-layer and mono-layer MoS₂, the electronic properties are predicted to change from semiconducting to metallic. These changes present very interesting possibilities for engineering the electronic properties of two-dimensional structures of MoS₂.      

Selective suspension in aqueous sodium dodecyl sulfate according to electronic structure type allows simple separation of metallic from semiconducting single-walled carbon nanotubes

Both density gradient centrifugation and gel electrophoresis have been reported to allow high throughput separation of metallic from semiconducting single-walled carbon nanotubes (SWNTs) when using aqueous sodium dodecyl sulphate (SDS) suspensions. We show here that both methods rely on an initial dispersion-by-sonication step, which is already selective with respect to electronic structure type. The corresponding aqueous SDS “starting” suspensions obtained after sonication and purification by simple centrifugation (70,000 g, 1 h) contain semiconducting SWNTs primarily in the form of small bundles whereas metallic SWNTs are predominantly suspended as individual tubes. Density gradient centrifugation then separates the bundles from the individual tubes on the basis of differences in their overall buoyant densities. Gel electrophoresis separates the longer bundles from the shorter individual tubes on the basis of their different mobilities. We also demonstrate that such starting suspensions can be fractionated according to electronic structure type by even simpler techniques such as size exclusion chromatography or gel filtration, thus opening the way for simple scale-up.      

Quantum transport properties of chemically functionalized long semiconducting carbon nanotubes

We present a first-principles study of the electronic transport properties of micrometer long semiconducting carbon nanotubes randomly covered with carbene functional groups. Whereas prior studies suggested that metallic tubes are hardly affected by such addends, we show here that the conductance of semiconducting tubes with standard diameter is, on the contrary, severely damaged. The configurational-averaged conductance as a function of tube diameter, with a coverage of up to one hundred molecules, is extracted. Our results indicate that the search for a conductance-preserving covalent functionalization route remains a challenging issue.      

Precise control of highly ordered arrays of nested semiconductor/metal nanotubes

Lithographically defined microporous templates in conjunction with the atomic layer deposition (ALD) technique enable remarkable control of complex novel nested nanotube structures. So far three-dimensional control of physical process parameters has not been fully realized with high precision resolution, and requires optimization in order to achieve a wider range of potential applications. Furthermore, the combination of composite insulating oxide layers alternating with semiconducting layers and metals can provide various types of novel applications and eventually provide unique and advanced levels of multifunctional nanoscale devices. Semiconducting TiO₂ nanotubes have potential applications in photovoltaic devices. The combination of nanostructured semiconducting materials with nested metal nanotubes has the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices. Platinum growth by ALD has been explored, covering the initial stages of the thin film nucleation process and the synthesis of high aspect ratio nanotube structures. The penetration depth of the Pt into porous templates having various pore sizes and aspect ratios has been investigated. Several multi-walled nested TiO₂-Pt nanotubes in series have been successfully fabricated using microporous Si templates. These innovative nested nanostructures have the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices in photovoltaic and sensing technologies.      

Modeling frequency- and temperature-invariant dissipative behaviors of randomly entangled carbon nanotube networks under cyclic loading

Recent experiments have shown that entangled networks of carbon nanotubes exhibit temperature- and frequency-invariant dissipative behaviors under cyclic loading. We have performed coarse-grained molecular dynamics simulations which show that these intriguing phenomena can be attributed to the unstable attachments/detachments between individual carbon nanotubes induced by van der Waals interactions. We show that this behavior can be described by a triboelastic constitutive model. This study highlights the promise of carbon nanomaterials for energy absorption and dissipation under extreme conditions.      

Theory and practice of “Striping” for improved ON/OFF Ratio in carbon nanonet thin film transistors

A new technique to reduce the influence of metallic carbon nanotubes (CNTs)—relevant for large-scale integrated circuits based on CNT-nanonet transistors—is proposed and verified. Historically, electrical and chemical filtering of the metallic CNTs have been used to improve the ON/OFF ratio of CNT-nanonet transistors; however, the corresponding degradation in ON-current has made these techniques somewhat unsatisfactory. Here, we abandon the classical approaches in favor of a new approach based on relocation of asymmetric percolation threshold of CNT-nanonet transistors by a technique called “striping”; this allows fabrication of transistors with ON/OFF ratio >1000 and ON-current degradation no more than a factor of 2. We offer first principle numerical models, experimental confirmation, and renormalization arguments to provide a broad theoretical and experimental foundation of the proposed method.      

Length-sorted semiconducting carbon nanotubes for high-mobility thin film transistors

We have developed a process for chemical purification of carbon nanotubes for solution-processable thin-film transistors (TFTs) having high mobility. Films of the purified carbon nanotubes fabricated by simple drop coating showed carrier mobilities as high as 164 cm²V⁻¹s⁻¹, normalized transconductances of 0.78 Sm⁻¹, and on/off current ratios of 10⁶. Such high performance requires the preparation of a suspension of micrometer-long and highly purified semiconducting single-walled carbon nanotubes (SWCNTs). Our purification process includes length and electronic-type selective trapping of SWCNTs using recycling gel filtration with a mixture of surfactants. The results provide an important milestone toward printed high-speed and large-area electronics with roll-to-roll and ink-jet device fabrication.      

Imaging electronic structure of carbon nanotubes by voltage-contrast scanning electron microscopy

We introduce voltage-contrast scanning electron microscopy (VC-SEM) for visual characterization of the electronic properties of single-walled carbon nanotubes. VC-SEM involves tuning the electronic band structure and imaging the potential profi le along the length of the nanotube. The resultant secondary electron contrast allows to distinguish between metallic and semiconducting carbon nanotubes and to follow the switching of semiconducting nanotube devices, as confi rmed by in situ electrical transport measurements. We demonstrate that high-density arrays of individual nanotube devices can be rapidly and simultaneously characterized. A leakage current model in combination with fi nite element simulations of the device electrostatics is presented in order to explain the observed contrast evolution of the nanotube and surface electrodes. This work serves to fill a void in electronic characterization of molecular device architectures. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Semiconducting carbon nanotube/fullerene blended heterojunctions for photovoltaic near-infrared photon harvesting

We demonstrate that the near-infrared (NIR) absorptivity of semiconducting single-walled carbon nanotubes (s-SWCNTs) can be harnessed in blended heterojunctions with the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Photogenerated charge separation is efficiently driven by the ultrahigh interfacial area of the blends and the favorable energy offsets between the two materials. NIR-sensitive photovoltaic and photodetector devices utilizing the stack (indium tin oxide/ca. 10 nm s-SWCNT:PCBM/100 nm C₆₀/10 nm 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag) were fabricated with NIR power conversion efficiencies >1.3% and peak, zero bias external quantum efficiency of 18% at λ = 1205 nm.      

Role of catalysts in the surface synthesis of single-walled carbon nanotubes

We demonstrate the role of catalysts in the surface growth of single-walled carbon nanotubes (SWNTs) by reviewing recent progress in the surface synthesis of SWNTs. Three effects of catalysts on surface synthesis are studied: type of catalyst, the relationship between the size of catalyst particles and carbon feeding rates, and interactions between catalysts and substrates. Understanding of the role of catalysts will contribute to our ability to control the synthesis of SWNTs on various substrates and facilitate the fabrication of nanotube-based devices.      

Direct growth of enclosed ZnO nanotubes

To date, wet syntheses of single-crystalline ZnO micro- and nanotubes have been carried out using a two-step indirect approach in which a selective dissolution step is required in order to create the vacant space in the tubular structures. In this work, we develop a direct growth process for preparation of single-crystal ZnO nanotubes and nanorods. We also report that a concave shaped crystal growth front is generally reactive and offers a large surface area for matter deposition during rapid expansion of unidirectional nanomaterials. Depending on the degree of supersaturation of nutrients in solution, the concave growth front can either remain unaltered or undergo a concave-to-convex transformation, leading to the growth of solid nanorods and/or hollow nanotubes. The observed volume inversion should, in principle, also be applicable to the nanoarchitecture of other one-dimensional wurtzite structured nanomaterials, although individual sets of synthesis parameters need to be developed for each target material. Electronic Supplementary Material  Supplementary material is available for this article at and is accessible for authorized users. This article is published with open access at Springerlink.com     

Polarized light microscopy and spectroscopy of individual single-walled carbon nanotubes

Polarized light microscopy (PLM) is used to image individual single-walled carbon nanotubes (SWNTs) suspended in air across a slit opening. The imaging contrast relies on the strong optical anisotropy typical of SWNTs. We combine PLM with a tunable light source to enable hyperspectral excitation spectroscopy and nanotube chirality assignment. Comparison with fluorescence microscopy and spectroscopy confirms the assignment made with PLM. This represents a versatile new approach to imaging SWNTs and related structures.      

Effect of the laser heating of nanotube nuclei on the nanotube type population

Many potential applications of carbon nanotubes are expected to benefit from the availability of single-walled carbon nanotube materials enriched in metallic species, and specifically armchair nanotubes. The present work focuses on the modification of the pulsed laser vaporization (PLV) technique to selectively produce certain carbon nanotube structures. Nanotube nuclei were “warmed-up” with an additional laser pulse, timed to coincide approximately with the nucleation event. The effect of the second laser on the carbon vapor temperature was studied by emission spectroscopy. Nanotube type populations with and without warm-up were compared by means of absorption, photoluminescence, and Raman spectroscopy. It was found that the warm-up of nanotube nuclei with a laser pulse has a noticeable, albeit small, effect on the nanotube population. The intensity of spectral features associated with (9,7) nanotube and its large chiral angle neighbors increased, while small chiral angle nanotubes decreased, with exception of the (15,0) tube. This experiment demonstrates that nanotube population during PLV synthesis can be manipulated in a controlled fashion.      

Ultra-thin double-walled carbon nanotubes: A novel nanocontainer for preparing atomic wires

Double-walled carbon nanotubes (DWCNTs) with high graphitization have been synthesized by hydrogen arc discharge. The obtained DWCNTs have a narrow distribution of diameters of both the inner and outer tubes, and more than half of the DWCNTs have inner diameters in the range 0.6–1.0 nm. Field electron emission from a DWCNT cathode to an anode has been measured, and the emission current density of DWCNTs reached 1 A/cm² at an applied field of about 4.3 V/μm. After high-temperature treatment of DWCNTs, long linear carbon chains (C-chains) can be grown inside the ultra-thin DWCNTs to form a novel C-chain@DWCNT nanostructure, showing that these ultra-thin DWCNTs are an appropriate nanocontainer for preparing truly one-dimensional nanostructures with one-atom-diameter.      

Output of an ultrasonic wave-driven nanogenerator in a confined tube

The output of an ultrasonic wave-driven nanogenerator (NG) has been found to depend on the excitation conditions and geometry. Incidence angle tests indicate that the effective area of an NG determines the amount of power that can be generated. The output power of an NG is also directly related to its distance from the ultrasonic source. A sinusoidal profile of the electrical output was observed when an NG was moved inside a long tube filled with water with the ultrasonic source located at one end. This is due to the oscillation of the wave intensity inside the tube as a function of the distance from the excitation source. This article is published with open access at Springerlink.com     

Short channel field-effect transistors from highly enriched semiconducting carbon nanotubes

Semiconducting single-walled carbon nanotubes (s-SWNTs) with a purity of ∼98% have been obtained by gel filtration of arc-discharge grown SWNTs with diameters in the range 1.2–1.6 nm. Multi-laser Raman spectroscopy confirmed the presence of less than 2% of metallic SWNTs (m-SWNTs) in the s-SWNT enriched sample. Measurement of ∼50 individual tubes in Pd-contacted devices with channel length 200 nm showed on/off ratios of >10⁴, conductances of 1.38–5.8 μS, and mobilities in the range 40–150 cm²·V/s. Short channel multi-tube devices with ∼100 tubes showed lower on/off ratios due to residual m-SWNTs, although the on-current was greatly increased relative to the devices made from individual tubes.      

Vertical oxide nanotubes connected by subsurface microchannels

We describe the fabrication of arrays of oxide nanotubes using a combination of bottom up and top down nanofabrication. The nanotubes are made from epitaxially grown semiconductor nanowires that are covered with an oxide layer using atomic layer deposition. The tips of the oxide-covered nanowires are removed by argon sputtering and the exposed semiconductor core is then selectively etched, leaving a hollow oxide tube. We show that it is possible to create fluidic connections to the nanotubes by a combination of electron beam lithography to precisely define the nanotube positions and controlled wet under-etching. DNA transport is demonstrated in the microchannel. Cells were successfully cultured on the nanotube arrays, demonstrating compatibility with cell-biological applications. Our device opens up the possibility of injecting molecules into cells with both spatial and temporal control.      

Defect- and dopant-controlled carbon nanotubes fabricated by self-assembly of graphene nanoribbons

Molecular dynamics simulations showed that a basal carbon nanotube can activate and guide the fabrication of single-walled carbon nanotubes (CNTs) on its internal surface by self-assembly of edge-unpassivated graphene nanoribbons with defects. Furthermore, the distribution of defects on self-assembled CNTs is controllable. The system temperature and defect fraction are two main factors that influence the success of self-assembly. Due to possible joint flaws formed at the boundaries under a relatively high constant temperature, a technique based on increasing the temperature is adopted. Self-assembly is always successful for graphene nanoribbons with relatively small defect fractions, while it will fail in cases with relatively large ones. Similar to the self-assembly of graphene nanoribbons with defects, graphene nanoribbons with different types of dopants can also be self-assembled into carbon nanotubes. The finding provides a possible fabrication technique not only for carbon nanotubes with metallic or semi-conductive properties but also for carbon nanotubes with electromagnetic induction characteristics.

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Ultralong aligned single-walled carbon nanotubes on flexible fluorphlogopite mica for strain sensors

Single-walled carbon nanotubes (SWNTs) are expected to be an ideal candidate for making highly efficient strain sensing devices owing to their unique mechanical, electronic, and electromechanical properties. Here we present the use of fluorphlogopite mica (F-mica) as a flexible, high-temperature-bearing and mechanically robust substrate for the direct growth of horizontally aligned ultra-long SWNT arrays by chemical vapor deposition (CVD), which in turn enables the straightforward, facile, and cost-effective fabrication of macro-scale SWNT-array-based strain sensors. Strain sensing tests of the SWNT-array devices demonstrated fairly good strain sensitivity with high ON-state current density.      

Tunable separation of single-walled carbon nanotubes by dual-surfactant density gradient ultracentrifugation

We present a systematic study of the effects of surfactants in the separation of single-walled carbon nanotubes (SWNTs) by density gradient ultracentrifugation (DGU). Through analysis of the buoyant densities, layer positions, and optical absorbance spectra of SWNT separation using the bile salt sodium deoxycholate (DOC) and the anionic salt sodium dodecyl sulfate (SDS), we clarify the roles and interactions of these two surfactants in yielding different DGU outcomes. The separation mechanism described here can also help in designing new DGU experiments by qualitatively predicting outcomes of different starting recipes, improving the efficacy of DGU and simplifying post-DGU fractionation.