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.     

Mechanically durable and highly conductive elastomeric composites from long single-walled carbon nanotubes mimicking the chain structure of polymers

By using long single-walled carbon nanotubes (SWNTs) as a filler possessing the highest aspect ratio and small diameter, we mimicked the chain structure of polymers in the matrix and realized a highly conductive elastomeric composite (30 S/cm) with an excellent mechanical durability (4500 strain cycles until failure), far superior to any other reported conductive elastomers. This exceptional mechanical durability was explained by the ability of long and traversing SWNTs to deform in concert with the elastomer with minimum stress concentration at their interfaces. The conductivity was sufficient to operate many active electronics components, and thus this material would be useful for practical stretchable electronic devices.     

Methods for enhanced control over the density and electrical properties of SWNT networks

The formation of manufacturable electronic materials that incorporate single-walled carbon nanotubes (SWNTs) will most likely involve the use of networks of these molecular wires, due to the enhanced current drive and reproducibility of such films. Therefore, control over the density of SWNTs during the deposition of 2-D networks is of critical importance for the development of numerous enhanced electronic materials. Room temperature deposition methods are of particular interest as they allow separation, purification, and/or chemical modification of SWNTs before deposition. This article reports three iterative liquid-deposition techniques that allow control over the properties of three distinct types of SWNT networks. First, density control was obtained for 2-D networks of unbundled, high-aspect ratio SWNTs. Such networks exhibited semiconductive behavior, with tunable on/off ratios. Second, electrically continuous 2-D clusters of high aspect ratio SWNTs were formed by allowing capillary forces to develop in a sessile suspension droplet. These constructs displayed tunable metallic conductivity, and may have the applications as interconnects in microelectronics. Finally, highly conductive, 3-D networks of bundled SWNTs were formed via an evaporation method. For these three types of networks, the density of SWNTs, and thus the macroscopic conductance, was readily controlled via the number of deposition cycles used in their formation.     

Spatially Resolved Analytical Electron Microscopy at Grain Boundaries of α—Al2O3

In this work the electronic structure and the impurity excess of the basal and rhombohedral twin grain boundaries are investigated,using electron energy loss spectroscopy(EELS) and energy dispersive X-ray spectroscopy(EDXS).The measurability of electronic structures of the twin grain boundaries are discussed by comparing theoretical density of states(DOS) from bulk material with interfacial DOS,obtained from local density functional theory(LDFT) calculations.     

Spatially Resolved Analytical Electron Microscopy at Grain Boundaries of а-Al2O3

In this work the electronic structure and the impurity excess of the basal and rhombohedral twin grain boundaries are investigated, using electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS).The measurability of electronic structures of the twin grain boundaries are discussed by comparing theoretical density of states (DOS) from bulk material with interfacial DOS, obtained from local density functional theory (LDFT)calculations.     

High-performance electronics using dense, perfectly aligned arrays of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have many exceptional electronic properties. Realizing the full potential of SWNTs in realistic electronic systems requires a scalable approach to device and circuit integration. We report the use of dense, perfectly aligned arrays of long, perfectly linear SWNTs as an effective thin-film semiconductor suitable for integration into transistors and other classes of electronic devices. The large number of SWNTs enable excellent device-level performance characteristics and good device-to-device uniformity, even with SWNTs that are electronically heterogeneous. Measurements on p- and n-channel transistors that involve as many as approximately 2,100 SWNTs reveal device-level mobilities and scaled transconductances approaching approximately 1,000 cm(2) V(-1) s(-1) and approximately 3,000 S m(-1), respectively, and with current outputs of up to approximately 1 A in devices that use interdigitated electrodes. PMOS and CMOS logic gates and mechanically flexible transistors on plastic provide examples of devices that can be formed with this approach. Collectively, these results may represent a route to large-scale integrated nanotube electronics.     

Half-Metallic Magnetism of Quaternary Heusler Compounds Co2Fe x Mn1−x Si(x =0,0.5, and 1.0): First-Principles Calculations

Using first-principles calculations, the structural, electronic, and magnetic properties of ferromagnetic half-metallic full-Heusler Co₂FeSi, Co₂MnSi and Co₂ Fe _0.5Mn _0.5Si alloy via the full-potential linearized augmented plane-wave (FP-LAPW) method in the generalized gradient (GGA) and GGA + U approximations are compared with other experimental and theoretical results. The electronic band structures and density of states (DOS) of the compounds indicate they are half metallic because of the existence of the energy gap in the minority spin (DOS and band structure), which yields perfect spin polarization. The half metallicity of the obtained material may prove useful for applications in spin-polarizers and spin-injectors of magnetic nanodevices. The calculated total spin magnetic moments are almost exactly that expected from the Slater-Pauling rule.     

Computational modeling of the thermal conductivity of single-walled carbon nanotube-polymer composites

A computational model was developed to study the thermal conductivity of single-walled carbon nanotube (SWNT)-polymer composites. A random walk simulation was used to model the effect of interfacial resistance on the heat flow in different orientations of SWNTs dispersed in the polymers. The simulation is a modification of a previous model taking into account the numerically determined thermal equilibrium factor between the SWNTs and the composite matrix material. The simulation results agreed well with reported experimental data for epoxy and polymethyl methacrylate (PMMA) composites. The effects of the SWNT orientation, weight fraction and thermal boundary resistance on the effective conductivity of composites were quantified. The present model is a useful tool for the prediction of the thermal conductivity within a wide range of volume fractions of the SWNTs, so long as the SWNTs are not in contact with each other. The developed model can be applied to other polymers and solid materials, possibly even metals.     

Single-walled carbon-nanotube spectroscopic and electronic field-effect transistor measurements: a combined approach

Spectroscopic and electronic field-effect transistor measurements reveal complimentary information about molecular interactions with single-walled carbon nanotubes (SWNTs). Here we demonstrate how these two complimentary techniques can be combined to further understand electronic modifications of the SWNTs. The complimentary nature of these techniques stems from the perturbation of the electronic structure of SWNTs upon electronic interaction with an electron-donating, or -accepting species.     

Correlation between electronic structure, mechanical properties and phase stability in intermetallic compounds

From the detailed electronic structure studies on intermetallic compounds, it has been found that these materials have low heat of formation and large glass-forming ability, if the Fermi level falls on the peak in the density of states (DOS) curve. On the other hand, if theE _F falls on the pseudogap in the DOS curve, the ordering energy will be larger and the system prefers to form an ordered alloy. The first principles electronic structure calculations performed on Ni₃Al show that it is possible to vary the filling of bonding, nonbonding and antibonding states in the DOS curve and this in turn shows gradual structural transformation as well as formation of multiphases by ternary alloying. Possibilities of tailoring the properties of materials by tuning the Fermi level is discussed in this paper.     

Electronic structure and optical properties of thorium monopnictides

We have calculated the electronic density of states (DOS) and dielectric function for the ThX (X = P, As and Sb) using the linear muffin tin orbital method within atomic sphere approximation (LMTO-ASA) including the combined correction terms. The calculated electronic DOS of ThSb has been compared with the available experimental data and we find a good agreement. The calculated optical conductivity for ThP and ThAs is increasing monotonically, while for ThSb a sharp peak has been found at 6–5 eV. Unfortunately there are no experimental data to compare with calculated optical properties, we hope our calculations will motivate some experimentalists.     

Why semiconducting single-walled carbon nanotubes are separated from their metallic counterparts

The separation of single-walled carbon nanotubes (SWNTs) according to their electronic structure has attracted much recent attention. In many cases, metallic SWNTs are separated from semiconducting SWNTs and enriched in the supernatant due to stronger interaction between metallic SWNTs and adsorbates. However, the inverse separation of semiconducting from metallic SWNTs is often observed. In this computational study, the underlying mechanism is elucidated by density functional theory. We show that the shape of an aromatic molecule, the degree of hybridization between a molecule and a SWNT, and the oxidative state of SWNTs can affect the type of enriched SWNTs. In principle, one can control the type of enriched SWNTs by selecting a structurally compatible aromatic molecule or changing the hole concentration of the SWNTs.     

Electronic and Vibration Spectra of Hydrogenated Carbon Single-Wall Nanotubes

Carbon single-wall nanotubes (SWNTs) were loaded with 5.4 wt.% H by exposing to a hydrogen pressure of 50 kbar at 500°C. Investigation of the optical transmission spectra showed that the hydrogenation significantly suppressed the high-frequency conductivity σ of free carries in the SWNTs and also eliminated the band-to-band electronic transitions. Instead, a narrow line of the C-H stretching vibrational mode appeared at 2845 cm^-1. A gradual removal of hydrogen from the hydrogenated SWNTs by vacuum annealing at T≥500°C resulted in an approximately linear decrease in the intensity of this line with decreasing hydrogen content. This evidenced that most H atoms in the hydrogenated SWNTs were covalently bonded to the carbon atoms. The complete removal of hydrogen by vacuum annealing at 700°C partly restored σ and the intensity of the electronic transitions characteristic of the initial SWNTs.     

Assessment of the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only approximately 1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications.     

Temperature-mediated growth of single-walled carbon-nanotube intramolecular junctions

Single-walled carbon nanotubes (SWNTs) possess superior electronic and physical properties that make them ideal candidates for making next-generation electronic circuits that break the size limitation of current silicon-based technology. The first critical step in making a full SWNT electronic circuit is to make SWNT intramolecular junctions in a controlled manner. Although SWNT intramolecular junctions have been grown by several methods, they only grew inadvertently in most cases. Here, we report well-controlled temperature-mediated growth of intramolecular junctions in SWNTs. Specifically, by changing the temperature during growth, we found that SWNTs systematically form intramolecular junctions. This was achieved by a consistent variation in the SWNT diameter and chirality with changing growth temperature even though the catalyst particles remained the same. These findings provide a potential approach for growing SWNT intramolecular junctions at desired locations, sizes and orientations, which are important for making SWNT electronic circuits.     

Colored semitransparent conductive coatings consisting of monodisperse metallic single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) are promising materials for transparent conduction as a result of their exceptional electrical, optical, mechanical, and chemical properties. However, since current synthetic methods yield polydisperse mixtures of SWNTs, the performance of SWNT transparent conductive films has previously been hindered by semiconducting species. Here, we describe the performance of transparent conductors produced using predominantly metallic SWNTs. Compared with unsorted material, films enriched in metallic SWNTs can enhance conductivity by factors of over 5.6 in the visible and 10 in the infrared. Moreover, by using monodisperse metallic SWNTs sorted with angstrom-level resolution in diameter, semitransparent conductive coatings with tunable optical transmittance can be produced.     

Synthesis and characterization of new polyaniline/nanotube composites

New polyaniline/nanotube (PANI/NT) composites have been synthesized by “in situ” polymerization processes using both multi-wall carbon nanotubes (MWNTs) and single-wall carbon nanotubes (SWNTs) in concentrations ranging from 2 to 50 wt.%. Although no structural changes are observed using MWNTs above a concentration of 20 wt.%, the in situ synthesis results in electronic interactions between nanotubes and the quinoid ring of PANI leading to enhanced electronic properties and thus to the formation of a genuine PANI/MWNT composite material. On the other hand, using SWNTs favors the formation of inhomogeneous mixtures rather than of a homogeneous composite materials, independent of the SWNT concentration. X-ray diffraction, Raman and transport measurements show the different behavior of both classes of nanotubes in PANI/NT materials. The difficulties in the formation of a true PANI/SWNT composite are related to the far more complex structure of the SWNT material itself, i.e. to the presence of entangled bundles of SWNTs, amorphous carbon and even catalytic metal particles.     

Formylation of single-walled carbon nanotubes

We report an improved, elegant method for the covalent formylation of single-wall carbon nanotubes (SWNTs) via formyl transfer from N-formylpiperidine, which could potentially open the gateway for more versatile chemical modification of carbon nanotube (CNT) walls than is possible via other reported functionalisation methods. The formylation reaction does not inflict damage upon the pristine CNT structure, unlike the currently commonly used carboxylation route, and involves much fewer steps, and takes considerably less time, than most other reported routes. The modified SWNTs have been characterised by Raman spectroscopy, ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy and "covalent tagging" with derivatising groups followed by thermogravimetric analysis-mass spectroscopy (TGA-MS). UV-vis-NIR spectroscopy shows that there is only limited disruption of the intrinsic electronic structure of the SWNTs. This is confirmed from estimates of the extent of functionalisation from TGA-MS, which suggest that it may be as low as 2 atomic per cent.