Properties of Single Walled Carbon Nanotube Films

<p>The preparation and physical properties of transparent, single-walled carbon nanotube (SWNT) networks fabricated from a novel, organic dispersion are described here for the first time. Characterisation of SWNT dispersions uncovered shifts in the radial breathing modes as a function of aggregation. These modes were redshifted in centrifuged butylamine dispersions by ~3cm -1. SWNT films cast using a simple, drop-deposition technique were annealed at 300'C after fabrication to remove solvent and surfactant residue. Annealed films with a sheet resistance of magnitude ~10 4 kOhms/square and transparency of ~85 % were fabricated in this study. The optoelectronic properties showed some inconsistency due to varying levels of oxygen doping and film thickness. Thin films annealed at 500'C were found to be preferentially depleted of nanotubes with high chiral angle and small diameter. Oxidative effects were also observed upon annealing at temperatures as low as 300'C. However, the reasons for this premature combustion are as yet uncertain. Temperature-dependent conduction studies revealed that the removal of adsorbed surfactant considerably reduced tunnelling barriers in annealed films. The dominant conduction mechanism in both unannealed and annealed films was found to be 3D variable range hopping. In the annealed films, a high temperature activation regime (with activation energy of 220 meV) was observed above 225 K. This regime is due to thermal activation over Schottky barriers within the nanotube network or electron activation over the pseudogap in armchair tubes.</p>

Properties of Single Walled Carbon Nanotube Films

<p>The preparation and physical properties of transparent, single-walled carbon nanotube (SWNT) networks fabricated from a novel, organic dispersion are described here for the first time. Characterisation of SWNT dispersions uncovered shifts in the radial breathing modes as a function of aggregation. These modes were redshifted in centrifuged butylamine dispersions by ~3cm -1. SWNT films cast using a simple, drop-deposition technique were annealed at 300'C after fabrication to remove solvent and surfactant residue. Annealed films with a sheet resistance of magnitude ~10 4 kOhms/square and transparency of ~85 % were fabricated in this study. The optoelectronic properties showed some inconsistency due to varying levels of oxygen doping and film thickness. Thin films annealed at 500'C were found to be preferentially depleted of nanotubes with high chiral angle and small diameter. Oxidative effects were also observed upon annealing at temperatures as low as 300'C. However, the reasons for this premature combustion are as yet uncertain. Temperature-dependent conduction studies revealed that the removal of adsorbed surfactant considerably reduced tunnelling barriers in annealed films. The dominant conduction mechanism in both unannealed and annealed films was found to be 3D variable range hopping. In the annealed films, a high temperature activation regime (with activation energy of 220 meV) was observed above 225 K. This regime is due to thermal activation over Schottky barriers within the nanotube network or electron activation over the pseudogap in armchair tubes.</p>

Essential Electronic Properties of Silicon Nanotubes

In this work, the various electronic properties of silicon nanotubes (SiNTs) were investigated by the density functional theory. The cooperative and competitive relationships between the chiral angle, periodic boundary conditions, and multi-orbital hybridizations create unusual narrow gaps and quasi-flat bands in the ultra-small armchair and zigzag tubes, respectively. The features varied dramatically with tube radii. Armchair SiNTs (aSiNTs) have an indirect-to-direct band gap transition as their radius is increased to a particular value, while zigzag SiNTs (zSiNTs) present a metal-semiconductor transition. The projected density of states was used to elucidate the critical transitions, and the evolution of p and s orbital mixing states during the process are discussed in detail. The information presented here provides a better understanding of the essential properties of SiNTs.

Strain Range Dependence of Young’s Modulus and Size Effects of Mechanical Properties for Chiral Graphene by Molecular Dynamics Simulations

The stress-strain response of pristine monolayer graphene under uniaxial loading/unloading over a larger size range (100nm×100nm) is studied by molecular dynamics (MD) simulations, which proves that graphene is perfectly elastic prior to the failure strain. Young’s modulus of graphene is calculated by selecting different strain ranges in the elastic region. It is found that Young’s modulus is strongly dependent on the strain range. When the selected strain ranges are increased from 0.5% to 8%, Young’s modulus of the armchair and zigzag graphene is reduced by approximately 60 Gpa and 150 Gpa, respectively. Based on the Pearson correlation coefficient method, the linearity of the stress-strain curve during graphene stretching is studied. The elastic region of the tensile curve is divided into the linear elastic region and non-linear elastic region. simultaneously, the linear elastic limit strains for the armchair and zigzag graphene are defined to be about 2.5% and 1.5%, respectively, and they are independent of the size of graphene. On this basis, the chirality dependence and size effects of Young’s modulus, failure strain, and fracture strength of pristine monolayer graphene are investigated. The results show that Young’s modulus is dependent on the chiral angle but insensitive to size. The failure strain and fracture strength depend on the chiral angle and have obvious size effects, which decrease with the increase of size range and size ratio.

One-dimensional van der Waals heterostructures: Growth mechanism and handedness correlation revealed by nondestructive TEM

We recently synthesized one-dimensional (1D) van der Waals heterostructures in which different atomic layers (e.g., boron nitride or molybdenum disulfide) seamlessly wrap around a single-walled carbon nanotube (SWCNT) and form a coaxial, crystalized heteronanotube. The growth process of 1D heterostructure is unconventional—different crystals need to nucleate on a highly curved surface and extend nanotubes shell by shell—so understanding the formation mechanism is of fundamental research interest. In this work, we perform a follow-up and comprehensive study on the structural details and formation mechanism of chemical vapor deposition (CVD)–synthesized 1D heterostructures. Edge structures, nucleation sites, and crystal epitaxial relationships are clearly revealed using transmission electron microscopy (TEM). This is achieved by the direct synthesis of heteronanotubes on a CVD-compatible Si/SiO2 TEM grid, which enabled a transfer-free and nondestructive access to many intrinsic structural details. In particular, we have distinguished different-shaped boron nitride nanotube (BNNT) edges, which are confirmed by electron diffraction at the same location to be strictly associated with its own chiral angle and polarity. We also demonstrate the importance of surface cleanness and isolation for the formation of perfect 1D heterostructures. Furthermore, we elucidate the handedness correlation between the SWCNT template and BNNT crystals. This work not only provides an in-depth understanding of this 1D heterostructure material group but also, in a more general perspective, serves as an interesting investigation on crystal growth on highly curved (radius of a couple of nanometers) atomic substrates.

Chirality Distributions for Semiconducting Single-Walled Carbon Nanotubes Determined by Photoluminescence Spectroscopy

To realize single-walled carbon nanotube (SWCNT) chiral selective growth, elucidating the mechanism of SWCNT chirality (n,m) selectivity is important. For this purpose, an accurate evaluation method for evaluating the chirality distribution of grown SWCNTs without post-growth processing or liquid-dispersion of SWCNTs is indispensable. Here, we used photoluminescence spectroscopy to directly measure the chirality distributions of individual semiconducting SWCNTs suspended on a pillar-patterned substrate. The number of chirality-assigned SWCNTs was up to 332 and 17 chirality types with the chiral angles ranging from 0° to 28.05° were detected. The growth yield of SWCNTs was confirmed to primarily depends on the chiral angle in accordance with the screw dislocation model. Furthermore, when higher-yield chiralities are selected, the chiral angle distribution with a peak corresponding to near-armchair SWCNTs is well fitted with a model that incorporates the thermodynamic effect at the SWCNT-catalyst interface into the kink growth-based kinetic model. Our quantitative and statistical data provide new insights into SWCNT growth mechanism as well as experimental confirmation of theoretical predictions.

Static and dynamic buckling behavior of CNTS with S-W defects

The excellent performance of carbon nanotubes (CNTs) allows them to be widely employed in various micro- and nano-electromechanical devices. However, different imperfections such as Stone–Wales (S-W) defects often arise in these structures during the preparation process. In this paper, special attention is paid to the effects of the number and location of defects as well as the diameter and chiral angle of CNTs on the static and dynamic buckling of CNTs with S-W defects. First, LAMMPS software is used to simulate the molecular dynamics (MDs) of CNTs with S-W defects, and their static buckling performances are discussed. Then based on the static buckling data, the dynamic buckling vibration performance of CNTs with S-W defects is analyzed in the context of the nonlocal elastic theory. Finally, the effective range of nonlocal parameters is established via the MD modeling. The results show that the existence of S-W defects will reduce the buckling performance and vibration characteristics of CNTs, and an increase in the number of defects will aggravate the influence of diameter and chiral angle on the buckling performance as well as the natural frequency and amplitude of the nanotube’s axial vibration.

Bacterial filamentation drives colony chirality

Chirality is ubiquitous in nature, with consequences at the cellular and tissue scales. As Escherichia coli colonies expand radially, an orthogonal component of growth creates a pinwheel-like pattern that can be revealed by fluorescent markers. To elucidate the mechanistic basis of this colony chirality, we investigated its link to left-handed, single-cell twisting during E. coli elongation. While chemical and genetic manipulation of cell width altered single-cell twisting handedness, colonies ceased to be chiral rather than switching handedness, and anaerobic growth altered colony chirality without affecting single-cell twisting. Chiral angle increased with increasing temperature even when growth rate decreased. Unifying these findings, we discovered that colony chirality was associated with the propensity for cell filamentation. Inhibition of cell division accentuated chirality under aerobic growth and generated chirality under anaerobic growth. Thus, regulation of cell division is intrinsically coupled to colony chirality, providing a mechanism for tuning macroscale spatial patterning.

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.

Chiral symmetry in Dirac equation and its effects on neutrino masses and dark matter

Chiral symmetry is included into the Dirac equation using the irreducible representations of the Poincaré group. The symmetry introduces the chiral angle that specifies the chiral basis. It is shown that the correct identification of these basis allows explaining small masses of neutrinos and predicting a new candidate for Dark Matter massive particle.