Narrow Nanotubes Research Articles

Ultrafast proton transport in sub-1-nm diameter carbon nanotube porins.

Proton transport plays an important role in many biological processes due to the ability of protons to rapidly translocate along chains of hydrogen-bonded water molecules. Molecular dynamics simulations have predicted that confinement in hydrophobic nanochannels should enhance the rate of proton transport. Here, we show that 0.8-nm-diameter carbon nanotube porins, which promote the formation of one-dimensional water wires, can support proton transport rates exceeding those of bulk water by an order of magnitude. The transport rates in these narrow nanotube pores also exceed those of biological channels and Nafion. With larger 1.5-nm-diameter nanotube porins, proton transport rates comparable to bulk water are observed. We also show that the proton conductance of these channels can be modulated by the presence of Ca(2+) ions. Our results illustrate the potential of small-diameter carbon nanotube porins as a proton conductor material and suggest that strong spatial confinement is a key factor in enabling efficient proton transport.

Salt Solutions in Carbon Nanotubes: The Role of Cation−π Interactions

Understanding the structure of aqueous electrolytes at interfaces is essential for predicting and optimizing device performance for a wide variety of emerging energy and environmental technologies. In this work, we investigate the structure of two common salt solutions, NaCl and KCl, at a hydrophobic interface within narrow carbon nanotubes (CNTs). Using a combination of first-principles and classical molecular dynamics simulations in conjunction with molecular orbital analysis, we find that the solvation structure of the cations in the CNTs can deviate substantially from the conventional weakly interacting hydrophobic picture. Instead, interactions between solvated ions and π orbitals of the CNTs are found to play a critically important role. Specifically, the ion solvation structure is ultimately determined by a complex interplay between cation−π interactions and the intrinsic flexibility of the solvation shell. In the case of K+, these effects result in an unusually strong propensity to partially desol...

Understanding Charge Transport in Mixed Networks of Semiconducting Carbon Nanotubes.

The ability to select and enrich semiconducting single-walled carbon nanotubes (SWNT) with high purity has led to a fast rise of solution-processed nanotube network field-effect transistors (FETs) with high carrier mobilities and on/off current ratios. However, it remains an open question whether it is best to use a network of only one nanotube species (monochiral) or whether a mix of purely semiconducting nanotubes but with different bandgaps is sufficient for high performance FETs. For a range of different polymer-sorted semiconducting SWNT networks, we demonstrate that a very small amount of narrow bandgap nanotubes within a dense network of large bandgap nanotubes can dominate the transport and thus severely limit on-currents and effective carrier mobility. Using gate-voltage-dependent electroluminescence, we spatially and spectrally reveal preferential charge transport that does not depend on nominal network density but on the energy level distribution within the network and carrier density. On the basis of these results, we outline rational guidelines for the use of mixed SWNT networks to obtain high performance FETs while reducing the cost for purification.

From dimers to collective dipoles: Structure and dynamics of methanol/ethanol partition by narrow carbon nanotubes.

Alcohol partitioning by narrow single-walled carbon nanotubes (SWCNTs) holds the promise for the development of novel nanodevices for diverse applications. Consequently, in this work, the partition of small alcohols by narrow tubes was kinetically and structurally quantified via molecular dynamics simulations. Alcohol partitioning is a fast process in the order of 10 ns for diluted solutions but the axial-diffusivity within SWCNT is greatly diminished being two to three orders of magnitude lower with respect to bulk conditions. Structurally, alcohols form a single-file conformation under confinement and more interestingly, they exhibit a pore-width dependent transition from dipole dimers to a single collective dipole, for both methanol and ethanol. Energetic analyses demonstrate that this transition is the result of a detailed balance between dispersion and electrostatics interactions, with the latter being more pronounced for collective dipoles. This transition fully modifies the reorientational dynamics of the loaded particles, generating stable collective dipoles that could find usage in signal-amplification devices. Overall, the results herein have shown distinct physico-chemical features of confined alcohols and are a further step towards the understanding and development of novel nanofluidics within SWCNTs.

Using Ag-embedded TiO2 nanotubes array as recyclable SERS substrate

A simple strategy for synthesizing Ag-loaded TiO2 nanotube film for use as multifunctional photocatalyst and recyclable surface-enhanced Raman scattering (SERS) substrate is introduced. Highly aligned TiO2 nanotube arrays (TNTA) prepared via electrochemical anodization were used as a 3D rough host for silver nanoparticles. Ag deposits were sputtered in a vacuum, and it was found that their morphologies were mainly influenced by the diameters of nanotubes and the UV irradiation induced aging process, especially the self-migration of silver along the tubular wall. SERS and the self-cleaning effect were observed using Rhodamine 6G (R6G) as the probe molecule. The results showed that narrow nanotube and silver nanoparticles embedment contributed significantly to both the phenomenal SERS and recyclability.

Interactions of anions and cations in carbon nanotubes.

We consider the insertion of alkali-halide ion pairs into a narrow (5,5) carbon nanotube. In all cases considered, the insertion of a dimer is only slightly exothermic. While the image charge induced on the surface of the tube favors insertion, it simultaneously weakens the Coulomb attraction between the two ions. In addition, the anion experiences a sizable Pauli repulsion. For a one dimensional chain of NaCl embedded in the tube the most favorable position for the anion is at the center, and for the cation near the wall. The phonon spectrum of such chains shows both an acoustic and an optical branch.

Giant Raman Response to the Encapsulation of Sulfur in Narrow Diameter Single-Walled Carbon Nanotubes.

Encapsulation of sulfur in HiPCO-SWNTs leads to large changes in the Raman spectra with the appearance of new peaks at 319, 395, and 715 cm(-1) which originate from the sulfur species within the SWNTs, while the high frequency SWNT bands (ν > 1200 cm(-1)) are decreased in intensity. The encapsulated species also shifts the near-IR interband electronic transitions to lower energy by more than 10%. These effects seem to originate with the van der Waals interaction of the confined sulfur species with the walls of the SWNTs which are not expected to be significant in the case of the previously studied large diameter SWNTs. We suggest that sulfur in the small diameter SWNTs exists as a helical polymeric sulfur chain that enters the SWNT interior in the form of S2 ((3)Σ(g)(-)) molecules which undergo polymerization to linear diradicals.

On the Energetics of Ions in Carbon and Gold Nanotubes

We investigate the insertion of halide and alkali atoms into narrow single-walled carbon nanotubes with diameters <9 Å by density functional theory; both chiral and non-chiral tubes are considered. The atoms are stored in the form of ions; the concomitant charge transfer affects the band structure and makes originally semiconducting tubes conducting. The electrostatic interaction between a charge and the walls of the tube is explicitly calculated. The insertion energies and the positions of the ions are determined by a competition between electrostatic energy and Pauli repulsion. For comparison, we consider ions in gold nanotubes. Alkali ions follow the same principles in gold as in carbon tubes, but chloride is specifically adsorbed inside gold tubes.

A simple model for charge storage in a nanotube

We consider a narrow nanotube in contact with an ionic liquid; the sizes of the ions are supposed to be so different that only one kind of ion can enter the tube. For a given number of N equal ions in the tube, the distance between the ions is determined by Coulomb repulsion. Using results obtained previously from density functional theory, we calculate the energy of an ensemble of equal ions, including vibrations about the equilibrium positions. For large tubes, in the thermodynamic limit, the resulting capacity diverges at zero charge for entropic reasons. It then increases rapidly, passes through a maximum, and becomes smaller as the distance between the ions decreases with increasing charge. Small nanotubes exhibit discrete charging steps reminiscent of quantum dots.

Mechanisms of Hydrogen Transport in Flexible‐Wall Narrow Carbon Nanotubes

Understanding the interaction between hydrogen and carbon nanotubes is crucial to enhancing the performance of hydrogen storage and nanofluidic carbon‐adsorbent systems. Accordingly, this study performs a series of molecular dynamics simulations to investigate the transport properties of hydrogen molecules confined within a flexible narrow carbon nanotube. The tube’s diameter is 10.8 Å at temperatures in the range of 100~800 K. The particle loadings inside carbon nanotubes are ranging from 0.01∼1 No/Å. The results show that the hydrogen molecules exhibit three distinct diffusion regimes, namely, single‐file, Fickian, and ballistic, depending on the value of the Knudsen number. In addition, it is shown that with the Knudsen number of less than 1, the tube‐wall long wavelength acoustic phonons induced Rayleigh traveling wave prompts a longitudinal wave slip and compression‐expansion of the hydrogen molecule crowds within the CNT, which leads to a significant increase in the mean square displacement of the molecules.

Nanotubes for charge storage – towards an atomistic model

We examine the insertion of alkali and halide ions into narrow nanotubes of graphite and gold by density functional theory (DFT). For tubes with diameters less than about 10 Åthe optimum position of the ion is in the center of the tube. Bader analysis and an analysis of the densities of states gave contradictory results for the charge on the halide ions, but we argue on physical grounds that they carry unit negative charge. We have calculated the energies of inserting the atoms into the tubes, where they are ionized. Because of the small system size the work function of the tubes changes during this process, which makes it difficult to interpret these energies. The surrounding tubes screen the ionic charge very effectively; thereby the ion-ion interactions are strongly reduced, which explains, why narrow tubes store charge more effectively than wider one. These effects are stronger for gold than for graphite tubes. We define an effective image radius of a cylindrical tube, and calculate the image energy experienced by the ions.

Electroabsorption in a narrow gap semiconductor nanotube in the field of uniformly charged ring

The electronic states as well as interband and intraband electrooptical transitions are considered for the narrow gap InSb nanotube in the field of homogeneously charged ring. The problem is solved in the framework of the variational approach. The field of the charged ring is brought to the field of the modified one dimensional Coulomb-like potential. Physically appropriate wave functions are suggested for this potential and the intensities of the threshold frequencies of the electrooptical transitions are calculated.

D magnetism in Ca doped narrow carbon nanotubes: First principle chirality effect study

Curvature has always had crucial effects on the physical properties of narrow carbon nanotubes (CNTs) and here spin-polarized density functional calculations were employed to study electronic and magnetic properties of calcium-decorated narrow (5,5) and (9,0)CNTs with close diameters (∼7 Å) and different chiralities. Our results showed that chirality had great impact on the electronic structure and magnetization of the doped CNTs. In addition, internally or externally doping of the calcium atoms was studied comparatively and although for the (9,0)CNT the internal doping was the most stable configuration, which involves a novel kind of spin-polarization originated from Ca-4s electrons, but for the (5,5)tube the external doping was the most stable one without any spin-polarization. On the other hand, calcium doping in the center of the (5,5)CNT was an endothermic process and led to the spin-polarization of unoccupied Ca-3d orbitals via direct exchange interaction between adjacent Ca atoms. In the considered systems, the existence of magnetization in the absence of any transition-metal elements was an example of valuable d0 magnetism title.

The effects of defects in CO2 diffusion through carbon nanotubes

The Letter concerns an investigation of CO2 flow through narrow carbon nanotubes (CNTs) at room temperature and the effects of CNT defects on CO2 diffusion. molecular dynamics (MD) simulations have been carried out for (6,6) and (8,8) CNTs, with Carbonyl groups being used as defects in the nano-structures. Different loadings have been investigated in connection with the transition mechanisms. The results show that both the different degrees of loadings and the presence of defects influence the diffusion mechanisms of CO2 through narrow CNTs.

Screening of ions in carbon and gold nanotubes — A theoretical study

We examine the insertion of alkali ions into narrow nanotubes of graphite and gold by density functional theory. The surrounding tubes screen the ionic charge very effectively; the resulting image energy compensates well for the loss of solvation. In addition, ion–ion interactions are strongly reduced. These effects are stronger for gold than for graphite tubes. Our results explain at an atomic level, why narrow tubes store charge so effectively.

Enhanced flow of core-softened fluids through narrow nanotubes.

We investigate through non-equilibrium molecular dynamic simulations the flow of anomalous fluids inside rigid nanotubes. Our results reveal an anomalous increase of the overall mass flux for nanotubes with sufficiently smaller radii. This is explained in terms of a transition from a single-file type of flow to the movement of an ordered-like fluid as the nanotube radius increases. The occurrence of a global minimum in the mass flux at this transition reflects the competition between the two characteristic length scales of the core-softened potential. Moreover, by increasing further the radius, another substantial change in the flow behavior, which becomes more evident at low temperatures, leads to a local minimum in the overall mass flux. Microscopically, this second transition is originated by the formation of a double-layer of flowing particles in the confined nanotube space. These nano-fluidic features give insights about the behavior of confined isotropic anomalous fluids.

Submerged arc discharge technique to explore novel non-carbon nanotubes: Syntheses of nanotubes from ZnO and BaTiO3

A unique reaction field using arc discharge in water can create novel nanostructures, where an extreme temperature drop is observed in bubbles around a hot arc plasma zone. Here, a Mo anode had a hole at its tip, into which ZnO or BaTiO3 powder was stuffed. The cathode received film products on its surface. It was revealed that BaTiO3 nanotubes were firstly synthesized as novel nanotubes when BaTiO3 powder was stuffed in the anode hole. When ZnO powder was stuffed in the anode hole, narrow ZnO nanotubes were synthesized. The diameter of the ZnO nanotubes synthesized was the smallest ever reported.

Arm Retraction Dynamics and Bistability of a Three-Arm Star Polymer in a Nanopore

Using molecular dynamics and Monte Carlo simulations as well as analytical considerations, we study the arm-retraction dynamics of a three-arm star polymer in a narrow nanotube as a function of arm length, N, and tube diameter, D. The system dynamics is analyzed and compared to the bistable collective behavior of a pair of polymer chains tethered in a nanopore. The bistability arises from alternate flipping of one arm of the star into the pore section occupied to that moment by a single arm only. We derive analytical expressions for the free-energy landscape of an arm flip and determine the barrier height as a function of N and D. In the related case of two chains in a narrow tube, we demonstrate that correlations lead to a bimodal distribution of the chain-end positions whereas in a polymer brush, made of equivalent chains at the same grafting density, one observes a single peak only. The residence time distribution between consecutive arm flips is shown to follow a power-exponential relationship, demonstrating good agreement between theory and simulation.

Deterministic modelling of carbon nanotube near-infrared solar cells

We investigate the optimal design of carbon nanotube near-IR solar cells, revealing a narrow density, thickness, and nanotube orientation space of maximum efficiency.

Toward an Accurate Description of Methane Physisorption on Carbon Nanotubes

We present a high-level ab initio investigation of the effects of model size and curvature on the exterior and interior binding energy of methane on single-walled carbon nanotubes. The interaction energies of methane with curved coronene were computed using complete basis set MP2 with a CCSD(T) correction. A variety of novel dispersion-including density functional approaches were then compared to the benchmark data. The top-performing functionals were used to calculate binding energies between methane and larger nanotube fragments all the way through infinite nanotubes. The methane binding energy of a narrow (9,0) carbon nanotube, compared to a flat graphene surface, is decreased by 32% on the exterior but increased by 185% on the interior.