Carbon nanotubes in benzene: internal and external solvation

The structure and dynamics of benzene inside and outside of single-walled carbon nanotubes (SWNTs) in the (n,n) armchair configuration are studied via molecular dynamics computer simulations. Irrespective of the nanotube diameter, benzene molecules form cylindrical solvation shell structures on the outside of the nanotubes. Their molecular planes near the SWNTs in the first external solvation shell are oriented parallel to the nanotube surface, forming a π-stacked structure between the two. By contrast, the benzene distributions in the interior of the SWNTs are found to vary markedly with the nanotube diameter. In the case of the (7,7) and (8,8) nanotubes, internal benzene forms a single-file distribution, either in a vertex-to-vertex (n = 7) or face-to-face (n = 8) orientation between two neighboring molecules. Inside a slightly wider (9,9) nanotube channel, however, a cylindrical single-shell distribution of benzene arises. A secondary solvation structure, which begins to appear inside (10,10), develops into a full structure separate from the first internal solvation shell in (12,12). The ring orientation of internal benzene is generally parallel to the nanotube wall for n = 9-12, while it becomes either slanted with respect to (n = 7), or perpendicular to (n = 8), the nanotube axis. The confinement inside the small nanotube pores exerts a strong influence on the dynamics of benzene. Both translational and rotational dynamics inside SWNTs are slower and more anisotropic than in liquid benzene. It is also found that reorientational dynamics of internal benzene deviate dramatically from the rotational diffusion regime and change substantially with the nanotube diameter.     

以荷电化碳纳米管构筑正渗透膜用于海水淡化的分子动力学模拟

受传统膜科学中分离膜的荷电化可提升膜盐水分离效能的启发,在前期工作基础上尝试以荷电化碳纳米管CNT(8,8)为水通道仿生构筑正渗透膜,利用分子动力学模拟的方法研究水分子在膜中的传递行为.模拟中,以0.5mo·lL-1氯化钠溶液模拟海水,1mo·lL-1的氯化镁溶液为汲取液,考察不同电量电荷修饰对碳纳米管正渗透膜中水分子密度分布、扩散系数以及水通量的影响.结果显示,电荷修饰对碳纳米管中水分子的密度分布和扩散速率以及水通量影响较显著,当碳纳米管管口荷电量为-0.3e时,碳纳米管膜可获得最大水通量.     

Water-soluble full-length single-wall carbon nanotube polyelectrolytes: preparation and characterization

HiPco single-wall carbon nanotubes (SWNTs) have been noncovalently modified with ionic pyrene and naphthalene derivatives to prepare water-soluble SWNT polyelectrolytes (SWNT-PEs), which are analogous to polyanions and polycations. The modified nanotubes have been characterized with UV-vis-NIR, fluorescence, Raman and X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The nanotube-adsorbate interactions consist of pi-pi stacking interactions between the aromatic core of the adsorbate and the nanotube surface and specific contributions because of the substituents. The interaction between nanotubes and adsorbates also involves charge transfer from adsorbates to SWNTs, and with naphthalene sulfonates the role of a free amino group was important. The ionic surface charge density of the modified SWNTs is constant and probably controlled by electrostatic repulsion between like charges. The linear ionic charge density of the modified SWNTs is similar to that of common highly charged polyelectrolytes.     

Aqueous dispersion, surface thiolation, and direct self-assembly of carbon nanotubes on gold

We report the efficient aqueous dispersion of pristine HiPco single-walled carbon nanotubes (SWNTs) with ionic liquid (IL)-based surfactants 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), the thiolation of nanotube sidewalls with 2, and the controlled self-assembly of positively charged SWNT-1,2 composites on gold. Optical absorption spectra and resonance Raman (RR) data of obtained aqueous SWNT-1,2 dispersions are consistent with debundled and noncovalently functionalized nanotubes whose electronic properties have not been disturbed. Additionally, the dispersion of pristine nanotube material with surfactants 1 and 2 leads to a high degree of purification from carbonaceous particles. The chiralities of the 14 smallest semiconducting HiPco SWNTs in resonance with Raman excitation at 1064 nm (1.165 eV) were determined in SWNT-2 aqueous dispersion using UV-vis-NIR and RR spectra. X-ray photoelectron spectroscopy (XPS) and surface-enhanced resonance Raman scattering (SERRS) spectroscopy of SWNT-2 submonolayers on gold verified the encapsulation of individualized SWNTs with IL surfactants, the cleavage of S-S disulfide bonds formed in aqueous SWNT-2 suspensions, and the direct chemisorption of the SWNT-2 composite on bare gold via the Au-S bond. Aqueous dispersions of SWNTs with IL-based surfactants add biofunctionality to carbon nanotubes by imparting the positive surface charge necessary for interactions with cell membranes. Our technique, which purifies pristine nanotube material and produces water-soluble, positively charged nanotubes with pendent surface-active thiol groups, may also be translated to other carbon nanotubes and carbon nanostructures. Self-assembled, positively charged submonolayers of SWNTs can be further used for applications in cell biology and sensor technology.     

Encapsulation of carbon chain molecules in single-walled carbon nanotubes

The vacuum space inside carbon nanotubes offers interesting possibilities for the inclusion, transportation, and functionalization of foreign molecules. Using first-principles density functional calculations, we show that linear carbon-based chain molecules, namely, polyynes (C(m)H(2), m = 4, 6, 10) and the dehydrogenated forms C(10)H and C(10), as well as hexane (C(6)H(14)), can be spontaneously encapsulated in open-ended single-walled carbon nanotubes (SWNTs) with edges that have dangling bonds or that are terminated with hydrogen atoms, as if they were drawn into a vacuum cleaner. The energy gains when C(10)H(2), C(10)H, C(10), C(6)H(2), C(4)H(2), and C(6)H(14) are encapsulated inside a (10,0) zigzag-shaped SWNT are 1.48, 2.04, 2.18, 1.05, 0.55, and 1.48 eV, respectively. When these molecules come inside a much wider (10,10) armchair SWNT along the tube axis, they experience neither an energy gain nor an energy barrier. They experience an energy gain when they approach the tube walls inside. Three hexane molecules can be encapsulated parallel to each other (i.e., nested) inside a (10,10) SWNT, and their energy gain is 1.98 eV. Three hexane molecules can exhibit a rotary motion. One reason for the stability of carbon chain molecules inside SWNTs is the large area of weak wave function overlap. Another reason concerns molecular dependence, that is, the quadrupole-quadrupole interaction in the case of the polyynes and electron charge transfer from the SWNT in the case of the dehydrogenated forms. The very flat potential surface inside an SWNT suggests that friction is quite low, and the space inside SWNTs serves as an ideal environment for the molecular transport of carbon chain molecules. The present theoretical results are certainly consistent with recent experimental results. Moreover, the encapsulation of C(10) makes an SWNT a (purely carbon-made) p-type acceptor. Another interesting possibility associated with the present system is the direction-controlled transport of C(10)H inside an SWNT under an external field. Because C(10)H has an electric dipole moment, it is expected to move under a gradient electric field. Finally, we derive the entropies of linear chain molecules inside and outside an open-ended SWNT to discuss the stability of including linear chain molecules inside an SWNT at finite temperatures.     

Surfactant‐Dependent Charge Transfer between Polyoxometalates and Single‐Walled Carbon Nanotubes: A Fluorescence Spectroscopic Study

Hybridizations of redox‐active polyoxometalates (POMs) with single‐walled carbon nanotubes (SWNTs) have been widely investigated for their diverse applications. For the purpose of constructing high‐quality electronic devices, controlling charge transfer within POM/SWNT hybrids is an inevitable issue. As determined by means of fluorescence spectroscopy, electron transfer between SWNTs and a common POM dopant, phosphomolybdic acid (PMo₁₂), can be tuned simply by an alteration of nanotube surfactant type from anionic to nonionic. The mechanism is attributed to the influence of surfactant type on the stabilization of the electron donor–acceptor hybrid and effect of surfactant–nanotube interactions. These results will be important to control charge‐transport behavior in nanohybrids consisting of carbon nanotubes.     

Low-temperature growth of single-walled carbon nanotubes by water plasma chemical vapor deposition

Preferential growth of pure single-walled carbon nanotubes (SWNTs) over multi-walled carbon nanotubes (MWNTs) was demonstrated at low temperature by water plasma chemical vapor deposition. Water plasma lowered the growth temperature down to 450 degrees C, and the grown nanotubes were single-walled without carbonaceous impurities and MWNTs. The preferential growth of pure SWNTs over MWNTs was proven with micro-Raman spectroscopy, high-resolution transmission electron microscopy, and electrical characterization of the grown nanotube networks.     

Electromodulated molecular transport in gold-nanotube membranes

We have developed a new class of synthetic membranes that contain monodisperse Au nanotubes with inside diameters of molecular dimensions (<1 nm). The Au nanotubes span the complete thickness of the membrane and can act as conduits for molecule and ion transport between solutions placed on either side of the membrane. We have recently become interested in the concept of electromodulating neutral molecule transport across these membranes. This communication describes a novel approach for accomplishing this objective. This approach makes use of an anionic surfactant which, when a positive potential is applied to the Au nanotube membrane, partitions into the nanotubes to charge the solution side of the electrical double layer at the tube walls. Because of the hydrophobic tail of the surfactant, this renders the nanotube interior hydrophobic, and the membrane now preferentially extracts and transports neutral hydrophobic molecules. Because the anionic surfactant can be expelled from the nanotubes by applying a negative potential, this provides a route for reversibly electromodulating neutral molecule transport in these membranes.     

Effects of electric fields on proton transport through water chains

Molecular dynamics simulations on quantum energy surfaces are carried out to study the effects of perturbing electric fields on proton transport (PT) in protonated water chains. As an idealized model of a hydrophobic cavity in the interior of a protein the water molecules are confined into a carbon nanotube (CNT). The water chain connects a hydrated hydronium ion (H3O+) at one end of the CNT and an imidazole molecule at the other end. Without perturbing electric fields PT from the hydronium proton donor to the imidazole acceptor occurs on a picosecond time scale. External perturbations to PT are created by electric fields of varying intensities, normal to the CNT axis, generated by a neutral pair of charges on the nanotube wall. For fields above approximately 0.5 VA, the hydronium ion is effectively trapped at the CNT center, and PT blocked. Fields of comparable strength are generated inside proteins by nearby polar/charged amino acids. At lower fields the system displays a rich dynamic behavior, where the excess charge shuttles back and forth along the water chain before reaching the acceptor group on the picosecond time scale. The effects of the perturbing field on the proton movement are analyzed in terms of structural and dynamic properties of the water chain. The implications of these observations on PT in biomolecular systems and its control by external perturbing fields are discussed.     

Electric-field-enhanced assembly of single-walled carbon nanotubes on a solid surface

We report a novel electric-field-enhanced chemical assembly approach for fabricating highly aligned SWNT arrays on a solid surface with remarkably improved efficiency and packing density, which is very important for the real applications of carbon nanotube arrays. With the enhancement of the electric field, the assembling kinetics of SWNTs is remarkably speeded up to effectively decrease the assembling time, and the packing density can even exceed the saturated density of conventional assembly method by four times within only half an hour. The molecular dynamics simulation results illustrated the alignment of SWNTs with their long axes along the electric flux in solution, leading to the increase of packing density and efficiency through overcoming the steric hindrance of the "giant" carbon nanotubes.     

水中受拉伸负载下单壁纳米碳管机械性质的分子动力学研究

利用分子动力学方法研究了(5,5)扶手椅型和(10,10)锯齿型纳米碳管在水中受拉伸负载下的机械性质.通过计算纳米碳管中氧和氢原子的局部密度分布研究了限制效应.结果表明,碳管在水中的杨式系数与在真空下相同,而碳管在水中的拉伸应力小于在真空中的.     

Investigating detailed mechanism of hydrogen molecules adsorbing on single-wall carbon nanotubes using fitted force field parameters containing carbon–carbon interactions

The nonbonded and bonded force field parameters for carbon atoms in single-wall carbon nanotubes (SWNT) are fitted by means of quantum chemistry calculations with considering the periodic boundary conditions. The nonbonded parameters between carbon atoms and hydrogen atoms are fitted as well. All the fitted parameters are verified by comparing to quantum chemistry results and by calculating Young's modulus. Adsorption of Hydrogen molecules are then carried out on a bundle of self-assembled SWNTs. The adsorption isotherms are consistent to the Freundlich equation. Both hydrogen molecules adsorbed outside and inside the SWNTs are counted. According to our result, hydrogen molecules adsorbed inside the SWNTs are more stable at a relatively high temperature and are playing an important part in total amount of the adsorbed molecules. While C(10,10) have the highest adsorption capacities in most of the temperatures, hydrogen molecules inside C(5,5) are the most stable of all the four kinds of SWNTs. Thus, balancing adsorption capacities and strength of interaction can be important in choosing SWNT for gas adsorption. Besides, we deduct an equation that can describe the relation between hydrogen pressure and amount of SWNTs based on our simulation results. The hydrogen pressure may decrease by adding SWNTs in the system. The fitting method in our system is valid to SWNTs and can be tested in further studies of similar systems. © 2018 Wiley Periodicals, Inc.     

DNA functionalization of carbon nanotubes for ultrathin atomic layer deposition of high kappa dielectrics for nanotube transistors with 60 mV/decade switching

For single-walled carbon nanotube (SWNT) field effect transistors, vertical scaling of high kappa dielectrics by atomic layer deposition (ALD) currently stands at approximately 8 nm with a subthreshold swing S approximately 70-90 mV/decade at room temperature. ALD on as-grown pristine SWNTs is incapable of producing a uniform and conformal dielectric layer due to the lack of functional groups on nanotubes and because nucleation of an oxide dielectric layer in the ALD process hinges upon covalent chemisorption on reactive groups on surfaces. Here, we show that by noncovalent functionalization of SWNTs with poly-T DNA molecules (dT40-DNA), one can impart functional groups of sufficient density and stability for uniform and conformal ALD of high kappa dielectrics on SWNTs with thickness down to 2-3 nm. This enables approaching the ultimate vertical scaling limit of nanotube FETs and reliably achieving S approximately 60 mV/decade at room temperature, and S approximately 50 mV/decade in the band-to-band tunneling regime of ambipolar transport. We have also carried out microscopy investigations to understand ALD processes on SWNTs with and without DNA functionalization.     

Controlling photochromism between fluoroalkyl end-capped oligomer/polyaniline and N,N'-diphenyl-1,4-phenylenediamine nanocomposites induced by UV-light-responsive titanium oxide nanoparticles

Debundling and selective dispersion of semiconducting single-walled carbon nanotubes (SWNTs) has been demonstrated using a neutral pH water soluble chitosan derivative, N-acetylated chitosan (NACHI), which is synthesized by controlled N-acetylation of chitosan using acetic anhydride. The SWNT-NACHI supernatant solution demonstrated semiconductor-enriched property owing to the preferential adsorption of N-groups of the NACHI on semiconducting nanotubes with a fairly weak charge transfer. The dispersion of nearly individualized SWNTs achieved by surface modification of nanotubes with a biocompatible polymer can be utilized for electronic and biomedical applications such as field effect transistor, biosensor, cell culture medium and SWNT-biomacromolecule hybrid materials.     

Density, distribution, and orientation of water molecules inside and outside carbon nanotubes

The behavior of water molecules inside and outside 1.1, 2.8, 6.9, and 10.4 nm diameter armchair carbon nanotubes (CNTs) is predicted using molecular dynamics simulations. The effects of CNT diameter on mass density, molecular distribution, and molecular orientation are identified for both the confined and unconfined fluids. Within 1 nm of the CNT surface, unconfined water molecules assume a spatially varying density profile. The molecules distribute nonuniformly around the carbon surface and have preferred orientations. The behavior of the unconfined water molecules is invariant with CNT diameter. The behavior of the confined water, however, can be correlated to tube diameter. Inside the 10.4 nm CNT, the molecular behavior is indistinguishable from that of the unconfined fluid. Within the smaller CNTs, surface curvature effects reduce the equilibrium water density and force water molecules away from the surface. This effect changes both the molecular distribution and preferred molecular orientations.     

Redox modulation of electroosmotic flow in a carbon nanotube membrane

Electroosmotic flow (EOF) is widely used to manipulate solutions in capillaries and microfluidic devices, and more recently in the nanotubes of a carbon nanotube membrane. In all of these applications it is important to control both the rate and direction of EOF through the system, independently of the electric field that drives EOF. For this reason, there has been considerable recent effort devoted to developing ways of modulating the rate and direction of EOF. We describe here a new method, and we use the carbon nanotube membrane (CNM) system to demonstrate this method. This new method entails coating the inside walls of the carbon nanotubes within the CNM with redox-active polymer films. The redox polymer, poly(vinylferrocene), can be reversibly electrochemically switched between an electrical neutral and a polycationic form. This provides a way for controlling both the magnitude and the sign of the surface charge on the nanotube walls, which in turn allows for control of both the rate and direction of EOF through the CNM.     

Controllable interconnection of single-walled carbon nanotubes under ac electric field

We demonstrated the controllable interconnection of single-walled carbon nanotubes (SWNTs) under alternating current (ac) electric field. The interconnected carbon nanotubes were found to be parallel with the electric flux and increased abruptly with deposition time following a self-accelerating process. Theoretical simulation indicates that the alignment and the interconnection of carbon nanotubes were induced by the dielectrophoresis force and the electric field redistribution at the nanotube apexes.     

Protonation of carbon single-walled nanotubes studied using 13C and 1H-13C cross polarization nuclear magnetic resonance and Raman spectroscopies

The reversible protonation of carbon single-walled nanotubes (SWNTs) in sulfuric acid and Nafion was investigated using solid-state nuclear magnetic resonance (NMR) and Raman spectroscopies. Magic-angle spinning (MAS) was used to obtain high-resolution 13C and 1H-13C cross polarization (CP) NMR spectra. The 13C NMR chemical shifts are reported for bulk SWNTs, H2SO4-treated SWNTs, SWNT-Nafion polymer composites, SWNT-AQ55 polymer composites, and SWNTs in contact with water. Protonation occurs without irreversible oxidation of the nanotube substrate via a charge-transfer process. This is the first report of a chemically induced change in a SWNT 13C resonance brought about by a reversible interaction with an acidic proton, providing additional evidence that carbon nanotubes behave as weak bases. Cross polarization was found to be a powerful technique for providing an additional contrast mechanism for studying nanotubes in contact with other chemical species. The CP studies confirmed polarization transfer from nearby protons to nanotube carbon atoms. The CP technique was also applied to investigate water adsorbed on carbon nanotube surfaces. Finally, the degree of bundling of the SWNTs in Nafion films was probed with the 1H-13C CP-MAS technique.     

Asymmetric deformation density analysis in carbon nanotubes

Electrons and holes as charge carriers appear when a molecular system is exposed to external electric field. For nonsymmetric molecules, the distribution of charge carriers is also nonsymmetric. Although symmetric distribution of charge carriers is expected when the molecular system is symmetric, as the present work shows, they are not symmetric unexpectedly; that is, electrons and holes are not each other's mirror images. In this respect, asymmetric deformation density analysis is introduced to measure the extent of asymmetric distribution of electrons and holes as charge carriers when the symmetric system is exposed to symmetric external potential. Segments of (5,0) carbon nanotubes with different lengths are selected as symmetric systems, and a linear electric field is applied along the principal axis as symmetric potential. Results show that, at high electric fields, electrons tend to localize at the ends, while holes tend to occupy the middle area of carbon nanotube segments. While charge carriers play a vital role in molecular conductivity, asymmetric distribution of electrons and holes in symmetric systems has not yet been reported.     

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.